Regulation of KCNMA1 transcription by Nrf2 in coronary arterial smooth muscle cells.

The large conductance Ca(2+)-activated K(+) (BK) channels, composed of the pore-forming alpha subunits (BK-alpha, encoded by KCNMA1 gene) and the regulatory beta1 subunits (BK-beta1, encoded by KCNMB1 gene), play a unique role in the regulation of coronary vascular tone and myocardial perfusion by linking intracellular Ca(2+) homeostasis with excitation-contraction coupling in coronary arterial smooth muscle cells (SMCs). The nuclear factor erythroid 2-related factor 2 (Nrf2) belongs to a member of basic leucine zipper transcription factor family that regulates the expression of antioxidant and detoxification enzymes by binding to the antioxidant response elements (AREs) of these target genes. We have previously reported that vascular BK-beta1 protein expression was tightly regulated by Nrf2. However, the molecular mechanism underlying the regulation of BK channel expression by Nrf2, particularly at transcription level, is unknown. In this study, we hypothesized that KCNMA1 and KCNMB1 are the target genes of Nrf2 transcriptional regulation. We found that BK channel protein expression and current density were diminished in freshly isolated coronary arterial SMCs of Nrf2 knockout (KO) mice. However, BK-alpha mRNA expression was reduced, but not that of BK-beta1 mRNA expression, in the arteries of Nrf2 KO mice. Promoter-Nrf2 luciferase reporter assay confirmed that Nrf2 binds to the ARE of KCNMA1 promoter, but not that of KCNMB1. Adenoviral expression and pharmacological activation of Nrf2 increased BK-alpha and BK-beta1 protein levels and enhanced BK channel activity in coronary arterial SMCs. Hence, our results indicate that Nrf2 is a key determinant of BK channel expression and function in vascular SMCs. Nrf2 facilitates BK-alpha expression through a direct increase in gene transcription, whereas that on BK-beta1 is through a different mechanism.