Oxidized
low-density lipoproteins (oxLDLs) trigger various biological responses potentially involved in
atherogenesis. Disturbing endoplasmic reticulum (ER) function results in ER stress and unfolded protein response, which tends to restore ER homeostasis but switches to apoptosis when ER stress is prolonged. We aimed to investigate whether ER stress is induced by oxLDLs and can be prevented by the ER-associated chaperone ORP150 (150-kDa oxygen-regulated protein). oxLDLs and the
lipid oxidation products
7-ketocholesterol and
4-hydroxynonenal induce ER stress in human endothelial cells (HMEC-1), characterized by the activation of ER stress sensors (phosphorylation of Ire1alpha and PERK, nuclear translocation of ATF6) and of their subsequent pathways (eukaryotic
initiation factor 2alpha phosphorylation, expression of XBP1/spliced XBP1, CHOP, and KDEL chaperones
GRP78,
GRP94, ORP150). ER stress was inhibited by the
antioxidant N-acetylcysteine. In advanced atherosclerotic lesions, phospho-Ire1alpha, KDEL, and ORP150 staining were localized in
lipid-rich areas with
4-hydroxynonenal adducts and CD68-positive macrophagic cells. By comparison, staining for
4-hydroxynonenal, phospho-Ire1alpha, KDEL, and ORP were faint and more diffuse in intimal
hyperplasia. ER stress takes part in the apoptotic effect of oxLDLs, through the Ire1alpha/
c-Jun N-terminal kinase pathway, as assessed by the protective effect of specific small interfering RNAs and
c-Jun N-terminal kinase inhibitor. Forced expression of the chaperone ORP150 reduced both
oxLDL-induced ER stress and apoptosis. ER stress markers and ORP150 chaperone are expressed in areas containing oxLDLs in atherosclerotic lesions and are induced by oxLDLs and oxidized
lipids in cultured cells. The forced expression of ORP150 highlights its new protective role against
oxLDL-induced ER stress and subsequent apoptosis.