Role of diacylglycerol (DAG) in hormonal induction of S phase in hepatocytes: the DAG-dependent protein kinase C pathway is not activated by epidermal growth factor (EGF), but is involved in mediating the enhancement of responsiveness to EGF by vasopressin, angiotensin II, and norepinephrine

CXC chemokine ligand-13 promotes metastasis via CXCR5-dependent signaling pathway in non-small cell lung cancer

The CXC chemokine ligand-13 (CXCL13) is a chemoattractant of B cells and has been implicated in the progression of many cancers. So far, CXCL13 and its related receptor CXCR5 have been proved to regulate cancer cell migration as well as tumour metastasis. However, the role of CXCL13-CXCR5 axis in metastasis of lung cancer is still poorly understood. In this study, we found that CXCL13 and CXCR5 were commonly up-regulated in lung cancer specimens compared with normal tissues among different cohorts. Our evidence showed that CXCL13 obviously promoted migration of lung cancer cells, and this effect was mediated by vascular cell adhesion molecule-1 (VCAM-1) expression. We also confirmed that CXCR5, the major receptor responsible for CXCL13 function, was required for CXCL13-promoted cell migration. We also test the candidate components which are activated after CXCL13 treatment and found that phospholipase C-β (PLCβ), protein kinase C-α (PKCα) and c-Src signalling pathways were involved in CXCL13-promoted cell migration and VCAM-1 expression in lung cancer cells. Finally, CXCL13 stimulated NF-κB transcription factor in lung cancer cells, contributing to VCAM-1 expression in translational level. These evidences propose a novel insight into lung cancer metastasis which is regulated by CXCL13.

Signal transduction mechanism for potentiation by α1- and β2-adrenoceptor agonists of L-ascorbic acid-induced DNA synthesis and proliferation in primary cultures of adult rat hepatocytes

We investigated the effects of α- and β-adrenoceptor agonists on L-ascorbic acid-induced hepatocyte DNA synthesis and proliferation in primary cultures of adult rat hepatocytes. The results showed that phenylephrine (10(-6) M) and metaproterenol (10(-6) M) alone did not induce hepatocyte DNA synthesis and proliferation. However, when combined with L-ascorbic acid (10(-6) M), these adrenoceptor agonists potentiated the hepatocyte DNA synthesis and proliferation induced by L-ascorbic acid. Then intracellular signal transduction mechanisms for the effects of phenylephrine and metaproterenol on L-ascorbic acid-induced hepatocyte mitogenesis were examined. Western blot analysis showed that phenylephrine and metaproterenol did not potentiate L-ascorbic acid-induced insulin-like growth factor I receptor tyrosine kinase phosphorylation. In contrast, they both significantly potentiated L-ascorbic acid-induced extracellular-signal regulated kinase-2 (ERK2) phosphorylation within 5 min. Moreover, cell-permeable second messenger analogs phorbol ester (10(-7) M) and 8-bromo cAMP (10(-7) M) mimicked the effects of phenylephrine and metaproterenol on L-ascorbic acid-induced ERK2 phosphorylation. The effects of these adrenoceptor agents were specifically antagonized by GF109203X and H-89, respectively. These results indicate that activation of ERK2 via protein kinas C and protein kinase A represents a mechanism for potentiation of L-ascorbic acid-induced hepatocyte DNA synthesis and proliferation in primary cultures of adult rat hepatocytes.

Activation of extracellular-signal regulated kinase by epidermal growth factor is potentiated by cAMP-elevating agents in primary cultures of adult rat hepatocytes

We investigated the effects of α- and β-adrenergic agonists on epidermal growth factor (EGF)-stimulated extracellular-signal regulated kinase (ERK) isoforms in primary cultures of adult rat hepatocytes. Hepatocytes were isolated and cultured with EGF (20 ng/ml) and/or α(1)-, α(2)- and β(2)-adrenergic agonists. Phosphorylated ERK isoforms (ERK1; p44 mitogen-activated protein kinase (MAPK) and ERK2; p42 MAPK) were detected by Western blotting analysis using anti-phospho-ERK1/2 antibody. The results show that EGF induced a 2.5-fold increase in ERK2-, but not ERK1-, phosphorylation within 3 min. This EGF-induced ERK2 activation was abolished by treatment with the EGF-receptor kinase inhibitor AG1478 (10(-7) M) or the MEK (MAPK kinase) inhibitor PD98059 (10(-6) M). The α(2)-adrenergic and β(2)-adrenergic agonists, UK14304 (10(-6) M) and metaproterenol (10(-6) M), respectively, had no effect in the absence of EGF, but metaproterenol significantly potentiated EGF-induced ERK2 phosphorylation. Moreover, the cell-permeable cAMP analog 8-bromo cAMP (10(-7) M), also potentiated EGF-induced ERK2 phosphorylation. The effects of these analogs were antagonized by the protein kinase A (PKA) inhibitor H-89 (10(-7) M). These results suggest that direct or indirect activation of PKA represents a positive regulatory mechanism for EGF stimulation of ERK2 induction.

Activation of extracellular-signal regulated kinase by platelet-derived growth factor is potentiated by phenylephrine in primary cultures of adult rat hepatocytes

We investigated the effects of the α(1)-adrenergic agonist phenylephrine on platelet-derived growth factor (PDGF)-stimulated extracellular signal-regulated kinase (ERK) in primary cultures of adult rat hepatocytes. Hepatocytes were isolated and cultured with PDGF (10 ng/ml) and/or α-adrenergic agonist. Phosphorylated ERK isoforms (ERK1 and ERK2) were detected by Western blotting analysis using anti-phospho mitogen-activated protein kinase (MAPK) antibody. PDGF stimulated phosphorylation of ERK2 (42 kDa MAPK) by 2.0-fold within 3-5 min. The PDGF-induced ERK activation was abolished by AG1296 (10(-7) M) or LY294002 (10(-7) M) treatment. MAPK kinase inhibitor, PD98059 (10(-6) M), completely inhibited the PDGF-induced increase in ERK activity. In addition, PDGF-induced mammalian target of rapamycin activity was completely inhibited by AG1296, LY294002, PD98059, or rapamycin treatment. Phenylephrine alone showed no effects on ERKs, but significantly increased phosphorylation of ERK2 induced by PDGF. Moreover, a synthetic analog of diacylglycerol (DG), phorbol 12-myristate 13 acetate (TPA; 10(-7) M), potentiated PDGF-induced ERK2 phosphorylation, while ionomycin had no effect (10(-6) M). The effects of phenylephrine and TPA were antagonized by the phospholipase C (PLC) inhibitor U73122 (10(-7) M), and the protein kinase C (PKC) inhibitor GF109203X (10(-7) M), respectively. Accordingly, PDGF-induced DNA synthesis and proliferation in the presence or absence of phenylephrine or TPA were completely inhibited by AG1296, LY294002, PD98059, or rapamycin treatment. These results suggest that activation of PLC/PKC by phenylephrine represent an indirect positive regulatory mechanism for stimulating ERK induced by 10 ng/ml PDGF.

Leptin stimulates protein synthesis-activating translation machinery in human trophoblastic cells

Leptin was originally considered as an adipocyte-derived signaling molecule for the central control of metabolism. However, pleiotropic effects of leptin have been identified in reproduction and pregnancy, particularly in placenta, where it may work as an autocrine hormone, mediating angiogenesis, growth, and immunomodulation. Leptin receptor (LEPR, also known as Ob-R) shows sequence homology to members of the class I cytokine receptor (gp130) superfamily. In fact, leptin may function as a proinflammatory cytokine. We have previously found that leptin is a trophic and mitogenic factor for trophoblastic cells. In order to further investigate the mechanism by which leptin stimulates cell growth in JEG-3 cells and trophoblastic cells, we studied the phosphorylation state of different proteins of the initiation stage of translation and the total protein synthesis by [(3)H]leucine incorporation in JEG-3 cells. We have found that leptin dose-dependently stimulates the phosphorylation and activation of the translation initiation factor EIF4E as well as the phosphorylation of the EIF4E binding protein EIF4EBP1 (PHAS-I), which releases EIF4E to form active complexes. Moreover, leptin dose-dependently stimulates protein synthesis, and this effect can be partially prevented by blocking mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3 kinase (PIK3) pathways. In conclusion, leptin stimulates protein synthesis, at least in part activating the translation machinery, via the activation of MAPK and PIK3 pathways.

Prostaglandins Enhance Epidermal Growth Factor-Induced DNA Synthesis in Hepatocytes by Stimulation of E Prostanoid 3 and F Prostanoid Receptors

Prostaglandins stimulate hepatocyte proliferation in vivo and in vitro. We have examined the role of E prostanoid (EP) and F prostanoid receptors (FP) in enhancing the growth-stimulatory effect of epidermal growth factor (EGF) in cultured hepatocytes. The EP2 receptor agonist butaprost had no significant effect on EGF-induced DNA synthesis. EP1 receptor-selective antagonists did not affect the enhancement by prostaglandin E(2) of EGF-stimulated DNA synthesis. Sulprostone, misoprostol, and fluprostenol strongly enhanced DNA synthesis and inhibited glucagon-stimulated cAMP accumulation, indicating that they all activated EP3 receptors. Sulprostone and fluprostenol, and to a lesser extent misoprostol, stimulated accumulation of inositol phosphates. The effects of fluprostenol and sulprostone on phospholipase C (PLC) were inhibited by the FP receptor antagonist AL-8810 [9 alpha, 15R-dihydroxy-11 beta-fluoro-15-(2,3-dihydro-1H-inden-2-yl)-16,17,18,19,20-pentanor-prosta-5Z, 13E-dien-1-oic acid], indicating that this effect was mediated by FP receptors. Inhibition of protein kinase C with GF109203X [2-[1-(3-dimetylaminopropyl)-1H-indol-3-yl]-maleimide] resulted in a partial reduction of the growth stimulation induced by fluprostenol, indicating a minor role of FP receptors. Combining fluprostenol with misoprostol, but not with sulprostone, resulted in partially additive effects on DNA synthesis, suggesting that both EP3 and FP receptors are involved. Combining sulprostone with misoprostol did not result in additive effects on DNA synthesis, suggesting that EP4 receptors were not involved. We conclude that, although a minor effect is exerted by FP receptors, the growth-stimulatory effects of prostaglandins in rat hepatocytes are mediated mainly by EP3 receptors. We have found no evidence of EP1 receptor involvement.

Activation of mitogen-activated protein kinase by hepatocyte growth factor is stimulated by both alpha1- and beta2-adrenergic agonists in primary cultures of adult rat hepatocytes

We investigated the effects of alpha(1)- and beta(2)-adrenergic agonists on hepatocyte growth factor (HGF)-stimulated mitogen-activated protein kinase (MAPK) isoforms in primary cultures of adult rat hepatocytes. Hepatocytes were isolated and cultured with HGF (5 ng/ml) and/or alpha- and beta-adrenergic agonists. Phosphorylated MAPK isoforms (p42 and p44 MAPK) were detected by Western blotting analysis using anti-phospho-MAPK antibody. The results show that HGF increased phosphorylation of p42 MAPK by 2.2-fold within 3 min. The HGF-induced MAPK activation was abolished by AG1478 treatment (10(-7) M). The MEK (MAPK kinase) inhibitor PD98059 (10(-6) M) completely inhibited the HGF-dependent increase in MAPK activity. Phenylephrine (10(-6) M) and metaproterenol (10(-6) M) alone had no effect in the absence of HGF, but significantly increased p42 MAPK induction by HGF. Moreover, the cell-permeable cAMP analog, 8-bromo cAMP (10(-7) M), and phorbol 12-myristate 13 acetate (10(-7) M) potentiated HGF-induced MAPK phosphorylation. The effects of these analogs were antagonized by the protein kinase A (PKA) inhibitor H-89 (10(-7) M) and the protein kinase C (PKC) inhibitor sphingosine (10(-6) M), respectively. These results suggest that direct or indirect activation of both PKA and PKC represent a positive regulatory mechanism for stimulating MAPK induction by HGF.

Effect of EGF on [3H]-thymidine incorporation and cell cycle regulatory proteins in primary cultured chicken hepatocytes: Involvement of Ca2+/PKC and MAPKs

The reported studies on the metabolism in chicken hepatocytes in comparison with those of mammals are quite different. Therefore, this study examined the effect of EGF on DNA synthesis along with its related signal cascades in primary cultured chicken hepatocytes. EGF stimulated DNA synthesis in a dose (> or =10 ng/ml)-dependent manner, which correlated with the increase in CDK-2 and CDK-4 expression. The EGF-induced increase in [3H]-thymidine incorporation was blocked by AG 1478 (an EGF receptor tyrosine kinase antagonist), genistein, and herbimycin A (tyrosine kinase inhibitors), suggesting a role in the activation and tyrosine phosphorylation of the EGF receptor. In addition, the EGF-induced stimulation of [3H]-thymidine incorporation was prevented by staurosporine, H-7, or bisindolylmaleimide I (protein kinase C inhibitors), suggesting a role of PKC. In addition, PD 98059 (a MEK inhibitor), SB 203580 (a p38 MAPK inhibitor), and SP 600125 (a JNK inhibitor) blocked the EGF-induced stimulation of [3H]-thymidine incorporation and CDK-2/4 expression. Indeed, EGF increased the translocation of PKC from the cytosol to the membrane fraction, and increased the activation of p44/42 MAPK, p38 MAPK, and JNK. Moreover, EGF increased the CDK-2, CDK-4, cyclin D1, and cyclin E expression levels but decreased the p21 and p27 expression levels. These EGF-induced increases were blocked by an EGF receptor antagonist, tyrosine kinase inhibitors, PKC inhibitors, and MAPKs inhibitors. In conclusion, EGF stimulates DNA synthesis of primary cultured chicken hepatocytes via Ca2+/PKC and the MAPKs signaling pathways.

The characterization and molecular structure of hepatoproliferin: a liver regeneration factor from rat hepatocytes

Hepatoproliferin (HPF) was purified from regenerating rat livers as an oligomeric entity (big-HPF) from which the monomeric form (small-HPF) could be obtained using disaggregating conditions. By using a solid-phase ion-exchange method, small-HPF was forced to dissociate into two charged ionic species, namely norepinephrine (NE) and a sulfonated disaccharide with a molecular structure consisting of D-glucuronic acid bound to glucosamine 2,6-disulfate by a beta-glycosidic linkage having a beta, 1 --> 4 configuration. Monomeric HPF stemmed from the formation of three electrostatic bonds between the protonated amine groups of three norepinephrines, of which two bind to the deprotonated sulfonic groups of glucosamine 2,6-disulfate and one to the deprotonated carboxylic group of glucuronic acid, to constitute a tightly associated complex with a molecular mass of 1046 Da. This represents one of the two purified isoforms of small-HPF. The other isoform, which has a lower molecular mass of 877 Da, lack one NE, leaving the weaker carboxylic group of glucuronic acid unoccupied, to constitute a more acidic form of HPF.

EGF-induced inhibition of glucose transport is mediated by PKC and MAPK signal pathways in primary cultured chicken hepatocytes

EGF is a regulator of a wide variety of processes in various cell systems. Hepatocytes are important sites in the body's metabolism and function. Glucose transporter 2 (GLUT2) is a major transporter that is expressed strongly in hepatocytes. Therefore, this study examined the effect of EGF on GLUT2 and its related signal cascades in primary cultured chicken hepatocytes. EGF decreased [(3)H]deoxyglucose uptake in a dose- and time-dependent manner (>10 ng/ml, 2 h). AG-1478 (an EGF receptor antagonist) and genistein and herbimycin A (tyrosine kinase inhibitors) blocked the EGF-induced decrease in [(3)H]deoxyglucose uptake, which correlated with the GLUT2 expression level. In addition, the EGF-induced decrease in GLUT2 protein expression was inhibited by staurosporine, H-7, or bisindolylmaleimide I (PKC inhibitors), PD-98059 (a MEK inhibitor), SB-203580 (a p38 MAPK inhibitor), and SP-600125 (a JNK inhibitor), suggesting a role of both PKC and MAPKs (p44/42 MAPK, p38 MAPK, and JNK). In particular, EGF increased the translocation of PKC isoforms (PKC-alpha, -beta(1), -gamma, -delta, and -zeta) from the cytosol to the membrane fraction and increased the activation of p44/42 MAPK, p38 MAPK, and JNK. Moreover, PKC inhibitors blocked the EGF-induced phosphorylation of three MAPKs. In conclusion, EGF decreases the GLUT2 expression level via the PKC-MAPK signal cascade in chicken hepatocytes.

Sustained diacylglycerol accumulation resulting from prolonged G protein-coupled receptor agonist-induced phosphoinositide breakdown in hepatocytes

Studies in various cells have led to the idea that agonist-stimulated diacylglycerol (DAG) generation results from an early, transient phospholipase C (PLC)-catalyzed phosphoinositide breakdown, while a more sustained elevation of DAG originates from phosphatidylcholine (PC). We have examined this issue further, using cultured rat hepatocytes, and report here that various G protein-coupled receptor (GPCR) agonists, including vasopressin (VP), angiotensin II (Ang.II), prostaglandin F2alpha, and norepinephrine (NE), may give rise to a prolonged phosphoinositide hydrolysis. Preincubation of hepatocytes with 1-butanol to prevent conversion of phosphatidic acid (PA) did not affect the agonist-induced DAG accumulation, suggesting that phospholipase D-mediated breakdown of PC was not involved. In contrast, the GPCR agonists induced phosphoinositide turnover, assessed by accumulation of inositol phosphates, that was sustained for up to 18 h, even under conditions where PLC was partially desensitized. Pretreatment of hepatocytes with wortmannin, to inhibit synthesis of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PIP2), prevented agonist-induced inositol phosphate and DAG accumulation. Upon VP stimulation the level of PIP) declined, but only transiently, while increases in inositol 1,4,5-trisphosphate (InsP3) and DAG mass were sustained, suggesting that efficient resynthesis of PIP2 allowed sustained PLC activity. This was confirmed when cells were pretreated with wortmannin to prevent resynthesis of PIP2. Furthermore, metabolism of InsP3 was rapid, compared to that of DAG, with a more than 20-fold difference in half-life. Thus, rapid metabolism of InsP3 and efficient resynthesis of PIP2 may account for the larger amount of DAG generated and the more sustained time course, compared to InsP3. The results suggest that DAG accumulation that is sustained for many hours in response to VP, Ang.II, NE, and prostaglandin F2alpha in hepatocytes is mainly due to phosphoinositide breakdown.

eNOS, nNOS, cGMP and protein kinase G mediate the inhibitory effect of pancreastatin, a chromogranin A-derived peptide, on growth and proliferation of hepatoma cells

Pancreastatin (PST), a chromogranin A-derived peptide, has an anti-insulin metabolic effect and inhibits growth and proliferation by producing nitric oxide (NO) in HTC rat hepatoma cells. When NO production is blocked, a proliferative effect prevails due to the activation a Galphaq/11-phospholipase C-beta (PLC-beta) pathway, which leads to an increase in [Ca2+]i, protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) activation. The aim of the present study was to investigate the NO synthase (NOS) isoform that mediates these effects of PST on HTC hepatoma cells and the possible roles of cyclic GMP (cGMP) and cGMP-dependent protein kinase. DNA and protein synthesis in response to PST were measured as [3H]-thymidine and [3H]-leucine incorporation in the presence of various pharmacological inhibitors: N-monomethyl-L-arginine (NMLA, nonspecific NOS inhibitor), L-NIO (endothelial nitric oxide synthase (eNOS) inhibitor), espermidine (neuronal nitric oxide synthase (nNOS) inhibitor), LY83583 (guanylyl cyclase inhibitor), and KT5823 (protein kinase G inhibitor, (PKG)). L-NIO, similarly to NMLA, reverted the inhibitory effect of PST on hepatoma cell into a stimulatory effect on growth and proliferation. Nevertheless, espermidine also prevented the inhibitory effect of PST, but there was no stimulation of growth and proliferation. When guanylyl cyclase activity was blocked, there was again a reversion of the inhibitory effect into a stimulatory action, suggesting that the effect of NO was mediated by the production of cGMP. PKG inhibition prevented the inhibitory effect of PST, but there was no stimulatory effect. Therefore, the inhibitory effect of PST on growth and proliferation of hepatoma cells may be mainly mediated by eNOS activation. In turn, the effect of NO may be mediated by cGMP, whereas other pathways in addition to PKG activation seem to mediate the inhibition of DNA and protein synthesis by PST in HTC hepatoma cells.

Brain phospholipase C–diacylglycerol lipase pathway is involved in vasopressin-induced release of noradrenaline and adrenaline from adrenal medulla in rats

Recently, we reported that intracerebroventricularly (i.c.v.) administered arginine-vasopressin evokes the release of noradrenaline and adrenaline from adrenal medulla by brain thromboxane A2-mediated mechanisms in rats. These results suggest the involvement of brain arachidonic acid in the vasopressin-induced activation of the central adrenomedullary outflow. Arachidonic acid is released mainly by two pathways: phospholipase A2 (PLA2)-dependent pathway; phospholipase C (PLC)- and diacylglycerol lipase-dependent pathway. In the present study, therefore, we attempted to identify which pathway is involved in the vasopressin-induced release of both catecholamines from adrenal medulla using urethane-anesthetized rats. Vasopressin (0.2 nmol/animal, i.c.v.)-induced elevation of plasma noradrenaline and adrenaline was dose-dependently reduced by neomycin [0.28 and 0.55 micromol (250 and 500 microg)/animal, i.c.v.] and 1-[6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U-73122) [5 and 10 nmol (2.3 and 4.6 microg)/animal, i.c.v.] (inhibitors of PLC), and also by 1,6-bis(cyclohexyloximinocarbonylamino)hexane (RHC-80267) [1.3 and 2.6 micromol (500 and 1000 microg)/animal, i.c.v.] (an inhibitor of diacylglycerol lipase). On the other hand, mepacrine [1.1 and 2.2 micromol (500 and 1000 microg)/animal, i.c.v.] (an inhibitor of PLA2) was largely ineffective on the vasopressin-induced elevation of plasma catecholamines. These results suggest that vasopressin evokes the release of noradrenaline and adrenaline from adrenal medulla by the brain PLC- and diacylglycerol lipase-dependent mechanisms in rats.

Contribution of PKC-dependent and -independent processes in temporal ERK regulation by ET-1, PDGF, and EGF in rat myometrial cells

Endothelin-1 (ET-1), platelet-derived growth factor (PDGF), and epidermal growth factor (EGF) stimulated thymidine incorporation with different efficiency (PDGF ≫ EGF = ET-1) in rat myometrial cells. They also stimulated ERK activation, which culminated at 5 min and then declined to reach a plateau (at 45 min: EGF > 90%, PDGF = 50%, and ET-1 < 10% of maximum). Inhibition and downregulation of PKC demonstrated that ERK activation at 5 min involved PKCδ and -ζ for ET-1 and PKCα plus another PKC isoform for PDGF. By contrast, the EGF response did not involve PKC. Stimulation of Ras was more important with EGF than with PDGF, with ET-1 being the weakest activator. The simultaneous incubation of the cells with EGF and ET-1 potentiated the ERK activation at 5 min and mimicked the plateau phase obtained with PDGF. Under these conditions thymidine incorporation was comparable to that induced by PDGF. Taken together, our results indicated that the kinetic profile of ERK activation and its impact on cell proliferation can be modulated by the differential involvement of PKC isoforms and the amplitude of Ras activation.

G protein-coupled receptor agonist-stimulated expression of ATF3/LRF-1 and c-myc and comitogenic effects in hepatocytes do not require EGF receptor transactivation

Several agonists acting on G protein-coupled receptors (GPCR) enhance the mitogenic effect of epidermal growth factor (EGF) in rat hepatocytes, through mechanisms that have only partially been clarified. Results in various cells have led to the idea that a major mechanism for GPCR-mediated stimulation of cell growth is transactivation of receptor tyrosine kinases, particularly the EGF receptor (EGFR), leading to rapid phosphorylation of the EGFR and activation of downstream signaling pathways. In the present study cultured rat hepatocytes were exposed to various GPCR agonists, including vasopressin, angiotensin II (Ang.II), norepinephrine, or prostaglandin F(2 alpha) (PGF(2 alpha)). None of these agents increased the phosphorylation of the EGFR or the docking protein Shc. Furthermore, we examined the effect of the GPCR agonists on the expression of two early response genes believed to be involved in growth activation. The GPCR agonists increased the mRNA expression of c-myc, and also of activating transcription factor 3 (ATF3)/liver regeneration factor-1 (LRF-1), which is a novel finding. Finally, the selective EGFR inhibitor AG1478 did not suppress the activation of extracellular signal-regulated kinase 1/2 (ERK1/2) or the induction of c-myc or ATF3/LRF-1 by the GPCR agonists, and did not prevent the comitogenic effects induced by these agents, while it blocked the effect of EGF on these responses. The results suggest that GPCR agonists induce expression of ATF3/LRF-1 and c-myc and exert comitogenic effects through mechanisms that do not require EGFR transactivation.

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.

Thyroid hormones regulate DNA-synthesis and cell-cycle proteins by activation of PKC? and p42/44 MAPK in chick embryo hepatocytes

The molecular mechanism by which thyroid hormones exert their effects on cell growth is still unknown. In this study, we used chick embryo hepatocytes at different stages of development as a model to investigate the effect of the two thyroid hormones, T3 and T4, and of their metabolite T2, on the control of cell proliferation. We observed that T2 provokes increase of DNA-synthesis as well as T3 and T4, independently of developmental stage. We found that this stimulatory effect on the S phase is reverted by specific inhibitors of protein kinase C (PKC) and p42/44 mitogen-activated protein kinase (p42/44 MAPK), Ro 31-8220 or PD 98059. Furthermore, the treatment with thyroid hormones induces the activation of PKCalpha and p42/44 MAPK, suggesting their role as possible downstream mediators of cell response mediated by thyroid hormones. The increase of DNA-synthesis is well correlated with the increased levels of cyclin D1 and cdk4 that control the G1 phase, and also with the activities of cell-cycle proteins involved in the G1 to S phase progression, such as cyclin E/A-cdk2 complexes. Interestingly, the activity of cyclin-cdk2 complexes is strongly repressed in the presence of PKC and p42/44 MAPK inhibitors. In conclusion, we demonstrated that the thyroid hormones could modulate different signaling pathways that are able to control cell-cycle progression, mainly during G1/S transition.

EGF receptor-mediated, c-Src-dependent, activation of Stat5b is downregulated in mitogenically responsive hepatocytes

Signal transducer and activator of transcription (STAT) proteins may be activated by epidermal growth factor (EGF), but their role in EGF receptor-mediated mitogenic signaling is not clear. We previously showed that Stat5b was activated by EGF in rat hepatocytes in primary monolayer culture. In the present study, we found that EGF induced tyrosine phosphorylation of Stat5b both on Tyr-699, which correlated with specific DNA binding activity, and also on other tyrosine residues. The Src tyrosine kinase inhibitor CGP77675 blocked the EGF-induced activation of Stat5b, but did not affect the Stat5b activation by growth hormone (GH) or prolactin (PRL). The Stat5b response to EGF was most pronounced soon (3 h) after plating (early G(1)) and at high cell density (50,000 hepatocytes per cm(2)). However, at this cell density EGF did not stimulate DNA synthesis. In hepatocytes at 24 h of culturing (mid/late G(1)) with 20,000 cells per cm(2), EGF induced strong phosphorylation of the EGF receptor, as well as Shc and ERK, and stimulated DNA synthesis, but did not activate Stat5b, although the Stat5b response to GH or PRL was retained. A strong GH-induced Stat5b activation neither influenced the DNA synthesis alone nor enhanced the mitogenic effect of EGF. The results show that EGF induces tyrosine phosphorylation and DNA-binding activity of Stat5b in a manner different from GH and PRL, apparently by a Src-dependent mechanism. The data also provide further evidence that Stat5b is not required for mitogenic signaling from the EGF receptor.

Retinoic acid maintains differentiated cell morphology and functions in long-term cultured porcine hepatocytes: obtaining functional cells for prolonged treatments with bioartificial liver

Porcine hepatocytes show several immunological characteristics and enzymatic activities of human liver, representing an ideal xenogenic source of cells as biological component of bioartificial liver (BAL). Isolated hepatocytes rapidly lose their specific metabolic activities and their typical morphology when cultured in the presence of serum. Since in BAL porcine hepatocytes are perfused by the patient's plasma, procedures able to minimize de-differentiation of cells could be useful for long-term treatment of acute liver failure (ALF). In this work we found that, in the presence of micromolar concentration of All trans-retinoic acid (ATRA), porcine parenchymal liver cells undergo to a lower extent the de-differentiating effects of long-term culture in the presence of serum. The evaluation of lidocaine metabolism showed that ATRA-treated cells retain specific hepatocyte function for a significantly longer time when compared to control hepatocytes. A tyrosine phosphorylation of PLC-gamma1 was observed in concomitance with the ATRA-induced maximal functional activity. An increased expression of PLC-beta3 and PKC-alpha and -beta2 was also evidentiated at the longer time points explored, when the effects of ATRA in preservation of the differentiated morphology were maximal. These results provide the first evidence that ATRA plays a differentiating role in adult porcine hepatocytes cultured under de-differentiating conditions. The administration of ATRA to isolated parenchymal cells from pig liver may provide functional hepatocytes for prolonged treatment with BAL.

Inhibitory effect of AP-1 complex on 5-aminolevulinate synthase gene expression through sequestration of cAMP-response element protein (CRE)-binding protein (CBP) coactivator

Activation protein-1 (AP-1) transcription factors are early response genes involved in a diverse set of transcriptional regulatory processes. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) is often used to induce AP-1 activity. The purpose of this work was to explore the molecular mechanisms involved in the TPA regulation of ubiquitous 5-aminolevulinate synthase (ALAS) gene expression, the first and rate-controlling step of the heme biosynthesis. Previous analysis of the 5'-flanking sequence of ALAS revealed the existence of two cAMP-response elements (CRE) required for basal and cAMP-stimulated expression. The fragment -833 to +42 in the 5'-flanking region of rat ALAS gene was subcloned into a chloramphenicol acetyltransferase (CAT) reporter vector. The expression vector pALAS/CAT produced a significant CAT activity in transiently transfected HepG2 human hepatoma cells, which was repressed by TPA. Sequence and deletion analysis detected a TPA response element (TRE), located between -261 and -255 (TRE-ALAS), that was critical for TPA regulation. We demonstrated that c-Fos, c-Jun, and JunD are involved in TPA inhibitory effect due to their ability to bind TRE-ALAS, evidenced by supershift analysis and their capacity to repress promoter activity in transfection assays. Repression of ALAS promoter activity by TPA treatment or Fos/Jun overexpression was largely relieved when CRE protein-binding protein or p300 was ectopically expressed. When the TRE site was placed in a different context with respect to CRE sites, it appeared to act as a transcriptional enhancer. We propose that the decrease in ALAS basal activity observed in the presence of TPA may reflect a lower ability of this promoter to assemble the productive pre-initiation complex due to CRE protein-binding protein sequestration. We also suggest that the transcriptional properties of this AP-1 site would depend on a spatial-disposition-dependent manner with respect to the CRE sites and to the transcription initiation site.

Flavanones structure-related inhibition on TPA-induced tumor promotion through suppression of extracellular signal-regulated protein kinases: involvement of prostaglandin E2 in anti-promotive process

Biological functions of flavanones have been studied extensively, however, the structure-related activities of flavanones on 12-o-tetradecanoylphorbol 13-acetate (TPA)-induced promotive effects are still unclear. In this study, flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone showed the most significant dose-dependent inhibition on TPA-induced proliferative effects among eight tested flavanones in NIH3T3 cells. TPA-induced mitogen activated protein kinases (MAPK) phosphorylation, ornithine decarboxylase (ODC), c-Jun, and cyclooxygenase 2 (COX-2) protein expressions in a time-dependent manner, and the maximal inductive time point is at 1 h for MAPK phosphorylation and 6 h for others. Flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone showed the dose-dependent inhibition on TPA-stimulated MAPK phosphorylation, COX-2, ODC, c-Jun protein expressions. Induction of, prostaglandin E(2) (PGE(2)) production was detected in TPA-treated NIH3T3 cells, and flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone inhibited significantly PGE(2) production induced by TPA. Addition of PGE(2) reverses the inhibitory activities of flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone on TPA-induced proliferation. And, PD98059, a specific inhibitor of ERKs, inhibited TPA-induced MAPK phosphorylation, accompanied by decreasing COX-2, c-Jun, and ODC protein expression, and showed dose-dependent inhibition on TPA-induced proliferation in cells. These results demonstrated that PGE(2) is an important mediator in TPA-induced proliferation, and MAPK phosphorylation was located at the upstream of COX-2, c-Jun, and ODC gene expressions in TPA-induced responses. Furthermore, flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone (100 microM) suppressed TPA-induced colony formation associated with blocking MAPK phosphorylation, ODC, c-Jun, and COX-2 proteins expression. And, 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay showed that flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone did not perform potent anti-radical activities among these eight tested compounds. In conclusion, this study provided molecular evidences to demonstrate that flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone were potent inhibitors on TPA-induced responses without notable cytotoxicity through suppression of PGE(2) production; and anti-radical activity of flavanones was not correlated with preventing the occurrence of tumor promotion. We proposed that blocking TPA-induced intracellular signaling responses might be involved in the anti-promotive mechanism of flavanones.

Differential role of MAP kinases in stimulation of hepatocyte growth by EGF and G-protein-coupled receptor agonists

Several agonists acting on G-protein-coupled receptors (GPCR) enhance the mitogenic effect of EGF in rat hepatocytes. Previous studies have shown that mitogen-activated protein (MAP) kinases are involved in the mitogenic effect of EGF. In the present study on cultured rat hepatocytes we show that although the comitogenic GPCR agonists prostaglandin F(2alpha), vasopressin, angiotensin II, and norepinephrine all activated ERK, blocking of the ERK pathway with the MEK inhibitor PD 98059 did not abolish their comitogenic effects. These GPCR agonists also activated p38, but the p38 blocker SB 203580 did not reduce the comitogenic effects. The mitogenic effect of EGF was inhibited completely by PD 98059 and partially by SB 203580. These results suggest that, in contrast to the mitogenic effect of EGF, the comitogenic effect of a group of GPCR agonists is independent of ERK and p38 in these cells.

Ca2+-mediated activation of ERK in hepatocytes by norepinephrine and prostaglandin F2 alpha: role of calmodulin and Src kinases

BACKGROUND

Previous studies have shown that several agents that stimulate heptahelical G-protein coupled receptors activate the extracellular signal regulated kinases ERK1 (p44mapk) and ERK2 (p42mapk) in hepatocytes. The molecular pathways that convey their signals to ERK1/2 are only partially clarified. In the present study we have explored the role of Ca2+ and Ca2+-dependent steps leading to ERK1/2 activation induced by norepinephrine and prostaglandin (PG)F2alpha.

RESULTS

Pretreatment of the cells with the Ca2+ chelators BAPTA-AM or EGTA, as well as the Ca2+ influx inhibitor gadolinium, resulted in a partial decrease of the ERK response. Furthermore, the calmodulin antagonists W-7, trifluoperazine, and J-8 markedly decreased ERK activation. Pretreatment with KN-93, an inhibitor of the multifunctional Ca2+/calmodulin-dependent protein kinase, had no effect on ERK activation. The Src kinase inhibitors PP1 and PP2 partially diminished the ERK responses elicited by both norepinephrine and PGF2alpha.

CONCLUSION

The present data indicate that Ca2+ is involved in ERK activation induced by hormones acting on G protein-coupled receptors in hepatocytes, and suggest that calmodulin and Src kinases might play a role in these signaling pathways.

Smad7 is induced by norepinephrine and protects rat hepatocytes from activin A-induced growth inhibition

Activin A induces growth arrest of rat hepatocytes in vitro and in vivo. The alpha(1)-adrenergic agonist, norepinephrine (NE), enhances epidermal growth factor-stimulated DNA synthesis and inhibits activin A-induced growth inhibition, but the mechanisms of these actions are unclear. Smad proteins have recently been identified as intracellular signaling mediators of transforming growth factor-beta family members. In the present study, we explored how NE modulates the Smad signaling pathway in rat cultured hepatocytes. We demonstrate that NE inhibits activin A-induced nuclear accumulation of Smad2/3 and that NE rapidly induces inhibitory Smad7 mRNA expression. Infection of Smad7 adenovirus into rat hepatocytes inhibited activin A-induced nuclear accumulation of Smad2/3, enhanced epidermal growth factor-stimulated DNA synthesis, and abolished the growth inhibitory effect of activin A. We also demonstrated that the induction of Smad7 by NE is dependent on nuclear factor-kappa B (NF-kappa B). The amount of active NF-kappa B complex rapidly increased after NE treatment. Preincubation of the cells with an NF-kappa B pathway inhibitor N-tosyl-l-phenylalanine chloromethyl ketone or infection of the cells with an adenovirus expressing an I kappa B super-repressor (Ad5I kappa B) abolished the NE-induced Smad7 expression. These results indicate a mechanism of transmodulation between the Smad and trimeric G protein signaling pathways in rat hepatocytes.

Pancreastatin, a chromogranin A-derived peptide, inhibits DNA and protein synthesis by producing nitric oxide in HTC rat hepatoma cells

BACKGROUND/AIMS

Pancreastatin, a chromogranin A-derived peptide, has a counter-regulatory effect on insulin action. We have previously characterized pancreastatin receptor and signalling in rat liver and HTC hepatoma cells. A G alpha(q/11)-PLC-beta pathway leads to an increase in [Ca2+]i, PKC and mitogen activated protein kinase (MAPK) activation. These data suggested that pancreastatin might have a role in growth and proliferation, similar to other calcium-mobilizing hormones.

METHODS

DNA and protein synthesis were measured as [3H]-thymidine and [3H]-leucine incorporation. Nitric oxide (NO) was determined by the Griess method and cGMP production was quantified by enzyme-linked immunoassay.

RESULTS

Contrary to the expected results, we have found that pancreastatin inhibits protein and DNA synthesis in HTC hepatoma cells. On the other hand, when the activity of NO synthase was inhibited by N-monomethyl-L-arginine (NMLA), the inhibitory effect of pancreastatin on DNA and protein synthesis was not only reverted, but a dose-dependent stimulatory effect was observed, probably due to MAPK activation, since it was prevented by PD98059. These data strongly suggested the role of NO in the inhibitory effect of pancreastatin on protein and DNA synthesis, which is overcoming the effect on MAPK activation. Moreover, pancreastatin dose-dependently increased NO production in parallel to cyclic guanosine monophosphate (cGMP). Both effects were prevented by NMLA. Finally, an indirect effect of pancreastatin through the induction of apoptosis was ruled out.

CONCLUSIONS

Therefore, the NO and the cGMP produced by the NO-activated guanylate cyclase may mediate the dose-dependent inhibitory effect of pancreastatin on growth and proliferation in HTC hepatoma cells.

G Protein-coupled Receptors Desensitize and Down-regulate Epidermal Growth Factor Receptors in Renal Mesangial Cells

Different types of plasma membrane receptors engage in various forms of cross-talk. We used cultures of rat renal mesangial cells to study the regulation of EGF receptors (EGFRs) by various endogenous G protein-coupled receptors (GPCRs). GPCRs (5-hydroxytryptamine(2A), lysophosphatidic acid, angiotensin AT(1), bradykinin B(2)) were shown to transactivate EGFRs through a protein kinase C-dependent pathway. This transactivation resulted in the initiation of multiple cellular signals (phosphorylation of the EGFRs and ERK and activation of cAMP-responsive element-binding protein (CREB), NF-kappaB, and E2F), as well as subsequent rapid down-regulation of cell-surface EGFRs and internalization and desensitization of the EGFRs without change in the total cellular complement of EGFRs. Internalization of the EGFRs and the down-regulation of cell-surface receptors in mesangial cells were blocked by pharmacological inhibitors of clathrin-mediated endocytosis and in HEK293 cells by transfection of cDNA constructs that encode dominant negative beta-arrestin-1 or dynamin. Whereas all of the effects of GPCRs on EGFRs were dependent to a great extent on protein kinase C, those initiated by EGF were not. These studies demonstrate that GPCRs can induce multiple signals through protein kinase C-dependent transactivation of EGFRs. Moreover, GPCRs induce profound desensitization of EGFRs by a process associated with the loss of cell-surface EGFRs through clathrin-mediated endocytosis.

Regulation of neuroblast mitosis is determined by PACAP receptor isoform expression

Although neurogenesis in the embryo proceeds in a region- or lineage-specific fashion coincident with neuropeptide expression, a regulatory role for G protein-coupled receptors (GPCR) remains undefined. Pituitary adenylate cyclase activating polypeptide (PACAP) stimulates sympathetic neuroblast proliferation, whereas the peptide inhibits embryonic cortical precursor mitosis. Here, by using ectopic expression strategies, we show that the opposing mitogenic effects of PACAP are determined by expression of PACAP receptor splice isoforms and differential coupling to the phospholipase C (PLC) pathway, as opposed to differences in cellular context. In embryonic day 14 (E14) cortical precursors transfected with the hop receptor variant, but not cells transfected with the short variant, PACAP activates the PLC pathway, increasing intracellular calcium and eliciting translocation of protein kinase C. Ectopic expression of the hop variant in cortical neuroblasts transforms the antimitotic effect of PACAP into a promitogenic signal. Furthermore, PACAP promitogenic effects required PLC pathway function indicated by antagonist U-73122 studies in hop-transfected cortical cells and native sympathetic neuroblasts. These observations highlight the critical role of lineage-specific expression of GPCR variants in determining mitogenic signaling in neural precursors.

Effects of pertussis toxin on extracellular signal-regulated kinase activation in hepatocytes by hormones and receptor-independent agents: evidence suggesting a stimulatory role of G(i) proteins at a level distal to receptor coupling

It was previously found that pertussis toxin (PTX) pretreatment inhibits the activation of extracellular signal-regulated kinases ERK1 (p44(mapk)) and ERK2 (p42(mapk)) in hepatocytes in response to either agonists that bind to heptahelical receptors or epidermal growth factor (EGF), suggesting a role of G(i) proteins in stimulatory mechanisms for ERK1/2. The present work shows that ERK1/2 is activated in a PTX-sensitive way not only by vasopressin, angiotensin II, prostaglandin (PG) F(2alpha), alpha(1)-adrenergic stimulation, and EGF but also by agents whose actions bypass receptors and stimulate protein kinase C (PKC) and/or elevate intracellular Ca(2+), such as 12-O-tetradecanoyl phorbol-13-acetate (TPA), exogenous phosphatidylcholine-specific phospholipase C (PC-PLC, from Bacillus cereus), thapsigargin, and the Ca(2+) ionophore A23187. Under the same conditions, PTX did not affect agonist stimulation of phosphoinositide-specific phospholipase C (PI-PLC) (IP(3) generation), and did not reduce the activation by these agents of phospholipase D (PLD). The results suggest that in hepatocytes a PTX-sensitive mechanism, presumably involving G(i) proteins, exerts a stimulatory effect on ERK at a level distal to receptor coupling, acting either as an integral part of the signaling pathway(s) or by a permissive, synergistic regulation.

The relationship between activation of phosphoinositide-specific phospholipase C and growth stimulation by Ca2+-mobilizing hormones in hepatocytes

Previous studies have shown that while vasopressin and angiotensin II are markedly more effective than norepinephrine and prostaglandin F2alpha in eliciting an acute elevation of inositol 1,4,5-trisphosphate (IP3), norepinephrine and prostaglandin F2alpha produce larger enhancement of DNA synthesis. This suggests that the initial activation of phosphoinositide-specific phospholipase C is not a common factor for the growth response to these agonists, but does not exclude a role of the integral of phospholipase C activity over a prolonged part of the prereplicative period, during which agonist-specific changes in responsiveness might occur. We show that vasopressin and angiotensin II also cause a prolonged elevation of cellular IP3 levels. which remain elevated for at least 60 min., while norepinephrine and prostaglandin F2alpha elevate IP3 levels slightly and transiently For vasopressin the dose-effect curves for IP3 accumulation and stimulation of DNA synthesis were closely parallel, while this was not the case for angiotensin II, norepinephrine, or prostaglandin F2alpha. After cultivation of the hepatocytes, hormone-stimulated IP3 accumulation rapidly declined, particularly in response to norepinephrine and prostaglandin F2alpha. When the IP3 response to norepinephrine and prostaglandin F2alpha was completely down-regulated, these agonists still enhanced the DNA synthesis. These results suggest that other mechanisms in addition to IP3 accumulation and Ca2+ release are likely to be involved in the growth stimulatory effects of the Ca2+-mobilizing agonists studied here, in particular for angiotensin II, norepinephrine, and prostaglandin F2alpha.