In this study, the microarray data of mitochondria-related genes in BC collected from GEO were used to identify the mrDEGs further validated in TCGA. Moreover, GO enrichment analysis and KEGG pathway mapping for validated mrDEGs were performed to explore the potential function of mrDEGs in breast carcinogenesis. Based on that, we constructed the PPI network, discovered the hub mrDEGs, and analyzed the correlation between hub mrDEGs and the overall survival of BC patients to investigate the influence of hub mrDEGs on BC prognosis.
Mitochondria play a critical role in multiple cell processes, and mitochondrial dysfunction may affect the occurrence and development of BC. The initiation and metastasis of BC could be altered by regulating the genetic background of mitochondria, making mrDEGs potential therapeutic targets (9). Here, we utilized multiple bioinformatics tools to analyze the microarray data of mitochondria-related genes, and found out 23 up-regulated and 71 down-regulated mrDEGs in BC. They were closely associated with mitochondrial dysfunction in breast carcinogenesis. GO enrichment analysis demonstrated that 94 mrDEGs were enriched in cancer-related biological processes, such as neural crest cell migration involved in autonomic nervous system development, regulation of cell migration, cell surface receptor signaling pathway, cell differentiation and regulation of cell communication. These biological processes conformed to tumor cell properties, including unlimited cell proliferation, cell invasion and migration, and reduced intercellular adhesion (10), suggesting that mrDEGs were tightly linked to breast carcinogenesis. KEGG pathway mapping showed that mrDEGs might participate in cancer-related regulation pathways, including PI3K-ALT pathway, TGF-beta pathway, evading apoptosis and resistance to chemotherapy. The relationship of them with BC could be listed as follows: (1) Inhibiting PI3K-ALT pathway may induce mitochondria-mediated cell apoptosis of BC (11); (2) Ligand-dependent or cell-autonomous activation of the TGF-β pathway in stromal cells could induce metabolic reprogramming, enhance oxidative stress, mitochondrial autophagy and aerobic glycolysis, and decrease Cav-1, which can spread to adjacent fibroblasts and maintain BC cell growth (12); (3). Regarding the well-known property of unlimited proliferation in BC cells, the GSTs gene mapped in evading apoptosis pathway could regulate cell apoptosis by its interaction with various protein partners (13); (4). MITF, a differential gene identified in our research, is able to enhance mitochondrial oxidative phosphorylation (14). It has been reported that enhanced mitochondrial oxidative phosphorylation may induce the resistance to chemotherapy of BC cells (15), thus these genes could be related to drug resistance of BC cells. Overall, the validated mrDEGs mentioned above might be enriched in the pathways for BC progression through regulating mitochondrial function.
PPI network analysis indicated that three interaction networks could be classical models to predict BC occurrence. Model 1 consisted of SEMA3F, EFNA3, SEMA3A and NRP1, which were mainly associated with the structure and function of nerve cells. EFNA3 was induced by HIF under anoxic conditions, and then Ephrin-A3 protein encoded by EFNA3 was aberrantly accumulated to promote the metastasis of BC cells (16). Model 2 was composed of BGN, DDR2 and COL5A2, which were mainly involved in extracellular matrix of cells. COL5A2 related to extracellular matrix remodeling was up-regulated during ductal carcinoma in situ developed to invasive ductal carcinoma, leading to BC progression (17). Model 3 included ZEB1, VIM and FN1, which participated in tissue development and gene expression. ZEB1 increased the expression of vascular endothelial growth factor (VEGF) via paracrine to stimulate angiogenesis in BC (18). ZEB1 also promoted epithelial mesenchymal transformation (EMT), proliferation and migration of BC (19). All the three models with different function took their parts in BC progression.
In our study, 9 hub mrDEGs were screened out based on MCC method, including up-regulated FN1, BGN, EFNA3, COL5A2 and SEMA3F as well as down-regulated RHOQ, SEMA3A, NRP1 and DDR2. Next, Kaplan Meier plotter was used to analyze the association between hub mrDEGs and the overall survival of BC patients. The results showed that only up-regulated FNA and SEMA3F and down-regulated DDR2 suggested poor BC prognosis (P < 0.05) with the potential to be a significant biomarker. FN1 has been demonstrated to be up-regulated in BC epithelial cells without mitochondria DNA (20). FN1 was also a core gene of mrDEGs network and its encoded fibronection distributed in BC cell matrix affecting tumor progression (20). Meanwhile, FN1 could regulate EMT of BC cells (21) and might be one of the key genes in BC invasion and migration (22). Therefore, FN1 has the potential to be a diagnostic biomarker and a molecular therapeutic target of BC. During the initial of breast neoplasms, SEMA3F could prevent tumor cells from spreading and attaching to stromal cells for further implantation (23). Additionally, SEMA3F may interact with NRP1 or NRP2 receptor to suppress the metastasis and invasion of BC cells (24). Hence, it could be speculated that up-regulated SEMA3F is likely to be a capable biomarker for BC detection. DDR2 was activated by fibrillar collagen to regulate the synthesis of extracellular matrix and wound healing (25), exerting important roles in microenvironment. DDR2 was involved in hypoxia-induced cancer metastasis by accelerating migration, invasion and EMT of BC cells (26), which might be a potential molecular therapeutic target. Above all, mitochondria-related hub genes may function in different stages of BC. Further investigations on these genes would help to elucidate BC etiology from the perspective of mitochondrial dysfunction, and thus to identify diagnostic and prognostic biomarkers and also molecular targets for BC targeted therapy.