Apolipoprotein B - Role in Lipoproteins and Atherosclerosis

Role in Lipoproteins and Atherosclerosis

APOB100 is found in lipoproteins originating from the liver (VLDL, IDL, LDL). Importantly, there is one APOB100 molecule per hepatic-derived lipoprotein. Hence, using that fact, one can quantify the number of lipoprotein particles by noting the total APOB100 concentration in the circulation. Since there is one and only one APOB100 per particle, the number of particles is reflected by the APOB100 concentration. The same technique can be applied to individual lipoprotein classes (e.g. LDL) and thereby enable one to count them as well.

It is well established that APOB100 levels are associated with coronary heart disease, and are even a better predictor of it than is LDL level. A naive way of explaining this observation is to use the idea that APOB100 reflects lipoprotein particle number (independent of their cholesterol content). In this way, one can infer that the number of APOB100-containing lipoprotein particles is a determinant of atherosclerosis and heart disease.

One way to explain the above is to consider that large numbers of lipoprotein particles, and, in particular large numbers of LDL particles, lead to competition at the APOB100 receptor (i.e. LDL receptor) of peripheral cells. Since such a competition will prolong the residence time of LDL particles in the circulation, it may lead to greater opportunity for them to undergo oxidation and/or other chemical modifications. Such modifications may lessen the particles' ability to be cleared by the classic LDL receptor and/or increase their ability to interact with so-called "scavenger" receptors. The net result is shunting of LDL particles to these scavenger receptors. Scavenger receptors typically are found on macrophages, with cholesterol laden macrophages being better known as "foam cells". Foam cells characterize atherosclerotic lesions. In addition to this possible mechanism of foam cell generation, an increase in the levels of chemically modified LDL particles may also lead to an increase in endothelial damage. This occurs as a result of modified-LDL's toxic effect on vascular endothelium as well its ability both to recruit immune effector cells and to promote platelet activation.

Recently, the INTERHEART study found that the ApoB100 / ApoA1 ratio is more effective at predicting heart attack risk, in patients who had had an acute myocardial infarction, than either the ApoB100 or ApoA1 measure alone. In the general population this remains unclear although in a recent study apoB was the strongest risk marker for cardiovascular events. A small study suggests that added to fluvastatatin treatment, omega 3 fatty acids daily, containing 460 mg of E-EPA and 380 mg of E-DHA (ethyl esters), may lower apo B48 in hyperlipemic type 2 diabetics.