Cell sub-journal: A new strategy for tumor therapy targeting the Hippo pathway

The Hippo signaling pathway is involved in different stages of tumorigenesis, development, and drug resistance, and is an ideal tumor therapy target
.
In recent years, the development of anticancer therapies targeting the Hippo pathway has become a research hotspot, but no related drugs have entered the clinic yet
.
Identifying the regulatory mechanism of the Hippo pathway is an important basis for the development of molecular targeted therapy
.
However, the research in the Hippo field in the past two decades has mainly focused on the identification and functional analysis of pathway components, and the molecular mechanism research is relatively lagging behind
.

The core part of the Hippo pathway is a kinase cascade: MST1/2 kinase (Hippo) - LATS1/2 kinase - YAP/TAZ transcriptional co-activator
.
For many years, the conclusion that MST1/2 can phosphorylate and activate LATS1/2 has been widely accepted, but the specific molecular mechanism by which MST1/2 activates LATS1/2 has received little attention
.
As the hub of the entire Hippo pathway, LATS1/2 kinases play a linking role.
Therefore, clarifying the molecular mechanism of upstream signals regulating LATS1/2 is a prerequisite for artificially regulating Hippo signal strength and developing targeted therapies
.

On April 15, 2022, the research group of Yu Faxing from our hospital published a long article entitled "WWC proteins mediate LATS1/2 activation by Hippo kinases and imply a tumor" in the journal Molecular Cell.
Research paper on suppression strategy
.
This study revealed the important function and molecular mechanism of the WWC protein family (WWC1/2/3) in the activation of LATS/2 by MST1/2, and based on this mechanism, the SuperHippo mini gene was developed, which can specifically activate LATS1/2 And significantly inhibit the occurrence of a variety of tumors
.

This study presents the MST1/2-SAV1-WWC1/2/3-LATS1/2 signaling model for the first time (Fig.
1)
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Among them, WWC1/2/3 directly binds to LATS1/2 and SAV1, and SAV1 then recruits MST1/2
.
Thus, WWC1/2/3 act as molecular platforms to bring together the core elements of the Hippo signaling pathway, thereby increasing the efficiency of the kinase cascade
.
For a long time, WWC family proteins were considered as common upstream regulators of Hippo pathway
.
This study shows that WWC1/2/3 is located in the core area of ​​the Hippo pathway, that is, between MST1/2 and LATS1/2, and plays a "neck" effect, and its protein level is highly correlated with the signal strength of the Hippo pathway
.

Figure 1.
Molecular mechanism of WWC1/2/3-mediated activation of LATS1/2 by MST1/2

The Hippo pathway was gradually established through a large number of genetic screens in model animals, and deletion of the pathway member genes resulted in excessive cell proliferation and increased organ size
.
However, after knockdown of different Hippo pathway genes, the phenotypic strength was not consistent
.
Based on the innovation of molecular mechanism, the study found that after liver-specific knockout of Sav1 and Wwc1/2 in mice, the organ size phenotype intensity was similar to that of Mst1/2 knockout; in addition, these two genotypes were also at similar time points.
Hepatocellular carcinoma (HCC) developed (Figure 2)
.
The Hippo pathway is highly conserved from Drosophila to mammals, and this study also yielded highly consistent results in Drosophila genetics experiments
.
This series of in vivo studies not only verified the molecular mechanism, but also discovered the first genotype (Sav1 and Wwc1/2 co-knockout) that can highly mimic Hippo kinase (Mst1/2) deletion in phenotype
.

Figure 2.
Co-knockout phenotype of Sav1 and Wwc1/2 mimics Mst1/2 knockout

The key site for binding of WWC family proteins to other Hippo core components is concentrated at its N-terminus (Fig.
1)
.
This study found that the N-terminal 200 amino acid sequence of WWC family proteins can effectively activate LATS1/2
.
A related minigene (named SuperHippo) was overexpressed in mouse liver, and its phenotype was very similar to that of Yap/Taz (a downstream effector of the Hippo pathway) knockout mice
.
Therefore, SuperHippo transgenic mice are also the first genotypes that can phenotypely mimic Yap/Taz deletion
.
The study also tested the anti-tumor efficacy of SuperHippo from various aspects, and found that SuperHippo could significantly inhibit YAP/TAZ transcriptional activity and effectively inhibit tumor development in various mouse tumor models (Figure 3)
.
Compared with the known Hippo pathway targeting solutions, SuperHippo has the following characteristics: 1) It is the first molecular tool to directly activate LATS1/2 kinase; 2) Its function is completely dependent on MST1/2 and SAV1, with extraordinary specificity; 3) Wide range of uses, not only can treat tumors caused by Hippo pathway disorder, but also have certain effects on cholangiocarcinoma, hepatocellular carcinoma, uveal melanoma and other tumors
.

Figure 3.
SuperHippo inhibits tumorigenesis in a mouse genetic model

In conclusion, this study not only revealed the molecular regulation mechanism of the upstream core components of the Hippo pathway, but also provided a new strategy for the development of tumor treatment programs targeting the Hippo pathway.
The research provides theoretical basis and application tools
.
It is reported that in order to strictly verify the ideas in the paper, the research team used a large number of complex knockout cells, Drosophila and mouse models
.

Qi Sixian, a 2017 doctoral student at the Institute of Biomedical Sciences of Fudan University, is the first author of this paper, and researcher Yu Faxing is the corresponding author
.
Researcher Zhang Lei from the Center for Excellence in Molecular and Cellular Science of the Chinese Academy of Sciences, Professor Zhou Dawang from Xiamen University and several professors from Fudan University made important contributions to this paper
.
This work was supported by research funds from the Ministry of Science and Technology, the Natural Science Foundation of China, and the Shanghai Municipal Science and Technology Commission
.

Original link: https://doi.
org/10.
1016/j.
molcel.
2022.
03.
027