Food Lipids: Chemistry, Nutrition, and Biotechnology

chemoattractant protein-1 (MCP-1), other chemoattractants, and inflammatory cytokines
by damaged endothelial cells and leukocytes attract leukocytes from the
circulation and initiate a local inflammation. Animal studies have shown that inflammatory
cell recruitment and activation is critical for the development of
atherosclerosis. For example, MCP-1 knockout mice and macrophage colony-stimulating
factor (M-CSF) knockout mice are less susceptible for the development of
atherosclerosis.
One of the earliest inflammatory steps in the atherogenesis is a slower rolling
of leukocytes along the vascular endothelium, which proceeds by a subsequent attachment
of rolling leukocytes to the vascular endothelium. In this process several
adhesion molecules such as vascular cell adhesion molecule 1 (VCAM-1), intercellular
adhesion molecule 1 (ICAM-1), and P-selectin play an important role. At least
in vitro, these adhesion molecules are rapidly synthesized in response to several
proinflammatory cytokines, such as tumor necrosis factor � (TNF-�) and interleukin-
1 (IL-1). Moreover, HDL cholesterol particles can inhibit the cytokine-induced expression
of adhesion molecules (VCAM-1, and E-selectin) on endothelial cells in
vitro [32]. This finding may be a possible link with the antiatherogenic effect of high
serum HDL cholesterol concentrations in vivo.
The presence of adhesion molecules on endothelial cells alone, however, is not
enough for attachment of leukocytes to the endothelium. For this interaction leukocytes
need to express ligands for these adhesion molecules. These ligands, expressed
on lymphocytes and monocytes, are known as integrins. For example, very late
antigen-4 (VLA-4), a �1-integrin is a ligand for VCAM-1 and lymphocyte function
associated-1 (LFA-1) a � 2-integrin for ICAM-1. After attachment, the proinflammatory
leukocytes infiltrate into the activated endothelium, followed by a further progressing
of the inflammatory response. Interestingly, blocking VLA-4 by antibodies
indeed decreased leucocyte entry and fatty streak formation in mice fed an atherogenic
diet [33], which shows that this integrin plays a causal role during atherosclerosis.
In conclusion, this process results in a continuous recruitment of new monocytes
and T lymphocytes to the place of oxidation in the endothelium and in
accumulation of macrophages filled with (oxidized) LDL in the arterial intima. Also,
several other processes are activated, which results in platelet aggregation, disturbance
of eicosanoid homeostasis, and release of growth factors. These factors cause
smooth muscle cell to proliferate and ultimately to migrate from the media to the
intima. All these mechanisms together will result in the formation of fatty streaks
and athersclerotic plaques (Fig. 10).
1. Measurement of LDL Oxidation
LDL oxidation is thought to be initiated and to proceed primarily in the endothelial
layer, while oxidized LDL particles are rapidly removed from the circulation. Thus,
it is very difficult to quantify the LDL oxidation process in vivo, and several in vitro
assays have been developed to measure LDL oxidation tendency in vitro.
a. In Vitro Copper-Mediated LDL Oxidation. Oxidative in vivo modification
of LDL can be mimicked by exposure of LDL particles in vitro to redox-active metal
ions, such as copper (Cu 2� ), or to reactive oxygen species, such as superoxide anion.
Esterbauer et al. [34] have developed a widely used method to determine in
vitro the susceptibility of LDL to oxidation by continuous monitoring of the for-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.