The ubiquitin-proteasome complex regulates many functions that are important in the development and progression of atherosclerosis. The aim of this study was to investigate the effect of proteasome inhibition on the composition of the atherosclerotic plaque in ApoE-deficient mice. The susceptibility of smooth muscle cells (SMCs) and peritoneal macrophages for the proapoptotic effects of bortezomib, a proteasome inhibitor, was examined in vitro. Treatment with bortezomib induced cell death of SMCs, but not of macrophages. The sensitivity of SMCs and macrophages for proteasome inhibition correlated with their protein synthesis rate. Bortezomib-induced cell death was associated with several signs of ER stress. Subsequently, ApoE-deficient mice were treated with bortezomib after induction of atherosclerotic plaques by cuff placement around the carotid artery. In vivo proteasome inhibition induced apoptotic cell death of SMCs in advanced atherosclerotic plaques of ApoE-deficient mice, which was associated with a significant decrease in collagen content and a significant enlargement of the necrotic core. Therefore, we conclude that the susceptibility of SMCs and macrophages for the proapoptotic effects of proteasome inhibition is determined by the protein synthesis rate. Proteasome inhibition induced several features of increased plaque instability, indicating that an impaired proteasome function can contribute to plaque destabilization.