The roles of glucagon-like peptide-2 and the intestinal epithelial insulin-like growth factor-1 receptor in regulating microvillus length

Evolution and therapeutic potential of glucagon-like peptide 2 analogs

Glucagon-like peptide 2 (GLP-2) is a proglucagon-derived peptide released by intestinal endocrine cells. However, its therapeutic potential is limited by rapid inactivation via dipeptidyl peptidase-IV. The elucidation of three-dimensional structures of G-protein-coupled receptors, including GLP-2 receptor, has facilitated the rational design of novel peptide therapeutics. Recent studies have explored various structural modifications based on the structure of GLP-2, such as amino acid substitution, lipidation, and fusion with proteins, to extend the half-life of GLP-2 and enhance its biological activity. One promising avenue involves the development of multifunctional molecules targeting multiple pharmacological systems to boost therapeutic efficacy. This paper reviews the recent advancements in understanding GLP-2, including its physiological roles and structure-activity relationships, and evaluates the development prospects of GLP-2 analogs.

Histological Changes of the Mucosal Epithelium in the Chicken Intestine during Pre- and Post-Hatching Stages

This study clarified the histological changes in the mucosal epithelium of the chicken intestine during the pre- and post-hatching stages. The duodenum, jejunum, ileum, and colorectum were collected from embryos at 15, 17, 18, 19, and 21 days of incubation and from chicks at 1 and 3 days after hatching. Paraffin sections prepared from tissue samples were stained with periodic acid-Schiff followed by alcian blue for histological analysis and to detect goblet cells. Villin and β-actin were detected using double immunofluorescence. Villi with finger-like shape were already observed in embryos after 15 days of incubation, and no obvious change in shape was observed even after hatching. Villous height increased in all intestinal regions as the developmental stage progressed, particularly a few days before and after hatching. Goblet cells first appeared in the epithelium of all intestinal regions after 18 days of incubation. The density of goblet cells rapidly increased from 18 to 21 days of incubation. Both villin and β-actin immunoreactivities were detected at the apical surface of the villous epithelium in all intestinal regions, and villin immunopositivity was stronger in the jejunum and ileum after hatching. These findings indicate that the villi and microvilli of the intestine of broiler chickens show histological changes during few days just before and after hatching. Additionally, the density of goblet cells rapidly increased for a few days before hatching.

The signaling landscape of insulin-like growth factor 1

The sheer amplitude of biological actions of insulin-like growth factor I (IGF-1) affecting all types of cells in all tissues suggests a vast signaling landscape for this ubiquitous humoral signal. While the canonical signaling pathways primarily involve the Ras/MAPK and PI3K/AKT cascades, the evolutionary conservation of insulin-like peptides (ILPs) and their pathways hints at the potential for novel functions to emerge over time. Indeed, the evolutionary trajectory of ILPs opens the possibility of either novel functions for these two pathways, novel downstream routes, or both. Evidence supporting this notion includes observations of neofunctionalization in bony fishes or crustaceans, and the involvement of ILPs pathways in invertebrate eusociality or in vertebrate bone physiology, respectively. Such evolutionary processes likely contribute to the rich diversity of ILPs signaling observed today. Moreover, the interplay between conserved signaling pathways, such as those implicated in aging (predominantly involving the PI3K-AKT route), and lesser known pathways, such as those mediated by biased G-protein coupled receptors and others even less known, may underpin the context-dependent actions characteristic of ILPs signaling. While canonical IGF-1 signaling is often assumed to account for the intracellular pathways utilized by this growth factor, a comprehensive analysis of all the pathways mediated by the IGF-1 receptor (IGF-1R) remains lacking. This review aims to explore both canonical and non-canonical routes of IGF-1R action across various cell types, offering a detailed examination of the mechanisms underlying IGF-1 signaling and highlighting the significant gaps in our current understanding.

Crosstalk Within the Intestinal Epithelium: Aspects of Intestinal Absorption, Homeostasis, and Immunity

Gut health is crucial in many ways, such as in improving human health in general and enhancing production in agricultural animals. To maximize the effect of a healthy gastrointestinal tract (GIT), an understanding of the regulation of intestinal functions is needed. Proper intestinal functions depend on the activity, composition, and behavior of intestinal epithelial cells (IECs). There are various types of IECs, including enterocytes, Paneth cells, enteroendocrine cells (EECs), goblet cells, tuft cells, M cells, and intestinal epithelial stem cells (IESCs), each with unique 3D structures and IEC distributions. Although the communication between IECs and other cell types, such as immune cells and neurons, has been intensively reviewed, communication between different IECs has rarely been addressed. The present paper overviews the networks among IECs that influence intestinal functions. Intestinal absorption is regulated by incretins derived from EECs that induce nutrient transporter activity in enterocytes. EECs, Paneth cells, tuft cells, and enterocytes release signals to activate Notch signaling, which modulates IESC activity and intestinal homeostasis, including proliferation and differentiation. Intestinal immunity can be altered via EECs, goblet cells, tuft cells, and cytokines derived from IECs. Finally, tools for investigating IEC communication have been discussed, including the novel 3D intestinal cell model utilizing enteroids that can be considered a powerful tool for IEC communication research. Overall, the importance of IEC communication, especially EECs and Paneth cells, which cover most intestinal functional regulating pathways, are overviewed in this paper. Such a compilation will be helpful in developing strategies for maintaining gut health.

The Long-Acting Glucagon-Like Peptide-2 Analog Apraglutide Enhances Intestinal Protection and Survival After Chemotherapy and Allogeneic Transplantation in Mice

BACKGROUND The gastrointestinal (GI) barrier can be damaged by chemotherapy or radiation therapy, causing fatigue, malnutrition, sepsis, dose-limiting toxicity, and, occasionally, death. Glucagon-like peptide-2 (GLP-2) promotes mucosal epithelium growth and repair in the GI tract. Here, we examined the GI-protective effects of apraglutide, a long-acting peptide GLP-2 analog, in murine models of chemotherapy, and total body irradiation followed by allogeneic transplantation. MATERIAL AND METHODS The impact of apraglutide on cytarabine or melphalan chemotherapy-induced intestinal damage was assessed in BALB/c mice, and the effect on allogeneic transplantation in BALB/cJ and C57BL/6J mice. Outcomes included survival, and changes in body weight, intestinal function and morphology, including colon length and bacterial composition of the intestinal microbiota. RESULTS Adding apraglutide to chemotherapy significantly improved survival rates and reduced weight loss, with no impact on leukocyte counts (and, therefore, no effect on chemotherapy-induced immunosuppression), compared with chemotherapy alone in mice. These benefits were associated with preservation of the morphological integrity of the GI mucosa, attenuation of the negative impact of cytarabine on the intestinal microbiota, and significant improvement in plasma levels of citrulline. In addition, in a model of irradiation followed by allogeneic transplantation, mice in groups receiving apraglutide had improved survival, reduced weight loss, and increased colon length compared with those that did not. CONCLUSIONS Apraglutide protects intestinal function and improves survival in mice following allogeneic transplantation or chemotherapy with cytarabine or melphalan. The potential effect of apraglutide on chemotherapy efficacy and on engraftment following allogeneic transplantation has been investigated in a parallel manuscript.

Enteroendocrine cells regulate intestinal homeostasis and epithelial function

Enteroendocrine cells (EECs) are well-known for their systemic hormonal effects, especially in the regulation of appetite and glycemia. Much less is known about how the products made by EECs regulate their local environment within the intestine. Here, we focus on paracrine interactions between EECs and other intestinal cells as they regulate three essential aspects of intestinal homeostasis and physiology: 1) intestinal stem cell function and proliferation; 2) nutrient absorption; and 3) mucosal barrier function. We also discuss the ability of EECs to express multiple hormones, describe in vitro and in vivo models to study EECs, and consider how EECs are altered in GI disease.

Gut hormone analogues and skeletal health in diabetes and obesity: Evidence from preclinical models

Diabetes mellitus and obesity are rapidly growing worldwide. Aside from metabolic disturbances, these two disorders also affect bone with a higher prevalence of bone fractures. In the last decade, a growing body of evidence suggested that several gut hormones, including ghrelin, gastrin, glucose-dependent insulinotropic polypeptide (GIP), glucagon, and glucagon-like peptide-1 and 2 (GLP-1 and GLP-2, respectively) may affect bone physiology. Several gut hormone analogues have been developed for the treatment of type 2 diabetes and obesity, and could represent a new alternative in the therapeutic arsenal against bone fragility. In the present review, a summary of the physiological roles of these gut hormones and their analogues is presented at the cellular level but also in several preclinical models of bone fragility disorders including type 2 diabetes mellitus, especially on bone mineral density, microarchitecture and bone material properties. The present review also summarizes the impact of GLP-1 receptor agonists approved for the treatment of type 2 diabetes mellitus and the more recent dual or triple analogue on bone physiology and strength.

Gut hormones and bone homeostasis: potential therapeutic implications

Bone resorption follows a circadian rhythm, with a marked reduction in circulating markers of resorption (such as carboxy-terminal telopeptide region of collagen type I in serum) in the postprandial period. Several gut hormones, including glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide 1 (GLP1) and GLP2, have been linked to this effect in humans and rodent models. These hormones are secreted from enteroendocrine cells in the gastrointestinal tract in response to a variety of stimuli and effect a wide range of physiological processes within and outside the gut. Single GLP1, dual GLP1-GIP or GLP1-glucagon and triple GLP1-GIP-glucagon receptor agonists have been developed for the treatment of type 2 diabetes mellitus and obesity. In addition, single GIP, GLP1 and GLP2 analogues have been investigated in preclinical studies as novel therapeutics to improve bone strength in bone fragility disorders. Dual GIP-GLP2 analogues have been developed that show therapeutic promise for bone fragility in preclinical studies and seem to exert considerable activity at the bone material level. This Review summarizes the evidence of the action of gut hormones on bone homeostasis and physiology.

Intestinal permeability, microbiota composition and expression of genes related to intestinal barrier function of broiler chickens fed different methionine sources supplemented at varying concentrations

Intestinal health of broiler chickens is influenced by the concentration of dietary amino acids but data are limited on the role of dietary methionine (Met). Two experiments were conducted to investigate the implications of different Met sources for performance, gut barrier function, and intestinal microbiota in broilers. In the first experiment, Ross 308 off-sex birds (n = 900) were assigned to 10 dietary treatments each replicated 9 times in a 35-day study. Three sources of Met included DL-Met, L-Met, or Met hydroxy analog free acid (MHA-FA), each supplemented at suboptimal (SUB) at 80%, adequate (ADE) at 100% and over-requirement (OVR) at 120% of the specifications against a deficient (DEF) diet with no added Met. The second experiment used 96 Ross 308 broilers in a 2 × 4 factorial arrangement. Four diets included 3 sources of Met supplemented at ADE level plus the DEF treatment. On d 17, 19, and 23, half of the birds in each dietary treatment were injected with dexamethasone (DEX) to induce leaky gut. In the first experiment, without an interaction, from d 0 to 35, birds fed DL-Met and L-Met performed similarly for BWG, feed intake, and FCR but birds fed MHA-FA had less feed intake and BWG (P < 0.05). At d 23, mRNA expression of selected tight junction proteins was not affected except for claudin 2. Ileal microbiota of DEF treatment was different from DL-MET or L-MET supplemented birds (P < 0.05). However, microbiota of MHA-FA treatments was only different at OVR from the DEF group. The abundance of Peptostreptococcus increased in DEF treatment whereas Lactobacillus decreased. In the second experiment, DEX independently increased (P < 0.001) intestinal permeability assayed by fluorescein isothiocyanate dextran, but diet had no effect. DL-Met and L-Met fed birds had a higher level of claudin 3 only in DEX-injected birds (P < 0.05). In conclusion, unlike the level of supplementation, DL-Met, L-Met, and MHA-FA were largely similar in their limited impacts on intestinal barrier function and gut microbiota in broilers.

Anthocyanin actions at the gastrointestinal tract: Relevance to their health benefits

Anthocyanins (AC) are flavonoids abundant in the human diet, which consumption has been associated to several health benefits, including the mitigation of cardiovascular disease, type 2 diabetes, non-alcoholic fatty liver disease, and neurological disorders. It is widely recognized that the gastrointestinal (GI) tract is not only central for food digestion but actively participates in the regulation of whole body physiology. Given that AC, and their metabolites reach high concentrations in the intestinal lumen after food consumption, their biological actions at the GI tract can in part explain their proposed local and systemic health benefits. In terms of mechanisms of action, AC have been found to: i) inhibit GI luminal enzymes that participate in the absorption of lipids and carbohydrates; ii) preserve intestinal barrier integrity and prevent endotoxemia, inflammation and oxidative stress; iii) sustain goblet cell number, immunological functions, and mucus production; iv) promote a healthy microbiota; v) be metabolized by the microbiota to AC metabolites which will be absorbed and have systemic effects; and vi) modulate the metabolism of GI-generated hormones. This review will summarize and discuss the latest information on AC actions at the GI tract and their relationship to overall health benefits.

Antibiotics in avian care and husbandry-status and alternative antimicrobials

Undoubtedly, the discovery of antibiotics was one of the greatest milestones in the treatment of human and animal diseases. Due to their over-use mainly as antibiotic growth promoters (AGP) in livestock farming, antimicrobial resistance has been reported with increasing intensity, especially in the last decades. In order to reduce the scale of this phenomenon, initially in the Scandinavian countries and then throughout the entire European Union, a total ban on the use of AGP was introduced, moreover, a significant limitation in the use of these feed additives is now observed almost all over the world. The withdrawal of AGP from widespread use has prompted investigators to search for alternative strategies to maintain and stabilize the composition of the gut microbiota. These strategies include substances that are used in an attempt to stimulate the growth and activity of symbiotic bacteria living in the digestive tract of animals, as well as living microorganisms capable of colonizing the host’s gastrointestinal tract, which can positively affect the composition of the intestinal microbiota by exerting a number of pro-health effects, i.e., prebiotics and probiotics, respectively. In this review we also focused on plants/herbs derived products that are collectively known as phytobiotic.

Comparative Effects of Allulose, Fructose, and Glucose on the Small Intestine

Despite numerous studies on the health benefits of the rare sugar allulose, its effects on intestinal mucosal morphology and function are unclear. We therefore first determined its acute effects on the small intestinal transcriptome using DNA microarray analysis following intestinal allulose, fructose and glucose perfusion in rats. Expression levels of about 8-fold more genes were altered by allulose compared to fructose and glucose perfusion, suggesting a much greater impact on the intestinal transcriptome. Subsequent pathway analysis indicated that nutrient transport, metabolism, and digestive system development were markedly upregulated, suggesting allulose may acutely stimulate these functions. We then evaluated whether allulose can restore rat small intestinal structure and function when ingested orally following total parenteral nutrition (TPN). We also monitored allulose effects on blood levels of glucagon-like peptides (GLP) 1 and 2 in TPN rats and normal mice. Expression levels of fatty acid binding and gut barrier proteins were reduced by TPN but rescued by allulose ingestion, and paralleled GLP-2 secretion potentially acting as the mechanism mediating the rescue effect. Thus, allulose can potentially enhance disrupted gut mucosal barriers as it can more extensively modulate the intestinal transcriptome relative to glucose and fructose considered risk factors of metabolic disease.

A Protective Role for Glucagon-like Peptide-2 in Heat-stable Enterotoxin b (STb)-Induced L-Cell Toxicity

Enterotoxigenic Escherichia coli (ETEC)-derived purified heat-stable enterotoxin b (STb) is responsible for secretory diarrhea in livestock and humans. STb disrupts intestinal fluid homeostasis, epithelial barrier function, and promotes cell death. Glucagon-like peptide-2 (GLP-2) is a potent intestinotrophic hormone secreted by enteroendocrine L cells. GLP-2 enhances crypt cell proliferation, epithelial barrier function, and inhibits enterocyte apoptosis. Whether STb can affect GLP-2 producing L cells remains to be elucidated. First, secreted-His-labeled STb from transformed E coli was collected and purified. When incubated with L-cell models (GLUTag, NCI-H716, and secretin tumor cell line [STC-1]), fluorescent immunocytochemistry revealed STb was internalized and was differentially localized in the cytoplasm and nucleus. Cell viability experiments with neutral red and resazurin revealed that STb was toxic in all but the GLUTag cells. STb stimulated 2-hour GLP-2 secretion in all cell models. Interestingly, GLUTag cells produced the highest amount of GLP-2 when treated with STb, demonstrating an inverse relationship in GLP-2 secretion and cell toxicity. To demonstrate a protective role for GLP-2, GLUTag-conditioned media (rich in GLP-2) blocked STb toxicity in STC-1 cells. Confirming a protective role of GLP-2, teduglutide was able to improve cell viability in cells treated with H2O2. In conclusion, STb interacts with the L cell, stimulates secretion, and may induce toxicity if GLP-2 is not produced at high levels. GLP-2 or receptor agonists have the ability to improve cell viability in response to toxins. These results suggest that GLP-2 secretion can play a protective role during STb intoxication. This work supports future investigation into the use of GLP-2 therapies in enterotoxigenic-related diseases.

Enteroendocrine cell differentiation and function in the intestine

PURPOSE OF REVIEW

The intestinal enteroendocrine cells (EECs) are specialized hormone-secreting cells that respond to both circulating and luminal cues. Collectively, EECs constitute the largest endocrine organ of the body and signal to a multitude of targets including locally to neighboring intestinal cells, enteric neurons, as well as systemically to other organs, such as the pancreas and brain. To accomplish their wide range of downstream signaling effects, EECs secrete multiple hormones; however, the mechanisms that influence EEC development in the embryo and differentiation in adults are not well defined.

RECENT FINDINGS

This review highlights the recent discoveries in EEC differentiation and function while also discussing newly revealed roles of transcription factors and signaling networks involved in the allocation of EEC subtypes that were discovered using a combination of novel intestinal model systems and genetic sequencing. We also discuss the potential of these new experimental models that study the mechanisms regulating EEC function and development both to uncover novel therapeutic targets.

SUMMARY

Several EEC hormones are being used to treat various metabolic disorders, such as type 2 diabetes and obesity. Therefore, understanding the signaling pathways and gene regulatory networks that facilitate EEC formation is paramount to the development of novel therapies.

Intra-amniotic administration of l-glutamine promotes intestinal maturation and enteroendocrine stimulation in chick embryos

Initial nutritional stimulation is a key driving force for small intestinal maturation. In chick embryos, administration of l-glutamine (Gln) into the amniotic fluid stimulates early development of the small intestinal epithelium by promoting enterocyte differentiation. In this study, we evaluated the effects of intra-amniotic administration of Gln on enterocyte morphology and function, and elucidated a potential enteroendocrine pathway through which Gln stimulates small intestinal maturation. Our results show that Gln stimulation at embryonic day 17 significantly increased enterocyte and microvilli dimensions by 10 and 20%, respectively, within 48 h. Post-hatch, enterocytes and microvilli were 20% longer in Gln-treated chicks. Correspondingly, Gln stimulation significantly upregulated mRNA expression of brush border nutrient transporters PepT-1 and SGLT-1 and tight junction proteins TJP-1 and TJP-2, before and after hatch (P < 0.05). Since GLP-2 signaling from intestinal L-cells is associated with enterocyte growth, functionality and integrity, we examined the effects of Gln stimulation on mRNA expression of key hormones and receptors within this enteroendocrine pathway and found significant increases in GLP-2R, IGF-1 and IGF-1R expression before and after hatch (P < 0.05). In conclusion, our findings link primary nutrient stimulation in the developing small intestine with enterocyte morphological and functional maturation and enteroendocrine signaling.

Present and Future Therapeutic Approaches to Barrier Dysfunction

There is converging and increasing evidence, but also uncertainty, for the role of abnormal intestinal epithelial barrier function in the origin and development of a growing number of human gastrointestinal and extraintestinal inflammatory disorders, and their related complaints. Despite a vast literature addressing factors and mechanisms underlying changes in intestinal permeability in humans, and its connection to the appearance and severity of clinical symptoms, the ultimate link remains to be established in many cases. Accordingly, there are no directives or clinical guidelines related to the therapeutic management of intestinal permeability disorders that allow health professionals involved in the management of these patients to carry out a consensus treatment based on clinical evidence. Instead, there are multiple pseudoscientific approaches and commercial propaganda scattered on the internet that confuse those affected and health professionals and that often lack scientific rigor. Therefore, in this review we aim to shed light on the different therapeutic options, which include, among others, dietary management, nutraceuticals and medical devices, microbiota and drugs, and epigenetic and exosomes-manipulation, through an objective evaluation of the scientific publications in this field. Advances in the knowledge and management of intestinal permeability will sure enable better options of dealing with this group of common disorders to enhance quality of life of those affected.

Distinct Identity of GLP-1R, GLP-2R, and GIPR Expressing Cells and Signaling Circuits Within the Gastrointestinal Tract

Enteroendocrine cells directly integrate signals of nutrient content within the gut lumen with distant hormonal responses and nutrient disposal via the production and secretion of peptides, including glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide 1 (GLP-1) and glucagon-like peptide 2 (GLP-2). Given their direct and indirect control of post-prandial nutrient uptake and demonstrated translational relevance for the treatment of type 2 diabetes, malabsorption and cardiometabolic disease, there is significant interest in the locally engaged circuits mediating these metabolic effects. Although several specific populations of cells in the intestine have been identified to express endocrine receptors, including intraepithelial lymphocytes (IELs) and αβ and γδ T-cells (Glp1r+) and smooth muscle cells (Glp2r+), the definitive cellular localization and co-expression, particularly in regards to the Gipr remain elusive. Here we review the current state of the literature and evaluate the identity of Glp1r, Glp2r, and Gipr expressing cells within preclinical and clinical models. Further elaboration of our understanding of the initiating G-protein coupled receptor (GPCR) circuits engaged locally within the intestine and how they become altered with high-fat diet feeding can offer insight into the dysregulation observed in obesity and diabetes.

Oral absorption of oxycodone in patients with short bowel syndrome

BACKGROUND

Short bowel syndrome is a disorder with several complications such as malnutrition and failure of drug therapy. Treatment with opioids is needed in many patients, and oral medication is preferred. However, optimal dosing is a difficult task as current guidelines are based on an intact gastrointestinal tract. Hence, the aim of this explorative case study was to assess the pharmacokinetics of orally administered oxycodone in patients with short bowel syndrome.

METHODS

Six patients with short bowel syndrome were administered 10 mg oral solution oxycodone after an overnight fast. Oxycodone plasma concentrations were determined over a 6-hour period. Pharmacokinetic profiles were visually inspected. Pharmacokinetic parameters: maximum plasma concentration, time of maximum concentration and area under the curve were calculated. Data were also compared to mean values obtained in healthy participants.

RESULTS

A clinically relevant concentration of oxycodone was found in all patients, although with large inter-individual variation. The absorption fraction tended to correlate positively with total intestinal length. Additionally, preservation of some or the entire colon seemed further to increase the absorption fraction. Time of maximum concentration varied from 30 min to approximately 90 min.

CONCLUSIONS

Oxycodone is absorbed in a clinically relevant extent in patients with short bowel syndrome, but bioavailability varies greatly between patients, which shall be taken into consideration. Absorption is related to functional small intestinal length, but preservation of colon is also beneficial. Still, optimal therapeutic dosing must be individualized, and other factors such as those related to malnutrition and motility shall also be taken into consideration.

Intraluminal nutrients acutely strengthen rat intestinal MRP2 barrier function by a glucagon-like peptide-2-mediated mechanism

AIM

MRP2 is an intestinal ABC transporter that prevents the absorption of dietary xenobiotics. The aims of this work were: (1) to evaluate whether a short-term regulation of intestinal MRP2 barrier function takes place in vivo after luminal incorporation of nutrients and (2) to explore the underlying mechanism.

METHODS

MRP2 activity and localization were assessed in an in vivo rat model with preserved irrigation and innervation. Nutrients were administered into distal jejunum. After 30-minutes treatments, MRP2 activity was assessed in proximal jejunum by quantifying the transport of the model substrate 2,4-dinitrophenyl-S-glutathione. MRP2 localization was determined by quantitative confocal microscopy. Participation of extracellular mediators was evaluated using selective inhibitors and by immunoneutralization. Intracellular pathways were explored in differentiated Caco-2 cells.

RESULTS

Oleic acid, administered intraluminally at dietary levels, acutely stimulated MRP2 insertion into brush border membrane. This was associated with increased efflux activity and, consequently, enhanced barrier function. Immunoneutralization of the gut hormone glucagon-like peptide-2 (GLP-2) prevented oleic acid effect on MRP2, demonstrating the participation of this trophic factor as a main mediator. Further experiments using selective inhibitors demonstrated that extracellular adenosine synthesis and its subsequent binding to enterocytic A2B adenosine receptor (A2BAR) take place downstream GLP-2. Finally, studies in intestinal Caco-2 cells revealed the participation of A2BAR/cAMP/PKA intracellular pathway, ultimately leading to increased MRP2 localization in apical domains.

CONCLUSION

These findings reveal an on-demand, acute regulation of MRP2-associated barrier function, constituting a novel physiological mechanism of protection against the absorption of dietary xenobiotics in response to food intake.

Insights into incretin-based therapies for treatment of diabetic dyslipidemia

Derangements in triglyceride and cholesterol metabolism (dyslipidemia) are major risk factors for the development of cardiovascular diseases in obese and type-2 diabetic (T2D) patients. An emerging class of glucagon-like peptide-1 (GLP-1) analogues and next generation peptide dual-agonists such as GLP-1/glucagon or GLP-1/GIP could provide effective therapeutic options for T2D patients. In addition to their role in glucose and energy homeostasis, GLP-1, GIP and glucagon serve as regulators of lipid metabolism. This review summarizes the current knowledge in GLP-1, glucagon and GIP effects on lipid and lipoprotein metabolism and frames the emerging therapeutic benefits of GLP-1 analogs and GLP-1-based multiagonists as add-on treatment options for diabetes associated dyslipidemia.

GLP-2, EGF, and the Intestinal Epithelial IGF-1 Receptor Interactions in the Regulation of Crypt Cell Proliferation

Glucagon-like peptide-2 (GLP-2) is an intestinotrophic hormone that promotes intestinal growth and proliferation through downstream mediators, including epidermal growth factor (EGF) and insulin-like growth factor-1 (IGF-1). EGF synergistically enhances the proliferative actions of IGF-1 in intestinal cell lines, and both of these factors are known to be essential for the trophic effects of GLP-2 in vivo. However, whether EGF and IGF-1 interact to mediate the proliferative actions of GLP-2 in vivo remains unknown. Normal and knockout (KO) mice lacking the intestinal epithelial IGF-1 receptor (IE-IGF-1R) were therefore treated chronically with EGF and/or long-acting human hGly2GLP-2, followed by determination of intestinal growth parameters. Intestines from control and IE-IGF-1R KO mice were also used to generate organoids (which lack the GLP-2 receptor) and were treated with EGF and/or IGF-1. Combination treatment with EGF and hGly2GLP-2 increased small intestinal weight and crypt-villus height in C57Bl/6 mice in an additive manner, whereas only hGly2GLP-2 treatment increased crypt cell proliferation. However, although combination treatment also increased small intestinal weight and crypt-villus height in IE-IGF-1R KO mice, the proliferative responses to hGly2GLP-2 alone or with EGF were diminished in these animals. Finally, IGF-1 treatment of organoids undergoing EGF withdrawal was not additive to the effect of EGF replacement on proliferation, but could restore normal proliferation in the absence of EGF. Together, these findings demonstrate that the intestinal proliferative effects of hGly2GLP-2 are augmented by exogenous EGF in a manner that is partially dependent upon IE-IGF-1R signaling.

Requirement for the intestinal epithelial insulin-like growth factor-1 receptor in the intestinal responses to glucagon-like peptide-2 and dietary fat

The intestinal hormone, glucagon-like peptide-2 (GLP-2), enhances the enterocyte chylomicron production. However, GLP-2 is known to require the intestinal-epithelial insulin-like growth factor-1 receptor (IE-IGF-1R) for its other actions to increase intestinal growth and barrier function. The role of the IE-IGF-1R in enterocyte lipid handling was thus tested in the GLP-2 signaling pathway, as well as in response to a Western diet (WD). IE-IGF-1R knockout (KO) and control mice were treated for 11 days with h(GLY² )GLP-2 or fed a WD for 18 weeks followed by a duodenal fat tolerance test with C¹⁴ -labeled triolein. Human Caco-2BBE cells were treated with an IGF-1R antagonist or signaling inhibitors to determine triglyceride-associated protein expression. The IE-IGF-1R was required for GLP-2-induced increases in CD36 and FATP-4 in chow-fed mice, and for expression in vitro; FATP-4 also required PI3K/Akt. Although WD-fed IE-IGF-1R KO mice demonstrated normal CD36 expression, the protein was incorrectly localized 2h post-duodenal fat administration. IE-IGF-1R KO also prevented the WD-induced increase in MTP and decrease in APOC3, increased jejunal mucosal C¹⁴ -fat accumulation, and elevated plasma triglyceride and C¹⁴ -fat levels. Collectively, these studies elucidate new roles for the IE-IGF-1R in enterocyte lipid handling, under basal conditions and in response to GLP-2 and WD-feeding.