C3G: a new player in the control of invasiveness, stemness and tumorigenesis in glioblastoma

Abstract

C3G is a guanine-nucleotide exchange factor (GEF) for some Ras family members, although it can act through GEF independent mechanisms. It is essential for embryonic development due to its role in adhesion and regulates different cellular functions. The role of C3G in human cancer is controversial, acting as either a tumour suppressor or mediator. Although the function of C3G in migration depends on the context, C3G was shown to inhibit migration in various cell types. We also found that C3G acts as a migratory and invasive repressor in MEFs and HCT116 cells through down-regulation of p38α MAPK activity. Based on this, we have studied the role played by C3G in human glioblastoma through gene silencing, using the U87 cell line as an in vitro model. We found that C3G knock-down enhances invasion and reduces cell adhesion. In fact, upon C3G silencing, U87 cells re-organize actin cytoskeleton, leading to neurite-like extensions. These functional and morphological changes correlate with the expression of transcription factors and proteins involved in the epithelial-mesenchymal transition (EMT). Moreover, C3G knock-down induces anchorage-dependent and -independent growth of U87 cells, suggesting that C3G could inhibit tumour growth. C3G may also play a role in the control of stemness and the acquisition of a glioblastoma initiating cell-like phenotype, as its silencing favours spheres formation. C3G silencing also induces changes in different signalling pathways, such as p38 MAPKs, ERKs, STAT3 or c-Met, which might contribute to regulate the observed functional changes. These data indicate that C3G is a new player in the regulation of glioblastoma progression, as it would inhibit tumour growth and invasion. Therefore, its down-regulation might promote tumour growth and the generation of metastasis through a mechanism involving an EMT process and stemness induction. However, more studies are required to fully characterize the function of C3G in glioblastoma, as well as to determine the molecular basis of this regulation and its relevance in clinic.