Consequences of Altered Short-Chain Carbon Metabolism in Heart Failure

Cardiovascular disease is currently the foremost cause of death within the United States. Heart failure (HF) is a syndrome defined by the inability of the heart to adequately execute requisite pump function in order to deliver nutrients and oxygen to peripheral tissues, irrespective of etiology. One of the most common causes of HF is chronic pressure overload due to hypertension. Ischemic heart disease is also a common driver of HF, often in conjunction with hypertension. Pressure overload initially causes compensatory metabolic changes. Structural changes follow shortly thereafter typically resulting in left ventricular hypertrophy. Eventually, the heart loses the ability to compensate for the aberrant hemodynamic load and begins failing. The failing heart is unable to supply adequate adenosine triphosphate (ATP) for contractile function as evidenced by falling phosphocreatine (PCr) levels. This energy deficit occurs concurrently with a metabolic re-programming that results in a fuel utilization pattern resembling the fetal heart. Notably, enzymes involved in catabolism of fatty acids, the chief fuel substrate for ATP generation in the normal adult heart, are downregulated in the failing heart. However, the extent to which alternative fuels compensate for decreased fatty acid oxidation (FAO) is not well-known. Furthermore, consequences of the fuel substrate switches that occur in heart failure are not well established. In this work, we discover a new paradigm for alternate fuel utilization in the failing heart and define consequences of altered fuel metabolism in HF. We discovered a post-translational modification resultant from an accumulation of acetyl groups (C2) present in a mouse model of early-stage HF and human HF. Mitochondrial proteins were found to be hyperacetylated in the failing heart, and at least some of these alterations result in diminished electron-transport chain (ETC) capacity as shown by mutagenesis studies on succinate dehydrogenase A (SDHA). We also found an accumulation of C4-OH carnitine, a by-product of ketone oxidation in HF. This metabolite aggregation occurred alongside an increase in b-hydroxybutyrate dehydrogenase 1 (BDH1) transcript and protein levels. This signature suggested that the failing heart shifted to ketone bodies as a fuel. Subsequent experiments confirmed increased capacity for myocardial ketone oxidation in compensated cardiac hypertrophy and in HF. The consequences of increased ketone oxidation were then assessed using a cardiac-specific BDH1 knockout (BDH1 KO) mouse. Despite not having any apparent defect at baseline, we found BDH1 KO mouse hearts are completely unable to oxidize 3-hydroxybutyrate. The deficit for ketone oxidation capacity became consequential upon subjugation to transverse aortic constriction with a small apical myocardial infarction (TAC/MI). The BDH1 KO mice exhibit altered pathological cardiac remodeling compared to wild-type controls. These latter data suggest the increased reliance on ketone oxidation in HF, mediated by BDH1, is an adaptive response. Together the results of these studies provide important information regarding the consequences of altered fuel metabolism in HF. Recent reports of reduced HF mortality and elevated circulating ketone levels in patients prescribed Empagliflozin make cardiac ketone metabolism research in this dissertation particularly apropos