Ileal glucagon gene expression: ontogeny and response to massive small bowel resection

A case of unexpected regeneration of small intestinal mucosal necrosis

If full-thickness mucosa, including the mucosal crypt, has been almost denuded, mucosa cannot regenerate as has been shown by animal models. The authors experienced an unusual mucosal regeneration exceed denuded bowel that occur in midgut volvulus of duration 2 days in a 6-day-old infant. Santulli's jejunostomy was performed using seriously denuded small bowel to prevent short bowel syndrome, despite the risks of leakage or stricture. Subsequently, stomal mucosa was fully regenerated when grossly identified 19 days after the second operation without any surgical complications.

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.

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.

The role of polyamines in glucagon-like peptide-2 prevention of TPN-induced gut hypoplasia

Total parenteral nutrition (TPN) of rats has been demonstrated to produce hypoplasia of gut mucosa, and to be associated with reduced immune response and elevated translocation of bacteria from gut to mesenteric lymph nodes, spleen and liver. Treatment of rats being maintained on TPN with the proglucagon fragment, glucagon-like peptide-2 (GLP-2), has been shown to totally prevent small intestine mucosal hypoplasia. In the present study, we found that depletion of polyamines with alpha-difluromethylornithine (DFMO) significantly reduced the efficacy of GLP-2 in preserving gut mucosa in rats maintained on TPN for 8 days. Co-infusion of GLP-2 with TPN prevented loss of protein and mucosa in duodenum, jejunum and ileum, but not in colon. Addition of DFMO to the infusate prevented the protective effects of GLP-2 in the duodenum and jejunum. In the jejunum, putrescine and spermidine were reduced in DFMO-treated rats, while the ileum exhibited reductions of these polyamines in rats infused with TPN or TPN plus GLP-2. DFMO infusion further reduced these polyamines in the ileum, while levels of spermine were increased. Concentrations of ornithine decarboxylase were elevated in jejunum of rats infused with TPN or TPN plus GLP-2, but were reduced significantly in DFMO-treated rats. These results suggest that normal levels of polyamines are necessary for the expression of GLP-2-induced hyperplasia. Differential effects of GLP-2 and DFMO across gut segments may relate to regional differences in proliferative and anti-apoptotic effects of the treatments.

Mechanisms of enteral nutrient-enhanced intestinal adaptation

The role of enteral nutrients in maintaining small intestinal structure and function is well established. Evidence that enteral nutrients induce intestinal adaptation include the structural and functional gradient along the length of the healthy intestine, the atrophy and functional compromise induced by fasting and parenteral nutrition, and the enhanced adaptive capacity of the distal intestine following partial enterectomy. Key mechanisms contributing to enteral nutrient-induced intestinal adaptation include nonspecific luminal stimulation and that provided by specific nutrients, "functional workload" induced by polymeric nutrients, potential stimulation of pancreaticobiliary secretions, secretion of humoral mediators, and induction of intestinal hyperemia.

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.

Late regeneration of infarcted small bowel mucosa: a case report

Ischemic injury of the small bowel may recover after revascularization, provided that full-thickness infarction did not occur. Animal studies showed that if the mucosal crypts remain viable, rapid mucosal restitution occurs hours after injury. The treatment of transmucosal infarction that does not extend to full wall thickness, however, was not investigated thoroughly. The patient presented had a mesenteric event leading to resection of about half of his small bowel. The unresected segment had severe ischemic injury, which seemed to cause transmucosal, but not transmural, infarction. Imaging of the remaining small bowel revealed a seromuscular layer denuded of mucosa. The ischemic damage was too deep to allow rapid regeneration, and the patient had short-bowel syndrome. A year later, during operation for stricture complications, new mucosa covered parts of the small-bowel surface, encouraging the surgeon to elect a conservative approach. Sixteen months after the injury, normal mucosa covered the entire small bowel, and enteral feeding resumed successfully. This report shows that infarcted small-bowel mucosa may regenerate even months after injury.

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.

Intestinal rehabilitation and the short bowel syndrome: part 1

The management of patients with intestinal failure due to short bowel syndrome (SBS) is complex, requiring a comprehensive approach that frequently necessitates long-term, if not life-long, use of parenteral nutrition (PN). Despite tremendous advances in the provision of PN over the past three decades, which have allowed significant improvements in the survival and quality of life of these patients, this mode of nutritional support carries with it significant risks to the patient, is very costly and, ultimately, does not attempt to improve the function of the remaining bowel. Intestinal rehabilitation refers to the process of restoring enteral autonomy and, thus, allowing freedom from parenteral nutrition, usually by means of dietary, medical, and, occasionally, surgical strategies. While recent investigations have focused on the use of trophic substances to increase the absorptive function of the remaining gut, whether intestinal rehabilitation occurs as a consequence of enhanced bowel adaptation or is simply a result of an optimized, comprehensive approach to the care of these patients remains unclear. In Part 1 of this review, an overview of SBS and pathophysiological considerations related to the remaining bowel anatomy in these patients will be provided. Additionally, a review of intestinal adaptation and factors that may enhance the adaptive process, focusing on evidence derived from animal studies, will also be discussed. In Part 2, relevant data on the development of intestinal adaptation in studies involving humans will be reviewed as will the general management of SBS. Lastly, the potential benefits of a multidisciplinary intestinal rehabilitation program in the care of these patients will also be discussed.

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.

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.

Influence of diet complexity on intestinal adaptation following massive small bowel resection in a preclinical model

AIMS

To investigate the effect of dietary complexity on intestinal adaptation using a preclinical model.

METHODS

Four-week-old piglets underwent a 75% proximal small bowel resection or transection operation (control). Post-operatively, animals received either pig chow (n = 15), polymeric formula (n = 9), polymeric formula plus fiber (n = 6), or elemental formula (n = 7).

RESULTS

The weight gain of all groups was reduced compared with controls that were fed the same diet. Animals that had a resection, which were fed elemental formula, had significantly reduced weight gain compared with the other groups (4.7 4.2 vs 30.7 7.1 kg chow and 11.5 1.3 kg polymeric formula). Villus height was increased in the jejunum, ileum and terminal ileum of resected animals compared with controls in animals fed with pig chow, polymeric formula and elemental formula. The animals that had a resection had a significant reduction in the transepithelial conductance (10.4 5.5 vs 25.4 6.5 mS/cm2) and 51Chromium-EDTA flux (2.8 1.9 vs 4.8 4.9 microL/h per cm2) compared with the controls.

CONCLUSIONS

A complex diet was found to be superior to an elemental diet in terms of the morphological and functional features of adaptation following massive small bowel resection.

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.

Intestinal adaptation in short bowel syndrome

Regaining enteral autonomy after extensive small bowel resection is dependent on intestinal adaptation. This adaptational process is characterized by hyperplastic growth of the remaining gut, which is accompanied by both an increase of cell division at the level of the crypt cells and by an increased rate of programmed cell death (apoptosis). Apart from the absorptive function, the small bowel also has a barrier function and plays an important role in interorgan metabolism. Also, these functions are greatly affected by a massive intestinal resection and subsequent recovery by intestinal adaptation. This review aims to give an overview of the debilitating effects of massive intestinal resection on gut function and subsequently discusses intestinal adaptation and possible factors stimulating adaptation.

Maintaining gut integrity during parenteral nutrition of tumor-bearing rats: effects of glucagon-like peptide 2

Maintaining tumor-bearing rats on total parenteral nutrition (TPN) for eight days significantly reduced mass, protein, and DNA in small intestine and colon. Coinfusion of glucagon-like peptide 2 (GLP-2) significantly increased each of these variables in the duodenum, jejunum, and ileum, but not in the colon. Histological analysis of tissue revealed normal mucosa thickness and villus height in the small intestine of GLP-2-treated rats, whereas non-treated rats maintained on TPN exhibited villus shortening and thinning of the mucosa. Compared with TPN alone, no significant effects of GLP-2 were noted on tumor growth, liver weight, or heart weight. Coinfusion of GLP-2 with TPN had no significant effect on TPN-associated immunosuppression, as measured by mitogen-induced proliferation of cultured splenocytes. Although translocation of bacteria to the mesenteric lymph nodes appeared to be reduced in GLP-2-treated rats, the difference between groups was not statistically significant. These results suggest that hormonal alterations may be more important than an absence of luminal nutrition in TPN-associated mucosa changes in tumor-bearing rats. Additionally, maintenance of gut integrity during TPN does not appear to be a sufficient condition for the avoidance of the negative sequelae associated with this route of supplemental nutrition.

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.

Glucagon-like peptide-2 stimulates gut mucosal growth and immune response in burned rats

Major burn trauma often leads to reduced gut barrier function, immunosuppression, and increased bacterial translocation. We hypothesized that treatments that maintain normal gut after burn trauma will also reduce immunosuppression and bacterial translocation. Recent studies suggest that treatment with glucagon-like peptide-2 (GLP-2), which is synthesized in the intestine and released after food intake, elicits mucosal hyperplasia in the small intestine of rodents and prevents parenteral nutrition-induced gut hypoplasia. Therefore, we determined whether GLP-2 would prevent loss of gut integrity after major burn trauma. Osmotic minipumps were implanted into the peritoneum of 22 adult, male, Sprague-Dawley rats to infuse saline (10 microl/hr; n = 14) or GLP-2 (1 microg/hr; n = 8). On the next day 8 saline-infused and 8 GLP-2-infused rats were subjected to a 25 sec duration 30% BSA open flame burn, with the remaining rats serving as sham-burn controls. Five days after burn, all rats were killed. Gut protein was assessed, and immunosuppression was estimated by the mitogenic response of cultured splenocytes to phytohemagglutinin, pokeweed, and concanavalin A. Bacterial translocation was determined by culturing the mesenteric lymph nodes. Although protein content was significantly decreased in the ileum of burned rats treated with saline, the burned rats treated with GLP-2 exhibited significant increases in protein levels in duodenum, jejunum. and ileum. Colon protein was not affected by GLP-2 infusion. Saline-treated burned rats also exhibited immunosuppression, as suggested by significantly decreased responses to each of the mitogens. Infusion of GLP-2 normalized the response by the burned rats to each of the mitogens. Lymph nodes taken from sham rats exhibited no colony forming units, whereas in both of the burn groups, 50% of the cultures were positive. However, more aggressive colonization may have occurred in the saline-infused burned rats as compared with the GLP-2-infused burned rats (81 +/- 63 vs 3 +/- 2 colony forming units). These results suggest that GLP-2 may stimulate gut mucosa and reduce immunosuppression in burned rats. However, there does not seem to be a statistically significant positive effect of GLP-2 on bacterial translocation. Thus, improving small intestine mucosa may increase immunity while being ineffective against bacterial translocation.

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.

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.

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.

Glucagonlike peptide-2 enhances small intestinal absorptive function and mucosal mass in vivo

PURPOSE

Glucagonlike peptide-2 (GLP-2) is a 33-amino acid peptide that appears to be highly tissue specific for the intestine. This study was designed to examine the effect of systemically administered GLP-2 on intestinal absorptive function and mucosal mass, and determine the in vivo dose-response curves for this new peptide.

METHODS

Twenty-five young adult male Sprague-Dawley rats had placement of jugular venous catheters connected to subcutaneously placed osmotic minipumps. The rats were divided into five groups based on the contents in the osmotic pump: group 1 (control, n = 5) normal saline and groups 2, 3, 4, and 5 (n = 5 each) were given GLP-2 at 5, 50, 250, and 500 microg/kg/d, respectively. After a 14-day infusion, [C14] galactose and [C14] glycine absorption were measured in a 10-cm segment of midsmall intestine using an in vivo closed-recirculation technique. Mucosal DNA content and protein content of the same small bowel segment were also determined for each group. Statistical analysis was performed by analysis of variance (ANOVA).

RESULTS

GLP-2 significantly increased galactose absorption at a dose of 50 (P<.01), 250 (P<.01), and 500 (P<.05) microg/kg/d and glycine absorption at a dose of 50, 250, and 500 microg/kg/d (P<.01). GLP-2 also significantly increased mucosal DNA content at a dose of 50 (P<.01) and 250 (P<.05) microg/kg/d and protein content at a dose of 50 and 250 microg/kg/d (P<.01).

CONCLUSIONS

These data demonstrate that GLP-2 can enhance normal rat small intestine mucosal mass and absorption in vivo with the maximum effect seen at 50 microg/kg/d.

Short bowel syndrome in pediatric patients

The treatment of infants and children with short bowel syndrome aims at restoring the intestinal continuity and at improving the physiological process of gut adaptation. Mucosal hyperplasia allows the remaining gut to ensure an adequate digestion and an absorption process leading to intestinal autonomy. During the period of adaptation, appropriate parenteral and/or enteral feeding must be directed at maintaining an optimal nutritional status. Delay of intestinal autonomy depends on the characteristics of the residual intestine: length, presence of the ileocecal valve and colon, and motor function. Bacterial overgrowth compromises intestinal adaptation and increases the risk of liver disorders. Few patients will remain long-term dependent on parenteral nutrition. All approaches aimed at achieving intestinal autonomy should be tried: use of trophic factors, intestinal tapering, and lengthening. In a few residual patients, permanent intestinal failure or extreme short bowel syndrome require intestinal transplantation.

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.

Intestinal response to growth factors administered alone or in combination with human [Gly2]glucagon-like peptide 2

The control of intestinal epithelial growth is regulated by interactions of growth factors in various cellular compartments of the small and large bowel. Little information is available on the intestinal growth response to combinations of growth factors. We studied the intestinotrophic properties of a dipeptidyl peptidase IV resistant glucagon-like peptide 2 (GLP-2) analog, human [Gly2]GLP-2 (h[Gly2]GLP-2), as well as of epidermal growth factor (EGF), long [Arg3]insulin-like growth factor I (LR3IGF-I), [Gly1]IGF-II, and human growth hormone (hGH), administered by subcutaneous injection alone or in combination in mice. At the doses tested, h[Gly2]GLP-2 was the most potent agent for increasing small and large bowel mass. Mice treated with h[Gly2]GLP-2 and either GH or IGF-I exhibited greater increases in histological parameters of small intestinal growth than did mice treated with h[Gly2]GLP-2 alone. Administration of all five growth factors together induced significant increases in crypt plus villus height and in small and large bowel length and weight. The results of these experiments define regional differences in both the cellular targets and relative activities of intestinotrophic molecules and raise the possibility that selective growth factor combinations may be useful for enhancement of intestinal adaptation in vivo.

A physiological level of rhubarb fiber increases proglucagon gene expression and modulates intestinal glucose uptake in rats

Previous work demonstrated that a high fiber diet upregulates proglucagon mRNA and secretion of glucagon-like peptide-1 [GLP-1(7-37)] and insulin compared with an elemental fiber-free diet. This study examined whether similar intakes of fibers differing in physiochemical and fermentative properties alter the expression of intestinal hormones and intestinal absorptive properties. Sprague-Dawley rats were fed either a 50 g/kg cellulose or rhubarb fiber diet for 14 d. Ileal proglucagon mRNA levels were significantly higher in rats fed rhubarb fiber than in those fed cellulose fiber (9.3 +/- 0.9 vs. 6.2 +/- 1.0 densitometer units). Proglucagon mRNA in the colon did not differ between diet treatments. Plasma c-peptide concentrations were significantly higher 30 min after an oral glucose tolerance test in the rhubarb vs. cellulose group (1627 +/- 67 vs. 1290 +/- 71 pmol/L). Passive permeability, measured by the uptake of L-glucose, was significantly higher in the jejunum of rats fed cellulose compared with those fed rhubarb fiber. Adjusting total glucose uptake for passive permeability and unstirred water layer resistance resulted in a higher Km being calculated for the jejunum and ileum of the cellulose fiber group. Jejunal and ileal carrier-mediated uptakes (Vmax) were not altered by diet and reflected the lack of difference between groups in sodium-dependent glucose cotransporter (SGLT-1) and sodium-independent glucose transporter (GLUT2) mRNA levels. Replacing cellulose fiber with rhubarb fiber in a diet upregulated ileal proglucagon mRNA and resulted in a reduced passive permeability but did not affect glucose transport of the small intestine. This work establishes the importance of dietary fiber fermentability in modulating intestinal proglucagon expression and possibly glucose homeostasis.

Diet and gene expression in the intestine

Gene expression is central to the pathogenesis of many disorders. An ability to alter the expression of genes would, if their relationship to disease processes were fully understood, constitute a new modality of treatment. This review examines the evidence that nutritional factors can regulate genes in the gastrointestinal epithelium and it discusses the physiological relevance of such alterations in gene expression. Dietary regulation of the genes expressed by the epithelium confers three fundamental advantages for mammals. It enables the epithelium to adapt to the luminal environment to digest and absorb food better; it provides the means whereby mother's milk can influence the development of the gastrointestinal tract; when the proteins expressed by the epithelium act on the immune system, it constitutes a signalling mechanism from the intestinal lumen to the body's defences. Each of these mechanisms is amenable to manipulation for therapeutic purposes.

Growth hormone and glutamine do not stimulate intestinal adaptation following massive small bowel resection in the rat

BACKGROUND

Certain nutrients and other trophic factors are highly sensitive stimulants of intestinal adaptation following short bowel syndrome. Growth hormone and glutamine in a modified diet have been shown to enhance nutrient absorption in patients with severe short bowel syndrome. However, neither growth hormone nor glutamine is capable of enhancing adaptation in an animal model. This study was conducted to determine if the combination of glutamine and growth hormone could enhance gut adaptation following massive small bowel resection in the rat.

METHODS

Thirty-four male rats received 70% jejunoileal resection. The first group received glycine and rat growth hormone, the second glutamine and rat growth hormone, and the third glycine but no growth hormone.

RESULTS

There was no evidence that the combination of glutamine and growth hormone could enhance mucosal mass, mucosal protein, or mucosal DNA levels relative to the other two control groups of animals. Likewise, sucrase activities were not enhanced by glutamine and growth hormone.

CONCLUSIONS

It is unlikely that the combination of glutamine and growth hormone will be of benefit in the treatment of patients with short bowel syndrome. The results in previous human studies can be alternatively explained by the long-term nonspecific effect of enteral nutrition on gut adaptation.

Intestinal function in mice with small bowel growth induced by glucagon-like peptide-2

Glucagon-like peptide-2 (GLP-2) stimulates small intestinal growth through induction of intestinal epithelial proliferation. To examine the physiology of GLP-2-induced bowel, mice were treated with GLP-2 (2.5 micrograms) or vehicle for 10 days. Small intestinal weight increased to 136 +/- 2% of controls in GLP-2-treated mice, in parallel with 1.4 +/- 0.1- and 1.9 +/- 0.5-fold increments in duodenal RNA and protein content, respectively (P < 0.05-0.001). Similarly, the activities of duodenal maltase, sucrase, lactase, glutamyl transpeptidase, and dipeptidyl-peptidase IV (215 +/- 28% of controls; P < 0.001) were increased by GLP-2. Oral or duodenal administration of glucose or maltose did not reveal any differences in the ability of GLP-2-treated mice to absorb these nutrients, possibly because of decreases in expression of the glucose transporters sodium-dependent glucose transporter-1 (SGLT-1) and GLUT-2. In contrast, absorption of leucine plus triolein was increased after duodenal administration in GLP-2-treated mice (P < 0.01-0.001). Finally, GLP-2 did not alter other markers of intestinal or pancreatic gene expression, including levels of mRNA transcripts for ornithine decarboxylase, multidrug resistance gene, amylase, proglucagon, proinsulin, and prosomatostatin. Thus induction of intestinal growth by GLP-2 in wild-type mice results in a normal-to-increased capacity for nutrient digestion and absorption in vivo.

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.

Short bowel syndrome in children and small intestinal transplantation

This article discusses the management of short bowel syndrome from the time of intestinal resection until the patient either recovers free of supplemental parenteral and enteral nutrition or progresses to the point of needing intestinal transplantation. The importance of aggressive use of enteral feedings is emphasized, especially in relation to the process of intestinal adaptation. An approach to the various complications of short bowel syndrome, especially small bowel bacterial overgrowth, is discussed. Surgical options short of transplantation also are described. Intestinal transplantation and its present and future roles in the management of patients with short bowel syndrome are discussed.

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.

Role of bombesin on gut mucosal growth

OBJECTIVE

The authors examined the effects of exogenous bombesin (BBS) on gut mucosal growth in chow-fed rats and the mucosal regeneration after gut atrophy brought about by feeding an elemental diet and after intestinal injury produced by methotrexate (MTX).

SUMMARY BACKGROUND DATA

Bombesin is one of many gastrointestinal peptides implicated in the regulation of gut mucosal growth. Although BBS is known to stimulate growth of normal pancreatic tissue, the trophic effect of BBS on gut mucosa is less clear and its exact role in gut mucosal regeneration and repair is not known.

METHODS

Rats were fed a regular chow diet (control) or an elemental diet plus either saline or BBS (10 micrograms/kg). In another experiment, rats fed a chow diet and treated with saline or BBS were given MTX (20 micrograms/kg) or a single intraperitoneal injection. In all experiments, small and large bowel mucosa and pancreas were removed and analyzed for BBS-mediated proliferation.

RESULTS

Bombesin produced significant mucosal proliferation of the small bowel at day 14, but not at day 7, in rats fed regular chow. In contrast, BBS treatment for 7 days produced significant proliferation in both the atrophic and injured gut mucosa of rats given elemental diet or MTX.

CONCLUSIONS

Bombesin may be an important enterotrophic factor for normal mucosal proliferation and may be clinically beneficial as an agent to restore or maintain gut mucosa during periods of atrophy or injury.

POU-domain gene expression in the gastrointestinal tract

The process of self-renewal which occurs in the gastrointestinal epithelium is greatly amplified and accelerated during the intestinal adaptation which occurs in the residual ileum after massive small bowel resection (MSBR). As with growth and development, these processes must involve the coordinated regulation of many genes. Several families of nuclear proteins are known to be involved in the control of gene expression during development including the POU-domain genes; their expression has not been characterized in the gastrointestinal tract during normal cellular renewal or adaptation, and POU-domain encoding cDNAs were cloned from ileal RNA. Three known genes were cloned: Oct-1, Brn-1 and Tst-1 but no novel members of this gene family were identified. The encoded sequence for rat Oct-1 differs from that previously reported. Oct-1 is relatively ubiquitously expressed with increased expression during both development and adaptation. Minimal expression of Tst-1 was observed. Brn-1 exhibits limited expression in the adult gastrointestinal tract but may play a role in the fetal gastrointestinal tract.

Trophic response of gut and pancreas after ileojejunal transposition

OBJECTIVE

The authors determined whether ileojejunal transposition (IJT) stimulates the growth of the pancreas or the nontransposed segment of small intestine, and ascertained whether this trophic effect is altered by the location of transposed gut segment.

SUMMARY BACKGROUND DATA

Transposition of the ileum to the proximal small intestine stimulates a marked mucosal growth of the transposed ileal segment; the cellular mechanisms responsible for this adaptive hyperplasia are not known.

METHODS

The distal quarter of the small intestine (distal ileum) was transposed into the proximal (Type I), middle (Type II), or distal (Type III) portions of the remaining small intestine. On postoperative day 28, the pancreas and scraped mucosa from the segments of transposed ileum, proximal ileum, and duodenum were obtained, weighed, and examined for DNA and protein content.

RESULTS

All types of IJT increased mucosal weight and DNA content of the transposed ileum. Types I and II IJT produced a significant proliferation of the pancreas and mucosa of the duodenum and proximal ileum. The magnitude of proliferative increases was greatest in Type I IJT.

CONCLUSIONS

Ileojejunal transposition appears to be an excellent model to examine the mechanisms by which intestinal epithelial cells proliferate in response to luminal nutrients or humoral factors.

Isolation of a rat amiloride-binding protein cDNA clone: tissue distribution and regulation of expression

Sodium transport across high-resistance epithelia involves both an apical amiloride-sensitive sodium channel and the basal Na(+)-K(+)-ATPase pump. Aldosterone regulates sodium transport by increasing the sodium permeability of the sodium channels. To study further the regulation of gene expression in sodium-transporting epithelia by corticosteroids, we have cloned an amiloride-binding protein (ABP) cDNA from rat descending colon and kidney. Identical 311 nucleotide cDNAs were amplified from both rat descending colon and kidney, and the predicted amino acid sequence exhibited 83% homology to the equivalent region of the human peptide sequence. Use of this cDNA as a probe resulted in detection of a transcript in both the small and large bowel, thymus, and seminal vesicle. The latter tissue exhibited the highest level of rat ABP expression. Low to undetectable levels of rat ABP were expressed in the descending colon and kidney. No regulation of rat ABP by either class of corticosteroids was observed. Levels of ABP were low at birth and increased gradually to adult levels just before weaning in the bowel. The distribution of rat ABP is not as would be predicted for an aldosterone-induced gene and is thus unlikely to be a component of the amiloride-sensitive electrogenic sodium channel.

Humoral regulation of intestinal adaptation

After the loss of small bowel through disease or surgery the residual bowel adapts by increasing its functional capacity. This process of adaptation involves dilatation, hypertrophy and mucosal hyperplasia, particularly distal to the area of bowel loss or disease. The response of the residual bowel is mediated by a complex interplay of factors including luminal nutrition, pancreaticobiliary secretions, luminal or local growth factors and also humoral or endocrine factors. The experimental model commonly used to characterize the adaptive response, massive small bowel resection (MSBR), involves 80% resection of the small bowel in the rat. Of the various putative humoral factors, most work has focused on the products of the ileal L cells: enteroglucagon and peptide YY. Plasma levels of both hormones are increased after MSBR and indeed their mRNA levels are also increased as a result of an increase in the amount of message per L cell. Whilst PYY probably serves as an 'ileal brake' to slow the movement of the luminal contents and hence increase their mucosal contact time, the role of the enteroglucagon is unresolved. The molecular cloning of the proglucagon gene has revealed, firstly, that there are a number of biologically active peptides which derive from the propeptide and, secondly, that tissue-specific differential processing occurs. Most studies do not clearly define which of these products of proglucagon is being measured and is termed as glucagon-like or enteroglucagon immunoreactivity. The insulin-like growth factors (IGF) have a potent mitogenic action on the bowel. Their role after MSBR is likely to be largely paracrine. Though IGF-I mRNA levels do not increase after MSBR, the precipitous and early fall in ileal IGF-binding protein-3 (IGFBP-3) mRNA levels suggests a fall in IGFBP-3 levels may increase local IGF-I bioactivity. Polyamine synthesis is a critical component of the adaptive response, although the stimulus to their dramatic increase in synthesis after MSBR remains to be elucidated. Other humoral factors such as cholecystokinin, neurotensin and bombesin probably have minor indirect roles in the adaptive response. Components of the epidermal growth factor/transforming growth factor alpha response pathway family of growth factors may be involved as paracrine regulators. There is thus strong evidence that humoral factors play an important role in intestinal adaptation; characterization of the nature of the humoral factors and their relationship with other influences such as luminal nutrition and pancreatic biliary secretions may facilitate the development of new therapeutic strategies for the short bowel syndromes.

Ileal proglucagon gene expression in the rat: characterization in intestinal adaptation using in situ hybridization

BACKGROUND

Proglucagon-derived peptides are potential mediators of the adaptive response of the terminal ileum to massive small bowel resection. Ileal proglucagon messenger RNA (mRNA) levels increase during ileal adaptation. The present study explored the cellular basis of this response.

METHODS

Sections of control ileum, ileum 4 days after resection, and pancreas were analyzed by in situ hybridization with 35S-labeled complementary RNA (cRNA) probes.

RESULTS

Both the proglucagon and the peptide YY cRNA probes hybridized to discrete cells in the ileal mucosa, the disposition of which corresponds to that reported for intestinal L cells. Four days after resection there was a marked increase in the intensity of the signal for both probes without an increase in cell number. Insulin and histone H3 probes were used as controls to confirm the specificity of the hybridization seen with the L-cell specific, proglucagon, and peptide YY probes.

CONCLUSIONS

The increase in proglucagon mRNA levels after massive small bowel resection is caused by an increase in the cellular content. The parallel increase in PYY mRNA levels implies an L cell--rather than a proglucagon gene--specific response.

Gradient expression of Cdx along the rat intestine throughout postnatal development

Rat genomic DNA was isolated by homology with Cdx1, a murine homeogene selectively expressed in intestinal cells of endodermal origin. Southern blot analysis indicated that the rat genome contains a single or a small number of closely related Cdx gene(s). A major 1.7 kb Cdx mRNA was detected in neonate, suckling and adult rats whereas a 6.5 kb mRNA was restricted to sucklings and adults. Both transcripts showed decreasing concentration from the colon towards the proximal part of the small intestine. No obvious correlation could be established with the patterns of expression of transcripts corresponding to markers of cell proliferation and cell differentiation during postnatal development.

Expression of ileal glucagon and peptide tyrosine-tyrosine genes. Response to inhibition of polyamine synthesis in the presence of massive small-bowel resection

Massive small-bowel resection results in a marked adaptive response in the residual terminal ileum. Increased polyamine synthesis is a necessary component of this response. The ileal L-cell-derived peptides enteroglucagon and peptide tyrosine tyrosine (PYY) have been implicated as humoral mediators of this response. We have previously reported a rapid and sustained increase in glucagon mRNA concentrations after massive small-bowel resection. In this study using an inhibitor of the rate-limiting enzyme in polyamine biosynthesis, ornithine decarboxylase, we have demonstrated that the response of the glucagon and PYY genes to massive small-bowel resection is dependent on polyamine biosynthesis. In addition, we have examined the response of both the ornithine decarboxylase and c-jun genes in this model of intestinal adaptation.

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

To assess potential mediators of adaptive bowel growth, ileal proglucagon messenger RNA (mRNA) ornithine decarboxylase (ODC) mRNA, plasma enteroglucagons, and plasma glucagonlike peptide I (GLP-I) were analyzed in rats soon after jejunoileal resection or control transection. Analyses were performed before and after refeeding to establish whether responses are nutrient dependent. The elevation of ileal proglucagon and ODC mRNAs within 12 hours after resection and before refeeding shows a nutrient-independent component of the adaptive response. The onset of adaptive growth of the ileum required luminal nutrient but occurred very rapidly, within 4 hours of refeeding. The onset of adaptive growth was accompanied by transient elevation of ileal ODC mRNAs. Ileal proglucagon mRNA and plasma GLP-I levels were also elevated, and these increases were sustained up to 8 days after resection. These early and sustained increases in proglucagon mRNA and plasma GLP-I indicate that in addition to the enteroglucagons, other intestinal proglucagon-derived peptides must be considered as potential mediators of adaptive growth after jejunoileal resection.