Chronic Atrial Fibrillation Increases MicroRNA-21 in Human Atrial Myocytes Decreasing L-Type Calcium Current

Abstract

Background— Atrial fibrillation is characterized by progressive atrial structural and electrical changes (atrial remodeling) that favor arrhythmia recurrence and maintenance. Reduction of L-type Ca 2+ current ( I Ca,L ) density is a hallmark of the electrical remodeling. Alterations in atrial microRNAs could contribute to the protein changes underlying atrial fibrillation–induced atrial electrical remodeling. This study was undertaken to compare miR-21 levels in isolated myocytes from atrial appendages obtained from patients in sinus rhythm and with chronic atrial fibrillation (CAF) and to determine whether L-type Ca 2+ channel subunits are targets for miR-21. Methods and Results— Quantitative polymerase chain reaction analysis showed that miR-21 was expressed in human atrial myocytes from patients in sinus rhythm and that its expression was significantly greater in CAF myocytes. There was an inverse correlation between miR-21 and the mRNA of the α1c subunit of the calcium channel (CACNA1C) expression and I Ca,L density. Computational analyses predicted that CACNA1C and the mRNA of the β2 subunit of the calcium channel (CACNB2) could be potential targets for miR-21. Luciferase reporter assays demonstrated that miR-21 produced a concentration-dependent decrease in the luciferase activity in Chinese Hamster Ovary cells transfected with CACNA1C and CACNB2 3′ untranslated region regions. miR-21 transfection in HL-1 cells produced changes in I Ca,L properties qualitatively similar to those produced by CAF (ie, a marked reduction of I Ca,L density and shift of the inactivation curves to more depolarized potentials). Conclusions— Our results demonstrated that CAF increases miR-21 expression in enzymatically isolated human atrial myocytes. Moreover, it decreases I Ca,L density by downregulating Ca 2+ channel subunits expression. These results suggested that this microRNA could participate in the CAF-induced I Ca,L downregulation and in the action potential duration shortening that maintains the arrhythmia.