TPA decreases 1,25(OH)2D3 binding and calbindin D-28K in renal (MDBK) cells

The effect of the phorbol ester TPA (12-O-tetradecanoylphorbol 13-acetate) on vitamin D receptors (VDRs) was studied in MDBK cells, a normal bovine renal epithelial cell line. 24 h treatment of MDBK cells with TPA resulted in down-regulation of VDR number, with no change in the binding affinity for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) or approximate molecular weight determined by fast protein liquid chromatography (FPLC). TPA treatment also reduced the level of calbindin D-28K, a vitamin D-dependent renal protein. 4 alpha-Phorbol 12,13-didecanoate (4 alpha-PDD), an inactive phorbol ester, did not affect either 1,25(OH)2D3 binding or calbindin D-28K levels. TPA elicited a significant decrease in membrane-associated protein kinase C (PKC) activity which coincided with the reduction in VDR number and calbindin D-28K. These data support a link between TPA, PKC activity and vitamin D actions in kidney.