Regulation of protein kinase C activity by phorbol ester, thrombin, parathyroid hormone and transforming growth factor-β2 in different types of osteoblastic cells

We investigated the role of protein kinase C (PKC) in osteoblast function using a set of putative PKC modulating factors and an in situ peptide substrate-based kinase assay in different types of osteoblastic cells. Primary calvarial rat osteoblastic cells (ROB) and ROS 17/2.8 osteosarcoma cells showed an equally high PKC activity when a maximal dose of PKC-activating phorbol ester was applied. The osteosarcoma cell line UMR 106-01 showed only 5–10% of this maximal PKC activity. All 3 cell types responded to 10 U/ml thrombin with a 2-fold stimulation of PKC activity. However, no distinct direct effects of parathyroid hormone (bPTH (1–34)) or transforming growth factor-β₂ (TGF-β₂) were found in either of the cell types. The thrombin-induced stimulation of PKC was associated with an increase in the PTH-mediated cAMP response of ROB. Down-regulation of PKC-activity was found when ROB were treated for 24 h with phorbol ester and, interestingly, also after a 24 h treatment with bPTH (1–34) and TGF-β₂. We conclude that differences in PKC activity exist among osteoblastic cell types, which may be related to their different proliferative activity. Direct PKC activation may lead to modulation of the cAMP signaling pathway. Down-regulation of PKC activity by bPTH (1–34) and TGF-β₂ provides an interesting possible mechanism for the long-term regulation of signal transduction.