High-density lipoprotein (HDL) formation and maturation are important for reverse cholesterol transport, a hypothesis that HDL transports cholesteryl ester (CE) from peripheral atherosclerotic lesions to the liver. This hypothesis is based on epidemiological studies that indicated an inverse correlation between the HDL cholesterol (HDL-C) levels and the prevalence of coronary heart disease (CHD) as an independent anti-atherogenic factor for CHD. It has been noted that despite a markedly low level of HDL-C and a loss in the process of HDL maturation, CHD is not a common complication in patients lacking lecithin:cholesterol acyltransferase (LCAT). In other patients with low HDL-C lacking ATP binding cassette AI (ABCAI), they are predisposed to atherosclerosis. Understanding the pathophysiology involved in HDL formation and tailoring specific targets to improve HDL-C function will lead to new treatments of coronary artery disease. ? ? LCAT is an enzyme that esterifies free cholesterol (FC) into cholesteryl ester (CE) in HDL. Human LCAT deficiency (LCAT-def) is characterized by corneal opacity, anemia, and proteinuria, with low levels of HDL and low density lipoprotein (LDL) and the accumulation of lipoprotein-X (LpX) but no increased atherosclerosis. LpX is a unique lipoprotein characterized by a balloon-like structure constructed of phospholipid, FC, apolipoprotein A-I, and albumin which is believed to cause the renal problems. HDL is markedly decreased, together with abnormalities in size and lipid composition in LCAT-def. Most of the HDL in LCAT-def is small and discoidal, a characteristic resembling newly synthesized HDL. In the absence of LCAT, newly synthesized HDL cannot esterify FC to CE in its core so that HDL cannot develop into its mature stable forms of either HDL2 or HDL3. We continue to investigate ways to increase or replace LCAT in an effort to treat patients who lack the enzyme but more importantly as a way to increase the functionality of HDL in the general population of patients at risk for atherosclerosis. ? ? Our goal was to elucidate the mechanism(s) responsible for the formation and maturation of HDL using in vitro cell cultures, animal models and correlating this in vivo in humans who are normal and have genetic mutations in specific lipid pathways. Studies are designed to formulate metabolic pathways in patients with defined genetic disorders of lipid metabolism as well as in healthy volunteers to provide new and novel insights into normal and pathologic metabolic pathways. All kinetic data is computer analyzed to provide quantitative data and facilitate direct comparison of multiple studies. Our early studies have laid the foundation for the understanding of many of the key metabolic pathways in lipid metabolism. Studying the effects of diet, drug, and genetic factors upon these different particles has permitted a better understanding of what modulates HDL levels.
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