Evaluation of tracer labelled methionine load test in vitamin B-12 deficient adolescent women.

BACKGROUND: Methionine loading test (MLT) has been used primarily to identify defects in transsulfuration of homocysteine in cystathionine beta synthase deficiency. It may not be as useful to evaluate remethylation pathway, in vitamin B-12 and folate deficiencies. OBJECTIVE: We used tracer isotope labelled MLT to interrogate transsulfuration and remethylation independently in vitamin B-12 deficiency. DESIGN: We studied vitamin B-12 deficient women with a tracer labelled MLT before and eleven months after treatment with vitamin B-12. The fractional contribution of [13C]homocysteine to breath CO2 was used as a measure of transsulfuration, and difference in the intracellular enrichment of [13C]methionine and that of [C2H3]methionine as a measure of remethylation of homocysteine. Combined pre- and post-treatment results were analyzed to investigate the association between plasma vitamin B-12 concentrations and measures of homocysteine metabolism. RESULTS: The subjects were 17 years old, with a BMI of 19.4 kg/m2. Treatment with vitamin B-12, 2µg/day increased plasma B-12 from 93 (78.7, 106.2) [median (25th, 75th centiles)] to 161.5 (125.5, 226.2) pmol/L; 44% were below <150pmol/L after treatment. Fasting homocysteine concentration was significantly lower and that of cysteine higher in subjects with B-12 levels >150pmol/L. The tracer estimated transsulfuration of homocysteine was lower and remethylation higher with B-12 levels >150pmol/L when compared with those <150pmol/L. CONCLUSIONS: The tracer labelled MLT in combination with fasting parameters is a robust way to estimate parameters of methionine metabolism and can be used in the field where prime-constant rate infusion studies cannot be done efficiently.

Arginine metabolic endotypes related to asthma severity.

AIMS: Arginine metabolism via inducible nitric oxide synthase (iNOS) and arginase 2 (ARG2) is higher in asthmatics than in healthy individuals. We hypothesized that a sub-phenotype of asthma might be defined by the magnitude of arginine metabolism categorized on the basis of high and low fraction of exhaled nitric oxide (FENO). METHODS: To test this hypothesis, asthmatics (n = 52) were compared to healthy controls (n = 51) for levels of FENO, serum arginase activity, and airway epithelial expression of iNOS and ARG2 proteins, in relation to clinical parameters of asthma inflammation and airway reactivity. In parallel, bronchial epithelial cells were evaluated for metabolic effects of iNOS and ARG2 expression in vitro. RESULTS: Asthmatics with high FENO (≥ 35 ppb; 44% of asthmatics) had higher expression of iNOS (P = 0.04) and ARG2 (P = 0.05) in the airway, indicating FENO is a marker of the high arginine metabolic endotype. High FENO asthmatics had the lowest FEV1% (P < 0.001), FEV1/FVC (P = 0.0002) and PC20 (P < 0.001) as compared to low FENO asthmatics or healthy controls. Low FENO asthmatics had near normal iNOS and ARG2 expression (both P > 0.05), and significantly higher PC20 (P < 0.001) as compared to high FENO asthmatics. In vitro studies to evaluate metabolic effects showed that iNOS overexpression and iNOS+ARG2 co-expression in a human bronchial epithelial cell line led to greater reliance on glycolysis with higher rate of pyruvate going to lactate. CONCLUSIONS: The high FENO phenotype represents a large portion of the asthma population, and is typified by greater arginine metabolism and more severe and reactive asthma.

Increased mitochondrial arginine metabolism supports bioenergetics in asthma.

High levels of arginine metabolizing enzymes, including inducible nitric oxide synthase (iNOS) and arginase (ARG), are typical in asthmatic airway epithelium; however, little is known about the metabolic effects of enhanced arginine flux in asthma. Here, we demonstrated that increased metabolism sustains arginine availability in asthmatic airway epithelium with consequences for bioenergetics and inflammation. Expression of iNOS, ARG2, arginine synthetic enzymes, and mitochondrial respiratory complexes III and IV was elevated in asthmatic lung samples compared with healthy controls. ARG2 overexpression in a human bronchial epithelial cell line accelerated oxidative bioenergetic pathways and suppressed hypoxia-inducible factors (HIFs) and phosphorylation of the signal transducer for atopic Th2 inflammation STAT6 (pSTAT6), both of which are implicated in asthma etiology. Arg2-deficient mice had lower mitochondrial membrane potential and greater HIF-2α than WT animals. In an allergen-induced asthma model, mice lacking Arg2 had greater Th2 inflammation than WT mice, as indicated by higher levels of pSTAT6, IL-13, IL-17, eotaxin, and eosinophils and more mucus metaplasia. Bone marrow transplants from Arg2-deficient mice did not affect airway inflammation in recipient mice, supporting resident lung cells as the drivers of elevated Th2 inflammation. These data demonstrate that arginine flux preserves cellular respiration and suppresses pathological signaling events that promote inflammation in asthma.

Sarcopenia associated with portosystemic shunting is reversed by follistatin.

BACKGROUND & AIMS: The distinct role of portosystemic shunting (PSS) in the pathogenesis of sarcopenia (skeletal muscle loss) that occurs commonly in cirrhosis is unclear. We have previously shown increased expression of myostatin (inhibitor of skeletal muscle mass) in the portacaval anastamosis (PCA) rat model of sarcopenia of PSS. The present study was performed to examine the mechanisms of sarcopenia following PCA. METHODS: In PCA and sham operated pair fed control rats, the phenylalanine flooding dose method was used to quantify the fractional and absolute protein synthesis rates in the skeletal muscle over time and in response to follistatin, a myostatin antagonist. The expression of myostatin and markers of satellite cell (myocyte precursors) proliferation and differentiation were quantified by real-time PCR and Western blot analyses. RESULTS: The absolute synthesis rate (ASR) was lower at 2, 4, and 6 weeks (p<0.05) and the fractional synthesis rate (FSR) of skeletal muscle protein was significantly lower (p<0.05) at week 2 in the PCA rats compared to control rats. Expression of myostatin was elevated while markers of satellite cell proliferation and differentiation were lower at 4 and 6 weeks after PCA. Follistatin increased skeletal muscle mass, muscle FSR and ASR, decreased expression of myostatin protein, and increased expression of markers of satellite cell function. CONCLUSIONS: Sarcopenia associated with PSS is caused by impaired protein synthesis and reduced satellite cell function due to increased myostatin expression. Confirming these alterations in human patients with cirrhosis will provide novel therapeutic targets for sarcopenia of liver disease.