PAX6 increases glioma cell susceptibility to detachment and oxidative stress

Glioblastoma multiforme (GBM) is an incurable malignant glioma which is very resistant to radiation and alkylating agent-based chemotherapy. Necrosis is a hallmark for GBM and the layer surrounding necrotic area is packed with cells which are now believed to be those migrating out of the necrotic area. Oxidative stress is a condition that GBM cells encounter in the necrotic zone, which is one of the stressful conditions that GBM cells need to resist in order to survive. Our previous studies revealed that low PAX6 expression is a favorable molecular trait for survival acquired by GBM cells because PAX6 could suppress cell invasion and tumorigenicity. Since detachment of cells from GBM is an early event for cell migration and subsequent invasion, we examined whether PAX6 is involved in cell survival after detachment.

PAX6 over-expression was achieved in glioma cells transiently (by adenoviral-mediated transient over-expression) or stably (by the establishment of stable cell lines after transfection). The effect of PAX6 over-expression on the survival and growth of glioma cells after detachment from the culture was determined.

Our data revealed that GBM cells (with their low PAX6 levels) survived the detachment procedure well. However, PAX6 over-expression attenuated GBM cell recovery of growth after detachment-induced stress. Importantly, intracellular reactive oxygen species (ROS) levels increased following cell detachment and that PAX6 over-expressing cells retained higher level of ROS than control cells. This may be partially responsible for the impaird growth rate after cell detachment. Addition of anti-oxidant improved the cell viability of PAX6 over-expressing cells, but did not restore their ability to proliferate.

To survive, GBM cells must resist oxidative stress in the necrotic zone as well as the intracellular ROS generated during detachment. Since PAX6 over-expression in low PAX6-expressing glioma cells attenuated cell survival and growth after detachment, these results suggest that a reduced PAX6 expression may be a molecular trait that gives glioma cells a real selection advantage over other cell types to survive in stressful conditions, thus resulting in expansion of their population.