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.
Footnotes
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This study was supported by the Swiss National Science Foundation [Grant 32-120463/1]; the Swiss Inflammatory Bowel Disease Cohort Study [SNF Grant 33CSC0-108792]; the Zurich Center of Integrative Human Physiology; the Center of Clinical Research at the University Hospital Zurich; and the Novartis Foundation for Biomedical Research.
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Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org.
doi:10.1124/mol.109.055392
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ABBREVIATIONS:
- PCFT
- proton-coupled folate transporter
- RFC
- reduced folate carrier
- vitamin D3
- 1,25-dihydroxyvitamin D3
- VDR
- vitamin D receptor
- VDRE
- vitamin D response element
- DR-3
- direct repeat-3
- RXRα
- retinoid X receptor-α
- PCR
- polymerase chain reaction
- EMSA
- electrophoretic mobility shift assay
- ChIP
- chromatin immunoprecipitation
- MES
- 2-(N-morpholino)ethanesulfonic acid
- HSV-TK
- herpes simplex virus thymidine kinase
- bp
- base pair(s)
- MTX
- methotrexate.
- Received February 9, 2009.
- Accepted August 7, 2009.
- © 2009 The American Society for Pharmacology and Experimental Therapeutics
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