Phosphatidylethanolamines Modified by γ-Ketoaldehyde (γKA) Induce Endoplasmic Reticulum Stress and Endothelial Activation

Abstract

Peroxidation of plasma lipoproteins has been implicated in the endothelial cell activation and monocyte adhesion that initiate atherosclerosis, but the exact mechanisms underlying this activation remain unclear. Lipid peroxidation generates lipid aldehydes, including the ␥-ketoaldehydes (␥KA), also termed isoketals or isolevuglandins, that readily modify the amine headgroup of phosphatidylethanolamine (PE). We hypothesized that aldehyde modification of PE could mediate some of the proinflammatory effects of lipid peroxidation. We found that PE modified by ␥KA (␥KA-PE) induced THP-1 monocyte adhesion to human umbilical cord endothelial cells. ␥KA-PE also induced expression of adhesion molecules and increased MCP-1 and IL-8 mRNA in human umbilical cord endothelial cells. To determine the structural requirements for ␥KA-PE activity, we tested several related compounds. PE modified by 4-oxo-pentanal induced THP-1 adhesion, but N-glutaroyl-PE and C 18:0 N-acyl-PE did not, suggesting that an N-pyrrole moiety was essential for cellular activity. As the N-pyrrole headgroup might distort the membrane, we tested the effect of the pyrrolePEs on membrane parameters. ␥KA-PE and 4-oxo-pentanal significantly reduced the temperature for the liquid crystalline to hexagonal phase transition in artificial bilayers, suggesting that these pyrrole-PE markedly altered membrane curvature. Additionally, fluorescently labeled ␥KA-PE rapidly internalized to the endoplasmic reticulum (ER); ␥KA-PE induced C/EBP homologous protein CHOP and BiP expression and p38 MAPK activity, and inhibitors of ER stress reduced ␥KA-PE-induced C/EBP homologous protein CHOP and BiP expression as well as EC activation, consistent with ␥KA-PE inducing ER stress responses that have been previously linked to inflammatory chemokine expression. Thus, ␥KA-PE is a potential mediator of the inflammation induced by lipid peroxidation.Lipid peroxidation has been implicated in a host of pathological processes, including inflammation and atherosclerosis, but there is still much that is unknown about the mechanisms linking lipid peroxidation to the activation of inflammatory responses. Lipid peroxidation produces a plethora of lipid aldehydes ( Fig. 1), including malondialdehyde, acrolein, and 4-hydroxynonenal (HNE).3 It also produces a large family of ␥-ketoaldehydes (␥KA), regioisomers that have been given the trivial name of isoketals or isolevuglandins. Exposure of vascular cells to various aldehydes results in endothelial dysfunction, secretion of cytokines, and recruitment of monocytes (1-7), all of which are key steps in the initiation of the chronic inflammatory conditions leading to atherosclerosis.Despite these potentially important inflammatory effects, the mechanisms whereby lipid aldehydes induce their effects on vascular cells are still poorly understood. Significant effort has focused on the effects of protein and DNA modification by lipid aldehydes. However, recent studies have shown that lipid aldehydes, including malondialdehyde, acrolein, HNE, and ␥KA, a...

Reactive gamma-ketoaldehydes as novel activators of hepatic stellate cells in vitro

“…In addition, previous experimental evidence obtained in in vitro studies has suggested that IsoLG-PE may have a limited incorporation into the cells leading to the need of employing higher doses in order to elicit biological effects [20]. Based on these considerations, we decided to employ a range of concentrations (picomolar or nanomolar concentrations) which are likely to be reached in conditions of sustained oxidative stress in vivo.…”

“…One of the triggers for autophagy is ER stress, and secondary activation of the unfolded protein response (UPR) [47,48]. Indeed activation of the unfolded protein response has previously been described in HUVEC cells following exposure to IsoLG [20].…”

“…IsoLGs have also been shown to form adducts with nucleic acids [19] and lipids, such as phosphatidyl-ethanolamine, resulting in the induction of endoplasmic reticulum stress, and mitochondrial dysfunction [20,21]. Other cellular consequences of IsoLGs exposure include increased expression of pro-inflammatory cytokines and adhesion molecules [20], and cell death [18,22].…”

“…In other cell types, IsoLGs exposure promotes a proinflammatory phenotype, via upregulation of adhesion molecules, chemokines/cytokines, and transcription factors [20,27,30]. Therefore, we investigated if incubation of HSCs with IsoLGs had similar effects by measuring mRNA levels of multiple pro-inflammatory mediators [37].…”

“…Other cellular consequences of IsoLGs exposure include increased expression of pro-inflammatory cytokines and adhesion molecules [20], and cell death [18,22].…”