Large tumor suppressors 1 and 2 regulate Aurora-B through phosphorylation of INCENP to ensure completion of cytokinesis

Differential requirement of Hippo cascade during CTNNB1 or AXIN1 mutation-driven hepatocarcinogenesis

BACKGROUND AND AIMS

Gain-of-function (GOF) mutations of CTNNB1 and loss-of-function (LOF) mutations of AXIN1 are recurrent genetic alterations in hepatocellular carcinoma (HCC). We aim to investigate the functional contribution of Hippo/YAP/TAZ in GOF CTNNB1 or LOF AXIN1 mutant HCCs.

APPROACH AND RESULTS

The requirement of YAP/TAZ in c-Met/β-Catenin and c-Met/sgAxin1-driven HCC was analyzed using conditional Yap , Taz , and Yap;Taz knockout (KO) mice. Mechanisms of AXIN1 in regulating YAP/TAZ were investigated using AXIN1 mutated HCC cells. Hepatocyte-specific inducible TTR-CreER T2KO system was applied to evaluate the role of Yap;Taz during tumor progression. Cabozantinib and G007-LK combinational treatment were tested in vitro and in vivo . Nuclear YAP/TAZ was strongly induced in c-Met/sgAxin1 mouse HCC cells. Activation of Hippo via overexpression of Lats2 or concomitant deletion of Yap and Taz significantly inhibited c-Met/sgAxin1 driven HCC development, whereas the same approaches had mild effects in c-Met/β-Catenin HCCs. YAP is the major Hippo effector in c-Met/β-Catenin HCCs, and both YAP and TAZ are required for c-Met/sgAxin1-dependent hepatocarcinogenesis. Mechanistically, AXIN1 binds to YAP/TAZ in human HCC cells and regulates YAP/TAZ stability. Genetic deletion of YAP/TAZ suppresses already formed c-Met/sgAxin1 liver tumors, supporting the requirement of YAP/TAZ during tumor progression. Importantly, tankyrase inhibitor G007-LK, which targets Hippo and Wnt pathways, synergizes with cabozantinib, a c-MET inhibitor, leading to tumor regression in the c-Met/sgAxin1 HCC model.

CONCLUSIONS

Our studies demonstrate that YAP/TAZ are major signaling molecules downstream of LOF AXIN1 mutant HCCs, and targeting YAP/TAZ is an effective treatment against AXIN1 mutant human HCCs.

The Hippo Signaling Pathway in Cancer: A Cell Cycle Perspective

Simple Summary

Cancer is increasingly viewed as a cell cycle disease in that the dysregulation of the cell cycle machinery is a common feature in cancer. The Hippo signaling pathway consists of a core kinase cascade as well as extended regulators, which together control organ size and tissue homeostasis. The aberrant expression of cell cycle regulators and/or Hippo pathway components contributes to cancer development, and for this reason, we specifically focus on delineating the roles of the Hippo pathway in the cell cycle. Improving our understanding of the Hippo pathway from a cell cycle perspective could be used as a powerful weapon in the cancer battlefield.

Abstract

Cell cycle progression is an elaborate process that requires stringent control for normal cellular function. Defects in cell cycle control, however, contribute to genomic instability and have become a characteristic phenomenon in cancers. Over the years, advancement in the understanding of disrupted cell cycle regulation in tumors has led to the development of powerful anti-cancer drugs. Therefore, an in-depth exploration of cell cycle dysregulation in cancers could provide therapeutic avenues for cancer treatment. The Hippo pathway is an evolutionarily conserved regulator network that controls organ size, and its dysregulation is implicated in various types of cancers. Although the role of the Hippo pathway in oncogenesis has been widely investigated, its role in cell cycle regulation has not been comprehensively scrutinized. Here, we specifically focus on delineating the involvement of the Hippo pathway in cell cycle regulation. To that end, we first compare the structural as well as functional conservation of the core Hippo pathway in yeasts, flies, and mammals. Then, we detail the multi-faceted aspects in which the core components of the mammalian Hippo pathway and their regulators affect the cell cycle, particularly with regard to the regulation of E2F activity, the G1 tetraploidy checkpoint, DNA synthesis, DNA damage checkpoint, centrosome dynamics, and mitosis. Finally, we briefly discuss how a collective understanding of cell cycle regulation and the Hippo pathway could be weaponized in combating cancer.

Yorkie and JNK Control Tumorigenesis in Drosophila Cells with Cytokinesis Failure

Cytokinesis failure may result in the formation of polyploid cells, and subsequent mitosis can lead to aneuploidy and tumor formation. Tumor suppressor mechanisms limiting the oncogenic potential of these cells have been described. However, the universal applicability of these tumor-suppressive barriers remains controversial. Here, we use Drosophila epithelial cells to investigate the consequences of cytokinesis failure in vivo. We report that cleavage defects trigger the activation of the JNK pathway, leading to downregulation of the inhibitor of apoptosis DIAP1 and programmed cell death. Yorkie overcomes the tumor-suppressive role of JNK and induces neoplasia. Yorkie regulates the cell cycle phosphatase Cdc25/string, which drives tumorigenesis in a context of cytokinesis failure. These results highlight the functional significance of the JNK pathway in epithelial cells with defective cytokinesis and elucidate a mechanism used by emerging tumor cells to bypass this tumor-suppressive barrier and develop into tumors.

LATS1/2 kinases trigger self-renewal of cancer stem cells in aggressive oral cancer

Cancer stem cells (CSCs), which play important roles in tumor initiation and progression, are resistant to many types of therapies. However, the regulatory mechanisms underlying CSC-specific properties, including self-renewal, are poorly understood. Here, we found that LATS1/2, the core Hippo pathway-kinases, were highly expressed in the oral squamous cell carcinoma line SAS, which exhibits high capacity of CSCs, and that depletion of these kinases prevented SAS cells from forming spheres under serum-free conditions. Detailed examination of the expression and activation of LATS kinases and related proteins over a time course of sphere formation revealed that LATS1/2 were more highly expressed and markedly activated before initiation of self-renewal. Moreover, TAZ, SNAIL, CHK1/2, and Aurora-A were expressed in hierarchical, oscillating patterns during sphere formation, suggesting that the process consists of four sequential steps. Our results indicate that LATS1/2 trigger self-renewal of CSCs by regulating the Hippo pathway, the EMT, and cell division.

High-Density Cell Arrays for Genome-Scale Phenotypic Screening

Due to high associated costs and considerable time investments of cell-based screening, there is a strong demand for new technologies that enable preclinical development and tests of diverse biologicals in a cost-saving and time-efficient manner. For those reasons we developed the high-density cell array (HD-CA) platform, which miniaturizes cell-based screening in the form of preprinted and ready-to-run screening arrays. With the HD-CA technology, up to 24,576 samples can be tested in a single experiment, thereby saving costs and time for microscopy-based screening by 75%. Experiments on the scale of the entire human genome can be addressed in a real parallel manner, with screening campaigns becoming more comfortable and devoid of robotics infrastructure on the user side. The high degree of miniaturization enables working with expensive reagents and rare and difficult-to-obtain cell lines. We have also optimized an automated imaging procedure for HD-CA and demonstrate the applicability of HD-CA to CRISPR-Cas9- and RNAi-mediated phenotypic assessment of the gene function.

Comparison of Cell Arrays and Multi-Well Plates in Microscopy-Based Screening

Multi-well plates and cell arrays enable microscopy-based screening assays in which many samples can be analysed in parallel. Each of the formats possesses its own strengths and weaknesses, but reference comparisons between these platforms and their application rationale is lacking. We aim to fill this gap by comparing two RNA interference (RNAi)-mediated fluorescence microscopy-based assays, namely epidermal growth factor (EGF) internalization and cell cycle progression, on both platforms. Quantitative analysis revealed that both platforms enabled the generation of data with the appearance of the expected phenotypes significantly distinct from the negative controls. The measurements of cell cycle progression were less variable in multi-well plates. The result can largely be attributed to higher cell numbers resulting in less data variability when dealing with the assay generating phenotypic cell subpopulations. The EGF internalization assay with a uniform phenotype over nearly the whole cell population performed better on cell arrays than in multi-well plates. The result was achieved by scoring five times less cells on cell arrays than in multi-well plates, indicating the efficiency of the cell array format. Our data indicate that the choice of the screening platform primarily depends on the type of the cellular assay to achieve a maximum data quality and screen efficiency.

Contribution of LATS1 and LATS2 promoter methylation in OSCC development

The aberrant DNA methylation of the tumor suppressor genes involved in DNA Damage Response (DDR) signaling and cell cycle regulation may lead to the tumorigenesis. Our purpose here is to analyze the promoter methylation and mRNA expression levels of LATS1 and LATS2 (LATS1/2) genes in OSCC. Promoter methylation status of LATS1/2 genes was evaluated in 70 OSCC paraffin-embedded tissues and 70 normal oral samples, using Methylation Specific PCR (MSP). LATS1/2 mRNA expression profiles were also investigated in 14 OSCC patients and 14 normal samples, using real-time PCR. In both candidate genes, promoter methylation assessment revealed significant relationship between cases and controls (OR = 2.24, 95 % CI = 1.40-3.54, P = 0.001; LATS1 and OR = 15.5, 95%CI = 3.64-64.76, P < 0.001; LATS2). As well as, the evaluation of mRNA expression levels showed decreased expression in OSCC tissues in compare to control tissues. (Mean ± SD 1.74 ± 0.14 in OSCC versus 2.10 ± 0.24 in controls, P < 0.001; LATS1 and Mean ± SD 1.36 ± 0.077 in OSCC versus 1.96 ± 0.096 in controls, P < 0.001; LATS2). To the best our knowledge, this is the first report regarding the down-regulation of LATS1/2 through promoter methylation in OSCC. It is suggested to explore the down-stream transcription factors of both genes for finding the molecular mechanism of this deregulation in OSCC.