Epidermal growth factor mimics insulin effects in rat hepatocytes

Regulation of Energy Metabolism by Receptor Tyrosine Kinase Ligands

Metabolic diseases, such as diabetes, obesity, and fatty liver disease, have now reached epidemic proportions. Receptor tyrosine kinases (RTKs) are a family of cell surface receptors responding to growth factors, hormones, and cytokines to mediate a diverse set of fundamental cellular and metabolic signaling pathways. These ligands signal by endocrine, paracrine, or autocrine means in peripheral organs and in the central nervous system to control cellular and tissue-specific metabolic processes. Interestingly, the expression of many RTKs and their ligands are controlled by changes in metabolic demand, for example, during starvation, feeding, or obesity. In addition, studies of RTKs and their ligands in regulating energy homeostasis have revealed unexpected diversity in the mechanisms of action and their specific metabolic functions. Our current understanding of the molecular, biochemical and genetic control of energy homeostasis by the endocrine RTK ligands insulin, FGF21 and FGF19 are now relatively well understood. In addition to these classical endocrine signals, non-endocrine ligands can govern local energy regulation, and the intriguing crosstalk between the RTK family and the TGFβ receptor family demonstrates a signaling network that diversifies metabolic process between tissues. Thus, there is a need to increase our molecular and mechanistic understanding of signal diversification of RTK actions in metabolic disease. Here we review the known and emerging molecular mechanisms of RTK signaling that regulate systemic glucose and lipid metabolism, as well as highlighting unexpected roles of non-classical RTK ligands that crosstalk with other receptor pathways.

Does epidermal growth factor participate in the regulation of glucose, insulin and glucagon levels?

BACKGROUND/AIMS

The presence of receptors for epidermal growth factor (EGF) on beta cells in the rat pancreatic islets has been established, but the physiological role remains to be settled. The aim of the present study was to evaluate the effect of EGF on glucose homeostasis.

METHODS

Fasted rats were treated with intraperitoneal injections of 10, 40 or 80 microg/kg body weight, either with EGF or 1% bovine serum albumin (controls). In a second experiment, fasted rats received an intraperitoneal injection of 1 mg glucose/kg body weight, followed by an injection of EGF or bovine serum albumin. Blood was drawn before the injections and at different time points afterwards. The plasma concentrations of glucose, insulin and glucagon were measured.

RESULTS

A modest elevation of the concentrations of glucose and insulin in plasma during the study was found in fasted rats in experiment 1. The increase in insulin concentration was attenuated by EGF, but after glucose injection this effect was reversed. Plasma glucagon levels were dose-dependently elevated by EGF and increased instead of decreased after glucose injection.

CONCLUSION

Our data suggest that EGF might play an important role in the regulation of glucagon secretion by preventing the lowering effect of glucose on plasma glucagon levels.

Role of epidermal growth factor receptor transactivation in alpha1B-adrenoceptor phosphorylation

Phosphorylation of G protein-coupled receptors is one of the earliest events that regulate their function. Current evidence indicates that homologous desensitization of these receptors mainly involves G protein-coupled receptor kinases whereas in heterologous desensitization second messenger-activated kinases play key roles. Recent data show that transactivation of EGF (epidermal growth factor) receptors may also play a role in receptor phosphorylation. The role of this process was studied for the alpha1B-adrenoceptor phosphorylation induced by agents acting through different processes using inhibitors to block the EGF receptor transactivation process at different levels. Experiments were performed using transfected rat-1 fibroblasts that express alpha1B-adrenoceptors in a stably fashion. A metalloproteinase inhibitor, an anti-heparin-binding-EGF-selective antibody, and a selective EGF-receptor kinase inhibitor blocked the alpha1B-adrenoceptor phosphorylation induced by noradrenaline or endothelin-1. Our results indicate that shedding of heparin-binding-EGF, transactivation of EGF receptors plays a more general role in alpha1B-adrenoceptor phosphorylation than previously anticipated. It is possible that other receptors/channels could be modulated through a similar pathway.

Insulin induces α1B-adrenergic receptor phosphorylation and desensitization

The ability of insulin to induce alpha1B-adrenoceptor phosphorylation and desensitization was tested in two model systems: rat-1 cells that stably express alpha1B-adrenoceptors, through transfection, and endogenously express insulin receptors and DDT1 MF2 cells that endogenously express both receptors. Insulin induced concentration-dependent increases in the phosphorylation state of the adrenergic receptors in the two models with similar EC50 values (0.5-2 nM). The effect was rapid in the two systems but it was sustained in rat-1 cells and transient in DDT1 MF2 cells. In both cell lines, the insulin-mediated phosphorylation of alpha1B-adrenoceptors was blocked by wortmannin and LY 294002, and by staurosporine and bisindolylmaleimide I, indicating that the effect involved phosphoinositide 3-kinase and protein kinase C activities. The adrenoceptor phosphorylation induced by insulin was associated to desensitization as evidences by a diminished elevation of intracellular calcium in response to noradrenaline. Inhibitors of phosphoinositide 3-kinase and protein kinase C blocked the functional desensitization induced by insulin.

Single Transmembrane Spanning Heterotrimeric G Protein-Coupled Receptors and Their Signaling Cascades

Heptahelical of serpentine receptors such as the adrenergic receptors are well known to mediate their actions via heterotrimeric GTP-binding proteins. Likewise, receptors that traverse the cell membrane once have been shown to mediate their biological actions by activating several different mechanisms including stimulation of their intrinsic tyrosine kinase activities or the kinase activities of other proteins. Some of these single transmembrane receptors have an intrinsic guanylyl cyclase activity and can stimulate the cyclic GMP second messenger system; however, over the last few years, several studies have shown the involvement of heterotrimeric GTP-binding proteins in mediating signals that eventually culminate in the biological actions of single transmembrane spanning receptors and proteins. These receptors include the receptor tyrosine kinases that mediate the actions of growth factors such as epidermal growth factor, insulin, insulin-like growth factor as well as receptors for atrial natiuretic hormone or the zona pellucida protein (ZP3) and integrins. In this review, the significance of the coupling of the single transmembrane spanning receptors to G proteins has been highlighted by providing several examples of the concept that signaling via these receptors may involve the activation of multiple signaling cascades.

The diacylglycerol and protein kinase C pathways are not involved in insulin signalling in primary rat hepatocytes

Diacylglycerol (DAG) and protein kinase C (PKC) isoforms have been implicated in insulin signalling in muscle and fat cells. We evaluated the involvement of DAG and PKC in the action of insulin in adult rat hepatocytes cultured with dexamethasone, but in the absence of serum, for 48 h. Our results show that although insulin stimulated glycolysis and glycogen synthesis, it had no effect on DAG mass or molecular species composition. Epidermal growth factor showed the expected insulin-mimetic effect on glycolysis, whereas ATP and exogenous phospholipase C acted as antagonists and abolished the insulin signal. Similarly to insulin, epidermal growth factor had no effect on DAG mass or molecular species composition. In contrast, both ATP and phospholipase C induced a prominent increase in several DAG molecular species, including 18:0/20:4, 18:0/20:5, 18:0/22:5 and a decrease in 18:1/18:1. These changes were paralleled by an increase in phospholipase D activity, which was absent in insulin-treated cells. By immunoblotting or by measuring PKC activity, we found that neither insulin nor ATP translocated the PKCalpha, -delta, -epsilon or -zeta isoforms from the cytosol to the membrane in cells cultured for six or 48 h. Similarly, insulin had no effect on immunoprecipitable PKCzeta. Suppression of the glycogenic insulin signal by phorbol 12-myristate 13-acetate, but not by ATP, could be completely alleviated by bisindolylmaleimide. Finally, insulin showed no effect on DAG mass or translocation of PKC isoforms in the perfused liver, although it reduced the glucagon-stimulated glucose output by 75%. Together these results indicate that phospholipases C and D or multiple PKC isoforms are not involved in the hepatic insulin signal chain.

Ionizing radiation stimulates existing signal transduction pathways involving the activation of epidermal growth factor receptor and ERBB-3, and changes of intracellular calcium in A431 human squamous carcinoma cells

Previous studies demonstrated that ionizing radiation activates the epidermal growth factor receptor (EGFR), as measured by Tyr autophosphorylation, and induces transient increases in cytosolic free [Ca2+], [Ca2+]f. The mechanistic linkage between these events has been investigated in A431 squamous carcinoma cells with the EGFR Tyr kinase inhibitor, AG1478. EGFR autophosphorylation induced by radiation at doses of 0.5-5 Gy or EGF concentrations of 1-10 ng/ml is inhibited by >75% at 100 nM AG1478. Activation of EGFR enhances IP3 production as a result of phospholipase C (PLC) activation. At the doses used, radiation stimulates Tyr phosphorylation of both, PLCgamma and erbB-3, and also mediates the association between erbB-3 and PLCgamma not previously described. The increased erbB-3 Tyr phosphorylation is to a significant extent due to transactivation by EGFR as >70% of radiation- and EGF-induced erbB-3 Tyr phosphorylation is inhibited by AG 1478. The radiation-induced changes in [Ca2+]f are dependent upon EGFR, erbB-3 and PLCgamma activation since radiation stimulated IP3 formation and Ca2+ oscillations are inhibited by AG1478, the PLCgamma inhibitor U73122 or neutralizing antibody against an extracellular epitope of erbB-3. These results demonstrate that radiation induces qualitatively and quantitatively similar responses to EGF in stimulation of the plasma membrane-associated receptor Tyr kinases and immediate downstream effectors, such as PLCgamma and Ca2+.

Epidermal growth factor-induced heterologous desensitization of the luteinizing hormone/choriogonadotropin receptor in a cell-free membrane preparation is associated with the tyrosine phosphorylation of the epidermal growth factor receptor

Epidermal growth factor (EGF) attenuated hCG-stimulated adenylyl cyclase activity in rat luteal and follicular membranes. H7, an equipotent serine/threonine protein kinase inhibitor of cAMP-dependent protein kinases, cGMP-dependent protein kinases, and lipid-dependent protein kinase C, did not effect the ability of EGF to decrease hCG-responsive adenylyl cyclase activity, suggesting that a serine/threonine phosphorylation event catalyzed by these kinases was not critically involved in EGF-induced desensitization. Likewise, pertussis toxin-catalyzed ADP-ribosylation of a 40-kDa luteal membrane protein, which exhibited immunoreactivity with an antibody against Gi alpha, did not hinder the ability of EGF to attenuate hCG-stimulated adenylyl cyclase activity, indicating that Gi did not mediate EGF-induced desensitization. Rather, EGF-induced heterologous desensitization of LH/CG receptor in ovarian membranes was closely associated with the specific and prominent tyrosine phosphorylation of the 170-kDa EGF receptor. Both EGF-stimulated autophosphorylation of EGF receptor and EGF-induced LH/CG receptor desensitization were attenuated by genistein, a tyrosine kinase inhibitor. These results suggest that tyrosine phosphorylation of the 170-kDa EGF receptor is a necessary component of the signaling pathway in EGF-induced heterologous desensitization of the LH/CG receptor.

Fast receptor-induced formation of glycerophosphoinositol-4-phosphate, a putative novel intracellular messenger in the Ras pathway

Glycerophosphoinositols are phosphoinositide metabolites whose levels are constitutively elevated in Ras-transformed cells. Here, we show that one of these compounds, glycerophosphoinositol-4-phosphate (GroPIns-4-P) responds acutely to the stimulation of the epidermal growth factor receptor, with a fast, massive and transient increase. The mechanism leading to GroPIns-4-P formation involves the activation of phosphoinositide-3 kinase and the small GTP-binding protein Rac, since GroPIns-4-P was neither formed in cells expressing the dominant negative form of Rac nor in cells treated with the phosphoinositide-3 kinase inhibitor wortmannin. GroPIns-4-P has been previously shown to inhibit adenylyl cyclase. Accordingly, epidermal growth factor also decreased the basal, cholera toxin-stimulated, and forskolin-stimulated cyclic AMP levels with kinetics similar to those of GroPIns-4-P formation, suggesting that GroPIns-4-P mediates this inhibitory effect. The hormone-induced formation of GroPIns-4-P was detected in several cell lines of various origin, suggesting that GroPIns-4-P is a novel intracellular messenger of the Ras pathway, possibly able to convey information from tyrosine kinase receptors to the cyclic AMP cascade.

Epidermal growth factor regulates adenylate cyclase activity via Gs and Gi1-2 proteins in pancreatic acinar membranes

In the present study, Western-blot and radioreceptor analyses have revealed the presence of the epidermal growth factor (EGF) receptor in pancreatic acinar membranes. Isolated pancreatic acinar membranes, which allow access of functional antibodies to individual components of the signal transduction cascade, were used to examine EGF-induced regulation of adenylate cyclase activity. Forskolin, vasoactive intestinal peptide (VIP) and to a smaller extent EGF increased cAMP production in pancreatic acinar membranes. Preincubation of the membranes with anti-GS alpha antibody abolished EGF- and VIP-induced cAMP production, but had no effect on forskolin-induced cAMP accumulation. In the presence of either VIP or forskolin, EGF inhibited the VIP- and forskolin-induced cAMP production with an IC50 of 5 nM. Anti-G alpha i1-2 protein antibody, but not anti-G alpha i3 antibody, increased basal cAMP production, indicating that Gi proteins exert an inhibitory influence on basal adenylate cyclase activity. Anti-G alpha i1-2 antibody, but not anti-G alpha i3 antibody, abolished the inhibitory effect of EGF on the forskolin- and VIP-induced cAMP accumulation. A peptide corresponding to the juxtamembrane region in the cytosolic domain of the rat EGF receptor increased cAMP production in pancreatic acinar membranes in an anti-G alpha s antibody-sensitive fashion, whereas the EGF receptor peptide did not mimic the inhibitory effect of the native EGF receptor. The tyrosine kinase inhibitors genistein and pp60v-src (137-157) inhibited both the stimulatory and the inhibitory effects of EGF on cAMP production. Thus the data of the present study show that EGF regulates adenylate cyclase via activation of Gs and Gi proteins by a tyrosine phosphorylation-dependent mechanism in pancreatic acinar membranes. This leads to stimulation of basal and inhibition of forskolin- and VIP-induced adenylate cyclase activity respectively.

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.

Expression of type V adenylyl cyclase is required for epidermal growth factor-mediated stimulation of cAMP accumulation

Previously, this laboratory has demonstrated that epidermal growth factor (EGF) increases adenylyl cyclase activity in cardiac membranes and elevates cAMP accumulation in hearts and cardiac myocytes. Since EGF does not increase cAMP accumulation in all tissues, we investigated the possibility that the expression of a specific isoform of adenylyl cyclase (AC) was necessary to observe EGF-elicited stimulation of cAMP accumulation. HEK 293 cells were transfected with different isoforms of AC, and the ability of EGF to increase AC activity as well as elevate cAMP accumulation was determined. In cells transfected with AC I, II, V, and VI cDNAs, neither the expression nor the amount of the two isoforms of Gs alpha (45 and 52 kDa) were altered. Similarly, EGF-elicited phosphorylation of cellular proteins on tyrosine residues in various transfectants was unaltered. However, EGF increased AC activity and elevated cAMP accumulation only in cells expressing the rat and canine ACV. EGF did not alter either AC activity or cAMP accumulation in cells overexpressing types I, II, and VI isozymes. As assessed by the ability of an anti-Gs alpha antibody to obliterate the effect, stimulation of AC activity in AC V transfectants involved the participation of Gs alpha, a finding consistent with previous data concerning EGF effects on cardiac AC (Nair, B. G., Parikh, B., Milligan, G., and Patel, T. B. (1990) J. Biol. Chem. 265, 21317-21322). Thus we conclude that the expression of AC V isoform confers specificity to the ability of EGF to stimulate AC activity.

Integrative effects of EGF on metabolism and proliferation in renal proximal tubular cells

This study examined the relationship between alterations in cellular metabolism and induction of proliferation in renal proximal tubular cells (RPTC) after epidermal growth factor (EGF) exposure. EGF treatment (10 ng/ml) of confluent RPTC cultures for 6 consecutive days increased monolayer DNA content 3.3-fold. EGF-stimulated proliferation of RPTC was preceded by a rapid (within 4 h) induction of glycolysis and a decrease in basal and ouabain-sensitive oxygen consumption (20 and 30%, respectively). EGF stimulated the pentose cycle by 58% and decreased gluconeogenesis by 48%. Supplementation of the culture medium with ribose-5-phosphate or ribose abolished the stimulation of glycolysis and the pentose cycle by EGF but had no effect on proliferation. These results show that EGF rapidly stimulates the pentose cycle, shifts glucose metabolism from gluconeogenesis to glycolysis, and decreases oxygen consumption before any increase in proliferation. The lack of an EGF effect on the pentose cycle and glycolysis in the presence of exogenous precursors for DNA synthesis suggests that the stimulation of these pathways before proliferation is due to increased demands for ribose for subsequent nucleic acid synthesis.

EGF-induced increase in diacylglycerol, choline release, and DNA synthesis is extracellular calcium dependent

Previous studies have demonstrated a strict extracellular Ca2+ dependence for the G0 to G1 and G1 to S transition in growth factor-treated T51B rat liver cells that is associated with increased levels of protein kinase C activity. Consequently, we have examined these cells for changes in phospholipid-derived second messengers in response to epidermal growth factor (EGF) and thrombin in order to determine which signals are generated during the initiation of the G0 to G1 transition. Thrombin is coupled to a phosphoinositide hydrolyzing phospholipase C, as we have found a rapid Ca(2+)-independent increase in the levels of inositol 1,4,5-trisphosphate (Ins[1,4,5]P3), inositol 1,4-bisphosphate (Ins[1,4]P2), and inositol 4-monophosphate (Ins[4]P), as well as a concomitant, transient elevation in diacylglycerol. No changes in either intracellular or extracellular choline metabolites, or an increase in DNA synthesis, were found in response to thrombin. By contrast, treatment of T51B cells with EGF results in a slower, more prolonged extracellular Ca(2+)-dependent increase in both [3H]-glycerol radiolabeled diacyl-glycerol, and diacylglycerol mass, an increase in choline release into the extracellular medium, and eventually a substantial DNA synthesis. We were, however, unable to detect any changes in phosphatidylinositol (PtdIns) turnover, either by accumulation of inositol phosphates or by changes in phospholipids in response to EGF. These results indicate that DNA synthesis can readily occur in the absence of stimulated PtdIns turnover, and that PtdIns turnover is not sufficient in itself or necessary to induce DNA synthesis and is not necessary for a Ca(2+)-dependent increase in diacylglycerol. Moreover, we have demonstrated that the extracellular Ca(2+)-dependent increase in diacylglycerol levels in response to EGF is associated with an increase in extracellular choline release, which is indicative of an activation of a phosphatidylcholine-linked phospholipase D. These results suggest that diacylglycerol sources other than PtdIns's may be important in the extracellular Ca(2+)-dependent regulation of EGF-mediated cell replication.

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.

A region in the cytosolic domain of the epidermal growth factor receptor antithetically regulates the stimulatory and inhibitory guanine nucleotide-binding regulatory proteins of adenylyl cyclase

Epidermal growth factor (EGF) stimulates adenylyl cyclase in the heart via activation of the stimulatory GTP-binding protein Gs. Therefore, employing peptides corresponding to regions in the cytosolic domain of the EGF receptor, we have investigated the ability of sequences within the EGF receptor to activate Gs. A 13-aa peptide (EGFR-13) corresponding to the juxtamembrane region in the cytosolic domain of the EGF receptor stimulated GTP binding and GTPase activity of Gs. This peptide did not stimulate GTP binding to Gi but increased the GTPase activity of this protein. Additionally, phosphorylation of the protein kinase C site (threonine residue) within EGFR-13 decreased the ability of the peptide to stimulate Gs and increase GTPase activity of Gi. Further, in functional assays of Gs employing S49 cyc- cell membranes, EGFR-13 increased the ability of Gs to stimulate adenylyl cyclase; phospho-EGFR-13 and a 14-aa peptide corresponding to a sequence in the cytosolic domain of the EGF receptor did not alter the functional activity of Gs. Hence, the juxtamembrane region of the EGF receptor can activate Gs and, by stimulating GTPase activity of Gi, inactivates this latter G protein. Phosphorylation of the threonine residue within this region attenuates the activity of the peptide as a modulator of G-protein function.

Effects of extracellular Ca2+ and HCO3- on epidermal growth factor-induced DNA synthesis in cultured rat hepatocytes

BACKGROUND/AIMS

The elevation of cytosolic free calcium concentration ([Ca2+]i) and intracellular pH mediate the growth factor-initiated proliferation of many cells, but it is not known if they trigger mitosis in resting hepatocytes. The maintenance of [Ca2+]i and intracellular pH depends partly on extracellular calcium concentration ([Ca2+]e) and extracellular bicarbonate concentration ([HCO3-]e). Therefore, the effects of [Ca2+]e and [HCO3-]e on hepatocyte proliferation were examined.

METHODS

Epidermal growth factor induced proliferation in primary cultures of rat hepatocytes. [3H]thymidine incorporation into DNA and nuclear labeling indices were measured.

RESULTS

Between 0.2 and 0.9 mmol/L of [Ca2+]e, the proliferative response to epidermal growth factor increased, and total hepatocellular Ca2+ content was increased. Increasing [HCO3-]e also stimulated DNA synthesis in a concentration-dependent manner, maximal at 35 mmol/L. Using optimal [Ca2+]e (0.9 mmol/L) and [HCO3-]e (35 mmol/L), a synergistic stimulation of hepatocellular DNA synthesis was shown. Voltage-dependent Ca2+ channel blockers failed to inhibit hepatocyte proliferation when administered in concentrations that inhibit proliferation in other cell types.

CONCLUSIONS

[Ca2+]e and [HCO3-]e are both essential for hepatocyte proliferation, and their effects are synergistic. The entry of extracellular Ca2+ is critical for epidermal growth factor-induced DNA synthesis in hepatocytes, but this is not mediated by voltage-dependent Ca2+ channels.

Characterization of signaling pathways to Na+/H+ exchanger activation with epidermal growth factor in hepatocytes

To investigate the signaling pathways to Na+/H+ exchanger activation with epidermal growth factor in hepatocytes, we measured changes in cytosolic free calcium and intracellular pH levels at the single-cell level using digital imaging fluorescence microscopy of fura-2- or BCECF-loaded hepatocytes in primary culture. Epidermal growth factor induced cytosolic free calcium oscillations consisting of periodic trains of spikes with a latency period of up to several minutes. These calcium responses were inhibited by tyrosine kinase inhibitor genistein (100 mumol/L) and abolished by emptying of intracellular Ca2+ pools with 3 mumol/L thapsigargin, an inhibitor of Ca(2+)-ATPase on the endoplasmic reticulum. Epidermal growth factor (1 nmol/L) induced an intracellular pH increase of 0.12 +/- 0.07 units from the basal level of 7.25 +/- 0.09 units after several minutes of latency. This effect was completely abolished by 1 mmol/L amiloride, an inhibitor of the Na+/H+ exchanger. The epidermal growth factor-induced intracellular pH increase was inhibited by pretreatment of hepatocytes with genistein (100 mumol/L), thapsigargin (3 mumol/L) or calmodulin inhibitor W-7 (25 mumol/L), but not with protein kinase C inhibitor H-7 (50 mumol/L) or with cyclic AMP-dependent kinase inhibitor H-8 (60 mumol/L). Phorbol ester PMA (phorbol 12-myristate 13-acetate), a potent activator of protein kinase C, induced a slight intracellular pH increase significantly smaller than that with epidermal growth factor, whereas this effect was completely blocked by pretreatment with H-7, indicating that PMA-induced intracellular pH increase is mediated by protein kinase C pathways, unlike epidermal growth factor.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Epidermal growth factor (EGF) counteracts the stimulation of glycogen synthesis by insulin in hepatocytes, but it is not known whether this is due to inhibition of glycogen synthesis or to inhibition of the insulin-signalling mechanism. This study investigates the mechanisms by which EGF affects the basal rate and the insulin stimulation of glycogen synthesis. The basal rate of glycogen synthesis is higher at low than at high cell density. EGF inhibits the basal rate of glycogen synthesis at low cell density but not in confluent cultures and abolishes the difference due to density. However, EGF inhibits the stimulation of glycogen synthesis by insulin irrespective of cell density. Increasing glycogen synthesis by increasing the [glucose] does not abolish the difference in rates of glycogen synthesis due to cell density, neither does it induce responsiveness to EGF at high cell density, establishing that responsiveness to EGF is a function of cell density and not of the basal rate and that inhibition of the insulin stimulation also cannot be accounted for by the higher rate of glycogen synthesis. Cytochalasin D and phalloidin, which alter cell morphology through interactions with the microfilament cytoskeleton, mimic the cell-density-dependent inhibition of glycogen synthesis by EGF. The inhibition of glycogen synthesis by EGF and cytochalasin D is additive and cytochalasin D potentiates the inhibition of glycogen synthesis by EGF, suggesting involvement of a cytoskeletal mechanism. Exogenous phospholipase C inhibits glycogen synthesis at both low and high cell density and the inhibition at low cell density is not additive with that caused by either EGF or cytochalasin D, suggesting that these agonists inhibit glycogen synthesis through changes in Ca2+ and/or diacylglycerol. The inhibition of glycogen synthesis by EGF in the absence of insulin stimulation is blocked by neomycin, which inhibits Ca2+ release from intracellular stores but not by antagonists of protein kinase C. It was also inhibited by pertussis toxin (50%), suggesting that it may involve GTP-binding-protein-mediated release of Ca2+ from intracellular stores. The inhibition of the stimulation of glycogen synthesis by insulin was not affected by neomycin and was only marginally inhibited by pertussis toxin or guanosine 5'-O-[3-thio]triphosphate (GTP[S]). We infer from these findings that the inhibition by EGF of the basal rate of glycogen synthesis and of the insulin stimulation are mediated by different mechanisms. The latter is pertussis toxin insensitive and independent of cell density, whereas the former is expressed only at low cell density, it is potentiated by cytochalasin D and inhibited by pertussis toxin.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Rapid and delayed effects of epidermal growth factor on gluconeogenesis

Most reports on the effects of epidermal growth factor (EGF) on gluconeogenesis have indicated that such effects depend on the substrate used and are only observable after a lag time of 30-40 min. Recently, an immediate and transient effect of EGF on glucose synthesis was described in a perfused liver system. Here we extend the study of the effect of EGF on gluconeogenesis to isolated hepatocytes from fasted rats. The delayed effect of EGF on gluconeogenesis was studied by adding the substrate 40 min after the peptide. Under these conditions EGF increased glucose synthesis from pyruvate, decreased it when the substrate was lactate or glycerol and did not modify gluconeogensis from fructose or dihydroxyacetone. EGF did not affect the metabolic flux through glycolysis, determined as the production of lactate+pyruvate from 30 mM glucose. Furthermore, EGF did not modify the metabolic flux through pyruvate kinase, determined as the production of lactate+pyruvate from 1 mM dihydroxyacetone. The differing effects of EGF on gluconeogenesis depending on the substrate used can be explained by the effects of EGF on the cytosolic redox state (measured as the lactate/pyruvate ratio). About 20 min after the addition of EGF, the mitochondrial redox state (measured as the 3-hydroxybutyrate/acetoacetate ratio) decreased. This effect of EGF was blocked by ammonium, which also abolished the effect of the peptide on gluconeogenesis. Thus the effect of EGF at the mitochondrial level appears to be necessary for its effects on gluconeogenesis. Taken together, our results indicate that the delayed effects of EGF on gluconeogenesis are secondary to the effects of the peptide at both the mitochondrial and cytosolic levels. In addition to these delayed effects, we observed that EGF rapidly and transiently stimulated glucose synthesis from lactate, decreased the cytosolic redox state and increased oxygen consumption. All of these rapid effects required the presence of extracellular calcium and disappeared in the presence of rotenone, suggesting that this rapid effect of EGF on gluconeogenesis is secondary to the stimulation of mitochondrial respiration.

Phospholipase D treatment enhances gonadotrophin receptor-coupled adenylate cyclase activity in isolated bovine luteal cells

LH-stimulated adenylate cyclase activity in membrane preparations of bovine luteal cells could be enhanced by treating the cells with either phospholipase D or its hydrolysis product, phosphatidic acid. Similar augmentary effects were also produced following treatment of the cells with EGF. Moreover, EGF could stimulate the formation of [3H]phosphatidic acid in [3H]myristic acid preloaded cells, suggesting that EGF is able to activate cellular phospholipase D. Also, PMA was able to increase the phosphatidic acid formation with a parallel increase in the adenylate cyclase activity. We propose, therefore, that phosphatidic acid may act as an intracellular second messenger linking EGF-mediated activation of phospholipase D with the sensitization of LH receptor-coupled adenylate cyclase signalling system.

Epidermal growth factor inhibits phosphoenolpyruvate carboxykinase gene expression in rat hepatocytes in primary culture

Epidermal growth factor (EGF) decreased the basal, and blocked the dibutyryl cyclic AMP (Bt2cAMP)-induced, expression of P-enolpyruvate carboxykinase (GTP) (PEPCK) and tyrosine aminotransferase (TAT) genes in both rat hepatocytes in primary culture and the FTO-2B hepatoma cell line. Treatment of hepatocytes with EGF in combination with phorbol ester (TPA) resulted in an additive decrease of PEPCK mRNA levels. Overnight pretreatment of hepatocytes with TPA, which is known to downregulate protein kinase C, abolished the TPA and reduced the EGF-mediated inhibition of PEPCK gene expression. These results suggested that EGF caused its effect, at least in part, through protein kinase C.

Epidermal growth factor interferes with the effect of adrenaline on glucose production and on hepatic lipase secretion in rat hepatocytes

We studied the interaction of epidermal growth factor (EGF) and adrenaline in the control of several metabolic functions in isolated hepatocytes from fed rats. EGF did not modulate glucose release, urea production or hepatic lipase secretion, but interfered with the stimulatory effect of adrenaline on both glucose and urea production and also with the inhibitory effect of this hormone on hepatic lipase secretion. EGF also interfered with the effect of both angiotensin II and vasopressin on glucose release and on hepatic lipase secretion. While the effect of EGF interfering with the action of adrenaline on glucose release was potentiated in the absence of extracellular calcium, the effect on the inhibition of hepatic lipase secretion was abolished. These results suggest that EGF interfered with catecholamine actions in the liver at a site distal from the generation of the calcium signal.

Effect of epidermal growth factor/transforming growth factor alpha on lactate production in porcine Sertoli cells: glucose transport and lactate dehydrogenase isozymes as potential sites of action

Germ cell development is dependent upon the delivery of essential nutriments such as lactate originating from Sertoli cells. Lactate production is under the systemic control but probably also under a local control exerted via certain growth factors. By using a model of porcine cultured Sertoli cells, we have characterized the action of epidermal growth factor (EGF) on lactate production and further delineated the potential biochemical mechanisms involved in the EGF action. EGF stimulated lactate production in a time and dose dependent manner with a half-maximal (ED50) and maximal effects, respectively with 3.8 (0.6 x 10(-9) M) and 22 ng/ml of EGF. Lactate formation involves several biochemical steps among which the glucose substrate uptake and transport system as well as the lactate dehydrogenase (LDH) activity appear to play key roles. We report here that EGF increased the uptake of glucose evaluated through that of 2-deoxy-D-glucose (2-DOG), a non-metabolizable glucose analog. Such an increase in glucose substrate uptake occurs both after a long term (48 h) and a short term treatment (ED50 = 6.4 ng/ml, 1.1 x 10(-9) M EGF). Moreover, EGF was also able to enhance the activity of the Sertoli cell LDH. The maximal effect of the growth factor on LDH activity was observed after a long term (24 h) treatment with an ED50 of 7 ng/ml (1.2 x 10(-9) M).(ABSTRACT TRUNCATED AT 250 WORDS)

Epidermal growth factor produces inotropic and chronotropic effects in rat hearts by increasing cyclic AMP accumulation

Previously we have shown that epidermal growth factor (EGF) stimulates cardiac adenylyl cyclase and increases cAMP accumulation in the rat heart (Nair et al., Biochem. J. 264, 563-571, 1989). Moreover, we have shown that the stimulation of adenylyl cyclase by EGF in heart is mediated via activation of the stimulatory GTP binding regulatory protein Gs alpha (Nair et al., J. Biol. Chem. 265, 21317-21322, 1990). Since cAMP increases the beating rate of hearts, studies were performed to investigate the effects of EGF on mechanical function of the heart and the role of cAMP in mediating the cardiac effects of EGF. In isolated perfused rat hearts EGF (15 nM) decreased perfusion pressure, increased ventricular contractility and heart rate in a manner similar to that observed with the beta-adrenergic receptor agonist isoproterenol (10 nM). In the presence of the adenosine A1 receptor agonist (-)-N6-(R-phenylisopropyl)-adenosine (PIA, 100 nM) which via activation of the inhibitory GTP binding protein Gi inhibits adenylyl cyclase, the effects of EGF on cAMP accumulation in the heart were markedly attenuated. PIA also decreased the ability of EGF and isoproterenol to alter cardiac contractility and beating rate. However, PIA did not attenuate the increase in heart rate and contractility induced by the alpha-adrenergic agonist phenylephrine which does not stimulate cAMP accumulation in the heart. These data suggest that EGF alters cardiac function by increasing cellular cAMP accumulation.

Effects of EGF on the mass of inositol 1,4,5-trisphosphate and SN(1,2)-diacylglycerol in freshly isolated rat hepatocytes: comparison with vasopressin

We measured the masses of inositol 1,4,5 trisphosphate (Ins(1,4,5)P3) and diacylglycerol (DAG) in hepatocytes in response to both epidermal growth factor (EGF) and vasopressin. EGF at 25 nM did not alter Ins(1,4,5)P3 content of hepatocytes. However, the combination of 100 nM EGF concentration and incubation with lithium did increase Ins(1,4,5)P3 content. This increase was only one tenth of that elicited by vasopressin in parallel incubations. This finding resolves a controversy concerning the ability of EGF to increase Ins(1,4,5)P3 in hepatocytes, and argues against a role for phosphoinositide hydrolysis in EGF action in hepatocytes. Both EGF and vasopressin caused a rapid (30 s) increase in DAG content. A delayed increase in DAG content, that was maximal after several minutes, was observed only for vasopressin. The rapid increase in DAG content implies an activation of protein kinase C for both EGF and vasopressin.

Modulation of cytosolic-[Ca2+] oscillations in hepatocytes results from cross-talk among second messengers. The synergism between the alpha 1-adrenergic response, glucagon and cyclic AMP, and their antagonism by insulin and diacylglycerol manifest themselves in the control of the cytosolic-[Ca2+] oscillations

Hepatocytes respond to stimulation by glycogenolytic agonists acting via phosphoinositide (PI) breakdown through oscillations of the free cytosolic concentration of Ca2+ ([Ca2+]cyt.). Since the second-messenger repertoire of hepatocytes includes many other factors besides Ca2+, we investigated to what degree the regulation of [Ca2+]cyt. oscillations is integrated into these other signalling systems. [Ca2+]cyt. was recorded in single rat hepatocytes by using the Ca(2+)-indicator fura-2. Parallel stimulation with phenylephrine (an alpha 1-adrenergic agonist of PI breakdown) and glucagon resulted in a synergistic stimulation of [Ca2+]cyt. oscillations. Direct activation of the cyclic-AMP-dependent pathway with several stimuli (forskolin, 8-bromo cyclic AMP, 8-CPT cyclic AMP) mimicked the response to glucagon. In contrast, [Ca2+]cyt. oscillations induced by various combinations of these agonists could be antagonized by the glycogenic hormone insulin. As one of the options in the insulin-signalling network, we tested a diacylglycerol activator of protein kinase C, DiC8. It also acted as an inhibitor of [Ca2+]cyt. oscillations. We investigated how these observations could be reconciled with our previously introduced model of [Ca2+]cyt. oscillations in hepatocytes [Somogyi and Stucki (1991) J. Biol. Chem. 266, 11068-11077]. First of all, the effect of calmodulin inhibitors (calmidazolium and CGS 9343 B), acting at the core of our model on the feedback of Ca2+ on Ins(1,4,5)P3-induced Ca2+ release, was not altered by the new modulators. In addition, all agonists and antagonists could be used interchangeably in combination and introduced no significant change in the oscillatory pattern or spike shape. Since the response was solely limited to frequency modulation, over- or understimulation of the oscillatory system, there is no need to create a new oscillator or to introduce further reaction steps into the core of the model. We conclude that the regulation of [Ca2+]cyt. via the explored second-messenger pathways can be embedded into the oscillatory system as modulation of rate constants already present in this model.

Epidermal growth factor induces dose-dependent calcium oscillations in single fura-2-loaded hepatocytes

Digital imaging fluorescence microscopy has been used to investigate epidermal growth factor-induced calcium responses of fura-2-loaded hepatocytes in primary culture at the single-cell level. Epidermal growth factor induced oscillations in cytosolic free calcium ([Ca2+]i) consisting of a periodic train of spikes unlike the monophasic elevation in cell suspensions reported previously. In this study, 79% of the cells in the microscopic field responded to 0.1 nmol/L epidermal growth factor, and 78% of the responsive cells displayed oscillations. However, the frequency of oscillations differed considerably from cell to cell. [Ca2+]i measurement in a cell population was simulated using these data, but only a slightly biphasic pattern was obtained, indicating the significance of single-cell measurement of [Ca2+]i. Because considerable heterogeneity existed in the sensitivity to epidermal growth factor between the cells, single hepatocytes were stimulated sequentially with increasing concentrations of epidermal growth factor to investigate the dose dependence of the oscillations. The frequency of the oscillations increased with increasing epidermal growth factor concentration, but the amplitude was similar for all concentrations, suggesting the existence of frequency-encoded information even in the pathway through tyrosine kinase for epidermal growth factor signaling. The pattern of the oscillations with epidermal growth factor, especially the latency, was considerably different from that with phenylephrine, which is known to use the phosphatidylinositol pathway, possibly because of the difference in the pathway toward phosphatidylinositol turnover between these agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of albumin expression in fetal rat hepatocytes cultured under proliferative conditions: role of epidermal growth factor and hormones

Sustained production of plasma proteins, notably albumin, is a reliable indicator of the differentiated state of hepatocytes. In this work, we have developed a fetal hepatocyte culture system where studying the regulation of albumin expression in proliferating liver cells. Our results show that under proliferative conditions (i.e., in the presence of EGF) fetal hepatocytes maintain albumin production above control quiescent non-treated cells. Glucagon and noradrenaline have no effect on the proliferation induced by EGF in cultured fetal hepatocytes; however, they act synergistically with the growth factor, increasing intracellular albumin levels. The maximum response is obtained by treatment of cells with EGF and noradrenaline. The stimulatory noradrenergic effect is mimicked by agents that increase cyclic AMP levels (forskolin plus IBMX). However, vasopressin or phorbol esters have no effect on albumin production, neither alone nor in combination with EGF. Dexamethasone, which does not alter the proliferative induction of EGF, increases albumin content. This effect is independent of the proliferative status of the cells and is not enhanced by glucagon, noradrenaline, or cyclic AMP increasing agents. The hormonal changes observed in albumin production partially correlate with changes in mRNA levels. This is the first time that cyclic AMP increasing agents are shown to act synergistically with EGF, increasing the expression of this liver specific gene.

Intracellular calcium as a second messenger for human hepatocyte growth factor in hepatocytes

Human hepatocyte growth factor is a newly discovered substance that stimulates DNA synthesis in vitro. In this study, we examined intracellular Ca2+ movement as one of the second messengers for human hepatocyte growth factor in primary-cultured hepatocytes. The addition of hHGF induced Ca2+ oscillation, but the frequency of oscillations varied from cell to cell. We also saw marked intercellular heterogeneity in the initial latent period for the Ca2+ response; the mean latent period was rather longer than those seen with phenylephrine and vasopressin. This difference in the initial latent period may be due to the difference in the pathways of Ca2+ elevation. Duration of culture determined the number of human hepatocyte growth factor-responsive cells; their number peaked at 2 to 5 hours of confluent culture, whereas the peak was earlier in a low-density culture. These changes in responsiveness during culture can be explained by the cell cycle-dependent sensitivity to human hepatocyte growth factor of hepatocytes. The Ca2+ response to human hepatocyte growth factor was dose dependent; 10(-10) mol/L hHGF gave the highest Ca2+ response, similar to the dose-response curve of DNA synthesis. We even observed the Ca2+ response in the Ca(2+)-free buffer, so the increase in Ca2+ was considered due to release from intracellular Ca2+ stores. These results suggest that human hepatocyte growth factor causes the intracellular Ca2+ elevation in the early stage of the cell cycle and that it plays important roles in the signal transduction systems for human hepatocyte growth factor and the proliferation of hepatocytes.

Transforming growth factor alpha inhibits glycogen synthesis and counteracts the stimulation by insulin in hepatocytes

Rat transforming growth factor alpha (TGF alpha) inhibits glycogen synthesis in rat and guinea pig hepatocyte cultures and counteracts the stimulation of glycogen deposition and activation of glycogen synthase caused by insulin. The EC50 for inhibition of glycogen deposition was 0.2nM. The inhibition of glycogen synthesis was also observed in the absence of extracellular Ca2+ and was not blocked by indomethacin, suggesting that it is not mediated by production of prostaglandins. Since TGF alpha is produced by hepatocytes during liver regeneration and by macrophages during endotoxin stimulation, it may have an autocrine/paracrine effect on hepatic carbohydrate metabolism in these states, and may account for the low hepatic glycogen levels during liver regeneration and the impaired glucose tolerance associated with sepsis.

Epidermal growth factor stimulates cAMP accumulation in cultured rat cardiac myocytes

We have previously shown that epidermal growth factor (EGF) augments cAMP accumulation in the heart and stimulates cardiac adenylyl cyclase via a G protein mediated mechanism (Nair et al., 1989). More recently, employing an antibody against the carboxy-terminus decapeptide of Gs alpha, we have demonstrated that Gs alpha mediates the effects of EGF on cardiac adenylyl cyclase (Nair et al., 1990). Since the heart comprises of a variety of cell types, the purpose of the studies presented here was to determine whether or not the effects of EGF on adenylyl cyclase were mediated in cardiac myocytes or noncardiomyocytes. Therefore, cultures of ventricular cardiomyocytes and noncardiomyocytes from neonatal rat hearts were established and characterized. Apart from the differences in cellular morphology, cardiomyocytes but not the noncardiomyocytes employed in our studies expressed the alpha- and beta-myosin heavy chain (MHC) mRNA and the beta-MHC protein. Additionally, as described previously, treatment of cardiomyocytes with thyroid hormone increased alpha-MHC mRNA and decreased the expression of beta-MHC mRNA, indicating that the cardiomyocytes employed in our studies were responding in a physiologically relevant manner. EGF in a time-dependent manner increased cAMP accumulation in the cardiomyocytes but not in noncardiomyocytes. Maximum and half-maximum effects were observed at 100 nM and 2 nM concentrations of EGF, respectively. As determined by the presence of immunoreactive EGF receptors and tyrosine phosphorylation of the 170 kDa protein in membranes of cardiomyocytes and noncardiomyocytes, both the cell populations contained functional EGF receptors. Therefore, the differential effects of EGF on cAMP accumulation in the two cell populations appear to be due to differential coupling of the EGF receptors to the adenylyl cyclase system rather than the absence of EGF receptors in noncardiomyocytes. Consistent with our previous findings in isolated membranes and perfused rat hearts, EGF-elicited increase in cAMP accumulation in cardiomyocytes did not involve activation of beta-adrenoreceptors and was abolished by prior treatment of cells with cholera toxin. Overall, our findings demonstrate that EGF-elicited increase in cAMP accumulation in the heart is the reflection of changes in cAMP content of cardiomyocytes and not noncardiomyocytes.

Comparison of effects of HGF and EGF on cellular calcium in rat hepatocytes

We compared the effects of HGF and EGF on cytoplasmic free calcium concentration, [Ca2+]c, and inositol trisphosphate production in rat hepatocytes. HGF induced a prompt and transient elevation of [Ca2+]c. EGF also induced an immediate increase in [Ca2+]c, the magnitude of which was greater than that by HGF. In contrast, in the presence of 1 microM extracellular calcium EGF increased [Ca2+]c to a lesser extent than HGF. When cells were pretreated with EGF, the effect of HGF on [Ca2+]c was greatly enhanced. However, such enhancement was not observed in medium containing 1 microM extracellular calcium. In hepatocytes prelabeled with [3H]-inositol, both HGF and EGF increased [3H]inositol trisphosphate. HGF and EGF acted synergistically to stimulate production of inositol trisphosphate. These results indicate that both HGF and EGF increase [Ca2+]c by a mechanism involving phosphoinositide turnover and that the actions of HGF and EGF on hepatocyte calcium metabolism are not totally identical.

Epidermal growth factor activation of rat parotid gland adenylate cyclase and mediation by a GTP-binding regulatory protein

Injection of rats with a single dose of epidermal growth factor (EGF) or isoproterenol increased parotid gland acinar cell levels of cyclic AMP (cAMP) significantly above control basal concentrations (34, 177 and 11.5 pmol/g tissue/100 g body weight, respectively). Following a chronic regimen of isoproterenol (3 days), EGF, bovine galactosyltransferase (Gal Tase, EC 2.4.1.22) and isoproterenol increased cAMP levels, albeit to a lower level than observed for the single dose (21, 17 and 51 pmol, respectively). Using isolated parotid gland membranes, EGF and bovine galactosyltransferase also stimulated adenylate cyclase (EC 2.7.4.3) activity in a concentration-dependent manner. Introduction of the beta-adrenergic receptor antagonist propranolol blocked isoproterenol-stimulated adenylate cyclase activity and cAMP accumulation, but not that observed with EGF or the transferase treatment. Growth factor-stimulated adenylate cyclase activity required the presence of the guanosine triphosphate (GTP) analogue, guanyl-5'-imidodiphosphate (p[NH]ppG), while cAMP accumulation could additionally be blocked by introducing the GDP analog, guanosine 5'[beta-thio]diphosphate (GDP[S]). The ability of EGF to activate adenylate cyclase was not affected by pretreatment of acinar cell membranes with pertussis toxin, whereas pretreatment with cholera toxin eliminated EGF-stimulated cyclase activity. The experimental results presented here expand to the parotid gland our knowledge of the ability of EGF to stimulate the cAMP second messenger signalling pathway via a G-binding regulatory protein, by a mechanism independent of beta-adrenergic receptor activation.

Epidermal growth factor inhibits both cholecystokinin octapeptide-induced inositol 1,4,5-trisphosphate production and [CA2+]i increase in rat pancreatic acinar cells

We have studied the effects of epidermal growth factor (EGF) on both cholecystokinin octapeptide (CCK-OP)-induced inositol-1,4,5 trisphosphate (IP3) production and on cytosolic free calcium concentrations [Ca2+]i by fluorescence measurements in fura-2-loaded pancreatic acini. Our data show that EGF inhibits CCK-OP induced IP3 production by 40 +/- 9% and decreases CCK-OP induced rise in cytosolic Ca2+ by 41 +/- 9%. These data indicate that activation of EGF receptors leads to inhibition of CCK-OP induced stimulation of phospholipase C (PLC).

Regulation of hepatic energy metabolism by epidermal growth factor

Employing the non-recirculating perfused rat liver preparation, we have investigated the regulation of hepatic gluconeogenesis, and metabolic fluxes through the tricarboxylic acid cycle and 2-oxoglutarate dehydrogenase reaction by epidermal growth factor (EGF) which mimics the actions of both insulin and Ca(2+)-mobilizing hormones (e.g. vasopressin). As monitored by the rate of 14CO2 production from [2-14C]pyruvate (0.5 mM), EGF (10 nM) transiently stimulated the activity of the tricarboxylic acid cycle. EGF also transiently stimulated hepatic gluconeogenesis from pyruvate. The transient stimulation of tricarboxylic acid cycle activity and gluconeogenesis were accompanied by an increase in perfusate Ca2+ content indicating that EGF also altered hepatic Ca2+ fluxes. EGF-elicited stimulation of gluconeogenesis was, at least in part, the result of a transient (50%) inhibition of pyruvate kinase activity. Likewise, EGF-mediated stimulation of tricarboxylic acid cycle activity can, in part, be attributed to EGF-elicited stimulation of metabolic flux through the mitochondrial, Ca(2+)-sensitive, 2-oxoglutarate dehydrogenase reaction. The regulation of hepatic metabolism by EGF appears to be the manifestation of alteration in cellular Ca2+ content since in experiments performed under conditions known to abolish the ability of EGF to alter cytosolic free-Ca2+ concentrations, i.e. in livers of pertussis-toxin-treated rats, EGF did not alter either perfusate Ca2+ content or any of the metabolic parameters monitored. Additionally, experiments involving pulsatile infusion of either EGF or phenylephrine into livers demonstrated that, unlike the alpha 1-adrenergic receptor, homologous desensitization of the EGF receptor occurs. Such a homologous desensitization of the EGF receptor can explain the transient nature of EGF-elicited stimulation of various metabolic processes. Since protein kinase C activation by EGF can lead to receptor desensitization, experiments were performed with phorbol esters which either activate or do not alter protein kinase C activity. While the inactive phorbol ester 4 alpha-phorbol 12,13-didecanoate did not modulate the hepatic actions of EGF, activation of protein kinase C by 4 beta-phorbol 12-myristate 13-acetate (70 nM) abolished the ability of EGF to stimulate gluconeogenesis, tricarboxylic acid cycle activity and metabolic flux through the 2-oxoglutarate dehydrogenase complex.

Epidermal growth factor stimulates the phosphorylation of pyruvate kinase in freshly isolated rat hepatocytes

Epidermal growth factor (EGF) has previously been shown to stimulate gluconeogenesis in rat liver by decreasing the activity of pyruvate kinase [(1988) Biochem. J. 255, 361-364]. Here we investigate the mechanism underlying the inactivation of the enzyme. EGF was found to increase the incorporation of phosphate into pyruvate kinase, with maximal phosphorylation achieved only after 10 min in the presence of the growth factor. The increase in phosphorylation was not additive with that caused by cyclic AMP. Phosphoamino acid analysis of pyruvate kinase isolated from cells treated with EGF indicated that EGF increases phosphorylation solely on serine residues. The exact site of EGF-mediated phosphorylation has yet to be identified.

Effects of cytokines on the liver

Cytokines are essential for the communication not only between the liver and extrahepatic sites but also within the liver itself. Cytokines regulate the intermediary metabolism of amino acids, proteins, carbohydrates, lipids and minerals. Cytokines partially interact with classical hormones such as glucocorticoids, resulting in a complex network of mutual control. Since many cytokines exert growth factor-like activities in addition to their specific proinflammatory effects, the distinction between cytokines and growth factors is somewhat artificial. The liver is an important site of synthesis and the major clearance organ for several cytokines. In liver disease, cytokines are involved in the onset of intrahepatic immune responses (e.g., during viral hepatitis), in liver regeneration (e.g., after partial hepatectomy) and in the fibrotic and cirrhotic transformation of the liver such as chronic chemical injury or viral infection. Further studies of cytokine actions may lead to a better understanding of liver diseases and to the development of new immunomodulating therapeutic options.

Effect of epidermal growth factor (EGF) on gluconeogenesis in isolated rat hepatocytes. Dependency on the red-ox state of the substrate

It was found that EGF decreased both the basal- and the glucagon-stimulated gluconeogenesis from lactate alone or from a high lactate/pyruvate ratio and that it enhanced both the basal- and the glucagon-inhibited glucose synthesis from pyruvate alone or from a low lactate/pyruvate ratio. These findings demonstrate that the effect of both EGF and glucagon on glucose production by isolated hepatocytes depends on the red-ox state of the substrate.

Changes of DNA methylation level during pre- and postnatal periods in mice

DNA methylation in an adult mammalian body shows tissue-specificity. But when and how the specificity is established in the process of development has not yet been elucidated. Here we have investigated age-dependent changes in the amount of 5-methyldeoxycytidine (5mdC) that DNA of various mouse tissues contains during the late-fetal and postnatal periods, using high-performance liquid chromatography. The tissue-specificity in the 5mdC level was observed in the late-fetal stage, and the level continued to change during the subsequent periods. The most pronounced alterations were observed in brain and liver, where similar biphasic changes were seen, but at different ages. At maturation, the 5mdC levels were high in thymus, spleen and brain, intermediate in lung, and low in liver and sperm. The data demonstrate the importance of the peri- and postnatal periods in establishment of tissue-specificity in 5mdC content.

Expression of epidermal growth factor in the kidney and submandibular gland during mouse postnatal development. An immunocytochemical and in situ hybridization study

Earlier work has demonstrated that the salivary glands and kidneys are the major sites of epidermal growth factor (EGF) synthesis in adult mice. The precise timing of the onset of endogenous EGF synthesis in these tissues is not yet clear. In the present study we assessed the ontogenesis of EGF expression in the Swiss-Webster mouse. Paraformaldehyde-fixed frozen sections of neonatal kidneys and salivary glands were probed with proEGF cRNA labelled with 35S for in situ hybridization and with rabbit antisera to mouse EGF for immunocytochemistry. Both EGF mRNA and immunoreactivity were first detected in the developing distal nephron between days 3 and 5 postpartum. Juxtamedullary nephrons underlying the superficial nephrogenic zone were the first to express EGF. During the 2nd week after birth, EGF-expressing tubules became more abundant and distributed to medullary as well as cortical regions, corresponding to the thick ascending limb of Henle and distal convoluted tubule. Initial EGF mRNA and immunoreactivity in the submandibular gland were first detected between days 18 and 20 postpartum and increased notably during the following weeks.

Control of glycolysis in cultured chick embryo hepatocytes. Fructose 2,6-bisphosphate content and phosphofructokinase-1 activity are stimulated by insulin and epidermal growth factor

Chick embryo hepatocytes were maintained in monolayer culture in a serum-free chemically defined medium for periods of up to 2 days. Over this time period, insulin provoked selective increases (up to 5-fold) in factors relevant to the control of glycolysis: the activities of phosphofructokinase-1 (PFK-1), phosphofructokinase-2 (PFK-2) and hexokinase isoenzymes and the content of fructose 2,6-bisphosphate (F26BP). Half-maximal effects of insulin on pFK-1 activity were in the physiological range (0.1 nM). Changes in enzyme activities and F26BP content in response to insulin were correlated with stimulation of glycolytic flux as estimated by radioisotopic flux. These data are discussed in relation to known changes which occur in hepatic glycolytic activity and PFK-1 activity in the intact chick around hatching. The effects of insulin on F26BP content, PFK-1 activity and glycolytic flux were mimicked by epidermal growth factor (EGF). In contrast, phorbol esters produced minimal actions on any of the above parameters. Our data indicate that protein kinase C is not involved in the actions of insulin or EGF in control of F26BP content or PFK-1 activity. This work indicates that the related tyrosyl kinase receptors of insulin and EGF may provoke identical responses within hepatocytes, but through the utilization of different transduction systems which merge to common control points.

Activation of calcium uptake in rat liver mitochondria by aminoglucoside antibiotics

Ca uptake in rat liver mitochondria is accelerated by various aminoglucoside antibiotics and, to a lesser degree, by triethylenetetramine and by protamine. With 1 mM Mg2+ and at a concentration of Ca2+ = 2 microM maximal (about six-fold) activation is achieved with 20 microM neomycin; at higher concentrations of the antibiotic the velocity of Ca uptake decreases. Activation to the same degree by gentamicin, kanamycin or streptomycin requires higher concentrations of the antibiotics. The reason for the acceleration of Ca uptake by neomycin is an allosteric alteration of the kinetics corresponding to that previously observed in the presence of spermine. The conformity with effects of spermine and possible inferences of that conformity are discussed.