In proximal tubular epithelial cells, AEG‑1/MTDH is important in TGF‑β1‑induced epithelial‑mesenchymal transition (EMT) through activation of p38 MAPK (33). A recent study has suggested that AEG‑1/MTDH contributes to the pathogenesis of diffuse large B‑cell lymphoma mediated through regulation of the Wnt/β‑catenin pathway (34).
Furthermore, in the carcinogenesis of acute myeloid leukemia (AML), a novel functional link has been revealed between AEG‑1/MTDH and Aurora A kinase (AURKA) with regard to Akt1 activation (35). In human AML cells, AEG‑1/MTDH overexpression is vital for the maintenance of the malignant state via upregulation of Akt1, which is mediated by AURKA activation (35). In breast cancer cells, AEG‑1/MTDH facilitates cancer proliferation and invasion by upregulating HER2/neu expression (36).
Angiogenesis and metastasis. AEG‑1/MTDH overexpression converts non‑tumorigenic human HCC cells into highly aggressive vascular tumors. In addition, AEG‑1/MTDH modulates the expression of genes associated with invasion, angiogenesis, metastasis, chemoresistance and senescence, as determined by microarray analysis (15). AEG‑1/MTDH has a dominant function in regulating oncogenic transformation and angiogenesis (37). AEG‑1/MTDH expression is also increased in multiple cancers and is crucial in oncogenic transformation and angiogenesis (38‑41). In a phage display study, Brown and Ruoslahti (10) identified that a lung homing domain (amino acids 378‑440 in mice and 381‑443 in humans) in AEG‑1/MTDH was a mediator of 4T1 mouse mammary tumor cell adhesion to the lung vasculature, and suggested that AEG‑1/MTDH is important in breast cancer metastasis. In CRC, Jiang et al (18) showed that AEG‑1/MTDH is overexpressed in liver metastasis patients compared with patients without liver metastasis. In addition, AEG‑1/MTDH may present as a potential novel biomarker for early liver metastasis. In a large proportion of epithelial ovarian cancer patients with peritoneal dissemination and/or lymph node metastasis, AEG‑1/MTDH is overexpressed and is a novel predictor of metastasis (42). In summary, AEG‑1/MTDH is crucial in lymph node metastasis (39,43‑45) and contributes to tumor progression, including transformation, the evasion of apoptosis, invasion and metastasis (13).
Chemoresistance. One of the important hallmarks of aggressive cancers is chemoresistance. Previous studies have suggested that AEG‑1/MTDH contributes to a broad spectrum of resistance to various chemotherapeutics, including 5‑fluorouracil, doxorubicin, paclitaxel, cisplatin and 4‑hydroxycyclophosphamide (16,46‑48). In human HCC cells, the gene expression profiles of overexpressed AEG‑1/MTDH have been identified in several drug‑metabolizing enzymes involved in chemoresistance, including dihydropyrimidine dehydrogenase, cytochrome P450B6, dihydrodiol dehydrogenase, ATP‑binding cassette transporter 11/MRP8 and transcription factor LSF/TFCP2 (15). AEG‑1/MTDH increases multidrug‑resistance gene 1 (MDR1) protein expression, which facilitates the association between MDR1 mRNA and polysomes, leading to increased translation, the inhibition of ubiquitination and the resultant proteasome‑mediated degradation of the MDR1 protein (47). The inhibition of AEG‑1/MTDH may be an effective method in HCC chemotherapy (47). Bhutia et al (49) also showed that protective autophagy is the cause of AEG‑1‑mediated chemoresistance, and that the inhibition of AEG‑1/MTDH results in a decrease in the protective autophagy and chemosensitization of cancer cells. Due to the multiple functions of AEG‑1/MTDH in drug resistance, AEG‑1/MTDH is a viable target as an anticancer agent for a wide range of cancer types (50).
Recent results have also indicated that AEG‑1/MTDH affects the radiosensitivity of cervical cancer cells (51). In summary, it has become apparent that AEG‑1/MTDH is an important oncogene, which is overexpressed in numerous human cancer types. Through a number of signaling cascades, AEG‑1/MTDH is involved in several crucial aspects of tumor progression, including transformation, proliferation, the evasion of apoptosis, cell survival, migration and invasion, angiogenesis, metastasis and chemoresistance (52). Future studies are required to evaluate the correlation between AEG‑1/MTDH function and signaling changes and interacting partners in order to highlight novel perspectives for AEG‑1/MTDH as a significant target for the clinical treatment of various cancers.