Gap junction expression and cell proliferation in differentiating cultures of Cx43 KO mouse hepatocytes

MiR-377 Regulates Inflammation and Angiogenesis in Rats After Cerebral Ischemic Injury

Ischemic stroke is the leading cause of disabilities worldwide. MicroRNA-377 (miR-377) plays important roles in ischemic injury. The present study focused on the mechanisms of miR-377 in protecting ischemic brain injury in rats. Cerebral ischemia was induced by middle cerebral artery occlusion (MCAO) in rats. Primary rat microglial cells and brain microvascular endothelial cells (BMECs) were exposed to oxygen-glucose deprivation (OGD). The concentrations of cytokines (TNF-α, IL-1β, IL-6, IFN-γ, TGF-β, MMP2, COX2, and iNOS) in the culture medium were measured by specific ELISA. Tube formation assay was for the in vitro study of angiogenesis. Luciferase reporter assay was performed to confirm whether VEGF and EGR2 were direct targets of miR-377. The MCAO rats were intracerebroventricular (ICV) injection of miR-377 inhibitor to assess its protective effects in vivo. MiR-377 levels were decreased in the rat brain tissues at 1, 3, and 7 d after MCAO. Both microglia cells and BMECs under OGD showed markedly lower expression levels of miR-377 while higher expression levels of EGR2 and VEGF compared to those under normoxia conditions. Knockdown of miR-377 inhibited microglial activation and the release of pro-inflammatory cytokines after OGD. Suppression of miR-377 promoted the capillary-like tube formation and cell proliferation and migration of BMECs. The anti-inflammation effect of EGR2 and the angiogenesis effect of VEGF were regulated by miR-377 after OGD. Inhibition of miR-377 decreased cerebral infarct volume and suppressed cerebral inflammation but promoted angiogenesis in MCAO rats. Knockdown of miR-377 lessened the ischemic brain injury through promoting angiogenesis and suppressing cerebral inflammation. J. Cell. Biochem. 119: 327-337, 2018. © 2017 Wiley Periodicals, Inc.

Connexins: junctional and non-junctional modulators of proliferation

Mounting evidence indicates that dysregulation of gap junctions and their structural subunits-connexins-often occurs in, and sometimes causes, a variety of proliferative disorders, including cancer. Connexin-mediated regulation of cell proliferation is complex and may involve modulation of gap junction intercellular communication (GJIC), hemichannel signalling, or gap junction-independent paths. However, the exact mechanisms linking connexins to proliferation remain poorly defined and a number of contradictory studies report both pro- and anti-proliferative effects, effects that often depend on the cell or tissue type or the microenvironment. The present review covers junctional and non-junctional regulation of proliferation by connexins, with a particular emphasis on their association with cancer.

Background lesions during a 24-month observation period in connexin 32-deficient mice

Connexin 32 (Cx32) is a major gap junction protein in the liver. Neoplastic and non-neoplastic lesions were examined in Cx32-deficient (Cx32KO) mice maintained for 24-month, and compared with those in wild-type mice as a corresponding control. In neoplastic lesions, hepatocellular carcinoma increased significantly only in male Cx32KO mice, suggesting that Cx32 deficiency may be related to their pathogenesis. For females, the incidence of pituitary adenoma in the pars distalis of Cx32KO mice was lower than that of wild-type mice. No non-neoplastic lesions related to Cx32-deficiency were observed in the Cx32KO mice. In conclusion, these results demonstrate that the incidence of hepatocellular carcinoma increases only in male Cx32KO mice, presumably due to enhanced tumor promotion and progression signals associated with Cx32 deficiency.

Connexins: sensors and regulators of cell cycling

It is nowadays well established that gap junctions are critical gatekeepers of cell proliferation, by controlling the intercellular exchange of essential growth regulators. In recent years, however, it has become clear that the picture is not as simple as originally anticipated, as structural precursors of gap junctions can affect cell cycling by performing actions not related to gap junctional intercellular communication. Indeed, connexin hemichannels also foresee a pathway for cell growth communication, albeit between the intracellular compartment and the extracellular environment, while connexin proteins as such can directly or indirectly influence the production of cell cycle regulators independently of their channel activities. Furthermore, a novel set of connexin-like proteins, the pannexins, have lately joined in as regulators of the cell proliferation process, which they can affect as either single units or as channel entities. In the current paper, these multifaceted aspects of connexin-related signalling in cell cycling are reviewed.

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.

Hepatic reconstruction from fetal porcine liver cells using a radial flow bioreactor

AIM

To examine the efficacy of the radial flow bioreactor (RFB) as an extracorporeal bioartificial liver (BAL) and the reconstruction of liver organoids using embryonic pig liver cells.

METHODS

We reconstructed the liver organoids using embryonic porcine liver cells in the RFB. We also determined the gestational time window for the optimum growth of embryonic porcine liver cells. Five weeks of gestation was designated as embryonic day (E) 35 and 8 wk of gestation was designated as E56. These cells were cultured for one week before morphological and functional examinations. Moreover, the efficacy of pulsed administration of a high concentration hepatocyte growth factor (HGF) was examined.

RESULTS

Both cell growth and function were excellent after harvesting on E35. The pulsed administration of a high concentration of HGF promoted the differentiation and maturation of these fetal hepatic cells. Microscopic examination of organoids in the RFB revealed palisading and showed that bile duct-like structures were well developed, indicating that the organoids were mini livers. Transmission electron microscopy revealed microvilli on the luminal surfaces of bile duct-like structures and junctional complexes, which form the basis of the cytoskeleton of epithelial tissues. Furthermore, strong expression of connexin (Cx) 32, which is the main protein of hepatocyte gap junctions, was observed. With respect to liver function, ammonia detoxification and urea synthesis were shown to be performed effectively.

CONCLUSION

Our system can potentially be applied in the fields of BAL and transplantation medicine.

Differential effects of hydroxamate histone deacetylase inhibitors on cellular functionality and gap junctions in primary cultures of mitogen-stimulated hepatocytes

Histone deacetylase (HDAC)-inhibitors are well known to induce proliferative blocks and concomitant differentiation boosts in a plethora of tumor cells. Despite their promising potential as clinical therapeutics, however, the biological outcome of HDAC-inhibitors in non-tumorous cells has been poorly documented. We previously reported that the HDAC-inhibitor trichostatin A (TSA) and its metabolically more stable structural analogue 5-(4-dimethylaminobenzoyl)-aminovaleric acid hydroxamide (4-Me2N-BAVAH) cause cell cycle arrests in primary cultures of mitogen-stimulated hepatocytes. The present study was set up to explore whether this proliferative block in non-tumorous cells is also associated with inducing effects on the differentiated hepatocellular phenotype, a scenario that is usually observed in tumorous cells. In particular, the molecular actions of TSA and 4-Me2N-BAVAH on hepatic functionality and gap junctions, gatekeepers of liver homeostasis, in primary cultures of mitogen-stimulated hepatocytes are investigated. Both HDAC-inhibitors were found to promote albumin secretion and CYP1A1 gene transcription and functionality, whereas CYP2B1 gene transcription and activity were only slightly enhanced. The protein production of the gap junction component Cx26 was downregulated, whereas Cx32 expression was upregulated in response to HDAC-inhibition. Furthermore, TSA increased protein levels of the non-specific hepatocellular Cx43, whereas 4-Me2N-BAVAH rather diminished its expression. These data provide new insight into the biological impact of HDAC-inhibitors on the homeostatic balance in hepatocytes, being major executors of xenobiotic biotransformation and primary targets of drug-induced toxicity.

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.

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.

Alginate/galactosylated chitosan/heparin scaffold as a new synthetic extracellular matrix for hepatocytes

Formation of multicellular hepatocyte spheroids in the three-dimensional culture is a potential approach for enhancing liver-specific functions in bioartificial liver (BAL) devices. In this study, as a synthetic extracellular matrix (ECM) for hepatocytes, a highly porous hydrogel (sponge-like) scaffold, 150-200 microm pore size in diameter, was fabricated with alginate (AL), galactosylated chitosan (GC), and heparin through electrostatic interaction. We attempt to select the best condition of AL/GC/heparin sponges for coculture with NIH3T3, as well as compare the liver-specific functions with monoculture. Cell adhesion to GC based on AL film was significantly increased with increasing GC concentration, but not to chitosan regardless of its concentration. The optimal concentration of GC and heparin in AL/GC/heparin sponges to perform the best liver-specific function was 1 and 6 wt% to AL contents, respectively, where albumin secretion were maintained with maximal rates. The mechanical properties in tensile strength of three types of sponges were very slightly different from one another. Cell viabilities performed on AL, AL/GC, and AL/GC/heparin sponges were 68.5, 83.3, and 90.4 % of control, respectively, after 15 days of incubation. Hepatocyte spheroids were more rapidly formed in the AL/GC and AL/GC/heparin sponges, with diameter enlarged to about 100 microm, than in AL sponges. Connexin32 and E-cadherin genes correlated with cell-to-cell adhesion were expressed in hepatocytes within AL/GC and AL/GC/heparin sponges at 36 h after incubation, but not in AL sponges. Treatment of a gap junctional intercellular communication (GJIC) inhibitor, 18beta-glycyrrhetinic acid, indicates that cell aggregation without GJIC does not perform the liver-specific functions for long periods. In the presence of HGF, the level of albumin secretion in AL/GC/heparin sponges was markedly elevated compared to that in AL/GC sponges. Coculture of hepatocytes in AL/GC/heparin sponges with NIH3T3 in a transwell insert resulted in significant increase of liver-specific functions, such as improved albumin secretion rates, ammonia elimination rates, and ethoxyresorufin-O-deethylase activity by cytochrome P4501A1 compared to those in hepatocyte monoculture. The results suggest that hepatocytes as stable spheroids enhance liver-specific functions in AL/GC/heparin sponges, providing a new synthetic ECM to design BAL devices.

Trichostatin a enhances gap junctional intercellular communication in primary cultures of adult rat hepatocytes

The effects of histone deacetylase inhibitor Trichostatin A (TSA) on connexin (Cx) expression and gap junctional intercellular communication (GJIC) were investigated in primary cultures of adult rat hepatocytes. GJIC was monitored by using the scrape-loading/dye transfer method. Immunoblotting and immunocytochemistry were used to investigate Cx protein levels and localization. Cx gene expression was studied by means of quantitative reverse transcriptase-polymerase chain reaction. TSA increased Cx32 protein levels and affected negatively the Cx26 protein levels. The latter was preferentially located in the cytosol of cultured cells. TSA also promoted the appearance of Cx43 in the nuclear compartment of primary cultured hepatocytes. Overall, this resulted in enhanced GJIC activity. It is important to note that the time of onset of TSA treatment was crucial for the extent of its outcome and that the effects of TSA on Cx protein levels occurred independently of transcriptional changes. TSA differentially affects Cx proteins in primary rat hepatocyte cultures, suggesting distinct regulation and/or distinct roles of the different Cx species in the control of hepatic homeostasis. TSA enhances GJIC between primary cultured rat hepatocytes, an interesting finding supporting its use to further optimize liver-based in vitro models for pharmacotoxicological purposes.

Alginate microcapsules prepared with xyloglucan as a synthetic extracellular matrix for hepatocyte attachment

In this study, xyloglucan (XG) was used as a new synthetic extracellular matrix (ECM) for primary mouse hepatocyte attachment in Ca-alginate (AL) capsules. The rates of hepatocytes adhesion onto collagen type I-, XG-coated and uncoated polystyrene (PS) surface were 89.1%, 91.1% and 25.5%, respectively, at 4 h after incubation at 37 degrees C. From the inhibition study in a cell adhesion assay, the adhesion rates of freshly isolated hepatocytes and preincubated hepatocytes with 20 mm galactose onto the XG-coated surface were 55.7 and 17.3%, respectively, after 30 min incubation at 37 degrees C. Flow cytometric analysis showed that the internalization of XG by freshly isolated hepatocytes was stronger than preincubated hepatocytes with 20 mm galactose. The concentration of XG in AL/XG capsules to perform the best liver-specific functions was 0.5 mg/ml, where the highest albumin secretion rates were obtained. The albumin secretion, ammonia elimination rates and cell viability of hepatocytes were slowly decreased with culture time in AL/XG capsules, whereas those were rapidly decreased in AL capsules, indication of the more rapid formation of hepatocyte spheroids in AL/XG capsules than in AL capsules. More than 70% of the seeded hepatocytes in AL/XG capsules participated in spheroid formation after 2 days, whereas most hepatocytes in AL capsules remained as single cells and only a few cells began to form aggregates after 3 days. Intercellular molecule genes, such as connexin (Cx) 32 and E-cadherin, of hepatocyte spheroids in AL or AL/XG capsules were detected by reverse transcriptase-polymerase chain reaction. Cx32 and E-cadherin genes in AL/XG capsules were more rapidly reexpressed and expressed, respectively, than in AL ones. The results suggest that the multicellular spheroid formation of hepatocytes can enhance the liver-specific functions in the three-dimensional space in the presence of XG as a new synthetic ECM owing to the specific interaction between the galactose moieties of XG and asialoglycoprotein receptors of hepatocytes.

Cx31 is assembled and trafficked to cell surface by ER-Golgi pathway and degraded by proteasomal or lysosomal pathways

Gap junctions, consisting of connexins, allow the exchange of small molecules (less than 1 KD) between adjacent cells, thus providing a mechanism for synchronizing the responses of groups of cells to environmental stimuli. Connexin 31 is a member of the connexin family. Mutations on connexin 31 are associated with erythrokeratodermia variabilis, hearing impairment and peripheral neuropathy. However, the pathological mechanism for connexin 31 mutants in these diseases are still unknown. In this study, we analyzed the assembly, trafficking and metabolism of connexin 31 in HeLa cells stably expressing connexin 31. Calcein transfer assay showed that calcein transfer was inhibited when cells were treated with Brefeldin A or cytochalasin D, but not when treated with nocodazole or a-glycyrrhetinic acid, suggesting that Golgi apparatus and actin filaments, but not microtubules, are crucial to the trafficking and assembly of connexin 31, as well as the formation of gap junction intercellular communication by connexin 31. Additionally, a-glycyrrhetinic acid did not effectively inhibit gap junctional intercellular communication formed by connexin 31. Pulse-chase assay revealed that connexin 31 had a half-life of about 6 h. Moreover, Western blotting and fluorescent staining demonstrated that in HeLa cells stably expressing connexin 31, the amount of connexin 31 was significantly increased after these cells were treated with proteasomal or lysosomal inhibitors. These findings indicate that connexin 31 was rapidly renewed, and possibly degraded by both proteasomal and lysosomal pathways.

Post-translational integration and oligomerization of connexin 26 in plasma membranes and evidence of formation of membrane pores: implications for the assembly of gap junctions

Gap-junction channels provide a widespread intercellular signalling mechanism. They are constructed of a family of connexin membrane proteins that thread across the membrane four times and oligomerize to generate hexameric gap-junction hemichannels. Using an in vitro cell-free transcription/translation system, we demonstrate that connexin (Cx) 26, one of the smallest connexins, is integrated directly in a post-translational manner into plasma membranes. Protein-cleavage studies of Cx26 integrated into plasma membranes indicate a similar native transmembrane topography to that of Cx26 integrated co-translationally into microsomes. Cx26 integrated post-translationally into plasma membranes oligomerizes and, when incorporated into liposomes, provides permeability to ascorbic acid, suggesting that gap-junction hemichannels are generated. The results provide the basis of a novel alternative mechanism for spontaneous assembly in plasma membranes of Cx26 gap-junction hemichannels that occurs independently of the conventional biogenesis of gap junctions involving connexin trafficking and oligomerization via membrane components of the secretory pathway.

Connexin family members target to lipid raft domains and interact with caveolin-1

Lipid rafts are cholesterol-sphingolipid-rich microdomains that function as platforms for membrane trafficking and signal transduction. Caveolae are specialized lipid raft domains that contain the structural proteins known as the caveolins. Connexins are a family of transmembrane proteins that self-associate to form cell-cell connections known as gap junctions and that are linked to cytosolic proteins, forming a protein complex or Nexus. To determine the extent to which these intracellular compartments intersect, we have systematically evaluated whether connexins are associated with lipid rafts and caveolin-1. We show that connexin 43 (Cx43) colocalizes, cofractionates, and coimmunoprecipitates with caveolin-1. A mutational analysis of Cx43 reveals that the hypothesized PDZ- and presumptive SH2/SH3-binding domains within the Cx43 carboxyl terminus are not required for this targeting event or for its stable interaction with caveolin-1. Furthermore, Cx43 appears to interact with two distinct caveolin-1 domains, i.e., the caveolin-scaffolding domain (residues 82-101) and the C-terminal domain (135-178). We also show that other connexins (Cx32, Cx36, and Cx46) are targeted to lipid rafts, while Cx26 and Cx50 are specifically excluded from these membrane microdomains. Interestingly, recombinant coexpression of Cx26 with caveolin-1 recruits Cx26 to lipid rafts, where it colocalizes with caveolin-1. This trafficking event appears to be unique to Cx26, since the other connexins investigated in this study do not require caveolin-1 for targeting to lipid rafts. Our results provide the first evidence that connexins interact with caveolins and partition into lipid raft domains and indicate that these interactions are connexin specific.