Vasopressin Inhibits Type-I Collagen and Albumin Gene Expression in Primary Cultures of Adult Rat Hepatocytes

c-Myb modulates transcription of the alpha-smooth muscle actin gene in activated hepatic stellate cells

Expression of alpha-smooth muscle actin (alpha-SMA) defines the phenotype of activated (myofibroblastic) hepatic stellate cells. These cells, but not quiescent stellate cells, have a high level of alpha-SMA and c-Myb expression, as well as increased c-Myb-binding activities to the proximal alpha-SMA E box. Therefore, we analyzed the role of c-Myb in alpha-SMA transcription and stellate cell activation. Activated primary rat stellate cells displayed a high expression of the -724 and -271 alpha-SMA/luciferase (LUC) chimeric genes, which contain c-Myb binding sites (-223/-216 bp). Alpha-SMA/LUC minigenes with mutation (-219/-217 bp), truncation (-224 bp), or deletion (-191 bp) of the c-Myb binding site were not efficiently transcribed. Transfection of wild-type c-Myb into quiescent stellate cells, which do not express endogenous c-Myb, induced a approximately 10-fold stimulation of -724 alpha-SMA/LUC expression. Conversely, expression of either a dominant-negative c-Myb basic domain mutant (Cys(43) --> Asp) or a c-Myb antisense RNA blocked transcription from the -724 alpha-SMA/LUC or -271 alpha-SMA/LUC in activated cells. Moreover, transfection of c-myb antisense, but not sense, RNA inhibited both expression of the endogenous alpha-SMA gene and stellate cell activation, whereas transfection of c-myb stimulated alpha-SMA expression in quiescent stellate cells. These findings suggest that c-Myb modulates the activation of stellate cells and that integrity of the redox sensor Cys(43) in c-Myb is required for this effect.

G proteins are involved in the suppression of collagen alpha 1 (I) gene expression in cultured rat hepatic stellate cells

We analyse the role of the G proteins in regulating collagen gene expression by measuring collagen alpha 1(I) mRNA levels in cultured hepatic stellate cells in basal conditions and after stimulating or inhibiting the major intracellular signalling pathways. Stimulation of Gs protein and adenylyl cyclase or the addition of 8Br-cAMP to the cells led to a decrease in collagen alpha 1(I) mRNA levels, while blocking protein kinase A abolished this effect. Blocking Gi protein, phospholipase A2 and C, calcium channels and calmodulin resulted in a significant increase in collagen mRNA levels. PKC stimulation led to a marked decrease in these levels. These results suggest that collagen gene expression is inhibited by a number of intracellular pathways. A Gs and a pertussis toxin-sensitive G protein seem to initiate cellular response. Transcription factors, acting in these pathways, must be identified. However, it seems that they do not need to be synthesised.

Iron overload and liver fibrosis

The pathogenesis of liver fibrosis in genetic haemochromatosis and other iron overload states remains enigmatic. Recent advances in the cellular and molecular pathogenesis of liver fibrosis have determined a central role for hepatic stellate cells. These become activated to a myofibroblastic phenotype following most forms of liver injury and are the major cellular source of collagens and other matrix proteins laid down in fibrotic liver. Similar changes have now been reported in the liver in genetic haemochromatosis, with activation of stellate cells becoming more prominent with increasing hepatic iron concentration. In contrast to other liver diseases, this apparently occurs in the absence of significant necroinflammatory change. Unravelling the mechanism of liver fibrogenesis in iron overload states may, therefore, provide important general insights into the pathogenesis of liver fibrosis. The present article reviews current knowledge of this field with emphasis on the role of lipid peroxidation, sideronecrosis of hepatocytes and spillover of iron to Kupffer cells. An attempt is made to draw these observations together with previous studies of the mechanisms of stellate cell activation in other models and diseases. A unifying hypothesis emerges that helps to define some of the next research questions in the pathogenic mechanisms of liver fibrosis in iron overload.

Rat hepatic lipocytes synthesize and secrete transin (stromelysin) in early primary culture

BACKGROUND & AIMS

Hepatic lipocyte proliferation and activation are pivotal in liver fibrosis. Disruption of normal lipocyte-matrix interactions may contribute to this process. The synthesis of transin, which degrades normal liver matrix, by culture-activated hepatic lipocytes was investigated.

METHODS

Lipocytes were isolated by pronase/collagenase perfusion, density gradient centrifugation, and centrifugal elutriation. Transin messenger RNA in lipocytes was analyzed by Northern blotting. Transin activity was analyzed by zymography, Western blotting, immunocytochemistry, and quantitative [14C]beta-casein degradation assay.

RESULTS

Transin messenger RNA was detected in early primary culture (3-5 days) but not in freshly isolated lipocytes or late primary culture. Zymography of lipocyte medium showed caseinolytic activity (relative molecular weight, 57 kilodaltons and 60 kilodaltons) inhibited by ethyl-enediaminetetraacetic acid but not thiol or serine protease inhibitors. Immunoblotting and immunocytochemistry confirmed the presence of transin in media and cells. Quantitative transin activity decreased progressively with increasing duration of primary lipocyte culture and myofibroblastic transformation.

CONCLUSION

Rat hepatic lipocytes express the transin gene and secrete its product during the early phase of lipocyte activation in primary culture. Because this enzyme degrades a wide spectrum of normal basement membrane proteins and activates progelatinase B and interstitial collagenase, it may have an important role in liver injury and fibrosis.

NF-I/Sp1 switch elements regulate collagen alpha 1(I) gene expression

The expression of type I collagen is regulated developmentally and tissue specifically. Two sets of binding sites for nuclear factor I (NF-I) and Sp1 transcription factors arrayed as an imperfect tandem repeat are critical for high activity of the murine alpha 1(I) collagen gene in NIH-3T3 fibroblasts and are conserved in evolution. Gel retardation analysis combined with methylation interference studies show that NF-I and Sp1 bind to overlapping sites in a mutually exclusive manner. Cotransfection studies using Drosophila Schneider L2 cells, which lack both transcription factors, demonstrate that each factor alone trans-activates the gene, while cotransfection of both factors results in the inhibition of the strong Sp1 trans-activation. In contrast, the herpes simplex virus thymidine kinase promoter, which contains functionally independent NF-I and Sp1 binding sites, is maximally transactivated by the cotransfection of both factors. Because the two NF-I/Sp1 binding sites overlap, the ratio of the activities of the two factors rather than their absolute concentrations determine alpha 1(I) gene expression, characterizing these promoter sequences as transcription factor switch elements.

Acetaldehyde selectively stimulates collagen production in cultured rat liver fat-storing cells but not in hepatocytes

Hepatocytes and fat-storing cells have been implicated in the production of collagen, under both normal and pathological conditions. In this study, short-term primary cultures of rat hepatocytes, maintained in a serum-free, hormonally defined medium without dexamethasone and cultured on a fibronectin-collagen type IV substratum, were used. Primary and passage 1 and 2 cultures of fat-storing cells maintained on tissue culture plastic were also studied. Hepatocytes produced significant amounts of collagen type III, but formation of collagen type I was not detectable. Laminin and collagen type IV production were very low. Hepatocytes maintained their ability to metabolize ethanol (at levels comparable to those observed at 2 hr) for at least 48 hr after plating and this metabolism was inhibited 86% to 95% by 4-methylpyrazole (1 mmol/L). Neither ethanol (50 mmol/L) nor acetaldehyde (175 mumol/L, initial concentration) had any effect on the production of collagen type III or laminin. Fat-storing cells (95% to 100% desmin-positive) produced significant amounts of both type I and type III collagen. Production of collagen type IV and laminin was very low. Metabolism of ethanol by these cultures was not detected. Addition of ethanol had no effect on collagen or laminin production in fat-storing cells. In contrast, acetaldehyde significantly increased the production of collagen type I, but did not alter the production of collagen type III, IV or laminin. Incorporation of 3H-proline into total protein was not affected by addition of ethanol or acetaldehyde to fat-storing cells or hepatocytes. Exposure of fat-storing cells to ethanol or acetaldehyde did not change 3H-collagen degrading activity in the media.(ABSTRACT TRUNCATED AT 250 WORDS)