Background: Gliomas in general and glioblastoma specifically, are among the most common primary tumors of the brain and skull base. The median survival of patients with glioblastoma has not changed in the last decade. Glioma cells are known for their ability to invade normal brain tissue, a process which makes them durable to standard treatment. The invasion process is not random and has preferred anatomical routs, which suggests that glioma cells interact with their microenvironment. We aimed to characterize the local cues occurring between glioma cells and astrocytes and to study its effect on tumor progression.
Methods and Results: Freshly dissociated astrocytes were harvested from newborn mice and the effect of their conditioned media on glioma cell lines was studied. Astrocytes conditioned media (ACM) significantly increased proliferation of glioma cells, as compared to controls. ACM also increased colony formation in soft agar assays and glioma migration, as demonstrated in transwell migration assay and in a wound healing assay.
The effect of ACM on migration of glioma was significantly reduced when astrocytes were transfected with siRNA directed against glial derived neurotrophic factor (GDNF), a protein secreted by glial cells and neurons, and promotes nerve survival and differentiation. The effect of ACM on proliferation and migration of glioma was also reduced when siRNA was directed against GDNF receptors, GFRα1 and RET, on glioma cells.
Western blotting analysis demonstrated that ACM induces glioma cells proliferation via activation of the PI3K-AKT signaling pathway. The AKT small molecule inhibitor Ly294002 abrogated the effect of astrocytes on glioma proliferation.
Conclusions: Astrocytes induce progression of glioma cells by secretion of GDNF, activation of GFRa1/RET receptors and down signaling through the PI3K-AKT pathway.
