Modified Low-Density Lipoproteins and High-Density Lipoproteins

It has long been known that the oxidative state of the various plasma lipoproteins modulates platelet aggregability, thereby contributing to atherogenesis. Low-density lipoprotein (LDL), occurring in vivo both in the native and oxidised forms, interacts directly with platelets, by binding to specific receptors. While the identity of the receptors for native LDL and some subfractions of high-density lipoproteins (HDL) remains disputed, apoE-containing HDL₂ binds to LRP8. The nature of these interactions as well as the distinction between candidate receptor proteins was elucidated using covalently modified apolipoproteins, which pointed to the participation of apolipoproteins in high affinity binding. However, the platelet effects initiated by binding of native lipoproteins remain controversial. Some of this ambiguity can be traced to the fact that native LDL inevitably undergoes substantial oxidisation upon modification, including by radiolabelling. The platelet-activating effects provoked by oxidised LDL are irrefutable, but many details remain unknown. The role of CD36 in platelet binding by oxidised LDL is well established, although additional receptors may exist. Much less is known about the interaction of oxidised HDL with platelets, since platelet activation was observed in some, but not all studies. Various frequently applied in vitro oxidation methods produce modified lipoprotein species that may not be relevant in vivo. Based on the reported modifications obtained by in vitro oxidation of LDL, early investigations focused mainly on the formation and the eventual effects of oxidised lipids. More recently, alterations to lipoproteins performed using hypochloric acid and myeloperoxidase redirected the attention to the role of modified apoproteins in triggering platelet responses.