Mucosal Protein Synthesis Research Articles

Effects of low protein diets with amino acids supplementation on biochemical and faeces parameters in weaned piglets

Effects of low protein diets with amino acids supplementation on biochemical and faeces parameters in weaned piglets

Enteral feeding induces profound early intestinal adaptation in a neonatal piglet model of short bowel syndrome

Our objective was to characterize small intestinal (SI) adaptation in a novel piglet model. Piglets (n = 32, 12–13 d old) were randomized to 1) 80% SI resection with total parenteral nutrition (TPN) feeding (R‐TPN), 2) 80% SI resection with TPN + enteral feeding (R‐EN) or 3) sham SI transection with TPN + enteral feeding (Sham‐EN). In R pigs, the distal 100 cm of ileum (residual SI) and 30 cm of proximal SI was left intact. All pigs received TPN after surgery. R‐EN and Sham‐EN piglets also received continuous gastric infusions of elemental diet starting on d 3 (40:60 TPN:EN). Piglets were killed 4, 6 or 10 d post‐surgery. Cell proliferation in residual SI, assessed by incorporation of 5‐bromo‐2′‐deoxyuridine (BrdU) was significantly greater in R‐EN animals (29.5 ± 4.7 %) compared to Sham‐EN (13.63 ± 5.7 %) and R‐TPN (12.97 ± 8.9 %) animals at d 4 (P<0.05). R‐EN at d 4 also had higher rates of mucosal protein synthesis than Sham‐EN (P<0.001) and R‐TPN animals (P<0.05), (87.2 ± 3.2 vs 53.2 ± 2.1 and 59.7 ± 9.7 %/d, respectively). Villous height and crypt depth were greater in R‐EN animals compared to Sham‐EN animals at d 6 and 10 (p<0.05). Less than 24 h of enteral feeding increased rates of cell proliferation and protein synthesis; this translated into greater villous height and crypt depth only 2 d later. This piglet model will be very useful in assessing the efficacy of nutritional interventions targeting SI adaptation. (SickKids Foundation)

W1190 Gastrointestinal Mucosal Protein Synthesis Rates in Pediatric Crohn's Disease

W1190 Gastrointestinal Mucosal Protein Synthesis Rates in Pediatric Crohn's Disease

Methotrexate induces intestinal mucositis and alters gut protein metabolism independently of reduced food intake

One of the main secondary toxic side effects of antimitotic agents used to treat cancer patients is intestinal mucositis. This one is characterized by compromised digestive and absorptive functions, barrier integrity, and immune competence. At the same time, food intake is decreased, which may induce intestinal damages per se. The aim of the study was to characterize which alterations are specific to methotrexate, independently of the anorexic effect of the drug. Male Sprague-Dawley rats received subcutaneously saline solution as control group or 2.5 mg/kg of methotrexate during 3 days (D0-D2). Methotrexate-treated rats were compared with ad libitum and pair-fed controls. Histological examinations and specific markers of the immune and nonimmune gut barrier function were assessed at D4 or D7. Compared with ad libitum and pair-fed controls, methotrexate induced at D4 villus atrophy associated with epithelial necrosis. Mucosal protein synthesis rate and mucin contents of methotrexate treated rats were reduced. At the same time, cathepsin D proteolytic activity was increased compared with ad libitum and pair-fed controls, whereas calpain activity was increased when compared with the only pair-fed controls. These intestinal lesions were associated with various metabolic disturbances such as increased TNF-alpha level and inflammation score in the jejunum but also disturbances of amino acid concentrations in the duodenum and plasma. At D7, these alterations were partially or completely normalized. In addition to the consequences of a low food intake, methotrexate further impairs different biological processes leading to a dramatic loss of gut homeostasis. Targeted nutritional management of chemotherapy receiving patients should be set up to prevent or limit such alterations.

Luminal Threonine Concentration Acutely Affects Intestinal Mucosal Protein and Mucin Synthesis in Piglets 3

Intestinal mucin synthesis is sensitive to dietary threonine supply, which suggests that the gut’s requirement for threonine may comprise a significant proportion of the whole body requirement. We used a continuously perfused gut loop model and intraluminal flooding dose technique in 6 young pigs to study the acute effects of varying luminal availability of threonine on intestinal protein and mucin syntheses. A complete amino acid mixture containing 0, 21, or 56 mg threonine/g total amino acids (whole body requirement ratio is ∼28 mg/g) was continuously perfused in isolated loops for 120 min, including a 30-min 3H-phenylalanine flooding dose. We measured fractional synthesis rates of total mucosal protein and mucin by analyzing 3H-phenylalanine incorporation. Fractional rates of total mucosal protein synthesis were higher in loops perfused with solutions containing threonine at 56 mg/g (66 ± 4%/d) compared with 0 mg/g (42 ± 9%/d) and 21 mg/g (53 ± 6%/d) (P < 0.05). For mucin, fractional rates of synthesis differed between 0 mg/g (323 ± 72%/d), 21 mg/g (347 ± 49%/d), and 56 mg/g (414 ± 31%/d) (P < 0.05). In addition, total proline and threonine concentrations in the protein hydrolysates increased with luminal threonine concentration (P < 0.05), indicating an increase in threonine- and proline-rich proteins. De novo synthesis of mucosal and mucin proteins is acutely sensitive to luminal threonine concentration, which demonstrates the importance of dietary amino acid supply to gut protein metabolism.

Effects of recombinant human growth hormone on enterocutaneous fistula patients

To explore the effects of recombinant human growth hormone (rhGH) on intestinal mucosal epithelial cell proliferation and nutritional status in patients with enterocutaneous fistula. Eight patients with enterocutaneous fistulas received recombinant human growth hormone (10 microg/d) for 7 d. Image analysis and immunohistochemical techniques were used to analyse the expression of proliferating cell nuclear antigen (PCNA) in intestinal mucosal epithelial cells in biopsy samples from the patients who had undergone an endoscopic biopsy through the fistula at day 0, 4 and 7. Body weights, nitrogen excretion, serum levels of total proteins, albumin, prealbumin, transferrin and fibronectin were measured at day 0, 4 and 7. Significant improvements occurred in the expression of PCNA in the intestinal mucosal epithelial cells at day 4 and 7 compared to day 0 (24.93 +/- 3.41%, 30.46 +/- 5.24% vs 12.92 +/- 4.20%, P < 0.01). These changes were accompanied by the significant improvement of villus height (500.54 +/- 53.79 microm, 459.03 +/- 88.98 microm vs 210.94 +/- 49.16 microm, P < 0.01), serum levels of total proteins (70.52 +/- 5.13 g/L, 74.89 +/- 5.16 g/L vs 63.51 +/- 2.47 g/L, P < 0.01), albumin (39.44 +/- 1.18 g/L, 42.39 +/- 1.68 g/L vs 35.74 +/- 1.75 g/L, P < 0.01) and fibronectin (236.3 +/- 16.5 mg/L, 275.8 +/- 16.9 mg/L vs 172.5 +/- 21.4 mg/L, P < 0.01) at day 4 and 7, and prealbumin (286.38 +/- 65.61 mg/L vs 180.88 +/- 48.28 mg/L, P < 0.05), transferrin (2.61 +/- 0.12 g/L vs 2.41 +/- 0.14 g/L, P < 0.05) at day 7. Nitrogen excretion was significantly decreased at day 7 (3.40 +/- 1.65 g/d vs 7.25 +/- 3.92 g/d, P < 0.05). No change was observed in the body weight. Recombinant human growth hormone could promote intestinal mucosal epithelial cell proliferation and protein synthesis in patients with enterocutaneous fistula.

Glucagon‐like Peptide‐2 Activates the mTOR Signaling Through a PI3‐kinase‐Akt‐ dependent Pathway

Glucagon-like peptide-2 (GLP-2) is a nutrient-responsive neuropeptide that exerts diverse actions in the gastrointestinal tract. We have shown that GLP-2-stimulated mucosal growth occurred in vivo with an increased rate of protein synthesis in the neonatal intestine, which was associated with up-regulated phosphorylation of the Akt kinase. Evidence indicates that protein translation is controlled by the mammalian target of rapamycine (mTOR) signaling pathway. Thus, we hypothesized that GLP-2 receptor activation intracellularly stimulates the mTOR signaling through the phosphatidylinositol 3 (PI3)-kinase-Akt pathway. The HEK 293 cells were transfected with a full-length cDNA of human GLP-2 receptor and treated with human GLP-2 with/without pre-treatment of a PI3-kinase inhibitor (LY294002). We found that GLP-2 dose-dependently stimulated Akt phosphorylation at Ser473 and Thr308, which were suppressed by LY294002 at 60% and 30%, respectively. Moreover, GLP-2 stimulated mTOR phosphorylation at Ser2448, thus activating p70S6 kinase and inactivating the elF4E inhibitor (4E-BP1) by inducing phosphorylation of the p70S6 at Thr389 and the 4E-BP1 at Thr37/46, which were suppressed by LY294002 at 35% and 20%, respectively. We concluded that GLP-2-mediated activation of the mTOR signaling might be involved in GLP-2-stimulated mucosal protein synthesis, and was partially mediated by the PI3-kinase-Akt signaling pathway. (Supported by NIH/NIDDK DK075489-01 and USDA/ARS 25337667381).

Dietary fiber decreases colonic epithelial cell proliferation and protein synthetic rates in human subjects

Although it has been proposed that high fiber consumption can prevent proliferative diseases of the colon, the clinical data to support this hypothesis have been inconsistent. To provide a more robust measure of the effects of fiber on colonic mucosal growth than previous studies, we evaluated both cell proliferation and colonic mucosal protein synthesis in nine healthy volunteers after they consumed a typical Western diet (<20 g fiber/day) or a Western diet supplemented with wheat bran (24 g/day) in a randomized crossover design. Biopsies taken from the sigmoid colon were used to assess mucosal proliferation by determining proliferating cell nuclear antigen (PCNA) in crypt cells and to assess mucosal protein synthetic rate using stable isotopically labeled leucine infusion. Fiber supplementation produced a 12% decrease in labeling index (%crypt cells stained with PCNA) (P < 0.001) and an 11% decrease in mucosal protein fractional synthetic rate (FSR; P < 0.05). Moreover, mucosal protein FSR correlated directly with labeling index (r2= 0.22, P < 0.05). These data demonstrate that increased wheat bran consumption decreases colonic mucosal proliferation and support the potential importance of dietary fiber in preventing proliferative diseases of the colon.

The Effect of Hyperglycemic Hyperinsulinemia on Small‐Intestinal Mucosal Protein Synthesis in Patients After Surgical Stress

Hyperglycemic hyperinsulinemia cannot stimulate intestinal protein synthesis in healthy individuals but does so in conditions characterized by an altered somatotropic axis such as diabetes. Only in a state of growth hormone resistance (high growth hormone but low insulin like growth factor [IGF-1] concentrations), extra insulin may acutely reverse the impaired, growth-hormone-induced IGF-1 release, thereby exerting anabolic actions at the intestinal tract. Growth hormone resistance can be also found in patients after surgical stress. Therefore, we wanted to test the hypothesis whether hyperglycemic hyperinsulinemia would stimulate ileal protein synthesis in the latter condition. Mass spectrometry techniques (capillary gas chromatography/combustion isotope ratio mass spectrometry) were used to directly determine the incorporation rate of 1-[(13)C]-leucine into ileal mucosal protein. All subjects had an ileostomy, which allowed easy access to the ileal mucosa, and consecutive sampling from the same tissue was performed during continuous isotope infusion (0.16 mumol/kg min). Isotopic enrichments and fractional protein synthesis were determined at baseline (period I) and after a 4-hour glucose infusion (170 mg/kg/h) or after infusion of saline (control group) (period II). In controls, ileal protein synthesis declined significantly during prolonged isotope infusion (period I: 1.11 +/- 0.14%/h, period II: 0.39 +/- 0.13%/h, p < .01). In contrast, ileal protein synthesis remained constant during glucose infusion (period I: 1.32 +/- 0.35%/h, period II: 1.33 +/- 0.21%/h, n.s. vs period I, but p < .005 vs the corresponding value at the end of period II in the control group). Using the continuous tracer infusion technique, ileal protein synthesis seemingly declines over a short time in control subjects. We found evidence that this artificial decline was due to mass effects of a rapidly turning over mucosa protein pool in which an isotopic plateau was reached during the experiment and of which the size amounted to approximately 4% of the total mixed protein pool. Maintenance of ileal protein synthesis during glucose infusion therefore indicates a rise of ileal protein synthesis in a slowly turning over protein pool. This effect in postsurgical patients would be compatible with the concept of intestinal insulin action to depend on the specific clinical state (eg, growth hormone resistance).

Feeding an Elemental Diet vs a Milk‐Based Formula Does Not Decrease Intestinal Mucosal Growth in Infant Pigs

We previously showed that the level of enteral nutrient intake determines the rate of intestinal growth in piglets. Our objective was to determine whether providing enteral nutrition in the form of elemental nutrients (glucose, amino acids, lipid [ED]) rather than cow's milk formula (lactose, protein, lipid [FORM]) reduces small intestinal growth and lactase activity. Three-week-old piglets were fed either ED (n = 7) intragastrically or FORM (n = 6) orally for 6 days. Intestinal protein and DNA masses, villus height, and crypt depth were not different in ED and FORM pigs. Crypt cell proliferation, measured by in vivo bromodeoxyuridine labeling, was significantly (p < .05) higher (+37%) in ED than in FORM pigs. Rates of mucosal protein synthesis (%/d), measured by in vivo 2H-leucine incorporation, were higher (p < .05) in ED than FORM (147 vs 89) pigs. Circulating concentrations (pmol/L) of the intestinotrophic peptide, glucagon-like peptide-2 (GLP-2), were also higher (p < .05) in ED than in FORM (148 vs 87) pigs. The mean lactase-specific activity (micromol/min/g) in proximal and distal segments was higher (p < .05) in FORM than in ED (124 vs 58) pigs. We conclude that intestinal mucosal growth and villus morphology are similar in pigs fed ED and FORM, despite higher cell proliferation and protein synthesis rates and lower lactase activity with ED. This implies that elemental diets may be as trophic as polymeric formulas to simultaneously provide nutrition and a stimulus for intestinal growth during bowel rest.

Effect of tumor removal on mucosal protein synthesis in patients with colorectal cancer.

It is currently controversial whether mucosal hyperproliferation is involved in colorectal cancerogenesis. The purpose of the present study was to examine protein synthetic rate as an indicator of potential tissue proliferation in grossly normal rectal mucosa from cancer-bearing subjects and to compare this rate with that in mucosa from subjects posttumor removal. Six postabsorptive patients with localized rectal cancer and five postsurgical control subjects received a primed constant infusion of [1-(13)C]leucine (0.16 micromol/kg min, 9.6 micromol/kg prime). Forceps biopsies from the mucosa were taken after 3 and 6 h. Protein synthesis was calculated from protein-bound leucine enrichment (determined by capillary GC-combustion IRMS) and from the enrichment of free intracellular leucine (determined by GC-quadrupole MS). In cancer-bearing subjects, mucosal protein synthesis amounted to 1.28 +/- 0.24%/h. This rate was significantly higher (P < 0.05) than the corresponding rate of mucosa from patients after cancer removal (0.69 +/- 0.09%/h). These findings do not support the concept that colorectal cancer originates from a proliferative disease of the whole colon. Increased mucosal protein synthesis appears to depend on the presence of the tumor itself and should therefore be considered a secondary phenomenon.

Intestinal protein and LPH synthesis in parenterally fed piglets receiving partial enteral nutrition and enteral insulinlike growth factor 1.

Providing partial enteral nutrition (PEN) supplemented with insulinlike growth factor-1 (IGF-1) to parenterally fed piglets increases lactase-phlorizin hydrolase (LPH) activity, but not LPH mRNA. The current aim was to investigate potential mechanisms by which IGF-1 up-regulates LPH activity. Newborn piglets (n = 15) received 100% parenteral nutrition (TPN), 80% parenteral nutrition + 20% parenteral nutrition (PEN), or PEN + IGF-1 (1.0 mg. kg-1. d-1) for 7 days. On day 7, [2H3]-leucine was intravenously administered to measure mucosal protein and brush border LPH (BB LPH) synthesis. Weight gain, nutrient intake, and jejunal weight and length were similar among the treatment groups. Partial enteral nutrition alone increased mucosal weight, villus width and cross-sectional area, LPH activity, mRNA expression, and high mannose LPH precursor (proLPHh) abundance compared with TPN (P<0.05). Insulinlike growth factor-1 further increased mucosal weight, LPH activity, and LPH activity per unit BB LPH approximately twofold over PEN alone (P < 0.05) but did not affect LPH mRNA or the abundance of proLPHh (one of the LPH isoforms) or mature LPH. Isotopic enrichment of [2H3]-leucine in plasma, mucosal protein, and LPH precursors, and the fractional and absolute synthesis rates of mucosal protein and LPH were similar among the treatment groups. Insulinlike growth factor-1 treatment increased total mucosal protein synthesis (60%, P < 0.05) but not LPH synthesis compared with the other two groups. Because IGF-1 did not affect the fractional synthesis rate of either mucosal protein or LPH, the authors suggest that enteral IGF-1 increases mucosal protein mass and LPH activity by suppressing mucosal proteolytic degradation.

Intestinal Protein and LPH Synthesis in Parenterally Fed Piglets Receiving Partial Enteral Nutrition and Enteral Insulinlike Growth Factor 1

ABSTRACTBackgroundProviding partial enteral nutrition (PEN) supplemented with insulinlike growth factor‐1 (IGF‐1) to parenterally fed piglets increases lactase–phlorizin hydrolase (LPH) activity, but not LPH mRNA. The current aim was to investigate potential mechanisms by which IGF‐1 up‐regulates LPH activity.MethodsNewborn piglets (n = 15) received 100% parenteral nutrition (TPN), 80% parenteral nutrition + 20% parenteral nutrition (PEN), or PEN + IGF‐1 (1.0 mg · kg−1 · d−1) for 7 days. On day 7, [2H3]‐leucine was intravenously administered to measure mucosal protein and brush border LPH (BB LPH) synthesis.ResultsWeight gain, nutrient intake, and jejunal weight and length were similar among the treatment groups. Partial enteral nutrition alone increased mucosal weight, villus width and cross‐sectional area, LPH activity, mRNA expression, and high mannose LPH precursor (proLPHh) abundance compared with TPN (P &lt;0.05). Insulinlike growth factor‐1 further increased mucosal weight, LPH activity, and LPH activity per unit BB LPH approximately twofold over PEN alone (P &lt; 0.05) but did not affect LPH mRNA or the abundance of proLPHh (one of the LPH isoforms) or mature LPH. Isotopic enrichment of [2H3]‐leucine in plasma, mucosal protein, and LPH precursors, and the fractional and absolute synthesis rates of mucosal protein and LPH were similar among the treatment groups. Insulinlike growth factor‐1 treatment increased total mucosal protein synthesis (60%, P &lt; 0.05) but not LPH synthesis compared with the other two groups.ConclusionsBecause IGF‐1 did not affect the fractional synthesis rate of either mucosal protein or LPH, the authors suggest that enteral IGF‐1 increases mucosal protein mass and LPH activity by suppressing mucosal proteolytic degradation.

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.

Effects of enteral glutamine on gut mucosal protein synthesis in healthy humans receiving glucocorticoids.

In hypercatabolic patients, the beneficial effects of glutamine on gut mucosa could be partly due to a stimulation of protein synthesis. The fractional synthesis rate (FSR) of gut mucosal protein was measured in four groups of healthy volunteers treated with glucocorticoids for 2 days. Two groups were studied in the postabsorptive state while receiving glutamine or a nitrogen equivalent (control) and two groups in the fed state with or without glutamine, using a 5-h intravenous infusion of [(13)C]leucine, [(2)H(5)]phenylalanine, and cortisone. After nutrient and tracer infusion, duodenal biopsies were taken. In the postabsorptive state, FSR of gut mucosal protein were 87 and 76%/day in the control group and 130% (P = 0.058 vs. control) and 104% (P = 0.17 vs. control)/day in the glutamine group, with leucine and phenylalanine as tracers, respectively. During feeding, FSR did not increase and no significant difference was observed between glutamine and control groups. Overall, FSR of the four groups were two- to threefold higher than those obtained previously in healthy humans, suggesting that glucocorticoids may increase gut mucosal protein synthesis. However, in this situation, a moderate enteral glutamine supply failed to demonstrate a significant effect on gut mucosal protein synthesis in the postabsorptive state and during feeding.

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

Small-intestinal mucosal protein synthesis and whole-body protein turnover in alcoholic liver disease

Small-intestinal mucosal protein synthesis and whole-body protein turnover in alcoholic liver disease

Effect of Intracerebroventricular Injection of Tumor Necrosis Factor α on Gut Mucosal Protein Turnover in Mice Fed Enterally

Objective. The objective of this study was to determine whether mucosal protein turnover is enhanced by intracerebroventricular (ICV) injection of tumor necrosis factor α (TNF-α) in mice fed enterally.Methods. Male B6C3F1 mice (n = 37, 20.0–25.0 g) were catheterized in the ICV space using stereotaxic coordination, and subsequently a catheter was inserted into the stomach for enteral feeding on Day 0. The animals were fed a standard mouse diet and water ad libitum for 4 days. On Day 4, enteral feeding was begun with a standard liquid diet, and either artificial cerebrospinal fluid (control) or recombinant human (Rh) TNF-α (5 ng/h) was continuously given via the ICV catheter using an osmotic pump. On either Day 5 or 7, the mice were sacrificed 10 min after injection of [U-14C]phenylalanine (5 μCi/mouse, ip). Fractional synthetic rates (FSRs) of jejunal mucosa and muscle were measured by the pool flooding technique. Interleukin-6 (IL-6) levels in the plasma were measured by ELISA.Results. Body weight was significantly reduced by ICV injection of TNF-α, because muscle FSR was decreased with TNF-α. There were no differences in IL-6 levels in the plasma between the two groups. The FSR of jejunal mucosa was elevated by infusion of TNF-α as compared with control mice on both Days 5 and 7 [Day 5: (control) 99.5 ± 17.6%/day vs (TNF) 150.5 ± 20.5%/day, P < 0.05; Day 7: (control) 54.8 ± 8.8%/day vs (TNF) 102.7 ± 19.1%/day, P < 0.05).Conclusion. ICV injection of TNF-α enhanced the mucosal protein synthesis rate without elevating IL-6 levels as compared with control mice, suggesting that the central nervous system links to the gut to regulate mucosal protein turnover through cytokine in the brain, but plasma IL-6 is not involved in this linkage.

Small-intestinal mucosal protein synthesis and whole-body protein turnover in alcoholic liver disease

We used stable-isotope-labelled amino acids to measure the effects of alcoholic liver disease (ALD) on whole-body protein turnover and small-intestinal mucosal protein synthesis. Groups comprising eight patients with ALD and eight healthy control subjects were studied. They received primed, continuous intravenous infusions of L-[1-(13)C]leucine after an overnight fast; after 4 h, duodenal biopsies were obtained via endoscopy. Protein synthesis was calculated from protein labelling relative to intracellular leucine enrichment. Rates of duodenal mucosal protein synthesis were 2. 58+/-0.32%.h(-1) (mean+/-S.D.) in the normal subjects and 2.04+/-0. 18%.h(-1) in the ALD patients (P<0.003), despite the fact that the protein synthetic capacity (microgram of RNA/mg of protein) was higher in ALD patients (160+/-14 compared with 137+/-6 microgram/mg; P<0.003). The mucosal cell size (protein/DNA ratio) was lower in ALD patients (9.23+/-0.91 compared with 13+/-2.2 microgram/mg; P<0.002). Although the mean rates of whole-body protein turnover were not significantly different between the two groups (204+/-18 and 196+/-44 micromol leucine.h(-1).kg(-1) for ALD and control subjects respectively), there was, in the ALD patients, an inverse relationship between the rate of small-intestinal mucosal protein synthesis and the severity of ALD; furthermore, there was a direct relationship between the rate of whole-body protein turnover and the severity of ALD. Thus there was an inverse relationship between the rate of small-intestinal mucosal protein synthesis and the rate of whole-body protein turnover in ALD patients, which was not seen in the normal subjects.

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.