The proteasome is responsible for the degradation of oxidized proteins, and proteasome inhibition has been shown to generate oxidative stress in vitro. Atherosclerosis is thought to be initiated as a consequence of increased endogenous oxidative stress. The current study was designed to assess whether chronic proteasome inhibition is associated with early coronary atherosclerosis. Female pigs, 3 months of age, were randomized to a normal (N) or high-cholesterol (HC) diet (2% cholesterol, 15% lard) without or with twice weekly subcutaneous injections of the proteasome inhibitor (PSI) MLN-273 (0.08 mg/kg, N+PSI and HC+PSI) for a period of 12 weeks (n=5 per group). Coronary vasorelaxation to bradykinin (10(-10.5) to 10(-6.5) mol/L) and sodium nitroprusside (10(-9) to 10(-5) mol/L) was assessed by in vitro organ chamber experiments, intima-media ratio by morphometric analysis of Elastica-van Gieson-stained slides, and intima superoxide production by dihydroethidium fluorescence. Vasorelaxation to 10(-6.5) mol/L bradykinin was reduced in HC compared with N (69+/-7 versus 90+/-2%, P<0.05) and further reduced in N+PSI and HC+PSI (57+/-6 and 48+/-13%, P<0.05 versus N and HC for each). Compared with N (0.03+/-0.01), intima-media ratio was higher in N+PSI (0.09+/-0.04, P<0.01) and HC+PSI (0.15+/-0.06, P<0.05). Compared with N (0.6+/-0.9% of intima area), dihydroethidium fluorescence was higher in HC, N+PSI, and HC+PSI (8.9+/-1.6, 6.0+/-3.5, and 7.2+/-3.9% of intima area, P<0.05 for all). Thus, chronic proteasome inhibition is associated with increased coronary artery oxidative stress and early atherosclerosis. These findings support the significance of the proteasome and related protein quality control for vascular biology and pathology.