Molecular Mechanisms of Intestinal Injury, Repair, and Growth

Enteral nutrition and keratinocyte growth factor regulate expression of glutathione-related enzyme messenger RNAs in rat intestine

BACKGROUND

Malnutrition is associated with increased reactive oxygen species (ROS) formation and depletion of the critical antioxidant glutathione (GSH) in the intestine. The malnutrition-induced decrease in gut GSH levels is prevented by recombinant keratinocyte growth factor (KGF) administration. We investigated whether enzymes that are induced by oxidants and modulate tissue GSH supply are regulated by enteral nutrients or KGF at the messenger RNA (mRNA) level.

METHODS

Adult rats were fasted for 3 days alone or fasted for 3 days then refed ad libitum. In a second model, rats were fasted for 3 days and then refed ad libitum or 25% of ad libitum intake with daily intraperitoneal saline or recombinant KGF (5 mg/kg/d) for 3 subsequent days. mRNA levels for gamma-glutamylcysteine synthetase (gamma-GCS), gamma-glutamyl transpeptidase (gamma-GT), glutathione-S-transferase Ya-subunit, gastrointestinal glutathione peroxidase (GI-GPx), and non-selenium-dependent glutathione peroxidase (ns-GPx) were determined in ileum and colon by ribonuclease protection assay.

RESULTS

Fasting increased ileal gamma-GCS, ns-GPx, and glutathione-S-transferase mRNAs (by 36%, 165%, and 130% of controls) and decreased GI-GPx mRNA (to 55% of controls). In the colon, mRNAs for GSH-related enzymes were unchanged by fasting or refeeding. Prolonged enteral nutrient restriction (25% refeeding after a 3-day fast) increased gamma-GCS and glutathione-S-transferase mRNAs (by >270% of controls), decreased GI-GPx mRNA (to <50% of controls) in ileum and colon and increased ns-GPx mRNA (by 180%) in colon. KGF treatment increased ns-GPx mRNA in the ileum and colon and glutathione-S-transferase mRNA in the colon (by >200% of controls).

CONCLUSIONS

Enteral nutrient intake regulates GSH-related enzyme mRNA levels in the intestine, which may contribute to the decrease in mucosal GSH during malnutrition. Increased ns-GPx and glutathione-S-transferase mRNA levels during malnutrition and with KGF administration may increase detoxifying functions in the gut under these conditions.

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.

Interactions between nutrients and peptide growth factors in intestinal growth, repair, and function

Several lines of evidence demonstrate that general nutritional status, specific nutrients (eg, zinc, glutamine), and certain trophic growth factors (eg, growth hormone, insulin-like growth factor I, keratinocyte growth factor, and glucagon-like peptide-2) have important interactions relevant for intestinal growth and function. Adequate nutritional status is critical for endogenous growth factor synthesis in the gut and other tissues and is an important mediator of organ responsiveness to exogenous growth factor administration. Both endogenously synthesized and exogenously administered growth factors upregulate nutrient uptake and utilization by gut mucosa, skeletal muscle, and other organs. Emerging data from both animal and human studies indicate that combinations of selected growth factors and specific nutrients may improve the growth, adaptation, and repair of the intestinal mucosa. Additional studies to determine basic mechanisms of nutrient-growth factor interactions and the safety and efficacy of treatment with combinations of specific nutrients and recombinant growth factors are needed. Results of these investigations should define new methods for support of the intestinal tract during short bowel syndrome (SBS), catabolic illness, and malnutrition.

Gut-trophic effects of keratinocyte growth factor in rat small intestine and colon during enteral refeeding

BACKGROUND

Keratinocyte growth factor (KGF) induces proliferation of gut epithelium in rat models, but KGF-nutrient interactions have not been studied. An experimental model of fasting-induced gut atrophy followed by different levels of enteral refeeding was used to investigate the influence of nutrient availability on the gut-trophic effects of exogenous KGF.

METHODS

After a 3-day fast, rats were enterally refed either ad libitum or at 25% of ad libitum intake for 3 subsequent days. Either intraperitoneal KGF (5 mg/kg/d) or saline was given in each dietary regimen. Wet weight, DNA, and protein content were measured as indices of full-thickness cellularity in duodenum, jejunum, ileum, and colon. Villus height in small bowel segments and crypt depth in all gut tissues were measured as specific indices of mucosal growth.

RESULTS

Refeeding at 25% of ad libitum intake significantly decreased full-thickness cellularity and mucosal growth indices in duodenum, jejunum, and ileum. In the colon, only protein content fell significantly and crypt depth was maintained. KGF administration during 25% refeeding did not alter full-thickness indices in any small bowel segment or affect jejunal mucosal growth. In contrast, KGF normalized duodenal villus height (p < .01) and duodenal and ileal crypt depth (p < .05) only in the 25%-refed model. KGF significantly increased ileal villus height in both ad libitum and 25%-refed rats (by 43% and 48%, respectively, p < .05) and markedly increased colonic cellularity and mucosal crypt depth with both levels of refeeding (p < .01).

CONCLUSIONS

Rat small bowel growth is more sensitive than colon to the level of enteral refeeding after a 3-day fast. KGF administration does not affect jejunal growth, but specifically prevents atrophy of duodenal and ileal mucosa during hypocaloric, hyponitrogenous refeeding. In ileum and colon, some KGF-mediated growth responses are independent of the level of enteral refeeding. Thus gut-trophic effects of KGF and KGF interactions with the level of nutrient intake are tissue-specific.

Pharmacological nutrition after burn injury

Burn patients develop pathophysiological alterations, which include extensive nitrogen loss, malnutrition, markedly increased metabolic rate and immunologic deficiency. This predisposes burn patients to frequent infections, poor wound healing, increased length of hospitalization and increased mortality. The nutritional support requires high protein and high energy diets preferably administered enterally soon after injury. The effects of increased dietary components such as glutamine, arginine and (n-3) fatty acids and related compounds have been evaluated in burn victims. These components, when supplied in quantities two to seven times of those in normal diets of healthy persons, appear to have beneficial pharmacological effects on the pathophysiological alterations associated with burns. However, the efficacy of immune-enhancing diets remains to be convincingly shown.

Nutrition and the insulin-like growth factor system

Nutritional status is a key regulator of the circulating and tissue insulin-like growth factor (IGF) system. IGF-I mRNA and protein levels decrease in tissues such as liver and intestine with fasting and are restored with refeeding. Additional studies suggest that the level of protein and calorie intake independently regulate plasma IGF-I concentrations in man. The level of nutrition effects the biological actions of recombinant growth hormone (GH) and IGF-I administration in humans. Limited data demonstrate that plasma and tissue levels of the insulin-like growth factor binding proteins (IGFBPs) are also sensitive to nutrient intake. Specific micronutrients, such as potassium, magnesium and zinc also appear to be important for optimal IGF-I synthesis and anabolic effects in animal models. Malnutrition is common in elderly patients, however, the interaction between specific nutrients, general nutritional status and the aging process on the IGF system is incompletely understood. Mechanisms of nutrient-IGF system interactions which may affect the biological actions of IGF-I, IGF-II, and the IGFBPs are increasingly being determined in basic studies. The effects of underlying nutritional status and responses to dietary intake will be important to evaluate in clinical studies of the IGF system and exogenous growth factor therapy.