Glycolysis in heart failure with preserved ejection fraction

This article refers to 'Landscape of glycolytic metabolites and their regulating proteins in myocardium from human heart failure with preserved ejection fraction' by N. Koleini et al., published in this issue on pages xxx. The heart has a very high energy demand that is primarily met by the production of adenosine triphosphate (ATP) from mitochondrial oxidative phosphorylation and glycolysis. In the failing heart, ATP production can be markedly impaired, primarily due to a decrease in mitochondrial oxidative metabolism.1 In heart failure with reduced ejection fraction (HFrEF), an increase in glycolytic ATP production partly compensates for the decrease in mitochondrial ATP production, although the heart can still face an energy deficit that contributes to the severity of contractile dysfunction.1 What happens to cardiac glycolytic rates in heart failure with preserved ejection fraction (HFpEF) has not been clearly defined. Part of the reason for this is that it has also not been clearly demonstrated what changes in mitochondrial oxidative metabolism occur in HFpEF. In HFrEF, mitochondrial ATP production decreases due to a decrease in the mitochondrial oxidation of glucose and/or fatty acids.1 However, in HFpEF while mitochondrial glucose oxidation is impaired,2, 3 there are conflicting studies as to whether mitochondrial fatty acid oxidation is increased2, 3 or decreased.4-6 Part of this confusion may be related to whether obesity or diabetes is present in the HFpEF patients, since both obesity and diabetes are associated with increased cardiac fatty acid oxidation rates.1, 7 Regardless, whether fatty acid oxidation is increased or decreased in HFpEF will have a big impact on what happens to glycolytic rates in the heart. In this issue of the Journal, Koleini et al.8 used a metabolomic approach to assess what changes in glycolysis and ancillary glucose metabolic pathways occur in human HFpEF patients. The Koleini et al.8 study examined what changes in glycolytic intermediates and their associated synthetic and catabolic enzymes occur in human HFpEF myocardium. They show that three major glycolytic intermediates (glucose-6-P, fructose-1,6bP, and 3-phosphoglycerate) were decreased, which was associated with a reduced expression of their respective synthesizing enzymes. They also show that metabolites in other ancillary pathways, such as the pentose phosphate pathway and the hexosamine biosynthetic pathway, that utilize proximal glycolytic intermediates as substrates, were also reduced. Curiously, pyruvate, the final glycolytic product, was actually higher in HFpEF myocardium. This would not necessarily be expected if glycolysis was decreased in HFpEF. However, the authors also observed a decrease in mitochondrial pyruvate carrier protein 1 and reduced mitochondrial metabolites. This suggests that in addition to a decrease in glycolysis, the subsequent mitochondrial oxidation of the pyruvate from glycolysis (glucose oxidation) was more dramatically reduced (Figure 1). These data support experimental studies demonstrating that glucose oxidation is decreased in HFpEF.2, 3 A clear understanding of what cardiac energy metabolic changes occur in human heart failure has been hindered by the inability to directly assess flux through the individual energy metabolic pathways. Experimental animal studies have used radiolabelled energy substrates to make these measurements,1-3 and although a similar approach has been used in humans in the past,9, 10 such studies are no longer practical or ethical. The use of positron labelled energy substrates and 13C-stable isotope studies can be used, but these approaches are complex and do not necessarily provide direct flux data.11 Metabolomics studies in which energy metabolites are assessed either from myocardial biopsies or from arterial-coronary sinus venous blood samples is one approach to studying cardiac-specific metabolic alterations in human heart failure. The study by Koleini et al.8 performed both a metabolomic and proteomic assessment on endomyocardial biopsies from HFpEF patients to uncover what specific alterations occur in the glycolysis pathway, as well as in ancillary pathways utilizing glycolytic intermediates. This study offers valuable insights into metabolic disruptions specific to the glycolytic pathway in HFpEF patients. However, although metabolomics is a valuable tool for identifying complex metabolic alterations, it provides only a static snapshot of metabolite levels at a specific moment and fails to capture the dynamic and fluctuating nature of metabolism over time. As the authors acknowledge, their study is no exception to this limitation. Cardiac metabolomics studies, including the present study, often focus on specific metabolite intermediates rather than a comprehensive analysis of the complete metabolic landscape. It is crucial to view cardiac metabolic pathways as interconnected networks, with consideration of both the upstream and downstream metabolites and pathways. Focusing solely on specific metabolite intermediates in isolation could potentially result in a misunderstanding of broader metabolic changes in heart failure. For example, glycolysis is just one part of the glucose metabolic pathway, and assessing the interconnections between glycolysis and glucose oxidation could offer a more comprehensive view of glucose metabolism. Although Koleini et al.8 examined glycolytic intermediates and pyruvate levels, the underlying connections between glucose uptake, glycolysis, and subsequent glucose oxidation remain somewhat underexplored. Similarly, metabolomics studies on fatty acid metabolism often focus on acylcarnitine levels without investigating final metabolic end products or upstream pathways. It is difficult to conclude that glycolysis is downregulated based on intermediate metabolite levels alone without direct measurements of glycolysis rates. This also applies to measurements of mRNA or protein levels in isolation. For instance, discrepancies between mRNA levels and enzyme activities can occur in failing hearts, as enzyme activities can be modified by post-translational modifications like acetylation. The decreased glycolysis intermediate levels with no changes in lactate alongside increased pyruvate and glycogen also raise questions about the coherence of the findings and the underlying metabolic mechanisms. Although Koleini et al.8 suggest an increased uptake for the elevated pyruvate levels, direct comparisons of the coronary sinus and arterial blood pyruvate levels or metabolic imaging studies are required to confirm this. These results, however, are intriguing for a more in-depth exploration of the mechanisms behind these changes. There are many confusions as to what happens to cardiac energy metabolism in heart failure, and only a few studies to date have investigated the alterations of cardiac metabolic profile in HFpEF, including the present one. In 2021 this same group12 performed myocardial transcriptomic analysis in right ventricular septum samples and identified uniquely upregulated pathways in HFpEF, being the mitochondrial ATP synthesis and oxidative phosphorylation pathways.1 Followed this, in 2023 the authors performed targeted liquid chromatography-mass spectrometry metabolomics to further elucidate the alterations of myocardial energy metabolism in HFpEF.5 The results showed that medium chain and long chain acylcarnitine levels, which are fatty acid metabolites, were reduced in HFpEF myocardium. Hahn et al.5 also observed a reduced gene expression of proteins involved with fatty acid uptake and oxidation. Like the present study, this paper also found significantly higher levels of pyruvate in HFpEF myocardium compared to non-failing controls. In addition, the authors also explored branched-chain amino acid (BCAA) metabolism. While myocardial levels of BCAAs were higher in HFpEF, the downstream BCAA catabolites were reduced, suggesting impaired BCAA catabolism. This cooperates experimental studies suggesting a decrease in BCAA oxidation in the failing heart.13, 14 As labelled flux studies are hard to conduct in humans, results from pre-clinical animal works can help provide insights in altered cardiac energy metabolism in HFpEF. Using isolated working heart perfusion with radiolabelled energy substrates, Sun et al.2, 3 measured rates of myocardial glucose oxidation and fatty acid oxidation in a mouse model of HFpEF. They showed impaired insulin-stimulated glucose oxidation coupled with higher fatty acid oxidation rates.2, 3 A study by Deng et al.15 using a '3-Hit' HFpEF mouse model found a 50% reduction in ketone oxidation with 13C nuclear magnetic resonance spectroscopy analysis. Increasing β-hydroxybutyrate levels via ketone ester treatment rescued HFpEF phenotype in mice. Importantly, ketone ester treatment resulted in marked suppression in fatty acid uptake-related genes, but with only modest changes in the expression of β-oxidation genes. Glucose metabolism involves three distinct steps: glucose uptake, glycolysis, and glucose oxidation/pyruvate metabolism. The study of Koleini et al.8 found that glucose uptake is higher in HFpEF myocardium, but with reduced glycolysis. We have also shown that in HFpEF mice, glucose oxidation is impaired.2, 3 Together, these studies challenge the concept that the heart is switching from fatty acids to glucose in heart failure. It is more likely that glucose metabolism is impaired in HFpEF. In addition, glucose and fatty acids are in a competitive relationship. As such, the impaired myocardial glycolysis in HFpEF myocardium observed from the present study could well indicate the inhibitory action from heightened fatty acid metabolism. Direct flux measurement have yet to be conducted in humans, but pre-clinical HFpEF mouse heart indeed supports this increased fatty acid oxidation by the heart.2, 3 In conclusion, the evidence from the Koleini et al.8 study suggests that glycolysis is impaired in human HFpEF myocardium, offering new insights into the metabolic disturbances associated with this condition. However, the field of cardiac energy metabolism in HFpEF is very broad and remains with many unknowns and contradictions. The study by Koleini et al.8 not only enhances our understanding of the altered metabolic profile in HFpEF, but also highlights the potential to target metabolic pathways as therapeutic strategies, and to tackle the unsolved clinical challenge of HFpEF. G.D.L. is funded by a Canadian Institutes for Health Research Foundation Grant. Q.S. is supported by the Alberta Diabetes Institute (ADI)–Helmholtz Research School for Diabetes (HRD) Graduate Studentship. E.B.K. was supported by a Maternal and Child Health (MatCH) scholarship programme, an Alberta Diabetes Institutes studentship and a Faculty of Medicine and Dentistry Graduate Studentship at the University of Alberta. Conflict of interest: GDL is a shareholder of Metabolic Modulators Research Ltd and has received grant support from Servier, Boehringer Ingelheim, Sanofi, and REMED Biopharmaceuticals. All other authors have nothing to disclose.