TRANSFORMING GROWTH FACTOR-BETA-1 INHIBITORY EFFECT OF PLATELET-DERIVED GROWTH-FACTOR INDUCED SIGNAL TRANSDUCTION ON HUMAN BONE-MARROW FIBROBLASTS - POSSIBLE INVOLVEMENT OF PROTEIN PHOSPHATASES

Transforming growth factor-beta-1 (TGF-beta-1) is a potent growth inhibitor for many cell types. On fibroblasts, TGF-beta-1 has been shown to inhibit human platelet-derived growth factor (PDGF)-induced mitogenicity. The mechanism implicated in this growth inhibition is unknown. In this work, we show on human bone marrow fibroblasts that TGF-beta-1, which inhibited PDGF-BB mitogenicity, was able to block PDGF-BB-induced early events such as polyphosphoinositide (PtdIns 4,5-P2, PtdIns 4-P, and PtdIns) breakdown and Ins 1,4,5-P3 formation. No significant modification by TGF-beta-1 of PDGF-BB binding (n1 = 200,000 vs. n2 = 195,000 sites per cell with TGF-beta-1; Kd1 = Kd2 = 0.5 x 10(-9)M) and of internalization kinetics was observed. In addition, TGF-beta-1 was shown to inhibit PDGF-BB receptor autophosphorylation either in intact cells or in partially isolated membranes and to partially inhibit PDGF-R tyrosine kinase activity. Since a dephosphorylation mechanism through protein phosphatases could be implicated, we used okadaic acid, a potent inhibitor of type 1 and 2A serine/threonine phosphatases and showed that okadaic acid restored PDGF-receptor autophosphorylation on tyrosine residues. Based on these data, we suggest that an alternative regulatory mechanism of PDGF tyrosine phosphorylation seems to involve serine/threonine phosphatase activation.