Role of protein kinase C in insulin's regulation of c-fos transcription.

One of insulin's actions is the induction of DNA synthesis and cell division, but little is known about the molecular mechanisms involved. Previous studies indicate that insulin stimulates cell division and regulates the expression of several genes in rat H4IIE (H4) hepatoma cells. One of these genes is the proto-oncogene c-fos, a cellular gene whose deregulation has been implicated in the process of cellular differentiation and division. We have shown that insulin induces transcription of the c-fos gene in H4 cells. In the present study, the phorbol ester, phorbol 12-myristate 13-acetate (PMA), stimulated c-fos transcription in a rapid and dose-dependent manner with an 800% increase in transcription following 15-30 min of addition. This increase in c-fos transcription was transitory, returning towards baseline transcription rates within 120 min. PMA stimulated the translocation of protein kinase C (PKC) from the cytoplasm to the membrane in H4 hepatoma cells, as evidenced by a 77% decrease in cytosolic PKC and a 29% increase in membrane PKC activity following 10 min of treatment. Insulin addition to H4 cells for 10 min also resulted in a 31% decrease in cytosolic PKC activity, suggesting a translocation response. When H4 cells were pretreated with PMA for 24 h, there was a decrease of 20-45% in both cytosolic and membrane PKC activity and a complete loss of PMA's induction of c-fos transcription. Thus, the cells were functionally desensitized to further PMA addition. When cells were pretreated with PMA for 24 h, the insulin-induced increase in transcription of c-fos was reduced by 50%. Western blot analysis indicated that the PKC-beta isozyme followed a translocation pattern almost identical with that of total PKC activity. These results suggest that a PMA-sensitive form of PKC is preferentially lost upon PMA pretreatment and that this PKC subtype may be necessary for insulin to fully induce c-fos gene expression.