Early Metabolic Markers of Dysglycemia and Type 2 Diabetes
Early Metabolic Markers of Dysglycemia and Type 2 Diabetes
With T2D prevalence continuing to increase worldwide, there is interest in identifying high-risk patients with more sensitivity than currently feasible. Because IR is a risk factor for IGT and T2D, we tested whether newly identified markers of insulin sensitivity and β-cell function could predict deterioration of glucose tolerance. We found α-HB and L-GPC were related to IR in an opposite manner—the former positive, the latter negative—in the RISC and Botnia cohorts, independently of known determinants of insulin sensitivity and of one another. Fasting concentrations of α-HB and L-GPC measured at baseline predicted worsening GT independently of classical predictors and with similar power as the 2-h plasma glucose level. Furthermore, changes in α-HB and L-GPC over time tracked with their ability to predict development of dysglycemia in the RISC cohort: at follow-up, α-HB had risen and L-GPC had fallen in subjects who progressed to dysglycemia compared with stable NGT subjects.
Strikingly, odds ratios for both metabolites for risk of progression to dysglycemia were virtually the same in RISC and Botnia (Fig. 1) despite differences in population characteristics, sample size, and end point. Accordingly, the metabolites added 0.028 and 0.017 units of ROC area to the standard model for predicting dysglycemia in RISC and Botnia, respectively. In contrast, fasting insulin, which has been shown to predict T2D in several studies, did not contribute to predictivity above the joint weight of α-HB and L-GPC in either study. Taken together, these results qualify these metabolites as disease biomarkers through their relation to underlying pathophysiological mechanisms. However, their actual value in routine clinical use requires additional studies, particularly intervention studies.
Because in RISC and Botnia an independent, inverse relationship also existed between α-HB levels and indices of β-cell function, we tested the direct impact of these metabolites on insulin secretion. In β-cell in vitro studies, α-HB dose-dependently inhibited overall glucose- and arginine-mediated insulin release. Although this effect must be confirmed in human islets and its mechanisms remain to be investigated, α-HB may mark IR and also β-cell function under diverse circumstances. This use would have utility for clinicians who have available surrogate indices of IR (e.g., homeostasis model assessment-IR or Matsuda index) but must choose among clinical tests of β-cell function (e.g., hyperglycemic clamp, acute insulin response to intravenous glucose, insulinogenic index) that are cumbersome and relatively incongruous with one another. In this regard, it is notable that L-GPC appeared to exert the opposite effect to α-HB on in vitro insulin release, thereby adding to the evidence that certain lipid-signaling molecules (including lysophospholipids) can stimulate glucose-dependent insulin release through lysophospholipid receptors such as G-protein–coupled receptor 119, which localized to pancreatic β-cells. Interestingly, a similar lipid-signaling pathway has been described for G-protein–coupled receptor 119, with its lipid agonist activation playing a role in glucagon-like peptide-1 release in intestinal l-cells. Moreover, lysophospholipids have been implicated in other signaling functions related to metabolism, including glucose uptake by adipocytes and myotubes. Declining L-GPC levels across IR, IGR, and T2D (Table 1 and Table 2) are likely a reflection of an important role for this metabolite in glucose metabolism.
Previous evidence has linked raised concentrations of selected amino acids with obesity and T2D (reviewed in). Newgard et al. described a metabolic signature related to BCAAs in obese humans. Fiehn et al. reported increased concentrations of leucine and valine—in addition to lipid substrates—in a small group of obese T2D women. Lower glycine concentrations have been reported in IR offspring of T2D patients, consistent with our observation of lower glycine associated with IR. However, compared with such reported analytes, α-HB and L-GPC were more sensitive markers of IR, e.g., able to discriminate insulin sensitive from IR individuals in both NGT and IGR ranges.
The pattern of metabolite changes we observed in the Botnia subgroup (Supplementar1y Table 1) can be organized into a coherent pathway (Supplementary Fig. 3) by assuming IR as a primary determinant of these changes. In this scenario, adipose tissue IR leads to elevated FFA concentrations, which feed into the TCA cycle and are oxidized at an increased rate, thereby producing an excess of reducing equivalents (NADH). Raised circulating FFA also reconstitutes phospholipids from circulating lipids such as L-GPC. Such overload of the TCA cycle leads to accumulation of amino acids such as glutamate and alanine, as well as α-ketobutyrate, substrate precursor of α-HB via propionyl-CoA. Oxidative stress and IR raise demand for glutathione synthesis, of which α-ketobutyrate and α-HB are by-products, and depletes glutathione constituents like glycine, whose levels are decreased in association with IR and T2D progression (Supplementary Table 1). Furthermore, by reducing amino acid transport and clearance, IR also raises BCAAs, which also feed into the TCA cycle, directly (leucine) or via propionyl-CoA. Thus, increased α-HB and decreased L-GPC levels serve as readouts of metabolic overload (elevated NADH-to-NAD ratio) and reduced glucose metabolism in both IR and the earliest phases of dysglycemia (Fig. 3). Finally, the abnormal α-HB and L-GPC levels and their biological activity on in vitro glucose-stimulated insulin release may translate into a burden on in vivo β-cell function.
Discussion
With T2D prevalence continuing to increase worldwide, there is interest in identifying high-risk patients with more sensitivity than currently feasible. Because IR is a risk factor for IGT and T2D, we tested whether newly identified markers of insulin sensitivity and β-cell function could predict deterioration of glucose tolerance. We found α-HB and L-GPC were related to IR in an opposite manner—the former positive, the latter negative—in the RISC and Botnia cohorts, independently of known determinants of insulin sensitivity and of one another. Fasting concentrations of α-HB and L-GPC measured at baseline predicted worsening GT independently of classical predictors and with similar power as the 2-h plasma glucose level. Furthermore, changes in α-HB and L-GPC over time tracked with their ability to predict development of dysglycemia in the RISC cohort: at follow-up, α-HB had risen and L-GPC had fallen in subjects who progressed to dysglycemia compared with stable NGT subjects.
Strikingly, odds ratios for both metabolites for risk of progression to dysglycemia were virtually the same in RISC and Botnia (Fig. 1) despite differences in population characteristics, sample size, and end point. Accordingly, the metabolites added 0.028 and 0.017 units of ROC area to the standard model for predicting dysglycemia in RISC and Botnia, respectively. In contrast, fasting insulin, which has been shown to predict T2D in several studies, did not contribute to predictivity above the joint weight of α-HB and L-GPC in either study. Taken together, these results qualify these metabolites as disease biomarkers through their relation to underlying pathophysiological mechanisms. However, their actual value in routine clinical use requires additional studies, particularly intervention studies.
Because in RISC and Botnia an independent, inverse relationship also existed between α-HB levels and indices of β-cell function, we tested the direct impact of these metabolites on insulin secretion. In β-cell in vitro studies, α-HB dose-dependently inhibited overall glucose- and arginine-mediated insulin release. Although this effect must be confirmed in human islets and its mechanisms remain to be investigated, α-HB may mark IR and also β-cell function under diverse circumstances. This use would have utility for clinicians who have available surrogate indices of IR (e.g., homeostasis model assessment-IR or Matsuda index) but must choose among clinical tests of β-cell function (e.g., hyperglycemic clamp, acute insulin response to intravenous glucose, insulinogenic index) that are cumbersome and relatively incongruous with one another. In this regard, it is notable that L-GPC appeared to exert the opposite effect to α-HB on in vitro insulin release, thereby adding to the evidence that certain lipid-signaling molecules (including lysophospholipids) can stimulate glucose-dependent insulin release through lysophospholipid receptors such as G-protein–coupled receptor 119, which localized to pancreatic β-cells. Interestingly, a similar lipid-signaling pathway has been described for G-protein–coupled receptor 119, with its lipid agonist activation playing a role in glucagon-like peptide-1 release in intestinal l-cells. Moreover, lysophospholipids have been implicated in other signaling functions related to metabolism, including glucose uptake by adipocytes and myotubes. Declining L-GPC levels across IR, IGR, and T2D (Table 1 and Table 2) are likely a reflection of an important role for this metabolite in glucose metabolism.
Previous evidence has linked raised concentrations of selected amino acids with obesity and T2D (reviewed in). Newgard et al. described a metabolic signature related to BCAAs in obese humans. Fiehn et al. reported increased concentrations of leucine and valine—in addition to lipid substrates—in a small group of obese T2D women. Lower glycine concentrations have been reported in IR offspring of T2D patients, consistent with our observation of lower glycine associated with IR. However, compared with such reported analytes, α-HB and L-GPC were more sensitive markers of IR, e.g., able to discriminate insulin sensitive from IR individuals in both NGT and IGR ranges.
The pattern of metabolite changes we observed in the Botnia subgroup (Supplementar1y Table 1) can be organized into a coherent pathway (Supplementary Fig. 3) by assuming IR as a primary determinant of these changes. In this scenario, adipose tissue IR leads to elevated FFA concentrations, which feed into the TCA cycle and are oxidized at an increased rate, thereby producing an excess of reducing equivalents (NADH). Raised circulating FFA also reconstitutes phospholipids from circulating lipids such as L-GPC. Such overload of the TCA cycle leads to accumulation of amino acids such as glutamate and alanine, as well as α-ketobutyrate, substrate precursor of α-HB via propionyl-CoA. Oxidative stress and IR raise demand for glutathione synthesis, of which α-ketobutyrate and α-HB are by-products, and depletes glutathione constituents like glycine, whose levels are decreased in association with IR and T2D progression (Supplementary Table 1). Furthermore, by reducing amino acid transport and clearance, IR also raises BCAAs, which also feed into the TCA cycle, directly (leucine) or via propionyl-CoA. Thus, increased α-HB and decreased L-GPC levels serve as readouts of metabolic overload (elevated NADH-to-NAD ratio) and reduced glucose metabolism in both IR and the earliest phases of dysglycemia (Fig. 3). Finally, the abnormal α-HB and L-GPC levels and their biological activity on in vitro glucose-stimulated insulin release may translate into a burden on in vivo β-cell function.
