Integrin α3 is overexpressed in glioma stem-like cells and promotes invasion.

Gliomas are the most common primary tumours of the central nervous system, with glioblastomas (GBMs) being the most malignant entity. The poor prognosis of GBM patients is largely due to the highly invasive nature of these tumours. These invading cells are extremely resistant to radiation and chemotherapy, and currently, there are no anti-invasive therapies available (Nakada et al, 2007, 2013). A better understanding of the glioma invasion mechanism will help in developing therapeutic strategies to combat GBM. An increasing body of evidence suggests that a subpopulation of tumour stem-like cell properties in glioma, called either glioma stem-like cells (GSCs) or glioma-initiating cells, is responsible for tumour formation, maintenance, and malignant progression (Singh et al, 2003; Tamase et al, 2009; Natsume et al, 2011). These rare tumour cells are characterised by their strong tumourigenic properties and self-renewal ability. It is critical to understand how the properties of GSCs make them particularly difficult to eradicate. From our data and data from other studies, it is clear that GSCs are primarily responsible for invasion (Liu et al, 2006; Beier et al, 2007; Tamase et al, 2009). However, the molecular features of GSCs that orchestrate the invasion process remain to be elucidated. If we can identify the responsible molecules that mediate GSC invasion, these molecules may represent promising targets for the development of novel anti-invasive therapies. In GSCs, the expression of some neural stem markers such as nestin, SOX2, and Musashi-1 has been reported (Ignatova et al, 2002). In addition, CD133 has been evaluated as an enrichment marker for GSCs; however, several studies have demonstrated their limitations as specific markers (Beier et al, 2007; Wang et al, 2008). To date, there is no perfect GSC marker that can isolate these cells alone. GSCs tend to form neurospheres in the specific culture medium containing various kinds of growth factors, such as epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF). The neurosphere assay is one of the reasonable methods for isolating GSCs. However, a previous report showed that the microenvironment in the neurosphere induces cell differentiation, suggesting that the neurosphere cannot maintain the stem cell properties on its own (Pollard et al, 2009). Therefore, it is difficult to obtain pure cell groups of GSC. However, it is reasonable to investigate the molecular characteristics of GSCs using cell groups that containing high numbers of GSCs. In this study, we used neurosphere methods and cell sorting with CD133 as a marker in order to investigate the biology of GSCs. Integrins are cell surface migration-promoting receptor glycoproteins that mediate various intracellular signals through interaction with the extracellular matrix (ECM). Integrins also have a significant role in the attachment of cells to the ECM, through the formation of cell adhesion complexes, consisting of integrins and many cytoplasmic proteins. In particular, in GBMs, integrins participate in the regulation of complex processes, such as invasion, tumour growth, and angiogenesis by interacting with the ECM in the brain (Nakada et al, 2007). A phase II clinical trial demonstrated that combining the integrin inhibitor cilengitide with standard chemoradiation improved the survival of patients with newly diagnosed GBMs (Nabors et al, 2012). Currently, cilengitide is under phase III clinical trials for patients with GBM and the results will be reported soon (Kurozumi et al, 2012). In this study, we report that GSCs attach to fibronectin and laminin and highly express integrin α3. The immunohistochemistry demonstrated that integrin α3 is localised in GBM cells, especially in invading cells and cells surrounding vessels in vivo. Additionally, overexpression of integrin α3 increases glioma migration and invasion, whereas downregulation of integrin α3 inhibits glioma invasion concomitant with a change in the phosphorylation level of the extracellular signal–regulated kinase (ERK) 1/2 pathway. These results suggest that integrin α3 have significant roles in the invasive behavior of GSCs through the activation of ERK1/2.