Curtis C. Hughey

5-Aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) Effect on Glucose Production, but Not Energy Metabolism, Is Independent of Hepatic AMPK in Vivo

Background: AMPK is implicated as the mediator of AICAR action on liver metabolism. Results: AICAR suppresses glucose production independent of AMPK. Regulation of mitochondrial function is AMPK-dependent. Conclusion: Nucleotide monophosphates rely on AMPK to regulate energy metabolism but not to suppress glucose production. Significance: Targeted AMPK activation will not lower glucose production in metabolic diseases but could improve hepatic energetics. Metabolic stress, as well as several antidiabetic agents, increases hepatic nucleotide monophosphate (NMP) levels, activates AMP-activated protein kinase (AMPK), and suppresses glucose production. We tested the necessity of hepatic AMPK for the in vivo effects of an acute elevation in NMP on metabolism. 5-Aminoimidazole-4-carboxamide 1-β-d-ribofuranoside (AICAR; 8 mg·kg−1·min−1)-euglycemic clamps were performed to elicit an increase in NMP in wild type (α1α2lox/lox) and liver-specific AMPK knock-out mice (α1α2lox/lox + Albcre) in the presence of fixed glucose. Glucose kinetics were equivalent in 5-h fasted α1α2lox/lox and α1α2lox/lox + Albcre mice. AMPK was not required for AICAR-mediated suppression of glucose production and increased glucose disappearance. These results demonstrate that AMPK is unnecessary for normal 5-h fasting glucose kinetics and AICAR-mediated inhibition of glucose production. Moreover, plasma fatty acids and triglycerides also decreased independently of hepatic AMPK during AICAR administration. Although the glucoregulatory effects of AICAR were shown to be independent of AMPK, these studies provide in vivo support for the AMPK energy sensor paradigm. AICAR reduced hepatic energy charge by ∼20% in α1α2lox/lox, which was exacerbated by ∼2-fold in α1α2lox/lox + Albcre. This corresponded to a ∼6-fold rise in AMP/ATP in α1α2lox/lox + Albcre. Consistent with the effects on adenine nucleotides, maximal mitochondrial respiration was ∼30% lower in α1α2lox/lox + Albcre than α1α2lox/lox livers. Mitochondrial oxidative phosphorylation efficiency was reduced by 25%. In summary, these results demonstrate that the NMP capacity to inhibit glucose production in vivo is independent of liver AMPK. In contrast, AMPK promotes mitochondrial function and protects against a more precipitous fall in ATP during AICAR administration.

Enhanced cardiac protein glycosylation (O-GlcNAc) of selected mitochondrial proteins in rats artificially selected for low running capacity

O-linked β-N-acetyl glucosamine (O-GlcNAc) is a posttranslational modification consisting of a single N-acetylglucosamine moiety attached by an O-β-glycosidic linkage to serine and threonine residues of both nuclear and cytosolic proteins. Analogous to phosphorylation, the modification is reversible and dynamic, changing in response to stress, nutrients, hormones, and exercise. Aims of this study were to examine differences in O-GlcNAc protein modification in the cardiac tissue of rats artificially selected for low (LCR) or high (HCR) running capacity. Hyperinsulinemic-euglycemic clamps in conscious animals assessed insulin sensitivity while 2-[(14)C] deoxyglucose tracked both whole body and tissue-specific glucose disposal. Immunoblots of cardiac muscle examined global O-GlcNAc modification, enzymes that control its regulation (OGT, OGA), and specific proteins involved in mitochondrial oxidative phosphorylation. LCR rats were insulin resistant disposing of 65% less glucose than HCR. Global tissue O-GlcNAc, OGT, OGA, and citrate synthase were similar between groups. Analysis of cardiac proteins revealed enhanced O-GlcNAcylation of mitochondrial Complex I, Complex IV, VDAC, and SERCA in LCR compared with HCR. These results are the first to establish an increase in specific protein O-GlcNAcylation in LCR animals that may contribute to progressive mitochondrial dysfunction and the pathogenesis of insulin resistance observed in the LCR phenotype.

Mass spectrometry-based microassay of 2H and 13C plasma glucose labeling to quantify liver metabolic fluxes in vivo

Mouse models designed to examine hepatic metabolism are critical to diabetes and obesity research. Thus, a microscale method to quantitatively assess hepatic glucose and intermediary metabolism in conscious, unrestrained mice was developed. [(13)C3]propionate, [(2)H2]water, and [6,6-(2)H2]glucose isotopes were delivered intravenously in short- (9 h) and long-term-fasted (19 h) C57BL/6J mice. GC-MS and mass isotopomer distribution (MID) analysis were performed on three 40-μl arterial plasma glucose samples obtained during the euglycemic isotopic steady state. Model-based regression of hepatic glucose and citric acid cycle (CAC)-related fluxes was performed using a comprehensive isotopomer model to track carbon and hydrogen atom transitions through the network and thereby simulate the MIDs of measured fragment ions. Glucose-6-phosphate production from glycogen diminished, and endogenous glucose production was exclusively gluconeogenic with prolonged fasting. Gluconeogenic flux from phosphoenolpyruvate (PEP) remained stable, whereas that from glycerol modestly increased from short- to long-term fasting. CAC flux [i.e., citrate synthase (VCS)] was reduced with long-term fasting. Interestingly, anaplerosis and cataplerosis increased with fast duration; accordingly, pyruvate carboxylation and the conversion of oxaloacetate to PEP were severalfold higher than VCS in long-term fasted mice. This method utilizes state-of-the-art in vivo methodology and comprehensive isotopomer modeling to quantify hepatic glucose and intermediary fluxes during physiological stress in mice. The small plasma requirements permit serial sampling without stress and the affirmation of steady-state glucose kinetics. Furthermore, the approach can accommodate a broad range of modeling assumptions, isotope tracers, and measurement inputs without the need to introduce ad hoc mathematical approximations.

Respirometric Oxidative Phosphorylation Assessment in Saponin-permeabilized Cardiac Fibers

Investigation of mitochondrial function represents an important parameter of cardiac physiology as mitochondria are involved in energy metabolism, oxidative stress, apoptosis, aging, mitochondrial encephalomyopathies and drug toxicity. Given this, technologies to measure cardiac mitochondrial function are in demand. One technique that employs an integrative approach to measure mitochondrial function is respirometric oxidative phosphorylation (OXPHOS) analysis. The principle of respirometric OXPHOS assessment is centered around measuring oxygen concentration utilizing a Clark electrode. As the permeabilized fiber bundle consumes oxygen, oxygen concentration in the closed chamber declines. Using selected substrate-inhibitor-uncoupler titration protocols, electrons are provided to specific sites of the electron transport chain, allowing evaluation of mitochondrial function. Prior to respirometric analysis of mitochondrial function, mechanical and chemical preparatory techniques are utilized to permeabilize the sarcolemma of muscle fibers. Chemical permeabilization employs saponin to selectively perforate the cell membrane while maintaining cellular architecture. This paper thoroughly describes the steps involved in preparing saponin-skinned cardiac fibers for oxygen consumption measurements to evaluate mitochondrial OXPHOS. Additionally, troubleshooting advice as well as specific substrates, inhibitors and uncouplers that may be used to determine mitochondria function at specific sites of the electron transport chain are provided. Importantly, the described protocol may be easily applied to cardiac and skeletal tissue of various animal models and human samples.

Mesenchymal stem cell transplantation for the infarcted heart: a role in minimizing abnormalities in cardiac-specific energy metabolism.

Intense interest has been focused on cell-based therapy for the infarcted heart given that stem cells have exhibited the ability to reduce infarct size and mitigate cardiac dysfunction. Despite this, it is unknown whether mesenchymal stem cell (MSC) therapy can prevent metabolic remodeling following a myocardial infarction (MI). This study examines the ability of MSCs to rescue the infarcted heart from perturbed substrate uptake in vivo. C57BL/6 mice underwent chronic ligation of the left anterior descending coronary artery to induce a MI. Echocardiography was performed on conscious mice at baseline as well as 7 and 23 days post-MI. Twenty-eight days following the ligation procedure, hyperinsulinemic euglycemic clamps assessed in vivo insulin sensitivity. Isotopic tracer administration evaluated whole body, peripheral tissue, and cardiac-specific glucose and fatty acid utilization. To gain insight into the mechanisms by which MSCs modulate metabolism, mitochondrial function was assessed by high-resolution respirometry using permeabilized cardiac fibers. Data show that MSC transplantation preserves insulin-stimulated fatty acid uptake in the peri-infarct region (4.25 ± 0.64 vs. 2.57 ± 0.34 vs. 3.89 ± 0.54 μmol·100 g(-1)·min(-1), SHAM vs. MI + PBS vs. MI + MSC; P < 0.05) and prevents increases in glucose uptake in the remote left ventricle (3.11 ± 0.43 vs. 3.81 ± 0.79 vs. 6.36 ± 1.08 μmol·100 g(-1)·min(-1), SHAM vs. MI + PBS vs. MI + MSC; P < 0.05). This was associated with an enhanced efficiency of mitochondrial oxidative phosphorylation with a respiratory control ratio of 3.36 ± 0.18 in MSC-treated cardiac fibers vs. 2.57 ± 0.14 in the infarct-only fibers (P < 0.05). In conclusion, MSC therapy exhibits the potential to rescue the heart from metabolic aberrations following a MI. Restoration of metabolic flexibility is important given the metabolic demands of the heart and the role of energetics in the progression to heart failure.

Myostatin inhibits proliferation and insulin-stimulated glucose uptake in mouse liver cells

Although myostatin functions primarily as a negative regulator of skeletal muscle growth and development, accumulating biological and epidemiological evidence indicates an important contributing role in liver disease. In this study, we demonstrate that myostatin suppresses the proliferation of mouse Hepa-1c1c7 murine-derived liver cells (50%; p < 0.001) in part by reducing the expression of the cyclins and cyclin-dependent kinases that elicit G1-S phase transition of the cell cycle (p < 0.001). Furthermore, real-time PCR-based quantification of the long noncoding RNA metastasis associated lung adenocarcinoma transcript 1 (Malat1), recently identified as a myostatin-responsive transcript in skeletal muscle, revealed a significant downregulation (25% and 50%, respectively; p < 0.05) in the livers of myostatin-treated mice and liver cells. The importance of Malat1 in liver cell proliferation was confirmed via arrested liver cell proliferation (p < 0.05) in response to partial Malat1 siRNA-mediated knockdown. Myostatin also significantly blunted insulin-stimulated glucose uptake and Akt phosphorylation in liver cells while increasing the phosphorylation of myristoylated alanine-rich C-kinase substrate (MARCKS), a protein that is essential for cancer cell proliferation and insulin-stimulated glucose transport. Together, these findings reveal a plausible mechanism by which circulating myostatin contributes to the diminished regenerative capacity of the liver and diseases characterized by liver insulin resistance.

Enhanced stem cell engraftment and modulation of hepatic reactive oxygen species production in diet-induced obesity

The impact of dietary‐induced obesity (DIO) on stem cell engraftment and the respective therapeutic potential of stem cell engraftment in DIO have not been reported. The objectives of this study were to examine the impact of DIO on the survival and efficacy of intravenous bone marrow‐derived mesenchymal stem cell (MSC) administration in the conscious C57BL/6 mouse.

Approach to assessing determinants of glucose homeostasis in the conscious mouse

AbstractObesity and type 2 diabetes lessen the quality of life of those afflicted and place considerable burden on the healthcare system. Furthermore, the detrimental impact of these pathologies is expected to persist or even worsen. Diabetes is characterized by impaired insulin action and glucose homeostasis. This has led to a rapid increase in the number of mouse models of metabolic disease being used in the basic sciences to assist in facilitating a greater understanding of the metabolic dysregulation associated with obesity and diabetes, the identification of therapeutic targets, and the discovery of effective treatments. This review briefly describes the most frequently utilized models of metabolic disease. A presentation of standard methods and technologies on the horizon for assessing metabolic phenotypes in mice, with particular emphasis on glucose handling and energy balance, is provided. The article also addresses issues related to study design, selection and execution of metabolic tests of glucose metabolism, the presentation of data, and interpretation of results.

O-GlcNAc modification is associated with insulin sensitivity in the whole blood of healthy young adult males

BackgroundHemoglobin A1c (HbA1c) is the predominant diagnostic tool for diabetes diagnosis and progression. However, it has proven to be insensitive at pre-diabetic threshold values. O-linked-β-N-acetylglucosamine (O-GlcNAc) modification has emerged as a sensitive biomarker. The purpose of this study was to explore the sensitivity of O-GlcNAc expression as a potential marker of early metabolic dysfunction in a young adult population. Healthy, young males (18–35 y) from the A ssessing I nherited M etabolic syndrome M arkers in the Y oung study (AIMMY), were divided into low (LH,0.60) or high (HH,1.61) homeostatic model assessment of insulin resistance (HOMA-IR) cohorts.FindingsThe relationships between a panel of anthropometric, metabolic measures and whole blood global protein O-GlcNAc was examined. O-GlcNAc and O-GlcNAc transferase (OGT) levels were quantified by immunoblotting and compared to anthropometric measures: body mass index (BMI), percentage body fat, aerobic fitness, blood glucose, triglycerides, HDL, insulin, and HbA1c. HOMA-IR cohorts showed no differences in BMI, blood glucose or HbA1c, but differed in percent body fat, plasma triglycerides, and circulating insulin. Greater O-GlcNAc expression was observed in the whole blood of HH compared to LH. Moreover, a positive association between HOMA-IR and O-GlcNAc emerged, while no relationship was found between HbA1c and HOMA-IR. This effect was not related to OGT expression.ConclusionsResults indicate that O-GlcNAc has a greater sensitivity to metabolic status compared to HbA1c in this population. O-GlcNAc has the potential to serve as a screening tool for predicting future metabolic disturbances in a young healthy adult population free of any clinically relevant pathologies.

Diminishing impairments in glucose uptake, mitochondrial content, and ADP-stimulated oxygen flux by mesenchymal stem cell therapy in the infarcted heart.

A constant provision of ATP is of necessity for cardiac contraction. As the heart progresses toward failure following a myocardial infarction (MI), it undergoes metabolic alterations that have the potential to compromise the ability to meet energetic demands. This study evaluated the efficacy of mesenchymal stem cell (MSC) transplantation into the infarcted heart to minimize impairments in the metabolic processes that contribute to energy provision. Seven and twenty-eight days following the MI and MSC transplantation, MSC administration minimized cardiac systolic dysfunction. Hyperinsulinemic-euglycemic clamps, coupled with 2-[(14)C]deoxyglucose administration, were employed to assess systemic insulin sensitivity and tissue-specific, insulin-mediated glucose uptake 36 days following the MI in the conscious, unrestrained, C57BL/6 mouse. The improved systolic performance in MSC-treated mice was associated with a preservation of in vivo insulin-stimulated cardiac glucose uptake. Conserved glucose uptake in the heart was linked to the ability of the MSC treatment to diminish the decline in insulin signaling as assessed by Akt phosphorylation. The MSC treatment also sustained mitochondrial content, ADP-stimulated oxygen flux, and mitochondrial oxidative phosphorylation efficiency in the heart. Maintenance of mitochondrial function and density was accompanied by preserved peroxisome proliferator-activated receptor-γ coactivator-1α, a master regulator of mitochondrial biogenesis. These studies provide insight into mechanisms of action that lead to an enhanced energetic state in the infarcted heart following MSC transplantation that may assist in energy provision and dampen cardiac dysfunction.