[Frontiers In Bioscience, Landmark, 23, 1721-1739, March 1, 2018] Glucose and amino acid in enterocyte: absorption, metabolism and maturation Cancan Chen1, Yulong Yin1,2, Qiang Tu3, Huansheng Yang1 1Animal Nutrition and Human Health Laboratory, School of Life Sciences, Hunan Normal University, Changsha, China, 2Key Laboratory of Agro-ecological Processes in Subtropical Region, Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha, China, 3Shandong University-Helmholtz Institute of Biotechnology, State Key Laboratory of Microbial Technology, School of Life Science, Shandong University, Jinan, China TABLE OF CONTENTS 1. Abstract 2. Introduction 3. Absorption of carbohydrate and amino acids in enterocytes 3.1. Hexose transporter 3.1.1. SGLT1 (SLC5A1) 3.1.2. Facilitative glucose transporters 3.1.2.1. GLUT2 (SLC2A2) 3.1.2.2. GLUT5 (SLC2A5) 3.1.2.3. GLUT7 (SLC2A7) 3.1.2.4. GLUT8 (SLC2A8) 3.1.2.5. GLUT9 (SLC2A9) 3.1.2.6. GLUT12 (SLC2A12) 3.2. Amino acids transporter 3.2.1. SLC1A family 3.2.1.1. EAAC1 (SLC1A1), GLT-1 (SLC1A2), and GLAST (SLC1A3) 3.2.1.2. ASCT2 (SLC1A5) 3.2.2. SLC6A family 3.2.2.1. B0AT1 (SLC6A19) 3.2.2.2. SIT1 (SLC6A20) 3.2.3. SLC7A family 3.2.3.1. CAT1 (SLC7A1) 3.2.3.2. y+LAT1 (SLC7A7) and y+LAT2 (SLC7A6) 3.2.3.3. LAT2 (SLC7A8) 3.2.3.4. b0, +AT (SLC7A9) 3.2.3.5. Asc-1 (SLC7A10) 3.2.3.6. 4F2hc (SLC3A2)-xCT(SLC7A11) 3.2.4. SLC16A family 3.2.4.1. TAT1 (SLC16A10) and B0, + (SLC16A14) 3.2.5. SLC38A family 3.2.5.1. ATA2 (SLC38A2) and SN2 (SLC38A5) 3.2.6. Others 3.2.6.1. PAT1 (SLC36A1) and PEPT1 (SLC15A1) 4. Glucose and AA metabolism along CVA 4.1. Glucose metabolism 4.2. AA metabolism 5. Effect of glucose and AA in enterocytes maturation and growth 6. Prospects 7. References 1721 Glucose and amino acid in enterocyte 1. ABSTRACT and amino acid transporters that are expressed in the small intestinal epithelial cells. We also provide The main function of the porcine intestinal a summary of how certain nutrients influence cell tract is nutrient digestion and absorption. This function metabolism and some signaling pathways for cell is performed by the absorptive enterocytes, which are maturation. differentiated from the intestinal stem cells residing at the bottom of the crypt. Nutrients such as glucose 3. ABSORPTION OF CARBOHYDRATES AND and amino acids are transported, absorbed by various AMINO ACIDS IN ENTEROCYTES transporters embedded on the membranes of these enterocytes. Metabolism occurs in each cell along the 3.1. Hexose transporters crypt-villus axis (CVA). Because the intestinal epithelial cells are the most vigorous, self-renewing cells, Over 50% of the energy needed for cell regenerating from the crypt bottom to the villus tip in maintenance, growth, and production is supplied by only three to five days, the CVA is an appealing organ dietary carbohydrates (9). for studying cell maturation. In this review, we examine the glucose and amino acid transporters expressed in In piglets, hexose absorption within the intestine the apical membrane, basolateral membrane, or the is achieved through either para-cellular or trans-cellular inside of the absorptive enterocytes. We also discuss routes (10, 11). Hexose transporters are expressed glucose and amino acid metabolism in small epithelial on the membrane of small porcine enterocytes. They cells, and show how these nutrients influence the can be categorized as sodium-dependent glucose proliferation and differentiation of an intestinal stem transporters (SGLTs) or facilitative glucose transporters cell into one specialized cell type when they migrate (GLUTs), and include SGLT1, GLUT2, GLUT5, GLUT7, from the bottom of the crypt to the tip of the villus. and GLUT12 (12, 13). In addition to their necessity for glucose absorption, these transporters have unique roles 2. INTRODUCTION in glucose sensing (14, 15). Along the CVA, expression of intestinal glucose transporters is regulated by various As the prime location of digestion in piglets, factors, e.g., substrate concentration, food deprivation the small intestine is part of the most significant or feed restriction, disease, diurnal cycle, local or organ for nutrient absorption. The uptake function is systemic hormone levels, and piglet developmental performed by the intestinal epithelium, which is folded stage (16-18). and arranged CVA. The epithelial cells are the most vigorous and self-renewing, regenerating from the 3.1.1. SGLT1 (SLC5A1) crypt bottom to the villus tip in only three to five days (1). Cells at the bottom of the crypt that are mainly In weaning piglets, SGLT1 protein is primarily proliferating are called stem cells. After 2 days of found on the apical membrane of villus cells and is division four to five times, they terminally differentiate detected in only negligible amounts on the apical into four specialized cell types: the crypt-sited Paneth membrane of crypt cells (17). Because it has limited cells and three types that cover the villus, i.e., the expression in other tissues, this protein is considered absorptive enterocytes, mucus-secreting goblet cells, to be the only sodium-glucose co-transporter against and hormone-secreting enteroendocrine cells (2). More the concentration gradient and the major route for specific designations have now been established for absorption of dietary glucose in the enterocytes those small intestinal cells, such as transit amplifying (Table 1)(12, 17). This protein has been cloned and cells, +4 stem cells, leucine-rich repeat-containing characterized in pigs; its mature size in weanling G-protein coupled receptor 5+ cells, stem cells, Tuft animals is approximately 75 kilodaltons (17, 19). High cells, or motilin secreting cells (1-3). To provide a activity by gut apical SGLT1 contributes to neonatal better description of their locations, researchers have glucose homeostasis (20). Absorption of SGLT1 peaks labelled the cells that are sequentially isolated from the along the CVA in the jejunum, and its mRNA abundance villus tip to the crypt bottom as F1 through F6, thereby is lower in crypt cells than in villus cells (6). Its activity yielding six “cell fractions” (4). Although cells can divide is governed by protein kinase C and mechanistic target in several ways along the CVA, e.g., into 3, 5, 10, or 12 of rapamycin (mTOR), which mediate intracellular sections (4-6), the most common of these sort orders signaling pathways (21, 22). Whereas the abundance is from villi (F1) to crypt (Fn, n = number of sections). of Solute Carrier (SLC) 5A1 mRNA increases during the process of aging, the abundance of SGLT1 protein in the Each of the four cell types occurs in a specific small intestine declines over time but always remains location. For example, the absorptive epithelial cells, or higher than that of SLC5A1 mRNA (23). Expression of enterocytes, covered approximately 90% of the crypt SGLT1 can be influenced by some amino acids, being and more than 95% of the villi, making them the most inhibited, for example, throughout the small intestine important for absorption, maturation, and growth along of animals that are fed an isoleucine-deficient diet the CVA (1, 7, 8). In this review, we present the glucose (24). As a glucose receptor, SGLT1 modulates diverse 1722 © 1996-2018 Glucose and amino acid in enterocyte Table 1. Hexose and amino acid transporters Distribution Transport HUGO Specific Classification Localization proportion Substance transported References family name transporter along CVA Sugar SGLT SLC5A1 SGLT1 AM V > C glucose 17 transporter GLUT SLC2A2 GLUT2 BM V > C glucose, fructose, and galactose 11, 29 SLC2A5 GLUT5 AM V > C Fructose 34, 36 SLC2A7 GLUT7 AM V > C glucose and galactose 11, 39, UD SLC2A8 GLUT8 I V > C glucose and fructose 44, 45, UD SLC2A9 GLUT9 AM & BM glucose and uric acid 46, 47, 49 SLC2A12 GLUT12 V > C D-glucose and 2-deoxy-D-glucose 53, UD AA transporter SLC1A SLC1A1 EAAC1 AM V > C glutamic acid 63 L-glutamic acid, L- and D-aspartic SLC1A2 GLT-1 AM V < C 65, 66, UD acid SLC1A3 GLAST AM V < C glutamic acid 62, UD alanine, serine, threonine, and SLC1A5 ASCT2 AM & I 62 cysteine SLC6A SLC6A19 B0AT1 AM V > C neutral AA 80, 76, 78 SLC6A20 SIT1 AM V < C proline 85, UD arginine, lysine, ornithine, and SLC7A SLC7A1 CAT1 AM & BM V < C 87, 88, UD histidine SLC7A6 y+LAT2 BM cationic and neutral AA 92, 93 SLC7A7 y+LAT1 BM V > C cationic and neutral AA 93, UD SLC7A8 LAT2 BM V > C aromatic neutral AA except proline 80, 95, UD SLC7A9 b0, +AT AM & BM V > C cationic and neutral AA 60, 99, UD L-alanine, L-serine, L-cysteine, SLC7A10 Asc-1 BM V < C 102, 103, UD glycine, L-threoninee SLC7A11 xCT BM V < C glutamine, and L-Cystine 104, 105, UD tryptophan, tyrosine, and SLC16A SLC16A10 TAT1 AM & BM V > C 109, UD phenylalanine SLC16A14 B0, + AM V < C neutral and cationic AA 111, UD serine, proline, glutamine, and SLC38A SLC38A2 ATA2 BM V < C 114, UD alanine glutamine, asparagine, and SLC38A5 SN2 AM or BM V < C 115, 116, UD histidine Other SLC36A1 PAT1 AM V > C proline, glycine, and alanine 118, UD SLC15A1 PepT1 AM V > C di- and tripeptides 125, UD AM: apical membrane, BM: basolateral membrane, I: intracellular, V: villi, C: crypt, UD: Unpublished Data cellular functions (14). Not only does it act as a sugar 3.1.2. Facilitative glucose transporters transporter to sustain energy production in the cells, but it also functions as a receptor for sensing extracellular Among the 14 currently identified members of glucose concentrations to control food intake (15). Via the facilitative glucose transporter family, only GLUT2, a signaling process: stimulation of the T1R2 and T1R3 GLUT5, GLUT7, GLUT9, and GLUT12 are known to taste receptor by sugars activates gustducin(25, 26).