Publications

• Functional identification of SLC5A8, a tumor suppressor down-regulated in colon cancer, as a Na(+)-coupled transporter for short-chain fatty acids.

  Miyauchi S., Gopal E., Fei Y.-J., Ganapathy V.

  SLC5A8, a tumor suppressor gene down-regulated in human colon cancer, codes for a transporter in the Na(+)/glucose cotransporter gene family, but the definitive functional identity of the transporter protein is not known. Since this gene is expressed abundantly in the colon where short-chain fatty ... >> More

  SLC5A8, a tumor suppressor gene down-regulated in human colon cancer, codes for a transporter in the Na(+)/glucose cotransporter gene family, but the definitive functional identity of the transporter protein is not known. Since this gene is expressed abundantly in the colon where short-chain fatty acids are generated by bacterial fermentation, we tested the hypothesis that it codes for a Na(+)-coupled transporter for these fatty acids. The coding region of SLC5A8 mRNA was amplified from human intestine and expressed heterologously in Xenopus laevis oocytes. Transport function was monitored by uptake of radiolabeled substrates and by substrate-induced currents under voltage-clamp conditions. Uptake of short-chain fatty acids (lactate, pyruvate, acetate, propionate, and butyrate) in oocytes expressing SLC5A8 was severalfold higher than in uninjected oocytes. Exposure of SLC5A8-expressing oocytes to these fatty acids induced inward currents under voltage-clamp conditions in a Na(+)-dependent manner. These currents were saturable and the substrate concentrations needed for half-maximal induction of the current were in the range of 0.08-2.5 mm. The substrate-induced currents decreased as the carbon chain length of the substrates increased. The Na(+)-activation kinetics indicated involvement of more than one Na(+) ion in the activation process. Direct measurements of substrate (propionate) and charge transfer showed that three positive charges are transferred into oocytes per substrate molecule. These studies establish the functional identity of SLC5A8 as a Na(+)-coupled transporter for short-chain fatty acids. << Less

  J. Biol. Chem. 279:13293-13296(2004) [PubMed] [EuropePMC]

  This publication is cited by 4 other entries.

• Expression of slc5a8 in kidney and its role in Na(+)-coupled transport of lactate.

  Gopal E., Fei Y.-J., Sugawara M., Miyauchi S., Zhuang L., Martin P., Smith S.B., Prasad P.D., Ganapathy V.

  We report here on the expression of slc5a8 in kidney and its relevance to Na(+)-coupled reabsorption of lactate. slc5a8 is the murine ortholog of SLC5A8, a candidate tumor suppressor gene, which we recently cloned from human intestine and demonstrated its functional identity as a Na(+)-coupled tra ... >> More

  We report here on the expression of slc5a8 in kidney and its relevance to Na(+)-coupled reabsorption of lactate. slc5a8 is the murine ortholog of SLC5A8, a candidate tumor suppressor gene, which we recently cloned from human intestine and demonstrated its functional identity as a Na(+)-coupled transporter for short-chain fatty acids and lactate. The slc5a8 cDNA, cloned from mouse kidney, codes for a protein consisting of 611 amino acids. When expressed heterologously in mammalian cells or Xenopus oocytes, slc5a8 mediates Na(+)-coupled electrogenic transport of lactate/pyruvate as well as short-chain fatty acids (e.g. acetate, propionate, and butyrate). The Na+/fatty acid stoichiometry varies depending on the fatty acid substrate (2:1 for lactate and 4:1 for propionate). This phenomenon of variable Na+/substrate stoichiometry depending on the fatty acid substrate is also demonstrable with human SLC5A8. In situ hybridization with sagittal sections of mouse kidney demonstrates abundant expression of the transcripts in the cortex as well as the medulla. Brush border membrane vesicles prepared from rabbit kidney are able to transport lactate in a Na(+)-coupled manner. The transport process exhibits the overshoot phenomenon, indicating uphill lactate transport in response to the transmembrane Na+ gradient. The Na(+)-coupled lactate transport in these membrane vesicles is inhibitable by short-chain fatty acids. We conclude that slc5a8 is expressed abundantly in the kidney and that it plays a role in the active reabsorption of lactate. slc5a8 is the first transporter known to be expressed in mammalian kidney that has the ability to mediate the Na(+)-coupled reabsorption of lactate. << Less

  J. Biol. Chem. 279:44522-44532(2004) [PubMed] [EuropePMC]

  This publication is cited by 4 other entries.

• Zebrafish Slc5a12 encodes an electroneutral sodium monocarboxylate transporter (SMCTn). A comparison with the electrogenic SMCT (SMCTe/Slc5a8).

  Plata C., Sussman C.R., Sindic A., Liang J.O., Mount D.B., Josephs Z.M., Chang M.-H., Romero M.F.

  We have identified and characterized two different sodium-coupled monocarboxylate cotransporters (SMCT) from zebrafish (Danio rerio), electrogenic (zSMCTe) and electroneutral (zSMCTn). zSMCTn is the 12th member of the zebrafish Slc5 gene family (zSlc5a12). Both zSMCT sequences have approximately 5 ... >> More

  We have identified and characterized two different sodium-coupled monocarboxylate cotransporters (SMCT) from zebrafish (Danio rerio), electrogenic (zSMCTe) and electroneutral (zSMCTn). zSMCTn is the 12th member of the zebrafish Slc5 gene family (zSlc5a12). Both zSMCT sequences have approximately 50% homology to human SLC5A8 (hSMCT). Transport function and kinetics were measured in Xenopus oocytes injected with zSMCT cRNAs by measurement of intracellular Na(+) concentration ([Na(+)](i)) and membrane potential. Both zSMCTs oocytes increased [Na(+)](i) with addition of monocarboxylates (MC) such as lactate, pyruvate, nicotinate, and butyrate. By using two electrode voltage clamp experiments, we measured currents elicited from zSMCTe after MC addition. MC-elicited currents from zSMCTe were similar to hSMCT currents. In contrast, we found no significant MC-elicited current in either zSMCTn or control oocytes. Kinetic data show that zSMCTe has a higher affinity for lactate, nicotinate, and pyruvate (K(m)(L-lactate) = 0.17 +/- 0.02 mM, K(m)(nicotinate) = 0.54 +/-0.12 mM at -150 mV) than zSMCTn (K(m)(L-lactate) = 1.81 +/-0.19 mM, K(m)(nicotinate) = 23.68 +/-4.88 mM). In situ hybridization showed that 1-, 3-, and 5-day-old zebrafish embryos abundantly express both zSMCTs in the brain, eyes, intestine, and kidney. Within the kidney, zSMCTn mRNA is expressed in pronephric tubules, whereas zSMCTe mRNA is more distal in pronephric ducts. zSMCTn is expressed in exocrine pancreas, but zSMCTe is not. Roles for Na(+)-coupled monocarboxylate cotransporters have not been described for the brain or eye. In summary, zSMCTe is the zebrafish SLC5A8 ortholog, and zSMCTn is a novel, electroneutral SMCT (zSlc5a12). Slc5a12 in higher vertebrates is likely responsible for the electroneutral Na(+)/lactate cotransport reported in mammalian and amphibian kidneys. << Less

  J. Biol. Chem. 282:11996-12009(2007) [PubMed] [EuropePMC]

  This publication is cited by 11 other entries.

• Establishing a definitive stoichiometry for the Na+/monocarboxylate cotransporter SMCT1.

  Coady M.J., Wallendorff B., Bourgeois F., Charron F., Lapointe J.Y.

  Several different stoichiometries have been proposed for the Na(+)/monocarboxylate cotransporter SMCT1, including variable Na(+)/substrate stoichiometry. In this work, we have definitively established an invariant 2:1 cotransport stoichiometry for SMCT1. By using two independent means of assay, we ... >> More

  Several different stoichiometries have been proposed for the Na(+)/monocarboxylate cotransporter SMCT1, including variable Na(+)/substrate stoichiometry. In this work, we have definitively established an invariant 2:1 cotransport stoichiometry for SMCT1. By using two independent means of assay, we first showed that SMCT1 exhibits a 2:1 stoichiometry for Na(+)/lactate cotransport. Radiolabel uptake experiments proved that, unlike lactate, propionic acid diffuses passively through oocyte membranes and, consequently, propionate is a poor candidate for stoichiometric determination by these methods. Although we previously determined SMCT1 stoichiometry by measuring reversal potentials, this technique produced erroneous values, because SMCT1 simultaneously mediates both an inwardly rectifying cotransport current and an outwardly rectifying anionic leak current; the leak current predominates in the range where reversal potentials are observed. We therefore employed a method that compared the effect of halving the external Na(+) concentration to the effect of halving the external substrate concentration on zero-current potentials. Both lactate and propionate were cotransported through SMCT1 using 2:1 stoichiometries. The leak current passing through the protein has a 1 osmolyte/charge stoichiometry. Identification of cotransporter stoichiometry is not always a trivial task and it can lead to a much better understanding of the transport activity mediated by the protein in question. << Less

  Biophys. J. 93:2325-2331(2007) [PubMed] [EuropePMC]

  This publication is cited by 3 other entries.