We here showed that increased expression of OCT4 and Nanog was significantly associated with aggressive behaviors of NPC including T classification, M classification and tumor stage. Furthermore, these two proteins were shown to be independent prognostic factors. Notably, both OCT4 and Nanog expression were predominantly observed in tumor cells at the invasive front, and correlated strongly with Nestin expression. As novel findings, OCT4 and Nanog expression might serve as valuable predictors of NPC patients.
Embryonic stem cells (ESCs) are defined as cells that have their ability to self-renew and to differentiate into a variety of adult tissues and cell types. It is generally considered that SOX2, OCT4 and Nanog are key transcription regulators that maintain the pluripotency and self-renewal properties of ESCs 
. Growing data demonstrates that stable expression of SOX2, OCT4 and Nanog could promote tumor cell growth, anti-apoptosis and metastasis in vitro
and in vivo
, therefore play an important role in carcinogenesis 
. Of importance, these ESCs-associated proteins were highly expressed in various cancers and contributed to tumor aggressiveness and poor outcome 
. However, little is known about the expression levels of these molecules and their correlations with clinical significance in NPC patients. Compared with non-tumoral epithelium, we observed that the expression levels of SOX2, OCT4 and Nanog were highly increased in NPC tissues, respectively, suggesting that these molecules might be involved in the pathogenesis of NPC. Furthermore, our results also revealed that the high expression of SOX2, OCT4 and Nanog was closely associated with tumor aggressive behaviors of NPC patients. For example, both OCT4 and Nanog expression correlated significantly with T classification, N classification and clinical stage. Furthermore, patients with coexpression of OCT4 and Nanog had significantly worse overall survival. Similar to our observations, coexpression of Oct4 and Nanog was found to link significantly with tumor aggressiveness and poor prognosis of several malignances including lung cancer, oral cancer and hepatocellular carcinoma 
. In fact, a direct link was investigated between OCT4 and Nanog, and they jointly controlled a cascade of pathways to govern the pluripotency and self-renewal characteristics of ESCs 
. In the present study, a significantly positive relationship between high expression of OCT4 and Nanog was also found. Based on these findings, we suggest that there might be a positive involvement of OCT4/Nanog signaling in tumor invasion and progression of NPC. However, functional impacts of coexpression of OCT4 and Nanog in NPC need to be further examined. Additionally, it is well known that EB-virus infection is strongly associated with NPC carcinogenesis. Therefore, it is of importance to further detect the potential correlation between EBV infection and ESCs-associated biomarkers.
The mesenchymal phenotypic changes by increased motility and invasiveness of epithelial tumor cells are known as the epithelial-mesenchymal transition (EMT) 
. As a feature of aggressive tumors, EMT is characterized by a switch from E-cadherin to N-cadherin expression, which has been found to correlate with tumor progression and metastasis 
. Our previous findings also showed that aberrant E/N-cadherin expression contributed to tumor progression and poor outcome of NPC 
. Of interest, we here showed that overexpression of SOX2, OCT4 and Nanog was significantly associated with high expression of N-cadherin, but adversely with low E-cadherin expression (except for SOX2). Additionally, overexpression of these proteins correlated strongly with the expression of Snail, a central transcription factor as E-cadherin repressor. In line with our observations, several studies in intro
reported that overexpression of SOX2, OCT4, and Nanog, individually or simultaneously, leaded to the induction of EMT 
. Particularly, we found that distributions of SOX2, OCT4 and Nanog staining were more frequently located in the invasive front of tumors, and these cells often exhibited a fibroblast-like, spindle-shaped phenotype. Recently, we have demonstrated that these spindle cells in the invasive tumor front correlated strongly with EMT in NPC tissues 
. Taken together, our findings strongly indicate that these stem cells-like cancer cells might strongly resemble cells that have undergone an EMT. As expected, high expression of these proteins in the invasive front correlated significantly with a majority of tumor aggressive behaviors in NPC, such as tumor infiltration, lymph node metastasis and distant metastasis. To the best of our knowledge, no reports have previously described the prognostic impacts of these ESCs proteins in the invasive front of tumors. Collectively, we speculate that tumor cells with stem-like properties in the invasive tumor front of NPC are capable of generating tumor invasion and metastases.
The niche concept was firstly defined as “stem cell niche”, which is composed of diverse stromal cells including mesenchymal and immune cells, and regulates self-renewal, proliferation, and apoptosis resistance of stem cells 
. Like normal stem cells, it is currently thought that CSCs also rely on a “CSC niche”, to maintain their exclusive abilities to self-renew and grow more differentiated cells 
. Of note, vascular niche has recently proved to be responsible for the induction of CSCs properties 
. For example, Calabrese et al. demonstrated that brain tumor CSCs might live in a “vascular niche” that stimulates their self-renewal. Disrupting this niche impaired CSC self-renewal and significantly inhibited tumor growth 
. Nestin, is an intermediate filament protein known as a stem/progenitor cell marker, which is normally expressed in undifferentiated central nervous system (CNS) cells, but also in endothelial cells 
. More recently, He H et al. reported that Nestin-positive blood vessels were crucial for maintaining the structure of the glioma stem cell niche 
. Interestingly, in the present study, Nestin expression was predominately found in endothelial cells at the invasive front of NPC. Furthermore, tumor cells exhibiting CSCs-like features correlated significantly with Nestin staining in the invasive front. Based on these findings, we postulate that vascular endothelial cells expressing Nestin might represent the CSCs niche in NPC. Surely, functional investigations are warranted to further elucidate this hypothesis.
In summary, our findings show first that OCT4 and Nanog expression might be independent prognostic predictors for patients with NPC. We also postulate that tumor cells with stem cell-like features in the invasive front could generate metastasic capability, and these stem-like features might be maintained by endothelial niches. These findings should be responsible for the clinical behaviors of NPC and could be valuable therapeutic targets.