Negatively charged L5 as a naturally occurring atherogenic low-density lipoprotein

Deranged metabolism of low-density lipoprotein (LDL) is considered the preeminent modifiable risk factor for atherosclerotic disease, and it is widely viewed as a chronic inflammatory disorder. Yet, the search for a circulating atherogenic LDL species continues, as the risk of coronary artery disease (CAD) cannot be measured by absolute LDL cholesterol concentrations in the plasma. Oxidized LDL (oxLDL) and small, dense LDL are associated with CAD, but neither has been retrieved from human plasma for mechanistic scrutiny. Electronegative LDL, a subclass of human plasma LDL, exhibits atherogenic properties in cultured vascular cells. L5, the most negatively charged subfraction of LDL, is an extreme form of electronegative LDL that we isolated through anion-exchange chromatography from the plasma of patients with increased cardiac risk (active smoking, hypercholesterolemia, type 2 diabetes mellitus, and metabolic syndrome). L5, which is scant in healthy normal patients, is as potent as artificially prepared oxLDL in inducing endothelial cell (EC) apoptosis by disrupting fibroblast growth factor 2 autoregulation that involves protein kinase B. Unlike oxLDL, however, L5 is not oxidized. Among subfractions L1-L5, which were separated by our chromatographic method, L1 is the most abundant and least negatively charged. It represents harmless normal LDL. Compared with L1, L5 has a greater content of total protein and triglycerides but a lesser amount of cholesteryl esters. Size exclusion chromatography and equilibrium density gradient ultracentrifugation indicated that L5 is neither smaller nor denser than L1. Negative charge on the particle surface has made L5 unrecognizable by the normal LDL receptor. Instead, L5 signals through and is internalized by lectin-like oxidized LDL receptor-1 (LOX-1), which has high affinity for negatively charged ligands. LOX-1 is also inducible by L5 but not L1. Through LOX-1, L5 disturbs homeostasis between the prosurvival and proapoptotic members of the Bcl-2 family, leading to mitochondrial destabilization. Additionally, it induces overexpression of various adhesion molecules and chemokines, thus promoting monocyte-EC adhesion, an early event during atherosclerosis development. Endothelial progenitor cells (EPCs) are important construction units for vascular repair and endothelial regeneration. Adding to the damage, L5 impairs EPC differentiation from mononuclear cells by inhibiting the induction of needed growth factor receptors. It also accelerates EPC senescence by suppressing the enzymatic activity of telomerase, which is essential for chromosome preservation. Thus, L5 is a naturally occurring, negatively charged but not oxidized LDL entity that is neither smaller nor denser than normal LDL but possesses a capacity for inducing a spectrum of atherogenic responses in vascular cells. Further investigation aimed at establishing its clinical relevance is warranted to confirm its atherogenic role. Subsequent efforts in L5 research will be directed toward the development of new diagnostic and treatment methods for CAD and other ischemic vascular diseases.