Buthionine sulfoximine sensitizes antihormone-resistant human breast cancer cells to estrogen-induced apoptosis.
BACKGROUND
Estrogen deprivation using aromatase inhibitors is one of the standard treatments for postmenopausal women with estrogen receptor (ER)-positive breast cancer. However, one of the consequences of prolonged estrogen suppression is acquired drug resistance. Our group is interested in studying antihormone resistance and has previously reported the development of an estrogen deprived human breast cancer cell line, MCF-7:5C, which undergoes apoptosis in the presence of estradiol. In contrast, another estrogen deprived cell line, MCF-7:2A, appears to have elevated levels of glutathione (GSH) and is resistant to estradiol-induced apoptosis. In the present study, we evaluated whether buthionine sulfoximine (BSO), a potent inhibitor of glutathione (GSH) synthesis, is capable of sensitizing antihormone resistant MCF-7:2A cells to estradiol-induced apoptosis.


METHODS
Estrogen deprived MCF-7:2A cells were treated with 1 nM 17beta-estradiol (E2), 100 microM BSO, or 1 nM E2 + 100 microM BSO combination in vitro, and the effects of these agents on cell growth and apoptosis were evaluated by DNA quantitation assay and annexin V and terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) staining. The in vitro results of the MCF-7:2A cell line were further confirmed in vivo in a mouse xenograft model.


RESULTS
Exposure of MCF-7:2A cells to 1 nM E2 plus 100 microM BSO combination for 48 to 96 h produced a sevenfold increase in apoptosis whereas the individual treatments had no significant effect on growth. Induction of apoptosis by the combination treatment of E2 plus BSO was evidenced by changes in Bcl-2 and Bax expression. The combination treatment also markedly increased phosphorylated c-Jun N-terminal kinase (JNK) levels in MCF-7:2A cells and blockade of the JNK pathway attenuated the apoptotic effect of E2 plus BSO. Our in vitro findings corroborated in vivo data from a mouse xenograft model in which daily administration of BSO either as a single agent or in combination with E2 significantly reduced tumor growth of MCF-7:2A cells.


CONCLUSIONS
Our data indicates that GSH participates in retarding apoptosis in antihormone-resistant human breast cancer cells and that depletion of this molecule by BSO may be critical in predisposing resistant cells to E2-induced apoptotic cell death. We suggest that these data may form the basis of improving therapeutic strategies for the treatment of antihormone resistant ER-positive breast cancer.
Lewis-Wambi
Joan S
JS
Department of Medical Sciences, Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, PA 19111, USA.
Kim
Helen R
HR
Wambi
Chris
C
Patel
Roshani
R
Pyle
Jennifer R
JR
Klein-Szanto
Andres J
AJ
Jordan
V Craig
VC
D000818:Animals; D017304:Annexin A5 / Q000378:metabolism; D000964:Antimetabolites, Antineoplastic / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D015153:Blotting, Western; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology*; D019328:Buthionine Sulfoximine / Q000494:pharmacology*; D049109:Cell Proliferation / Q000187:drug effects; D045304:Cytochromes c / Q000378:metabolism; D019008:Drug Resistance, Neoplasm; D004357:Drug Synergism; D004958:Estradiol / Q000494:pharmacology; D004967:Estrogens / Q000172:deficiency / Q000494:pharmacology*; D005260:Female; D051858:Forkhead Transcription Factors / Q000502:physiology; D005978:Glutathione / Q000378:metabolism; D006801:Humans; D007124:Immunoenzyme Techniques; D020287:In Situ Nick-End Labeling; D048031:JNK Mitogen-Activated Protein Kinases / Q000378:metabolism; D053078:Membrane Potential, Mitochondrial / Q000187:drug effects; D051379:Mice; D008819:Mice, Nude; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering / Q000494:pharmacology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014407:Tumor Cells, Cultured; D051028:bcl-2-Associated X Protein / Q000378:metabolism
D017304:Annexin A5; D000964:Antimetabolites, Antineoplastic; D004967:Estrogens; D051858:Forkhead Transcription Factors; D019253:Proto-Oncogene Proteins c-bcl-2; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D051028:bcl-2-Associated X Protein; D004958:Estradiol; D019328:Buthionine Sulfoximine; D045304:Cytochromes c; D048031:JNK Mitogen-Activated Protein Kinases; D005978:Glutathione