title: Deregulation of the cell cycle by breast tumor kinase (Brk).
abstract: Brk is a cytoplasmic nonreceptor tyrosine kinase that is overexpressed in breast tumors but undetectable in normal or benign mammary tissues. Brk promotes proliferation of human mammary epithelial cells and tumor growth in a mouse model, but the role of Brk in cell cycle regulation is not known. In this study, we describe the mechanism of Brk-induced deregulation of the cell cycle. We provide evidence that Brk antagonizes the transcriptional activity of the transcription factor FoxO family of proteins by inhibiting its nuclear localization. As a result, the cell cycle inhibitor p27, a FoxO target gene, is down-regulated. This event is accompanied by G1/S cell cycle progression of quiescent cells. As p27 is a key regulator of the G1/S cell cycle checkpoint, these data suggest that perturbation of p27 expression induced by Brk causes S phase entrance. Deregulation of the cell cycle is a key event in neoplasia, and thus, the mechanism presented here likely contributes to breast cancer development.
authors.affiliation: Department of Pediatric Hematology/Oncology, State University of New York at Stony Brook, Stony Brook, New York 11794, USA.
authors.forename: Edward
authors.initials: E
authors.lastname: Chan
authors.forename: Anjaruwee S
authors.initials: AS
authors.lastname: Nimnual
mesh_terms: D001943:Breast Neoplasms / Q000201:enzymology* / Q000235:genetics / Q000473:pathology*; D002453:Cell Cycle / Q000502:physiology*; D048708:Cell Growth Processes / Q000502:physiology; D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000378:metabolism; D050760:Cyclin-Dependent Kinase Inhibitor p27 / Q000096:biosynthesis / Q000378:metabolism; D015536:Down-Regulation; D005260:Female; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D016193:G1 Phase / Q000502:physiology; D006801:Humans; D009363:Neoplasm Proteins / Q000235:genetics / Q000378:metabolism*; D011505:Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000008:administration & dosage / Q000235:genetics; D016196:S Phase / Q000502:physiology; D014162:Transfection
chemical_list: C499517:FOXO3 protein, human; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; D009363:Neoplasm Proteins; D034741:RNA, Small Interfering; D050760:Cyclin-Dependent Kinase Inhibitor p27; D011505:Protein-Tyrosine Kinases; C088375:PTK6 protein, human