Differential regulation of the mitogen-activated protein and stress-activated protein kinase cascades by adrenergic agonists in quiescent and regenerating adult rat hepatocytes

HGF/c-Met: A Key Promoter in Liver Regeneration

Hepatocyte growth factor (HGF) is a peptide-containing multifunctional cytokine that acts on various epithelial cells to regulate cell growth, movement and morphogenesis, and tissue regeneration of injured organs. HGF is sequestered by heparin-like protein in its inactive form and is widespread in the extracellular matrix of most tissues. When the liver loses its average mass, volume, or physiological and biochemical functions due to various reasons, HGF binds to its specific receptor c-Met (cellular mesenchymal-epithelial transition) and transmits the signals into the cells, and triggers the intrinsic kinase activity of c-Met. The downstream cascades of HGF/c-Met include JAK/STAT3, PI3K/Akt/NF-κB, and Ras/Raf pathways, affecting cell proliferation, growth, and survival. HGF has important clinical significance for liver fibrosis, hepatocyte regeneration after inflammation, and liver regeneration after transplantation. And the development of HGF as a biological drug for regenerative therapy of diseases, that is, using recombinant human HGF protein to treat disorders in clinical trials, is underway. This review summarizes the recent findings of the HGF/c-Met signaling functions in liver regeneration.

The Role of Catecholamines in Pathophysiological Liver Processes

Over the last few years, the number of research publications about the role of catecholamines (epinephrine, norepinephrine, and dopamine) in the development of liver diseases such as liver fibrosis, fatty liver diseases, or liver cancers is constantly increasing. However, the mechanisms involved in these effects are not well understood. In this review, we first recapitulate the way the liver is in contact with catecholamines and consider liver implications in their metabolism. A focus on the expression of the adrenergic and dopaminergic receptors by the liver cells is also discussed. Involvement of catecholamines in physiological (glucose metabolism, lipids metabolism, and liver regeneration) and pathophysiological (impact on drug-metabolizing enzymes expression, liver dysfunction during sepsis, fibrosis development, or liver fatty diseases and liver cancers) processes are then discussed. This review highlights the importance of understanding the mechanisms through which catecholamines influence liver functions in order to draw benefit from the adrenergic and dopaminergic antagonists currently marketed. Indeed, as these molecules are well-known drugs, their use as therapies or adjuvant treatments in several liver diseases could be facilitated.

Proliferation‑inhibiting pathways in liver regeneration (Review)

Liver regeneration, an orchestrated process, is the primary compensatory mechanism following liver injury caused by various factors. The process of liver regeneration consists of three stages: Initiation, proliferation and termination. Proliferation‑promoting factors, which stimulate the recovery of mitosis in quiescent hepatocytes, are essential in the initiation and proliferation steps of liver regeneration. Proliferation‑promoting factors act as the 'motor' of liver regeneration, whereas proliferation inhibitors arrest cell proliferation when the remnant liver reaches a suitable size. Certain proliferation inhibitors are also expressed and activated in the first two steps of liver regeneration. Anti‑proliferation factors, acting as a 'brake', control the speed of proliferation and determine the terminal point of liver regeneration. Furthermore, anti‑proliferation factors function as a 'steering‑wheel', ensuring that the regeneration process proceeds in the right direction by preventing proliferation in the wrong direction, as occurs in oncogenesis. Therefore, proliferation inhibitors to ensure safe and stable liver regeneration are as important as proliferation‑promoting factors. Cytokines, including transforming growth factor‑β and interleukin‑1, and tumor suppressor genes, including p53 and p21, are important members of the proliferation inhibitor family in liver regeneration. Certain anti‑proliferation factors are involved in the process of gene expression and protein modification. The suppression of liver regeneration led by metabolism, hormone activity and pathological performance have been reviewed previously. However, less is known regarding the proliferation inhibitors of liver regeneration and further investigations are required. Detailed information regarding the majority of known anti‑proliferation signaling pathways also remains fragmented. The present review aimed to understand the signalling pathways that inhbit proliferation in the process of liver regeneration.

The eukaryotic translation initiation factor 3f (eIF3f) interacts physically with the alpha 1B-adrenergic receptor and stimulates adrenoceptor activity

Background

eIF3f is a multifunctional protein capable of interacting with proteins involved in different cellular processes, such as protein synthesis, DNA repair, and viral mRNA edition. In human cells, eIF3f is related to cell cycle and proliferation, and its deregulation compromises cell viability.

Results

We here report that, in native conditions, eIF3f physically interacts with the alpha 1B-adrenergic receptor, a plasma membrane protein considered as a proto-oncogene, and involved in vasoconstriction and cell proliferation. The complex formed by eIF3f and alpha 1B-ADR was found in human and mouse cell lines. Upon catecholamine stimulation, eIF3f promotes adrenoceptor activity in vitro, independently of the eIF3f proline- and alanine-rich N-terminal region.

Conclusions

The eIF3f/alpha adrenergic receptor interaction opens new insights regarding adrenoceptor-related transduction pathways and proliferation control in human cells. The eIf3f/alpha 1B-ADR complex is found in mammals and is not tissue specific.

The effect of sorafenib on liver regeneration and angiogenesis after partial hepatectomy in rats

BACKGROUND

Liver regeneration is vital for the survival of patients submitted to extensive liver resection as a treatment of hepatocellular carcinoma (HCC). Sorafenib is a multikinase inhibitor of angiogenesis and cell division, both of which are integral components of liver regeneration. We investigated the effect of preoperative treatment with sorafenib, a drug used for the treatment of HCC, on liver regeneration and angiogenesis in healthy rats, after two-thirds partial hepatectomy (PH2/3).

METHODS

In total 48 Wistar rats received intragastric injections of sorafenib (30 mg/kg/d) or vehicle, underwent PH2/3, and were sacrificed at 48, 96 or 168 hours after that. The regenerative index of the liver remnant was studied, as well as the mitotic index. DNA synthesis and angiogenesis were estimated by immunohistochemistry for the Ki-67 and CD34 antigens, respectively.

RESULTS

Sorafenib reduced significantly the regenerative index at all time points but not the mitotic index at 48, 96 or 168 hours. Deoxyribonucleic acid (DNA) synthesis and angiogenesis were not affected significantly either.

CONCLUSIONS

Sorafenib, when administered preoperatively, reduces incompletely and transiently the regeneration of the liver after PH2/3 in rats. This could mean that sorafenib can be used as neoadjuvant treatment of patients with HCC prior to liver resection, but further experimental and clinical studies are needed to establish the safety of this treatment. Hippokratia 2015; 19 (3): 249-255.

Pyroglutamic acid stimulates DNA synthesis in rat primary hepatocytes through the mitogen-activated protein kinase pathway

We purified pyroglutamic acid from human placental extract and identified it as a potent stimulator of rat primary hepatocyte DNA synthesis. Pyroglutamic acid dose-dependently stimulated DNA synthesis, and this effect was inhibited by PD98059, a dual specificity mitogen-activated protein kinase kinase 1 (MAP2K1) inhibitor. Therefore, pyroglutamic acid stimulated DNA synthesis in rat primary hepatocytes via MAPK signaling.

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.

Regulation of signal transduction and role of platelets in liver regeneration

Among all organs, the liver has a unique regeneration capability after sustaining injury or the loss of tissue that occurs mainly due to mitosis in the hepatocytes that are quiescent under normal conditions. Liver regeneration is induced through a cascade of various cytokines and growth factors, such as, tumor necrosis factor alpha, interleukin-6, hepatocyte growth factor, and insulin-like growth factor, which activate nuclear factor κB, signal transducer and activator of transcription 3, and phosphatidyl inositol 3-kinase signaling pathways. We previously reported that platelets can play important roles in liver regeneration through a direct effect on hepatocytes and collaborative effects with the nonparenchymal cells of the liver, including Kupffer cells and liver sinusoidal endothelial cells, which participate in liver regeneration through the production of various growth factors and cytokines. In this paper, the roles of platelets and nonparenchymal cells in liver regeneration, including the associated cytokines, growth factors, and signaling pathways, are described.

Advances in the regulation of liver regeneration

Liver regeneration is known to be a process involving highly organized and ordered tissue growth triggered by the loss of liver tissue, and remains a fascinating topic. A large number of genes are involved in this process, and there exists a sequence of stages that results in liver regeneration, while at the same time inhibitors control the size of the regenerated liver. The initiation step is characterized by priming of quiescent hepatocytes by factors such as TNF-α, IL-6 and nitric oxide. The proliferation step is the step during which hepatocytes enter into the cell cycle's G1 phase and are stimulated by complete mitogens including HGF, TGF-α and EGF. Hepatic stimulator substance, glucagon, insulin, TNF-α, IL-1 and IL-6 have also been implicated in regulating the regeneration process. Inhibitors and stop signals of hepatic regeneration are not well known and only limited information is available. Furthermore, the effects of other factors such as VEGF, PDGF, hypothyroidism, proliferating cell nuclear antigen, heat shock proteins, ischemic-reperfusion injury, steatosis and granulocyte colony-stimulating factor on liver regeneration are also systematically reviewed in this article. A tissue engineering approach using isolated hepatocytes for in vitro tissue generation and heterotopic transplantation of liver cells has been established. The use of stem cells might also be very attractive to overcome the limitation of donor liver tissue. Liver-specific differentiation of embryonic, fetal or adult stem cells is currently under investigation.

Effect of sorafenib on murine liver regeneration

Hepatocellular carcinoma (HCC) is a common cause of cancer-related death. Sorafenib prolongs survival of patients with advanced disease and is approved for the systemic treatment of unresectable HCC. It possesses antiangiogenic and antiproliferative properties by way of inhibition of the receptor tyrosine kinases vascular endothelial growth factor receptor 2 (VEGFR-2) and platelet-derived growth factor receptor-beta 1/2 (PDGFR-β) and the kinase RAF. Sorafenib represents a candidate compound for adjuvant therapy in HCC patients. The aim of our study was to investigate whether sorafenib affects liver regeneration. C57BL6 mice received sorafenib orally at 30 mg/kg/day or its vehicle either for 14 days until the day before hepatectomy or starting the day after surgery or both. Animals were sacrificed 24, 72, and 120 hours after hepatectomy. Liver regeneration was calculated as a percent of initial liver weight. Bromodeoxyuridine (BrdU) incorporation and phospho-extracellular signal-regulated kinase (pERK1/2) were determined by immunohistochemistry on liver sections. VEGF-A, PDGF-BB, and hepatocyte growth factor (HGF) levels were measured in liver tissue homogenates. Histological analysis of scar tissue was performed. Treatment stopped 1 day before surgery had no impact on liver regeneration. Continuous sorafenib treatment and treatment started 1 day after surgery had statistically significant effects on liver regeneration at 120 hours compared to vehicle-treated control animals (72% ± 12 versus control 88% ± 15 and 70% ± 13 versus control 86% ± 5 at 120 hours, both P ≤ 0.02). BrdU incorporation showed decreased numbers of positive nuclei in both groups receiving sorafenib after surgery. Phospho-ERK levels were reduced in sorafenib-treated animals. An increase of VEGF-A levels was observed in mice receiving sorafenib. Wound-healing complications were observed in animals receiving sorafenib after surgery and confirmed on histological sections.

CONCLUSION

This preclinical study shows that sorafenib did not impact on liver regeneration when ceased before surgery; however, administration after hepatectomy affected late liver regeneration.

Prostate growth inhibition by subtype-selective alpha(1)-adrenoceptor antagonist naftopidil in benign prostatic hyperplasia

BACKGROUND

Recently, alpha(1)-adrenoceptors (alpha(1)-ARs) have been reported to play a prominent role in the growth of a variety of cells; however, little is known about prostate growth and subtype-specific effects on cell proliferation. We examined the role of alpha(1d)-AR in prostate growth and the effect of subtype-selective alpha(1)-AR antagonist, naftopidil, which has relatively higher affinity for alpha(1d)-AR, on prostate growth in vitro and in vivo.

METHODS

First, we examined the effect of naftopidil on the cell proliferation of PrEC, PrSC, and PrSMC using WST-1 assay. Second, we performed real-time RT-PCR to quantify each alpha(1)-AR subtype mRNA expression level in a benign prostate hyperplasia (BPH) model rat, which was recently established to pathologically resemble human BPH patients. In addition, naftopidil was given to this model orally for 21 days and the proliferative and apoptotic indexes measured. Third, 18 BPH patients were administered naftopidil for 12 weeks and the proliferative and apoptotic indexes were compared before and after naftopidil administration.

RESULTS

Naftopidil significantly inhibited cell proliferation dose-dependently in all cell lines that expressed alpha(1d)-AR mRNA. The expression level of alpha(1d)-AR during the growth process of the prostate in the BPH model rat was significantly higher than that in the normal prostate (P < 0.001). Naftopidil administration inhibited cell proliferation without apoptosis in the BPH model rat and BPH patients.

CONCLUSIONS

alpha(1d)-AR may play an important role in the regulation of cellular proliferation in the prostate, and alpha(1d)-AR blockage by naftopidil may not only improve lower urinary tract symptoms but also inhibit prostate growth in BPH patients.

DNA synthesis of rat bone marrow mesenchymal stem cells through alpha1-adrenergic receptors

Multipotential bone marrow mesenchymal stem cells (BMSCs) are important in maintaining the microenvironment of the bone marrow (BM). Sympathetic nerves histologically innervate the BM; however, their role remains unclear. In this study, the effects of norepinephrine on DNA synthesis and the related signaling molecules involved in rBMSCs were examined. mRNA levels of the alpha1-adrenergic receptor subtypes increased following norepinephrine stimulation (10(-5) M for 30 min). DNA synthesis increased in dose- and time-dependent manners as determined by [(3)H]thymidine incorporation. Intracellular Ca(2+) concentration and translocation of protein kinase C from the cytosol to the membrane were also found to be elevated in rBMSCs. Phentolamine was able to suppress translocation of PKC. Norepinephrine also induced phosphorylation of ERK1/2, which was prevented by staurosporine treatment. Pretreatment with PD98059 inhibited ERK1/2 phosphorylation and DNA synthesis in rBMSCs. These findings indicate that norepinephrine stimulates DNA synthesis via alpha1-adrenergic receptors and downstream Ca(2+)/PKC and ERK1/2 activation in rBMSCs.

Are angiotensin II receptor antagonists useful strategies in steatotic and nonsteatotic livers in conditions of partial hepatectomy under ischemia-reperfusion?

We examined whether angiotensin (Ang) II receptor antagonists could be considered a therapeutic strategy in steatotic and nonsteatotic livers in conditions of partial hepatectomy under ischemia-reperfusion (I/R), which is commonly applied in clinical practice to reduce blood loss. We report that Ang II type I receptor (AT1R) antagonist, but not Ang II type II receptor (AT2R) antagonist, increased regeneration in nonsteatotic livers. In the presence of steatosis, both AT1R and AT2R antagonists increased liver regeneration. This effect was stronger when the two were combined. Neither of the Ang II receptor antagonists protected nonsteatotic livers against damage. Only the AT1R antagonist, through nitric oxide inhibition, reduced damage in steatotic livers. The combination of the AT1R and AT2R antagonists in steatotic livers conferred a similar degree of protection to AT1R antagonist alone. Herein, we show that p38 mitogen-activated protein kinase (p38) was a key mechanism in the regeneration induced by the Ang II receptor antagonists in both liver types because when this signaling pathway was inhibited, the beneficial effects of the Ang II receptor antagonists on liver regeneration disappeared, regardless of hepatocyte growth factor or transforming growth factor beta-hepatic levels. In conclusion, in conditions of partial hepatectomy under I/R, the AT1R antagonist for nonsteatotic livers and the AT1R and AT2R antagonists for steatotic livers improved regeneration in the remnant liver through p38 activation. In addition, the combination of the AT1R and AT2R antagonists in steatotic livers led to stronger liver regeneration than either antagonists used separately and also provided the same protection against damage as that afforded by AT1R antagonist alone.

Epinephrine increases DNA synthesis via ERK1/2s through cAMP, Ca(2+)/PKC, and PI3K/Akt signaling pathways in mouse embryonic stem cells

Epinephrine is a catecholamine that plays important roles in regulating a wide variety of physiological systems by acting through the adrenergic receptors (ARs). The cellular responses to AR stimulation are mediated through various signaling pathways. Therefore, this study examined the effects of epinephrine on DNA synthesis and related signaling molecules in mouse embryonic stem cells (ESCs). Epinephrine increased DNA synthesis in a dose- and time-dependent manner, as determined by the level of [(3)H]-thymidine incorporation. AR subtypes (alpha1(A), alpha2(A), beta1, beta2, and beta3) were expressed in mouse ESCs and their expression levels were increased by epinephrine. In this experiment, epinephrine increased cAMP levels, intracellular Ca(2+) concentration ([Ca(2+)](i)), and translocation of protein kinase C (PKC) from the cytosol to the membrane compartment. In addition, we observed Akt phosphorylation in response to epinephrine; this was stimulated by phosphorylation of the epidermal growth factor receptor (EGFR). Epinephrine also induced phosphorylation of ERK1/2 (p44/42 MAPKs), while inhibition of PKC or Akt blocked this phosphorylation. Epinephrine increased the mRNA levels of proto-oncogenes (c-fos, c-jun, c-myc), while inhibition of ERK1/2 decreased these mRNA levels. In experiments aimed at examining the involvement of cell cycle regulatory proteins, epinephrine increased the levels of cyclin E/cyclin-dependent kinase 2 (CDK2) and cyclin D1/cyclin-dependent kinase 4 (CDK4). In conclusion, epinephrine stimulates DNA synthesis via ERK1/2 through cAMP, Ca(2+)/PKC, and PI3K/Akt signaling pathways in mouse ESCs.

Alpha-1 adrenergic receptor transactivates signal transducer and activator of transcription-3 (Stat3) through activation of Src and epidermal growth factor receptor (EGFR) in hepatocytes

Hepatocytes express adrenergic receptors (ARs) that modulate several functions, including liver regeneration, hepatocyte proliferation, glycogenolysis, gluconeogenesis, synthesis of urea and fatty acid metabolism. Adrenergic hepatic function in adults is mainly under the control of alpha(1)-ARs; however, the mechanism through which they influence diverse processes remains incompletely understood. This study describes a novel alpha(1)-AR-mediated transactivation of signal transducer and activator of transcription-3 (Stat3) in primary and transformed hepatocytes. Treatment of primary rat hepatocytes with the alpha(1)-AR agonist, phenylephrine (PE), induced a rapid phosphorylation of Stat3. PE also increased Stat3 phosphorylation, DNA binding and transcription activity in transformed human hepatocellular carcinoma cells (Hep3B). The PE-induced Stat3 phosphorylation, DNA binding and reporter activity were completely blocked by the selective alpha(1)-AR antagonist, prazosin. In addition, transfection of Hep3B cells with human alpha(1B)-AR expression vector also enhanced Stat3 phosphorylation and reporter activity. Moreover, overexpression of RGS2, a protein inhibitor of G(q/11) signaling, blocked PE-induced Stat3 phosphorylation and reporter activity. The observations that PE induced the formation of c-Src-Stat3 binding complex and phosphorylation of epidermal growth factor receptor (EGFR) and that inhibiting Src and EGFR prevented PE-induced Stat3 activation indicate the involvement of Src and EGFR. Taken together, these observations demonstrate a novel alpha(1)-AR-mediated Stat3 activation that involves G(q/11), Src, and EGFR in hepatic cells.

Platelets promote liver regeneration in early period after hepatectomy in mice

BACKGROUND

Platelets contain several growth factors, including platelet-derived growth factor and hepatocyte growth factor.

MATERIALS AND METHODS

We examined the effects of platelet increment on liver regeneration after 70% hepatectomy. Hepatectomies were carried out in male BALB/c mice, and subsequently divided into three groups: (i) untreated mice, (ii) thrombocytotic mice induced with thrombopoietin, and (iii) thrombocytopenic mice induced with anti-platelet antibody. Growth kinetics in the liver were analyzed as a function of the liver/body weight ratio, the mitotic index, the proliferating cell nuclear antigen labeling index and Ki-67 labeling index. Activation of signal transduction pathways relating to cell proliferation were examined, including the STAT3, Akt, and ERK1/2 pathways. Platelet accumulation in the residual liver was quantified by immunohistochemistry and transmission electron microscopy.

RESULTS

In thrombocytotic and thrombocytopenic mice, liver/body weight ratios and Ki-67 labeling indices were significantly increased and significantly decreased, respectively, compared with untreated mice 48 hours post-hepatectomy. The Akt pathway was strongly activated, and platelet accumulation was significantly increased in thrombocytotic group 5 minutes post-hepatectomy compared with normal and thrombocytopenic groups. After hepatectomy platelets accumulated in the sinusoids of liver and promoted hepatocyte proliferation in early period after hepatectomy.

CONCLUSION

By increasing or decreasing the platelet, marked changes in liver regeneration can occur, due to differences in cellular signaling and mitosis.

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.

Frequent reduced expression of alpha-1B-adrenergic receptor caused by aberrant promoter methylation in gastric cancers

Recent studies have suggested that epigenetic inactivation of tumour-related genes by promoter methylation participates in the development of gastric cancer. We newly identified the frequently aberrant promoter methylation of alpha-1B-adrenergic receptor (ADRA1B) in colorectal cancer by methylation-sensitive representational difference analysis (MS-RDA) and examined the methylation status of the ADRA1B promoter in 34 paired samples of colorectal cancer and surrounding epithelial tissue, and 34 paired samples of gastric cancer and surrounding epithelial tissue. In colorectal cancers, only four of 34 (11.8%) tumours showed ADRA1B promoter methylation. In contrast, ADRA1B promoter methylation was detected in 24 of 34 (70.6%) gastric cancers and in 14 of 34 (41.2%) surrounding epithelial tissues. The frequency of ADRA1B promoter methylation was higher in gastric epithelial tissues with intestinal metaplasia (41.6%) than in those without intestinal metaplasia (25.0%). Reverse transcription–PCR detected reduced ADRA1B expression in 12 of 18 (66.7%) gastric cancers, and its promoter methylation was detected in 11 of these 12 (91.7%) gastric cancers with reduced ADRA1B expression. Thus, ADRA1B promoter is frequently methylated in gastric cancer. Our results suggest that the ADRA1B gene is an important tumour-related gene frequently involved in the development and progression of gastric cancer.

Role of ischaemic preconditioning in liver regeneration following major liver resection and transplantation

Liver ischaemic preconditioning (IPC) is known to protect the liver from the detrimental effects of ischaemic-reperfusion injury (IRI), which contributes significantly to the morbidity and mortality following major liver surgery. Recent studies have focused on the role of IPC in liver regeneration, the precise mechanism of which are not completely understood. This review discusses the current understanding of the mechanism of liver regeneration and the role of IPC in this setting. Relevant articles were reviewed from the published literature using the Medline database. The search was performed using the keywords “liver”, “ischaemic reperfusion”, “ischaemic preconditioning”, “regeneration”, “hepatectomy” and “transplantation”. The underlying mechanism of liver regeneration is a complex process involving the interaction of cytokines, growth factors and the metabolic demand of the liver. IPC, through various mediators, promotes liver regeneration by up-regulating growth-promoting factors and suppresses growth-inhibiting factors as well as damaging stresses. The increased understanding of the cellular mechanisms involved in IPC will enable the development of alternative treatment modalities aimed at promoting liver regeneration following major liver resection and transplantation.

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.

Low p38 MAPK and JNK activation in cultured hepatocytes of DRH rats; a strain highly resistant to hepatocarcinogenesis

DRH rats are a hepatocarcinogenesis-resistant strain isolated from hepatocarcinogenesis-sensitive Donryu rats, and the liver of DRH shows less histological damage and fewer/smaller neoplastic hepatic lesions by the treatment with hepatocarcinogens. To investigate the mechanism of the resistance, the properties of hepatocytes of DRH and Donryu were compared. In primary culture, DRH hepatocytes exhibited higher proliferation and less apoptosis than Donryu hepatocytes in the presence of EGF and insulin. However, such difference was not correlated to the degree of DNA damage associated with cell culture or cell cycle checkpoint function. Although the mitogen-activated protein kinases [EGF receptor (EGFR) and extracellular signal regulating kinases (ERK1/2)] were activated to the same degree, the stress-activated protein kinases [p38 mitogen-activated protein kinase (p38) and c-jun N-terminal kinase (JNK)] were activated to a lesser degree in the DRH hepatocytes. Treatment with 2-acetylaminofluorene (2-AAF) in vivo also resulted in less JNK and p38 activation in the DRH livers. Furthermore, apoptosis signal-regulating kinase 1 (ASK1) was inhibited by the lysate from the DRH but not by the Donryu hepatocytes. The low activation of the stress-activated protein kinases may be linked to the resistance to cellular stress, which may underlie the hepatocarcinogenesis-resistance in DRH rats.

p38 mitogen-activated protein kinase plays an inhibitory role in hepatic lipogenesis

Hepatic lipogenesis is the principal route to convert excess carbohydrates into fatty acids and is mainly regulated by two opposing hormones, insulin and glucagon. Although insulin stimulates hepatic lipogenesis, glucagon inhibits it. However, the mechanism by which glucagon suppresses lipogenesis remains poorly understood. In this study, we have observed that p38 mitogen-activated protein kinase plays an inhibitory role in hepatic lipogenesis. Levels of plasma triglyceride and triglyceride accumulation in the liver were both elevated when p38 activation was blocked. Expression levels of central lipogenic genes, including sterol regulatory element-binding protein-1 (SREBP-1), fatty acid synthase, hydroxy-3-methylglutaryl coenzyme A reductase, farnesyl pyrophosphate synthase, and cytochrome P-450-51, were decreased in liver by fasting and in primary hepatocytes by glucagon but increased by the inhibition of p38. In addition, we have shown that p38 can inhibit insulin-induced expression of key lipogenic genes in isolated hepatocytes. Our results in hepatoma cells demonstrate that p38 plays an inhibitory role in the activation of the SREBP-1c promoter. Finally, we have shown that transcription of the PGC-1beta gene, a key coactivator of SREBP-1c, was reduced in liver by fasting and in isolated hepatocytes by glucagon. This reduction was significantly reversed by the blockade of p38. Insulin-induced expression of the PGC-1beta gene was enhanced by the inhibition of p38 but suppressed by the activation of p38. Together, we have identified an inhibitory role for p38 in the transcription of central lipogenic genes, SREBPs, and PGC-1beta and hepatic lipogenesis.

Signalling pathways evoked by alpha1-adrenoceptors in human melanoma cells

The biological effects of catecholamines in mammalian pigment cells are poorly understood, but in poikilothermic vertebrates they regulate the translocation of pigment granules. We have previously demonstrated in SK-Mel 23-human melanoma cells the presence of low affinity alpha(1)-adrenoceptors, which mediate a decrease in cell proliferation and increase in tyrosinase activity, with no change of tyrosinase expression. In this report, we investigated the signalling pathways involved in these responses. Calcium mobilization in response to phenylephrine (PHE), an alpha(1)-adrenergic agonist, was investigated by confocal microscopy, and no change of fluorescence during the treatment was observed, suggesting that calcium is not involved in the signalling pathway activated by alpha(1)-adrenoceptors in SK-Mel 23 cells. cAMP levels, determined by enzyme-immunoassay, were significantly increased by PHE (10(-5)-10(-4)M), that could be blocked by the alpha(1)-adrenergic antagonist benoxathian (10(-5)-10(-4)M). Several biological assays were then performed with PHE, for 72 h, in the absence or presence of various signalling pathway inhibitors, in an attempt to determine the intracellular messengers involved in the responses of proliferation and tyrosinase activity. Our results suggest the participation of p38 and ERKs in PHE-induced decrease of proliferation, and possibly also of cAMP and protein kinase A. Regarding PHE-induced increase of tyrosinase activity, it is suggested that the following signalling components are involved: cAMP/PKA, PKC, PI3K, p38 and ERKs.

The phosphoinositide 3-kinase/Akt-signal pathway mediates proliferation and secretory function of hepatic sinusoidal endothelial cells in rats after partial hepatectomy

OBJECTIVE

To investigate the role of AKT signaling pathway in hepatic sinusoidal endothelial cells (SECs) early after partial hepatectomy in rats and the regulatory mechanisms involved.

METHODS

The animal model of 70% hepatectomy was made. Hepatic SECs were isolated and cultured according to Braet et al.'s method with some modifications. The cultured hepatic SECs were divided into two groups: 70% partial hepatectomy groups and LY294002 group (LY). We observed the expressions of AKT and NF-kappaB in cultured hepatic SECs by Western blot, measured the levels of NO, NOs, IL-6, and HGF in the supernatants of hepatic SEC cultures and [3H]thymidine incorporation, and analyzed cell cycle of cultured hepatic SECs by flow cytometer. The relationship of the Akt pathway with secretions and proliferation of hepatic SECs after partial hepatectomy was probed.

RESULTS

The levels of Akt protein expression increased significantly after partial hepatectomy in OG group and with a peak at 24 h post operation. Meanwhile, there was a markedly increase in phosphorylated Akt protein during 2-72 h after operation. But the expression and activity of Akt protein did not change significantly after partial hepatectomy in the LY group. So, partial hepatectomy can marked induce Akt expression and result in rapid and marked phosphorylation of Akt from 2 to 72 h thereafter. The changes of NF-kappaB expression in cultured hepatic SECs were similar to those of Akt expression after operation. The concentrations of HGF and IL-6 in the supernatants of cultured hepatic SECs were relatively low in the LY group, and were markedly increased after partial hepatectomy, with a peak at 24 h in the OG group. There were significant differences between the OG and LY groups at 6 and 24 h (P < 0.05). Both NO and NOS secretion was increased in the OG group compared to the LY group within 24 h after partial hepatectomy. But the secretion of NO and NOS was increased more markedly in the LY group than that in the OG beyond 24 h. These findings suggest that the secretion of the cytokines by hepatic SECs is mediated by Akt signaling. Akt signaling pathway in relationship with proliferation of hepatic SECs and suppression of apoptosis. In OG group, the hepatic SECs in S and G2/M obviously increased. The proliferative index of hepatic SECs in OG group had significant differences with that in LY group at 6, 24, and 72 h, P < 0.05. Meanwhile, the cells of apoptosis in OG group were very low, and the cells in LY group gradually increased.

CONCLUSIONS

These results suggest that AKT signaling pathway plays a crucial role in mediating proliferating and secreted signals in hepatic SECs. AKT has been suggested to play a pivotal role in early liver regeneration involved in the induction of secreted cytokines and proliferation of hepatic SECs.

Ethanol reduces p38 kinase activation and cyclin D1 protein expression after partial hepatectomy in rats

BACKGROUND/AIMS

Chronic ethanol consumption inhibits liver regeneration. We examined the effects of chronic ethanol consumption on two mitogen-activated protein kinases in relation to induction of cell cycle proteins after partial hepatectomy (PH).

METHODS

Male Wistar rats were ethanol-fed (EF) or pair-fed (PF) for 16 weeks before PH. Hepatic activation of extracellular signal regulated kinase (ERK)1/2, p38 kinase and expression of cyclinD1, cyclin-dependent kinase-4 (cdk4) and proliferating cell nuclear antigen (PCNA) were studied.

RESULTS

In PF rats, PH-induced p38 activation was evident at 2h and was maximal at 12h. There was a close temporal relationship between p38 activation, cyclin D1 and PCNA expression. Alcohol exposure reduced p38 activation, cyclin D1 and PCNA, each by approximately 50%. ERK1/2 activation occurred during the first 2h post-PH in both EF and PF rats, and there was no later increase in PF rats. In vivo inhibition of p38 suppressed PCNA expression whereas the effect of ERK1/2 inhibition was inconsistent.

CONCLUSIONS

p38 kinase activation is linked temporally with cyclin D1 expression after PH and appears to exert cell cycle control in the adult liver. p38 signaling also appears to be a target for the inhibitory effect of chronic alcohol on liver regeneration.

Conjugated bile acids promote ERK1/2 and AKT activation via a pertussis toxin-sensitive mechanism in murine and human hepatocytes

Several studies have argued that G-protein-coupled receptors (GPCR) have the capacity to promote activation of receptor tyrosine kinases. The current studies were performed to examine the regulation of the extracellular regulated kinase (ERK)1/2 and AKT pathways by conjugated and unconjugated bile acids in primary hepatocytes. Deoxycholic acid (DCA), chenodeoxycholic acid (CDCA), taurodeoxycholic acid (TDCA), glycodeoxycholic acid (GDCA), taurochenodeoxycholic acid (TCDCA), glycochenodeoxycholic acid (GCDCA), taurocholic acid (TCA), glycocholic acid (GCA), and tauroursodeoxycholic acid (TUDCA) all activated ERK1/2 in primary rat hepatocytes that was abolished by inhibition of ERBB1, and significantly reduced by ROS quenching agents. Bile acid-induced AKT activation was blunted by preventing ERBB1 activation and ROS generation. Treatment of rat hepatocytes with pertussis toxin (PTX) did not alter ERK1/2 and AKT activation induced by DCA or CDCA but abolished pathway activations by conjugated bile acids. Similar data to those with PTX were obtained when a dominant negative form of G(i1alpha) was overexpressed. Treatment of rat hepatocytes with TDCA and TCA promoted guanosine triphosphate (GTP) loading of G(i1alpha), G(i2alpha), and G(i3alpha) in vitro. Treatment of rat hepatocytes with PTX abolished TDCA-induced tyrosine phosphorylation of ERBB1. Similar findings to those in rat hepatocytes were also obtained in primary mouse and human hepatocytes, but not in established rodent or human hepatoma cell lines. In conclusion, collectively our findings demonstrate that unconjugated bile acids activate hepatocyte receptor tyrosine kinases and intracellular signaling pathways in a ROS-dependent manner. In contrast, conjugated bile acids primarily activate receptor tyrosine kinases and intracellular signaling pathways in a GPCR (G(ialpha))-dependent and ROS-dependent manner.

p38 Mitogen-activated protein kinase plays a stimulatory role in hepatic gluconeogenesis

Hepatic gluconeogenesis is essential for maintaining blood glucose levels during fasting and is the major contributor to postprandial and fasting hyperglycemia in diabetes. Gluconeogenesis is a classic cAMP/protein kinase A-dependent process initiated by glucagon, which is elevated in the blood during fasting and in diabetes. In this study, we have shown that p38 mitogen-activated protein kinase (p38) was activated in liver by fasting and in primary hepatocytes by glucagon or forskolin. Fasting plasma glucose levels were reduced upon blockade of p38 with either a chemical inhibitor or small interference RNA in mice. In examining the mechanism, inhibition of p38 suppressed gluconeogenesis in liver, along with expression of key gluconeogenic genes, including phosphoenolpyruvate carboxykinase and glucose-6-phosphatase. Peroxisome proliferator-activated receptor gamma coactivator 1alpha and cAMP-response element-binding protein have been shown to be important mediators of hepatic gluconeogenesis. We have shown that inhibition of p38 prevented transcription of the PPARgamma coactivator 1alpha gene as well as phosphorylation of cAMP-response element-binding protein. Together, our results from in vitro and in vivo studies define a model in which cAMP-dependent activation of genes involved in gluconeogenesis is dependent upon the p38 pathway, thus adding a new player to our evolving understanding of this physiology.

p38 MAP-kinase regulates function of gap and tight junctions during regeneration of rat hepatocytes

BACKGROUND/AIMS

Hepatocyte regeneration is considered to be associated with adaptive changes in expression of gap and tight junctions through multiple signal transduction pathways including p38 MAP-kinase. The role of the stress responsitive MAP-kinase, p38 MAP-kinase, signaling pathway in function of gap and tight junctions was examined during regeneration of rat hepatocytes in vivo and in vitro.

METHODS

We examined changes in formation, expression and function of gap and tight junctions in rat livers after 70% partial hepatectomy and in primary cultures of rat hepatocytes, by using a p38 MAP-kinase inhibitor, SB203580.

RESULTS

When p38 MAP-kinase was activated during partial hepatectomy, down-regulation of Cx32 and up-regulation of claudin-1 were observed. By SB203580 treatment, the down-regulation of Cx32 was inhibited and the up-regulation of claudin-1 was enhanced, well maintaining the structures of gap and tight junctions. SB203580 treatment did not affect the increase of hepatocyte proliferation. In EGF induced proliferative rat hepatocytes treated with SB203580, the expression and function of Cx32 and claudin-1 were increased.

CONCLUSIONS

Dynamic changes of formation of gap and tight junctions during regeneration of rat hepatocytes in vivo and in vitro are in part controlled via a p38 MAP-kinase signaling pathway, and are independent of cell growth.

Thyroid Status Affects Rat Liver Regeneration After Partial Hepatectomy by Regulating Cell Cycle and Apoptosis

In rats, various growth factors and hormones, as well as partial hepatectomy (PH) are able to trigger the proliferative response of hepatocytes. Although recent evidence highlights the important role of thyroid hormones and thyroid status in regulating the growth of liver cells in vitro and in vivo models, the mechanism involved in the pro-proliferative effects of thyroid hormones is still unclear. Here we have investigated how in rats made hypo- and hyperthyroid after prolonged treatment respectively with propylthiouracil (PTU) and triiodothyronine (T3), the thyroid status affects liver regeneration after PH by regulating cell cycle and apoptosis proteins. Our results show that both in control and partially hepatectomized animals hyperthyroidism increases the cyclin D1, E and A levels and the activity of cyclin-cdk complexes, and decreases the levels of cdk inhibitors such as p16 and p27. On the contrary hypothyroidism induces a down-regulation of the activity of cyclin cdk complexes decreasing cyclin levels. Thyroid hormones control also p53 and p73, two proteins involved in apoptosis and growth arrest which are induced by PH. In particular, hypothyroidism increases and T3 treatment decreases p73 levels. The analysis of the phosphorylated forms of p42/44 and p38 MAPK revealed that they are induced during hepatic regeneration in euthyroid and hyperthyroid rats whereas they are negatively regulated in hypothyroid rats. In conclusion our data demonstrate that thyroid status can affects liver regeneration, altering the expression and the activity of the proteins involved in the control of cell cycle and growth arrest.

Met provides essential signals for liver regeneration

Genetic analysis in mice has demonstrated a crucial role of the Met tyrosine kinase receptor and its ligand, hepatocyte growth factor/scatter factor (HGF/SF), in development of the liver, muscle, and placenta. Here, we use conditional mutagenesis in mice to analyze the function of Met during liver regeneration, using the Mx-cre transgene to introduce the mutation in the adult. After partial hepatectomy in mice carrying the Mx-cre-induced Met mutation, regeneration of the liver is impaired. Comparison of signal transduction pathways in control and mutant livers indicates that Met and other signaling receptors cooperate to fully activate particular signaling molecules, for instance, the protein kinase Akt. However, activation of the Erk1/2 kinase during liver regeneration depends exclusively on Met. Signaling crosstalk is thus an important aspect of the regulation of liver regeneration. Analysis of cell cycle progression of hepatocytes in conditional Met mutant mice indicates a defective exit from quiescence and diminished entry into S phase. Impaired liver regeneration is accompanied by compensatory physiological responses that include prolonged up-regulation of HGF/SF and IL-6 in peripheral blood. Our data demonstrate that the HGF/SF/Met signaling system is essential not only during liver development but also for the regeneration of the organ in the adult.

MAP kinase signaling in diverse effects of ethanol

Chronic ethanol abuse is associated with liver injury, neurotoxicity, hypertension, cardiomyopathy, modulation of immune responses and increased risk for cancer, whereas moderate alcohol consumption exerts protective effect on coronary heart disease. However, the signal transduction mechanisms underlying these processes are not well understood. Emerging evidences highlight a central role for mitogen activated protein kinase (MAPK) family in several of these effects of ethanol. MAPK signaling cascade plays an essential role in the initiation of cellular processes such as proliferation, differentiation, development, apoptosis, stress and inflammatory responses. Modulation of MAPK signaling pathway by ethanol is distinctive, depending on the cell type; acute or chronic; normal or transformed cell phenotype and on the type of agonist stimulating the MAPK. Acute exposure to ethanol results in modest activation of p42/44 MAPK in hepatocytes, astrocytes, and vascular smooth muscle cells. Acute ethanol exposure also results in potentiation or prolonged activation of p42/44MAPK in an agonist selective manner. Acute ethanol treatment also inhibits serum stimulated p42/44 MAPK activation and DNA synthesis in vascular smooth muscle cells. Chronic ethanol treatment causes decreased activation of p42/44 MAPK and inhibition of growth factor stimulated p42/44 MAPK activation and these effects of ethanol are correlated to suppression of DNA synthesis, impaired synaptic plasticity and neurotoxicity. In contrast, chronic ethanol treatment causes potentiation of endotoxin stimulated p42/44 MAPK and p38 MAPK signaling in Kupffer cells leading to increased synthesis of tumor necrosis factor. Acute exposure to ethanol activates pro-apoptotic JNK pathway and anti-apoptotic p42/44 MAPK pathway. Apoptosis caused by chronic ethanol treatment may be due to ethanol potentiation of TNF induced activation of p38 MAPK. Ethanol induced activation of MAPK signaling is also involved in collagen expression in stellate cells. Ethanol did not potentiate serum stimulated or Gi-protein dependent activation of p42/44 MAPK in normal hepatocytes but did so in embryonic liver cells and transformed hepatocytes leading to enhanced DNA synthesis. Ethanol has a 'triangular effect' on MAPK that involve direct effects of ethanol, its metabolically derived mediators and oxidative stress. Acetaldehyde, phosphatidylethanol, fatty acid ethyl ester and oxidative stress, mediate some of the effects seen after ethanol alone whereas ethanol modulation of agonist stimulated MAPK signaling appears to be mediated by phosphatidylethanol. Nuclear MAPKs are also affected by ethanol. Ethanol modulation of nuclear p42/44 MAPK occurs by both nuclear translocation of p42/44 MAPK and its activation in the nucleus. Of interest is the observation that ethanol caused selective acetylation of Lys 9 of histone 3 in the hepatocyte nucleus. It is plausible that ethanol modulation of cross talk between phosphorylation and acetylations of histone may regulate chromatin remodeling. Taken together, these recent developments place MAPK in a pivotal position in relation to cellular actions of ethanol. Furthermore, they offer promising insights into the specificity of ethanol effects and pharmacological modulation of MAPK signaling. Such molecular signaling approaches have the potential to provide mechanism-based therapy for the management of deleterious effects of ethanol or for exploiting its beneficial effects.

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Call volume and insulin signaling

Perturbations of cell hydration as provoked by changes in ambient osmolarity or under isoosmotic conditions by hormones, second messengers, intracellular substrate accumulation, or reactive oxygen intermediates critically contribute to the physiological regulation of cell function. In general an increase in cell hydration stimulates anabolic metabolism and proliferation and provides cytoprotection, whereas cellular dehydration leads to a catabolic situation and sensitizes cells to apoptotic stimuli. Insulin produces cell swelling by inducing a net K+ and Na+ accumulation inside the cell, which results from a concerted activation of Na+/H+ exchange, Na+/K+/2Cl- symport, and the Na+/K(+)-ATPase. In the liver, insulin-induced cell swelling is critical for stimulation of glycogen and protein synthesis as well as inhibition of autophagic proteolysis. These insulin effects can largely be mimicked by hypoosmotic cell swelling, pointing to a role of cell swelling as a trigger of signal transduction. This article discusses insulin-induced signal transduction upstream of swelling and introduces the hypothesis that cell swelling as a signal amplifyer represents an essential component in insulin signaling, which contributes to the full response to insulin at the level of signal transduction and function. Cellular dehydration impairs insulin signaling and may be a major cause of insulin resistance, which develops in systemic hyperosmolarity, nutrient deprivation, uremia, oxidative challenges, and unbalanced production of insulin-counteracting hormones. Hydration changes affect cell functions at multiple levels (such as transcriptom, proteom, phosphoproteom, and the metabolom) and a system biological approach may allow us to develop a more holistic view on the hydration dependence of insulin signaling in the future.

Role of the p38 MAP-kinase signaling pathway for Cx32 and claudin-1 in the rat liver

Liver regeneration and cholestasis are associated with adaptive changes in expression of gap and tight junctions through signal transduction. The roles of stress responsitive MAP-kinase, p38 MAP-kinase, in the signaling pathway for gap junction protein, Cx32, and tight junction protein, claudin-1, were examined in rat liver in vivo and in vitro, including regeneration following partial hepatectomy and cholestasis after common bile duct ligation. Changes in the expression and function of Cx32 and claudin-1 in hepatocytes in vivo were studied using the p38 MAP-kinase inhibitor SB203580. Following partial hepatectomy and common bile duct ligation, down-regulation of Cx32 protein was inhibited by SB203580 treatment. Up-regulation of claudin-1 protein was enhanced by SB203580 treatment after partial hepatectomy but not common bile duct ligation. However, no change of the Ki-67 labeling index (which is a marker for cell proliferation) in the livers treated with SB203580, was observed compared to that without SB203580 treatment. In primary cultures of rat hepatocytes, however, treatment with a p38 MAP-kinase activator, anisomycin, decreased Cx32 and claudin-1 protein levels. p38 MAP-kinase may be an important signaling pathway for regulation of gap and tight junctions in hepatocytes. Changes of gap and tight junctions during liver regeneration and cholestasis are shown to be in part controlled via the p38 MAP-kinase signaling pathway and are independent of cell growth.

Glucagon induces the plasma membrane insertion of functional aquaporin-8 water channels in isolated rat hepatocytes

Although glucagon is known to stimulate the cyclic adenosine monophosphate (cAMP)-mediated hepatocyte bile secretion, the precise mechanisms accounting for this choleretic effect are unknown. We recently reported that hepatocytes express the water channel aquaporin-8 (AQP8), which is located primarily in intracellular vesicles, and its relocalization to plasma membranes can be induced with dibutyryl cAMP. In this study, we tested the hypothesis that glucagon induces the trafficking of AQP8 to the hepatocyte plasma membrane and thus increases membrane water permeability. Immunoblotting analysis in subcellular fractions from isolated rat hepatocytes indicated that glucagon caused a significant, dose-dependent increase in the amount of AQP8 in plasma membranes (e.g., 102% with 1 micromol/L glucagon) and a simultaneous decrease in intracellular membranes (e.g., 38% with 1 micromol/L glucagon). Confocal immunofluorescence microscopy in cultured hepatocytes confirmed the glucagon-induced redistribution of AQP8 from intracellular vesicles to plasma membrane. Polarized hepatocyte couplets showed that this redistribution was specifically to the canalicular domain. Glucagon also significantly increased hepatocyte membrane water permeability by about 70%, which was inhibited by the water channel blocker dimethyl sulfoxide (DMSO). The inhibitors of protein kinase A, H-89, and PKI, as well as the microtubule blocker colchicine, prevented the glucagon effect on both AQP8 redistribution to hepatocyte surface and cell membrane water permeability. In conclusion, our data suggest that glucagon induces the protein kinase A and microtubule-dependent translocation of AQP8 water channels to the hepatocyte canalicular plasma membrane, which in turn leads to an increase in membrane water permeability. These findings provide evidence supporting the molecular mechanisms of glucagon-induced hepatocyte bile secretion.

Impaired hepatocyte DNA synthetic response posthepatectomy in insulin-like growth factor binding protein 1-deficient mice with defects in C/EBP beta and mitogen-activated protein kinase/extracellular signal-regulated kinase regulation

After a two-thirds hepatectomy, normally quiescent liver cells are stimulated to reenter the cell cycle and proliferate to restore the original liver mass. One of the most rapidly and highly induced genes and proteins in regenerating liver is insulin-like growth factor binding protein 1 (IGFBP-1), a secreted protein that may modulate the activities of insulin-like growth factors (IGFs) or signal via IGF-independent mechanisms. To assess the functional role of IGFBP-1 in liver regeneration, mice with a targeted disruption of the IGFBP-1 gene were generated. Although IGFBP-1(-/-) mice demonstrated normal development, they had abnormal liver regeneration after partial hepatectomy, characterized by liver necrosis and reduced and delayed hepatocyte DNA synthesis. The abnormal regenerative response was associated with blunted activation of mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) and a reduced induction of C/EBP beta protein expression posthepatectomy. Like cell cycle abnormalities observed in hepatectomized C/EBP beta(-/-) mice, cyclin A and cyclin B1 expression was delayed and reduced in IGFBP-1(-/-) livers, whereas cyclin D1 expression was normal. Treatment of IGFBP-1(-/-) mice with a preoperative dose of IGFBP-1 induced MAPK/ERK activation and C/EBP beta expression, suggesting that IGFBP-1 may support liver regeneration at least in part via its effect on MAPK/ERK and C/EBP beta activities. These findings are the first demonstration of the involvement of IGFBP-1 in the regulation of in vivo mitogenic signaling pathways.

alpha 1-Adrenergic receptor subtypes differentially control the cell cycle of transfected CHO cells through a cAMP-dependent mechanism involving p27Kip1

Three distinct subtypes of alpha(1)-adrenergic receptors (alpha(1)A-, alpha(1)B-, and alpha(1)D-AR) play a prominent role in cell growth. However, little is known about subtype-specific effects on cell proliferation. The activation of alpha(1)A- or alpha(1)B-AR inhibits serum-promoted cell proliferation, whereas alpha(1)D-AR activation does not show such an inhibitory effect. Notably, cell-cycle progression was blocked at G(1)/S transition after activation of alpha(1)A/alpha(1)B-AR but not of alpha(1)D-AR. In agreement with the differential cell proliferation effect, cAMP production was increased after activation of alpha(1)A/alpha(1)B-AR but not alpha(1)D-AR, whereas all alpha(1)-AR subtypes are associated with inositol 1,4,5-trisphosphate production and mitogen-activated protein kinase activation in a similar fashion. Furthermore, the serum-induced reduction in the levels of the cyclin-dependent kinase inhibitor, p27(Kip1), was blocked after activation of alpha(1)A/alpha(1)B-AR but not alpha(1)D-AR. These results show that alpha(1)-AR subtypes differentially activate the cAMP/p27(Kip1) pathway and thereby have differential inhibitory effects on cell proliferation. Subtype-dependent effects should be taken into consideration when assessing the physiological response of native cells where alpha(1)-AR subtypes are generally co-expressed.

PI3K-FRAP/mTOR pathway is critical for hepatocyte proliferation whereas MEK/ERK supports both proliferation and survival

Growth factors are known to favor both proliferation and survival of hepatocytes. In this work, we investigated the role of 2 main signaling pathways, phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK), in these processes. First, evidence was provided that the PI3K cascade as well as the MEK/ERK cascade is a key transduction pathway controlling hepatocyte proliferation, as ascertained by arrest of DNA synthesis in the presence of LY294002, a specific PI3K inhibitor. Inhibition of FRAP/mTOR by rapamycin also abrogated DNA replication and protein synthesis induced by growth factor. We showed that expression of cyclin D1 at messenger RNA (mRNA) and protein levels was regulated by this pathway. We highlighted that 4E-BP1 phosphorylation was not activated by epidermal growth factor (EGF) but was under an insulin-regulation mechanism through a PI3K-FRAP/mTOR activation that could account for the permissive role of insulin on hepatocyte proliferation. No interference between the MEK/ERK pathway and 4E-BP1 phosphorylation was detected, whereas p70S6K phosphorylation induced by EGF was under a U0126-sensitive regulation. Last, we established that the antiapoptotic function of EGF was dependent on MEK, whereas LY294002 and rapamycin had no direct effect on cell survival. Taken together, these data highlight the regulation and the role of 2 pathways that mediate growth-related response by acting onto distinct steps. In conclusion, hepatocyte progression in late G1 phase induced by EGF generates survival signals depending on MEK activation, whereas PI3K and MEK/ERK cascades are both necessary for hepatocyte replication.

Identification of alpha1-adrenergic receptors and their involvement in phosphoinositide hydrolysis in the frog heart

The aim of this study was to characterize alpha(1)-adrenergic receptors in frog heart and to examine their related signal transduction pathway. alpha(1)-Adrenergic binding sites were studied in purified heart membranes using the specific alpha(1)-adrenergic antagonist [(3)H]prazosin. Analysis of the binding data indicated one class of binding sites displaying a K(d) of 4.19 +/- 0.56 nM and a B(max) of 14.66 +/- 1.61 fmol/mg original wet weight. Adrenaline, noradrenaline, or phenylephrine, in the presence of propranolol, competed with [(3)H]prazosin binding with a similar potency and a K(i) value of about 10 microM. The kinetics of adrenaline binding was closely related to its biological effect. Adrenaline concentration dependently increased the production of inositol phosphates in the heart in the presence or absence of propranolol. Maximal stimulation was about 8.5-fold, and the half-maximum effective concentration was 30 and 21 microM in the absence and presence of propranolol, respectively. These data clearly show that alpha(1)-adrenergic receptors are coupled to the phosphoinositide hydrolysis in frog heart. To our knowledge, this is the first direct evidence supporting the presence of functional alpha(1)-adrenergic receptors in the frog heart.

Pro-inflammatory cytokines tumor necrosis factor alpha and interleukin-6 and survival factor epidermal growth factor positively regulate the murine GSTA4 enzyme in hepatocytes

We hypothesized that glutathione transferases could be induced and may participate to cellular defenses against the oxidative stress occurring during liver regeneration. Here, we evidenced that murine GSTA1 (mGSTA1), A4, Pi, and Mu are up-regulated during mouse liver regeneration, exhibiting a biphasic pattern of induction correlating early G(1) phase and G(1)/S transition of the cell cycle. Using confocal microscopy immunolocalization and subcellular fractionation, mGSTA4 was demonstrated in both mitochondria and cytosol and found preferentially increased in cytosol during liver regeneration. In addition, mGSTA4 was induced in vivo and in cultured hepatocytes by tumor necrosis factor alpha (TNFalpha), interleukin-6 (IL-6), and epidermal growth factor (EGF), factors that play crucial roles in hepatocyte survival and proliferation during liver regeneration. However, the mitogenic effect of EGF was not responsible for the induction of mGSTA4. In transient transfections, IL-6 and EGF, but not TNFalpha, transactivated the human GSTA4 (hGSTA4) promoter cloned upstream of the luciferase reporter gene suggesting that IL-6 and EGF up-regulated hGSTA4 at a transcriptional level, whereas TNFalpha could rather act at a post-transcriptional level. The inhibition of phosphoinositide 3-kinase, p38 MAPK, and MEK/ERK signaling pathways, using specific inhibitors, prevented EGF-dependent induction of mGSTA4 and transactivation of hGSTA4 promoter. Altogether, these data favor the conclusion that, in regenerating hepatocytes, several GST isoforms are induced and that cytokines TNFalpha and IL-6 and survival factor EGF positively regulate mGSTA4 via survival signaling pathways.

Recent progress in alpha 1-adrenoceptor pharmacology

The adrenoceptors (ARs) play a key role in the modulation of sympathetic nervous system activity and are a site of action for many clinically important therapeutic agents. The alpha1-adrenoceptor subtypes (alpha1A-, alpha1B-, and alpha1D-AR) play a prominent role in regulating vascular tone and hypertrophic growth of smooth muscle and cardiac cells. Their functional characteristics with respect to ligand binding and second messenger utilization have been well described. Here, we review recent progress on subtype-specific subcellular localization, participation in signaling cascades, and the pivotal function of alpha1-ARs, as delineated through studies on genetically engineered animals. Together, these findings will provide new insights into the physiological and pathophysiological roles of the alpha1-ARs.

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.

Retinoic acid inhibits hepatic Jun N-terminal kinase-dependent signaling pathway in ethanol-fed rats

Retinoic acid (RA) supplementation suppresses ethanol-enhanced hepatocyte hyperproliferation in rats; however, little is known about the mechanism(s). Here, we investigated whether RA affects the protein kinase signaling pathways in the liver tissues of rats fed with a high dose of ethanol for a prolonged period of time (6 months). Results show that there were greater levels of phosphorylated Jun N-terminal kinase (JNK) and phosphorylated c-Jun protein, but not total JNK protein, in livers of ethanol-fed rats vs those of controls. Moreover, ethanol feeding to rats increased the levels of phosphorylated mitogen-activated protein kinase kinase-4 (MKK-4) and decreased the levels of mitogen-activated kinase phosphatase-1 (MKP-1) in liver tissue. However, hepatic levels of phosphorylated-p38 protein and total-p38 protein were not altered by the ethanol treatment. In contrast, all-trans-RA supplementation at two doses in ethanol-fed rats greatly attenuated the ethanol-induced hepatic phosphorylation of MKK-4, phosphorylated-JNK and c-Jun proteins. The level of MKP-1 was increased in ethanol-fed rats supplemented with all-trans-RA. Further, ethanol-induced hepatocyte hyperproliferation, measured by immunostaining for proliferating cell nuclear antigen, were markedly decreased by all-trans-RA supplementation. Interestingly, hepatic apoptosis in the liver of ethanol-fed rats after 6 months of treatment decreased significantly. This decrease of hepatic apoptosis in ethanol-fed rats was prevented by all-trans-RA supplementation in a dose-dependent manner. The results from these studies indicate that restoration of RA homeostasis is critical for the regulation of JNK-dependent signaling pathway and apoptosis in the liver of ethanol-fed rats.

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.

Growth-related signaling regulates activation of telomerase in regenerating hepatocytes

Although there have been many reports on the relationship between activation of telomerase and carcinogenesis, the role of telomerase in normal cellular growth is still unclear. In this study, we analyzed the relationship between upregulation of telomerase activity and cell cycle progression during the liver regeneration process by using an in vivo mouse two-thirds partial hepatectomy (PH) model as well as by using in vitro hepatocyte culture systems. Furthermore, we also investigated the effects of growth factors on telomerase activity during liver regeneration and the influence of MAPK pathway inhibitors (MEK inhibitors PD98059 and U0126; p38 MAPK inhibitor SB203580) on the telomerase activity of regenerating hepatocytes in vitro. An upregulation of the telomerase activity was found at 24 h after PH, and thereafter an increase in the S-phase fraction was observed at 36-48 h. There was no remarkable change in the telomere length after PH. Preoperative treatment with EGF and HGF increased the in vivo telomerase activity. In a hepatocyte primary culture, the upregulation of the telomerase activity required the presence of EGF, and this upregulation was accelerated by the addition of HGF. A remarkable activation of p44/42 MAPK was seen but no such activation of p38 MAPK was observed at 48 h after PH. Although SB203580 had no effect on the telomerase activity of regenerating hepatocytes, treatment with MEK inhibitors (PD 98059, U0126) significantly repressed the telomerase activity. In conclusion, the telomerase activity is upregulated before hepatocytes enter the S phase, and both EGF and HGF play important roles in this step. In addition, the activation of the p44/42 MAPK pathway seems to play an essential role in telomerase upregulation during the liver regeneration process.

Hepatitis C virus core protein induces cell proliferation and activates ERK, JNK, and p38 MAP kinases together with the MAP kinase phosphatase MKP-1 in a HepG2 Tet-Off cell line

Hepatitis C virus (HCV) core protein is a multifunctional protein interacting with cellular and viral proteins and promoters. A tetracycline-regulated system was used to generate a HepG2 Tet-Off cell line allowing regulated expression of a full-length (191 aa) and an N(c)-truncated core protein (160 aa). In this system HCV core protein expression activates extracellular signal-regulated kinase (ERK), c-jun N-terminal kinase (JNK), and p38 mitogen-activated protein (MAP) kinase, induces MAP kinase phosphatase MKP-1 expression, and increases cell proliferation. This was accompanied by an activation of c-Jun and ATF-2, but not Elk-1 and c-Fos. Furthermore, AP-1 activation was independent of c-Fos. Full-length and N(c)-truncated HCV core proteins exerted similar effects.

Ras adenoviruses modulate cyclin E protein expression and DNA synthesis after partial hepatectomy

Ras-genes encode for proteins important for transmitting extracellular signals from the cytoplasm to the nucleus. In this study we investigated the impact of Ras on cell cycle progression after hepatectomy by using adenoviral vectors (adv) expressing beta-galactosidase (beta-gal), a dominant-negative (Ras N17) or a dominant-active (Ras 61L) form of H-Ras. Partial hepatectomy was performed in mice treated with the different adenoviruses and cell cycle progression was studied by analysing factors involved in cell cycle control during liver regeneration. After hepatectomy, adv Ras 61L increases DNA synthesis significantly in comparison to the other treatment groups. Higher Ras activity results in an early increase of transcriptional active E2F-3, which is associated with higher cyclin E, but almost unchanged cyclin D protein expression. However, Northern blot analysis and cyclin E promoter experiments indicate that, besides transcriptional mechanisms also post-transcriptional mechanisms are involved in regulating cyclin E protein expression after partial hepatectomy in mice treated with adv Ras 61L. Cyclin E phosphorylation studies demonstrate that adv Ras 61L results in hypophosphorylation of cyclin E compared to the control group at early time points after hepatectomy, when cyclin E protein expression strongly increases and there is only a minor effect on cyclin E mRNA levels. Our experiments indicate adv Ras 61L in vivo increases Cyclin E expression by higher transcription via E2F and a post-transcriptional mechanism. These mechanisms result in an earlier activation of an active CDK2/Cyclin E complex which, in turn, triggers DNA synthesis.

Evidence for the involvement of Gi2 in activation of extracellular signal-regulated kinases in hepatocytes

BACKGROUND

Activation of the extracellular signal-regulated kinases ERK1 and ERK2 in hepatocytes by prostaglandin (PG)F2alpha was recently found to be inhibited by pertussis toxin (PTX) suggesting a role for Gi proteins.

RESULTS

Targeting the Gi2alpha expression by a specific ribozyme inhibited the PGF2alpha -induced ERK1/2 activation in hepatocytes. On the other hand a non-cleaving form of the Gi2alpha ribozyme did not significantly decrease the ERK1/2 activation. In ribozyme-treated cells the Gi2alpha protein level was reduced, while the Gqalpha level was not affected thus confirming the specificity of the ribozyme.

CONCLUSION

The present data suggest an important role of Gi2 in PGF2alpha -induced ERK1/2 signaling in hepatocytes.

Ethanol inhibits the JAK-STAT signaling pathway in freshly isolated rat hepatocytes but not in cultured hepatocytes or HepG2 cells: evidence for a lack of involvement of ethanol metabolism

OBJECTIVES

To understand the molecular mechanism underlying alcoholic liver injury, effects of acute ethanol on the Janus kinase-signal transducer and activator transcription factor (JAK-STAT) signaling in hepatic cells were studied.

DESIGNS AND METHODS

Effects of acute ethanol on the JAK-STAT signaling in freshly isolated, cultured rat hepatocytes, and HepG2 cells were explored.

RESULTS

Acute ethanol exposure inhibited IL-6- or IFN-activated STAT in freshly isolated hepatocytes but not in cultured hepatocytes, HepG2 cells, or HepG2 cells transfected with alcohol dehydrogenase (ADH) or cytochrome P450(2E1). The inhibitory action of ethanol in freshly isolated hepatocytes was not antagonized by the ADH inhibitor 4-methylpyrazole (4-MP). Acute exposure of hepatocytes to acetaldehyde or hydrogen peroxide did not suppress STAT activation. Further studies indicated that the loss of response to the inhibitory effect of ethanol was not due to hepatocyte proliferation and collagen contact.

CONCLUSIONS

Freshly isolated hepatocytes are more susceptible to the inhibitory action of ethanol on the JAK-STAT signaling than cultured hepatocytes or HepG2 cells, which may be implicated in pathogenesis and progression of alcoholic liver disease.