VDR-dependent transcriptional activity of vitamin D derivatives in ATDC5 cells and analysis of the effect of binding to DBP.

(A) Concentration dependence of the effects of vitamin D derivatives on VDR-dependent transcriptional activity of CYP24A1 in ATDC5 cells. When 1α,25D3 was added, CYP24A1 transcriptional activity increased in a concentration-dependent manner. Although no transcriptional activity was noted upon treatment with ALF and 25D3, ELD markedly induced transcriptional activity at 10−8 M. (B) Effects of vitamin D derivatives on VDR-dependent transcriptional activity of CYP24A1 in the presence or absence of DBP in ATDC5 cells. Although ALF was not affected by DBP, 1α,25D3 increased CYP24A1 transcriptional activity in the presence of DBP. However, ELD and 25D3 showed decreased transcriptional activity in a dose-dependent manner in the presence of DBP. (C, D) Megalin-mediated VDR-dependent transcriptional activity of CYP24A1 mediated by vitamin D derivatives in the presence of DBP in ATDC5 cells. (C) Megalin mRNA expression in si-megalin-transfected ATDC5 cells. In si-megalin transfected ATDC5 cells, megalin mRNA expression was knocked down by approximately 90%. (D) VDR-dependent transcriptional activity of CYP24A1 in megalin-knockdown ATDC5 cells. In megalin-knockdown ATDC5 cells, while 1α,25D3 and ALF had no effect, ELD and 25D3 significantly lowered CYP24A1 transcriptional activity.