The Role of Lipids and Lipoproteins in Atherosclerosis

Atherosclerosis is the underlying cause of heart attack and stroke. Early observations that cholesterol is a key component of arterial plaques gave rise to the cholesterol hypothesis for the pathogenesis of atherosclerosis. Population studies have demonstrated that elevated levels of LDL cholesterol and apolipoprotein B (apoB) 100, the main structural protein of LDL, are directly associated with risk for atherosclerotic cardiovascular events (ASCVE). Indeed, infiltration and retention of apoB containing lipoproteins in the artery wall is a critical initiating event that sparks an inflammatory response and promotes the development of atherosclerosis. Arterial injury causes endothelial dysfunction promoting modification of apoB containing lipoproteins and infiltration of monocytes into the subendothelial space. Internalization of the apoB containing lipoproteins by macrophages promotes foam cell formation, which is the hallmark of the fatty streak phase of atherosclerosis. Macrophage inflammation results in enhanced oxidative stress and cytokine/chemokine secretion, causing more LDL/remnant oxidation, endothelial cell activation, monocyte recruitment, and foam cell formation. HDL, apoA-I, and endogenous apoE prevent inflammation and oxidative stress and promote cholesterol efflux to reduce lesion formation. Macrophage inflammatory chemoattractants stimulate infiltration and proliferation of smooth muscle cells. Smooth muscle cells produce the extracellular matrix providing a stable fibrous barrier between plaque prothrombotic factors and platelets. Unresolved inflammation results in formation of vulnerable plaques characterized by enhanced macrophage apoptosis and defective efferocytosis of apoptotic cells resulting in necrotic cell death leading to increased smooth muscle cell death, decreased extracellular matrix production, and collagen degradation by macrophage proteases. Rupture of the thinning fibrous cap promotes thrombus formation resulting in clinical ischemic ASCVE. Surprisingly, native LDL is not taken up by macrophages in vitro but has to be modified to promote foam cell formation. Oxidative modification converts LDL into atherogenic particles that initiate inflammatory responses. Uptake and accumulation of oxidatively modified LDL (oxLDL) by macrophages initiates a wide range of bioactivities that may drive development of atherosclerotic lesions. Lowering LDL-cholesterol with statins reduces risk for cardiovascular events, providing ultimate proof of the cholesterol hypothesis. All of the apoB containing lipoproteins are atherogenic, and both triglyceride rich remnant lipoproteins and Lp(a) promote atherothrombosis. Non-HDL cholesterol levels capture all of the apoB containing lipoproteins in one number and are useful in assessing risk in the setting of hypertriglyceridemia. Measures of apoB and LDL-P are superior at predicting risk for ASCVE, when levels of LDL-C and LDL-P are discordant. Here, we also describe the current landscape of HDL metabolism. Epidemiological studies have consistently shown that HDL-C levels are inversely related to ASCVE. We highlight recent clinical trials aimed at raising HDL-C that failed to reduce CVE and the shifting clinical targets of HDL-C, HDL particle numbers, and HDL function (e.g. cholesterol efflux capacity). Furthermore, we describe many beneficial properties of HDL that antagonize atherosclerosis and how HDL dysfunction may promote cardiometabolic disease.