Induction of apoptosis by the retinoid inducible growth regulator RIG1 depends on the NC motif in HtTA cervical cancer cells

Background: Retinoid-inducible gene 1 (RIG1), also known as tazarotene-induced gene 3 or retinoic-acid receptor responder 3, is a growth regulator, which induces apoptosis and differentiation. RIG1 is classified into the NC protein family. This study investigated functional domains and critical amino acids associated with RIG1-mediated cell death and apoptosis. Results: Using enhanced green fluorescence protein (EGFP)-tagged RIG1 variants, RIG1 proteins with deletion at the NC domain significantly decreased cell death induced by RIG1, and fusion variants containing only the NC domain significantly induced apoptosis of HtTA cervical cancer cells. The EGFP-RIG1-induced apoptosis was significantly decreased in cells expressing (NC113)-C-112 motif double- (NC -> FG) or triple- (NCR -> FGE) mutated RIG1 variants. Using dodecapeptides, nuclear localization and profound cell death was observed in HtTA cells expressing wild type RIG1(111-123) or Leu(121)-mutated RIG1(111-123):L -> C peptide, but peptides double- or triple-mutated at the NC motif alone, RIG1(111-123):NC -> FG or RIG1(111-123):NCR -> FGE, were cytoplasmically localized and did not induce apoptosis. The RIG1(111-123) also induced apoptosis of A2058 melanoma cells but not normal human fibroblasts. Conclusion: The NC domain, especially the NC motif, plays the major role in RIG1-mediated proapoptotic activity. The RIG1(111-123) dodecapeptide exhibited strong pro-apoptotic activity and has potential as an anticancer drug.