Abstract C2: Cross-talk between the O(6)-Methylguanine-DNA methyltransferase (MGMT) and p53 in glioblastoma multiforme.

Abstract

Abstract Background. Glioblastoma multiforme (GBM) is a fatal malignant primary brain tumor in adults with a median survival time of approximately 14.6 months, despite treatment with surgery, concurrent radiation therapy (RT) and the alkylating agent temozolomide (TMZ). O(6)-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein involved in resistance to TMZ in GBM. Our group showed that MGMT is a critical regulator of GBM angiogenesis, invasion and response to antiangiogenic treatment (sunitinib). While sunitinib treatment significantly decreased invasion in MGMT-positive cell lines, it led to a significant increase of invasion in MGMT-negative cell lines. Our group launched the first phase II clinical trial of concurrent sunitinib with RT and TMZ for selected patients with MGMT-positive tumors. The p53 tumor suppressor protein is involved in proliferation, angiogenesis and response to RT. Previous studies showed the effect of p53 in regulating MGMT levels. The role of MGMT and p53 in response to RT and sunitinib is unknown. We hypothesize that MGMT may affect levels of p53 and response to RT and sunitinib treatment. Results. We used isogenic human GBM cell lines isogenic for MGMT and assessed p53 levels by Western blotting and immunofluorescence. We showed for the first time that stable transfection of GBM cell line negative for MGMT (U87MG) and harboring wild type TP53 with a vector encoding for MGMT (U87MGMT) led to significant increase of p53 expression and its nuclear localization. Strikingly, increased p53 in U87MGMT cells did not lead to increased p21 protein, one of the main targets of p53. As shown by western blotting analysis, ionizing radiation (6 and 10 Gy) did not increase p21 levels compared to sham control. Interestingly, it was previously shown that irradiation does not lead to p21 activation in GBM cells with basal overexpression of p21 mRNA and can even lead to its repression as a mechanism of resistance to RT. We suggest that MGMT could be an upstream regulator of this resistance. Conclusion. Our study revealed a novel mechanism for regulation of p53 through MGMT, which may affect response to RT in GBM. We will further establish the relationship between MGMT/p53 status and response to combined RT and sunitinib in GBM cell lines and samples from patients enrolled in our clinical trial. This will lead to more efficient strategies of treatment for patients with tumors unresponsive to combined RT and sunitinib. Grant acknowledgments. This work has been funded by the Cedar Cancer Foundation, Research Institute of McGill University Health Center and the McGill-CIHR Drug Development Training Program (DDTP). Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C2. Citation Format: Mariia Patyka, Bassam Abdulkarim, Yaoxian Xu, Siham Sabri. Cross-talk between the O(6)-Methylguanine-DNA methyltransferase (MGMT) and p53 in glioblastoma multiforme. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C2.