title: Activation of mitogen-activated protein kinase in xenografts and cells during prolonged treatment with aromatase inhibitor letrozole.
abstract: Ovariectomized mice bearing tumor xenografts grown from aromatase-transfected estrogen receptor (ER)-positive human breast cancer cells (MCF-7Ca) were injected s.c. with 10 microg/d letrozole for up to 56 weeks. Western blot analysis of the tumors revealed that ERs (ERalpha) were increased at 4 weeks but decreased at weeks 28 and 56. Expression of erbB-2 and p-Shc increased throughout treatment, whereas growth factor receptor binding protein 2 (Grb2) increased only in tumors proliferating on letrozole (weeks 28 and 56). In cells isolated from tumors after 56 weeks and maintained as a cell line (LTLT-Ca) in 1 micromol/L letrozole, ERalpha was also decreased whereas erbB-2, adapter proteins (p-Shc and Grb2), and the signaling proteins in the mitogen-activated protein kinase (MAPK) cascade were increased compared with MCF-7Ca cells. Growth was inhibited in LTLT-Ca cells but not in MCF-7Ca cells treated with MAPK kinase 1/2 inhibitors U0126, and PD98059 (IC(50) approximately 25 micromol/L). PD98059 (5 micromol/L) also reduced MAPK activity and increased ERalpha to the levels in MCF-7Ca cells. Epidermal growth factor receptor kinase inhibitor, gefitinib (ZD1839) inhibited growth of LTLT-Ca cells (IC(50) approximately 10 micromol/L) and restored their sensitivity to tamoxifen and anastrozole. In xenografts, combined treatment with ER down-regulator fulvestrant and letrozole, prevented increases in erbB-2 and activation of MAPK and was highly effective in inhibiting tumor growth throughout 29 weeks of treatment. These results indicate that blocking both ER- and growth factor-mediated transcription resulted in the most effective inhibition of growth of ER-positive breast cancer cells.
authors.affiliation: Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA.
authors.forename: Danijela
authors.initials: D
authors.lastname: Jelovac
authors.forename: Gauri
authors.initials: G
authors.lastname: Sabnis
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Long
authors.forename: Luciana
authors.initials: L
authors.lastname: Macedo
authors.forename: Olga G
authors.initials: OG
authors.lastname: Goloubeva
authors.forename: Angela M H
authors.initials: AM
authors.lastname: Brodie
mesh_terms: D000818:Animals; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000494:pharmacology; D047072:Aromatase Inhibitors / Q000008:administration & dosage / Q000494:pharmacology*; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000201:enzymology*; D045744:Cell Line, Tumor; D004334:Drug Administration Schedule; D004789:Enzyme Activation / Q000187:drug effects; D047628:Estrogen Receptor alpha / Q000378:metabolism; D005260:Female; D006801:Humans; D000077289:Letrozole; D020935:MAP Kinase Signaling System / Q000187:drug effects; D051379:Mice; D008807:Mice, Inbred BALB C; D008819:Mice, Nude; D020928:Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D009570:Nitriles / Q000008:administration & dosage / Q000494:pharmacology*; D013629:Tamoxifen / Q000008:administration & dosage; D014230:Triazoles / Q000008:administration & dosage / Q000494:pharmacology*; D023041:Xenograft Model Antitumor Assays
chemical_list: D047072:Aromatase Inhibitors; D047628:Estrogen Receptor alpha; D009570:Nitriles; D014230:Triazoles; D013629:Tamoxifen; D000077289:Letrozole; D020928:Mitogen-Activated Protein Kinases