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SummaryBecause 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.

Background/Aims: Transforming growth factor-β (TGF-β) is considered to be an important mediator in the development of fibrosis in several chronic liver diseases. To understand the mechanism(s) by which TGF-β exerts its action(s), we investigated the cellular distribution of TGF-β 1,2,3 transcripts in normal and carbon tetrachloride (CCl 4 )-induced fibrotic rat liver. Methods: Parenchymal, sinusoidal endothelial, Kupffer and stellate cells were isolated and purified. The exact cellular composition of each isolate was determined by transmission electron microscopy. Expression of TGF-β 1,2,3 transcripts was investigated using Northern hybridization analysis. Hybridization signals were quantified by scanning densitometry and corrected for: (i) differences in extractable RNA per cell type, (ii) signal contribution from contaminating cells, and (iii) differences in loading, capillary transfer and hybridization. Results: In normal liver, TGF-β 1 mRNA was predominantly expressed in Kupffer cells, exhibiting values approximately 9-fold higher than those in stellate cells. No expression was found in endothelial and parenchymal cells. Signals for TGF-β 2 and TGF-β 3 were much weaker when compared to TGF-β 1 . In Kupffer cells, the level of TGF-β 2 was approximately 4-fold higher than in stellate cells. Little expression was found in endothelial cells. TGF-β 3 expression could only be detected in stellate cells. TGF-β 2 and TGF-β 3 was not expressed in parenchymal cells. In fibrotic liver, TGF-β 1 mRNA was strongly expressed in all the sinusoidal cells. TGF-β 2 and TGF-β 3 could no longer be detected. When compared to the level of expression in normal stellate cells, the level of TGF-β 1 increased 12-fold in stellate cells from fibrotic livers, and 6-fold in endothelial cells. In Kupffer cells, the level of expression remained unchanged. Conclusions: (i) In both normal and fibrotic liver, TGF-β 1 is the most abundant isoform, (ii) in normal liver, TGF-β 1 is expressed strongly by Kupffer cells and moderately by stellate cells, TGF-β 2 expression is highest in Kupffer cells, followed by stellate cells and endothelial cells. TGF-β 3 is expressed by stellate cells, (iii) in fibrotic liver, the level of TGF-β 1 expression increases selectively in stellate cells and endothelial cells. This suggests an important role, not only for stellate, but also for endothelial cells in fibrogenesis.

Background/Aims: The effects of Trichostatin A (TSA), a drug candidate for cancer therapy, on proliferation and survival of primary hepatocytes, the major site of xenobiotic biotransformation and primary target of drug-induced toxicity, were investigated. Methods: DNA replication was measured using [methyl- 3 H]-thymidine incorporation. Cell cycle markers were analyzed by Western and Northern blottings. Necrosis and apoptosis were monitored by LDH release, caspase-3activation, respectively. Results: We identified two distinct cell cycle arrests, prior DNA replication, in two experimental conditions. First, perfusion of the liver in presence of TSA, prevented c-jun and cyclin D1 induction, characteristic for G1 entry and progression through late G1, respectively. Secondly, TSA treatment of isolated hepatocytes, located in early G1, led to an early S-phase arrest evidenced by the absence of the S/G2/M marker, CDK1. TSA upregulated the expression of the anti-apoptotic protein BclxL and did not increase caspase-3-activity and LDH release. Conclusions: TSA inhibits hepatocyte proliferation at different steps of the cell cycle. Our data suggest that this inhibition may involve downregulation of distinct subsets of genes. TSA does not induce apoptosis in primary hepatocytes, in contrast to what has been observed in hepatoma cells. This finding supports its use in the treatment of proliferative disorders. q 2003 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved.