Abnormal mitochondrial function is proposed to contribute to the hepatic insulin resistance and steatosis associated with obesity and type 2 diabetes. Recently, diet-induced obesity in mice was found to be associated with reduced rates of hepatic mitophagy, a mitochondrial quality control pathway that regulates selective removal of damaged mitochondria from the cell. Mitophagy signaling and targeting of damaged mitochondria relies on generation of a phosphorylated ubiquitin signaling motif on the outer mitochondrial membrane that is produced through the coordinated activation of the ubiquitin E3 ligase PARKIN and the serine/threonine kinase PINK1. We hypothesize that obesity-associated liver metabolic disease arises, in part, due to a loss of mitochondrial homeostasis resulting from impaired PINK1-PARKIN-mediated signaling and subsequently reduced mitophagy. We are currently testing this hypothesis using a novel mouse line with liver-specific deletion of PARKIN at baseline and during diet-induced obesity using a breadth of in vivo and ex vivo approaches designed to evaluate hepatic mitochondrial oxidative metabolism, reactive oxygen species production, hepatic insulin sensitivity, rates of mitophagic flux, and changes in hepatic insulin signaling and stress signaling pathways. These studies will define how genetic inhibition of PARKIN-mediated mitophagy in the liver impacts mitochondrial biology and the pathogenesis of obesity-associated liver metabolic disease. Additionally, we are evaluating key aspects of PINK1-PARKIN-mediated mitophagy signaling in lean and obese mice using a combination of approaches that includes quantitative mass spectrometry-based proteomics, confocal microscopy, and mutagenesis followed by in vivo and in vitro functional characterization using primary and stable cell lines and cell free models. These studies will define how nutritional stress in the obese liver contributes to post-translational modifications of mitochondrial proteins that then interfere with recently defined phosphorylation and ubiquitination signaling events that activate the mitophagy pathway. They will provide further insight into how PINK1 and PARKIN dynamics are affected in the obese liver and may account for the reduced mitophagy observed in liver from diet-induced obese mice. Overall, these studies will explore a novel pathway by which nutritional stress in the liver may contribute to the pathogenesis of hepatic steatosis, insulin resistance, and mitochondrial dysfunction. This new knowledge will ultimately improve our understanding of the biochemical and molecular events that contribute to these diseases, which may lend themselves to the development of effective lifestyle interventions or targeted therapeutics to treat obesity-associated liver disease.
Research Focus 2:
The number of overweight and obese individuals in the United States has increased dramatically over the last two decades. With this rise, the prevalence of a number of obesity-associated metabolic diseases, such as type 2 diabetes (T2D) and non-alcoholic fatty liver disease (NAFLD), has similarly sky rocketed. NAFLD encompasses a range of hepatocellular alterations, including simple steatosis and steatosis with inflammation (NASH), which can lead to fibrosis, cirrhosis, and hepatocellular carcinoma. Hepatic insulin resistance, changes in mitochondrial metabolism, inflammatory signaling, extracellular vesicle signaling, and/or altered intestinal microbial diversity are all proposed to contribute the pathogenesis of NAFLD and NASH. However, the mechanistic basis for the pathogenesis of NAFLD remains incompletely understood and there are currently no approved treatments for NAFLD. 5′-AMP-activated protein kinase (AMPK) is a cellular energy sensor that coordinates metabolic pathways to balance energy demand with production, and growing evidence suggests that loss of AMPK activity and its downstream signaling pathways contributes to NAFLD. Current work in the Jurczak Lab will test the hypothesis that increased phosphorylation-dependent, ubiquitin/proteasomal-mediated degradation of AMPK contributes to the pathogenesis of obesity-associated NAFLD. We are also working to establish the identity of the F-box protein that mediates the specific recognition of phosphorylated AMPK by the Skp-Cullin1-Rbx1 E3 ligase complex. Finally, current work in the lab will address how modulating the expression or activity of the putative F-box protein during the pathogenesis of NAFLD or after development of established NASH impacts major outcomes associated with these diseases, and whether any potential changes require AMPK. These goals are being pursued through a combination of approaches including cell biology, amino acid and peptide chemistry, animal physiology, metabolic isotope tracing, and homology modeling- and molecular docking-based drug design and optimization.
Research Focus 3:
While numerous therapeutic advances have reduced acute mortality from cardiovascular events, heart disease remains the leading cause of death in the United States. One of the largest recent breakthroughs in heart failure treatment has been the development of SGLT2 inhibitors, which have been shown in several major clinical trials to reduce all-cause cardiovascular events in heart failure patients. While SGLT2 inhibitors show clear protective outcomes, we still do not fully understand their mechanism of action in preventing cardiac disease. This gap in our knowledge prevents us from developing new treatments that may target and enhance this cardioprotective pathway. Current work in our lab seeks to investigate and characterize a novel molecular pathway that drives the cardioprotective functions of SGLT2 inhibitors. During an unbiased metabolomic screen of the hearts from obese mice treated with the SGLT2 inhibitor empagliflozin, we identified a dramatic reduction in the abundance of a sugar phosphate metabolite involved in cardiac immune cell activity. Based on our preliminary data and recent published studies, we propose that SGLT2 inhibitors reduce cardiovascular disease events by reducing the maladaptive activation of immune cells in the heart after injury or stress. We are now determining whether empagliflozin regulates the attachment and cardiac infiltration of immune cells in response to myocardial infarction in mice via reduced immune cell glycosylation. We are also determining the mechanism by which empagliflozin regulates the in vivo abundance of the sugar phosphate metabolite, as well as whether targeting the sugar phosphate-dependent post-translational modification pathways regulated by empagliflozin directly can protect from cardiac injury. We believe that our work defines a new molecular target for future translational investigation.
