title: p53-independent dephosphorylation and cleavage of retinoblastoma protein during tamoxifen-induced apoptosis in human breast carcinoma cells.
abstract: We have investigated several molecular events that occur during the process of tamoxifen-induced apoptosis in human breast carcinoma cells. We show that the treatment of either MCF-7 (containing wild-type p53) or MDA-MB-231 cells (containing mutant p53) with tamoxifen resulted in apoptotic nuclear changes and an increase in the pre-G1 apoptotic population. This was accompanied by activation of the caspase enzymes, as evidenced by specific cleavage of poly(ADP-ribose) polymerase and retinoblastoma (RB) protein. The RB protein was cleaved at both an interior and carboxyl terminus cleavage site. In addition, dephosphorylation of RB was found at an early stage of tamoxifen-induced apoptosis in both cell lines. However, neither induction of p53 in MCF-7 cells nor induction of p21 in either cell line was detected, suggesting that tamoxifen-induced RB dephosphorylation and apoptosis are independent of the p53/p21 pathway. We also observed an increase in levels of the pro-apoptotic Bax protein, the inhibitory cytokine TGF-beta1 and the transcription factor c-Myc in tamoxifen-treated MDA-MB-231 cells, suggesting the possible involvement of these proteins during apoptosis in this system.
authors.affiliation: Department of Pharmacology, University of Pittsburgh School of Medicine and University of Pittsburgh Cancer Institute, PA 15213-2582, USA.
authors.forename: C L
authors.initials: CL
authors.lastname: Fattman
authors.forename: B
authors.initials: B
authors.lastname: An
authors.forename: L
authors.initials: L
authors.lastname: Sussman
authors.forename: Q P
authors.initials: QP
authors.lastname: Dou
mesh_terms: D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology*; D017209:Apoptosis / Q000235:genetics; D001943:Breast Neoplasms / Q000378:metabolism*; D053938:DNA Fragmentation; D004273:DNA, Neoplasm / Q000187:drug effects; D005260:Female; D006801:Humans; D009363:Neoplasm Proteins / Q000378:metabolism*; D010766:Phosphorylation; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism; D016160:Retinoblastoma Protein / Q000378:metabolism*; D013629:Tamoxifen / Q000494:pharmacology*; D013997:Time Factors; D016212:Transforming Growth Factor beta / Q000378:metabolism; D014407:Tumor Cells, Cultured / Q000187:drug effects; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism; D051028:bcl-2-Associated X Protein
chemical_list: D018931:Antineoplastic Agents, Hormonal; C494826:BAX protein, human; D004273:DNA, Neoplasm; D009363:Neoplasm Proteins; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D016271:Proto-Oncogene Proteins c-myc; D016160:Retinoblastoma Protein; D016212:Transforming Growth Factor beta; D016159:Tumor Suppressor Protein p53; D051028:bcl-2-Associated X Protein; D013629:Tamoxifen; D011065:Poly(ADP-ribose) Polymerases