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

The retinoic acid (RA) receptor beta isoform (RAR beta) 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 a cetate (TPA) induce RAR beta gene expression in neuroblastoma SH-SY5Y cells . IGF-1 and TPA caused a marked induction of RAR beta 2 promoter activity a nd had a synergistic effect with RA that also upregulates transcription. Th e effect of RA is mediated by two RA responsive elements (RAREs), whereas t he 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 pathway s stimulated by TPA and IGF-1 could modify the components assembled at the core RAR beta 2 promoter and activate transcription. Expression of Ras(Val1 2) mimics the effect of IGF-1 and TPA on the promoter, and a dominant negat ive Ras mutant abrogates activation. A dominant negative Raf also blocks ac tivation showing that the Ras-Raf pathway mediates stimulation of the RAR b eta 2 promoter. Our results show that neuronal differentiation induced by n on-retinoid agents that activate Ras is accompanied by increased transcript ion of the RAR beta gene.