Dual inhibition of mTOR and estrogen receptor signaling in vitro induces cell death in models of breast cancer.
OBJECTIVE
RAD001 (everolimus), a mammalian target of rapamycin (mTOR) pathway inhibitor in phase II clinical trials in oncology, exerts potent antiproliferative/antitumor activities. Many breast cancers are dependent for proliferation on estrogens synthesized from androgens (i.e., androstenedione) by aromatase. Letrozole (Femara) is an aromatase inhibitor used for treatment of postmenopausal women with hormone-dependent breast cancers. The role of the mTOR pathway in estrogen-driven proliferation and effects of combining RAD001 and letrozole were examined in vitro in two breast cancer models.


METHODS
The role of the mTOR pathway in estrogen response was evaluated in aromatase-expressing MCF7/Aro breast cancer cells by immunoblotting. Effects of RAD001 and letrozole (alone and in combination) on the proliferation and survival of MCF7/Aro and T47D/Aro cells were evaluated using proliferation assays, flow cytometry, immunoblotting, and apoptosis analyses.


RESULTS
Treatment of MCF7/Aro cells with estradiol or androstenedione caused modulation of the mTOR pathway, a phenomenon reversed by letrozole or RAD001. In MCF7/Aro and T47D/Aro cells, both agents inhibited androstenedione-induced proliferation; however, in combination, this was significantly augmented (P < 0.001, two-way ANOVA, synergy by isobologram analysis). Increased activity of the combination correlated with more profound effects on G1 progression and a significant decrease in cell viability (P < 0.01, two-way ANOVA) defined as apoptosis (P < 0.05, Friedman test). Increased cell death was particularly evident with optimal drug concentrations.


CONCLUSIONS
mTOR signaling is required for estrogen-induced breast tumor cell proliferation. Moreover, RAD001-letrozole combinations can act in a synergistic manner to inhibit proliferation and trigger apoptotic cell death. This combination holds promise for the treatment of hormone-dependent breast cancers.
Boulay
Anne
A
Novartis Institutes for BioMedical Research Basel, Oncology Research, Novartis Pharma AG, Basel, Switzerland.
Rudloff
Joelle
J
Ye
Jingjing
J
Zumstein-Mecker
Sabine
S
O'Reilly
Terence
T
Evans
Dean B
DB
Chen
Shiuan
S
Lane
Heidi A
HA
D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D001943:Breast Neoplasms / Q000473:pathology*; D002277:Carcinoma / Q000473:pathology*; D049109:Cell Proliferation; D002470:Cell Survival; D004347:Drug Interactions; D004967:Estrogens / Q000502:physiology; D000068338:Everolimus; D005260:Female; D006801:Humans; D007166:Immunosuppressive Agents / Q000494:pharmacology*; D000077289:Letrozole; D009570:Nitriles / Q000494:pharmacology*; D011494:Protein Kinases / Q000502:physiology*; D011960:Receptors, Estrogen / Q000502:physiology*; D015398:Signal Transduction; D020123:Sirolimus / Q000031:analogs & derivatives* / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D014230:Triazoles / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
D000970:Antineoplastic Agents; D004967:Estrogens; D007166:Immunosuppressive Agents; D009570:Nitriles; D011960:Receptors, Estrogen; D014230:Triazoles; D000077289:Letrozole; D000068338:Everolimus; D011494:Protein Kinases; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases; D020123:Sirolimus