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Estrogens induce breast tumor cell proliferation by directly regulating gene expression via the estrogen receptor (ER) transcriptional activity and by affecting growth factor signaling pathways such as mitogen-activated protein kinase (MAPK) and AKT/mammalian target of rapamycin Complex1 (mTORC1) cascades. In this study we demonstrated the preclinical therapeutic efficacy of combining the aromatase inhibitor letrozole with the multi-kinase inhibitor sorafenib in aromatase-expressing breast cancer cell lines. Treatment with letrozole reduced testosterone-driven cell proliferation, by inhibiting the synthesis of estrogens. Sorafenib inhibited cell proliferation in a concentration-dependent manner; this effect was not dependent on sorafenib-mediated inhibition of Raf1, but involved the down-regulation of mTORC1 and its targets p70S6K and 4E-binding protein 1 (4E-BP1). At concentrations of 5-10 μM the growth-inhibitory effect of sorafenib was associated with the induction of apoptosis, as indicated by release of cytochrome c and Apoptosis-Inducing Factor into the cytosol, activation of caspase-9 and caspase-7, and PARP-1 cleavage. Combination of letrozole and sorafenib produced a synergistic inhibition of cell proliferation associated with an enhanced accumulation of cells in the G(0)/G(1) phase of the cell cycle and with a down-regulation of the cell cycle regulatory proteins c-myc, cyclin D1, and phospho-Rb. In addition, longer experiments (12 weeks) demonstrated that sorafenib may be effective in preventing the acquisition of resistance towards letrozole. Together, these results indicate that combination of letrozole and sorafenib might constitute a promising approach to the treatment of hormone-dependent breast cancer.

Original publication




Journal article


Breast Cancer Res Treat

Publication Date





79 - 88


Adaptor Proteins, Signal Transducing, Antineoplastic Combined Chemotherapy Protocols, Apoptosis, Apoptosis Inducing Factor, Aromatase, Aromatase Inhibitors, Benzenesulfonates, Breast Neoplasms, Caspase 7, Caspase 9, Cell Cycle, Cell Line, Tumor, Cell Proliferation, Cyclin D1, Cytochromes c, Dose-Response Relationship, Drug, Drug Resistance, Neoplasm, Drug Synergism, Estradiol, Female, Humans, Letrozole, Mechanistic Target of Rapamycin Complex 1, Multiprotein Complexes, Niacinamide, Nitriles, Phenylurea Compounds, Phosphoproteins, Phosphorylation, Poly (ADP-Ribose) Polymerase-1, Poly(ADP-ribose) Polymerases, Protein Kinase Inhibitors, Proteins, Proto-Oncogene Proteins c-myc, Pyridines, Retinoblastoma Protein, Ribosomal Protein S6 Kinases, 70-kDa, Sorafenib, TOR Serine-Threonine Kinases, Testosterone, Time Factors, Transfection, Triazoles