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Targeting Metabolic Dysregulation in Metabolic Dysfunction-Associated Steatotic Liver Disease
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- Author / Creator
- Saed, Christina Thabet
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Metabolic dysfunction-associated steatotic liver disease (MASLD) is a condition characterized by the accumulation of fat in the liver of persons who do not consume alcohol, commonly associated with metabolic dysregulation. MASLD is rising globally, particularly in Canada, impacting more than 7 million people. MASLD is a serious condition that can elevate the likelihood of developing heart disease and type 2 diabetes (T2D). No medication has yet received approval to treat this serious illness. Recent research indicates that glucose metabolism is disrupted in MASLD, and the rate-limiting enzyme in glucose metabolism, pyruvate dehydrogenase (PDH, gene Pdha1), is suppressed. Thus, we posit that enhancing PDH activity to boost glucose metabolism in the liver could alleviate the impact of MASLD. Our laboratory has demonstrated that ranolazine, a medication prescribed for heart disease, can effectively treat MASLD in obese mice, which was correlated with elevated PDH activity. We aim to investigate whether ranolazine exhibits similar beneficial effects in T2D. We induced experimental T2D in male mice (15 weeks of high-fat diet along with a single low-dose injection of streptozotocin [75 mg/kg] at 4 weeks). The mice were administered either a vehicle control or ranolazine (50 mg/kg) once daily through oral gavage. We evaluated blood glucose levels by monitoring glucose tolerance, insulin tolerance, and pyruvate tolerance, while hepatic steatosis was evaluated by measuring triacylglycerols (TAGs) content. Ranolazine did not affect blood glucose levels in mice with experimental T2D or influence the amount of fat stored in the liver. Ranolazine’s beneficial effects on MASLD may be restricted to obese persons and not extend to those who are obese with T2D.
MASLD raises the risk of developing insulin resistance and T2D associated with obesity. Recent research from our laboratory has shown that activating PDH may mitigate MASLD caused by obesity using the antianginal drug ranolazine. Our goal was to investigate if ranolazine’s capacity to alleviate obesity-induced MASLD and hyperglycemia depends on the enhancement of hepatic PDH activity. Therefore, we generated hepatic PDH knockout mice (Pdha1Liver-/-) and fed a high-fat diet for 12 weeks to promote obesity, while lean controls were given a low-fat diet. Pdha1Liver−/− mice and their AlbCre littermates were randomly assigned to receive either a vehicle control or ranolazine (50 mg/kg) once daily through oral gavage for the last 5 weeks. Subsequently, we evaluated body composition, glucose tolerance, and pyruvate tolerance. Hepatic TAGs were measured using the Bligh and Dyer lipid extraction technique. Pdha1Liver-/- mice did not show any noticeable variations in physical characteristics (such as fat mass, body weight, and glucose tolerance) compared to their AlbCre littermates, but a little tendency towards a modest deterioration in pyruvate tolerance was noted. Ranolazine therapy improved glucose tolerance and slightly reduced hepatic TAG concentration in obese AlbCre mice but not in Pdha1Liver-/- obese mice. The former was not influenced by alterations in hepatic mRNA levels of genes that control lipogenesis. Ultimately, liver-specific PDH loss does not effectively induce an MASLD phenotype. However, hepatic PDH activity plays a role in the mechanism by which the antianginal drug ranolazine improves glucose tolerance and reduces hepatic steatosis in obesity.
We have shown that pimozide, an antipsychotic drug that typically inhibits dopamine 2 receptors (D2R), can effectively reduce glucose levels in mice with an experimental obesity/T2D model. The observations were linked to the unconventional effects of pimozide in binding and inhibiting succinyl-CoA:3-ketoacid CoA transferase (SCOT) in skeletal muscle. Additionally, the Similarity Ensemble Approach (SEA) was utilized, identifying X-box binding protein 1 (XBP1) as a promising target for pimozide. Due to the significant role of XBP1 in the unfolded protein response (UPR) and its influence on endoplasmic reticulum (ER) stress regulation, we shifted our attention to inositol-requiring enzyme 1 (IRE1). We aimed to determine the extent of this connection using both in vitro and in vivo experiments. HepaRG cells were grown in Williams’ Medium supplemented with 2 mM oleic acid and treated with either a vehicle or pimozide (12.5 nm) for 16 hours. We induced experimental T2D in 8-week-old male C57BL/6J mice by feeding them a high-fat/high-sucrose diet for 10 weeks. Additionally, a single dose of streptozotocin (STZ; 75 mg/kg) was given to the animals fed the high-fat diet at 4 weeks. After 8 weeks of the regimen, all mice were randomly assigned to receive either a vehicle control or pimozide (10 mg/kg) every other day via oral gavage until the study ended. Two weeks after administering pimozide, we evaluated glucose regulation using a glucose tolerance test and assessed hepatic steatosis by measuring TAG levels. HepaRG cells treated with pimozide showed a notable reduction in steatosis and increased levels of IRE1 and XBP1. Our data showed that pimozide enhanced glucose levels in mice with experimental T2D without affecting hepatic TAG content. These results indicate that pimozide may be beneficial in reducing hepatic steatosis, as shown by its significant influence on steatosis in HepaRG cells. The improvement in blood glucose levels without changing the amount of fat in the liver of mice with experimental T2D suggests that pimozide has a complex effect that warrants more investigation into its processes and potential use in treating metabolic diseases.
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- Graduation date
- Fall 2024
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Library with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.