INSERM U55 , Paris , France .
Exp Cell Res 182 : 129-43 ( 1989)
Abstract
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 .