Targeting DNA-PKcs and telomerase in brain tumour cells

BACKGROUND: Patients suffering from brain tumours such as glioblastoma and medulloblastoma have poor prognosis with a median survival of less than a year. Identifying alternative molecular targets would enable us to develop different therapeutic strategies for better management of these tumours. METHODS: Glioblastoma (MO59K and KNS60) and medulloblastoma cells (ONS76) were used in this study. Telomerase inhibitory effects of MST-312, a chemically modified-derivative of epigallocatechin gallate, in the cells were assessed using telomere repeat amplification protocol. Gene expression analysis following MST-312 treatment was done by microarray. Telomere length was measured by telomere restriction fragments analysis. Effects of MST-312 on DNA integrity were evaluated by single cell gel electrophoresis, immunofluorescence assay and cytogenetic analysis. Phosphorylation status of DNA-PKcs was measured with immunoblotting and effects on cell proliferation were monitored with cell titre glow and trypan blue exclusion following dual inhibition. RESULTS: MST-312 showed strong binding affinity to DNA and displayed reversible telomerase inhibitory effects in brain tumour cells. In addition to the disruption of telomere length maintenance, MST-312 treatment decreased brain tumour cell viability, induced cell cycle arrest and double strand breaks (DSBs). DNA-PKcs activation was observed in telomerase-inhibited cells presumably as a response to DNA damage. Impaired DNA-PKcs in MO59J cells or in MO59K cells treated with DNA-PKcs inhibitor, NU7026, caused a delay in the repair of DSBs. In contrast, MST-312 did not induce DSBs in telomerase negative osteosarcoma cells (U2OS). Combined inhibition of DNA-PKcs and telomerase resulted in an increase in telomere signal-free chromosomal ends in brain tumour cells as well. Interestingly, continual exposure of brain tumour cells to telomerase inhibitor led to population of cells, which displayed resistance to telomerase inhibition-mediated cell arrest. DNA-PKcs ablation in these cells, however, confers higher cell sensitivity to telomerase inhibition, inducing cell death. CONCLUSIONS: Efficient telomerase inhibition was achieved with acute exposure to MST-312 and this resulted in subtle but significant increase in DSBs. Activation of DNA-PKcs might indicate the requirement of NHEJ pathway in the repair telomerase inhibitor induced DNA damage. Therefore, our results suggest a potential strategy in combating brain tumour cells with dual inhibition of telomerase and NHEJ pathway.