Background
Infiltrative tendency is such a prominent feature of malignant gliomas that tumor cells migrate far from the tumoral epicenter through the surrounding parenchyma [
1,
2]. In addition to cytological atypia and mitotic activity that are needed for histopathological definition of lower-grade gliomas, microvascular proliferation and/or necrosis are specific defining attributes of glioblastoma, which is the most malignant primary brain tumor [
3]. Even lower-grade gliomas assorted as World Health Organization (WHO) grade II to III are characterized by propensity for diffuse infiltration and for the malignant transformation to higher-grade tumors. The hurdle in therapeutic resistance of gliomas is intimately connected to the infiltrative phenotype. The infiltrative feature is an essential part of the clinical aggravation of malignant glioma, making surgical resection incomplete and promoting regrowth of residual tumor cells [
1‐
3]. Infiltrative phenotypes in epithelial malignancies have been unequivocally linked to the phenomenon of epithelial-mesenchymal transition (EMT) that manifests as tumor recurrence, metastasis and therapeutic intractability [
4]. By contrast, only recently has the EMT process in non-epithelial tumors been highlighted as an important player in tumor progression [
5‐
7]. Considering that the various downstream pathways of EMT are related with cancer invasion or metastasis, and therapeutic resistance in non-epithelial human cancer, EMT/EMT-like process can be addressed as a possible therapeutic target [
8].
EMT, as a complicated cellular machinery provoked by various circumstantial factors, leads cellular and biochemical acquisition of motile mesenchymal properties from immobile epithelial cells [
9]. In recent glioma research, EMT have been named as a key player of tumor progression and invasion. In this respect, the recently defined mesenchymal subgroup of glioblastomas reinforce the idea that the EMT-like process has prognostic consequence for malignant brain tumors [
10,
11]. In line with the regulation of stem cell features, EMT may contribute to tumor progression and chemoresistance, and in tumor relapse after treatment as well [
8].
Although the contribution of EMT in glioma progression is not as clear as that in epithelial malignancies, glial-mesenchymal transition as a counterpart of EMT has been revealed as an essential process in glioma invasion [
12‐
16]. EMT principally involves an E-cadherin to N-cadherin shift. Still, the clinical impact of E- and N-cadherin including their effect on patient survival rates remains unknown. It is conceivable that expression of E-cadherin and N-cadherin affects the clinical aspect of glioma patients in terms of survival rates. This study assessed the expression of the classic EMT markers in human glioma samples including formalin-fixed paraffin embedded tissues, fresh frozen glioma samples and human and mouse glioma cell lines, and analyzed the implication in the context of patient survival by comparing the data with the statistics from the large cohort NCI Repository for Molecular Brain Neoplasia Database (REMBRANDT).
Discussion
In the present study, expression of the representative EMT markers, E- and N-cadherin, was investigated in a series of gliomas consisting of WHO grade I through IV tumors to explore the clinical implication with regard to patient survival. Epithelial phenotypes indicated by E-cadherin expression were rarely identified in both low-grade and high-grade tumors, as intuitively expected in non-epithelial malignancies. In comparison, mesenchymal phenotypes denoted by N-cadherin expression were observed in the majority of gliomas through grade I through IV. Although the survival benefit in terms of PFS in the patient group with down-regulated N-cadherin expression showed marginal significance, the survival advantage in the patient group with low N-cadherin expression was increased in the larger REMBRANDT cohort. The current results suggest that the gain of mesenchymal traits in gliomas is boosted by increased N-cadherin expression that is not balanced by E-cadherin alteration.
Glial tumors that lack epithelial phenotypes intrinsically have been observed to rearrange the cytoskeleton, dissimilar to classical EMT of epithelial tumors manifested by E-cadherin to N-cadherin shift [
8,
13]. Similar to our data, a prior study reported that the majority of glioblastomas did not show intrinsic E-cadherin expression in a previous study [
14]. It has been a very rare occasion to encounter malignant gliomas with E-cadherin expression [
17].
Malignant glioma is notable for biological heterogeneity and extreme fatalness, which is fairly connected to its infiltrative attribute. [
1,
2]. EMT is a crucial component in early developmental course, tissue repair process and restructuring [
18], and takes an important part of tumor advancement [
19] and metastasis [
4]. Certain transcription factors such as Slug, Snai1, Twist and matrix metalloproteinases, have been reported to be implicated in EMT, and to promote glioma cell migration and invasion [
19‐
22]. Epithelial cell plasticity is rigorously redirected to display increased mesenchymal cadherins including N-cadherin or cadherin-11 and to convert immotile parent epithelial cells to motile cells with enhanced invasive properties [
23,
24].
In cancerous transformations, the acquisition of EMT traits is closely linked to the proceeding of dedifferentiation and gain of stem cell status [
4]. As supported by important experimental findings in epithelial malignancies including colon, pancreatic and breast cancer, induction of EMT can co-induce stem cell properties, thereby connecting cell motility and stem cell-like programs [
4,
25,
26]. Malignant gliomas turned out to have cancer stem cell population recently [
27]. However, the co-existence of stem cell and EMT features during the progression of glioblastoma has been described lately [
9]. Aside from metastasis, which is rare in gliomas, dedifferentiated phenotype in the residual tumor cell population at the invasive front is coupled with malignant transformation in the recurrent tumor. Despite removal of extensive tumor volume based on grossly detectable levels, microscopic foci of remnant tumor cells beyond the resection margins bring about eventual tumor recurrence, not infrequently accompanied by advance into a higher-grade glioma. With the most infiltrative phenotype of the cells in the invasion front far off the resection margins, the tumor cells are prone to proliferate and to progress after a variable dormant period by means of stem cell characteristics in the remaining population. A previous study reported the enhanced expression of stem cell factors in paired primary and recurrent glioma samples by unsupervised clustering analysis of gene expression profiling [
28]. The concurrence of a stem cell status with EMT features in glioblastoma, either via the β–catenin pathway or KITENIN mediation, has been recently described [
12,
13].
Increased mesenchymal traits are considered to be a pivotal molecular event that leads to enhanced malignancy in gliomas [
8]. Based on genome-scale analysis of large cohorts of glioblastomas, four different subgroups were identified that are dependent on neural differentiation [
10,
11]. Glioblastomas categorized into the mesenchymal subtype displayed by far shorter overall and progression-free survival periods, related with extensive aggressiveness indicated by multifocality or therapeutic resistance against radio- and chemo-therapy [
11]. Accordingly, the clinical consequence of invasive phenotype induced by EMT is evident. A previous study proposed that a small population of glioma cells undergo molecular events that bring about cytoskeletal reorganization and apoptotic resistance. As a result, the tumor cells become highly motile and invasive and then evolve into the treatment-resistant condition [
8]. Compared to metastases of most carcinomas in which the eventual process recapitulate the organization of the primary tumors [
29], the evolutional changes in glioma progression do not involve the organization represented by restoration of E-cadherin expression. Instead, glioma progression seems to incorporate strengthened mesenchymal phenotypes in relevance to up-regulated N-cadherin expression and resultant therapeutic resistance. In addition, glioblastoma clusters other than the mesenchymal subtype acquire mesenchymal traits over recurrent episodes [
11]. An evolutional change with increased mesenchymal phenotype appears to be a frequent episode with regard to disease progression, alike cancer cells using EMT mechanism during the advance to a more aggressive status [
8,
30]. However, further investigation needs be performed to reveal the relation between N-cadherin or other EMT markers/inducers and key parameters for new 2016 WHO glioma classification, such as IDH1 mutation, 1p/19q co-deletion or MGMT promoter methylation [
31].