Atherosclerosis is a chronic vascular disease characterized by lipid deposition and fibrous tissue proliferation in the arterial wall, and it serves as the primary pathological foundation for cardiovascular diseases (CVDs) (1). Epidemiological data indicate that most cardiovascular-related deaths are attributable to atherosclerosis-associated conditions, with coronary artery disease (49.2%) and stroke (approximately one-quarter) being the most common outcomes (2). The pathological lesions in atherosclerosis typically begin in the intima of the arteries, where lipid accumulation and inflammatory responses play pivotal roles. Under the influence of risk factors such as hyperlipidemia, hypertension, smoking, and elevated low-density lipoprotein (LDL) levels, endothelial cells undergo varying degrees of damage (3). This leads to the recruitment of vascular smooth muscle cells and circulating monocytes, which engulf oxidized low-density lipoprotein (ox-LDL), forming lipid-laden foam cells and promoting plaque formation (3, 4). Macrophages, as the predominant immune cells in plaques, play a critical role in modulating pro-inflammatory and anti-inflammatory mechanisms at various stages of plaque development (5).

Classic cell death pathways, including necrosis, apoptosis, and autophagy, contribute significantly to the pathogenesis of atherosclerosis by expanding the necrotic core and releasing pro-inflammatory cytokines (6). However, these traditional mechanisms do not fully explain the iron metabolism abnormalities, lipid peroxidation, and lipid core expansion observed in atherosclerosis. Ferroptosis, an iron-dependent regulatory form of cell death, has gained increasing attention in recent years (7). Ferroptosis not only alters the function of endothelial and smooth muscle cells but also, by modulating macrophage activity, promotes foam cell formation, further destabilizing plaques and profoundly influencing the progression of atherosclerosis (6, 7).

Among the regulatory factors of ferroptosis, heme oxygenase-1 (HO-1) has recently emerged as a key player, garnering significant attention (8). Known for its role in antioxidant stress, HO-1 catalyzes the degradation of heme to release free iron, carbon monoxide, and biliverdin, helping to maintain iron homeostasis and exert antioxidative effects (9). In atherosclerosis-related cells, HO-1 plays a key role in inhibiting disease progression through its antioxidative effects, improving endothelial cell function, modulating macrophage polarization, and inhibiting the proliferation and migration of vascular smooth muscle cells (10, 11). However, growing clinical and pathological evidence suggests that excessive upregulation of HO-1 can result in iron accumulation, promote oxidative stress, and trigger ferroptosis, destabilizing plaques (12, 13). Moreover, HO-1 expression is positively correlated with the severity of atherosclerosis (12). Under conditions of high iron load and chronic inflammation—particularly in late-stage triggering receptor expressed on myeloid cells 2 (TREM2^low) foam macrophages, which are more prone to ferroptosis—overexpression of HO-1 can lead to mitochondrial dysfunction, increasing cellular vulnerability to oxidative damage (cell viability reduced by approximately 30%) (14). Recent studies have shown that in the later stages of atherosclerosis, HO-1 participates in a non-classical ferroptotic pathway, contributing to the expansion of the necrotic core and raising the risk of plaque rupture (15, 16).

This review will explore the dual role of HO-1 in ferroptosis within atherosclerosis, with an emphasis on its involvement in iron metabolism, oxidative stress, and plaque stability. These insights provide a deeper understanding of atherosclerosis pathophysiology and highlight potential targets for novel therapeutic strategies.

Ferroptosis, a form of iron-dependent programmed cell death identified in 2012, is characterized by the accumulation of lipid hydroperoxides, ultimately leading to cellular damage and death (7, 17). This process arises from an imbalance between oxidative stress and cellular antioxidant defenses, particularly through the inhibition of glutathione peroxidase 4 (GPX4). As a critical enzyme involved in the detoxification of lipid hydroperoxides, the inhibition of GPX4 leads to the accumulation of lipid peroxides, which damage cell membranes and trigger ferroptotic cell death (6, 18). Morphologically, ferroptosis is marked by mitochondrial shrinkage, increased membrane density, and the loss or reduction of mitochondrial cristae (7).

The classical ferroptosis pathway is regulated by the Cystine/GSH/GPX4 axis. Disruption of this pathway induces oxidative stress and lipid peroxide accumulation, ultimately resulting in cell death (19, 20) (Figure 1). In contrast, non-classical ferroptosis operates independently of this axis. Although iron accumulation still plays a critical role in non-classical ferroptosis, the mechanisms of oxidative stress are more complex and involve a wider array of regulatory molecules and signaling pathways. For instance, iron transporters, lipid peroxidases, and inflammatory cytokines are key regulatory molecules in this pathway, collectively contributing to lipid peroxidation and ferroptosis (21). While significant progress has been made in understanding non-classical ferroptosis, it remains an emerging area of research, and a consensus definition for this process has not yet been established.

In macrophages, ferroptosis is regulated by several interrelated pathways, including the Keap1-Nrf2, system Xc^-/GSH/GPX4, FSP1-CoQ10-NAD(P)H, and GTP-GCH1-BH4 pathways (22). In addition to these pathways, various factors influence ferroptosis in different cell types, affecting the process in both physiological and pathological contexts. Stressors such as elevated uric acid levels and autophagy deficiencies have been shown to promote ferroptosis by inhibiting the expression of protective genes regulated by the key transcription factor nuclear factor erythroid 2–related factor 2 (Nrf2) (23, 24). In contrast, compounds like micheliolide, interleukin-37, melatonin, and certain Chinese herbal medicines exert protective effects by activating Nrf2, thereby enhancing the expression of GPX4 and reduced glutathione (GSH) and inhibiting ferroptosis (25–33). Conversely, agents such as ox-LDL, ferric ammonium citrate, and lnc-MRGPRF-6:1 interfere with GPX4 function or modulate ferroptosis-related signaling pathways, facilitating lipid peroxidation and ferroptotic cell death (34–41) (Figure 2).

In-depth exploration of these molecular mechanisms provides critical insights into macrophage ferroptosis and identifies potential therapeutic targets for atherosclerosis.

HO-1, encoded by HMOX1, is a heme-metabolizing enzyme predominantly expressed in the spleen and reticuloendothelial system (42). Under inflammatory, infectious, or oxidative stress stimuli, HO-1 expression is significantly upregulated, leading to increased synthesis and release (43). In addition to its essential role in maintaining homeostasis through the regulation of inflammatory and immune responses, HO-1 is also closely associated with genetic polymorphisms that influence physiological equilibrium (9, 43).

Heme, an iron-rich porphyrin compound, is widely distributed across various cell types and represents the most significant source of iron ions aside from ferritin (44). Its iron ions participate in oxygen binding and electron transfer via redox reactions. However, excessive accumulation of free heme can cause cellular damage, necessitating strict regulation of its intracellular concentration and localization (44, 45). HO-1 plays a crucial role in this regulatory process by catalyzing the oxidative cleavage of heme at the α-methene bridge carbon, yielding biliverdin, carbon monoxide, and free iron ions, thereby precisely modulating the dynamic balance of the intracellular heme pool (9, 43) (Figure 1). Biliverdin is converted to bilirubin, a potent antioxidant, while carbon monoxide modulates vasodilation and anti-inflammatory processes (44). In macrophages, both HO-1 and its isoform HO-2 participate in the degradation of heme derived from senescent or damaged red blood cells, thereby playing a crucial role in maintaining systemic heme homeostasis through their enzymatic activities (46). Beyond their central role in maintaining systemic heme homeostasis, HO-1 is also closely linked to ferroptosis. A sequencing-based study has identified HMOX1 as a crucial ferroptosis-related differentially expressed gene, suggesting that alterations in its expression levels may impact cell survival (8). Notably, HO-1 exerts either cytoprotective or cytotoxic effects depending on the pathological context, thus being characterized as a “beneficial” or “detrimental” molecule (43). An experiment conducted on Apo E knockout mice fed a high-fat diet to induce atherosclerosis, along with in vitro studies on macrophage-derived foam cells, has confirmed that HO-1 plays a dual role in ferroptosis: low expression enhances cell viability and reduces lipid peroxidation, whereas high expression can induce ferroptosis in macrophage-derived foam cells (13).

As research progresses, more transcription factors have been identified as regulators of HO-1 expression, with increasing potential for clinical applications. Nrf2 is a key regulator, promoting HO-1 expression by activating the oxidative stress response (47). In contrast, BTB and CNC homology 1 (BACH1) suppresses HO-1 transcription under normal conditions, and stress signals lead to its degradation, allowing Nrf2 to initiate HO-1 expression (48, 49). Additionally, activator protein 1 (AP-1), heat shock factor 1 (HSF1), nuclear factor kappa B (NF-κB), and hypoxia-inducible factor 1 alpha (HIF-1α) contribute to the activation of HO-1 under various stress conditions (49).

In conclusion, the dual role of HO-1 is determined by a complex network of regulatory factors. Currently, its dual effects lack fixed boundaries or a clear definition. Further research is needed to fully elucidate the context-dependent effects of HO-1.

HO-1 plays a dual role in atherosclerosis, particularly in the regulation of ferroptosis, which presents both therapeutic opportunities and challenges (13). On one hand, moderate HO-1 activity exerts protective antioxidative effects, maintaining cellular homeostasis and potentially promoting plaque stability in atherosclerosis. On the other hand, overexpression of HO-1 leads to pathological iron overload, enhanced oxidative stress, and ferroptotic cell death, which can destabilize plaques and increase the risk of adverse cardiovascular events (13, 14). Due to the complexity and vastness of the HO-1 regulatory network, the dual role of HO-1 has yet to be clearly defined or delineated. Both excessive inhibition and activation of HO-1 may carry therapeutic risks, highlighting the difficulty of balancing its dual effects. Effectively suppressing the ferroptotic consequences of HO-1 overexpression, while preserving its beneficial antioxidative function, remains a major challenge in atherosclerosis treatment.

HO-1 can exert both beneficial and harmful effects at different pathological stages, making the regulation of its temporal and spatial expression crucial. For instance, in the late stages of atherosclerosis, inhibition of HO-1 expression effectively mitigates its detrimental effects, such as oxidative stress imbalance, mitochondrial damage, and ferroptosis (12, 14). Additionally, directly targeting ferroptosis or the HMOX1 gene can significantly alleviate the negative impacts of HO-1. However, targeting HO-1 at any stage may compromise its antioxidant function. Therefore, it is essential to develop targeted therapeutic strategies that selectively modulate HO-1 activity without undermining its protective effects. Current research focuses on counteracting HO-1-mediated ferroptosis through the following strategies: using HO-1 inhibitors (e.g., zinc protoporphyrin-IX (ZnPP)), ferroptosis inhibitors (e.g., ferrostatin-1, iron chelators), modulating key signaling pathways (e.g., targeting the Nrf2/Keap1 axis), and microRNA-mediated inhibition (e.g., miR-24, miR-200c) (69). Additionally, future approaches might integrate advanced drug delivery systems, such as nanoparticles, to locally deliver HO-1 inhibitors to atherosclerotic plaques, thereby improving treatment specificity and efficacy. This approach would allow for precise regulation of HO-1, inhibiting its harmful effects while preserving its antioxidative function. It is important to note that HO-1-mediated ferroptosis has been shown to inhibit the progression of diseases such as glioblastoma, further revealing the potential therapeutic value of HO-1 in other pathological contexts (95). Future research should further investigate the multifaceted functions of HO-1 and assess its therapeutic potential in different pathological backgrounds.

Given that ferroptosis results in cellular rupture and exacerbates inflammation, it remains unclear whether HO-1-induced ferroptosis could establish a positive feedback loop that drives disease progression. While this mechanism is theoretically plausible, the role of HO-1 in different cell types and microenvironments remains controversial, as experimental findings are sometimes inconsistent (9, 12, 15). Furthermore, in the pathological context of atherosclerosis, the interactions between HO-1 and other forms of regulated cell death, such as apoptosis, necroptosis, and necrosis, are primarily confined to the regulatory role of HO-1. Although theoretically, crosstalk between HO-1-driven ferroptosis and these alternative cell death pathways is conceivable, direct experimental evidence remains scarce. Therefore, although HO-1 has demonstrated promising therapeutic potential in cellular and animal models, the clinical application of HO-1 remains uncertain and warrants further experimental investigation.

In conclusion, HO-1 holds significant potential as a therapeutic target for atherosclerosis, but its clinical application faces many challenges. Future research should further clarify the specific roles of HO-1 in different cell types and pathological conditions, and combine clinical studies to assess its therapeutic value in atherosclerosis. Through more comprehensive experimental studies, we aim to provide a theoretical basis for the clinical modulation of HO-1, thus advancing its practical application in atherosclerosis treatment.