| The anti-atherogenic role of high-density lipoprotein (HDL) is believed to include protection of LDL against oxidation, removal of cholesterol from peripheral cells and prevention of activation of the endothelium. Maintenance of this atheroprotective role depends on the integrity of structure and function, which is dependent on HDL lipids, apolipoproteins, and associated enzymes. The antioxidant function of HDL has been attributed mainly to apolipoprotein AI (apoAI) and paraoxonase-1 (PON-1); however, this has not been critically tested with peroxynitrite, a strong oxidant.;I hypothesized (1) that, like LDL, HDL undergoes exposure to oxidation in the vessel wall, increasing the formation of oxo-phospholipids that activate the endothelium, modify apoAI and promote uptake of oxidized HDL by THP-1 macrophages; and (2) these effects are minimized by PON-1.;This thesis demonstrated significant increases in the accumulation of hydroperoxides, isoprostanes, core aldehyde derivatives of PC, and lysoPC following incubation of native HDL with SIN-1, compared to HDL lipids or HDL treated with trypsin. Unexpectedly, apoAI promoted the formation of PC core aldehydes during oxidation of PC proteoliposomes. Oxidation of apoAI proteoliposomes, in the presence of purified PON-1, decreased PC hydroperoxides, minimized accumulation of PC core aldehydes and PC isoprostanes, as well as increased lysoPC. Incubation of oxidized HDL with human umbilical vein endothelial cells (HUVECs) increased THP-1 macrophage binding to HUVECs and enhanced THP-1 macrophage chemotaxis. This was shown to be due to PC core aldehydes derived from linoleoyl and arachidonoyl glycerophosphocholine. PON-1 inhibited the activation of HUVECs by oxidized PCs by hydrolysis of PC core aldehydes to IysoPC.;It is concluded that oxidation of HDL increased the formation of PC core aldehydes and their binding to apoAI via Schiff base adducts. ApoAI-Schiff base adducts enhanced binding and uptake of oxidized HDL by THP-1 macrophages.;It is proposed that apoAI and PON-1 serve as a coupled system that normally functions as a protective system by hydrolyzing oxo-PCs to lysoPC, minimizing the accumulation of harmful bioactive products of PC oxidation. Overexposure of HDL to peroxynitrite replicates pro-atherogenic events previously ascribed to oxidation of LDL. (Abstract shortened by UMI.). |
