All-trans retinoic acid activity in Merkel cell carcinoma cells: implication of the retinoid pathway

Background: Merkel cell carcinoma (MCC) is a rare but aggressive skin cancer. About 80% of MCCs, are linked to oncogenic Merkel cell polyomavirus (MCPyV). The currently available MCC therapeutic options are unsatisfactory, therefore novel therapeutic approaches are required. The biological activity of all-trans retinoic acid (ATRA) is mediated by RAR/RXR receptors that activate genes crucial for cell differentiation. Dysregulations of RAR/RXR receptors lead to carcinogenesis. ATRA displays a strong in vitro/in vivo antitumor activity in different carcinoma types, but its effect in MCC is currently unknown. Herein, we investigated cell death effects of ATRA in MCC cells. Methods: For this purpose, in vitro in MCPyV-positive (MCCP), i.e., PeTa and WaGa, and -negative (MCCN), i.e., MCC13 and MCC26, MCC cell lines and control, normal human lung fibroblasts MRC-5 were tested with ATRA. The effect of ATRA was evaluated by testing MCC cell proliferation, migration and colony formation abilities. Apoptosis/cell death were evaluated via Annexin-V/P.I. assays. Apoptosis was evaluated by RT2 Profiler PCR mRNA array and by western blot (WB) analysis. Retinoid pathway was evaluated by RT2 Profiler PCR mRNA array. Results: ATRA treatment led to a significant reduction in MCC cell proliferation, migration and clonogenicity, while increasing apoptosis/cell death in MCC cell lines compared to untreated cells. MCCP cells were slightly more ATRA-sensitive compared to MCCN cells. No significant effects have been found in the ATRA-treated control cell line. Gene expression array indicated a significant overexpression of several pro-apoptotic genes in MCC cells. Consistently, high levels of pro-apoptotic proteins have been found following ATRA treatments in MCC cells, while being almost undetectable in untreated cells. Pro-apoptotic markers were almost undetectable in ATRA-treated MRC-5. Numerous retinoic signaling genes were differentially expressed in ATRA-treated MCC cells compared to untreated cells. Conclusions: Overall, in vitro data indicate that ATRA is effective in reducing MCC cell growth, while presenting strong pro-apoptotic effects and favoring cell death, by modulating the retinoic receptor pathway. These results, for the first time, point to ATRA as a potential novel effective antineoplastic drug for the MCC therapy.