Asaronic Acid Inhibits ER Stress and Ameliorates Impaired ERAD in 7Β-Hydroxycholesterol-Loaded Macrophages

Misfolded proteins were formed in the endoplasmic reticulum (ER) due to diverse stresses including metabolic stress and oxidative stress. Accumulation of unfolded proteins in the ER stimulates chaperone expression and ER-associated degradation (ERAD) process. This process involves the recognition of misfolded proteins to maintain the protein quality control, which in turn eliminates in association with the ER membrane. Upregulation of ubiquitination enzymes is an essential mechanism by which ER stress enhances ERAD. Asaronic acid (2,4,5-trimethoxybenzoic acid), identified as one of purple perilla constituents, has anti-diabetic and anti-inflammatory effects. This study attempted to examine whether asaronic acid attenuated the 7Β-hydroxycholesterol-elicited ER stress of macrophages. J774A.1 murine macrophage was incubated with 28 μM 7Β-hydroxycholesterol in absence and presence of 1–20 μΜ asaronic acid up to 24 h. Cytotoxicity was assessed by MTT assay. Expression levels of ER stress-responsive chaperones and ERAD biomarkers were measured by Western blot analysis and immunocytochemical staining with a specific antibody. Asaronic acid at 1–20 μM had a cytoprotective effect on macrophages against 7Β-hydroxycholesterol-induced toxicity. Asaronic acid diminished the induction and activation of ER stress sensors such as Grp/BiP, IRE1, and PERK in macrophages exposed to 7Β-hydroxycholesterol. Also, asaronic acid positively influenced the induction of ERAD process-linked components of EDEM1, OS9, SEl1L, HRD1, and VCP1/p97. Furthermore, asaronic acid promoted subsequent degradation reduced by 7Β-hydroxycholesterol via the cytosolar ubiquitin-proteasome system of macrophages. These results demonstrate that asaronic acid attenuated 7Β-hydroxycholesterol-induced ER stress and improved impaired ER stress-mediated degradation systems. Therefore, asaronic acid may be a potent agent protecting macrophages against pathological ER stress damage. This work was supported by the BK21 FOUR(Fostering Outstanding Universities for Research, 4220200913807) funded by the National Research Foundation of Korea (NRF).