Serum response element and flanking sequences mediate the synergistic transcriptional activation of c-fos by 12-O-tetradecanoylphorbol-13-acetate and cholera toxin in AKR-2B cells.

12-O-Tetradecanoylphorbol-13-acetate (TPA) and cholera toxin have been shown previously to act synergistically to stimulate traverse of G0-G1 and entry into S phase in quiescent mouse fibroblasts. These agents also have a synergistic effect on the induction of the endogenous c-fos gene, as well as a transfected reporter construct containing the mouse fos promoter/enhancer region from -397 to +1 cloned upstream of luciferase. A detailed mutational analysis of the c-fos-regulatory region revealed that the synergy between TPA and cholera toxin requires multiple discrete elements, including the binding sites for the serum response factor (-308 to -299), p62/Elk-1 (-316 to -309), on the 5' side of the serum response element, and a CCAAT or E box-binding protein(s) on the 3'-flanking side of the serum response element (-303 to -295 or -297 to -292, respectively). The putative cyclic AMP response element (-65 to -58), shown to be activated in a number of cell types after increases in cyclic AMP levels, mediated an induction by TPA but not by cholera toxin in AKR-2B cells, and was not required for the synergistic transactivation induced by the combination of TPA and cholera toxin.