α-Hydroxybutyric Acid Is a Selective Metabolite Biomarker of Impaired Glucose Tolerance

OBJECTIVE

Plasma metabolites that distinguish isolated impaired glucose tolerance (iIGT) from isolated impaired fasting glucose (iIFG) may be useful biomarkers to predict IGT, a high-risk state for the development of type 2 diabetes.

RESEARCH DESIGN AND METHODS

Targeted metabolomics with 23 metabolites previously associated with dysglycemia was performed with fasting plasma samples from subjects without diabetes at time 0 of an oral glucose tolerance test (OGTT) in two observational cohorts: RISC (Relationship Between Insulin Sensitivity and Cardiovascular Disease) and DMVhi (Diabetes Mellitus and Vascular Health Initiative). Odds ratios (ORs) for a one-SD change in the metabolite level were calculated using multiple logistic regression models controlling for age, sex, and BMI to test for associations with iIGT or iIFG versus normal. Selective biomarkers of iIGT were further validated in the Botnia study.

RESULTS

α-Hydroxybutyric acid (α-HB) was most strongly associated with iIGT in RISC (OR 2.54 [95% CI 1.86-3.48], P value 5E-9) and DMVhi (2.75 [1.81-4.19], 4E-5) while having no significant association with iIFG. In Botnia, α-HB was selectively associated with iIGT (2.03 [1.65-2.49], 3E-11) and had no significant association with iIFG. Linoleoyl-glycerophosphocholine (L-GPC) and oleic acid were also found to be selective biomarkers of iIGT. In multivariate IGT prediction models, addition of α-HB, L-GPC, and oleic acid to age, sex, BMI, and fasting glucose significantly improved area under the curve in all three cohorts.

CONCLUSIONS

α-HB, L-GPC, and oleic acid were shown to be selective biomarkers of iIGT, independent of age, sex, BMI, and fasting glucose, in 4,053 subjects without diabetes from three European cohorts. These biomarkers can be used in predictive models to identify subjects with IGT without performing an OGTT.

A novel test for IGT utilizing metabolite markers of glucose tolerance

The oral glucose tolerance test (OGTT) is the only method to diagnose patients having impaired glucose tolerance (IGT), but its use has diminished considerably in recent years. Metabolomic profiling studies have identified a number of metabolites whose fasting levels are associated with dysglycemia and type 2 diabetes. These metabolites may serve as the basis of an alternative test for IGT. Using the stable isotope dilution technique, quantitative assays were developed for 23 candidate biomarker metabolites. These metabolites were measured in fasting plasma samples taken just prior to an OGTT from 1623 nondiabetic subjects: 955 from the Relationship between Insulin Sensitivity and Cardiovascular Disease Study (RISC Study; 11.7% IGT) and 668 subjects from the Diabetes Mellitus and Vascular Health Initiative (DMVhi) cohort from the DEXLIFE project (11.8% IGT). The associations between metabolites, anthropometric, and metabolic parameters and 2hPG values were assessed by Pearson correlation coefficients and Random Forest classification analysis to rank variables for their ability to distinguish IGT from normal glucose tolerance (NGT). Multivariate logistic regression models for estimating risk of IGT were developed and evaluated using AUCs calculated from the corresponding ROC curves. A model based on the fasting plasma levels of glucose, α-hydroxybutyric acid, β-hydroxybutyric acid, 4-methyl-2-oxopentanoic acid, linoleoylglycerophosphocholine, oleic acid, serine and vitamin B5 was optimized in the RISC cohort (AUC = 0.82) and validated in the DMVhi cohort (AUC = 0.83). A novel, all-metabolite-based test is shown to be a discriminate marker of IGT. It requires only a single fasted blood draw and may serve as a more convenient surrogate for the OGTT or as a means of identifying subjects likely to be IGT.

Calcineurin inhibition normalizes beta-adrenergic responsiveness in the spontaneously hypertensive rat

Calcineurin, a Ca(2+)-regulated protein phosphatase, links myocardial Ca(2+) signaling with hypertrophic gene transcription. Calcineurin abundance increases in pressure-overload hypertrophy and may reduce agonist-mediated phospholamban (PLB) phosphorylation to underlie blunted beta-adrenergic receptor (beta-AR) responsiveness in hypertension. This hypothesis was tested by measuring the effects of calcineurin inhibition on changes in cardiac contractility caused by beta-adrenergic stimulation in spontaneously hypertensive rats (SHR). Female SHR (age: 7 mo) and age-matched female Wistar-Kyoto rats (WKY) were studied. Heart weight-to-body weight ratio (P < 0.01) and systolic blood pressure (P < 0.01) were greater in SHR compared with WKY and were associated with increased myocardial calcineurin mRNA (CnAbeta) and activity (P < 0.05). beta-AR stimulation with isoproterenol (Iso) increased calcineurin activity (P < 0.05) in both WKY and SHR hearts, and this activity was suppressed with cyclosporin A (CsA) treatment. In SHR, CsA improved left ventricular whole heart and isolated myocyte beta-AR responsiveness by normalizing PLB phosphorylation at Ser(16) and Thr(17) (P < 0.05). These CsA-induced, PLB-mediated effects were associated with an augmentation in cardiomyocyte peak Ca(2+) and a reduced rate (time constant of isovolumic pressure relaxation, tau) and magnitude of diastolic Ca(2+) during beta-AR stimulation. In conclusion, CsA normalized the blunted beta-AR responsiveness associated with hypertension, in part, by mitigating calcineurin activity while improving PLB phosphorylation and subsequent sarcoplasmic reticulum Ca(2+) regulation.

Vascular beta-adrenergic receptor system is dysfunctional after myocardial infarction

We identified abnormalities in the vascular beta-adrenergic receptor (beta-AR) signaling pathway in heart failure after myocardial infarction (MI). To examine these abnormalities, we measured beta-AR-mediated hemodynamics, vascular reactivity, and the vascular beta-AR molecular signaling components in rats with heart failure after MI. Six weeks after MI, these rats had an increased left ventricular (LV) end-diastolic pressure, decreased LV systolic pressure, and decreased rate of LV pressure change (dP/dt). LV dP/dt responses to isoproterenol were shifted downward, although the responses for systemic vascular resistance were shifted upward in heart failure rats (P < 0.05). Isoproterenol- and IBMX-induced vasorelaxations were blunted in heart failure rats (P < 0.05) with no change in the forskolin-mediated vasorelaxation. These changes were associated with the following alterations in beta-AR signaling (P < 0.05): decreases in beta-AR density (aorta: 58.7 +/- 6.0 vs. 35.7 +/- 1.9 fmol/mg membrane protein; carotid: 29.6 +/- 5.6 vs. 18.0 +/- 3.9 fmol/mg membrane protein, n = 5), increases in G protein-coupled receptor kinase activity levels (relative phosphorimage counts of 191 +/- 39 vs. 259 +/- 26 in the aorta and 115 +/- 30 vs. 202 +/- 7 in the carotid artery, n = 5), and decreases in cGMP and cAMP in the carotid artery (0.85 +/- 0.10 vs. 0.31 +/- 0.06 pmol/mg protein and 2.3 +/- 0.3 vs. 1.2 +/- 0.1 pmol/mg protein, n = 5) with no change in Galpha(s) or Galpha(i )in the aorta. Thus in heart failure there are abnormalities in the vascular beta-AR system that are similar to those seen in the myocardium. This suggests a common neurohormonal mechanism and raises the possibility that treatment in heart failure focused on the myocardium may also affect the vasculature.

Oxidized collagen stimulates proliferation of vascular smooth muscle cells

We hypothesize that the vascular smooth muscle proliferation after lung injury results from oxidative damage to the matrix proteins in the walls of pulmonary blood vessels. The smooth muscle cells (SMC) isolated from rat aorta were cultured on the surface coated with oxidized and nonoxidized (control) collagen of type I. Oxidation of collagen was induced by UV irradiation and characterized by fluorescence tridimensional spectral arrays and by gel electrophoresis. From day 1 to 6 of the experiment, SMC proliferated more rapidly on the oxidized collagen than on the control surface. At high SMC population densities (day 9 of experiment) the difference disappeared. After 10 min of trypsinization the cells growing on oxidized collagen rounded and detached completely from the growth surface. The control cells on nonoxidized collagen detached only after 30 min of trypsinization. We conclude that oxidation of collagen of vascular wall matrix may participate in stimulation of SMC proliferation after oxidant tissue injury.

Enhanced leukotriene C4 production by peripheral eosinophilic granulocytes from children with asthma

Granulocytes and mononuclear cells were isolated from the blood of asthmatic and healthy children. Stimulation with ionophore A 23187 induced a significantly higher leukotriene C4 (LTC4) generation from granulocytes of asthmatic children than from granulocytes of healthy controls. In contrast, mononuclear cells from patients and controls did not differ in their ability to produce LTC4. Additional analysis showed that the difference in LTC4 generation of granulocytes was due to increased formation but not to decreased oxidative degradation of LTC4. Analysis of LTC4 generation of purified neutrophils and eosinophils revealed that LTC4 was generated almost exclusively by eosinophils and, in particular, the hypodense population. Granulocytes from patients with a history of severe asthma displayed a higher LTC4 formation than granulocytes from patients with less severe disease.