Regulation of the antioncogenic Chk2 kinase by the oncogenic Wip1 phosphatase

Gain of chromosome 17 is an early genetic abnormality in neuroblastoma with PPM1D emerging as a strong candidate oncogene driving tumor progression

Segmental gain of chromosome 17q is the most common genetic aberration in high-risk neuroblastoma, but its role in disease progression is poorly understood. This study aims to address the contribution of 17q gain to neuroblastoma malignancy. We analyzed the genetic and transcriptional landscape of 417 neuroblastoma patients across various risk groups and clinical stages using multi-omic approaches. Single-cell RNA/DNA sequencing and SNP arrays were combined to characterize genomic aberrations, while evolutionary trajectories were mapped to explore the accumulation of genetic changes in patients with neuroblastoma. Additionally, DNA and RNA sequencing were used to assess mutational burden and gene expression patterns. Our findings suggest that chromosome 17 gain is an early genetic event acquired during neuroblastoma development, correlating with the accumulation of additional chromosomal aberrations and poor prognosis. Increased segmental gains of chromosome 17q were observed during clonal evolution, relapse disease and metastasis. We identified PPM1D, a p53-inducible Ser/Thr phosphatase located on chr17q22.3, as a key player activated by segmental 17q-gain, gene-fusion, or gain-of-function somatic and germline mutations, further promoting neuroblastoma development/progression. Gain of chromosome 17 is an early driver of genetic instability in neuroblastoma, with PPM1D emerging as a potential candidate gene implicated in high-risk disease progression.

Wip1 phosphatase activator QGC-8-52 specifically sensitizes p53-negative cancer cells to chemotherapy while protecting normal cells

PP2C serine-threonine phosphatase Wip1 plays an important role in normal tissue homeostasis, stress signaling and pathogenesis of various human diseases. It is an attractive drug target for cancer treatment and inhibition of its expression or activity constitute a novel therapeutic intervention strategy to prevent the development of various cancers. However, previous strategies for Wip1 suppression may be ineffective in cancers lacking p53. Here, we have characterized the activity of a novel Wip1 phosphatase activator, QGC-8-52, in preclinical models of breast malignancies. QGC-8-52 significantly sensitizes the cancer cell lines with p53 deletion to chemotherapeutic agents. This effect was mediated by the Wip1-FOXO3a interaction and subsequent dephosphorylation of Thr487 that resulted, in response to anticancer treatment, in enhancing the transcription activity of FOXO3a on the proapoptotic TRAIL gene. The sensitizing effect of Wip1 activation on chemotherapeutic drugs only targeted cancer cells lacking p53. The activation of Wip1 in normal cells provided protection from anticancer drug-induced apoptosis by reducing the strength of upstream signaling to p53. Therefore, during the treatment of anticancer drugs, the activated Wip1 phosphatase boosts the apoptosis of p53-negative tumors and protects normal tissues. Our findings may represent an effective and safe therapeutic strategy for cancers with p53 deletion.

PP2C phosphatases-terminators of suicidal thoughts

Cell death and related signaling pathways are essential during development and in various physiological and pathological conditions. Post-translational modifications such as ubiquitination and phosphorylation play an important role in these signaling pathways. The involvement of kinases - enzymes that catalyze protein phosphorylation - in cell death signaling has been extensively studied. On the other hand, not many studies have been devoted to analyzing the role in cell death of phosphatases, enzymes involved in the removal of phosphorylated residues added to proteins by kinases. Obviously, the two opposite reactions, phosphorylation and dephosphorylation, are equally important in the regulation of protein functions and subsequently in the execution of the cell death program. Here, we have summarized recent work on the involvement of serine-threonine PP2C phosphatases in cell death pathways, senescence and autophagy, focusing in particular on the most studied phosphatase PPM1D (PP2Cδ) as an example of the regulatory role of PP2Cs in cell death. The review should help to draw attention to the importance of PP2C family phosphatases in cell death checkpoints and to discover new targets for drug development.

Schlafen 11 further sensitizes BRCA-deficient cells to PARP inhibitors through single-strand DNA gap accumulation behind replication forks

The preferential response to PARP inhibitors (PARPis) in BRCA-deficient and Schlafen 11 (SLFN11)-expressing ovarian cancers has been documented, yet the underlying molecular mechanisms remain unclear. As the accumulation of single-strand DNA (ssDNA) gaps behind replication forks is key for the lethality effect of PARPis, we investigated the combined effects of SLFN11 expression and BRCA deficiency on PARPi sensitivity and ssDNA gap formation in human cancer cells. PARPis increased chromatin-bound RPA2 and ssDNA gaps in SLFN11-expressing cells and even more in cells with BRCA1 or BRCA2 deficiency. SLFN11 was co-localized with chromatin-bound RPA2 under PARPis treatment, with enhanced recruitment in BRCA2-deficient cells. Notably, the chromatin-bound SLFN11 under PARPis did not block replication, contrary to its function under replication stress. SLFN11 recruitment was attenuated by the inactivation of MRE11. Hence, under PARPi treatment, MRE11 expression and BRCA deficiency lead to ssDNA gaps behind replication forks, where SLFN11 binds and increases their accumulation. As ovarian cancer patients who responded (progression-free survival >2 years) to olaparib maintenance therapy had a significantly higher SLFN11-positivity than short-responders (<6 months), our findings provide a mechanistic understanding of the favorable responses to PARPis in SLFN11-expressing and BRCA-deficient tumors. It highlight the clinical implications of SLFN11.

Targeting WIP1 phosphatase promotes partial remission in experimental collapsing glomerulopathy

Collapsing focal segmental glomerulosclerosis (FSGS), also known as collapsing glomerulopathy (CG), is the most aggressive variant of FSGS and is characterized by a rapid progression to kidney failure. Understanding CG pathogenesis represents a key step for the development of targeted therapies. Previous work implicated the telomerase protein component TERT in CG pathogenesis, as transgenic TERT expression in adult mice resulted in a CG resembling that seen in human primary CG and HIV-associated nephropathy (HIVAN). Here, we used the telomerase-induced mouse model of CG (i-TERTci mice) to identify mechanisms to inhibit CG pathogenesis. Inactivation of WIP1 phosphatase, a p53 target acting in a negative feedback loop, blocked disease initiation in i-TERTci mice. Repression of disease initiation upon WIP1 deficiency was associated with senescence enhancement and required transforming growth factor-β functions. The efficacy of a pharmacologic treatment to reduce disease severity in both i-TERTci mice and in a mouse model of HIVAN (Tg26 mice) was then assessed. Pharmacologic inhibition of WIP1 enzymatic activity in either the telomerase mice with CG or in the Tg26 mice promoted partial remission of proteinuria and ameliorated kidney histopathologic features. Histological as well as high-throughput sequencing methods further showed that selective inhibition of WIP1 does not promote kidney fibrosis or inflammation. Thus, our findings suggest that targeting WIP1 may be an effective therapeutic strategy for patients with CG.

Calciprotein Particles Induce Cellular Compartment-Specific Proteome Alterations in Human Arterial Endothelial Cells

Calciprotein particles (CPPs) are indispensable scavengers of excessive Ca2+ and PO43- ions in blood, being internalised and recycled by liver and spleen macrophages, monocytes, and endothelial cells (ECs). Here, we performed a pathway enrichment analysis of cellular compartment-specific proteomes in primary human coronary artery ECs (HCAEC) and human internal thoracic artery ECs (HITAEC) treated with primary (amorphous) or secondary (crystalline) CPPs (CPP-P and CPPs, respectively). Exposure to CPP-P and CPP-S induced notable upregulation of: (1) cytokine- and chemokine-mediated signaling, Ca2+-dependent events, and apoptosis in cytosolic and nuclear proteomes; (2) H+ and Ca2+ transmembrane transport, generation of reactive oxygen species, mitochondrial outer membrane permeabilisation, and intrinsic apoptosis in the mitochondrial proteome; (3) oxidative, calcium, and endoplasmic reticulum (ER) stress, unfolded protein binding, and apoptosis in the ER proteome. In contrast, transcription, post-transcriptional regulation, translation, cell cycle, and cell-cell adhesion pathways were underrepresented in cytosol and nuclear compartments, whilst biosynthesis of amino acids, mitochondrial translation, fatty acid oxidation, pyruvate dehydrogenase activity, and energy generation were downregulated in the mitochondrial proteome of CPP-treated ECs. Differentially expressed organelle-specific pathways were coherent in HCAEC and HITAEC and between ECs treated with CPP-P or CPP-S. Proteomic analysis of mitochondrial and nuclear lysates from CPP-treated ECs confirmed bioinformatic filtration findings.

Maintaining Genome Integrity: Protein Kinases and Phosphatases Orchestrate the Balancing Act of DNA Double-Strand Breaks Repair in Cancer

DNA double-strand breaks (DSBs) are the most lethal DNA damages which lead to severe genome instability. Phosphorylation is one of the most important protein post-translation modifications involved in DSBs repair regulation. Kinases and phosphatases play coordinating roles in DSB repair by phosphorylating and dephosphorylating various proteins. Recent research has shed light on the importance of maintaining a balance between kinase and phosphatase activities in DSB repair. The interplay between kinases and phosphatases plays an important role in regulating DNA-repair processes, and alterations in their activity can lead to genomic instability and disease. Therefore, study on the function of kinases and phosphatases in DSBs repair is essential for understanding their roles in cancer development and therapeutics. In this review, we summarize the current knowledge of kinases and phosphatases in DSBs repair regulation and highlight the advancements in the development of cancer therapies targeting kinases or phosphatases in DSBs repair pathways. In conclusion, understanding the balance of kinase and phosphatase activities in DSBs repair provides opportunities for the development of novel cancer therapeutics.

Radiotherapy and radio-sensitization in H3K27M -mutated diffuse midline gliomas

BACKGROUND

H3^K27M mutated diffuse midline gliomas (DMGs) are extremely aggressive and the leading cause of cancer-related deaths in pediatric brain tumors with 5-year survival <1%. Radiotherapy is the only established adjuvant treatment of H3^K27M DMGs; however, the radio-resistance is commonly observed.

METHODS

We summarized current understandings of the molecular responses of H3^K27M DMGs to radiotherapy and provide crucial insights into current advances in radiosensitivity enhancement.

RESULTS

Ionizing radiation (IR) can mainly inhibit tumor cell growth by inducing DNA damage regulated by the cell cycle checkpoints and DNA damage repair (DDR) system. In H3K27M DMGs, the aberrant genetic and epigenetic changes, stemness genotype, and epithelial-mesenchymal transition (EMT) disrupt the cell cycle checkpoints and DDR system by altering the associated regulatory signaling pathways, which leads to the development of radio-resistance.

CONCLUSIONS

The advances in mechanisms of radio-resistance in H3^K27M  DMGs promote the potential targets to enhance the sensitivity to radiotherapy.

PPM1D suppresses p53-dependent transactivation and cell death by inhibiting the Integrated Stress Response

The p53 transcription factor is a master regulator of cellular stress responses inhibited by repressors such as MDM2 and the phosphatase PPM1D. Activation of p53 with pharmacological inhibitors of its repressors is being tested in clinical trials for cancer therapy, but efficacy has been limited by poor induction of tumor cell death. We demonstrate that dual inhibition of MDM2 and PPM1D induces apoptosis in multiple cancer cell types via amplification of the p53 transcriptional program through the eIF2α-ATF4 pathway. PPM1D inhibition induces phosphorylation of eIF2α, ATF4 accumulation, and ATF4-dependent enhancement of p53-dependent transactivation upon MDM2 inhibition. Dual inhibition of p53 repressors depletes heme and induces HRI-dependent eIF2α phosphorylation. Pharmacological induction of eIF2α phosphorylation synergizes with MDM2 inhibition to induce cell death and halt tumor growth in mice. These results demonstrate that PPM1D inhibits both the p53 network and the integrated stress response controlled by eIF2α-ATF4, with clear therapeutic implications.

MicroRNAs in doxorubicin-induced cardiotoxicity: The DNA damage response

Doxorubicin (DOX) is a chemotherapeutic drug widely used for cancer treatment, but its use is limited by cardiotoxicity. Although free radicals from redox cycling and free cellular iron have been predominant as the suggested primary pathogenic mechanism, novel evidence has pointed to topoisomerase II inhibition and resultant genotoxic stress as the more fundamental mechanism. Recently, a growing list of microRNAs (miRNAs) has been implicated in DOX-induced cardiotoxicity (DIC). This review summarizes miRNAs reported in the recent literature in the context of DIC. A particular focus is given to miRNAs that regulate cellular responses downstream to DOX-induced DNA damage, especially p53 activation, pro-survival signaling pathway inhibition (e.g., AMPK, AKT, GATA-4, and sirtuin pathways), mitochondrial dysfunction, and ferroptosis. Since these pathways are potential targets for cardioprotection against DOX, an understanding of how miRNAs participate is necessary for developing future therapies.

Checkpoint Kinase 2 Inhibition Can Reverse Tamoxifen Resistance in ER-Positive Breast Cancer

Breast cancer is a heterogeneous disease. Tamoxifen is frequently used to treat ER-positive breast cancer. Our team has identified a novel splice variant of NCOR2, BQ323636.1 (BQ), that mediates tamoxifen resistance. However, the upstream factors that modulate BQ expression are not apparent. This study reveals that tamoxifen treatment causes induction of DNA damage which can enhance BQ expression. We show that DNA damage can activate the ATM/CHK2 and ATR/CHK1 signalling cascades and confirm that ATM/CHK2 signalling is responsible for enhancing the protein stability of BQ. siRNA or a small inhibitor targeting CHK2 resulted in the reduction in BQ expression through reduced phosphorylation and enhanced poly-ubiquitination of BQ. Inhibition of CHK2 by CCT241533 could reverse tamoxifen resistance in vitro and in vivo. Using clinical samples in the tissue microarray, we confirmed that high p-CHK2 expression was significantly associated with high nuclear BQ expression, tamoxifen resistance and poorer overall and disease-specific survival. In conclusion, tamoxifen treatment can enhance BQ expression in ER-positive breast cancer by activating the ATM/CHK2 axis. Targeting CHK2 is a promising approach to overcoming tamoxifen resistance in ER-positive breast cancer.

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.

PPM1D in Solid and Hematologic Malignancies: Friend and Foe?

In the face of constant genomic insults, the DNA damage response (DDR) is initiated to preserve genome integrity; its disruption is a classic hallmark of cancer. Protein phosphatase Mg2+/Mn2+-dependent 1D (PPM1D) is a central negative regulator of the DDR that is mutated or amplified in many solid cancers. PPM1D overexpression is associated with increased proliferative and metastatic behavior in multiple solid tumor types and patients with PPM1D-mutated malignancies have poorer prognoses. Recent findings have sparked an interest in the role of PPM1D in hematologic malignancies. Acquired somatic mutations may provide hematopoietic stem cells with a competitive advantage, leading to a substantial proportion of mutant progeny in the peripheral blood, an age-associated phenomenon termed "clonal hematopoiesis" (CH). Recent large-scale genomic studies have identified PPM1D to be among the most frequently mutated genes found in individuals with CH. While PPM1D mutations are particularly enriched in patients with therapy-related myeloid neoplasms, their role in driving leukemic transformation remains uncertain. Here, we examine the mechanisms through which PPM1D overexpression or mutation may drive malignancy by suppression of DNA repair, cell-cycle arrest, and apoptosis. We also discuss the divergent roles of PPM1D in the oncogenesis of solid versus hematologic cancers with a view to clinical implications and new therapeutic avenues.

Development of Antibody-like Proteins Targeting the Oncogenic Ser/Thr Protein Phosphatase PPM1D

PPM1D, a protein Ser/Thr phosphatase, is overexpressed in various cancers and functions as an oncogenic protein by inactivating the p53 pathway. Therefore, molecules that bind PPM1D are expected to be useful anti-cancer agents. In this study, we constructed a phage display library based on the antibody-like small molecule protein adnectin and screened for PPM1D-specific binding molecules. We identified two adnectins, PMDB-1 and PMD-24, that bind PPM1D specific B-loop and PPM1D430 as targets, respectively. Specificity analyses of these recombinant proteins using other Ser/Thr protein phosphatases showed that these molecules bind to only PPM1D. Expression of PMDB-1 in breast cancer-derived MCF-7 cells overexpressing endogenous PPM1D stabilized p53, indicating that PMDB-1 functions as an inhibitor of PPM1D. Furthermore, MTT assay exhibited that MCF-7 cells expressing PMDB-1 showed inhibition of cell proliferation. These data suggest that the adnectin PMDB-1 identified in this study can be used as a lead compound for anti-cancer drugs targeting intracellular PPM1D.

Co-targeting WIP1 and PARP induces synthetic lethality in hepatocellular carcinoma

Background

Hepatocellular carcinoma (HCC) is one of the most fatal cancers. Due to limited strategies for effective treatments, patients with advanced HCC have a very poor prognosis. This study aims to identify new insights in HCC to develop novel strategies for HCC management.

Methods

The role of WIP1 (wild type p53 induced protein phosphatase1) in HCC was analyzed in HCC cells, xenograft model, DEN (Diethylnitrosamine) induced mice liver cancer model with WIP1 knockout mice, and TCGA database. DNA damage was evaluated by Gene Set Enrichment Analysis, western blotting, comet assay, and Immunofluorescence.

Results

High expression of WIP1 is associated with the poor prognosis of patients with HCC. Genetically and chemically suppression of WIP1 drastically reduced HCC cell proliferation. Besides, WIP1 knockout retarded DEN induced mice hepato-carcinogenesis. Mechanically, WIP1 inhibition induced DNA damage by increasing H2AX phosphorylation (γH2AX). Therefore, suppression of WIP1 and PARP induced synthetic lethality in HCC in vitro and in vivo by augmenting DNA damage.

Conclusion

WIP1 plays an oncogenic effect in HCC development, and targeting WIP1-dependent DNA damage repair alone or in combination with PARP inhibition might be a reasonable strategy for HCC management.

MiR-145-5p Inhibits the Invasion of Prostate Cancer and Induces Apoptosis by Inhibiting WIP1

Prostate cancer (PCa) is a common malignant tumor of the male genitourinary system that seriously affects the quality of life of patients. Studying the pathogenesis and therapeutic targets of PCa is important. In this study, we investigated the role of miR-145-5p in PCa and its potential molecular mechanisms. The expression levels of miR-145-5p in PCa tissues and adjacent control tissues were detected by real-time quantitative polymerase chain reaction. The effects of miR-145-5p overexpression on PCa were studied using cell proliferation, migration, and invasion experiments. Furthermore, WIP1 was the target gene of miR-145-5p through the bioinformatics website and dual-luciferase reporter gene experiment. Further studies found that WIP1 downregulation could inhibit the proliferation, invasion, and cloning of PCa cells. Overexpression of WIP1 reversed the anticancer effects of miR-145. The anticancer effect of miR-145 was achieved by inhibiting the PI3K/AKT signaling pathway and upregulating ChK2 and p-p38MAPK. Taken together, these results confirmed that miR-145-5p inhibited the growth and metastasis of PCa cells by inhibiting the expression of proto-oncogene WIP1, thereby playing a role in tumor suppression in PCa and may become a potential therapeutic target for the treatment of PCa.

PPM1D Is a Therapeutic Target in Childhood Neural Tumors

Simple Summary

Medulloblastoma and neuroblastoma are childhood tumors of the central nervous system or the peripheral nervous system, respectively. These are the most common and deadly tumors of childhood. A common genetic feature of medulloblastoma and neuroblastoma is frequent segmental gain or amplification of chromosome 17q. Located on chromosome 17q23.2 is PPM1D which encodes WIP1, a phosphatase that acts as a regulator of p53 and DNA repair. Overexpression of WIP1 correlates with poor patient prognosis. We investigated the effects of genetic or pharmacologic inhibition of WIP1 activity and found that medulloblastoma and neuroblastoma cells were strongly dependent on WIP1 expression for survival. We also tested a number of small molecule inhibitors of WIP1 and show that SL-176 was the most effective compound suppressing the growth of medulloblastoma and neuroblastoma in vitro and in vivo.

Abstract

Childhood medulloblastoma and high-risk neuroblastoma frequently present with segmental gain of chromosome 17q corresponding to aggressive tumors and poor patient prognosis. Located within the 17q-gained chromosomal segments is PPM1D at chromosome 17q23.2. PPM1D encodes a serine/threonine phosphatase, WIP1, that is a negative regulator of p53 activity as well as key proteins involved in cell cycle control, DNA repair and apoptosis. Here, we show that the level of PPM1D expression correlates with chromosome 17q gain in medulloblastoma and neuroblastoma cells, and both medulloblastoma and neuroblastoma cells are highly dependent on PPM1D expression for survival. Comparison of different inhibitors of WIP1 showed that SL-176 was the most potent compound inhibiting medulloblastoma and neuroblastoma growth and had similar or more potent effects on cell survival than the MDM2 inhibitor Nutlin-3 or the p53 activator RITA. SL-176 monotherapy significantly suppressed the growth of established medulloblastoma and neuroblastoma xenografts in nude mice. These results suggest that the development of clinically applicable compounds inhibiting the activity of WIP1 is of importance since PPM1D activating mutations, genetic gain or amplifications and/or overexpression of WIP1 are frequently detected in several different cancers.

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.

A novel assay for screening WIP1 phosphatase substrates in nuclear extracts

Upon exposure to genotoxic stress, cells activate DNA damage response (DDR) that coordinates DNA repair with a temporal arrest in the cell cycle progression. DDR is triggered by activation of ataxia telangiectasia mutated/ataxia telangiectasia and Rad3-related protein kinases that phosphorylate multiple targets including tumor suppressor protein tumor suppressor p53 (p53). In addition, DNA damage can activate parallel stress response pathways [such as mitogen-activated protein kinase p38 alpha (p38)/MAPK-activated protein kinase 2 (MK2) kinases] contributing to establishing the cell cycle arrest. Wild-type p53-induced phosphatase 1 (WIP1) controls timely inactivation of DDR and is needed for recovery from the G2 checkpoint by counteracting the function of p53. Here, we developed a simple in vitro assay for testing WIP1 substrates in nuclear extracts. Whereas we did not detect any activity of WIP1 toward p38/MK2, we confirmed p53 as a substrate of WIP1. Inhibition or inactivation of WIP1 in U2OS cells increased phosphorylation of p53 at S15 and potentiated its acetylation at K382. Further, we identified Deleted in breast cancer gene 1 (DBC1) as a new substrate of WIP1 but surprisingly, depletion of DBC1 did not interfere with the ability of WIP1 to regulate p53 acetylation. Instead, we have found that WIP1 activity suppresses p53-K382 acetylation by inhibiting the interaction between p53 and the acetyltransferase p300. Newly established phosphatase assay allows an easy comparison of WIP1 ability to dephosphorylate various proteins and thus contributes to identification of its physiological substrates.

Protein phosphatases in TLR signaling

Toll-like receptors (TLRs) are critical sensors for the detection of potentially harmful microbes. They are instrumental in initiating innate and adaptive immune responses against pathogenic organisms. However, exaggerated activation of TLR receptor signaling can also be responsible for the onset of autoimmune and inflammatory diseases. While positive regulators of TLR signaling, such as protein serine/threonine kinases, have been studied intensively, only little is known about phosphatases, which counterbalance and limit TLR signaling. In this review, we summarize protein phosphorylation events and their roles in the TLR pathway and highlight the involvement of protein phosphatases as negative regulators at specific steps along the TLR-initiated signaling cascade. Then, we focus on individual phosphatase families, specify the function of individual enzymes in TLR signaling in more detail and give perspectives for future research. A better understanding of phosphatase-mediated regulation of TLR signaling could provide novel access points to mitigate excessive immune activation and to modulate innate immune signaling.

Sustained CHK2 activity, but not ATM activity, is critical to maintain a G1 arrest after DNA damage in untransformed cells

BACKGROUND

The G1 checkpoint is a critical regulator of genomic stability in untransformed cells, preventing cell cycle progression after DNA damage. DNA double-strand breaks (DSBs) recruit and activate ATM, a kinase which in turn activates the CHK2 kinase to establish G1 arrest. While the onset of G1 arrest is well understood, the specific role that ATM and CHK2 play in regulating G1 checkpoint maintenance remains poorly characterized.

RESULTS

Here we examine the impact of ATM and CHK2 activities on G1 checkpoint maintenance in untransformed cells after DNA damage caused by DSBs. We show that ATM becomes dispensable for G1 checkpoint maintenance as early as 1 h after DSB induction. In contrast, CHK2 kinase activity is necessary to maintain the G1 arrest, independently of ATM, ATR, and DNA-PKcs, implying that the G1 arrest is maintained in a lesion-independent manner. Sustained CHK2 activity is achieved through auto-activation and its acute inhibition enables cells to abrogate the G1-checkpoint and enter into S-phase. Accordingly, we show that CHK2 activity is lost in cells that recover from the G1 arrest, pointing to the involvement of a phosphatase with fast turnover.

CONCLUSION

Our data indicate that G1 checkpoint maintenance relies on CHK2 and that its negative regulation is crucial for G1 checkpoint recovery after DSB induction.

Epigenetic suppression of SLFN11 in germinal center B-cells during B-cell development

Background

SLFN11 has recently been reported to execute cancer cells harboring replicative stress induced by DNA damaging agents. However, the roles of SLFN11 under physiological conditions remain poorly understood. Germinal center B-cells (GCBs) undergo somatic hypermutations and class-switch recombination, which can cause physiological genotoxic stress. Hence, we tested whether SLFN11 expression needs to be suppressed in GCBs during B-cell development.

Objective

To clarify the expression profile of SLFN11 in different developmental stages of B-cells and B-cell-derived cancers.

Methods

We analyzed the expression of SLFN11 by mining cell line databases for different stages of normal B-cells and various types of B-cell-derived cancer cell lines. We performed dual immunohistochemical staining for SLFN11 and B-cell specific markers in normal human lymphatic tissues. We tested the effects of two epigenetic modifiers, an EZH2 inhibitor, tazemetostat (EPZ6438) and a histone deacetylase inhibitor, panobinostat (LBH589) on SLFN11 expression in GCB-derived lymphoma cell lines. We also examined the therapeutic efficacy of these drugs in combination with cytosine arabinoside and the effects of SLFN11 on the efficacy of cytosine arabinoside in SLFN11-overexpressing cells.

Results

SLFN11 mRNA level was found low in both normal GCBs and GCB-DLBCL (GCB like-diffuse large B-cell lymphoma). Immunohistochemical staining showed low SLFN11 expression in GCBs and high SLFN11 expression in plasmablasts and plasmacytes. The EZH2 and HDAC epigenetic modifiers upregulated SLFN11 expression in GCB-derived lymphoma cells and made them more susceptible to cytosine arabinoside. SLFN11 overexpression further sensitized GCB-derived lymphoma cells to cytosine arabinoside.

Conclusions

The expression of SLFN11 is epigenetically suppressed in normal GCBs and GCB-derived lymphomas. GCB-derived lymphomas with low SLFN11 expression can be treated by the combination of epigenetic modifiers and cytosine arabinoside.

Wip1 phosphatase deficiency impairs spatial learning and memory

Spatial learning and memory are typically assessed to evaluate hippocampus-dependent cognitive and memory functions in vivo. Protein phosphorylation and dephosphorylation by kinases and phosphatases play critical roles in spatial learning and memory. Here we report that the Wip1 phosphatase is essential for spatial learning, with knockout mice lacking Wip1 phosphatase exhibiting dysfunctional spatial cognition. Aberrant phosphorylation of the Wip1 substrates p38, ATM, and p53 were observed in the hippocampi of Wip1^-/- mice, but only p38 inhibition reversed impairments in long-term potentiation in Wip1-knockout mice. p38 inhibition consistently ameliorated the spatial learning dysfunction caused by Wip1 deficiency. Our results demonstrate that deletion of Wip1 phosphatase impairs hippocampus-dependent spatial learning and memory, with aberrant downstream p38 phosphorylation involved in this process and providing a potential therapeutic target.

CHEK2 Germline Variants in Cancer Predisposition: Stalemate Rather than Checkmate

Germline alterations in many genes coding for proteins regulating DNA repair and DNA damage response (DDR) to DNA double-strand breaks (DDSB) have been recognized as pathogenic factors in hereditary cancer predisposition. The ATM-CHEK2-p53 axis has been documented as a backbone for DDR and hypothesized as a barrier against cancer initiation. However, although CHK2 kinase coded by the CHEK2 gene expedites the DDR signal, its function in activation of p53-dependent cell cycle arrest is dispensable. CHEK2 mutations rank among the most frequent germline alterations revealed by germline genetic testing for various hereditary cancer predispositions, but their interpretation is not trivial. From the perspective of interpretation of germline CHEK2 variants, we review the current knowledge related to the structure of the CHEK2 gene, the function of CHK2 kinase, and the clinical significance of CHEK2 germline mutations in patients with hereditary breast, prostate, kidney, thyroid, and colon cancers.

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.

Radio-Resistance and DNA Repair in Pediatric Diffuse Midline Gliomas

Malignant gliomas (MG) are among the most prevalent and lethal primary intrinsic brain tumors. Although radiotherapy (RT) is the most effective nonsurgical therapy, recurrence is universal. Dysregulated DNA damage response pathway (DDR) signaling, rampant genomic instability, and radio-resistance are among the hallmarks of MGs, with current therapies only offering palliation. A subgroup of pediatric high-grade gliomas (pHGG) is characterized by H3K27M mutation, which drives global loss of di- and trimethylation of histone H3K27. Here, we review the most recent literature and discuss the key studies dissecting the molecular biology of H3K27M-mutated gliomas in children. We speculate that the aberrant activation and/or deactivation of some of the key components of DDR may be synthetically lethal to H3K27M mutation and thus can open novel avenues for effective therapeutic interventions for patients suffering from this deadly disease.

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.

Metal-dependent Ser/Thr protein phosphatase PPM family: Evolution, structures, diseases and inhibitors

Protein phosphatases and kinases control multiple cellular events including proliferation, differentiation, and stress responses through regulating reversible protein phosphorylation, the most important post-translational modification. Members of metal-dependent protein phosphatase (PPM) family, also known as PP2C phosphatases, are Ser/Thr phosphatases that bind manganese/magnesium ions (Mn²⁺/Mg²⁺) in their active center and function as single subunit enzymes. In mammals, there are 20 isoforms of PPM phosphatases: PPM1A, PPM1B, PPM1D, PPM1E, PPM1F, PPM1G, PPM1H, PPM1J, PPM1K, PPM1L, PPM1M, PPM1N, ILKAP, PDP1, PDP2, PHLPP1, PHLPP2, PP2D1, PPTC7, and TAB1, whereas there are only 8 in yeast. Phylogenetic analysis of the DNA sequences of vertebrate PPM isoforms revealed that they can be divided into 12 different classes: PPM1A/PPM1B/PPM1N, PPM1D, PPM1E/PPM1F, PPM1G, PPM1H/PPM1J/PPM1M, PPM1K, PPM1L, ILKAP, PDP1/PDP2, PP2D1/PHLPP1/PHLPP2, TAB1, and PPTC7. PPM-family members have a conserved catalytic core region, which contains the metal-chelating residues. The different isoforms also have isoform specific regions within their catalytic core domain and terminal domains, and these regions may be involved in substrate recognition and/or functional regulation of the phosphatases. The twenty mammalian PPM phosphatases are involved in regulating diverse cellular functions, such as cell cycle control, cell differentiation, immune responses, and cell metabolism. Mutation, overexpression, or deletion of the PPM phosphatase gene results in abnormal cellular responses, which lead to various human diseases. This review focuses on the structures and biological functions of the PPM-phosphatase family and their associated diseases. The development of specific inhibitors against the PPM phosphatase family as a therapeutic strategy will also be discussed.

Frequency switching between oscillatory homeostats and the regulation of p53

Homeostasis is an essential concept to understand the stability of organisms and their adaptive behaviors when coping with external and internal assaults. Many hormones that take part in homeostatic control come in antagonistic pairs, such as glucagon and insulin reflecting the inflow and outflow compensatory mechanisms to control a certain internal variable, such as blood sugar levels. By including negative feedback loops homeostatic controllers can exhibit oscillations with characteristic frequencies. In this paper we demonstrate the associated frequency changes in homeostatic systems when individual controllers -in a set of interlocked feedback loops- gain control in response to environmental changes. Taking p53 as an example, we show how Per2, ATM and Mdm2 feedback loops -interlocked with p53- gain individual control in dependence to the level of DNA damage, and how each of these controllers provide certain functionalities in their regulation of p53. In unstressed cells, the circadian regulator Per2 ensures a basic p53 level to allow its rapid up-regulation in case of DNA damage. When DNA damage occurs the ATM controller increases the level of p53 and defends it towards uncontrolled degradation, which despite DNA damage, would drive p53 to lower values and p53 dysfunction. Mdm2 on its side keeps p53 at a high but sub-apoptotic level to avoid premature apoptosis. However, with on-going DNA damage the Mdm2 set-point is increased by HSP90 and other p53 stabilizers leading finally to apoptosis. An emergent aspect of p53 upregulation during cell stress is the coordinated inhibition of ubiquitin-independent and ubiquitin-dependent degradation reactions. Whether oscillations serve a function or are merely a by-product of the controllers are discussed in view of the finding that homeostatic control of p53, as indicated above, does in principle not require oscillatory homeostats.

WIP1 dephosphorylation of p27Kip1 Serine 140 destabilizes p27Kip1 and reverses anti-proliferative effects of ATM phosphorylation

The phosphoinositide-3-kinase like kinases (PIKK) such as ATM and ATR play a key role in initiating the cellular DNA damage response (DDR). One key ATM target is the cyclin-dependent kinase inhibitor p27Kip1 that promotes G1 arrest. ATM activates p27Kip1-induced arrest in part through phosphorylation of p27Kip1 at Serine 140. Here we show that this site is dephosphorylated by the type 2C serine/threonine phosphatase, WIP1 (Wildtype p53-Induced Phosphatase-1), encoded by the PPM1D gene. WIP1 has been shown to dephosphorylate numerous ATM target sites in DDR proteins, and its overexpression and/or mutation has often been associated with oncogenesis. We demonstrate that wildtype, but not phosphatase-dead WIP1, efficiently dephosphorylates p27Kip1 Ser140 both in vitro and in cells and that this dephosphorylation is sensitive to the WIP1-specific inhibitor GSK 2830371. Increased expression of wildtype WIP1 reduces stability of p27Kip1 while increased expression of similar amounts of phosphatase-dead WIP1 has no effect on p27Kip1 protein stability. Overexpression of wildtype p27Kip1 reduces cell proliferation and colony forming capability relative to the S140A (constitutively non-phosphorylated) form of p27. Thus, WIP1 plays a significant role in homeostatic modulation of p27Kip1 activity following activation by ATM.

Cell Cycle and DNA Repair Regulation in the Damage Response: Protein Phosphatases Take Over the Reins

Cells are constantly suffering genotoxic stresses that affect the integrity of our genetic material. Genotoxic insults must be repaired to avoid the loss or inappropriate transmission of the genetic information, a situation that could lead to the appearance of developmental abnormalities and tumorigenesis. To combat this threat, eukaryotic cells have evolved a set of sophisticated molecular mechanisms that are collectively known as the DNA damage response (DDR). This surveillance system controls several aspects of the cellular response, including the detection of lesions, a temporary cell cycle arrest, and the repair of the broken DNA. While the regulation of the DDR by numerous kinases has been well documented over the last decade, the complex roles of protein dephosphorylation have only recently begun to be investigated. Here, we review recent progress in the characterization of DDR-related protein phosphatases during the response to a DNA lesion, focusing mainly on their ability to modulate the DNA damage checkpoint and the repair of the damaged DNA. We also discuss their protein composition and structure, target specificity, and biochemical regulation along the different stages encompassed in the DDR. The compilation of this information will allow us to better comprehend the physiological significance of protein dephosphorylation in the maintenance of genome integrity and cell viability in response to genotoxic stress.

Physiologically relevant orthogonal assays for the discovery of small-molecule modulators of WIP1 phosphatase in high-throughput screens

WT P53-Induced Phosphatase 1 (WIP1) is a member of the magnesium-dependent serine/threonine protein phosphatase (PPM) family and is induced by P53 in response to DNA damage. In several human cancers, the WIP1 protein is overexpressed, which is generally associated with a worse prognosis. Although WIP1 is an attractive therapeutic target, no potent, selective, and bioactive small-molecule modulator with favorable pharmacokinetics has been reported. Phosphatase enzymes are among the most challenging targets for small molecules because of the difficulty of achieving both modulator selectivity and bioavailability. Another major obstacle has been the availability of robust and physiologically relevant phosphatase assays that are suitable for high-throughput screening. Here, we describe orthogonal biochemical WIP1 activity assays that utilize phosphopeptides from native WIP1 substrates. We optimized an MS assay to quantify the enzymatically dephosphorylated peptide reaction product in a 384-well format. Additionally, a red-shifted fluorescence assay was optimized in a 1,536-well format to enable real-time WIP1 activity measurements through the detection of the orthogonal reaction product, P_i. We validated these two optimized assays by quantitative high-throughput screening against the National Center for Advancing Translational Sciences (NCATS) Pharmaceutical Collection and used secondary assays to confirm and evaluate inhibitors identified in the primary screen. Five inhibitors were further tested with an orthogonal WIP1 activity assay and surface plasmon resonance binding studies. Our results validate the application of miniaturized physiologically relevant and orthogonal WIP1 activity assays to discover small-molecule modulators from high-throughput screens.

Replication protein A, the laxative that keeps DNA regular: The importance of RPA phosphorylation in maintaining genome stability

The eukaryotic ssDNA-binding protein, Replication protein A (RPA), was first discovered almost three decades ago. Since then, much progress has been made to elucidate the critical roles for RPA in DNA metabolic pathways that help promote genomic stability. The canonical RPA heterotrimer (RPA1-3) is an essential coordinator of DNA metabolism that interacts with ssDNA and numerous protein partners to coordinate its roles in DNA replication, repair, recombination and telomere maintenance. An alternative form of RPA, termed aRPA, is formed by a complex of RPA4 with RPA1 and RPA3. aRPA is expressed differentially in cells compared to canonical RPA and has been shown to inhibit canonical RPA function while allowing for regular maintenance of cell viability. Interestingly, while aRPA is defective in DNA replication and cell cycle progression, it was shown to play a supporting role in nucleotide excision repair and recombination. The binding domains of canonical RPA interact with a growing number of partners involved in numerous genome maintenance processes. The protein interactions of the RPA-ssDNA complex are not only governed by competition between the binding proteins but also by post-translation modifications such as phosphorylation. Phosphorylation of RPA2 is an important post-translational modification of the RPA complex, and is essential for directing context-specific functions of the RPA complex in the DNA damage response. Due to the importance of RPA in cellular metabolism, it was identified as an appealing target for chemotherapeutic drug development that could be used in future cancer treatment regimens.

NF-κB-induced WIP1 expression promotes colorectal cancer cell proliferation through mTOR signaling

Colorectal cancer (CRC) is one of the major causes of cancer deaths worldwide. Wild-type p53-induced protein 1 (WIP1) is overexpressed in multiple human cancers and acted as an oncogene. This study was aimed to investigate the effect of WIP1 in colorectal cancer growth and analyzed underlying mechanisms. Herein, we determined WIP1 expression in CRC tissues and cell lines, as well as evaluated its detailed function in CRC cell proliferation. Several factors have been reported to mediate WIP1 effects; herein, we examined the involvement of mTOR and p21 in WIP1 regulation of CRC cell proliferation. Moreover, NF-κB has been regarded as a positive transcriptional regulator of WIP1 to activate its expression. NF-κB knockdown suppressed CRC cell proliferation, which could be reversed by WIP1 overexpression, through p21 and mTOR. Further, we examined the binding of NF-κB to the promoter region of WIP1. In CRC tissues, NF-κB expression was significantly up-regulated, and positively correlated with WIP1 expression, suggesting that inhibiting NF-κB expression to attenuate its activating effect on WIP1 expression presented a promising strategy of controlling excess proliferation of CRC cell. In summary, WIP1 promotes CRC proliferation through p21 and mTOR, both downstream targets of p53; NF-κB served as a positive transcriptional regulator of WIP1 to activate its expression and affect its function in CRC cells. Our finding provided a novel strategy for treatment for CRC.

Chemical Features Important for Activity in a Class of Inhibitors Targeting the Wip1 Flap Subdomain

The wild-type p53 induced phosphatase 1, Wip1 (PP2Cδ), is a protein phosphatase 2C (PP2C) family serine/threonine phosphatase that negatively regulates the function of the tumor suppressor p53 and several of its positive regulators such as ATM, Chk1, Chk2, Mdm2, and p38 MAPK. Wip1 dephosphorylates and inactivates its protein targets, which are critical for cellular stress responses. Additionally, Wip1 is frequently amplified and overexpressed in several human cancer types. Because of its negative role in regulating the function of tumor suppressor proteins, Wip1 has been identified as a potential therapeutic target in various types of cancers. Based on a recently reported Wip1 inhibitor (G-1), we performed an extensive structure-activity relationship (SAR) analysis. This led us to interesting findings in SAR trends and to the discovery of new chemical analogues with good specificity and bioavailability.

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.

Cooperation of Nutlin-3a and a Wip1 inhibitor to induce p53 activity

Targeting the Mdm2 oncoprotein by drugs has the potential of re-establishing p53 function and tumor suppression. However, Mdm2-antagonizing drug candidates, e. g. Nutlin-3a, often fail to abolish cancer cell growth sustainably. To overcome these limitations, we inhibited Mdm2 and simultaneously a second negative regulator of p53, the phosphatase Wip1/PPM1D. When combining Nutlin-3a with the Wip1 inhibitor GSK2830371 in the treatment of p53-proficient but not p53-deficient cells, we observed enhanced phosphorylation (Ser 15) and acetylation (Lys 382) of p53, increased expression of p53 target gene products, and synergistic inhibition of cell proliferation. Surprisingly, when testing the two compounds individually, largely distinct sets of genes were induced, as revealed by deep sequencing analysis of RNA. In contrast, the combination of both drugs led to an expression signature that largely comprised that of Nutlin-3a alone. Moreover, the combination of drugs, or the combination of Nutlin-3a with Wip1-depletion by siRNA, activated p53-responsive genes to a greater extent than either of the compounds alone. Simultaneous inhibition of Mdm2 and Wip1 enhanced cell senescence and G2/M accumulation. Taken together, the inhibition of Wip1 might fortify p53-mediated tumor suppression by Mdm2 antagonists.

Wip1 phosphatase: between p53 and MAPK kinases pathways

Cells undergoing oncogenic transformation frequently inactivate tumor suppressor pathways that could prevent their uncontrolled growth. Among those pathways p53 and p38MAPK pathways play a critical role in regulation of cell cycle, senescence and cell death in response to activation of oncogenes, stress and DNA damage. Consequently, these two pathways are important in determining the sensitivity of tumor cells to anti-cancer treatment. Wild type p53-induced phosphatase, Wip1, is involved in governance of both pathways. Recently, strategies directed to manipulation with Wip1 activity proposed to advance current day anticancer treatment and novel chemical compounds synthesized to improve specificity of manipulation with Wip1 activity. Here we reviewed the history of Wip1 studies in vitro and in vivo, in genetically modified animal models that support Wip1 role in tumorigenesis through regulation of p53 and p38MAPK pathways. Based on our knowledge we propose several recommendations for future more accurate studies of Wip1 interactions with other pathways involved in tumorigenesis using recently developed tools and for adoption of Wip1 manipulation strategies in anti-cancer therapy.

Inhibition of Ser/Thr phosphatase PPM1D induces neutrophil differentiation in HL-60 cells

Protein phosphatase Magnesium-dependent 1, Delta (PPM1D) is a wild-type p53-inducible Ser/Thr phosphatase that acts as a negative regulator of the p53 tumor suppressor. Gene amplification and overexpression of PPM1D have been reported in various cancers including leukemia and neuroblastoma. Therefore, PPM1D is a promising target in cancer therapy. It has been reported that PPM1D knockout mice exhibit neutrophilia in blood and show a defective immune response. Here, we found that inhibition of PPM1D induced neutrophil differentiation of human promyelocytic leukemia cell line HL-60. The combination of a PPM1D inhibitor and all-trans retinoic acid significantly increased their differentiation efficiency. The PPM1D inhibitor also induced G1 arrest in HL-60 cells. Our results suggest that PPM1D may be a potential therapeutic target for blood cell diseases including leukemia.

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.

Phosphatase Wip1 in Immunity: An Overview and Update

Wild-type p53-induced phosphatase 1 (Wip1) is a newly identified serine/threonine phosphatase, which belongs to the PP2C family. Due to its involvement in stress-induced networks and overexpression in human tumors, primary studies have mainly focused on the role of Wip1 in tumorigenesis. It now has also been implicated in regulating several other physiological processes such as organism aging and neurogenesis. Recent evidence highlights a new role of Wip1 in controlling immune response through regulating immune cell development and function, as well as through the interplay with inflammatory signaling pathways such NF-κB and p38 mitogen-activated protein kinase. In this short review, we will give an overview of Wip1 in immunity to better understand this important phosphatase.

Wild-type p53-induced phosphatase 1 is a prognostic marker and therapeutic target in bladder transitional cell carcinoma

Wild-type p53-induced phosphatase (Wip1) is an established oncogene and is associated with development of multiple forms of human cancer. However, the expression and role of Wip1 in human bladder transitional cell carcinoma (TCC) remains to be elucidated. In the present study, immunohistochemistry demonstrated that Wip1 was overexpressed in bladder TCC tissues compared with corresponding normal bladder tissues in 106 bladder TCC cases (P<0.0001). Furthermore, high expression levels of Wip1 were significantly associated with increasing tumor size (P=0.002), pathological grade (P=0.025), clinical T stage (P=0.001) and lymph nodal metastasis (P=0.003). Kaplan-Meier survival analysis identified that patients with high Wip1 expression levels exhibited a lower overall survival time (P<0.0001), and Cox proportional hazards regression model analysis demonstrated that Wip1 expression was an independent prognostic factor in patients with bladder TCC (P=0.025). In addition, downregulation of Wip1 expression by transfection with small interfering RNA in bladder cancer cells inhibited cell proliferation, invasion and migration (P<0.05), along with the upregulation of p53 protein levels (P<0.05). These findings suggest that Wip1 may function as a potential prognostic marker and therapeutic target in bladder cancer.

Rapid Discovery and Structure-Activity Relationships of Pyrazolopyrimidines That Potently Suppress Breast Cancer Cell Growth via SRC Kinase Inhibition with Exceptional Selectivity over ABL Kinase

Novel pyrazolopyrimidines displaying high potency and selectivity toward SRC family kinases have been developed by combining ligand-based design and phenotypic screening in an iterative manner. Compounds were derived from the promiscuous kinase inhibitor PP1 to search for analogs that could potentially target a broad spectrum of kinases involved in cancer. Phenotypic screening against MCF7 mammary adenocarcinoma cells generated target-agnostic structure–activity relationships that biased subsequent designs toward breast cancer treatment rather than to a particular target. This strategy led to the discovery of two potent antiproliferative leads with phenotypically distinct anticancer mode of actions. Kinase profiling and further optimization resulted in eCF506, the first small molecule with subnanomolar IC₅₀ for SRC that requires 3 orders of magnitude greater concentration to inhibit ABL. eCF506 exhibits excellent water solubility, an optimal DMPK profile and oral bioavailability, halts SRC-associated neuromast migration in zebrafish embryos without inducing life-threatening heart defects, and inhibits SRC phosphorylation in tumor xenografts in mice.

Squalene Inhibits ATM-Dependent Signaling in γIR-Induced DNA Damage Response through Induction of Wip1 Phosphatase

Ataxia telangiectasia mutated (ATM) kinase plays a crucial role as a master controller in the cellular DNA damage response. Inhibition of ATM leads to inhibition of the checkpoint signaling pathway. Hence, addition of checkpoint inhibitors to anticancer therapies may be an effective targeting strategy. A recent study reported that Wip1, a protein phosphatase, de-phosphorylates serine 1981 of ATM during the DNA damage response. Squalene has been proposed to complement anticancer therapies such as chemotherapy and radiotherapy; however, there is little mechanistic information supporting this idea. Here, we report the inhibitory effect of squalene on ATM-dependent DNA damage signals. Squalene itself did not affect cell viability and the cell cycle of A549 cells, but it enhanced the cytotoxicity of gamma-irradiation (γIR). The in vitro kinase activity of ATM was not altered by squalene. However, squalene increased Wip1 expression in cells and suppressed ATM activation in γIR-treated cells. Consistent with the potential inhibition of ATM by squalene, IR-induced phosphorylation of ATM effectors such as p53 (Ser15) and Chk1 (Ser317) was inhibited by cell treatment with squalene. Thus, squalene inhibits the ATM-dependent signaling pathway following DNA damage through intracellular induction of Wip1 expression.

CHK2 stability is regulated by the E3 ubiquitin ligase SIAH2

The serine threonine checkpoint kinase 2 (CHK2) is a critical protein involved in the DNA damage-response pathway, which is activated by phosphorylation inducing cellular response such as DNA repair, cell-cycle regulation or apoptosis. Although CHK2 activation mechanisms have been amply described, very little is known about degradation control processes. In the present study, we identify the ubiquitin E3 ligase SIAH2 as an interaction partner of CHK2, which mediates its ubiquitination and proteasomal degradation. CHK2 degradation is independent of both its activation and its kinase activity, but also of the phosphorylation in S456. We show that SIAH2-deficient cells present CHK2 accumulation together with lower ubiquitination levels. Accordingly, SIAH2 depletion by siRNA increases CHK2 levels. In response to DNA damage induced by etoposide, interaction between both proteins is disrupted, thus avoiding CHK2 degradation and promoting its stabilization. We also found that CHK2 phosphorylates SIAH2 at three residues (Thr26, Ser28 and Thr119), modifying its ability to regulate certain substrates. Cellular arrest in the G2/M phase induced by DNA damage is reverted by SIAH2 expression through the control of CHK2 levels. We observed that hypoxia decreases CHK2 levels in parallel to SIAH2 induction. Similarly, we provide evidence suggesting that resistance to apoptosis induced by genotoxic agents in cells subjected to hypoxia could be partly explained by the mutual regulation between both proteins. These results indicate that SIAH2 regulates CHK2 basal turnover, with important consequences on cell-cycle control and on the ability of hypoxia to alter the DNA damage-response pathway in cancer cells.

2-Indolinone a versatile scaffold for treatment of cancer: a patent review (2008-2014)

INTRODUCTION

2-Indolinone is a well-known aromatic heterocyclic organic compound. A lot of work has been done on this bicyclic structure by academic and company researchers to synthesize compounds directed to a plethora of molecular targets in order to discover new drug leads. This review presents up-to-date information in the field of cancer therapy research based on this small building block.

AREAS COVERED

The present review gives an account of the recent patent literature (2008-2014) describing the discovery of 2-indolinone derivatives with selected therapeutic activities. In this period, a large amount of patents were published on this topic. We have limited the analysis to 37 patents on 2-indolinone derivatives having potential clinical application as chemotherapeutic agents. In this review, the therapeutic applications of 2-indolinone derivatives for the treatment of cancer reported in international patents have been discussed.

EXPERT OPINION

2-Indolinone is the scaffold of the compounds considered from a medicinal chemistry perspective. Many of them have been developed and marketed for therapeutic use. In cancer chemotherapy, progress has been made in designing selective 2-indolinone derivatives. Some of them show preclinical efficacy. However, 2-indolinone has not exhausted all of its potential in the development of new compounds for clinical applications and remains a great tool for future research.

DNA damage-induced regulatory interplay between DAXX, p53, ATM kinase and Wip1 phosphatase

Death domain-associated protein 6 (DAXX) is a histone chaperone, putative regulator of apoptosis and transcription, and candidate modulator of p53-mediated gene expression following DNA damage. DAXX becomes phosphorylated upon DNA damage, however regulation of this modification, and its relationship to p53 remain unclear. Here we show that in human cells exposed to ionizing radiation or genotoxic drugs etoposide and neocarzinostatin, DAXX became rapidly phosphorylated in an ATM kinase-dependent manner. Our deletion and site-directed mutagenesis experiments identified Serine 564 (S564) as the dominant ATM-targeted site of DAXX, and immunofluorescence experiments revealed localization of S564-phosphorylated DAXX to PML nuclear bodies. Furthermore, using a panel of human cell types, we identified the p53-regulated Wip1 protein phosphatase as a key negative regulator of DAXX phosphorylation at S564, both in vitro and in cells. Consistent with the emerging oncogenic role of Wip1, its DAXX-dephosphorylating impact was most apparent in cancer cell lines harboring gain-of-function mutant and/or overexpressed Wip1. Unexpectedly, while Wip1 depletion increased DAXX phosphorylation both before and after DNA damage and increased p53 stability and transcriptional activity, knock-down of DAXX impacted neither p53 stabilization nor p53-mediated expression of Gadd45a, Noxa, Mdm2, p21, Puma, Sesn2, Tigar or Wip1. Consistently, analyses of cells with genetic, TALEN-mediated DAXX deletion corroborated the notion that neither phosphorylated nor non-phosphorylated DAXX is required for p53-mediated gene expression upon DNA damage. Overall, we identify ATM kinase and Wip1 phosphatase as opposing regulators of DAXX-S564 phosphorylation, and propose that the role of DAXX phosphorylation and DAXX itself are independent of p53-mediated gene expression.