SUMO-deficient progesterone receptors promote increased cell proliferation and decreased apoptosis. (A) The proliferative potential of T47D cell lines expressing inducible PR was measured using MTT assays in the presence of progestin (R5020) and inducer, AP21967 (AP) (B) Western blot showing that inducible PR expression is sustained for at least five days following the addition of AP21967 to the cell culture media, ERK1/2 western blotting was performed as a loading control. (C) Apoptosis occurring in cells expressing inducible PRs was detected by western blotting for poly (ADP)-ribose polymerase 1 (PARP) cleavage. Cells were treated with progestin and/or doxorubicin before protein harvest. (D) Proliferation and apoptosis was measured in cells constitutively expressing PR using cell viability luciferase assays, where day 4 luminescence was normalized to day 0. Pooled data are represented as mean of n = 6 +/- SD and significance calculated using Student's t-test. MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; n, number; PR, progesterone receptor; SD, standard deviation; SUMO, small ubiquitin-like modifier.