Low doses of γ-rays can induce the expression of mdr gene

We reported earlier that human cervical carcinoma HeLa cells exposed to 30 fractions of 0.5 Gy γ-rays became resistant to cisplatin, methotrexate and vincristine, but retained the same sensitivity to γ-rays and ultraviolet light. The aim of this study was to examine whether a small number of γ-ray fractions, with a lower daily dose, may also change the sensitivity of preirradiated cells to different cytotoxic drugs. Using the modified MTT staining procedure, we found that cells preirradiated with 5 or 10 daily fractions of only 0.17 Gy γ-rays did not alter their sensitivity to mitomycin, cisplatin, methotrexate, 5-fluorouracil, etoposide and doxorubicin. However, 10 fractions of γ-rays induced resistance to vincristine and vinblastine. Our immunocytochemical experiments using monoclonal antibody JSB-1 show that the plasma membrane P-glycoprotein is involved in the induced resistance to Vinca alkaloids