title: Activation of the estrogen-signaling pathway by p21(WAF1/CIP1) in estrogen receptor-negative breast cancer cells.
abstract: BACKGROUND
  Estrogen stimulates the proliferation of cells in normal mammary glands and most estrogen receptor (ER)-positive mammary carcinomas by binding to the ER and promoting the transcription of ER-responsive genes. In cells with functional ERs, estrogen mediates the transition of cells from the G(1) to S phase of the cell cycle. Several cell cycle regulatory proteins have been implicated in the ER-signaling pathway involved in estrogen-mediated growth stimulation and antiestrogen-mediated growth arrest. We sought to determine whether p21, a cyclin-dependent kinase inhibitor, is a component of this pathway and, if so, whether it can mediate estrogen's action in ER-negative breast cancer cells.
  
  
  METHODS
  We overexpressed p21 with a tetracycline-inducible system in ER-negative, p21-negative breast cancer cells. Activity of the ER-signaling pathway was monitored in transient transfection assays by using constructs in which the ER promoter or the estrogen-response element (ERE) controls Luciferase expression. The growth-modulating effects of estradiol and antiestrogens on p21-overexpressing clones were assessed. All P: values are from two-sided tests.
  
  
  RESULTS
  A strong positive association was found between the expression of p21 and ER in nine breast cancer cell lines and in tumor samples from 60 patients with breast cancer (P:<. 001). Overexpression of p21 in a p21-negative, ER-negative cell line induced both the ER and ERE promoters in an estrogen-responsive manner. Last, stable p21 clones that also lack the expression of wild-type ER were responsive to the growth-inhibitory effects of ICI 182,780, a potent antiestrogen, and the growth-stimulatory effects of 17beta-estradiol.
  
  
  CONCLUSIONS
  The ability of p21 to mediate the activation of the estrogen-signaling pathway in ER-negative tumor cells suggests that p21 plays a novel role in this pathway, a finding that also has important clinical implications.
authors.affiliation: Division of Molecular Medicine, Wadsworth Center, Albany, NY, USA.
authors.forename: X
authors.initials: X
authors.lastname: Chen
authors.forename: C
authors.initials: C
authors.lastname: Danes
authors.forename: M
authors.initials: M
authors.lastname: Lowe
authors.forename: T W
authors.initials: TW
authors.lastname: Herliczek
authors.forename: K
authors.initials: K
authors.lastname: Keyomarsi
mesh_terms: D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology; D015153:Blotting, Western; D001943:Breast Neoplasms / Q000188:drug therapy / Q000378:metabolism*; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000187:drug effects / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000378:metabolism*; D004958:Estradiol / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D020847:Estrogen Receptor Modulators / Q000494:pharmacology; D004967:Estrogens / Q000378:metabolism*; D005434:Flow Cytometry; D000077267:Fulvestrant; D006801:Humans; D008156:Luciferases / Q000032:analysis; D011960:Receptors, Estrogen / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D014162:Transfection; D014407:Tumor Cells, Cultured; D015854:Up-Regulation / Q000187:drug effects
chemical_list: D018931:Antineoplastic Agents, Hormonal; C493956:CDKN1A protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D004791:Enzyme Inhibitors; D020847:Estrogen Receptor Modulators; D004967:Estrogens; D011960:Receptors, Estrogen; D000077267:Fulvestrant; D004958:Estradiol; D008156:Luciferases