Abstract
Apolipoprotein A-I (apoA-I), the major protein of HDL, is probably among the most intensively studied of all proteins. It functions in HDL assembly, in the removal of excess cholesterol from cells, and in the transport of cholesterol from peripheral tissues to the liver, a process known as “reverse cholesterol transport.” It is a cofactor for lecithin:cholesterol acyl transferase (LCAT), the enzyme responsible for cholesterol esterification in HDL, and it stabilizes certain antioxidant enzymes, such as serum paraoxonase, that are carried on HDL.1,2 Rare null mutations of apoA-I in human populations are associated with early coronary heart disease,3 and overexpression of apoA-I in transgenic mice protects against atherogenesis.4 An exciting recent development was the 4-angstrom-resolution x-ray structure for the helical part of apoA-I, encompassing residues 44 to 243 (reviewed by Segrest et al5). See page 1977 In contrast, apolipoprotein A-II (apoA-II), the second most abundant protein of HDL, has no known function, and it has been the subject of few detailed studies. Its presence in HDL has been reported to decrease,6 or have no influence on, cholesterol efflux from cells,7,8 nor is it a cofactor for any known enzyme. Rare individuals lacking apoA-II appear to have normal plasma lipids.9 In fact, all of the known effects of apoA-II are deleterious. In studies with mice and humans, apoA-II levels have been associated with increased susceptibility to atherosclerosis,10–13 increased free fatty acid levels,14,15 increased body fat,15,16 and increased insulin resistance.15,17 Undoubtedly, apoA-II has some unknown beneficial functions, possibly related to fatty acid metabolism or host defense. In this issue of Arteriosclerosis, Thrombosis, and Vascular Biology , …
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