Abdominal aortic aneurysm (AAA) is a leading cause of sudden death in aged people. Activation of ubiquitin proteasome system (UPS) plays a critical role in the protein quality control and various diseases. However, the functional role of UPS in AAA formation remains unclear. In this study, we found that the proteasome activities and subunit expressions in AAA tissues from human and angiotensin II (Ang II)-infused apolipoprotein E knockout (Apo E-/-) mice were significantly increased. To investigate the effect of proteasome activation on the AAA formation, Apo E-/- mice were cotreated with bortezomib (BTZ) (a proteasome inhibitor, 50 mu g/kg, 2 times per week) and Ang II (1000 ng/kg/min) up to 28 days. Ang II infusion significantly increased the incidence and severity of AAA in Apo E-/- mice, whereas BTZ treatment markedly inhibited proteasome activities and prevented AAA formation. Furthermore, BTZ treatment significantly reduced the inflammation, inhibited the metal matrix metalloprotease activity, and reversed the phenotypic SMC modulation in AAA tissue. In conclusion, these results provide a new evidence that proteasome activation plays a critical role in AAA formation through multiple mechanisms, and suggest that BTZ might be a novel therapeutic target for treatment of AAA formation.