Sequential changes in intercellular junctions between hepatocytes during the course of acute liver injury and restoration after thioacetamide treatment

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.

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.

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.

Polarisation and functional characterisation of hepatocytes derived from human embryonic and mesenchymal stem cells

Adult hepatocytes are polarised with their apical and basolateral membranes separated from neighbouring cells by tight junction proteins. Although efficient differentiation of pluripotent stem cells to hepatocytes has been achieved, the formation of proper polarisation in these cells has not been thoroughly investigated. In the present study, human embryonic stem cells (hESCs) and human mesenchymal stem cells (hMSCs) were differentiated to hepatocyte-like cells and the derived hepatocytes were characterised for mature hepatocyte markers. The secretion of hepatic proteins, expression of hepatic genes and the functional hepatic polarisation of stem cell-derived hepatocytes, foetal hepatocytes and the HepG2 hepatic cell line were evaluated and the different lines were compared. The results indicate that hESC-derived hepatocytes are phenotypically more robust and functionally more efficient compared with the hMSC-derived hepatocytes, suggesting their suitability for toxicity studies.

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.

Involvement of cell junctions in hepatocyte culture functionality

In liver, like in other multicellular systems, the establishment of cellular contacts is a prerequisite for normal functioning. In particular, well-defined cell junctions between hepatocytes, including adherens junctions, desmosomes, tight junctions, and gap junctions, are known to play key roles in the performance of liver-specific functionality. In a first part of this review article, we summarize the current knowledge concerning cell junctions and their roles in hepatic (patho)physiology. In a second part, we discuss their relevance in liver-based in vitro modeling, thereby highlighting the use of primary hepatocyte cultures as suitable in vitro models for preclinical pharmaco-toxicological testing. We further describe the actual strategies to regain and maintain cell junctions in these in vitro systems over the long-term.

IL-1beta regulates expression of Cx32, occludin, and claudin-2 of rat hepatocytes via distinct signal transduction pathways

The functions of gap and tight junctions are perturbed during the acute-phase response to liver injury. To elucidate the mechanism of pro-inflammatory cytokine IL-1beta responsible for regulation of hepatic gap and tight junctions, we analyzed expression and function of gap and tight junctions using a rat liver injury model and primary cultures of rat hepatocyte. In rat liver lobules at 24 h after thioacetamide (TAA) treatment, where some IL-1beta-positive non-parenchymal cells existed, disappearance of connexin32-positive spots at cell borders of the hepatocytes and increases of claudin-2 and occludin immunoreactivities in bile canalicular regions were observed. In primary cultures of rat hepatocytes, IL-1beta caused the disappearance of connexin32, which was reciprocal to the induction and localization of claudin-2 to cell membranes. The downregulated connexin32 expression was inhibited by treatment with a MAP-kinase inhibitor (PD98059), whereas the upregulated claudin-2 expression was blocked by p38 MAP and PI3-kinase inhibitors (SB203580 and LY294002). The changes of connexin32 and claudin-2 may be controlled at the transcriptional level via NF-kappaB, HNF-1alpha, and CDX2. Occludin was hyperphosphorylated by IL-1beta treatment and was inhibited by treatment with a PI3-kinase inhibitor. These results demonstrate that MAP-kinase, p38 MAP-kinase, and PI3-kinase are distinctly involved in the regulation of hepatic gap and tight junctions during the acute-phase response to IL-1beta.