Transforming growth factor-β gene expression in normal and fibrotic rat liver

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-β1,2,3 transcripts in normal and carbon tetrachloride (CCl4)-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-β1,2,3 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-β1 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-β2 and TGF-β3 were much weaker when compared to TGF-β1. In Kupffer cells, the level of TGF-β2 was approximately 4-fold higher than in stellate cells. Little expression was found in endothelial cells. TGF-β3 expression could only be detected in stellate cells. TGF-β2 and TGF-β3 was not expressed in parenchymal cells. In fibrotic liver, TGF-β1 mRNA was strongly expressed in all the sinusoidal cells. TGF-β2 and TGF-β3 could no longer be detected. When compared to the level of expression in normal stellate cells, the level of TGF-β1 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-β1 is the most abundant isoform, (ii) in normal liver, TGF-β1 is expressed strongly by Kupffer cells and moderately by stellate cells, TGF-β2 expression is highest in Kupffer cells, followed by stellate cells and endothelial cells. TGF-β3 is expressed by stellate cells, (iii) in fibrotic liver, the level of TGF-β1 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.