Protein kinase D mediates the synergistic effects of BMP-7 and IGF-I on osteoblastic cell differentiation

We previously showed that exogenous insulin-like growth factor-I (IGF-I) and bone morphogenetic protein-7 (BMP-7) synergistically stimulated osteoblast differentiation in fetal rat calvaria (FRC) cells. We have now shown that BMP-7 alone and the BMP-7 and IGF-I combination synergistically stimulated protein kinase D (PKD) phosphorylation at Ser744/748 and Ser916. Transfection of FRC cells with a constitutively active PKD stimulated marker expression, while transfection with a catalytically inactive PKD did not. Moreover, G#x00F6;6976, which inhibits protein kinase C (PKC)#x03B1; and#x03B2;1, blocked PKD phosphorylation and the synergistic action of the BMP-7 and IGF-I combination on osteoblast differentiation, whereas G#x00F6;6983, which inhibits PKC#x03B1;,#x03B2;,#x03B3;,#x03B4;, and#x03B6;, did not. Our results suggest that the FRC cell differentiation induced by BMP-7 and the BMP-7 and IGF-I combination requires stimulation of PKD activity. Our results are consistent with a novel mechanism in which combined BMP-7 and IGF-I signaling activates upstream novel PKC(s), which then phosphorylates and activates PKD, leading to enhanced osteoblast differentiation.