Received for publication February 18, 2009.
Revised May 19, 2009.
Accepted for publication May 21, 2009.

Involvement of vitamin D receptor in the intestinal induction of human ABCB1

Abstract

ABCB1 (P-glycoprotein) is an efflux transporter that limits the cellular uptake levels of various drugs in intestine, brain and other tissues. The expression of human ABCB1 has recently been reported to be under control of nuclear receptor NR1I subfamily members, pregnane X receptor (PXR; NR1I2) and constitutive androstane receptor (CAR; NR1I3). Here we have investigated the involvement of another NR1I member, vitamin D receptor (VDR; NR1I1), in the ABCB1 expression. In human colorectal adenocarcinoma cell line LS174T, which abundantly expresses VDR, both 1,25-dihydroxyvitamin D3 (1,25-VD3) and lithocholic acid (LCA) increased ABCB1 mRNA levels. Reporter gene assays in LS174T cells with constructs containing various length of the ABCB1 regulatory region revealed that the region containing multiple nuclear receptor binding motifs located at -7.8 kb (termed NURREM), to which PXR and CAR also bind, is essential for the VDR-mediated ABCB1 transactivation. Further reporter assays with constructs containing truncated NURREM and gel shift assays suggested simultaneous binding of multiple VDR/retinoid X receptor hetrerodimers to NURREM. Furthermore, knockdown of VDR expression in LS174T cells blocked the LCA- as well as 1,25-VD3-induced transcription of ABCB1 reporter genes. In human hepatoma HepG2 cells, in contrast to LS174T cells, 1,25-VD3 activated the ABCB1 transcription only in the presence of ectopically expressed VDR. These results suggest that the NR1I subfamily members regulate the ABCB1 expression sharing the binding sites within NURREM and that the physiologically produced LCA and 1,25-VD3 may modulate the ABCB1 expression in human intestines, possibly associated with interindividual variations of ABCB1 expression.

Key words: ABC transporters, induction, nuclear receptors, transcriptional regulation, vitamin D