Apolipoprotein A-II, HDL metabolism and atherosclerosis.
Apolipoprotein (Apo) A-I and apo A-II are the major apolipoproteins of HDL. It is clearly demonstrated that there are inverse relationships between HDL-cholesterol and apo A-I plasma levels and the risk of coronary heart disease (CHD) in the general population. On the other hand, it is still not clearly demonstrated whether apo A-II plasma levels are associated with CHD risk. A recent prospective epidemiological (PRIME) study suggests that Lp A-I (HDL containing apo A-I but not apo A-II) and Lp A-I:A-II (HDL containing apo A-I and apo A-II) were both reduced in survivors of myocardial infarction, suggesting that both particles are risk markers of CHD. Apo A-II and Lp A-I:A-II plasma levels should be rather related to apo A-II production rate than to apo A-II catabolism. Mice transgenic for both human apo A-I and apo A-II are less protected against atherosclerosis development than mice transgenic for human apo A-I only, but the results of the effects of trangenesis of human apo A-II (in the absence of a co-transgenesis of human apo A-I) are controversial. It is highly suggested that HDL reduce CHD risk by promoting the transfer of peripherical free cholesterol to the liver through the so-called 'reverse cholesterol transfer'. Apo A-II modulates different steps of HDL metabolism and therefore probably alters reverse cholesterol transport. Nevertheless, some effects of apo A-II on intermediate HDL metabolism might improve reverse cholesterol transport and might reduce atherosclerosis development while some other effects might be deleterious. In different in vitro models of cell cultures, Lp A-I:A-II induce either a lower or a similar cellular cholesterol efflux (the first step of reverse cholesterol transport) than Lp A-I. Results depend on numerous factors such as cultured cell types and experimental conditions. Furthermore, the effects of apo A-II on HDL metabolism, beyond cellular cholesterol efflux, are also complex and controversial: apo A-II may inhibit lecithin-cholesterol acyltransferase (LCAT) (potential deleterious effect) and cholesteryl-ester-transfer protein (CETP) (potential beneficial effect) activities, but may increase the hepatic lipase (HL) activity (potential beneficial effect). Apo A-II may also inhibit the hepatic cholesteryl uptake from HDL (potential deleterious effect) probably through the SR-BI depending pathway. Therefore, in terms of atherogenesis, apo A-II alters the intermediate HDL metabolism in opposing ways by increasing (LCAT, SR-BI) or decreasing (HL, CETP) the atherogenicity of lipid metabolism. Effects of apo A-II on atherogenesis are controversial in humans and in transgenic animals and probably depend on the complex effects of apo A-II on these different intermediate metabolic steps which are in weak equilibrium with each other and which can be modified by both endogenous and environmental factors. It can be suggested that apo A-II is not a strong determinant of lipid metabolism, but is rather a modulator of reverse cholesterol transport.
Tailleux
Anne
A
Faculté de Pharmacie, Département d'athérosclérose et INSERM U 545, Institut Pasteur, Université Lille 2, 1, rue du Professeur Calmette, 59019 Cedex, Lille, France. anne.tailleux@pasteur.lille.fr
Duriez
Patrick
P
Fruchart
Jean-Charles
JC
Clavey
Véronique
V
D000818:Animals; D016632:Apolipoprotein A-I / Q000235:genetics / Q000378:metabolism*; D016633:Apolipoprotein A-II / Q000235:genetics / Q000378:metabolism*; D001161:Arteriosclerosis / Q000453:epidemiology / Q000235:genetics / Q000378:metabolism*; D008076:Cholesterol, HDL / Q000378:metabolism*; D003327:Coronary Disease / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism; D006801:Humans; D008071:Lipoprotein Lipase / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic
D016632:Apolipoprotein A-I; D016633:Apolipoprotein A-II; D008076:Cholesterol, HDL; D008071:Lipoprotein Lipase; D005947:Glucose