The BC morbidity rate continues to increase steadily and has become a major public health concern, accounting for 25% of cancer incidence and 15% of cancer-related deaths in women(1). Although mammographic screenings are still helping diagnose breast cancer, the current prevention and treatment strategies are ineffective (30). A genetic risk factor, including pathogenic mutations in genes such as checkpoint kinase 2, considerably impacts BC development; inherited mutations in BRCA1 and BRCA2 account for the majority of cases (31). There are no underlying mechanisms that are capable of preventing or treating breast cancer effectively. In light of mutations of genes, the microenvironment of tumors has been altered, and we can enhance the immune system's role in the fight against cancer to benefit patients. Melanoma, renal cell carcinoma, and non-small cell lung cancer are successfully combated by activating T cells and blocking CTLA-4 and PD-1/PD-L1 axes with monoclonal antibodies (32, 33). It is also possible to study some critically important inhibitors to enhance an immune-activated tumor microenvironment and speed up breast cancer elimination by investigating potential BC biomarkers.
BC patients with bone metastases have low survival rates, with less than 30% surviving over 5 years (34). The majority of BC metastases occur in the bones, except for the liver, brain, and lung. It is defined as the dissemination of cancer cells from primary cancer to secondary sites, which is the evolution of BC metastasis to bone (35). When cancer cells enter the bone marrow, they destroy bone tissue by interacting with osteoblasts, osteoclasts and bone stromal cells, and the cancer-associated stromal fibroblasts release various growth factors stored in bone tissue, causing cancer cells to proliferate continuously to form metastases (36). The function of vascular endothelial cells, hematopoietic stem cells, osteoblasts, osteoclasts, fibroblasts and immune cells is critical during the process of metastasis and the control of tumor dormancy at secondary sites (37–39). It may be beneficial to target these cells and the bone environment to develop novel approaches to prevent and treat bone metastatic BC in female patients.
Bioinformatics can exert a useful tool to understand diseases (40, 41). In our study, we identified the molecules involved in BC bone metastasis based on the data from multiple BC cohorts. Then, we comprehensively investigated the effect pattern and underlying biological role of these molecules. We found that in the identified molecules, the EMP1, ACKR3, ITGA10, MMP13, COL11A1, and THY1 were significantly correlated with patient prognosis and mainly expressed in CAFs. Therefore, we explored the CAFs in BC microenvironment. Results showed that CAFs could activate multiple carcinogenic pathways and most of these pathways play important role in cancer metastasis. Meanwhile, we noticed the interaction between CAFs and malignant, endothelial and M2 macrophage cells. Moreover, we found that CAFs could induce the remodeling of BC microenvironment and promote the malignant behavior of BC cells. Then, we identified MMP13 for further analysis. The result showed that MMP13 could enhance the malignant phenotype of BC cells. Meanwhile, biological enrichment and immune infiltration analysis were conducted to present the effect pattern of MMP13 in BC.
CAFs are a heterogeneous stromal cell population and the most important component of the tumor microenvironment. Our result showed that bone metastasis-related genes were mainly expressed in CAFs. Also, in BC microenvironment, the patients with high CAFs infiltration might have a higher activity of multiple pathways involved in cancer metastasis, including EMT, angiogenesis and coagulation. Meanwhile, CAFS can inhibit the inflammatory response. In colon cancer, Hu et al. found that exosomes secreted by CAFs could enhance chemotherapy resistance and metastasis ability through regulating EMT pathways and tumor stemness (42). Kashima et al. showed that in esophageal cancer, CAFs could promote lymph node metastasis (43). In BC, Wen et al. noticed that the IL32 generated by CAFs could enhance the invasion and metastasis ability through integrin β3-p38 MAPK signaling (44). Zheng et al. found that the biglycan secreted by CAFs in BC can exert a cancer-promoting factor by inducing the immunosuppressive microenvironment (45). Our results indicated that the CAFs might contribute to the bone metastasis process of BC, making it a potential target for clinical applications. Moreover, we found that CAFs could significantly promote the invasion, migration and proliferation ability of BC cells.
MMP13 is a member of matrix metalloprotease. It is reported that MMP is an interesting gene associated with cancer progression, angiogenesis promotion, metastasis and immune surveillance avoidance (46). Zhang et al. indicated that in ovarian cancer, HIF-1α could promote cancer invasion and migration by targeting MMP13 and inducing the hypoxia microenvironment (47). Kumamoto et al. noticed that ING2 can facilitate colon cancer progression through increasing the expression level of MMP13 (48). Also, Ma et al. found that MMP3 and MMP13 are the hub genes of anaplastic thyroid cancer based on transcriptome sequencing (49).
Although we have carried out a lot of analysis to elaborate on the possible mechanism of bone metastasis of BC and obtained some trustworthy results, some shortcomings still need to be mentioned. Firstly, underlying race bias is inevitable. Detailed, the enrolled sample were mostly Western populations. However, there are still differences in genomics among different races, which may reduce the reliability of our conclusions. Secondly, follow-up biological research is still needed to validate our conclusions.