The NOTCH-RIPK4-IRF6-ELOVL4 Axis Suppresses Squamous Cell Carcinoma

SIMPLE SUMMARY: The aim of our study is to understand the signaling mechanisms behind the tumor suppressive function of RIPK4 (receptor-interacting serine/threonine protein kinase 4) in squamous cell carcinomas (SCCs) of skin, head, and neck regions. SCCs are some of the deadliest cancers, with poor survival rate, and understanding molecular mechanisms behind the tumor formation will help us to develop effective therapies. Through gene profiling and targeted CRISPR screening approaches in mouse models of cancer, we identified ELOVL4, (elongation of very-long-chain fatty acid-4) as a downstream target of RIPK4. Previous studies have shown that RIPK4 is a NOTCH target gene and RIPK4 phosphorylates IRF6 (interferon regulatory factor 6). Our data from in vitro and in vivo mouse experiments show that the NOTCH-RIPK4-IRF6-ELOVL4 signaling axis functions as a potent tumor suppressor in SCCs. ABSTRACT: Receptor-interacting serine/threonine protein kinase 4 (RIPK4) and its kinase substrate the transcription factor interferon regulatory factor 6 (IRF6) play critical roles in the development and maintenance of the epidermis. In addition, ourselves and others have previously shown that RIPK4 is a NOTCH target gene that suppresses the development of cutaneous and head and neck squamous cell carcinomas (HNSCCs). In this study, we used autochthonous mouse models, where the expression of Pik3ca(H1047R) oncogene predisposes the skin and oral cavity to tumor development, and show that not only loss of Ripk4, but also loss of its kinase substrate Irf6, triggers rapid SCC development. In vivo rescue experiments using Ripk4 or a kinase-dead Ripk4 mutant showed that the tumor suppressive function of Ripk4 is dependent on its kinase activity. To elucidate critical mediators of this tumor suppressive pathway, we performed transcriptional profiling of Ripk4-deficient epidermal cells followed by multiplexed in vivo CRISPR screening to identify genes with tumor suppressive capabilities. We show that Elovl4 is a critical Notch-Ripk4-Irf6 downstream target gene, and that Elovl4 loss itself triggers SCC development. Importantly, overexpression of Elovl4 suppressed tumor growth of Ripk4-deficient keratinocytes. Altogether, our work identifies a potent Notch1-Ripk4-Irf6-Elovl4 tumor suppressor axis.