Author ORCID Identifier

https://orcid.org/0000-0002-9462-656X

Date Available

8-15-2023

Year of Publication

2022

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Pharmacy

Department/School/Program

Pharmaceutical Sciences

First Advisor

Dr. Gregory A. Graf

Abstract

Despite decades of advances in research, death due to cardiometabolic disease remains the largest contributor to mortality in the US. While present therapies oppose “forward cholesterol transport,” and atherosclerotic plaque progression, they do not remove cholesterol from established atherosclerotic plaques. Reverse cholesterol transport (RCT) is the active process of mobilizing peripheral cholesterol for excretion through the hepatobiliary (transhepatic cholesterol elimination, THCE) or intestinal (transintestinal cholesterol elimination, TICE) pathways. Currently, there are no clinically approved therapeutics that target RCT. Elucidation of the mechanisms that govern RCT and TICE, and the subsequent identification of novel targets is of importance. The present work focuses on mechanisms in the liver and intestine that maintain sterol homeostasis in response to alterations in the biliary sterol transporter and plasma lipoproteins. The ABCG5/ABCG8 transporter heterodimers (G5/G8) account for >70% of biliary cholesterol elimination, the major route of cholesterol elimination. Despite the loss of biliary cholesterol secretion, by obstructive cholestasis or genetic inactivation of G5/G8, whole-body sterol balance is reportedly maintained. This signifies the presence of an adaptive, non-biliary route of cholesterol elimination in the gastrointestinal tract, TICE. The first chapter investigates the time course, degree, and location of these adaptive changes in cholesterol efflux when G5/G8 is inactivated in the liver of adult mice. We show adaptive changes to maintain sterol balance happen rapidly, and the first segment of the small intestine is the most active for TICE. Based on in vivo simultaneous bile collection and intestinal perfusion studies, we conclude that TICE, and the adaptation to disruptions in biliary cholesterol secretion, are dependent on the cholesterol concentration of the luminal content rather than a physiological adaptive process that accelerates TICE to maintain sterol balance. TICE relies on the plasma compartment for delivery of cholesterol, presumably though lipoprotein carriers. However, the exact source of this cholesterol is controversial. The ApoB100 transgenic mouse displays reduced cholesterol clearance compared with ApoB48, and when crossed with a Cholesterol Ester Transferase Protein (CETP) transgenic, exhibits a humanized lipoprotein profile on Western Diet. Using a previously published surgical procedure to measure simultaneous biliary and intestinal cholesterol secretion, we investigate how changes in the lipoprotein profile affect routes of cholesterol elimination. In both females and males, CETP/ApoB100 and ApoB100 transgenic mice showed a robust increase in the LDL fraction and increased concentration of total plasma cholesterol. This alteration did not lead to any significant differences in body weight, adiposity, total fecal cholesterol secretion, or total biliary cholesterol secretion and composition. Gender differences were observed when examining routes of cholesterol elimination, yet significant differences across the four genotypes were not detected. Based on the present data, we conclude that alterations in lipoprotein profile do not alter routes of cholesterol elimination.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2022.343

Funding Information

Pharmaceutical Sciences Excellence in Graduate Achievement Fellowship Award, UK College of Pharmacy. 01/21 – 01/22

Nutritional Sciences and Pharmacology T32 (DK007778), UK College of Medicine. 10/18 – 10/20

National Institutes of Health - National Institute of Diabetes and Digestive and Kidney Disease (1R01DK113625). 09/17-05/22

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