We have reported previously that parathyroid hormone (PTH) acts on cultured bone cells to stimulate creatine kinase (CK) activity and [3H]thymidine incorporation into DNA via phosphoinositide turnover, in addition to its other actions via increased cyclic AMP production. We also found that mid-region fragments of PTH stimulate [3H]thymidine incorporation into avian chondrocytes. In the present study of mammalian systems, we demonstrate differential effects of defined synthetic PTH fragments on CK activity and DNA synthesis, as compared with cyclic AMP production, in osteoblast-enriched embryonic rat calvaria cell cultures, in an osteoblast-like clone of rat osteosarcoma cells (ROS 17/2.8) and in chondroblasts from rat epiphysial cartilage cell cultures. Unlike full-length bovine (b)PTH-(1-84) or the fully effective shorter fragment human (h)PTH-(1-34), fragments lacking the N-terminal region of the hormone did not increase cyclic AMP formation, whereas they did stimulate increases in both DNA synthesis and CK activity. Moreover, the PTH fragment hPTH-(28-48) at 10 microM inhibited the increase in cyclic AMP caused by 10 nM-bPTH-(1-84). The increase of CK activity in ROS 17/2.8 cells caused by bPTH-(1-84) or hPTH-(28-48) was completely inhibited by either cycloheximide or actinomycin D, as was shown previously for rat calvaria cell cultures. These results indicated the presence of a functional domain of PTH in the central part of the molecule which exerts its mitogenic-related effects on osteoblast- and chondroblast-like cells in a cyclic AMP-independent manner. Since cyclic AMP formation by PTH leads to bone resorption, specific mid-region fragments of PTH might prove suitable for use in vivo to induce bone formation without concomitant resorption.

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