I. David Goldman, M.D.

Regulation, structure/function, physiological and pharmacological roles of a proton-coupled folate transporter and the genetics of hereditary folate malabsorption

This laboratory recently cloned a novel proton-coupled folate transporter (PCFT- SLC46A1), required for the intestinal absorption of folates, and demonstrated that PCFT is mutated in the autosomal recessive disorder hereditary folate malabsorption (HFM) (Cell, 127:917-928, 2006; Blood, 110: 1147-1152, 2007). Since folates are key one-carbon donors essential for DNA and RNA synthesis and methylation reactions, understanding the properties, role and regulation of this transporter is of considerable physiological relevance.

Structure-function studies of PCFT are geared towards identifying residues required for the maintenance of tertiary structure, the translocation pathway, folate and proton binding, and oscillation of the carrier's alternative conformational states. These studies are informed by analysis of PCFT mutations in patients with HFM, by site-directed mutagenesis and applications of substituted cysteine scanning mutagenesis.  Transport studies employ both electrophysiological measurements in Xenopus oocytes and radiolabeled folate flux determinations in cell lines.  The tertiary PCFT structure is being developed using homology modeling based upon bacterial facilitative transporter structures (J Biol Chem, 284:17846-17857, 2009; Am J Physiol Cell Physiol, 297:C66-74, 2009).   PCFT is also required for transport of folates into the brain, essential for neural development in infancy. Studies are exploring the role PCFT plays at the vascular blood-brain-barrier and at the choroid plexus.

PCFT is a member of one of the seven families of proton-coupled transporters that operate most efficiently at low-pH. These transporters are all required for the absorption of their substrates (peptides, amino acids, monocarboxylic acids, divalent metal ions, etc) in the acidic microclimate of the proximal small intestine. Proton-coupled transporters also mediate export of their substrates from acidified endosomes during receptor-mediated endocytosis. This is also the case for PCFT in a process under study in this laboratory (J Biol Chem, 284:4267-74, 2009). Other studies are exploring regulation of PCFT and the impact of methylation on the expression of this gene (Mol Cancer Ther, 8: 2424-31, 2009).

As families are identified world-wide with HFM, and studied in this laboratory, the genetics of this disorder and its manifestations are being characterized along with genotype-phenotype relationships.

Finally, structural analogs of folates are employed for the treatment of cancer and autoimmune diseases. Membrane transport is a key determinant of the effectiveness of these drugs and impaired transport is an important element in drug resistance. A new-generation antifolate, pemetrexed, approved for the treatment of lung cancer and mesothelioma, has a high affinity for PCFT, and this transporter preserves the activity of this drug in the absence of all other folate transporters. Studies are exploring the role of PCFT in the activity of this and other antifolate agents (Mol Pharm, 74:854-862, 2008). For additional information on publications from this laboratory go to PubMed and search: Goldman id.