Abstract 1502: PRAME modulates the effect of retinoids on keratinocyte differentiation and cell cycle progression in basal cell carcinoma and cutaneous squamous cell carcinoma

Abstract Substantial research supports the use of retinoids as prophylactics and treatments for keratinocyte carcinomas (KC). However, the practical applications of these compounds for KC management are limited by a poor understanding of the molecular basis of retinoid resistance. Preferentially Expressed Antigen in Melanoma (PRAME) is a cancer-testis antigen that functions as a retinoic acid receptor repressor and as a substrate recognition subunit for Cullin-2 E3 ubiquitin ligase, targeting degradation of cell cycle regulatory factors. PRAME is ectopically expressed in both types of KC: basal cell carcinoma (BCC) and cutaneous squamous cell carcinoma (SCC). The functions and clinicopathological relevance of PRAME expression in BCC and SCC are unknown. The purpose of this study is to investigate the effect of PRAME on retinoid sensitivity and the pathogenesis of BCC and SCC. Cell lines representative of human BCC, SCC and immortalized keratinocytes were subjected to shRNA-mediated PRAME knockdown, PRAME knockout using CRISPR-Cas9, and ORF overexpression. Cells were treated with retinoids (all-trans retinoic acid, acitretin or tazarotene), and subjected to assays for cell proliferation (e.g., flow cytometric cell cycle analysis, label-free confluence tracking, Ki-67 immunofluorescence, etc.), cell death/survival (e.g., clonogenic assay, caspase staining, etc.) and for retinoid-induced gene expression changes (RT-qPCR and immunoblotting, etc.). PRAME overexpression attenuated retinoid-induced changes in cytokeratin expression in immortalized keratinocytes and SCC cells. PRAME knockdown in BCC and SCC cells augmented retinoid-induced changes in cytokeratin expression. Importantly, PRAME overexpression attenuated the pro-differentiation synergy between high calcium culture conditions and retinoid treatment in immortalized keratinocytes. PRAME also altered expression of several cell cycle regulatory proteins (p14/ARF, p16/INK4A, p21/Waf, p27/Kip1, etc.), which was in turn reflected accelerated cell proliferation dynamics in some PRAME-overexpressing cell lines. PRAME also conferred increased resistance to retinoid-induced cell cycle arrest in SCC cells, which was restored by PRAME knockdown. Furthermore, PRAME expression was inversely correlated with the pro-apoptotic protein Tazarotene Inducible Gene 3 (TIG3) in SCC cells. Our results implicate PRAME in KC tumorigenesis and differentiation, and suggest that PRAME could serve as a therapeutic target to optimize the use of retinoids to manage these cancers. We propose future studies to determine the clinicopathological relevance of PRAME expression in KC, PRAME-targeting strategies to enhance retinoid sensitivity, and the use skin tissue organoid models to study the effect of PRAME on epidermal differentiation dynamics in premalignant skin and KCs. Citation Format: Brandon Liam Ramchatesingh, Ivan Litvinov. PRAME modulates the effect of retinoids on keratinocyte differentiation and cell cycle progression in basal cell carcinoma and cutaneous squamous cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1502.