Therapeutic approaches for the modification of high-density lipoproteins

Current understanding of the metabolism and biological actions of HDL

PURPOSE OF REVIEW

A low concentration of HDL-cholesterol is an important risk factor for coronary heart disease. The purpose of this review is to summarize the novel functions of HDL that may protect not only from atherosclerosis, but also from inflammation-induced organ damage.

RECENT FINDINGS

HDL interacts with several cellular receptors and lipid transporters. The interactions of HDL or apolipoprotein A-I with the scavenger receptor BI or adenosine triphosphate binding cassette transporters A1, G1 and G4 induce cholesterol efflux. Apolipoproteins and enzymes carried by HDL exert antioxidative functions. Some oxidative modifications of apolipoprotein A-I, for example of tyrosine residues, may however interfere with anti-atherosclerotic activities. The interactions of HDL and lysosphingolipids therein with scavenger receptor BI and sphingolipid receptors, respectively, elicit signals activating the protein kinase Akt, which in turn is a regulator of apoptosis in beta, endothelial and smooth muscle cells as well as a regulator of nitric oxide production and adhesion molecule expression in endothelial cells. Other signal transduction cascades are also elicited by HDL, some of which induce cholesterol efflux or activate mitogen-activated protein kinases.

SUMMARY

Properties with respect to cytokine production, lipid oxidation, cholesterol efflux and reverse cholesterol transport make HDL a protective agent and thus an interesting therapeutic target in atherosclerosis and inflammation-induced organ damage.

Modulation of high-density lipoprotein cholesterol metabolism and reverse cholesterol transport

Low high-density lipoprotein (HDL)-cholesterol (C) is an important risk factor for coronary heart disease. In vitro, HDL exerts several potentially anti-atherogenic effects including reverse cholesterol transport (RCT) from peripheral cells to the liver. Hence, raising HDL-C has become an interesting target for anti-atherosclerotic drug therapy. Levels of HDL-C and the composition of HDL subclasses in plasma are regulated by apolipoproteins, lipolytic enzymes, lipid transfer proteins, receptors, and cellular transporters. The interplay of these factors leads to RCT and determines the composition and thereby the anti-atherogenic properties of HDL. Recent findings suggest that the mechanism of HDL modification rather than a sole increase in HDL-C determines the efficacy of anti-atherosclerotic drug therapy. In several controlled and prospective intervention studies, patients with low HDL-C and additional risk factors benefited from treatment with fibrates or statins. However, in only some of the fibrate trials was prevention of coronary events in patients with low HDL-C and hypertriglyceridaemia related to an increase in HDL-C. This may be because currently available drugs increase HDL-C levels only moderately and because HDL levels per se do not necessarily correlate with the functionality of HDL. However, several novel targets to modify RCT have emerged from the recent understanding of HDL synthesis, maturation and catabolism. The four major targets for an anti-atherogenic strategy in HDL metabolism include stimulation of apoA-I synthesis and secretion, the stimulation of ABCA1 expression, the inhibition of cholesterol ester transfer protein, and the up-regulation of scavenger receptor BI. These and other modulations of HDL metabolism are thought to result in improved RCT making them attractive targets for the development of new regimens of anti-atherogenic drug therapy.