N -aldehyde modified phosphatidylethanolamines formed by lipid peroxidation inhibit efferocytosis
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Background
Impairments in efferocytosis, the phagocytic clearance of apoptotic cells, contribute to the development of chronic inflammation and promote the formation of large necrotic cores within atherosclerotic plaques. A better understanding of the factors that impair efferocytosis could lead to improved interventions. Many chronic inflammatory conditions feature excessive lipid peroxidation, which generates reactive lipid aldehydes that can potentially form N -aldehyde-modified phosphatidylethanolamines (NAMPs). We therefore investigated the extent to which levels of NAMPs are elevated under conditions of oxidative stress and whether exposure of macrophages to NAMPs inhibit efferocytosis.
Methods
The effects of synthetic NAMPs and lipid peroxidation on efferocytosis were measured using the Incucyte Live-Cell Analysis System to monitor rate of ingestion of CypHer-labeled apoptotic cells by cultured bone marrow-derived macrophages in real time. The extent of NAMP formation under various conditions was examined using liquid chromatography coupled with mass spectrometry.
Results
Several representative species of NAMPs ( N -isolevuglandin-phosphatidylethanolamine, N -IsoLG-PE; N -4-hydroxynonenal-phosphatidylethanolamine, N -HNE-PE; and N -azeloyl-phosphatidylethanolamine, N-Aze-PE) inhibited efferocytosis in a concentration-dependent manner. Hydrolysis of NAMPs by recombinant N -acyl phosphatidylethanolamine hydrolyzing phospholipase D (NAPE-PLD) abolished their inhibitory activity, whereas genetic deletion of macrophage NAPE-PLD enhanced NAMP-mediated inhibition of efferocytosis. Blocking the inhibitor of lipid peroxidation, glutathione peroxidase 4 (GPX4), increased macrophage NAMP concentrations and impaired efferocytosis. Unmodified high-density lipoprotein (HDL) promoted efferocytosis, but exposing HDL to reactive lipid aldehydes or peroxidizing agents markedly increased NAMPs and this modified HDL inhibited efferocytosis. HDL isolated from subjects with familial hypercholesterolemia had elevated levels of multiple NAMP species. NAMPs impaired macrophage cholesterol efflux, which is required for continual efferocytosis.
Conclusions
NAMPs accumulate under conditions associated with lipid peroxidation and inhibit the ability of macrophages to carry out efferocytosis.
What are the clinical implications of this study?
Our finding that NAMPs inhibit efferocytosis by macrophages suggests that interventions that reduce NAMP formation such as lipid aldehyde scavenging or increasing NAPE-PLD expression or activity to enhance NAMP degradation represent novel therapeutic strategies to restore efferocytosis in atherosclerosis and other chronic inflammatory diseases.
