Hepatic control of metabolism in health and disease

Abstract

Non-alcoholic fatty liver disease (NAFLD) puts one quarter of the global population at risk for a life-threatening disease. When NAFLD progresses, it can cause liver cell death, fibrosis, and organ failure, and it will soon become the leading cause of liver transplantation. But, many people with NAFLD never see progression and will run a benign clinical course. Therefore, if we could better understand the processes that cause progression, treatment options could be targeted to the right people. NALFD is a deposition of fats within the liver suspected to impede its function. Systemic disruption of the metabolic process often results in metabolic syndrome, a condition characterized by the cooccurrence of metabolic abnormalities. As the liver is the nexus of metabolism, metabolic syndrome manifests within the liver as NAFLD. Herein, we set out to understand disruptions of hepatic function in relation to NAFLD in a three-pronged approach. We start by focusing on the connection between NAFLD and dyslipidemia by examining the mechanism by which the gut derived metabolite, trimethylamine-N-oxide (TMAO), causes metabolic syndrome. We use a combination of inhibitor and genetic knock down of TMAO’s proposed signaling effector and examine the resulting physiologic response, although we find inconsistent results of TMAO activity. We additionally directly study hepatic lysosomes challenged with a diet high in fats and cholesterol that is known to cause advanced NAFLD in animal models. We employ direct immunostaining alongside a genetic tag to rapidly and specifically isolate lysosomes from hepatocytes, and we perform multi-omic profiling to assess their function following a high lipid load. We find an immediate change in lysosomal morphology to become larger and more aggregated when the liver is lipid challenged, and we further document a zonal and spatial regulation of hepatic lysosomal activity. Lastly, we focus attention on a central coordinator of metabolic processes, the mechanistic target of rapamycin complex 1 (mTORC1). We perform both pan-liver and zone-specific knock down to assess resulting liver function, as we attempt to define zone-specific signaling elements. We report a surprising zonal pattern of mTORC1 signaling, and we show that knock down actually increases hepatic lipids on a normal diet. In total, we present a novel model of hepatic function in relation to lipid metabolism and handling.