What is apolipoprotein B?
Apolipoproteins are proteins that carry lipids in our blood. Apolipoprotein B (ApoB) is insoluble, which means that it cannot exist freely in the plasma. It is always part of a lipoprotein particle. These particles are important for the transportation of “bad” lipids between various tissues. The role of ApoB molecules is to maintain the structure of the atherogenic (artery-clogging) lipoproteins, to enable the transport of lipids in the plasma, and to control lipoprotein metabolism. Every atherogenic particle contains one molecule of apoB, so concentrations of apoB are a direct measure of the total number of lipoproteins causing atherosclerosis (plaque-filled artery). The measurement of apoB is an estimate of low-density lipoprotein (LDL) particle number because more than 90% of ApoB in plasma is associated with LDL.
What is LDL?
The higher the number of LDL particles, the higher the risk of atherosclerosis. If there are high levels of LDL particles in the blood over a long period of time, they may cross the arterial wall in greater amounts than usual. In this way, the LDL particles are exposed to changes, such as oxidation. These oxidized LDL particles induce a local inflammatory and immune response. It leads to the proliferation of the smooth muscle cells in the arterial wall and the formation of the fibrous cap of the atherosclerotic plaque. The plaque is unstable and can rupture easily, which may lead to vascular stenosis. It means that the lumen of the vein is narrowed, so the blood cannot reach the target organs adequately. If the lumen is totally blocked, it may lead to a stroke.
Artery-clogging lipoproteins can cause the build-up of plaques and consequently limit blood flow. This may lead to heart attacks, strokes, and other cardiovascular diseases.
LDL vs ApoB as Biomarkers
Both ApoB levels and LDL levels are used to predict the risk of atherosclerosis. Usually, they correlate with each other. However, in some individuals with normal or even low LDL concentrations, atherosclerotic events occur. ApoB levels are a far better predictor of atherosclerotic risk than LDL concentrations because the cholesterol content in the particles can vary widely between individuals. Small, dense LDL particles contain less cholesterol than larger ones. It means that the apoB levels are high, but the LDL concentration can be normal or even low. It is common in patients with diabetes and metabolic syndrome. Studies have shown that cardiovascular risk is more closely associated with the number of particles than with the mass of cholesterol. Taking this into account, ApoB is widely regarded as a more accurate measurement for identifying individuals with an increased risk of atherosclerosis than LDL.
Sources
Mehta, A., & Shapiro, M.D. (2022). Apolipoproteins in vascular biology and atherosclerotic disease. Nature Reviews Cardiology, 19, 168-179. https://doi.org/10.1038/s41569-021-00613-5
Langlois, M.R., & Sniderman, A.D. (2020). Non-HDL Cholesterol or apoB: Which to Prefer as a Target for the Prevention of Atherosclerotic Cardiovascular Disease?. Current Cardiology Reports, 22, 67. https://doi.org/10.1007/s11886-020-01323-z
