Kegg Pathway: Tyrosine metabolism

Rapid Commun Mass Spectrom. 2008 Aug 21; 22(18): 2882-2888
Lu Y, A J, Wang G, Hao H, Huang Q, Yan B, Zha W, Gu S, Ren H, Zhang Y, Fan X, Zhang M, Hao K

Metabonomics is a systematic approach to the study of in vivo metabolic profiles and therefore allows deep insight into and a better understanding of the pathogenesis of disease. To characterize the development of hypertension, a hypertensive animal model, the spontaneously hypertensive rat (SHR), and its normotensive control, the Wistar Kyoto (WKY) rat, were investigated and their blood plasma analyzed using the high-throughput metabolomic tool, gas chromatography/time-of-flight mass spectrometry (GC/TOFMS). A total of 187 peaks were quantitatively determined after deconvolution, and 78 of them were identified. Principal components analysis (PCA) and projection to latent structure partial least-squares discriminant analysis (PLS-DA) were used to process the GC/TOFMS data. The resulting mathematical models were further validated by cross-validation. Plasma compositional differences of many identified compounds showed hypertension-related variation between SHR and WKY rats, and age-related changes from 10 to 18 weeks for both the SHR and WKY rats. These compositional changes involved compounds such as hexadecanoic acid, linoleic acid, oleic acid, stearic acid, 3-hydroxybutyric acid, citric acid, threonic acid, Tyrosine, tryptophan, threonine, phenylalanine, serine, ornithine, methionine, 3-hydroxyproline, creatinine, erythrose, myo-inositol, D-methylglucopyranoside, tocopherol, sitosterol, and nonesterified cholesterol. Significantly elevated free fatty acids (FFA) were observed in SHR relative to those in WKY rats, and their levels increased as the SHR aged from 10 to 18 weeks. The close correlation between FFA and hypertension suggests that FFA are potential biomarker candidates for hypertension and they may play an important role in the development of hypertension in SHR. It is also indicated that GC/TOFMS-based metabonomics is a powerful approach to identifying potential biomarkers and investigating the pathological processes of hypertension and the physiological developments of aging. Copyright (c) 2008 John Wiley & Sons, Ltd.

Am J Physiol Endocrinol Metab. 2008 Aug 19;
Laviola L, Natalicchio A, Perrini S, Giorgino F

IGF-I action is essential for the regulation of tissue formation and remodeling, bone growth, prenatal growth, brain development, and muscle metabolism. Cellular effects of IGF-I are mediated through the IGF-I receptor, a trans-membrane Tyrosine kinase, which phosphorylates intracellular substrates, resulting in the activation of multiple intracellular signaling cascades. Dysregulation of IGF-I actions due to impairment in the post-receptor signaling machinery may contribute to multiple diseases in humans. This article will review current information on IGF-I signaling and illustrate recent results demonstrating how impaired IGF-I signaling and action may contribute to the pathogenesis of human diseases, including osteoporosis, neurodegenerative disorders, and reduced fetal growth in utero. Key words: IGF-I receptor, IGF-I signaling, osteoporosis, neurodegenerative disorders, IUGR.

J Nutr Biochem. 2008 Aug 15;
Carneiro EM, Latorraca MQ, Araujo E, Beltrá M, Oliveras MJ, Navarro M, Berná G, Bedoya FJ, Velloso LA, Soria B, Martín F

Taurine is a conditionally essential amino acid for human that is involved in the control of glucose homeostasis; however, the mechanisms by which the amino acid affects blood glucose levels are unknown. Using an animal model, we have studied these mechanisms. Mice were supplemented with taurine for 30 d. Blood glucose homeostasis was assessed by intraperitoneal glucose tolerance tests (IPGTT). Islet cell function was determined by insulin secretion, cytosolic Ca(2+) measurements and glucose metabolism from isolated islets. Islet cell gene expression and translocation was examined via immunohistochemistry and quantitative real-time polymerase chain reaction. Insulin signaling was studied by Western blot. Islets from taurine-supplemented mice had: (i) significantly higher insulin content, (ii) increased insulin secretion at stimulatory glucose concentrations, (iii) significantly displaced the dose-response curve for glucose-induced insulin release to the left, (iv) increased glucose metabolism at 5.6 and 11.1-mmol/L concentrations; (v) slowed cytosolic Ca(2+) concentration ([Ca(2+)]i) oscillations in response to stimulatory glucose concentrations; (vi) increased insulin, sulfonylurea receptor-1, glucokinase, Glut-2, proconvertase and pancreas duodenum homeobox-1 (PDX-1) gene expression and (vii) increased PDX-1 expression in the nucleus. Moreover, taurine supplementation significantly increased both basal and insulin stimulated Tyrosine phosphorylation of the insulin receptor in skeletal muscle and liver tissues. Finally, taurine supplemented mice showed an improved IPGTT. These results indicate that taurine controls glucose homeostasis by regulating the expression of genes required for glucose-stimulated insulin secretion. In addition, taurine enhances peripheral insulin sensitivity.

metabolism. 2008 Sep; 57(9): 1173-80
Saito M, Lessard SJ, Rivas DA, Reeder DW, Hawley JA, Yaspelkis BB

We determined whether sustained aerobic exercise reverses high-fat diet-induced impairments in the c-Cbl associated protein (CAP)/Casitas b-lineage lymphoma (c-Cbl) signaling cascade in rodent skeletal muscle. Sprague-Dawley rats were placed into either control (n = 16) or high-fat-fed (n = 32) diet groups for 4 weeks. During a subsequent 4-week experimental period, 16 high-fat-fed rats remained sedentary, 16 high-fat-fed rats completed 4 weeks of exercise training, and control animals were sedentary and remained on the control diet. After the intervention period, animals were subjected to hind limb perfusions in the presence (n = 8 per group) or absence (n = 8 per group) of insulin. In the plasma membrane fractions, neither high-fat feeding nor exercise training altered adaptor protein with PH and SH2 domains, (APS), c-Cbl, or TC10 protein concentrations. In contrast, CAP protein concentration and insulin-stimulated plasma membrane c-Cbl Tyrosine phosphorylation were reduced by high-fat feeding; but exercise training reversed these impairments. Of note was that insulin-stimulated atypical protein kinase Czeta kinase activity toward TC10 was reduced by high-fat feeding but normalized by exercise training. We conclude that sustained (4 weeks) exercise training can reverse high-fat diet-induced impairments on the CAP/c-Cbl pathway in high-fat-fed rodent skeletal muscle. We also provide the first evidence that the CAP/c-Cbl insulin signaling cascade in skeletal muscle may directly interact with components of the classic (phosphoinositide 3-kinase dependent) insulin signaling cascade.

Am J Physiol Endocrinol Metab. 2008 Aug 12;
Luiking YC, Hallemeesch MM, de Jonge WJ, Lamers WH, Deutz NE

The amino acid arginine is the sole precursor for nitric oxide (NO) synthesis. We recently demonstrated that an acute reduction of circulating arginine does not compromise basal or lipopolysaccharide (LPS)-inducible NO production in mice. In the present study, we investigated the importance of citrulline availability in ornithine transcarbamoylase-deficient spf(ash) (OTCD) mice on NO production, using stable isotope techniques and C57BL6/J (wild-type) mice controls. Plasma amino acids and tracer-tracee ratios were measured by LC-MS. NO production was measured as the in vivo conversion of L-[guanidino -(15)N2]arginine to L-[guanidine-(15)N]citrulline; de novo arginine production as conversion of L-[ureido-(13)C-5,5-(2)H2]citrulline to L-[guanidino -(13)C-5,5-(2)H2]arginine. Protein metabolism was measured using L-[ring-(2)H5]phenylalanine and L-[ring-(2)H2]Tyrosine. OTC deficiency caused a reduction of systemic citrulline concentration and production to 30-50% (P<0.001), reduced de novo arginine production (P<0.05), reduced whole-body NO production to 50% (P<0.005) and increased net protein breakdown by a factor 2-4 (P<0.001). NO production was 2-fold higher in female than in male OTCD mice, in agreement with the X-linked location of the OTC gene. In response to LPS treatment (10 mg/kg i.p.), circulating arginine increased in all groups (P<0.001), and NO production was no longer affected by the OTC deficiency due to increased net protein breakdown as a source for arginine. Our study shows that reduced citrulline availability is related to reduced basal NO production via reduced de novo arginine production. Under basal conditions this is probably c-NOS mediated NO production. When sufficient arginine is available after LPS stimulated net protein breakdown, NO production is unaffected by OTC deficiency. Key words: arginine, OTC, sepsis.

Int J Oncol. 2008 Sep; 33(3): 443-52
Pantaleo MA, Nannini M, Lopci E, Castellucci P, Maleddu A, Lodi F, Nanni C, Allegri V, Astorino M, Brandi G, Di Battista M, Boschi S, Fanti S, Biasco G

The widespread use of several new non-cytotoxic drugs and the significant improvements in functional imaging highlights a number of difficulties in monitoring, interpreting and predicting treatment response in clinical practice. Certain guidelines for disease assessment after therapy are already available: the traditional Response Evaluation Criteria in Solid Tumours guidelines based on tumour size variations using conventional imaging technologies, the recent combined method developed by Choi and colleagues in gastrointestinal stromal tumour treated with Tyrosine kinase inhibitors based on tumour density variations using computed tomography (CT), and the European Organization for Research and Treatment of Cancer criteria based on tumour glucose metabolism variations using fluorodeoxyglucose (FDG) positron emission tomography (PET). At the moment combined PET/CT response criteria are still not available. A number of new PET compounds other than FDG are also currently being developed to visualize specific cellular and molecular tumour pathways but their role in assessment and prediction of cancer treatment response has not yet been thoroughly investigated in a large series. However, in clinical practice many oncologists treat cancer patients with targeted therapies or chemotherapy and evaluate the response using conventional or functional imaging without appropriate and standardized guidelines. The aim of this study was to present a selection of clinical cases that illustrate the usefulness of new PET tracers and efficacy evaluation of new drugs. In the era of molecular imaging and molecular therapies, these cases highlight the urgency to develop new criteria for treatment assessment and the exigency of correctly interpreting the biological information obtained from new technologies, and introduce new concepts that require further investigation in clinical trials.

Am J Pathol. 2008 Aug 18;
Goldoni S, Seidler DG, Heath J, Fassan M, Baffa R, Thakur ML, Owens RT, McQuillan DJ, Iozzo RV

Decorin, a member of the small leucine-rich proteoglycan gene family, down-regulates members of the ErbB receptor Tyrosine kinase family and attenuates their signaling, leading to growth inhibition. We investigated the effects of decorin on the growth of ErbB2-overexpressing mammary carcinoma cells in comparison with AG879, an established ErbB2 kinase inhibitor. Cell proliferation and anchorage-independent growth assays showed that decorin was a potent inhibitor of breast cancer cell growth and a pro-apoptotic agent. When decorin and AG879 were used in combination, the inhibitory effect was synergistic in proliferation assays but only additive in both colony formation and apoptosis assays. Active recombinant human decorin protein core, AG879, or a combination of both was administered systemically to mice bearing orthotopic mammary carcinoma xenografts. Primary tumor growth and metabolism were reduced by approximately 50% by both decorin and AG879. However, no synergism was observed in vivo. Decorin specifically targeted the tumor cells and caused a significant reduction of ErbB2 levels in the tumor xenografts. Most importantly, systemic delivery of decorin prevented metastatic spreading to the lungs, as detected by novel species-specific DNA detection and quantitative assays. In contrast, AG879 failed to have any effect. Our data support a role for decorin as a powerful and effective therapeutic agent against breast cancer due to its inhibition of both primary tumor growth and metastatic spreading.

Pediatr Crit Care Med. 2008 Aug 1;
Checchia PA, Schierding W, Polpitiya A, Dixon D, Macmillan S, Muenzer J, Stromberg P, Coopersmith CM, Buchman TG, Cobb JP

BACKGROUND:: Age influences outcome of sepsis and septic shock. The mechanism of this age-dependent vulnerability to sepsis remains largely unknown. Because much of the mortality and morbidity associated with sepsis and septic shock is the result of severe derangements in the cardiovascular system, it is possible that the myocardium responds to injury in a developmentally influenced manner. We hypothesized that analysis of cardiac RNA expression profiles may differentiate between the myocardial response to sepsis in young and old mice. METHODS AND RESULTS:: Sixteen FVB/N male mice were stratified based on age. Young animals were 6 wks old, correlating to 4 to 6 human years, and aged animals were 20 months old correlating to 70 to 80 human years. Animals underwent either cecal ligation and puncture to produce polymicrobial sepsis or a sham operation. Both ventricles were excised after kill at 24 hrs. There were 53 genes that differed in RNA abundance between the four groups (false discovery rate of 0.005, p < 0.00001). Additionally, four genes were associated with an age-dependent response to sepsis: CYP2B2 (cytochrome P450, family 2, subfamily B, polypeptide 6), VGLL2 (vestigial like 2), and PAH (phenylalanine hydroxylase). The fourth gene is an expressed sequence tag, the function of which is related to the cytochrome P450 family. These genes play roles in phenylalanine, Tyrosine, tryptophan, and fatty acid metabolism. CONCLUSIONS:: This report describes the transcriptional response of the heart to sepsis. In addition, our findings suggest that these differences are in part age-dependent and serve as hypothesis generation.

Neuroscience. 2008 Jul 8;
Hoard JL, Hoover DB, Mabe AM, Blakely RD, Feng N, Paolocci N

Half of the cholinergic neurons of human and primate intrinsic cardiac ganglia (ICG) have a dual cholinergic/noradrenergic phenotype. Likewise, a large subpopulation of cholinergic neurons of the mouse heart expresses enzymes needed for synthesis of norepinephrine (NE), but they lack the vesicular monoamine transporter type 2 (VMAT2) required for catecholamine storage. In the present study, we determined the full scope of noradrenergic properties (i.e. synthetic enzymes and transporters) expressed by cholinergic neurons of mouse ICG, estimated the relative abundance of neurons expressing different elements of the noradrenergic phenotype, and evaluated the colocalization of cholinergic and noradrenergic markers in atrial nerve fibers. Stellate ganglia were used as a positive control for noradrenergic markers. Using fluorescence immunohistochemistry and confocal microscopy, we found that about 30% of cholinergic cell bodies contained Tyrosine hydroxylase (TH), including the activated form that is phosphorylated at Ser-40 (pSer40 TH). Dopamine beta-hydroxylase (DBH) and norepinephrine transporter (NET) were present in all cholinergic somata, indicating a wider capability for dopamine metabolism and catecholamine uptake. Yet, cholinergic somata lacked VMAT2, precluding the potential for NE storage and vesicular release. In contrast to cholinergic somata, cardiac nerve fibers rarely showed colocalization of cholinergic and noradrenergic markers. Instead, these labels were closely apposed but clearly distinct from each other. Since cholinergic somata expressed several noradrenergic proteins, we questioned whether these neurons might also contain trophic factor receptors typical of noradrenergic neurons. Indeed, we found that all cholinergic cell bodies of mouse ICG, like noradrenergic cell bodies of the stellate ganglia, contained both tropomyosin-related kinase A (TrkA) and p75 neurotrophin receptors. Collectively, these findings demonstrate that mouse intrinsic cardiac neurons (ICNs), like those of humans, have a complex neurochemical phenotype that goes beyond the classical view of cardiac parasympathetic neurons. They also suggest that neurotrophins and local NE synthesis might have important effects on neurons of the mouse ICG.

Toxicol Appl Pharmacol. 2008 Jul 12;
Bradford BU, O'Connell TM, Han J, Kosyk O, Shymonyak S, Ross PK, Winnike J, Kono H, Rusyn I

Metabolomic evaluation of urine and liver was conducted to assess the biochemical changes that occur as a result of alcohol-induced liver injury. Male C57BL/6J mice were fed an isocaloric control- or alcohol-containing liquid diet with 35% of calories from corn oil, 18% protein and 47% carbohydrate/alcohol for up to 36 days ad libitum. Alcohol treatment was initiated at 7 g/kg/day and gradually reached a final dose of 21 g/kg/day. Urine samples were collected at 22, 30 and 36 days and, in additional treatment groups, liver and serum samples were harvested at 28 days. Steatohepatitis was induced in the alcohol-fed group since a 5-fold increase in serum alanine aminotransferase activity, a 6-fold increase in liver injury score (necrosis, inflammation and steatosis) and an increase in lipid peroxidation in liver were observed. Liver and urine samples were analyzed by nuclear magnetic resonance spectroscopy and electrospray infusion/Fourier transform ion cyclotron resonance-mass spectrometry. In livers of alcohol-treated mice the following changes were noted. Hypoxia and glycolysis were activated as evidenced by elevated levels of alanine and lactate. Tyrosine, which is required for l-DOPA and dopamine as well as thyroid hormones, was elevated possibly reflecting alterations of basal metabolism by alcohol. A 4-fold increase in the prostacyclin inhibitor 7,10,13,16-docosatetraenoic acid, a molecule important for regulation of platelet formation and blood clotting, may explain why chronic drinking causes serious bleeding problems. Metabolomic analysis of the urine revealed that alcohol treatment leads to decreased excretion of taurine, a metabolite of glutathione, and an increase in lactate, n-acetylglutamine and n-acetylglycine. Changes in the latter two metabolites suggest an inhibition of the kidney enzyme aminoacylase I and may be useful as markers for alcohol consumption.

J Thorac Oncol. 2008 Aug; 3(8): 938-41
De Pas T, Toffalorio F, Colombo P, Trifirò G, Pelosi G, Vigna PD, Manzotti M, Agostini M, de Braud F

Malignant solitary fibrous tumor (MSFT) of the pleura is a rare neoplasm, with unpredictable biologic behavior and a low sensitivity to chemotherapy. To the authors' knowledge, no other effective medical treatment is available for this disease. Imatinib mesylate is a Tyrosine kinase inhibitor targeting the platelet-derived growth factor (PDGFR-alpha and PDGFR-beta), the BCR-ABL, and c-KIT receptors. We report the first evidence of the activity of imatinib in a symptomatic patient with a chemo- and radio-resistant advanced MSFT, who obtained a 21-months lasting major clinical benefit with a consistent reduction in tumor metabolism. Immunostaining of tumor cells demonstrated the positivity for PDGFR-alpha and PDGFR-beta and the absence of c-KIT over-expression, in the absence of c-KIT and PDGRFR mutations; all the cells strongly and diffusely expressed the ligand PDGF A in the cytoplasm. This profile suggests that the observed tumor response was mediated through the inhibition of the Tyrosine kinase activity of PDGFR. Treatment with imatinib should be considered for patients with recurrent or unresectable MSFTs with PDGFR expression.

Curr Clin Pharmacol. 2006 Sep; 1(3): 223-8
Ng R, Chen EX

Sorafenib (BAY 43-9006) is a novel oral bis-aryl urea compound originally developed as an inhibitor to RAF kinase for its anti-proliferative property. It also inhibits receptor Tyrosine kinases of multiple pro-angiogenic factors such as VEGFR-2/3, Flt-3/ and PDGFR-beta. The combination of both its anti-proliferative and anti-angiogenic properties makes sorafenib an attractive agent in cancer treatment. Phase I studies demonstrated that sorafenib was well tolerated, and the recommended phase II dose was 400 mg twice daily continuously. Common toxicities included skin toxicity (rash and hand-foot syndrome), gastrointestinal toxicities (nausea and diarrhea) and fatigue. Anti-tumor activities were observed in multiple tumors types including renal cell carcinoma and hepatocellular carcinoma. Randomized phase III studies in these tumor types are ongoing, and results are eagerly waited.

Br J Cancer. 2008 Aug 5; 99(3): 520-6
Ponnusamy MP, Singh AP, Jain M, Chakraborty S, Moniaux N, Batra SK

The mucin MUC4 is a high molecular weight transmembrane glycoprotein. It consists of a mucin-type subunit (MUC4alpha) and a transmembrane growth factor-like subunit (MUC4beta). The mucin MUC4 is overexpressed in many epithelial malignancies including ovarian cancer, suggesting a possible role in the pathogenesis of these cancers. In this study, we investigated the functional role of MUC4 in the human ovarian cancer cell line SKOV3. The mucin MUC4 was ectopically expressed by stable transfection, and its expression was examined by western blot and confocal microscopy analyses. The in vitro studies demonstrated an enhanced motility of MUC4-expressing SKOV3 cells compared with the vector-transfected cells. The mucin MUC4 expression was associated with apparent changes in actin organisation, leading to the formation of microspike, lammelopodia and filopodia-like cellular projections. An enhanced protein expression and activation of HER2, a receptor Tyrosine kinase, was also seen, although no significant change was observed in HER-2 transcript levels in the MUC4-transfected SKOV3 cells. Reciprocal co-immunoprecipitation revealed an interaction of MUC4 with HER2. Further, the MUC4-overexpressing SKOV3 cells exhibited an increase in the phosphorylation of focal adhesion kinase (FAK), Akt and ERK, downstream effectors of HER2. Taken together, our findings demonstrate that MUC4 plays a role in ovarian cancer cell motility, in part, by altering actin arrangement and potentiating HER2 downstream signalling in these cells.

Yao Xue Xue Bao. 2008 Apr; 43(4): 323-34
Mao YJ, Li HH, Li JF, Shen JS

Intracellular signal transduction plays an important role in the process of cellular metabolism, segmentation, differentiation, biological behaviour and cell death. Overactive signal transduction relates to tumor development and progression. Signaling pathways operated by protein Tyrosine kinases (PTKs) will be illuminated here briefly. The Ras/Raf/MAPK and PI-3K/Akt pathways through receptor protein Tyrosine kinases (RTKs), the Src, Bcr-Abl and JAK/STAT pathways by non-receptor protein Tyrosine kinases (nrPTKs) are shown separately. Antitumor agents targeting the key proteins involved in the above five signalling routes are also summarized in this review.

J Exp Med. 2008 Aug 4; 205(8): 1775-88
Bezman NA, Lian L, Abrams CS, Brass LF, Kahn ML, Jordan MS, Koretzky GA

Src homology 2 domain-containing leukocyte phosphoprotein of 76 kD (SLP76), an adaptor that plays a critical role in platelet activation in vitro, contains three N-terminal Tyrosine residues that are essential for its function. We demonstrate that mice containing complementary Tyrosine to phenylalanine mutations in Y145 (Y145F) and Y112 and Y128 (Y112/128F) differentially regulate integrin and collagen receptor signaling. We show that mutation of Y145 leads to severe impairment of glycoprotein VI (GPVI)-mediated responses while preserving outside-in integrin signaling. Platelets from Y112/128F mice, although having mild defects in GPVI signaling, exhibit defective actin reorganization after GPVI or alpha IIb beta 3 engagement. The in vivo consequences of these signaling defects correlate with the mild protection from thrombosis seen in Y112/128F mice and the near complete protection observed in Y145F mice. Using genetic complementation, we further demonstrate that all three phosphorylatable Tyrosines are required within the same SLP76 molecule to support platelet activation by GPVI.

Am J Physiol Gastrointest Liver Physiol. 2008 Jul 24;
Tessari P, Kiwanuka E, Vettore M, Barazzoni R, Zanetti M, Cecchet D, Orlando R

We explored the mechanism(s) of increased aromatic amino acids concentrations in liver cirrhosis using Phe and Tyr isotope infusions in male patients with compensated cirrhosis (five in Child Class A, three in B) and in eight matched healthy controls, both in post-absorptive and fed states. After a baseline period, a standard liquid mixed-meal was fed continuously over 4-hr. Both a "plasma" and an intracellular (ic) model were employed. In the patients, steady-state Phe and Tyr concentrations were approximately 30%-50% greater, and rates of Phe appearance (Ra) (plasma model), Tyr Ra, and Phe hydroxylation (both models), were approximately 25%- >100% greater than in controls in both states. Meal ingestion increased (p<0.05 or less vs. basal) Phe and Tyr concentrations, Phe and Tyr Ra, Phe hydroxylation, and % Tyr Ra not deriving from Hy in both groups. Hy and Tyr Ra remained >50% greater (p<0.04 -p<0.01) in patients, whereas Phe Ra was more modestly increased. Phe utilization for protein synthesis increased similarly in both groups. Tyr clearance was normal, whereas Phe clearance tended to be lower (p=0.09, ic model) in the patients. In summary, in compensated liver cirrhosis studied under fasted and fed states: 1) Tyr Ra is increased; 2) Phe hydroxylation and Phe Ra (plasma model) are increased: 3) Tyr clearance is normal; 4) Phe clearance is slightly decreased. In conclusion, in cirrhosis increased total Tyrosine Ra and hydroxylation contribute to fasting and post-meal hyperTyrosinemia, whereas the mechanism(s) responsible for the hyperphenylalaninemia may include both increased production and decreased disposal. Key words: Compensated liver cirrhosis , mixed meal , Tyrosine hydroxylation, stable isotopes.

Endocrinology. 2008 Jul 24;
Hu J, Klein JD, Du J, Wang XH

Protein degradation is increased by both insulin deficiency and insulin resistance in humans and animal models. In skeletal muscle, this insulin-dependent increase in protein degradation involves activation of both caspase-3 and the ubiquitin-proteasome system. The influence of abnormal insulin signaling on protein metabolism in cardiac muscle is not well understood; therefore, we measured protein degradation in cardiac muscle of mice with streptozotocin-induced diabetes. Insulin deficiency increased both total muscle proteolysis (measured as Tyrosine release in muscle slices or extracts) and the degradation of the myofibrillar protein actin (measured as the appearance of a 14-kDa actin fragment). Expression of ubiquitin mRNA and chymotrypsin-like activity in the proteasome were increased, indicating activation of the ubiquitin-proteasome system in diabetic mouse heart. We also evaluated possible signaling pathways that might regulate cardiac muscle proteolysis. Insulin receptor substrate-1 (IRS-1) Tyrosine phosphorylation and Akt phosphorylation were decreased. Insulin replacement prevented the decrease in IRS-1/Akt phosphorylation, the increase in proteolysis, and attenuated the increase in ubiquitin mRNA. We conclude that insulinopenia accelerates proteolysis in cardiac muscle by reducing IRS-1/Akt signaling, which leads to activation of the ubiquitin-proteasome proteolytic pathway.

J Int Soc Sports Nutr. 2008; 5: 10
Oben J, Kothari SC, Anderson ML

ABSTRACT: BACKGROUND: Current research suggests that protein intake of 1.5 - 2.8 g/kg/day (3.5 times the current recommended daily allowance) is effective and safe for individuals trying to increase or maintain lean muscle mass. To achieve these levels of daily protein consumption, supplementing the diet with processed whey protein concentrate (WPC) in liquid form has become a popular choice for many people. Some products have a suggested serving size as high as 50 g of protein. However, due to possible inhibition of endogenous digestive enzymes from over-processing and rapid small intestine transit time, the average amount of liquid WPC that is absorbed may be only 15 g. The combined effect of these factors may contribute to incomplete digestion, thereby limiting the absorption rate of protein before it reaches the ceacum and is eliminated as waste. The purpose of this study was to determine if Aminogen(R), a patented blend of digestive proteases from Aspergillus niger and Aspergillus oryzae, would significantly increase the in-vivo absorption rate of processed WPC over control values. It also investigated if any increase would be sufficient to significantly alter nitrogen (N2) balance and C-reactive protein (CRP) levels over control values as further evidence of increased WPC absorption rate. METHODS: Two groups of healthy male subjects were assigned a specified balanced diet before and after each of two legs of the study. Subjects served as their own controls. In the first leg each control group (CG) was dosed with 50 g of WPC following an overnight fast. Nine days later each test group (TG) was dosed following an overnight fast with 50 g of WPC containing either 2.5 g (A2.5) or 5 g (A5) of Aminogen(R). Blood samples were collected during each leg at 0 hr, 0.5 hr, 1 hr, 2 hr, 3 hr, 3.5 hr and 4 hr for amino acid (AA) and CRP analyses. The following 18 AAs were quantified: alanine, arginine, aspartic acid, cysteine, glutamic acid, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, Tyrosine and valine. Urine was collected for 24 hours from 0 hr for total N2 analysis. Results are expressed as means +/- SEM. All significance and power testing on results was done at a level of alpha = 0.05. Area under the concentration time curve (AUC) was calculated using the trapezoidal rule. One-way analysis of variance (ANOVA-1) was done between CGs, between TGs and between time points. One-way repeated measures analysis of variance (ANOVA-1-RM) was done to compare CGs and TGs. Two-way analysis of variance (ANOVA-2) was performed on total serum amino acid (TSAA) levels, urine N2 levels and CRP levels between each CG and TG. RESULTS: After baseline subtraction the mean AUC was significantly (p