The Role of Hepatocyte D-Β-Hydroxybutyrate Dehydrogenase In Ketone Body Metabolism And Liver Health

Abstract

Throughout the last decade, interest has intensified in intermittent fasting, ketogenic diets, and exogenous ketone therapies as prospective health-promoting, therapeutic, and performance-enhancing agents. However, the regulatory roles of ketogenesis and ketone metabolism on liver homeostasis remain unclear. This thesis seeks to develop a better understanding of the metabolic consequences of hepatic ketone body metabolism by focusing on the redox-dependent interconversion of acetoacetate (AcAc) and D-β-hydroxybutyrate (D-βOHB). Using targeted and isotope tracing high-resolution liquid chromatography-mass spectrometry, dual stable isotope tracer nuclear magnetic resonance spectroscopy-based metabolic flux modeling, dietary-induced mouse models of nonalcoholic fatty liver disease (NAFLD), and complementary physiological approaches in novel cell type-specific knockout mice, the roles of hepatocyte D-β-hydroxybutyrate dehydrogenase (BDH1), a mitochondrial enzyme required for NAD+/NADH-dependent oxidation/reduction of ketone bodies, are quantified. Exogenously administered AcAc is reduced to D-βOHB, and increases hepatic NAD+/NADH ratio, reflecting hepatic BDH1 activity. Livers of hepatocyte-specific BDH1 deficient mice produced no D-βOHB, but due to extrahepatic BDH1, these mice nonetheless remained capable of AcAc/D-βOHB interconversion. Compared to littermate controls, hepatocyte specific BDH1 deficient mice, maintained on either a chow or NAFLD-inducing western-style diet, showed diminished liver tricarboxylic acid (TCA) cycle flux and impaired gluconeogenesis but normal overall hepatic energy charge. Furthermore, the livers of knockout mice maintained on a 60% high fat diet were less fibrotic, with reduced markers of oxidative stress, than littermate controls.

Collectively, this thesis illustrates how ketone bodies and BDH1 activity influence liver homeostasis and health. While liver BDH1 is not required for whole body equilibration of AcAc and D-βOHB, loss of the ability to interconvert these ketone bodies in hepatocytes results in impaired TCA cycle flux and glucose production, with a beneficial effect on liver fibrosis. Therefore, BDH1 is a significant contributor to hepatic mitochondrial redox, liver physiology, and organism-wide ketone body homeostasis, and augmentation of hepatic BDH1 activity could prove beneficial in the treatment of NAFLD.