Augmented cancer resistance and DNA damage response phenotypes in PPM1D null mice

WIP1 is a novel specific target for growth hormone action

DNA damage repair (DDR) is mediated by phosphorylating effectors ATM kinase, CHK2, p53, and γH2AX. We showed earlier that GH suppresses DDR by suppressing pATM, resulting in DNA damage accumulation. Here, we show GH acting through GH receptor (GHR) inducing wild-type p53-inducible phosphatase 1 (WIP1), which dephosphorylated ATM and its effectors in normal human colon cells and three-dimensional human intestinal organoids. Mice bearing GH-secreting xenografts exhibited induced colon WIP1 with suppressed pATM and γH2AX. WIP1 was also induced in buffy coats derived from patients with elevated GH from somatotroph adenomas. In contrast, decreased colon WIP1 was observed in GHR-/- mice. WIP1 inhibition restored ATM phosphorylation and reversed GH-induced DNA damage. We elucidated a novel GH signaling pathway activating Src/AMPK to trigger HIPK2 nuclear-cytoplasmic relocation and suppressing WIP1 ubiquitination. Concordantly, blocking either AMPK or Src abolished GH-induced WIP1. We identify WIP1 as a specific target for GH-mediated epithelial DNA damage accumulation.

High Expression of PPM1D Induces Tumors Phenotypically Similar to TP53 Loss-of-Function Mutations in Mice

Simple Summary

Aberrant expression of the PPM1D gene which encodes a phosphatase called WIP1 is frequently observed in cancers of different origins. WIP1 is a negative regulator of the tumor suppressor p53. Improper inactivation of p53 results in genomic instability and can induce neoplastic transformation. We show that overexpression of PPM1D induces tumors in mice similar to cancers harboring p53 mutations. Our results suggest that PPM1D can act as an oncogenic driver by inducing genomic instability, impaired growth arrest, and apoptotic escape that can result in neoplastic transformation and malignant tumor development.

Abstract

PPM1D is a negative regulator of p53 and genomic aberrations resulting in increased activity of PPM1D have been observed in cancers of different origins, indicating that PPM1D has oncogenic properties. We established a transgenic mouse model overexpressing PPM1D and showed that these mice developed a wide variety of cancers. PPM1D-expressing mice developed tumors phenotypically and genetically similar to tumors in mice with dysfunctional p53. T-cell lymphoblastic lymphoma was the most frequent cancer observed in these mice (55%) followed by adenocarcinomas (24%), leukemia (12%) and other solid tumors including neuroblastoma. Characterization of T-cell lymphomas in mice overexpressing PPM1D demonstrates Pten-deletion and p53-accumulation similar to mice with p53 loss-of-function. Also, Notch1 mutations which are recurrently observed in T-cell acute lymphoblastic lymphoma (T-ALL) were frequently detected in PPM1D-transgenic mice. Hence, PPM1D acts as an oncogenic driver in connection with cellular stress, suggesting that the PPM1D gene status and expression levels should be investigated in TP53 wild-type tumors.

Inhibition of the DNA damage response phosphatase PPM1D reprograms neutrophils to enhance anti-tumor immune responses

PPM1D/Wip1 is a negative regulator of the tumor suppressor p53 and is overexpressed in several human solid tumors. Recent reports associate gain-of-function mutations of PPM1D in immune cells with worse outcomes for several human cancers. Here we show that mice with genetic knockout of Ppm1d or with conditional knockout of Ppm1d in the hematopoietic system, in myeloid cells, or in neutrophils all display significantly reduced growth of syngeneic melanoma or lung carcinoma tumors. Ppm1d knockout neutrophils infiltrate tumors extensively. Chemical inhibition of Wip1 in human or mouse neutrophils increases anti-tumor phenotypes, p53-dependent expression of co-stimulatory ligands, and proliferation of co-cultured cytotoxic T cells. These results suggest that inhibition of Wip1 in neutrophils enhances immune anti-tumor responses.

Protein phosphatase, Mg2+/Mn2+-dependent 1D (PPM1D) mutations in haematological cancer

Until recently, the protein phosphatase, Mg²⁺/Mn²⁺-dependent 1D (PPM1D) gene had not been examined in haematological cancer, but several studies have now explored the functional role of this gene and its aberrations. It is often mutated in the context of clonal haemopoiesis (including in patients with lymphoma, myeloproliferative neoplasms and myelodysplastic syndrome) and mutations have been associated with exposure to cytotoxic and radiation therapy, development of therapy-related neoplasms and inferior survival. The vast majority of PPM1D mutations found in haematopoietic cells are of the nonsense or frameshift type and located within terminal exon 6. These genetic defects are rarely found in the blood of healthy individuals. PPM1D encodes the PPM1D phosphatase [also named wild-type p53-induced phosphatase 1 (WIP1)], which negatively regulates signalling molecules within the DNA damage response pathway, including tumour suppressor p53. Clonal expansion of PPM1D mutant haematopoietic cells can potentially be prevented with inhibitors; however, human trials are awaited. In the present review, we provide a review of the literature regarding PPM1D and its role in haematological cancer.

Development of Specific Inhibitors for Oncogenic Phosphatase PPM1D by Using Ion-Responsive DNA Aptamer Library

(1) Background: Ser/Thr protein phosphatase PPM1D is an oncogenic protein. In normal cells, however, PPM1D plays essential roles in spermatogenesis and immune response. Hence, it is necessary to develop novel PPM1D inhibitors without side effects on normal cells. Stimuli-responsive molecules are suitable for the spatiotemporal regulation of inhibitory activity. (2) Methods: In this study, we designed an ion-responsive DNA aptamer library based on G-quadruplex DNA that can change its conformation and function in response to monovalent cations. (3) Results: Using this library, we identified the PPM1D specific inhibitor M1D-Q5F aptamer. The M1D-Q5F aptamer showed anti-cancer activity against breast cancer MCF7 cells. Interestingly, the induction of the structural change resulting in the formation of G-quadruplex upon stimulation by monovalent cations led to the enhancement of the inhibitory activity and binding affinity of M1D-Q5F. (4) Conclusions: These data suggest that the M1D-Q5F aptamer may act as a novel stimuli-responsive anti-cancer agent.

miR129-1 regulates protein phosphatase 1D protein expression under hypoxic conditions in non-small cell lung cancer cells harboring a TP53 mutation

Protein phosphatase 1D (PPM1D), which functions as an oncogene, is a known target of the tumor suppressor p53 and is involved in p53-regulated genomic surveillance mechanisms. PPM1D dephosphorylates both p53 and its ubiquitin ligase mouse double minute 2 homolog, as well as the RNA-binding protein (RBM)38, which turns RBM38 from an inducer to inhibitor of TP53 translation. In addition, RBM38 induces PPM1D translation. Hence, the PPM1D-RBM38-p53 axis is important in maintaining genomic integrity and is often altered during tumorigenesis. TP53, which encodes p53, is deleted or mutated in >50% of cancer types, including lung cancer. Mutant p53 has been revealed to complex with hypoxia-inducible factor 1α (HIF1α) and upregulate transcription of pro-metastatic genes. However, the mechanism underlying the action of the PPM1D-RBM38-p53 axis in the context of mutant p53 under normoxic and hypoxic conditions is yet to be elucidated. In the present study, using non-small cell lung cancer (NSCLC) cell lines harboring wild-type (A549 cells) or hot-spot mutant (NCI-H1770 and R249WΔ-TP53-A549 cells) TP53, it was demonstrated that in cells harboring mutant p53, RBM38 was not the primary regulator of PPM1D translation under hypoxic conditions. Knockdown of RBM38 in TP53 mutant cells did not affect the PPM1D protein expression under hypoxic conditions. Instead, in NCI-H1770 cells maintained under normoxic conditions, PPM1D was revealed as a target of micro RNA (miR)-129-1-3p, a known tumor suppressor in lung cancer. Hypoxia resulted in the downregulation of miR-129-1-3p expression, and thus, in the downregulation of PPM1D messenger RNA (mRNA) translation. In NCI-H1770 cells grown under hypoxic conditions, the transient transfection of miR-129-1-3p mimic, and not control mimic, repressed the expression of a reporter containing wild-type, but not miR-129-1-3p binding mutant, of the PPM1D 3'-untranslated region (UTR). Analysis of NSCLC cell lines from the Broad Institute Cancer Cell Encyclopedia and patients with NSCLC from The Cancer Genome Atlas dataset revealed significant co-occurrence of PPM1D/RBM38 and PPM1D/HIF1A mutations. However, there was no significant difference in the overall survival of patients with NSCLC with or without genomic alterations in TP53, RBM38, PPM1D and HIF1A. In summary, the current study demonstrated hypoxia-dependent miR-129-1-3p-mediated regulation of PPM1D protein expression in NSCLC cell line harboring mutant TP53.

Wip1 regulates Smad4 phosphorylation and inhibits TGF-β signaling

The tumor suppressor Smad4, a key mediator of the TGF-β/BMP pathways, is essential for development and tissue homeostasis. Phosphorylation of Smad4 in its linker region catalyzed by the mitogen-activated protein kinase (MAPK) plays a pivotal role in regulating its transcriptional activity and stability. In contrast, roles of Smad4 dephosphorylation as a control mechanism of TGF-β/BMP signaling and the phosphatases responsible for its dephosphorylation remain so far elusive. Here, we identify Wip1 as a Smad4 phosphatase. Wip1 selectively binds and dephosphorylates Smad4 at Thr277, a key MAPK phosphorylation site, thereby regulating its nuclear accumulation and half-life. In Xenopus embryos, Wip1 limits mesoderm formation and favors neural induction by inhibiting TGF-β/BMP signals. Wip1 restrains TGF-β-induced growth arrest, migration, and invasion in human cells and enhances the tumorigenicity of cancer cells by repressing the antimitogenic activity of Smad4. We propose that Wip1-dependent dephosphorylation of Smad4 is critical for the regulation of TGF-β signaling.

PP2Cδ inhibits p300-mediated p53 acetylation via ATM/BRCA1 pathway to impede DNA damage response in breast cancer

Although nuclear type 2C protein phosphatase (PP2Cδ) has been demonstrated to be pro-oncogenic with an important role in tumorigenesis, the underlying mechanisms that link aberrant PP2Cδ levels with cancer development remain elusive. Here, we found that aberrant PP2Cδ activity decreases p53 acetylation and its transcriptional activity and suppresses doxorubicin-induced cell apoptosis. Mechanistically, we show that BRCA1 facilitates p300-mediated p53 acetylation by complexing with these two proteins and that S1423/1524 phosphorylation is indispensable for this regulatory process. PP2Cδ, via dephosphorylation of ATM, suppresses DNA damage-induced BRCA1 phosphorylation, leading to inhibition of p300-mediated p53 acetylation. Furthermore, PP2Cδ levels correlate with histological grade and are inversely associated with BRCA1 phosphorylation and p53 acetylation in breast cancer specimens. C23, our newly developed PP2Cδ inhibitor, promotes the anticancer effect of doxorubicin in MCF-7 xenograft-bearing nude mice. Together, our data indicate that PP2Cδ impairs p53 acetylation and DNA damage response by compromising BRCA1 function.

Targeting 17q23 amplicon to overcome the resistance to anti-HER2 therapy in HER2+ breast cancer

Chromosome 17q23 amplification occurs in ~11% of human breast cancers. Enriched in HER2+ breast cancers, the 17q23 amplification is significantly correlated with poor clinical outcomes. In addition to the previously identified oncogene WIP1, we uncover an oncogenic microRNA gene, MIR21, in a majority of the WIP1-containing 17q23 amplicons. The 17q23 amplification results in aberrant expression of WIP1 and miR-21, which not only promotes breast tumorigenesis, but also leads to resistance to anti-HER2 therapies. Inhibiting WIP1 and miR-21 selectively inhibits the proliferation, survival and tumorigenic potential of the HER2+ breast cancer cells harboring 17q23 amplification. To overcome the resistance of trastuzumab-based therapies in vivo, we develop pH-sensitive nanoparticles for specific co-delivery of the WIP1 and miR-21 inhibitors into HER2+ breast tumors, leading to a profound reduction of tumor growth. These results demonstrate the great potential of the combined treatment of WIP1 and miR-21 inhibitors for the trastuzumab-resistant HER2+ breast cancers.

The 17q23 amplicon containing the WIP1 oncogene is frequently amplified in HER2+ breast cancer. Here they find MIR21 to be present in WIP1-containing amplicons, and report nanoparticle based co-delivery of WIP1 and miR-21 inhibitors to be effective in trastuzumab-resistant HER2+ breast cancer.

PPM1D Functions as Oncogene and is Associated with Poor Prognosis in Esophageal Squamous Cell Carcinoma

Mounting evidence has demonstrated that PPM1D participates in the development and progression of a wide variety of tumors. However, its precise roles in esophageal squamous cell carcinoma (ESCC) remain under investigation. Here, UALCAN, an interactive web-portal to perform the expression analyses of PPM1D using TCGA gene expression data, and PPM1D high expression was exhibited in primary esophageal cancer. Further investigation revealed that PPM1D expression was obviously higher in ESCC tissues than in normal tissues (P < 0.01), which was consistent with the results from real-time qPCR and Western blotting in ESCC tissues and paired normal esophageal tissues. Besides, PPM1D expression was closely correlated with TNM staging, tumor differentiation and lymph node metastasis (P < 0.01), but not related to the patients' gender and age (P > 0.05). Notably, PPM1D expression in metastatic ESCC patients was markedly higher than that in non-metastatic ESCC patients (P < 0.01), and the ESCC patients with high PPM1D expression predicted poor prognosis. Multivariate assay demonstrated that PPM1D and lymph node metastasis were considered as independent prognostic factors for the ESCC patients. These findings suggest PPM1D plays a pivotal important role in onset and progression of ESCC, and may be a new biomarker for metastasis and prognosis of the ESCC patients.

p63 Silencing induces reprogramming of cardiac fibroblasts into cardiomyocyte-like cells

OBJECTIVE

Reprogramming of fibroblasts into induced cardiomyocytes represents a potential new therapy for heart failure. We hypothesized that inactivation of p63, a p53 gene family member, may help overcome human cell resistance to reprogramming.

METHODS

p63 Knockout (^-/-) and knockdown murine embryonic fibroblasts (MEFs), p63^-/- adult murine cardiac fibroblasts, and human cardiac fibroblasts were assessed for cardiomyocyte-specific feature changes, with or without treatment by the cardiac transcription factors Hand2-Myocardin (HM).

RESULTS

Flow cytometry revealed that a significantly greater number of p63^-/- MEFs expressed the cardiac-specific marker cardiac troponin T (cTnT) in culture compared with wild-type (WT) cells (38% ± 11% vs 0.9% ± 0.9%, P < .05). HM treatment of p63^-/- MEFs increased cTnT expression to 74% ± 3% of cells but did not induce cTnT expression in wild-type murine embryonic fibroblasts. shRNA-mediated p63 knockdown likewise yielded a 20-fold increase in cTnT microRNA expression compared with untreated MEFs. Adult murine cardiac fibroblasts demonstrated a 200-fold increase in cTnT gene expression after inducible p63 knockout and expressed sarcomeric α-actinin as well as cTnT. These p63^-/- adult cardiac fibroblasts exhibited calcium transients and electrically stimulated contractions when co-cultured with neonatal rat cardiomyocytes and treated with HM. Increased expression of cTnT and other marker genes was also observed in p63 knockdown human cardiac fibroblasts procured from patients undergoing procedures for heart failure.

CONCLUSIONS

Downregulation of p63 facilitates direct cardiac cellular reprogramming and may help overcome the resistance of human cells to reprogramming.

Wip1 is associated with tumorigenity and metastasis through MMP-2 in human intrahepatic cholangiocarcinoma

Wip1 has been shown to correlate with the metastasis/invasion of several tumors. This study was designed to investigate the clinical significance and biological function of Wip1 in intrahepatic cholangiocarcinoma (ICC). The expression of Wip1 was investigated in sixty human ICC biopsy samples by immunohistochemistry. Transient and stable knockdown of Wip1 in two human ICC cells (ICC-9810 and SSP25) were established using short hairpin RNA expression vector. Immunohistochemistry revealed that Wip1 was up-regulated in human ICC tissues (47/60, 78.3%). High levels of Wip1 in human ICC correlated with metastasis to the lymph metastasis (P=0.022). Genetic depletion of Wip1 in ICC cells resulted in significantly inhibited proliferation and invasion compared with controls. Most importantly, Wip1 down-regulation impaired tumor migration capacity of ICC cells in vivo. Subsequent investigations revealed that matrix metalloproteinase-2 (MMP-2) is an important target of Wip1. Consistently, in human ICC tissues, Wip1 level was positively correlated with MMP-2 expression. Taken together, our founding indicates that Wip1 may be a crucial regulator in the tumorigenicity and invasion of human ICC, Wip1 exerts its pro-invasion function at least in part through the MMP-2 signaling pathway, suggesting Wip1 as a potential therapeutic target for ICC.

WIP1 phosphatase as pharmacological target in cancer therapy

DNA damage response (DDR) pathway protects cells from genome instability and prevents cancer development. Tumor suppressor p53 is a key molecule that interconnects DDR, cell cycle checkpoints, and cell fate decisions in the presence of genotoxic stress. Inactivating mutations in TP53 and other genes implicated in DDR potentiate cancer development and also influence the sensitivity of cancer cells to treatment. Protein phosphatase 2C delta (referred to as WIP1) is a negative regulator of DDR and has been proposed as potential pharmaceutical target. Until recently, exploitation of WIP1 inhibition for suppression of cancer cell growth was compromised by the lack of selective small-molecule inhibitors effective at cellular and organismal levels. Here, we review recent advances in development of WIP1 inhibitors and discuss their potential use in cancer treatment.

LZAP is a novel Wip1 binding partner and positive regulator of its phosphatase activity in vitro

The phosphatase Wip1 attenuates the DNA damage response (DDR) by removing phosphorylation marks from a number of DDR proteins (p53, MDM2, Chk1/2, p38). Wip1 also dephosphorylates and inactivates RelA. Notably, LZAP, a putative tumor suppressor, has been linked to dephosphorylation of several of these substrates, including RelA, p38, Chk1, and Chk2. LZAP has no known catalytic activity or functional motifs, suggesting that it exerts its effects through interaction with other proteins. Here we show that LZAP binds Wip1 and stimulates its phosphatase activity. LZAP had been previously shown to bind many Wip1 substrates (RelA, p38, Chk1/2), and our results show that LZAP also binds the previously identified Wip1 substrate, MDM2. This work identifies 2 novel Wip1 substrates, ERK1 and HuR, and demonstrates that HuR is a binding partner of LZAP. Pleasingly, LZAP potentiated Wip1 catalytic activity toward each substrate tested, regardless of whether full-length substrates or phosphopeptides were utilized. Since this effect was observed on ERK1, which does not bind LZAP, as well as for each of 7 peptides tested, we hypothesize that LZAP binding to the substrate is not required for this effect and that LZAP directly binds Wip1 to augment its phosphatase activity.

WIP1 modulates responsiveness to Sonic Hedgehog signaling in neuronal precursor cells and medulloblastoma

High-level amplification of the protein phosphatase PPM1D (WIP1) is present in a subset of medulloblastomas (MBs) that have an expression profile consistent with active Sonic Hedgehog (SHH) signaling. We found that WIP1 overexpression increased expression of Shh target genes and cell proliferation in response to Shh stimulation in NIH3T3 and cerebellar granule neuron precursor cells in a p53-independent manner. Thus, we developed a mouse in which WIP1 is expressed in the developing brain under control of the Neurod2 promoter (ND2:WIP1). The external granule layer (EGL) in early postnatal ND2:WIP1 mice exhibited increased proliferation and expression of Shh downstream targets. MB incidence increased and survival decreased when ND2:WIP1 mice were crossed with an Shh-activated MB mouse model. Conversely, Wip1 knockout significantly suppressed MB formation in two independent mouse models of Shh-activated MB. Furthermore, Wip1 knockdown or treatment with a WIP1 inhibitor suppressed the effects of Shh stimulation and potentiated the growth inhibitory effects of SHH pathway-inhibiting drugs in Shh-activated MB cells in vitro. This suggests an important cross-talk between SHH and WIP1 pathways that accelerates tumorigenesis and supports WIP1 inhibition as a potential treatment strategy for MB.

Gamma-H2AX upregulation caused by Wip1 deficiency increases depression-related cellular senescence in hippocampus

The PP2C family member Wild-type p53-induced phosphatase 1 (Wip1) critically regulates DNA damage response (DDR) under stressful situations. In the present study, we investigated whether Wip1 expression was involved in the regulation of DDR-induced and depression-related cellular senescence in mouse hippocampus. We found that Wip1 gene knockout (KO) mice showed aberrant elevation of hippocampal cellular senescence and of γ-H2AX activity, which is known as a biomarker of DDR and cellular senescence, indicating that the lack of Wip1-mediated γ-H2AX dephosphorylation facilitates cellular senescence in hippocampus. Administration of the antidepressant fluoxetine had no significant effects on the increased depression-like behaviors, enriched cellular senescence, and aberrantly upregulated hippocampal γ-H2AX activity in Wip1 KO mice. After wildtype C57BL/6 mice were exposed to the procedure of chronic unpredictable mild stress (CUMS), cellular senescence and γ-H2AX activity in hippocampus were also elevated, accompanied by the suppression of Wip1 expression in hippocampus when compared to the control group without CUMS experience. These CUMS-induced symptoms were effectively prevented following fluoxetine administration in wildtype C57BL/6 mice, with the normalization of depression-like behaviors. Our data demonstrate that Wip1-mediated γ-H2AX dephosphorylation may play an important role in the occurrence of depression-related cellular senescence.

Deficiency of the tumor promoter gene wip1 induces insulin resistance

Diabetes is a growing health care issue, and prediabetes has been established as a risk factor for type 2 diabetes. Prediabetes is characterized by deregulated glucose control, and elucidating pathways which govern this process is critical. We have identified the wild-type (WT) p53-inducible phosphatase (WIP1) phosphatase as a regulator of glucose homeostasis. Initial characterization of insulin signaling in WIP1 knockout (WIP1(KO)) murine embryo fibroblasts demonstrated reduced insulin-mediated Ak mouse transforming activation. In order to assess the role of WIP1 in glucose homeostasis, we performed metabolic analysis on mice on a low-fat chow diet (LFD) and high fat diet (HFD). We observed increased expression of proinflammatory cytokines in WIP1(KO) murine embryo fibroblasts, and WIP1(KO) mice fed a LFD and a HFD. WIP1(KO) mice exhibited glucose intolerance and insulin intolerance on a LFD and HFD. However, the effects of WIP1 deficiency cause different metabolic defects in mice on a LFD and a HFD. WIP1(KO) mice on a LFD develop hepatic insulin resistance, whereas this is not observed in HFD-fed mice. Mouse body weights and food consumption increase slightly over time in LFD-fed WT and WIP1(KO) mice. Leptin levels are increased in LFD-fed WIP1(KO) mice, compared with WT. In contrast, HFD-fed WIP1(KO) mice are resistant to HFD-induced obesity, have decreased levels of food consumption, and decreased leptin levels compared with HFD-WT mice. WIP1 has been shown to regulate the nuclear factor kappa-light-chain-enhancer of activated B cells pathway, loss of which leads to increased inflammation. We propose that this increased inflammation triggers insulin resistance in WIP1(KO) mice on LFD and HFD.

Association between overexpression of Wip1 and prognosis of patients with non-small cell lung cancer

Wild-type p53-induced phosphatase 1 (Wip1), also termed PPM1D, is a member of the protein phosphatase 2C family, which is characterized by distinctive oncogenic properties. Overexpression of Wip1 is observed in certain types of human tumors that are associated with significantly poor prognosis. The present study aimed to detect the expression of Wip1 in non-small cell lung cancer (NSCLC) and to analyze its prognostic value in such patients. The protein expression level of Wip1 was compared between NSCLC and normal lung tissue specimens using by immunohistochemistry, and it was found that Wip1 was highly expressed in NSCLCs but was absent or weakly expressed in normal lung tissues. Detailed clinical and demographic information of patients were retrospectively collected pre- and postoperatively, and Kaplan-Meier survival and Cox's regression analyses were performed to evaluate the prognosis of patients. Survival analysis revealed that the overall survival rate for patients in the Wip1-positive expression group was significantly lower than that of the Wip1-negative group, and Cox multivariate analysis indicated that positive Wip1 expression, pN classification and pathological stage were significant prognostic predictors. The results of the current study suggest that Wip1 may be associated with pathological diagnosis and prognostic evaluation of NSCLC.

PPM1D phosphatase, a target of p53 and RBM38 RNA-binding protein, inhibits p53 mRNA translation via dephosphorylation of RBM38

PPM1D phosphatase, also called wild-type p53-induced phosphatase 1 (Wip1), promotes tumor development by inactivating the p53 tumor suppressor pathway. RBM38 RNA-binding protein, also called RNPC1 and a target of p53, inhibits p53 mRNA translation, which can be reversed by GSK3 protein kinase via phosphorylation of RBM38 at serine 195. Here we showed that ectopic expression of RBM38 increases, whereas knockdown of RBM38 inhibits, PPM1D mRNA translation. Consistent with this, we found that RBM38 directly binds to PPM1D 3' untranslated region (3’UTR) and promotes expression of a heterologous reporter gene that carries PPM1D 3’UTR in a dose-dependent manner. Interestingly, we showed that PPM1D directly interacts with and dephosphorylates RBM38 at serine 195. Furthermore, we showed that PPM1D modulates p53 mRNA translation and p53-dependent growth suppression through dephosphorylation of RBM38. These findings provide evidence that the crosstalk between PPM1D and RBM38, both of which are targets and modulators of p53, plays a critical role in p53 expression and activity.

WIP1 phosphatase as a potential therapeutic target in neuroblastoma

The wild-type p53-induced phosphatase 1 (WIP1) is a serine/threonine phosphatase that negatively regulates multiple proteins involved in DNA damage response including p53, CHK2, Histone H2AX, and ATM, and it has been shown to be overexpressed or amplified in human cancers including breast and ovarian cancers. We examined WIP1 mRNA levels across multiple tumor types and found the highest levels in breast cancer, leukemia, medulloblastoma and neuroblastoma. Neuroblastoma is an exclusively TP53 wild type tumor at diagnosis and inhibition of p53 is required for tumorigenesis. Neuroblastomas in particular have previously been shown to have 17q amplification, harboring the WIP1 (PPM1D) gene and associated with poor clinical outcome. We therefore sought to determine whether inhibiting WIP1 with a selective antagonist, GSK2830371, can attenuate neuroblastoma cell growth through reactivation of p53 mediated tumor suppression. Neuroblastoma cell lines with wild-type TP53 alleles were highly sensitive to GSK2830371 treatment, while cell lines with mutant TP53 were resistant to GSK2830371. The majority of tested neuroblastoma cell lines with copy number gains of the PPM1D locus were also TP53 wild-type and sensitive to GSK2830371A; in contrast cell lines with no copy gain of PPM1D were mixed in their sensitivity to WIP1 inhibition, with the primary determinant being TP53 mutational status. Since WIP1 is involved in the cellular response to DNA damage and drugs used in neuroblastoma treatment induce apoptosis through DNA damage, we sought to determine whether GSK2830371 could act synergistically with standard of care chemotherapeutics. Treatment of wild-type TP53 neuroblastoma cell lines with both GSK2830371 and either doxorubicin or carboplatin resulted in enhanced cell death, mediated through caspase 3/7 induction, as compared to either agent alone. Our data suggests that WIP1 inhibition represents a novel therapeutic approach to neuroblastoma that could be integrated with current chemotherapeutic approaches.

Overexpression of wip1 is associated with biologic behavior in human clear cell renal cell carcinoma

Wild-type p53-induced phosphatase (Wip1 or PPM1D) has been reported to be aberrantly expressed in various cancers and correlated with the malignant behavior of cancer cells. However, the function of Wip1 in RCC remains unclear. The present study investigated its abnormal expression and dysfunctions in clear cell renal cell carcinoma (ccRCC) in vitro. With the combination of immunohistochemistry, western blotting, immunofluorescence, qRT-PCR, and cell proliferation, migration and invasion assays, we found that levels of Wip1 mRNA and protein were dramatically increased in human ccRCC tissues (P<0.001 for both), and upregulation of Wip1 was significantly associated with depth of invasion (P<0.001), Distant metastasis (P = 0.001), lymph node status (P<0.001) and Fuhrman grade (P<0.001). Wip1 knockdown inhibited the proliferation, migration and invasion of 786-O and RLC-310 cells, whereas Wip1 overexpression promoted the growth and aggressive phenotype of 786-O and RLC-310 cells in vitro. The uni- and multivariate analyses indicated that expression of Wip1 was an independent predictor for survival of ccRCC patients (P = 0.003, P = 0.027 respectively). Wip1- negative patients had a higher tumor-free/overall survival rate than patients with high Wip1 expression (P = 0.001, P = 0.002 respectively). Overexpression of Wip1 is useful in the prediction of survival in ccRCC patients.

PPM1D silencing by RNA interference inhibits the proliferation of lung cancer cells

BACKGROUND

PPM1D (protein phosphatase, Mg2+/Mn2+ dependent, 1D) has been reported to be involved in multiple human tumors. This study was designed to investigate the functional role of PPM1D in lung cancer cells.

METHODS

Expression levels of PPM1D were analyzed in A549 and H1299 cells by real-time PCR and Western blotting. Lentivirus-mediated short hairpin RNA (shRNA) was used to knock down PPM1D expression in both cell lines. The effects of PPM1D on lung cancer cell growth were investigated by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide), colony formation and flow cytometry assays.

RESULTS

Knockdown of PPM1D in lung cancer cells resulted in decreased cell proliferation and impaired colony formation ability. Moreover, flow cytometry analysis showed that knockdown of PPM1D arrested cell cycle at the G0/G1 phase. Furthermore, PPM1D silencing downregulated the expression of cyclin B1 in H1299 cells. Therefore, it is reasonable to speculate that the mechanisms by which PPM1D knockdown alleviates cell growth may be partly via the induction of cell cycle arrest due to the suppression of cyclin B1.

CONCLUSIONS

These results suggest that PPM1D silencing by RNA interference (RNAi) may be a potential therapeutic approach for the treatment of lung cancer.

Wip1 abrogation decreases intestinal tumor frequency in APC(Min/+) mice irrespective of radiation quality

Low-linear energy transfer (low-LET) γ-ray exposure is a risk factor for colorectal cancer (CRC). Due to their high-LET nature, energetic iron ions found in space are expected to pose greater CRC risks to astronauts undertaking long-duration space missions beyond low Earth orbit. Wild-type p53-induced phosphatase 1 (Wip1) is important for cellular DNA damage response and its abrogation has been shown to inhibit spontaneous intestinal tumorigenesis in APC(Min/+) mice, a well-studied mouse model of human CRC. However, the relationship of Wip1 to radiation-induced intestinal tumorigenesis, especially by energetic iron ions, has not been investigated in APC(Min/+) mice. We have previously reported that there is a greater intestinal tumorigenic potential of iron-ion radiation relative to (137)Cs γ rays, so the purpose of the current study was to investigate whether Wip1 abrogation could influence high-LET dependent intestinal tumorigenesis in APC(Min/+) mice. Intestinal tumor frequency and grade were assessed in APC(Min/+)/Wip1(-/-) mice and results were compared to those in APC(Min/+)/Wip1(+/+) mice after exposure to a mean absorbed dose of 2 Gy from (137)Cs γ rays or 1.6 Gy from 1 GeV/n iron ions. Cellular differentiation and proliferation were also assessed in the intestinal tumors of sham-irradiated and irradiated mice. Decreased tumor frequency and lower tumor grade were observed in APC(Min/+)/Wip1(-/-) relative to APC(Min/+)/Wip1(+/+) mice. Notably, a similar decrease (∼6-fold in both groups) in tumor number was observed in sham-irradiated and γ-irradiated APC(Min/+)/Wip1(-/-) relative to APC(Min/+)/Wip1(+/+) mice. However, tumorigenesis in the energetic iron-ion exposed group was reduced ∼8-fold in APC(Min/+)/Wip1(-/-) relative to APC(Min/+)/Wip1(+/+) mice. A significantly lower proliferation/differentiation index in tumors of iron-ion exposed APC(Min/+)/Wip1(-/-) relative to APC(Min/+)/Wip1(+/+) mice suggests that reduced proliferation and enhanced differentiation as a result of Wip1 abrogation maybe involved. In conclusion, the current study demonstrated that the absence of Wip1 blocked radiation-induced intestinal tumorigenesis irrespective of radiation quality and has implications for developing preventive strategies against the tumorigenic potential of radiation exposure on earth and in outer space.

The WIP1 oncogene promotes progression and invasion of aggressive medulloblastoma variants

Recent studies suggest that medulloblastoma, the most common malignant brain tumor of childhood, is comprised of four disease variants. The WIP1 oncogene is overexpressed in Group 3 and 4 tumors, which contain medulloblastomas with the most aggressive clinical behavior. Our data demonstrate increased WIP1 expression in metastatic medulloblastomas, and inferior progression-free and overall survival of patients with WIP1 high-expressing medulloblastoma. Microarray analysis identified up-regulation of genes involved in tumor metastasis, including the G protein-coupled receptor CXCR4, in medulloblastoma cells with high WIP1 expression. Stimulation with the CXCR4 ligand SDF1ααactivated PI-3 kinase signaling, and promoted growth and invasion of WIP1 high-expressing medulloblastoma cells in a p53-dependent manner. When xenografted into the cerebellum of immunodeficient mice, medulloblastoma cells with stable or endogenous high WIP1 expression exhibited strong expression of CXCR4 and activated AKT in primary and invasive tumor cells. WIP1 or CXCR4 knock-down inhibited medulloblastoma growth and invasion. WIP1 knock-down also improved the survival of mice xenografted with WIP1 high-expressing medulloblastoma cells. WIP1 knock-down inhibited cell surface localization of CXCR4 by suppressing expression of the G protein receptor kinase 5, GRK5. Restoration of wild-type GRK5 promoted Ser339 phosphorylation of CXCR4 and inhibited the growth of WIP1-stable medulloblastoma cells. Conversely, GRK5 knock-down inhibited Ser339 phosphorylation of CXCR4, increased cell surface localization of CXCR4, and promoted the growth of medulloblastoma cells with low WIP1 expression. These results demonstrate cross-talk among WIP1, CXCR4, and GRK5, which may be important for the aggressive phenotype of a subclass of medulloblastomas in children.

Wild-type p53-induced phosphatase 1 (Wip1) forestalls cellular premature senescence at physiological oxygen levels by regulating DNA damage response signaling during DNA replication

Wip1 (protein phosphatase Mg(2+)/Mn(2+)-dependent 1D, Ppm1d) is a nuclear serine/threonine protein phosphatase that is induced by p53 following the activation of DNA damage response (DDR) signaling. Ppm1d(-/-) mouse embryonic fibroblasts (MEFs) exhibit premature senescence under conventional culture conditions; however, little is known regarding the role of Wip1 in regulating cellular senescence. In this study, we found that even at a representative physiological concentration of 3% O2, Ppm1d(-/-) MEFs underwent premature cellular senescence that depended on the functional activation of p53. Interestingly, Ppm1d(-/-) MEFs showed increased H2AX phosphorylation levels without increased levels of reactive oxygen species (ROS) or DNA base damage compared with wild-type (Wt) MEFs, suggesting a decreased threshold for DDR activation or sustained DDR activation during recovery. Notably, the increased H2AX phosphorylation levels observed in Ppm1d(-/-) MEFs were primarily associated with S-phase cells and predominantly dependent on the activation of ATM. Moreover, these same phenotypes were observed when Wt and Ppm1d(-/-) MEFs were either transiently or chronically exposed to low levels of agents that induce replication-mediated double-stranded breaks. These findings suggest that Wip1 prevents the induction of cellular senescence at physiological oxygen levels by attenuating DDR signaling in response to endogenous double-stranded breaks that form during DNA replication.

Genetic variants and mutations of PPM1D control the response to DNA damage

The Wip1 phosphatase is an oncogene that is overexpressed in a variety of primary human cancers. We were interested in identifying genetic variants that could change Wip1 activity. We identified 3 missense SNPs of the human Wip1 phosphatase, L120F, P322Q, and I496V confer a dominant-negative phenotype. On the other hand, in primary human cancers, PPM1D mutations commonly result in a gain-of-function phenotype, leading us to identify a hot-spot truncating mutation at position 525. Surprisingly, we also found a significant number of loss-of-function mutations of PPM1D in primary human cancers, both in the phosphatase domain and in the C terminus. Thus, PPM1D has evolved to generate genetic variants with lower activity, potentially providing a better fitness for the organism through suppression of multiple diseases. In cancer, however, the situation is more complex, and the presence of both activating and inhibiting mutations requires further investigation to understand their contribution to tumorigenesis.

Gain-of-function mutations of PPM1D/Wip1 impair the p53-dependent G1 checkpoint

Mutations in PPM1D/Wip1 phosphatase impair the DNA damage-induced checkpoint and may predispose cells to tumorigenesis.

The DNA damage response (DDR) pathway and its core component tumor suppressor p53 block cell cycle progression after genotoxic stress and represent an intrinsic barrier preventing cancer development. The serine/threonine phosphatase PPM1D/Wip1 inactivates p53 and promotes termination of the DDR pathway. Wip1 has been suggested to act as an oncogene in a subset of tumors that retain wild-type p53. In this paper, we have identified novel gain-of-function mutations in exon 6 of PPM1D that result in expression of C-terminally truncated Wip1. Remarkably, mutations in PPM1D are present not only in the tumors but also in other tissues of breast and colorectal cancer patients, indicating that they arise early in development or affect the germline. We show that mutations in PPM1D affect the DDR pathway and propose that they could predispose to cancer.

Molecular determinants of ovarian cancer chemoresistance: new insights into an old conundrum

Ovarian cancer is the most lethal gynecological malignancy. Cisplatin and its derivatives are first-line chemotherapeutics, and their resistance is a major hurdle in successful ovarian cancer treatment. Understanding the molecular dysregulation underlying chemoresistance is important for enhancing therapeutic outcome. Here, we review two established pathways in cancer chemoresistance. p53 is a major tumor suppressor regulating proliferation and apoptosis, and its mutation is a frequent event in human malignancies. The PI3K/Akt axis is a key oncogenic pathway regulating survival and tumorigenesis by controlling several tumor suppressors, including p53. The interplay between these pathways is well established, although the oncogenic phosphatase PPM1D adds a new layer to this intricate relationship and provides new insights into the processes determining cell fate. Inhibition of the PI3K/Akt pathway by functional food compounds as an adjunct to chemotherapeutics may tip the balance in favor of apoptosis rather than survival, enhancing therapeutic efficacy, and reducing side effects.

Wip1 and p53 contribute to HTLV-1 Tax-induced tumorigenesis

Background

Human T-cell Leukemia Virus type 1 (HTLV-1) infects 20 million individuals world-wide and causes Adult T-cell Leukemia/Lymphoma (ATLL), a highly aggressive T-cell cancer. ATLL is refractory to treatment with conventional chemotherapy and fewer than 10% of afflicted individuals survive more than 5 years after diagnosis. HTLV-1 encodes a viral oncoprotein, Tax, that functions in transforming virus-infected T-cells into leukemic cells. All ATLL cases are believed to have reduced p53 activity although only a minority of ATLLs have genetic mutations in their p53 gene. It has been suggested that p53 function is inactivated by the Tax protein.

Results

Using genetically altered mice, we report here that Tax expression does not achieve a functional equivalence of p53 inactivation as that seen with genetic mutation of p53 (i.e. a p53^−/− genotype). Thus, we find statistically significant differences in tumorigenesis between Tax⁺p53^+/+versus Tax⁺p53^−/− mice. We also find a role contributed by the cellular Wip1 phosphatase protein in tumor formation in Tax transgenic mice. Notably, Tax⁺Wip1^−/− mice show statistically significant reduced prevalence of tumorigenesis compared to Tax⁺Wip1^+/+ counterparts.

Conclusions

Our findings provide new insights into contributions by p53 and Wip1 in the in vivo oncogenesis of Tax-induced tumors in mice.

WIP1 phosphatase modulates the Hedgehog signaling by enhancing GLI1 function

The Hedgehog-GLI (HH-GLI) signaling plays a critical role in controlling growth and tissue patterning during embryogenesis and is implicated in a variety of human malignancies, including those of the skin. Phosphorylation events have been shown to regulate the activity of the GLI transcription factors, the final effectors of the HH-GLI signaling pathway. Here, we show that WIP1 (or PPM1D), an oncogenic phosphatase amplified/overexpressed in several types of human cancer, is a positive modulator of the HH signaling. Mechanistically, WIP1 enhances the function of GLI1 by increasing its transcriptional activity, nuclear localization and protein stability, but not of GLI2 nor GLI3. We also find that WIP1 and GLI1 are in a complex. Modulation of the transcriptional activity of GLI1 by WIP1 depends on the latter's phosphatase activity and, remarkably, does not require p53, a known WIP1 target. Functionally, we find that WIP1 is required for melanoma and breast cancer cell proliferation and self-renewal in vitro and melanoma xenograft growth induced by activation of the HH signaling. Pharmacological blockade of the HH pathway with the SMOOTHENED antagonist cyclopamine acts synergistically with inhibition of WIP1 in reducing growth of melanoma and breast cancer cells in vitro. Overall, our data uncover a role for WIP1 in modulating the activity of GLI1 and in sustaining cancer cell growth and cancer stem cell self-renewal induced by activation of the HH pathway. These findings open a novel therapeutic approach for human melanomas and, possibly, other cancer types expressing WIP1 and with activated HH pathway.

WIP1 enhances tumor formation in a sonic hedgehog-dependent model of medulloblastoma

BACKGROUND

A significant number of medulloblastomas (MBs) originate from abnormal activation of the sonic hedgehog/patched (SHH/PTC) signaling pathway. Although p53 deficiency enhances tumor formation in mice, inactivation of the p53 gene is seen in a minority of MBs. Wild-type p53-induced phosphatase 1 (WIP1) downregulates p53 expression and has been shown to be overexpressed in MBs.

OBJECTIVE

We tested the hypothesis that overexpression of WIP1 enhances tumor formation in an SHH-dependent model of MB.

METHODS

We used the RCAS/Ntv-a system to study the effect of WIP1 in vitro and in vivo. We transfected A375-TVA cells with RCAS-WIP1 and then exposed these cells to cisplatin to determine the effect on p53 expression. We modeled ectopic WIP1 expression independently and in combination with SHH in the cerebella of newborn mice to assess the effect on tumor formation. Mice were observed for 12 weeks or until neurological symptoms developed. The brains were examined for tumor formation.

RESULTS

A375-TVA cells infected with RCAS-WIP1 demonstrated reduced p53 expression after exposure to cisplatin compared with controls. We detected tumors in 12 of 35 mice (34%) injected with RCAS-WIP1 and RCAS-SHH. Tumors were detected in 3 of 40 mice (8%) injected with RCAS-SHH alone. The difference in tumor formation rates was significant (χ(2) test, P = < .01). Tumors did not form in mice injected with RCAS-WIP1 alone.

CONCLUSION

We show that ectopic expression of WIP1 cooperates with SHH to enhance formation of MB, although it is insufficient to induce tumors independently. Our results verify the role of WIP1 in MB formation and provide a crucial link to the inactivation of p53 in MBs.

Crosstalk between the DNA damage response pathway and microRNAs

MicroRNAs (miRNAs) are a family of small, non-coding RNAs that control gene expression at the post-transcriptional level by destabilizing and inhibiting translation of their target messenger RNAs. MiRNAs are involved in the regulation of a number of fundamental biological processes, and their dysregulation is thought to contribute to several disease processes. Emerging evidence suggests that miRNAs also play a critical role in protecting the heritable genome by contributing to the regulation of the DNA damage response. Consequently, much recent investigative effort has been directed towards an improved understanding of how miRNAs are regulated in response to DNA damage. In this review, we discuss the most recent findings regarding the regulation of miRNA expression and the functional roles of miRNAs in the DNA damage response.

Using mice to unveil the genetics of cancer resistance

In the UK, four in ten people will develop some form of cancer during their lifetime, with an individual's relative risk depending on many factors, including age, lifestyle and genetic make-up. Much research has gone into identifying the genes that are mutated in tumorigenesis with the overwhelming majority of genetically-modified (GM) mice in cancer research showing accelerated tumorigenesis or recapitulating key aspects of the tumorigenic process. Yet if six out of ten people will not develop some form of cancer during their lifetime, together with the fact that some cancer patients experience spontaneous regression/remission, it suggests there are ways of 'resisting' cancer. Indeed, there are wildtype, spontaneously-arising mutants and GM mice that show some form of 'resistance' to cancer. Identification of mice with increased resistance to cancer is a novel aspect of cancer research that is important in terms of providing both chemopreventative and therapeutic options. In this review we describe the different mouse lines that display a 'cancer resistance' phenotype and discuss the molecular basis of their resistance.

HDM2 promotes WIP1-mediated medulloblastoma growth

Medulloblastoma is the most common malignant childhood brain tumor. The protein phosphatase and oncogene WIP1 is over-expressed or amplified in a significant number of primary human medulloblastomas and cell lines. In the present study, we examine an important mechanism by which WIP1 promotes medulloblastoma growth using in vitro and in vivo models. Human cell lines and intracerebellar xenografted animal models were used to study the role of WIP1 and the major TP53 regulator, HDM2, in medulloblastoma growth. Stable expression of WIP1 enhances growth of TP53 wild-type medulloblastoma cells, compared with cells with stable expression of an empty-vector or mutant WIP1. In an animal model, WIP1 enhances proliferation and reduces the survival of immunodeficient mice bearing intracerebellar xenografted human medulloblastoma cells. Cells with increased WIP1 expression also exhibit increased expression of HDM2. HDM2 knockdown or treatment with the HDM2 inhibitor Nutlin-3a, the active enantomer of Nutlin-3, specifically inhibits the growth of medulloblastoma cells with increased WIP1 expression. Nutlin-3a does not affect growth of medulloblastoma cells with stable expression of an empty vector or of mutant WIP1. Knockdown of WIP1 or treatment with the WIP1 inhibitor CCT007093 results in increased phosphorylation of known WIP1 targets, reduced HDM2 expression, and reduced growth specifically in WIP1 wild-type and high-expressing medulloblastoma cells. Combined WIP1 and HDM2 inhibition is more effective than WIP1 inhibition alone in blocking growth of WIP1 high-expressing medulloblastoma cells. Our preclinical study supports a role for therapies that target WIP1 and HDM2 in the treatment of medulloblastoma.

Downregulation of Wip1 phosphatase modulates the cellular threshold of DNA damage signaling in mitosis

Cells are constantly challenged by DNA damage and protect their genome integrity by activation of an evolutionary conserved DNA damage response pathway (DDR). A central core of DDR is composed of a spatiotemporally ordered net of post-translational modifications, among which protein phosphorylation plays a major role. Activation of checkpoint kinases ATM/ATR and Chk1/2 leads to a temporal arrest in cell cycle progression (checkpoint) and allows time for DNA repair. Following DNA repair, cells re-enter the cell cycle by checkpoint recovery. Wip1 phosphatase (also called PPM1D) dephosphorylates multiple proteins involved in DDR and is essential for timely termination of the DDR. Here we have investigated how Wip1 is regulated in the context of the cell cycle. We found that Wip1 activity is downregulated by several mechanisms during mitosis. Wip1 protein abundance increases from G(1) phase to G(2) and declines in mitosis. Decreased abundance of Wip1 during mitosis is caused by proteasomal degradation. In addition, Wip1 is phosphorylated at multiple residues during mitosis, and this leads to inhibition of its enzymatic activity. Importantly, ectopic expression of Wip1 reduced γH2AX staining in mitotic cells and decreased the number of 53BP1 nuclear bodies in G(1) cells. We propose that the combined decrease and inhibition of Wip1 in mitosis decreases the threshold necessary for DDR activation and enables cells to react adequately even to modest levels of DNA damage encountered during unperturbed mitotic progression.

Wip1-dependent signaling pathways in health and diseases

Spatial and temporal regulation of protein phosphorylation is key to the control of different molecular networks. This regulation is achieved in part through dephosphorylation of numerous signaling molecules, and emerging evidence highlights the importance of a new member of the PP2C family of phosphatase, Wild-type p53 induced phosphatase 1 (Wip1), in regulating stress-induced and DNA damage-induced networks. In recent years, analysis of Wip1 has focused primarily on its role in tumorigenesis because of its overexpression in human tumors and a profound tumor-resistant phenotype of Wip1-deficient mice. Recently, Wip1 has also been shown to play an important role in several physiological processes including adult neurogenesis and organismal aging. This review addresses how Wip1 phosphatase regulates different signaling networks in a spatial and temporal manner and how these differences contribute to various biological outcomes in the context of physiological and pathological conditions.

A small molecule inhibitor of p53-inducible protein phosphatase PPM1D

PPM1D is a p53-inducible Ser/Thr protein phosphatase. PPM1D gene amplification and overexpression have been reported in a variety of human tumors, including breast cancer and neuroblastoma. Because the phosphatase activity of PPM1D is essential for its oncogenic role, PPM1D inhibitors should be viable anti-cancer agents. In our current study, we showed that SPI-001 was a potent and specific PPM1D inhibitor. SPI-001 inhibited PPM1D phosphatase activity in PPM1D-overexpressing human breast cancer cells and increased phosphorylation of p53. Furthermore, SPI-001 suppressed cell proliferation by inducing apoptosis. Our present study suggested that SPI-001 was a potential lead compound in developing anti-cancer drugs.

Checkpoint control and cancer

DNA-damaging therapies represent the most frequently used non-surgical anticancer strategies in the treatment of human tumors. These therapies can kill tumor cells, but at the same time they can be particularly damaging and mutagenic to healthy tissues. The efficacy of DNA-damaging treatments can be improved if tumor cell death is selectively enhanced, and the recent application of poly-(ADP-ribose) polymerase inhibitors in BRCA1/2-deficient tumors is a successful example of this. DNA damage is known to trigger cell-cycle arrest through activation of DNA-damage checkpoints. This arrest can be reversed once the damage has been repaired, but irreparable damage can promote apoptosis or senescence. Alternatively, cells can reenter the cell cycle before repair has been completed, giving rise to mutations. In this review we discuss the mechanisms involved in the activation and inactivation of DNA-damage checkpoints, and how the transition from arrest and cell-cycle re-entry is controlled. In addition, we discuss recent attempts to target the checkpoint in anticancer strategies.

Absence of Wip1 partially rescues Atm deficiency phenotypes in mice

Wild-type p53-induced phosphatase 1 (WIP1) is a serine/threonine phosphatase that dephosphorylates proteins in the ataxia telangiectasia mutated (ATM)-initiated DNA damage response pathway. WIP1 may have a homeostatic role in ATM signaling by returning the cell to a normal pre-stress state following completion of DNA repair. To better understand the effects of WIP1 on ATM signaling, we crossed Atm-deficient mice to Wip1-deficient mice and characterized phenotypes of the double knockout progeny. We hypothesized that the absence of Wip1 might rescue Atm deficiency phenotypes. Atm null mice, like ATM-deficient humans with the inherited syndrome ataxia telangiectasia, exhibit radiation sensitivity, fertility defects, and are T-cell lymphoma prone. Most double knockout mice were largely protected from lymphoma development and had a greatly extended lifespan compared with Atm null mice. Double knockout mice had increased p53 and H2AX phosphorylation and p21 expression compared with their Atm null counterparts, indicating enhanced p53 and DNA damage responses. Additionally, double knockout splenocytes displayed reduced chromosomal instability compared with Atm null mice. Finally, doubly null mice were partially rescued from gametogenesis defects observed in Atm null mice. These results indicate that inhibition of WIP1 may represent a useful strategy for cancer treatment in general and A-T patients in particular.

Phosphatase assay for multi-phosphorylated substrates using phosphatase specific-motif antibody

Protein phosphorylation plays central roles in a wide variety of signal transduction pathways and most phosphorylated proteins contain multi-phosphorylated sites. PPM1 type Ser/Thr protein phosphatase family is known to show rigid substrate specificity unlike other Ser/Thr phosphatase PPP family including PP1, PP2A and PP2B. PPM1 type phosphatases are reported to play important roles in growth regulation and in cellular stress signalling. In this study, we developed a phosphatase assay of PPM1D using phosphatase motif-specific antibody. PPM1D is a member of PPM1 type Ser/Thr phosphatase and known to dephosphorylate Ser(P)-Gln sequence. The gene amplification and overexpression of PPM1D were reported in many human cancers. We generated the monoclonal antibody specific for the Ser(P)-Gln sequence, named 3G9-H11. The specificity of this method using ELISA enables the convenient measurement of the dephosphorylation level of only PPM1D target residues of substrate peptides with multiple phosphorylated sites in the presence of multiple phosphatases. In addition, the antibody was applicable to immunoblotting assay for PPM1D function analysis. These results suggested that this method should be very useful for the PPM1D phosphatase assay, including high-throughput analysis and screening of specific inhibitors as anti-cancer drugs. The method using phosphatase motif-specific antibody can be applied to other PPM1 phosphatase family.

Oncogenic phosphatase Wip1 is a novel prognostic marker for lung adenocarcinoma patient survival

DNA damage response pathways are important for maintaining genomic stability. The oncogenic phosphatase Wip1 plays a crucial role in DNA damage response by inhibiting several cell cycle proteins, including p53. Although Wip1 gene amplification has been reported in various primary tumors, including lung cancer, its biological significance for survival of primary lung tumor patients remains unclear. We investigated the expression of Wip1 in cancer epithelial cells immunohistochemically in 84 consecutive resected cases of lung adenocarcinoma. Increased Wip1 expression was observed in 54 (64.3%) of the 84 cases. Wip1 expression was found to be correlated significantly with two clinicopathological factors: γ-H2AX expression, and invasion to the pulmonary vein. A univariate analysis and log-rank test indicated a significant association between Wip1 expression and lower overall survival rate (P = 0.019 and P = 0.0099, respectively). A multivariate analysis also indicated a statistically significant association between increased Wip1 expression and lower overall survival rate (hazard ratio, 4.3; P = 0.026). The Ki67 index level was higher in the Wip1-positive group than in the negative group (P < 0.04, Mann-Whitney U-test). Moreover, in a subgroup analysis of only stage I patients, increased Wip1 expression was also significantly associated with a lower overall survival rate (P = 0.023, log-rank test). These results indicate that the increased expression of Wip1 in cancer epithelial cells has significant value for tumor progression and the clinical prognosis of patients with primary lung adenocarcinoma.

Control of p53 and NF-κB signaling by WIP1 and MIF: role in cellular senescence and organismal aging

The stress-activated signaling pathways, p53 and NF-κB, have a major role in the regulation of cellular senescence and organismal aging. These ancient signaling networks display functional antagonism via negative autoregulatory circuits. WIP1 (wildtype p53-induced phosphatase 1) and MIF (macrophage migration inhibitory factor) are signaling molecules which link together the p53 and NF-κB pathways via positive and negative feedback loops. It seems that the efficiency of the p53 signaling pathway declines during aging whereas that of NF-κB is clearly enhanced. Moreover, p53 is an important trigger of cellular senescence while NF-κB signaling seems to be involved in the induction of the senescence-associated secretory phenotype (SASP). MIF is a pro-inflammatory cytokine which inhibits the function of p53 signaling whereas it is linked to NF-κB signaling via a positive feedback loop. MIF knockout mice are healthier and live longer than their wild-type counterparts. An increased level of MIF can support inflammatory responses via enhancing NF-κB signaling and repressing the function of p53. p53 is an inducer of the expression of WIP1 which can subsequently inhibit NF-κB signaling. Several observations indicate that the activity of WIP1 decreases during the aging process, this being probably attributable to the decline in p53 function. Decreased WIP1 activity potentiates the activity of p38MAPK and NF-κB signaling leading to premature cellular senescence as well as low-level chronic inflammation. We will review the findings linking WIP1 and MIF to specific signaling responses of p53 and NF-κB and discuss their role in the regulation of cellular senescence and organismal aging.

Dephosphorylation of γ-H2AX by WIP1: an important homeostatic regulatory event in DNA repair and cell cycle control

DNA double strand breaks are a particularly toxic form of DNA damage and the mammalian cell has evolved an intricate set of responses to repair this type of DNA lesion. A key early event in the DNA damage response (DDR) is ATM phosphorylation of the histone variant H2AX at serine 139 at the site of the DNA break. Phosphorylated S139 H2AX, or γ-H2AX, forms a docking site for binding of MDC1, leading to sustained recruitment of other DNA repair factors that mediate the repair of the DNA double strand break.  Moreover, recruitment of MDC1 to the break site activates cell cycle checkpoints, protecting the cell from replication of damaged DNA templates. While the molecular events leading to DNA double strand break repair have been well described, the deactivating or homeostatic mechanisms following completion of repair remain largely unexplored. Recent publications by our laboratories and the Medema laboratory shed new light on this issue. Both publications showed that the Wild-type p53-Induced Phosphatase 1 (WIP1) directly dephosphorylates γ-H2AX. WIP1 migrates to the sites of irradiation-induced foci (IRIF), though at a delayed rate relative to MDC1 and mediates γ-H2AX dephosphorylation, presumably after DNA repair is complete. This prevents recruitment of other repair factors such as MDC1 and 53BP1 to the DNA damage sites and promotes the dissolution of IRIF.  In addition, overexpression of WIP1 has a suppressive effect on DNA double strand break repair. Taken together, these reports further implicate WIP1 as a critical homeostatic regulator of the DDR.

Wip1 directly dephosphorylates gamma-H2AX and attenuates the DNA damage response

The integrity of DNA is constantly challenged throughout the life of a cell by both endogenous and exogenous stresses. A well-organized rapid damage response and proficient DNA repair, therefore, become critically important for maintaining genomic stability and cell survival. When DNA is damaged, the DNA damage response (DDR) can be initiated by alterations in chromosomal structure and histone modifications, such as the phosphorylation of the histone H2AX (the phosphorylated form is referred to as gamma-H2AX). gamma-H2AX plays a crucial role in recruiting DDR factors to damage sites for accurate DNA repair. On repair completion, gamma-H2AX must then be reverted to H2AX by dephosphorylation for attenuation of the DDR. Here, we report that the wild-type p53-induced phosphatase 1 (Wip1) phosphatase, which is often overexpressed in a variety of tumors, effectively dephosphorylates gamma-H2AX in vitro and in vivo. Ectopic expression of Wip1 significantly reduces the level of gamma-H2AX after ionizing as well as UV radiation. Forced premature dephosphorylation of gamma-H2AX by Wip1 disrupts recruitment of important DNA repair factors to damaged sites and delays DNA damage repair. Additionally, deletion of Wip1 enhances gamma-H2AX levels in cells undergoing constitutive oncogenic stress. Taken together, our studies show that Wip1 is an important mammalian phosphatase for gamma-H2AX and shows an additional mechanism for Wip1 in the tumor surveillance network.

The oncogenic phosphatase WIP1 negatively regulates nucleotide excision repair

Nucleotide excision repair (NER) is the only mechanism in humans to repair UV-induced DNA lesions such as pyrimidine (6-4) pyrimidone photoproducts and cyclobutane pyrimidine dimers (CPDs). In response to UV damage, the ataxia telangiectasia mutated and Rad3-related (ATR) kinase phosphorylates and activates several downstream effector proteins, such as p53 and XPA, to arrest cell cycle progression, stimulate DNA repair, or initiate apoptosis. However, following the completion of DNA repair, there must be active mechanisms that restore the cell to a prestressed homeostatic state. An important part of this recovery must include a process to reduce p53 and NER activity as well as to remove repair protein complexes from the DNA damage sites. Since activation of the damage response occurs in part through phosphorylation, phosphatases are obvious candidates as homeostatic regulators of the DNA damage and repair responses. Therefore, we investigated whether the serine/threonine wild-type p53-induced phosphatase 1 (WIP1/PPM1D) might regulate NER. WIP1 overexpression inhibits the kinetics of NER and CPD repair, whereas WIP1 depletion enhances NER kinetics and CPD repair. This NER suppression is dependent on WIP1 phosphatase activity, as phosphatase-dead WIP1 mutants failed to inhibit NER. Moreover, WIP1 suppresses the kinetics of UV-induced damage repair largely through effects on NER, as XPD-deficient cells are not further suppressed in repairing UV damage by overexpressed WIP1. Wip1 null mice quickly repair their CPD and undergo less UV-induced apoptosis than their wild-type counterparts. In vitro phosphatase assays identify XPA and XPC as two potential WIP1 targets in the NER pathway. Thus WIP1 may suppress NER kinetics by dephosphorylating and inactivating XPA and XPC and other NER proteins and regulators after UV-induced DNA damage is repaired.

Wild-type p53-induced phosphatase 1 dephosphorylates histone variant gamma-H2AX and suppresses DNA double strand break repair

In response to DNA double strand breaks, the histone variant H2AX at the break site is phosphorylated at serine 139 by DNA damage sensor kinases such as ataxia telangiectasia-mutated, forming gamma-H2AX. This phosphorylation event is critical for sustained recruitment of other proteins to repair the break. After repair, restoration of the cell to a prestress state is associated with gamma-H2AX dephosphorylation and dissolution of gamma-H2AX-associated damage foci. The phosphatases PP2A and PP4 have previously been shown to dephosphorylate gamma-H2AX. Here, we demonstrate that the wild-type p53-induced phosphatase 1 (WIP1) also dephosphorylates gamma-H2AX at serine 139 in vitro and in vivo. Overexpression of WIP1 reduces formation of gamma-H2AX foci in response to ionizing and ultraviolet radiation and blocks recruitment of MDC1 (mediator of DNA damage checkpoint 1) and 53BP1 (p53 binding protein 1) to DNA damage foci. Finally, these inhibitory effects of WIP1 on gamma-H2AX are accompanied by WIP1 suppression of DNA double strand break repair. Thus, WIP1 has a homeostatic role in reversing the effects of ataxia telangiectasia-mutated phosphorylation of H2AX.