Osteopontin in Atrial Fibrillation and Atrial Remodeling

Abstract

Atrial fibrillation is the most prevalent sustained cardiac arrhythmia, characterized by limited therapeutic options because of incompletely understood mechanisms of its development. The key underlying pathogenetic mechanisms involve atrial myocardial inflammation and fibrosis in response to pressure or volume overload, myocardial infarction, as well as metabolic alterations. Recent studies have highlighted the crucial role of osteopontin in diverse cardiovascular diseases, including atrial fibrillation. Osteopontin is a multifunctional extracellular matrix protein that has recently emerged as a potential mediator of atrial remodeling and arrhythmogenesis. Human studies have revealed that circulating osteopontin levels can serve as a valuable biomarker for atrial fibrillation and its progression. Furthermore, in vitro studies have demonstrated that osteopontin modulates several key cellular processes involved in atrial inflammation and fibrosis, including fibroblast activation, extracellular matrix remodeling, and proinflammatory cytokine production. This review critically examines the available evidence on the role of osteopontin in atrial remodeling and arrhythmia starting from the molecular mechanisms by which osteopontin contributes to structural and electrical remodeling of the atria, to its potential as a biomarker for atrial fibrillation.

Nonstandard Abbreviations and Acronyms

AKT

protein kinase B

ECM

extracellular matrix

ERK

extracellular signal‐regulated kinase

PLK2

polo‐like kinase 2

SPP1

secreted phosphoprotein 1

Atrial fibrillation (AF) is the most common sustained arrhythmia that is characterized by uncoordinated atrial electrical activity, resulting in ineffective and uncoordinated atrial contraction. ¹ Globally, the incidence and prevalence of AF are on the rise. The FHS (Framingham Heart Study) reported a 3‐fold increase in AF prevalence over the past 50 years, establishing it as the most prevalent cardiac arrhythmia worldwide. ² , ³ Current data indicate that >52 million individuals globally live with AF and atrial flutter. ⁴ European projections estimate an increase in AF incidence to 14 million by 2060, up from ≈8 million in 2016. ⁵ AF is associated with severe complications, including stroke, heart failure, cognitive impairment, and cardiac arrest, significantly impacting patient quality of life. ³

Structural and electrical atrial remodeling plays a pivotal role in the initiation and progression of various atrial arrhythmias. ⁶ Atrial electrical remodeling in AF is characterized by changes in the quantity and distribution of ion channels and gap junction proteins. These alterations result in a reduction of the atrial effective refractory period and an increase in atrial effective refractory period dispersion. ⁷ Structural remodeling is marked by the progressive accumulation of collagen and infiltration of the tissue with various immune cells, leading to atrial fibrosis, which is a consequence of cardiac fibroblast activation and dysfunction in the metabolism of the extracellular matrix (ECM). ⁸ Cardiovascular conditions associated with atrial myocardial stretching attributable to pressure and volume overload undermine atrial structural integrity, thus promoting atrial remodeling and dysfunction. ⁹ , ¹⁰ This disruption augments the risk of electrical remodeling, including the emergence of ectopic electrical activities. ³ , ¹¹ As a result, further degradation of atrial structure and contractile function occurs, precipitating arrhythmic activity such as AF. ³

Currently, AF is managed through minimally invasive procedures, surgical interventions, and pharmacologic treatments. However, more than half of patients undergoing ablation experience recurrence. ¹² , ¹³ The left atrial volume was shown to be the predicting factor for the success of catheter ablation. ¹⁴ Furthermore, medications fail to prevent AF onset or progression in 85% of patients and may increase the risk of other arrhythmias. ¹ These data highlight that the pathogenesis of AF remains incompletely understood, limiting the development of effective treatment strategies and suggesting that further studies are necessary to elucidate its pathogenesis and develop effective therapies.

Circulating factors, such as inflammatory mediators, neurohormonal agents, and metabolic factors, have been identified as significant contributors to the development of AF. ¹⁵ , ¹⁶ Emerging research indicates that modifications in the inflammatory and immune systems play a pivotal role in AF, with alterations in immune and inflammatory profiles being a critical factor in atrial electropathology. ¹⁷ These changes in immune cell composition and function within atrial tissue involve the mobilization and activation of immune cells, as well as modifications in immune molecular components, thereby establishing a new immune environment within the atrial myocardium and systemically. ¹⁷ , ¹⁸

However, there is currently an unmet need of knowledge on the factors that orchestrate these processes, including atrial tissue inflammation, fibrosis, and ion channel alterations, which collectively promote atrial tissue remodeling and arrhythmia. Many of the current therapeutic approaches primarily target the symptomatic manifestations of AF rather than targeting the underlying pathophysiological mechanisms, thereby contributing to high rate of recurrence. Therefore, a comprehensive understanding of the molecular mechanisms driving atrial remodeling and arrhythmia is essential for identifying causative targets and developing effective therapeutic strategies based on these mechanisms.

Recent preclinical and clinical studies indicate that osteopontin is involved in a range of cardiovascular and noncardiovascular diseases, where it may contribute to pathogenic mechanisms, serve as a biomarker of disease activity, and represent a potential therapeutic target. ¹⁹ , ²⁰ , ²¹ , ²² , ²³ Specifically, elevated level of osteopontin have been observed in several cardiovascular diseases, including dilated cardiomyopathy, atherosclerosis, chronic kidney disease, and coronary artery disease. ²⁴ , ²⁵ , ²⁶ , ²⁷ Recent evidence indicates that osteopontin participates in structural and electrical remodeling of the atria, thereby contributing to initiation and perpetuation of atrial arrhythmias. ²⁸ , ²⁹ , ³⁰ However, the molecular mechanisms linking osteopontin to atrial pathology remain incompletely characterized, its biomarker potential remains inadequately validated, and its therapeutic targetability remains unexplored. In the present narrative review, we address these gaps by synthesizing current knowledge on molecular, cellular, biomarker, and therapeutic relevance of osteopontin in atrial pathology and arrhythmogenesis. This narrative review synthesizes current knowledge on these aspects by targeted searches of PubMed and Google Scholar (inception to December 2023) using terms including “osteopontin” (and recognized synonyms: secreted phosphoprotein‐1 [SPP1]; bone sialoprotein‐1; and early T‐lymphocyte activation‐1), “atrial fibrillation,” “atrial remodeling,” “atrial fibrosis,” and “arrhythmia.” The collected evidence converges on 3 principal themes: (1) Circulating and atrial tissue osteopontin concentrations are altered in patients with paroxysmal and persistent AF, correlate with atrial functional and structural characteristics, and predict recurrence after catheter ablation. (2) Autocrine and paracrine osteopontin signaling in different atrial cell types is involved in hypertrophy, fibrosis, inflammation, and channelopathies in vitro. (3) Translational studies in animal models demonstrate positive correlation between myocardial osteopontin abundance and atrial fibrosis, inflammation, and inducible atrial arrhythmia.

BIOLOGY OF OSTEOPONTIN

Osteopontin, also known as SPP1, bone sialoprotein‐1, and early T‐lymphocyte activation‐1, is a multifunctional ECM protein that also acts as a cytokine, playing a key role in regulating immunity, inflammation, fibrogenesis, and tissue remodeling. ³¹ It is encoded by the SPP1 (secreted phosphoprotein 1) gene. Fibroblasts, osteoblasts, osteoclasts, macrophages, T lymphocytes, endothelial cells, and epithelial cells synthesize osteopontin at low levels under homeostatic conditions; however, inflammatory cytokines, mechanical stress, hypoxia, or tissue injury rapidly upregulate its expression during wound healing, tissue remodeling, fibrosis, development, and neoplasia. ³² , ³³ Several transcription factors regulate osteopontin expression in a cell type– and disease‐specific manner, including RUNX2 (Runt‐related transcription factor 2), ³⁴ c‐Jun/AP‐1 (Transcription factor Jun/Activator protein‐1), ³⁵ GLI1 (GLI Family Zinc Finger 1), ³⁶ NF‐κB (Nuclear Factor Kappa B), ³⁷ AP‐1, ³⁸ and Smad2/3 (Small mothers against decapentaplegic homolog 2/3). ³⁹ Furthermore, posttranslational modifications, including phosphorylation, O‐glycosylation, and sulfation, determine tertiary structure, receptor affinity, and half‐life. ⁴⁰ , ⁴¹ , ⁴² , ⁴³ Alternative splicing of the osteopontin gene generates distinct isoforms with differing receptor affinities and functions. ⁴⁴ , ⁴⁵ Moreover, full‐length osteopontin undergoes cleavage by thrombin, matrix metalloproteinases, plasmin, and cathepsin D, generating fragments of varying sizes with distinct biological activities. ⁴⁶ , ⁴⁷ , ⁴⁸ Osteopontin binds different receptors in a context‐ and cell type–specific manner to exert its effects. For example, the RGD motif binds integrins αvβ1, αvβ3, and αvβ5; the SVVYGLR motif interacts with α4β1 and α9β1; and the C‐terminus binds certain CD44 variants. ⁴⁹ , ⁵⁰ , ⁵¹ These interactions promote cell adhesion, contraction, and migration in physiological and pathologic contexts, including wound healing, inflammation, fibrosis, tissue remodeling, and tumorigenesis, and involve multiple upstream and downstream regulatory factors (Figure 1). Taken together, osteopontin is a stress‐responsive glycoprotein, which undergoes alternative splicing, proteolytic cleavage, and integrin/CD44 engagement to effect cell adhesion, migration, and remodeling in wound healing, fibrosis, inflammation, and cancer.

Figure 1. Osteopontin (SPP1) interaction with other associated genes related to atrial fibrosis retrieved from GeneCards.

Each node represents the gene, and the edges (lines connecting the nodes) represent the interaction between 2 genes. The central node is osteopontin (SPP1), and the surrounding nodes are MMP3, MGP, MMP7, ITGA8, VTN, BGLAP, ITGAV, MMP9, IBSP, CD44, ITGA4, ITGB1, RUNX2, BMP2, ITGB3, FN1, and GSTM1. The black lines are the interaction of SPP1 with other genes, whereas the gray lines in the background are the interaction among all other genes among themselves. The interaction map was generated using the STRING database and visualized in Cytoscape. BGLAP indicates bone γ‐carboxyglutamate protein; BMP2, bone morphogenetic protein 2; CD44, CD44 molecule (in blood group); FN1, fibronectin 1; GSTM1, glutathione S‐transferase Mu 1; IBSP, integrin‐binding sialoprotein; ITGA4, integrin subunit α 4; ITGA8, integrin subunit α 8; ITGAV, integrin subunit α V; ITGB1, integrin subunit β 1; ITGB3, integrin subunit β 3; MGP, matrix Gla protein; MMP, matrix metalloproteinase; RUNX2, RUNX family transcription factor 2; SPP, secreted phosphoprotein 1; and VTN, vitronectin.

OSTEOPONTIN REGULATION IN HUMAN ATRIAL TISSUES DURING AF

Multiple studies have demonstrated higher osteopontin expression in atrial tissue samples from patients with AF compared with those in sinus rhythm ⁵² , ⁵³ , ⁵⁴ (Figure 1). Transcriptomic analyses of atrial tissues from patients with AF identified osteopontin as a central hub gene that promotes fibrosis via the phosphoinositide 3‐kinase/protein kinase B (Akt)/cyclin‐dependent kinase inhibitor 1B signaling axis. ³⁰ Atrial macrophages, particularly the TREM2⁺ (Triggering receptor expressed on myeloid cells 2) subset, are a major source of osteopontin in fibrotic atrial tissue. ⁵⁴ Single‐cell RNA sequencing on left atrial tissue from patients with AF showed osteopontin from macrophages as one of the top upregulated genes. ⁵³ Osteopontin from these macrophages activates cardiac fibroblasts, promoting the production of ECM components, thus promoting tissue fibrosis. This structural remodeling impairs electrical conduction, facilitating the development and persistence of AF. ⁵³ , ⁵⁴ Osteopontin expression in human ex vivo atrial myocardium is increased by angiotensin II, suggesting that angiotensin II–induced osteopontin expression contributes to adverse atrial remodeling. ⁵⁵ Increased osteopontin expression correlates with the degree of atrial tissue fibrosis. ²⁹ In line with these data, osteopontin induced dose‐ and time‐dependent increases in collagen I and fibronectin expression in cultured human atrial fibroblasts, and it also promoted atrial fibroblast proliferation. ²⁹ Furthermore, analyses of atrial tissues from patients with AF demonstrated downregulation of polo‐like kinase 2 (PLK2) concurrent with elevated osteopontin plasma levels compared with sinus rhythm controls, ⁵² establishing an inverse correlation between PLK2 and osteopontin levels. In contrast, analyses of atrial tissues from patients with postoperative AF show no significant difference in osteopontin expression compared with those maintaining sinus rhythm after cardiac surgery. ⁵⁶ Collectively, these findings identify osteopontin as a pivotal molecule in AF pathogenesis, with elevated atrial tissue expression consistently linked to the condition. However, regional variations in osteopontin expression in the atria, regulation by angiotensin II, and inverse correlations with PLK2 levels highlight the complexity of its role in atrial remodeling. Osteopontin interacts as a central mediator with multiple ECM proteins, integrins, and signaling molecules (Figure 1), highlighting its connectivity with key ECM regulators, such as integrin subunits (ITGB1/3, ITGAV, ITGA4/8), matrix metalloproteinases (3/7/9, RUNX family transcription factor 2 [RUNX2], and fibronectin, which are all playing a role in promoting tissue fibrosis and structural remodeling.

CIRCULATING OSTEOPONTIN AS A BIOMARKER OF ATRIAL REMODELING AND ARRHYTHMIA

Emerging evidence supports circulating osteopontin as a potential biomarker for atrial remodeling and arrhythmia pathogenesis. Several existing studies have shown the key role of osteopontin in atrial arrhythmia and remodeling (Table). ²⁹ , ⁵² , ⁵⁷ , ⁵⁸ , ⁵⁹ , ⁶⁰ , ⁶¹ For example, elevated circulating osteopontin concentrations measured by ELISA have been documented in patients with AF compared with healthy controls, with progressive increases observed during disease progression from paroxysmal to persistent forms. ²⁹ In this cohort, osteopontin levels demonstrated a direct correlation with electroanatomic mapping‐derived markers of atrial fibrosis, ²⁹ which was recently confirmed in another study where osteopontin levels were high in patients with AF with atrial fibrosis, as determined in a cardiac electrophysiological study, compared with patients with sinus rhythm. ⁵² Notably, a subsequent study confirmed that elevated circulating osteopontin was observed specifically in patients with AF with histologically confirmed atrial fibrosis, differentiating them from controls with sinus rhythm. ⁵²

Table 1.

Summary of the Studies Assessing Circulating Osteopontin as a Biomarker in Patients With AF

Reference	Subjects	Aim of the study	Samples (assay)	Findings	Summary
Güneş 2017 57	Patients with paroxysmal AF (n=47) and persistent AF (n=13). Average age=54±11 y; 40% female, 60% male	To investigate the relationship between osteopontin level and AF recurrence after AF cryoablation	Serum osteopontin (ELISA)	Postprocedure osteopontin levels were significantly higher than preprocedure levels (32.18 vs 15.58 ng/mL; P<0.00) Patients with AF recurrence had significantly higher preprocedure osteopontin levels compared with those without recurrence (19.65 vs 13.29 ng/mL; P=0.035) Preprocedure osteopontin levels independently predicted AF recurrence (β=0.059; P=0.048; OR, 1.061 [95% CI, 1.001–1.125])	Higher preprocedural serum osteopontin levels and persistent AF type were independently associated with increased risk of AF recurrence following cryoballoon catheter ablation in patients with paroxysmal or persistent AF
Namba et al, 2017 58	Patients with persistent or chronic AF (n=21), aged 60–80 y, with hypertension, treated with warfarin for at least 12 mo, and scheduled to switch to rivaroxaban	To evaluate the effects of switching to rivaroxaban from warfarin on measured biomarkers	Serum osteopontin (ELISA)	Osteopontin levels significantly decreased after 6 mo of rivaroxaban treatment compared with baseline (268.3±46.8 to 253.4±47.1 ng/mL; P<0.01) Osteopontin reduction was accompanied by significant improvements in arterial stiffness markers (augmentation index and pulse‐wave velocity)	Switching from warfarin to rivaroxaban in patients with AF was associated with a significant decrease in osteopontin levels and improvements in arterial stiffness markers
Hijazi et al, 2020 59	Patients with AF from the ARISTOTLE trial (n=4124 without ischemic stroke/systemic embolism, n=282 with ischemic stroke/systemic embolism) and the RE‐LY trial (n=1062 without ischemic stroke/systemic embolism, n=149 with ischemic stroke/systemic embolism). All subjects had AF and were receiving oral anticoagulation	To evaluate the association between 268 plasma proteins and subsequent ischemic stroke in patients with AF to identify biomarkers that could predict ischemic stroke/systemic embolism	Plasma samples (Olink proteomic profiling [268 proteins measured])	Osteopontin levels were significantly associated with ischemic stroke/systemic embolism in both the ARISTOTLE and RE‐LY cohorts	Osteopontin was associated with increased ischemic stroke/systemic embolism risk across 2 studied cohorts
Molvin et al,2020 60	Swedish population‐based Malmö preventive project (n=1694) with mean age 69.5 y and 29.3% female, followed up for 9.7±3.1 y. Incident cases of AF (n=278)	To identify novel biomarkers and underlying pathophysiological mechanisms for incident AF using a multiplex proteomics	Plasma samples (Olink proteomic profiling [92 proteins measured])	Osteopontin was significantly associated with incident AF (adjusted for traditional risk factors and NT‐proBNP)	Osteopontin was independently associated with incident AF
Lin et al, 2020 29	Patients with AF (n=60), including paroxysmal AF (n=30) and persistent AF (n=30), healthy controls (n=30)	To determine the involvement of osteopontin in atrial fibrosis and elucidate the underlying mechanisms during this pathologic remodeling process	Circulating osteopontin (ELISA; specific values reported graphically as ng/mL but no exact values given in text)	Osteopontin levels were higher in patients with AF compared with controls, increased with AF progression, and positively correlated with low‐voltage areas indicative of atrial fibrosis.	Osteopontin was elevated in patients with AF
Siegbahn et al, 2021 61	Patients with AF from ARISTOTLE (n=4200, including 204 cases with major bleedings) and RE‐LY (replication cohort, n=1368, including 344 cases with major bleedings) studies. Median follow‐up of 1.74 y in ARISTOTLE and 1.96 y in RE‐LY	To explore associations between a wide range of biomarkers and bleeding risk in patients with AF on oral anticoagulation	Plasma samples (Olink proteomic profiling [276 proteins measured])	Osteopontin was independently associated with major bleeding in patients with AF in both ARISTOTLE and RE‐LY cohorts	Osteopontin was identified as an independent biomarker associated with major bleeding risk in patients with AF on oral anticoagulation
Künzel et al, 2021 52	Healthy control (n=4), AF undergoing pulmonary vein isolation with (n=8) or without (n=9) electrophysiologically determined LVZs	To assess whether atrial fibrosis severity correlates with plasma osteopontin in patients with AF with and without LA LVZs (fibrosis marker)	Plasma samples (ELISA)	Patients with AF with LVZs (fibrosis) had higher levels of circulating osteopontin	Patients with AF with atrial fibrosis have increased circulating osteopontin

AF indicates atrial fibrillation; ARISTOTLE, Apixaban for Reduction in Stroke and Other Thromboembolic Events in Atrial Fibrillation; LA, left atrial; LVZ, low‐voltage zone; NT‐proBNP, N‐terminal pro‐B‐type natriuretic peptide; OR, odds ratio; and RE‐LY, Randomized Evaluation of Long‐Term Anticoagulation Therapy.

Elevated preprocedure serum osteopontin concentrations measured by ELISA, when combined with persistent AF, were identified as independent predictors of arrhythmia recurrence following cryoballoon ablation. ⁵⁷ Although postprocedure osteopontin concentrations increased significantly across all patients, no differences were observed between those with or without arrhythmia recurrence. This suggests that baseline osteopontin levels, rather than postablation fluctuations, may influence clinical outcomes. ⁵⁷ A longitudinal study using multiplex proteomics (92 proteins measured) further demonstrated that elevated plasma osteopontin levels were independently associated with incident AF risk after adjustment for age, sex, body mass index, blood pressure, smoking, diabetes, coronary disease, heart failure, and NT‐proBNP (N‐terminal pro‐B‐type natriuretic peptide). ⁶⁰ However, no significant association was observed between osteopontin and prevalent AF following multivariable adjustment. ⁶⁰

Osteopontin has also been shown to associate with clinical outcomes in patients with AF. For example, analyses from the ARISTOTLE (Apixaban for Reduction in Stroke and Other Thromboembolic Events in Atrial Fibrillation) and RE‐LY (Randomized Evaluation of Long‐Term Anticoagulation Therapy) study cohorts using large‐scale proteomic profiling (268 and 276 proteins, respectively) identified osteopontin as a top biomarker linked to major bleeding events in patients with AF receiving anticoagulation therapy over 1 to 3 years, potentially mediated by its role in vascular calcification, ⁵⁹ , ⁶¹ supported by evidence indicating a direct osteopontin contribution to vascular calcification pathways. ⁶²

As these studies have shown, ⁵⁹ , ⁶⁰ , ⁶¹ the strength of osteopontin as a reliable candidate biomarker stems from its identification through hypothesis‐free proteomic profiling approaches, enabling unbiased discovery of associations between osteopontin and AF pathophysiology among numerous other measured proteins. However, reliance on proteomic assays necessitates independent validation using ELISA methods. Moreover, heterogeneity in sample matrices (plasma versus serum) and measurement techniques highlight the need for standardized, AF‐specific biomarker validation studies incorporating ELISA confirmation, longitudinal stability assessment, and clinically relevant cutoff values to establish the biomarker role of osteopontin in AF.

Collectively, these studies suggest that circulating osteopontin exhibits a strong clinical utility as a biomarker of atrial remodeling, with concentrations increasing proportionally to the severity of progressive atrial fibrosis and correlating with progression from paroxysmal to persistent AF. Its prognostic value includes predicting AF recurrence following ablation and identifying incident AF risk independent of conventional confounding factors. Furthermore, elevated osteopontin levels correlate with major bleeding events in anticoagulated patients, potentially mediated by vascular calcification pathways. These findings establish osteopontin as a valuable biomarker for tracking arrhythmia progression and predicting adverse clinical outcomes, necessitating further validation to establish its role in risk stratification and therapeutic decision‐making.

CELL‐SPECIFIC ROLES OF OSTEOPONTIN IN ATRIAL REMODELING

The contribution of osteopontin to atrial remodeling arises from its expression across a variety of atrial cell types, including fibroblasts, cardiomyocytes, endothelial cells, and inflammatory cell lineages (Figure 2). ⁶³ The distinct role of these cellular sources is explored in the following sections.

Figure 2. Upstream and downstream regulators of OPN expression in atrial remodeling.

PLK2 loss in cardiac fibroblasts increases OPN synthesis, promoting myofibroblast differentiation and a profibrotic phenotype. Rapid electrical stimulation upregulates OPN expression in atrial cardiomyocytes. SPP1⁺ macrophages represent the dominant cellular source of OPN in atrial fibrillation. In cardiac fibroblasts, OPN induces proliferation, myofibroblast differentiation, extracellular matrix production, and expression of collagen I, fibronectin, and α‐SMA. OPN promotes atrial macrophage proliferation and cardiac fibroblast activation. Circulating OPN may affect atrial tissue, whereas atrial tissue may secrete OPN into circulation. ECM indicates extracellular matrix; OPN, osteopontin; PLK2, polo‐like kinase 2; α‐SMA, α‐smooth muscle actin; and SPP1, secreted phosphoprotein 1. Created with BioRender.com.

CARDIAC FIBROBLAST–DERIVED OSTEOPONTIN

Under physiological conditions, atrial fibroblasts maintain ECM homeostasis and provide structural support to cardiomyocytes. In pathologic states, atrial fibroblasts serve as key mediators of atrial remodeling by their regulation of ECM turnover and composition, a process critical in the progression of atrial fibrosis and AF. ⁶⁴ For instance, atrial pressure or volume overload directly stimulates fibroblast proliferation and activation, increasing secretion of ECM proteins, such as collagen and matrix metalloproteinases, which drive atrial remodeling. ⁶⁵

Multiple growth factors, such as transforming growth factor‐β1, connective tissue growth factor, and platelet‐derived growth factor, activate atrial fibroblasts during disease progression, thereby promoting atrial remodeling. ⁶⁶ Similarly, inflammatory mediators, such as tumor necrosis factor, interleukin‐1β, interleukin‐6, and interleukin‐8, have also been implicated in atrial fibroblast activation and fibrosis. ⁶⁷ Furthermore, osteopontin signaling is critically involved in the pathologic activation of atrial fibroblasts. ²⁹ It activates the Akt/glycogen synthase kinase‐3β/β‐catenin signaling pathway in cardiac fibroblasts, promoting their proliferation, differentiation into myofibroblasts, and increased ECM production. ²⁹ , ³⁰ , ⁵² Interestingly, osteopontin induced the expression of collagen I and fibronectin in human atrial fibroblasts in a concentration‐ and time‐dependent manner. ²⁹ Similarly, another study showed that osteopontin induced the expression of collagen I and α‐smooth muscle actin via the phosphoinositide 3‐kinase/Akt signaling pathway in human cardiac fibroblasts in a concentration‐dependent manner. ³⁰

Although multiple signaling pathways may influence osteopontin signaling in atrial fibroblasts, emerging evidence indicates that PLK2 may play a key role, at least in part, in its upstream regulation. Atrial tissues from patients with AF exhibited reduced PLK2 expression and elevated osteopontin levels compared with tissues from controls with sinus rhythm. ⁵² Pharmacologic inhibition or genetic ablation of PLK2 increased spontaneous myofibroblast differentiation, suggesting that PLK2 suppresses fibroblast activation. ⁶⁸ Indeed, fibroblasts isolated from PLK2 knockout mice demonstrated spontaneous myofibroblast differentiation and increased osteopontin expression relative to wild‐type fibroblasts. ⁶⁹ Furthermore, genetic deletion or pharmacologic inhibition of PLK2 in cardiac fibroblasts induced a profibrotic phenotype shown by excessive osteopontin synthesis and secretion. ⁵² Similarly, myocardial tissue hypoxia was implicated in modulating PLK2 expression within the atrial myocardium. For example, hypoxia‐induced PLK2 promoter methylation via hypoxia‐inducible factor‐1 signaling was reported to contribute to PLK2 downregulation. ⁵² Subsequent loss of PLK2 activated the extracellular signal‐regulated kinase (ERK) 1/2 pathway, leading to increased osteopontin expression in human atrial fibroblasts. ⁵²

Taken together, these findings suggest that atrial fibroblasts are central to ECM remodeling and AF progression, with osteopontin emerging as a critical mediator of their pathologic activation. Osteopontin drives fibroblast proliferation, differentiation, and profibrotic factor synthesis via involving various signaling pathways, exacerbating fibrosis. Myocardial tissue hypoxia‐mediated PLK2 downregulation, observed in patients with AF, disrupts its suppressive role on fibroblast activation, leading to elevated osteopontin levels and fibrosis, highlighting atrial PLK2 and osteopontin as potential therapeutic targets to mitigate atrial remodeling.

ATRIAL CARDIOMYOCYTE–DERIVED OSTEOPONTIN

In AF, dysfunction of atrial cardiomyocytes, in conjunction with alterations in other cellular populations of the atria, contributes to extensive structural and functional atrial remodeling. ⁷⁰ Multiple signaling cascades mediate this dysfunction, including oxidative stress, ⁷⁰ altered calcium homeostasis, ⁷¹ inflammation, ⁷² and metabolic dysregulation in atrial cardiomyocytes. ⁷³ The relationship between these processes and the upstream or downstream signaling pathways of atrial cardiomyocyte osteopontin signaling remains undetermined. Nevertheless, osteopontin expression was significantly upregulated in cardiomyocytes isolated from rats subjected to rapid electrical stimulation in a rapid atrial pacing model, ³⁰ suggesting a potential role of osteopontin in atrial cardiomyocyte abnormalities. Despite well‐documented evidence of cardiomyocytes being the source of osteopontin and its association with diverse cardiac pathologies, ²⁰ , ⁷⁴ its specific functions and molecular mechanisms in atrial cardiomyocyte dysfunction underlying atrial remodeling and arrhythmia remain underexplored and warrant comprehensive investigation.

ENDOTHELIAL CELL–DERIVED OSTEOPONTIN

Although there are currently no studies directly demonstrating the role of osteopontin in endothelial cell function during atrial remodeling and arrhythmia, evidence from other cardiovascular contexts suggests a potential involvement of endothelial‐mediated osteopontin effects in such conditions. For instance, osteopontin expressed in endothelial cells has shown to be induced by various factors, including interferon‐γ, angiotensin‐II, and interleukin‐1β. ²⁰ , ⁷⁵ Systemic endothelial dysfunction persists in AF and is characterized by alterations in endothelial dysfunction markers, such as von Willebrand factor, soluble thrombomodulin, nitric oxide production, and soluble E‐selectin. ⁷⁶ Endothelial cell function regulation by osteopontin is dependent on its attachment with the integrin α_vβ₃, CD44, as well as the activation of ERK1/2 phosphorylation pathways. ⁷⁷ However, future studies will be required to study the precise mechanisms linking osteopontin to endothelial cell function in atrial remodeling and arrhythmia.

INFLAMMATORY CELL–DERIVED OSTEOPONTIN

Inflammatory cells, particularly macrophages, play a central role in the progression of AF. Several studies have demonstrated increased macrophage infiltration in the atrial tissue of patients with AF. ⁵³ , ⁷⁸ , ⁷⁹ Recent transcriptomic analyses showed a significant expansion of a distinct macrophage subpopulation characterized by elevated osteopontin expression (SPP1⁺ macrophages) in the atria of patients with AF. ⁵³ , ⁵⁴ The SPP1⁺ macrophage cluster was the dominant source of osteopontin expression in the atria of patients with AF. ⁵³ Functionally, SPP1⁺ macrophages were primarily recruited monocyte‐derived macrophages and exhibited enhanced expression of genes involved in lipid storage, intracellular cholesterol accumulation, and proinflammatory cytokine expression. ⁸⁰ Subsequently, osteopontin expressed by SPP1⁺ macrophages has been shown to increase atrial macrophage proliferation and fibroblast activation. ³⁰ In addition, osteopontin derived from circulating leukocytes may contribute to systemic inflammation and promote recruitment of inflammatory cells to the myocardium in various cardiac conditions. ⁸¹ Taken together, these findings suggest that osteopontin‐expressing macrophages regulate the inflammatory signaling networks that promote atrial remodeling in AF.

OSTEOPONTIN REGULATION IN IN VIVO MODELS OF ATRIAL REMODELING

The role of osteopontin in atrial remodeling and arrhythmogenesis remains insufficiently explored. Nevertheless, recent in vivo rodent studies indicate that osteopontin may significantly influence atrial remodeling through the modulation of atrial tissue fibrosis and inflammation. In the obese spontaneously hypertensive rat (SHR) model, a comorbidity‐driven model reflecting hypertension and obesity‐associated atrial pathology, both left atrial Spp1 gene expression and plasma osteopontin concentrations were significantly elevated compared with lean SHR rats and normotensive control rats. ⁸² This increase in osteopontin expression in obese SHR rats correlated with enhanced ECM formation, atrial fibrosis, conduction abnormalities, and increased susceptibility to AF compared with SHR‐lean and controls. ⁸² Similarly, in the murine Hypertension, obesity and mitral valve regurgitation (HOMER) model of atrial disease, which integrates hypertension, obesity, and mitral valve regurgitation to mimic human AF comorbidities, a marked expansion of Spp1⁺ macrophage subpopulations was observed compared with controls. ⁵³ These Spp1⁺ macrophage clusters coexpressed Trem2 and Cd9, analogous to the Spp1⁺ macrophage cluster identified in human patients with AF. ⁵³ Deletion of Spp1 in bone marrow–derived cells using bone marrow transplantation, which disrupts Spp1⁺ macrophage development that populates the atria, resulted in reduced AF inducibility and burden in HOMER mice, compared with control mice. ⁵³ Another study has shown that genetic deletion or pharmacologic inhibition of PLK2 induced a profibrotic phenotype in cardiac fibroblasts, which was associated with osteopontin overexpression. ⁵² Consequently, PLK2‐deficient mice developed atrial fibrosis and exhibited increased susceptibility to inducible atrial arrhythmia through upregulation of osteopontin expression. ⁵² Moreover, treatment with mesalazine, an ERK1/2 inhibitor, prevented osteopontin overexpression and reversed atrial remodeling in PLK2 knockout mice. ⁵² Furthermore, cell–cell interaction analysis suggested that macrophage‐derived osteopontin interacts with various immune and stromal cells in the atria through integrins, CD44, and prostaglandin E2 receptor 4, functioning as a pleiotropic amplifier of atrial inflammation and fibrosis. ⁵³ Additionally, studies in angiotensin II–infused mice demonstrated that osteopontin is required for activator protein 1 activation, miR‐21 induction, ERK/AKT signaling activation, PTEN (Phosphatase and tensin homolog)/SMAD7 suppression, and subsequent collagen accumulation in vivo, supporting its role as a regulator of profibrotic structural remodeling pathways. ⁸³ These studies collectively highlight the crucial role of osteopontin in atrial remodeling in in vivo settings. However, additional research is necessary to explore the effects of osteopontin overexpression or deletion in a cell‐specific manner to determine the precise cells and processes modulated by osteopontin in in vivo atrial remodeling models.

DISCUSSION

A growing body of evidence supports the role of osteopontin as a critical mediator in the pathogenesis of atrial remodeling and arrhythmia, integrating proinflammatory signaling with fibrotic remodeling and electrical‐physiological alterations. Converging evidence from human studies, in vitro models, as well as preclinical research highlights involvement of osteopontin in structural transformation of atria and its potential utility as a diagnostic and prognostic biomarker. Increased expression in atrial tissues and elevated circulating levels of osteopontin are closely associated with the severity of AF, particularly with progressive fibrosis and inflammation in atrial tissue. Human atrial biopsies from patients with AF show association of the upregulated osteopontin with fibroblast activation via the phosphoinositide 3‐kinase/Akt pathways, collagen deposition, and macrophage‐driven inflammation. The interaction between osteopontin and angiotensin II highlights its role in pressure‐ and volume‐overload–induced atrial remodeling, whereas its inverse relationship with PLK2 reveals a novel regulatory axis, in which PLK2 suppression exacerbates atrial tissue fibrosis. Elevated circulating osteopontin levels further stratify progression of AF, predicting postablation recurrence and bleeding risks in anticoagulated patients through partially elucidated pathways. In vivo models, such as obese SHR rats and HOMER mice, confirm the contribution of osteopontin to conduction abnormalities and arrhythmogenicity, notably through SPP1⁺ macrophages that enhance atrial inflammation.

Certain limitations remain in this area of research. An important limitation of current biomarker evidence is the inability to distinguish causation from association. Although elevated circulating osteopontin correlates with AF presence, progression, and adverse outcomes, the temporal relationship remains incompletely characterized. However, human atrial tissue studies suggest osteopontin functions as both a pathogenic mediator and a disease severity marker. Osteopontin secreted by TREM2⁺ atrial macrophages actively promotes fibroblast activation and ECM production, driving structural remodeling. Parallel dysregulation in atrial tissue and circulation, combined with dose‐dependent fibrotic effects and its identification as a transcriptomic hub gene, suggests a mechanistic link between local pathology and systemic biomarker elevation. Whether circulating osteopontin predominantly reflects spillover from active remodeling or contributes to atrial remodeling processes remains unresolved. There is also a lack of cell‐specific mechanistic data, which would aid in elucidating precise cell‐specific contributions of osteopontin. PLK2‐osteopontin interactions are implicated in fibroblasts; however, the upstream regulators and downstream effectors remain inadequately characterized. Furthermore, the role of PLK2‐osteopontin interactions in atrial cardiomyocytes or immune cells is yet to be explored. The utility of circulating osteopontin as a biomarker is compromised by the point about its causative role in AF, such as whether it directly drives remodeling or merely reflects upstream pathology. Although preclinical models offer valuable insights, the role of osteopontin in atrial remodeling was studied in a limited number of in vivo experiments, which do not fully replicate the complexity of human AF, particularly in aging or comorbid conditions, such as diabetes, thereby limiting its translational applicability. Recent acknowledgment of the significance of inflammatory and fibrotic pathways in atrial remodeling focused on various fibrotic and inflammatory mediators. The identification of osteopontin, which displays both inflammatory and fibrotic properties, as a hub gene in transcriptomic analyses provides a significant advance, positioning it as a multifunctional orchestrator of structural and electrical alterations. Although available reports investigating osteopontin in AF are promising, this protein is not specific to atrial pathologies. For example, osteopontin has been implicated in diverse cardiac conditions, ranging from myocarditis to cardiac hypertrophy and heart failure. ²⁰ , ²³ , ⁸⁴ Consequently, a significant challenge remains in investigating the atrial‐specific roles of osteopontin through carefully designed in vitro and in vivo studies, detailed histologic analyses of atrial tissues from patients with atrial arrhythmias, and large‐scale biomarker studies focusing on AF development, recurrence, and therapeutic response.

RESEARCH GAPS AND FUTURE DIRECTIONS

Several research gaps necessitate future studies, which include the following points. First, although osteopontin has emerged as a candidate biomarker through unbiased proteomic approaches in several studies, specific biomarker studies remain limited. Large‐scale analytical validation and prospective AF cohort studies are needed to establish standardized osteopontin assays, reference ranges, and clinically validated cutoffs for risk stratification. Such validation would enable osteopontin measurement to inform clinical decision‐making, including patient selection for ablation procedures and intensity of anticoagulation monitoring. Second, it remains unclear whether osteopontin directly affects atrial cardiomyocytes that contribute to the ectopic or abnormal atrial electrical activity underlying the development and progression of atrial arrhythmias. Elucidating these direct electrophysiological effects could reveal novel therapeutic targets for rhythm control strategies. Third, the interaction of osteopontin with metabolic dysregulation in obesity and metabolic disorders, which are critical in arrhythmia development, is unclear. Understanding these interactions could inform personalized prevention strategies in high‐risk metabolic populations. Fourth, despite substantial evidence demonstrating osteopontin dysregulation in atrial arrhythmia and structural remodeling, specific therapeutic approaches, including small‐molecule inhibitors or neutralizing antibodies directed against osteopontin signaling, have not yet been systematically evaluated in animal models of atrial remodeling to establish whether this strategy will yield a clinically meaningful benefit. Successful preclinical validation of osteopontin‐targeted therapies could help to develop mechanism‐based AF treatment. Finally, further studies focused on the development of cell‐specific interventions could elucidate crucial for atrial remodeling cell‐specific roles of osteopontin, while minimizing off‐target effects. Such precision approaches would be essential for clinical translation, given diverse physiological roles of osteopontin across multiple organ systems. In summary, addressing these gaps could elucidate osteopontin‐mediated mechanisms in AF pathogenesis and inform targeted therapies.

SUMMARY

This review consolidates the central role of osteopontin in the pathophysiology of atrial remodeling and arrhythmia, emphasizing its dual utility as both a biomarker and a therapeutic target. Elevated osteopontin levels in atrial tissues and circulation correlate with fibrosis severity, progressive arrhythmia, and adverse clinical outcomes, including postablation recurrence and bleeding associated with anticoagulants. Mechanistically, osteopontin facilitates fibroblast activation, macrophage‐mediated inflammation, and ECM remodeling through pathways such as phosphoinositide 3‐kinase/Akt and PLK2/ERK, and its interaction with angiotensin II and hypoxia indicates responsiveness to stress factors. The translational potential of osteopontin lies in its ability to refine AF management. As a biomarker, it offers prognostic insights along with established traditional clinical parameters, potentially informing decisions about ablation candidacy or anticoagulation strategies, although further validation studies are required for such strategies. Therapeutically, targeting osteopontin signaling, through either its direct inhibition or upstream/downstream modulation using specific therapeutic approaches, including small‐molecule inhibitors or neutralizing antibodies, may mitigate fibrosis and inflammation, addressing the underlying causes of remodeling. Future research should prioritize cell‐specific mechanistic studies, including all key cell types of atria. Integrating multiomics approaches may elucidate the osteopontin interactome, identifying key coplayers driving atrial remodeling processes. Addressing these gaps will ultimately enhance our understanding on the role of osteopontin in atrial remodeling and rhythmogenesis.

Disclosures

None.