Abstract 2037: The butyrate transporter SLC5A8 is a tumor suppressor in colon linked to dietary fiber content
Authors: Ganapathy Vadivel , Gurav Ashish , Singh Nagendra 
Published: August 2015

Abstract
Dietary fiber has long been known to protect against colonic inflammation and colon cancer. Short-chain fatty acids (SCFA) such as butyrate generated in colonic lumen by bacterial fermentation of dietary fiber mediate most of these protective effects, but the underlying molecular mechanisms are poorly understood. We examined the role of the plasma membrane transporter Slc5a8 in the beneficial effects of these bacterial metabolites against colitis and colon cancer. Slc5a8 is expressed on the luminal membrane of colonic epithelial cells and also in mucosal immune cells including the antigen-presenting dendritic cells. The transporter mediates the entry of SCFAs into colonic epithelium from the lumen and also into dendritic cells in the lamina propria. We interrogated the role of the transporter in colon by comparing the function of dendritic cells and progression of experimentally induced colitis and colon cancer between wild type mice and Slc5a8-null mice. Since the amount of SCFAs generated in colonic lumen depends on the fiber content in the diet, we used two different diets, one with optimal fiber content (FC diet) and the other with no fiber (FF diet). These studies have shown that Slc5a8 is obligatory for butyrate-dependent inhibition of histone deacetylases in colonic epithelium and dendritic cells and also for the maintenance of the intestinal barrier function; but this obligatory need for the transporter is evident only when the mice are fed FF diet. The transporter is dispensable with FC diet. Compared to wild type mice, Slc5a8-null mice exhibit increased susceptibility to dextran sulfate sodium-induced colitis and azoxymethane/dextran sulfate sodium-induced colon cancer, again only with FF diet and not with FC diet. Butyrate induces the immunosuppressive enzymes indoleamine dioxygenase 1 (IDO1) and aldehyde dehydrogenase 1A2 (Aldh1A2) in dendritic cells in an Slc5a8-dependent manner. This is corroborated by the decreased expression of these two enzymes in the colon of germ-free mice compared to conventional mice. IDO1 and Aldh1A2 play a critical role in dendritic cells to maintain an immunosuppressive phenotype. Butyrate, which induces the expression of both enzymes, promotes the ability of dendritic cells to convert naive T cells into FoxP3-positive immunosuppressive Tregs and also their ability to suppress interferon-gamma-secreting pro-inflammatory T cells. As such, Slc5a8-null mice have decreased Tregs and increased IFN-gamma-positive T cells in colon. We conclude that Slc5a8 may be dispensable in colon under dietary conditions associated with high fiber content when the luminal concentrations of SCFAs are high, but the transporter is indispensable for protection against colonic inflammation and colon cancer with a diet containing suboptimal fiber which results in decreased concentrations of SCFAs in the lumen. Thus, Slc5a8 is a conditional tumor suppressor in colon linked to dietary fiber content.
Citation Format: Vadivel Ganapathy, Ashish Gurav, Nagendra Singh. The butyrate transporter SLC5A8 is a tumor suppressor in colon linked to dietary fiber content. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2037. doi:10.1158/1538-7445.AM2015-2037