Changing concepts of atherogenesis

Abstract

This review discusses three stages in the life history of an atheroma: initiation, progression and complication. Recruitment of mononuclear leucocytes to the intima characterizes initiation of the atherosclerotic lesion. Specific adhesion molecules expressed on the surface of vascular endothelial cells mediate leucocyte adhesion: the selectins and members of the immunoglobulin superfamily such as vascular cell adhesion molecule-1 (VCAM-1). Once adherent, the leucocytes enter the artery wall directed by chemoattractant chemokines such as macrophage chemoattractant protein-1 (MCP-1). Modified lipoproteins contain oxidized phospholipids which can elicit expression of adhesion molecule and cytokines implicated in early atherogenesis. Progression of atheroma involves accumulation of smooth muscle cells which elaborate extracellular matrix macromolecules. These processes appear to result from an eventual net positive balance of growth stimulatory versus growth inhibitory stimuli, including proteins (cytokines and growth factors) and small molecules (e.g. prostanoids and nitric oxide). The clinically important complications of atheroma usually involve thrombosis. Arterial stenoses by themselves seldom cause acute unstable angina or acute myocardial infarction. Indeed, sizeable atheroma may remain silent for decades or produce only stable symptoms such as angina pectoris precipitated by increased demand. Recent research has furnished new insight into the molecular mechanisms that cause transition from the chronic to the acute phase of atherosclerosis. Thrombus formation usually occurs because of a physical disruption of atherosclerotic plaque. The majority of coronary thromboses result from a rupture of the plaque's protective fibrous cap, which permits contact between blood and the highly thrombogenic material located in the lesion's lipid core, e.g. tissue factor. Interstitial collagen accounts for most of the tensile strength of the plaque's fibrous cap. The amount of collagen in the lesion's fibrous cap depends upon its rate of biosynthesis stimulated by factors released from platelets (e.g. transforming growth factor beta or platelet-derived growth factor), but inhibited by gamma interferon, a product of activated T cells found in plaques. Degradation by specialized enzymes (matrix metalloproteinases) also influences the level of collagen in the plaque's fibrous cap. Such studies illustrate how the application of cellular and molecular approaches has fostered a deeper understanding of the pathogenesis of atherosclerosis. This increased knowledge of the basic mechanisms enables us to understand how current therapies for atherosclerosis may act. Moreover, the insights derived from recent scientific advances should aid the discovery of new therapeutic targets that would stimulate development of novel treatments. Such new treatments could further reduce the considerable burden of morbidity and mortality due to this modern scourge, and reduce reliance on costly technologies that address the symptoms rather than the cause of atherosclerosis.