SYNERGISTIC INCREASE IN C-FOS EXPRESSION BY SIMULTANEOUS ACTIVATION OF THE RAS RAF/MAP KINASE-A AND PROTEIN-KINASE-A SIGNALING PATHWAYS IS MEDIATED BY THE C-FOS AP-1 AND SRE SITES/

Expression of the c-Sos proto-oncogene is induced by numerous stimuli some of which are transmitted through the Ras/Raf/MAP kinase or the cA MP-dependent protein kinase (PKA) pathways. The effect of cell-specifi c interactions between these pathways on c-fos expression was investig ated by exposing quiescent NIH3T3 cells to serum, forskolin, or a comb ination. Go-stimulation with serum and forskolin resulted in a more th an additive increase in c-fos transcription. Synergistic increase in c -fos promoter activity was also observed in transient transfection stu dies after co-stimulation with serum plus forskolin or co-transfection with c-Raf and PKA expression plasmids. Analysis of the cAMP signalin g pathway revealed that the synergy was neither due to an increase in PKA activity nor to Ser-133 phosphorylation/activation of CREB. The ac tivation status of the MAP kinases ERK1 and ERK2 in co-treated cells w as comparable to that in serum-treated cells. Go-stimulation with fors kolin did not alter the phosphorylation state of Elk-l compared to ser um-induced phosphorylation of Elk-1. Deletion of c-fos promoter elemen ts previously shown to be important for regulation of c-fos expression in response to mitogens indicates a role for SRE and FAP-1 elements. (C) 1998 Elsevier Science B.V.