A low-pH culture condition enhances the radiosensitizing effect of wortmannin.

Abstract

The radiosensitizing effect of wortmannin on human tumor cells in a low-pH microenvironment was compared with that in a neutral-pH environment. A172 human glioblastoma cells, A549 human lung adenocarcinoma cells, and HMV-1 human melanoma cells were treated with 20 microM wortmannin 2 h before irradiation, and cell survival was examined. A low-pH microenvironment was simulated by exposing cells to low-pH culture medium for 24 h before wortmannin treatment. The effects of wortmannin on the repair of DNA double-strand breaks (dsbs) after 50-Gy irradiation in both low- and neutral-pH conditions were measured by pulsed-field gel electrophoresis. Expression of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) in low-pH conditions was also compared with that in neutral-pH conditions by Western blot analysis. The radiosensitizing effect of wortmannin was greater in low-pH cultures than in neutral-pH cultures for all cell lines. The fast-rejoining component of DNA dsb repair was inhibited more strongly in low-pH than in neutral-pH conditions, although there was little difference in DNA-PKcs expression between groups. The low-pH culture condition, which was designed to mimic the microenvironment of the central tumor mass in actively proliferating solid tumors, enhanced the radiosensitizing effect of wortmannin by inhibiting the fast-rejoining component of DNA dsb repair and by prolonging the retention of nonrejoined DNA dsbs.