© 1999 American Heart Association, Inc.
Editorial |
Monocyte Rolling in Early Atherogenesis
Vital Role in Lesion Development
From the Department of Molecular and Cellular Physiology, Louisiana State University Medical Center, Shreveport, La.
Correspondence to David J. Lefer, PhD, Department of Molecular and Cellular Physiology, LSU Medical Center, 1501 Kings Hwy, Shreveport, LA 71130. E-mail dlefer{at}lsumc.edu
Key Words: monocyte • gene-targeted mice • hypercholesterolemia • cell adhesion molecule
The process of atherosclerotic lesion formation represents a complex interaction of a number of circulating blood cells with cells that reside within the arterial wall. Understanding the cellular mechanisms involved in this process is vital for the development of novel therapeutic strategies for prevention and treatment of coronary and carotid occlusive disease. In this issue of Circulation Research, Ramos and colleagues1 describe a novel in vitro model system for visualization and quantification of mononuclear cell rolling in large arteries that are prone to development of atherosclerotic lesions in apolipoprotein E (ApoE)–deficient mice. This novel model system was used to define the contribution of different adhesion molecules to monocyte rolling along atherosclerotic lesions. Their findings are important and should be considered relative to the existing body of published data on leukocyte-endothelial cell interactions in atherosclerosis.
Cell Adhesion Molecules and Atherogenesis
Data generated from animal models of hypercholesterolemia and atherosclerosis suggest that circulating leukocytes (ie, monocytes) contribute to the development of atherosclerotic lesions.2 3 Previous studies have demonstrated that the expression of both leukocyte and endothelial cell adhesion molecules (CAMs) is enhanced in the setting of hypercholesterolemia.4 5 6 7 Furthermore, atherosclerotic lesions in both animals and humans are a rich source of several endothelial CAMs, including P-selectin, intracellular cell adhesion molecule-1 (ICAM-1), and vascular cell adhesion molecule-1 (VCAM-1).8 9 10 11 More recently, gene-targeted mice that are deficient in leukocyte (CD18) or endothelial (P-selectin or ICAM-1) CAMs have been placed on a high-fat diet and the extent of atherosclerosis compared with that produced in their wild-type counterparts placed on the same diet.12 It was noted that mice deficient in ICAM-1, P-selectin, or CD18 exhibited a 50% to 75% reduction in atherosclerotic fatty streak lesions. Other investigators13 have addressed the same issue using P-selectin–deficient mice that are crossbred with mice lacking the LDL receptor (LDLr-KO), which are known to readily develop atherosclerotic lesions on a high-fat diet. This experimental strategy revealed a delayed rate of fatty streak development in P-selectin–deficient/LDLr-KO mice, such that smaller streaks were noted in the double mutants (compared with LDLr-KO) after 8 to 20 weeks of high-fat feeding. However, at 37 weeks on the high-cholesterol diet, fatty streaks progressed to fibrous plaques, and this process was similar in the LDLr-deficient animals independent of P-selectin levels.
In the current issue of Circulation Research, Ramos and
colleagues1 describe the rolling of
fluorescently labeled human and murine monocyte cell lines in
isolated mouse carotid arteries using intravital videomicroscopy, a
technique that has already been used to study the kinetics of
leukocyte-endothelial cell adhesion in arterioles and
postcapillary venules of atherogenic mice.7 The authors
clearly demonstrate that monocyte rolling is markedly enhanced in mice
fed a high-cholesterol diet for 4 to 5 weeks.
Administration of monoclonal antibodies directed against P-selectin or
the P-selectin leukocyte ligand, PSGL-1, significantly attenuated
monocyte rolling and adhesion. Furthermore, monocyte rolling and
adherence were abrogated after administration of a monoclonal antibody
that neutralizes 
4 integrin. Hence, these
elegant studies clearly define the role of specific
leukocyte-endothelial CAMs to the monocyte rolling
observed in the early stages of
hypercholesterolemia preceding the development
of atherosclerotic lesions. The study by Ramos et al1 also
confirms the results of an earlier study of ApoE-deficient mice,
wherein it was noted that accumulation of labeled macrophages
in atherosclerotic plaques is diminished after inhibition of ICAM-1 and
4 integrin.8 An advantage of the
intravital microscopic approach used by Ramos et al1 is
the potential for aquisition of real-time kinetics of leukocyte
interactions with the wall of lesion-prone areas of large arteries.
Monocytes and Atherogenesis
One of the characteristic features of early atherosclerotic lesions is the localized accumulation of monocyte/macrophages and T lymphocytes within the arterial intima.14 15 16 17 18 Monocytes are transformed into macrophages that steadily accumulate cholesterol esters and are subsequently transformed into foam cells. T lymphocytes that accumulate in the intima secrete a variety of inflammatory mediators that serve to activate vascular cells, thus contributing to atherosclerotic lesion formation.14 15 16 17 18
In primates, monocyte-endothelial cell adhesion and endothelial transmigration have been shown to occur within 1 week after placement on a high-cholesterol diet.19 20 Monocyte accumulation resulted in intimal lesions containing macrophage-derived foam cells as well as T lymphocytes. Additional evidence for the role of monocytes/macrophages in atherogenesis is provided by a recent study demonstrating that atherosclerosis is significantly retarded in mice that are genetically deficient in both macrophage colony-stimulating factor and ApoE.21
By using in vitro adhesion assay systems, many investigators22 23 24 25 26 have attempted to define the role of monocytes in the progression of atherosclerotic vascular disease. These studies have also served to define the cellular and molecular mechanisms responsible for monocyte/macrophage adhesive interactions with the vessel wall. It is now well appreciated that lipoproteins such as ßVLDL and LDL can stimulate monocyte adhesion to vascular endothelial cells in vitro.22 23 24 25 In addition, the oxidation status of lipoproteins appears to be of critical importance in the regulation of monocyte adhesion in vitro.17 23 24 Tsao et al26 have also demonstrated that the adhesion of monocytes derived from hypercholesterolemic rabbits to vascular endothelium is highly influenced by nitric oxide (NO) production. Previous studies27 28 29 have demonstrated that one of the earliest effects of hypercholesterolemia is a reduction in endothelial cell NO generation. Tsao et al26 reported that augmentation of NO levels with L-arginine can effectively blunt the hyperadhesivity of monocytes derived from hypercholesterolemic animals. Although all of the aforementioned studies have greatly extended our understanding of monocyte interactions with the vessel wall, additional work in animal models is needed to confirm and extend what is already known about this dynamic inflammatory process.
Gene-Targeted Mice as Models of Hypercholesterolemia and Atherosclerosis
Targeted disruption, deletion, or insertion of specific genes that regulate lipoprotein metabolism has resulted in the generation of a variety of novel murine models of hypercholesterolemia and atherosclerosis.30 31 32 33 34 35 36 Many of these animals develop arterial lesions similar in composition to those observed in humans, especially when placed on a high-fat diet.30 36 Consequently, the gene-targeted mouse models of hypercholesterolemia are considered to be more relevant to the hypercholesterolemia in humans, when compared with some of the other animal models (eg, rat) of hypercholesterolemia.
In recent years, a number of studies have used gene-targeted mice that suffer from alterations in lipoprotein metabolism to further our understanding of the pathogenesis of atherosclerotic lesion formation. The present study by Ramos et al1 uses the ApoE-deficient mice in combination with a high-fat Western diet as a model of hypercholesterolemia. These investigators emphasize that their model is focused on a time frame that precedes the formation of fatty lesions in large arteries, in view of the fact that the mice were maintained on an atherogenic diet for a very short period of time. Hence, this brief hypercholesteremic insult provides a unique opportunity for investigation of monocyte-endothelial cell adhesive interactions during the early stages of the atherogenic process.
Future Directions
The study by Ramos et al1 underscores the promise for an improved understanding of the atherogenic process that is offered by the application of intravital microscopic techniques to large arteries in mutant mice. Before this report, most of the available information regarding monocyte adhesion to endothelium in hypercholesterolemia and atherosclerosis was gleaned from studies of leukocyte accumulation at fixed time points during the atherogenic process or from in vitro adhesion assays that do not include the influence of shear. The dynamic images of leukocyte rolling in intact arteries exposed to physiological shear stress provide unique insights into the atherogenic process that were not previously attainable. However, there are several avenues for improvement of this powerful new tool. Highest priority should be given to applying this experimental approach to intact, arterial vessels perfused with whole, homologous blood. A recently published preliminary report,37 which demonstrated P-selectin–dependent leukocyte rolling in the intact aorta of cytokine-challenged mice, supports the feasibility of applying this approach to atherogenic mice. The development of mutant mice that express marker fluorochromes (such as green fluorescent protein) only in specific leukocyte populations (eg, monocytes) and the crossbreeding of these mutants with atherogenic mutants (eg, ApoE-deficient mice) should also enhance the utility and validity of this model system. Indeed, with the introduction of this technology to atherogenesis research, the possibilities for advancement in this important field of investigation now appear endless.
Footnotes
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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