Background/aims: Transforming growth factor-beta (TGF-beta) 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-beta exerts its action(s), we investigated the cellular distribution of TGF-beta(1,2,3) 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-beta(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-beta1 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-beta2 and TGF-beta3 were much weaker when compared to TGF-beta1. In Kupffer cells, the level of TGF-beta2 was approximately 4-fold higher than in stellate cells. Little expression was found in endothelial cells. TGF-beta3 expression could only be detected in stellate cells. TGF-beta2 and TGF-beta3 was not expressed in parenchymal cells. In fibrotic liver, TGF-beta1 mRNA was strongly expressed in all the sinusoidal cells. TGF-beta2 and TGF-beta3 could no longer be detected. When compared to the level of expression in normal stellate cells, the level of TGF-beta1 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-beta1 is the most abundant isoform, (ii) in normal liver, TGF-beta1 is expressed strongly by Kupffer cells and moderately by stellate cells, TGF-beta2 expression is highest in Kupffer cells, followed by stellate cells and endothelial cells. TGF-beta3 is expressed by stellate cells, (iii) in fibrotic liver, the level of TGF-beta1 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.