Expression of sex-determining region Y-box protein 2 in breast cancer and its clinical significance

2.1 SOX2 and the development of breast cancer (Figure 1)

To study the role of SOX2 in the development of breast tumors, Chen et al26 found that overexpression of SOX2 increases the population of S + G2/M phase cells and decreases the G0/G1 phase cell population. They used functional acquisition experiments and loss of function experiments to examine the effect of SOX2 overexpression or low expression on cell cycle regulation in breast cancer cells via retroviral-mediated overexpression and low expression strategies. Similarly, using siRNA to decrease the expression of SOX2 results in cell arrest at the G0/G1 phase. This finding indicates that the accumulation of G0/G1 cells is specifically correlated with the loss of the SOX2 gene and SOX2 protein. Thus, SOX2 might promote cell proliferation and tumorigenesis of breast cancer by accelerating the G1/S transition of cell cycle, and SOX2 and b-catenin both regulate the transcription of cyclin D1 (CCND1) in breast cancer. Cyclin D1 may also play a certain role in the development of the disease. However, the specific mechanisms still need to be further explored.

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Figure 1

Specific mechanism of SOX2 in the development, invasion and metastasis, diagnosis and treatment, recurrence, and resistance of breast cancer.

2.2 SOX2 with the invasion and metastasis of breast cancer

The recruitment of tumor-associated macrophages promotes the metastasis of breast cancer. Sex-determining region Y-box protein 2-positive breast cancer cells can regulate the secretion of many cytokines such as MIP-1a/CCL3 and ICAM. It also regulates the activation of the NF-kB and STAT3 signaling pathway to play an important role in the recruitment of tumor-associated macrophages.27 Li et al28 found that SOX2 promotes epithelial–mesenchymal transition to promote breast cancer metastasis by regulating the WNT/b-catenin signaling pathway in breast cancer cells. Lengerke et al22 reported similar associations in a mate analysis of the relationship between SOX2 and clinicopathological parameters in patients with breast cancer. Increasing SOX2 expression in early breast cancer might promote tumor metastasis. Although there is no conclusive evidence that SOX2 is associated with lymph node metastasis, recent studies have demonstrated that SOX2-positive patients present significantly higher lymph node metastasis rates than those with SOX2-negative disease.29 Michifuri et al30 stated that SOX2 is essential for invasion and proliferation of breast cancer, and SOX2-positive cancer cells have a higher ability and probability to transfer to the lymph nodes than SOX2-negative cancer cells; in addition, down regulation of SOX2 expression significantly reduces angiogenesis and lymphangiogenesis of breast cancer. Chinese studies have shown that SOX2 might promote angiogenesis and lymphangiogenesis by promoting vascular endothelial growth factor A secretion to affect tumor lymph node metastasis.31 These studies suggested that SOX2 plays a role in breast cancer metastasis through a variety of mechanisms. Sex-determining region Y-box protein 2 indicates poor prognosis, so it is a viable marker for prognosis in patients with breast cancer.

At present, the problems of SOX2 and invasive breast cancer remain controversial. Fang et al32 found that downregulation of SOX2 expression significantly increases invasiveness of breast cancer such as MCF7 and ZR751 in a study on the relationship between SOX2 and breast cancer cell invasion. Sex-determining region Y-box protein 2 inhibits the invasiveness of breast cancer cells through a mechanism that is dependent on Twist-1 and the transcriptional activity of SOX2. The latest Chinese studies revealed that SOX2-positive cells suggest high tumor invasion and lead to tumor lymph node metastasis by promoting blood vessels and lymphangiogenesis. Experimental results show that SOX2 plays a certain role in angiogenesis.33 The relationship between SOX2 and invasive ability of breast cancer cells needs further confirmation.

2.3 SOX2 with the diagnosis and treatment of breast cancer

Li et al34 detected the regulation effect of SOX2 in the expression of FOXA1 by reverse transcription polymerase chain reaction (RT-PCR) and Western blot. Their results revealed that SOX2 acts as a negative upstream regulator of the FOXA1 (Forkhead Box A1) gene and inhibits FOXA1 expression. They also found significantly higher tumor incidence because SOX2 and FOXA1 were both expressed abnormally compared with that in which only a single gene was expressed abnormally in invasive breast cancer. Immunohistochemical staining further revealed that SOX2 and FOXA1 demonstrates complementation in the diagnosis of human breast cancer. Therefore, SOX2 and FOXA1 can be used as effective markers for the diagnosis of breast cancer. The use of FOXA1, as a cofactor of SOX2, can improve the diagnostic efficiency of breast cancer.

In addition, studies found that SOX2-related serum, SOX2 autoantibody (SOX2-Abs), can be used as a serum marker of breast cancer. Sex-determining region Y-box protein 2-Abs was first reported in 2012. It is related to high tumor grade and positive lymph node status in breast cancer patients. Sun et al35 determined the presence of SOX2-Ab in 282 breast cancer patients, and they discovered it in 78 patients with benign breast disease and 194 healthy women via indirect enzyme-linked immunosorbent assay. SOX2-Abs was generally found in breast cancer patients (18.4%) than among healthy women (2.6%) and patients with benign breast disease (6.4%). Comparing the sensitivity and specificity of SOX2Ab and serum CEA, TPS, CA125, and CA153 in patients with benign and malignant breast cancer proved that the sensitivity of the SOX2-Ab assay (18.4%) was significantly higher than that of CEA, CA125, and CA153 (all <5%), and its specificity was superior to that of the TPS assay. Furthermore, SOX2-Abs measurement was more effective than serum CEA, TPS, CA 125, and CA 153 determination upon comparing the distinction between benign and malignant breast disease. Therefore, SOX2-Abs measurements were more powerful diagnostic tools than these clinically acceptable diagnostic markers. This study was the first report of the presence of SOX2-Abs, and its potential clinical applications may provide a noninvasive and simple serological marker for the diagnosis and prognosis of breast cancer.

The high expression of SOX2 is related to the occurrence of breast cancer. The inhibition of high SOX2 expression may provide a novel idea for the treatment of breast cancer. Schaefer et al36 found that Ser/Thr-kinase AKT is an upstream regulator of SOX2 protein, and ectopic expression of SOX2 enhances the ability of clones of tumor cells and restores the cellular tumorigenicity of cells that were inhibited by AKT inhibitors in vivo. AKT inhibitors can effectively inhibit the growth of tumor stem cells with SOX2 expression on which conventional chemotherapeutic agents usually act. Therefore, AKT inhibitors can inhibit the expression of SOX2 protein. Novel treatment methods for breast cancer may be obtained by studying effective AKT inhibitors. In a recent study, Liu et al37 demonstrated that a new SOX2-mediated regulatory axis plays an important role in the proliferation, invasion, and metastasis of breast cancer cells, thereby providing a novel direction for the treatment of breast cancer.

2.4 SOX2 with the recurrence and resistance of breast cancer

Sex-determining region Y-box protein 2 is associated with early recurrence of breast cancer. Finicelli et al38 observed the effect of SOX2 on early recurrence in patients with breast cancer via multivariate analysis. The risk of recurrence with SOX2-positive disease (82%) was higher than that in SOX2-negative disease (42%) in 117 breast cancer samples. Therefore, patients with SOX2-positive disease will relapse earlier than patients with SOX2-negative disease.

Drug resistance of breast cancer has always been a clinical problem. CSCs play a role in tumor radiotherapy and chemotherapy resistance. Tamoxifen, an estrogen receptor antagonist, is the basic therapy of patients with breast cancer. Piva et al39 found that tamoxifen-resistant cells are enriched in tumor stem/progenitor cells, and they express high levels of stem cell marker SOX2. The sensitivity of tamoxifen is negatively related to the ectopic expression of SOX2. The tumor stem/progenitor cell population decreases and sensitivity to tamoxifen is restored if the expression of SOX2 gene is knocked out. Compared with patients who were successful with endocrine therapy, the SOX2 level was higher in primary tumors of patients with failure to endocrine therapy. Gene expression profiles showed that the Wnt signaling pathway is activated in SOX2-expressing cells. It can restore sensitivity of drug-resistant cells to tamoxifen through inhibition of Wnt signaling. These results indicate that the development of tamoxifen resistance is mediated by SOX2-dependent activation of Wnt signaling in cancer stem/progenitor cells.