Mechanisms of membrane transport of folates into cells and across epithelia - PubMed (original) (raw)

Review

Mechanisms of membrane transport of folates into cells and across epithelia

Rongbao Zhao et al. Annu Rev Nutr. 2011.

Abstract

Until recently, the transport of folates into cells and across epithelia has been interpreted primarily within the context of two transporters with high affinity and specificity for folates, the reduced folate carrier and the folate receptors. However, there were discrepancies between the properties of these transporters and characteristics of folate transport in many tissues, most notably the intestinal absorption of folates, in terms of pH dependency and substrate specificity. With the recent cloning of the proton-coupled folate transporter (PCFT) and the demonstration that this transporter is mutated in hereditary folate malabsorption, an autosomal recessive disorder, the molecular basis for this low-pH transport activity is now understood. This review focuses on the properties of PCFT and briefly addresses the two other folate-specific transporters along with other facilitative and ATP-binding cassette (ABC) transporters with folate transport activities. The role of these transporters in the vectorial transport of folates across epithelia is considered.

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Figures

Figure 1

Figure 1

The pH profile of reduced folate carrier (RFC)- and proton-coupled folate transporter (PCFT)-mediated methotrexate (MTX) influx. These data were obtained from wild-type Hela cells and Hela cells in which RFC was deleted from the genome. From Reference .

Figure 2

Figure 2

How changes in pH affect folate influx kinetics parameters mediated by the proton-coupled folate transporter (PCFT). (Top panel) The pattern of changes in influx Km is compared among folic acid, methotrexate (MTX), and pemetrexed. (Bottom panel) The pattern of changes in influx Vmax for folic acid and pemetrexed. From References and .

Figure 3

Figure 3

The genomic location, organization, and topology of the proton-coupled folate transporter (PCFT) and residues that play an important role in function. The top panel is the location of PCFT on chromosome 17, the middle panel is the organization of the PCFT gene, and the lower panel is the confirmed PCFT topology. The colored components illustrate how the five exons code for different regions of the protein. Residues highlighted include Glu185 that is required for proton coupling; His281 that plays a role in proton binding and, allosterically, folate binding; His247 and Ser172 (connected by the interrupted line) that appear to be in proximity in the tertiary structure and play a role controlling substrate access to the folate binding pocket through the aqueous translocation pathway; Asp109 and Arg113 in the first intracellular loop that may be required for carrier oscillation between its inward- and outward-facing conformations; Arg376 that plays a role in folate substrate binding; and Asp156 that is important for protein stability. The topological basis for this structure was reported in References , , and .

Figure 4

Figure 4

Figure 4**A.** Folate transport across the enterocyte of the proximal small intestine. The pH at the microenvironment of the apical brush-border is 5.8–6.0. The reduced folate carrier (RFC) is expressed at the apical membrane; however, its pH optimum is 7.4. RFC does not contribute significantly to folate transport across this membrane since it cannot compensate for loss of the proton-coupled folate transporter (PCFT) function in subjects with hereditary folate malabsorption (HFM) on a normal diet. The multidrug resistance-associated protein (MRP)3 plays a role in mediating transport of folates across the basolateral membrane of the proximal small intestine. MRP1 and MRP5 are also expressed at this membrane, but their role in transport of folates has not been demonstrated. Not shown is MRP2/ATP-binding cassette (ABC)G2 along with OATPB (SLC21A9) expressed at the apical membrane. Their impact on folate transport at this site is not clear. Figure 4**B.** Folate transport across the choroid plexus. Both the proton-coupled folate transporter (PCFT) and folate receptor α(FRα) are required for transport of folates from blood to cerebrospinal fluid across choroid plexus ependymal cells, based upon studies in human subjects in whom there are loss-of-function mutations in these transporters. However, the bulk of FRα expression, and all reduced folate carrier (RFC) expression, is at the apical brush-border membrane. The pH at the basolateral membrane, where PCFT is expressed, is not known. Not shown are multidrug resistance-associated protein (MRP)1 and MRP4 that are expressed at the basolateral membrane. Their impact on the transport of folates is not clear. Figure 4**C.** Folate transport across the proximal renal tubule. Folate receptor α(FRα) is highly expressed at the apical membrane and plays an important role in the reabsorption of filtered folates. The proton-coupled folate transporter (PCFT) is also highly expressed in the kidney and appears to be expressed in the apical membrane based upon the dominant low-pH transport activity in membrane vesicles from this segment of the kidney; however, its role is not clear. Although the reduced folate carrier (RFC) is located at the basolateral membrane, and could contribute to transport into the peritubular compartment, this transporter favors transport into the cells. Not shown are multidrug resistance-associated protein (MRP)2, MRP4, and ATP-binding cassette (ABC)G2 expressed at the apical membrane, and Oat1–4 expressed at the basolateral membrane. Their contributions relative to the folate-specific transporters is not clear, although one or more of the MRPs may account for secretion of methotrexate.

Figure 4

Figure 4

Figure 4**A.** Folate transport across the enterocyte of the proximal small intestine. The pH at the microenvironment of the apical brush-border is 5.8–6.0. The reduced folate carrier (RFC) is expressed at the apical membrane; however, its pH optimum is 7.4. RFC does not contribute significantly to folate transport across this membrane since it cannot compensate for loss of the proton-coupled folate transporter (PCFT) function in subjects with hereditary folate malabsorption (HFM) on a normal diet. The multidrug resistance-associated protein (MRP)3 plays a role in mediating transport of folates across the basolateral membrane of the proximal small intestine. MRP1 and MRP5 are also expressed at this membrane, but their role in transport of folates has not been demonstrated. Not shown is MRP2/ATP-binding cassette (ABC)G2 along with OATPB (SLC21A9) expressed at the apical membrane. Their impact on folate transport at this site is not clear. Figure 4**B.** Folate transport across the choroid plexus. Both the proton-coupled folate transporter (PCFT) and folate receptor α(FRα) are required for transport of folates from blood to cerebrospinal fluid across choroid plexus ependymal cells, based upon studies in human subjects in whom there are loss-of-function mutations in these transporters. However, the bulk of FRα expression, and all reduced folate carrier (RFC) expression, is at the apical brush-border membrane. The pH at the basolateral membrane, where PCFT is expressed, is not known. Not shown are multidrug resistance-associated protein (MRP)1 and MRP4 that are expressed at the basolateral membrane. Their impact on the transport of folates is not clear. Figure 4**C.** Folate transport across the proximal renal tubule. Folate receptor α(FRα) is highly expressed at the apical membrane and plays an important role in the reabsorption of filtered folates. The proton-coupled folate transporter (PCFT) is also highly expressed in the kidney and appears to be expressed in the apical membrane based upon the dominant low-pH transport activity in membrane vesicles from this segment of the kidney; however, its role is not clear. Although the reduced folate carrier (RFC) is located at the basolateral membrane, and could contribute to transport into the peritubular compartment, this transporter favors transport into the cells. Not shown are multidrug resistance-associated protein (MRP)2, MRP4, and ATP-binding cassette (ABC)G2 expressed at the apical membrane, and Oat1–4 expressed at the basolateral membrane. Their contributions relative to the folate-specific transporters is not clear, although one or more of the MRPs may account for secretion of methotrexate.

Figure 4

Figure 4

Figure 4**A.** Folate transport across the enterocyte of the proximal small intestine. The pH at the microenvironment of the apical brush-border is 5.8–6.0. The reduced folate carrier (RFC) is expressed at the apical membrane; however, its pH optimum is 7.4. RFC does not contribute significantly to folate transport across this membrane since it cannot compensate for loss of the proton-coupled folate transporter (PCFT) function in subjects with hereditary folate malabsorption (HFM) on a normal diet. The multidrug resistance-associated protein (MRP)3 plays a role in mediating transport of folates across the basolateral membrane of the proximal small intestine. MRP1 and MRP5 are also expressed at this membrane, but their role in transport of folates has not been demonstrated. Not shown is MRP2/ATP-binding cassette (ABC)G2 along with OATPB (SLC21A9) expressed at the apical membrane. Their impact on folate transport at this site is not clear. Figure 4**B.** Folate transport across the choroid plexus. Both the proton-coupled folate transporter (PCFT) and folate receptor α(FRα) are required for transport of folates from blood to cerebrospinal fluid across choroid plexus ependymal cells, based upon studies in human subjects in whom there are loss-of-function mutations in these transporters. However, the bulk of FRα expression, and all reduced folate carrier (RFC) expression, is at the apical brush-border membrane. The pH at the basolateral membrane, where PCFT is expressed, is not known. Not shown are multidrug resistance-associated protein (MRP)1 and MRP4 that are expressed at the basolateral membrane. Their impact on the transport of folates is not clear. Figure 4**C.** Folate transport across the proximal renal tubule. Folate receptor α(FRα) is highly expressed at the apical membrane and plays an important role in the reabsorption of filtered folates. The proton-coupled folate transporter (PCFT) is also highly expressed in the kidney and appears to be expressed in the apical membrane based upon the dominant low-pH transport activity in membrane vesicles from this segment of the kidney; however, its role is not clear. Although the reduced folate carrier (RFC) is located at the basolateral membrane, and could contribute to transport into the peritubular compartment, this transporter favors transport into the cells. Not shown are multidrug resistance-associated protein (MRP)2, MRP4, and ATP-binding cassette (ABC)G2 expressed at the apical membrane, and Oat1–4 expressed at the basolateral membrane. Their contributions relative to the folate-specific transporters is not clear, although one or more of the MRPs may account for secretion of methotrexate.

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