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We recently showed that osteogenic protein-1(OP-1), a bone morphogenetic protein member of TGF-beta superfamily, induces endochondral bone formation in vivo, and stimulates growth and differentiation of osteoblasts in rat calvarial-derived cell cultures. In the present study, we examined the effect of OP-1 on cell growth and expression of markers that are characteristic of osteoblast phenotype using the clonal rat osteosarcoma cells (ROS 17/2.8). A comparison of OP-1 and TGF-beta 1 effects on cell growth showed that, both OP-1 and TGF-beta 1 inhibited DNA synthesis up to 90 percent and 60 percent of the controls at concentrations of 10 ng/ml and 1 ng/ml, respectively, in serum-free medium. In the presence of 5% serum, TGF-beta 1 did not have any significant inhibitory effects while 40 ng OP-1/ml inhibited the DNA synthesis up to 80% of the controls. Examination of collagen synthesis showed that 40 ng OP-1/ml increased the expression of type I collagen mRNA, and thus increased collagen synthesis (4-fold), as examined by collagenase-digestible protein. Evaluation of markers that are characteristic of the osteoblast phenotype demonstrated that OP-1 stimulated cAMP production in response to PTH (10-fold at 200 ng/ml), alkaline phosphatase specific activity (ALPase) (4-fold at 80 ng/ml), and osteocalcin (OC) synthesis (4.5-fold at 40 ng/ml). Northern blot analysis revealed that OP-1 increased mRNA expression for both ALPase and OC in a dose-dependent manner. These data collectively demonstrate that OP-1 suppresses cell proliferation and stimulates the expression of markers characteristic of osteoblast phenotype in rat clonal osteoblastic osteosarcoma cells (ROS 17/2.8).