PNAS: How liver cancer hijacks the circadian clock mechanism within cells

One of the most common types of liver cancer, hepatocellular carcinoma (HCC), is already the third leading cause of cancer-related death worldwide, and cases are on the rise in the United States and around the
world.
While chemotherapy, surgery, and liver transplants can help some patients, targeted therapies against HCC could save millions of lives
.

Recent research provides clues to a potential target: a circadian clock protein within cells, which helps coordinate changes
in body functions throughout the day.
But much of the study only hinted at an indirect link between circadian clock function and HCC, such as observing that cells collected from liver cancer patients disrupted circadian rhythms
.

Now, a study led by researchers at the Keck School of Medicine of USC not only directly links clock proteins to liver cancer, but also shows precisely how cancer cells hijack the clock mechanism to divide and spread
.
The study, just published in the Proceedings of the National Academy of Sciences, also found that inhibiting key clock proteins prevents cancer cells from multiplying
.

"Early research didn't really give us a grasp of how to use specific treatments to target processes
within liver cancer cells.
In this paper, we are taking the first steps in that direction," said Steve A.
Kay, Ph.
D.
, senior author of the study, a university and provost professor of neurology, biomedical engineering, and quantitative computational biology at the Keck School of Medicine of USC and director of
the USC Michelson Center for Polymer Biosciences.

The study was conducted in collaboration with cell biology experts and clinicians at the USC Norris Comprehensive Cancer Center, known for being a leader in clinical trials for
a variety of cancers, including HCC.

Heinz-Josef Lenz, M.
D.
, professor of medicine and preventive medicine, associate director of clinical research, and co-leader of the USC Norris Gastrointestinal Cancer Program, said, "We are very excited to find a new, innovative treatment strategy that may ultimately improve outcomes for liver cancer patients
.
" "By targeting the circadian clock, we target not only tumor cells, but also the area around the tumor, which helps improve the efficacy
of other targeted therapies.
"

Interrupt the cell cycle

To elucidate the role of clock proteins in HCC, Kay, Lenz and their colleagues conducted a series of experiments combining cell culture, genomic analysis, and animal models
.

First, the researchers discovered that two key clock proteins, clock and BMAL1, are critical
for replication of liver cancer cells in cell culture.
When CLOCK and BMAL1 are inhibited, the replication process of cancer cells is interrupted, eventually leading to cell death or apoptosis
.
Triggering apoptosis, where cells stop dividing and then self-destruct, is the goal of
many modern cancer treatments.

Next, the team used their toolbox of genomic samples to build on years of research on circadian clock proteins in vivo to further understand the role of
clock and BMAL1.
Among other findings, they found that eliminating clock proteins lowered levels of the enzyme Wee1 and increased levels of
the enzyme inhibitor P21.

"That's exactly what you want, because when it comes to cancer cell proliferation, P21 is a brake and Wee1 is a gas pedal.
"

Finally, the researchers tested their findings in living organisms.
Mice injected with unmodified human liver cancer cells grew large tumors, but mice injected with modified cells that inhibit CLOCK and BMAL1 had little tumor growth
.

Development of targeted therapies

Understanding how cancer cells hijack circadian clock proteins is a big step toward stopping liver cancer from spreading, but researchers have more questions to answer
.
For example, Kay and his team hope to explore the relationship between the
Circiadian clock protein Wee1 and P53 genes.
The gene helps prevent the growth of tumors in the body, and mutations in P53 have long been linked to
an increased risk of various cancers.

"We really need to understand this relationship in order to better determine which patients might benefit the most
from targeted therapies targeting CLOCK and BMAL1," Kay said.

He and his team also hope to begin testing experimental drugs
for CLOCK and BMAL1 in liver cancer patients.
The work was part of a larger study that analyzed circadian clock proteins in several cancers, including glioblastoma, leukemia, and colorectal cancer
.