Atherogenesis on the chopping block

Ethanol extract of propolis protects macrophages from oxidized low density lipoprotein-induced apoptosis by inhibiting CD36 expression and endoplasmic reticulum stress-C/EBP homologous protein pathway

Background

Ethanol extract of propolis (EEP), rich in flavones, has been known for various biological activities including antioxidant, antiinflammatory and antibiotic activities. Our previous studies have shown that EEP protects endothelial cells from oxidized low-density lipoprotein (ox-LDL)-induced apoptosis and inhibits atherosclerotic lesion development. In this present study, we explored the protective effect of EEP on ox-LDL-induced cytotoxicity in macrophages and specifically the endoplasmic reticulum (ER) stress-C/EBP homologous protein (CHOP) pathway-mediated apoptosis.

Methods

EEP was prepared and the total flavonoids content of EEP was determined by the colorimetric method of Chinese Standard (GB/T 20574-2006). The effects of EEP on lipid accumulation, cytotoxicity and apoptosis in RAW264.7 cells induced by ox-LDL or tunicamycin (TM, an ER stress inducer) were assayed using oil red O staining, MTT assay, flow cytometric analysis and so on. Immunofluorescence, Western blot and real time-PCR analysis were then used to further investigate the molecular mechanisms by which EEP protects macrophages from ox-LDL-induced apoptosis. 4-phenylbutyric acid (PBA), an ER stress inhibitor, was used as a positive control.

Results

EEP (7.5, 15 and 30 mg/L) not only attenuated ox-LDL-induced lipid accumulation in RAW264.7 macrophages in a dose-dependent manner but also inhibited the decreased cell viability and the increased lactate dehydrogenase (LDH) leakage, caspase-3 activation and apoptosis induced by ox-LDL or tunicamycin (TM, a classical ER stress inducer), which were similar to 4-phenylbutyric acid (PBA, an inhibitor of ER stress) treatment. In addition, like PBA, EEP significantly suppressed the ox-LDL- or TM-induced activation of ER stress signaling pathway including the phosphorylation of double-stranded RNA-activated protein kinase-like ER kinase (PERK) and eukaryotic translation initiation factor 2α (eIF2α) as well as upregulation of glucose regulated protein 78 (GRP78) and the pro-apoptotic protein CHOP. Furthermore, EEP significantly suppressed ox-LDL intake by macrophages and the upregulation of CD36 induced by ox-LDL.

Conclusion

These data indicate that EEP may protect macrophages from ox-LDL-induced apoptosis and the mechanism at least partially involves its ability to suppress the CD36-mediated ox-LDL intake and subsequent activation of ER stress-CHOP signalling pathway.

Quercetin protects macrophages from oxidized low-density lipoprotein-induced apoptosis by inhibiting the endoplasmic reticulum stress-C/EBP homologous protein pathway

Quercetin (QUE), a member of the bioflavonoid family, has been proposed to have antioxidative, anti-inflammatory and antihypertensive properties. This study was designed to investigate the protective effect of QUE on oxidized low-density lipoprotein (ox-LDL)-induced cytotoxicity in RAW264.7 macrophages and specifically the endoplasmic reticulum (ER) stress-C/EBP homologous protein (CHOP) pathway-mediated apoptosis. Our results showed that treatment with QUE (20, 40 and 80 μmol/L) significantly attenuated ox-LDL-induced cholesterol accumulation in macrophages and foam cell formation in a dose-dependent manner. Similar to tunicamycin (TM), a classical ER stress inducer, ox-LDL reduced cell viability and induced apoptosis in RAW264.7 macrophages. The cytotoxic effects of ox-LDL and TM were significantly inhibited by QUE treatment. Interestingly, we found that QUE also significantly suppressed the ox-LDL- and TM-induced activation of ER stress signaling events, including the phosphorylation of inositol-requiring enzyme 1 (IRE1), translocation of activating transcription factor 6 (ATF6) from the cytoplasm to the nucleus and upregulation of X-box-binding protein 1. In addition, exposure of RAW264.7 macrophages to ox-LDL or TM resulted in a significant increase in the expression of CHOP, a transcription factor regulated by IRE1 and ATF6 under conditions of ER stress, as well as a decrease in Bcl-2 transcript and protein concentrations. QUE blocked these effects in a dose-dependent manner. These data indicate that QUE can protect RAW264.7 cells from ox-LDL-induced apoptosis and that the mechanism at least partially involves its ability to inhibit the ER stress-CHOP signaling pathway.

A reporter for tracking the UPR in vivo reveals patterns of temporal and cellular stress during atherosclerotic progression

Progression of human arteriosclerosis is associated with and promoted by induction of the endoplasmic reticulum (ER) stress pathway known as the unfolded protein response (UPR). Most studies that assess UPR markers in atherosclerosis rely on methodologies that suffer from low signal sensitivity, nonspecific immunohistochemistry, or inability to resolve differences between cellular subsets. To accurately monitor the UPR independently of artifacts generated postmortem, we describe here the first in vivo reporter for ER stress during atherosclerosis. Mice transgenic for the fluorescent XBP-1 ER stress indicator Erai were bred onto the Ldlr(-/-) background and fed an atherogenic diet. Subsequently, ERAI fluorescence at aortic roots was quantified and colocalized with lesional cell type. We found that the ERAI fluorescent signal increased as a function of time on the atherogenic diet and, in advanced lesions, was found close to necrotic cores. The majority of ERAI fluorescence localized to macrophages, and to a lesser extent, to intimal smooth muscle cells and patches of endothelial cells. These mice provide a valuable tool to monitor activation of the UPR in atherosclerosis and will be useful for future studies investigating relationships between pharmacologic and genetic modulators of UPR and atherosclerosis.