Breast cancer is the most frequently diagnosed cancer in women. It has been a significant health problem and represents a top biomedical research priority [16, 17]. With increases in breast cancer incidence and mortality in recent years [18, 19], it is essential to diagnose and treat breast cancer in the early stages. Magnetic resonance imaging, ultrasound, mammography, positron emission tomography, and biopsy are currently available for breast cancer diagnosis [20, 21]. However, these methods have some limitations include expensive, low sensitivity, and time consuming. Biomarkers are promising diagnostic targets for rapid and cost-effective early breast cancer detection. Breast cancer patients are currently available for multimodal treatment, including surgery, radiotherapy, chemotherapy, and biological therapy [22]. Although many efforts have been made to treat breast cancer, the survival rate remains low over the past few years. Thus, it is necessary to explore novel targets for early diagnosis and optimal therapies for breast cancer.
PAX2 encodes paired box gene 2, one of many human homologs of the Drosophila melanogaster gene prd. It has been reported that PAX2 is commonly overexpressed in epithelial tumors of the kidney and female genital tract, which may be a diagnostic biomarker for renal epithelial neoplasms and epithelial tumors of the female [23]. Naoto Kuroda et al. indicated that the combined panel of PAX2 and PAX8 is a good marker for diagnosing metastatic renal cell carcinoma [24]. Moreover, Yan Feng et al. suggested that PAX2 may promote ovarian cancer progression through fatty acid metabolic reprogramming [25]. Hence, we hypothesized that PAX2 might play a regulatory role in breast cancer.
In this study, the expression and function of PAX2 in breast cancer were explored by data mining, bioinformatics analysis, and cell culture experiments. The result of data mining and bioinformatics analysis indicated that the PAX2 was overexpressed in breast cancer patients. Furthermore, PAX2 was upregulated in breast cancer tissue samples, and its expression was associated with menopause. The downregulation of PAX2 in MCF7 and MB231 cells significantly enhanced cell proliferation, migration and invasion. Hence, PAX2 might be an early diagnostic marker and a therapeutic target for breast cancer. Our study suggested that PAX2 is a tumor suppressor gene in breast cancer. Intriguingly, overexpression of PAX2 has also been described in breast cancer. Further experiments are needed to elucidate when and why PAX2 overexpression occurs.
In our study, WGCNA analysis and genes in the greenyellow module were obtained. These genes have similar gene expression patterns as PAX2. Hyaluronan synthase 3 (HAS3) is involved in the synthesis of an unbranched glycosaminoglycan, hyaluronan, a main component of the extracellular matrix. It has been reported that downregulating HAS3 could inhibit gastric cancer cell proliferation, colony‑forming, migration, and invasion[26]. Moreover, Uma Thanigai Arasu et al. indicates that the overexpression of HAS3 could induce extracellular vesicle shedding, which could cause cell proliferation and epithelial-to-mesenchymal transition in cancer cells[27]. Thus, we hypothesized that PAX2 could be involved in breast cancer development by targeting HAS3, but the results need to be examined by further experiments. Thymic stromal lymphopoietin (TSLP) is a hemopoietic cytokine acting on myeloid cells to induce T cell-attracting chemokines release from monocytes and enhance CD11c (+) dendritic cell maturation. TSLP also plays essential roles in maintaining immune homeostasis and regulating inflammatory responses. It has been reported that cancer cells could manipulate the immune response through TSLP and subsequently change the ability of immune cells to recognize and effectively remove tumors [28, 29]. Shadmehr Demehri et al. indicated that genetic and chemical induction of TSLP at a distant site could result in antitumor immunity against breast carcinogenesis in mice. TSLP is also expressed in breast tumor cells to block breast cancer progression[30]. Thus, we also hypothesized that PAX2 might have a tumor suppressor role in breast cancer through the TSLP genes. However, additional studies are needed to elucidate the molecular mechanisms.
In this study, we found that PAX2 overexpression significantly suppressed cell proliferation. Cell proliferation is an integral part of cancer development and progression. The constitutive activation of many signaling pathways could stimulate the cell proliferation [31, 32]. It was worth noting that the MAPK signaling pathway was significantly enriched in the GSEA analysis. Many studies have shown that the MAPK signaling pathway is downstream of many growth factor receptors, and its activation plays key roles in cancer cell proliferation[33, 34]. Xinying Zhu et al. indicates that MicroRNA-188-5p could inhibit cell proliferation of breast cancer cells via the MAPK signaling pathway [35]. Thus, we hypothesized that PAX2 suppressed breast cancer cell proliferation by regulating the MAPK signaling pathway. However, the results need to be examined by further investigations with larger samples. We also found that PAX2 overexpression could inhibit the migration and invasion of breast cancer cells. Migration and invasion of cancer cells are important initial steps in metastasis modulated by multiple signaling pathways[36, 37]. The Hippo signaling and MAPK signaling pathways were significantly enriched in the GSEA analysis. It has been reported that Sushi repeat containing protein X-linked 2 could promote cell migration and invasion in osteosarcoma through regulating Hippo Signaling Pathway [38]. G M Sharif et al. also indicated that Hippo Signaling Pathway could modulate cancer cell invasion [39]. Moreover, studies have shown that the activated MAPK signaling pathway could promote cell migration and invasion[40, 41]. Thus, we hypothesized that PAX2 inhibited the migration and invasion of breast cancer cells by regulating the Hippo signaling pathway and MAPK signaling pathway.
In conclusion, our study revealed a tumor suppressing effect of PAX2 in breast cancer, which might provide a novel therapeutic approach and early diagnosis target for breast cancer.