Autocrine secretion of transforming growth factor-β in cultured rat mesangial cells

Abstract

Transforming growth factor-β (TGF-β) recently has been shown to modulate mesangial cell growth and to stimulate mesangial matrix synthesis by mesangial cells. Here we examined whether mesangial cells expressed TGF-β mRNA and secreted mature TGF-β, and we investigated the role of TGF-β in mesangial cell growth. Cultured rat mesangial cells expressed 2.5 kb TGF-βmRNA, and removal of fetal calf serum (FCS) for two days decreased the TGF-β mRNA level, which was then stimulated by re-addition of 17% FCS reaching the maximum at nine hours. 12-O-tetradecanoyl phorbol-13-acetate (TPA), one of the phorbol esters, markedly increased the mRNA level and reached the maximum at six or nine hours. Immunoblot analysis of the conditioned media using specific anti-TGF-β1 antibodies revealed single 12.5 kDa proteins, the size compatible with mature TGF-β subunits. By means of bioassay using CCL-64 cell line, TGF-β production rate by mesangial cells was estimated to be 22.1 ± 6.5 (mean ± SD) ng/106 cells/24 hours, 96% of which was in latent forms. Exogenously added TGF-β inhibited mesangial cell growth at 10 pM or higher. Moreover, addition of anti-TGF-β neutralizing antibodies augmented mesangial cell growth, indicating that the secreted TGF-β actually exerted a growth-inhibitory action. In summary, mesangial cells produce and secrete substantial amounts of TGF-β but mostly in latent forms, and the secreted TGF-β may regulate mesangial cell growth and differentiation. We conclude that TGF-β may function as an autocrine factor in mesangial cells.