Parathyroid hormone-related protein (PTHrP) is produced by osteoblasts (OB), but its role in bone cell growth and differentiation is not completely understood. The C-terminal 107-111 sequence of PTHrP (osteostatin) is a potent inhibitor of osteoclastic bone resorption. The aim of our study was to evaluate the effect of osteostatin on the proliferation of human (h)OB in primary culture, and its action on hOB alkaline phosphatase activity, and on osteocalcin and procollagen secretion by these cells. We compared these effects with those induced by bovine (b)PTH (1-34) and [Tyr 3 6 ]hPTHrP (1-36).Human OB, isolated from trabecular bone from patients undergoing knee surgery, were grown in DME/20% fetal calf serum (FCS) until cell confluence 4-6 weeks). Then, hOB (10 4 cells/cm 2 ) were plated, and agonists were added every 3 days for 13 days. Agonists' mitogenicity was assessed by cell counting every 3 days, and by [ 3 H]thymidine (2 μCi/ml) incorporation for the last 24h. Osteocalcin (OC, Nichols), and C-terminal type I collagen propeptide (PICP, Metra) were measured in the cell conditioned media at day 13 after seeding. Alkaline phosphatase (AP) activity was measured in cellular extracts at the same time period.Immunocytochemistry, using antibodies against N-and C-terminal epitopes of PTHrP, showed cytoplasmic staining in hOB with both antibodies. Osteostatin (10 - 8 -10 - 1 4 M) consistently inhibited [ 3 H]thymidine incorporation and growth in hOB by 30%. Human PTHrP (107-139), bPTH (1-34), and [Tyr 3 6 ]hPTHrP (1-36) were also found to be inhibitory to these cells, in the same concentration range. In addition, the effects of osteostatin and [Tyr 3 6 ]hPTHrP (1-36) on hOB growth were not additive. OC production by hOB was unaffected by 10 - 6 -10 - 1 0 M of the aforementioned peptides. In contrast, osteostatin (10 - 8 -10 - 1 1 M) inhibited PICP secretion and cellular AP activity in hOB by 40%.Our results indicate that a C-terminal fragment of PTHrP which is an inhibitor of osteoclast-dependent bone resorption also decreases hOB growth and differentiation. Our data point to a regulatory role of PTHrP synthesized by hOB in bone turnover.