Mifepristone induces growth arrest, caspase activation, and apoptosis of estrogen receptor-expressing, antiestrogen-resistant breast cancer cells.
OBJECTIVE
A major clinical problem in the treatment of breast cancer is the inherent and acquired resistance to antiestrogen therapy. In this study, we sought to determine whether antiprogestin treatment, used as a monotherapy or in combination with antiestrogen therapy, induced growth arrest and active cell death in antiestrogen-resistant breast cancer cells.


METHODS
MCF-7 sublines were established from independent clonal isolations performed in the absence of drug selection and tested for their response to the antiestrogens 4-hydroxytamoxifen (4-OHT) and ICI 182,780 (fulvestrant), and the antiprogestin mifepristone (MIF). The cytostatic (growth arrest) effects of the hormones were assessed with proliferation assays, cell counting, flow cytometry, and a determination of the phosphorylation status of the retinoblastoma protein. The cytotoxic (apoptotic) effects were analyzed by assessing increases in caspase activity and cleavage of poly(ADP-ribose) polymerase.


RESULTS
All of the clonally derived MCF-7 sublines expressed estrogen receptor and progesterone receptor but showed a wide range of antiestrogen sensitivity, including resistance to physiological levels of 4-OHT. Importantly, all of the clones were sensitive to the antiprogestin MIF, whether used as a monotherapy or in combination with 4-OHT. MIF induced retinoblastoma activation, G(1) arrest, and apoptosis preceded by caspase activation.


CONCLUSIONS
We demonstrate that: (a) estrogen receptor(+)progesterone receptor(+), 4-OHT-resistant clonal variants can be isolated from an MCF-7 cell line in the absence of antiestrogen selection; and (b) MIF and MIF plus 4-OHT combination therapy induces growth arrest and active cell death of the antiestrogen-resistant breast cancer cells. These preclinical findings show potential for a combined hormonal regimen of an antiestrogen and an antiprogestin to combat the emergence of antiestrogen-resistant breast cancer cells and, ultimately, improve the therapeutic index of antiestrogen therapy.
Department of Cellular Biology and Anatomy, Medical College of Georgia, Augusta, Georgia 30912, USA.
Virgil T
VT
Gaddy
John T
JT
Barrett
Jennifer N
JN
Delk
Andre M
AM
Kallab
Alan G
AG
Porter
Patricia V
PV
Schoenlein
D017209:Apoptosis; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000473:pathology*; D020169:Caspases / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D004396:Coloring Agents / Q000494:pharmacology; D053938:DNA Fragmentation; D019008:Drug Resistance, Neoplasm; D004789:Enzyme Activation; D004958:Estradiol / Q000031:analogs & derivatives* / Q000494:pharmacology; D020847:Estrogen Receptor Modulators / Q000378:metabolism / Q000494:pharmacology*; D000077267:Fulvestrant; D016193:G1 Phase; D006727:Hormone Antagonists / Q000494:pharmacology; D006801:Humans; D015151:Immunoblotting; D015735:Mifepristone / Q000494:pharmacology*; D010766:Phosphorylation; D011960:Receptors, Estrogen / Q000378:metabolism; D011980:Receptors, Progesterone / Q000378:metabolism; D016192:Resting Phase, Cell Cycle; D013629:Tamoxifen / Q000031:analogs & derivatives* / Q000494:pharmacology; D013778:Tetrazolium Salts / Q000494:pharmacology; D013844:Thiazoles / Q000494:pharmacology; D013997:Time Factors
D004396:Coloring Agents; D020847:Estrogen Receptor Modulators; D006727:Hormone Antagonists; D011960:Receptors, Estrogen; D011980:Receptors, Progesterone; D013778:Tetrazolium Salts; D013844:Thiazoles; D013629:Tamoxifen; C016601:afimoxifene; D000077267:Fulvestrant; D015735:Mifepristone; D004958:Estradiol; D020169:Caspases; C022616:thiazolyl blue