Cholesterol and fatty acid homeostasis is maintained by a complex network of regulatory mechanisms that control the biosynthesis and deposition of lipids over diverse physiological conditions. However, these processes can become dysregulated and uncoupled from energy metabolism by metabolic stress such as a hyper-caloric diet and physical inactivity; eventually manifesting as risk factors associated with atherosclerotic cardiovascular disease (ASCVD), Type 2 diabetes (T2D), and/or non-alcoholic fatty liver disease (NAFLD). AMP-activated protein kinase (AMPK) is a sensor of cellular energy status that promotes metabolic homeostasis by mediating effects on multiple cellular processes including cholesterol and fatty acid synthesis biosynthesis. However, the mechanisms linking AMPK to lipid metabolism under normal and pathological conditions, remain undefined. In these studies, we identify a novel nutrient sensing mechanism whereby the coenzyme A (CoA) activated esters of long-chain fatty acids (LCFA-CoA) directly activate AMPK via specific interactions within the β1-regulatory subunit involving a Ser108 residue previously shown only with synthetic activators. We demonstrate the physiological relevance for this mechanism in an acute setting by showing that fatty acid oxidation was attenuated in mice harboring an AMPKβ1-S108A knock-in mutation compared to WT mice. We then demonstrated that β1-selctive AMPK activation is mimicked by the CoA conjugated form of bempedoic acid, a synthetic small molecule lipid synthesis inhibitor in clinical development for lowering elevated levels of low-density lipoprotein cholesterol (LDL-C). The importance of this mechanism was determined by assessing multiple disease outcomes in Ampkβ1-/-/Apoe-/- double knockout (DKO) mice fed a high fat-high cholesterol (HFHC) diet ± bempedoic acid. In these studies, bempedoic acid treatment reduced plasma LDL-C and atherosclerosis in both Apoe-/- and DKO mice, while no differences in disease outcomes was detected between the two genotypes in response to HFHC feeding. Further mechanistic investigations in rodent and primary human hepatocytes, revealed that the CoA conjugate of bempedoic acid suppressed lipid synthesis via competitive inhibition of ATP-citrate lyase (ACL), which promoted LDL receptor upregulation and associated reductions in LDL-C. We then integrate these findings with published literature in a written synthesis aimed to evaluate the role of ACL in metabolism, and its potential utility as a therapeutic target to treat ASCVD and metabolic disorders in humans. Although several questions remain regarding the metabolic role of AMPK activation by LCFA-CoAs, these studies have expanded our understanding of how cells acutely integrate lipid and energy signals to maintain lipid homeostasis, and identified ACL as a promising strategy to treat hypercholesterolemia, ASCVD, and associated metabolic disorders. / Thesis / Doctor of Philosophy (PhD) / The dysregulation of cholesterol and triglyceride metabolism can manifest as risk factors for life-threating diseases such as atherosclerotic cardiovascular diseases (ASCVD), Type-2 diabetes (T2D), and nonalcoholic fatty liver disease (NAFLD). However, the underlying mechanisms controlling lipid homeoastasis in health and disease are not completely understood. ATP-citrate lyase (ACL) and AMP-activated protein kinase (AMPK) are emerging as key nodes in metabolism that integrate lipid metabolism with signals of nutrient availability and cellular energy status, respectively. These strategic positions in metabolism suggest that both these enzymes could play an important role in the underlying pathophysiology of lipid-related diseases, and are therefore, prime candidates for therapeutic intervention. In these studies, we expand our understanding of the role of AMPK in metabolism beyond energy sensing by identifying specific lipid metabolites as direct allosteric activators of kinase activity. We also evaluate the therapeutic utility of targeting both AMPK and ACL in novel models of hypercholesterolemia and metabolic disease, and demonstrate that ACL inhibition offers a promising strategy to address multiple unmet medical needs.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/23456 |
| Date | 12 1900 |
| Creators | Pinkosky, Stephen |
| Contributors | Steinberg, Gregory, Medical Sciences |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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