Platelet-activating factor stimulates gene expression and synthesis of matrix proteins in cultured rat and human mesangial cells: role of TGF-beta.

Platelet-activating factor (PAF) is a potent inflammatory mediator that participates in the pathogenesis of proteinuria and glomerular damage. However, the role of this lipid in glomerular sclerosis remains unknown. This study examines the effect of PAF on the regulation of extracellular matrix proteins by rat and human mesangial cells. PAF increased in a dose-dependent manner the gene expression of fibronectin and type IV collagen, but not type I collagen. Moreover, an increase in cell-associated and soluble fibronectin synthesis was also seen. These effects were abolished by BN52021 and WEB2086, two different PAF receptor antagonists. Because transforming growth factor (TGF)-beta has been considered a profibrogenic cytokine, this study also evaluated whether PAF effects might be mediated by the production of endogenous TGF-beta. PAF caused an increase in TGF-beta1 mRNA expression (by a protein kinase C-dependent pathway) and TGF-beta activity. Moreover, PAF-induced fibronectin synthesis was totally abolished when an anti-TGF-beta-neutralizing antibody was added to the culture medium, suggesting that PAF stimulates fibronectin synthesis, at least in part, through the induction of TGF-beta. Addition of cycloheximide, a protein synthesis inhibitor, upregulated PAF-induced fibronectin mRNA expression but downregulated PAF-induced TGF-beta1 gene expression, suggesting the existence of different regulatory transcriptional factors of the two proteins. These results suggest that PAF may be implicated in matrix accumulation during renal injury and therefore contribute to the pathogenesis of glomerulosclerosis.