Intercellular communication via gap junctions in activated rat hepatic stellate cells

Accumulation of α-synuclein in hepatocytes in nonalcoholic steatohepatitis and its usefulness in pathological diagnosis

BACKGROUNDS

Nonalcoholic steatohepatitis (NASH) is characterized by fat deposition, inflammation, and hepatocellular damage. The diagnosis of NASH is confirmed pathologically, and hepatocyte ballooning is an important finding for definite diagnosis. Recently, α-synuclein deposition in multiple organs was reported in Parkinson's disease. Since it was reported that α-synuclein is taken up by hepatocytes via connexin 32, the expression of α-synuclein in the liver in NASH is of interest. The accumulation of α-synuclein in the liver in NASH was investigated. Immunostaining for p62, ubiquitin, and α-synuclein was performed, and the usefulness of immunostaining in pathological diagnosis was examined.

METHODS

Liver biopsy tissue specimens from 20 patients were evaluated. Several antibodies against α-synuclein, as well as antibodies against connexin 32, p62, and ubiquitin were used for immunohistochemical analyses. Staining results were evaluated by several pathologists with varying experience, and the diagnostic accuracy of ballooning was compared.

RESULTS

Polyclonal α-synuclein antibody, not the monoclonal antibody, reacted with eosinophilic aggregates in ballooning cells. Expression of connexin 32 in degenerating cells was also demonstrated. Antibodies against p62 and ubiquitin also reacted with some of the ballooning cells. In the pathologists' evaluations, the highest interobserver agreement was obtained with hematoxylin and eosin (H&E)-stained slides, followed by slides immunostained for p62 and α-synuclein, and there were cases with different results between H&E staining and immunostaining CONCLUSION: These results indicate the incorporation of degenerated α-synuclein into ballooning cells, suggesting the involvement of α-synuclein in the pathogenesis of NASH. The combination of immunostaining including polyclonal α-synuclein may contribute to improving the diagnosis of NASH.

Gap junction intercellular communication mediates cadmium-induced apoptosis in hepatocytes via the Fas/FasL pathway

As a common environmental pollutant, cadmium (Cd) causes damage to many organs of the body. Gap junction intercellular communication (GJIC) represents one of the most important routes of rapid signaling between cells. However, the mechanisms underlying GJIC's role in hepatotoxicity induced by Cd remain unknown. We established a Cd poisoning model in vitro by co-culturing Cd-exposed and unexposed hepatocytes and found that 18β-glycyrrhetinic acid (GA), a GJIC inhibitor, can effectively reduce the apoptosis rate of healthy cells co-cultured with apoptotic cells treated with Cd. We also found that anti-FasL antibody had the same effect. However, in mono-cultured cells, GA treatment in combination with Cd was found to aggravate the damage induced by Cd exposure, increase the level of oxidative stress and protein expression of HO-1, decrease the mitochondrial membrane potential, incur more serious morphological damage to mitochondria than Cd treatment alone. Moreover, compared with Cd-only exposure, GA and Cd co-treatment further increased the expression levels of the apoptosis-related proteins Fas, FasL, FADD and the ratio of Bax/Bcl-2, inhibited the protein expression of ASK1 and Daxx. We also found that the protein expression of Daxx in siFADD + Cd hepatocytes was significantly higher than in Cd-treated cells. Thus, our study suggests that gap junction inhibition may play a dual role in Cd-induced cell damage by inhibiting the transmission of death signals from damaged cells to healthy cells but also aggravating the transmission of death signals between damaged cells, and that the Fas/FasL-mediated death receptor pathway may play an important role in this process.

Mechanisms of SARS-CoV-2 and Male Infertility: Could Connexin and Pannexin Play a Role?

The impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on male infertility has lately received significant attention. SARS-CoV-2, the virus that causes coronavirus disease (COVID-19) in humans, has been shown to impose adverse effects on both the structural components and function of the testis, which potentially impact spermatogenesis. These adverse effects are partially explained by fever, systemic inflammation, oxidative stress, and an increased immune response leading to impaired blood-testis barrier. It has been well established that efficient cellular communication via gap junctions or functional channels is required for tissue homeostasis. Connexins and pannexins are two protein families that mediate autocrine and paracrine signaling between the cells and the extracellular environment. These channel-forming proteins have been shown to play a role in coordinating cellular communication in the testis and epididymis. Despite their role in maintaining a proper male reproductive milieu, their function is disrupted under pathological conditions. The involvement of these channels has been well documented in several physiological and pathological conditions and their designated function in infectious diseases. However, their role in COVID-19 and their meaningful contribution to male infertility remains to be elucidated. Therefore, this review highlights the multivariate pathophysiological mechanisms of SARS-CoV-2 involvement in male reproduction. It also aims to shed light on the role of connexin and pannexin channels in disease progression, emphasizing their unexplored role and regulation of SARS-CoV-2 pathophysiology. Finally, we hypothesize the possible involvement of connexins and pannexins in SARS-CoV-2 inducing male infertility to assist future research ideas targeting therapeutic approaches.

Expression and Functionality of Connexin-Based Channels in Human Liver Cancer Cell Lines

Liver cancer cell lines are frequently used in vitro tools to test candidate anti-cancer agents as well as to elucidate mechanisms of liver carcinogenesis. Among such mechanisms is cellular communication mediated by connexin-based gap junctions. The present study investigated changes in connexin expression and gap junction functionality in liver cancer in vitro. For this purpose, seven human liver cancer cell lines, as well as primary human hepatocytes, were subjected to connexin and gap junction analysis at the transcriptional, translational and activity level. Real-time quantitative reverse transcription polymerase chain reaction analysis showed enhanced expression of connexin43 in the majority of liver cancer cell lines at the expense of connexin32 and connexin26. Some of these changes were paralleled at the protein level, as evidenced by immunoblot analysis and in situ immunocytochemistry. Gap junctional intercellular communication, assessed by the scrape loading/dye transfer assay, was generally low in all liver cancer cell lines. Collectively, these results provide a full scenario of modifications in hepatocyte connexin production and gap junction activity in cultured liver cancer cell lines. The findings may be valuable for the selection of neoplastic hepatocytes for future mechanistic investigation and testing of anti-cancer drugs that target connexins and their channels.

Connexin-Based Channel Activity Is Not Specifically Altered by Hepatocarcinogenic Chemicals

Connexin-based channels play key roles in cellular communication and can be affected by deleterious chemicals. In this study, the effects of various genotoxic carcinogenic compounds, non-genotoxic carcinogenic compounds and non-carcinogenic compounds on the expression and functionality of connexin-based channels, both gap junctions and connexin hemichannels, were investigated in human hepatoma HepaRG cell cultures. Expression of connexin26, connexin32, and connexin43 was evaluated by means of real-time reverse transcription quantitative polymerase chain reaction analysis, immunoblot analysis and in situ immunostaining. Gap junction functionality was assessed via a scrape loading/dye transfer assay. Opening of connexin hemichannels was monitored by measuring extracellular release of adenosine triphosphate. It was found that both genotoxic and non-genotoxic carcinogenic compounds negatively affect connexin32 expression. However, no specific effects related to chemical type were observed at gap junction or connexin hemichannel functionality level.

GJA1 promotes hepatocellular carcinoma progression by mediating TGF-β-induced activation and the epithelial-mesenchymal transition of hepatic stellate cells

Introduction

Gap junction protein, alpha 1 (GJA1), which is correlated with recurrences and unfavorable prognoses in hepatocellular carcinomas (HCCs), is one of the specific proteins expressed by activated hepatic stellate cells (HSCs).

Methods

Expression of GJA1 was compared between HCCs and nontumor tissues (NTs), between hepatic cirrhosis and NTs, and between primary and metastatic HCCs using transcriptomic datasets from the Gene Expression Omnibus and the Integrative Molecular Database of Hepatocellular Carcinoma. The in vitro activities of GJA1 were investigated in cultured HSCs and HCC cells. The underlying mechanism was characterized using Gene Set Enrichment Analysis and validated by western blotting.

Results

The expression of GJA1 was significantly increased in HCCs and hepatic cirrhosis compared to that in NTs. GJA1 was also overexpressed in pulmonary metastases from HCCs when compared with HCCs without metastasis. Overexpression of GJA1 promoted while knockdown of GJA1 inhibited proliferation and transforming growth factor (TGF)-β-mediated activation and migration of cultured HSCs. Overexpression of GJA1 by lentivirus infection promoted proliferation and migration, while conditioned medium from HSCs overexpressing GJA1 promoted migration but inhibited proliferation of Hep3B and PLC-PRF-5 cells. Lentivirus infection with shGJA1 or conditioned medium from shGJA1-infected HSCs inhibited the proliferation and migration of HCCLM3 cells that had a high propensity toward lung metastasis. Mechanistically, GJA1 induced the epithelial–mesenchymal transition (EMT) in HSCs and HCCLM3 cells.

Conclusion

GJA1 promoted HCC progression by inducing HSC activation and the EMT in HSCs. GJA1 is potentially regulated by TGF-β and thus may be a therapeutic target to inhibit HCC progression.

Human umbilical cord-derived mesenchymal stem cells affect urea synthesis and the cell apoptosis of human induced hepatocytes by secreting IL-6 in a serum-free co-culture system

BACKGROUND

Bioartificial livers (BALs) are emerging as a potential supportive therapy for liver diseases. However, the maintenance of hepatocyte function and viability is a major challenge. Mesenchymal stem cells (MSCs) have attracted extensive attention for providing trophic support to hepatocytes, but only few studies have explored the interaction between human MSCs and human hepatocytes, and very little is known about the underlying molecular mechanisms whereby MSCs affect hepatocyte function, especially in serum-free medium (SFM).

CONCLUSION

The SFM co-culture strategy showed major advantages in maintaining hiHep function and viability, which is of great significance for the clinical application of hiHeps in BALs. This article is protected by copyright. All rights reserved.

Cholestasis Differentially Affects Liver Connexins

Connexins are goal keepers of tissue homeostasis, including in the liver. As a result, they are frequently involved in disease. The current study was set up to investigate the effects of cholestatic disease on the production of connexin26, connexin32 and connexin43 in the liver. For this purpose, bile duct ligation, a well-known trigger of cholestatic liver injury, was applied to mice. In parallel, human hepatoma HepaRG cell cultures were exposed to cholestatic drugs and bile acids. Samples from both the in vivo and in vitro settings were subsequently subjected to assessment of mRNA and protein quantities as well as to in situ immunostaining. While the outcome of cholestasis on connexin26 and connexin43 varied among experimental settings, a more generalized repressing effect was seen for connexin32. This has also been observed in many other liver pathologies and could suggest a role for connexin32 as a robust biomarker of liver disease and toxicity.

Targeting gap junctional intercellular communication by hepatocarcinogenic compounds

Gap junctions in liver, as in other organs, play a critical role in tissue homeostasis. Inherently, these cellular constituents are major targets for systemic toxicity and diseases, including cancer. This review provides an overview of chemicals that compromise liver gap junctions, in particular biological toxins, organic solvents, pesticides, pharmaceuticals, peroxides, metals and phthalates. The focus in this review is placed upon the mechanistic scenarios that underlie these adverse effects. Further, the potential use of gap junctional activity as an in vitro biomarker to identify non-genotoxic hepatocarcinogenic chemicals is discussed.

Vitamin E Blocks Connexin Hemichannels and Prevents Deleterious Effects of Glucocorticoid Treatment on Skeletal Muscles

Glucocorticoids are frequently used as anti-inflammatory and immunosuppressive agents. However, high doses and/or prolonged use induce undesired secondary effects such as muscular atrophy. Recently, de novo expression of connexin43 and connexin45 hemichannels (Cx43 HCs and Cx45 HCs, respectively) has been proposed to play a critical role in the mechanism underlying myofiber atrophy induced by dexamethasone (Dex: a synthetic glucocorticoid), but their involvement in specific muscle changes promoted by Dex remains poorly understood. Moreover, treatments that could prevent the undesired effects of glucocorticoids on skeletal muscles remain unknown. In the present work, a 7-day Dex treatment in adult mice was found to induce weight loss and skeletal muscle changes including expression of functional Cx43/Cx45 HCs, elevated atrogin immunoreactivity, atrophy, oxidative stress and mitochondrial dysfunction. All these undesired effects were absent in muscles of mice simultaneously treated with Dex and vitamin E (VitE). Moreover, VitE was found to rapidly inhibit the activity of Cx HCs in freshly isolated myofibers of Dex treated mice. Exposure to alkaline pH induced free radical generation only in HeLa cells expressing Cx43 or Cx45 where Ca²⁺ was present in the extracellular milieu, response that was prevented by VitE. Besides, VitE and two other anti-oxidant compounds, Tempol and Resveratrol, were found to inhibit Cx43 HCs in HeLa cells transfectants. Thus, we propose that in addition to their intrinsic anti-oxidant potency, some antioxidants could be used to reduce expression and/or opening of Cx HCs and consequently reduce the undesired effect of glucocorticoids on skeletal muscles.

How does Helicobacter pylori cause gastric cancer through connexins: An opinion review

Helicobacter pylori (H. pylori) is a Gram-negative bacterium with a number of virulence factors, such as cytotoxin-associated gene A, vacuolating cytotoxin A, its pathogenicity island, and lipopolysaccharide, which cause gastrointestinal diseases. Connexins function in gap junctional homeostasis, and their downregulation is closely related to gastric carcinogenesis. Investigations into H. pylori infection and the fine-tuning of connexins in cells or tissues have been reported in previous studies. Therefore, in this review, the potential mechanisms of H. pylori-induced gastric cancer through connexins are summarized in detail.

Gap junctions in liver disease: Implications for pathogenesis and therapy

In the normal liver, cells interact closely through gap junctions. By providing a pathway for the trafficking of low molecular mass molecules, these channels contribute to tissue homeostasis and maintenance of hepatic function. Thus, dysfunction of gap junctions affects a wide variety of liver processes, such as differentiation, cell death, inflammation and fibrosis. In fact, dysfunctional gap junctions have been implicated, for more than a decade, in cholestatic disease, hepatic cancer and cirrhosis. Additionally, in recent years there is an increasing body of evidence that these channels are also involved in other relevant and prevalent liver pathological processes, such as non-alcoholic fatty liver disease, acute liver injury and portal hypertension. In parallel to these new clinical implications the available data include controversial observations. Thus, a comprehensive overview is required to better understand the functional complexity of these pores. This paper will review the most recent knowledge concerning gap junction dysfunction, with a special focus on the role of these channels in the pathogenesis of relevant clinical entities and on potential therapeutic targets that are amenable to modification by drugs.

Connexin and Pannexin (Hemi)Channels: Emerging Targets in the Treatment of Liver Disease

Connexin proteins are the building blocks of hemichannels, which dock further between adjacent cells to form gap junctions. Gap junctions control the intercellular exchange of critical homeostasis regulators. By doing so, gap junctions control virtually all aspects of the hepatic life cycle. In the last decade, it has become clear that connexin hemichannels also provide a pathway for cellular communication on their own independent of their role as structural precursors of gap junctions, namely between the cytosol of an individual cell and its extracellular environment. In contrast to gap junctions, connexin hemichannels become particularly active in liver disease by facilitating inflammation and cell death. This equally holds true for cellular channels composed of pannexins, being connexin-like proteins recently identified in the liver that gather in structures reminiscent of hemichannels. This paper gives an overview of the involvement of connexin-based and pannexin-based channels in noncancerous liver disease.

The Role of Connexins in Gastrointestinal Diseases

Gap junctions are hexagonal arrays of protein molecules in the plasma membrane and were first described in Mauthner cell synapses of goldfish. They form pathways for coupling between cells, allowing passive, electrotonic spread of ions and also passage of larger molecules such as amino acids and nucleotides. They are expressed in both excitable and non-excitable tissues. Each gap junction is made of two connexons, which are hexameric proteins of the connexin subunit. In this review, the roles that connexins play in gastrointestinal motility, the mechanisms of altered connexin expression leading to inflammatory bowel disease, gastrointestinal infections, and gastrointestinal symptoms in autistic spectrum disorder are discussed in detail.

TAT-Gap19 and Carbenoxolone Alleviate Liver Fibrosis in Mice

Although a plethora of signaling pathways are known to drive the activation of hepatic stellate cells in liver fibrosis, the involvement of connexin-based communication in this process remains elusive. Connexin43 expression is enhanced in activated hepatic stellate cells and constitutes the molecular building stone of hemichannels and gap junctions. While gap junctions support intercellular communication, and hence the maintenance of liver homeostasis, hemichannels provide a circuit for extracellular communication and are typically opened by pathological stimuli, such as oxidative stress and inflammation. The present study was set up to investigate the effects of inhibition of connexin43-based hemichannels and gap junctions on liver fibrosis in mice. Liver fibrosis was induced by administration of thioacetamide to Balb/c mice for eight weeks. Thereafter, mice were treated for two weeks with TAT-Gap19, a specific connexin43 hemichannel inhibitor, or carbenoxolone, a general hemichannel and gap junction inhibitor. Subsequently, histopathological analysis was performed and markers of hepatic damage and functionality, oxidative stress, hepatic stellate cell activation and inflammation were evaluated. Connexin43 hemichannel specificity of TAT-Gap19 was confirmed in vitro by fluorescence recovery after photobleaching analysis and the measurement of extracellular release of adenosine-5′-triphosphate. Upon administration to animals, both TAT-Gap19 and carbenoxolone lowered the degree of liver fibrosis accompanied by superoxide dismutase overactivation and reduced production of inflammatory proteins, respectively. These results support a role of connexin-based signaling in the resolution of liver fibrosis, and simultaneously demonstrate the therapeutic potential of TAT-Gap19 and carbenoxolone in the treatment of this type of chronic liver disease.

Inhibitors of connexin and pannexin channels as potential therapeutics

While gap junctions support the exchange of a number of molecules between neighboring cells, connexin hemichannels provide communication between the cytosol and the extracellular environment of an individual cell. The latter equally holds true for channels composed of pannexin proteins, which display an architecture reminiscent of connexin hemichannels. In physiological conditions, gap junctions are usually open, while connexin hemichannels and, to a lesser extent, pannexin channels are typically closed, yet they can be activated by a number of pathological triggers. Several agents are available to inhibit channels built up by connexin and pannexin proteins, including alcoholic substances, glycyrrhetinic acid, anesthetics and fatty acids. These compounds not always strictly distinguish between gap junctions, connexin hemichannels and pannexin channels, and may have effects on other targets as well. An exception lies with mimetic peptides, which reproduce specific amino acid sequences in connexin or pannexin primary protein structure. In this paper, a state-of-the-art overview is provided on inhibitors of cellular channels consisting of connexins and pannexins with specific focus on their mode-of-action and therapeutic potential.

The role of connexin and pannexin containing channels in the innate and acquired immune response

Connexin (Cx) and pannexin (Panx) containing channels - gap junctions (GJs) and hemichannels (HCs) - are present in virtually all cells and tissues. Currently, the role of these channels under physiological conditions is well defined. However, their role in the immune response and pathological conditions has only recently been explored. Data from several laboratories demonstrates that infectious agents, including HIV, have evolved to take advantage of GJs and HCs to improve viral/bacterial replication, enhance inflammation, and help spread toxicity into neighboring areas. In the current review, we discuss the role of Cx and Panx containing channels in immune activation and the pathogenesis of several infectious diseases. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

The stellate cell system (vitamin A-storing cell system)

Past, present, and future research into hepatic stellate cells (HSCs, also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells, or Ito cells) are summarized and discussed in this review. Kupffer discovered black-stained cells in the liver using the gold chloride method and named them stellate cells (Sternzellen in German) in 1876. Wake rediscovered the cells in 1971 using the same gold chloride method and various modern histological techniques including electron microscopy. Between their discovery and rediscovery, HSCs disappeared from the research history. Their identification, the establishment of cell isolation and culture methods, and the development of cellular and molecular biological techniques promoted HSC research after their rediscovery. In mammals, HSCs exist in the space between liver parenchymal cells (PCs) or hepatocytes and liver sinusoidal endothelial cells (LSECs) of the hepatic lobule, and store 50-80% of all vitamin A in the body as retinyl ester in lipid droplets in the cytoplasm. SCs also exist in extrahepatic organs such as pancreas, lung, and kidney. Hepatic (HSCs) and extrahepatic stellate cells (EHSCs) form the stellate cell (SC) system or SC family; the main storage site of vitamin A in the body is HSCs in the liver. In pathological conditions such as liver fibrosis, HSCs lose vitamin A, and synthesize a large amount of extracellular matrix (ECM) components including collagen, proteoglycan, glycosaminoglycan, and adhesive glycoproteins. The morphology of these cells also changes from the star-shaped HSCs to that of fibroblasts or myofibroblasts.

Connexin-based signaling and drug-induced hepatotoxicity

Being critical mediators of liver homeostasis, connexins and their channels are frequently involved in liver toxicity. In the current paper, specific attention is paid to actions of hepatotoxic drugs on these communicative structures. In a first part, an overview is provided on the structural, regulatory and functional properties of connexin-based channels in the liver. In the second part, documented effects of acetaminophen, hypolipidemic drugs, phenobarbital and methapyriline on connexin signaling are discussed. Furthermore, the relevance of this subject for the fields of clinical and in vitro toxicology is demonstrated.

Relevance for patients: The role of connexin signaling in drug-induced hepatotoxicity may be of high clinical relevance, as it offers perspectives for the therapeutic treatment of such insults by interfering with connexin channel opening.

Dexamethasone-induced muscular atrophy is mediated by functional expression of connexin-based hemichannels

Long-term treatment with high glucocorticoid doses induces skeletal muscle atrophy. However, the molecular mechanism of such atrophy remains unclear. We evaluated the possible involvement of connexin-based hemichannels (Cx HCs) in muscle atrophy induced by dexamethasone (DEX), a synthetic glucocorticoid, on control (Cx43(fl/fl)Cx45(fl/fl)) and Cx43/Cx45 expression-deficient (Cx43(fl/fl)Cx45(fl/fl):Myo-Cre) skeletal myofibers. Myofibers of Cx43(fl/fl)Cx45(fl/fl) mice treated with DEX (5h) expressed several proteins that form non-selective membrane channels (Cx39, Cx43, Cx45, Panx1, P2X7 receptor and TRPV2). After 5h DEX treatment in vivo, myofibers of Cx43(fl/fl)Cx45(fl/fl) mice showed Evans blue uptake, which was absent in myofibers of Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice. Similar results were obtained in vitro using ethidium as an HC permeability probe, and DEX-induced dye uptake in control myofibers was blocked by P2X7 receptor inhibitors. DEX also induced a significant increase in basal intracellular Ca(2+) signal and a reduction in resting membrane potential in Cx43(fl/fl)Cx45(fl/fl) myofibers, changes that were not elicited by myofibers deficient in Cx43/Cx45 expression. Moreover, treatment with DEX induced NFκB activation and increased mRNA levels of TNF-α in control but not in Cx43/Cx45 expression-deficient myofibers. Finally, a prolonged DEX treatment (7days) increased atrogin-1 and Murf-1 and reduced the cross sectional area of Cx43(fl/fl)Cx45(fl/fl) myofibers, but these parameters remained unaffected in Cx43(fl/fl)Cx45(fl/fl):Myo-Cre myofibers. Therefore, DEX-induced expression of Cx43 and Cx45 plays a critical role in early sarcolemma changes that lead to atrophy. Consequently, this side effect of chronic glucocorticoid treatment might be avoided by co-administration with a Cx HC blocker.

Connexins and their channels in inflammation

Inflammation may be caused by a variety of factors and is a hallmark of a plethora of acute and chronic diseases. The purpose of inflammation is to eliminate the initial cell injury trigger, to clear out dead cells from damaged tissue and to initiate tissue regeneration. Despite the wealth of knowledge regarding the involvement of cellular communication in inflammation, studies on the role of connexin-based channels in this process have only begun to emerge in the last few years. In this paper, a state-of-the-art overview of the effects of inflammation on connexin signaling is provided. Vice versa, the involvement of connexins and their channels in inflammation will be discussed by relying on studies that use a variety of experimental tools, such as genetically modified animals, small interfering RNA and connexin-based channel blockers. A better understanding of the importance of connexin signaling in inflammation may open up towards clinical perspectives.

Connexin32 deficiency exacerbates carbon tetrachloride-induced hepatocellular injury and liver fibrosis in mice

OBJECTIVE

Liver fibrosis results from the perpetuation of the normal wound healing response to several types of injury. Despite the wealth of knowledge regarding the involvement of intracellular and extracellular signaling pathways in liver fibrogenesis, information about the role of intercellular communication mediated by gap junctions is scarce.

METHODS

In this study, liver fibrosis was chemically induced by carbon tetrachloride in mice lacking connexin32, the major liver gap junction constituent. The manifestation of liver fibrosis was evaluated based on a series of read-outs, including collagen morphometric and mRNA analysis, oxidative stress, apoptotic, proliferative and inflammatory markers.

RESULTS

More pronounced liver damage and enhanced collagen deposition were observed in connexin32 knockout mice compared to wild-type animals in experimentally triggered induced liver fibrosis. No differences between both groups were noticed in apoptotic signaling nor in inflammation markers. However, connexin32 deficient mice displayed decreased catalase activity and increased malondialdehyde levels.

CONCLUSION

These findings could suggest that connexin32-based signaling mediates tissue resistance against liver damage by the modulation of the antioxidant capacity. In turn, this could point to a role for connexin32 signaling as a therapeutic target in the treatment of liver fibrosis.

Structure, Regulation and Function of Gap Junctions in Liver

Gap junctions are a specialized group of cell-to-cell junctions that mediate direct intercellular communication between cells. They arise from the interaction of two hemichannels of adjacent cells, which in turn are composed of six connexin proteins. In liver, gap junctions are predominantly found in hepatocytes and play critical roles in virtually all phases of the hepatic life cycle, including cell growth, differentiation, liver-specific functionality and cell death. Liver gap junctions are directed through a broad variety of mechanisms ranging from epigenetic control of connexin expression to post-translational regulation of gap junction activity. This paper reviews established and novel aspects regarding the architecture, control and functional relevance of liver gap junctions.

Connexins and pannexins in liver damage

Connexins and pannexins are key players in the control of cellular communication and thus in the maintenance of tissue homeostasis. Inherent to this function these proteins are frequently involved in pathological processes. The present paper reviews the role of connexins and pannexins in liver toxicity and disease. As they act both as sensors and effectors in these deleterious events connexins and pannexins could represent a set of novel clinical diagnostic biomarkers and drug targets.

Connexins, Pannexins, and Their Channels in Fibroproliferative Diseases

Cellular and molecular mechanisms of wound healing, tissue repair, and fibrogenesis are established in different organs and are essential for the maintenance of function and tissue integrity after cell injury. These mechanisms are also involved in a plethora of fibroproliferative diseases or organ-specific fibrotic disorders, all of which are associated with the excessive deposition of extracellular matrix components. Fibroblasts, which are key cells in tissue repair and fibrogenesis, rely on communicative cellular networks to ensure efficient control of these processes and to prevent abnormal accumulation of extracellular matrix into the tissue. Despite the significant impact on human health, and thus the epidemiologic relevance, there is still no effective treatment for most fibrosis-related diseases. This paper provides an overview of current concepts and mechanisms involved in the participation of cellular communication via connexin-based pores as well as pannexin-based channels in the processes of tissue repair and fibrogenesis in chronic diseases. Understanding these mechanisms may contribute to the development of new therapeutic strategies to clinically manage fibroproliferative diseases and organ-specific fibrotic disorders.

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.

The Renal Connexome and Possible Roles of Connexins in Kidney Diseases

Connexins are membrane-spanning proteins that allow for the formation of cell-to-cell channels and cell-to-extracellular space hemichannels. Many connexin subtypes are expressed in kidney cells. Some mutations in connexin genes have been linked to various human pathologies, including cardiovascular, neurodegenerative, lung, and skin diseases, but the exact role of connexins in kidney disease remains unclear. Some hypotheses about a connection between genetic mutations, endoplasmic reticulum (ER) stress, and the unfolded protein response (UPR) in kidney pathology have been explored. The potential relationship of kidney disease to abnormal production of connexin proteins, mutations in their genes together with ER stress, or the UPR is still a matter of debate. In this scenario, it is tantalizing to speculate about a possible role of connexins in the setting of kidney pathologies that are thought to be caused by a deregulated podocyte protein expression, the so-called podocytopathies. In this article, we give examples of the roles of connexins in kidney (patho)physiology and propose avenues for further research concerning connexins, ER stress, and UPR in podocytopathies that may ultimately help refine drug treatment.

Connexin and pannexin signaling in gastrointestinal and liver disease

Gap junctions, which mediate intercellular communication, are key players in digestive homeostasis. They are also frequently involved in gastrointestinal and liver pathology. This equally holds true for connexin (Cx) hemichannels, the structural precursors of gap junctions, and pannexin (Panx) channels, Cx-like proteins assembled in a hemichannel configuration. Both Cx hemichannels and Panx channels facilitate extracellular communication and drive a number of deteriorative processes, such as cell death and inflammation. Cxs, Panxs, and their channels underlie a wide spectrum of gastrointestinal and liver diseases, including gastritis and peptic ulcer disease, inflammatory intestinal conditions, acute liver failure, cholestasis, hepatitis and steatosis, liver fibrosis and cirrhosis, infectious gastrointestinal pathologies, and gastrointestinal and liver cancer. This could open promising perspectives for the characterization of new targets and biomarkers for therapeutic and diagnostic clinical purposes in the area of gastroenterology and hepatology.

Roles of connexins and pannexins in digestive homeostasis

Connexin proteins are abundantly present in the digestive system. They primarily form gap junctions, which control the intercellular exchange of critical homeostasis regulators. By doing so, gap junctions drive a plethora of gastrointestinal and hepatic functional features, including gastric and gut motility, gastric acid secretion, intestinal innate immune defense, xenobiotic biotransformation, glycogenolysis, bile secretion, ammonia detoxification and plasma protein synthesis. In the last decade, it has become clear that connexin hemichannels, which are the structural precursors of gap junctions, also provide a pathway for cellular communication, namely between the cytosol and the extracellular environment. Although merely pathological functions have been described, some physiological roles have been attributed to connexin hemichannels, in particular in the modulation of colonic motility. This equally holds true for cellular channels composed of pannexins, connexin-like proteins recently identified in the intestine and the liver, which have become acknowledged key players in inflammatory processes and that have been proposed to control colonic motility, secretion and blood flow.

Hepatocyte-ductal transdifferentiation is mediated by reciprocal repression of SOX9 and C/EBPα

Primary hepatocytes rapidly dedifferentiate when cultured in vitro. We have studied the mechanism of hepatocyte dedifferentiation by using two culture media: one that maintains hepatocytes in a differentiated state and another that allows dedifferentiation. We show that dedifferentiation involves partial transformation of hepatocytes into cells that resemble biliary epithelial cells. Lineage labeling and time-lapse filming confirm that the dedifferentiated cells are derived from hepatocytes and not from contaminating ductal or fibroblastic cells in the original culture. Furthermore, we establish that the conversion of hepatocytes to biliary-like cells is regulated by mutual antagonism of CCAAT/enhancer binding protein alpha (C/EBPα) and SOX9, which have opposing effects on the expression of hepatocyte and ductal genes. Thus, hepatocyte dedifferentiation induces the biliary gene expression program by alleviating C/EBPα-mediated repression of Sox9. We propose that reciprocal antagonism of C/EBPα and SOX9 also operates in the formation of hepatocytes and biliary ducts from hepatoblasts during normal embryonic development. These data demonstrate that reprogramming of differentiated cells can be used to model the acquisition and maintenance of cell fate in vivo.

Role of gap junctions modulating hepatic vascular tone in cirrhosis

BACKGROUND & AIMS

Gap junctions are formed by connexins (Cx), a family of proteins that couple endothelial and smooth muscle cells in systemic vessels. In this context, Cx allow the transmission of signals modulating vascular tone. Recently, vascular Cx have been observed in liver cells implicated in liver blood flow regulation. Here, we investigated the role of Cx in the regulation of intrahepatic vascular tone in cirrhosis.

METHODS

Livers of Sprague-Dawley control and cirrhotic (common bile duct ligation-CBDL and CCl4 ) rats were perfused, and concentration-effect curves in response to acetylcholine (ACh) precontracted with methoxamine were obtained in the presence of the specific Cx inhibitor 18-alpha-glycyrrhetinic acid or vehicle. Cx expression was assessed by immunofluorescence, western blot and reverse-transcription polymerase chain reaction in liver tissue, hepatic stellate cells, sinusoidal endothelial cells and hepatocytes isolated from control and cirrhotic rat livers. Cx protein expression was also determined in cirrhotic human tissue.

RESULTS

Gap junction blockade markedly attenuated relaxation of hepatic vasculature in response to ACh in control (maximal relaxation, -55 ± 10.5% vs. -95.3 ± 10% with vehicle; P < 0.01) and CBDL rats (50.9 ± 18.5% vs. -18.7 ± 5.5% with vehicle; P = 0.01). Livers from CBDL rats and patients with cirrhosis exhibited Cx overexpression. By contrast, CCl4 -cirrhotic rats did not show attenuated relaxation of hepatic vasculature after blockade and Cx expression was significantly lower than in controls.

CONCLUSIONS

Gap junctions may contribute to modulating portal pressure and intrahepatic vascular relaxation. Liver gap junctions may represent a new therapeutic target in cirrhotic portal hypertension.

Connexin and pannexin (hemi)channels in the liver

The liver was among the first organs in which connexin proteins have been identified. Hepatocytes harbor connexin32 and connexin26, while non-parenchymal liver cells typically express connexin43. Connexins give rise to hemichannels, which dock with counterparts on adjacent cells to form gap junctions. Both hemichannels and gap junctions provide pathways for communication, via paracrine signaling or direct intercellular coupling, respectively. Over the years, hepatocellular gap junctions have been shown to regulate a number of liver-specific functions and to drive liver cell growth. In the last few years, it has become clear that connexin hemichannels are involved in liver cell death, particularly in hepatocyte apoptosis. This also holds true for hemichannels composed of pannexin1, a connexin-like protein recently identified in the liver. Moreover, pannexin1 hemichannels are key players in the regulation of hepatic inflammatory processes. The current paper provides a concise overview of the features of connexins, pannexins and their channels in the liver.

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.

Connexins regulate cell functions in pancreatic stellate cells

OBJECTIVES

Pancreatic stellate cells (PSCs) play a pivotal role in pancreatic fibrosis associated with chronic pancreatitis and pancreatic cancer. Connexins (Cxs) allow direct intercellular communications as components of gap junction but also play important roles in the regulation of cell proliferation, cell differentiation, and tissue development. We here examined the expression of Cxs and Cx-mediated regulation of cell functions in PSCs.

METHODS

Human PSCs were isolated from patients undergoing operation for chronic pancreatitis or pancreatic cancer. The expression of Cxs was examined by reverse transcription polymerase chain reaction, Western blotting, and immunofluorescent staining. The roles of Cxs in PSC functions were examined by using carbenoxolone, a broad-spectrum Cx inhibitor, and small interfering RNA for Cx43.

RESULTS

Human activated PSCs expressed a variety of Cxs including Cx43 both in vitro and in vivo. Carbenoxolone inhibited platelet-derived growth factor-BB-induced proliferation and migration, and type I collagen expression in PSCs. In addition, carbenoxolone inhibited the activation of quiescent PSCs to a myofibroblastlike phenotype. Decreased Cx43 expression by small interfering RNA resulted in decreased proliferation and type I collagen expression.

CONCLUSIONS

Pancreatic stellate cells expressed a variety of Cxs. Connexins, especially Cx43, might regulate the cell functions and activation of PSCs.

Gap junctions and non-neoplastic liver disease

Because of their critical role as goalkeepers of hepatic homeostasis, gap junctions are frequent targets in liver disease. This concept has been demonstrated on many occasions in the light of hepatocarcinogenesis. Relatively little focus has been put on the fate of gap junctions in other liver pathologies, including hepatitis, liver fibrosis and cirrhosis, cholestasis and hepatic ischemia and reperfusion injury. The present paper provides an in-depth description of the multiple changes in expression, localization and function of connexins, the molecular constituents of gap junctions. The use of connexins as biomarkers and therapeutic targets in liver disease is also illustrated.

Systemic administration of autologous adipose-derived mesenchymal stem cells alleviates hepatic ischemia-reperfusion injury in rats

OBJECTIVES

Mesenchymal stem cells have previously been shown to offer significant therapeutic benefit in ischemic organ injuries. This study aimed at investigating the therapeutic role of adipose tissue-derived mesenchymal stem cells in hepatic ischemia-reperfusion injury and the underlying mechanisms.

DESIGN

Adult male Fisher rats (n = 30) were equally divided into three groups (group 1: Sham-operated normal controls; group 2: Ischemia-reperfusion injury with intravenous fresh culture medium; group 3: Ischemia-reperfusion injury with intravenous adipose tissue-derived mesenchymal stem cells). Ischemia-reperfusion injury was induced by occluding the vascular supplies of left lobe liver for 60 minutes followed by reperfusion for 72 hrs. Adipose tissue-derived mesenchymal stem cells (1.2 × 106) were administered through tail vein immediately after reperfusion and at 6 hrs and 24 hrs after reperfusion in group 3. All animals were sacrificed 72 hrs after reperfusion.

SETTING

Animal laboratory at a medical institute.

MEASUREMENTS AND MAIN RESULTS

Histologic features, plasma aspartate aminotransferase, hepatic cytokine profile, oxidative stress, and terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling were analyzed. Seventy-two hrs after reperfusion, plasma aspartate aminotransferase, hepatic oxidative stress, messenger RNA expressions of tumor necrosis factor-a, transforming growth factor-b, interleukin-1b, interleukin-6, endothelin-1, matrix metalloproteinase-9, plasminogen activator inhibitor-1, Bax and caspase-3, protein expression of intercellular adhesion molecule as well as the number of apoptotic nuclei were significantly increased in group 2 compared with group 3, whereas messenger RNA expressions of endothelial nitric oxide synthase, Bcl-2, interleukin-10, protein expressions of reduced nicotinamide-adenine dinucleotide phosphate:quinone oxidoreductase 1, and heme oxygenase-1 were lower in group 2 than group 3.

CONCLUSIONS

The results showed that systemic adipose tissue-derived mesenchymal stem cell administration significantly preserved hepatocyte integrity and suppressed inflammatory responses, oxidative stress, and apoptosis in a rodent model of hepatic ischemia-reperfusion injury.

Modulation of connexin signaling by bacterial pathogens and their toxins

Inherent to their pivotal tasks in the maintenance of cellular homeostasis, gap junctions, connexin hemichannels, and pannexin hemichannels are frequently involved in the dysregulation of this critical balance. The present paper specifically focuses on their roles in bacterial infection and disease. In particular, the reported biological outcome of clinically important bacteria including Escherichia coli, Shigella flexneri, Yersinia enterocolitica, Helicobacter pylori, Bordetella pertussis, Aggregatibacter actinomycetemcomitans, Pseudomonas aeruginosa, Citrobacter rodentium, Clostridium species, Streptococcus pneumoniae, and Staphylococcus aureus and their toxic products on connexin- and pannexin-related signaling in host cells is reviewed. Particular attention is paid to the underlying molecular mechanisms of these effects as well as to the actual biological relevance of these findings.

Safety of higher dosages of Viscum album L. in animals and humans--systematic review of immune changes and safety parameters

BACKGROUND

Viscum album L extracts (VAE, mistletoe) and isolated mistletoe lectins (ML) have immunostimulating properties and a strong dose-dependent cytotoxic activity. They are frequently used in complementary cancer treatment, mainly to improve quality of life, but partly also to influence tumour growth, especially by injecting VAE locally and in high dosage. The question is raised whether these higher dosages can induce any harm or immunosuppressive effects.

METHODS

Systematic review of all experiments and clinical studies investigating higher dosages of VAE in animals and humans (Viscum album > 1 mg in humans corresponding to > 0.02 mg/kg in animals or ML > 1 ng/kg) and assessing immune parameters or infections or adverse drug reactions.

RESULTS

69 clinical studies and 48 animal experiments reported application of higher doses of VAE or ML and had assessed immune changes and/or harm. In these studies, Viscum album was applied in dosages up to 1500 mg in humans and 1400 mg/kg in animals, ML was applied up to 6.4 μg/kg in humans and in animals up to 14 μg/kg subcutaneously, 50 μg/kg nasally and 500 μg/kg orally. A variety of immune parameters showed fluctuating or rising outcomes, but no immunosuppressive effect. Side effects consisted mainly of dose-dependent flu-like symptoms (FLS), fever, local reactions at the injection site and various mild unspecific effects. Occasionally, allergic reactions were reported. After application of high doses of recombinant ML, reversible hepatotoxicity was observed in some cases.

CONCLUSIONS

Application of higher dosages of VAE or ML is not accompanied by immunosuppression; altogether VAE seems to exhibit low risk but should be monitored by clinicians when applied in high dosages.

Cardiac fibroblasts in cell culture systems: myofibroblasts all along?

The cytoarchitecture of the working myocardium is characterized by densely packed cardiomyocytes that are embedded in a three-dimensional network of numerous fibroblasts. Although the importance of cardiac fibroblasts in maintaining an orderly structured extracellular matrix is well recognized, less is known about their potential paracrine and electrotonic interactions with cardiomyocytes. This is partly the result of the complex intermingling of both cell types in vivo that tends to preclude a direct investigation of heterocellular crosstalk. It is for that reason that most of our present knowledge regarding stromal-parenchymal cell interactions is based on culture systems that permit direct access to either cell type. An often disregarded feature of such studies is that cardiac fibroblasts in standard two-dimensional cell culture have a pronounced tendency to undergo a phenotype switch to myofibroblasts. This cell type typically appears in injured hearts where it contributes importantly to fibrotic remodeling. The present review focuses on recent insights into electrical and paracrine crosstalk between myofibroblasts and cardiomyocytes while acknowledging that a comprehensive understanding of stromal-parenchymal cell interactions will depend on future methodological developments that permit retaining the fibroblast phenotype in cell culture systems and that will, most importantly, allow direct investigations of heterocellular crosstalk in intact tissue.

S-Adenosylmethionine regulates connexins sub-types expressed by hepatocytes

Intercellular communication via GAP Junctions plays an important role in tissue homeostasis, apoptosis, carcinogenesis, cell proliferation and differentiation. Hepatocyte connexins (Cx) 26 and 32 levels are decreased during the de-differentiation process of primary hepatocytes in culture, a situation that is also characterized by a decrease in S-Adenosylmethionine (SAMe) levels. In this current study, we show that SAMe supplementation in cultured hepatocytes every 12h, leads to an up-regulation of Cx26 and 32 mRNA and protein levels and blocks culture-induced Cx43 expression, although it failed to increase Cx26 and 32 membrane localization and GAP junction intracellular communication. SAMe reduced nuclear β-catenin accumulation, which is known to stimulate the TCF/LEF-dependent gene transcription of Cx43. Moreover SAMe-induced reduction in Cx43 and β-catenin was prevented by the proteasome inhibitor MG132, and was not mediated by GSK3 activity. SAMe, and its metabolite 5'-methylthioadenosine (MTA) increased Cx26 mRNA in a process partially mediated by Adenosine A(2A) receptors but independent of PKA. Finally livers from MAT1A knockout mice, characterized by low hepatic SAMe levels, express higher Cx43 and lower Cx26 and 32 protein levels than control mice. These results suggest that SAMe maintains a characteristic expression pattern of the different Cxs in hepatocytes by differentially regulating their levels.

Morphological and molecular pathology of CCL4-induced hepatic fibrosis in connexin43-deficient mice

Gap junction channels, formed by connexins (Cx), are involved in the maintenance of tissue homeostasis, cell growth, differentiation, and development. Several studies have shown that Cx43 is involved in the control of wound healing in dermal tissue. However, it remains unknown whether Cx43 plays a role in the control of liver fibrogenesis. Our study investigated the roles of Cx43 heterologous deletion on carbon tetrachloride (CCl(4))-induced hepatic fibrosis in mice. We administered CCl(4) to both Cx43-deficient (Cx43(+/-)) and wild-type mice and examined hepatocellular injury and collagen deposition by histological and ultrastructural analyses. Serum biochemical analysis was performed to quantify liver injury. Hepatocyte proliferation was analyzed immunohistochemically. Protein and messenger RNA (mRNA) expression of liver connexins were evaluated using immunohistochemistry as well as immunoblotting analysis and quantitative real-time PCR. We demonstrated that Cx43(+/-) mice developed excessive liver fibrosis compared with wild-type mice after CCl(4) -induced chronic hepatic injury, with thick and irregular collagen fibers. Histopathological evaluation showed that Cx43(+/-) mice present less necroinflammatory lesions in liver parenchyma and consequent reduction of serum aminotransferase activity. Hepatocyte cell proliferation was reduced in Cx43(+/-) mice. There was no difference in Cx32 and Cx26 protein or mRNA expression in fibrotic mice. Protein expression of Cx43 increased in CCl(4)-treated mice, although with aberrant protein location on cytoplasm of perisinusoidal cells. Our results demonstrate that Cx43 plays an important role in the control and regulation of hepatic fibrogenesis.

Hedgehog pathway activation and epithelial-to-mesenchymal transitions during myofibroblastic transformation of rat hepatic cells in culture and cirrhosis

Myofibroblastic hepatic stellate cells (MF-HSC) are derived from quiescent hepatic stellate cells (Q-HSC). Q-HSC express certain epithelial cell markers and have been reported to form junctional complexes similar to epithelial cells. We have shown that Hedgehog (Hh) signaling plays a key role in HSC growth. Because Hh ligands regulate epithelial-to-mesenchymal transition (EMT), we determined whether Q-HSC express EMT markers and then assessed whether these markers change as Q-HSC transition into MF-HSC and whether the process is modulated by Hh signaling. Q-HSC were isolated from healthy livers and cultured to promote myofibroblastic transition. Changes in mRNA and protein expression of epithelial and mesenchymal markers, Hh ligands, and target genes were monitored in HSC treated with and without cyclopamine (an Hh inhibitor). Studies were repeated in primary human HSC and clonally derived HSC from a cirrhotic rat. Q-HSC activation in vitro (culture) and in vivo (CCl(4)-induced cirrhosis) resulted in decreased expression of Hh-interacting protein (Hhip, an Hh antagonist), the EMT inhibitors bone morphogenic protein (BMP-7) and inhibitor of differentiation (Id2), the adherens junction component E-cadherin, and epithelial keratins 7 and 19 and increased expression of Gli2 (an Hh target gene) and mesenchymal markers, including the mesenchyme-associated transcription factors Lhx2 and Msx2, the myofibroblast marker alpha-smooth muscle actin, and matrix molecules such as collagen. Cyclopamine reverted myofibroblastic transition, reducing mesenchymal gene expression while increasing epithelial markers in rodent and human HSC. We conclude that Hh signaling plays a key role in transition of Q-HSC into MF-HSC. Our findings suggest that Q-HSC are capable of transitioning between epithelial and mesenchymal fates.

HuR regulates gap junctional intercellular communication by controlling beta-catenin levels and adherens junction integrity

Gap junctional intercellular communication (GJIC) plays a critical role in the regulation of tissue homeostasis and carcinogenesis and is modulated by the levels, subcellular localization, and posttranslational modification of gap junction proteins, the connexins (Cx). Here, using oval cell-like rat liver epithelial cells, we demonstrate that the RNA-binding protein HuR promotes GJIC through two mechanisms. First, HuR silencing lowered the levels of Cx43 protein and Cx43 messenger RNA (mRNA), and decreased Cx43 mRNA half-life. This regulation was likely due to the direct stabilization of Cx43 mRNA by HuR, because HuR associated directly with Cx43 mRNA, a transcript that bears signature adenylate-uridylate-rich (AU-rich) and uridylate-rich (U-rich) sequences in its 3'-untranslated region. Second, HuR silencing reduced both half-life and the levels of beta-catenin mRNA, also a target of HuR; accordingly, HuR silencing lowered the levels of whole-cell and membrane-associated beta-catenin. Coimmunoprecipitation experiments showed a direct interaction between beta-catenin and Cx43. Small interfering RNA (siRNA)-mediated depletion of beta-catenin recapitulated the effects of decreasing HuR levels: it attenuated GJIC, decreased Cx43 levels, and redistributed Cx43 to the cytoplasm, suggesting that depletion of beta-catenin in HuR-silenced cells contributed to lowering Cx43 levels at the membrane. Finally, HuR was demonstrated to support GJIC after exposure to a genotoxic agent, doxorubicin, or an inducer of differentiation processes, retinoic acid, thus pointing to a crucial role of HuR in the cellular response to stress and in physiological processes modulated by GJIC.

CONCLUSION

HuR promotes gap junctional intercellular communication in rat liver epithelial cells through two related regulatory processes, by enhancing the expression of Cx43 and by increasing the expression of beta-catenin, which, in turn, interacts with Cx43 and is required for proper positioning of Cx43 at the plasma membrane.

Hedgehog pathway activation and epithelial-to-mesenchymal transitions during myofibroblastic transformation of rat hepatic cells in culture and cirrhosis

Myofibroblastic hepatic stellate cells (MF-HSC) are derived from quiescent hepatic stellate cells (Q-HSC). Q-HSC express certain epithelial cell markers and have been reported to form junctional complexes similar to epithelial cells. We have shown that Hedgehog (Hh) signaling plays a key role in HSC growth. Because Hh ligands regulate epithelial-to-mesenchymal transition (EMT), we determined whether Q-HSC express EMT markers and then assessed whether these markers change as Q-HSC transition into MF-HSC and whether the process is modulated by Hh signaling. Q-HSC were isolated from healthy livers and cultured to promote myofibroblastic transition. Changes in mRNA and protein expression of epithelial and mesenchymal markers, Hh ligands, and target genes were monitored in HSC treated with and without cyclopamine (an Hh inhibitor). Studies were repeated in primary human HSC and clonally derived HSC from a cirrhotic rat. Q-HSC activation in vitro (culture) and in vivo (CCl(4)-induced cirrhosis) resulted in decreased expression of Hh-interacting protein (Hhip, an Hh antagonist), the EMT inhibitors bone morphogenic protein (BMP-7) and inhibitor of differentiation (Id2), the adherens junction component E-cadherin, and epithelial keratins 7 and 19 and increased expression of Gli2 (an Hh target gene) and mesenchymal markers, including the mesenchyme-associated transcription factors Lhx2 and Msx2, the myofibroblast marker alpha-smooth muscle actin, and matrix molecules such as collagen. Cyclopamine reverted myofibroblastic transition, reducing mesenchymal gene expression while increasing epithelial markers in rodent and human HSC. We conclude that Hh signaling plays a key role in transition of Q-HSC into MF-HSC. Our findings suggest that Q-HSC are capable of transitioning between epithelial and mesenchymal fates.

Gap junctional intercellular communication as a target for liver toxicity and carcinogenicity

Direct communication between hepatocytes, mediated by gap junctions, constitutes a major regulatory platform in the control of liver homeostasis, ranging from hepatocellular proliferation to hepatocyte cell death. Inherent to this pivotal task, gap junction functionality is frequently disrupted upon impairment of the homeostatic balance, as occurs during liver toxicity and carcinogenicity. In the present paper, the deleterious effects of a number of chemical and biological toxic compounds on hepatic gap junctions are discussed, including environmental pollutants, biological toxins, organic solvents, pesticides, pharmaceuticals, peroxides, metals and phthalates. Particular attention is paid to the molecular mechanisms that underlie the abrogation of gap junction functionality. Since hepatic gap junctions are specifically targeted by tumor promoters and epigenetic carcinogens, both in vivo and in vitro, inhibition of gap junction functionality is considered as a suitable indicator for the detection of nongenotoxic hepatocarcinogenicity.

TGF-beta1 down-regulates connexin 43 expression and gap junction intercellular communication in rat hepatic stellate cells

Intercellular communication is an important tool used by the cells to effectively regulate concerted responses. Hepatic stellate cells (HSCs) communicate to each other through functional gap junctions composed of connexin 43 (Cx43) proteins. We show that exogenous human TGF-beta1 (hTGF-beta1), a pro-fibrotic stimulus, decreases Cx43 mRNA and protein in a rat HSC cell line and primary HSCs. Furthermore, hTGF-beta1 increases the phosphorylation of Cx43 at serine 368. These effects lead to a decrease in the gap junction intercellular communication between the HSCs, as shown by gap-FRAP analysis. We also observe the binding of Snai1, from the nuclear extract of HSCs, to a Snai1 consensus sequence in the Cx43 promoter. In the same context, Snai1 siRNA transfection results in an up-regulation of Cx43 suggesting that TGF-beta1 may regulate Cx43 via Snai1. In addition, we demonstrate that the knockdown of Cx43 by siRNA transfection results in a slower proliferation of HSCs. These findings illuminate a new effect of TGF-beta1 in HSCs, namely the regulation of intercellular communication by affecting the expression level and the phosphorylation state of Cx43 through Snai1 signaling.

Hepatitis C virus infection sensitizes human hepatocytes to TRAIL-induced apoptosis in a caspase 9-dependent manner

Apoptosis of infected cells represents a key host defense mechanism against viral infections. The impact of apoptosis on the elimination of hepatitis C virus (HCV)-infected cells is poorly understood. The TRAIL has been implicated in the death of liver cells in hepatitis-infected but not in normal liver cells. To determine the impact of TRAIL on apoptosis of virus-infected host cells, we studied TRAIL-induced apoptosis in a tissue culture model system for HCV infection. We demonstrated that HCV infection sensitizes primary human hepatocytes and Huh7.5 hepatoma cells to TRAIL induced apoptosis in a dose- and time-dependent manner. Mapping studies identified the HCV nonstructural proteins as key mediators of sensitization to TRAIL. Using a panel of inhibitors targeting different apoptosis pathways, we demonstrate that sensitization to TRAIL is caspase-9 dependent and mediated in part via the mitochondrial pathway. Sensitization of hepatocytes to TRAIL-induced apoptosis by HCV infection represents a novel antiviral host defense mechanism that may have important implications for the pathogenesis of HCV infection and may contribute to the elimination of virus-infected hepatocytes.

Biology and pathobiology of gap junctional channels in hepatocytes

The present review provides the state of the art of the current knowledge concerning gap junctional channels and their roles in liver functioning. In the first part, we summarize some relevant biochemical properties of hepatic gap junctional channels, including their structure and regulation. In the second part, we discuss the involvement of gap junctional channels in the occurrence of liver cell growth, liver cell differentiation, and liver cell death. We further exemplify their relevance in hepatic pathophysiology. Finally, a number of directions for future liver gap junctional channel research are proposed, and the up-regulation of gap junctional channel activity as a novel strategy in (liver) cancer therapy is illustrated.