Breast cancer has become the most common cause of cancer-related deaths in women worldwide, and the current treatment for breast cancer mainly relies on surgical resection, supplemented by radiotherapy, chemotherapy, and targeted therapy. Although there are various treatment options, the number of deaths due to breast cancer has not decreased, which may be due to the increase in the prevalence of breast cancer; therefore, the current primary task is to explore the pathogenesis of breast cancer and discover new biomolecular markers and therapeutic targets to effectively predict the progression and prognosis of breast cancer, then achieve early detection, diagnosis, and treatment.
YAP is a transcriptional co-activator, and it is a major regulator of mammalian organ size and cell proliferation, which is highly conserved in mammals and vertebrates. Moreover, YAP is a downstream effector of the mammalian Hippo signaling pathway, which is composed of a core kinase. The activated mammalian sterile 20-like kinase 1 (MST1) or 2 (MST2) binds and phosphorylates the scaffold protein (SAV1). The active MST–SAV1 complex subsequently phosphorylates and activates the large tumor suppressor 1/2 (LATS1/2) and the scaffold protein MOB kinase activator 1A and 1B (MOB1A and MOB1B). Then, the activated LATS–MOB complex phosphorylates and inhibits YAP and the PDZ-binding motif (TAZ), resulting in the inhibition of the Hippo pathway. When the Hippo pathway is inhibited, non-phosphorylated YAP/TAZ can be transported freely to the nucleus, where they can promote the transcription of their target genes, thereby promoting tumorigenesis, proliferation, migration, and apoptosis[17,18]. Some studies have confirmed that YAP is highly expressed in cancers of the digestive system, such as esophageal, gastric, liver, colon and pancreatic cancers, so YAP is considered to be an oncogene. In breast cancer research, studies have reported that YAP is highly expressed in breast cancer, but this has always been controversial because some scholars have found that YAP expression is downregulated or absent in breast cancer tissues. Thus, there are still differences in whether YAP is oncogene or tumor suppressor gene. The results of this study show that the expression level of YAP in normal breast tissues is higher than that in breast cancer tissues, which is consistent with the findings of Yuan et al. The study of Lamar et al. showed that abnormal YAP expression can activate and drive tumor formation and metastasis, which makes YAP a possible new target for the treatment of breast cancer. However, YAP plays an important role in human tissues, so directly targeting YAP may have side effects. Thus, it is necessary to look for target factors that are associated with YAP. In this study, the correlation analysis between YAP and the clinicopathological characteristics of breast cancer patients showed that YAP is related to the Her-2, ER, and PR. This suggests that YAP has an important role in the endocrine or targeted therapy of invasive breast cancer. This study also further tested the expression of YAP in different breast cancer cell lines, and the results showed that YAP was expressed more highly in MCF-7 cells than in MDA-MB-231 cells. MCF-7 is a type of cell line expressing the ER, which further suggests the significance of YAP in the endocrine therapy of breast cancer, and this has been verified by Zhou et al.; therefore, the results of our study indicate that YAP plays a role as a tumor suppressor in the occurrence and development of invasive breast cancer and plays an important role in the treatment of breast cancer.
Wnt signaling is a conservative pathway in the evolution of multicellular animals. Nearly 40 years have passed since the discovery of the Int-1 proto-oncogene (now called Wnt-1) as the integration site of the mouse breast mammary virus. In contrast, breast cancer is the first cancer related to Wnt signaling. Over the past few decades, several studies have shown that Wnt signaling is involved in cell proliferation and metastasis, immune microenvironment regulation, stem cell maintenance, and treatment resistance in cancer, among others, showing strong anti-cancer potential. β-catenin is the key part of the classic Wnt signaling pathway. When there is no Wnt outside the cell, the Wnt signaling pathway is inhibited, whereas when Wnt exists, Wnt binds to specific Frizzled (FZD) receptors and activates the signaling pathway. The reactive protein binds to leucine-rich repeat-containing G protein-coupled receptor 5 (LGR5) and induces membrane clearance, and with the help of the two, FZD recruits the Disheveled (DVL) protein to the plasma membrane to start the signal transduction in the cell, thus destroying the "destruction complex" composed of adenomatous polyposis coli (APC), Axin, casein kinase 1α (CK1α), and glycogen synthase kinase-3β (GSK-3β). Consequently, the activity of GSK-3β is inhibited, making it unable to phosphorylate β-catenin and causing β-catenin to be unrecognized and degraded. As a result, β-catenin accumulates in the cytoplasm and undergoes nuclear transfer, which will replace the original protein to form a new complex, and finally trigger the expression of the target gene. The high expression of β-catenin in this study was mainly located in the cytoplasmic nucleus, and the expression level in the breast cancer tissues was higher than that in the normal breast tissues. These results were similar to those of Jang et al., wherein compared with normal tissues, the Wnt/β-catenin signal activity of the breast cancer tissues was enhanced. In this study, β-catenin was related to Her-2, suggesting the potential of the Wnt/β-catenin signaling pathway in breast cancer-targeted therapy, although no Wnt inhibitor has currently been approved for the treatment of breast cancer. However, the Wnt/β-catenin pathway can still provide potential solutions for the basic research and clinical treatment of breast cancer.
The Hedgehog signaling pathway is also a highly conserved signaling pathway, which is essential for normal embryogenesis. The Hedgehog signaling pathway has been confirmed to be abnormally activated and highly expressed in many malignant tumors, including small cell lung cancer, medulloblastoma, glioma, melanoma, prostate cancer, ovarian cancer, gastric cancer, and pancreatic cancer. Moreover, the Hedgehog pathway has been found to be involved in the regulation of the induction of embryonic mammary glands, the development of duct structures, and the differentiation of the breast during lactation. Therefore, abnormal or dysregulation of the Hedgehog signaling pathway is closely related to the ocurrence, development, and metastasis of breast cancer. The SMO is one of the core components of the Hedgehog pathway. When the activity of SMO is inhibited, the downstream pathway mediated by SMO will be blocked, and the Hedgehog signaling pathway is in a static state. In contrast, when SMO is activated, the Hedgehog signaling pathway will be further activated. The Hedgehog pathway promotes the growth and development of tumor cells. The core protein, including SMO in the Hedgehog signaling pathway, is highly expressed in breast cancer, which is consistent with the experimental results of our study. In this study, the correlation analysis between SMO and the clinicopathological characteristics of breast cancer patients showed that SMO was related to Her-2, suggesting that SMO may also be of great significance in the process of targeted therapy of invasive breast cancer.
In our study, the expression of YAP, β-catenin, and SMO in the adjacent breast cancer tissues were analyzed, and the results showed that the expression levels of the three proteins in the adjacent breast cancer tissues was X between those of breast cancer tissues and normal tissues, and the results were not statistically different (P > 0.05). However, the results suggest that the expression of protein in paracancerous tissue may be of great significance in breast cancer patients undergoing breast-conserving surgery. The results of a prospective multicenter randomized controlled trial conducted by Dupont et al. showed that removing the extra tissue around the tumor can reduce the marginal positive rate by 50 %, and retaining the negative margin is essential to reduce local recurrence. Therefore, clarifying the protein expression in the adjacent tissues of the cancer may help clinically confirm an optimal range of surgical resection, which has a significant impact on reducing the recurrence of breast cancer after breast-conserving surgery and promoting the prognosis.
In recent years, the cross-talk between the Hippo and Wnt signaling pathways has been confirmed. These pathways can antagonize tissue cell growth by regulating common components such as YAP and TAZ. For example, phosphorylated YAP/TAZ (through LATS) binds or isolates proteins in the cytoplasm β-catenin, while dephosphorylated YAP/TAZ promote the nuclear translocation of β-catenin nuclear translocation[27,28]. Some studies have also found that YAP/TAZ is essential for the formation of β-catenin’s "destruction complex" in the absence of Wnt pathway ligands. The connection between the Hippo and Wnt pathway is related to the development of breast cancer, but the key intersection point has not yet been fully determined. The experimental results of our study show that there is no significant correlation between YAP and β-catenin, indicating that the occurrence and development of breast cancer are very complex and may be affected by many factors. Further research is needed on the mechanism of Hippo and Wnt signaling pathway. Our study proved that the expression of YAP and SMO in breast cancer tissues is negatively correlated, suggesting that there may be an interaction between the Hippo and Hedgehog signaling pathways in breast cancer. Zheng et al. found that YAP attenuated dependent glycolysis and tumor growth by promoting the expression of the breast cancer anti-estrogen resistance 4 (BCAR-4). Then, it coordinates with the hedgehog signal to enhance the transcription of glycolysis activators hexokinase 2 (HK2) and 6-phosphofructo-2-kinase/fructose-2, 6-biphosphatase 3 (PFKFB3) and targets BCAR-4 to lock the therapeutic delivery of nucleic acid (locked nucleic acid, LNA). The experimental results of Tariki et al. showed that overexpression of YAP blocks the Hedgehog signaling, and the knockout of YAP can enhance Hedgehog activity, which further verified the results of this experiment. The co-activation of Hedgehog and Wnt signaling has been demonstrated in some cancers, including basal cell carcinoma and pancreatic ductal adenocarcinoma; however, some studies have shown that the activity of the Wnt/β-catenin pathway can be inhibited by the participants of the Hedgehog pathway. Therefore, the mutual mechanism of the Hedgehog and Wnt signaling pathways is not yet fully clear, and the results of our study have not found a correlation between the two.