Identification of c-fos sequences involved in induction by insulin and phorbol esters

We evaluated the mechanism of insulin and phorbol ester induction of the proto-oncogene c-fos in Chinese hamster ovary fibroblasts stably transformed with high levels of genes expressing normal or truncated human insulin receptors. Both insulin and the tumor-promoting phorbol ester phorbol 12-myristate 13-acetate (PMA) induced c-fos mRNA accumulation in cells expressing high numbers of normal human insulin receptors; PMA but not insulin was effective in the cells expressing the mutant receptor. Transient expression studies with plasmid constructions containing c-fos 5'-flanking sequences ligated to the bacterial chloramphenicol acetyltransferase gene indicated that sequences corresponding to the serum response element were required for induction of c-fos transcription by both insulin and PMA. The insulin-sensitive cells contained a nuclear factor, presumably a protein, which bound specifically to this sequence of the c-fos gene; the apparent affinity of this factor to the normal serum response element was not affected by prior treatment of the cells with insulin or PMA. This c-fos binding factor may prove to be important in the regulation of c-fos expression by insulin and activators of protein kinase C.