S-Adenosylmethionine regulates connexins sub-types expressed by hepatocytes

Connexin-Based Channels in the Liver

Connexin proteins oligomerize in hexameric structures called connexin hemichannels, which then dock to form gap junctions. Gap junctions direct cell-cell communication by allowing the exchange of small molecules and ions between neighboring cells. In this way, hepatic gap junctions support liver homeostasis. Besides serving as building blocks for gap junctions, connexin hemichannels provide a pathway between the intracellular and the extracellular environment. The activation of connexin hemichannels is associated with acute and chronic liver pathologies. This article discusses the role of gap junctions and connexin hemichannels in the liver. © 2022 American Physiological Society. Compr Physiol 12:1-17, 2022.

Therapeutic effect of adenosylmethionine on viral hepatitis and related factors inducing diseas

OBJECTIVE

To analyze the therapeutic efficacy of adenosylmethionine on viral hepatitis and the related factors inducing disease.

METHODS

From May 2018 to April 2019, 137 patients with viral hepatitis who received treatment in our hospital were selected and assigned to two groups according to different treatment methods. In the control group (CG), 61 cases were treated with routine liver protection and enzyme reduction. In the research group, 76 cases were treated with adenosylmethionine on the basis of the CG. After therapy, the total response rate was analyzed in both groups, and the adverse reactions were observed during the treatment. The liver function indexes [albumin (ALB), alanine aminotransferase (ALT), glutamic acid transaminase (AST) and total bilirubin (TBIL)], liver fiber indicators [hyaluronic acid (HA), laminin (LN), type III procollagen (PCIII), type IV collagen (IV-C)], inflammatory factors [interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α)] were compared in both groups before and after therapy. ELISA was applied to detect inflammatory factors in both groups before and after treatment. Logistic analysis was applied to analyze the independent risk factors affecting the curative effect of patients with viral hepatitis.

RESULTS

After therapy, the total response rate of patients in RG was obviously higher than that in CG; The total incidence of adverse effects in RG was obviously lower than that in CG; The improvement of liver function indexes and liver fiber indicators in RG was better than that in CG; The expression of inflammatory factors in RG was obviously lower than that in CG. Logistic analysis revealed that patients' age (>40 years old), drinking history, family history, low improvement of hepatic function and hepatic fibrosis, high level of inflammatory cytokines and routine treatment were independent prognostic factors affecting patients with viral hepatitis.

CONCLUSION

Adenosylmethionine intervention can promote disease recovery, reduce inflammation level and improve liver function damage for patients with viral hepatitis.

Connexin 32 dysfunction promotes ethanol-related hepatocarcinogenesis via activation of Dusp1-Erk axis

There is abundant epidemiological evidence that heavy alcohol intake contributes to hepatocellular carcinoma (HCC) development. Previous reports indicated that connexin 32 (Cx32), which is a major hepatocyte gap junction protein, is down-regulated in chronic liver disease and has a protective role in hepatocarcinogenesis. However, functions of Cx32 in alcohol-related hepatocarcinogenesis have not been clarified. To evaluate them, 9-week-old Cx32 dominant negative transgenic (Tg) rats and their wild-type (Wt) littermates were given 1 % or 5 % ethanol (EtOH) or water ad libitum, for 16 weeks after an intraperitoneal injection of diethylnitrosamine (200 mg/kg). EtOH significantly increased the incidence and multiplicity of HCC and total tumors in a dose-dependent manner in Tg rats, but not in Wt rats. Although the number and area of glutathione S-transferase placental form (GST-P) positive foci were not significantly different between the groups, EtOH increased the Ki-67 labeling indices in GST-P positive foci only in Tg rats. EtOH up-regulated phosphorylated Erk1/2 with decrease of the Erk1/2 inhibitor, dual specificity protein phosphatase 1 (Dusp1) in whole livers of Tg and Wt rats. Immunofluorescence staining and quantitative RT-PCR revealed that EtOH significantly increased nucleolar localization of phosphorylated Erk1/2 and contrastingly reduced Dusp1 protein and mRNA expression in GST-P positive foci and HCC of Tg rats as compared to those of Wt rats. These findings suggest that Cx32 dysfunction like in chronic liver disease promoted EtOH-associated hepatocarcinogenesis through dysregulation of Erk-Dusp1 signaling.

Models and methods for in vitro testing of hepatic gap junctional communication

Inherent to their pivotal roles in controlling all aspects of the liver cell life cycle, hepatocellular gap junctions are frequently disrupted upon impairment of the homeostatic balance, as occurs during liver toxicity. Hepatic gap junctions, which are mainly built up by connexin32, are specifically targeted by tumor promoters and epigenetic carcinogens. This renders inhibition of gap junction functionality a suitable indicator for the in vitro detection of nongenotoxic hepatocarcinogenicity. The establishment of a reliable liver gap junction inhibition assay for routine in vitro testing purposes requires a cellular system in which gap junctions are expressed at an in vivo-like level as well as an appropriate technique to probe gap junction activity. Both these models and methods are discussed in the current paper, thereby focusing on connexin32-based gap junctions.

S-Adenosylmethionine and methylthioadenosine inhibit β-catenin signaling by multiple mechanisms in liver and colon cancer

S-Adenosylmethionine (SAMe), the principal methyl donor that is available as a nutritional supplement, and its metabolite methylthioadenosine (MTA) exert chemopreventive properties against liver and colon cancer in experimental models. Both agents reduced β-catenin expression on immunohistochemistry in a murine colitis-associated colon cancer model. In this study, we examined the molecular mechanisms involved. SAMe or MTA treatment in the colitis-associated cancer model lowered total β-catenin protein levels by 47 and 78%, respectively. In an orthotopic liver cancer model, increasing SAMe levels by overexpressing methionine adenosyltransferase 1A also reduced total β-catenin levels by 68%. In both cases, lower cyclin D1 and c-Myc expression correlated with lower β-catenin levels. In liver (HepG2) and colon (SW480, HCT116) cancer cells with constitutively active β-catenin signaling, SAMe and MTA treatment inhibited β-catenin activity by excluding it from the nuclear compartment. However, in liver (Huh-7) and colon (RKO) cancer cells expressing wild-type Wnt/β-catenin, SAMe and MTA accelerated β-catenin degradation by a glycogen synthase kinase 3-β-dependent mechanism. Both agents lowered protein kinase B activity, but this was not mediated by inhibiting phosphoinositide 3-kinase. Instead, both agents increased the activity of protein phosphatase 2A, which inactivates protein kinase B. The effect of MTA on lowering β-catenin is direct and not mediated by its conversion to SAMe, as blocking this conversion had no influence. In conclusion, SAMe and MTA inhibit Wnt/β-catenin signaling in colon and liver cancer cells regardless of whether this pathway is aberrantly induced, making them ideal candidates for chemoprevention and/or chemotherapy in these cancers.

Importance of Connexin-43 based gap junction in cirrhosis and acute-on-chronic liver failure

BACKGROUND & AIMS

In cirrhosis, superimposed inflammation often culminates in acute-on-chronic liver failure (ACLF) but the mechanism underlying this increased sensitivity is not clear. Cx43 is a ubiquitous gap junction protein that allows transmission of signals between cells at a much higher rate than the constitutively expressed gap junctions. The aims of the study were to test the hypothesis that inflammation drives the increased expression of hepatic Cx43 and to determine its role by Cx43 inhibition.

METHODS

Four weeks after bile-duct ligation (BDL) or sham operation, rats were treated with an anti-TNF antibody, or saline; with or without LPS (1mg/kg); given 3h prior to termination. Biochemistry and cytokines were measured in the plasma and hepatic protein expression (NFkB, TNFα, iNOS, 4HNE, Cx26, 32, and 43) and confocal microscopy (Cx26, 32, and 43) were performed. The effect of a Cx43-specific inhibitory peptide was studied in a mouse BDL model.

RESULTS

BDL animals administered LPS developed typical features of ACLF but animals administered infliximab were relatively protected. Cx26/32 expression was significantly decreased in BDL animals while Cx43 was significantly increased and increased further following LPS. Infliximab treatment prevented this increase. However, inhibiting Cx43 in BDL mice produced detrimental effects with markedly greater hepatocellular necrosis.

CONCLUSIONS

The results of this study show for the first time an increased expression of hepatic Cx43 in cirrhosis and ACLF, which was related to the severity of inflammation. This increased Cx43 expression is likely to be an adaptive protective response of the liver to allow better cell-to-cell communication.

Lupus autoimmunity altered by cellular methylation metabolism

Modifications of both DNA and protein by methylation are key factors in normal T and B cell immune responses as well as in the development of autoimmune disease. For example, the failure to maintain the methylation status of CpG dinucleotides in DNA triggers T cell autoreactivity. Methylated proteins are known targets of autoimmunity, including the symmetrical dimethylarginine residues of SmD1 and SmD3 in SLE. Herein, we demonstrate that altering the metabolism of S-adenosylmethionine (SAM), the major methyl donor for transmethylation reactions, can suppress T cell immunity. A by-product of SAM metabolism, 5'-deoxy-5'-methylthioadenosine (MTA), and an indirect inhibitor of methyltransferases, inhibits T cell responses including T cell activation markers, Th1/Th2 cytokines and TCR-related signaling events. Moreover, treatment of the lupus-prone MRL/lpr mouse with MTA markedly ameliorates splenomegaly, lymphadenopathy, autoantibody titers as well as IgG deposition and cellular infiltration in the kidney. Incubation of cells with SAM, which increases intracellular MTA levels, inhibits both TCR-mediated T cell proliferation and BCR (anti-IgM)-triggered B cell proliferation in a dose-dependent manner. These studies define the central role of MTA and SAM in immune responses and provide a simple approach to altering lymphocyte transmethylation and T cell mediated autoimmune syndromes.

Connexin43 signaling contributes to spontaneous apoptosis in cultures of primary hepatocytes

Primary hepatocyte cultures suffer from the progressive occurrence of dedifferentiation followed by spontaneous apoptosis. This is associated with modifications in the expression of connexins (Cxs), which are the building stones of hemichannels that in turn form gap junctions between neighboring cells. Specifically, a shift is observed from the adult hepatocellular Cx32 species toward the fetal Cx43 isoform. The current study was set up to investigate the role of Cx43 in spontaneous apoptosis taking place in primary hepatocyte cultures. For this purpose, freshly isolated adult rat hepatocytes were cultivated in conventional conditions for 4 days with daily monitoring of Cx expression, Cx localization, and gap junction channel and hemichannel functionality. Gap junction activity was low shortly after isolation, whereas the inverse was observed for hemichannel functionality. Both channel types displayed high activity near the end stages of the cultivation period. The Cx32-to-Cx43 switch became progressively manifested at the translational level. At the transcriptional level, a fivefold decrease in Cx32 messenger RNA abundance and a twofold increase in Cx43 expression were noticed within the first 24 h of cultivation. Throughout the cultivation period, Cx32 was mainly located at the plasma membrane surface, whereas Cx43 immunostaining was more diffuse. Application of three Cx43 inhibitors resulted in the downregulation of both hemichannel functionality and gap junction activity. This was paralleled by decreased expression and activity of caspase 3 as well as by reduced expression of Bid. Collectively, these data show that Cx43 signaling actively contributes to the occurrence of spontaneous apoptosis in cultures of primary hepatocytes.