Hiroko Fujimoto

Regulation of the antioncogenic Chk2 kinase by the oncogenic Wip1 phosphatase

The antioncogenic Chk2 kinase plays a crucial role in DNA damage-induced cell-cycle checkpoint regulation. Here we show that Chk2 associates with the oncogenic protein Wip1 (wild-type p53-inducible phosphatase 1) (PPM1D), a p53-inducible protein phosphatase. Phosphorylation of Chk2 at threonine68 (Thr68), a critical event for Chk2 activation, which is normally induced by DNA damage or overexpression of Chk2, is inhibited by expression of wild-type (WT), but not a phosphatase-deficient mutant (D314A) of Wip1 in cultured cells. Furthermore, an in vitro phosphatase assay revealed that Wip1 (WT), but not Wip1 (D314A), dephosphorylates Thr68 on phosphorylated Chk2 in vitro, resulting in the inhibition of Chk2 kinase activity toward glutathione S-transferase-Cdc25C. Moreover, inhibition of Wip1 expression by RNA interference results in abnormally sustained Thr68 phosphorylation of Chk2 and increased susceptibility of cells in response to DNA damage, indicating that Wip1 acts as a negative regulator of Chk2 in response to DNA damage.

Hypoxia-inducible factor-1alpha induces cell cycle arrest of endothelial cells.

Background: Hypoxia can induce tissue injury, including apoptosis of endothelial cells. However, little is known about the effects of hypoxia on endothelial cell function. We assessed the effects of hypoxia inducible factor (HIF)‐1α on the functional characteristics of endothelial cells, particularly on cell cycle regulators, by cationic liposome‐mediated transfection of HIF‐1α‐expression vector into the cells.

Membrane‐bound form of fractalkine induces IFN‐γ production by NK cells

Natural killer (NK) cells participate in both innate and adaptive immunity, in part by their prompt secretion of cytokines including IFN‐γ, a pro‐inflammatory cytokine with an important role in Th1 polarization. To assess the involvement of fractalkine in inflammatory processes, we examined the effect of fractalkine on IFN‐γ production by NK cells. Although soluble chemokines, including MCP‐1 and RANTES as well as fractalkine, had a negligible effect on IFN‐γ production, immobilized fractalkine markedly induced IFN‐γ production by NK cells in a dose‐dependent manner. Pretreatment of NK cells with the phosphatidylinositol 3‐kinase (PI 3‐K) inhibitor, wortmannin, completely inhibited the production of IFN‐γ induced by fractalkine, and pretreatment with the protein tyrosine kinase inhibitor, herbimycin A, partially suppressed the response, suggesting that augmentation of IFN‐γ production in response to fractalkine treatment of NK cells involves signaling through PI 3‐K and protein tyrosine kinases. Furthermore, co‐culture of NK cells with fractalkine‐transfected 293E cells markedly enhanced IFN‐γ production by NK cells compared with co‐culture with control 293E cells. These findings may indicate a paracrine feedback loop system in which endothelial cells may be activated to produce more fractalkine, and also suggest a role for fractalkine expressed on endothelial cells in Th1 polarization through the stimulation of IFN‐γ production by NK cells.

Intrinsic kinase activity and SQ/TQ domain of Chk2 kinase as well as N-terminal domain of Wip1 phosphatase are required for regulation of Chk2 by Wip1

The anti-oncogenic Chk2 kinase plays a crucial role in DNA damage-induced cell cycle checkpoint regulation. Recently, we have shown that Chk2 associates with the oncogenic Wip1 (PPM1D) phosphatase and that Wip1 acts as a negative regulator of Chk2 during DNA damage response by dephosphorylating phosphorylated Thr-68 in activated Chk2 (Fujimoto, H., Onishi, N., Kato, N., Takekawa, M., Xu, X. Z., Kosugi, A., Kondo, T., Imamura, M., Oishi, I., Yoda, A., and Minami, Y. (2006) Cell Death Differ. 13, 1170-1180). Here, we performed structure-function analyses of Chk2 and Wip1 by using a series of deletion or amino acid-substituted mutant proteins of Chk2 and Wip1. We show that nuclear localization of both Chk2 and Wip1 is required for their association in cultured cells and that the serine-glutamine (SQ)/threonine-glutamine (TQ) domain of Chk2, containing Thr-68, and the N-terminal domain of Wip1, comprising about 100 amino acids, are necessary and sufficient for the association of both molecules. However, it was found that an intrinsic kinase activity of Chk2, but not phosphatase activity of Wip1, is required for the association of fulllength Chk2 and Wip1. Interestingly, we also show that the mutant Wip1 proteins, bearing the N-terminal domain of Wip1 alone or lacking an intrinsic phosphatase activity, exhibit dominant negative effects on the functions of the wild-type Wip1, i.e. ectopic expression of either of these Wip1 mutants inhibits dephosphorylation of Thr-68 in Chk2 by Wip1 and anti-apoptotic function of Wip1. These results provide a molecular basis for developing novel anti-cancer drugs, targeting oncogenic Wip1 phosphatase.

Nucleolin Participates in DNA Double-Strand Break-Induced Damage Response through MDC1-Dependent Pathway

H2AX is an important factor for chromatin remodeling to facilitate accumulation of DNA damage-related proteins at DNA double-strand break (DSB) sites. In order to further understand the role of H2AX in the DNA damage response (DDR), we attempted to identify H2AX-interacting proteins by proteomics analysis. As a result, we identified nucleolin as one of candidates. Here, we show a novel role of a major nucleolar protein, nucleolin, in DDR. Nucleolin interacted with γ-H2AX and accumulated to laser micro-irradiated DSB damage sites. Chromatin Immunoprecipitation assay also displayed the accumulation of nucleolin around DSB sites. Nucleolin-depleted cells exhibited repression of both ATM-dependent phosphorylation following exposure to γ-ray and subsequent cell cycle checkpoint activation. Furthermore, nucleolin-knockdown reduced HR and NHEJ activity and showed decrease in IR-induced chromatin accumulation of HR/NHEJ factors, agreeing with the delayed kinetics of γ-H2AX focus. Moreover, nucleolin-knockdown decreased MDC1-related events such as focus formation of 53 BP1, RNF168, phosphorylated ATM, and H2A ubiquitination. Nucleolin also showed FACT-like activity for DSB damage-induced histone eviction from chromatin. Taken together, nucleolin could promote both ATM-dependent cell cycle checkpoint and DSB repair by functioning in an MDC1-related pathway through its FACT-like function.

Critical role of Caenorhabditis elegans homologs of Cds1 (Chk2)-related kinases in meiotic recombination

ABSTRACT Although chromosomal segregation at meiosis I is the critical process for genetic reassortment and inheritance, little is known about molecules involved in this process in metazoa. Here we show by utilizing double-stranded RNA (dsRNA)-mediated genetic interference that novel protein kinases (Ce-CDS-1 and Ce-CDS-2) related to Cds1 (Chk2) play an essential role in meiotic recombination inCaenorhabditis elegans. Injection of dsRNA into adult animals resulted in the inhibition of meiotic crossing over and induced the loss of chiasmata at diakinesis in oocytes of F1animals. However, electron microscopic analysis revealed that synaptonemal complex formation in pachytene nuclei of the same progeny of injected animals appeared to be normal. Thus, Ce-CDS-1 and Ce-CDS-2 are the first example of Cds1-related kinases that are required for meiotic recombination in multicellular organisms.

Regulation of Chk2 gene expression in lymphoid malignancies: involvement of epigenetic mechanisms in Hodgkin's lymphoma cell lines.

AbstractThe tumor suppressor Chk2 kinase plays crucial roles in regulating cell-cycle checkpoints and apoptosis following DNA damage. We investigated the expression levels of the genes encoding Chk2 and several cell-cycle regulators in nine cell lines from lymphoid malignancies, including three Hodgkins lymphoma (HL) lines. We found that all HL cell lines exhibited a drastic reduction in Chk2 expression without any apparent mutation of the Chk2 gene. However, expression of Chk2 in HL cells was restored following treatment with the histone deacetylase inhibitors trichostatin A (TsA) and sodium butyrate (SB), or with the DNA methyltransferase inhibitor 5-aza-2′-deoxycytidine (5Aza-dC). Chromatin-immunoprecipitation (Chip) assays revealed that treatment of HL cells with TsA, SB or 5Aza-dC resulted in increased levels of acetylated histones H3 and H4, and decreased levels of dimethylated H3 lysine 9 at the Chk2 promoter. These results indicate that expression of the Chk2 gene is downregulated in HL cells via epigenetic mechanisms.

Down‐regulation of α6 integrin, an anti‐oncogene product, by functional cooperation of H‐Ras and c‐Myc

The molecular basis of cooperation of H‐Ras and c‐Myc in regulating cellular behaviour, such as cell adhesiveness, is still poorly understood. To investigate the role of H‐Ras and c‐Myc in cell adhesiveness, a constitutively active H‐RasV12 (H‐RasV12) and c‐Myc were stably expressed, singly or in combination in a haematopoietic cell line, and the expression and activity of cell adhesion molecules were monitored.

Role of NBS1 in DNA damage response and its relationship with cancer development

Nijmegen breakage syndrome (NBS) is a recessive genetic disorder characterized by an elevated sensitivity to ionizing radiation, chromosome instability, and a high frequency of malignancies phenotypes similar to those of ataxia-telangiectasia (A-T). NBS1, the product of the causative gene in NBS, contains several protein-interaction motifs in the N-terminus and C-terminus. The N-terminal FHA/BRCT domains interact with MDC1, γ-H2AX, TopBP1, and WRN, and the C-terminal motifs are indispensable for binding MRE11, ataxia telangiectasia mutated (ATM), RAD18, and RNF20. NBS1 typically forms a complex with the hMRE11/hRAD50 nuclease and functions in homologous recombination repair for DNA double-strand breaks (DSBs). The interactions of NBS1 with ATM, MDC1, γ-H2AX, and TopBP1 are important for focus formation at the sites of DSB and the activation of the ATM/ATR-dependent cell cycle checkpoint following DNA damage. NBS1 also participates in the regulation of RAD18/Polη-dependent translesion DNA synthesis (TLS) through its interaction with RAD18 and WRN. Recently, the ATM/ATR-dependent checkpoints have been reported to function as an anti-cancer barrier in human tumorigenesis. As NBS1 is an important factor in the regulation of ATM/ATR activation, a relationship between NBS1 and cancer development is probable. In fact, some studies have shown an association between several single-nucleotide polymorphisms (SNPs) or mutations in the N-terminus of NBS1 and the risk of lymphoma or breast cancer. However, the contribution of C-terminal SNPs or mutations to cancer development has not yet been reported. Therefore, further studies of the C-terminus are required to clarify the role of NBS1 in cancer development.

Update of thermotolerant genes essential for survival at a critical high temperature in Escherichia coli

Previous screening of a single-gene knockout library consisting of 3,908 disrupted-mutant strains allowed us to identify 51 thermotolerant genes that are essential for survival at a critical high temperature (CHT) in Escherichia coli [Murata M, Fujimoto H, Nishimura K, Charoensuk K, Nagamitsu H, Raina S, Kosaka T, Oshima T, Ogasawara N, Yamada M (2011) PLoS ONE 6: e20063]. In this study, we identified another 21 thermotolerant genes. E. coli thus has 72 thermotolerant genes in total. The genes are classified into 8 groups: genes for energy metabolism, outer membrane organization, DNA double-strand break repair, tRNA modification, protein quality control, translation control, cell division and transporters. This classification and physiological analysis indicate the existence of fundamental strategies for survival at a CHT, which seems to exclude most of the heat shock responses.