Mechanism of retinoid‐induced activation of latent transforming growth factor‐β in bovine endothelial cells

Abstract

Cell-associated plasmin is a putative physiological activator of latent transforming growth factor-β (LTGF-β). Since retinoids enhance the production of plasminogen activator (PA) and thereby increase cell-associated plasmin activity, we tested the possibility that retinoids might induce the activation of LTGF-β using bovine endothelial cells (ECs) as a model system. ECs treated with physiological concentrations of retinol or retinoic acid formed active TGF-β in the culture media in a dose- and time-dependent fashion. Cells were treated with 2 μM retinol for 24 h, and the amount of TGF-β produced during a subsequent 12-h incubation period was measured. Out of a total of 14 pM LTGF-β secreted, 0.7 pM was converted to active TGF-β. Northern blot analyses showed that mRNA levels for TGF-β2 but not for TGF-β1 increased in cells treated with retinol. Inclusion of either inhibitors of PA or of plasmin or antibody against PA in the culture medium as well as depletion of plasminogen from the serum blocked the formation of TGF-β, suggesting that PA, plasminogen, and the resulting plasmin are essential for activation of LTGF-β in retinoid-stimulated cells. Antibody against the LTGF-β binding protein blocked activation implying that localization of LTGF-β through its binding protein may be important. However, inhibition of binding of LTGF-β to the cell surface mannose 6-phosphate receptor did not prevent activation. These data indicate that retinoids up-regulate the production of LTGF-β in ECs and induce activation of LTGF-β, perhaps, by increasing PA and plasmin levels. Thus, TGF-β might be a local mediator of some of the biological activities of retinoids both in vivo and in vitro.