Regulation of Ki-ras expression in Reuber H35 cells.

Glucagon at a low concentration has a stimulatory effect on Ki-ras expression, whereas, at high concentrations the hormone suppresses the level of the Ki-ras transcripts. Incubation of the hepatoma cells with 10 microM dibutyryl cyclic AMP results in suppression of Ki-ras expression but the phorbol ester, 21-O-tetradecanoylphorbol 13-acetate (TPA) causes an increase. Down regulation of protein kinase C by prolonged exposure of hepatoma cells to TPA causes a dramatic decrease in the glucagon-stimulated effect on Ki-ras expression. The presence of diacylglycerol for 2 h in the culture medium results in a significant increase in Ki-ras expression, while treatment of the cells with 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine, a potent inhibitor of protein kinase C, leads to a dramatic reduction. The calcium ionophore, A23187 is able to stimulate Ki-ras expression, whereas, addition of verapamil or EGTA results in its suppression. The present findings suggest that the inductive effect of glucagon on Ki-ras expression at low concentrations is via the activation of protein kinase C which causes phosphorylation of some regulatory proteins that may eventually affect the level of Ki-ras mRNA. The suppressive effect of glucagon at higher concentrations is via an increase in cAMP through activation of adenylate cyclase.