TRANSFORMING GROWTH-FACTOR-BETA-1 INHIBITS MITOGEN-ACTIVATED PROTEIN-KINASE INDUCED BY BASIC FIBROBLAST GROWTH-FACTOR IN SMOOTH-MUSCLE CELLS

Stimulation of smooth muscle cells with basic fibroblast growth factor (bFGF) results in the activation of the mitogen-activated protein kin ase (MAP kinase) cascade and leads to cell proliferation. We show that transforming growth factor beta 1 (TGF-beta 1), at concentrations tha t completely inhibited bFGF-induced mitogenic activity, decreased bFGF -induced MAP kinase activity. Under these conditions, tyrosine and thr eonine phosphorylations of MAP kinase were differentially affected dep ending on the time period of TGF-beta 1 pretreatment. After a short (3 0 min) TGF-beta 1 pretreatment, the bFGF-mediated increase in phosphor ylation of p42(mapk) on threonine was inhibited, with no effect on the level of phosphotyrosine or decrease in the electrophoretic mobility of p42(mapk). This suggests that TGF-beta 1 inhibited MAP kinase activ ity through the action of a serine/threonine phosphatase. In contrast, a longer TGF-beta 1 pretreatment (4 h) partly inhibited the bFGF-indu ced MAP kinase mobility shift and correlated with the inhibition of ph osphorylation on both threonine and tyrosine, suggesting that long-ter m TGF-beta 1 treatment prevented activation of the MAP kinase cascade or directly blocked MAP kinase. The ability of long-term (4 h) but not short-term (30 min) TGF-beta 1 pretreatment to inhibit MAP kinase act ivity was completely dependent on protein synthesis and suggests that TGF-beta 1 inhibits MAP kinase activity by two distinct mechanisms. Th ese findings provide a molecular basis for the growth-inhibitory actio n of TGF-beta 1 on bFGF-induced mitogenic activity.