Phosphorylation and degradation of MdmX is inhibited by Wip1 phosphatase in the DNA damage response

Cellular, Structural Basis, and Recent Progress for Targeting Murine Double Minute X (MDMX) in Tumors

Murine double minute X (MDMX) is an oncoprotein that mainly has a negative regulatory effect on the tumor suppressor p53 to induce tumorigenesis. As MDMX is highly expressed in various types of tumor cells, targeting and inhibiting MDMX are becoming a promising strategy for treating cancers. However, the high degree of structural homology between MDMX and its homologous protein murine double minute 2 (MDM2) is a great challenge for the development of MDMX-targeted therapies. This review introduces the structure, distribution, and regulation of the MDMX, summarizes the structural features and structure-activity relationships (SARs) of MDMX ligands, and focuses on the differences between MDMX and MDM2 in these aspects. Our purpose of this work is to propose potential strategies to achieve the specific targeting of MDMX.

Low WIP1 Expression Accelerates Ovarian Aging by Promoting Follicular Atresia and Primordial Follicle Activation

Our previous study demonstrated that ovarian wild-type P53-induced phosphatase 1 (WIP1) expression decreased with age. We hypothesized that WIP1 activity was related to ovarian aging. The role of WIP1 in regulating ovarian aging and its mechanisms remain to be elucidated. Adult female mice with or without WIP1 inhibitor (GSK2830371) treatment were divided into three groups (Veh, GSK-7.5, GSK-15) to evaluate the effect of WIP1 on ovarian endocrine and reproductive function and the ovarian reserve. In vitro follicle culture and primary granulosa cell culture were applied to explore the mechanisms of WIP1 in regulating follicular development. This study revealed that WIP1 expression in atretic follicle granulosa cells is significantly lower than that in healthy follicles. Inhibiting WIP1 phosphatase activity in mice induced irregular estrous cycles, caused fertility declines, and decreased the ovarian reserve through triggering excessive follicular atresia and primordial follicle activation. Primordial follicle depletion was accelerated via PI3K-AKT-rpS6 signaling pathway activation. In vitro follicle culture experiments revealed that inhibiting WIP1 activity impaired follicular development and oocyte quality. In vitro granulosa cell experiments further indicated that downregulating WIP1 expression promoted granulosa cell death via WIP1-p53-BAX signaling pathway-mediated apoptosis. These findings suggest that appropriate WIP1 expression is essential for healthy follicular development, and decreased WIP1 expression accelerates ovarian aging by promoting follicular atresia and primordial follicle activation.

DNA damage response revisited: the p53 family and its regulators provide endless cancer therapy opportunities

Antitumor therapeutic strategies that fundamentally rely on the induction of DNA damage to eradicate and inhibit the growth of cancer cells are integral approaches to cancer therapy. Although DNA-damaging therapies advance the battle with cancer, resistance, and recurrence following treatment are common. Thus, searching for vulnerabilities that facilitate the action of DNA-damaging agents by sensitizing cancer cells is an active research area. Therefore, it is crucial to decipher the detailed molecular events involved in DNA damage responses (DDRs) to DNA-damaging agents in cancer. The tumor suppressor p53 is active at the hub of the DDR. Researchers have identified an increasing number of genes regulated by p53 transcriptional functions that have been shown to be critical direct or indirect mediators of cell fate, cell cycle regulation, and DNA repair. Posttranslational modifications (PTMs) primarily orchestrate and direct the activity of p53 in response to DNA damage. Many molecules mediating PTMs on p53 have been identified. The anticancer potential realized by targeting these molecules has been shown through experiments and clinical trials to sensitize cancer cells to DNA-damaging agents. This review briefly acknowledges the complexity of DDR pathways/networks. We specifically focus on p53 regulators, protein kinases, and E3/E4 ubiquitin ligases and their anticancer potential.

Resistance mechanisms to inhibitors of p53-MDM2 interactions in cancer therapy: can we overcome them?

Since the discovery of the first MDM2 inhibitors, we have gained deeper insights into the cellular roles of MDM2 and p53. In this review, we focus on MDM2 inhibitors that bind to the p53-binding domain of MDM2 and aim to disrupt the binding of MDM2 to p53. We describe the basic mechanism of action of these MDM2 inhibitors, such as nutlin-3a, summarise the determinants of sensitivity to MDM2 inhibition from p53-dependent and p53-independent points of view and discuss the problems with innate and acquired resistance to MDM2 inhibition. Despite progress in MDM2 inhibitor design and ongoing clinical trials, their broad use in cancer treatment is not fulfilling expectations in heterogenous human cancers. We assess the MDM2 inhibitor types in clinical trials and provide an overview of possible sources of resistance to MDM2 inhibition, underlining the need for patient stratification based on these aspects to gain better clinical responses, including the use of combination therapies for personalised medicine.

Pharmacological Inhibition of WIP1 Sensitizes Acute Myeloid Leukemia Cells to the MDM2 Inhibitor Nutlin-3a

In acute myeloid leukemia (AML), the restoration of p53 activity through MDM2 inhibition proved efficacy in combinatorial therapies. WIP1, encoded from PPM1D, is a negative regulator of p53. We evaluated PPM1D expression and explored the therapeutic efficacy of WIP1 inhibitor (WIP1i) GSK2830371, in association with the MDM2 inhibitor Nutlin-3a (Nut-3a) in AML cell lines and primary samples. PPM1D transcript levels were higher in young patients compared with older ones and in core-binding-factor AML compared with other cytogenetic subgroups. In contrast, its expression was reduced in NPM1-mutated (mut, irrespective of FLT3-ITD status) or TP53-mut cases compared with wild-type (wt) ones. Either Nut-3a, and moderately WIP1i, as single agent decreased cell viability of TP53-wt cells (MV-4-11, MOLM-13, OCI-AML3) in a time/dosage-dependent manner, but not of TP53-mut cells (HEL, KASUMI-1, NOMO-1). The drug combination synergistically reduced viability and induced apoptosis in TP53-wt AML cell line and primary cells, but not in TP53-mut cells. Gene expression and immunoblotting analyses showed increased p53, MDM2 and p21 levels in treated TP53-wt cells and highlighted the enrichment of MYC, PI3K-AKT-mTOR and inflammation-related signatures upon WIP1i, Nut-3a and their combination, respectively, in the MV-4-11 TP53-wt model. This study demonstrated that WIP1 is a promising therapeutic target to enhance Nut-3a efficacy in TP53-wt AML.

Phosphatase magnesium-dependent 1 δ (PPM1D), serine/threonine protein phosphatase and novel pharmacological target in cancer

Aberrations in DNA damage response genes are recognized mediators of tumorigenesis and resistance to chemo- and radiotherapy. While protein phosphatase magnesium-dependent 1 δ (PPM1D), located on the long arm of chromosome 17 at 17q22-23, is a key regulator of cellular responses to DNA damage, amplification, overexpression, or mutation of this gene is important in a wide range of pathologic processes. In this review, we describe the physiologic function of PPM1D, as well as its role in diverse processes, including fertility, development, stemness, immunity, tumorigenesis, and treatment responsiveness. We highlight both the advances and limitations of current approaches to targeting malignant processes mediated by pathogenic alterations in PPM1D with the goal of providing rationale for continued research and development of clinically viable treatment approaches for PPM1D-associated diseases.

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.

The biological function and the regulatory roles of wild-type p53-induced phosphatase 1 in immune system

Wild-type p53-induced phosphatase 1 (WIP1) belongs to the protein phosphatase 2C (PP2C) family and is a mammalian serine/threonine specific protein phosphatase to dephosphorylate numerous signaling molecules. Mammalian WIP1 regulates a wide array of targeting molecules and plays key regulatory roles in many cell processes such as DNA damage and repair, cell proliferation, differentiation, apoptosis, and senescence. WIP1 promotes the formation and development of tumors as an oncogene and a negative regulator of p53. It is also involved in the regulation of aging, neurological diseases and immune diseases. Recent studies demonstrated the critical roles of WIP1 in the differentiation and function of immune cells including T cells, neutrophils and macrophages. In the present manuscript, we briefly summarized the expression patterns, biological function and the target molecules and signal pathways of WIP1 and mainly discussed the latest advances on the regulatory effects of WIP1 in the immune system. WIP1 may be a potential target molecule to treat cancers and immune diseases such as allergic asthma.

The long and the short of it: the MDM4 tail so far

The mouse double minute 4 (MDM4) is emerging from the shadow of its more famous relative MDM2 and is starting to steal the limelight, largely due to its therapeutic possibilities. MDM4 is a vital regulator of the tumor suppressor p53. It restricts p53 transcriptional activity and also, at least in development, facilitates MDM2's E3 ligase activity toward p53. These functions of MDM4 are critical for normal cell function and a proper response to stress. Their importance for proper cell maintenance and proliferation identifies them as a risk for deregulation associated with the uncontrolled growth of cancer. MDM4 tails are vital for its function, where its N-terminus transactivation domain engages p53 and its C-terminus RING domain binds to MDM2. In this review, we highlight recently identified cellular functions of MDM4 and survey emerging therapies directed to correcting its dysregulation in disease.

Inhibition of WIP1 phosphatase sensitizes breast cancer cells to genotoxic stress and to MDM2 antagonist nutlin-3

PP2C family serine/threonine phosphatase WIP1 acts as a negative regulator of the tumor suppressor p53 and is implicated in silencing of cellular responses to genotoxic stress. Chromosomal locus 17q23 carrying the PPM1D (coding for WIP1) is commonly amplified in breast carcinomas and WIP1 was proposed as potential pharmacological target. Here we employed a cellular model with knocked out PPM1D to validate the specificity and efficiency of GSK2830371, novel small molecule inhibitor of WIP1. We have found that GSK2830371 increased activation of the DNA damage response pathway to a comparable level as the loss of PPM1D. In addition, GSK2830371 did not affect proliferation of cells lacking PPM1D but significantly supressed proliferation of breast cancer cells with amplified PPM1D. Over time cells treated with GSK2830371 accumulated in G1 and G2 phases of the cell cycle in a p21-dependent manner and were prone to induction of senescence by a low dose of MDM2 antagonist nutlin-3. In addition, combined treatment with GSK2830371 and doxorubicin or nutlin-3 potentiated cell death through a strong induction of p53 pathway and activation of caspase 9. We conclude that efficient inhibition of WIP1 by GSK2830371 sensitizes breast cancer cells with amplified PPM1D and wild type p53 to chemotherapy.

Role of wild-type p53-induced phosphatase 1 in cancer

Wild-type p53-induced phosphatase (Wip1) is a member of the protein phosphatase type 2C family and is an established oncogene due to its dephosphorylation of several tumor suppressors and negative control of the DNA damage response system. It has been reported to dephosphorylate p53, ataxia telangiectasia mutated, checkpoint kinase 1 and p38 mitogen activated protein kinases, forming negative feedback loops to inhibit apoptosis and cell cycle arrest. Wip1 serves a major role in tumorigenesis, progression, invasion, distant metastasis and apoptosis in various types of human cancer. Therefore, it may be a potential biomarker and therapeutic target in the diagnosis and treatment of cancer. Furthermore, previous evidence has revealed a new role for Wip1 in the regulation of chemotherapy resistance. In the present review, the current knowledge on the role of Wip1 in cancer is discussed, as well as its potential as a novel target for cancer treatment and its function in chemotherapy resistance.

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.

Intra molecular interactions in the regulation of p53 pathway

The p53 tumor suppressor is highly regulated at the level of protein degradation and transcriptional activity. The key players of the pathway, p53, MDM2, and MDMX are present at multiple conformational states that are responsive to regulation by post-translational modifications and protein-protein interactions. The structures of major functional domains of these proteins have been determined, but the mechanisms of several intrinsically disordered regions remain unclear despite their critical roles in signaling and regulation. Recent studies suggest that these disordered regions function in part by dynamic intra molecular interactions with the structured domains to regulate p53 DNA binding, MDM2 ubiquitin E3 ligase activity, and MDMX-p53 binding. These findings provide new insight on how p53 is controlled by various stress signals, and suggest potential targets for the search of allosteric regulators of the p53 pathway.

MDMX under stress: the MDMX-MDM2 complex as stress signals hub

The tumor suppressor p53 plays a central role in safeguarding cellular homeostasis. Upon various types of stress signals such as DNA damage or oncogenic stress, p53 is promptly activated to prevent and repair damages that can threaten the genome stability. The two major negative regulators of p53 are MDM2 and MDMX, two homolog proteins that control p53 activity and turnover, hence keeping it in check during normal cell cycling. In the event of cellular stress, they have to be inhibited in order to relieve p53 from their suppression and allow its activation. As the essential upstream modulator of p53, the MDMX-MDM2 complex integrates multiple signaling pathways regulating p53 response to perturbations of cellular homeostasis. Given its predominantly cytoplasmic localization in normal conditions, we hypothesize that MDMX, rather than MDM2, is the first recipient of signaling cues directed towards the MDMX-MDM2 complex and aimed at modulating p53. In this review we give a synthetic overview of the phosphorylation sites of MDMX that are known to affect its degradation, ubiquitination, intracellular localization and interaction with MDM2 and p53, ultimately modulating the stability and activity of p53. The role of MDMX in response to the main types of cellular stress is also briefly discussed, along with the potential of the MDMX-MDM2 complex as therapeutic target to restore p53 activity.

Chemical Inhibition of Wild-Type p53-Induced Phosphatase 1 (WIP1/PPM1D) by GSK2830371 Potentiates the Sensitivity to MDM2 Inhibitors in a p53-Dependent Manner

Sensitivity to MDM2 inhibitors is widely different among responsive TP53 wild-type cell lines and tumors. Understanding the determinants of MDM2 inhibitor sensitivity is pertinent for their optimal clinical application. Wild-type p53-inducible phosphatase-1 (WIP1) encoded by PPM1D, is activated, gained/amplified in a range of TP53 wild-type malignancies, and is involved in p53 stress response homeostasis. We investigated cellular growth/proliferation of TP53 wild-type and matched mutant/null cell line pairs, differing in PPM1D genetic status, in response to Nutlin-3/RG7388 ± a highly selective WIP1 inhibitor, GSK2830371. We also assessed the effects of GSK2830371 on MDM2 inhibitor-induced p53(Ser15) phosphorylation, p53-mediated global transcriptional activity, and apoptosis. The investigated cell line pairs were relatively insensitive to single-agent GSK2830371. However, a non-growth-inhibitory dose of GSK2830371 markedly potentiated the response to MDM2 inhibitors in TP53 wild-type cell lines, most notably in those harboring PPM1D-activating mutations or copy number gain (up to 5.8-fold decrease in GI50). Potentiation also correlated with significant increase in MDM2 inhibitor-induced cell death endpoints that were preceded by a marked increase in a WIP1 negatively regulated substrate, phosphorylated p53(Ser15), known to increase p53 transcriptional activity. Microarray-based gene expression analysis showed that the combination treatment increases the subset of early RG7388-induced p53 transcriptional target genes. These findings demonstrate that potent and selective WIP1 inhibition potentiates the response to MDM2 inhibitors in TP53 wild-type cells, particularly those with PPM1D activation or gain, while highlighting the mechanistic importance of p53(Ser15) and its potential use as a biomarker for response to this combination regimen.

Targeting tumor suppressor genes for cancer therapy

Cancer drugs are broadly classified into two categories: cytotoxic chemotherapies and targeted therapies that specifically modulate the activity of one or more proteins involved in cancer. Major advances have been achieved in targeted cancer therapies in the past few decades, which is ascribed to the increasing understanding of molecular mechanisms for cancer initiation and progression. Consequently, monoclonal antibodies and small molecules have been developed to interfere with a specific molecular oncogenic target. Targeting gain-of-function mutations, in general, has been productive. However, it has been a major challenge to use standard pharmacologic approaches to target loss-of-function mutations of tumor suppressor genes. Novel approaches, including synthetic lethality and collateral vulnerability screens, are now being developed to target gene defects in p53, PTEN, and BRCA1/2. Here, we review and summarize the recent findings in cancer genomics, drug development, and molecular cancer biology, which show promise in targeting tumor suppressors in cancer therapeutics.

Regulation of MDM2 Stability After DNA Damage

Cells in our body are constantly exposed to various stresses and threats to their genomic integrity. The tumor suppressor protein p53 plays a critical role in successful defense against these threats by inducing apoptotic cell death or cell cycle arrest. In unstressed conditions, p53 levels and activity must be kept low to prevent lethal activation of apoptotic and senescence pathways. However, upon DNA damage or other stressors, p53 is released from its inhibitory state to induce an array of apoptosis and cell cycle genes. Conversely, inactivation of p53 could promote unrestrained tumor proliferation and failure to appropriately undergo apoptotic cell death, which could, in turn, lead to carcinogenesis. The ubiquitin E3 ligase MDM2 is the most critical inhibitor of p53 that determines the cellular response to various p53-activating agents, including DNA damage. MDM2 activity is controlled by post-translational modifications, especially phosphorylation. However, accumulating evidence suggests that MDM2 is also regulated at the level of protein stability, which is controlled by the ubiquitin-proteasome pathway. Here, we discuss how MDM2 can be regulated in response to DNA damage with particular focus on the regulation of MDM2 protein stability.

Wip1 phosphatase in breast cancer

Understanding the factors contributing to tumor initiation, progression and evolution is of paramount significance. Among them, wild-type p53-induced phosphatase 1 (Wip1) is emerging as an important oncogene by virtue of its negative control on several key tumor suppressor pathways. Originally discovered as a p53-regulated gene, Wip1 has been subsequently found amplified and more recently mutated in a significant fraction of human cancers including breast tumors. Recent development in the field further uncovered the utility of anti-Wip1-directed therapies in delaying tumor onset or in reducing the tumor burden. Furthermore, Wip1 could be an important factor that contributes to tumor heterogeneity, suggesting that its inhibition may decrease the rate of cancer evolution. These effects depend on several signaling pathways modulated by Wip1 phosphatase in a spatial and temporal manner. In this review we discuss the recent development in understanding how Wip1 contributes to tumorigenesis with its relevance to breast cancer.

Akt confers cisplatin chemoresistance in human gynecological carcinoma cells by modulating PPM1D stability

Ovarian cancer (OVCA) and cervical cancer (CECA) are lethal gynecological malignancies. Cisplatin (CDDP) and platinum derivatives are first line chemotherapeutics and their resistance impedes successful treatment. Understanding the molecular dysregulation underlying chemoresistance is important in developing rational therapeutic strategies. We have established that Protein Phosphatase Magnesium-dependent 1 D (PPM1D) confers CDDP resistance in gynecological cancer cells by deactivating p53. However, whether CDDP regulates intra-cellular PPM1D localization and whether this regulation is different between chemosensitive and chemoresistant cancer cells is unknown. Moreover, whether Akt regulates PPM1D in the context of CDDP resistance has not been studied. To illustrate the role of PPM1D in gynecological cancer cell chemoresistance and its regulation by Akt we have demonstrated that: (a) CDDP induced PPM1D down-regulation through proteasomal degradation in sensitive CECA cells; (b) CDDP induced PPM1D nuclear localization in resistant CECA cells, and nuclear exclusion in sensitive CECA cells and OVCA xenografts; (c) Over-expression of active Akt in sensitive CECA cells stabilized PPM1D content through inhibition of CDDP-induced PPM1D down-regulation; (d) Inhibition of Akt activity in resistant OVCA cells leads to decreased PPM1D stability and CDDP-induced down-regulation in resistant CECA cells; and (e) PPM1D is highly expressed in human ovarian tumor subtypes and in a tissue microarray panel of human ovarian tumors. In conclusion, we have established that PPM1D plays an important role in promoting CDDP resistance and as a novel downstream target of Akt, PPM1D mediates its action in conferring CDDP resistance in gynecological cancer cells.

Ionizing radiation-induced responses in human cells with differing TP53 status

Ionizing radiation triggers diverse responses in human cells encompassing apoptosis, necrosis, stress-induced premature senescence (SIPS), autophagy, and endopolyploidy (e.g., multinucleation). Most of these responses result in loss of colony-forming ability in the clonogenic survival assay. However, not all modes of so-called clonogenic cell "death" are necessarily advantageous for therapeutic outcome in cancer radiotherapy. For example, the crosstalk between SIPS and autophagy is considered to influence the capacity of the tumor cells to maintain a prolonged state of growth inhibition that unfortunately can be succeeded by tumor regrowth and disease recurrence. Likewise, endopolyploid giant cells are able to segregate into near diploid descendants that continue mitotic activities. Herein we review the current knowledge on the roles that the p53 and p21(WAF1) tumor suppressors play in determining the fate of human fibroblasts (normal and Li-Fraumeni syndrome) and solid tumor-derived cells after exposure to ionizing radiation. In addition, we discuss the important role of WIP1, a p53-regulated oncogene, in the temporal regulation of the DNA damage response and its contribution to p53 dynamics post-irradiation. This article highlights the complexity of the DNA damage response and provides an impetus for rethinking the nature of cancer cell resistance to therapeutic agents.

Cisplatin causes cell death via TAB1 regulation of p53/MDM2/MDMX circuitry

The interdependence of p53 and MDM2 is critical for proper cell survival and cell death. Zhu et al. find that TAB1, an activator of TAK1 and p38α, inhibits the E3 ligase activity of MDM2 toward p53 and its homolog, MDMX. Cisplatin-induced cell death is mitigated by TAB1 knockdown. TAB1 stabilizes MDMX and activates p38α to phosphorylate p53, allowing p53 target induction. TAB1 levels are relatively low in cisplatin-resistant clones of ovarian cells and in ovarian tumors, implicating TAB1 as a tumor suppressor.

The interdependence of p53 and MDM2 is critical for proper cell survival and cell death and, when altered, can lead to tumorigenesis. Mitogen-activated protein kinase (MAPK) signaling pathways function in a wide variety of cellular processes, including cell growth, migration, differentiation, and death. Here we discovered that transforming growth factor β-activated kinase 1 (TAK1)-binding protein 1 (TAB1), an activator of TAK1 and of p38α, associates with and inhibits the E3 ligase activity of MDM2 toward p53 and its homolog, MDMX. Depletion of TAB1 inhibits MDM2 siRNA-mediated p53 accumulation and p21 induction, partially rescuing cell cycle arrest induced by MDM2 ablation. Interestingly, of several agents commonly used as DNA-damaging therapeutics, only cell death caused by cisplatin is mitigated by knockdown of TAB1. Two mechanisms are required for TAB1 to regulate apoptosis in cisplatin-treated cells. First, p38α is activated by TAB1 to phosphorylate p53 N-terminal sites, leading to selective induction of p53 targets such as NOXA. Second, MDMX is stabilized in a TAB1-dependent manner and is required for cell death after cisplatin treatment. Interestingly TAB1 levels are relatively low in cisplatin-resistant clones of ovarian cells and in ovarian patient's tumors compared with normal ovarian tissue. Together, our results indicate that TAB1 is a potential tumor suppressor that serves as a functional link between p53–MDM2 circuitry and a key MAPK signaling pathway.

The chromatin response to DNA breaks: leaving a mark on genome integrity

Genetic, biochemical, and cellular studies have uncovered many of the molecular mechanisms underlying the signaling and repair of chromosomal DNA breaks. However, efficient repair of DNA damage is complicated in that genomic DNA is packaged, through histone and nonhistone proteins, into chromatin. The DNA repair machinery has to overcome this physical barrier to gain access to damaged DNA and repair DNA lesions. Posttranslational modifications of chromatin as well as ATP-dependent chromatin remodeling factors help to overcome this barrier and facilitate access to damaged DNA by altering chromatin structure at sites of DNA damage. Here we review and discuss our current knowledge of and recent advances in chromatin changes induced by chromosome breakage in mammalian cells and their implications for genome stability and human disease.

MDM2, MDMX and p53 in oncogenesis and cancer therapy

The MDM2 and MDMX (also known as HDMX and MDM4) proteins are deregulated in many human cancers and exert their oncogenic activity predominantly by inhibiting the p53 tumour suppressor. However, the MDM proteins modulate and respond to many other signalling networks in which they are embedded. Recent mechanistic studies and animal models have demonstrated how functional interactions in these networks are crucial for maintaining normal tissue homeostasis, and for determining responses to oncogenic and therapeutic challenges. This Review highlights the progress made and pitfalls encountered as the field continues to search for MDM-targeted antitumour agents.

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.

Protein phosphatase 1 inhibits p53 signaling by dephosphorylating and stabilizing Mdmx

The activation and stabilization of the p53 protein play a major role in the DNA damage response. Protein levels of p53 are tightly controlled by transcriptional regulation and a number of positive and negative posttranslational modifiers, including kinases, phosphatases, E3 ubiquitin ligases, deubiquitinases, acetylases and deacetylases. One of the primary p53 regulators is Mdmx. Despite its RING domain and structural similarity with Mdm2, Mdmx does not have an intrinsic ligase activity, but inhibits the transcriptional activity of p53. Previous studies reported that Mdmx is phosphorylated and destabilized in response to DNA damage stress. Three phosphorylation sites identified are Ser342, Ser367, and Ser403. In the present study, we identify protein phosphatase 1 (PP1) as a negative regulator in the p53 signaling pathway. PP1 directly interacts with Mdmx and specifically dephosphorylates Mdmx at Ser367. The dephosphorylation of Mdmx increases its stability and thereby inhibits p53 activity. Our results suggest that PP1 is a crucial component in the ATM-Chk2-p53 signaling pathway.

Attractor landscape analysis reveals feedback loops in the p53 network that control the cellular response to DNA damage

The protein p53 functions as a tumor suppressor and can trigger either cell cycle arrest or apoptosis in response to DNA damage. We used Boolean network modeling and attractor landscape analysis to analyze the state transition dynamics of a simplified p53 network for which particular combinations of activation states of the molecules corresponded to specific cellular outcomes. Our results identified five critical interactions in the network that determined the cellular response to DNA damage, and simulations lacking any of these interactions produced states associated with sustained p53 activity, which corresponded to a cell death response. Attractor landscape analysis of the cellular response to DNA damage of the breast cancer cell line MCF7 and the effect of the Mdm2 (murine double minute 2) inhibitor nutlin-3 indicated that nutlin-3 would exhibit limited efficacy in triggering cell death, because the cell death state was not induced to a large extent by simulations with nutlin-3 and instead produced a state consistent with oscillatory p53 dynamics and cell cycle arrest. Attractor landscape analysis also suggested that combining nutlin-3 with inhibition of Wip1 would synergize to stimulate a sustained increase in p53 activity and promote p53-mediated cell death. We validated this synergistic effect in stimulating p53 activity and triggering cell death with single-cell imaging of a fluorescent p53 reporter in MCF7 cells. Thus, attractor landscape analysis of p53 network dynamics and its regulation can identify potential therapeutic strategies for treating cancer.

The Roles of MDM2 and MDMX Phosphorylation in Stress Signaling to p53

The p53 tumor suppressor is highly responsive to different physiological stresses such as abnormal cell proliferation, nutrient deprivation, and DNA damage. Distinct signaling mechanisms have evolved to activate p53, which in turn modulate numerous pathways to enhance fitness and survival of the organism. Elucidating the molecular mechanisms of these signaling events is critical for understanding tumor suppression by p53 and development of novel therapeutics. Studies in the past decade have established that MDM2 and MDMX are important targets of signaling input from different pathways. Here, we focus our discussion on MDM2 and MDMX phosphorylation, which is important for p53 activation by DNA damage. Investigations in this area have generated new insight into the inner workings of MDM2 and MDMX and underscore the importance of allosteric communication between different domains in achieving an efficient response to phosphorylation. It is likely that MDM2 and MDMX regulation by phosphorylation will share mechanistic similarities to other signaling hub molecules. Phosphorylation-independent p53 activators such as ARF and ribosomal proteins ultimately achieve the same outcome as phosphorylation, suggesting that they may induce similar changes in the structure and function of MDM2 and MDMX through protein-protein interactions.

Regulation of p53: a collaboration between Mdm2 and Mdmx

p53 plays an important role in the regulation of the cell cycle, DNA repair, and apoptosis and is an attractive cancer therapeutic target. Mdm2 and Mdmx are recognized as the main p53 negative regulators. Although it is still unknown why Mdm2 and Mdmx both are required for p53 degradation, a model has been proposed whereby these two proteins function independent of one another; Mdm2 acts as an E3 ubiquitin ligase that catalyzes the ubiquitination of p53 for degradation, whereas Mdmx inhibits p53 by binding to and masking the transcriptional activation domain of p53, without causing its degradation. However, Mdm2 and Mdmx have been shown to function collaboratively. In fact, recent studies have pointed to a more important role for an Mdm2/Mdmx co-regulatory mechanism for p53 regulation than previously thought. In this review, we summarize current progress in the field about the functional and physical interactions between Mdm2 and Mdmx, their individual and collaborative roles in controlling p53, and inhibitors that target Mdm2 and Mdmx as a novel class of anticancer therapeutics.

A prognostic signature of defective p53-dependent G1 checkpoint function in melanoma cell lines

Melanoma cell lines and normal human melanocytes (NHM) were assayed for p53-dependent G1 checkpoint response to ionizing radiation (IR)-induced DNA damage. Sixty-six percent of melanoma cell lines displayed a defective G1 checkpoint. Checkpoint function was correlated with sensitivity to IR with checkpoint-defective lines being radio-resistant. Microarray analysis identified 316 probes whose expression was correlated with G1 checkpoint function in melanoma lines (P≤0.007) including p53 transactivation targets CDKN1A, DDB2, and RRM2B. The 316 probe list predicted G1 checkpoint function of the melanoma lines with 86% accuracy using a binary analysis and 91% accuracy using a continuous analysis. When applied to microarray data from primary melanomas, the 316 probe list was prognostic of 4-yr distant metastasis-free survival. Thus, p53 function, radio-sensitivity, and metastatic spread may be estimated in melanomas from a signature of gene expression.

p53-Independent expression of wild-type p53-induced phosphatase 1 (Wip1) in methylmethane sulfonate-treated cancer cell lines and human tumors

Wild-type p53-induced phosphatase 1 (Wip1, PPM1D) is induced by p53 in response to various stressors and dephosphorylates cellular target proteins involved in DNA repair and cell cycle checkpoint pathways. The Wip1 gene is frequently amplified or overexpressed in human cancers, promoting tumor growth by switching off major checkpoint kinases and p53. To explore wild-type p53-independent Wip1 induction, Wip1 promoter activity and its transcript level were evaluated by luciferase assay and real-time PCR, after methylmethane sulfonate (MMS) treatment in breast cancer cell lines and p53-null cell lines. Wip1 promoter activities in response to UV irradiation and various anti-cancer agents were compared between wild-type and a p53-response element (p53RE) mutated construct. Wip1 expression and its effects were examined in primary non-small cell lung cancer (NSCLC) and colon tumor cells by using Wip1-specific siRNA. MMS induced Wip1 promoter activity in Hs578T, MDA-MB-231, and SK-BR-3 cells expressing DNA binding-deficient p53 mutants. A549-E6 and HCT116 (p53(-/-)) cells retained substantial Wip1 induction. Wip1 promoter activity was reduced, but not eliminated, in cells expressing a promoter containing a mutated p53-response element. Wip1 induction was not blocked by SB202190 or SP600125. MMS increased Wip1 expression in primary non-small cell lung cancer cells expressing a p53 R175H mutant. Our data indicate that Wip1 is induced in the absence of functional p53, like p38 MAPK and JNK, as a stress response terminator.

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.

Regulation of the Wip1 phosphatase and its effects on the stress response

Wip1 (PPM1D) is a stress responsive PP2C phosphatase that plays a key role in stress signaling. Although originally identified as a gene induced by p53 after genotoxic stress, we now know that Wip1 expression is additionally regulated by other mechanisms. Wip1 is not only a target of p53, but is also a target of other transcription factors, including Estrogen Receptor-alpha and NF-kappaB. Additionally, Wip1 expression is regulated by post-transcriptional mechanisms such as mRNA stabilization and alternative splicing. Upon induction, Wip1 dampens the stress response by dephosphorylating and inactivating proteins such as p53, p38 MAPK, and ATM, usually as part of a negative feedback loop. As a result, Wip1 functions to abrogate cell cycle checkpoints and inhibit senescence, apoptosis, DNA repair, and the production of inflammatory cytokines. Furthermore, Wip1 is overexpressed in several types of human cancers and has oncogenic functions. The regulation of Wip1, the role of Wip1 in stress signaling, and the cooperation of Wip1 with oncogenes in promoting tumorigenesis will be discussed in this review.

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.

Abrogation of Wip1 expression by RITA-activated p53 potentiates apoptosis induction via activation of ATM and inhibition of HdmX

Inactivation of the p53 tumour suppressor, either by mutation or by overexpression of its inhibitors Hdm2 and HdmX is the most frequent event in cancer. Reactivation of p53 by targeting Hdm2 and HdmX is therefore a promising strategy for therapy. However, Hdm2 inhibitors do not prevent inhibition of p53 by HdmX, which impedes p53-mediated apoptosis. Here, we show that p53 reactivation by the small molecule RITA leads to efficient HdmX degradation in tumour cell lines of different origin and in xenograft tumours in vivo. Notably, HdmX degradation occurs selectively in cancer cells, but not in non-transformed cells. We identified the inhibition of the wild-type p53-induced phosphatase 1 (Wip1) as the major mechanism important for full engagement of p53 activity accomplished by restoration of the ataxia telangiectasia mutated (ATM) kinase-signalling cascade, which leads to HdmX degradation. In contrast to previously reported transactivation of Wip1 by p53, we observed p53-dependent repression of Wip1 expression, which disrupts the negative feedback loop conferred by Wip1. Our study reveals that the depletion of both HdmX and Wip1 potentiates cell death due to sustained activation of p53. Thus, RITA is an example of a p53-reactivating drug that not only blocks Hdm2, but also inhibits two important negative regulators of p53 - HdmX and Wip1, leading to efficient elimination of tumour cells.

Checkpoint recovery in cells: how a molecular understanding can help in the fight against cancer

Dysregulation of the cell cycle is the underlying mechanism of neoplasia. Healthy cells prevent propagation of DNA mutations to progeny by activation of cellular checkpoints, which allows time for DNA repair. On the other hand, activation of the DNA damage response is also the general principle of many current cancer treatments. Thus, recent advances in understanding how checkpoints in the cell cycle work at the molecular level open the door to new approaches to antitumor therapy.

USP4 inhibits p53 through deubiquitinating and stabilizing ARF-BP1

Tumour suppressor p53 levels in the cell are tightly regulated by controlled degradation through ubiquitin ligases including Mdm2, COP1, Pirh2, and ARF-BP1. The ubiquitination process is reversible via deubiquitinating enzymes, such as ubiquitin-specific peptidases (USPs). In this study, we identified ubiquitin-specific peptidase 4 (USP4) as an important regulator of p53. USP4 interacts directly with and deubiquitinates ARF-BP1, leading to the stabilization of ARF-BP1 and subsequent reduction of p53 levels. Usp4 knockout mice are viable and developmentally normal, but showed enhanced apoptosis in thymus and spleen in response to ionizing radiation. Compared with wild-type mouse embryonic fibroblasts (MEFs), Usp4-/- MEFs exhibited retarded growth, premature cellular senescence, resistance to oncogenic transformation, and hyperactive DNA damage checkpoints, consistent with upregulated levels and activity of p53 in the absence of USP4. Finally, we showed that USP4 is overexpressed in several types of human cancer, suggesting that USP4 is a potential oncogene.

PPM1D silencing by lentiviral-mediated RNA interference inhibits proliferation and invasion of human glioma cells

To construct a lentiviral shRNA vector targeting human protein phosphatase 1D magnesium-dependent (PPM1D) gene and detect its effectiveness of gene silencing in human gliomas, specific siRNA targets with short hairpin frame were designed and synthesized. DNA oligo was cloned into the pFU-GW-iRNA lentiviral expression vector, and then PCR and sequencing analyses were conducted to verify the constructs. After the verified plasmids were transfected into 293T cells, the lentivirus was produced and the titer of virus was determined. Real-time quantitative PCR and Western blot were performed to detect the PPM1D expression level in the infected glioma cells. PCR and Western blot analyses revealed the optimal interfering target, and the virus with a titer of 6×10(8) TU/mL was successfully packaged. The PPM1D expression in human glioma cells was knocked down at both mRNA and protein levels by virus infection. The expression of PPM1D mRNA and protein was decreased by 76.3% and 87.0% respectively as compared with control group. The multiple functions of human glioma cells after PPM1D RNA interference were detected by flow cytometry and cell counting kit-8 (CCK-8). Efficient down-regulation of PPM1D resulted in significantly increased cell apoptosis and reduced cell proliferation and invasion potential in U87-MG cells. We have successfully constructed the lentiviral shRNA expression vector capable of stable PPM1D gene silencing at both mRNA and protein levels in glioma cells. And our data gave evidence that the reduced cell growth observed after PPM1D silencing in glioma cells was at least partly due to increased apoptotic cell death.

ATM-dependent and -independent dynamics of the nuclear phosphoproteome after DNA damage

The double-strand break (DSB) is a cytotoxic DNA lesion caused by oxygen radicals, ionizing radiation, and radiomimetic chemicals. Cells cope with DNA damage by activating the DNA damage response (DDR), which leads either to damage repair and cellular survival or to programmed cell death. The main transducer of the DSB response is the nuclear protein kinase ataxia telangiectasia mutated (ATM). We applied label-free quantitative mass spectrometry to follow the dynamics of DSB-induced phosphoproteome in nuclear fractions of the human melanoma G361 cells after radiomimetic treatment. We found that these dynamics are complex, including both phosphorylation and dephosphorylation events. In addition to identifying previously unknown ATM-dependent phosphorylation and dephosphorylation events, we found that about 40% of DSB-induced phosphorylations were ATM-independent and that several other kinases are potentially involved. Sustained activity of ATM was required to maintain many ATM-dependent phosphorylations. We identified an ATM-dependent phosphorylation site on ATM itself that played a role in its retention on damaged chromatin. By connecting many of the phosphorylated and dephosphorylated proteins into functional networks, we highlight putative cross talks between proteins pertaining to several cellular biological processes. Our study expands the DDR phosphorylation landscape and identifies previously unknown ATM-dependent and -independent branches. It reveals insights into the breadth and complexity of the cellular responses involved in the coordination of many DDR pathways, which is in line with the critical importance of genomic stability in maintenance of cellular homeostasis.

c-Abl phosphorylation of Mdm2 facilitates Mdm2-Mdmx complex formation

Mdm2 and Mdmx are oncoproteins that have essential yet nonredundant roles in development and function as part of a multicomponent ubiquitinating complex that targets p53 for proteasomal degradation. However, in response to DNA damage, Mdm2 and Mdmx are phosphorylated and protect p53 through various mechanisms. It has been predicted that Mdm2-Mdmx complex formation modulates Mdm2 ligase activity, yet the mechanism that promotes formation of Mdm2-Mdmx complexes is unknown. Here, we show that optimal Mdm2-Mdmx complex formation requires c-Abl phosphorylation of Mdm2 both in vitro and in vivo. In addition, Abl phosphorylation of Mdm2 is required for efficient ubiquitination of Mdmx in vitro, and eliminating c-Abl signaling, using c-Abl(-/-) knock-out murine embryonic fibroblasts, led to a decrease in Mdmx ubiquitination. Further, p53 levels are not induced as efficiently in c-Abl(-/-) murine embryonic fibroblasts following DNA damage. Overall, these results define a direct link between genotoxic stress-activated c-Abl kinase signaling and Mdm2-Mdmx complex formation. Our results add an important regulatory mechanism for the activation of p53 in response to DNA damage.

PPM1D gene amplification and overexpression in breast cancer: a qRT-PCR and chromogenic in situ hybridization study

PPM1D (protein phosphatase magnesium-dependent 1δ) maps to the 17q23.2 amplicon and is amplified in ∼8% of breast cancers. The PPM1D gene encodes a serine threonine phosphatase, which is involved in the regulation of several tumour suppressor pathways, including the p53 pathway. Along with others, we have recently shown that PPM1D is one of the drivers of the 17q23.2 amplicon and a promising therapeutic target. Here we investigate whether PPM1D is overexpressed when amplified in breast cancers and the correlations between PPM1D overexpression and amplification with clinicopathological features and survival of breast cancer patients from a cohort of 245 patients with invasive breast cancer treated with therapeutic surgery followed by adjuvant anthracycline-based chemotherapy. mRNA was extracted from representative sections of tumours containing >50% of tumour cells and subjected to TaqMan quantitative real-time PCR using primers for PPM1D and for two housekeeping genes. PPM1D overexpression was defined as the top quartile of expression levels. Chromogenic in situ hybridization with in-house-generated probes for PPM1D was performed. Amplification was defined as >50% of cancer cells with >5 signals per nucleus/large gene clusters. PPM1D overexpression and amplification were found in 25 and 6% of breast cancers, respectively. All cases harbouring PPM1D amplification displayed PPM1D overexpression. PPM1D overexpression was inversely correlated with expression of TOP2A, EGFR and cytokeratins 5/6 and 17. PPM1D amplification was significantly associated with HER2 overexpression, and HER2, TOP2A and CCND1 amplification. No association between PPM1D gene amplification and PPM1D mRNA overexpression with survival was observed. In conclusion, PPM1D is consistently overexpressed when amplified; however, PPM1D overexpression is more pervasive than gene amplification. PPM1D overexpression and amplification are associated with tumours displaying luminal or HER2 phenotypes. Co-amplification of PPM1D and HER2/TOP2A and CCND1 are not random events and may suggest the presence of a 'firestorm' genetic profile.

Oncogenic Wip1 phosphatase is inhibited by miR-16 in the DNA damage signaling pathway

Wild-type p53-induced phosphatase 1 (Wip1) was identified as an oncogene amplified and overexpressed in several human cancers. Recent evidence suggested that Wip1 is a critical inhibitor in the ATM/ATR-p53 DNA damage signaling pathway. Wip1 dephosphorylates several key DNA damage-responsive proteins and reverses DNA damage-induced cell cycle checkpoints. Previous reports showed that Wip1 was transcriptionally induced by p53 at the early stage of the DNA damage response. To investigate the temporal and functional regulation of Wip1, we identified a microRNA, miR-16, that specifically targets the mRNA of Wip1 and thus negatively regulates the expression level of Wip1. miR-16 itself is induced immediately after DNA damage. Therefore, the increase in Wip1 protein level is significantly postponed compared with that of its mRNA level, preventing a premature inactivation of ATM/ATR signaling and allowing a functional completion of the early DNA damage response. To better understand miR-16 biological functions in the context of cancer cells, we examined its expression in mammary tumor stem cells and found it to be markedly downregulated in mammary tumor stem cells. Overexpression of miR-16 or inhibition of Wip1 suppresses the self-renewal and growth of mouse mammary tumor stem cells and sensitizes MCF-7 human breast cancer cells to the chemotherapeutic drug doxorubicin. Together, our results suggest an important role of miR-16 in the regulation of Wip1 phosphatase in the DNA damage response and mammary tumorigenesis.

Regulation of the p53 pathway by ubiquitin and related proteins

The p53 tumour suppressor protein is subject to many levels of control, including modification with ubiquitin and related proteins such as SUMO and NEDD8. These modifications regulate p53 at a number of levels, including control of protein turnover, alterations in sub-cellular localization and changes in the ability to regulate gene expression. Numerous E3 ligases that can mediate these modifications of p53 have been described, some of which promote conjugation with more than one ubiquitin-like protein. Understanding the complexity of this mechanism of p53 regulation will help in the development of therapeutic drugs that function to modulate these events.

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.