Effects of the multidrug transporter P-glycoprotein on cellular responses to ionizing radiation.

Ionizing radiation induces apoptosis, mitotic catastrophe, and senescence-like terminal proliferation arrest in tumor cells. We investigated the effect of the MDR1 P-glycoprotein (Pgp), recently shown to inhibit caspase-mediated apoptosis, on cellular responses to radiation. Pgp strongly inhibited radiation-induced apoptosis in a HeLa-derived cell line with inducible MDR1 expression and in NIH 3T3 cells transduced with a MDR1-expressing retroviral vector. The inhibition of apoptosis by Pgp was associated, however, with increases in radiation-induced mitotic catastrophe and senescence and produced only a marginal change in the survival of irradiated cells. Pgp had no effect on radiation responses in apoptosis-resistant HT1080 cells. These results indicate that Pgp inhibits radiation-induced apoptosis, but this effect of Pgp provides no substantial increase in radiation resistance of the tested cell lines because apoptosis-resistant cells die from mitotic catastrophe or undergo senescence-like terminal proliferation arrest.