title: Cyclopentenyl cytosine induces senescence in breast cancer cells through the nucleolar stress response and activation of p53.
abstract: The induction of senescence has emerged as a potentially important contributor to the effects of chemotherapeutic agents against tumors. We have demonstrated that depletion of CTP induced by cyclopentenyl cytosine (CPEC; NSC 375575), a specific inhibitor of the enzyme CTP synthetase, induces irreversible growth arrest and senescence characterized by altered morphology and expression of senescence-associated β-galactosidase activity in MCF-7 breast cancer cells expressing wild-type p53. In contrast, differentiation in the absence of senescence resulted from CPEC treatment in MDA-MB-231 breast cancer cells that express a mutated p53. Both senescence of MCF-7 cells and differentiation of MDA-MB-231 cells were prevented by repletion of CTP through the cytidine salvage pathway. Senescence in MCF-7 cells was associated with a G(2)- and S-phase arrest, whereas differentiation in MDA-MB-231 cells was associated with arrest in G(1) phase at 5 days. Mechanistic studies revealed that CTP depletion induced a rapid translocation of nucleolar proteins, including nucleostemin and nucleolin into the nucleoplasm. This nucleolar stress response resulted in a sustained elevation of p53 and the p53 target genes, p21 and Mdm2, in cells with wild-type p53. Furthermore, short interfering RNA-induced knockdown of p53 in MCF-7 cells treated with CPEC prevented cellular senescence and increased apoptotic cell death. We conclude that CTP depletion and the resulting nucleolar stress response results in a senescence-like growth arrest through activation of p53, whereas cells with mutated p53 undergo differentiation or apoptotic cell death.
authors.affiliation: Department of Medicine, Divisions of Oncology and Hematology, and the Stanford Cancer Center, Stanford University School of Medicine, Stanford, California 94305-5458, USA.
authors.forename: Min
authors.initials: M
authors.lastname: Huang
authors.forename: Patrick
authors.initials: P
authors.lastname: Whang
authors.forename: Patrick
authors.initials: P
authors.lastname: Lewicki
authors.forename: Beverly S
authors.initials: BS
authors.lastname: Mitchell
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology; D001943:Breast Neoplasms / Q000378:metabolism / Q000473:pathology*; D045744:Cell Line, Tumor; D002466:Cell Nucleolus / Q000502:physiology*; D016922:Cellular Senescence / Q000187:drug effects*; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000378:metabolism; D003562:Cytidine / Q000031:analogs & derivatives* / Q000494:pharmacology; D003570:Cytidine Triphosphate / Q000378:metabolism; D005260:Female; D020868:Gene Silencing; D016158:Genes, p53; D006657:Histones / Q000378:metabolism; D006801:Humans; D007150:Immunohistochemistry; D034741:RNA, Small Interfering
chemical_list: D000970:Antineoplastic Agents; C493956:CDKN1A protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; C468783:H2AX protein, human; D006657:Histones; D034741:RNA, Small Interfering; D003562:Cytidine; D003570:Cytidine Triphosphate; C045911:cyclopentenyl cytosine