Apo A-I inhibits foam cell formation in Apo E-deficient mice after monocyte adherence to endothelium

Genetics of lipoprotein abnormalities associated with coronary artery disease susceptibility

Coronary heart disease is a complex genetic disease with many genes involved, environmental influences, and important gene-environment interactions. This review discusses the genetic basis of the principal lipoprotein abnormalities associated with coronary heart disease susceptibility in the general population. Individual sections discuss genes regulating LDL cholesterol, HDL cholesterol, and triglyceride levels. A section is included on the effects of the common apo E genetic variation on lipoprotein levels, as well as sections on the genetic regulation of lipoprotein(a) levels, genes regulating the inverse relationship between triglyceride-rich lipoproteins and HDL cholesterol levels, and our current understanding of the genetic basis of familial combined hyperlipidemia. It is clear that the field has progressed, with early studies focused mainly on the association of candidate gene RFLPs with phenotypes, later studies of candidate genes in both parametric and nonparametric linkage studies, and now more and more studies combining linkage analysis with genome scans to identify new loci that influence lipoprotein phenotypes. The future should provide us with the capability to perform reasonable genetic profiling for lipoprotein abnormalities associated with coronary heart disease susceptibility.

Inability of plasma high-density lipoproteins to inhibit cell adhesion molecule expression in human coronary artery endothelial cells

High-density lipoproteins (HDL) have several antiatherogenic actions, including the ability to sequester cellular cholesterol, to protect low-density lipoproteins from oxidation and to inhibit platelet aggregation. An early event in atherogenesis is the adhesion and recruitment of blood monocytes, a process mediated by cell adhesion molecules (CAMs), including vascular cell adhesion molecule-1 (VCAM-1) which is rapidly synthesized by endothelial cells in response to cytokines. It has been reported that HDL limits CAM expression in cultured human umbilical vein endothelial cells (HUVECs), implying that HDL also protects at an early stage in lesion development. Here, we have studied HDL suppression of CAM induction in human coronary artery endothelial cells (HCAECs), a model directly relevant to blood vessels susceptible to atherosclerosis. Arterial endothelial cells were preincubated with increasing amounts of total HDL, or different subfractions, and then activated with the inflammatory cytokine, tumor necrosis factor-alpha (TNF-alpha). Flow cytometric analysis failed to detect any downregulation of VCAM-1 or E-selectin expression by HDL in this model of vascular endothelium. Moreover, we were unable to confirm that HDL could suppress CAM induction in well-characterized, low-passage HUVECs, even though positive controls, 17beta-estradiol or a nitric oxide donor, did cause downregulation and factors such as variability in donors and HDL preparation, or culture conditions, were excluded. We tentatively conclude that, as isolated HDL did not downregulate CAM expression in cultured HCAECs or HUVECs, attenuation of CAM induction in arterial endothelium is unlikely to contribute to HDL antiatherogenic actions in vivo.

High density lipoproteins and arteriosclerosis. Role of cholesterol efflux and reverse cholesterol transport

High density lipoprotein (HDL) cholesterol is an important risk factor for coronary heart disease, and HDL exerts various potentially antiatherogenic properties, including the mediation of reverse transport of cholesterol from cells of the arterial wall to the liver and steroidogenic organs. Enhancement of cholesterol efflux and of reverse cholesterol transport (RCT) is considered an important target for antiatherosclerotic drug therapy. Levels and composition of HDL subclasses in plasma are regulated by many factors, including apolipoproteins, lipolytic enzymes, lipid transfer proteins, receptors, and cellular transporters. In vitro experiments as well as genetic family and population studies and investigation of transgenic animal models have revealed that HDL cholesterol plasma levels do not necessarily reflect the efficacy and antiatherogenicity of RCT. Instead, the concentration of HDL subclasses, the mobilization of cellular lipids for efflux, and the kinetics of HDL metabolism are important determinants of RCT and the risk of atherosclerosis.

HDL-associated PAF-AH reduces endothelial adhesiveness in apoE-/- mice

Macrophage infiltration into the subendothelial space at lesion prone sites is the primary event in atherogenesis. Inhibition of macrophage homing might therefore prevent atherosclerosis. Since HDL levels are inversely correlated with cardiovascular risk, their effect on macrophage homing was assessed in apoE-deficient (apoE-/-) mice. Overexpression of human apolipoprotein AI in apoE-/- mice increased HDL levels 3-fold and reduced macrophage accumulation in an established assay of leukocyte homing to aortic root endothelium 3.2-fold (P<0.005). This was due to reduced in vivo betaVLDL oxidation, reduced betaVLDL triggered endothelial cytosolic Ca2+ signaling through PAF-like bioactivity, lower ICAM-1 and VCAM-1 expression, and diminished ex vivo leukocyte adhesion. Adenoviral gene transfer of human PAF-acetylhydrolase (PAF-AH) in apoE-/- mice increased PAF-AH activity 1.5-fold (P<0.001), reduced betaVLDL-induced ex vivo macrophage adhesion 3.5-fold (P<0.01), and reduced in vivo macrophage homing 2.6-fold (P<0.02). These inhibitory effects were observed in the absence of increased HDL cholesterol levels. In conclusion, HDL reduces macrophage homing to endothelium by reducing oxidative stress via its associated PAF-AH activity. This protective mechanism is independent of the function of HDL as cholesterol acceptor. Modulation of lipoprotein oxidation by PAF-AH may prevent leukocyte recruitment to the vessel wall, a key feature in atherogenesis.

How high-density lipoprotein protects against the effects of lipid peroxidation

The protective effect of HDL against the development of atherosclerosis appears to be multifaceted involving a number of mechanisms. One of the major mechanisms is, however, the ability of HDL to decrease, directly or indirectly, the lipid peroxidation of LDL. The hydrolysis of lipid peroxides by PON1 makes a major contribution to this effect of HDL. Evidence is accumulating that the PON1 activity of human serum can be modulated by a variety of natural compounds and that these may increase or decrease the protective ability of PON1 and therefore of HDL on which it is exclusively located. Modulations of PON1 that enhance its activity may help to delay the atherosclerotic process.

The value of apolipoprotein E knockout mice for studying the effects of dietary fat and cholesterol on atherogenesis

The ability of the apolipoprotein E-deficient mouse to develop spontaneous atherosclerosis, which resembles the human process, is an excellent model in which to assess the impact of dietary factors. This review discusses the role of several nutrients in the development of atherosclerosis and the mechanisms through which they act.