C. Randall Fuller

Enhanced growth of small bowel in transgenic mice overexpressing bovine growth hormone

BACKGROUND Transgenic mice with a bovine growth hormone gene linked to a mouse metallothionein I promoter (growth hormone transgenics) are a model of chronic growth hormone excess. METHODS Growth of small bowel mucosa in ad libitum-fed growth hormone transgenics and wild type littermates and in growth hormone transgenics pair fed with wild-type littermates were compared. RESULTS In both groups, body weight and small bowel weight were greater in growth hormone transgenics. Similarly, mucosal mass was 50%-100% greater in growth hormone transgenics, and the effect was greatest in proximal bowel. Villus height, measured in jejunum, was also greater in growth hormone transgenics. Measurements of mucosal proliferation did not differ between the growth hormone transgenics and wild type. Abundance of insulin-like growth factor-I messenger RNA in bowel was greater in growth hormone transgenics. CONCLUSIONS Chronic growth hormone excess results in increased growth of small bowel mucosa. This effect appears to be specific because it occurred in ad libitum-fed and diet-restricted growth hormone transgenics, influenced villus height, and was more pronounced in upper than lower small bowel. The effect of chronic growth hormone excess does not appear to be secondary to an increase in the rate of mucosal proliferation, suggesting an effect on lifespan of mucosal cells.

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.

Postnatal growth responses to insulin-like growth factor I in insulin receptor substrate-1-deficient mice.

Organ weight was compared in adult mice with deletion of one (IRS-1−/+) or both (IRS-1−/−) copies of the insulin receptor substrate-1 (IRS-1) gene and IRS-1+/+ littermates. IRS-1−/+ mice showed modest reductions in weight of most organs in proportion to a decrease in body weight. IRS-1−/− mice showed major reductions in weight of heart, liver, and spleen that were directly proportional to a decrease in body weight. In IRS-1−/− mice, kidney and particularly small intestine and brain exhibited proportionately smaller weight reductions, and gastrocnemius muscle showed a proportionately greater weight reduction than the decrease in body weight. Growth deficits in IRS-1−/− mice could reflect impaired actions of multiple hormones or cytokines that activate IRS-1. To assess the requirement for IRS-1 in insulin-like growth factor I (IGF-I)-dependent postnatal growth, IRS-1−/+ mice were cross-bred with mice that widely overexpress a human IGF-I transgene (IGF+) to generate IGF+ and wild-type mice on an IRS-1+/+, I...

Effects of Fasting, Refeeding, and Intraluminal Triglyceride on Proglucagon Expression in Jejunum and Ileum

Intestinal proglucagon is thought to be synthesized primarily by the distal gut, although the role of proglucagon-derived glucagon-like peptide I (GLP-I) as a major physiological incretin would seem to be associated with production in proximal small bowel. To better characterize the sites of production of proglucagon and GLP-I in the small intestine and evaluate nutrient regulation of small bowel proglucagon and derived peptides, we evaluated the effects of fasting for 72 h and subsequent refeeding or jejunal infusion of long-chain triglyceride (LCT) for 24 h on local expression of proglucagon in proximal and distal small bowel. Proglucagon mRNA abundance and cellular localization were determined and correlated with wet weight of bowel. In jejunum, proglucagon mRNA abundance decreased by 40% with fasting (P < 0.005) and increased with refeeding to levels similar to those of ad libitum-fed animals. In ileum, fasting resulted in a 20% decrease in proglucagon mRNA (P < 0.005); in contrast to jejunum, refeeding did not result in a significant rise in ileal proglucagon mRNA abundance from fasting values. In jejunum, signal intensity of proglucagon mRNA per cell, determined by in situ hybridization, decreased with fasting (P < 0.05) and increased with refeeding (P < 0.005) in proportion to changes in mRNA abundance. Plasma enteroglucagon and GLP-I levels correlated with jejunal proglucagon mRNA. Intrajejunal infusion of LCT increased expression of proglucagon to a greater extent in jejunum than in ileum. In conclusion, enteral nutrient intake stimulates small bowel proglucagon expression; this effect is greater in jejunum than ileum, consistent with greater intraluminal nutrient exposure and the role of jejunum as a source of the major incretin GLP-I.

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 fedad 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 growth but cannot stimulate growth when ornithine decarboxylase activity is inhibited.

Oral Transforming Growth Factor-α Enhances Jejunal Mucosal Recovery and Electrical Resistance in Piglet Rotavirus Enteritis

ABSTRACT: A randomized, investigator-masked trial determined the effects of oral recombinant human transforming growth factor-α (TGFα) on jejunal mucosal recovery in 75 piglets with rotavirus diarrhea. Rotavirus inoculation of artificially reared piglets induced subtotal (∼50%) villus atrophy and watery diarrhea. Dietary TGFα was associated with significant restoration of villus surface area by 4 d postinoculation (p.i.) and complete restoration by 8 d p.i., whereas saline-treated animals required 12 d for recovery. Jejuna segments from clinically recovered TGFα-treated piglets showed an increase in electrical resistance across the epithelial barrier in vitro which was proportional to villus height. TGFα treatment for 12 d also produced a 30–50% increase in jejunal mucosal mass (protein content and wet weight), compared with the corresponding values in salinetreated piglets and in uninfected controls. However, oral TGFα did not hasten the resolution of diarrhea, enhance the specific activities of jejunal mucosal digestive enzymes, or increase jejunal glucose-stimulated Na+ absorption in vitro. We conclude that dietary TGFα stimulates jejunal mucosal hypertrophy, improves barrier function, and enhances regrowth of villi in rotavirus enteritis; however, it does not facilitate the restoration of functional activity or mucosal digestive enzymes. Oral TGFα can facilitate intestinal epithelial recovery in diseases associated with mucosal damage.

Rectal mucosal ornithine decarboxylase activity is not a useful marker of risk for colorectal neoplasia

Rectal mucosal ornithine decarboxylase activity has been reported to distinguish patients with adenomas from normal controls. In order to further explore this association, we assayed biopsy samples from 119 unselected individuals undergoing routine colonoscopic examinations. The overall mean ODC activity was 127.4(±93.1sd) pmol/mg protein/hr. There were no differences by age, sex, or race. Tissue handling and storage influenced ODC activity. Specimens collected and transported on Dry Ice had higher ODC activity than specimens initially frozen in a −20°C freezer. After adjusting for storage and collection method, the activity was similar in subjects with adenomas (126.3 pmol/mg/hr) compared to those without adenomas (128.8 pmol/mg/hr). We conclude that variations in assay technique make comparisons between laboratories difficult. Patients with largebowel adenomas do not necessarily have higher ODC activity in uninvolved rectal mucosa. Further study of the environmental and genetic factors that influence rectal mucosal proliferation may improve our understanding of carcinogenesis in the large bowel.