The control of melanoma by the Hippo pathway

Abstract

Melanoma is an aggressive cancer with extremely unfavourable prognosis. Two main types of melanoma include cutaneous melanoma (CM) accounting for around 95% and uveal melanoma (UM) around 5%. In Australia, melanoma is in the top five most commonly diagnosed cancers, estimated to contribute to over 10% of all new cancer diagnoses in 2017 (Cancer Australia, 2018.). While the overall death rate caused by all cancer is decreasing, the mortality of melanoma has increased in recent years (Howlader et al., 2012; AIHW, 2017). Patients diagnosed only with primary melanoma have relatively high survival rates, whereas when patients are diagnosed with metastatic melanoma, the survival rate is very low (Gershenwald et al., 2017). Currently, the mechanisms that drive melanoma progression and metastasis remain poorly understood; but better therapies are definitely required. BRAF mutations are most common in melanoma, occurring in around 50% of this disease (Akbani et al., 2015), which provides a possibility for targeted therapy. Indeed, the United States Food and Drug Administration (USFDA) has approved BRAF inhibitors (BRAFi) and MEK inhibitors (MEKi) as the standard treatment for metastatic melanoma patients harbouring BRAF mutations. However, drug resistance occurs in the majority of these patients within two years of treatment (Long et al., 2016). Therefore there is an urgent need to understand the mechanism of BRAFi and MEKi resistance, and find new therapeutic strategies for melanoma. One gene that has been linked to BRAFi resistance is the YAP, which is the key downstream effector of a pathway called the Hippo pathway. The Hippo pathway is an important regulator of organ growth in development. Deregulation of the Hippo pathway stimulates the activity of the YAP oncoprotein, which can cause several human cancers (Zanconato, Cordenonsi and Piccolo, 2016). However, the impacts of YAP deregulation in melanoma are not thoroughly understood.

In this project, the roles of YAP in melanoma were examined. Firstly, the impacts of knockdown, overexpression, and activation of YAP on anchorage-independent growth of melanoma cells were assessed using soft agar assays. The results showed that either YAP activation or overexpression promotes colony formation, whilst YAP knockdown reduces this, suggesting potential influences of YAP on melanoma tumorigenesis. Secondly, the effects of YAP in melanoma invasion and metastasis were investigated. Melanoma cells stably expressing an active YAP mutant (YAP-5SA) have a greater invasive ability, as determined with transwell invasion assays. A spontaneous murine metastasis model was used to investigate the impact of YAP on metastasis. The results demonstrated that YAP-5SA promotes metastasis to multiple organs such as the lung and the liver; YAP-5SA enhances vascularity and necrosis of primary melanoma. Thirdly, mechanisms responsible for YAP-induced invasion were explored. Four potential target genes of YAP, derived from RNA-sequencing data, were found crucial, as well as the key YAP transcription factor partners, TEAD1-4. Finally, a lipid-lowering drug called simvastatin was found to kill melanoma cells and inhibits YAP activity in vitro. A post-translational modification, geranylgeranylation, was found to be essential in the statin-induced melanoma cell death and YAP inactivation; RhoA and other geranylgeranylated proteins might be important in these phenotypes.

To conclude, this study explored the role of YAP in melanoma metastatic progression, and identified crucial transcription factors and target genes that mediate YAP-induced impacts on melanoma invasion. Additionally, inhibition of YAP and its mechanism in melanoma cells was preliminarily assessed using simvastatin. Understanding the molecular mechanism of melanoma metastasis and inhibition may help us establish more effective therapies for this disease.