Proton-coupled folate transporter

The proton-coupled folate transporter is a protein that in humans is encoded by the SLC46A1 gene.[5][6][7] The major physiological roles of PCFTs are in mediating the intestinal absorption of folate (Vitamin B9), and its delivery to the central nervous system.

SLC46A1
Identifiers
AliasesSLC46A1, G21, HCP1, PCFT, solute carrier family 46 member 1
External IDsOMIM: 611672 MGI: 1098733 HomoloGene: 41693 GeneCards: SLC46A1
Gene location (Human)
Chr.Chromosome 17 (human)[1]
Band17q11.2Start28,394,642 bp[1]
End28,407,197 bp[1]
RNA expression pattern
More reference expression data
Orthologs
SpeciesHumanMouse
Entrez

113235

52466

Ensembl

ENSG00000076351

ENSMUSG00000020829

UniProt

Q96NT5

Q6PEM8

RefSeq (mRNA)

NM_001242366
NM_080669

NM_026740

RefSeq (protein)

NP_001229295
NP_542400

NP_081016

Location (UCSC)Chr 17: 28.39 – 28.41 MbChr 11: 78.47 – 78.47 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

Structure

PCFT is located on chromosome 17q11.2 and consists of five exons encoding a protein with 459 amino acids and a MW of ~50kDa. PCFT is highly conserved, sharing 87% identity to the mouse and rat PCFT and retaining more than 50% amino acid identity to the frog (XP415815) and zebrafish (AAH77859) proteins.[8] Structurally, there are twelve transmembrane helices with the N- and C- termini directed to the cytoplasm and a large internal loop that divides the molecule in half.[9][10] There are two glycosylation sites (N58, N68) and a disulfide bond connecting residues C66, in the 1st and C298 in the 4th, external loop. Neither glycosylation nor the disulfide bond are essential for function.[9][11] Residues have been identified that play a role in proton-coupling, proton binding, folate binding and oscillation of the carrier between its conformational states.[12] PCFT forms oligomers and some of the linking residues have been identified.[13][14]

Properties and Regulation

PCFT-mediated transport into cells is optimal at pH 5.5. The low-pH activity and the structural specificity of PCFT (high affinity for folic acid, and low affinity for PT523 - a non-polyglutamable analog of aminopterin) distinguishes this transporter functionally from the other major folate transporter, the reduced folate carrier[15] (optimal activity at pH 7.4, very low affinity for folic acid and very high affinity for PT523), another member (SLC19A1) of the superfamily of solute transporters.[8][15][16] Influx mediated by PCFT is electrogenic and can be assessed by current, cellular acidification, and radiotracer uptake.[8][16][17][18] Influx has a Km range of 0.5 to 3µM for most folates and antifolates at pH 5.5. The influx Km rises and the influx Vmax falls as the pH is increased, least so for the antifolate, pemetrexed.[19] The transporter is specific for the monoglutamyl forms of folates.[16] A variety of organic anions inhibit PCFT-mediated transport at extremely high ratio of inhibitor to folate, the most potent are sulfobromophthalein, p-aminobenzylglutamate, and sulfathalazine.[18][20] This may have pharmacological relevance in terms of the inhibitory effect of these agents on the intestinal absorption of folates. The PCFT minimal promoter has been defined[21][22] and contains an NRF1 response element.[23] There is also evidence for a role of vitamin D in the regulation of PCFT with a VDR response element upstream of the minimal promoter.[24] PCFT mRNA was reported to be increased in folate-deficient mice.[16]

Pattern of Expression

PCFT is expressed in the proximal jejunum with a lower level of expression elsewhere in the intestine.[8][16][25] Expression is localized to the apical membrane of intestinal [16][18][25] and polarized MDCK dog kidney cells.[26] PCFT is also expressed at the basolateral membrane of the choroid plexus. In view of the low levels of folate in the cerebrospinal fluid (CSF) in PCFT-null humans,[27] PCFT must play a role in transport of folates across the choroid plexus into the CSF; however, the underlying mechanism for this has not been established.[28] PCFT is expressed at the sinusoidal (basolateral) membrane of the hepatocyte, the apical brush-border membrane of the proximal tubule of the kidney, the basolateral membrane of the retinal pigment epithelium and the placenta.[9][29][30] There is a prominent low-pH folate transport activity in the cells and/or membrane vesicles derived from these tissues which, in some cases, has been shown to be indicative of a proton-coupled folate transport process.[31][32][33][34][35] However, it is unclear as to the extent that PCFT contributes to folate transport across these epithelia.

Consequences of the loss-of-function of PCFT

The physiological role of PCFT is known based upon the phenotype of subjects with loss-of-function mutations of this gene – the rare autosomal hereditary disorder, hereditary folate malabsorption (HFM).[8][27][36] These subjects have two major abnormalities: (i) severe systemic folate deficiency and (ii) a defect in the transport of folates from blood across the choroid plexus into the CSF with very low CSF folate levels even when the blood folate level is corrected or supranormal.[37] Severe anemia, usually macrocytic, always accompanies the folate deficiency. Sometimes there is pancytopenia and/or hypogammaglobulinemia and/or T-cell dysfunction which can result in infections such as Pneumocystis jirovecii pneumonia. There can be GI signs including diarrhea and mucositis. The CNS folate deficiency is associated with a variety of neurological findings including developmental delays and seizures. The phenotype of the PCFT-null mouse has been reported and mirrors many of the findings in humans.[38] PCFT was initially reported to be a low-affinity heme transporter.[25] However, a role for PCFT in heme and iron homeostasis is excluded by the observation that humans or mice with loss-of-function PCFT mutations are not iron or heme deficient and the anemia, and all other systemic consequences of the loss of this transporter, are completely corrected with high-dose oral, or low-dose, parenteral folate.[27][36]

Potential pharmacological role

Because of the Warburg effect, and a compromised blood supply, human epithelial cancers grow within an acidic milieu, as lactate is produced during anaerobic glycolysis. Because PCFT activity is optimal at low pH, and its expression and a prominent low-pH transport activity are present in human cancers,[39][40] there is interest in exploiting these properties by the development of antifolates that have a high affinity for this transporter and a very low affinity for the reduced folate carrier which delivers antifolates to normal tissues and thereby mediates the toxicity of these agents.[41] A novel class of inhibitors of one carbon incorporation into purines is being developed with these properties.[41] Pemetrexed, an antifolate inhibitor primarily of thymidylate synthase, is a good substrate for PCFT even at neutral pH as compared to other antifolates and folates.[19]

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References
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  2. GRCm38: Ensembl release 89: ENSMUSG00000020829 - Ensembl, May 2017
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  4. "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. "Entrez Gene: PCFT proton-coupled folate transporter".
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  8. Qiu, A; Jansen, M; Sakaris, A; Min, SH; Chattopadhyay, S; Tsai, E; Sandoval, C; Zhao, R; Akabas, MH; Goldman, ID (1 December 2006). "Identification of an intestinal folate transporter and the molecular basis for hereditary folate malabsorption". Cell. 127 (5): 917–28. doi:10.1016/j.cell.2006.09.041. PMID 17129779.
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  38. Salojin, KV; Cabrera, RM; Sun, W; Chang, WC; Lin, C; Duncan, L; Platt, KA; Read, R; Vogel, P; Liu, Q; Finnell, RH; Oravecz, T (5 May 2011). "A mouse model of hereditary folate malabsorption: deletion of the PCFT gene leads to systemic folate deficiency". Blood. 117 (18): 4895–904. doi:10.1182/blood-2010-04-279653. PMID 21346251.
  39. Kugel Desmoulin, S; Wang, L; Hales, E; Polin, L; White, K; Kushner, J; Stout, M; Hou, Z; Cherian, C; Gangjee, A; Matherly, LH (December 2011). "Therapeutic targeting of a novel 6-substituted pyrrolo [2,3-d]pyrimidine thienoyl antifolate to human solid tumors based on selective uptake by the proton-coupled folate transporter". Molecular Pharmacology. 80 (6): 1096–107. doi:10.1124/mol.111.073833. PMC 3228537. PMID 21940787.
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