Clinical Significance and Treatment of Hypertriglyceridemia
Several lines of evidence suggest that triglyceride-rich lipoproteins contribute significantly to the development of atherosclerosis. However, the relationship between cardiovascular disease and plasma levels of triglyceride remains complex due to the presence of two major confounders: (1) the inverse relationship between plasma triglyceride levels and high-density lipoprotein-cholesterol and (2) the heterogeneity in triglyceride-rich lipoprotein size, number, and composition between individuals. Plasma apo-B measurement is recommended for patients with high triglyceride levels to identify their risk category. The goals of lipid-modifying therapy for these patients are to reduce the atherogenic lipoprotein number and to increase HDL particle number.
Introduction
Over the past several decades, basic research and clinical studies have led to a better understanding of the atherosclerotic process. It appears that lipoproteins, particularly apolipoprotein-B (apo-B)-containing lipoproteins, first accumulate in the intima of the artery wall. As these lipoproteins become modified, they induce the production of local cytokines, adhesion molecules, and chemoattractants stimulating transendothelial migration of monocytes and their subsequent transformation into macrophages and foam cells. Smooth muscle cells and lymphocytes have also been shown to accumulate in the growing atherosclerotic plaque, which then become vulnerable to fissuring and rupturing.1 Despite the fact that LDL particles play a central role in the development of atherosclerosis, several lines of evidence suggest that triglyceride-rich lipoproteins contribute significantly to this process. For example, it is well established that type 2 diabetic individuals have elevated plasma levels of triglyceride-rich lipoproteins that may impair endothelial function, enter subendothelial space of the artery wall, and promote the development of atherosclerosis.2 It is therefore of considerable interest to review briefly the metabolism of triglyceride-rich lipoproteins to understand better the relationship between plasma triglyceride levels and cardiovascular risk.
Metabolism of Triglyceride-rich Lipoproteins
Synthesis of triglyceride-rich lipoproteins occurs in the small intestine and the liver in the form of chylomicrons and very low density lipoprotein (VLDL), respectively. Triglyceride is the major lipid in chylomicrons and VLDL and serves as energy substrates in the liver and peripheral tissues, particularly muscle. Once in the plasma, the vast majority of triglyceride molecules in chylomicrons and VLDL are hydrolyzed by the action of lipases (lipoprotein and hepatic lipases), leading to the formation of smaller, denser, triglyceride-rich lipoproteins (called remnants) and ultimately to LDL particles in the case of apo-B-100-containing lipoproteins. The fatty acids released by these reactions are taken up by liver, muscles, and adipocytes. Because of their smaller size and increased cholesteryl ester content, remnant lipoproteins of both chylomicron and VLDL are considered more atherogenic than their respective precursors. At the artery wall, remnant lipoproteins of both VLDL and chylomicron have the potential to interact with endothelial cells, causing changes that may enhance transendothelial passage of the remnants and circulating monocytes into the intima, leading to an acceleration of the development of atherosclerosis.2-4
The relationship between cardiovascular disease and plasma levels of triglyceride is complex. In part, this is related to the presence of two major confounders: (1) the inverse relationship between plasma triglyceride levels and high-density lipoprotein (HDL)-cholesterol and (2) the heterogeneity in triglyceride-rich lipoprotein size, number, and composition between individuals. HDL-cholesterol is widely accepted as being protective against cardiovascular disease and plays a crucial role in the reverse cholesterol transport pathway by delivering cellular cholesterol to the liver for excretion. Several lines of evidence indicate that increased levels of triglyceride-rich lipoproteins stimulate the exchange of their triglyceride for cholesterol in HDL, a process mediated in plasma by