Role for Transforming Growth Factor β in Thromboxane-Induced Increases in Mesangial Cell Fibronectin Synthesis

Thromboxane A₂ (TXA₂) has been implicated in the pathogenesis of progressive glomerulosclerosis and stimulates the synthesis of matrix protein by mesangial cells (MCs). This study examined the role of transforming growth factor β (TGF-β) in the mediation of the action of the stable TXA₂/prostaglandin (PG) endoperoxide analog U-46619 to stimulate fibronectin (Fn) synthesis in cultured rat MC. Exogenous TGF-β increased Fn synthesis by MC in a concentration- and time-dependent fashion, as reflected by incorporation of [³⁵S]methionine into immunoprecipitable Fn. Submaximal concentrations of TGF-β (1–2.5 pmol/l) increased Fn synthesis two- to threefold, a response comparable in magnitude to that observed with a maximal stimulatory concentration (1 μmol/l) of U-46619. Anti-TGF-β antibody, but not isotypic IgG, blocked the increases in Fn synthesis induced by both U-46619 and exogenous TGF-β. Endogenous TGF-β bioactivity in MC culture media, assessed by the mink lung epithelial cell system, was significantly increased by 1 μmol/l U-46619 (1.7 ± 0.3 pmol/l) compared with that of control media (0.6 ± 0.1 pmol/l, P < 0.05). Total (active plus latent) TGF-β bioactivity, assayed after heat activation of latent TGF-β, was also significantly higher in media of MCs cultured with U-46619 (45 ± 4 pmol/l) compared with control (24 ± 4 pmol/l). Thus, U-46619 increased endogenous TGF-β bioactivity to a level sufficient to account for the enhancement of Fn synthesis observed with U-46619, as reflected by the Fn synthetic response to exogenous TGF-β. This finding, together with the inhibitory effect of anti-TGF-β antibody on Fn synthesis, implicates TGF-β in expression of this action of U-46619.