Transforming growth factor-beta (TGF-beta) 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-beta mRNA and secreted mature TGF-beta, and we investigated the role of TGF-beta in mesangial cell growth. Cultured rat mesangial cells expressed 2.5 kb TGF-beta mRNA, and removal of fetal calf serum (FCS) for two days decreased the TGF-beta 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-beta 1 antibodies revealed single 12.5 kDa proteins, the size compatible with mature TGF-beta subunits. By means of bioassay using CCL-64 cell line, TGF-beta production rate by mesangial cells was estimated to be 22.1 +/- 6.5 (mean +/- SD) ng/10(6) cells/24 hours, 96% of which was in latent forms. Exogenously added TGF-beta inhibited mesangial cell growth at 10 pM or higher. Moreover, addition of anti-TGF-beta neutralizing antibodies augmented mesangial cell growth, indicating that the secreted TGF-beta actually exerted a growth-inhibitory action. In summary, mesangial cells produce and secrete substantial amounts of TGF-beta but mostly in latent forms, and the secreted TGF-beta may regulate mesangial cell growth and differentiation. We conclude that TGF-beta may function as an autocrine factor in mesangial cells.