title: BRCA1-dependent Chk1 phosphorylation triggers partial chromatin disassociation of phosphorylated Chk1 and facilitates S-phase cell cycle arrest.
abstract: Chk1 phosphorylation by the PI3-like kinases ATR and ATM is critical for its activation and its role in prevention of premature mitotic entry in response to DNA damage or stalled replication. The breast and ovarian tumor suppressor, BRCA1, is among several checkpoint mediators that are required for Chk1 activation by ATM and ATR. Previously we showed that BRCA1 is necessary for Chk1 phosphorylation and activation following ionizing radiation. BRCA1 has been implicated in S-phase checkpoint control yet its mechanism of action is not well characterized. Here we report that BRCA1 is critical for Chk1 phosphorylation in response to inhibition of replication by either cisplatin or hydroxyurea. While Chk1 phosphorylation of S317 is fully dependent on BRCA1, additional proteins may mediate S345 phosphorylation at later time points. In addition, we show that a subset of phosphorylated Chk1 is released from the chromatin in a BRCA1-dependent manner which may lead to the phosphorylation of Chk1 substrate, Cdc25C, on S216 and to S-phase checkpoint activation. Inhibition of Chk1 kinase by UCN-01 or expression of Chk1 phosphorylation mutants in which the serine residues were substituted with alanine residues abrogates BRCA1-dependent cell cycle arrest in response replication inhibition. These data reveal that BRCA1 facilitates Chk1 phosphorylation and its partial chromatin dissociation following replication inhibition that is likely to be required for S-phase checkpoint signaling.
authors: Laboratory of Genomic Applications, Department of Surgical Oncology, Sheba Medical Center, Tel-Hashomer 52621, Israel. riy2@georgetown.edu
authors: Ronit I
authors: RI
authors: Yarden
authors: Sally
authors: S
authors: Metsuyanim
authors: Itay
authors: I
authors: Pickholtz
authors: Shabana
authors: S
authors: Shabbeer
authors: Hadass
authors: H
authors: Tellio
authors: Moshe Z
authors: MZ
authors: Papa
mesh_terms: D019313:BRCA1 Protein / Q000378:metabolism*; D059447:Cell Cycle Checkpoints / Q000187:drug effects / Q000528:radiation effects; D045744:Cell Line, Tumor; D000071877:Checkpoint Kinase 1; D002843:Chromatin / Q000187:drug effects / Q000378:metabolism*; D004249:DNA Damage; D004261:DNA Replication / Q000187:drug effects / Q000528:radiation effects; D004789:Enzyme Activation / Q000187:drug effects / Q000528:radiation effects; D006801:Humans; D006918:Hydroxyurea / Q000494:pharmacology; D050505:Mutant Proteins / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects / Q000528:radiation effects; D010768:Phosphoserine / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism*; D011839:Radiation, Ionizing; D016196:S Phase / Q000187:drug effects / Q000528:radiation effects; D015398:Signal Transduction / Q000187:drug effects / Q000528:radiation effects; D013312:Stress, Physiological / Q000187:drug effects / Q000528:radiation effects
chemical_list: D019313:BRCA1 Protein; D002843:Chromatin; D050505:Mutant Proteins; D010768:Phosphoserine; D011494:Protein Kinases; C000605867:CHEK1 protein, human; D000071877:Checkpoint Kinase 1; D006918:Hydroxyurea