PROJECT SUMMARY Kinases are essential to development and deregulation of their activity contributes to cancer progression in multiple human malignancies. We recently demonstrated that the receptor tyrosine kinase rearranged during transfection (RET) enforces the undifferentiated state in epidermal progenitor cells and is aberrantly activated in cutaneous squamous cell carcinoma (cSCC). Our long-term goal is to help develop topical selective RET inhibitors for effective therapeutic prevention of cSCC. The overall objectives of this proposal are to (i) elucidate the molecular mechanism(s) of RET regulation in human epidermis and (ii) define RET-regulated impacts on the tumor immune microenvironment while demonstrating the therapeutic efficacy of targeting RET for cSCC prevention. Our central hypothesis is that sustained expression of RET is an early and actionable event in cSCC development that results from disruption of E3 ligase interaction, enabling deregulation of downstream kinases and promoting immune escape through downregulation of MHC expression. The rationale for this project is that a determination of RET regulation and activation in the epidermis will create a strong conceptual framework for development of therapeutic strategies aimed at controlling RET signaling, such as inhibiting RET for effective secondary prevention of cSCC in at-risk persons. In the first Aim of our proposal, we will test the central hypothesis by characterizing the RET-Siah1 interaction and defining the key downstream branches regulated by epidermal RET. In the second Aim, we will investigate the timing and consequences of RET activation during the progression of human cSCC from normal skin to carcinoma in situ and evaluate the efficacy of targeting RET through genetic ablation or pharmacologic inhibition on photocarcinogenesis in vivo. The research proposed in this application is innovative, in our opinion, because it focuses on inhibiting a novel molecular target to therapeutically restore terminal maturation as a means of skin cancer prevention. The proposed research is significant because it is expected to justify preclinical and clinical studies on RET inhibition for secondary prevention of cSCC. Ultimately, such knowledge has the potential to reduce the incidence of skin cancer in this country.