Catalase: The golden key to regulate oxidative stress in breast cancer

Abstract

Catalase (CAT) is a kind of tetrameric protein in the human body, play as a key regulator for controlling oxidative stress. The main function of CAT is to regulate the concentration of hydrogen peroxide (H₂O₂) by catalyzing the decomposition of H₂O₂. At present, it is reported that CAT is also involved in regulating the oxidative stress in tumor cells, and its expression level is significantly related to the development of breast cancer (BC). In addition, CAT with different expression patterns, was related in the proliferation, invasion, treatment and prognosis of BC cells. Meanwhile, BC is a common and well-known cancer among women worldwide, and its incidence has been increasing in recent years. Therefore, in-depth study of CAT in the pathogenesis and progression of BC is of great significance for the future treatment and diagnosis. The present review summarized the effects of oxidative stress on cancer cells, and emphasized the key role of CAT in the development of BC, which provides a key clue for promoting research on BC and selecting therapeutic targets.

Key Words: Catalase; Breast cancer; Hydrogen peroxide; Oxidative stress; Treatment

Core Tip: Catalase (CAT) plays a critical role in breast cancer (BC) by regulating oxidative stress. Its expression is regulated by hormone receptors and epigenetics and is closely related to the metastatic site and prognosis of BC. Therapeutic strategies targeting CAT provides a potential promising new avenue for advancing BC treatment.

INTRODUCTION

The most common type of cancer in females, breast cancer (BC) has a high mortality rate all over the world, with increased incidence and mortality, brought huge health burden and economic pressure to the people[1]. However, the survival rate of patients with BC at early stage, can be greatly improved after active treatment[2]. Therefore, research on early diagnosis biomarkers for BC is of great significance for patients.

On the other hand, the production of reactive oxygen species (ROS) was indicated as a risk for the incidence of invasive BC with bidirectional effect[3]. It is indicated that ROS has multiple functions that can promote the development of malignant tumors[4], while overproduction of ROS can disrupt the antioxidant system of tumor cells, leading to cell death[5]. Thus, ROS is also thought to be potential targets of invasive BC, while antioxidants prevent such damage caused by ROS. As part of the antioxidant system in the body, many antioxidant enzymes (AE) also enhance immune defenses and reduce the risk of disease and cancer.

Catalase (CAT) is one of the important AE in the human body, which protect the body from damage caused by oxidative stress. It is found that AE represented by CAT are highly expressed to protect cells from excessive formation of ROS[6]. It is well-known that the main functions of CAT are the catalytic decomposition of hydrogen peroxide (H₂O₂) into water and molecular oxygen (O₂)[7], and the concentration of H₂O₂ in human body adjustment plays an important role. Due to its close association with oxidative stress in the human body, CAT exhibits distinct expression patterns in tumor cells compared to normal cells. Excessive or insufficient expression of CAT can exert varying impacts on tumor growth. It is revealed that there is a significant relationship between the expression level of CAT and BC cells[8], closely related to the proliferation and invasion ability of BC cells[9], which provides new clues for further research on the pathogenesis of BC. This article summarized the effect of CAT on oxidative stress in tumor cells, and discussed its application in the diagnosis and prognosis of BC.

DUAL EFFECTS OF ROS ON MALIGNANCIES

The balance between oxidative damage and antioxidant protection is important to keep homeostasis in normal aerobic cells. Insufficient antioxidant clearance or excessive formation of O₂ free radical (FR) results in a condition called oxidative stress. The original definition of oxidative stress was the disruption of the balance between oxidants and antioxidants, which is conducive to the production of oxidants[10]. It is proved that a small amount of oxidant produced endogenously or externally is essential for maintaining balance in cells, tissues, and organisms[11,12]. Hence, this physiological oxidative stress can be regarded as a good factor under certain circumstances[13]. However, when the level of oxidants in the body exceeds normal physiological levels, it can lead to pathological change in the organism. Therefore, the current definition of oxidative stress is added that it can also cause interruption of cellular redox signaling and control, leading to damage[14,15].

ROS are highly reactive molecules produced by organisms in response to normal cellular metabolism and various environmental factors, which can disrupt the structure of nucleic acids, lipids, and proteins, thereby altering their function. Most intracellular ROS originate from the FR superoxide formed by the acquisition of a single electron from O₂via the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase or electron leakage in the mitochondrial electron transport chain. In addition, two superoxide molecules can be converted to one non-radical ROS molecule H₂O₂ and one water molecule by superoxide dismutase (SOD). H₂O₂ can also accept an electron from free Fe²⁺ through the Fenton reaction to become a hydroxyl radical[16]. In vivo, ROS are mainly derived from inflammatory cells, mitochondria and peroxisomes. The organelles producing the most H₂O₂ and superoxide anions, peroxisomes also contain large amounts of antioxidants to balance the ROS produced. In addition, NADPH oxidase is also a major source of intracellular ROS, which catalyzes O₂ and NADPH to produce superoxide[17]. Oncogenes have been found to stimulate the production of NADPH oxidase-dependent ROS, necessary for cellular proliferation[18].

Despite the presence of a multifaceted antioxidant enzyme system in human body, there is broad consensus in many studies that redox balance is altered in cancer[19,20]. In the process of cellular metabolism, many short-term and long-term ROS are produced, the ROS produced in this process are relatively stationary, playing an important role in the normal physiological activities of the human body. But in cancer cells, the increase in metabolic rate and relative hypoxia can increase the production of ROS. In addition, oncogenes can also provide the necessary conditions for cell proliferation by stimulating NADPH oxidase-dependent ROS production[18]. Meanwhile, the excessive ROS often induces gene mutations and alternations during transcription processes, ultimately leading to the oncogenesis[21].

ROS is proved to activate a variety of signal transduction pathways that promote tumorigenesis, enhance cell survival, proliferation, migration, and chemotherapy resistance, and lead to DNA damage and genetic instability[22,23]. For example, in inflamed cells, persistent inflammation can cause cells to secrete a large amount of ROS to recruit more activated immune cells, and lead to the imbalance of intracellular immune balance, which eventually leads to precancerous lesions. On this basis, if the amount of ROS produced by cells is higher than the endogenous antioxidant response, it will cause irreversible damage on nucleic acids, proteins and lipids, driving the initiation of carcinogenesis[24]. Compared with normal tissues, the lipid peroxidation in BC tissues was significantly increased, along with increased enzymatic and non-enzymatic antioxidants[25].

Irreversible DNA changes, like point mutations or chromosomal aberrations, drive tumor initiation[26-28]. The 8-hydroxy-deoxyguanosine (8-OHdG), with a mutagenic effect was found to be increased in many different types of cancers[29-31]. The 8-OHdG level in Michigan Cancer Foundation-7 (MCF-7) cells with positive estrogen receptor (ER) was also dramatically higher than that MDA-MB-231 cells with negative ER (9.3-fold)[32], suggesting the important role of ROS in the early stage of carcinogenesis[33].

Importantly, ROS can also interact with surface and intracellular receptors to modulate and interfere with native signaling pathways[34]. Phosphoinositide 3-kinase (PI3K) pathway, a central signaling pathway, was found to be overactivated in many cancers[35]. It is well-known that activation of protein kinase B (Akt) serves as a critical component in the PI3K signaling pathway, promoting cell proliferation and suppressing apoptosis. Among them, phosphatidylinositol (3,4,5)-trisphosphate (PIP3) enables its translocation to the plasma membrane, by binding Akt. Concurrently, phosphatase and tensin homolog deleted on chromosome ten (PTEN), a negative regulator of this pathway, exhibits constitutive phosphatase activities toward PIP3, converting it into the inactive form, phosphatidylinositol (4,5)-bisphosphate[16]. It is found that ROS can oxidize Cys124, the active site of PTEN, leading to the formation of another protein (Cys71) by disulfide bonds. This conformational change leads to the suppression of PTEN and unblocks the inhibition of PI3K, leaving the PI3K pathway permanently activated, which is a key driver for the development of cancers[36,37].

The expression of selenium-dependent glutathione peroxidase was negatively correlated with ER content in BC cells[38]. It is proved that estrogen is associated with the progression of BC[39,40], which can increase ROS amount in cells[41,42]. Uncoupling proteins (UCPs) can prevent mitochondrial ROS production, serving as a protective factor for normal cells[43,44]. It is explored that estrogen can induce BC by reducing the levels of UCPs and CAT in BC MCF-7 cells, increasing ROS production in mitochondria, indicating that estrogen participates in cancer development by inducing reactive O₂ species[45].

On the other hand, it has been found that elevated levels of ROS can also promote anti-tumor signaling under certain conditions, leading to increased oxidative stress and inducing cancer cell death[46,47]. Although ROS typically maintains at a stable high concentration in cancer cells, further increasing ROS can greatly impair their antioxidant capacity and ultimately lead to death driven by oxidative damage. In this case, as normal cells typically have lower levels of oxidants, additional concentrations of ROS are likely to preferentially kill cancer cells rather than non-tumor cells. So-called pro-oxidant therapies take advantage of this important feature[48]. The cytotoxic effect of myricetin on triple-negative BC (TNBC) cells is due to oxidative stress caused by extracellular H₂O₂ generated by myricetin auto oxidation, leading to the production of ROS through Fenton reaction inside the cells[49]. Interestingly, the induced growth inhibition and ROS generation by fertilized soy milk product can be inhibited by CAT and deferoxamine[50]. In addition, ROS production was also observed in cancer cells treated with paclitaxel[51]. Generally, cancer cells also have the ability to adapt to the elevated ROS levels and clear excess ROS with activating antioxidants for self-protection[52,53]. Overexpression of antioxidants has been found in some tumors to make cancer cells more resistant to subsequent oxidative damage[54,55].

Intrinsic AE are key to protecting cells from FR damage in the human body. Inducible levels of AE in tumor blood vessels can protect the host from high FRs and ROS caused by the tumor[56]. Adenovirus-mediated SOD overexpression can inhibit the growth of BC cells through combination with 1,3-bis(2-chloroethyl)-1-nitrosourea, and achieve complete tumor remission in vivo, providing a new combined treatment regimen for BC treatment[57]. As mentioned previously, ROS can serve as intracellular signaling cascade in the cell of the second messenger, can also play various roles during the oncogenesis[34,48]. In a word, oxidative stress, an imbalance between the production and elimination of ROS, plays an important role in the pathogenesis of various diseases and pathophysiological processes such as BC[58,59]. Therefore, effective utilization and the development of pro-oxidant therapy is a promising strategy for the treatment of malignances.

Undesirable oxidative stress can damage cellular components and lead to the development of cancer[60]. The most important components are the enzymatic antioxidants, which act as the endogenous antioxidant defense system[61]. Among them, CAT and SOD are the important antioxidants in the body, acting as the first line of defense against ROS-mediated damage. When cells are exposed to oxidative stress, SOD can be rapidly induced to catalyze superoxide into O₂ and H₂O₂[62], while CAT can then neutralize H₂O₂ by splitting it into molecular O₂ and water[63].

CAT IN HUMAN BODIES

The human CAT gene consists of 12 introns and 13 exons, located on chromosome 11[64]. In human bodies, CAT belongs to a typical heme-containing monofunctional enzyme with an iron protoporphyrin IX prosthetic group that reacts with H₂O₂. This enzyme is located in the peroxisome and has a molecular weight of about 220-240 kDa[65]. Each subunit, in the tetrameric protein, contains four different domains, that is N-terminal threading arm, C-terminal helices, wrapping loop, and β barrel, as well as a heme group[66]. Each subunit has a hydrophobic core, which is composed of eight antiparallel β barrels surrounded by α-helices. The N-terminal threaded arm of one subunit (residues 5-70) connects the two subunits in a complex way by a long wrapping loop (residues 380-438) surrounding the other subunit (Figure 1)[67]. Finally, on β barrel, there is a helical domain consisting of four C-terminal helices that is important for NADPH binding[7,68].

Figure 1  The location division of each subunit domain in catalase.

CAT is often first oxidized to a high valent iron intermediate, as compound I, and then reduced to a stable state by the second H₂O₂ molecule[69]. Compound I is a kind of iron oxide porphyrin cationic radical with a cationic group in its porphyrin[69]. It has been found that with single-electron donors and low H₂O₂ concentrations, the high valent iron intermediate (compound I) undergoes a one-electron reduction and become an inactive compound intermediate, as compound II. And transition returns to the resting state by another one-electron reduction reaction[70]. In compound I, the porphyrin has a cationic group, while in compound II it lacks a porphyrin cationic group, so compound II does not belong to the conventional iron oxide group[68]. However, at higher H₂O₂ concentrations, NADPH was found to prevent the formation of this inactive compound II by two-electron reduction process[71]. In the presence of higher concentrations of H₂O₂ molecules, this inactive compound will also be converted to another inactive intermediate, as compound III, which is intermediate state with an oxyferrous state of iron[72]. And the intermediate, compound III will then return to a quiescent state (Figure 2)[73].

Figure 2 The diagram of catalase reaction mechanism under different status. CAT: Catalase; H₂O₂: Hydrogen peroxide; NADPH: Nicotinamide adenine dinucleotide phosphate; OH: Reactive hydroxyl radicals; O₂: Oxygen.

CAT is expressed in almost all tissues of the human body with variable degrees, especially in the liver, kidney and red blood cells, to protect cells against oxidative stress[74]. The expression of human CAT is mainly mediated by peroxisome targeting signals, mainly located in peroxisomes[75]. Functional tetrameric CAT has also been detected in the cytoplasm of human skin fibroblasts[76]. In addition, the functional CAT is also found in cancer and chronic lymphocytic leukemia cell membrane surface, suggesting that CAT production does not necessarily occur exclusively within peroxisomes[77,78]. And this local high CAT level on tumor cell membrane has been suggested to be highly likely related to the dependence of tumor in vivo on increased resistance to exogenous H₂O₂[79]. Therefore, it is believed that the presence of extracellular CAT is related to the transformation state of malignant cells[80]. An increasing number of studies have also proved that membrane-associated CAT and extracellular CAT are common features of cancer cells, which prevent apoptosis induced by excessive intercellular ROS[81-83]. These observations also open the way for novel anticancer therapies using specific antibodies to target CAT. In addition to membrane-associated CAT, tumor cells have also been found to release soluble CAT, which is suggested with protective effect for tumor cells[84].

CAT is very important for the physiological health of human body. CAT deficiency or dysfunction is associated with different types of diseases. For example, acatalasemia is a rare disease characterized by a deficiency of CAT in red blood cell, which was first identified in 1948 with two mutations in the CAT gene[85-87]. Acatalasemia is usually a benign disease, but sometimes it may lead to the occurrence of other diseases, such as oral gangrene ulcers or essential hypertension[86,88], which may be promoted by phagocytes/bacteria-produced H₂O₂. Soon, it is pointed out that the mutation of CAT gene results in the change of structure of CAT protein structure, leading to Hungarian acatalasemia[89], while the family members of the Hungarian acatalasemic patient would have a high risk of developing diabetes, which also need further biochemical and genetic analysis[90].

Polymorphisms in the CAT promoter gene are also involved in pathogenesis[91]. The most common ones are -262C/T and -844G/A or -844C/T, affecting the transcription activities of CAT gene, as well as CAT activities[92]. Among them, the variant at -262C/T has significant functional significance, which can affect the binding of AP-2 and Sp-1, and the expression of CAT in red blood cell[93]. In addition, -262C/T polymorphism is also reported to be associated with BC. Compared with individuals with TT genotype, individuals with CC genotype tend to have lower levels of blood CAT, which will lead to oxidative stress, thereby promoting the occurrence of type 1 diabetes (T1D) and making them more susceptible to T1D[93]. Compared with TT and TC genotypes, the activities of CAT in red blood cell with CC genotype is increased, which may reduce the risk of BC[94]. Another one, -844C/T or -844G/A, may reduce the level of CAT by affecting the transcription frequency, and it is closely related to the occurrence of hypertension, but underlying mechanism is not completely clear[95].

In summary, CAT plays a key role in combating oxidative stress in normal and malignant cells, as well as in regulating H₂O₂ levels. The alterations in CAT expression may have widespread effects on the mechanisms of cancer development and pathological processes, which are of high importance to human life[96]. However, the molecular mechanism involved is still unclear, which needs further research.

REGULATORY ROLE OF CAT IN BC

Due to the altered redox balance in cancer, the expression of CAT and other AE is often altered in cancer cell. Transcriptional regulation mainly plays a role through affecting the transcriptional activities of the CAT promoter[68]. In addition, the epigenetic changes represented by DNA methylation and histone modification can also regulate the activity of CAT in tumor cells[97]. In transcriptional regulation, transcription factors generally act through the regulation of chromatin remodeling, which can regulate CAT expression[98]. Studies have found that chromatin remodeling is also a major regulator of CAT expression in BC cells[98,99]. It is confirmed that retinoic acid receptor α (RARα) and Jun B proto-oncogene (JunB) transcription factors function in the control of chromatin remodeling and CAT expression in BC cells[98,100].

The BC MCF-7 resistant to oxidative stress, the so-called Resox cells, was first established by exposing MCF-7 cells to H₂O₂-generating systems for a long period[101]. These cells exhibited reduced basal levels of ROS and increased CAT expression. On this basis, it was found that there was a new promoter region, at -1518/-1226 site, responsible for regulating CAT expression in Resox cells. And activating protein-1 family members JunB and RARα can mediate the activation and repression of CAT transcription by recruiting coactivator complexes and histone deacetylase-dependent mechanisms to control chromatin remodeling[98]. Therefore, regulation of intracellular oxidative stress by transcription factors through chromatin remodeling in BC may become a novel mechanism for targeting cancer cells.

Due to the bidirectional effect of ROS on tumor cells, CAT also have a similar effect in tumor cells due to its ability to regulate oxidative stress in tumor cells. Firstly, CAT plays a role in preventing the excessive accumulation of oxidants in human bodies, so it can protect cells from the occurrence and development of tumors. It is also found that BC cells with human CAT gene have lower ROS concentration and stronger resistance to H₂O₂, which confirms that CAT gene also has the ability to eliminate intracellular ROS and maintain intracellular oxidation level[9]. CAT activity is closely related to the growth of human BC cells. Silencing CAT expression resulted in a further increase in the steady-state level of H₂O₂, which was also accompanied with the increasd growth rate of human BC cells. It is mainly due to the inhibition of protein phosphatase 2A (PP2A) activities by excessive H₂O₂, leading to the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK 1/2) and Akt, and subsequent proliferation of BC cells. Therefore, overexpression of CAT can induce PP2A-dependent apoptosis of BC cells, thereby inhibiting the occurrence of BC[102].

On the contrary, CAT can also perform a protective or even promoting effect on tumor cells in some cases. In general, high concentrations of H₂O₂ are toxic to most eukaryotic and prokaryotic cells. Taking advantage of this property, anti-tumor antibiotics containing quinones, such as doxorubicin, can destroy tumor cells by stimulating the production of H₂O₂, in which CAT also plays an important role[103]. MCF7/H₂O₂ BC cell line is a H₂O₂-resistant MCF-7 variant cell line, with a 5-fold increase in H₂O₂ resistance and a 3-fold increase in CAT activities. In the study of oxidative stress resistance, it was found that the resistance of MCF7/H₂O₂ resulted from the increased CAT activity, which was directly related to the enhanced mRNA stability, possibly through the enhanced polyribosome binding and changes in ribonuclease activity. This suggests that the post-transcriptional regulatory mechanism of CAT also plays an important role in drug resistance, and excessive CAT activity may enhance the resistance of tumor cells to chemotherapeutic drugs such as doxorubicin[104]. In addition, due to high ROS levels in tumor cells contributing to their sensitivity to glucose deficiency induced cytotoxicity and oxidative stress, tumor cells often perform increased glucose metabolism to compensate for excessive ROS metabolism. However, under certain conditions, overexpression of MnSOD and mitochondrial targeted CAT can significantly protect tumor cells from glucose deficiency induced cytotoxicity and oxidative stress[105].

The decreased biological activities of CAT in BC are often accompanied with the increased steady-state level of H₂O₂ in cells[102]. So increased oxidative stress markers and diminished antioxidant defense systems are also considered factors associated with the occurrence and progression of BC[106]. The expression of CAT in BC is affected by many factors and is closely related to the development and prognosis of BC (Figure 3)[73].

Figure 3 The reported role of catalase in breast cancers. CAT: Catalase; HDAC: Histone deacetylase; H₂O₂: Hydrogen peroxide; JunB: Jun B proto-oncogene; PP2A: Protein phosphatase 2A; RARα: Retinoic acid receptor α; ROS: Reactive oxygen species.

However, it remains unclear whether there are significant differences in the activity of CAT among various molecular subtypes of BC. It can be speculated that CAT activity may be closely associated with the levels of oxidative stress, metabolic properties, and clinical prognosis of each subtype. Maycotte et al[107] found that TNBC cells exhibited lower levels of CAT compared with MCF7 cells. The low CAT activity may be related to the high level of oxidative stress in TNBC cells[107]. Since TNBC cells generally have a high capacity for ROS generation, low CAT activity may limit their ability to scavenge H₂O₂, resulting in elevated levels of intracellular oxidative stress. However, the specific reasons and results need further research.

CAT decreases the invasiveness of primary BCs

Normally, when ROS produced within cells increases, the cells are exposed to a pro-oxidative environment for a long time, which is closely related to tumor invasiveness[108,109]. It is proved that ROS at a certain concentration can act as a second messenger to participate in and stimulate tumor growth and metastasis pathways [p38/mitogen-activated protein kinase (MAPK)][4]. The concentration of AE such as CAT in the mitochondria of cells can reduce the oxidative stress produced by mitochondria, thereby controlling the concentration of ROS[110]. And this may also affect the aggressive phenotype of BC to a certain extent.

However, at present, the correlation between CAT and BC aggressiveness is not clear. By comparing the tumor tissues of mitochondrially targeted CAT (mCAT)-positive and mCAT-negative PyMT mice with different parameters, it was found that only 13% of the tumors from the McAt-positive PyMT mice exhibited a grade 3 histological aggressive phenotype, whereas 63% of the tumors from the McAt-negative PyMT mice had a grade 3 histological aggressive phenotype[9]. This indicated that mitochondrial CAT has the ability to reduce matrix infiltration and cell motility and migration to a certain extent. It is speculated that mCAT may activate different factors by altering ROS-dependent signaling pathways, thereby promoting epithelial-mesenchymal transition and tumor metastasis[111]. However, its specific mechanism may need further research.

The expression pattern of CAT was site-specific in BCs

Distant metastasis of advanced BC is often the main cause of death in patients with BC[112]. The main affected organs of metastatic BC are lung, brain, liver and bone[113,114]. According to previous studies, different metastatic cancers show different characteristics when the metastatic sites are different[115,116]. In metastatic BC, the expression of CAT and ROS-related proteins can also be affected by the metastatic site[117]. Interestingly, it is found that the expression of CAT is low in bone metastases, and the expression of stromal glutathione S-transferase π is high in the metastatic BC in bone and liver, along with high proportion of ER expression and inactive ROS states[116]. According to previous studies, the expression patterns of BC biomarkers (ER, PR, and human epidermal growth factor receptor 2) also vary according to the site of metastasis[118]. The majority of bone metastases predominantly exhibit hormone receptor positive[119]. Importantly, the inhibition of ER expression can lead to increased ROS production in BC cells[120]. Consequently, the lower CAT expression observed in bone metastases and the variations in ROS status across different metastatic sites are likely attributable to differences in hormone receptor status.

CAT expression correlates with prognosis in patients with BCs

As an important modulator in ROS regulation, it is found that the negative expression of CAT has a great correlation with the short overall survival (OS) of the patients[121]. The gene mutation caused by oxidative stress will also change the prognosis of BC after treatment. It has been demonstrated that BC patients with genotypes leading to high levels of ROS exhibit superior OS compared to those with genotypes associated with lower ROS levels[122]. This suggests that genetic variants contributing to increased oxidative stress may potentiate the efficacy of chemotherapy or radiotherapy, thereby enhance treatment outcomes and improve patient survival.

Progestins are equally important for BC progression. It is reported that progesterone and various progestin-potent anabolic steroids (MPA and tibolone) can effectively induce CAT activity, which can counteract the H₂O₂ induced cell growth in BC cells and normal human breast epithelial cells. Among them, progesterone inhibits cell proliferation mainly by mediating progesterone receptor (PR) B subtype to effectively induce CAT activity[123]. Therefore, in order to provide better treatment and prognosis of BC, the evaluation of PR is essential. In addition, ER-α also can mediate estradiol to reduce the CAT activity through antioxidant effect of BC cells, indicating the potential role of steroid hormones in ROS metabolism and oncogenesis[124,125].

Regarding the common polymorphism, a C262T substitution, in the upstream of CAT transcription start site, the T allele leads to higher levels of CAT expression in red blood cell by enhancing promoter activity, which may influence the host's response to oxidative stress[126]. However, the specific key transcription factors need to be further identified. Interestingly, genetic polymorphisms, the CAT TT and MnSOD CC genotypes were found to decrease mortality risk in BC patients[122], potentially due to heightened oxidative stress and cytotoxicity associated with these polymorphisms. And the specific mechanism needs to be further studied. Collectively, these findings indicate that patients undergoing radiotherapy and chemotherapy for BC may achieve improved survival outcomes. Importantly, it is indicated that patients with TNBC, the aggressive molecular type of BC, who exhibit high CAT expression tend to have a lower N stage, reduced tumor recurrence rates, and prolonged OS[127]. It is plausible that CAT overexpression in TNBC protects normal cells from elevated ROS-induced toxicity and enhances therapeutic sensitivity, thereby improving patient survival[127].

PROSPECTS FOR THE THERAPEUTIC APPLICATION OF CAT

As mentioned above, oxidative stress is caused by the imbalance of ROS, participating in the pathogenesis of various diseases, including BC[128]. The high production of O₂ FRs and low CAT activities in BC indicate the high oxidative stress in BC patients, predicting AE (especially CAT) as the promising therapeutic strategy for BC[129]. Therefore, as a ROS-associated protein, the expression of CAT has been regarded as a potential strategy for treating patients with BC.

Most chemotherapeutic agents utilized in BC therapy exert their effects, by modulating ROS homeostasis. Taxanes, including paclitaxel and docetaxel, are frequently employed for the therapy of patients with BC[130]. Their primarily function by stabilizing the GDP-tubulin complex, thereby inhibiting microtubule dynamics and inducing mitotic arrest and apoptosis. Additionally, taxanes can disrupt mitochondrial respiratory chain function and impair normal electron transport, leading to the generation of superoxide radicals[131]. Consequently, excessive superoxide production may enhance the cytotoxic effects of these drugs on tumor cells, contributing to their therapeutic efficacy.

Platinum complexes, such as cisplatin, are widely used in clinical settings for the management of TNBC[130]. Their primary mechanism of action involves the formation of DNA adducts, which impede replication and trigger apoptosis. Moreover, platinum complexes can induce substantial ROS production via mitochondrial or NADPH oxidase pathways, resulting in significant side effects in patients[132].

Anthracyclines, including doxorubicin and epirubicin, represent standard therapeutic options for BC patients[130]. These agents inhibit topoisomerase II, thereby disrupting the synthesis of DNA and RNA, subsequently inducing cell death[48]. Furthermore, doxorubicin penetrates the inner mitochondrial membrane, competing with coenzyme Q₁₀ in the electron transport chain, promoting the formation of superoxide radicals. The elevated ROS levels may augment drug-induced cytotoxicity against tumor cells[133].

Although the current understanding of CAT function and mechanism in tumorigenesis is still limited, the reported features of CAT in BC formation and progression make it a central factor in many therapeutic modalities.

CAT inhibitors

The accumulation of ROS caused by enzyme inhibition is beneficial to the death of cancer cells and can be used as an anti-cancer drug. However, only a few CAT inhibitor drugs have been reported, which primarily interact with key amino acid residues in the catalytic site or the cofactor NADPH. Ferroptosis is a kind of cell death caused by excessive lipid peroxidation induced by ROS[134]. Recently, a novel CAT inhibitor, benzaldehyde thiourea derivatives, a derivative of benzaldehyde thioaminolone, has been designed to inhibit CAT activity by binding to NADPH-binding sites. This interaction induces endoplasmic reticulum stress and subsequent autophagy in DU145 castrate-resistant prostate cancer cells, leading to the degradation of ferritin heavy chain 1 and Fe²⁺ accumulation. The results promote the Fenton reaction and reactive hydroxyl radicals (•OH) production, ultimately inducing ferroptosis in tumor cells. And when the same experiment was performed in BC cells (MCF-7), similar results were obtained. Therefore, the inhibition of CAT may become a new strategy to induce siderosis of cancer cells through the dual regulation of ROS levels and ferroptosis[135].

Flavonoids are polyphenolic substances widely present in fruits and vegetables, with antioxidant, anti-inflammatory and anti-cancer activities[136,137]. The enzyme inhibitory properties of flavonoids have been studied recently. The investigation found that when the CAT interact with flavonoids, its alpha helix structure will be lost, which inhibits CAT activity[138]. In addition, in flavonols such as myricetin and quercetin, the higher number of hydroxyl groups in the B ring may contribute to their stronger inhibitory ability to CAT. This observed inhibition of CAT can lead to a rise in intracellular ROS levels and ultimately trigger a higher rate of apoptosis[138]. This can also be an important anti-cancer mechanism and provide a basis for the development of new anticancer drugs. Besides, it is suggested that Halymenia durvillei (HD), which is consumed in the diet of Korean women, has a BC risk reduction efficacy[139]. Recently, HD ethanolic extract was found to reduce the expression of CAT mRNA levels and induce ROS production in tumor cells, leading to cell cycle arrest in BC[140]. To some extent, it adds a new understanding of CAT in the targeted therapy of BC cells.

As a classic CAT inhibitor, 3-aminotriazole (3-AT) has been shown to exert anti-tumor effects through multiple pathways in cancer. Early studies have shown that 3-AT can inhibit γ-radiation-induced lymphoma and neutron radiation-induced ovarian tumors in animal models[141]. It may be due to the inhibition of CAT expression by 3-AT, which promotes the increase of oxidative stress level and enhances the sensitivity of tumor to radiation. Moreover, 3-AT could delay the appearance of murine mammary tumor virus (MUMTV)-driven breast tumors and even achieve long-term tumor-free status in animal models[141]. This suggests that the effect of 3-AT is background-dependent. By itself, it causes cancer by inhibiting CAT leading to H₂O₂ accumulation, but when combined with other carcinogenic factors (such as MuMTV and radiation), it can exert anti-tumor effect. In addition, with the development of nanoparticle technology, the combination of nanoparticles and 3-AT has further promoted the treatment of tumors in recent years. For example, nano-metal organic frameworks loaded 3-AT targeted delivery system can release 3-AT and chemotherapeutic drugs through pH response. Among them, 3-AT can promote the efficacy of chemodynamic therapy by inhibiting the activity of CAT to achieve the dual synergistic effect of chemodynamic therapy and chemotherapy[142]. It has shown efficient tumor suppression in models such as BC. Although 3-AT has shown potential in preclinical studies, its long-term toxicity and optimal dosing regimen are still unclear and need to be further explored.

CAT activators

There are also compounds with the ability to enhance CAT activities. On this basis, they can also play a role in the treatment of tumors. For example, Autocrine human growth hormone (hGH) can specifically regulate the gene expression of CAT by activating the p44/42 MAPK (MAPK, ERK 1/2) signaling pathway. It has been shown that binding of hGH to its transmembrane receptor induces sustained activation of ERK 1/2 through a Janus kinase 2-dependent phosphorylation cascade[143]. Activated ERK 1/2 further translocalizes to the nucleus and binds to transcription factors in the promoter region of the CAT gene, thereby enhancing its transcriptional activities[144]. Notably, the selective mitogen-activated protein kinase inhibitor PD098059 completely abolished hGH-induced transcriptional stimulation of CAT genes by inhibiting the phosphorylation of ERK 1/2 (IC₅₀ = 10 μM), confirming the necessity of ERK signaling in this regulation.

As one of the polyphenols, resveratrol was initially identified as a potential CAT activator due to its ability to activate sirtuins and fork head transcription factors of the O class transcription factors. At specific concentrations, resveratrol has been shown to enhance CAT activity and inhibit cell proliferation in human cancer cell lines[145]. Furthermore, metformin has been demonstrated to form hydrogen bonds with CAT and interacts with it. This interaction results in elevated CAT activity in mouse liver and provides protection against CCl4-induced liver injury[146]. Notably, recent studies have revealed that the combination of metformin and resveratrol exhibits a synergistic effect, which can mitigate the progression of TNBC by enhancing CAT activity[147].

In addition, non-enzymatic antioxidants such as curcumin, vitamin C, and plant polyphenols can also play a role in treating cancer by blocking FR chain reactions[148]. For example, curcumin has antioxidant activity and antiproliferation effect, which can significantly reduce the level of oxidative stress by regulating CAT activities, thus inhibiting the progress of BC[149]. Further research has found that the anticancer effect of curcumin is to some extent related to the copper mediated ROS induced selective cell death mechanism in cancer cells[150].

Since cancer cells generally exhibit more ROS than normal cellular tissues[151], pro-oxidant therapies in cancer treatment are also increasingly being developed. This selectivity arises because normal cells, which typically maintain low oxidative stress levels, are equipped to manage increased ROS without experiencing oxidative damage or cell death[48]. Consistently, high doses of vitamin C exhibit effective cytotoxicity toward cancer cells due to their pro-oxidative effects. Specifically, high-dose vitamin C treatment has been shown to elevate the levels of upstream metabolites in the glycolytic pathway and the tricarboxylic acid cycle, while simultaneously reducing adenosine triphosphate levels and adenylate energy charge in MCF-7 cells[152]. One of the pro-oxidant therapies is a combination of ascorbic acid and menadione (ASC/MEN). Interestingly, overexpression of CAT instead protects cancer cells from ASC/MEN-mediated cell death, thereby reducing patient survival[8]. In addition, other studies have found that arsenic trioxide can also reduce the level of CAT in cancer cells and increase the sensitivity of BC cells to ASC/MEN therapy[153]. Therefore, for therapy strategy, the effect of CAT overexpression on the resistance of pro-oxidant drugs should also be considered.

CAT-related nanotechnology therapy

Recently, the strategy of nanotechnology based on AE has shown breakthrough potential in the treatment of BC, which can specifically destroy the redox defense system through the precise delivery of functionalized nanoparticles and multiple mechanisms of action. For example, anti-SOD2 antibody-modified gold nanoparticles (AuNPs), loaded with siRNA to impair the antioxidant defense of tumor cells by silencing SOD 2 gene expression, can localize BC tissues by dual antibody-mediated active targeting and enhanced permeation retention effect. At the same time, the local surface plasmon resonance effect generated by AuNPs under near-infrared light excitation can directly trigger the excessive production of mitochondrial ROS in BC cells, and realize the spatiotemporal synergy of gene silencing and physical therapy[154].

According to the acidic microenvironment of BC cells, superparamagnetic iron oxide nanoparticles (SPIONs) can be enriched in the tumor site under the guidance of magnetic targeting and release the iron ions in response to the pH value and synergies with β-lapachone lead to accumulation of H₂O₂, thereby increasing the oxidative stress in cancer cells. In addition, SPIONs can further amplify the toxicity of H₂O₂ through the Fenton reaction and generate highly •OH, leading to DNA damage and cell death in cancer cells. However, the expression of CAT in cells or the administration of iron chelators can block the therapeutic synergy[61,155]. Therefore, the combination of SPION and CAT inhibitory drugs (such as 3-AT) may significantly enhance the level of oxidative stress in cells and improve the therapeutic effect to BC. This synergistic effect provides a new strategy for the targeted therapy of BC.

In addition to affecting the oxidative toxicity of SPIONs to tumor cells, the combination of cat and nanoparticles can also improve the therapeutic effect of chemotherapy and radiotherapy to a certain extent. The tumor microenvironment (such as hypoxia) is usually the main factor limiting the two cancer treatments, chemotherapy and radiotherapy[156,157]. It is reported that CAT encapsulated in liposomes composed of cisplatin (IV)-prodrug coupled phospholipids to form CAT^@Pt (IV)-liposomes, can be used to enhance chemoradiotherapy for cancer. CAT^@Pt (IV) is loaded into liposomes, where the enzyme activity is preserved and well protected, and is able to trigger the breakdown of H₂O₂ produced by tumor cells, thereby generating additional O₂ to alleviate hypoxia. In vivo tumor treatment further demonstrated that the use of these CAT^@Pt (IV) liposomal nanoparticles could significantly improve the therapeutic efficacy of chemoradiotherapy[158].

Developed multifunctional integrated nanoplatforms (such as Au^@Fe₃O₄ Janus particles) are engineered to not only enhance penetration into dense BC tissues, but also integrate multiple mechanisms such as antioxidant enzyme inhibition, cascade catalysis, and photothermal/photodynamic therapy. Precise disruption of the redox network of BC cells at the single-cell level[159]. These strategies systematically interfere with the antioxidant defense system of BC cells through spatially and temporally resolved drug release and energy conversion, providing innovative therapeutic paradigms for reversing tumor drug resistance and targeting metabolic vulnerability.

Generally, the special role of CAT in BC has been paid more and more attention by scholars. Based on the expression of CAT in tumor cells, more and more therapeutic strategies are being proposed. With the continuous development of related fields, CAT is likely to become a potential therapeutic target in the future.

CONCLUSION

Due to changes in CAT expression and activity can lead to changes in the level of ROS in cells, so many studies have shown that CAT plays a dual role in cancer, it can be used as tumor suppressor proteins, and also can be used as tumor growth protein in promoting survival. At present, investigation of CAT in BC are deepening and expanding the understanding of its molecular mechanism and principle. As the understanding of CAT function continues to deepen, it is believed that CAT is likely to become a new choice for BC treatment in the future. In conclusion, the study of CAT in the context of BC is an expanding field that will enhance our understanding of BC. A further understanding of the complex roles of CAT in cancer is a major challenge for future studies of CAT, especially its dual regulation in different cancer types and its impact on resistance to pro-oxidant drugs.