Vitamin D3 and Its Nuclear Receptor Increase the Expression and Activity of the Human Proton-Coupled Folate Transporter

Abstract

Folates are essential for nucleic acid synthesis and are particularly required in rapidly proliferating tissues, such as intestinal epithelium and hemopoietic cells. Availability of dietary folates is determined by their absorption across the intestinal epithelium, mediated by the proton-coupled folate transporter (PCFT) at the apical enterocyte membranes. Whereas transport properties of PCFT are well characterized, regulation of PCFT gene expression remains less elucidated. We have studied the mechanisms that regulate PCFT promoter activity and expression in intestine-derived cells. PCFT mRNA levels are increased in Caco-2 cells treated with 1,25-dihydroxyvitamin D(3) (vitamin D(3)) in a dose-dependent fashion, and the duodenal rat Pcft mRNA expression is induced by vitamin D(3) ex vivo. The PCFT promoter region is transactivated by the vitamin D receptor (VDR) and its heterodimeric partner retinoid X receptor-alpha (RXRalpha) in the presence of vitamin D(3). In silico analyses predicted a VDR response element (VDRE) in the PCFT promoter region -1694/-1680. DNA binding assays showed direct and specific binding of the VDR:RXRalpha heterodimer to the PCFT(-1694/-1680), and chromatin immunoprecipitations verified that this interaction occurs within living cells. Mutational promoter analyses confirmed that the PCFT(-1694/-1680) motif mediates a transcriptional response to vitamin D(3). In functional support of this regulatory mechanism, treatment with vitamin D(3) significantly increased the uptake of [(3)H]folic acid into Caco-2 cells at pH 5.5. In conclusion, vitamin D(3) and VDR increase intestinal PCFT expression, resulting in enhanced cellular folate uptake. Pharmacological treatment of patients with vitamin D(3) may have the added therapeutic benefit of enhancing the intestinal absorption of folates.