Metabolism of the Heart in Failure

Cardiac lipid metabolism, mitochondrial function, and heart failure

A fine balance between uptake, storage, and the use of high energy fuels, like lipids, is crucial in the homeostasis of different metabolic tissues. Nowhere is this balance more important and more precarious than in the heart. This highly energy-demanding muscle normally oxidizes almost all the available substrates to generate energy, with fatty acids being the preferred source under physiological conditions. In patients with cardiomyopathies and heart failure, changes in the main energetic substrate are observed; these hearts often prefer to utilize glucose rather than oxidizing fatty acids. An imbalance between uptake and oxidation of fatty acid can result in cellular lipid accumulation and cytotoxicity. In this review, we will focus on the sources and uptake pathways used to direct fatty acids to cardiomyocytes. We will then discuss the intracellular machinery used to either store or oxidize these lipids and explain how disruptions in homeostasis can lead to mitochondrial dysfunction and heart failure. Moreover, we will also discuss the role of cholesterol accumulation in cardiomyocytes. Our discussion will attempt to weave in vitro experiments and in vivo data from mice and humans and use several human diseases to illustrate metabolism gone haywire as a cause of or accomplice to cardiac dysfunction.

Lipid Droplet-Associated Proteins in Cardiomyopathy

BACKGROUND

The heart requires a high rate of fatty-acid oxidation (FAO) to meet its energy needs. Neutral lipids are the main source of energy for the heart and are stored in lipid droplets (LDs), which are cytosolic organelles that primarily serve to store neutral lipids and regulate cellular lipid metabolism. LD-associated proteins (LDAPs) are proteins either located on the surface of the LDs or reside in the cytosol and contribute to lipid metabolism. Therefore, abnormal cardiac lipid accumulation or FAO can alter the redox state of the heart, resulting in cardiomyopathy, a group of diseases that negatively affect the myocardial function, thereby leading to heart failure and even cardiac death.

SUMMARY

LDs, along with LDAPs, are pivotal for modulating heart lipid homeostasis. The proper cardiac development and the maintenance of its normal function depend largely on lipid homeostasis regulated by LDs and LDAPs. Overexpression or deletion of specific LDAPs can trigger myocardial dysfunction and may contribute to the development of cardiomyopathy. Extensive connections and interactions may also exist between LDAPs. Key Message: In this review, the various mechanisms involved in LDAP-mediated regulation of lipid metabolism, the association between cardiac development and lipid metabolism, as well as the role of LDAPs in cardiomyopathy progression are discussed.

Racial and socioeconomic disparity associates with differences in cardiac DNA methylation among men with end-stage heart failure

Heart failure (HF) is a multifactorial syndrome that remains a leading cause of worldwide morbidity. Despite its high prevalence, only half of patients with HF respond to guideline-directed medical management, prompting therapeutic efforts to confront the molecular underpinnings of its heterogeneity. In the current study, we examined epigenetics as a yet unexplored source of heterogeneity among patients with end-stage HF. Specifically, a multicohort-based study was designed to quantify cardiac genome-wide cytosine-p-guanine (CpG) methylation of cardiac biopsies from male patients undergoing left ventricular assist device (LVAD) implantation. In both pilot (n = 11) and testing (n = 31) cohorts, unsupervised multidimensional scaling of genome-wide myocardial DNA methylation exhibited a bimodal distribution of CpG methylation found largely to occur in the promoter regions of metabolic genes. Among the available patient attributes, only categorical self-identified patient race could delineate this methylation signature, with African American (AA) and Caucasian American (CA) samples clustering separately. Because race is a social construct, and thus a poor proxy of human physiology, extensive review of medical records was conducted, but ultimately failed to identify covariates of race at the time of LVAD surgery. By contrast, retrospective analysis exposed a higher all-cause mortality among AA (56.3%) relative to CA (16.7%) patients at 2 yr following LVAD placement (P = 0.03). Geocoding-based approximation of patient demographics uncovered disparities in income levels among AA relative to CA patients. Although additional studies are needed, the current analysis implicates cardiac DNA methylation as a previously unrecognized indicator of socioeconomic disparity in human heart failure outcomes.

NEW & NOTEWORTHY A bimodal signature of cardiac DNA methylation in heart failure corresponds with racial differences in all-cause mortality following mechanical circulatory support. Racial differences in promoter methylation disproportionately affect metabolic signaling pathways. Socioeconomic factors are associated with racial differences in the cardiac methylome among men with end-stage heart failure.

Listen to this article’s corresponding podcast at https://ajpheart.podbean.com/e/racial-socioeconomic-determinants-of-the-cardiac-epigenome/.

Lipid Use and Misuse by the Heart

The heart utilizes large amounts of fatty acids as energy providing substrates. The physiological balance of lipid uptake and oxidation prevents accumulation of excess lipids. Several processes that affect cardiac function, including ischemia, obesity, diabetes mellitus, sepsis, and most forms of heart failure lead to altered fatty acid oxidation and often also to the accumulation of lipids. There is now mounting evidence associating certain species of these lipids with cardiac lipotoxicity and subsequent myocardial dysfunction. Experimental and clinical data are discussed and paths to reduction of toxic lipids as a means to improve cardiac function are suggested.

Preferential oxidation of triacylglyceride-derived fatty acids in heart is augmented by the nuclear receptor PPARalpha

RATIONALE

Long chain fatty acids (LCFAs) are the preferred substrate for energy provision in hearts. However, the contribution of endogenous triacylglyceride (TAG) turnover to LCFA oxidation and the overall dependence of mitochondrial oxidation on endogenous lipid is largely unstudied.

OBJECTIVE

We sought to determine the role of TAG turnover in supporting LCFA oxidation and the influence of the lipid-activated nuclear receptor, proliferator-activated receptor (PPAR)alpha, on this balance.

METHODS AND RESULTS

Palmitoyl turnover within TAG and palmitate oxidation rates were quantified in isolated hearts, from normal mice (nontransgenic) and mice with cardiac-specific overexpression of PPARalpha (MHC-PPARalpha). Turnover of palmitoyl units within TAG, and thus palmitoyl-coenzyme A recycling, in nontransgenic (4.5+/-2.3 micromol/min per gram dry weight) was 3.75-fold faster than palmitate oxidation (1.2+/-0.4). This high rate of palmitoyl unit turnover indicates preferential oxidation of palmitoyl units derived from TAG in normal hearts. PPARalpha overexpression augmented TAG turnover 3-fold over nontransgenic hearts, despite similar fractions of acetyl-coenzyme A synthesis from palmitate and oxygen use at the same workload. Palmitoyl turnover within TAG of MHC-PPARalpha hearts (16.2+/-2.9, P<0.05) was 12.5-fold faster than oxidation (1.3+/-0.2). Elevated TAG turnover in MHC-PPARalpha correlated with increased mRNA for enzymes involved in both TAG synthesis, Gpam (glycerol-3-phosphate acyltransferase, mitochondrial), Dgat1 (diacylglycerol acetyltransferase 1), and Agpat3 (1-acylglycerol-3-phospate O-acyltransferase 3), and lipolysis, Pnliprp1 (pancreatic lipase related protein 1).

CONCLUSIONS

The role of endogenous TAG in supporting beta-oxidation in the normal heart is much more dynamic than previously thought, and lipolysis provides the bulk of LCFA for oxidation. Accelerated palmitoyl turnover in TAG, attributable to chronic PPARalpha activation, results in near requisite oxidation of LCFAs from TAG.

Is the failing heart energy depleted?

This article takes three different approaches to the question of whether the failing heart is in an energy-starved state. A brief historical overview introduces the issue and points out problems in both models and methods. Second, current information regarding the energetic state of the failing heart is examined. Finally, the mechanistic and therapeutic implications of a defect in energy production are described.