Discussion
This study showed that BRD4 was positively correlated with atrial fibrosis, AF, postoperative recurrence of AF, LVAs, and LAAPD, and was negatively correlated with LVEF. BRD4 may be involved in atrial fibrosis and remodeling through the TGF-β1/Smad signaling pathway, and thus participate in the occurrence and maintenance of AF.
Association between LAAPD and AF
Studies have shown that atrial muscle fibrosis is significantly aggravated in patients with AF, which was positively correlated with LAAPD [25]. Left atrial fibrosis and structural remodeling were more obvious in patients with AF with left atrial enlargement [26, 27]. Clinically, LAAPD is usually measured by echocardiography or cardiac magnetic resonance(CMR) to reflect the size of the left atrium. Only LAAPD has been widely used in large-scale clinical studies that could be reviewed. LAAPD can accurately estimate the left atrial volume through the nonlinear equation of the elliptic model [28]. Multiple studies have shown that LAAPD enlargement is an independent predictor of recurrence after AF ablation [29-31]. Our study showed that the mean values of LAAPD were significantly higher in participants of the AF group, patients with persistent AF, and participants in the reAF group than those in the PSVT group, patients with paroxysmal AF, and participants in the NreAF group, and all the differences were statistically significant. It has been suggested that the left atrial enlargement is more obvious in patients with AF, especially those with persistent AF. Moreover, patients with AF with left atrial enlargement had an increased rate of postoperative recurrence.
Association between LAA-PEV and AF
Some studies have found that LAA is one of the potential trigger points of spontaneous AF [32]. Increased LAA volume and decreased LAA ejection fraction were predictors of recurrence after ablation of AF [33-36]. The decrease in LAA-PEV was positively correlated with postoperative recurrence of AF [37]. Transesophageal echocardiography could accurately determine LAA-PEV in patients with AF. In this study, the LAA-PEV of patients in the reAF group was significantly lower than that of patients in the NreAF group, and the difference was statistically significant. This result suggests that patients with AF with reduced LAA-PEV had an increased postoperative recurrence rate.
Association between left atrial fibrosis and AF
Excessive synthesis and irregular deposition of extracellular matrix (ECM) proteins are thought to be involved in the onset and maintenance of AF [38]. These collagen-based scars (collagen type I and III) could form barriers that blocked electrical conduction and separated well-connected syncytes, thus directly interfering with electrical conduction among cardiomyocytes [39]. Additionally, fibroblasts and myofibroblasts could form cell membrane fusions with cardiomyocytes to form intercellular gap junctions via connexin 40, 43, and 45 (Cx40, Cx43, and Cx45) [40, 41]. These material bases, through changing the electrical conduction velocity among cardiac myocytes and increasing the electrical conduction heterogeneity, lead to the formation of complex electrical conduction reentry in the atrium, and ultimately lead to the occurrence and maintenance of AF [42-44]. Atrial remodeling is a key factor linking all the mechanisms associated with AF, and atrial fibrosis was the most prominent feature of atrial remodeling [45]. In the rat AF model, compared to the CTL group, the degree of left atrial fibrosis and collagen volume fraction in the AF + CTL group were significantly higher. Our results showed that compared to the CTL group, the level of type III collagen was significantly higher and the level of Cx43 was significantly lower in the AF + CTL group.
Association between LVAs and AF
AF and atrial fibrosis are cause and effect and promote each other. The presence of left atrial fibrosis increased the recurrence rate of AF after ablation. Cardiac MRI (CMR) is a well-established method for detecting left atrial fibrosis and can be used to noninvasively identify the scar matrix of the left atrium, which is known as the fibrotic area [46-48]. However, CMR was found to be inaccurate in evaluating left atrial fibrosis in patients with persistent AF [49, 50]. CMR and left atrial bipolar voltage mapping were used to detect left atrial fibrosis and left atrial low-voltage region (bipolar voltage < 0.5 mV)/scar area (bipolar voltage < 0.05 mV) in participants with sinus rhythm, and the left atrial low voltage/scar area was highly consistent with the left atrial fibrosis area in CMR in Jeong’s study [51]. Subsequent studies also confirmed that LVAs measured by left atrial bipolar voltage mapping were consistent with the left atrial fibrosis regions in CMR [52, 53]. Therefore, it is reasonable to use LVAs to represent left atrial fibrosis in clinical practice. Several studies had shown that the presence of LVAs is an independent predictor of postoperative AF recurrence [54-57]. Our results showed that the mean values of LVAs were significantly lower in participants with paroxysmal AF and those in the NreAF group than those with persistent AF and those in the reAF group. These results suggest that the decrease in left atrial voltage was more obvious in patients with persistent AF. Moreover, LAVs were positively correlated with postoperative recurrence of AF.
Association among LAAPD, LVEF, and LAVs
The relationship among LAAPD, LVEF, and LAVs has not been reported previously. Studies have shown that left atrial fibrosis was more pronounced in patients with AF with enlarged left atrium. In patients with AF with reduced LVEF, the pressure of the left atrium was increased, which promoted left atrial enlargement and left atrial fibrosis. Therefore, it is reasonable to use LVAs to represent left atrial fibrosis in clinical practice. We speculated that LAAPD and LVEF were correlated with LAVs. The results of this study showed that the mean values of LAAPD were significantly lower in participants in the NLAV group than those in the LAV group. However, the mean values of LVEF were significantly higher in participants in the NLAV group than those in the LAV group. These findings suggest that LAAPD is positively correlated with LAVs, while LVEF is negatively correlated with LAVs.
Association between TGF-β/Smads and AF
Previous studies have confirmed that TGF-β is closely associated with the occurrence of tissue fibrosis [58] and plays a crucial role in the process of myocardial fibrosis. TGF-β activates the synthesis of ECM protein through the Smads signal transduction pathway to cause excessive deposition of ECM and myocardial fibrosis. Atrial fibrosis affects the development of AF through the TGF-β1/Smads pathway [59]. TGF-β expression was mainly completed through TGF-β/Smads signaling pathway. Multiple studies have shown that downregulation of the TGF-β/Smads signaling pathway could decrease TGF-β expression, attenuate the degree of myocardial fibrosis, and reduce the incidence of AF [60-62]. Therefore, TGF-β is closely related to AF and atrial fibrosis. Our results showed that compared to the CTL group, the levels of TGF-β1,type III collagen,P-Smad2/3, and Smad2/3 were significantly higher, and the level of Smad7 was significantly lower in the AF + CTL group.
Association between BRD4 and AF
There are few studies on BRD4 in cardiovascular diseases, especially in AF. It was found that BRD4 played an important role in the pathogenesis of high glucose-induced ventricular fibrosis through the TGF-β/SMAD pathway [18], and that inhibition of BRD4 could reduce TGF-β-induced ventricular fibrosis [19]. Considering that atrial fibrosis and ventricular fibrosis share common pathways, atrial fibrosis and AF have mutual causation, and that the TGF-β/SMAD pathway plays an important role in the pathogenesis of atrial fibrosis and AF, we speculated that BRD4 might be involved in the occurrence and development of atrial fibrosis and AF through the TGF-β/SMAD pathway. Additionally, at the 40th Annual Meeting of the American Heart Rhythm Society, Professor Yigang Li’s team first reported that BRD4 was highly expressed in the left atrial tissue of patients with valvular AF, and found that inhibition of BRD4 expression could attenuate atrial fibrosis and reduce the incidence of AF [20]. This research indicated, for the first time, that BRD4 inhibitors might be used to prevent and treat atrial fibrosis and AF.
JQ-1 is a specific BRD4 inhibitor that binds to the bromine domain of BET protein with high shape complementarity and nanoscale affinity. This interaction leads to a strong, competitive, and transient replacement of BRD4 from acetylated chromatin, and thereby inhibited signaling events downstream of Pol II, inhibiting the biological effects of BRD4 [63, 64]. Therefore, (+)JQ-1 was applied to the SD rats in the AF + JQ1 group in this study.
Our study showed that the mean BRD4 level was significantly higher in participants in the AF group, reAF group, LAV group, and in patients with persistent AF than those in the PSVT group, NreAF group, NLAV group, and patients with paroxysmal AF. Moreover, an elevated level of BRD4 was confirmed in patients with non-valvular and non-ischemic cardiomyopathy AF, suggesting that BRD4 is associated with AF. These results suggest that the BRD4 level is significantly positively correlated with LAVs and postoperative recurrence of AF, and that a high BRD4 level is a predictor of LAVs and postoperative recurrence of AF. The results of this study showed that the mean incidence and duration of AF were significantly higher in SD rats in the AF + CTL group than those in the AF + JQ1 group (P < 0.05), suggesting that BRD4 inhibition could reduce the incidence and burden of AF.
Our study showed that the mean collagen volume fraction of the left atrium was significantly higher in SD rats in the AF + CTL group than those in the AF + JQ1 and CTL groups. The mean collagen volume fraction of the left atrium was significantly higher in SD rats in the AF + JQ1 group than that in the CTL group. The area of fibrotic collagen, which was stained blue, was largest in the AF + CTL group, smallest in the CTL group, and second largest in the AF + JQ1 group. These results suggest that BRD4 inhibition could attenuate the degree of left atrial fibrosis. Our study showed that the mean values of BRD4, TGF-β1, P-Smad2/3, Smad2/3, and type III collagen were significantly higher in SD rats in the AF + CTL group than those in the AF + JQ1 and CTL groups. The mean values of Smad7 and CX43 were significantly lower among SD rats in the AF + CTL group than those in the AF + JQ1 and CTL groups. These results suggest that inhibition of BRD4 expression could reduce the expression of atrial fibrosis-related proteins. BRD4 may be involved in atrial fibrosis and remodeling through the TGF-β1/Smad signaling pathway, and thus participate in the occurrence and maintenance of AF.