Nerve growth factor activates the RARβ2 promoter by a Ras-dependent mechanism

Abstract

Nerve growth factor (NGF) and retinoic acid (RA) exert important actions on PC12 cells. We have previously shown that incubation with NGF induces retinoic acid receptor β (RARβ) binding to a hormone response element in PC12 cells. In this study we show that NGF increases RARβ protein levels by enhancing basal RARβ2 promoter activity, and potentiates stimulation by RA in transient transfection assays. The effect of RA is mediated by a RA response element (RARE) located at −37/−53 and mutation of this element abolishes activation by the retinoid, as well as cooperation with NGF. However, the action of NGF is independent of the RARE and is mediated by sequences overlapping the TATA box and the INR comprising nucleotides −59 to +14. NGF produces a strong decrease in some of the complexes that bind to the INR. These results suggest that the RARβ2 gene could be in a basal repressed state and NGF could increase RARβ2 transcription by inducing the release of some inhibitory factors from the INR. Functional Ras is required for RARβ2 promoter activation by NGF because expression of oncogenic Ras increases promoter activity and a dominant inhibitory Ras mutant blocks the effect of NGF. Oncogenic Raf also mimics the effect of NGF on the promoter. Other ligands of tyrosine kinase receptors that stimulate Ras also cause RARβ2 promoter activation and act cooperatively with RA. These results indicate the existence of cross-coupling of the Ras–Raf signal transduction pathway with retinoid receptor pathways which could increase sensitivity to RA and be important for PC12 cell function.