Apoptosis induced by synthetic retinoic acid CD437 on human melanoma A375 cells involves RIG-I pathway

Human malignant melanoma is notoriously resistant to currently available pharmacological modulation. Our aim was to evaluate the anti-tumor effect of a novel synthetic retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carbo-xylic acid (CD437) on melanoma cell line A375. Analysis of cell morphology showed that CD437 promoted marked apoptosis in A375 cells. To explore the mechanisms of CD437-induced apoptosis, an NF-κB-luciferase reporter assay was performed, demonstrating that apoptosis induction by CD437 required activation of transcription factor NF-κB. Importantly, based on the findings that RIG-I (retinoic acid inducible gene I) can be induced by retinotic acid and can activate NF-κB through a CARD-containing adaptor protein VISA, we proposed a hypothesis that RIG-I was involved in the signal pathway of NF-κB activation induced by CD437 through the adaptor protein VISA. By specially cleaving VISA with hepatitis C virus (HCV) non-structural (NS)3/4A, the RIG-I pathway was blocked, with subsequent simultaneous inhibition of CD437-induced NF-κB activation and cell apoptosis in A375 cells. These results support our hypothesis and suggest that RIG-I may be a useful intermediate biologic marker for retinoid chemoprevention and treatment studies.