Abstract 5417: Inducible 20S β Subunit LMP-2 is Down-Regulated in db/db Mouse Hearts with Diabetic Cardiomyopathy

Background: The ubiquitin-proteasome system (UPS) plays an important role in cell growth, differentiation, and survival. Low molecular mass polypeptide (LMP)-2 is an inducible 20S beta subunit and its role in diabetic cardiomyopathy has not been previously studied. We investigated the hypothesis that LMP-2 is down-regulated in diabetic hearts and is involved in diabetic cardiomyopathy. Methods: db/db mice and control db/+ littermates were studied at 20 weeks of age. Cardiac function was measured by echocardiography. Chymotryptic-like activity of the 20S proteasome was analyzed by fluorogenic assay using the substrate Suc-LLVY-aminomethycoumarin (-AMC). Results: db/db mice had a severe increase in blood glucose and a decrease in heart weight compared with db/+ mice (135±6 vs. 168±7 mg, n = 4, p < 0.01). Left ventricular function was decreased with a lower fractional shortening in db/db mice (48.7±3.2 vs. 60.7±0.4 %, n = 4, p < 0.05). LMP-2 was constitutively expressed in both db/+ and db/db mouse hearts, but its level was severely depressed in the db/db hearts (78±11 vs. 245±18 AU, n = 4, p < 0.01). Consistent with the change in LMP-2, chymotryptic-like activity was also significantly down-regulated in these diabetic hearts (3286±112 vs. 3598±52 AU, n = 3, p < 0.05). Furthermore, the role of LMP-2 in regulating 20S proteasome function and cardiac mass was determined in LMP-2 knockout and wildtype mice. Chymotryptic-like activity was significantly decreased in LMP-2 knockout mouse hearts (11156±360 vs. 18280±454 AU, n = 4, p < 0.01), and cardiac atrophy was found in these knockout mice (147±7.1 vs 178±10.5 mg, n=10; p < 0.01). Taken together, these results suggest that LMP-2 is necessary for normal proteasome function in the heart and its down-regulation may contribute to the development of diabetic cardiomyopathy.