Plasma membrane cholesterol homeostasis

The level of free intracellular cholesterol is strictly controlled by a wellcharacterized set of homeostatic mechanisms. In contrast, the mechanisms that control the intracellular distribution of sterols between various membranes within the cell are poorly understood. The majority (>95%) of cellular cholesterol resides in the plasma membrane and in endocytic vesicles derived from the plasma membrane. Here I describe my work designed to elucidate the mechanisms by which this intracellular distribution is developed and maintained.; In Chapter 1, I review what is known about the intracellular distribution of cholesterol and the transport mechanisms that maintain these distributions. In Chapter 2, I describe the characterization of NPC fibroblasts, which have a defect in the transport of LDL-derived cholesterol from the endo/lysosomal compartment to the plasma membrane. I also describe the development and use of an assay to measure this transport process in vitro. In Chapter 3, I describe the identification of a protein (Hpr6.6) that is over expressed in NPC cells and is required for maintaining normal levels of plasma membrane sterols. In Chapter 4, I describe the characterization of luteolin, a metabolite isolated from artichoke that has the unique ability to inhibit both cholesterol biosynthesis and fatty acid biosynthesis. Our studies suggest that luteolin prevents cholesterol precursors and short chain fatty acids from reaching the endoplasmic reticulum (ER) where the enzymes that catalyze the final steps in cholesterol and long-chain fatty acid biosynthesis reside. These studies suggest that luteolin may be useful as a dietary supplement in treating patients at risk for coronary heart disease. In Chapter 5, I summarize what we have learned from these studies as well as future studies that will need to be performed in order to completely understand these important processes of intracellular cholesterol transport.