Regulation of DNA damage tolerance by the AhR pathway and its role in glioma progressio

Glioblastoma multiforme (GBM) is the most deadly primary brain tumor and patients with GBM have a median survival time of 12 months, due in large part to poor responses to chemo- and radio-therapy. These tumors exhibit high levels of DNA damage response activation before treatment and this likely plays a role in the inherent chemo- and radio-resistance of GBM. Y-family polymerases aid cells in bypassing genotoxic damage, but they are mis-regulated in many cancers. Over-expression of the Y-family enzyme human polymerase kappa (hpol kappa), which is up-regulated in GBM patients, has been linked to an increase in genomic instability. The factors promoting over-expression of hpol kappa in GBM are unknown, but previous work implicates the aryl hydrocarbon receptor (AhR) in the regulation of hpol kappa levels. Up-regulation of the AhR pathway through aberrant production of the endogenous ligand kynurenine is associated with malignant progression and shorter survival in glioma patients. Using in vitro cell culture models, we tested the hypothesis that the AhR signaling pathway transcriptionally regulates hpol kappa levels in gliomas. Exposure to the AhR agonist 3-methylcholanthrene increases hpol kappa levels in GBM cells. Furthermore, when the enzyme tryptophan 2,3-dioxygenase (TDO), which is responsible for aberrant kynurenine production in gliomas, is inhibited by the small molecule 680C91, hpol kappa expression decreased in three GBM cell lines. We then explored the idea that stimulation of hpol kappa expression by the AhR pathway contributes to the inherent genomic instability seen in gliomas. Indeed, TDO inhibition leads to a decrease in spontaneous micronuclei formation in three GBM cell lines. Additionally, treatment with an AhR antagonist decreases micronuclei formation in T98G cells, as does knockdown of hpol kappa expression. Epistasis assays supported our hypothesis that TDO activity, AhR signaling, and hpol kappa expression act in the same pathway to promote genomic instability in gliomas. Moreover, we found that TDO inhibition also significantly increases sensitivity of T98G cells to cis-diamminedichloroplatinum (CDDP) and bis-chloroethylnitrosourea (BCNU). Our study reveals how activation of AhR signaling impacts replication dynamics in gliomas and provides insight into the mechanisms leading to the genomic instability and chemoresistance often inherent to this disease.