Short chain fatty acids (SCFAs) are produced in the colonic lumen by bacterial fermentation of dietary fiber. SCFAs serve a key role as the metabolic fuel for the colonocytes and are important for the integrity of colonic epithelium. A major consequence of reduction in intracellular SCFA oxidation results in metabolic starvation and mucosal atrophy. In fact, local starving of the colon plays a critical role in the etiology of inflammatory bowel diseases. This warrants the need for a better understanding of the regulation of their absorption. Over the last 15 years our group has been involved in understanding the membrane and molecular basis of SCFA absorption in the human intestine. Studies during the previous funding cycles have demonstrated critical role of the monocarboxylate transporter 1 (MCT1) in SCFA absorption and has defined the mechanisms of its regulation. Our extensive preliminary studies, in the current application, showed a role of a novel nutrient- sensing mechanism involving GPR109a (a G-protein coupled SCFA receptor in human colonocyte apical membranes) and leading to translocation of MCT1 to the apical membrane domains in response to high levels of luminal SCFAs. Our preliminary data utilizing polarized C2BBE cells showed that acute treatment with butyrate (10 mM, 60-120 min) increased MCT1 function and association with CD147 (the ancillary chaperon protein), enhanced apical surface MCT1 level, and decreased intracellular cAMP levels. Further, MCT1 function was reduced by cAMP agonists forskolin and dibutyryl cAMP and enhanced by nicotinic acid (a GPR109A agonist). Ex vivo studies in normal and soluble fiber pectin fed rats also showed evidence for nutrient sensing. In fiber fed rats, the MCT1 expression was increased along with a marked increase in its translocation to the apical membranes. Therefore, we propose a novel hypothesis that the activation of GPR109A by high levels of luminal SCFAs represents a nutrient-sensing mechanism that increases the level of MCT1 on the apical membranes and involves distinct signaling and trafficking pathways. We will test this hypothesis by systematic set of approaches utilizing both in vitro and in vivo models and state-of- the-art imaging and advanced molecular biology techniques. Studies proposed in Specific Aim 1 will investigate the mechanisms of acute regulation of MCT1 by SCFAs in model human intestinal C2BBE and rat intestinal IEC-6 cell monolayers;
Specific Aim 2 will focus on elucidating mechanisms of substrate-induced trafficking of MCT1 in polarized C2BBE/ IEC-6 cells; and studies in Specific Aim 3 will examine the effects of short-term as well as long-term SCFAs on MCT1 function and membrane targeting mechanisms in in vivo rat models. These studies should enhance our understanding of cellular pathways upregulating MCT1 function in response to substrates and provide the basis for its variable membrane localization. In addition, given that MCT1 is down-regulated in inflammation, defining the molecular mechanisms involved in the up-regulation of MCT1 as proposed here could identify novel targets for the treatment of intestinal inflammatory disorders.
Fiber in our diet is broken down into short chain fatty acids (SCFAs) by normal bacteria in the large intestine (colon). These short chain fatty acids serve as important nutrients for maintaining the health of the colonic wall. Deficiency of these nutrients has been shown to result in local starvation of the colonic wall and contributes to the development of inflammatory bowel diseases. The studies proposed in this application will test a novel hypothesis that SCFAs can regulate the targeting of the SCFA transporter Mono Carboxylate Transporter-1 via distinct trafficking mechanisms. These studies may identify novel targets for the treatment of inflammatory bowel diseases.
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