TRANSFORMING GROWTH-FACTOR-BETA GENE-EXPRESSION IN NORMAL AND FIBROTIC RAT-LIVER

Background/Aims: Transforming growth factor-beta (TGF-beta) is conside red to be an important mediator in the development of fibrosis in seve ral chronic liver diseases. To understand the mechanism(s) by which TG F-beta exerts its action(s), we investigated the cellular distribution of TGE-beta(1,2,3) transcripts in normal and carbon tetrachloride (CC l4)-induced fibrotic rat liver. Methods: Parenchymal, sinusoidal endot helial, Kupffer and stellate cells were isolated and purified. The exa ct cellular composition of each isolate was determined by transmission electron microscopy. Expression of TGF-beta(1,2,3) transcripts was in vestigated using Northern hybridization analysis. Hybridization signal s were quantified by scanning densitometry and corrected for: (i) diff erences in extractable RNA per cell type, (ii) signal contribution fro m contaminating cells, and (iii) differences in loading, capillary tra nsfer and hybridization. Results: In normal liver, TGF-beta 1 mRNA was predominantly expressed in Kupffer cells, exhibiting values approxima tely 9-fold higher than those in stellate cells. No expression was fou nd in endothelial and parenchymal cells. Signals for TGF-beta(2) and T GF-beta(3) were much weaker when compared to TGF-beta(1). In Kupffer c ells, the level of TGF-beta(2) was approximately 4-fold higher than in stellate cells. Little expression was found in endothelial cells. TGF -beta(3) expression could only be detected in stellate cells. TGF-beta (2) and TGF-beta(3) was not expressed in parenchymal cells. In fibroti c liver, TGF-beta(1) mRNA was strongly expressed in all the sinusoidal cells. TGF-beta(2) and TGF-beta(3) could no longer be detected. When compared to the level of expression in normal stellate cells, the leve l of TGF-beta(1) increased 12-fold in stellate cells from fibrotic liv ers, and 6-fold in endothelial cells. In Kupffer cells, the level of e xpression remained unchanged. Conclusions: (i) In both normal and fibr otic liver, TGF-beta(1) is the most abundant isoform, (ii) in normal l iver, TGF-beta(1) is expressed strongly by Kupffer cells and moderatel y by stellate cells, TGF-beta(2) expression is highest in Kupffer cell s, followed by stellate cells and endothelial cells. TGF-beta(3) is ex pressed by stellate cells, (iii) in fibrotic liver, the level of TGF-b eta(1) expression increases selectively in stellate cells and endothel ial cells. This suggests an important role, not only for stellate, but also for endothelial cells in fibrogenesis.