Modification Of Hdl By Reactive Aldehydes Impairs Hdl'S Athero-Protective Functions

High density lipoproteins (HDL) are athero-protective particles that promote the removal of excess cholesterol from lipid-loaded macrophages and stimulate their migration in order to protect against foam cell formation, a precursor to atherosclerotic plaque build-up. Recently, studies have shown that oxidative modification of HDL prevents HDL from protecting against atherosclerosis; however, the exact mechanisms by which this occurs are not well defined. We hypothesize that oxidative modification of HDL by reactive aldehydes such as acrolein (a major component of cigarette smoke) and 4-hydroxynonenal (HNE; a product of lipid peroxidation) impairs HDL’s athero-protective effects in macrophages. We tested our hypothesis using three different assays. First, we determined that modified forms of HDL upregulate mRNA levels of pro-atherogenic scavenger receptors such as cluster of differentiation 36 (CD36), a known oxidized LDL receptor. Incubation of macrophages with native HDL did not exert similar effects. Second, we tested the ability of oxidized HDL to prevent foam cell formation. Peritoneal macrophages isolated from WT C57Bl/J mice were cholesterol-loaded and incubated with native HDL, acrolein-modified HDL (acro-HDL), or HNE-modified HDL (HNE-HDL). Oil Red-O staining demonstrated that 24% of macrophages had foam cell formation upon incubation with native HDL, whereas 61% and 49% foam cell formation was observed for acro- and HNE-HDL, respectively. Preliminary data suggests this may be CD36-dependent. Finally, using a Boyden chamber assay, we demonstrated that both acro- and HNE-HDL, but not native HDL, had an impaired ability to promote macrophage migration (43% and 72% of HDL cell migration levels, respectively). We determined that the inability of acro- and/or HNE-HDL to stimulate macrophage migration may be due to an impaired ability of these modified lipoproteins to activate the PI3K pathway, as shown by decreased levels of phosphorylated protein kinase B (Akt). In conclusion, we have identified three independent mechanisms by which modification of HDL with acrolein or HNE impairs HDL’s cardio-protective effects and, instead, generates a particle that promotes pathways that lead to atherosclerosis.