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

This study was designed to explore the inductive effect of glycated high-density lipoprotein (gly-HDL) on endoplasmic reticulum (ER) stress-C/EBP homologous protein (CHOP)-mediated macrophage apoptosis and its relationship with autophagy. Our results showed that gly-HDL caused macrophage apoptosis with concomitant activation of ER stress pathway, including nuclear translocation of activating transcription factor 6, phosphorylation of protein kinase-like ER kinase (PERK) and eukaryotic translation initiation factor 2α, and CHOP up-regulation, which were inhibited by 4-phenylbutyric acid (PBA, an ER stress inhibitor) and the gene silencing of PERK and CHOP. Similar data were obtained from macrophages treated by HDL isolated from diabetic patients. Gly-HDL induced macrophage autophagy as assessed by up-regulation of beclin-1, autophagy-related gene 5 and microtubule-associated protein one light chain 3-II, which were depressed by PBA and PERK siRNA. Gly-HDL-induced apoptosis, PERK phosphorylation and CHOP up-regulation were suppressed by rapamycin (an autophagy inducer), whereas aggravated by 3-methyladenine (an autophagy inhibitor) and beclin-1 siRNA. Administration of diabetic apoE-/- mice with rapamycin attenuated MOMA-2 and CHOP up-regulation and apoptosis in atherosclerotic lesions. These data indicate that gly-HDL may induce macrophage apoptosis through activating ER stress-CHOP pathway and ER stress mediates gly-HDL-induced autophagy, which in turn protects macrophages against apoptosis by alleviating CHOP pathway.

[Activating transcription factor 6-C/EBP homologous protein pathway mediates advanced glycated albumin-induced macrophage apoptosis].

The purpose of this study was to investigate whether activating transcription factor 6 (ATF6), a sensor to endoplasmic reticulum stress (ERS), would mediate advanced glycated albumin (AGE-alb)-induced macrophage apoptosis and to elucidate the possible molecular mechanisms. RAW264.7 macrophages were cultured in vitro and treated with AGE-alb (2, 4 and 6 g/L), normal control albumin or tunicamycin (TM, 4 mg/L) for 24 h. ATF6 small interfering RNA (siRNA) was transfected to RAW264.7 cells by Lipofectamine 2000. Cell viability and apoptosis were determined by MTT method and Annexin V-FITC/propidium iodide apoptosis detection kit, respectively. The activities of lactate dehydrogenase (LDH) in medium and caspase-3 in cells were measured by corresponding detection kits. ATF6 nuclear translocation was detected by Western blot and immunofluorescence cytochemistry. Protein and mRNA levels of C/EBP homologous protein (CHOP, a key-signaling component of ERS-induced apoptosis) were detected by Western blot and real-time fluorescence quantitative PCR, respectively. The results showed that similar to TM, AGE-alb increased the expression of CHOP at both the protein and mRNA levels in a concentration dependent manner. ATF6, as a factor that positively regulates CHOP expression, was activated by AGE-alb in a concentration dependent manner. siRNA-mediated knockdown of ATF6 significantly inhibited AGE-alb-induced macrophage injury, as indicated by the increased cell viability and the decreased LDH release, apoptosis and caspase-3 activation. Additionally, ATF6 siRNA attenuated AGE-alb-induced CHOP upregulation at both the protein and mRNA levels. These results suggest that ATF6 and its downstream molecule CHOP are involved in AGE-alb-induced macrophage apoptosis.