Histochemical demonstration of a Na+-coupled transporter for short-chain fatty acids (Slc5a8) in the intestine and kidney of the mouse

Abstract

Short-chain fatty acids in the intestinal lumen affect colonic cell proliferation as well as function as an energy source for intestinal epithelial cells. A novel transporter of monocarboxylates, Slc5a8, is expressed abundantly in the colon, where it may participate in the Na + -coupled absorption of short-chain fatty acids produced by bacterial fermentation of dietary fiber. The present study examined the cellular localization of Slc5a8 in the murine gastrointestinal tract and kidney by in situ hybridization and immunohistochemistry. The hybridization signals were recognized in the terminal ileum and whole length of the large intestine, and were especially intense in the distal colon and rectum. The immunoreactivity of Slc5a8 was restricted to the striated border (the brush border) of enterocytes, and was not present in goblet cells, Paneth cells, or lamina propria cells. In the kidney, proximal tubules of both the cortex and the outer stripe of the outer medulla intensely expressed Slc5a8 mRNA, while the distal portions, including the loop of Henle, lacked the signals. The renal Slc5a8 immunoreactivity was localized only in the brush border of proximal tubules, not along the basolateral membrane. Thyroid follicular cells were immunoreactive for Slc5a8, with predominant labeling on the apical membrane. No other organs, including the esophagus, stomach, liver, pancreas, and salivary glands contained any notable signals of Slc5a8. These findings on the cellular and subcellular localization of Slc5a8 under normal conditions are helpful for understanding the physiological and pathological roles of Slc5a8.Short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate are generated abundantly in the large intestine by bacterial fermentation of dietary fiber and unabsorbed carbohydrates, and intestinal epithelia derive 60-70% of their energy supply from SCFAs, particularly butyrate (16,19,20). It has also been shown that SCFAs prevent colonic cell proliferation and reduce the incidence of colon cancer (4,5,21). Slc5A8, a tumor suppressor gene down-regulated in colon cancer, codes for a Na + -coupled transporter for SCFAs and other monocarboxylates such as lactate and pyruvate (6,9,15). Transfection of the Slc5a8 gene into Xenopus laevis oocytes increased the uptake of these monocarboxylates Na + -dependently, the rates of increase being especially high for lactate, pyruvate, and propionate (15). The comparative affinities of SCFAs for Slc5a8 were found to be in the following order: butyrate > propionate >> acetate (15). Similarly, the substrate-induced current in Slc5a8-expressing oocytes was markedly smaller upon superfusion with formic

Cellular expression of monocarboxylate transporters (MCT) in the digestive tract of the mouse, rat, and humans, with special reference to slc5a8

“…In contrast, the expression of slc5a8 mRNA was very intense in the distal colon, in which the signals gathered in the middle region of crypts (Fig. 3d), as reported previously (28). Signals for MCT2 mRNA were restricted to the parietal cell region in the acid-secreting area of the stomach (Fig.…”

“…1f). As reported previously (28), transcripts of slc5a8 were found throughout the large intestine, with the most intense expression in the distal colon, followed by the cecum. Noteworthily, the expression levels in the proximal half of colo-rectum were lower compared with those in the distal half.…”

“…The down-regulation of slc5a8 expression in a variety of cancers including colon cancer suggests that this transporter may have a tumor suppressive role (18,23); this can explain partially the anti-tumourigenic effects of butyrate. In a previous study (28), we revealed the selective localization of slc5a8 on the brush border in the terminal ileum and large intestine of mice. Therefore, a comparative expression of slc5a8 and MCT, especially the topographical relationship of the two transporters, is important for a good understanding of a transporting system of SCFA produced by the luminal fermentation.…”

“…They were pretreated both with 0.3% Triton X-100-containing PBS (pH 7.2) to enhance the penetration of antibodies and with 0.03% H 2 O 2 in methanol to block endogeneous peroxidase activities. After preincubation with normal goat serum, the sections were incu bated overnight with chicken anti-MCT1 antibody (AB1286, Chemicon International, Temecula, CA; diluted 2,000 fold), rabbit anti-MCT1 antibody (Biogenesis, Poole, UK; diluted 1, 600 fold), or a rabbit anti-human SLC5A8 serum (RY1617; diluted 6,000 fold) (28). They were then incubated with biotinylated goat anti-chicken IgY (Santa Crruz Biotechnology, Santa Cruz, CA) or goat anti-rabbit immunoglobulins (Nichirei, Tokyo, Japan) followed by incubation with the avidin-peroxidase complex (Vestastain ABC kit; Vector, Burlimgame, CA), each for 1 h. The antigen-antibody reaction was visualized by incubation in 0.05 M Tris-HCl buffer (pH 7.6) containing 0.01% 3,3'-diaminobenzidine and 0.001% H 2 O 2 .…”

“…Noteworthily, the expression levels in the proximal half of colo-rectum were lower compared with those in the distal half. Another site showing an intense expression of slc5a8 mRNA was the terminal ileum, where the expression was restricted to the distal 10 cm segment of the ileum (28).…”