title: Elevated expression of the estrogen receptor prevents the down-regulation of p21Waf1/Cip1 in hormone dependent breast cancer cells.
abstract: Expression of an estrogen receptor alpha (ER) transgene in hormone independent breast cancer and normal breast epithelial cells arrests cell cycling when estradiol is added. Although endogenously expressed ER does not typically affect estradiol-induced cell cycling of hormone dependent breast cancer cells, we observed that elevated expression of a green fluorescent protein fused to ER (GFP-ER) hindered entry of estrogen treated MCF-7 cells into S phase of the cell cycle. In analyses of key cell-cycle regulating proteins, we observed that GFP-ER expression had no affect on the protein levels of cyclin D1, cyclin E, or p27, a cyclin dependent kinase (Cdk) inhibitor. However, at 24 h, p21 (Waf1, Cip1; a Cdk2 inhibitor) protein remained elevated in the high GFP-ER expressing cells but not in non-GFP-ER expressing cells. Elevated expression of p21 inhibited Cdk2 activity, preventing cells from entering S phase. The results show that elevated levels of ER prevented the down-regulation of p21 protein expression, which is required for hormone responsive cells to enter S phase.
authors.affiliation: Manitoba Institute of Cell Biology, 675 McDermot Avenue, Winnipeg, Manitoba, Canada R3E 0V9.
authors.forename: Helen
authors.initials: H
authors.lastname: Zhao
authors.forename: Jenny
authors.initials: J
authors.lastname: Yu
authors.forename: Cheryl P
authors.initials: CP
authors.lastname: Peltier
authors.forename: James R
authors.initials: JR
authors.lastname: Davie
mesh_terms: D001943:Breast Neoplasms / Q000378:metabolism*; D042846:CDC2-CDC28 Kinases / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000378:metabolism*; D019938:Cyclin D1 / Q000378:metabolism; D019927:Cyclin E / Q000378:metabolism; D051357:Cyclin-Dependent Kinase 2; D050759:Cyclin-Dependent Kinase Inhibitor p21; D050760:Cyclin-Dependent Kinase Inhibitor p27; D015536:Down-Regulation / Q000502:physiology*; D047628:Estrogen Receptor alpha / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006801:Humans; D016196:S Phase / Q000502:physiology*; D014407:Tumor Cells, Cultured; D025521:Tumor Suppressor Proteins / Q000378:metabolism
chemical_list: C493956:CDKN1A protein, human; D018797:Cell Cycle Proteins; D019927:Cyclin E; D050759:Cyclin-Dependent Kinase Inhibitor p21; D047628:Estrogen Receptor alpha; D025521:Tumor Suppressor Proteins; D019938:Cyclin D1; D049452:Green Fluorescent Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2