Metabolic effects of glutamine on the heart: Anaplerosis versus the hexosamine biosynthetic pathway

Abstract

Rationale and Goal Glutamine, the most abundant amino acid in plasma, has attracted considerable interest for its cardioprotective properties. The primary effect of glutamine in the heart is commonly believed to be mediated via its anaplerotic metabolism to citric acid cycle (CAC) intermediates; however, there is little direct evidence to support this concept. Another potential candidate is the hexosamine biosynthetic pathway (HBP), which has recently been shown to modulate cardiomyocyte function and metabolism. Therefore, the goal of this study was to evaluate the contribution of anaplerosis and the HBP to the acute metabolic effects of glutamine in the heart. Methods Normoxic ex vivo working rat hearts were perfused with 13C-labeled substrates to assess relevant metabolic fluxes perfused with a physiological mixture of carbohydrates and a fatty acid (control) or under conditions of restricted pyruvate anaplerosis. Results Addition of a physiological concentration of glutamine (0.5 mM) had no effect on contractile function of hearts perfused under the control condition, but improved that of hearts perfused under restricted pyruvate anaplerosis. Changes in CAC intermediate concentrations as well as 13C-enrichment from [U-13C]glutamine did not support a major role of glutamine anaplerosis under any conditions. Under the control condition, however, glutamine significantly increased the contribution of exogenous oleate to β-oxidation, 1.6-fold, and triglyceride formation, 2.8-fold. Glutamine had no effect on malonyl-CoA or AMPK kinase activity levels; however, it resulted in a higher plasma membrane level of the fatty acid transporter CD36. These metabolic effects of glutamine were reversed by azaserine, which inhibits glucose entry into the HPB. Conclusion Our results reveal a metabolic role of physiological concentration of glutamine in the healthy working heart beyond anaplerosis. This role appears to involve the HBP and regulation of fatty acid entry and metabolism via CD36.

Cardiac glutaminolysis: a maladaptive cancer metabolism pathway in the right ventricle in pulmonary hypertension

“…We show the pathway is only induced in RVH. Thus, consistent with Lauzier’s findings[30] we conclude there is little glutaminolysis under control condition (Fig 1). However in RVH, the increased glutaminolysis impairs cardiac output and shortens treadmill distance.…”

Assessing Cardiac Metabolism

“…Both techniques have been used to trace processes and fluxes in intermediary metabolism of the heart, with emphasis on the contributions of fuels to energy substrate metabolism and anaplerosis. Fluxes are assessed by use of (1) the labeling of acetyl-CoA or the acetyl moiety of citrate, assayed by MS (142–145) versus (2) the labeling of C-4,5 of glutamate, a proxy of acetyl-CoA labeling, assayed by MRS. 146 In addition, MS assesses other complex processes in the intact heart, such as (1) simultaneous lactate and pyruvate release (from glycolysis), lactate/pyruvate uptake (for oxidation in the Krebs cycle), and pyruvate partitioning between oxidative decarboxylation and carboxylation; (2) the reversibility of the isocitrate dehydrogenase reaction (recently renamed as reductive carboxylation of glutamate); (3) fumarate metabolism though the reductive pathway; (4) the partitioning of LCFAs between mitochondrial, peroxisomal β-oxidation, and TG synthesis (for reviews, see references 142, 144, and 145); (5) the contribution of glutamine to anaplerosis 147 ; and (6) the source of acetyl-CoA oxidized in the Krebs cycle 148,149 or used for FA elongation. 150 These labeling studies using MS have been conducted in isolated rat and mouse heart study models, as well as in pig hearts, 142–146 but they may also be applied to isolated cardiomyocytes (although rates of energy substrate metabolism are lower than in the intact beating heart).…”

Enzymatic passaging of human embryonic stem cells alters central carbon metabolism and glycan abundance

“…Glycans are also the key components that comprise the glycocalyx, which surrounds the cell membrane of some cells [12]. Through the cleavage of extracellular proteins and their associated glycans, enzymatic digestion could affect the pheno‐type of hESCs by impacting their metabolic demands or ability to respond to extracellular signaling cues [13–17].…”