Metformin Hydrochloride Suppresses Achilles Tendon Ossification via Nr4a1/Wnt/β-catenin Inhibition

Study Background and Research Question

Heterotopic ossification (HO) is defined by abnormal bone formation within soft tissues, such as tendons, muscles, and ligaments. This pathological process frequently follows trauma or surgery, and leads to pain, impaired mobility, and a substantial reduction in quality of life. Particularly in orthopedic settings, up to 14–28% of patients undergoing Achilles tendon repair develop tendon-related HO, and the incidence after procedures like anterior cruciate ligament reconstruction ranges from 10–20% (reference study). Despite the clinical prevalence, effective nonsurgical interventions remain elusive, largely due to incomplete understanding of the molecular mechanisms driving HO.

Recent evidence has highlighted the importance of local signaling cascades in tendon calcification and HO. The Wnt/β-catenin pathway is particularly implicated in osteogenic gene expression and ectopic bone formation. Additionally, nuclear receptor subfamily 4 group A member 1 (Nr4a1) has emerged as a potential modulator of bone and soft tissue calcification. However, the specific interplay between these factors in tendon-derived stem cells (TDSCs) and their modulation by pharmacological agents remain insufficiently characterized.

Metformin Hydrochloride (Metformin HCl), a well-characterized AMPK signaling pathway modulator and inhibitor of hepatic gluconeogenesis, has demonstrated pleiotropic effects beyond glycemic control, including anti-inflammatory and bone-protective properties. This prompted the central research question: Can metformin prevent or attenuate heterotopic ossification in tendon tissue, and if so, through which molecular mechanisms?

Key Innovation from the Reference Study

The reference study (Danxia Zheng et al., 2026) delivers a significant advance by demonstrating that metformin not only suppresses pathological ossification in a mouse Achilles tendon model, but does so via selective inhibition of the Nr4a1/Wnt/β-catenin signaling axis. While the metabolic and anti-inflammatory actions of metformin are established, this work identifies a specific non-metabolic pathway—downregulation of Nr4a1 and its downstream effect on Wnt/β-catenin—as a principal mechanism for reducing osteogenic differentiation in TDSCs. This positions metformin as a tool for dissecting molecular crosstalk in tendon calcification and offers a prospective avenue for targeted HO interventions.

Methods and Experimental Design Insights

To dissect the effect of metformin on heterotopic ossification, the authors employed both in vivo and in vitro models:

• In vivo: A mouse Achilles tendon HO model was established, followed by metformin administration. Ectopic bone formation was quantified using micro-computed tomography (micro-CT) and histological analysis.
• In vitro: Tendon-derived stem cells (TDSCs) were isolated and subjected to osteogenic differentiation protocols. Metformin was added in graded concentrations to assess dose-dependent effects. The deposition of calcium nodules (alizarin red staining) and expression of osteogenic markers (e.g., Runx2, ALP, Ocn) were measured via qPCR and immunoblotting.
• Transcriptomics: RNA-seq was performed to identify differentially expressed genes, focusing on Nr4a1, Wnt4, and β-catenin.
• Functional validation: The role of Nr4a1 was probed by both chemical agonism and genetic knockdown in TDSCs, followed by assessment of osteogenic differentiation and downstream signaling.

This multifaceted design allowed the dissection of metformin’s molecular impact at both tissue and cellular levels, while integrating unbiased transcriptomic discovery with mechanistic validation.

Core Findings and Why They Matter

• Metformin significantly reduced HO in vivo, as evidenced by decreased ectopic bone volume and lower expression of osteogenic genes in treated tendons (reference study).
• In vitro, metformin suppressed TDSC osteogenic differentiation in a dose-dependent manner, reducing both calcium nodule formation and the expression of key osteogenic markers.
• Transcriptomic analysis revealed marked downregulation of Nr4a1 in metformin-treated HO samples. Experimental activation of Nr4a1 enhanced osteogenesis, whereas knockdown suppressed it, confirming its pro-osteogenic role.
• Metformin also suppressed Wnt4 and β-catenin expression, indicating that the inhibition of HO is mediated by downregulation of the Nr4a1/Wnt/β-catenin axis.

These findings are notable because they clarify a mechanistic link between metformin and the suppression of pathological bone formation. The study establishes that targeting Nr4a1 in TDSCs disrupts Wnt/β-catenin-driven osteogenic programming, suggesting a tractable therapeutic target for HO beyond traditional metabolic endpoints.

Comparison with Existing Internal Articles

This work aligns with and extends insights from recent internal resources. For example, "Metformin HCl Inhibits Achilles Tendon Ossification via Nr4a1/Wnt Pathway" previously summarized how metformin attenuates HO via this signaling axis, but the reference study provides in-depth transcriptomic and functional validation, adding rigor and translational relevance.

Similarly, protocol guides highlight the use of Metformin HCl for interrogating the Nr4a1/Wnt/β-catenin pathway in both metabolic and musculoskeletal contexts, while reviews discuss its dual role as an AMPK signaling pathway modulator and as an inhibitor of hepatic gluconeogenesis. What distinguishes the present study is the focus on tendon-specific stem cell differentiation and the direct demonstration of gene regulatory effects in vivo and in vitro, bridging metabolic research and musculoskeletal translational science.

Limitations and Transferability

Despite the robust design, several limitations should be considered. First, the study is restricted to a mouse model and in vitro TDSCs, which may not fully recapitulate human tendon biology or clinical HO. Second, while the results suggest a central role for Nr4a1/Wnt/β-catenin, the possibility of additional signaling pathways or tissue context factors cannot be excluded. Third, dosing regimens, timing, and long-term effects of metformin in the context of musculoskeletal disease remain to be optimized and validated in higher-order models or clinical trials.

Transferability is promising for basic research into tendon calcification and related pathologies, but direct clinical application will require further study, especially regarding safety, efficacy, and optimal administration protocols in non-diabetic patients or other populations.

Protocol Parameters

• In vivo administration: Metformin HCl is typically given by oral gavage or intraperitoneal injection at doses ranging from 100–300 mg/kg/day in mouse models of HO; dosing and duration should be tailored to the experimental design and animal health status (see protocol).
• In vitro application: For TDSC osteogenic differentiation assays, metformin is commonly used at concentrations between 0.5–5 mM, with solubilization in DMSO or water, and solutions freshly prepared to maintain stability (internal guide).
• Transcriptomic profiling: Tissue or cell samples should be collected after sufficient metformin exposure (typically 7–14 days in vivo; 3–7 days in vitro) for RNA-seq or qPCR analysis of key genes (Nr4a1, Wnt4, β-catenin, Runx2, ALP, Ocn).
• TDSC functional validation: Use both chemical agonists and siRNA-mediated knockdown to dissect the role of candidate signaling nodes in differentiation.

Research Support Resources

Researchers seeking to replicate or extend these findings can utilize Metformin Hydrochloride (Metformin HCl) (SKU B1970) for both in vitro and in vivo HO protocols. This reagent is supplied as a solid, with recommended solubilization in DMSO or water and immediate use to ensure activity. For optimized protocols, troubleshooting strategies, and additional mechanistic background, see the referenced internal protocol guide and mechanism review. APExBIO provides quality-controlled Metformin HCl for laboratory research, supporting reproducibility in studies targeting the Nr4a1/Wnt/β-catenin signaling axis and related pathways.