Regulation of the SK3 channel by microRNA-499-Potential role in atrial fibrillation

BACKGROUND MicroRNAs are important regulators of gene expression, including those involving electrical remodeling in atrial fibrillation (AF). Recently, KCNN3, the gene that encodes the small-conductance calcium-activated potassium channel 3 (SK3), was found to be strongly associated with AF. OBJECTIVES To evaluate the changes in atrial myocardial microRNAs in patients with permanent AF and to determine the role of microRNA on the regulation of cardiac SK3 expression. METHODS Atrial tissue obtained during cardiac surgery from patients (4 sinus rhythm and 4 permanent AF) was analyzed by using microRNA arrays. Potential targets of microRNAs were predicted by using software programs. The effects of specific microRNAs on target gene expression were evaluated in HL-1 cells from a continuously proliferating mouse hyperplastic atrial cardiomyocyte cell line. Interactions between microRNAs and targets were further evaluated by using luciferase reporter assay and by using Argonaute pull-down assay. RESULTS Twenty-one microRNAs showed significant (>2-fold) changes in AF. MicroRNA 499 (miR-499) was upregulated by 2.33-fold (P <.01) in AF atria, whereas SK3 protein expression was downregulated by 46% (P <.05). Transfection of miR-499 mimic in HL-1 cells resulted in the downregulation of SK3 protein expression, while that of miR-499 inhibitor upregulated SK3 expression. Binding of miR-499 to the 3' untranslated region of KCNN3 was confirmed by luciferase reporter assay and by the increased presence of SK3 mRNA in Argonaute pulled-down microRNA-induced silencing complexes after transfection with miR-499. CONCLUSION Atrial miR-499 is significantly upregulated in AF, leading to SK3 downregulation and possibly contributing to the electrical remodeling in AF.