Phosphorylation of the human 1,25-dihydroxyvitamin D₃ receptor by cAMP-dependent protein-kinase, In Vitro, and in transfected COS-7 cells

We report that the human 1,25-dihydroxyvitamin D₃ receptor is an efficient substrate for cAMP-dependent protein kinase, in vitro. This phosphorylation reaction is rapid and neither dependent upon nor significantly affected by the presence of the 1,25-dihydroxyvitamin D₃ ligand. Preliminary mapping experiments utilizing C-terminal truncation mutants reveal that the primary site(s) of phosphorylation, in vitro, is localized between amino acids 133 and 201. Cotransfection of the catalytic subunit of murine cAMP-dependent protein kinase and the human 1,25-dihydroxyvitamin D₃ receptor into monkey kidney (COS-7) cells not only results in a dramatic kinase-dependent increase in receptor phosphorylation but also elicits an attenuation in 1,25-dihydroxyvitamin D₃-dependent transcriptional activation of a reporter gene. These observations suggest a potential role for cAMP-dependent protein kinase in the modulation of 1,25-dihydroxyvitamin D₃ receptor-mediated gene regulation.