RAS-MEDIATED 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 mi togenic effects in PC12 cells, NFG and FGF, which-cause morphological differentiation, as well as EGF, that induces cell growth, produce a s ignificant increase of TGF-beta 1 transcripts in PC12 cells. Sequences responsible for the transcriptional effects of the growth factors are located in the 5'-flanking region of the TGF-beta 1 gene, The TGF-bet a 1 gene has two promoters and the growth factors significantly enhanc e the activity of constructs containing either the first or the second promoter, A functional p21(ras) is required for the regulation of TGF -beta 1 by ligands of tyrosine kinase receptors since expression of on cogenic I as in PC12 cells also increases TGF-beta 1 transcripts, and a dominant inhibitory ras mutant blocks activation of TGF-beta 1 gene expression by NGF, Oncogenic raf stimulates the activity of both promo ters and a dominant negative raf also significantly inhibits growth fa ctor activation. As determined by Mv1Lu cell proliferation inhibition assay, PC12 cells release a significant amount of TGF-beta 1 in a late nt form and incubation with growth factors or expression of oncogenic ras further increase TGF-beta 1 production. These results suggest that during proliferation or growth factor-induced differentiation of symp athetic neurons there is an increase in TGF-beta 1 that could be an im portant mediator of neural cells function.