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 voltageclamp 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.

The human orthologue of the H+‐coupled amino acid transporter (hPAT1) was cloned from the human intestinal cell line Caco‐2 and its functional characteristics evaluated in a mammalian cell heterologous expression system. The cloned hPAT1 consists of 476 amino acids and exhibits 85 % identity with rat PAT1. Among the various human tissues examined by Northern blot, PAT1 mRNA was expressed most predominantly in the intestinal tract. When expressed heterologously in mammalian cells, hPAT1 mediated the transport of α‐(methylamino)isobutyric acid (MeAIB). The cDNA‐induced transport was Na+‐independent, but was energized by an inwardly directed H+ gradient. hPAT1 interacted with glycine, l‐alanine, l‐proline, α‐aminoisobutyrate (AIB) and γ‐aminobutyrate (GABA), as evidenced from direct transport measurements and from competition experiments with MeAIB as a transport substrate. hPAT1 also recognized the d‐isomers of alanine and proline. With serine and cysteine, though the l‐isomers did not interact with hPAT1 to any significant extent, the corresponding d‐isomers were recognized as substrates. With proline and alanine, the affinity was similar for l‐ and d‐isomers. However, with cysteine and serine, the d‐isomers showed 6‐ to 8‐fold higher affinity for hPAT1 than the corresponding l‐isomers. These functional characteristics of hPAT1 closely resemble those that have been described previously for the H+‐coupled amino acid transport system in Caco‐2 cells. Furthermore, there was a high degree of correlation (r2= 0.93) between the relative potencies of various amino acids to inhibit the H+‐coupled MeAIB transport measured with native Caco‐2 cells and with hPAT1 in the heterologous expression system. Immunolocalization studies showed that PAT1 was expressed exclusively in the apical membrane of Caco‐2 cells. These data suggest that hPAT1 is responsible for the H+‐coupled amino acid transport expressed in the apical membrane of Caco‐2 cells.

Abstract. SLC5A8 and SLC5A12 are sodium-coupled monocarboxylate transporters (SMCTs), the former being a high-affinity type and the latter a low-affinity type. Both transport a variety of monocarboxylates in a Na + -coupled manner. They are expressed in the gastrointestinal tract, kidney, thyroid, brain, and retina. SLC5A8 is localized to the apical membrane of epithelial cells lining the intestinal tract and proximal tubule. In the brain and retina, its expression is restricted to neurons and the retinal pigment epithelium. The physiologic functions of SLC5A8 include absorption of short-chain fatty acids in the colon and small intestine, reabsorption of lactate and pyruvate in the kidney, and cellular uptake of lactate and ketone bodies in neurons. It also transports the B-complex vitamin nicotinate. SLC5A12 is also localized to the apical membrane of epithelial cells lining the intestinal tract and proximal tubule. In the brain and retina, its expression is restricted to astrocytes and Müller cells. SLC5A8 also functions as a tumor suppressor; its expression is silenced in tumors of colon, thyroid, stomach, kidney, and brain. The tumor-suppressive function is related to its ability to mediate concentrative uptake of butyrate, propionate, and pyruvate, all of which are inhibitors of histone deacetylases. SLC5A8 can also transport a variety of pharmacologically relevant monocarboxylates, including salicylates, benzoate, and g-hydroxybutyrate. Non-steroidal anti-inflammatory drugs such as ibuprofen, ketoprofen, and fenoprofen, also interact with SLC5A8. These drugs are not transportable substrates for SLC5A8, but instead function as blockers of the transporter. Relatively less is known on the role of SLC5A12 in drug transport.

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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.L-Lactate is present in blood at a concentration of ϳ90 mg/ liter (ϳ1.5 mM), but the urinary excretion of L-lactate is very low (100 -600 mg/day). With the normal glomerular filtration rate of 120 ml/min, the fractional reabsorption rate for L-lactate in mammalian kidney is Ͼ95% (1). The molecular identity of the transport system that is responsible for such an effective absorption process in the kidney has not yet been established. L-Lactate transport across mammalian cell plasma membrane is mediated by monocarboxylate transporters (MCTs) 1 (2, 3).MCTs are H ϩ -coupled transporters and, therefore, the direction of lactate flux in mammalian cells depends on the net chemical gradients for H ϩ and lactate across the membrane. The transport process is electroneutral because of the H ϩ / lactate stoichiometry of 1:1. There are several members within the MCT gene family that are expressed differentially in different tissues (2, 3). Many of the MCT gene family members are expressed in the kidney (2-5), but the exact locations of these transporters in terms of cell type and the apical membrane versus the basolateral membrane of the tubular cells are not known. Because the lumen-facing brush border membrane of the renal tubular cells mediates the first step in the reabsorption of solutes present in the glomerular filtrate, studies have been carr...