Desensitization of prostaglandin F2 alpha-stimulated inositol phosphate generation in NIH-3T3 fibroblasts transformed by overexpression of normal c-Ha-ras-1, c-Ki-ras-2 and c-N-ras genes

Glycogenolytic and antiglycogenolytic prostaglandin E2 actions in rat hepatocytes are mediated via different signalling pathways

Prostaglandin E2 has been reported both to stimulate glycogen-phosphorylase activity (glycogenolytic effect) and to inhibit the glucagon-stimulated glycogen-phosphorylase activity (antiglycogenolytic effect) in rat hepatocytes. It was the purpose of this study to resolve this apparent contradiction and to characterize the signalling pathways and receptor subtypes involved in the opposing prostaglandin E2 actions. Prostaglandin E2 (10 microM) increased glucose output, glycogen-phosphorylase activity and inositol trisphosphate formation in hepatocyte cell culture and/or suspension. In the same systems, prostaglandin E2 decreased the glucagon-stimulated (1 nM) glycogen-phosphorylase activity and cAMP formation. The signalling pathway leading to the glycogenolytic effect of PGE2 was interrupted by incubation of the hepatocytes with 4 beta-phorbol 12-myristate 13-acetate (100 nM) for 10 min, while the antiglycogenolytic effect of prostaglandin E2 was not attenuated. The signalling pathway leading to the antiglycogenolytic effect of prostaglandin E2 was interrupted by an incubation of cultured hepatocytes with pertussis toxin (100 ng/ml) for 18 h, whereas the glycogenolytic effect of prostaglandin E2 was enhanced. The EP1/EP3 prostaglandin-E2-receptor-specific prostaglandin E2 analogue Sulproston had a stronger glycogenolytic potency than the EP3 prostaglandin-E2-receptor-specific prostaglandin E2 analogue Misoprostol. The antiglycogenolytic potency of both agonists was equal. It is concluded that the glycogenolytic and the antiglycogenolytic effects of prostaglandin E2 are mediated via different signalling pathways in hepatocytes possibly involving EP1 and EP3 prostaglandin E2 receptors, respectively.

Inhibition by the protein kinase C activator 4 beta-phorbol 12-myristate 13-acetate of the prostaglandin F2 alpha-mediated and noradrenaline-mediated but not glucagon-mediated activation of glycogenolysis in rat liver

In perfused rat livers, infusion of prostaglandin F2 alpha (PGF2 alpha) or noradrenaline increased glucose and lactate output and reduced flow. Glucagon increased glucose output and decreased lactate output without influence on flow. Infusion of phorbol 13-myristate 14-acetate (PMA) for 20 min prior to these stimuli strongly inhibited the metabolic and hemodynamic effects of noradrenaline, reduced the metabolic actions of PGF2 alpha but did not alter the effects of glucagon. In isolated rat hepatocytes PGF2 alpha, noradrenaline and glucagon activated glycogen phosphorylase but only PGF2 alpha and noradrenaline increased intracellular inositol 1,4,5-trisphosphate (InsP3). The noradrenaline- or PGF2 alpha-elicited activation of glycogen phosphorylase and increase in InsP3 were largely reduced after preincubation of the cells for 10 min with PMA, whereas the glucagon-mediated enzyme activation was not affected. In contrast to PMA, the phorbol ester 4 alpha-phorbol 13,14-didecanoate, which does not activate protein kinase C, did not attenuate the PGF2 alpha- and noradrenaline-elicited stimulation of glucose output, glycogen phosphorylase and InsP3 formation. Stimulation of InsP3 formation by AlF4-, which activates phospholipase C independently of the receptor, was not attenuated by prior incubation with PMA. Plasma membranes purified from isolated hepatocytes had both a high-capacity, low-affinity and a low-capacity, high-affinity binding site for PGF2 alpha. The Kd of the high-capacity, low-affinity binding site was close to the concentration of PGF2 alpha that increased glycogen phosphorylase activity half-maximally. Binding to the high-capacity, low-affinity binding site was enhanced by guanosine 5'-O-(3-thio)triphosphate (GTP[S]). This high-capacity, low-affinity site might thus represent the receptor. The Bmax and Kd of the high-capacity site, as well as the enhancement by GTP[S] of PGF2 alpha binding to this site, remained unaffected by PMA treatment. It is concluded that, in hepatocytes, activation of protein kinase C by PMA interrupted the InsP3-mediated signal pathway from PGF2 alpha via a PGF2 alpha receptor and phospholipase C to glycogen phosphorylase at a point distal of the receptor prior to phospholipase C.

NIH-3T3 cells transformed with a ras oncogene exhibit a protein kinase C-mediated inhibition of agonist-stimulated Ca2+ inflow

1. The ability of bombesin or platelet-derived growth factor (PDGF) to stimulate Ca2+ inflow (assessed by measuring changes in the intracellular free Ca2+ concentration in cells loaded with fura-2) in NIH-3T3 cells transformed with the EJ/T24-Ha-ras-1 oncogene is inhibited when compared with the action of the agonists on wild-type cells. 2. The effects of transformation with the ras oncogene are associated with complete inhibition of the ability of bombesin to release Ca2+ from intracellular stores, a substantial decrease in the number of bombesin receptors, no change in the ability of foetal calf serum or ionomycin to release Ca2+ from intracellular stores and the activation of protein kinase C. 3. The effects of transformation with the H-ras oncogene on the ability of bombesin or PDGF to stimulate Ca2+ inflow were mimicked by a 30 min exposure of wild-type cells to phorbol dibutyrate. This action of phorbol dibutyrate was completely blocked by prior treatment of wild-type cells for 24 h with the phorbol ester. 4. It is concluded that one of the actions of the H-ras oncogene in fibroblasts is to inhibit agonist-stimulated Ca2+ inflow by a mechanism which involves the activation of protein kinase C.