Objective—To elucidate whether and how the endoplasmic reticulum (ER) stress-C/EBP homologous protein (CHOP) pathway in macrophages is involved in the rupture of atherosclerotic plaques.

Methods and Results—Increases in macrophage-derived foam cell death in coronary atherosclerotic plaques cause the plaque to become vulnerable, thus resulting in acute coronary syndrome. The ER stress–CHOP/growth arrest and DNA damage-inducible gene-153 (GADD153) pathway is induced in the macrophage-derived cells in atherosclerotic lesions and is involved in plaque formation. However, the role of CHOP in the final stage of atherosclerosis has not been fully elucidated. Many CHOP-expressing macrophages showed apoptosis in advanced ruptured atherosclerotic lesions in wild-type mice, whereas few apoptotic cells were observed in Chop^−/− mice. The rupture of atherosclerotic plaques was significantly reduced in high cholesterol–fed Chop^−/−/Apoe^−/− mice compared with Chop^+/+/Apoe^−/− mice. Furthermore, using mice that underwent bone marrow transplantation, we showed that expression of CHOP in macrophages significantly contributes to the formation of ruptures. By using primary cultured macrophages, we further showed that unesterified free cholesterol derived from incorporated denatured low-density lipoprotein was accumulated in the ER and induced ER stress-mediated apoptosis in a CHOP-Bcl2-associated X protein (Bax) pathway-dependent manner.

Conclusion—The ER stress-CHOP-Bax–mediated apoptosis in macrophages contributes to the instability of atherosclerotic plaques.