The role of galectin-3 in atrial fibrillation

Numerous risk factors for atrial fibrillation (AF) progression have been identified. However, the biomarkers mentioned in the guidelines do not have any clinically relevant predictive value. Some research groups investigated the potential utility of galectin-3 (gal-3) as a diagnostic, prognostic, and predictive biomarker in AF. In this review, we have thoroughly summarized the current data on the role of gal-3 in AF based on the original research in this field. Patients suffering from AF present with increased levels of gal-3. The concentration of gal-3 differs between patients with AF depending on the type of AF — it is higher in patients with persistent AF than in patients with paroxysmal AF. Multiple studies investigating the reappearance of AF in patients who underwent ablation have shown that gal-3 is a promising biomarker to predict the outcome of this therapy. Patients with increased levels of gal-3 are at higher risk of AF recurrence. Although the research considered in this work addressed many aspects of the role of gal-3 in AF, most of it has been conducted on a small group of patients. Therefore, further research and extensive clinical trials confirming described findings are highly warranted.


Introduction
According to the European Society of Cardiology (ESC) definition, atrial fibrillation (AF) is a supraventricular tachyarrhythmia with uncoordinated atrial electrical activation and consequently ineffective atrial contraction [1].More than 60% of patients suffering from AF have a decreased quality of life.AF is a significant risk factor for ischaemic stroke and heart failure, increasing mortality [2].Currently, the estimated prevalence of AF in adults is between 2 and 4%, being the most frequent cardiac arrhythmia nowadays [1,3].AF incidence and prevalence have increased over the last 20 years and will likely continue rising over the next 30 years [3].
AF progression from paroxysmal to non-paroxysmal is associated with multiple risk factors, including age, and many diseases, i.e., heart failure, hypertension, chronic kidney disease, chronic pulmonary diseases, diabetes mellitus, and previous stroke [1,4].However, the biomarkers mentioned in the guidelines do not have clinically relevant predictive value [1].Because of that, further research on the role of biomarkers in AF is highly warranted [5,6].Knowledge about the pathogenesis of the disease should always be the rationale for the search for potential biomarkers.The ongoing fibrosis in the left atrium is one of the leading causes of AF since it delays electromechanical conduction and creates a substrate for AF [7].Multiple proteins involved in fibrosis have been investigated so far.Recently, much scientific attention has been drawn to galectin-3 (gal-3).
Gal-3 is a β-galactoside-binding protein belonging to the lectin family.It plays a vital role in many physiological cellular functions, including cellular growth, differentiation, proliferation, apoptosis, cellular adhesion, and tissue repair [8].The concentration of gal-3 levels can be measured with the use of immunoassays.Enzyme-linked immunosorbent assay (ELISA) is the most commonly used technique.Nevertheless, other variants of immunoassays can also be used, including enzyme-linked fluorescent assay (ELFA) or chemiluminescent microparticle immunoassay (CMIA).Although there are some differences in the specificity and sensitivity of these tests, price, and availability seem to be decisive factors for test choice, favoring ELISA.Grzegorz Procyk, Aleksandra Czapla, and Kamila Jałocha contributed equally to this work and are first co-authors.
How gal-3 is implicated in the pathogenesis of various cardiovascular diseases has been extensively explored, mainly referring to its involvement in inflammation and tissue fibrosis processes [8,9].Much research has focused on its role in AF.
Some research groups investigated the potential utility of gal-3 as a diagnostic and disease severity/prognostic biomarker in AF.In contrast, others studied the correlation between gal-3 concentrations and the possibility of AF recurrence after different interventions, such as ablation [10,11].The understanding of the role of gal-3 in the induction and the progression of AF may lead to better management of patients as well as may provide new targets for treatment.
In this review, we have thoroughly summarized the current data on the role of gal-3 in AF based on the original research in this field.We aim to contribute to setting objectives for future research on gal-3 and its use as a biomarker in daily clinical practice.

Galectin-3 as a potential biomarker in atrial fibrillation
Numerous clinical studies investigating the role of gal-3 in AF have already been conducted.In our review, we included only original clinical research.Reviews, letters to the editors, and commentaries were not included.We searched PubMed Database by the query: "(galectin-3) AND (atrial fibrillation)" which yielded a total of 117 records.Excluding unsuitable titles, article types, or abstracts, we retrieved in complete form and assessed 51 studies.We evaluated the complete data reports for eligibility and excluded 17 studies irrelevant to the field.We eventually included 34 original clinical research relevant to the discussed area (Fig. 1).
We divided these studies into the following parts: (i) differences in the levels of gal-3 between patients with AF and patients in sinus rhythm, (ii) differences in the levels of gal-3 between patients with different types of AF, (iii) gal-3 in patients with AF undergoing cardioversion, (iv) gal-3 as a predictor of AF recurrence after ablation and (v) gal-3 in patients with AF undergoing surgeries and other invasive procedures (Fig. 2).

Differences in the levels of galectin-3 between patients with atrial fibrillation and patients in sinus rhythm
Ho et al. studied the relationship between gal-3 levels and the probability of AF occurrence in participants of the Framingham Offspring cohort.They discovered that higher concentrations of circulating gal-3 were associated with an increased risk of developing AF [12].Another research group compared the levels of gal-3 between patients with non-valvular AF and age-matched patients in sinus rhythm.It was shown that patients with AF had significantly higher concentrations of this biomarker than the control group.Moreover, the authors found a positive correlation between the concentration of gal-3 in blood and the left atrial volume index [13].Consistently, Pauklin et al. analyzed the levels of gal-3 in patients suffering from paroxysmal or persistent AF admitted for electrical cardioversion or pulmonary vein isolation (PVI) and compared them with healthy controls.The study revealed that the levels of gal-3 were higher in patients with AF [14].
Similarly, Selcoki et al. analyzed gal-3 levels in patients with paroxysmal AF and preserved left ventricular systolic function and compared them with healthy controls matched by age and gender.It was shown that patients with AF had higher levels of gal-3 than controls.Moreover, the authors found a positive correlation between left atrial diameter and serum gal-3 levels [15].
Contrary results to these studies were found by Begg et al.They considered gal-3 level a marker of fibrosis in patients who underwent ablation.Therefore, they analyzed patients suffering from AF (paroxysmal or non-paroxysmal) who underwent ablation and compared them to non-AF control patients.They proved no significant difference in gal-3 levels between these groups [16].However, the authors found that peripheral gal-3 levels were significantly higher than left atrial gal-3 levels.
Fashanu et al. studied the relationship between gal-3 levels and the probability of an AF incident.It was shown that patients with gal-3 levels ≥ 90th percentile had a higher risk of an AF incident than those with gal-3 levels < 90th percentile [17].In another study, Shen et al. compared two groups of patients who underwent single mitral valve repair/replacement.The first group consisted of patients suffering from persistent AF, while the second group had sinus rhythm patients.It was proven that the levels of gal-3 measured in serum collected from coronary sinus blood and gal-3 measured in atrial tissues were higher in the first group than in the second one [18].
Hernández-Romero et al. analyzed gal-3 levels in patients with permanent AF who underwent cardiac surgery and compared them to those with aortic valve or ischaemic heart diseases who also underwent cardiac surgery.They observed differences in serum concentrations of gal-3 between the groups-gal-3 levels were higher in patients with permanent AF.Moreover, high levels of gal-3 were found to be an independent predictor of fibrosis [19].All studies discussed in this section with additional information have been presented in Table 1.

Differences in the levels of galectin-3 between patients with different types of atrial fibrillation
Gurses et al. analyzed gal-3 levels in persistent or paroxysmal AF patients with preserved left ventricular systolic function and compared them to the age-and-gender-matched controls.It was proved that gal-3 levels and left atrial volume index were higher in AF patients than in the controls.Moreover, gal-3 levels were significantly higher in patients with persistent AF than in those with paroxysmal AF [20].Accordingly, Chen et al. compared gal-3 levels between new-onset AF patients and patients with chronic AF.The study revealed that patients with new-onset AF had higher gal-3 levels than those with pre-existing, chronic AF [21].
Tang et al. compared patients with all three types of AF: paroxysmal, persistent, and permanent.It was shown that patients with persistent or permanent AF had higher levels of gal-3 compared to patients with paroxysmal AF.Moreover, gal-3 was an independent determinant of LAA thrombus in AF patients [22].
Another interesting research was held by Arbault-Biton et al.They divided the group of AF patients based on the AF duration: there were 47 patients with AF ≤ 48 h and 51 patients with AF > 48 h.It was proved that gal-3 concentrations did not differ between the groups [23].
Interestingly, Zaslavskaya et al. compared gal-3 levels in patients suffering from metabolic syndrome (MS) with coexisting paroxysmal/persistent AF or without arrhythmia to healthy controls.They showed that gal-3 levels were higher in patients with coexisting MS and AF than in patients with only MS and healthy controls.Moreover, gal-3 levels were higher in patients with persistent AF than in those with paroxysmal AF [24].
Wang et al. studied patients suffering from paroxysmal AF, dividing them based on the progression to persistent AF.It was demonstrated that patients with AF progression to a persistent state had higher gal-3 levels than those without AF progression.Moreover, gal-3 concentration was associated with AF progression [25].All studies discussed in this section with additional information have been presented in Table 2.

Galectin-3 in patients with atrial fibrillation undergoing cardioversion
Begg et al. examined gal-3 levels as a potential prognostic factor of AF recurrence after direct current cardioversion (DCCV).They included patients who underwent DCCV for AF and compared them to healthy controls.They demonstrated that there was no difference in gal-3 levels between these groups.Moreover, gal-3 levels did not predict AF recurrence after DCCV [26].Consistently, Kisheva et al. examined gal-3 levels as a predictive factor of AF recurrence in patients suffering from AF after sinus rhythm restoration.
It was shown that gal-3 levels did not affect the number of AF recurrences after sinus rhythm restoration [27].
A similar study was conducted by Gürses et al.They evaluated the group of 90 patients suffering from persistent AF who underwent DCCV.As presented, 28 patients experienced early AF recurrence within 3 months.Contrary to the studies above, this research group showed that the patients with AF recurrence had higher gal-3 levels compared to those who did not experience AF recurrence after the procedure [28].
Wałek et al. examined correlations between the levels of gal-3 and the echocardiographic parameters of the left atrium and left ventricle in patients suffering from persistent AF with left atrial enlargement qualified for DCCV.The authors observed negative correlations between the concentrations of gal-3 and left atrial: dimensions, volume, contractility, and compliance.Moreover, negative correlations were found between the levels of gal-3 and left ventricular: volume and contractility [29].All studies discussed in this section with additional information have been presented in Table 3.

Galectin-3 as a predictor of atrial fibrillation recurrence after ablation
Wu et al. analyzed plasma concentrations of gal-3 in patients with persistent AF without coexisting structural heart disease who underwent first-time catheter ablation and compared them to healthy controls.The levels of gal-3 were higher in patients suffering from persistent AF.Furthermore, the concentrations of gal-3 were increased in patients with recurrence of AF after ablation, compared to patients without AF recurrence.Moreover, gal-3 was an independent predictor of AF reappearance after catheter ablation [30].Similarly, Ruan et al. analyzed  preoperative level of gal-3 and AF recurrence in patients undergoing radiofrequency catheter ablation (RFCA).It was shown that patients who experienced the recurrence of AF after the RFCA presented higher levels of gal-3 (measured at baseline) than patients who did not develop AF again after the procedure.It was proved that gal-3 could be considered an independent predictor of AF reappearance after the procedure [31].Consistently, Lee et al. investigated the correlation between levels of gal-3 and the risk of atrial tachyarrhythmias recurrence in patients who underwent ablation for AF.Increased levels of gal-3 were associated with an increased risk of atrial tachyarrhythmias recurrence [32].Takemoto et al. compared the levels of gal-3 measured in intracardiac serum obtained from the coronary sinus and left atrium between patients with persistent AF and patients with paroxysmal AF.Higher levels of this biomarker were found in patients with persistent AF.They also showed that gal-3 was an independent predictor of atrial tachyarrhythmia recurrence after a single ablation procedure [33].

the relationship between the
Clementy et al. investigated gal-3 levels and left atrial diameter (LAD) as potential predictive factors of AF recurrence in patients suffering from AF who underwent ablation.Patients who experienced AF recurrence after the procedure appeared to have higher gal-3 levels and larger left atrium.Gal-3 levels and LAD were independent predictors of AF recurrence [34].
Contrary results were obtained by Kornej et al.They conducted a study including AF patients who underwent catheter ablation and compared them to AF-free controls.It was shown that gal-3 levels were higher in AF patients compared to AF-free controls.However, the authors concluded that gal-3 did not help predict the rhythm outcome of the catheter ablation [35].Another research team had similar findings: Celik et al. analyzed gal-3 levels as a predictive factor of AF recurrence in patients who underwent PVI.It was proven that gal-3 levels were not associated with AF recurrence.Moreover, PVI did not significantly affect serum gal-3 levels [36].
Berger et al. studied the relationship between gal-3 levels and the probability of AF recurrence in patients undergoing thoracoscopic surgery for AF.A higher recurrence rate was observed in patients with increased gal-3 levels after ablation compared to baseline than in patients with unchanged or decreased gal-3 level [37].
Yalcin et al. evaluated patients suffering from paroxysmal AF undergoing cryoballoon-based AF ablation.They studied the relationship between the levels of gal-3 and left atrium fibrosis assessed by delayed-enhancement magnetic resonance imaging (DE-MRI) and atrial electromechanical delay (AEMD).They proved that the concentrations of gal-3 were independently correlated with the extent of left atrium fibrosis detected with DE-MRI.Moreover, the levels of gal-3 correlated with intra-left and inter-AEMD [38].Aksan et al. investigated the association between gal-3 level and low voltage areas (LVA) severity in patients suffering from paroxysmal AF who underwent PVI.It was proved that gal-3 levels were higher in paroxysmal AF patients with moderate and severe LVA compared to those with mild LVA.Moreover, gal-3 levels were higher in paroxysmal AF patients with LVA than in those without LVA [39].All studies discussed in this section with additional information have been presented in Table 4.

Galectin-3 in patients with atrial fibrillation undergoing surgeries and other invasive procedures
Szadkowska et al. evaluated patients with first acute myocardial infarction who underwent primary percutaneous coronary intervention with stent implantation.The authors showed that new-onset AF was independently associated with increased levels of gal-3 (particularly > 16 ng/mL) [40].Similarly, Wang et al. evaluated patients admitted with acute myocardial infarction (AMI).The authors compared patients who developed post-AMI new-onset AF to those who did not.It was evidenced that the first group had increased levels of gal-3.Furthermore, the plasma concentration of gal-3 was found as an independent predictor of post-AMI new-onset AF [41].
Richter et al. hypothesized that gal-3 level could independently predict postoperative atrial fibrillation (POAF).Based on these assumptions, they enrolled patients undergoing elective cardiac surgeries.They proved that the levels of gal-3 were higher in patients who developed POAF compared to those who did not develop it.Moreover, gal-3 levels were an independent predictor of POAF and mortality after cardiac surgery [42].Erdem et al. obtained similar results, investigating patients undergoing coronary artery bypass graft (CABG) surgery.It was shown that gal-3 could predict POAF possessing high specificity and sensitivity [43].
Aksan et al. investigated gal-3 levels as a marker of AF recurrence expressed as atrial high-rate episodes (AHRE) in patients who underwent cardiac resynchronization therapy.The study revealed that gal-3 levels were higher in patients with AHRE than those without AHRE.Also, there was a positive correlation between gal-3 levels and the percent of time spent in total AHRE [44].
Tan et al. investigated patients suffering from heart failure (HF), dividing them based on the coexistence of AF.It was proved that gal-3 predicted HF-related hospitalization,  but only in patients with coexisting AF [45].All studies discussed in this section with additional information have been presented in Table 5.

Conclusions and future perspectives
Many studies regarding gal-3 as a biomarker of AF have been conducted so far.The main findings from these studies are that (i) there is a correlation between higher levels of gal-3 and activation of the profibrotic pathway, (ii) the concentration of this biomarker differs between patients depending on the type of AF (paroxysmal, persistent, and permanent), (iii) gal-3 is a promising biomarker to predict the recurrence of AF in patients after ablation and POAF after cardiac surgery.Almost all research groups confirmed higher levels of gal-3 in patients with AF compared to non-AF controls [13-15, 18, 19].Begg et al. obtained contrary results in 2 studies: (i) comparing AF patients undergoing ablation to healthy controls and (ii) comparing AF patients undergoing DCCV to age-and-disease-matched controls [16,26].The potential rationale for this observed difference may be found in the characteristics of the included population.Patients undergoing ablation are most commonly relatively young, and their AF cannot be permanent.Similarly, patients undergoing DCCV are often diagnosed quite recently, and thus the fibrosis process is less advanced than it would be after, e.g., 20 years of AF occurrence.
The next conflicting findings regard the predictive value of gal-3 for AF recurrence after DCCV.Most research found no predictive value of this biomarker in this use [26,27].Nevertheless, Gürses et al. showed that patients with AF recurrence after DCCV had higher levels of gal-3 [28].Moreover, in the multivariate analysis, they found that serum gal-3 level was independently associated with early recurrence of AF after a successful DCCV.This discrepancy with other studies requires further investigation and a search for an explanation.
Although the research considered in this manuscript addressed many aspects of the role of gal-3 in AF, most of it was conducted on small groups of patients.Therefore, more extensive research, optimally a randomized clinical trial, would be of great value to standardize and confirm the previous findings, particularly the thesis regarding the role of gal-3 as a predictive biomarker of AF recurrence after ablation (Fig. 3).Further improvement in our knowledge about the role of gal-3 in AF would lead to a better understanding of the pathogenesis of this disease.Significantly, it could contribute to the earlier diagnosis of AF, even before the physical signs of an irregular heart rhythm.As paroxysmal AF is very difficult to spot during the electrocardiographic examination, establishing a biomarker enabling the diagnosis of AF based on its concentration would help diagnose AF at its earliest stages.
Gal-3 acts on fibroblasts'/myofibroblasts' migration and proliferation.Macrophages are critical mediators of this process since gal-3 is most expressed in tissue-resident macrophages [46].This plays a crucial role in activating a pro-fibrotic phenotype of macrophages and the following fibroblasts'/myofibroblasts' activation [47].Although much is known, there are still gaps in evidence regarding the exact role of gal-3 in the fibrosis process.Therefore, further research on the effects of gal-3 inhibition in preclinical and clinical settings may contribute to a better understanding of the gal-3 role in fibrosis and AF pathogenesis and a possible introduction of a new targeted therapy in AF.In conclusion, future studies are recommended to fully confirm the role of gal-3 as an AF biomarker and open doors for the new targeted therapy in this disease.

Fig. 1
Fig. 1 The flowchart for the selection process; n-a number of studies

Fig. 2
Fig. 2 Graphical presentation of clinical studies included in this review, divided into subtopics; AF-atrial fibrillation; n-a number of included clinical studies in a given field; SRsinus rhythm

Fig. 3
Fig. 3 Graphical summarization of the role of galectin-3 as a potential diagnostic, prognostic, and predictive biomarker in atrial fibrillation; AF-atrial fibrillation; DCCV-direct current electrical cardioversion Ref. Year Population

Table 1
Summary of recent research investigating differences in the levels of galectin-3 between patients with atrial fibrillation and patients in sinus rhythm

Table 2
Summary of recent research investigating differences in the levels of galectin-3 between patients with different types of atrial fibrillation ↑ increased, AF atrial fibrillation,ELISA enzyme-linked immunosorbent assay, gal-3 galectin-3, h hours, LAA left atrial appendage, LAVI left atrial volume index, LV left ventricular, lvl levels, MS metabolic syndrome, NOAF new-onset atrial fibrillation, pts patients, ref. reference, SR sinus rhythm, w/o without

Table 3
Summary of recent research investigating the role of galectin-3 in patients with atrial fibrillation undergoing cardioversion ↑ increased, AF atrial fibrillation, DCCV direct current electrical cardioversion, ELISA enzyme-linked immunosorbent assay, gal-3 galectin-3, LA left atrial, LV left ventricular, lvl levels, pts patients, ref. reference, SR sinus rhythm, w/o without

Table 4
Summary of recent research investigating galectin-3 as a predictor of arrhythmia recurrence after ablation in patients with atrial fibrillation

Table 5
Summary of recent research investigating the role of galectin-3 in patients with atrial fibrillation undergoing surgeries and other invasive procedures ↑ increased, AF atrial fibrillation, AHRE atrial high-rate episode, AMI acute myocardial infarction, CABG coronary artery bypass graft, CRT cardiac resynchronization therapy, ELFA Enzyme-Linked Fluorescent Assay, ELISA enzyme-linked immunosorbent assay, gal-3 galectin-3, HF heart failure, lvl levels, NOAF new-onset atrial fibrillation, POAF postoperative atrial fibrillation, pPCI primary percutaneous coronary intervention, pts patients, Ref. reference, w/o without