Oxidized high density lipoprotein induces macrophage apoptosis via toll-like receptor 4-dependent CHOP pathway

Abstract

This article is available online at http://www.jlr.org Atherosclerosis, one of the prime causes of heart attack and stroke, has been well known as a lipid-driven inflammatory disease characterized by macrophage accumulation in subendothelial space. Apoptosis of macrophage-derived foam cells, especially in advanced atherosclerotic lesions where phagocytic clearance of dead and dying cells is defective, results in acellular necrotic lesions that are prone to plaque rupture and thrombosis formation, leading to the majority of acute vascular events (1). Thus, deeper understanding of the mechanisms of macrophage apoptosis may pave the way for developing novel therapeutic strategies to effectively prevent atherosclerotic plaque rupture and subsequent clinical complications (2). Oxidized LDL (ox-LDL) has been recognized as a major inducer, promoting foam cell formation and apoptosis in macrophages (3). In contrast, HDL exerts antiatherogenic functions such as promoting reverse cholesterol transport, suppressing inflammation, inhibiting LDL oxidation, and increasing endothelial nitric oxide production (4). However, accumulating evidence suggests that HDL, like LDL, is also susceptible to oxidative modification. Oxidized HDL Abstract Oxidized HDL (ox-HDL), unlike native HDL that exerts antiatherogenic effects, plays a proatherogenic role. However, the underlying mechanisms are not completely understood. This study was designed to explore the inductive effect of ox-HDL on endoplasmic reticulum (ER) stress-CCAAT-enhancer-binding protein homologous protein (CHOP)-mediated macrophage apoptosis and its upstream mechanisms. Our results showed that ox-HDL could be ingested by macrophages, causing intracellular lipid accumulation. As with tunicamycin (an ER stress inducer), ox-HDL induced macrophage apoptosis with concomitant activation of ER stress pathway, including nuclear translocation of activating transcription factor 6, phosphorylation of protein kinase-like ER kinase and eukaryotic translation initiation factor 2, and upregulation of glucose-regulated protein 78 and CHOP, which were inhibited by 4-phenylbutyric acid (PBA, an ER stress inhibitor) and CHOP gene silencing. Additionally, diphenyleneiodonium (DPI, an oxidative stress inhibitor), probucol (a reactive oxygen species scavenger), and toll-like receptor 4 (TLR4) silencing reduced ox-HDL-induced macrophage apoptosis, oxidative stress, and CHOP upregulation. Moreover, HDL isolated from patients with metabolic syndrome induced macrophage apoptosis, oxidative stress, and CHOP upregulation, which were blocked by PBA and DPI. These data indicate that ox-HDL may activate ER stress-CHOP-induced apoptotic pathway in macrophages via enhanced oxidative stress and that this pathway may be mediated by TLR4.

High Mobility Group B-1 (HMGB-1) Promotes Apoptosis of Macrophage-Derived Foam Cells by Inducing Endoplasmic Reticulum Stress

“…These data provided additional evidence to the hypothesis that HMGB-1 may promote oxLDL-induced foam cell apoptosis via ERS. CHOP is a well-known apoptosisrelated signaling protein in macrophage-derived foam cells, contributing to the instability of atherosclerosis plaques [2,36]. In the current study, Western blot analysis showed that pretreatment with 4-PBA attenuated the HMGB-1-induced activation of ERS/CHOP signaling Cellular Physiology and Biochemistry Cellular Physiology and Biochemistry pathway, which was accompanied by a similar trend with the incidence of macrophage apoptosis.…”

4‐Phenylbutyrate protects against atherosclerotic lesion growth by increasing the expression of HSP25 in macrophages and in the circulation of Apoe −/− mice

“…Here, we report the novel findings that: showed a significant reduction in atherosclerotic lesion growth in 17-wk-old Apoe 2/2 mice fed a chow diet, 2) 4-PBA treatment increased expression of HSP25 and HSP27 in peritoneal macrophages isolated from Apoe 2/2 mice and in THP-1-derived macrophages, 3) 4-PBA treatment and reduced lesion size were associated with increased plasma levels of HSP25 in Apoe 2/2 mice, 4) 4-PBA treatment induced nuclear localization of HSF1, a transcription factor for HSP27, in THP-1 derived macrophages, and 5) both HSP27 and HSF1 siRNA treatments inhibited the protective effect of 4-PBA on cell attachment. Previous studies have demonstrated that ER stress contributes to atherogenesis and plaque progression (8)(9)(10)(34)(35)(36)(37)(38)(39). In support of these findings, hematopoietic cells deficient in microRNA 21 promoted CHOP expression, apoptosis, and lesion growth in LDLR 2/2 mice fed a Western diet for 12 wk (40).…”

“…In support of these findings, hematopoietic cells deficient in microRNA 21 promoted CHOP expression, apoptosis, and lesion growth in LDLR 2/2 mice fed a Western diet for 12 wk (40). Alternatively, inhibiting ER stress has been shown to decrease apoptosis and cell death in multiple cell types that modulate lesion growth, including endothelial cells (35), smooth muscle cells (36,37), and macrophages (38,39). 4-PBA treatment inhibited both oxidized HDL-induced expression of ER stress markers such as GRP78, CHOP, and P-PERK and apoptosis in macrophages (39).…”

“…Alternatively, inhibiting ER stress has been shown to decrease apoptosis and cell death in multiple cell types that modulate lesion growth, including endothelial cells (35), smooth muscle cells (36,37), and macrophages (38,39). 4-PBA treatment inhibited both oxidized HDL-induced expression of ER stress markers such as GRP78, CHOP, and P-PERK and apoptosis in macrophages (39). Furthermore, overexpression of the adipokine vaspin inhibited both the expression of proapoptotic CHOP and progression of atherosclerotic plaques in Apoe 2/2 mice fed a high-cholesterol diet; this protective effect against lesion growth was associated with a reduction in ER stress-induced macrophage apoptosis (38).…”

Endoplasmic reticulum stress‐dependent autophagy inhibits glycated high‐density lipoprotein‐induced macrophage apoptosis by inhibiting CHOP pathway

“…35 Furthermore, oxidized HDL may become pro-atherogenic by promoting ROS production, increasing proinflammatory factor expression, 36 inducing the proliferation and migration of vascular smooth muscle cells, 37 and causing apoptosis in endothelial cells 38 and macrophages. 16 In addition to oxidative modification, HDL can also be glycated in diabetes and the biological properties of HDL are altered including the reduced paraoxonase activity and anti-inflammatory function, and the decreased capacity to mediate reverse cholesterol transport. 5,6,39 Moreover, gly-HDL has been shown to induce endothelial apoptosis, 7 and cause cellular senescence and foam cell formation.…”