Cell metabo: a review of the cancer metabolic blueprint

To compete for growth space in the tumor microenvironment (TME) with limited nutrient supply and accumulation of metabolic waste, cunning tumor cells undergo many metabolic adaptive shifts and tend to produce more biomass, which promotes their proliferation
.
As tumors grow and metastasize, even the nutrient distribution and metabolism of the entire body are affected
.
To this end, on March 1 this year, Professor Craig B.
Thompson and his team at Sloan Kettering Cancer Center published a review entitled "The hallmarks of cancer metabolism: Still emerging" in Cell metabolism (IF=31.
373) to present their latest understanding of
cancer metabolism.

summary

Cancer cells are metabolized for the production and proliferation
of biomass.
Due to limited metabolic resources within local tissues, this can lead to nutrient depletion and accumulation
of metabolic waste.
In order to maintain growth under these conditions, cancer cells adapt using a variety of metabolisms, the properties of which are determined
by the physiology of the cells of origin, the characteristics of the lesions, and the tissues in which the cancer cells reside.
In addition, some metabolites not only serve as substrates for energy and biomass production, but also regulate the expression of genes and proteins and influence the behavior
of non-transformed cells near tumors.
As tumors grow and metastasize, they affect the distribution of nutrients in the body and are also affected
by them.
In this paper, recent research advances are incorporated into a conceptual framework that may help guide further research efforts
to explore cancer cell metabolism.

Brief introduction

Single-celled organisms determine biomass and proliferation
based on the availability and quality of nutrient sources in their environment.
In contrast, the growth and proliferation of individual cells in multicellular organisms is regulated
in a non-cellularly autonomous manner through a combination of tissue-specific soluble growth factors and biophysical cues.
The integration of these signaling inputs enables cells to regulate the input of essential nutrients and use these nutrients to produce the necessary biomass components
.
In normal cells, the extent and duration of these signals are limited
by the tissue's homeostatic needs.
Conversely, transformed cells accumulate selective and epigenetic alterations that allow them to be in a constant "on" state by maintaining the main pro-survival and proliferative signals, thereby evading tissue control
over their proliferation.
This, in turn, leads to the establishment of a persistent pro-anabolic state in the affected cells and allows the transformed cells to aggregate uncontrollably and tumor expansion
to occur.

In quiescent cells, the input nutrients are mainly used to produce energy
.
This process involves the gradual oxidation
of carbon atoms from a nutrient source.
Electrons extracted from these oxidation reactions are deposited onto the electron carrier molecules NAD+ and FAD to generate NADH and FADH2, which are then delivered to individual components of the mitochondrial electron transport chain (ETC) and finally to molecular oxygen
.
Electrons continuously pass through the ETC to maintain the production of adenosine triphosphate (ATP) while also regenerating NAD+ and FAD vectors
.
Instead, cells signaled to proliferate not only take in more nutrients from their surroundings, but also tune their metabolic network to use the rest of the nutrients for production and accumulation of biomass
.

While proliferative signals themselves can remain "on" in transformed cells indefinitely, the need for nutrients for continued tumor expansion will eventually exceed the nutrient resources
available to local tissues.
Depletion of nutrients in the extracellular tumor microenvironment (TME) is often accompanied by elevated
levels of anabolic byproducts.
To sustain themselves, transformed cells rely on a range of different metabolic adaptations to break through the constraints
on nutrient supply.
As a result, some cancer cells engulf and break down extracellular macromolecules, or even whole cells, as a source of
nutrient deficiency.
In addition, some types of transformed cells adapt to lower levels of nitrogenous nutrient consumption in their environment (which are in high demand in proliferating cells due to nitrogen's role in building proteins and nucleic
acids) by utilizing free ammonium and remodeling metabolism to preferentially use nitrogen donors for nucleotide and non-essential amino acid synthesis.

In addition to structural components, some critical biosynthetic reactions require a reducing force source in the form of an electron donor, known as NADPH.

NADPH's regeneration from NADP+ comes from the oxidation of carbon atoms, a special set of reactions
embedded in central carbon metabolism.
In addition to NADPH, several metabolic responses critical to tumor cell survival and/or growth are correspondingly driven
by the oxidative or reducing ability provided by NAD+ or NADH cofactors.
In anabolically active cells, NADH/NAD+ redox pairs are not only used to promote oxidative phosphorylation but also to support biosynthesis, especially in glycolysis and tricarboxylic acid (TCA) cycles, where NAD+ is needed as an electron acceptor to maintain the flux
of the pathway.
Therefore, the balance between these reduced and oxidized cofactors is crucial
.
In fact, although NADH production is favored by the increase in the influx of oxidizable carbon into cells due to the activation of pro-growth signals, excessively high a NADH/NAD+ ratio may interfere with those reactions
that require NAD+.
In particular, the excess supply of NADH may exceed the NAD+ regenerative capacity of the ETC, especially when
the oxygen that acts as the final electron acceptor becomes limited as the tumor grows.
Therefore, transformed cells must rely on strong oxidative stress defense mechanisms to mitigate the damage
caused by free electron radicals to cellular structures.

While the cell's input and utilization of nutrients is controlled by signaling pathways, it is already clear that some metabolic intermediates are themselves effective signal modulators
.
Therefore, changes in the levels of certain metabolites can cause comprehensive changes in cellular genes and protein expression by controlling epigenetic changes
in cellular DNA, RNA, and histones, or even directly regulating protein production.
In addition, studies have shown that as tumors grow, changes in the composition of metabolites in their surrounding environment can act as a significant signal that modulates the behavior of various non-transformed cell types in their vicinity, thereby further promoting tumor growth
.
Finally, there is also a growing recognition that tumors not only interact metabolically with the cells in their surrounding environment, but also affect the metabolic economy
of the entire organism.

A century ago, German biochemist Otto Warburg first described the link between
tumorigenesis and metabolic dysregulation.
Over the past 20 years, the field of cancer metabolism has entered a new era
with detailed new understanding of the genetic and epigenetic mechanisms of cell transformation, as well as the development of modern experimental techniques.
Five years ago we came up with several emerging concepts, but there have been some new, paradigm-shifting discoveries in cancer cell metabolism
.
The original concept of metabolic characteristics of cancer has also been extended
.

Glucose and amino acid uptake disorders

Glucose is the main source of
carbon consumed by mammalian cells.
The breakdown of glucose through glycolysis and the TCA cycle not only powers the production of ATP, but also produces carbon intermediates
that support macromolecular biosynthesis.
In mammalian cells, glucose uptake is non-cellularly regulated by growth factor signals and positional cues
.
Therefore, normal cells can only obtain enough glucose to support growth and proliferation
when stimulated by cell type-specific growth factors such as insulin, platelet-derived growth factor (PDGF), or epidermal growth factor (EGF).
Stimulation of growth factors can lead to the activation of downstream signaling events, including the receptor tyrosine kinase (RTK) PI3K-AKT1 (also known as protein kinase B) cascade
.
The RTK-PI3K-AKT1 axis promotes the expression of glucose transporter 1 (GLUT1) and its transfer from the inner cell membrane to the cell surface to promote glucose uptake
.
In addition, activation of AKT1 increases hexokinase activity so that the input glucose can be phosphorylated and captured for glycolysis and downstream metabolic pathways (Figure 1).

In addition to soluble growth factors, inputs from mechanical wire signals, including the biophysical properties of the cell to the extracellular matrix (ECM) and matrix, are also gatekeepers for normal cellular uptake and utilization of glucose
.

As a direct and indirect consequence of oncogenic mutations, cancer cells often manifest as an increased
ability to take up glucose from the extracellular environment.
Amplification of RTK-coding genes such as EGFR, ERBB2, and c-Met
is often observed in human cancers.
Similarly, the oncogenic mutation of PI3K and the genetic deletion of its negative regulators PTEN and INPP4B are often considered to be the driving events
of tumorigenesis.
These genetic alterations focus on the PI3K-AKT1 signaling cascade and allow cells to take up sufficient amounts of glucose independently of the signal input provided by the external signal input to achieve sustained growth and proliferation
.

Transformed cells with an aggressive phenotype increase the energy requirements required by the cells to maintain actin migration, thus further increasing the need
for glucose uptake and breakdown.
In turn, it has recently been found that the degradation of hyaluronic acid, one of the ECM components associated with invasion, increases glucose uptake and up-regulation of glycolysis in cells, which helps to produce sufficient amounts of ATP to support the invasion
of cancer cells.
In addition, a decrease in local cell density itself can increase the expression of the GLUT1 transporter and increase glucose uptake and utilization
.

The shift to anabolism allocates a large portion of the remaining carbon into biosynthetic
pathways.
Thus, glucose-6-phosphate, an intermediate product of glycolysis, can be transferred directly to the pentose phosphate pathway (PPP) to support nucleotide biosynthesis
.
In this process, the glucose backbone is gradually oxidized, enabling cells to regenerate a reduced form of NADPH from its oxidized form (NADP+) to support reducing metabolic reactions in cellular fluids, such as those
required for fatty acid biosynthesis.
Other downstream glycolytic intermediates can also be used as precursors for anabolism
.
These include fructose-6-phosphate, which produces glucose-amino-6-phosphate, which is a component of the synthesis of glucosaminoglycans, and dihydroxyacetone phosphate (DHAP, which produces glycerol and is also the backbone of diglycerides and triglycerides).

In addition, glyceryl 3-phosphate produced in the glycolytic pathway can be transferred to serine biosynthesis by a reaction catalyzed by phosphoglyceril ester dehydrogenase (PHGDH) (Figure 1).

Cancer cells need not only serine as a building block of proteins, but also phospholipids containing phosphatidylserine to assemble cell membranes and serve as carbon donors for nucleotide production, as well as an electron source
for mitochondrial NADPH production.
Therefore, genome amplification of PHGDH is often observed in human malignancies, including breast cancer and melanoma, which are needed to promote tumorigenesis
.

In addition to glucose, the need for anabolism increases glutamine input
.
Specifically, proliferative stimuli trigger upregulation
of the glutamine transporters ASCT2/SLC1A5 and SN2/SLC38A5 in plasma.
The expression of these two transporters is positively regulated by the proliferation-driven transcription factor c-myc; In addition, ASCT2 expression was also promoted
by another proliferation-related transcription factor, E2F-3.
In addition, EGFR signaling also contributes to the plasma membrane localization
of ASCT2 proteins.
In addition to being involved in protein synthesis, glutamine also plays a variety of anabolic roles
in cells.
Specifically, a variety of cytosol-localized biosynthases utilize the amide group of glutamine to promote the production of nucleotides, aminohexose units, and asparagine, while the α-amine group of glutamine is bound to other de novo non-essential amino acids
.
Glutamine can also be broken down in mitochondria by glutaminase 1 (GLS1), which is also positively regulated by c-myc (Figure 1).

GLS1 is essential for providing a mitochondrial source of α-ketoglutarate for the TCA cycle; In addition, GLS1 and its cytosol-based counterpart, GLS2, are involved in strengthening cells' oxidative stress defenses
by providing glutamate for glutathione production.
In addition to supporting biosynthesis and bioenergy, intracellular glutamine output also facilitates extracellular uptake of some essential amino acids
via SLC5A7/SLC3A2 heterodimer bidirectional transporters.

Utilizes central carbon metabolism to support biosynthesis

The TCA cycle is traditionally thought of as a catabolic process in which a carbon substrate is oxidized to produce energy
.
In fact, as recent quantitative analyses of glucose outcomes in vivo have revealed, in most adult tissues, the main TCA circulating substrate that is oxidized to support oxidative phosphorylation is lactic acid, which is found in about 1 mM
in extracellular fluid.
Metabolically active cells also use intermediates of the TCA cycle as precursors for the synthesis of macromolecules (Figure 2A).

Partially stimulated by increased levels of glucose uptake and intracellular pyruvate synthesis, the resulting carbon surplus allows the intermediates of the TCA cycle to leave the center of carbon metabolism and are consumed
in various biosynthetic reactions.
This, in turn, not only provides a new source of biomass, but also allows metabolically active cells to avoid producing too much NADH beyond the ability of
ETC to convert it to NAD+.
Therefore, the ability to release some TCA carbon to support biosynthesis can be considered as a necessity for
cells to receive pro-growth signaling stimulation and thus experience excess oxidizable nutrient metabolism.

In cells where anabolism occurs, citrate produced in the TCA cycle can be exported from mitochondria and used to produce building blocks
that synthesize fatty acids and cholesterol in the cytoplasm.
Since lipids are the main components of cell membranes, activation by lipid-derived transcription programs coordinated by SREBP1 transcription factor is also a key part
of the cell-promoting anabolic program coordinated by mTORC1 activation.
In addition, in the ATP-citrase (ACLY)-catalyzed reaction, citrate is converted to oxaloacetate and acetyl CoA, which are the initiation steps in de novo synthesis of fatty acyl chains and cholesterol and are directly regulated
by AKT1.
In addition, the most important fatty acid synthase, fatty acid synthase (FASN), is often upregulated and plays a crucial role
in the development of tumors.

Fat production is a highly NADPH-consuming process, for example, 14 molecules of NADPH
are required to make one molecule of palmitic acid.
To help balance these demands with the supply of cellular NADPH, in an ALYS-mediated reaction, oxaloacetate produced can be converted to malic acid, which can re-enter mitochondria or be oxidized to pyruvate in a reaction catalyzed by malate 1 (ME1), producing NADPH.

NADPH can also be converted from citrate in cytoplasm to isocitrate and then to α-ketoglutarate production
by cytoplasmic-localized isocitrate dehydrogenase (IDH1).
To emphasize the role of these NADPH-producing reactions in promoting de novo fat synthesis, depletion of ME1 or IDH1 in various cellular environments has been shown to inhibit de novo synthesis of fat and inhibit tumor growth
in vivo.

In addition, in addition to the ability to reconvert NADP+ to NADPH, the total size of the cellular NADPH pool can also be dynamically regulated according to the metabolic needs of the cell (Figure 2B).

Two NAD kinase isotypes, cytoplasmic-localized NADK1 and mitochondria-localized NADK2, enable cells to convert NAD+ to NADP+
in their respective cellular compartments.
NADK1 activity is directly activated by AKT1-mediated phosphorylation, which may allow cells to adjust their ability to carry out NADPH-driven cellular biosynthetic reactions based on PI3K/AKT1-driven signal inputs, such as fat de novo synthesis
.
Similarly, NADK2 was found essential for de novo biosynthesis of NADPH-dependent proline, a reaction that takes place in the mitochondrial region
.

Another TCA circulation intermediate is α-ketoglutarate, which produces glutamate
.
Glutamic acid in turn can act as a precursor to several other non-essential amino acids, including proline
.
It is worth noting that in addition to serving as an outlet for TCA circulating carbon, the production of a molecule of proline also requires the consumption of ATP, NADH and NADPH, which further inhibits the electron load
of mitochondria.
Blocking the output of citrate or the synthesis of proline, supported by the vent hypothesis, increases oxidative stress levels
.
Finally, the TCA intermediate oxaloacetate can also be released from the TCA cycle into the synthesis of aspartic acid, a precursor to pyrimidine bases and asparagine
.

Although glucose uptake can be increased by proliferative stimulation, a smaller portion of this carbon enters the mitochondria for oxidation
.
To regulate pyruvate carbon into the TCA cycle, pyruvate dehydrogenase, the rate-limiting enzyme that converts pyruvate to acetyl CoA, is significantly negatively regulated, both through the allosteric of its products and by inhibitory phosphorylation of four pyruvate dehydrogenase kinases (PDKs
).
Notably, overexpression of PDK1 or depletion of PDH phosphatase (PDP2) can lead to dysregulated PDH activity, which triggers oxidative stress and the initiation
of senescence in KRAS-transformed cells.

As an alternative to oxidation in mitochondria, pyruvate produced by glycolysis can be reduced to lactic acid
in a cytoplasmic lactate dehydrogenase (LDH) driven reaction.
Importantly, the production of lactic acid regenerates an NAD+ equivalent from NADH
.
In addition, because lactic acid can be rapidly balanced with the extracellular space through monocarboxylate transporters (MCTs), this metabolic pathway keeps redox neutral throughout the glycolytic cascade
.
With no NADH consumption or production, glucose-to-lactic acid metabolism completely bypasses the need for ETC and molecular oxygen, while still producing 2 molecules of ATP
.

The metabolism of glucose to lactic acid is also necessary under hypoxic conditions, where the ability of the ETC to unload NADH electrons is limited
by hypoxia.
To this end, HIF1α, as the main coordination factor of hypoxic reactions, can simultaneously upregulate the glucose transporter GLUT1, LDH and PDH inhibitory kinase PDK1 to achieve this process
.

Hypoxia is not the only case where
cells preferentially convert glucose into lactic acid.
In fact, nearly 100 years ago, Otto Warburg discovered that tumors consume more glucose than normal tissue and preferentially convert it to lactic acid rather than oxidizing it in mitochondria (even in the presence of large amounts of oxygen).

Since choosing not to follow the glycolytic carbon oxidation route loses 95 percent of the energy in glucose carbon, Warburg believes that this truncated form of glycolysis reflects the damage caused to mitochondria by carcinogens and hypothesizes that it is the underlying cause of
cancer.
While Warburg's findings have become the basis for the field of cancer metabolism, his own explanation for the effects has since been refuted
.
In fact, most cancer cells continue to oxidize carbon in the mitochondria and require functional ETC to maintain growth
.
In addition, non-transformed cells, including lymphocytes and endothelial cells, also exhibit the Warburg effect when stimulated by proproliferative signals while still retaining the need for oxidative phosphorylation, revealing that the Warburg effect is a universal metabolic strategy
that accompanies cell proliferation.

The adaptive utility of the Warburg effect is still debated
today.
First, aerobic glycolysis allows cells to regenerate ATP faster
than the TCA cycle.
Second, the accumulation of lactic acid and the accompanying acidification of the extracellular environment also play an important role
in establishing a microenvironment conducive to tumorigenesis.
Third, pyruvate is preferentially converted to lactic acid, which not only keeps glucose-derived pyruvate from oxidation in the mitochondria, but also helps reduce the electron load
by regenerating NAD+ directly from NADH.
Fourth, switching to aerobic glycolysis enables cells to increase their ability to produce glycolytic intermediates to safely support the accumulation of biomass without the risk of
excess electrons overwling ETC.

A series of recent studies has challenged
the long-held view that lactic acid is a byproduct of tumor metabolism.
In fact, tracking the transformation of injected 13C isotope-labeled glucose and lactate in cancer patients and genetically engineered mouse tumor models, it was found that circulating lactic acid is an important contributor to
the tumor TCA cycle.
When patients with lung and brain tumors received systemic infusion of 13C isotope-labeled glucose, it was found that glucose-derived 13C carbons contributed more to lactic acid and citric acid, a circulating intermediate of TCA, in tumors than they contributed to upstream glycolytic intermediates, such as glycerides
3-phosphate.
This pattern of labeling is consistent with these tumor types, not only directly utilizing glucose, but also absorbing and metabolizing lactic acid
produced elsewhere in the body.
In addition, when a similar infusion was performed with 13C-labeled lactic acid, it was found that the carbon of lactic acid was involved in the intermediatees
of the TCA cycle in both tumor and non-transformed tissues.
Assessing the extent to which lacate-derived carbon contributes to the TCA cycle in various tissues remains a hotly debated topic; In addition, the applicability of the lactate paradigm in different tumor types, as well as the properties of cells and tissue types that produce and consume lactic acid, are still under active research
.

Given the prevalence of MCT1 transporters that transport lactic acid, these latest findings position the role of lactic acid in the systemic carbon economy as a fixed source of oxidable carbon and maintain equilibrium
at the body level.
Instead, glucose uptake is a highly controlled event that is closely related
to the proliferative state of cells.
The source of oxidable carbon provided by extracellular lactate may be particularly appropriate
when considering the glucose deprivation microenvironment in advanced tumors.
In this case, the utilization of lactic acid, along with potentially other non-glucose sources of oxidable carbon, while increasing the ETC-mediated load of NAD+ regeneration from NADH, provides cells with a key advantage of allocating the limited glucose supply to biomass production
.
While the range of heterogeneity in the selection of carbon sources in tumor cells is still under active research, these studies highlight the interrelated metabolic trade-offs
that cells must contend with when performing anabolic procedures in the challenging environment of tumor metabolism.

Use opportunism for nourishment

In addition to the main low-molecular-weight nutrients, namely glucose and amino acids, cancer cells are able to take advantage of a wide range of alternative nutrient sources whose needs can be driven
by specific metabolic environments.
Autophagy involves capture and lysosome-mediated degradation
of intracellular proteins or entire intracellular structures such as ribosomes, mitochondria, and parts of the endoplasmic reticulum [ER].
When the uptake of low molecular weight metabolic substrates is affected, the nutrients provided through autophagy are critical
to cell survival.
In addition to serving as an emergency nutrient supply, organelle-specific autophagy can help clear damaged intracellular structures and even alter biophysical properties
inside cells by reducing molecular aggregation.

Despite the immense and multi-layered importance of autophagy in cell physiology, one thing that autophagy cannot do is to provide enough material for cell proliferation to produce new biomass
.
However, mammalian cells have also been found to use their lysosomes to indiscriminately digest macromolecular material
of extracellular origin in a single-celled eukaryotic manner.
This is achieved through the extension of the membrane protrusion and the formation of macropinosomes, a process known as macropinocytosis
.
Macromolecules engulfed in this way are then transported to lysosomes for degradation
.
PI3K, located downstream of growth factor signaling, is required
to activate membrane ruffling and macropodium shutdown.
Genetic alterations observed in cancer, such as the cancer-mutated form of Ras GTPases, increase the rate and volume of macropinocytosis without being
associated with stimulation of growth factors.
Thus, cancer cells carrying carcinogenic Ras mutations have shown an enhanced ability to utilize extracellular proteins through macropinostosis, enabling them to maintain cell survival and proliferation in amino acid-depleted TMEs (Figure 3B).

Internalized macrosisomes are additionally regulated because they send "cargo" to lysosomes for degradation
.
The active mTORC1 complex inhibits the breakdown of extracellular proteins as a source of amino acids; Correspondingly, inhibiting the activity of mTORC1 in the face of nutrient depletion is beneficial for Ras transformed cells to use extracellular white protein to maintain survival and growth
under the condition of amino acid depletion.
Consistently, when free amino acids are abundant, activation of mTORC1 by AKT1 inhibits the utilization of internalized albumin (Figure 3A).

In contrast, small GTPases Rac1 and phospholipase C (PLC) downstream of PI3K activity promote cell growth, which relies on the breakdown of extracellular proteins
through macropinostomiasis.

Although the use of serum albumin as an amino acid source has been studied most for macropinostasis, other extracellular macromolecules can also be absorbed
by macropinostone.
Pancreatic ductal adenocarcinoma (PDAC) cells are normally present in a pro-fibrotic microenvironment
containing a dense collagen network.
PDAC cells have been reported to be able to utilize extracellular collagen
in part through macropinocytosis under conditions of limited glucose or glutamate.
Proline is a major amino acid component of
collagen.
Thus, collagen internalized in PDAC cells can provide cells with a source of proline, which can be further broken down by proline dehydrogenase (PRODH) as an electron source for energy production and a TCA cycle substrate
.
In addition to the utilization of proteins, macropinocystis has also been found to be involved in the removal of other extracellular components such as exosomes and lysophospholipids
.

It is worth noting that under hypoxic conditions, the clearance of extracellular lipids becomes higher, which helps restore the balance
of saturated (SFA) and monounsaturated fatty acids (MUFA) in the cell membrane.
In fact, the introduction of double bonds into the acyl chain, which converts the SFA molecule into the MUFA counterpart, is driven by stearoyl-CoA desaturase 1 (SCD1), which requires molecular oxygen as an electron acceptor
.
The hypoxic condition inhibits SCD1, thereby changing the SFA/UFA ratio in favor of SFA
.
This, in turn, changes the biophysical properties of cell membranes, making them less
fluid and flexible.
ER membranes appear to be particularly affected by SFA/MUFFA imbalances, in part due to the complex
shape of ER membranes compared to other cell membrane structures.
In fact, hypoxia-related SCD1 inhibition was found to trigger ER stress in cells with high activity mTORC1 and therefore high translation load
.
Therefore, when a cell's ability to desaturate fatty acids is compromised by hypoxia, adaptive measures such as the absorption of extracellular lipids or, in other cases, the release of lipids from internal lipid reservoirs can maintain fatty acid balance
.

In addition to the pattern of removing large amounts of nutrients through macropinostomiasis, cells can internalize some macromolecules
from the extracellular environment through selective, receptor-mediated endocytosis.
For example, low-density lipoprotein (LDL) is the main carrier
of extracellular cholesterol.
Mammalian cells capture LDL through LDL receptors on the plasma membrane, which are transported to the endolysosomal region, releasing cholesterol required for cell membrane assembly (Figure 3B).

Abnormal activation of RTK-PI3K-AKT1 signaling in cancer cells was found to upregulate LDL receptors
in part through the activity of mTORC1 and SREBP.
In addition, exogenous cholesterol can enter cells
in its high-density lipoprotein (HDL) form through the scavenger receptor B1 (SCARB1).
Clear cell renal cell carcinoma was found to be nutritionally deficient in cholesterol and relied primarily on SCARB1-mediated HDL input as a source of
cholesterol.
In addition, iron is essential for various metabolic activities, such as heme synthesis groups and biosynthesis
of iron-sulfur clusters.
Endocytosis of transferrin (Tfn) receptors on extracellular siderophore Tfn is a major source of intracellular iron and is essential
in regulating iron-dependent cell iron death.

Finally, cells are able to engulf whole living cells and/or dying cells, a non-phagocytic process known as "entosis" that can be induced by metabolic stress, including glucose deficiency
.
Similarly, PTEN-deficient prostate cancer cells have been reported to remove necrotic cell debris by macropinocytosis in an AMPK-dependent manner (Figure 3C).

Similar to the phagocytosis of soluble proteins, nutrients from phagocytosed live or dead cells can be recycled in the lysosomal digestive system to further support cell proliferation
.

Expanding demand for electron acceptors

Carbon metabolism through glycolysis and the TCA cycle requires NAD+ as an electron acceptor
.
The enhanced glycolytic activity of transformed cells makes them dependent on the continuous regeneration
of NAD+.
The Warburg effect is a distinctive feature
of anabolically active cells when they regenerate NAD+ from NADH through LDH-mediated responses.
Therefore, lactic acid secretion is a cancer cell characteristic
that indicates a high glycolysis rate.
Cytosolic electrons in the form of NADH can also be transported to mitochondria through specialized electron shuttles, such as malic acid-aspartate shunts and glycerophosphate shunts, to facilitate the regeneration of NAD+ in ETC (Figure 4).

Recently, MCART1 (encoded by SLC25A51) has been identified as the NAD+ transporter of mitochondria, whose function is also to transmit NAD+ and NADH
between the cytoplasm and mitochondria.

Dissecting the specific role of mitochondrial respiration in supporting anabolism and cell proliferation, it was found that an essential function of ETC is to regenerate NAD+ to support the biosynthesis
of aspartic acid.
In fact, aspartic acid production may be inhibited by inhibition
of complex I or complex III of ETC.
Conversely, supplementation with electron acceptor substrates, such as pyruvate or α-ketobutyric acid, helps increase the ratio of NAD+/NADH, restores aspartate biosynthesis, and promotes cell proliferation
even in cells where ETC is inhibited.
The availability of asparagine, a product of aspartic acid, is also affected
by ETC inhibitors.
Interestingly, when ETC inhibitors are administered in low doses, asparagine supplementation is sufficient to restore nucleotide synthesis and cell proliferation, but does not "salvage" the abundance of aspartate in cells
.

The electron flux of ETC not only contributes to the regeneration of NAD+ and FAD, but also directly links
the energy production capacity of mitochondria to the biosynthesis of pyrimidine through the activity of dihydroorotic acid dehydrogenase (DHODH).
To this end, ETC complex III regenerates oxidative ubiquinone as a key electron acceptor supporting DHODH function, thereby supporting pyrimidine synthesis in proliferating cells (Figure 4).

In summary, cancer cells in the proliferative phase have a continuously high demand for the regeneration of electron receptors to synthesize precursors
for protein and nucleic acid biosynthesis.

Increased dependence on protective mechanisms of oxidative stress

Dysregulated anabolism, combined with TME-related restrictions, exposes transformed cells to higher levels of reducing stress (e.
g.
, high NADH, low oxygen).

In fact, as proliferating cancer cells enter more carbon sources, the supply of NADH may increase, exceeding the capacity of ETC treatment, especially if
oxygen or ADP are restricted.
Some ways to reduce the NADH/NAD+ ratio to
avoid reductive stress have been outlined above.
Cancer cells are also affected
by oxidative stress.
In the presence of environmental oxidants such as H2O2, NO and even oxygen, macromolecular oxides have the ability to damage components within cells, such as lipids, which in turn can lead to loss of cell integrity
.
Cancer cells rely on multiple antioxidant defense mechanisms, including glutathione (GSH) and thioredoxin (TRX) systems, to protect themselves from this oxidative damage
.
Therefore, genetic and metabolic alterations
favorable to these protective mechanisms are often found in tumors.

The NRF2 (encoded by NFE2L2) transcription factor and its mutated form of its E3 ubiquitin ligase KEAP1 are present in solid tumors and are most prevalent in lung cancer, which is characterized by elevated levels of oxidative stress, in part due to the presence of high levels of extracellular oxygen
in lung tissue.
Both NRF2 and KEAP1 mutations disrupt the binding of KEAP1 to the NRF2 protein, making the latter a target for degradation
.
This, in turn, increases NRF2 protein levels, facilitating NRF2-driven transcription programs that lead to upregulated expression of enzymes involved in GSH biosynthesis (Figure 5).

Whereas GSH increases require more restrictive substrates and cysteine supplies
.
Thus, NRF2-driven transcriptional reactions also increase the expression of xCT (encoded by SLC7A11), prompting the oxidized form of cysteine, cysteine, to enter cells
.
The depletion of intracellular cysteine stores during oxidative stress leads to a decrease in its levels and is sensed
by GCN2 kinase.
GCN2, in turn, triggers the accumulation
of ATF4 (activating transcription factor 4).
The increase of ATF4 further promotes the expression of xCT and promotes the absorption
of cystine.
When extracellular cystine levels are depleted, as is often seen in TMEs, ATF4 can also upregulate enzymes involved in de novo synthesis of cysteine from methionine via the transsulfur pathway (Figure 5).

In addition to upregulating the production of GSH, transformed cells often respond to oxidative stress by altering their metabolism to maintain the antioxidant capacity
of these molecules that scavenge reactive oxygen species (ROS).
A common challenge that cancer cells encounter during tumorigenesis is oxidative stress
caused by ECM detachment.
The study found that activation of receptor tyrosine kinases and the downstream PI3K pathway promotes non-anchored growth by supporting NADPH regeneration from NADP+ by increasing glucose uptake and its utilization in PPP (Figure 5).

RTK-driven tyrosine phosphorylation of IDH1 can also regulate the shuttle
of NADPH between the cytosol and mitochondria.
Studies have reported that tyrosine phosphorylation of IDH1 favors the reducing citric acid of glutamate-derived α-ketoglutarate, thereby depleting cytosolic NADPH.

Citrate is then retrograde transported to mitochondria, where IDH2, the counterpart of IDH1, can convert the input citrate back to α-ketoglutaric acid, thereby regenerating NADPH
in the mitochondrial interval.
It has also been reported that in ETC-damaged cells, IDH2 consumes mitochondrial NADPH to catalyze the reduction of α-ketoglutaric acid to citrate, forming a reverse cycle
.

When transferred cells enter the bloodstream, exposure to a higher oxygen content may further increase the oxidative stress burden
.
One of the adaptations used by circulating tumor cells to oxidative stress is to rely on the tendency of circulating tumor cells to
aggregate.
Thus, clumped cells create a hypoxic pocket in their core section, which, through the accumulation of HIF1α, may limit the oxidative metabolism of carbon in favor of a redox-neutral glycolytic pathway, as well as trigger mitochondrial autophagy to remove oxidatively damaged mitochondria
.

Studies in vivo tumorigenesis models further support the importance of
strong ROS defenses in promoting metastasis.
In the melanoma model, higher levels of oxidative stress were found in circulating tumor cells and from metastatic nodular cells than in subcutaneous tumors; Correspondingly, higher throughput through the one-carbon pathway and increased lactate uptake were found to be metabolic determinants
of successful metastatic growth.
Both of these adaptations contribute to enhancing the cells' NADPH production capacity, either through the one-carbon pathway-mediated NADPH production and by saving glucose carbon for the oxidative branching reaction of PPP (Figure 5).

Unlike these findings, studies from other tumorigenic environments have shown that elevated ROS levels may actually promote metastasis
.
Studies from mouse models of pancreatic cancer have shown that the loss of TIGAR expression inhibits the ability of cells to use PPP as a source of NADPH, increases ROS levels in cells, promotes cell aggressiveness, and promotes colonization
of lung metastases.
Whether differences in the level or location of ROS produced, or whether the cell lineage background itself contributed to these different results, remains to be further elucidated
.

Oxidative stress, particularly oxidative damage to cellular lipid components, can lead to cell iron death
.
Recent studies have revealed complex regulatory patterns of iron death by cellular metabolic activity, including iron storage and release, selenium balance, phospholipid peroxidation, cysteine and GSH availability
.
Interestingly, transport from the lymphatic system was found to facilitate metastasis of melanoma before entering the bloodstream environment, as it gives melanoma cells a better ability to defend against a higher risk of
iron death in an iron- and oxygen-rich blood environment.
This protective effect of transport through the lymphatic environment may have multiple mechanisms, including the production of anti-iron-dead properties of oleic acid and glutathione, which are particularly abundant
in lymph.
The protective effect of oleic acid during iron death is a subject that needs further study, but it may be at least partly due to the fact that oleic acid, as a MUFA, competes with PUFA on cell membranes, thereby reducing PUFA on cell membranes
.
In fact, because PUFA contains multiple double bonds in its chemical structure, it is subject to oxidative damage caused by iron death, which negatively affects
membrane function and cellular integrity and survival.

In preclinical studies, targeting metabolic mechanisms that protect cells from the effects of iron death has emerged as a promising therapeutic intervention
.
For example, blocking SREBP-mediated lipogenesis has been found to work synergistically with interference with GPX4 defense mechanisms to induce iron death
in PI3K-mutated cancer cells.
In addition to GPX4-dependent iron death defense mechanisms, FSP1 has recently been found to produce ubiquinone from ubiquinol as a ROS scavenger, revealing glutathione-independent iron death defense mechanisms
.
Correspondingly, DHODH-mediated ubiquinone synthesis has also been reported as a targetable weakness
in GPX4 low-expression cancer cells.
In summary, the tumorigenic process tends to expose cancer cells to higher levels of oxidative stress, which may be used as a therapeutic site
in the future.

Increased demand for nitrogen

Carbon sources can be used as structural intermediates in biosynthesis or oxidized to generate energy or support NADPH production
.
Conversely, reducing nitrogen can be used in macromolecular synthesis or converted into waste
.
Increased nitrogen demand is specifically linked
to proliferation.
Therefore, the dysplasia of cancer cells preferentially depletes the preferred nitrogen donor to support growth in the microenvironment, most prominently glutamine
.
Therefore, in order to maintain growth and proliferation after exhausting the supply of preferred exogenous reducing nitrogen, cancer cells must rely on adaptations that enable them to use and reuse alternative sources
of nitrogen.

Glutamine is the nitrogen nutrient of choice in animals
.
Given its high demand, glutamine levels are maintained at about 600-700 μM in circulation, almost an order of magnitude
higher than other amino acids.
During injury or systemic infection, circulating glutamine is depleted; In addition, local depletion
of glutamine levels is observed in both solid tumors and healed wounds.
For example, a recent study found free glutamine levels in the interstitial fluid of allografts breast tumors as low as 100 μM
.
However, not all tumors have glutamine-limited glutamine levels
.
For example, in some genetically engineered mouse tumor models, the concentration of glutamine in the tumor interstitial fluid is the same
as in the blood circulation.
It has also been reported that tumors can reproduce glutamine from glutamate and free ammonium through a reaction driven by glutamine synthase (also known as glutamate-ammonia ligase, or GLUL) (Figure 6A).

Some transcription factors involved in tumorigenesis, including c-myc and Yap, can upregulate GLUL expression
in various cellular contexts.
In addition to being regulated at gene expression levels, depletion of glutamine also leads to accumulation of GLUL
at the protein level.
In fact, in genetically engineered mouse models, tumor-specific GLUL deletion has been shown to significantly delay tumor formation
.

Under hypoxic conditions, the demand for glutamine rises further when the level of oxygen molecules is below the level required to sustain the regeneration of electron flow from NADH to NAD+ through ETC
.
In this metabolic situation, glutamine-derived α-ketoglutarate can take advantage of the reversibility of the IDH1-catalyzed reaction, called the reducing carboxylation reaction, to provide cancer cells with a source of citrate to support de novo fat synthesis
.
According to the study, due to phosphorylation of the Y42 and Y391 tyrosine residues of EGFR-driven IDH1, mutant EGFR-driven non-small cell lung cancer cell lines can participate in the reduction
from α-ketoglutaric acid to citric acid even in the presence of oxygen.
While this adaptation increases the need for glutamine, it may also help reduce the reducing stress
generated by the TCA cycle.

Cells have more than one option
for converting glutamate to α-ketoglutarate.
One option is to oxidize glutamate to α-ketoglutarate through a reaction catalyzed by glutamate dehydrogenase (GDH), thereby releasing free ammonium
.
In addition, α-ketoglutarate can be produced by transamination, in which the amino group of glutamic acid is transferred to the ketoacid receptor by transaminases to produce non-essential amino acids such as aspartic acid, serine, and alanine (Figure 6B).

Notably, the study found that breast epithelial cells in the proliferative phase tend to favor the transammonia pathway rather than GDH-driven deammonia pathways, while cells in the quiescent phase tend to choose the GDH pathway
.
This adaptation may allow proliferating cells to maximize their available amino acid reserves to optimally support the synthesis
of non-essential amino acids.

The α-ketoglutarate produced by the TCA cycle can also be reversibly produced by GDH catalyzed reaction to produce glutamate
.
Ammonium often accumulates in TME, and the concentration can even reach 3mM
.
In this case of ammonium accumulation, it was found that cells can use GDH to drive the opposite direction of the reaction, which allows them to produce glutamate and use it to facilitate the synthesis of other non-essential amino acids (Figure 6C).

A characteristic of ammonium scavenging specific to transformed cells carrying specific carcinogenic lesions, which involves the non-classical route of carbamoyl phosphate synthesis, the initial step
of pyrimidine base synthesis.
Proliferating cells synthesize carbamoyl phosphate by a multifunctional CAD enzyme, a reaction that requires glutamine as an amide donor
.
However, non-small cell lung cancer cells carrying the mutated KRAS allele and tumor suppressor LKB1 deletion have been found to express a different carbamoyl phosphate synthase, CPS1
.
It is worth noting that CPS1 produces carbamoyl phosphate in a more economical way than CAD enzymes: CAD requires glutamine as a nitrogen source, while CPS1 can use ammonium ions as an alternative (Figure 6D).

Normally, CPS1 is only expressed in the liver and promotes ammonium clearance
through the urea cycle.
However, in the absence of LKB1, CPS1 is abnormally upregulated and used by cancer cells to perform key synthetic roles
.
Choosing to forgo the "expensive" CAD catalytic reaction in favor of a more economical CPS1 catalytic route allows cells to reduce some of their glutamine requirements and release the remaining glutamine for other biosynthetic needs
.

Some tumor cells can also increase pyrimidine synthesis by reducing the flux of aspartate into the urea cycle (Figure 6E).

Most normal cells can utilize aspartate to initiate the removal of excess reducing nitrogen from the urea cycle, a reaction catalyzed
by ASS1.
However, many solid tumors, including pancreatic, prostate, and melanoma, have lost ASS1 expression, largely due to epigenetic silencing
.
Extracellular environmental acidification associated with hypoxia can further promote the downregulation
of ASS1 in a non-cellularly autonomous manner.
Without consuming aspartate to initiate the removal of excess reducing nitrogen, proliferating cancer cells can use more of the available aspartate for pyrimidine biosynthesis
.
However, ASS1 expression loss comes at a cost because it hinders the ability of cells to regenerate from citrulline to arginine in the urea cycle, making them more susceptible to exogenous arginine depletion
.
Therefore, therapeutic strategies for depleting arginine, such as the use of PEGylated recombinant arginase, as a potential metabolic therapeutic intervention
, are currently being explored.
In addition, ASS1 or other urea cycle enzyme downregulated cancer cells exhibit a typical pyrimidine/purine ratio imbalance, which in turn is associated with a higher mutational load in this cancer subtype, a feature that makes transformed cells more sensitive to immune attack on tumors and thus susceptible to immune checkpoint inhibitor therapy
.

Heterogeneity of metabolic adaptations

Cancer can arise from a variety of tissue and cell types and have significant physiological differences in metabolism; In addition, tumors may also have different combinations of
cancer-causing lesions.
The combination of these factors can help form tumors
with unique metabolic characteristics.
When cancer cells leave the primary tissue and colonize other organs, they may be constrained
by the microenvironment of the organ tissue where they metastasize.
Therefore, the diversity of metabolic signatures associated with cancer is determined by a combination of intrinsic factors of cancer cells and the specifics of the metabolic environment in which they are located (Figure 7).

Some of the cancer-causing lesions carried by cancer cells can directly modulate specific metabolic pathways
in a way that promotes tumorigenesis.
There have been multiple examples, including pseudohypoxic states
caused by HIF1α and HIF2α transcription factor stabilization in VHL-deficient renal cell carcinoma.
In tumors that lose the expression of KEAP1 inhibitory factor, glutamate supply dependent on glutamine hydrolysis is more necessary to facilitate cystine input, and in EGFR-transformed non-small cell lung cancer subtypes, the reductive metabolism of α-ketoglutarate is activated
.
Even in the context of the same tissue origin, the expression of different oncogenes sometimes leads not only to different, but also to the opposite metabolic results
.
For example, c-Met receptor tyrosine kinase-transformed liver tumors were shown to upregulate GLUL and accumulate glutamine, while c-myc-transformed tumors of the same origin were found to deplete glutamine
from their environment.

Tissue of origin is another important factor
that determines how cancer cells participate in their metabolic pathways and support growth.
As mentioned above, the expression of transgenic C-MYC drives liver tumors to consume glutamine; In contrast, c-myc-driven lung tumorigenesis is associated
with GLUL expression and glutamine synthesis.
Similarly, lung tumors driven by the combination of KRAS oncogene and TRP53 inactivation rely on the catabolism of branched-chain amino acids (BCAAs) as nutrients, while pancreatic tumors driven by the same combination of transforming genes do not break down BCAAs
.

Further analysis of human tumors and the corresponding normal tissue transcriptome confirmed evidence
from genetically engineered animal tumor models.
These studies showed that transformed cells retained metabolic gene expression patterns
unique to their primary tissues.
Thus, despite the accumulation of some genetic perturbations, tumors retain the original metabolic features
of their primary tissue.
However, during the conversion process, these pre-existing metabolic properties are reused to support anabolism
.
In some tumors, these pre-existing metabolic phenotypes are genetically amplified
.
For example, PHGDH is a de novo rate-limiting enzyme synthesized by serine, and its amplification often occurs in tumors from basaloid breast epithelial cell subtypes, which are characterized by high expression of PHGDH; Similarly, NARPT, an enzyme that drives de novo synthesis of NAD+, can be found in cancer tissues with high NARPT expression
.
Instead, some metabolic enzymes act as true tumor suppressors
in specific tissues.
For example, loss of expression of the rate-limiting enzyme FBP1 occurs in tumors originating in both the liver and kidneys, both of which have gluconeogenesis
.
Deletion of FBP1 appears to exert a wide range of pro-tumor effects, including the ability to
promote aerobic glycolysis, activate HIF1α transcription factor, and culture tumor-promoting properties in other cells near the tumor, such as hepatic astrocytes.

A prominent example of heterogeneity in metabolic adaptations is the specific preference
for anaplerosis substrates in the TCA cycle.
Two distinct replenishment strategies dominate in transformed cells, one is to replenish the TCA cycle at the point where glutamine catabolism produces α-ketoglutarate; The other is to supplement oxaloacetate, an intermediate substance in TCA, by pyruvate carboxylase (PC)-driven pyruvate
carboxylation reaction.

Although glutamine-mediated replenishment reactions are almost ubiquitous in cultured cells, when glutamine is depleted, cells can reorient their carbon flow to a PC-driven replenishment reaction
.
For example, in vivo studies with glucose infusions containing the 13C carbon isotope have shown that preference for a particular replenishment pathway depends on the tumor type
.
In fact, both pancreatic and lung tumors tend to have a PC-mediated backfill pathway and need it to support growth
.
Conversely, studies of 13C carbon infusion of colorectal-derived xenografts have shown that these tumors can utilize glutamine as a source of
their replenishment.
Reliance on specific compensation strategies may further illustrate the metabolic characteristics of the primary tissue and the link between
it and its tumor.
For example, lung tumors and normal lung tissue are not net (net) consumers of glutamine, and this pattern is also consistent
with the role of lung tissue to provide the body with de novo synthesis of glutamine when circulating glutamine levels are affected.

Resuming growth in newly colonized organs requires metastatic cancer cells to adapt to a different trophic microenvironment
from their primary tissue.
Depending on the environment, this adaptation may require taking advantage of the abundance of substrates available at the metastatic site while accommodating the finite nature
of other nutrients.
For example, nutrients (such as lipids) in the central nervous system are relatively scarce compared to other metastatic niches
.
As a result, tumors that metastasize to the brain rely on de novo synthesis
of fatty acids.
In addition, the ability to synthesize asparagine de novo has also been shown to be necessary for breast cancer cells to metastasize to the lungs, but not for the growth of the primary tumor
.

Not only restriction, but also excess of certain nutrients help shape the metabolic characteristics
of metastatic cancer cells.
For example, breast tumors metastasized to the lungs or bones are highly dependent on PGC1α expression and mitochondrial carbon metabolism pathways, while their counterparts in the liver are highly glycolytic and rely on high expression of PDK1, thereby reducing glucose carbon
entering mitochondria.
The highly glycolytic nature of liver metastases may indicate that cells utilize the abundant glucose supply in
liver tissue.
In contrast, colorectal tumors that metastasize to the liver show the ability to metabolize fructose, which is abundant
in the gut.
To do this, liver metastatic clones from colorectal tumors are selected to express high levels of aldolase B (ALDOB), which allows them to utilize fructose as a glycolytic substrate
.
Similarly, breast tumor cells that metastasize to the lungs increase their use of pyruvate as a replenishment substrate compared with primary tumors, allowing them to take advantage of high pyruvate levels in
lung tissue.
This finding suggests that not only primary lung tumors, but also metastatic tumors can also take advantage of pyruvate in lung tissue with higher levels, further emphasizing the role
of nutrients in the surrounding environment in shaping tumor metabolism.

Metabolites can drive changes in signaling events

The activity of various regulatory pathways can be affected
by the availability and abundance of specific metabolites.
For example, cells can take advantage of the AMP/ATP ratio and release signaling events
through kinases such as AMPK.
Glutamine and glycolysis intermediate, fructose 6-phosphate, are essential substrates
for the aminohexose biosynthesis pathway.
The levels of these metabolites directly affect the production of UDP-N-ACETYLGLUCOSAMINE (UDP-GlcNAc), which is essential
for protein glycosylation and expression on the surface of certain growth factor receptors.
In addition, O-linked GlcNActransferase (OGT) is typically upregulated in multiple cancer types and maintains the carcinogenic signaling pathway
through stable glycosylation at serine/threonine sites, proteins involved in signal transduction.

Nutrient supply can also influence DNA and histone modifications, thereby epigenetically regulating gene expression
.
Acetyl-CoA and S-adenosylmethionine (SAM) are essential substrates for acetyltransferase and methyltransferase, respectively, both involved in post-translational modification of histones and regulate the transcription
of associated DNA.
Other modifiable histone and non-histone metabolites include succinyl-CoA, crotonyl-CoA, and lactic acid, although their physiological significance remains to be further determined
.
During histone deacetylation, NAD+ is an essential reactant to support the Sirtuin family of deacetylases in catalyzing
reactions.
Similarly, methylation markers of histones and DNA can be reversed; α-Ketoglutaric acid is required for dioxygenase function, including TET DNA hydroxylase/demethylase and JMJD histone demethylase (Figure 8).

Disturbances in these metabolism-dependent chromatin modification processes can also promote tumorigenesis
.
Due to their structural similarity to α-ketoglutarate, succinate and fumarate act as competitive inhibitors
of α-ketoglutarate-dependent dioxygenase.
Mutations in succinate dehydrogenase (SDH) are more common in familial paragangliomas, and fumarate hydration enzyme (FH) acts as a tumor suppressor and is usually absent
in uterine fibroids and renal cell carcinomas.
Cancer-associated mutations in these genes can lead to accumulation of succinate and fumarate, respectively, as well as inhibition of DNA and histone demethylase
.
Similarly, oncogenic mutations of isocitrate dehydrogenase 1 (IDH1) or IDH2 are prevalent
in gliomas, acute myeloid leukemia (AML), cholangiocarcinoma, and chondrosarcoma.
Mutant IDH converts α-ketoglutaric acid to D-2-hydroxyglutaric acid (D-2-HG), and D-2-HG competitively inhibits α-ketoglutarate-dependent dioxygenase (Figure 8).

As a result, these cancer types often manifest as genomic hypermethylation and inhibit cell differentiation and promote tumorigenesis
.
The accumulation of D-2-HG enantiomer L-2-HG can trigger similar dioxygenase inhibition and associated hypermethylation epigenetic features
.
The accumulation of L-2-HG is not related to the activity of isocitrate dehydrogenase, but is driven by low-pH-related changes specific to substrates of at least two dehydrogenases (specifically lactic acid and malate dehydrogenase), and in renal cell cancer cells, the loss of L-2-HG dehydrogenase (L2HGDH), which is necessary
to reduce L-2-HG levels.

Metabolite-driven regulation of gene expression also extends to RNA
modifications.
After methanization, the ratio of SAM to S-adenosylhomocysteine (SAM/SAH) decreases, and also reduces the activity of the N6-adenosylmethyltransferase METTL16, which methylates mRNA and facilitates translation
.
Conversely, mTORC1 activation downstream of growth factors or carcinogenic signals can stimulate m6A modification of mRNA through WTAP expression and SAM synthesis, which in turn enhances mRNA translation and promotes tumor cell growth
.
Interestingly, like DNA and proteins, RNA m6A demethylase, FTO, and ALKBH5 are α-ketoglutarate-dependent double oxygenases whose activity α-ketoglutarate, iron, and oxygen all promote their activity and are inhibited by D-2-HG produced by mutant IDH enzymes (Figure 8).

Protein synthesis can also be regulated
by nutrient supply independent of post-transcriptional mRNA modifications.
In an amino acid-starved environment, glutamine-specific tRNA isomers become selectively uncharged, while other tRNAs retain the charge
of their homologous amino acids.
Decreased charged tRNA Gln triggers preferential depletion of polyglutamine-containing proteins, which are found in a high proportion of gene transcription mechanism components, so this may alter the transcriptional output
of cells.
In summary, trophic status and metabolite abundance cough regulate intracellular signaling cascades and gene expression at multiple levels, and therefore can promote tumorigenesis
.

Metabolic interaction with the tumor microenvironment

Metazoan (understood as multicellular) tissues contain not only cells that perform major tissue functions, but also a variety of accessory or stromal cell types, including fibroblasts, immune cells, and endothelial cells
.
Stromal cells play a supportive role, helping to maintain tissue homeostasis and coordinating tissue repair
in the event of injury.
Transformed cells do not cut ties to this support network, instead, their proproliferative state can influence stromal cell behavior, often collaborating with stromal cells to participate in tissue repair activities, thereby promoting cancer cell survival and expansion/invasion
.
While a complex messaging system of multiple growth factors and cytokines is associated with this crosstalk, it is also increasingly recognized that metabolic factors within TME also play an important role in establishing and maintaining wound healing-like states in growing tumors (Figure 9).

Tissue repair can be thought of as a massive (immense) anabolic event, and transformed cells are not the only cells
in TME with active anabolism.
In fact, in some tumorigenic environments, transformed cells make up only a minority of tumor clumps in a
growing state.
Therefore, the metabolic activity of matrix components can act as a powerful shaping force
in TME.
However, while the metabolic goal of transformed cells is to build more of themselves, the metabolic phenotype of tumor-associated stromal cells is determined by the limited role they play in tissue homeostasis
.
For example, many solid tumors contain an abundance of cancer-associated fibroblasts (CAFs) that are recruited from a local tissue fibroblast population and synthesize large amounts of ECM
.
Interestingly, matrix proteins are highly rich in proline and glycine, and the production of the tumor matrix just needs to consume a large amount of these amino acids
.
TGFβ is a potent inducer of ECM production by fibroblasts and has been shown to coordinate the anabolic processes of these cells to increase the absorption of glucose and glutamine, promote the synthesis of glycine from glucose through the serine synthesis pathway, and produce proline
from glutamine.
Thus, the anabolism of CAFs can lead to glucose and glutamine depletion
in tumors in many growing states.

In some cases, tissue-specific fibroblasts can also provide nutritional support
to tumors by synthesizing and/or releasing non-essential amino acids or ketoacids in essential amino acids.
For example, ovarian CAFs can synthesize and release glutamine into TME, while pancreatic cancer-associated fibroblasts, or stellate cells, have been found to provide alanine and BCAAs of ketoacids
for TME.
To support the production and/or recovery of amino acids and ketoacids in TME, it appears that some fibroblasts in growing tumors also participate in macropinocytosis of soluble extracellular proteins to help increase the supply of amino acids for the synthesis of the matrix and release excess amino acids and ketoacids into TME
.

The deposition of the dense lattices of the ECM can also alter the biophysical properties of TME, causing the matrix immobilized by the cell to harden
.
Encountering a hard rather than soft matrix can act as a physical factor in changing the properties
of embedded cancer cells.
Excessive ECM buildup can lead to the collapse of the existing capillary network within the TME, further exacerbate the limited exchange
of solutes and gases between the tumor and the circulation.
In particular, solid tumors have lower oxygen availability compared to surrounding tissues, and availability is a particularly important parameter
that shapes the metabolic characteristics of TME.
Among its many effects, insufficient oxygen supply promotes glucose consumption and its conversion
to lactic acid.
In addition, the dependence on glutamine reductive carboxylation increases glutamine consumption, acidifies the extracellular environment of the tumor, and leads to epigenetic changes
.

The lactic acid-rich hypoxic niche induces resident innate immune cells such as type II macrophages to release VEGF, a growth factor
that stimulates endothelial cell recruitment and new capillary growth and branching.
In addition to VEGF, endothelial cells integrate some TME-related metabolic stimuli into cellular activity
.
Among metabolic stimulants that promote angiogenesis, the accumulation of gas transmitters such as arginine-derived NO and cysteine-derived hydrogen sulfide, as well as the consumption
of amino acids in TME.
In summary, the diversity of stimuli that shape TME and their effects leads to a complex dynamic in which the activity of multiple TME-related factors can contribute to the depletion or restoration of nutrient supplies
to tumors.

For effector T cells, a population of immune cells at the forefront of anti-tumor immunity, the metabolic status of TME nutrient depletion is particularly restrictive
.
Stimulated by antigens, T lymphocytes transition from a quiescent state to a significant anabolic state, an adaptation that provides the power
for their proliferation and differentiation into effector cells that kill tumors.
Therefore, nutrient depletion is more likely to disrupt T cell activation and function
than other cell types.
For example, T cells significantly rely on the ability to
absorb exogenous non-essential amino acids, such as serine and alanine, in place of de novo synthesis of these nutrients.
T cell activation is also very sensitive
to the depletion of glutamine and glucose.
In addition, effector T cells are also sensitive to increased levels of oxidative stress in TME and can trigger so-called depletion phenotypes
.
In addition, transformed cells and their non-transformed TME cells, such as type II macrophages, can further help inhibit the T cell population
by participating in the consumption of nutrients needed to maintain T cell effector function.
To this end, both cancer cells and tumor-associated macrophages have been shown to secrete the amino acid metabolase IDO1 to degrade tryptophan, which further hinders the effector function of T cells and, similar to glucose depletion, also leads to the emergence of immunosuppressive Treg cell populations
.

In view of the fact that tumors infiltrated with low effect T cell populations do not respond to immune checkpoint inhibitor therapy, it is of great significance
to improve the success rate and durability of immunotherapy by regulating the metabolic state of TME, inhibiting the tumor promoting cell population, and enhancing the tumicicidal activity of T cells.
A recent study showed that blocking the utilization of glutamine by breast tumor cells through GLS1 deletion can increase glutamine levels in TME by nearly an order of magnitude, thereby promoting the antioxidant capacity of T cells and enhancing their infiltration in tumors
.
Similarly, IDO1 inhibitors that mediate tryptophan degradation have also been reported to have tumor-inhibiting effects
.

In summary, TME represents a complex metabolic ecosystem in which multiple metabolically active cell types contribute to the emergence of tumor-specific metabolic environments, which in turn regulate the behavior of TME cellular components and promote tumor growth
.
Although studies have found many metabolic interactions between tumor components, the complete rules of TME metabolic economy have yet to be elucidated
.
For example, recent in vivo metabolic tracing experiments in mouse models of colorectal cancer have shown that TME cells use nutrients unevenly, for example, transformed cells are the main consumers of glutamine, but most of the glucose is consumed
by myeloid cells such as macrophages.
Finally, new cell types have been added to the list of TMEs that support anabolism in transformed cells
.
These cell types include cancer-associated fat cells, which provide fatty acids for melanoma, and tumor-associated neurons, which provide an additional source of
serine for pancreatic cancer cells.

Metabolic economy integrated into the whole body

There is growing evidence that carcinogenicity is influenced
by the body's metabolic regulation.
Therefore, metabolic changes associated with obesity and type 2 diabetes may also play an important role
in promoting tumorigenesis.
In addition, studies have found that an increase in serum methylmalonic acid associated with aging can directly promote the aggressive behavior
of cancer cells.
In addition, some tumors have also demonstrated the ability to influence the metabolic state of the entire body through mechanisms (Figure 10).

Identifying the mechanisms behind these phenomena may help guide cancer prevention, early detection, and new therapeutic intervention strategies
.

Epidemiological research evidence suggests that obesity and type 2 diabetes are significant risk factors
for many cancers.
Fasting blood glucose levels are significantly increased in patients with type 2 diabetes mellitus (from a healthy 4-5 mM to 7 mM).

On its own, this increase is unlikely to provide cancer cells with extra glucose
.
This is because GLUT1 is the main glucose transporter expressed by cancer cells and is already operating at its maximum capacity
, even at the physiological level of glucose.
Therefore, abnormally elevated blood sugar is unlikely to directly affect cancer cells' access
to glucose.

Even in the case of high extracellular glucose levels, glucose entry into most cells is primarily mediated by ligands of cell- and tissue-specific receptor tyrosine kinases, which in turn regulate the relative expression and localization
of GLUT1 on the cell surface 。 While tissue-specific metabolism is controlled by locally produced growth factors such as EGF, PDGF, or FGF, body-wide nutrient utilization is controlled
by systemically circulating factors that act simultaneously on various tissues.
These factors include insulin, which is produced almost exclusively by the pancreas, and insulin-like growth factor (IGF-I), produced by the liver and various local tissues, and are affected
by growth hormone in the central nervous system.

In patients with metabolic syndrome and type 2 diabetes, circulating insulin and IGF-I levels are compensated by
impaired ability of insulin to facilitate the entry of glucose into insulin-dependent tissues, such as muscles.
Genetic and pharmacological evidence suggests that systemic upregulation of IGF-I plays an important role
in tumorigenesis.
In fact, although IGF-I receptors are rarely mutated, they are often overactive
in tumors.
For example, studies of up to 50 percent of breast cancers have signs of increased IGF-I receptor activity, which is associated
with low survival.
In addition, in mice with elevated IGF-I levels, orthotopic transplantation and carcinogen-induced tumor growth were also enhanced
.
Genetic evidence from functional deficits in animal models and populations also further supports the involvement of systemic IGF-I signaling in tumorigenesis
.
Homozygous deletion of IGF-I receptors or double deletions of IRS1 and IRS2 of proximal effectors have also been shown to significantly delay the occurrence
of Kras-driven tumors in mice.
Evidence from population-based studies also suggests that IGF-I signaling is associated
with tumorigenesis.
In fact, people with congenital IGF-I deficiency syndromes, such as Laron syndrome, develop cancer
almost never occur.

Since systemic insulin and IGF-I release is largely governed by food intake, dietary adjustments can help reduce circulating levels
of these two factors.
In fact, Peyton Rous' 1914 study of underfed animals provided early experimental evidence that reducing caloric intake had the potential to slow tumor growth
.
It is worth noting that caloric restriction-mediated retardation of tumorigenesis in animals can be easily reversed by injection of recombinant IGF-I, which further suggests that the reduction of IGF-I levels is a key factor
in diet-mediated tumor growth retardation.
Dietary strategies for insulin/IGF-I normalization, such as the ketogenic diet, have also been shown to have similar tumor suppressive effects
in various mouse tumor models.
At the population level, treatments that lower extracellular glucose and insulin levels, such as metformin, have been shown to reduce cancer incidence
in people with diabetes.
However, not all tumors are sensitive to
caloric restriction or reduced glucose levels.
In particular, tumors with mutations in PI-3 kinase activity were not affected by dietary restriction in animals, suggesting that the genetic properties of the tumor may play a key role
in the extent to which the tumor depends on systemic nutrient absorption signals.

On the one hand, tumors are affected by metabolic control factors throughout the body, on the other hand, some tumors also seem to affect metabolic set points throughout the body
.
The most prominent manifestation of tumor regulation of systemic metabolism is the cachexia state
of multi-organ metabolism.
Cachexia is characterized by the loss of insulin sensitivity in many tissues; progressive wasting of skeletal muscle, heart, liver, fat, and brain tissue; No appetite; And suppress the immune system
.
Although cachexia is not a phenomenon that is exclusively associated with cancer, and severe trauma and AIDS are also associated with cachexia, advanced stages of cancer are often accompanied by cachexia phenomena
.
It is said that at least 20% of patients die from cachexia
as a direct cause of cancer.
Cachexia can be induced by a number of circulating factors, including TNFalpha, inflammatory cytokines, and glucocorticoids
.
Whether these or other factors directly or indirectly contribute to cachexia associated with a particular cancer type still needs further in-depth study
.

Recent studies have found that tumors can affect the body's metabolic state
by disrupting the normal circadian rhythm of metabolic gene expression in liver and adipose tissue.
Whether disrupting the metabolic cycle that accompanies the circadian cycle promotes tumor growth or the catabolic state of the liver, muscle, and fat during cancer development remains to be further determined
.

conclusion

Metabolic changes associated with tumorigenesis enable transformed cells to maintain abnormal accumulation and colonization of different tissues
by evading tissue homeostatic defenses and in collaboration with internal signaling mechanisms and local tissue and systemic resources.
Importantly, not only the transformed cells themselves, but also the stromal cells within TME, as well as the metabolic balance of the entire body, will be remodeled during the cancer process, jointly promoting the accumulation and spread of cancer cells, reducing the ability of the immune system to fight tumor growth, and directly leading to cancer-related death
.
Dissecting tumors promotes metabolic adaptations to these changes and maintain growth in metabolically disadvantaged environments, helps guide new treatments, and may help guide dietary combinations
that work synergistically with existing therapeutic interventions such as chemotherapy, targeted inhibitors, and immune checkpoint blockade therapies.

Original source:

Pavlova NN, Zhu J, Thompson CB.
The hallmarks of cancer metabolism: Still emerging.