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Hypoxia is known to be a major factor of resistance to cancer treatment, specially to radiotherapy. Many tumours known to be hypoxic are aggressive and radioresistant. One of the best example is the glioblastoma which is a high grade glioma of very poor prognosis. We have previously shown that the in vivo inhibition of RhoB in glioblastoma xenografts led to significant radiosensitization and oxygenation. RhoB is a small GTPase involved in cytoskeleton actin organization. The aim of the present work was to determine the involvement of RhoB in the control of in vitro hypoxia in U87 glioblastoma cells. To test this hypothesis, we used the SiRNA method in order to specifically inhibit RhoB in U87 glioblastoma cells submitted to hypoxic conditions in an anaerobic chamber. We first showed that inhibiting RhoB led to a decrease of intracellular hypoxia inducible factor (HIF-1α) in hypoxic cells, which could be prevented by incubating cells with proteasome inhibitors. We then investigated the signalling pathways that might be involved in the control of HIF-1α and focused our attention on the Akt/GSK3 pathway. Inhibiting RhoB induced a decrease in Akt phosphorylation and of the GSK3 inactive form level that participates when activated, to the degradation of HIF-1α. Furthermore, the treatment of U87 with GSK3 inhibitor reversed the RhoB-induced reduction of HIF-1α stabilisation. Taken together, our results demonstrate for the first time that the small GTPase RhoB is involved in the control of HIF-1α accumulation via Akt and GSK3. These results suggest that RhoB which controls tumor radioresistance by regulating signaling pathways of intrinsic radioresistance, angiogenesis and hypoxia could be an important therapeutic target for an adjuvant treatment combined with radiotherapy for treating glioblastoma.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]