Differentiation of neuroblastoma cells by phorbol esters and insulin-like growth factor 1 is associated with induction of retinoic acid receptor beta gene expression.

The retinoic acid (RA) receptor beta isoform (RARbeta) plays an important role in RA-induced differentiation of human neuroblastoma. In this study we show that insulin-like growth factor 1 (IGF-1) and tetradecanoyl phorbol acetate (TPA) induce RARbeta gene expression in neuroblastoma SH-SY5Y cells. IGF-1 and TPA caused a marked induction of RARbeta2 promoter activity and had a synergistic effect with RA that also upregulates transcription. The effect of RA is mediated by two RA responsive elements (RAREs), whereas the IGF-1 and TPA actions are independent of the RAREs and map to sequences that overlap the TATA box. These results suggest that the signaling pathways stimulated by TPA and IGF-1 could modify the components assembled at the core RARbeta2 promoter and activate transcription. Expression of RasVal12 mimics the effect of IGF-1 and TPA on the promoter, and a dominant negative Ras mutant abrogates activation. A dominant negative Raf also blocks activation showing that the Ras-Raf pathway mediates stimulation of the RARbeta2 promoter. Our results show that neuronal differentiation induced by non-retinoid agents that activate Ras is accompanied by increased transcription of the RARbeta gene.