Breast cancer remains one of the most common cancers among women worldwide and a leading cause of death (1). It encompasses a variety of pathological types, ranging from noninvasive tumors to invasive tumors. In situ breast cancers, including ductal carcinoma in situ (DCIS) and lobular carcinoma in situ (LCIS), represent noninvasive types (2). In contrast, a classification of invasive breast cancer indicates that cancer cells have penetrated the basal membrane of the breast ducts or glands, with the potential to spread to other parts of the body (3). The treatment approach and prognosis for different types of breast cancer are determined by molecular subtypes on the basis of the expression of oestrogen receptor (ER), progesterone receptor (PR), and HER2 proteins (4). The incidence of this disease is influenced by the complex interplay of genetic predispositions and environmental factors (5). Treatment for breast cancer typically involves surgery, radiotherapy, chemotherapy, hormone therapy, and targeted therapy. However, there are challenges in treatment, such as resistance, individual variation, and recurrence. The progression of breast cancer involves various cellular signaling pathways and molecular mechanisms (6, 7). A deeper understanding of these complex processes may reveal new therapeutic targets, offering new directions for treatment.

Cathepsins, a type of lysosomal protease, have been implicated in the progression of several types of cancer, including breast cancer. These enzymes, categorized based on the amino acid type at their active sites, include cysteine proteases (such as cathepsins B, L, and K), serine proteases (such as cathepsins A and G), and aspartic proteases (such as cathepsins D and E). They play significant roles in immune response regulation, lipid metabolism, and tumor progression (8). Their expression is upregulated in various cancers, including breast, lung, colorectal, and liver cancers, and they can promote cancer cell invasion and metastasis by facilitating the degradation of the extracellular matrix (ECM) (9–11). Previous studies has linked cathepsins B, D, and L to breast cancer metastasis, and cathepsin D activity is known to be regulated by oestrogen (12). Cathepsin D deficiency can inhibit the development of breast cancer by blocking signal transduction mediated by the mTORC1 complex (13, 14). Research at the mechanistic level concerning the relationship between cathepsins and breast cancer has been growing; however, while there is extensive research on cysteine and aspartic cathepsins in breast cancer, the specific roles of different cathepsins across various breast cancer subtypes remain largely unexplored. A comprehensive understanding of how different members of the cathepsin family contribute to the progression of different subtypes of breast cancer is still needed. Using Mendelian randomization (MR) analysis—a statistical approach that uses genetic variants as instrumental variables to explore the causal relationship between an exposure and an outcome—this study aims to elucidate the relationship between cathepsins and breast cancer (15, 16). The availability of data from Genome-Wide Association Studies (GWAS) databases enables such in-depth studies (17). The complex role of cathepsins and other proteases in cancer highlights their potential as therapeutic targets. Our study, by examining the causal link between different types of breast cancer and cathepsins, provides a new perspective that may advance our understanding of the molecular basis of breast cancer and offer insights for future diagnostic and therapeutic developments.

Previous studies have demonstrated an association between specific cathepsins and tumor progression, as well as immune infiltration in the tumor microenvironment, suggesting their potential as biomarkers for predicting tumor prognosis (11). Several cathepsins have been shown to promote tumorigenesis in cancers such as liver and pancreatic cancer through various mechanisms (9, 20). In breast cancer, anti-cathepsin D therapy has been found to activate both innate and adaptive immunity, indicating that cathepsin D is a potential target for tumor immunotherapy (21). The expression and function of cathepsin D vary among different subtypes of breast cancer. Cathepsin D expression is elevated in patients with the luminal B subtype and in patients with clinical metastasis, affecting the expression of HER2 and ER proteins (22). A recent Mendelian randomization study by Deng et al. (23) suggested a potential role for cathepsin O in breast cancer, highlighting the value of MR in providing causal insights into cathepsin-mediated cancer risk (23). Previous studies, such as the one by Yang Wang et al. (24), have used P/TWAS combined with Mendelian randomization to identify plasma proteins causally associated with breast cancer. Their finding that CTSF is causally linked to breast cancer supports our results and highlights the role of cathepsins in the disease (24). Additionally, another study also suggested that CTSF could be a potential therapeutic target for breast cancer, further emphasizing the importance of cathepsins as targets for future drug development (25). In summary, cathepsins may have high value in breast cancer treatment and prognosis assessment.

This study utilized a two-sample MR approach to assess the causal relationships between nine types of cathepsins and breast cancer. It was found that cathepsins E and F increased the risk of malignant breast cancer and in situ breast cancer, respectively, whereas cathepsin Z had a protective effect against in situ breast cancer. Notably, cathepsins E and D are both members of the protease A1 family and are highly expressed in the immune system, gastrointestinal system, and cancer cells, and they share structural similarities, suggesting that they might influence breast cancer risk through similar mechanisms (26). Cathepsin Z is involved in cancer progression and inflammatory processes and has been shown to have protective effects in inflammatory gastric diseases (27, 28). Analysis of peripheral blood samples from breast cancer patients revealed that the methylation level of cathepsin Z DNA is associated with the incidence of breast cancer, especially early-stage breast cancer. The methylation of cathepsin Z DNA holds promise as a biomarker for early-stage breast cancer (29). Interestingly, the current study revealed that the protein expression level of cathepsin Z is a protective factor for early-stage breast cancer; hence, the methylation level of cathepsin Z DNA may be related to the development of early-stage breast cancer and might serve as a topic of future research.

To further explore the role of cathepsins in breast cancer progression, we integrated data from expression quantitative trait loci (eQTLs), a step that allowed us to understand from a gene expression level how cathepsins act as molecular mediators in the regulation of specific genes in the context of breast cancer progression. Through this approach, we were able to identify key genes that influence the development of breast cancer by regulating the expression or activity of cathepsins, revealing a molecular pathway from gene expression to cathepsin regulation and then to breast cancer development. The discovery of this molecular mechanism not only emphasizes the regulatory role of cathepsins in the progression of breast cancer but also reveals the differences in pathological mechanisms between different breast cancer subtypes. Accurately distinguishing the HER2 status of breast cancer is crucial to ensuring that patients receive the most appropriate treatment plan for their disease characteristics, maximizing therapeutic efficacy, reducing the risk of recurrence, and improving prognosis (30). Our findings not only aid in mechanistically distinguishing between HER2+ and HER2- breast cancer but are also highly important for the formulation of clinical diagnostic approaches and personalized treatment strategies.

Although this study revealed the significant role of cathepsins in the onset and progression of breast cancer, providing new insights and directions for observational research, it still has certain limitations. For example, owing to database constraints, not all subtypes of breast cancer and cathepsins were covered. Therefore, further research is needed to explore the relationships between more cathepsins and breast cancer subtypes. Additionally, the results of MR analysis need to be validated through independent biological experiments to ensure that the discovered causal relationships have biological significance. Although eQTL data provides valuable insights into gene expression regulation, it has limitations. Gene expression does not always correlate with protein expression, which is the functional molecule driving disease mechanisms. Therefore, future studies incorporating proteomic data are needed to validate these findings. Finally, this study focused primarily on the role of cathepsins as molecular mediators in the progression of breast cancer without delving into other potential molecular mechanisms and pathways, which may limit our comprehensive understanding of the complex pathophysiology of breast cancer.

In summary, our research suggests that cathepsins play a significant role in the progression of breast cancer. These findings provide new insights into the potential role of cathepsins as molecular mediators in breast cancer development. Additionally, they highlight the varying molecular mechanisms of cathepsins across different breast cancer subtypes. Our study lays the groundwork for further research into cathepsins as potential targets for therapeutic strategies in breast cancer.

This study explored the potential causal relationship between cathepsins and breast cancer using a two-sample Mendelian randomization approach. Our findings indicate that specific cathepsins, such as cathepsins E and F, are associated with an increased risk of malignant and in situ breast cancer, while cathepsin Z appears to have a protective effect against in situ breast cancer. Furthermore, we examined the mechanisms by which gene expression regulates cathepsin levels, influencing breast cancer onset. The regulatory roles of cathepsins vary among different types and molecular subtypes of breast cancer, warranting further investigation.