YAP Signaling is Involved in Keratinocyte Proliferation and Tumorigenesis Mediated by RIPK4 Knockdown

Abstract

Cutaneous squamous cell carcinoma (cSCC) is the second most common cancer of the skin, with approximately 800,000 new cases diagnosed yearly in the US, and its incidence is continuing to increase. Although local cSCC is curable with conventional treatments, cSCC exhibits a significant risk of metastasis and no effective treatment is currently available for patients with the metastatic disease, leading to a survival rate of less than 20% over 10 years. Thus, a better understanding of the molecular mechanisms of cSCC pathogenesis is urgently needed for developing novel effective therapies to combat this disease. Recently, we have found that RIPK4 mRNA levels are significantly lower in human cSCC samples than in normal human keratinocytes. Interestingly, newly genomic studies identified the protein kinase RIPK4 (also known as PKK) as a recurrently mutated gene in aggressive and metastatic cSCCs. We previously demonstrated that knockdown of RIPK4 expression increased the proliferation of human cSCC cells in vitro as well as tumor growth of human cSCC in a mouse xenograft model. In addition, we showed that conditional knockout of RIPK4 in mouse epidermis greatly enhanced the development of mouse cSCC induced by chemical carcinogen treatment, suggesting RIPK4 functions as a tumor suppressor. Although we showed that the NFkB and p63 pathways are involved in the RIPK4-mediated cSCC development, the link of RIPK4 with the Hippo pathway has not been reported. In our current study, we investigated whether YAP1, a transcription regulator downstream of the Hippo pathway, is involved in RIPK4-mediated tumor formation. By using small-hairpin RNA (shRNA) targeting the RIPK4 gene and small interfering RNA targeting the YAP1 gene, we demonstrated that suppression of RIPK4 led to tumor development in keratinocytes via upregulating YAP signaling. More strikingly, silencing YAP1 expression led to a dramatic decrease in the size of colonies formed due to RIPK4 depletion. Our results may provide a novel therapeutic strategy by targeting the YAP signaling pathway in RIPK4-deficient cSCC, potentially improving outcomes for patients with this aggressive and metastatic disease.