REVIEW CURRENT OPINION Gastrointestinal defense mechanisms Hyder Said a,b and Jonathan D. Kaunitzb,c Purpose of review To summarize and illuminate the recent findings regarding gastroduodenal mucosal defense mechanisms and the specific biomolecules involved in regulating this process, such as glucagon-like peptides (GLPs). Recent findings There has been a growing interest in luminal nutrient chemosensing and its physiological effects throughout the digestive system. From the ingestion of food in the oral cavity to the processing and absorption of nutrients in the intestines, nutrient chemosensing receptors signal the production and release of numerous bioactive peptides from enteroendocrine cells, such as the proglucagon-derived peptides. There has been a major emphasis on two proglucagon-derived peptides, namely GLP-1 and GLP-2, due to their apparent beneficial effect on gut structure, function, and on metabolic processes. As an incretin, GLP-1 not only enhances the effect and release of insulin on pancreatic bcells but also has been implicated in having trophic effects on the intestinal epithelium. In addition, GLP-2, the other major proglucagon-derived peptide, has potent intestinotrophic effects, such as increasing the rate of mucosal stem cell proliferation, mucosal blood flow, and fluid absorption, as well as augmenting the rate of duodenal bicarbonate secretion to improve gastric mucosal health and longevity. Summary Understanding the mechanisms underlying nutrient chemosensing and how it relates to GLP release can further elucidate how the gut functions in response to cellular changes and disturbances. Furthermore, a more in-depth comprehension of GLP release and its tissue-specific effects will help improve the utility of GLP-1 and GLP-2 receptor agonists in clinical settings. This, in turn, should help patients suffering from intestinal failure, malabsorption, and mucosal injury. Keywords free fatty acid receptors, gastric mucosal defense, glucagon-like peptides, gut taste receptors, nutrient chemosensing, trophic feeds INTRODUCTION To address this point, it is important to Over the past decade, many findings and pheno- understand the interplay between trophic feeding mena have been discovered regarding luminal and nutrient chemosensing, the cellular process of nutrient absorption and the clinical intervention recognizing luminal nutrients present in the gut known as ‘trophic feeding’, used to perfuse lumen that involves specific taste receptors. Unlike nutrients, often at low rates, into the gut lumen the oral cavity, which contains five distinct classes and thus promote gut health. More recent studies of taste receptors, the intestines only express three: have suggested that the luminal nutrients contained umami (proteinaceous), sweet, and bitter, the latter in these feeds trigger a signaling cascade that containing a multitude of subclasses. Each of these releases gut hormones, which subsequently lead to the mucosal protective effects observed in the aCollege of Letters & Sciences, University of California, Los Angeles, gastrointestinal tract [1]. In this review, we aim b c to support this hypothesis through a thorough Research Service, West Los Angeles VAMC and David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, summary of recent findings regarding gut hormones California, USA and their release following mucosal exposure to Correspondence to Jonathan D. Kaunitz, MD, Research Service, West luminal nutrients. These findings are important Los Angeles VAMC, 11301 Wilshire Blvd (115/217E), Los Angeles, CA not only for establishing the mechanism behind 90073, USA. Tel: +1 310 268 3879; fax: +1 310 268 4811; the success of trophic feeds but also for identifying e-mail: [email protected] which nutrients may optimally regenerate entero- Curr Opin Gastroenterol 2016, 32:461–466 cytes and reestablish overall gut health. DOI:10.1097/MOG.0000000000000316 0267-1379 Copyright ß 2016 Wolters Kluwer Health, Inc. All rights reserved. www.co-gastroenterology.com Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved. Stomach and duodenum endogenously-produced FAs may be sufficient to KEY POINTS release gut hormones, further implicating exogen- Glucagon-like peptides (GLPs) play an important role in ous SCFAs as possible therapeutic mucosal agents intestinal regeneration and growth as well as with the ultimate aim of repairing intestinal glucose metabolism. damage. Additional studies that parallel Kripke’s findings also support the concept of SCFA-induced Taste receptors (TASRs) are expressed not only in the intestinal mucosal growth. In a piglet model, SCFA- oral microflora but throughout the gastrointestinal tract and have been implicated in incretin release and induced trophic effects occurred in portions of promoting gut health. the intestine not originally exposed to the SCFA infusions, suggesting that these trophic effects did Free fatty acid receptors (FFARs) have been co- not require direct mucosal exposure [7]. Although localized to enteroendocrine cells with GLPs and help there is ample evidence to support the notion that mediate their release as a part of a wider nutrient chemosensing process. SCFAs signal mucosal protective effects, it has been previously widely accepted that the trophic effects Bile acid receptors expressed on L cells modulate TASR of SCFAs, specifically butyrate, are mainly derived and FFAR-dependent GLP release. from fueling enterocyte growth [8]. This school of thought was based on prior observations that epi- thelial cells absorb SCFAs and are damaged when receptors is a G-protein coupled receptor (GPCR) directly exposed to high concentrations SCFAs in heterodimer, comprising combinations of members vitro, before the discovery of SCFA-specific GPCRs. of taste receptor families (TAS1R and TAS2R), which The concept of trophic feeds has subsequently been result in distinct and discrete perceptions of taste widely established and implemented clinically; [2&]. Umami receptors comprise TAS1R1 and TAS1R3, luminal exposure to small amounts of SCFAs may whereas sweet taste receptors combine TAS1R2 and thus be useful in patients receiving parenteral nutri- TAS1R3 [3]. In addition to this first family (TAS1R) of tion due to their prevention of the mucosal atrophy taste receptors, there is a second type (TAS2R) that is that occurs in the absence of luminal nutrients. utilized exclusively by bitter taste receptors. There Two major similarities between TASRs and free are over 25 known human subtypes of TAS2Rs, FA receptors (FFARs) are the observation that their conferring over 300 possible GPCR heterodimers activation reverses mucosal atrophy and maintains [4]. Similar to the oral taste receptors, taste receptors appropriate glucose metabolism when activated, in the gut are responsible for maintaining energy effects that are at least partially mediated by gluca- balance and glycemic control during digestive gon-like peptides (GLPs). These biomolecules processes. To accomplish this goal, taste receptors are proteolytically cleaved from proglucagon via signal the release of a large variety of paracrine prohormone convertase 1/3, a process responsible and endocrine gut hormones that elicit functional for the production of the two peptides expressed physiological responses throughout the gastrointes- in enteroendocrine L-cells: GLP-1 and GLP-2 [8]. tinal tract [5]. Together, taste receptors coupled to GLP-1 is highly expressed in enteroendocrine L-cells hormone release help maintain metabolic processes, throughout the gastrointestinal tract, where it release gut hormones, and ultimately promote serves as an incretin that helps maintain glycemic intestinal health. control. Like GLP-1, GLP-2, also expressed in L-cells, In addition to taste receptors, there are a wide has many intestinotrophic properties, such as variety of biomolecules that elicit a physiological increasing the rate of crypt enterocyte proliferation, response through activation of nutrient chemosen- increasing the rate of mesenteric artery blood flow, sors. One such group of molecules is fatty acids and increasing crypt-villus depth [9&]. The GLP-2 (FAs), which are sensed by specific and distinct receptor (GLP-2R) is expressed in subepithelial peri- GPCRs based on the length of the respective carbon cryptal myofibroblasts and enteric neurons, which tail. Two such classes are short-chain FAs (SCFAs) further support its proposed function as major inter- and long-chain FAs (LCFAs), which are shorter mediate in many cell-signaling pathways [10,11]. than eight and longer than 16 carbons, respectively. Amongst other functions, many of these pathways An important early study conducted by Kripke’s are believed to decrease inflammation in gastroin- group in 1989 [6] suggested that exogenous SCFA testinal, hepatic, pulmonary, and muscular tissues infusion was trophic for the intestine, in particular [12&]. The proteolytic enzyme dipeptidyl peptidase by increasing the rate of mucosal cell growth and IV (DPPIV) has major specificity for GLP hormones, accelerating maturation of the villous cells. Because governing their plasma t1/2. DPPIV inhibition, by SCFAs are generated by the oral microflora in increasing the plasma t1/2 of GLP-1, is used clinically addition to the gut flora, an acute load of these in the treatment of type 2 diabetes [13,14&,15&] and 462 www.co-gastroenterology.com Volume 32 Number 6 November 2016 Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved. Gastrointestinal defense mechanisms