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 D₃ (vitamin D₃) in a dose-dependent fashion, and the duodenal rat Pcft mRNA expression is induced by vitamin D₃ ex vivo. The PCFT promoter region is transactivated by the vitamin D receptor (VDR) and its heterodimeric partner retinoid X receptor-α (RXRα) in the presence of vitamin D₃. 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:RXRα 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 D₃. In functional support of this regulatory mechanism, treatment with vitamin D₃ significantly increased the uptake of [³H]folic acid into Caco-2 cells at pH 5.5. In conclusion, vitamin D₃ and VDR increase intestinal PCFT expression, resulting in enhanced cellular folate uptake. Pharmacological treatment of patients with vitamin D₃ may have the added therapeutic benefit of enhancing the intestinal absorption of folates.