The role of hepatic lipids in hepatic insulin resistance and type 2 diabetes

Non-alcoholic fatty liver disease and its downstream sequelae, hepatic insulin resistance and type 2 diabetes, are rapidly growing epidemics, which lead to increased morbidity and mortality rates, and soaring health-care costs. Developing interventions requires a comprehensive understanding of the mechanisms by which excess hepatic lipid develops and causes hepatic insulin resistance and type 2 diabetes. Proposed mechanisms implicate various lipid species, inflammatory signalling and other cellular modifications. Studies in mice and humans have elucidated a key role for hepatic diacylglycerol activation of protein kinase Cε in triggering hepatic insulin resistance. Therapeutic approaches based on this mechanism could alleviate the related epidemics of non-alcoholic fatty liver disease and type 2 diabetes

Assessment of Hepatic Mitochondrial Oxidation and Pyruvate Cycling in NAFLD by (13)C Magnetic Resonance Spectroscopy

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease and there is great interest in understanding the potential role of alterations in mitochondrial metabolism in its pathogenesis. To address this question we assessed rates of hepatic mitochondrial oxidation in subjects with and without NAFLD by monitoring the rate of (13)C labeling in hepatic [5-(13)C]glutamate and [1-(13)C]glutamate by (13)C MRS during an infusion of [1-(13)C]acetate. We found that rates of hepatic mitochondrial oxidation were similar between NAFLD and Control subjects. We also assessed rates of hepatic pyruvate cycling during an infusion of [3-(13)C]lactate by monitoring the (13)C label in hepatic [2-(13)C]alanine and [2-(13)C]glutamate and found that this flux also was similar between groups and more than 10-fold lower than previously reported. Contrary to previous studies we show that hepatic mitochondrial oxidation and pyruvate cycling are not altered in NAFLD and do not account for the hepatic fat accumulation

Propionate Increases Hepatic Pyruvate Cycling and Anaplerosis and Alters Mitochondrial Metabolism

In mammals, pyruvate kinase (PK) plays a key role in regulating the balance between glycolysis and gluconeogenesis; however, in vivo regulation of PK flux by gluconeogenic hormones and substrates is poorly understood. To this end, we developed a novel NMR-liquid chromatography/tandem-mass spectrometry (LC-MS/MS) method to directly assess pyruvate cycling relative to mitochondrial pyruvate metabolism (V(Pyr-Cyc)/V(Mito)) in vivo using [3-(13)C]lactate as a tracer. Using this approach, V(Pyr-Cyc)/V(Mito) was only 6% in overnight fasted rats. In contrast, when propionate was infused simultaneously at doses previously used as a tracer, it increased V(Pyr-Cyc)/V(Mito) by 20–30-fold, increased hepatic TCA metabolite concentrations 2–3-fold, and increased endogenous glucose production rates by 20–100%. The physiologic stimuli, glucagon and epinephrine, both increased hepatic glucose production, but only glucagon suppressed V(Pyr-Cyc)/V(Mito). These data show that under fasting conditions, when hepatic gluconeogenesis is stimulated, pyruvate recycling is relatively low in liver compared with V(Mito) flux and that liver metabolism, in particular pyruvate cycling, is sensitive to propionate making it an unsuitable tracer to assess hepatic glycolytic, gluconeogenic, and mitochondrial metabolism in vivo

Hypophosphatemia promotes lower rates of muscle ATP synthesis

Hypophosphatemia can lead to muscle weakness and respiratory and heart failure, but the mechanism is unknown. To address this question, we noninvasively assessed rates of muscle ATP synthesis in hypophosphatemic mice by using in vivo saturation transfer [(31)P]-magnetic resonance spectroscopy. By using this approach, we found that basal and insulin-stimulated rates of muscle ATP synthetic flux (V(ATP)) and plasma inorganic phosphate (P(i)) were reduced by 50% in mice with diet-induced hypophosphatemia as well as in sodium-dependent P(i) transporter solute carrier family 34, member 1 (NaPi2a)-knockout (NaPi2a(−/−)) mice compared with their wild-type littermate controls. Rates of V(ATP) normalized in both hypophosphatemic groups after restoring plasma P(i) concentrations. Furthermore, V(ATP) was directly related to cellular and mitochondrial P(i) uptake in L6 and RC13 rodent myocytes and isolated muscle mitochondria. Similar findings were observed in a patient with chronic hypophosphatemia as a result of a mutation in SLC34A3 who had a 50% reduction in both serum P(i) content and muscle V(ATP). After oral P(i) repletion and normalization of serum P(i) levels, muscle V(ATP) completely normalized in the patient. Taken together, these data support the hypothesis that decreased muscle ATP synthesis, in part, may be caused by low blood P(i) concentrations, which may explain some aspects of muscle weakness observed in patients with hypophosphatemia.—Pesta, D. H., Tsirigotis, D. N., Befroy, D. E., Caballero, D., Jurczak, M. J., Rahimi, Y., Cline, G. W., Dufour, S., Birkenfeld, A. L., Rothman, D. L., Carpenter, T. O., Insogna, K., Petersen, K. F., Bergwitz, C., Shulman, G. I. Hypophosphatemia promotes lower rates of muscle ATP synthesis