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Protein kinase C (PKC) isoforms have been shown to have specific expression profiles and individual isoforms are believed to play distinct roles in the cells in which they are found. The goal here was to determine which specific isoform(s) is involved in proliferation of primary human osteoblasts. In primary human osteoblasts, 10 M of acute sphingosine-1-phosphate (S1P) treatment induced an increase in proliferation that correlated with an increase in PKC␣ and PKC expression. To further delineate which isoforms are involved in osteoblastic cell proliferation, the effect of low versus high serum culture conditions on PKC isoform expression was determined. Likewise, the effect of antisense oligodeoxynucleotides (ODNs) to specific PKC isoforms on proliferation and MAPK activation was studied. The effect of S1P on intracellular translocation of activated PKC isoforms was also evaluated. The results indicated that in primary human osteoblasts, PKC␣ was not expressed under conditions of low proliferative rate while PKC␦ and PKC expression was not affected. The specific inhibition of PKC␣ by antisense ODNs resulted in inhibition of MAPK activity leading to a significant decrease in proliferation. S1P up-regulated antisense ODN inhibited PKC␣ expression and MAPK activity and led to an increase in proliferation. Subsequent experiments using platelet-derived growth factor (PDGF) as an additional mitogen generated similar data. PDGF stimulation resulted in a significant increase in proliferation that correlated with an up-regulation of inhibited PKC␣ expression in antisense ODN-treated cells. Immunofluorescence methods showed that mitogenic stimulation of PKC␣ resulted in nuclear translocation. Our findings present original data that PKC␣ is the isoform specifically involved in the proliferation of primary human osteoblasts. (J Bone Miner Res 2002;17:1968 -1976