title: The estrogen receptor influences microtubule-associated protein tau (MAPT) expression and the selective estrogen receptor inhibitor fulvestrant downregulates MAPT and increases the sensitivity to taxane in breast cancer cells.
abstract: BACKGROUND
  Microtubule-associated protein tau (MAPT) inhibits the function of taxanes and high expression of MAPT decreases the sensitivity to taxanes. The relationship between estrogen receptor (ER) and MAPT in breast cancer is unclear. In this study, we examined the correlation of MAPT expression with the sensitivity of human breast cancer cells to taxanes, and the relationship between ER and MAPT.
  
  
  METHODS
  The correlation between MAPT expression and sensitivity to taxanes was investigated in 12 human breast cancer cell lines. Alterations in cellular sensitivity to taxanes were evaluated after knockdown of MAPT expression. ER expression was knocked down or stimulated in MAPT- and ER-positive cell lines to examine the relationship between ER and MAPT. The cells were also treated with hormone drugs (tamoxifen and fulvestrant) and taxanes.
  
  
  RESULTS
  mRNA expression of MAPT did not correlate with sensitivity to taxanes. However, expression of MAPT protein isoforms of less than 70 kDa was correlated with a low sensitivity to taxanes. Downregulation of MAPT increased cellular sensitivity to taxanes. MAPT protein expression was increased by stimulation with 17-beta-estradiol or tamoxifen, but decreased by ER downregulation and by fulvestrant, an ER inhibitor. The combination of fulvestrant with taxanes had a synergistic effect, whereas tamoxifen and taxanes had an antagonistic effect.
  
  
  CONCLUSIONS
  Expression of MAPT protein isoforms of less than 70 kDa is correlated with a low sensitivity to taxanes in breast cancer cells. ER influences MAPT expression and fulvestrant increases the sensitivity to taxanes in MAPT- and ER-positive breast cancer cells.
authors.affiliation: Department of Cancer and Thoracic Surgery, Okayama University Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences, 2-5-1 Shikata-cho, Okayama-city, Okayama, Japan. yashima_hirokuni@msn.com
authors.forename: Hirokuni
authors.initials: H
authors.lastname: Ikeda
authors.forename: Naruto
authors.initials: N
authors.lastname: Taira
authors.forename: Fumikata
authors.initials: F
authors.lastname: Hara
authors.forename: Takeo
authors.initials: T
authors.lastname: Fujita
authors.forename: Hiromasa
authors.initials: H
authors.lastname: Yamamoto
authors.forename: Junichi
authors.initials: J
authors.lastname: Soh
authors.forename: Shinichi
authors.initials: S
authors.lastname: Toyooka
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Nogami
authors.forename: Tadahiko
authors.initials: T
authors.lastname: Shien
authors.forename: Hiroyoshi
authors.initials: H
authors.lastname: Doihara
authors.forename: Shinichiro
authors.initials: S
authors.lastname: Miyoshi
mesh_terms: D015153:Blotting, Western; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000235:genetics* / Q000473:pathology; D045744:Cell Line, Tumor; D015536:Down-Regulation; D004357:Drug Synergism; D004958:Estradiol / Q000031:analogs & derivatives* / Q000494:pharmacology; D004965:Estrogen Antagonists / Q000494:pharmacology*; D020847:Estrogen Receptor Modulators / Q000494:pharmacology; D005260:Female; D005455:Fluorescent Antibody Technique; D000077267:Fulvestrant; D006801:Humans; D020033:Protein Isoforms; D012333:RNA, Messenger / Q000235:genetics; D034741:RNA, Small Interfering / Q000494:pharmacology; D011960:Receptors, Estrogen / Q000037:antagonists & inhibitors / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013629:Tamoxifen / Q000494:pharmacology; D043823:Taxoids / Q000494:pharmacology*; D016875:tau Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*
chemical_list: D004965:Estrogen Antagonists; D020847:Estrogen Receptor Modulators; C054369:MAPT protein, human; D020033:Protein Isoforms; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D011960:Receptors, Estrogen; D043823:Taxoids; D016875:tau Proteins; D013629:Tamoxifen; D000077267:Fulvestrant; D004958:Estradiol