Protein kinase C isozyme distribution and down-regulation in relation to insulin-stimulated c-fos induction

Insulin can stimulate the expression of c-fos and other immediate early genes in many insulin-sensitive cell types. We found previously that this effect of insulin was essentially normal in cells in which protein kinase C (PKC) was down-regulated by 16 h of exposure to 16 microM phorbol 12-myristate 13-acetate (PMA). However, recent studies by other groups have suggested that much of the insulin response was lost in down-regulated cells and that at least part of the remaining response could be due to a species of PKC beta that is resistant to down-regulation. To resolve these discrepancies, we performed PKC enzyme assays and immunoblots on HIRc-B, H4IIEC3, and BC3H-1 cells before and after down-regulation with PMA. PKC enzyme activity was undetectable in the first two cell types after down-regulation; in addition, in all three cells the expressed PKC isozymes other than PKC zeta were completely down-regulated by the PMA exposure. Neither PKC beta 1 nor beta 2 was expressed in any of the cells, as determined by immunoblotting with isotype-specific antibodies. As in our previous studies, c-Fos mRNA accumulation in response to insulin was essentially normal in the down-regulated HIRc-B and H4IIEC3 cells, whereas the response to re-added PMA was completely abolished. PKC zeta was expressed in all three cell types and was not down-regulated by PMA; these and other considerations leave open the possibility that this and other "atypical" PKC isotypes could play a role in insulin action.