Signal transduction initiated by TGFbeta1 and OP-1 was studied in MG63 human osteosarcoma cells and in normal human bone cells (HBCs) in the presence of inhibitors of signal transduction events, using insulinlike growth factor binding protein-3 (IGFBP-3) production as an end point. Treatment of serum-free MG63 cells and normal HBCs with TGFbeta1 increased IGFBP-3 protein level several fold in the conditioned medium. This effect of TGFbeta1 was mediated by increased de novo synthesis because mRNA level increased to the same extent as protein level and TGFbeta1 treatment had very little effect on IGFBP-3 protease activity. The stimulatory effect of TGFbeta1 on IGFBP-3 production was inhibited in a dose-dependent manner by pretreatment with staurosporine, a protein kinase C inhibitor, or with vanadate, a phosphotyrosyl protein phosphatase inhibitor in both MG63 cells and normal HBCs. In addition, pretreatment with okadoic acid, an inhibitor of serine/threonine protein phosphatase, counteracted TGFbeta1 induction of IGFBP-3 production. Interestingly, pretreatment of MG63 cells or HBCs with staurosporine, vanadate, or okadoic acid augmented OP-1 stimulation of IGFBP3 production. Staurosporine- or vanadate-induced changes in IGFBP-3 protein levels in the presence of TGFbeta1 and OP-1 were associated with corresponding changes in IGFBP-3 mRNA levels in MG63 cells. These findings are consistent with the hypothesis that TGFbeta1 and OP-1 increase IGFBP-3 expression via distinct intracellular signal transduction pathways.