Abstract 836: Downregulation of microRNA-1/microRNA-133 and Overexpression of Sp1 activates Re-expression of Pacemaker Channel Genes HCN2 and HCN4 in Hypertrophic Heart
The pacemaker current If, carried by hyperpolarization-activated channels encoded mainly by HCN2 and HCN4 genes in the heart, plays an important role in rhythmogenesis. Their expressions reportedly increase in hypertrophic and failing hearts, contributing to arrhythmogenesis under these pathological conditions. However, how their expressions are controlled remained unclear. We performed a mechanistic study on transcriptional and posttranscriptional regulation of HCN2 and HCN4 by transcription factors and microRNAs. We identified the transcription start sites and core promoter regions of these genes, and revealed striking similarities between HCN2 and HCN4 promoter regions and ubiquitous Sp1 protein as a common transactivator of HCN2/HCN4. We experimentally established HCN2 mRNA as a target for repression by the muscle-specific miRNAs miR-1 and miR-133, and HCN4 as a target for miR-1 only. We further unraveled robust increases in HCN2/HCN4 transcripts and protein levels in a rat model of left ventricular hypertrophy and in angiotensin II-induced neonatal ventricular hypertrophy. The upregulation of HCN2/HCN4 was accompanied by pronounced elevations of Sp1 and reduction of miR-1/miR-133. Silencing of Sp1 by siRNA or overexpression of miR-1/miR-133 by transfection prevented overexpression of HCN2/HCN4 in hypertrophic cardiomyocytes. Our data indicate that while Sp1 drives HCN2/HCN4 transcription, miR-1/miR-133 act to limit overexpression of HCN2/HCN4 at protein level, and the dynamic balance of the two opposing actions determines the functional level of HCN2/HCN4 and abnormal enhancement of HCN2/HCN4 expression thereby If activities in hypertrophic hearts. Our study also sheds new light on the cellular function and pathological role of miR-1/miR-133 in the heart.