Anti-MDA-LDL monoclonal antibody, clone 23F8

Oxidised low-density lipoprotein refers to substantial content of unsaturated fatty acids in low-density lipoprotein (LDL) which is under the action of excessive free radicals or other oxygen-generating factors, leading to peroxidation and producing MDA. MDA can bind to apolipoprotein B on the surface of LDL. Studies have found that LDL is a spherical lipoprotein particle that is responsible for transporting cholesterol to peripheral tissue cells. Further research results showed that the diameter of LDL is 22nm, and the core of its particles contains 1,500 cholesterol esters, 800 phospholipids and 500 free cholesterol molecules. Under normal physiological conditions, most LDL in plasma is recognized by LDL receptors, combined with cells, and metabolized in lysosomes. When lipid metabolism is abnormal, LDL increases and receptors are relatively reduced, resulting in LDL aggregation under the endothelium. Under some inducements, such as smoking, diabetes, hypertension, etc., the body produces a large number of oxygen free radicals (O), and the LDL aggregated under the endothelium undergoes peroxidation and further generates oxLDL. It is divided into two steps. Firstly, LDL is slightly modified and oxidized. The fatty acid that leads to the oxidation of LDL further undergoes more oxidation reactions to produce a series of lipid peroxides, including important biomarkers of cardiovascular risk such as malondialdehyde (MDA) and 4-hydroxy enoic acid (4-HNE), which can bind with the benzene ring of apoB to form new antigenic determinants, and the formation of these new antigenic determinants are the compounds that eventually induce inflammation. This is the minimally modified LDL (MM-LDL), which is modified at the first step, and it can still be recognised by the LDL receptor. In the second step, the MM-LDL is completely oxidised to oxLDL, and MDA and 4-HNE are decomposed from the lipid peroxides and then bound to apoB to form new antigenic determinants, which is different from normal metabolic pathways of LDL, so that they can then bind to the scavenger receptors on macrophages and smooth muscle cells. The main product of complete oxidation is the lysophosphatidylcholine (LPC). Among them, LPC is the main component with pathogenicity.

Figure 1. Sites of modification in LDL apoB-100-containing lipoproteins. (Sources: Lorey MB, et al. 2022)

Atherosclerosis (AS) is a chronic systemic disease characterized by disorder of lipid metabolism, vascular endothelial injury, deposition of lipids in blood vessel wall, proliferation and acceleration of mononuclear macrophage and formation of atherosclerotic plaques. As the leading cause of death and disability worldwide, its polygenic etiology and complex pathogenesis make the disease difficult to treat. Among numerous factors, oxLDL plays a pivotal role in the pathogenesis of AS. The pathological stages of AS include lipid spots, lipid streaks, pre-plaque stage, atherosclerotic plaque stage, fibrous plaque stage, and complex lesion stage. oxLDL plays an important role in different stages of AS formation. AS begins with endothelial dysfunction and foam cell formation. On the one hand, oxLDL directly damages endothelial cells through cytotoxic effects, resulting in increased endothelial cell gaps, increased vascular endothelial permeability, and promoted lipid deposition in the subendothelial layer. On the other hand, oxLDL induces endothelial cell apoptosis and necrosis by activating the caspase family (cysteinylaspartate-specific proteinase, caspase) or inhibiting autophagy, further increasing vascular permeability. Secondly, oxLDL binds to the lectin-like oxidized low density lipoprotein receptor 1 (LOX-1) to produce superoxide anions, which reduces the production of nitric oxide, further aggravates endothelial cell damage, and leads to endothelial dysfunction. In the process of foam cell formation, oxLDL stimulates vascular endothelial cells to express chemokines, induces monocytes to adhere to the vascular endothelium, and chemotaxis to the subintimal layer, followed by differentiation into macrophages. In addition, after LDL is oxidatively modified, the structure of the antigenic determinant changes, and it cannot be recognized by normal LDL receptors, but can be recognized by oxLDL receptors (such as scavenger receptors, Toll-like receptors, etc.), so that it is phagocytosed by macrophages to form foam cells. Foam cells and a small number of smooth muscle cells are focally aggregated under the endothelium to form fatty streaks. The formation of fatty streaks is an early sign of AS. During the formation of fibrous plaques, on the one hand, oxLDL can still activate nuclear factor κB, caspase-9 and caspase-3 to cause endothelial cell apoptosis, leading to further enlargement of the endothelial cell gap and aggravated lipid deposition. On the other hand, oxLDL activates signaling pathways such as mitogen-activated protein kinase, upregulates the expression of genes such as adhesion molecules, inflammatory factors, chemokines and osteopontin, promotes the migration and proliferation of smooth muscle cells and fibroblasts, and enhances the synthesis of collagen, thereby promoting connective tissue hyperplasia and fibrous capsule formation. The hyperplastic connective tissue and a large number of foam cells in the intima constitute fibrous plaques. Finally, oxLDL also plays an important role in promoting the instability and thrombosis of atherosclerotic plaques. On the one hand, the concentration of oxLDL in the plaques in the late stage of AS continues to increase, and high concentrations of oxLDL can induce apoptosis of vascular smooth muscle cells, resulting in decreased plaque stability and easy rupture. In addition, high concentrations of oxLDL can also upregulate the expression of LOX-1 receptors, causing increased activity of proteolytic enzymes such as matrix metalloproteinases in plaques, leading to degradation of the extracellular matrix of the fibrous cap, thinning of the plaque fibrous cap, and accelerated plaque rupture. On the other hand, oxLDL activates platelets and promotes platelet aggregation by binding to CD36 receptors, and reduces fibrinolysis by upregulating the expression of plasminogen activator inhibitor-1, further promoting thrombosis. Fibrous plaques develop complex lesions of hemorrhage, necrosis, ulceration, calcification, and mural thrombosis.

Oxidized LDL (oxLDL)
MDA-modified LDL
Lipid peroxidation LDL