Regulation of Large Conductance Ca2+ -activated K+ ( BK) Channel β1 Subunit Expression by Muscle RING Finger Protein 1 in Diabetic Vessels

Background: Impaired BK channel function in diabetic vessels is associated with decreased BK channel 1 subunit (BK-1) expression. Results: Muscle RING finger protein 1 (MuRF1) physically interacts with BK-1 and accelerates BK-1 proteolysis. Conclusion: Increased MuRF1 expression is a novel mechanism underlying diabetic BK channelopathy and vasculopathy. Significance: MuRF1 is a potential therapeutic target of BK channel dysfunction and vascular complications in diabetes. The large conductance Ca2+-activated K+ (BK) channel, expressed abundantly in vascular smooth muscle cells (SMCs), is a key determinant of vascular tone. BK channel activity is tightly regulated by its accessory 1 subunit (BK-1). However, BK channel function is impaired in diabetic vessels by increased ubiquitin/proteasome-dependent BK-1 protein degradation. Muscle RING finger protein 1 (MuRF1), a muscle-specific ubiquitin ligase, is implicated in many cardiac and skeletal muscle diseases. However, the role of MuRF1 in the regulation of vascular BK channel and coronary function has not been examined. In this study, we hypothesized that MuRF1 participated in BK-1 proteolysis, leading to the down-regulation of BK channel activation and impaired coronary function in diabetes. Combining patch clamp and molecular biological approaches, we found that MuRF1 expression was enhanced, accompanied by reduced BK-1 expression, in high glucose-cultured human coronary SMCs and in diabetic vessels. Knockdown of MuRF1 by siRNA in cultured human SMCs attenuated BK-1 ubiquitination and increased BK-1 expression, whereas adenoviral expression of MuRF1 in mouse coronary arteries reduced BK-1 expression and diminished BK channel-mediated vasodilation. Physical interaction between the N terminus of BK-1 and the coiled-coil domain of MuRF1 was demonstrated by pulldown assay. Moreover, MuRF1 expression was regulated by NF-B. Most importantly, pharmacological inhibition of proteasome and NF-B activities preserved BK-1 expression and BK-channel-mediated coronary vasodilation in diabetic mice. Hence, our results provide the first evidence that the up-regulation of NF-B-dependent MuRF1 expression is a novel mechanism that leads to BK channelopathy and vasculopathy in diabetes.