Vitamin D receptor is not required for the rapid actions of 1,25-dihydroxyvitamin D₃ to increase intracellular calcium and activate protein kinase C in mouse osteoblasts

The rapid, non-genomic actions of 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] have been well described, however, the role of the nuclear vitamin D receptor (VDR) in this pathway remains unclear. To address this question, we used VDR(+/+) and VDR(-/-) osteoblasts isolated from wild-type and VDR null mice to study the increase in intracellular calcium ([Ca²⁺]_i) and activation of protein kinase C (PKC) induced by 1,25(OH)₂D₃. Within 1 min of 1,25(OH)₂D₃ (100 nM) treatment, an increase of 58 and 53 nM in [Ca²⁺]_i (n = 3) was detected in VDR(+/+) and VDR(-/-) cells, respectively. By 5 min, 1,25(OH)₂D₃ caused a 2.1- and 1.9-fold increase (n=6) in the phosphorylation of PKC substrate peptide acetylated-MBP_4-14 in VDR(+/+) and VDR(-/-) osteoblasts. The 1,25(OH)₂D₃-induced phosphorylation was abolished by GF109203X, a general PKC inhibitor, in both cell types, confirming that the secosteroid induced PKC activity. Moreover, 1,25(OH)₂D₃ treatment resulted in the same degree of translocation of PKC-α and PKC-δ, but not of PKC-ζ, from cytosol to plasma membrane in both VDR(+/+) and VDR(-/-) cells. These experiments demonstrate that the 1,25(OH)₂D₃-induced rapid increases in [Ca²⁺]_i and PKC activity are neither mediated by, nor dependent upon, a functional nuclear VDR in mouse osteoblasts. Thus, VDR is not essential for these rapid actions of 1,25(OH)₂D₃ in osteoblasts.