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1 Medicine and Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York, United States
2 Medicine, Albert Einstein College of Medicine, Bronx, New York, United States
3 Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, New York, United States
4 Developmental Therapeutics Program, Barbara Ann Karmanos Cancer Institute, United States
5 Developmental Therapeutics Program, Barbara Ann Karmanos Cancer Institute, United States; Pharmacology, Wayne State University School of Medicine, United States
6 Medicine, Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, New York, United States
* To whom correspondence should be addressed. E-mail: igoldman{at}aecom.yu.edu.
This laboratory recently identified a human gene that encodes a novel folate transporter (HsPCFT; SLC46A1) required for intestinal folate absorption. This study focuses on mouse and rat PCFTs (MmPCFT and RnPCFT) and addresses their secondary structure, specificity, tissue expression, and regulation by dietary folates. Both rodent PCFT proteins traffic to the cell membrane with the N- and C- termini accessible to antibodies targeted to these domains only in permeabilized HeLa cells. This, together with computer-based topological analyses, is consistent with a model in which the rodent PCFT proteins likely contain twelve transmembrane domains. Transport of [3H]folates was optimal at pH 5.5, decreasing with increasing pH due to an increase in Km and decrease in Vmax. At pH 7.0, folic acid and methotrexate influx was negligible, but there was residual (6S)5-methyltetrahydrofolate transport. Uptake of folates in PCFT-injected Xenopus oocytes was electrogenic and pH-dependent. Folic acid influx Kms of MmPCFT and RnPCFT, assessed electrophysiologically, were 0.7 and 0.3 µM at pH 5.5, and 1.1 and 0.8 µM at pH 6.5, respectively. Rodent PCFTs were highly specific for monoglutamyl, but not polyglutamyl methotrexate. MmPCFT mRNA was highly expressed in duodenum, proximal jejunum, liver, and kidney with lesser expression in brain and other tissues. MmPCFT protein was localized to the apical brush-border membrane of duodenum and proximal jejunum. MmPCFT mRNA levels increased ~13 fold in the proximal small intestine in mice fed a folate-deficient vesus a folate-replete diet, consistent with the critical role that PCFT plays in intestinal folate absorption.
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V. S. Subramanian, J. C. Reidling, and H. M. Said Differentiation-dependent regulation of the intestinal folate uptake process: studies with Caco-2 cells and native mouse intestine Am J Physiol Cell Physiol, September 1, 2008; 295(3): C828 - C835. [Abstract] [Full Text] [PDF]
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J. F. Collins Novel insights into intestinal and renal folate transport. Focus on "Apical membrane targeting and trafficking of the human proton-coupled folate transporter in polarized epithelia" Am J Physiol Cell Physiol, February 1, 2008; 294(2): C381 - C382. [Full Text] [PDF]
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