ATM kinase inhibition preferentially sensitizes p53-mutant glioma to ionizing radiation

Predictability, efficacy and safety of radiosensitization of glioblastoma-initiating cells by the ATM inhibitor KU-60019

We have previously shown that pharmacological inhibition of ataxia telangiectasia mutated (ATM) protein sensitizes glioblastoma-initiating cells (GICs) to ionizing radiation (IR). Herein, we report the experimental conditions to overcome GIC radioresistance in vitro using the specific ATM inhibitor KU-60019, two major determinants of the tumor response to this drug and the absence of toxicity of this treatment in vitro and in vivo. Repeated treatments with KU-60019 followed by IR substantially delayed GIC proliferation in vitro and even eradicated radioresistant cells, whereas GIC treated with vehicle plus radiation recovered early and expanded. The tumor response to the drug occurred under a cutoff level of expression of TP53 and over a cutoff level of expression of phosphatidylinositol 3-kinase (PI3K). No increased clastogenicity or point mutagenicity was induced by KU-60019 plus radiation when compared to vehicle plus radiation. No significant histological changes to the brain or other organs were observed after prolonged infusion into the brain of KU-60019 at millimolar concentrations. Taken together, these findings suggest that GIC-driven tumors with low expression of TP53 and high expression of PI3K might be effectively and safely radiosensitized by KU-60019.

Targeting nuclear kinases in cancer: development of cell cycle kinase inhibitors

Cellular proliferation is a tightly controlled set of events that is regulated by numerous nuclear protein kinases. The proteins involved include checkpoint kinases (CHK), cyclin-dependent kinases (CDK), which regulate the cell cycle and aurora kinases (AURK) and polo-like kinases (PLK), which regulate mitosis. In cancer, these nuclear kinases are often dysregulated and cause uncontrolled cell proliferation and growth. Much work has gone into developing novel therapeutics that target each of these protein kinases in cancer but none have been approved in patients. In this review we provide an overview of the current compounds being developed clinically to target these nuclear kinases involved in regulating the cell cycle and mitosis.

Silencing of poly(ADP-ribose) glycohydrolase sensitizes lung cancer cells to radiation through the abrogation of DNA damage checkpoint

Poly(ADP-ribose) glycohydrolase (PARG) is a major enzyme that plays a role in the degradation of poly(ADP-ribose) (PAR). PARG deficiency reportedly sensitizes cells to the effects of radiation. In lung cancer, however, it has not been fully elucidated. Here, we investigated whether PARG siRNA contributes to an increased radiosensitivity using 8 lung cancer cell lines. Among them, the silencing of PARG induced a radiosensitizing effect in 5 cell lines. Radiation-induced G2/M arrest was largely suppressed by PARG siRNA in PC-14 and A427 cells, which exhibited significantly enhanced radiosensitivity in response to PARG knockdown. On the other hand, a similar effect was not observed in H520 cells, which did not exhibit a radiosensitizing effect. Consistent with a cell cycle analysis, radiation-induced checkpoint signals were not well activated in the PC-14 and A427 cells when treated with PARG siRNA. These results suggest that the increased sensitivity to radiation induced by PARG knockdown occurs through the abrogation of radiation-induced G2/M arrest and checkpoint activation in lung cancer cells. Our findings indicate that PARG could be a potential target for lung cancer treatments when used in combination with radiotherapy.

Autotaxin Inhibition with PF-8380 Enhances the Radiosensitivity of Human and Murine Glioblastoma Cell Lines

Purpose: Glioblastoma multiforme (GBM) is an aggressive primary brain tumor that is radio-resistant and recurs despite aggressive surgery, chemo, and radiotherapy. Autotaxin (ATX) is over expressed in various cancers including GBM and is implicated in tumor progression, invasion, and angiogenesis. Using the ATX specific inhibitor, PF-8380, we studied ATX as a potential target to enhance radiosensitivity in GBM.

Methods and Materials: Mouse GL261 and Human U87-MG cells were used as GBM cell models. Clonogenic survival assays and tumor transwell invasion assays were performed using PF-8380 to evaluate role of ATX in survival and invasion. Radiation dependent activation of Akt was analyzed by immunoblotting. Tumor induced angiogenesis was studied using the dorsal skin fold model in GL261. Heterotopic mouse GL261 tumors were used to evaluate the efficacy of PF-8380 as a radiosensitizer.

Results: Pre-treatment of GL261 and U87-MG cells with 1 μM PF-8380 followed by 4 Gy irradiation resulted in decreased clonogenic survival, decreased migration (33% in GL261; P = 0.002 and 17.9% in U87-MG; P = 0.012), decreased invasion (35.6% in GL261; P = 0.0037 and 31.8% in U87-MG; P = 0.002), and attenuated radiation-induced Akt phosphorylation. In the tumor window model, inhibition of ATX abrogated radiation induced tumor neovascularization (65%; P = 0.011). In a heterotopic mouse GL261 tumors untreated mice took 11.2 days to reach a tumor volume of 7000 mm³, however combination of PF-8380 (10 mg/kg) with irradiation (five fractions of 2 Gy) took more than 32 days to reach a tumor volume of 7000 mm³.

Conclusion: Inhibition of ATX by PF-8380 led to decreased invasion and enhanced radiosensitization of GBM cells. Radiation-induced activation of Akt was abrogated by inhibition of ATX. Furthermore, inhibition of ATX led to diminished tumor vascularity and delayed tumor growth. These results suggest that inhibition of ATX may ameliorate GBM response to radiotherapy.