ATM-NFκB axis-driven TIGAR regulates sensitivity of glioma cells to radiomimetics in the presence of TNFα

Gliomas are resistant to radiation therapy, as well as to TNF alpha induced killing. Radiation-induced TNF alpha triggers Nuclear factor kappa B (NF kappa B)-mediated radioresistance. As inhibition of NF kappa B activation sensitizes glioma cells to TNF alpha-induced apoptosis, we investigated whether TNF alpha modulates the responsiveness of glioma cells to ionizing radiation-mimetic Neocarzinostatin (NCS). TNF alpha enhanced the ability of NCS to induce glioma cell apoptosis. NCS-mediated death involved caspase-9 activation, reduction of mitochondrial copy number and lactate production. Death was concurrent with NF kappa B, Akt and Erk activation. Abrogation of Akt and NF kappa B activation further potentiated the death inducing ability of NCS in TNF alpha cotreated cells. NCS-induced p53 expression was accompanied by increase in TP53-induced glycolysis and apoptosis regulator (TIGAR) levels and ATM phosphorylation. siRNA-mediated knockdown of TIGAR abrogated NCS-induced apoptosis. While DN-I kappa B abrogated NCS-induced TIGAR both in the presence and absence of TNF alpha, TIGAR had no effect on NF kappa B activation. Transfection with TIGAR mutant (i) decreased apoptosis and gamma H2AX foci formation (ii) decreased p53 (iii) elevated ROS and (iv) increased Akt/Erk activation in cells cotreated with NCS and TNF alpha. Heightened TIGAR expression was observed in GBM tumors. While NCS induced ATM phosphorylation in a NF kappa B independent manner, ATM inhibition abrogated TIGAR and NF kappa B activation. Metabolic gene profiling indicated that TNF alpha affects NCS-mediated regulation of several genes associated with glycolysis. The existence of ATM-NF kappa B axis that regulate metabolic modeler TIGAR to overcome prosurvival response in NCS and TNF alpha cotreated cells, suggests mechanisms through which inflammation could affect resistance and adaptation to radiomimetics despite concurrent induction of death.