Intestinal Mucosal Protein Synthesis Research Articles

Anabolic and catabolic mediators of intestinal protein turnover: a new experimental approach.

Studies on regulation of protein turnover in skeletal muscle have revealed the important contributions of protein synthesis and catabolism to tissue protein balance, and have identified a host of specific anabolic and catabolic stimuli and biochemical mechanisms that regulate these processes. This knowledge is critical to current efforts designed to promote anabolism and limit atrophy. Of the tissues with a potentially large contribution to whole-body amino acid metabolism, protein turnover of the intestine stands out as being poorly understood. The intestine is subject to complexities in regulation of its metabolism that are not apparent for other tissues. The study of intestinal protein turnover also entails some important technical challenges. We recently developed an in-situ experimental system for study of intestinal mucosal protein synthesis with the following unique features: multiple observations within an animal; controlled delivery of nutritional stimuli to the apical side, basolateral side, or both; and luminal delivery of tracer in a flooding dose for determination of protein synthesis. We have begun to use the system to test the specific roles of individual luminal nutrients in regulation of mucosal protein synthesis. We have also used protease gene expression as an index of potential regulation of catabolic pathways.

The effect of insulin on human small intestinal mucosal protein synthesis

Background & Aims: Insulin deficiency was recently shown to stimulate splanchnic protein synthesis in vivo, whereas insulin enhances small intestinal mucosal cell proliferation in vitro. Because insulin is a postprandial hormone, it was hypothesized that it has an important role in regulating small intestinal protein synthesis in humans. Methods: Small intestinal mucosal protein synthesis was measured in C-peptide–negative patients with type 1 diabetes mellitus during insulin deprivation (n = 6) and during insulin treatment (n = 6) and in nondiabetic control subjects (n = 6). Mucosal protein synthesis was measured from the increment of [ 13C]leucine enrichment in endoscopically obtained duodenal mucosa samples during a primed continuous infusion of L-[1- 13C]leucine. Results: During insulin treatment, the rate of mucosal protein synthesis in patients with type 1 diabetes was similar (1.32% ± 0.05%/h) to that of nondiabetic controls (1.33% ± 0.06%/h). However, during insulin deprivation, the mucosal protein synthesis rate in patients with type 1 diabetes was significantly lower (1.15% ± 0.33%/h) than during either insulin treatment ( P = 0.01) or in nondiabetic controls ( P = 0.04). Conclusions: These studies show that insulin is required for the maintenance of normal rates of protein synthesis in small intestinal mucosa. Because protein synthesis is an essential component of the remodeling process of this fast turning over tissue, the decline in the synthesis rate of small intestinal mucosa during insulin deprivation may be a contributing factor in the development of gastrointestinal complications that occur in poorly controlled type 1 diabetic patients. GASTROENTEROLOGY 2000;118:299-306

Luminal Amino Acids Acutely Decrease Intestinal Mucosal Protein Synthesis and Protease mRNA in Piglets

Because parenteral feeding is associated with negative N balance and reduced rates of protein synthesis in intestinal mucosa, we hypothesized that luminal exposure to specific amino acids or energy fuels would stimulate intestinal protein synthesis. We studied the acute effects of luminal nutrients on mucosal protein synthesis in the absence of systemic influences. Multiple jejunal segments constructed in piglets deprived of food overnight (n = 6) were randomly assigned to luminal perfusion with saline, 30 mmol/L amino acid mixture with or without 50 mmol/L glucose, or 30 mmol/L glutamine for 90 min. Protein synthesis was then measured by luminal perfusion with L-[2,6-3H]-phenylalanine. Energy substrates (glucose, short-chain fatty acids or β-hydroxybutyrate) had no effect on mucosal protein synthesis. Relative to saline, a 30 mmol/L amino acid mixture or 30 mmol/L glutamine suppressed mucosal protein synthesis by 20–25% (P < 0.05). On the basis of these surprising results, we speculated that a coordinate reduction of proteolytic processes would be required to maintain positive intestinal N balance. Although intestinal protein catabolism cannot be assessed directly, the 30 mmol/L amino acid mixture acutely suppressed mucosal levels of mRNA encoding ubiquitin, 14-kDa ubiquitin conjugating enzyme and the C9 subunit of the proteasome by 20–30% (P < 0.05), demonstrating the sensitivity of components of the ATP-ubiquitin proteolytic pathway to acute regulation by nutrients. The suppression of protein synthesis by luminal amino acids in the absorptive state might lower intestinal utilization of amino acids to ensure efficient allocation of absorbed nutrients to nonintestinal tissues.

P.117Effect of an enteral glutamine supplementationon intestinal mucosal protein synthesis of hypercatabolic subjects

P.117Effect of an enteral glutamine supplementationon intestinal mucosal protein synthesis of hypercatabolic subjects

O.15 Small intestinal mucosal protein synthesis is depressed in patients with alcoholic liver disease (ALD) and correlates with whole body breakdown

O.15 Small intestinal mucosal protein synthesis is depressed in patients with alcoholic liver disease (ALD) and correlates with whole body breakdown

Intestinal mucosa in diabetes: synthesis of total proteins and sucrase-isomaltase.

The effects of insulin deficiency on nitrogen metabolism in muscle and liver have been extensively studied with recent in vivo demonstration of impaired protein synthesis in rats with streptozotocin-induced diabetes. Despite the significant contribution of small intestinal mucosa to overall protein metabolism, the effects of insulin deficiency on intestinal protein synthesis have not been completely defined. We studied the effects of streptozotocin-induced diabetes on total protein synthesis by small intestinal mucosa and on synthesis of a single enzyme protein of the enterocyte brush-border membrane sucrase-isomaltase. We used the flooding-dose technique of McNurlan, Tomkins, and Garlick (Biochem. J. 178: 373-379, 1979) to minimize the difficulties of measuring specific radioactivity of precursor phenylalanine and determined incorporation into mucosal proteins and sucrase-isomaltase 20 min after injection of the labeled amino acid. Diabetes did not alter mucosal mass as determined by weight and content of protein and DNA during the 5 days after injection of streptozotocin. Increased rates of sucrase-isomaltase synthesis developed beginning on day 3, and those of total protein developed on day 5. Thus intestinal mucosal protein synthesis is not an insulin-sensitive process.

Effects of feeding t-2 toxin on rna, dna and protein contents of liver and intestinal mucosa of rats

Young male albino rats were fed 1.5 mg T-2 toxin/kg body weight daily for 4 days and the effects on body weight, liver weight and protein, RNA, and DNA contents of liver and intestinal mucosa were studied. A significant decrease in body weight and an increase in liver weight were observed. Liver protein and DNA and intestinal mucosal protein contents were significantly decreased, whereas intestinal mucosal RNA content was increased. Decreased liver and intestinal mucosal protein synthesis in T-2 toxin-fed rats was inferred from the [ 14C]leucine incorporation studies.

Increased intestinal uptake of cobalamin in pregnancy does not require synthesis of new receptors

Increased intrinsic factor cobalamin binding to receptors present in ileal mucosa from mice in the late stages of pregnancy is regulated by placental lactogen. In mice at day 18–20 of pregnancy given an intraperitoneal injection of cycloheximide, 0.5 mg/kg, receptor binding was reduced from 0.42 ng/mg protein to 0.18 ng/mg protein 4 h later. Intestinal mucosal protein synthesis was less than 20% of control values after this dose of cycloheximide. Although this result could be interpreted to mean that the increase in receptors in pregnancy was due to new protein synthesis, cycloheximide-treated mice also had reduced concentrations of placental lactogen in serum. Supplementation with the hormone in cycloheximide-treated mice maintained receptor binding at pregnant levels. Analysis of binding data showed receptor number to be 3.1·10 11/mg protein and the binding constant ( K a) to be 0.5·10 12 M −1, which were similar to values found in untreated pregnant mice. It is concluded that, because the increase in receptors cannot be explained on the grounds of new protein synthesis, placental lactogen may recruit cryptic intrinsic factor cobalamin receptors.