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

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.

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.

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.

(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.