Nuclear factor erythroid 2–related factor 2 (Nrf2) is a pivotal member of the Cap ‘n’ Collar (CNC) family of transcription factors. As a key regulator of the cellular antioxidant defense system, Nrf2 exerts its biological functions through the coordinated action of multiple functional domains (Karunatilleke et al., 2021). Among these, the Neh2 domain, located at the N-terminus, is critically involved in the interaction with Kelch-like ECH-associated protein 1 (Keap1), which facilitates the ubiquitination and proteasomal degradation of Nrf2 under basal conditions (Wang M. et al., 2019). In contrast, the C-terminal basic leucine zipper (bZIP) domain is essential for DNA binding and transcriptional activation (Zhang S. et al., 2022).

Under physiological (non-stressed) conditions, Keap1—primarily localized in the cytoplasm—acts as a major negative regulator of Nrf2. It forms a part of an E3 ubiquitin ligase complex, with Cullin3 (Cul3) serving as the scaffold protein, promoting continuous ubiquitin-dependent degradation of Nrf2 and thereby maintaining its low basal expression (Alonso-Piñeiro et al., 2021). However, upon exposure to oxidative stress, electrophilic agents, or metabolic perturbations, key cysteine residues in Keap1 (notably Cys151, Cys273, and Cys288) undergo oxidation or covalent modification. These modifications induce conformational changes in Keap1, impairing its ability to bind and target Nrf2 for degradation (Pribil Pardun et al., 2024).

As a result, stabilized Nrf2 accumulates in the cytoplasm and translocates into the nucleus, where it forms heterodimers with small Maf proteins. This complex specifically binds to the antioxidant response element (ARE) within the promoter regions of target genes, thereby initiating the transcriptional activation of a wide array of cytoprotective genes, including heme oxygenase-1 (HO-1), NAD(P)H quinone dehydrogenase 1 (NQO1), glutamate–cysteine ligase modifier subunit (GCLM), and ferritin heavy chain 1 (FTH1), among others (Zhai et al., 2022). Taken together, Nrf2 is widely regarded as the master regulator of intracellular redox homeostasis. Its activation plays a central role in orchestrating antioxidant responses, regulating metal ion metabolism, and suppressing various forms of regulated cell death—including ferroptosis—thereby maintaining cellular integrity under stress conditions.

Heme oxygenase-1 (HO-1) is one of the most prominent target genes in the Nrf2 transcriptional regulatory network (Song and Long, 2020). It encodes a rate-limiting enzyme responsible for the degradation of heme into three key metabolites: carbon monoxide (CO), ferrous iron (Fe²⁺), and biliverdin (BV). Biliverdin can subsequently be converted into bilirubin (BR) by the enzyme biliverdin reductase (Li et al., 2023b).

These metabolites—CO, Fe²⁺, and BV—exert significant pleiotropic effects in maintaining cellular homeostasis and modulating stress responses. As an intracellular signaling molecule, CO has anti-apoptotic, anti-inflammatory, and vasoregulatory properties (Wang et al., 2020). Meanwhile, BV and its derivative BR are fat-soluble antioxidants that can efficiently scavenge peroxides within the cell membrane’s phospholipid bilayer, thus mitigating oxidative damage (Xie F. et al., 2023). In terms of iron metabolism, the proteins ferritin, ferroportin (FPN), and transferrin receptor 1 (TfR1) constitute the iron homeostasis system, ensuring the safe transport, storage, and regulation of intracellular iron levels (Bogdan et al., 2016).

However, under conditions of sustained high expression of HO-1 or when iron regulation is impaired, excessive accumulation of Fe²⁺ can exacerbate oxidative stress. This is due to the Fenton reaction, where Fe²⁺ catalyzes the generation of highly reactive hydroxyl radicals (•OH), promoting lipid peroxidation and acting as a key driver of ferroptosis (Chen B. et al., 2024).

Thus, HO-1 exhibits a dose-dependent or context-dependent dual regulatory role in cellular stress defense and ferroptosis induction. Moderate activation of HO-1 confers protective effects against cellular damage, while excessive activation or dysregulation of downstream pathways can transform HO-1 into a pathogenic factor, contributing to cellular dysfunction and ferroptotic cell death.

The Nrf2/HO-1 signaling pathway is not only a central axis of the cellular antioxidant response but also plays a pivotal role in the regulation of several fundamental physiological processes. It orchestrates a tightly coordinated cytoprotective network that includes the following core functions: (1) Maintenance of Redox Homeostasis: Nrf2 mitigates the accumulation of ROS by upregulating a range of antioxidant enzymes, including HO-1, glutathione synthetase, superoxide dismutase (SOD), and catalase (CAT). Through this regulation, Nrf2 prevents mitochondrial dysfunction, DNA fragmentation, and protein oxidation, serving as the first line of defense against oxidative stress-induced cellular injury (Bu et al., 2023). (2) Iron Metabolism Remodeling and Homeostatic Regulation: Nrf2 modulates the expression of multiple genes involved in iron metabolism, such as ferritin, ferroportin (FPN), transferrin receptor 1 (TfR1), and hepcidin. These genes work in concert with Fe²⁺ released via HO-1-mediated heme degradation to maintain iron homeostasis (Kajarabille and Latunde-Dada, 2019). This regulatory mechanism is essential in protecting cells from iron overload-induced cytotoxicity, particularly under conditions of high metabolic activity or pathological iron accumulation. (3) Lipid Peroxidation Defense and Ferroptosis Suppression: In addition to its role in redox and iron balance, Nrf2 also regulates the expression of lipid peroxidation defense genes, such as glutathione peroxidase 4 (GPX4), ferritin heavy chain 1 (FTH1), and SLC7A11. These factors are essential for suppressing lipid ROS accumulation, preserving membrane integrity, and thereby inhibiting ferroptosis (Zhang et al., 2025). Notably, Nrf2 deficiency is often associated with GPX4 downregulation and increased phospholipid-derived ROS, which are critical initiating events in the ferroptotic cascade (Liao et al., 2024) (Figure 1).

Therefore, the Nrf2/HO-1 pathway serves as an integrated protective mechanism in response to various cellular stressors, regulating redox homeostasis, iron balance, and lipid peroxidation networks in a synergistic manner. However, its biological function exhibits significant tissue-specific and pathology-dependent variations. Some studies indicate that, under certain stimuli, this pathway may shift from a protective to a disease-promoting role, highlighting the complexity of its regulatory mechanisms.

Bone homeostasis depends on the precise coupling between osteogenesis and bone resorption (Lin et al., 2022; Huo G. et al., 2024). As a regulated way of cell death, ferroptosis shows significant heterogeneity in the response patterns of different bone-related cells (Figures 2 C,D).

In osteoblasts, the Nrf2/HO-1 signaling axis, as a core pathway regulating cellular antioxidant capacity and iron metabolism, is critical for inhibiting ferroptosis and protecting osteogenic function (Wu and Huang, 2024; Tonelli et al., 2018). Nrf2 transcriptionally activates a series of antioxidant enzymes, including GPX4, to effectively remove lipid peroxides in cell membrane phospholipids, thereby limiting the occurrence of ferroptosis (Fan et al., 2017; Yang et al., 2016). As the core enzyme of ferroptosis defense system, GPX4 can reduce phospholipid peroxides by GSH, which is a key factor to maintain the membrane integrity and functional stability of osteoblasts (Fang et al., 2020; Huang et al., 2024).

In addition to lipid oxidation, Nrf2 can also upregulate the expression of ferritin heavy chain 1 (FTH1), enhance the intracellular iron storage capacity, reduce the level of free Fe²⁺, and inhibit the production of hydroxyl radicals (•OH) caused by Fenton reaction, thus blocking the iron-catalyzed lipid oxidation reaction chain (Zeng et al., 2023; Liu et al., 2020). This mechanism fundamentally curbs the risk of activation of ferroptosis in osteoblasts.

In addition, HO-1, as a classical target gene of Nrf2, catalyzes the degradation of heme to produce products-CO, BV and Fe²⁺, which can participate in anti-apoptosis and anti-inflammatory processes under certain conditions. At the same time, it cooperates with the iron homeostasis system to regulate the Fe²⁺ load level (Martínez-Casales et al., 2021; Lv et al., 2024). Although the activation of HO-1 has ferroptosis potential in some contexts, its mild induction in osteoblasts is more likely to be biased towards inhibiting lipid peroxidation and ROS accumulation, thereby exerting a protective effect (Figure 3).

In summary, the Nrf2/HO-1 pathway constructs a multi-level barrier of osteoblasts to ferroptosis stress by integrating and regulating redox homeostasis, lipid antioxidant system and iron metabolism pathway. Its activation can not only slow down cell damage and apoptosis during bone formation, but also enhance osteogenic ability and promote bone matrix deposition and mineralization, which is expected to become a potential target for maintaining bone homeostasis and preventing bone loss.

The role of Nrf2/HO-1 pathway in osteoclasts shows a certain bidirectionality. Moderate Nrf2 activation has a protective effect on osteoclasts, but excessive HO-1 metabolites may promote osteoclast activity and lead to excessive bone resorption (Pan et al., 2021; Fang et al., 2024).

Moderate activation of Nrf2 can upregulate the expression of antioxidant enzymes and reduce the accumulation of ROS, thereby reducing the damage of osteoclasts caused by oxidative stress (Li et al., 2018). Oxidative stress is an important regulator of osteoclast function. Excessive ROS can cause osteoclast damage and aggravate bone resorption (Ji et al., 2023; XiaHumulus lupulus et al., 2023). Therefore, Nrf2 plays a protective role in bone metabolism by maintaining the antioxidant capacity of osteoclasts (Figure 4A).

However, excessive HO-1 metabolites, such as Fe²⁺, may have adverse effects. Ferroptosis is triggered by free radicals generated by Fenton reaction, which promote lipid peroxidation (Cai et al., 2024; Chen et al., 2023). Ferroptosis is a form of osteoclast death, and excessive Fe²⁺ can lead to excessive bone resorption (Chen Y. et al., 2024; Qu et al., 2024). Therefore, the role of Nrf2/HO-1 pathway in osteoclasts requires fine regulation. Moderate HO-1 activity has a protective effect on bone resorption, while excessive HO-1 activation may promote bone resorption and affect bone homeostasis (Figure 4B).

Taken together, this two-way regulatory mechanism reveals the importance of the precise balance of the Nrf2/HO-1 signaling axis in osteoclasts. Moderate activation has anti-ferroptosis and homeostasis maintenance effects, while excessive activation promotes ferroptosis and osteoclast dysfunction. This finding provides a new perspective for understanding the molecular mechanism of abnormal osteoclast activity in various bone metabolic diseases, and also lays a foundation for the development of precise treatment strategies for ferroptosis and bone resorption imbalance.

HO-1 produces a series of metabolites by degrading heme, including CO, Fe²⁺ and BV (Tun et al., 2020). These metabolites play an important role in the regulation of bone cell fate.

As one of the products of HO-1, CO has antioxidant properties (Zhang et al., 2020). At low concentrations, CO can protect bone cells by reducing the accumulation of ROS, improve mitochondrial function, and enhance the antioxidant capacity of cells (Wang et al., 2024). The inhibitory effect of CO on oxidative stress in osteocytes helps maintain bone homeostasis (Van Phan et al., 2013). Fe²⁺ is another important product of HO-1. Although it is essential for the physiological function of bone cells, excessive Fe²⁺ will generate free radicals through Fenton reaction, induce lipid peroxidation, and eventually lead to ferroptosis (Yang and Shang, 2022). As a form of iron-dependent cell death, ferroptosis poses a threat to the survival of bone cells. Therefore, the excessive accumulation of Fe²⁺ may have a negative impact on the health of bone cells. BV is another product produced during the degradation of heme by HO-1, which has a strong antioxidant effect (Wu and Hsieh, 2022). BV can remove excessive ROS and reduce the damage of oxidative stress to bone cells, thereby maintaining bone homeostasis (Zhou et al., 2021). Its antioxidant properties make it a key factor in protecting bone cells in HO-1 metabolites (Figure 5).

In summary, the regulation of HO-1 metabolites on the fate of osteocytes is highly environmentally dependent, and produces distinct biological effects under different cell types and different concentrations, which constitutes the molecular basis for the fine regulation of bone homeostasis. This environment-specific and cell-specific mode of action provides a new theoretical framework for understanding the pathological mechanism of bone metabolic diseases and developing targeted treatment strategies.

The role of Nrf2/HO-1 pathway in bone metabolic diseases, especially in osteoporosis and osteonecrosis, has received extensive attention (Li et al., 2019; Hu et al., 2022). In these diseases, decreased expression or functional inactivation of Nrf2 is often accompanied by increased ferroptosis, excessive bone resorption, and bone loss.

In the animal model of estrogen deficiency-induced osteoporosis, the expression of Nrf2 is significantly decreased, which is closely related to the inhibition of osteogenesis and the decrease of bone mineralization function (Liu et al., 2024). Studies have shown that Nrf2 activation can upregulate the expression of antioxidant enzymes, reduce oxidative stress and ferroptosis, thereby inhibiting bone resorption, promoting bone formation, improving bone structure, and slowing the process of osteoporosis and osteonecrosis (Xue et al., 2019). In addition, HO-1, as a downstream effector molecule of the Nrf2 pathway, its inducer can promote bone repair by reducing ROS accumulation and inhibiting bone resorption in the early stage (Han et al., 2022; Liu et al., 2025). Therefore, the regulation of Nrf2/HO-1 pathway has potential application prospects in the treatment of bone metabolic diseases such as osteoporosis and osteonecrosis (Figure 6).

Therefore, Nrf2 dysfunction caused by estrogen deficiency or glucocorticoid treatment leads to increased oxidative stress, iron metabolism disorders and lipid peroxidation, and ultimately damages bone structure. Based on these findings, multi-target treatment strategies from Nrf2 activators to ferroptosis inhibitors have shown clinical application prospects. However, there are still many challenges in this field. The regulatory mechanism of Nrf2/HO-1 pathway in different bone cells has not been fully elucidated. The targeted delivery and long-term safety of therapeutic drugs need to be verified, and the interaction with other bone metabolic networks needs further study.

Osteoporosis, characterized by decreased bone density and increased fracture risk, has emerged as a primary target for Nrf2/HO-1 pathway modulation. Nrf2 agonists have shown remarkable efficacy in preclinical osteoporosis models. Sulforaphane, through alkylation of Keap1 Cys151 residues, promotes Nrf2 nuclear translocation and ARE activation, leading to increased HO-1 and NQO1 expression (Lin et al., 2014). In ovariectomized rat models, sulforaphane treatment significantly improved BMD and trabecular architecture (Thaler et al., 2016).

Melatonin represents another promising therapeutic agent for diabetic osteoporosis. By activating the MT2-ERK/AKT-Nrf2/HO-1 cascade, melatonin reduces ROS levels, upregulates SLC7A11 expression, and increases GPX4 activity, thereby protecting osteoblasts from ferroptosis and improving bone mineral density (Hu et al., 2022) (Yan et al., 2022). Similarly, traditional Chinese medicine components have demonstrated efficacy: quercetin enhances osteoblast mineralization through PI3K/AKT-mediated Nrf2 activation, while simultaneously suppressing NF-κB inflammatory signaling (Xiao et al., 2023; Mao et al., 2024).

For postmenopausal osteoporosis, α-tocopherol (Vitamin E) functions as a lipid-chain termination agent, reinforcing membrane antioxidant barriers and improving implant biocompatibility (Lovati et al., 2018). These diverse approaches underscore the multifaceted therapeutic potential of targeting the Nrf2/HO-1 pathway in osteoporosis management.

Osteonecrosis, particularly glucocorticoid-induced osteonecrosis, involves excessive ferroptosis and compromised bone vascularization. Iron chelators have emerged as crucial therapeutic agents in this context. Deferoxamine (DFO) blocks the Fenton reaction by chelating Fe²⁺, leading to HIF-1α stabilization and enhanced VEGF release, thereby promoting vascularization and osteogenesis in bone defect models (Guo et al., 2023; Shen et al., 2024).

The nano-delivery system has shown particular promise for osteonecrosis treatment. Curcumin-loaded nanoparticles enhance bone defect repair through p62-mediated Keap1 degradation, resulting in sustained Nrf2 activation and dual antioxidant/anti-inflammatory effects (Wang, 2024; Astaneh et al., 2024). This targeted delivery approach improves bioavailability while minimizing systemic side effects (Cheng et al., 2017; Chen Y. J. et al., 2024).

Combined therapeutic strategies have proven especially effective. The synergistic use of DFO with lipid antioxidants like Ferrostatin-1 addresses both iron overload and lipid peroxidation, providing comprehensive protection against ferroptotic cell death in osteonecrosis (Guo et al., 2023; Valanezhad et al., 2021).

Inflammatory bone diseases, including rheumatoid arthritis and osteoarthritis, present unique therapeutic challenges due to the interplay between inflammation and oxidative stress. Dimethyl fumarate targets this dual pathology through Keap1 alkylation, promoting GSH synthesis while inhibiting ferroptosis. In osteolytic models, this approach reduces osteoclast activity and upregulates Runx2 expression (Sánchez-de-Diego et al., 2021; Yamaguchi et al., 2018).

For rheumatoid arthritis, HO-1 modulation requires careful balance. Tin protoporphyrin IX, a competitive HO-1 inhibitor, reduces CO production and suppresses excess HO-1-mediated bone resorption, effectively inhibiting bone loss in arthritis models (Ibáñez et al., 2011). This highlights the importance of context-dependent HO-1 regulation (Kajarabille and Latunde-Dada, 2019; Li et al., 2020; Yang et al., 2022; Castany et al., 2016).

Tanshinone IIA offers a multifaceted approach for radiation-induced bone injury through SIRT1-mediated Nrf2 deacetylation, driving ARE-dependent antioxidant expression while suppressing p38/MAPK inflammatory signaling. This dual action helps restore bone marrow microenvironment (Cao et al., 2024; Cheng et al., 2024).

Chronic iron overload conditions, such as those seen in transfusion-dependent thalassemia, require specialized therapeutic approaches. Deferiprone provides small-molecule chelation of labile iron, effectively preserving bone density in transfusion-induced iron overload (Shen et al., 2024). Similarly, deferasirox coordinates bidentate Fe³⁺ binding to reduce bone-matrix iron accumulation, significantly decreasing fracture risk in chronic iron overload patients (Casale et al., 2014).

These iron-specific interventions demonstrate the critical importance of maintaining iron homeostasis in bone health, particularly in patients with systemic iron metabolism disorders.

Novel therapeutic agents continue to emerge. Bardoxolone methyl (CDDO-Me), currently in Phase I trials, represents an ultra-potent Nrf2 activator through Keap1 Cys151 binding, offering potential trabecular bone protection with combined antioxidant and anti-inflammatory properties (Yang et al., 2023).

Advanced delivery systems are revolutionizing treatment approaches. Bone-targeted nanoparticle formulations enable precise delivery of Nrf2 agonists or traditional medicine components directly to affected bone tissue, maximizing therapeutic efficacy while minimizing off-target effects (Astaneh et al., 2024; Cheng et al., 2017; Chen Y. J. et al., 2024).

The integration of combination therapies—utilizing Nrf2 activators (Xiao et al., 2023; Cao et al., 2024), iron chelators (Guo et al., 2023), and ferroptosis inhibitors (Valanezhad et al., 2021)—represents the future of personalized bone disease treatment, allowing clinicians to address the specific pathological mechanisms underlying each patient’s condition.