In this study, we examined the effect of activation of protein kinase C (PKC) pathways on the regulation of 1,25-dihydroxyvitamin D receptors (VDR) in rat osteosarcoma (ROS) 17/2.8 cells. Activation of PKC with phorbol 12-myristate 13-acetate (PMA) resulted in a time- and dose-dependent increase in VDR expression in ROS cells. Treatment of ROS cells with 4alpha-phorbol 12,13-dedeconate, a PKC-inactive phorbol ester, had no effect on VDR expression. Oleoyl acetyl glycerol (OAG), a synthetic diacylglycerol, stimulated VDR up-regulation in ROS cells. The PKC inhibitors (H-7, staurosporin, and sphingosine) all blocked PMA-mediated up-regulation of VDR in a dose-dependent manner. We next examined the interaction of 1,25(OH)2D3 and PKC activation by PMA on the regulation of VDR in ROS cells. We found that PMA or 1,25(OH)2D3 treatment alone resulted in a 50 and 200% increase in VDR, respectively. PMA treatment alone resulted in a 50% increase in VDR protein and a marginal 20% increase in VDR mRNA. 1,25(OH)2D3 up-regulation of VDR was associated with a 2-fold increase in VDR mRNA. In contrast, co-treatment of ROS cells with PMA and 1,25(OH)2D3 resulted in a synergistic 10-fold induction of VDR mRNA and the appearance of a 7.2 kb VDR transcript. VDR protein was also synergistically up-regulated by combined PMA and 1,25(OH)2D3 treatment of ROS cells. Scatchard analysis demonstrated that the synergistic effect of PMA and 1,25(OH)2D3 on VDR protein expression was not associated with any change in the affinity of VDR for 1,25(OH)2D3. The synergistic effect of 1,25(OH)2D3 and PMA on VDR expression supports a link between PKC signal pathways and the function of VDR.