Inhibition of glycogen synthesis by epidermal growth factor in hepatocytes. The role of cell density and pertussis toxin-sensitive GTP-binding proteins

Regulation of glycogen synthase in rat hepatocytes. Evidence for multiple signaling pathways

We examined the signaling pathways regulating glycogen synthase (GS) in primary cultures of rat hepatocytes. The activation of GS by insulin and glucose was completely reversed by the phosphatidylinositol 3-kinase inhibitor wortmannin. Wortmannin also inhibited insulin-induced phosphorylation and activation of protein kinase B/Akt (PKB/Akt) as well as insulin-induced inactivation of GS kinase-3 (GSK-3), consistent with a role for the phosphatidylinositol 3-kinase/PKB-Akt/GSK-3 axis in insulin-induced GS activation. Although wortmannin completely inhibited the significantly greater level of GS activation produced by the insulin-mimetic bisperoxovanadium 1,10-phenanthroline (bpV(phen)), there was only minimal accompanying inhibition of bpV(phen)-induced phosphorylation and activation of PKB/Akt, and inactivation of GSK-3. Thus, PKB/Akt activation and GSK-3 inactivation may be necessary but are not sufficient to induce GS activation in rat hepatocytes. Rapamycin partially inhibited the GS activation induced by bpV(phen) but not that effected by insulin. Both insulin- and bpV(phen)-induced activation of the atypical protein kinase C (zeta/lambda) (PKC (zeta/lambda)) was reversed by wortmannin. Inhibition of PKC (zeta/lambda) with a pseudosubstrate peptide had no effect on GS activation by insulin, but substantially reversed GS activation by bpV(phen). The combination of this inhibitor with rapamycin produced an additive inhibitory effect on bpV(phen)-mediated GS activation. Taken together, our results indicate that the signaling components mammalian target of rapamycin and PKC (zeta/lambda) as well as other yet to be defined effector(s) contribute to the modulation of GS in rat hepatocytes.

Control of glycogen synthesis in cultured human muscle cells

The regulation of glycogen synthesis and associated enzymes was studied in human myoblasts and myotubes maintained in culture. Both epidermal growth factor (EGF) and insulin stimulated glycogen synthesis approximately 2-fold, this stimulation being accompanied by a rapid and stable activation of the controlling enzyme glycogen synthase (GS). EGF also caused inhibition of glycogen synthase kinase 3 (GSK-3) and activation of the alpha isoform of protein kinase B (PKB) with the time-course and magnitude of its effects being similar to those induced by insulin. An inhibitor of the mitogen-activated protein (MAP) kinase pathway did not prevent stimulation of GS by EGF, suggesting that this pathway is not essential for the effect. A partial decrease in the fold activation of GS was, however, observed when p70(S6k) activation was blocked with rapamycin, suggesting a contribution of this pathway to the control of GS by either hormone. Wortmannin, a selective inhibitor of phosphatidylinositol 3'-kinase (PI-3 kinase) completely blocked the effects of both EGF and insulin in these cells. These results demonstrate that EGF, like insulin, activates glycogen synthesis in muscle, acting principally via the PKB/GSK-3 pathway but with a contribution from a rapamycin-sensitive component that lies downstream of PI-3 kinase.

Transforming growth factor beta increases the activity of phosphatidate phosphohydrolase-1 in rat hepatocytes

Phosphatidic acid (PA) is a potent second messenger arising from growth factor-induced stimulation of phospholipase D which hydrolyses phosphatidylcholine. PA is hydrolysed to diacylglycerol by PA phosphohydrolase (PAP) which exists in two forms: PAP-1 and PAP-2. In rat hepatocyte cultures, overnight (20h) incubation with transforming growth factor (TGF) beta (1 ng/ml) increased PAP-1 activity two-fold. This effect was concentration and time dependent and was greatest at low cell density. The TGFbeta effect on PAP-1 was additive to stimulation induced by dexamethasone but not by glucagon and it reversed the inhibition by insulin. Epidermal growth factor had no effect on PAP-1 activity. None of the above hormones or growth factors affected the subcellular distribution of PAP-1. Stimulation of PAP-1 by TGFbeta may be involved in mediating some of its biological effects.

EGF responsiveness of hepatocytes after partial hepatectomy

We investigate the effect of EGF on IP3 production, PLC gamma phosphorylation, calcium transients in rat hepatocytes isolated in quiescent liver (G0 phase of cell cycle) and at 4 h (G1 phase of cell cycle) and 24 h (M phase of cell cycle) after partial hepatectomy. Our results show that EGF does not utilize IP3 and calcium as its signal transduction molecules when the hepatocytes are in vivo stimulated to entry in the cell cycle. In particular the growth factor does not phosphorylate PLC gamma and induces a decrease in IP3 content. These data suggest that EGF utilizes different signal transduction to send information from receptor to nucleus during PH with respect to the quiescent liver.

Epidermal growth factor administration decreases liver glycogen and causes mild hyperglycaemia in mice

Several laboratories report different effects of epidermal growth factor (EGF) on glycogen metabolism in hepatocytes. The discrepancies may be attributed to differences in the experimental conditions. It is therefore important to establish the actual effect of EGF in vivo. Because large physiological variations of EGF concentration in plasma occur in mice, we used this species to address this question. In freshly isolated mouse hepatocytes, EGF increased glycogen degradation in a dose-dependent manner. The maximal effect (36% increase over basal glycogenolysis) was smaller than maximal effects of classical glycogenolytic hormones like adrenaline or glucagon (more than 150% increase over basal). This is in keeping with the smaller effect of EGF on phosphorylase a activity. In contrast with these hormones, EGF did not inhibit glycolysis. Thus these effects of EGF in mouse hepatocytes are similar to those recently described by us in rat hepatocytes [Quintana, Grau, Moreno, Soler, Ramirez and Soley (1995) Biochem J 308, 889-894]. When administered to whole animals, EGF increased phosphorylase a activity, decreased the glycogen content in the liver and caused mild hyperglycaemia. Taking together the results obtained for isolated cells and for whole animals, we suggest that the glucosyl residues released from glycogen are used mostly by the liver rather than released to the circulation. This would be different from the action of the classical glycogenolytic hormones, adrenaline and glucagon.

The early stimulation of glycolysis by epidermal growth factor in isolated rat hepatocytes is secondary to the glycogenolytic effect

We have studied the relationship between the effect of epidermal growth factor (EGF) on glycogen metabolism and its effect on glycolysis, in rat hepatocyte suspensions. Although 10 nM glucagon or 10 microM adrenaline increased glycogen degradation by more than 120%, 10 nM EGF increased glycogenolysis by less than 20% in hepatocytes incubated in glucose-free medium. Both glucagon and adrenaline increased phosphorylase a activity by more than 130%; EGF increased this activity by about 30%. Under basal conditions, 65% of the glucosyl residues were released as free glucose and about 30% ended up as C3 molecules (lactate and pyruvate). Both glucagon and adrenaline decreased the proportion of glucosyl units that rendered glycolysis end-products (to 2% for glucagon and 6% for adrenaline) and increased the proportion that ended up as free glucose (to 94% and 88% of the glucosyl residues for glucagon and adrenaline respectively). EGF increased the production of both free glucose and lactate+pyruvate, but the proportion of glucosyl residues that ended up as free glucose or glycolysis end-products was unchanged. In glycogen-depleted hepatocytes incubated in the presence of 25 mM glucose, EGF affected neither glycogen deposition nor glycolysis. EGF increased cytosolic free Ca2+, and neomycin decreased both the Ca2+ signal and the glycogenolytic effect. In conclusion, our results indicate that the effect of EGF on glycolysis is secondary to the Ca(2+)-mediated stimulation of glycogenolysis in rat hepatocyte suspensions.

Role of heterotrimeric G-proteins in epidermal growth factor signalling

Since in 1986 it was reported that a pertussis toxin-sensitive substrate was involved in the Ca2+ signal induced by epidermal growth factor (EGF) in rat hepatocytes, much evidence accumulated to implicate heterotrimeric G-proteins in EGF action. EGF can also induce a cyclic AMP signal, but while the generation of a Ca2+ signal appears to be quite general in EGF action, the increase in cyclic AMP occurs only in few cell types. In non-transformed cell types these effects appear to involve G-proteins. EGF not only induces cell proliferation but also interacts with hormones in the short-term control of cell function in quiescent cells. Most of the known interactions are on cyclic AMP mediated hormone effects, and in many cases, the interaction between EGF and hormones involves G-proteins. Here we review the evidence accumulated in recent years that implicate G-proteins in EGF action. An understanding of the mechanisms involved may reveal new mechanisms of G-protein regulation and will contribute to our knowledge of EGF function and signal transduction.

Cytochalisin D exerts stimulatory and inhibitory effects on insulin-induced glucokinase mRNA expression in hepatocytes

The microfilament cytoskeleton is postulated to have a role in the localization, transport and anchorage of certain specific mRNAs. We investigated the effects of cytochalasin D, a fungal metabolite that binds to actin and disrupts the microfilament structure, on insulin-induced expression of glucokinase mRNA in rat hepatocyte cultures. Cytochalasin-D significantly potentiates insulin-induced glucokinase mRNA expression at 100 nM concentration but counteracts glucokinase expression at 2-20 microM. The latter effect is at least in part due to an increase in glucokinase mRNA degradation. This effect of cytochalasin D cannot be accounted for by an increase in cAMP and is also not due to a non-specific effect on mRNA degradation since albumin mRNA levels were not affected by cytochalasin-D and actin mRNA and tubulin mRNA levels were increased. Measurement of glucokinase mRNA release from digitonin-permeabilized hepatocytes suggests that cytochalasin D does not cause acute dissociation of glucokinase mRNA from its binding site. The increased degradation of glucokinase mRNA suggests involvement of the cytoskeleton in glucokinase mRNA stability. However, an additional effect of cytochalasin D on the insulin signalling mechanism cannot be excluded.