Mitogenic activity of phorbol esters and insulin-like growth factor 1 in chemically transformed mouse fibroblasts BP-A31: independent effects and differential sensitivity to inhibition by 3-isobutyl-1-methyl xanthine.

Mitogenic effects of agents activating either the protein kinase C (PDGF; phorbol esters) or the insulin-like growth factor 1 (IGF1)-receptor pathway were studied in quiescent chemically transformed mouse fibroblasts (BP-A31), by evaluating the rate of [3H]thymidine incorporation. Each of these pathways alone was found to be sufficient to sustain progression through the entire cell division cycle. The mitogenic activity of phorbol 12-myristate 13-acetate (PMA) but not that of insulin was blocked by staurosporine (an inhibitor of protein kinase C), in support of the notion that protein kinase C activation was required for the PMA-induced cell cycle progression. The mitogenic effects of PMA were potentiated by cycloheximide pretreatment, and they were abolished by 3-isobutyl-1-methyl xanthine (IBMX; a cyclic nucleotide phosphodiesterase inhibitor). PDGF (known to activate the phospholipase C-protein kinase C pathway) also displayed mitogenic activity in the cycloheximide-pretreated BP-A31 cells, and its effects were prevented by IBMX. In contrast, the mitogenic effects of insulin (at concentrations where it activates the IGF1 receptor) or of IGF1 neither were notably influenced by cycloheximide pretreatment nor were inhibited by IBMX (in the presence of IBMX, the onset of S-phase was delayed by several hours). The expression of the c-fos gene was absent at quiescence; its induction by growth factors was not proportional to their mitogenic potency. Thus, c-fos expression was strongly induced by PMA but only weakly by insulin. IBMX was a powerful inducer of c-fos gene expression but caused a decrease in the level of c-myc mRNA.