COMPLEMENT C4 PROTEIN EXPRESSION BY RAT HEPATIC STELLATE CELLS

Stellate cells play an important role in the production and turnover o f the normal extracellular matrix of the liver and are key effector ce lls in the hepatic fibrogenesis that occurs in response to liver injur y, In the present study, we used a rat model of long term dietary iron supplementation to identify stellate cell genes that are expressed du ring chronic hepatic iron overload, Using a subtraction cloning strate gy, we identified a rat isoform of the complement C4 protein gene whos e expression was strongly induced in stellate cells after iron overloa d, Highly purified, cultured stellate cells synthesized the C4 precurs or protein and released its subunits into the culture medium, The C4 p rotein secreted in vitro was biologically active in a C4-specific hemo lytic assay, C4 mRNA expression was minimal in freshly isolated stella te cells and increased between days 3 and 7 of primary culture, coinci dent with the expression of smooth muscle alpha-actin (alpha-SMA), a m arker of cellular activation, C4 expression was absent in strongly alp ha-SMA-positive, passaged cells, but was induced by lFN-gamma, which s imultaneously inhibited alpha-SMA expression. Our studies establish he patic stellate cells as a previously unrecognized source of C4 and rai se the possibility that complement protein expression by the cells pla ys a role in the hepatic injury response and in fibrogenesis. Our in v itro data point to the presence of two distinct stimulatory pathways f or C4 expression in stellate cells that differ with regard to their se nsitivity to IFN-gamma and their relationship to cellular activation.