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Ganapathy
Transport of Nicotinate and Structurally Related Compounds by Human SMCT1 (SLC5A8) and Its Relevance to Drug Transport in the Mammalian Intestinal Tract.
Gopal E, Miyauchi S, Martin PM, Ananth S, Roon P, Smith SB, Ganapathy V.
Pharm Res. 2007 Jan 24; [Epub ahead of print]
[abstract only]
"PURPOSE: To examine the involvement of human SMCT1, a Na(+)-coupled transporter for short-chain fatty acids, in the transport of nicotinate/structural analogs and monocarboxylate drugs, and to analyze its expression in mouse intestinal tract.
MATERIALS AND METHODS: We expressed human SMCT1 in X. laevis oocytes and monitored its function by [(14)C]nicotinate uptake and substrate-induced inward currents. SMCT1 expression in mouse intestinal tract was examined by immunofluorescence.
RESULTS: [(14)C]Nicotinate uptake was several-fold higher in SMCT1-expressing oocytes than in water-injected oocytes. The uptake was inhibited by short-chain/medium-chain fatty acids and various structural analogs of nicotinate. Exposure of SMCT1-expressing oocytes to nicotinate induced Na(+)-dependent inward currents. Measurements of nicotinate flux and associated charge transfer into oocytes suggest a Na(+):nicotinate stoichiometry of 2:1. Monocarboxylate drugs benzoate, salicylate, and 5-aminosalicylate are also transported by human SMCT1. The transporter is expressed in the small intestine as well as colon, and the expression is restricted to the lumen-facing apical membrane of intestinal and colonic epithelial cells.
CONCLUSIONS: Human SMCT1 transports not only nicotinate and its structural analogs but also various monocarboxylate drugs. The transporter is expressed on the luminal membrane of the epithelial cells lining the intestinal tract. SMCT1 may participate in the intestinal absorption of monocarboxylate drugs."
SLC5A8 triggers tumor cell apoptosis through pyruvate-dependent inhibition of histone deacetylases.
Thangaraju M, Gopal E, Martin PM, Ananth S, Smith SB, Prasad PD, Sterneck E, Ganapathy V.
Cancer Res. 2006 Dec 15;66(24):11560-4.
[abstract only]
"Tumor cells up-regulate glycolysis but convert pyruvate into lactate instead of oxidizing it. Here, we show that pyruvate, but not lactate, is an inhibitor of histone deacetylases (HDAC) and an inducer of apoptosis in tumor cells and that SLC5A8, a Na(+)/monocarboxylate cotransporter, is obligatory for this process. We found that SLC5A8 is expressed in nontransformed breast epithelial cell lines but silenced by DNA methylation in tumor cell lines. The down-regulation of the gene is also evident in primary breast tumors. When MCF7 breast tumor cells are transfected with SLC5A8 cDNA, the cells undergo pyruvate-dependent apoptosis. Butyrate and propionate also induce apoptosis in SLC5A8-expressing cells, whereas lactate does not. The differential ability of these monocarboxylates to cause apoptosis in SLC5A8-expressing MCF7 cells correlates with their ability to inhibit HDACs. Apoptosis induced by SLC5A8/pyruvate in MCF7 cells is associated with up-regulation of p53, Bax, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), TRAIL receptor (TRAILR) 1, and TRAILR2 and down-regulation of Bcl2 and survivin. Lactate dehydrogenase isozymes are differentially expressed in nontransformed cells and tumor cells such that the latter convert pyruvate into lactate. Silencing of SLC5A8 coupled with conversion of pyruvate into lactate in tumor cells correlates with increased HDAC activity in these cells compared with nontransformed cells. Our studies thus identify pyruvate as a HDAC inhibitor and indicate that the Na(+)-coupled pyruvate transport underlies the tumor-suppressive role of SLC5A8. We propose that tumor cells silence SLC5A8 and convert pyruvate into lactate as complementary mechanisms to avoid pyruvate-induced cell death."
SLC5A8 (SMCT1)-mediated transport of butyrate forms the basis for the tumor suppressive function of the transporter.
Gupta N, Martin PM, Prasad PD, Ganapathy V.
Life Sci. 2006 Apr 18;78(21):2419-25. Epub 2005 Dec 20. Review.
"The identification of
SLC5A8 as a tumor suppressor gene in colorectal cancer marks, for the
first time, the association of a plasma membrane transporter with
tumor suppressive properties. The subsequent establishment of the
functional identity of SLC5A8 as a Na+-coupled transporter for
short-chain monocarboxylates provides a mechanism for the tumor
suppressive function of the transporter. Butyrate, a substrate for the
transporter, is a histone deacetylase inhibitor and protective against
colorectal cancer. This fatty acid is produced in the colonic lumen by
bacterial fermentation of dietary fiber. SLC5A8 mediates the
concentrative entry of butyrate from the lumen into colonocytes.
Consequently, the transport function of SLC5A8 has the ability to
influence the acetylation status of histones and hence gene expression
in colonocytes. The ability of SLC5A8 to deliver butyrate into colonic
epithelial cells most likely underlies the tumor suppressive role of
this transporter."
Biological functions of SLC5A8, a candidate tumour suppressor.
Ganapathy V, Gopal E, Miyauchi S, Prasad PD.
Biochem Soc Trans. 2005 Feb;33(Pt 1):237-40. Review.
"SLC5A8 is a candidate
tumour suppressor gene that is silenced in colon cancer, gastric
cancer and possibly other cancers in humans. This gene codes for a
transporter belonging to the Na(+)/glucose co-transporter gene family
(SLC5). The cancer-associated silencing of the gene involves
hypermethylation of CpG islands present in exon 1 of the gene. SLC5A8
is expressed in colon, ileum, kidney and thyroid gland. The protein
coded by the gene mediates the Na(+)-coupled and electrogenic
transport of a variety of monocarboxylates, including short-chain
fatty acids, lactate and nicotinate. It may also transport iodide. The
normal physiological function of this transporter in the intestinal
tract and kidney is likely to facilitate the active absorption of
short-chain fatty acids, lactate and nicotinate. One of the
short-chain fatty acids that serves as a substrate for SLC5A8 is
butyrate. This fatty acid is an inhibitor of histone deacetylases and
is known to induce apoptosis in a variety of tumours including colonic
tumour. Since butyrate is produced in the colonic lumen at high
concentrations by bacterial fermentation of dietary fibre, we
speculate that the ability of SLC5A8 to mediate the entry of this
short-chain fatty acid into colonic epithelial cells underlies the
potential tumour suppressor function of this transporter."
Expression of SLC5A8 in kidney and its role in Na(+)-coupled transport of lactate.
Gopal E, Fei YJ, Sugawara M, Miyauchi S, Zhuang L, Martin P, Smith SB, Prasad PD, Ganapathy V.
J Biol Chem. 2004 Oct 22;279(43):44522-32. Epub 2004 Aug 17.
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."