title: mTOR inhibitors counteract tamoxifen-induced activation of breast cancer stem cells.
abstract: Breast cancer cells with stem cell characteristics (CSC) are a distinct cell population with phenotypic similarities to mammary stem cells. CSCs are important drivers of tumorigenesis and the metastatic process. Tamoxifen is the most widely used hormonal therapy for estrogen receptor (ER) positive cancers. In our study, tamoxifen was effective in reducing proliferation of ER + adherent cancer cells, but not their CSC population. We isolated, expanded and incubated CSC from seven breast cancers with or without tamoxifen. By genome-wide transcriptional analysis we identified tamoxifen-induced transcriptional pathways associated with ribosomal biogenesis and mRNA translation, both regulated by the mTOR-pathway. We observed induction of the key mTOR downstream targets S6K1, S6RP and 4E-BP1 in-patient derived CSCs by tamoxifen on protein level. Using the mTOR inhibitors rapamycin, everolimus and PF-04691502 (a dual PI3K/mTOR inhibitor) and in combination with tamoxifen, significant reduction in mammosphere formation was observed. Hence, we suggest that the CSC population play a significant role during endocrine resistance through activity of the mTOR pathway. In addition, tamoxifen further stimulates the mTOR-pathway but can be antagonized using mTOR-inhibitors.
authors.affiliation: Department of Oncology and Pathology, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Govindasamy-Muralidharan
authors.initials: GM
authors.lastname: Karthik
authors.affiliation: Department of Oncology and Pathology, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Ran
authors.initials: R
authors.lastname: Ma
authors.affiliation: Department of Oncology and Pathology, Karolinska Institutet, Stockholm, Sweden.
authors.forename: John
authors.initials: J
authors.lastname: Lövrot
authors.affiliation: Department of Oncology and Pathology, Karolinska Institutet, Stockholm, Sweden; Department of Clinical Pathology/Cytology, Karolinska University Hospital, 17177 Stockholm, Sweden.
authors.forename: Lorand Levente
authors.initials: LL
authors.lastname: Kis
authors.affiliation: Department of Oncology and Pathology, Karolinska Institutet, Stockholm, Sweden; Department of Clinical Pathology/Cytology, Karolinska University Hospital, 17177 Stockholm, Sweden.
authors.forename: Claes
authors.initials: C
authors.lastname: Lindh
authors.affiliation: Advita Plastic Surgery, St Görans Hospital, Stockholm, Sweden.
authors.forename: Lennart
authors.initials: L
authors.lastname: Blomquist
authors.affiliation: Department of Breast and Endocrine Surgery, Karolinska Institutet, Stockholm, Sweden; Department of Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Irma
authors.initials: I
authors.lastname: Fredriksson
authors.affiliation: Department of Oncology and Pathology, Karolinska Institutet, Stockholm, Sweden; Radiumhemmet - Karolinska Oncology, Karolinska University Hospital, 17176 Stockholm, Sweden.
authors.forename: Jonas
authors.initials: J
authors.lastname: Bergh
authors.affiliation: Department of Oncology and Pathology, Karolinska Institutet, Stockholm, Sweden; Department of Clinical Pathology/Cytology, Karolinska University Hospital, 17177 Stockholm, Sweden. Electronic address: Johan.Hartman@ki.se.
authors.forename: Johan
authors.initials: J
authors.lastname: Hartman
mesh_terms: D018931:Antineoplastic Agents, Hormonal / Q000633:toxicity*; D001943:Breast Neoplasms / Q000201:enzymology* / Q000235:genetics / Q000473:pathology; D002469:Cell Separation / Q000379:methods; D002470:Cell Survival / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D019008:Drug Resistance, Neoplasm / Q000187:drug effects; D004789:Enzyme Activation; D004965:Estrogen Antagonists / Q000633:toxicity*; D000068338:Everolimus; D005260:Female; D020869:Gene Expression Profiling / Q000379:methods; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D061986:MCF-7 Cells; D014411:Neoplastic Stem Cells / Q000187:drug effects* / Q000201:enzymology / Q000473:pathology; D047428:Protein Kinase Inhibitors / Q000494:pharmacology*; D011728:Pyridones / Q000494:pharmacology; D011743:Pyrimidines / Q000494:pharmacology; D012333:RNA, Messenger / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D020123:Sirolimus / Q000031:analogs & derivatives / Q000494:pharmacology; D018874:Spheroids, Cellular; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D013629:Tamoxifen / Q000633:toxicity*; D014158:Transcription, Genetic; D015533:Transcriptional Activation; D014407:Tumor Cells, Cultured
chemical_list: C570662:2-amino-8-(4-(2-hydroxyethoxy)cyclohexyl)-6-(6-methoxypyridin-3-yl)-4-methylpyrido(2,3-d)pyrimidin-7(8H)-one; D018931:Antineoplastic Agents, Hormonal; D004965:Estrogen Antagonists; D047428:Protein Kinase Inhibitors; D011728:Pyridones; D011743:Pyrimidines; D012333:RNA, Messenger; D013629:Tamoxifen; D000068338:Everolimus; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases; D020123:Sirolimus
keywords: Breast cancer stem cells; Tamoxifen resistance; mTOR pathway activation