Year of Publication
Doctor of Philosophy (PhD)
Dr. Gregory A. Graf
The collective presence of central obesity, low HDL-cholesterol, and elevated triglycerides, blood pressure, and fasting blood glucose constitutes Metabolic Syndrome (MetS), a disease state that increases the risk of cardiovascular disease (CVD) and Type 2 Diabetes Mellitus (T2DM). Nonalcoholic fatty liver disease (NAFLD), present in up to 90% of obese adults, is also linked to MetS. As in CVD, disruptions in cholesterol metabolism play a contributing role in the development of T2DM and NAFLD. Genes involved in cholesterol synthesis, secretion, and catabolism are diurnally regulated in the liver and adipose. Disruptions in the sleep-wake cycle are thought to potentiate metabolic disorders associated with CVD, thus revealing a potential role of disrupted circadian rhythms in the development of MetS phenotypes.
We initially observed that a group of clock-controlled genes in the Par bZip family were downregulated in adipose tissue of obese mice and humans. Further studies revealed that deletion of the core clock gene, Bmal1, from adipose tissue alone (ABKO mice) or in combination with the liver (LABKO mice) altered feeding behavior and locomotor activity. Obesity was increased in LABKO mice but there were no further detrimental effects in either ABKO or LABKO mice. Interestingly, high-fat diet suppressed Bmal1 transcript levels in adipose tissue suggesting that the muted effects observed with genetic Bmal1 deletion were likely due to the overpowering effects of a high-fat diet.
Cholesterol metabolizing and bile acid synthesizing enzymes oscillate diurnally at the transcriptional level. Promoting cholesterol elimination through bile acid synthesis and biliary secretion is essential for reducing hepatic cholesterol, a perpetrating factor in the progression of NAFLD. Previous studies in animal models demonstrated that deletion of the hepatic cholesterol transporters, Abcg5 and Abcg8 (G5G8), decreased biliary cholesterol secretion and increased obesity and hepatic steatosis. Ursodiol (Urso) increased G5G8 protein expression and in combination with ezetimibe, a cholesterol-lowering agent, showed promise as a therapeutic for NAFLD. However, bile acid synthesis was suppressed in the presence of Urso and ezetimibe, suggesting that the overall effect on cholesterol elimination was minimized. The increase in G5G8 and decrease in bile acid synthesis was associated with an increase in Fgf15/19, a suppressor of bile acid synthesis. In order to determine if and how FGF15/19 regulates G5G8, mice were injected with FGF19. G5G8 protein expression and biliary total cholesterol were increased. Additionally, G5G8 localized to the canalicular surface of hepatocytes. Urso caused a similar localization of G5G8 that appeared to be dependent of the FGF15/19-FGFR4 signaling pathway. Interestingly, G5G8 was not required for maintaining cholesterol homeostasis in the presence of FGF15/19.
Data from murine models suggest Urso and ezetimibe promote cholesterol elimination, though the effects may be limited by the suppression of bile acid synthesis. Nonetheless, there may be a therapeutic window in which optimal cholesterol elimination can be reached. Additionally, the effects of Urso and ezetimibe are variable between mice and humans. Given the clinical availability of the two drugs, the translatability into humans and the potential to address whether Urso and ezetimibe can minimize NAFLD is great.
Digital Object Identifier (DOI)
Pijut, Sonja S., "Factors Regulating Features of Metabolic Syndrome" (2017). Theses and Dissertations--Pharmacy. 70.