Nonsmall cell lung cancer (NSCLC) is a major type of lung cancer. In current study, we aim to evaluate whether the combination of Ku70/80 heterodimer protein inhibitor STL127705 and gemcitabine would be more favorable approach for the treatment of NSCLC compared with monotreatment with gemcitabine. Clongenic survival assay was used to determine the survival and sensitivity to irradiation. H1299 was stained with fluorescein isothiocyanate-Annexin V, and cell apoptosis was measured by flow cytometry. H1299 cells were transfected with nonhomologous end-joining (NHEJ) repair reporter, and stable cell line was selected by puromycin. NHEJ activity was determined based on the intensity of green fluorescent protein. DNA double-strand breaks (DSBs) were determined by the fluorescence intensity of γH2AX using flow cytometry. The mRNA expressions of Ku70 and Ku80 were determined using quantitative real-time PCR. Combination of STL127705 enhanced sensitivity of NSCLC cell lines to irradiation when compared with treatment with gemcitabine alone. However, small cell lung cancer cell line was not affected. H1299 cells treated with STL127705 in combination with gemcitabine showed a significantly increased apoptosis compared with H1299 cells treated with gemcitabine alone. Moreover, STL127705 treatment dramatically reduced NHEJ activity in H1299 cells when compared with gemcitabine single treatment. Increased DSBs were consistently observed in H1299 when treated with the combination of STL127705 and gemcitabine. However, the mRNA levels of Ku70 and Ku80 were upregulated by the combination treatment. It demonstrated that STL127705 enhanced antitumor activity of gemcitabine. Mechanistically, treatment with STL127705 enhanced DNA damage via inhibiting NHEJ pathway, blocking DNA-PK, and forming Ku70/80 heterodimer, eventually leading to tumor cells apoptosis.
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