Ras- and Raf-mediated regulation of transforming growth factor beta 1 gene expression by ligands of tyrosine kinase receptors in PC12 cells.

Different ligands of tyrosine kinase receptors have neurotrophic or mitogenic effects in PC12 cells. NFG and FGF, which cause morphological differentiation, as well as EGF, that induces cell growth, produce a significant increase of TGF-beta1 transcripts in PC12 cells. Sequences responsible for the transcriptional effects of the growth factors are located in the 5'-flanking region of the TGF-beta1 gene. The TGF-beta1 gene has two promoters and the growth factors significantly enhance the activity of constructs containing either the first or the second promoter. A functional p21ras is required for the regulation of TGF-beta1 by ligands of tyrosine kinase receptors since expression of oncogenic ras in PC12 cells also increases TGF-beta1 transcripts, and a dominant inhibitory ras mutant blocks activation of TGF-beta1 gene expression by NGF. Oncogenic raf stimulates the activity of both promoters and a dominant negative raf also significantly inhibits growth factor activation. As determined by Mv1Lu cell proliferation inhibition assay, PC12 cells release a significant amount of TGF-beta1 in a latent form and incubation with growth factors or expression of oncogenic ras further increase TCF-beta1 production. These results suggest that during proliferation or growth factor-induced differentiation of sympathetic neurons there is an increase in TGF-beta1 that could be an important mediator of neural cells function.