The novel histone deacetylase inhibitor 4-Me2N-BAVAH differentially affects cell junctions between primary 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.

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.

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.

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.

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.

Regulation of connexin signaling by the epigenetic machinery

Connexins and their channels are involved in the control of all aspects of the cellular life cycle, ranging from cell growth to cell death, by mediating extracellular, intercellular and intracellular communication. These multifaceted aspects of connexin-related cellular signaling obviously require strict regulation. While connexin channel activity is mainly directed by posttranslational modifications, connexin expression as such is managed by classical cis/trans mechanisms. Over the past few years, it has become clear that connexin production is equally dictated by epigenetic actions. This paper provides an overview of the role of major determinants of the epigenome, including DNA methylation, histone acetylation and microRNA species, in connexin expression.

Histone deacetylase inhibition reduces cardiac connexin43 expression and gap junction communication

Histone deacetylase inhibitors (HDACIs) are being investigated as novel therapies for cancer, inflammation, neurodegeneration, and heart failure. The effects of HDACIs on the functional expression of cardiac gap junctions (GJs) are essentially unknown. The purpose of this study was to determine the effects of trichostatin A (TSA) and vorinostat (VOR) on functional GJ expression in ventricular cardiomyocytes. The effects of HDAC inhibition on connexin43 (Cx43) expression and functional GJ assembly were examined in primary cultured neonatal mouse ventricular myocytes. TSA and VOR reduced Cx43 mRNA, protein expression, and immunolocalized Cx43 GJ plaque area within ventricular myocyte monolayer cultures in a dose-dependent manner. Chromatin immunoprecipitation experiments revealed altered protein interactions with the Cx43 promoter. VOR also altered the phosphorylation state of several key regulatory Cx43 phospho-serine sites. Patch clamp analysis revealed reduced electrical coupling between isolated ventricular myocyte pairs, altered transjunctional voltage-dependent inactivation kinetics, and steady state junctional conductance inactivation and recovery relationships. Single GJ channel conductance was reduced to 54 pS only by maximum inhibitory doses of TSA (≥ 100 nM). These two hydroxamate pan-HDACIs exert multiple levels of regulation on ventricular GJ communication by altering Cx43 expression, GJ area, post-translational modifications (e.g., phosphorylation, acetylation), gating, and channel conductance. Although a 50% downregulation of Cx43 GJ communication alone may not be sufficient to slow ventricular conduction or induce arrhythmias, the development of class-selective HDACIs may help avoid the potential negative cardiovascular effects of pan-HDACI.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

Role of connexin-related signalling in hepatic homeostasis and its relevance for liver-based in vitro modelling

Direct intercellular communication mediated by gap junctions constitutes a major regulatory platform in the control of hepatic homeostasis. Hepatocellular gap junctions are composed of two hemichannels of adjacent cells which are built up by connexin proteins,

in casu Cx32. Mathieu Vinken, Pofessor at the Department of Toxicology of the Free University Brussels-Belgium, was one of the first investigators to demonstrate that hepatic connexin expression is controlled by epigenetic mechanisms. In particular, he found that inhibitors of histone deacetylase enzymes enhance Cx32 production and gap junction activity in cultures of primary hepatocytes, a finding that is of importance for liver-based in vitro modelling. Professor Dr. Mathieu Vinken’s recent work is focussed on the elucidation of the role of connexin proteins and their channels in the hepatocyte life cycle. Specific attention is paid to apoptosis in this context, whereby it has been found that Cx32 hemichannels control the termination of induced cell death in cultures of primary hepatocytes. Overall, Professor Dr. Mathieu Vinken’s research can be considered as an important contribution to the field of hepatic connexin physiology.

Preservation of hepatocellular functionality in cultures of primary rat hepatocytes upon exposure to 4-Me2N-BAVAH, a hydroxamate-based HDAC-inhibitor

Great efforts are being put in the development/optimization of reliable and highly predictive models for high-throughput screening of efficacy and toxicity of promising drug candidates. The use of primary hepatocyte cultures, however, is still limited by the occurrence of phenotypic alterations, including loss of xenobiotic biotransformation capacity. In the present study, the differentiation-stabilizing effect of a new histone deacetylase inhibitor 5-(4-dimethylaminobenzoyl)-aminovaleric acid hydroxamide (4-Me(2)N-BAVAH), a structural Trichostatin A (TSA)-analogue with a more favourable pharmaco-toxicological profile, was studied at a genome-wide scale by means of microarray analysis. Several genes coding for xenobiotic biotransformation enzymes were found to be positively regulated upon exposure to 4-Me(2)N-BAVAH. For CYP1A1/2B1/3A2, these observations were confirmed by qRT-PCR and immunoblot analysis. In addition, significantly higher 7-ethoxyresorufin-O-deethylase and 7-pentoxyresorufin-O-dealkylase activity levels were measured. These effects were accompanied by an increased expression of CCAAT/enhancer binding protein alpha and hepatic nuclear factor (HNF)4α, but not of HNF1α. Finally, 4-Me(2)N-BAVAH was found to induce histone H3 acetylation at the proximal promoter of the albumin, CYP1A1 and CYP2B1 genes, suggesting that chromatin remodelling is directly involved in the transcriptional regulation of these genes. In conclusion, histone deacetylase inhibitors prove to be efficient agents for better maintaining a differentiated hepatic phenotype in rat hepatocyte cultures.

DNA methyltransferase 3a expression decreases during apoptosis in primary cultures of hepatocytes

Although it has been acknowledged that DNA methylation is a key factor in the epigenetic control of liver homeostasis, its role in the occurrence of hepatocyte cell death has yet been poorly documented. We therefore have investigated the expression pattern of the effectors of DNA methylation, namely DNA methyltransferase (DNMT) isoenzymes, during Fas-mediated apoptotic cell death in primary hepatocyte cultures. Cell death was assessed by in situ stainings with Annexin V, Hoechst 33342 and Propidium iodide, and measurement of caspase 3-like activity and lactate dehydrogenase release. Similar to the hepatic in vivo situation, DNMT1, DNMT2 and DNMT3b could not be detected, whereas relatively high levels of DNMT3a protein were observed in the in vitro setting, as studied by immunoblotting. Upon induction of cell death, a progressive decrease in DNMT3a protein amount was noticed, reaching a minimum level towards the final stages of the cell death process. This was preceded by parallel changes in DNMT3a mRNA production, measured by qRT-PCR analysis, which became already evident during the early stages of apoptosis. We conclude that downregulated DNMT3a protein production during Fas-mediated hepatocyte apoptosis results from inhibition of DNMT3a gene transcription. This finding further substantiates the existence of an epigenetic signature of apoptosis.

Role of epigenetics in liver-specific gene transcription, hepatocyte differentiation and stem cell reprogrammation

Controlling both growth and differentiation of stem cells and their differentiated somatic progeny is a challenge in numerous fields, from preclinical drug development to clinical therapy. Recently, new insights into the underlying molecular mechanisms have unveiled key regulatory roles of epigenetic marks driving cellular pluripotency, differentiation and self-renewal/proliferation. Indeed, the transcription of genes, governing cell-fate decisions during development and maintenance of a cell's differentiated status in adult life, critically depends on the chromatin accessibility of transcription factors to genomic regulatory and coding regions. In this review, we discuss the epigenetic control of (liver-specific) gene-transcription and the intricate interplay between chromatin modulation, including histone (de)acetylation and DNA (de)methylation, and liver-enriched transcription factors. Special attention is paid to their role in directing hepatic differentiation of primary hepatocytes and stem cells in vitro.