Sheau-Yann Shieh

Ataxia telangiectasia mutated and checkpoint kinase 2 regulate BRCA1 to promote the fidelity of DNA end-joining.

Homologous recombination (HR) and nonhomologous end-joining (NHEJ) are the two mechanisms responsible for repairing DNA double-strand breaks (DSBs) and act in either a collaborative or competitive manner in mammalian cells. DSB repaired by NHEJ may be more complicated than the simple joining of the ends of DSB, because, if nucleotides were lost, it would result in error-prone repair. This has led to the proposal that a subpathway of precise NHEJ exists that can repair DSBs with higher fidelity; this is supported by recent findings that the expression of the HR gene, BRCA1, is causally linked to in vitro and in vivo precise NHEJ activity. To further delineate this mechanism, the present study explored the connection between NHEJ and the cell-cycle checkpoint proteins, ataxia telangiectasia mutated (ATM) and checkpoint kinase 2 (Chk2), known to be involved in activating BRCA1, and tested the hypothesis that ATM and Chk2 promote precise end-joining by BRCA1. Support for this hypothesis came from the observations that (a) knockdown of ATM and Chk2 expression affected end-joining activity; (b) in BRCA1-defective cells, precise end-joining activity was not restored by a BRCA1 mutant lacking the site phosphorylated by Chk2 but was restored by wild-type BRCA1 or a mutant mimicking phosphorylation by Chk2; (c) Chk2 mutants lacking kinase activity or with a mutation at a site phosphorylated by ATM had a dominant negative effect on precise end-joining in BRCA1-expressing cells. These results suggest that the other two HR regulatory proteins, ATM and Chk2, act jointly to regulate the activity of BRCA1 in controlling the fidelity of DNA end-joining by precise NHEJ.

DDX3 Regulates Cell Growth through Translational Control of Cyclin E1

ABSTRACT DDX3 belongs to the DEAD box family of RNA helicases, but the details of its biological function remain largely unclear. Here we show that knockdown of DDX3 expression impedes G1/S-phase transition of the cell cycle. To know how DDX3 may act in cell cycle control, we screened for cellular mRNA targets of DDX3. Many of the identified DDX3 targets encoded cell cycle regulators, including G1/S-specific cyclin E1. DDX3 depletion specifically downregulates translation of cyclin E1 mRNA. Moreover, our data suggest that DDX3 participates in translation initiation of targeted mRNAs as well as in cell growth control via its RNA helicase activity. Consistent with these findings, we show that in the temperature-sensitive DDX3 mutant hamster cell line tsET24, cyclin E1 expression is downregulated at a nonpermissive temperature that inactivates mutant DDX3. Taken together, our results indicate that DDX3 is critical for translation of cyclin E1 mRNA, which provides an alternative mechanism for regulating cyclin E1 expression during the cell cycle.

Chk2-dependent phosphorylation of XRCC1 in the DNA damage response promotes base excision repair

The DNA damage response (DDR) has an essential function in maintaining genomic stability. Ataxia telangiectasia‐mutated (ATM)‐checkpoint kinase 2 (Chk2) and ATM‐ and Rad3‐related (ATR)‐Chk1, triggered, respectively, by DNA double‐strand breaks and blocked replication forks, are two major DDRs processing structurally complicated DNA damage. In contrast, damage repaired by base excision repair (BER) is structurally simple, but whether, and how, the DDR is involved in repairing this damage is unclear. Here, we demonstrated that ATM‐Chk2 was activated in the early response to oxidative and alkylation damage, known to be repaired by BER. Furthermore, Chk2 formed a complex with XRCC1, the BER scaffold protein, and phosphorylated XRCC1 in vivo and in vitro at Thr284. A mutated XRCC1 lacking Thr284 phosphorylation was linked to increased accumulation of unrepaired BER intermediate, reduced DNA repair capacity, and higher sensitivity to alkylation damage. In addition, a phosphorylation‐mimic form of XRCC1 showed increased interaction with glycosylases, but not other BER proteins. Our results are consistent with the phosphorylation of XRCC1 by ATM‐Chk2 facilitating recruitment of downstream BER proteins to the initial damage recognition/excision step to promote BER.

The candidate tumor suppressor BTG3 is a transcriptional target of p53 that inhibits E2F1

Proper regulation of cell cycle progression is pivotal for maintaining genome stability. In a search for DNA damage‐inducible, CHK1‐modulated genes, we have identified BTG3 (B‐cell translocation gene 3) as a direct p53 target. The p53 transcription factor binds to a consensus sequence located in intron 2 of the gene both in vitro and in vivo, and depletion of p53 by small interfering RNA (siRNA) abolishes DNA damage‐induced expression of the gene. Furthermore, ablation of BTG3 by siRNA in cancer cells results in accelerated exit from the DNA damage‐induced G2/M block. In vitro, BTG3 binds to and inhibits E2F1 through an N‐terminal domain including the conserved box A. Deletion of the interaction domain in BTG3 abrogates not only its growth suppression activity, but also its repression on E2F1‐mediated transactivation. We also present evidence that by disrupting the DNA binding activity of E2F1, BTG3 participates in the regulation of E2F1 target gene expression. Therefore, our studies have revealed a previously unidentified pathway through which the activity of E2F1 may be guarded by activated p53.

TTK/hMps1 mediates the p53-dependent postmitotic checkpoint by phosphorylating p53 at Thr18.

ABSTRACT Upon prolonged arrest in mitosis, cells undergo adaptation and exit mitosis without cell division. These tetraploid cells are either eliminated by apoptosis or arrested in the subsequent G1 phase in a spindle checkpoint- and p53-dependent manner. p53 has long been known to be activated by spindle poisons, such as nocodazole and Taxol, although the underlying mechanism remains elusive. Here we present evidence that stabilization and activation of p53 by spindle disruption requires the spindle checkpoint kinase TTK/hMps1. TTK/hMps1 phoshorylates the N-terminal domain of p53 at Thr18, and this phosphorylation disrupts the interaction with MDM2 and abrogates MDM2-mediated p53 ubiquitination. Phosphorylation at Thr18 enhances p53-dependent activation of not only p21 but also Lats2, two mediators of the postmitotic checkpoint. Furthermore, a phospho-mimicking substitution at Thr18 (T18D) is more competent than the phospho-deficient mutant (T18A) in rescuing the tetraploid checkpoint defect of p53-depleted cells. Our findings therefore provide a mechanism connecting the spindle checkpoint with p53 in the maintenance of genome stability.

p53-Mediated transactivation of LIMK2b links actin dynamics to cell cycle checkpoint control

The p53 tumor suppressor protein is widely known for its role as a sequence-specific transcription factor that regulates the expression of stress response genes. Here, we report the identification of LIMK2, which encodes a kinase that regulates actin dynamics through phosphorylation of cofilin, as a p53 target upregulated by DNA damage. Interestingly, the splice variant LIMK2b, but not LIMK2a, was induced in a p53-dependent manner through an intronic consensus p53-binding site. Depletion of LIMK2b leads to early exit of G2/M arrest after DNA damage, whereas its overexpression prolongs the arrest. These responses are recapitulated by ectopic expression of the active cofilin S3A mutant and the inactive cofilin S3D mutant, respectively, suggesting that LIMK2b may modulate G2/M arrest through cofilin phosphorylation. Furthermore, in support of its potential role as a tumor suppressor, LIMK2b was downregulated in esophageal and thyroid cancers, as well as in a number of established cancer cell lines, and its expression suppresses cancer cell migration. Taken together, our results unveil a novel pathway whereby LIMK2b, acting downstream of p53, ensures proper execution of checkpoint arrest by modulating the dynamics of actin polymerization.

The cell cycle checkpoint kinase CHK2 mediates DNA damage-induced stabilization of TTK/hMps1

Cell cycle progression is monitored constantly to ensure faithful passage of genetic codes and genome stability. We have demonstrated previously that, upon DNA damage, TTK/hMps1 activates the checkpoint kinase CHK2 by phosphorylating CHK2 at Thr68. However, it remains to be determined whether and how TTK/hMps1 responds to DNA damage. In this report, we present evidence that TTK/hMps1 can be induced by DNA damage in normal human fibroblasts. Interestingly, the induction depends on CHK2 because CHK2-targeting small interfering RNA or a CHK2 inhibitor abolishes the increase. Such induction is mediated through phosphorylation of TTK/hMps1 at Thr288 by CHK2 and requires the CHK2 SQ/TQ cluster domain/forkhead-associated domain. In cells, TTK/hMps1 phosphorylation at Thr288 is induced by DNA damage and forms nuclear foci, which colocalize partially with γ-H2AX. Reexpression of TTK/hMps1 T288A mutant in TTK/hMps1-knockdown cells causes a defect in G2/M arrest, suggesting that phosphorylation at this site participates in the proper checkpoint execution. Our study uncovered a regulatory loop between TTK/hMps1 and CHK2 whereby DNA damage-activated CHK2 may facilitate the stabilization of TTK/hMps1, therefore maintaining the checkpoint control.

Loss of the candidate tumor suppressor BTG3 triggers acute cellular senescence via the ERK–JMJD3–p16 INK4a signaling axis

The B-cell translocation gene 3 (BTG3) is a member of the antiproliferative BTG gene family and a downstream target of p53. BTG3 also binds and inhibits E2F1. Although it connects functionally two major growth-regulatory pathways, the physiological role of BTG3 remains largely uncharacterized. Here, we present evidence that loss of BTG3 in normal cells induced cellular senescence, which was correlated with enhanced ERK–AP1 signaling and elevated expression of the histone H3K27me3 demethylase JMJD3/KDM6B, leading to acute induction of p16INK4a. Importantly, we also found that BTG3 expression is specifically downregulated in prostate cancer, thus providing a physiological link with human cancers. Our data suggest that BTG3 may have a fail-safe role against tumorigenic progression.

Breast Cancer Amplified Sequence 2, a Novel Negative Regulator of the p53 Tumor Suppressor

Breast cancer amplified sequence 2 (BCAS2) was reported previously as a transcriptional coactivator of estrogen receptor. Here, we report that BCAS2 directly interacts with p53 to reduce p53 transcriptional activity by mildly but consistently decreasing p53 protein in the absence of DNA damage. However, in the presence of DNA damage, BCAS2 prominently reduces p53 protein and provides protection against chemotherapeutic agent such as doxorubicin. Deprivation of BCAS2 induces apoptosis in p53 wild-type cells but causes G(2)-M arrest in p53-null or p53 mutant cells. There are at least two apoptosis mechanisms induced by silencing BCAS2 in wild-type p53-containing cells. Firstly, it increases p53 retention in nucleus that triggers the expression of apoptosis-related genes. Secondly, it increases p53 transcriptional activity by raising p53 phosphorylation at Ser(46) and decreases p53 protein degradation by reducing p53 phosphorylation at Ser(315). We show for the first time that BCAS2, a small nuclear protein (26 kDa), is a novel negative regulator of p53 and hence a potential molecular target for cancer therapy.

The GAS41-PP2Cβ Complex Dephosphorylates p53 at Serine 366 and Regulates Its Stability

The p53 tumor suppressor is principally regulated by post-translational modifications and proteasome-dependent degradation. Various kinases have been shown to phosphorylate p53, but little is known about the counteracting phosphatases. We demonstrate here that the newly identified complex GAS41-PP2Cβ, and not PP2Cβ alone, is specifically required for dephosphorylation of serine 366 on p53. Ectopic expression of GAS41 and PP2Cβ reduces UV radiation-induced p53 up-regulation, thereby increasing the cell survival upon genotoxic DNA damage. To our knowledge, the GAS41-PP2Cβ complex is the first example in which substrate specificity of a PP2C family member is controlled by an associated regulatory subunit. Because GAS41 is frequently amplified in human gliomas, our finding illustrates a novel oncogenic mechanism of GAS41 by p53 dephosphorylation.