Phosphorylation is involved in transcriptional activation by the 1,25-dihydroxyvitamin D3 receptor

The 1,25-dihydroxyvitamin D3 receptor becomes phosphorylated upon treatment with 1,25-dihydroxyvitamin D3. We have investigated the role of phosphorylation in the transcriptional activity induced by 1,25-dihydroxyvitamin D3 through its receptor. An active 1,25-dihydroxyvitamin D3-dependent transcription system was reconstituted in CV-1 cells by co-transfection of plasmids containing the rat 1,25-(OH)2D3 receptor DNA and a functional vitamin D response element (DRE) in a reporter gene construct. Treatment of these transiently transfected CV-1 cells with modulators of protein kinase A (8-Br-cAMP, PKIA and H-9) and phosphatases (Okadaic acid) resulted in mimicking or abolishing the transcriptional activity of 1,25-dihydroxyvitamin D3 in a receptor-dependent fashion. These modulators directly altered 1,25-dihydroxyvitamin D3 receptor phosphorylation. Therefore, the present results strongly suggest that phosphorylation plays a central role in the transcriptional activity of the 1,25-dihydroxyvitamin D3 receptor.