Altered expression of micro-RNA 199a and increased levels of cardiac SIRT1 protein are associated with the occurrence of atrial fibrillation after coronary artery bypass graft surgery

Abstract

Background

Postoperative atrial fibrillation (POAF) is a potentially life-threatening complication after coronary artery bypass graft (CABG) surgery. The expression of the cardioprotective SIRT1 protein with its antioxidant activity is increased in cardiac tissue of patients suffering from POAF. So far, information is lacking about the relationship between SIRT1 regulating micro RNAs (miRs), SIRT1 protein and the occurrence of POAF.

Methods

A total of 63 patients undergoing CABG were recruited, and biopsies were obtained from the right atrial appendage during cannulation. Postoperative, all patients were rhythm-monitored until discharge and randomized to POAF (n = 20) or sinus rhythm (n = 43). The expression of the micro RNAs miR-199a and miR-195 was quantified by real-time PCR. SIRT1 protein was detected by western blot analysis.

Results

The relative expression of miR-199a in the POAF group was significantly decreased compared to the control group (0.77 ± 0.27 vs. 1.11 ± 0.69, P = .022) Accordingly, SIRT 1 protein was significantly induced in tissue probes of patients with POAF (P < .001).

Conclusion

Altered expression of the SIRT1 protein regulating miR-199a in human atrial tissue was found to be related to the occurrence of POAF, indicating its usefulness as a biomarker for cardiac surgery management.

Introduction

Atrial fibrillation (AF) is the most common type of arrhythmia, affecting 5% of the population older than 65 years and 7.1% of those older than 85 years [1]. Approximately 70% to 80% of patients with AF have structural heart disease, including coronary artery disease and valvular heart disease [2].

It is associated with significant morbidity and mortality, including a quadrupled risk of heart failure and a nearly doubled risk of death [3].

Postoperative atrial fibrillation (POAF) is a common complication after coronary artery bypass surgery (CABG); a third of all patients suffer from POAF during the early postoperative period [4], [5]. The mechanism POAF is not well defined and is probably multifactorial. There are several risk factors, which are associated with POAF, such as advanced age, male gender; gene polymorphism; history of chronic heart failure (CHF) or AF, chronic obstructive pulmonary disease, chronic renal insufficiency, diabetes mellitus, rheumatic heart disease; previous cardiac surgery, metabolic syndrome, obesity; severe proximal right coronary artery stenosis; increased left atrial size; preoperative increase in P wave duration on surface (> 116 ms) or on signal averaged (> 140 ms) EKG [16], [17] and blood transfusion before surgery [6]. Prolonged mechanical ventilation, atrial ischemia, hypokalemia and hypomagnesemia are other known risk factors for POAF. There is conflicting data whether increased aortic cross-clamp and cardiopulmonary bypass time increase POAF [6].

Despite several basic and clinical studies, the precise underlying mechanism of onset and persistence of AF has not been completely elucidated. The following pathophysiological factors might play an important role: atrial factors like atrial dilatation, hypertrophy and fibrosis; postoperative inflammation; electrical remodeling (shortening of the effective refractory period); autonomic imbalance and alterations in atrial oxidative stress [6].

It is of great importance to understand the early and causative changes in AF pathogenesis to avoid persistence and recurrence and initiate targeted prophylaxis [3].

The expression of the cardio-protective protein SIRT1 is induced in cardiac disease such as AF and CAD, indicating a compensatory mechanism to overcome the disease-related pathologies such as oxidative stress [7], [8].

SIRT1, known as a longevity gene, protects cells against oxidative stress and promotes DNA stability by binding and deacetylating several substrates. In the cardiovascular system, SIRT1 activation exerts multiple protective effects through distinct metabolic and stress-response pathways. Importantly, SIRT1 has been recognized as a key regulator of vascular endothelial homeostasis and also regulates angiogenesis, endothelial senescence and dysfunction [9], [10]. It prevents atherosclerosis by improving endothelium relaxation through up-regulating eNOS expression and production of nitric oxide [11].

In cardiomyocytes, due to its antioxidant activity, nuclear SIRT1 increases the resistance of myoblast to oxidative stress by enhancing the MnSOD expression through p53 deacetylation [12]. Protection of cardiomyocytes from oxidative stress is also regulated by overexpression of SIRT1 protein and activation of FoxO1-dependent pathway [13]. The activation of this pathway also reduces cardiac infarct volume and improves functional recovery after ischemia/reperfusion in mice [14].

The expression of SIRT1 protein in the cardiac and vascular tissue is inter alia regulated by the micro RNAs 195 and 199a [15].

MicroRNAs are small noncoding RNAs composed of 21–25 ribonucleotides, which control the expression of complementary target messenger RNAs [16], [17], [18]. These small nonprotein-coding RNAs began to compose an important role in the cardiovascular system, and recent research indicates the potential of miRNAs as a novel mechanism for AF [16], [17], [18], [19]. However, published studies that focused on miRNAs and AF are sparse, and most investigated chronic AF, partially using animal models. As a result, information regarding the relationship between miRNA expression in human atrial tissue and new onset of AF is not available.

In cardiomyocytes, SIRT1 is controlled by miR-195 and -199a. The free fatty acid palmitate up-regulates miR-195 expression, which inhibits SIRT1 and promotes apoptosis [20]. MiR-199a inhibits the expression of both SIRT1 and HIF-1α [21]. Hypoxia or cardiac ischemia decreases miR-199a, permitting an increase in SIRT1 in cardiocmyocytes. SIRT1 in turn down-regulates prolylhydroxylase2 (PHD2), which stabilizes HIF-1α and induces HIF-related signaling. Thus, miR-199a increases HIF-1α in two ways: first by regulating HIF-1α directly, second by a SIRT1–PHD2–HIF-1 pathway [21]. As enhanced SIRT1 expression is associated with the occurrence of AF [7], we investigated whether SIRT1 regulating miR-195 and -199a are involved in the pathogenesis of AF.