Effects of 1α,25-dihydroxyvitamin D3 on transport and metabolism of adefovir dipivoxil and its metabolites in Caco-2 cells

Effects of 1 alpha,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3), natural ligand of the VDR, on the fates of adefovir dipivoxil (P-gp substrate) and its metabolites, mono(POM)-PMEA and adefovir (MRP4 substrate), were investigated in Caco-2 cells. After 1,25(OH)(2)D-3-treatment, higher apical efflux of adefovir was observed after a 60 min incubation of adefovir divipoxil. Changes in these washout studies were predicted by a catenary model for the Caco-2 monolayer that described a higher MRP4 activity with 1,25(OH)(2)D-3 treatment, as confirmed by Western blotting. Moreover, 1,25(OH)(2)D-3 treatment (100 nM for 3 days) resulted in increased basolateral (B) to apical (A) (B-to-A) transport of adefovir dipivoxil but an unchanged A-to-B flux, rendering an elevated efflux ratio (EfR) (from 1.97 to 3.19). The EfR values in control and 1,25(OH)(2)D-3-treated groups in these transport studies were reduced to 1.32 and 1.57, respectively, in the presence of verapamil (50 mu M), the P-gp inhibitor. The B-to-A transport of the metabolite, adefovir, was increased in 1,25(OH)(2)D-3-treated cells in the presence of verapamil, whereas the A-to-B and B-to-A transport of mono(POM)-PMEA remained unchanged. But the verapamil and 1,25(OH)(2)D-3 treatments failed to alter rates of sequential metabolism of adefovir dipivoxil in cell lysate. The composite data established that 1,25(OH)(2)D-3 treatment increased both P-gp and MRP4 transport activities without affecting the metabolism of adefovir dipivoxil by esterases. Moreover, an asymmetric appearance of metabolites, being higher with apical application, was observed. According to the catenary model, the asymmetry is suggestive that esterases are predominantly localized on the apical membrane and within the cell. (C) 2012 Elsevier B.V. All rights reserved.