p53-expressing tumor cells circumvent mitosis, express markers consistent with a G1-like state, and become senescent in response to continuous chemotherapeutic treatment. p53-expressing tumor cells circumvent mitosis, express markers consistent with a G1-like state, and become senescent in response to continuous chemotherapeutic treatment. A, p53-expressing cells show a decrease in the G2 markers, Cdc2, Cyclin B1, and Cyclin A, and an increase in the G1 marker, cyclin E, in response to treatment with a chemotherapeutic agent. The U2OS derivatives, clone 1 (expressing control shRNA) or clone 7 (expressing p53 shRNA) were either untreated or treated with 0.05 μg/mL doxorubicin for the indicated times. Cell extracts were analyzed by immunoblotting for levels of the indicated proteins. Actin levels serve as a protein loading control. B, both p53-expressing and p53-null cells fail to proliferate in response to continuous treatment with a chemotherapeutic agent. Clone 1 (expressing control shRNA) or clone 7 (expressing p53 shRNA) cells were either untreated or treated with 0.05 μg/mL doxorubicin for 3 weeks and then stained with Giemsa. C, p53-expressing cells persist but do not proliferate with two distinct morphologies in response to continuous treatment with a chemotherapeutic agent. Cells treated as in B were examined by phase-contrast microscopy. D, p53-expressing cells contain enhanced senescent-associated β-galactosidase activity in response to treatment with a chemotherapeutic agent. Clone 1 (expressing control shRNA) or clone 7 (expressing p53 shRNA) were either untreated or treated with 0.05 μg/mL doxorubicin for 7 days. Cells were then stained for senescence-associated β-galactosidase activity. Dana J. Lukin et al. Mol Cancer Res 2015;13:16-28 ©2015 by American Association for Cancer Research