Modulation of Retinoic Acid Receptor Signaling Pathway via All-Trans Retinoic Acid in Merkel Cell Carcinoma Cells ^†

Abstract

The biological activity of retinoic acid or all-trans retinoic acid (ATRA) is mediated by retinoic receptors, which are ligand-dependent transcription factors that activate genes crucial for cell differentiation. Dysregulations of retinoic receptor signaling pathway led to carcinogenesis. A strong in vitro/in vivo antitumor activity of ATRA by modulating the retinoic pathway has been proved in carcinoma of different histotypes. However, the effect of this molecule in Merkel cell carcinoma (MCC), a rare but aggressive skin neoplasm of viral origin in 80% of cases, is unknown. Herein, we investigated the antineoplastic effect of ATRA in Merkel cell polyomavirus (MCPyV)-positive/-negative MCC cells and in human fibroblasts as controls. The antineoplastic effect of ATRA was evaluated at day 3 of treatment by testing MCC cell proliferation, migration, and clonogenicity. Apoptosis/cell death and cell cycle were evaluated via Annexin-V/propidium iodide (P.I.) and TALI assays, respectively. Apoptotic and retinoic pathways were evaluated by RT² Profiler PCR mRNA array, which allows the analysis of pro/anti-apoptotic and retinoic pathway genes (84 + 84 genes), as well as by Western blot (WB) analysis. ATRA treatment led to a strong reduction in MCC cell proliferation, migration and clonogenicity, while inducing cell cycle arrest and promoting apoptosis/death in MCC cells, with a more pronounced effect in MCPyV-positive MCC cells. A significant overexpression of various pro-apoptotic markers in ATRA-treated MCC cells compared to untreated cells was determined by gene expression array and WB analyses. No phenotypic and molecular effects were identified in ATRA-treated fibroblast control cells. Upon ATRA treatments in MCC cells, numerous retinoic signaling genes, such as BMP2, FOXA1, MAFB, RBP4, OLIG2, UCP1 were found to be differentially expressed compared to untreated cells. Our in vitro data indicate that ATRA is effective in reducing MCC cell growth while presenting strong pro-apoptotic effects and favoring cell cycle arrest/death via retinoic receptor signaling pathway regulation.