Protein kinase C-independent activation of protein kinase D is involved in BMP-2-induced activation of stress mitogen-activated protein kinases JNK and p38 and osteoblastic cell differentiation.

An important role for JNK* and p38 has recently been discovered in the differentiating effect of bone morphogenetic protein 2 (BMP-2) on osteoblastic cells. In this study, we investigated the molecular mechanism by which BMP-2 activates JNK and p38 in MC3T3-E1 osteoblastic cells. Activation of JNK and p38 induced by BMP-2 was blocked by the protein kinase C/protein kinase D (PKC/PKD) inhibitor Go6976 but not by the related compound, Go6983, a selective inhibitor of conventional PKCs. Associated with this inhibitory effect of Go6976, BMP-2 induced a selective and a dose-dependent Ser916 phosphorylation/activation of PKD, which was also blocked by Go6976. In contrast to the recently described PKC-dependent molecular mechanism involved in activation of PKD by G protein-coupled receptor agonists, BMP-2 did not induce a phosphorylation of PKD on Ser744/748. To further document an implication of PKD in activation of JNK and p38 induced by BMP-2, we constructed MC3T3-E1 cells stably expressing PKD antisense oligonucleotide (AS-PKD). In AS-PKD clones having low PKD levels, activation of JNK and p38 by BMP-2, but not of Smad1/5, was markedly impaired compared with empty vector transfected (V-PKD) cells. Analysis of osteoblastic cell differentiation in AS-PKD compared with V-PKD cells showed that mRNA and protein expressions of alkaline phosphatase and osteocalcin induced by BMP-2 were markedly reduced in AS-PKD. In conclusion, results presented in this study indicate that BMP-2 can induce activation of PKD in osteoblastic cells by a PKC-independent mechanism and that this kinase is involved in activation of JNK and p38 induced by BMP-2. Thus, this pathway, in addition to Smads, appears to be essential for the effect of BMP-2 on osteoblastic cell differentiation.