Deubiquitinating enzyme USP3 controls CHK1 chromatin association and activation.

Checkpoint kinase 1 (CHK1), a Ser/Thr protein kinase, is modified by the K63-linked ubiquitin chain in response to genotoxic stress, which promotes its nuclear localization, chromatin association, and activation. Interestingly, this bulky modification is linked to a critical residue, K132, at the kinase active site. It is unclear how this modification affects the kinase activity and how it is removed to enable the release of CHK1 from chromatin. Herein, we show that the K63-linked ubiquitin chain at CHK1's K132 residue has an inhibitory effect on the kinase activity. Furthermore, we demonstrate that this modification can be removed by ubiquitin-specific protease 3 (USP3), a deubiquitinating enzyme that targets K63-linked ubiquitin chains. Wild-type USP3, but not the catalytically defective or nuclear localization sequence-deficient mutants, reduced CHK1 K63-linked ubiquitination. Conversely, USP3 knockdown elevated K63-linked ubiquitination of the kinase, leading to prolonged CHK1 chromatin association and phosphorylation. Paradoxically, by removing the bulky ubiquitin chain at the active site, USP3 also increased the accessibility of CHK1 to its substrates. Thus, our findings on the dual roles of USP3 (namely, one to release CHK1 from the chromatin and the other to open up the active site) provide further insights into the regulation of CHK1 following DNA damage.

CK1α ablation in keratinocytes induces p53-dependent, sunburn-protective skin hyperpigmentation.

Casein kinase 1α (CK1α), a component of the ß-catenin destruction complex, is a critical regulator of Wnt signaling; its ablation induces both Wnt and p53 activation. To characterize the role of CK1α (encoded by Csnk1a1) in skin physiology, we crossed mice harboring floxed Csnk1a1 with mice expressing K14-Cre-ERT2 to generate mice in which tamoxifen induces the deletion of Csnk1a1 exclusively in keratinocytes [single-knockout (SKO) mice]. As expected, CK1α loss was accompanied by ß-catenin and p53 stabilization, with the preferential induction of p53 target genes, but phenotypically most striking was hyperpigmentation of the skin, importantly without tumorigenesis, for at least 9 mo after Csnk1a1 ablation. The number of epidermal melanocytes and eumelanin levels were dramatically increased in SKO mice. To clarify the putative role of p53 in epidermal hyperpigmentation, we established K14-Cre-ERT2 CK1α/p53 double-knockout (DKO) mice and found that coablation failed to induce epidermal hyperpigmentation, demonstrating that it was p53-dependent. Transcriptome analysis of the epidermis revealed p53-dependent up-regulation of Kit ligand (KitL). SKO mice treated with ACK2 (a Kit-neutralizing antibody) or imatinib (a Kit inhibitor) abrogated the CK1α ablation-induced hyperpigmentation, demonstrating that it requires the KitL/Kit pathway. Pro-opiomelanocortin (POMC), a precursor of α-melanocyte-stimulating hormone (α-MSH), was not activated in the CK1α ablation-induced hyperpigmentation, which is in contrast to the mechanism of p53-dependent UV tanning. Nevertheless, acute sunburn effects were successfully prevented in the hyperpigmented skin of SKO mice. CK1α inhibition induces skin-protective eumelanin but no carcinogenic pheomelanin and may therefore constitute an effective strategy for safely increasing eumelanin via UV-independent pathways, protecting against acute sunburn.

Requirement for human Mps1/TTK in oxidative DNA damage repair and cell survival through MDM2 phosphorylation.

Human Mps1 (hMps1) is a protein kinase essential for mitotic checkpoints and the DNA damage response. Here, we present new evidence that hMps1 also participates in the repair of oxidative DNA lesions and cell survival through the MDM2-H2B axis. In response to oxidative stress, hMps1 phosphorylates MDM2, which in turn promotes histone H2B ubiquitination and chromatin decompaction. These events facilitate oxidative DNA damage repair and ATR-CHK1, but not ATM-CHK2 signaling. Depletion of hMps1 or MDM2 compromised H2B ubiquitination, DNA repair and cell survival. The impairment could be rescued by re-expression of WT but not the phospho-deficient MDM2 mutant, supporting the involvement of hMps1-dependent MDM2 phosphorylation in the oxidative stress response. In line with these findings, localization of RPA and base excision repair proteins to damage foci also requires MDM2 and hMps1. Significantly, like MDM2, hMps1 is upregulated in human sarcoma, suggesting high hMps1 and MDM2 expression may be beneficial for tumors constantly challenged by an oxidative micro-environment. Our study therefore identified an hMps1-MDM2-H2B signaling axis that likely plays a relevant role in tumor progression.

Candidate tumor suppressor BTG3 maintains genomic stability by promoting Lys63-linked ubiquitination and activation of the checkpoint kinase CHK1.

B-cell translocation gene 3 (BTG3) is a member of the antiproliferative BTG/ Transducer of ErbB2 gene family and is induced by genotoxic stress in a p53- and Checkpoint kinase 1 (CHK1)-dependent manner. Down-regulation of BTG3 has been observed in human cancers, suggesting that it plays an important role in tumor suppression, although the underlying mechanisms are unclear. Here, we report that BTG3 interacts with CHK1, a key effector kinase in the cell cycle checkpoint response, and regulates its phosphorylation and activation. Upon interaction, BTG3 mediates K63-linked ubiquitination of CHK1 at Lys132 through the cullin-RING ligase 4(Cdt2) E3 complex, thus facilitating CHK1 chromatin association. We show that BTG3-depleted cells phenocopy those CHK1-deficient cells, exhibiting increased cell death after replication block and impaired chromosome alignment and segregation. These defects could be corrected by wild-type BTG3 but not by a mutant impaired in CHK1 interaction. We propose that BTG3-dependent CHK1 ubiquitination contributes to its chromatin localization and activation and that a defect in this regulation may increase genome instability and promote tumorigenesis.

Phosphorylation at threonine 288 by cell cycle checkpoint kinase 2 (CHK2) controls human monopolar spindle 1 (Mps1) kinetochore localization.

Human Mps1 (hMps1) is a mitotic checkpoint kinase responsible for sensing the unattached and tensionless kinetochore. Despite its importance in safeguarding proper chromosome segregation, how hMps1 is recruited to the kinetochore remains incompletely understood. Here, we demonstrate that phosphorylation at Thr-288 by the cell cycle checkpoint kinase CHK2 is involved in this process. We discovered that the phosphorylation-deficient T288A mutant has an impaired ability to localize to the kinetochore and cannot reestablish the mitotic checkpoint in hMps1-depleted cells. In support, we found that nocodazole induced hMps1 phosphorylation at the previously identified CHK2 site Thr-288 and that this could be detected at the kinetochore in a CHK2-dependent manner. Mechanistically, phosphorylation at Thr-288 promoted the interaction with the KMN (KNL1-Mis12-Ndc80 network) protein HEC1. Forced kinetochore localization corrected the defects associated with the T288A mutant. Our results provide evidence of a newly identified hMps1 phosphorylation site that is involved in the mitotic checkpoint and that CHK2 contributes to chromosomal stability through hMps1.

A keratinocyte-adipocyte signaling loop is reprogrammed by loss of BTG3 to augment skin carcinogenesis.

Obesity is endemic to many developed countries. Overweight or obesity is associated with an increased risk of developing cancer. Dysfunctional adipose tissue alters cancer cell proliferation and migration; however, whether and how neoplastic epithelial cells communicate with adipose tissue and the underlying mechanism are less clear. BTG3 is a member of the anti-proliferative BTG/Tob family and functions as a tumor suppressor. Here, we demonstrated that BTG3 levels are downregulated in basal cell carcinoma and squamous cell carcinoma compared to normal skin tissue, and Btg3 knockout in mice augmented the development of papilloma in a mouse model of DMBA/TPA-induced skin carcinogenesis. Mechanistically, BTG3-knockout keratinocytes promoted adipocyte differentiation mainly through the release of IL1α, IL10, and CCL4, as a result of elevated NF-κB activity. These adipocytes produced CCL20 and FGF7 in a feedback loop to promote keratinocyte migration. Thus, our findings showcased the role of BTG3 in guarding the interplay between keratinocytes and adjacent adipocytes, and identified the underlying neoplastic molecular mediators that may serve as possible targets in the treatment of skin cancer.

The GAS41-PP2Cbeta 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-PP2Cß, and not PP2Cß alone, is specifically required for dephosphorylation of serine 366 on p53. Ectopic expression of GAS41 and PP2Cß reduces UV radiation-induced p53 up-regulation, thereby increasing the cell survival upon genotoxic DNA damage. To our knowledge, the GAS41-PP2Cß 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.

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 Thr(284). A mutated XRCC1 lacking Thr(284) 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.

JAK2-CHK2 signaling safeguards the integrity of the mitotic spindle assembly checkpoint and genome stability.

Checkpoint kinase 2 (CHK2) plays an important role in safeguarding the mitotic progression, specifically the spindle assembly, though the mechanism of regulation remains poorly understood. Here, we identified a novel mitotic phosphorylation site on CHK2 Tyr156, and its responsible kinase JAK2. Expression of a phospho-deficient mutant CHK2 Y156F or treatment with JAK2 inhibitor IV compromised mitotic spindle assembly, leading to genome instability. In contrast, a phospho-mimicking mutant CHK2 Y156E restored mitotic normalcy in JAK2-inhibited cells. Mechanistically, we show that this phosphorylation is required for CHK2 interaction with and phosphorylation of the spindle assembly checkpoint (SAC) kinase Mps1, and failure of which results in impaired Mps1 kinetochore localization and defective SAC. Concordantly, analysis of clinical cancer datasets revealed that deletion of JAK2 is associated with increased genome alteration; and alteration in CHEK2 and JAK2 is linked to preferential deletion or amplification of cancer-related genes. Thus, our findings not only reveal a novel JAK2-CHK2 signaling axis that maintains genome integrity through SAC but also highlight the potential impact on genomic stability with clinical JAK2 inhibition.

Determination of CHK1 Cellular Localization by Immunofluorescence Microscopy.

Many proteins involved in the DNA damage pathway shuttle between the cytoplasm and nucleus, and their localizations are important for functions. In that regard, immunofluorescence microscopy has been widely used to delineate the temporal and spatial regulation of proteins. Here, we describe an unconventional method for studying the cellular localization of CHK1, a cell cycle checkpoint kinase that undergoes shuttling from the cytoplasm to the nucleus in response to genotoxic stress. In this study, we included an acid extraction step to better reveal the nuclear localization of CHK1.

Loss of the tumor suppressor BTG3 drives a pro-angiogenic tumor microenvironment through HIF-1 activation.

B-cell translocation gene 3 (BTG3) is a member of the antiproliferative BTG gene family and is a downstream target of p53. Here, we show that senescence triggered by BTG3 depletion was accompanied by a secretome enriched with cytokines, growth factors, and matrix-remodeling enzymes, which could promote angiogenesis and cell scattering in vitro. We present evidence that at least part of these activities can be explained by elevated HIF-1α activity. Mechanistically, the BTG3 C-terminal domain competes with the coactivator p300 for binding the HIF-1α transactivation domain. The angiogenic promoting effect of BTG3 knockdown was largely diminished upon co-depletion of HIF-1α, indicating that HIF-1α is a major downstream target of BTG3 in the control of angiogenesis. In vivo, ectopic expression of BTG3 suppresses angiogenesis in xenograft tumors; and syngenic tumor growth and metastasis were enhanced in Btg3-null mice. Moreover, analysis of clinical datasets revealed that a higher BTG3/VEGFA expression ratio correlates with improved patient survival in a number of cancer types. Taken together, our findings highlight the non-autonomous regulation of tumor microenvironment by BTG3 while suppressing tumor progression.

CHK2-mediated regulation of PARP1 in oxidative DNA damage response.

Poly(ADP-ribose) polymerase 1 (PARP1) is a DNA damage sensor, which upon activation, recruits downstream proteins by poly(ADP-ribosyl)ation (PARylation). However, it remains largely unclear how PARP1 activity is regulated. Interestingly, the data obtained through this study revealed that PARP1 was co-immunoprecipitated with checkpoint kinase 2 (CHK2), and the interaction was increased after oxidative DNA damage. Moreover, CHK2 depletion resulted in a reduction in overall PARylation. To further explore the functional relationship between PARP1 and CHK2, this study employed H2O2 to induce an oxidative DNA damage response in cells. Here, we showed that CHK2 and PARP1 interact in vitro and in vivo through the CHK2 SCD domain and the PARP1 BRCT domain. Furthermore, CHK2 stimulates the PARylation activity of PARP1 through CHK2-dependent phosphorylation. Consequently, the impaired repair associated with PARP1 depletion could be rescued by re-expression of wild-type PARP1 and the phospho-mimic but not the phospho-deficient mutant. Mechanistically, we showed that CHK2-dependent phosphorylation of PARP1 not only regulates its cellular localization but also promotes its catalytic activity and its interaction with XRCC1. These findings indicate that CHK2 exerts a multifaceted impact on PARP1 in response to oxidative stress to facilitate DNA repair and to maintain cell survival.

Characterization and transcriptional analysis of an ECF sigma factor from Xanthomonas campestris pv. campestris.

The genomic DNA segment encoding the rpoE gene and its flanking region was cloned from Xanthomonas campestris pv. campestris strain 11 (Xc11). The transcriptional start site of rpoE was located at nucleotide G, which is 33 nucleotides preceding the putative translation initiation codon of rpoE, and a extracytoplasmic function sigma factors (sigma(E))-dependent promoter was identified with -35 (5'-GAACTT-3') and -10 (5'-TCTCA-3') consensus sequences. The protein encoded by rpoE gene acted as a sigma (sigma) factor and was sufficient to direct core RNA polymerase to the rpoE promoter and to stimulate initiation of transcription in vitro. The specific binding of the reconstituted Esigma(E) holoenzyme with the Xc11 rpoE promoter was demonstrated by gel retardation assay and DNAse I footprint analysis. This study clearly demonstrated that the rpoE-rseA-mucD genomic organization of X. campestris is similar to that found in Xylella fastidiosa; however, expression of rpoE in X. campestris is autoregulated by its own sigma(E)-dependent promoter.

p53 C-terminal phosphorylation by CHK1 and CHK2 participates in the regulation of DNA-damage-induced C-terminal acetylation.

The tumor suppressor protein p53 mediates stress-induced growth arrest or apoptosis and plays a major role in safeguarding genome integrity. In response to DNA damage, p53 can be modified at multiple sites by phosphorylation and acetylation. We report on the characterization of p53 C-terminal phosphorylation by CHK1 and CHK2, two serine/threonine (Ser/Thr) protein kinases, previously implicated in the phosphorylation of the p53 N terminus. Using tryptic phosphopeptide mapping, we have identified six additional CHK1 and CHK2 sites residing in the final 100 amino acids of p53. Phosphorylation of at least three of these sites, Ser366, Ser378, and Thr387, was induced by DNA damage, and the induction at Ser366 and Thr387 was abrogated by small interfering RNA targeting chk1 and chk2. Furthermore, mutation of these phosphorylation sites has a different impact on p53 C-terminal acetylation and on the activation of p53-targeted promoters. Our results demonstrate a possible interplay between p53 C-terminal phosphorylation and acetylation, and they provide an additional mechanism for the control of the activity of p53 by CHK1 and CHK2.

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.

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 G(1)/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 G(1)/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.

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

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 G(1) 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.

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 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.