Vitamin D receptor-dependent regulation of colon multidrug resistance-associated protein 3 gene expression by bile acids

The multidrug resistance-associated protein 3 (MRP3) is a multispecific anion transporter that is capable of transporting a number of conjugated and unconjugated bile acids. Expression of the MRP3 gene is increased during pathological states associated with elevated bile acid (BA) concentrations. Thus, we hypothesized that BA activated nuclear receptors could directly regulate expression of the MRP3 gene. To test this hypothesis, the murine MRP3 promoter was cloned, characterized and tested for regulation by nuclear receptors. MRP3 mRNA levels in murine tissues exposed to bile acids revealed the highest relative expression in the colon followed in order by the liver, duodenum, jejunum, ileum, and kidney. Functional analysis of a murine MRP3 promoter reporter construct revealed vitamin D receptor (VDR)-dependent activation by 1,25-dihydroxyvitamin D3 (VD3), 9-cis-retinoic acid (RA), and the cholestatic secondary bile acid, lithocholic acid (LCA). Using a series of deletion constructs combined with sequence analysis, a candidate VDR response element (VDRE) was identified between- 1028 and -1014 bp of the MRP3 promoter. Activation of the MRP3 promoter in response to VD3, RA, or LCA, as well as binding of VDR/RXR heterodimers, was attenuated substantially by mutation of this VDRE. Treatment of mice with VD3 or LCA demonstrated in vivo modulation of the MRP3 gene in colon but not in the liver. Reduction of endogenous VDR expression in colon adenocarcinoma MCA-38 cells by siRNA transfection was associated with reduced constitutive and inducible expression of the MRP3 gene. These data support a regulatory role for the VDR in the regulation of MRP3 expression that may aid in protecting the colon from the toxicity and tumor promoting effects associated with BAs.