REGULATION OF PROTEIN-KINASE-C ACTIVITY BY PHORBOL ESTER, THROMBIN, PARATHYROID-HORMONE AND TRANSFORMING GROWTH FACTOR-BETA(2), IN DIFFERENT TYPES OF OSTEOBLASTIC CELLS

We investigated the role of protein kinase C (PKC) in osteoblast funct ion using a set of putative PKC modulating factors and an in situ pept ide substrate-based kinase assay in different types of osteoblastic ce lls. Primary calvarial rat osteoblastic cells (ROB) and ROS 17/2.8 ost eosarcoma cells showed an equally high PKC activity when a maximal dos e of PKC-activating phorbol ester was applied. The osteosarcoma cell l ine UMR 106-01 showed only 5-10% of this maximal PKC activity. All 3 c ell types responded to 10 U/ml thrombin with a 2-fold stimulation of P KC activity. However, no distinct direct effects of parathyroid hormon e (bPTH (1-34)) or transforming growth factor-beta(2) (TGF-beta(2)) we re found in either of the cell types. The thrombin-induced stimulation of PKC was associated with an increase in the PTH-mediated cAMP respo nse of ROB. Down-regulation of PKC-activity was found when ROB were tr eated for 24 h with phorbol ester and, interestingly, also after a 24 h treatment with bPTH (1-34) and TGF-beta(2). We conclude that differe nces in PKC activity exist among osteoblastic cell types, which may be related to their different proliferative activity. Direct PKC activat ion may lead to modulation of the cAMP signaling pathway. Down-regulat ion of PKC activity by bPTH (1-34) and TGF-beta(2) provides an interes ting possible mechanism for the long-term regulation of signal transdu ction.