Date of Award
cholesterol, StARD4, cholesterol metabolism, sterol transport, HDL, StARD4 knockout mice
Cholesterol is crucial for mammalian survival by playing important roles, such as regulating membrane fluidity and as a precursor for the synthesis of steroid and sex hormones, bile acids, and Vitamin D. In addition, cellular and organismal regulation of cholesterol is important for health. For example, increased levels of plasma LDL cholesterol are a risk factor for coronary heart disease and stroke. Intracellular cholesterol levels are regulated by a variety of mechanisms, but numerous studies indicate a very important role for transcriptional regulation by Sterol Regulatory Binding Proteins (SREBPs), Liver X Receptors (LXRs) and Unfolded Protein Response (UPR) or ER stress families of transcription factors. StARD4 is regulated by SREBPs and we have chosen to make a mouse knockout model to characterize its role in-vivo. StARD4, expressed primarily in liver and macrophages, is a known intracellular cholesterol transporter previously shown to be down-regulated ~2 fold in liver, by high cholesterol feeding. It is thought to be involved in the dynamics of cholesterol movement between ER, plasma membrane, endosomes and lipid droplets. Based on these observations, I hypothesized that a knockout of StARD4 in a mouse model would show altered intracellular cholesterol sorting, and figuring out the basis of such a defect would provide insight into the general mechanisms of intracellular sterol transport. To my surprise, StARD4 knockouts were viable and for the most part phenotypically normal. They showed no alteration in plasma or liver cholesterol or triglycerides. In addition, no abnormalities were found in glucose metabolism, macrophage cholesterol efflux, or atherosclerosis susceptibility. Based on these observations, I hypothesize that in-vivo, the absence of StARD4 is compensated for by other genes and/or pathways. In the future, it will be necessary to identify these compensatory mechanism(s) to truly understand the physiological role of StARD4.I also studied another aspect of cholesterol metabolism related to its transport in plasma in high density lipoproteins (HDLs). HDL is involved in the reverse cholesterol transport mechanism, whereby excess cholesterol is removed from peripheral tissues and transported to the liver for excretion. The major protein of HDL is apoA-I and in mouse models it has been shown that animals transgenic for apoA-I have increased HDL levels. This suggests increased apoA-I transcription as a mechanism for increasing HDL, which might be preventive or therapeutic for coronary heart disease. With this as a goal, as part of my thesis I studied the epigenetic regulation of apoA-I transcription. I found that increased apoA-I transcription in liver cell culture cell lines was associated with highly unmethylated CpGs in the apoA-I promoter, and the reverse, in cultures with poor apoA-I expression. I also found histone marks associated with apoA-I expression. This project was discontinued in favor of the StARD4 knockout mouse project. However, it might be continued in the future to reveal drug targets that alter epigenetic regulation of apoA-I in a manner that raises HDL levels.
Riegelhaupt, Joshua, "Characterization of the Functional Role of the Intracellular Cholesterol Transporter StARD4 in Knockout Mice, and Investigation of Epigenetic Modulation of ApoA-I Transcription" (2010). Student Theses and Dissertations. 70.