title: c-Myc suppresses p21WAF1/CIP1 expression during estrogen signaling and antiestrogen resistance in human breast cancer cells.
abstract: Estrogen rapidly induces expression of the proto-oncogene c-myc. c-Myc is required for estrogen-stimulated proliferation of breast cancer cells, and deregulated c-Myc expression has been implicated in antiestrogen resistance. In this report, we investigate the mechanism(s) by which c-Myc mediates estrogen-stimulated proliferation and contributes to cell cycle progression in the presence of antiestrogen. The MCF-7 cell line is a model of estrogen-dependent, antiestrogen-sensitive human breast cancer. Using stable MCF-7 derivatives with inducible c-Myc expression, we demonstrated that in antiestrogen-treated cells, the elevated mRNA and protein levels of p21(WAF1/CIP1), a cell cycle inhibitor, decreased upon either c-Myc induction or estrogen treatment. Expression of p21 blocked c-Myc-mediated cell cycle progression in the presence of antiestrogen, suggesting that the decrease in p21 is necessary for this process. Using RNA interference to suppress c-Myc expression, we further established that c-Myc is required for estrogen-mediated decreases in p21(WAF1/CIP1). Finally, we observed that neither c-Myc nor p21(WAF1/CIP1) is regulated by estrogen or antiestrogen in an antiestrogen-resistant MCF-7 derivative. The p21 levels in the antiestrogen-resistant cells increased when c-Myc expression was suppressed, suggesting that loss of p21 regulation was a consequence of constitutive c-Myc expression. Together, these studies implicate p21(WAF1/CIP1) as an important target of c-Myc in breast cancer cells and provide a link between estrogen, c-Myc, and the cell cycle machinery. They further suggest that aberrant c-Myc expression, which is frequently observed in human breast cancers, can contribute to antiestrogen resistance by altering p21(WAF1/CIP1) regulation.
authors.affiliation: Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, Michigan 48824, USA.
authors.forename: Shibani
authors.initials: S
authors.lastname: Mukherjee
authors.forename: Susan E
authors.initials: SE
authors.lastname: Conrad
mesh_terms: D001943:Breast Neoplasms / Q000235:genetics / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000037:antagonists & inhibitors* / Q000096:biosynthesis / Q000235:genetics; D045744:Cell Line, Tumor; D050759:Cyclin-Dependent Kinase Inhibitor p21; D015536:Down-Regulation / Q000502:physiology*; D020847:Estrogen Receptor Modulators / Q000378:metabolism / Q000494:pharmacology*; D004967:Estrogens / Q000235:genetics / Q000502:physiology*; D015972:Gene Expression Regulation, Neoplastic / Q000502:physiology*; D006801:Humans; D000090063:Proto-Oncogene Mas; D016271:Proto-Oncogene Proteins c-myc / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D034622:RNA Interference; D015398:Signal Transduction / Q000235:genetics
chemical_list: C493956:CDKN1A protein, human; D018797:Cell Cycle Proteins; D050759:Cyclin-Dependent Kinase Inhibitor p21; D020847:Estrogen Receptor Modulators; D004967:Estrogens; C000715470:MAS1 protein, human; D000090063:Proto-Oncogene Mas; D016271:Proto-Oncogene Proteins c-myc