Nutrient-independent increases in proglucagon and ornithine decarboxylase messenger RNAs after jejunoileal resection

Mechanisms of intestinal adaptation

Following loss of functional small bowel surface area due to surgical resection for therapy of Crohn's disease, ischemia, trauma or other disorders, the remnant gut undergoes a morphometric and functional compensatory adaptive response which has been best characterized in preclinical models. Increased crypt cell proliferation results in increased villus height, crypt depth and villus hyperplasia, accompanied by increased nutrient, fluid and electrolyte absorption. Clinical observations suggest that functional adaptation occurs in humans. In the immediate postoperative period, patients with substantial small bowel resection have massive fluid and electrolyte loss with reduced nutrient absorption. For many patients, the adaptive response permits partial or complete weaning from parenteral nutrition (PN), within two years following resection. However, others have life-long PN dependence. An understanding of the molecular mechanisms that regulate the gut adaptive response is critical for developing novel therapies for short bowel syndrome. Herein we present a summary of key studies that seek to elucidate the mechanisms that regulate post-resection adaptation, focusing on stem and crypt cell proliferation, epithelial differentiation, apoptosis, enterocyte function and the role of growth factors and the enteric nervous system.

Morphological, kinetic, membrane biochemical and genetic aspects of intestinal enteroplasticity

The process of intestinal adaptation (“enteroplasticity”) is complex and multifaceted. Although a number of trophic nutrients and non-nutritive factors have been identified in animal studies, successful, reproducible clinical trials in humans are awaited. Understanding mechanisms underlying this adaptive process may direct research toward strategies that maximize intestinal function and impart a true clinical benefit to patients with short bowel syndrome, or to persons in whom nutrient absorption needs to be maximized. In this review, we consider the morphological, kinetic and membrane biochemical aspects of enteroplasticity, focus on the importance of nutritional factors, provide an overview of the many hormones that may alter the adaptive process, and consider some of the possible molecular profiles. While most of the data is derived from rodent studies, wherever possible, the results of human studies of intestinal enteroplasticity are provided.

Exogenous glucagon-like peptide-2 (GLP-2) augments GLP-2 receptor mRNA and maintains proglucagon mRNA levels in resected rats

BACKGROUND

Glucagon-like peptide-2 (GLP-2) is a nutrient-dependent proglucagon-derived hormone that stimulates intestinal adaptive growth. Our aim was to determine whether exogenous GLP-2 increases resection-induced adaptation without diminishing endogenous proglucagon and GLP-2 receptor expression.

METHODS

Rats underwent transection or 70% jejunoileal resection +/- GLP-2 infusion (100 microg/kg body weight/d) and were fed a semipurified diet with continuous infusion of GLP-2 or saline by means of jugular catheter. After 7 days, body weight, mucosal cellularity (dry mass, protein and DNA), crypt-villus height, and crypt cell proliferation (by bromodeoxyuridine staining) were determined. Plasma bioactive GLP-2 (by radioimmunoassay), proglucagon and GLP-2 receptor mRNA expression (by Northern blot and real-time reverse transcriptase quantitative polymerase chain reaction) were measured. GLP-2 receptor was colocalized to neuroendocrine markers by immunohistochemistry.

RESULTS

Low-dose exogenous GLP-2 increased mucosal cellularity and crypt-villus height in the duodenum, jejunum, and ileum; enterocyte proliferation in the jejunal crypt; and duodenal and jejunal sucrase segmental activity. Plasma bioactive GLP-2 concentration increased 70% upon resection, with an additional 54% increase upon GLP-2 infusion in resected rats (P < .05). Ileal proglucagon mRNA expression increased with resection, and exogenous ileum GLP-2 failed to blunt this response. Exogenous GLP-2 increased ileum GLP-2 receptor expression 3-fold in resected animals and was colocalized to vasoactive intestinal peptide-positive and endothelial nitric oxide synthase-expressing enteric neurons and serotonin-containing enteroendocrine cells in the jejunum and ileum of resected rats.

CONCLUSIONS

Exogenous GLP-2 augments adaptive growth and digestive capacity of the residual small intestine in a rat model of mid-small bowel resection by increasing plasma GLP-2 concentrations and GLP-2 receptor expression without diminishing endogenous proglucagon expression.

Impairment of arginine metabolism in rats after massive intestinal resection: effect of parenteral nutrition supplemented with citrulline compared with arginine

Arginine homoeostasis is impaired in short bowel syndrome, but its supplementation in short bowel syndrome patients remains controversial. Recently, we demonstrated the benefits of citrulline supplementation by the enteral route in resected rats. Since the first step in managing short bowel syndrome is to initiate total parenteral nutrition, we hypothesized that parenteral citrulline supplementation would be more appropriate in this situation than arginine supplementation. In the present study, 24 rats were assigned to four groups. The sham group underwent transection whereas the three other groups underwent resection (R) of 80% of the small intestine. All rats were then fed exclusively by total parenteral nutrition as follows: supplementation with citrulline (R+CIT), with arginine (R+ARG) or no supplementation (R). All of the rats received isocaloric and isonitrogenous nutrition for 4 days. Nitrogen balance was measured daily. Rats were then killed and the blood was collected and the intestinal mucosa and extensor digitorum longus muscle were removed for amino acid and protein analysis. Citrulline and arginine increased mucosal protein content in the ileum (compared with sham and R, P<0.05). However, only citrulline prevented extensor digitorum longus atrophy (R+CIT, 130±3 mg compared with R, 100±6 mg and R+ARG, 110±2 mg, P<0.05). In addition, arginine worsened nitrogen balance (R+ARG, 104±46 mg/72 h compared with R, 249±69 mg/72 h, P<0.05). Only citrulline was able to prevent muscle atrophy and it was achieved independently from any noticeable effect on the gut in particular because citrulline and arginine share the same effect on mucosal ileal protein content. These results suggest that citrulline should be considered as a potential supplement for total parenteral nutrition of short bowel syndrome patients.

Dietary lipids alter the effect of steroids on the transport of fructose following intestinal resection in rats

BACKGROUND

Glucocorticosteroids alter intestinal morphology and transport. We tested the hypothesis that the desired intestinal adaptive response following intestinal resection may be enhanced further by the locally active steroid budesonide, and by feeding a saturated as compared with a polyunsaturated fatty acid diet.

METHODS

An in-vitro uptake method was used to assess intestinal fructose uptake by rats of semisynthetic diets enriched in saturated or polyunsaturated fatty acids, and injected with budesonide or control solution.

RESULTS

Budesonide increased ileal fructose uptake in chow and PUFA-fed animals, but reduced jejunal fructose uptake in rats fed SFA. GLUT5 and GLUT2 protein and mRNA did not correlate with changes in fructose uptake. Steroids reduced jejunal proglucagon expression in animals fed chow. Animals fed SFA and given budesonide had a reduction in jejunal ODC mRNA compared with those fed PUFA or chow.

CONCLUSIONS

(1) budesonide increases ileal fructose uptake following intestinal resection, and this beneficial effect is prevented by feeding SFA rather than PUFA; (2) fructose uptake does not correlate with GLUT5 and GLUT2 protein and mRNA; (3) ODC and proglucagon may be involved in this adaptive response.

Biology of incretins: GLP-1 and GIP

This review focuses on the mechanisms regulating the synthesis, secretion, biological actions, and therapeutic relevance of the incretin peptides glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). The published literature was reviewed, with emphasis on recent advances in our understanding of the biology of GIP and GLP-1. GIP and GLP-1 are both secreted within minutes of nutrient ingestion and facilitate the rapid disposal of ingested nutrients. Both peptides share common actions on islet beta-cells acting through structurally distinct yet related receptors. Incretin-receptor activation leads to glucose-dependent insulin secretion, induction of beta-cell proliferation, and enhanced resistance to apoptosis. GIP also promotes energy storage via direct actions on adipose tissue, and enhances bone formation via stimulation of osteoblast proliferation and inhibition of apoptosis. In contrast, GLP-1 exerts glucoregulatory actions via slowing of gastric emptying and glucose-dependent inhibition of glucagon secretion. GLP-1 also promotes satiety and sustained GLP-1-receptor activation is associated with weight loss in both preclinical and clinical studies. The rapid degradation of both GIP and GLP-1 by the enzyme dipeptidyl peptidase-4 has led to the development of degradation-resistant GLP-1-receptor agonists and dipeptidyl peptidase-4 inhibitors for the treatment of type 2 diabetes. These agents decrease hemoglobin A1c (HbA1c) safely without weight gain in subjects with type 2 diabetes. GLP-1 and GIP integrate nutrient-derived signals to control food intake, energy absorption, and assimilation. Recently approved therapeutic agents based on potentiation of incretin action provide new physiologically based approaches for the treatment of type 2 diabetes.

Local glutathione redox status does not regulate ileal mucosal growth after massive small bowel resection in rats

Glutathione (GSH) concentration affects cell proliferation and apoptosis in intestinal and other cell lines in vitro. However, in vivo data on gut mucosal GSH redox status and cell turnover are limited. We investigated the effect of altered GSH redox status on the ileal mucosa in a rat model of short bowel syndrome following massive small bowel resection (SBR). Rats underwent 80% mid-jejunoileal resection (RX) or small bowel transection (TX; as operative controls), with administration of either saline or D, L-buthionine-sulfoximine (BSO), a specific inhibitor of cellular GSH synthesis. Ileal mucosal redox, morphology, and indices of cell proliferation and apoptosis were determined at different days after surgery. Ileal GSH redox status was assessed by GSH and GSH disulfide (GSSG) concentrations and the redox potential of GSH/GSSG (Eh). Ileal lipid peroxidation [free malondialdehyde (MDA)] was measured as an index of lipid peroxidation. BSO markedly decreased ileal mucosal GSH, oxidized GSH/GSSG Eh, and increased MDA content without inducing morphological damage as assessed by light or electron microscopy. As expected, SBR stimulated adaptive growth of ileal villus height and total mucosal height at 7 d after surgery, but this response was unaffected by BSO treatment despite a modest increase in crypt cell apoptosis. Ileal cell proliferation (crypt cell bromodeoxyuridine incorporation) increased at 2 d after SBR but was unaffected by BSO. Collectively, our in vivo data show that marked depletion of ileal GSH and oxidation of the GSH redox pool does not alter indices of ileal epithelial proliferation or SBR-induced ileal mucosal adaptive growth.

Vagal afferents are essential for maximal resection-induced intestinal adaptive growth in orally fed rats

Small bowel resection stimulates intestinal adaptive growth by a neuroendocrine process thought to involve both sympathetic and parasympathetic innervation and enterotrophic hormones such as glucagon-like peptide-2 (GLP-2). We investigated whether capsaicin-sensitive vagal afferent neurons are essential for maximal resection-induced intestinal growth. Rats received systemic or perivagal capsaicin or ganglionectomy before 70% midjejunoileal resection or transection and were fed orally or by total parenteral nutrition (TPN) for 7 days after surgery. Growth of residual bowel was assessed by changes in mucosal mass, protein, DNA, and histology. Both systemic and perivagal capsaicin significantly attenuated by 48-100% resection-induced increases in ileal mucosal mass, protein, and DNA in rats fed orally. Villus height was significantly reduced in resected rats given capsaicin compared with vehicle. Sucrase specific activity in jejunal mucosa was not significantly different; ileal mucosal sucrase specific activity was significantly increased by resection in capsaicin-treated rats. Capsaicin did not alter the 57% increase in ileal proglucagon mRNA or the 150% increase in plasma concentration of bioactive GLP-2 resulting from resection in orally fed rats. Ablation of spinal/splanchnic innervation by ganglionectomy failed to attenuate resection-induced adaptive growth. In TPN rats, capsaicin did not attenuate resection-induced mucosal growth. We conclude that vagal afferents are not essential for GLP-2 secretion when the ileum has direct contact with luminal nutrients after resection. In summary, vagal afferent neurons are essential for maximal resection-induced intestinal adaptation through a mechanism that appears to involve stimulation by luminal nutrients.

Gene expression in the adapting small bowel after massive small bowel resection

BACKGROUND

Intestinal adaptation occurs in the residual bowel following the loss or resection of a proportion of the small bowel. The purpose of the adaptive response is to return absorptive and digestive properties to near normal levels. This study employed a rat model of massive small bowel resection (MSBR) to study the adaptive response in the residual terminal ileum and the jejunum. The time points were chosen to reflect changes in gene expression early on in the response, because these are the genes that alter to initiate and maximize the response observed during adaptation.

METHODS

Sprague Dawley rats underwent an 80% resection. Differential display polymerase chain reaction (DD-PCR) analysis was performed on mRNA extracted from the remnant ileum and jejunum 0, 1, 2, 4, and 7 days post-MSBR.

RESULTS

DD-PCR identified 11 genes that were possibly regulated following MSBR. Genes confirmed to be regulated were 16S ribosomal RNA, lymphocyte antigen 6 (LY6)-like molecule, Krüppel-like factor-3 (KLF-3), G-protein-binding protein (CRFG), system A transporter 2 (SAT2), and an intestine-specific gene (similar to mKIAA0493).

CONCLUSIONS

DD-PCR analysis showed regulation of a number of genes not previously known to be involved in adaptation after MSBR or previously characterized in the intestine. These genes may be important in bringing about the complement of changes seen during the adaptive response.

Intestinal mucosal adaptation

Intestinal failure is a condition characterized by malnutrition and/or dehydration as a result of the inadequate digestion and absorption of nutrients. The most common cause of intestinal failure is short bowel syndrome, which occurs when the functional gut mass is reduced below the level necessary for adequate nutrient and water absorption. This condition may be congenital, or may be acquired as a result of a massive resection of the small bowel. Following resection, the intestine is capable of adaptation in response to enteral nutrients as well as other trophic stimuli. Identifying factors that may enhance the process of intestinal adaptation is an exciting area of research with important potential clinical applications.

Intestinal Adaptation in Short Bowel Syndrome

Short bowel syndrome occurs when there is insufficient length of the small intestine to maintain adequate nutrition and/or hydration status without supplemental support. This syndrome most frequently occurs following extensive surgical resection of the intestine, and the extent of adaptation depends on the anatomy of the resected bowel and the amount of bowel remaining. Following resection, the intestinal tissue undergoes morphologic and functional changes to compensate for the lost function of the resected bowel. These changes are mediated by multiple interactive factors, including intraluminal and parenteral nutrients, gastrointestinal secretions, hormones, cytokines, and growth factors, many of which have been well characterized in animal models. The amount of small bowel remaining is the most important predictor of adaptive potential; neither structural nor functional adaptative changes have been demonstrated in humans or animal models with more extreme resections resulting in an end-jejunostomy. The current understanding of these processes has led to the recent use of supplemental hormones, such as growth hormone and glucagon-like peptide 2, in intestinal rehabilitation programs and may lead to the development of pharmacologic agents designed to augment the innate adaptive response.

Diet and age affect intestinal morphology and large bowel fermentative end-product concentrations in senior and young adult dogs

The objective of this study was to determine the effects of age and diet on intestinal morphology and large bowel fermentative end-product concentrations in healthy dogs. Small intestinal villus width, height, and area, and small intestinal and colonic crypt depth were measured. Large bowel digesta samples were analyzed for ammonia, SCFAs, and branched-chain fatty acids (BCFAs). SCFAs are considered to be beneficial fermentative end-products in the intestine because they exert trophic effects on intestinal cells. Twelve senior (age = 11.1 y +/- 0.6 at baseline; 6 male, 6 female) and 12 young adult (age = 8 wk old at baseline; 6 male, 6 female) beagles were randomly assigned to 1 of 2 dietary treatments, an animal product-based diet (APB) and a plant product-based diet (PPB). Diets were fed for 12 mo. Jejunal (P = 0.03) and ileal (P = 0.02) villus height, and duodenal (P = 0.04) villus width were greater for dogs consuming the PPB diet. Young dogs had greater (P = 0.04) jejunal villus height, whereas senior dogs had greater (P < 0.001) colonic crypt depth. Ammonia concentrations decreased (P = 0.03) from proximal to distal colon and were higher in dogs consuming APB (P = 0.03). Age and treatment affected butyrate concentrations, with senior dogs (P = 0.04) and dogs consuming APB (P = 0.04) having higher concentrations. Both diet and age affected small and large intestinal morphology, and colonic fermentative end-product concentrations in dogs.

Enterocyte metabolism during early adaptation after extensive intestinal resection in a rat model

OBJECTIVE

A better knowledge of intestinal adaptation after resection is required to improve the nutritional support that is given to patients. The aim of this study was to understand the metabolic changes underlying early adaptation after massive intestinal resection.

METHODS

Rats were assigned to either 80% intestinal resection or transection. All animals received the same intragastric nutrition. On day 8, plasma glutamine turnover was measured. Substrate use was determined on isolated enterocytes that were incubated in the presence of D-[U-(14)C] glucose (2 mmol/L), L-[U-(14)C] glutamine (2 mmol/L), L-[U-(14)C] arginine (1 mmol/L), or L-[1-(14)C] ornithine (1 mmol/L).

RESULTS

Plasma glutamine turnover was similar in both groups. The rate of enterocyte glutamine use was significantly increased in the resection group, although the maximal glutaminase activity was unchanged. Glutathione generation was enhanced 3-fold in remnant intestine as compared with transected intestine (P <.05). L-ornithine decarboxylation was increased markedly in resected animals (P <.05), without any detectable change of maximal ornithine decarboxylase activity.

CONCLUSION

The early phase of intestinal adaptation after resection induces changes in enterocyte glutamine and ornithine metabolism that may be related, in part, to increased de novo polyamine synthesis. This observation suggests that a supplementation of artificial nutrition by nutrients that lead to the generation of trophic agents may be of potential interest.

A serum factor(s) after small bowel resection induces intestinal epithelial cell proliferation: effects of timing, site, and extent of resection

BACKGROUND/PURPOSE

After small bowel resection (SBR), serum induces proliferation in rat intestinal epithelial cells (RIEC-6). This study was designed to elucidate the effects of postoperative time interval, site, and magnitude of SBR on RIEC-6 proliferation.

METHODS

Serum was collected from rats at various times after a 75% mid-SBR or sham operation and added to RIEC-6 cells and growth determined over 5 days. In other experiments, cell growth was recorded in the presence of serum from rats after 25%, 50%, or 75% SBR, or after jejunal or ileal SBR.

RESULTS

SBR serum enhanced RIEC-6 cell proliferation as early as 12 hours after resection. The extent of SBR directly correlated with the level of adaptation; however, the effects on cell growth by the serum were similar. SBR serum induced proliferation equally after either proximal or distal resection.

CONCLUSIONS

Serum contains a factor that stimulates intestinal cell proliferation soon after SBR but independent of the degree or site of intestinal resection. Although humoral factor(s) play a role in the early induction of enterocyte proliferation after SBR, further modulation of adaptation to varied lengths or sites of intestinal resection are probably governed by mechanisms independent of factors that circulate in the serum.

Role of luminal nutrients and endogenous GLP-2 in intestinal adaptation to mid-small bowel resection

To elucidate the role of luminal nutrients and glucagon-like peptide-2 (GLP-2) in intestinal adaptation, rats were subjected to 70% midjejunoileal resection or ileal transection and were maintained with total parenteral nutrition (TPN) or oral feeding. TPN rats showed small bowel mucosal hyperplasia at 8 h through 7 days after resection, demonstrating that exogenous luminal nutrients are not essential for resection-induced adaptation when residual ileum and colon are present. Increased enterocyte proliferation was a stronger determinant of resection-induced mucosal growth in orally fed animals, whereas decreased apoptosis showed a greater effect in TPN animals. Resection induced significant transient increases in plasma bioactive GLP-2 during TPN, whereas resection induced sustained increases in plasma GLP-2 during oral feeding. Resection-induced adaptive growth in TPN and orally fed rats was associated with a significant positive correlation between increases in plasma bioactive GLP-2 and proglucagon mRNA expression in the colon of TPN rats and ileum of orally fed rats. These data support a significant role for endogenous GLP-2 in the adaptive response to mid-small bowel resection in both TPN and orally fed rats.

Dietary lipids alter the effect of steroids on transport of glucose after intestinal resection: Part II. Signalling of the response

BACKGROUND/PURPOSE

Glucocorticosteroids alter the function of the intestine. Budesonide (Bud) increases the jejunal D-glucose uptake, and this effect is prevented through a polyunsaturated fatty acid (PUFA) diet. This study was undertaken to assess the possible signalling effect of budesonide, prednisone (Pred), or dexamethasone (Dex) in animals with a 50% intestinal resection and fed chow or a diet enriched with saturated (SFA) or polyunsaturated fatty acids.

METHODS

Northern blots were performed.

RESULTS

Steroids reduced the jejunal but not the ileal expression of proglucagon. Ornithine decarboxylase (ODC) expression was reduced in the jejunum.

CONCLUSIONS

c-jun, ODC, and proglucagon may be involved in the adaptive response that occurs with steroids and variations in dietary lipids after intestinal resection.

Glucagon-like peptide 2 function in domestic animals

Glucagon-like peptide 2 (GLP-2) is a member of family of peptides derived from the proglucagon gene expressed in the intestines, pancreas and brain. Tissue-specific posttranslational processing of proglucagon leads to GLP-2 and GLP-1 secretion from the intestine and glucagon secretion from the pancreas. GLP-2 and GLP-1 are co-secreted from the enteroendocrine L-cells located in distal intestine in response to enteral nutrient ingestion, especially carbohydrate and fat. GLP-2 secretion is mediated by direct nutrient stimulation of the L-cells and indirect action from enteroendocrine and neural inputs, including GIP, gastrin-releasing peptide (GRP) and the vagus nerve. GLP-2 is secreted as a 33-amino acid peptide and is rapidly cleaved by dipeptidylpeptidase IV (DPP-IV) to a truncated peptide which acts as a weak agonist with competitive antagonistic properties. GLP-2 acts to enhance nutrient absorption by inhibiting gastric motility and secretion and stimulating nutrient transport. GLP-2 also suppresses food intake when infused centrally. The trophic actions of GLP-2 are specific for the intestine and occur via stimulation of crypt cell proliferation and suppression of apoptosis in mucosal epithelial cells. GLP-2 reduces gut permeability, bacterial translocation and proinflammatory cytokine expression under conditions of intestinal inflammation and injury. The effects of GLP-2 are mediated by a G-protein-linked receptor that is localized to the intestinal mucosa and hypothalamus. The intestinal localization of the GLP-2R to neural and endocrine cells, but not enterocytes, suggests that its actions are mediated indirectly via a secondary signaling mechanism. The implications of GLP-2 in domestic animal production are largely unexplored. However, GLP-2 may have therapeutic application in treatment of gastrointestinal injury and diarrheal diseases that occur in developing neonatal and weanling animals.

Locally and systemically active glucocorticosteroids modify intestinal absorption of sugars in rats

Glucocorticosteroids enhance digestive and absorptive functions of the intestine of weaning and adult rats. This study was undertaken to assess the influence of treatment of weaning male rats with budesonide (Bud), prednisone (Pred), or control vehicle on the in vitro jejunal and ileal uptake of glucose and fructose. Bud and Pred had no effect on the uptake of d-glucose by sodium glucose transporter-1. In contrast, the uptake of d-fructose by GLUT-5 was similarly increased with Bud and with Pred. The increases in the uptake of fructose were not due to variations in the weight of the intestinal mucosa, food intake, or in GLUT-5 protein or mRNA expression. There were no steroid-associated changes in mRNA expression of c-myc, c-jun, c-fos, proglucagon, or selected cytokines. However, the abundance of ileal ornithine decarboxylase mRNA was increased with Pred. Giving postweaning rats 4 wk of Bud or Pred in doses equivalent to those used in clinical practice increases fructose but not glucose uptake. This enhanced uptake of fructose was likely regulated by posttranslational processes.

Glucagon-like peptide 2 enhances intestinal epithelial restitution

BACKGROUND

Glucagon-like peptide 2 (GLP-2) is a potent intestinotrophic peptide that enhances recovery following intestinal injury. We tested the hypothesis that GLP-2 acutely enhances intestinal epithelial restitution.

MATERIAL AND METHODS

Rat jejunal segments were mounted in Ussing chambers. HCl (10 mM) was applied to the mucosal surfaces for 10 min to induce injury. Tissues were then lavaged with modified Ringer's solution and maintained in the chambers for an additional 3 h. Tissues were treated with 10 microM GLP-2 or vehicle alone (control). Electrical parameters were recorded, and tissues were salvaged for morphometric analysis.

RESULTS

GLP-2-treated tissues exhibited a significantly greater recovery of potential difference and resistance (P < 0.05) than did controls. Morphometric analysis revealed that columnar cells covered a greater percentage of the epithelial surface in GLP-2-treated tissues than in controls (P < 0.05).

CONCLUSIONS

These results suggest that GLP-2 acutely enhances intestinal epithelial restitution following acid-induced injury. This novel biological action of GLP-2 may contribute to its therapeutic effect in models of intestinal disease.

Enhancing bowel adaptation in short bowel syndrome

Malabsorption of both nonessential and essential nutrients, fluid, and electrolytes will, if not compensated for by increased intake, lead to diminished body stores and to subclinical and eventually clinical deficiencies. By definition, intestinal failure prevails when parenteral support is necessary to maintain nutritional equilibrium. After intestinal resection, adaptation, a progressive recovery from the malabsorptive disorder, may be seen. Research has focused on optimizing remnant intestinal function through dietary or pharmacologic interventions. In this review, factors responsible for the morphologic and functional changes in the adaptive processes are described. Results of clinical trials employing either growth hormone and glutamine or glucagon-like peptide-2 in short bowel patients are presented.

Biological actions and therapeutic potential of the glucagon-like peptides

The glucagon-like peptides (GLP-1 and GLP-2) are proglucagon-derived peptides cosecreted from gut endocrine cells in response to nutrient ingestion. GLP-1 acts as an incretin to lower blood glucose via stimulation of insulin secretion from islet beta cells. GLP-1 also exerts actions independent of insulin secretion, including inhibition of gastric emptying and acid secretion, reduction in food ingestion and glucagon secretion, and stimulation of beta-cell proliferation. Administration of GLP-1 lowers blood glucose and reduces food intake in human subjects with type 2 diabetes. GLP-2 promotes nutrient absorption via expansion of the mucosal epithelium by stimulation of crypt cell proliferation and inhibition of apoptosis in the small intestine. GLP-2 also reduces epithelial permeability, and decreases meal-stimulated gastric acid secretion and gastrointestinal motility. Administration of GLP-2 in the setting of experimental intestinal injury is associated with reduced epithelial damage, decreased bacterial infection, and decreased mortality or gut injury in rodents with chemically induced enteritis, vascular-ischemia reperfusion injury, and dextran sulfate-induced colitis. GLP-2 also attenuates chemotherapy-induced mucositis via inhibition of drug-induced apoptosis in the small and large bowel. GLP-2 improves intestinal adaptation and nutrient absorption in rats after major small bowel resection, and in humans with short bowel syndrome. The actions of GLP-2 are mediated by a distinct GLP-2 receptor expressed on subsets of enteric nerves and enteroendocrine cells in the stomach and small and large intestine. The beneficial actions of GLP-1 and GLP-2 in preclinical and clinical studies of diabetes and intestinal disease, respectively, has fostered interest in the potential therapeutic use of these gut peptides. Nevertheless, the actions of the glucagon-like peptides are limited in duration by enzymatic inactivation via cleavage at the N-terminal penultimate alanine by dipeptidyl peptidase IV (DP IV). Hence, inhibitors of DP IV activity, or DP IV-resistant glucagon-like peptide analogues, may be alternative therapeutic approaches for treatment of human diseases.

Locally and systemically active glucocorticosteroids modify intestinal absorption of lipids in rats

Orally administered systemically active steroids enhance the digestive and absorptive functions of the intestine, but their effect on lipid uptake is unknown. The effect of the locally acting steroid budesonide on intestinal absorptive function also is unknown. Accordingly, this study was undertaken to assess the influence of 4 wk of treatment of weaning male rats with a daily oral gavage of budesonide (BUD), prednisone (PRED), or control vehicle on the jejunal and ileal uptake of fatty acids and cholesterol. BUD enhanced jejunal uptake of oleic acid and ileal uptake of linoleic acid. PRED increased jejunal uptake of cholesterol and ileal uptake of lauric, palmitic, linoleic, and linolenic acids. Higher doses of BUD (up to 1 mg/kg) given to adult rats for 2 wk further increased the uptake of some lipids. The changes in the uptake of lipids were not due to variations in the weight of the intestinal mucosa or in the animals' food intake. Ileal ornithine decarboxylase mRNA expression was increased with PRED, but there were no steroid-associated changes in the expression of the mRNA of the early response genes c-myc, c-jun, or c-fos or of proglucagon, the liver fatty acid-binding protein (FABP), the ileal lipid-binding protein, tumor necrosis factor alpha, interleukin 2 (IL-2), IL-6, or IL-10. In summary, treatment of weanling rats with BUD and PRED enhances the uptake of some lipids by a process that is independent of the effects of early response genes and genes encoding cytokines, proglucagon, and FABP.

Intestinal ornithine decarboxylase in short bowel syndrome patients with oral diet

The major consequence of extensive intestinal resection is loss of absorptive surface area, which results in malabsorption of nutrients; this condition is known as short-bowel syndrome (SBS). Patients with extensive small intestinal resection and colectomy leading to jejunostomy have the most severe SBS. Ornithine decarboxylase (ODC) plays a central role in cell proliferation and in the process of gut adaptation. Polyamine synthesis in crypt cells mediates the action of extracellular growth factors on DNA synthesis and mitotic activity. The aim of this study was to examine ODC expression and activity, diamine oxidase (DAO) activity and polyamine levels in the jejunal mucosa and red blood cells of SBS patients with a jejunostomy. The study group consisted of 6 patients (4 men and 2 women, mean age 55.8+/-9.8 years), who had undergone extensive small bowel resection and colectomy. All patients were maintained on cyclic parenteral nutrition and non-restricted oral nutrition. Two groups of patients operated on for unrelated reasons were included as the jejunum control group (n=6) and the ileum control group (n=13). Non statistical differences were observed in polyamine levels of red blood cells versus the control group (spermidine: 21.0+/-3.6 vs. 17.7+/-1.1 and spermine: 17.1+/-8.6 vs. 13.2+/-1.6 nmol/ml RBC, respectively). No significant decreases in putrescine and spermidine levels were observed between the groups, but spermine levels in SBS jejunum were significantly lower than the controls (P<0.05). In SBS patients a significant decrease in ODC and DAO activity were observed vs jejunum. A significant decrease in ODC-mRNA abundance was found for the SBS patients as compared to the two control groups (P<0.05). These results suggest that in SBS patients with jejunostomy intestinal adaptation may be impaired.

Glucagon-like peptide 2 improves nutrient absorption and nutritional status in short-bowel patients with no colon

BACKGROUND & AIMS

Glucagon-like peptide 2 (GLP-2) is intestinotrophic, antisecretory, and transit-modulating in rodents, and it is mainly secreted from the intestinal mucosa of the terminal ileum and colon after food ingestion. We assessed the effect of GLP-2 on the gastrointestinal function in patients without a terminal ileum and colon who have functional short-bowel syndrome with severe malabsorption of wet weight (>1.5 kg/day) and energy (>2.3 MJ/day) and no postprandial secretion of GLP-2.

METHODS

Balance studies were performed before and after treatment with GLP-2, 400 microg subcutaneously twice a day for 35 days, in 8 patients (4-17 years from last bowel resection; 6 with Crohn's disease). Four patients received home parenteral nutrition (mean residual jejunum, 83 cm), and 4 did not (mean ileum resection, 106 cm). Biopsy specimens were taken from jejunal/ileal stomas, transit was measured by scintigraphy, and body composition was measured by dual-energy x-ray absorptiometry.

RESULTS

Treatment with GLP-2 improved the intestinal absorption of energy 3.5% +/- 4.0% (mean +/- SD) from 49.9% to 53.4% (P = 0.04), wet weight 11% +/- 12% from 25% to 36% (P = 0.04), and nitrogen 4.7% +/- 5.4% from 47.4% to 52.1% (P = 0.04). Body weight increased 1.2 +/- 1.0 kg (P = 0.01), lean body mass increased 2.9 +/- 1.9 kg (P = 0.004), fat mass decreased 1.8 +/- 1.3 kg (P = 0.007), and 24-hour urine creatinine excretion increased (P = 0.02). The time to 50% gastric emptying of solids increased 30 +/- 16 minutes from 89 to 119 minutes (P < 0.05). Small bowel transit time was not changed. Crypt depth and villus height were increased in 5 and 6 patients, respectively.

CONCLUSIONS

Treatment with GLP-2 improves intestinal absorption and nutritional status in short-bowel patients with impaired postprandial GLP-2 secretion in whom the terminal ileum and the colon have been resected.

Luminal and systemic signals trigger intestinal adaptation in the juvenile python

Juvenile pythons undergo large rapid upregulation of intestinal mass and intestinal transporter activities upon feeding. Because it is also easy to do surgery on pythons and to maintain them in the laboratory, we used a python model to examine signals and agents for intestinal adaptation. We surgically isolated the middle third of the small intestine from enteric continuity, leaving its mesenteric nerve and vascular supply intact. Intestinal continuity was restored by an end-to-end anastomosis between the proximal and distal thirds. Within 24 h of the snake's feeding, the reanastomosed proximal and distal segments (receiving luminal nutrients) had upregulated amino acid and glucose uptakes by up to 15-fold, had doubled intestinal mass, and thereby soon achieved total nutrient uptake capacities equal to those of the normal fed full-length intestine. At this time, however, the isolated middle segment, receiving no luminal nutrients, experienced no changes from the fasted state in either nutrient uptakes or in morphology. By 3 days postfeeding, the isolated middle segment had upregulated nutrient uptakes to the same levels as the reanastomosed proximal and distal segments, but it still lacked any appreciable morphological response. These contrasting results for the reanastomosed intestine and for the isolated middle segment suggest that luminal nutrients and/or pancreatic biliary secretions are the agents triggering rapid upregulation of transporters and of intestinal mass and that systemic nerve or hormonal signals later trigger transporter regulation but no trophic response.

Humoral factors in intestinal adaptation

The small bowel has a remarkable ability to adapt after injury, inflammation or resection. It has long been suggested that humoral factors, particularly enteroglucagon, epidermal growth factor, neurotensin and growth hormone/insulin-like growth factor I, might stimulate bowel growth. Of particular interest is the recent finding that glucagon-like peptide 2 (GLP-2), a product of the gene encoding proglucagon, exerts a trophic effect on the intestinal epithelium via a specific G-protein-coupled receptor. GLP-2 and/or these other trophic peptides might prove to have a role in the treatment of bowel diseases associated with structural or functional loss of the small bowel.

Analysis of intestinal adaptation gene expression by cDNA expression arrays

BACKGROUND

As a tool for determining gene expression on a genomic scale, cDNA microarrays are a promising new technology that can be applied to the study of complex physiologic processes. The objective of this study was to characterize the expression of individual genes and patterns of gene expression that might provide insight into the mechanism of intestinal adaptation after massive small bowel resection.

METHODS

Male ICR mice underwent a 50% proximal small bowel resection (SBR) or sham operation. After 3 days, the remnant ileum was harvested, weighed, and RNA extracted. Changes in gene expression were detected utilizing Clontech Atlas mouse cDNA expression arrays. Some of these changes were confirmed by reverse transcriptase-polymerase chain reactions (RT-PCR) and Northern blots.

RESULTS

Analysis of these cDNA arrays revealed changes in the expression of multiple genes, including those involved in cell cycle regulation, apoptosis, DNA synthesis, and transcriptional regulation. The patterns of expression were consistent with the increased cell proliferation and apoptosis observed during intestinal adaptation. A large number of genes not previously associated with intestinal adaptation were identified.

CONCLUSIONS

This technology may facilitate the elucidation of the intricate cellular mechanisms underlying intestinal adaptation.

New frontiers in the biology of GLP-2

Glucagon-like peptide-2 (GLP-2) is a 33 amino acid peptide hormone released from the intestinal endocrine cells following nutrient ingestion. GLP-2 exerts trophic effects on the small and large bowel epithelium via stimulation of cell proliferation and inhibition of apoptosis. GLP-2 also upregulates intestinal glucose transporter activity, and reduces gastric emptying and gastric acid secretion. The activity of GLP-2 is regulated in part via renal clearance and cleavage by the aminopeptidase dipeptidyl peptidase IV. In experimental models of intestinal disease, GLP-2 reversed parenteral nutrition-induced mucosal atrophy and accelerated the process of endogenous intestinal adaptation in rats following major small bowel resection. GLP-2 also markedly attenuated intestinal injury and weight loss in mice with chemically-induced colitis, and significantly reduced mortality, bacterial infection and intestinal mucosal damage in mice with indomethacin-induced enteritis. The actions of GLP-2 are transduced by a recently cloned glucagon-like peptide-2 receptor (GLP-2R) that represents a new member of the G protein-coupled receptor superfamily. The GLP-2R is expressed in a highly tissue-specific manner predominantly in the gastrointestinal tract and GLP-2R activation is coupled to increased adenylate cyclase activity. The available evidence suggests that the biological properties of GLP-2 merit careful therapeutic assessment in selected human diseases characterized by injury and defective repair of the gastrointestinal epithelium.

Ontogeny of intestinal nutrient transport

Children born prematurely lack the ability to digest and to absorb nutrients at rates compatible with their nutritional needs. As a result, total parenteral nutrition may need to be given. While this nutritional support may be life-saving, the baby who receives this therapy is exposed to the risks of possible sepsis, catheter dysfunction, and liver disease. The rodent model of postnatal development provides a useful framework to investigate some of the cellular features of human intestinal development. The up-regulation of intestinal gene expression and precocious development of intestinal nutrient absorption can be achieved by providing growth factor(s) or by modifying the composition of the maternal diet during pregnancy and nursing or the weaning diet of the infant. Accelerating the digestive and absorptive functions of the intestine would thereby allow for the maintenance of infant nutrition through oral food intake, and might possibly eliminate the need for, and risks of, total parenteral nutrition. Accordingly, this review was undertaken to focus on the adaptive processes available to the intestine, to identify what might be the signals for and mechanisms of the modified nutrient absorption, and to speculate on approaches that need to be studied as means to possibly accelerate the adaptive processes in ways which would be beneficial to the newborn young.Key words: absorption, adaptation, diet, peptides.

Differential jejunal and colonic adaptation due to resection and IGF-I in parenterally fed rats

Patients with severe short-bowel syndrome (SBS) often require long-term total parenteral nutrition (TPN) to maintain their nutritional status because of limited intestinal adaptation. Growth factors, including insulin-like growth factor I (IGF-I), are under investigation to promote intestinal adaptation and tolerance to oral feeding. We investigated structural and functional adaptation of the jejunum and colon in four groups of rats maintained with TPN for 7 days after a 60% jejunoileal resection and cecectomy or sham surgery and treatment with IGF-I or vehicle. Resection alone did not stimulate jejunal growth. IGF-I significantly increased jejunal mucosal mass, enterocyte proliferation, and migration rates. IGF-I decreased jejunal sucrase specific activity and reduced active ion transport and ionic permeability; resection alone had no effect. In contrast, resection significantly increased colonic mass and crypt depth but had no effect on active ion transport or ionic permeability. IGF-I had minimal effects on colonic structure. IGF-I but not resection stimulates jejunal adaptation, whereas resection but not IGF-I stimulates colonic growth in rats subjected to a model for human SBS. IGF-I treatment may improve intestinal adaptation in humans with SBS.

Keratinocyte growth factor enhances early gut adaptation in a rat model of short bowel syndrome

OBJECTIVE

To evaluate the effects of keratinocyte growth factor (KGF) on intestinal adaptation after resection of 85% of the small intestine and consider its potential application in short bowel syndrome (SBS).

STUDY DESIGN

Experimental study using a known model of SBS.

ANIMAL POPULATION

Thirty male Sprague Dawley rats.

METHODS

Four groups of animals were designated. Two groups underwent 85% resection of the small intestine, while the other two groups were sham-operated, undergoing transection and reanastomosis. Resected and sham-operated groups then received either 3 mg/kg KGF or vehicle subcutaneously daily for 3 days. Gut adaptation was evaluated by measurements of mucosal cellularity and biochemical activity in duodenal, jejunal, and ileal segments.

RESULTS

Significant small intestinal growth after bowel resection alone was confirmed in resected versus sham-operated rats. KGF further augmented this growth in the resected animals. Mucosal wet weight of the small intestine increased with resection and was further increased (by 20% or more) with KGF administration. Mucosal thickness, villus length, and crypt depth exhibited similar patterns of response. The KGF-induced increase in mucosal morphology was accompanied by increased mucosal DNA and protein content, followed by a trend toward increased mucosal enzyme activity. Histology demonstrated an increase in goblet cells in KGF-treated animals. In situ hybridization analysis demonstrated that KGF markedly increased mucosal expression of intestinal trefoil protein (ITF) mRNA.

CONCLUSIONS

KGF enhances gut growth, differentiation, and gene regulation during adaptation in rat small intestine after massive resection.

CLINICAL RELEVANCE

KGF may be beneficial in the management of veterinary and human patients undergoing massive intestinal resection.

mRNA levels of dipeptidyl peptidase IV decrease during intestinal adaptation

BACKGROUND

Glucagon-like peptide 2 (GLP-2) has recently been shown to be a potent enterotrophic factor that may mediate mucosal hyperplasia during intestinal adaptation. The intestinal brush-border protease dipeptidyl peptidase IV (DPP IV) cleaves GLP-2 to an inactive form. It has been postulated that DPP IV activity limits the enterotrophic activity of GLP-2 in rats and humans. Massive small bowel resection (MSBR) in rats is an animal model of intestinal adaptation that has been used successfully to characterize factors involved in the modulation of adaptation.

METHODS

Total RNA was extracted from normal terminal ileum or terminal ileum post-MSBR from Sprague-Dawley rats which were sacrificed 2, 4, and 7 days postresection. A partial rat DPP IV clone was isolated by reverse transcription polymerase chain reaction, and Northern blot analysis of rat DPP IV mRNA levels in normal small bowel and small bowel post-MSBR was performed.

RESULTS

Within normal small bowel, DPP IV mRNA levels were greatest in the terminal ileum; levels in the duodenum and jejunum were approximately 50% of those in the terminal ileum. DPP IV mRNA levels decreased in terminal ileum post-MSBR 2, 4, and 7 days after resection.

CONCLUSION

The decreased DPPIV gene expression suggests a novel mechanism by which the effects on mucosal growth of GLP-2 may be further enhanced, and further that GLP-2 may be a more useful therapeutic agent in humans than currently anticipated.

Biologic properties and therapeutic potential of glucagon-like peptide-2

BACKGROUND

Glucagon-like peptide-2 (GLP-2), a 33 amino acid, proglucagon-derived peptide with intestinotrophic activity, is secreted from enteroendocrine cells in the small and large intestine.

METHODS

This review describes recent advances in our understanding of GLP-2 physiology from rodent experiments in vivo.

RESULTS

GLP-2 administration induces mucosal epithelial proliferation in small and large bowel and stomach. GLP-2 is rapidly degraded by the enzyme dipeptidyl peptidase IV (DPP-IV) to produce the biologically inactive form GLP-2(3-33), however, GLP-2 analogs that confer resistance to DPP-IV exhibit enhanced biologic activity in vivo. GLP-2-treated bowel retains normal to enhanced functional absorptive capacity. Furthermore, GLP-2 infusion prevents total parenteral nutrition (TPN)-associated intestinal hypoplasia, and enhances bowel adaptation and nutrient absorption in rats following small bowel resection. GLP-2 also reverses weight loss and improves histologic and biochemical parameters of disease activity in mice with experimental colitis.

CONCLUSIONS

GLP-2 is an intestine-derived peptide with intestinotrophic properties that may be therapeutically useful in diseases characterized by intestinal damage or insufficiency.

Prototypic G protein-coupled receptor for the intestinotrophic factor glucagon-like peptide 2

Glucagon-like peptide 2 (GLP-2) is a 33-aa proglucagon-derived peptide produced by intestinal enteroendocrine cells. GLP-2 stimulates intestinal growth and up-regulates villus height in the small intestine, concomitant with increased crypt cell proliferation and decreased enterocyte apoptosis. Moreover, GLP-2 prevents intestinal hypoplasia resulting from total parenteral nutrition. However, the mechanism underlying these actions has remained unclear. Here we report the cloning and characterization of cDNAs encoding rat and human GLP-2 receptors (GLP-2R), a G protein-coupled receptor superfamily member expressed in the gut and closely related to the glucagon and GLP-1 receptors. The human GLP-2R gene maps to chromosome 17p13.3. Cells expressing the GLP-2R responded to GLP-2, but not GLP-1 or related peptides, with increased cAMP production (EC50 = 0.58 nM) and displayed saturable high-affinity radioligand binding (Kd = 0.57 nM), which could be displaced by synthetic rat GLP-2 (Ki = 0.06 nM). GLP-2 analogs that activated GLP-2R signal transduction in vitro displayed intestinotrophic activity in vivo. These results strongly suggest that GLP-2, like glucagon and GLP-1, exerts its actions through a distinct and specific novel receptor expressed in its principal target tissue, the gastrointestinal tract.

Early response of ornithine decarboxylase activity and energy metabolism to postsurgery refeeding in rat small intestine

INTRODUCTION

Enterocyte proliferation and cellular energy status are important to intestinal integrity after starvation and trauma. The proliferative response to nutrients is expressed in the activity of ornithine decarboxylase (ODC), but ODC activity and ATP level in the intestinal mucosa the first hours after surgery and immediate refeeding are not known.

METHODS

Male Wistar rats (240-280 g) were starved for 48 h and submitted to laparotomy with distal ileal transection, gastrostomy and jejunal instillation of either enteral formula or saline. The ODC activity and ATP content of the jejunal mucosa were analysed in samples taken at 1, 2, 4 and 6 h after surgery.

RESULTS

ODC activity increased and reached the highest peak at 2 h in the refed animals. ATP concentration and energy charge of jejunal mucosa were significantly reduced 6 h after surgery compared to initial levels, but there were no differences between animals that were refed or not. Intestinal transection did not stimulate ODC activity.

CONCLUSION

ATP levels in intestinal mucosa decreased after surgery, and early enteral feeding did not seem to prevent this decrease during the first 6 h. Refeeding immediately after surgery elicits an early but transient increase of ODC activity in rat jejunal mucosa.

Intestinal adaptation after extensive small bowel resection: differential changes in growth and insulin-like growth factor system messenger ribonucleic acids in jejunum and ileum

The distal small bowel exhibits greater adaptive growth than proximal segments after partial small intestine resection. To explore this process, we evaluated adaptive cellularity, intestinal insulin-like growth factor (IGF) system messenger RNA (mRNA) transcripts, and effects of recombinant IGF-I treatment in jejunum and ileum of adult rats. Gastrostomy-fed animals underwent 80% jejuno-ileal resection or intestinal transection and reanastomosis without resection, followed by infusion of human recombinant IGF-I (2.4 mg/kgXday) or vehicle. After 7 days, resected rats demonstrated modest adaptive growth in jejunum and marked cell proliferation in ileum. Resection increased IGF-I mRNA in both jejunum (183%) and ileum (249%) and up-regulated IGFBP-4 mRNA levels in both tissues. IGFBP-3 mRNA fell significantly in ileum after resection. IGF-I infusion modestly increased ileal cellularity after resection, but had no effect in jejunum. IGF-I markedly increased IGFBP-3 mRNA levels in jejunum after both transection and resection. These data confirm that bowel resection induces greater adaptive growth in ileum than jejunum. IGF-I administration modestly increases ileal, but not jejunal, growth after resection. Increased levels of intestinal IGF-I and IGFBP-4 mRNA suggest roles for IGF-I and IGFBP-4 in mediating small bowel adaptation. Higher levels of jejunal IGFBP-3 mRNA may be related to limited jejunal vs. ileal growth after extensive jejuno-ileal resection.

Tissue distribution of rat glucagon receptor and GLP-1 receptor gene expression

The regulation of glucose metabolism by glucagon and GLP-1 is well established, but novel functions for these and other proglucagon-derived peptides are less well defined. This paper highlights the diversity of both GLP-1 and glucagon activity by studying the tissue distribution of glucagon and GLP-1 receptor gene expression by both Southern blot analysis of RT-PCR products and nuclease protection assays. By Southern blot analysis of RT-PCR products, GLP-1 receptor mRNA was detected in lung, hypothalamus, hippocampus, cerebral cortex, kidney, pancreas, and throughout the gastrointestinal tract. Glucagon receptor expression was detected in liver, kidney, spleen, thymus, adrenal glands, pancreas, cerebral cortex, lung, and throughout the gastrointestinal tract. Nuclease protection assay revealed glucagon receptor expression to be highest in liver and kidney, whereas GLP-1 receptor expression was only detected by protection assay in lung, stomach, and large bowel. Despite previous evidence that other receptors for proglucagon-derived peptides may exist, no evidence of novel receptors or multiple isoforms of the glucagon and GLP-1 receptors was found, indicating that the two cloned receptors may mediate all the effects of proglucagon-derived peptides, or that novel receptors may share less homology with the glucagon and GLP-1 receptors than previously anticipated.

Trophic effects of glicentin on rat small-intestinal mucosa in vivo and in vitro

To define the role of glicentin the active site of enteroglucagon, we evaluated the trophic effects of recombinant rat glicentin on rat small intestine and IEC-6 cells. In vivo, a significant increase was observed in jejunal wet weight, protein content, DNA content, and alkaline phosphatase activity after the subcutaneous administration of 100 micrograms/kg per day of glicentin for 2 weeks. In the ileum, however, there were no significant differences between the control versus glicentin groups in any of these parameters. Ornithine decarboxylase (ODC) activity 3.5 h after an intraperitoneal injection of glicentin was increased in the jejunal mucosa, but not in the ileal mucosa. In vitro, glicentin, at a dose of more than 100 ng/ml, significantly increased both tritium-thymidine incorporation and the number of IEC-6 cells. These findings indicate that glicentin exerts direct trophic effects on the rat small-intestinal mucosa and on the rat small-intestinal cell line, IEC-6, and that this peptide appears to be an active site of enteroglucagon.

Development of morphological changes and ileal glucagon gene expression in the small intestine of lambs infected with Trichostrongylus colubriformis

To investigate the consequences of subclinical Trichostrongylus colubriformis infection on the intestinal mucosa and the associated changes in entero-glucagon gene expression, sheep were infected with 30000 larvae and killed 5, 10, 15 or 20 days after infection. Histological and cytological changes were examined. In the main site of infection, the upper duodenum, villous atrophy associated with crypt hyperplasia developed gradually. Cytological changes in the enterocytes appeared concurrently, characterized by a progressive reduction in brush border and in the number of ribosomes in the cytoplasm, changes in the internal structure of mitochondria, and enlargement of the intercellular spaces. Neither histological nor cytological modifications were found before day 15. At the same time, villous hypertrophy developed distally, beyond the main site of infection; this was interpreted as an adaptive response to parasitism. Enteroglucagon gene expression in the ileum was measured in parallel with the mucosal changes but did not reveal any difference between infected and control sheep. The results indicate that this gastrointestinal hormone does not have a major role in the response to nematode parasitism.

Short-chain fatty acids increase proglucagon and ornithine decarboxylase messenger RNAs after intestinal resection in rats

BACKGROUND

Intestinal adaptation is a complex physiological process that is not completely understood. Systemic administration of short-chain fatty acids (SCFAs) has been shown to facilitate adaptation to small bowel resection; however the mechanisms underlying this phenomena are unknown.

METHODS

Forty-six male Sprague-Dawley rats underwent an 80% jejunoileal resection and jugular catheterization. After surgery, rats were randomly assigned to receive standard total parenteral nutrition (TPN) or an isoenergetic, isonitrogenous TPN supplemented with SCFAs. On day 3 or 7 after surgery, ileal samples were removed for determination of mucosal wet weight, DNA, RNA, and protein concentrations. Total cellular RNA was extracted for use in Northern blot analysis to quantify proglucagon and ornithine decarboxylase messenger RNAs (mRNAs).

RESULTS

Total, mucosal, and submucosal weights were increased (p < .05) in the SCFA group both 3 and 7 days after surgery. Ileal DNA and RNA concentrations were increased (p < .05) in the SCFA group at both time points; however ileal protein concentration did not differ between groups until 7 days after resection. Levels of proglucagon and ornithine decarboxylase messenger RNAs were higher (p < .05) in the SCFA group at both time points.

CONCLUSION

The upregulation of proglucagon and ornithine decarboxylase gene expression may be the mechanism by which SCFAs facilitate intestinal adaptation.

Induction of intestinal epithelial proliferation by glucagon-like peptide 2

Injury, inflammation, or resection of the small intestine results in severe compromise of intestinal function. Nevertheless, therapeutic strategies for enhancing growth and repair of the intestinal mucosal epithelium are currently not available. We demonstrate that nude mice bearing subcutaneous proglucagon-producing tumors exhibit marked proliferation of the small intestinal epithelium. The factor responsible for inducing intestinal proliferation was identified as glucagon-like peptide 2 (GLP-2), a 33-aa peptide with no previously ascribed biological function. GLP-2 stimulated crypt cell proliferation and consistently induced a marked increase in bowel weight and villus growth of the jejunum and ileum that was evident within 4 days after initiation of GLP-2 administration. These observations define a novel biological role for GLP-2 as an intestinal-derived peptide stimulator of small bowel epithelial proliferation.

Altered polyamine biosynthesis with aging after massive proximal small bowel resection in rat

We examined the effect of aging on polyamine biosynthesis in the small intestine. Two groups of male Wistar rats (young; 10-week-old, n = 40; old; 24-month-old, n = 40) underwent either a jejunal transection and reanastomosis or 90% proximal small bowel resection. The rats were sacrificed on the 1st, 2nd, 4th, and 7th postoperative day (POD). The mucosa was submitted for histological examination, weighed, and assayed for protein, DNA, RNA, and polyamine content. Ornithine decarboxylase (ODC) activity was measured and ODC mRNA in the mucosa was determined by Northern blot analysis. Compared with the values for wet weight and protein content in old rats, young rats showed significantly higher values for wet weight on the 1st and 2nd POD, and for protein content on the 1st POD, but there were no differences between young and old rats after the 4th POD. The values for ODC activity and ODC mRNA were significantly lower in old rats than in young rats on the 1st POD, but there were no differences between young and old rats after the 2nd POD. The value for putrescine in old rats was significantly lower on the 2nd POD, but was significantly higher on the 4th POD than that in young rats. The present study showed that, in old rats, the residual intestine after small bowel resection preserved sufficient adaptive capacity, but that the adaptive response was decreased. The findings in this study also suggest that a decrease in ODC mRNA expression is involved in the decreased adaptive response that occurs with aging.

Time course of adaptive regulatory peptide changes following massive small bowel resection in the dog

Basal and postprandial concentrations of gastrointestinal hormones were measured in 12 dogs before and at one and three months after a 75% small bowel resection. Five animals were studied again at six months. Concentrations of enteric hormones and neuropeptides, measured in the proximal jejunum and distal ileum adjacent to the anastomotic site at the time of euthanasia, were compared with concentrations in control tissues taken from each animal at the time of resection. Increased basal and postprandial levels of gastrin (P < 0.05), cholecystokinin (CCK, P < 0.05), glucose-dependent insulinotropic peptide (GIP, P < 0.01), peptide YY (PYY, P < 0.001), and enteroglucagon (P < 0.001), were seen at one month after small bowel resection. In contrast, no significant changes were seen in concentrations of secretin, motilin, neurotensin, somatostatin, PP, or glucagon. Concentrations of enteroglucagon, GIP, and PYY remained high throughout the six-month study period. In contrast, gastrin and CCK had normalized by three months. Thus, only enteroglucagon, PYY, and GIP showed sustained elevations following enterectomy; the gastrin and CCK changes were transient. Following enterectomy, concentrations of vasoactive intestinal polypeptide (VIP) were reduced by about 50% in mucosal (P < 0.001) and muscle (P < 0.05) layers of proximal and distal gut. In contrast, calcitonin gene-related peptide (CGRP) was increased by about twofold in jejunal and ileal mucosa (P < 0.05), and CGRP elevations were even more marked in the muscle layers (P < 0.001). Somatostatin and neuropeptide Y (NPY) concentrations were similar to controls in all areas except for a small decrease in NPY in ileal mucosa (P < 0.05). These findings suggest that the increased motilin and PP concentrations previously reported after bowel resection in man are more likely to reflect underlying inflammatory bowel disease rather than enterectomy. The normalization of hypergastrinemia explains why the increased acid secretion after small bowel resection is transient. These results provide evidence for independent secretory control of enteroglucagon and PYY, which are both products of intestinal L cells. In addition, these studies reveal marked changes in enteric neuropeptide concentrations following bowel resection. VIP, which is thought to be a major inhibitory transmitter in the gut, is markedly reduced, while CGRP, which is mainly localized in sensory afferent fibers, is increased. These major neuropeptide changes are likely to be of importance in the adaptive responses to massive small bowel resection.

Increased ileal proglucagon expression after jejunectomy is not suppressed by inhibition of bowel growth

After jejunectomy, a rapid and sustained increase in the abundance of proglucagon mRNA occurs in residual ileum and is accompanied by increases in plasma intestinal proglucagon-derived peptides. This response may be a component of adaptive growth, or proglucagon-derived peptides may regulate adaptive growth. To distinguish these possibilities, rats were treated with difluoromethylornithine, blocking ornithine decarboxylase activity and thereby adaptive bowel growth. Three groups fed ad libitum were compared: (1) resect: rats with 80% proximal small bowel resection; (2) resect + difluoromethylornithine: resected rats given difluoromethylornithine in drinking water; and (3) transect: transected controls. Six days after surgery, the resect + difluoromethylornithine group demonstrated inhibition of adaptive bowel growth. Abundance of ileal proglucagon mRNA in resect and resect + difluoromethylornithine groups was double that in the transect group (P < 0.02), whereas ornithine decarboxylase mRNA levels did not differ. Plasma enteroglucagon and glucagon-like peptide-I levels were greater in resect than transect groups (P < 0.002) and did not differ between resect and resect + difluoromethylornithine groups. The rise in ileal proglucagon mRNA after proximal small bowel resection is not inhibited by difluoromethylornithine despite blocking bowel growth and, therefore, is not merely a component of adaptive growth. Proglucagon-derived peptides are possible modulators of adaptive bowel but cannot stimulate growth when ornithine decarboxylase activity is inhibited.

Enteroglucagon, but not CCK, plays an important role in pancreatic hyperplasia after proximal small bowel resection

The present study was performed to examine the role played by pancreatotrophic factors, especially enteroglucagon and cholecystokinin (CCK), in the compensatory pancreatic hyperplasia observed after proximal small bowel resection (PSBR). Male Wistar rats were randomized into two groups, receiving either PSBR or transection (TRC). Five animals from each group were randomly selected for treatment with FK-480, a novel CCK antagonist. Four weeks after the operation, plasma levels of gastrin, CCK and enteroglucagon were measured. Pancreatic wet weight and protein, DNA, RNA and enzyme content were also determined. The wet weight and content of protein, DNA and RNA were significantly higher in PSBR rats than in TRC rats, regardless of whether they received FK-480. FK-480 had no suppressive effects on adaptive pancreatic growth after PSBR. Plasma enteroglucagon levels rose significantly in PSBR rats, and there were positive correlations between plasma enteroglucagon levels and pancreatic protein, DNA and RNA content. These findings demonstrated that plasma CCK was not the major trophic factor operating in the pancreas after PSBR, and showed that enteroglucagon plays an important role in the pancreatic hyperplasia that occurs after PSBR.

Purification and sequence of rat oxyntomodulin

Structural information about rat enteroglucagon, intestinal peptides containing the pancreatic glucagon sequence, has been based previously on cDNA, immunologic, and chromatographic data. Our interests in testing the physiological actions of synthetic enteroglucagon peptides in rats required that we identify precisely the forms present in vivo. From knowledge of the proglucagon gene sequence, we synthesized an enteroglucagon C-terminal octapeptide common to both proposed enteroglucagon forms, glicentin and oxyntomodulin, but sharing no sequence overlap with glucagon. We then developed a radioimmunoassay using antibodies raised against the octapeptide that was specific for enteroglucagon peptides without cross-reacting with glucagon. Rat intestine was extracted, and one presumptive enteroglucagon form was purified by following the enteroglucagon C-terminal octapeptide-like immunoreactivity through several HPLC purification steps. Structural characterization of the material by amino acid composition, microsequence, and mass spectral analyses identified the peptide as rat oxyntomodulin. The 37-residue peptide consists of pancreatic glucagon plus the C-terminal extension, Lys-Arg-Asn-Arg-Asn-Asn-Ile-Ala. This now permits synthesis of an unambiguous duplicate of endogenous rat oxyntomodulin for physiological studies.