The structure and expression of the murine wildtype p53-induced phosphatase 1 (Wip1) gene

The human wildtype p53-induced phosphatase 1 (Wip1; GenBank symbol Ppm1d) gene encodes a type 2C protein phosphatase (PP2C) that is induced by ionizing radiation in a p53-dependent manner. We have cloned and sequenced the mouse Wip1 gene and its encoded mRNA. The mouse Wip1 gene is composed of six exons and spans over 36 kb of DNA. The mouse cDNA sequence predicts a 598-amino-acid protein with a molecular mass of roughly 66 kDa. Comparison of human and mouse Wip1 sequences revealed 83% overall identity at the amino acid level. The 5'-flanking region of exon 1 had promoter elements characteristic of a housekeeping gene. The Wip1 coding sequences share conserved functional regions with other PP2Cs from a diverse array of species. Expression of Wip1 mRNA was detected ubiquitously in adult and embryonic tissues, though expression in the testis was much higher than in other tissues. Wip1 has been mapped near the p53 gene on mouse chromosome 11.     

Development of a substrate-based cyclic phosphopeptide inhibitor of protein phosphatase 2Cdelta, Wip1

The wild-type p53-induced phosphatase, Wip1 (PP2Cdelta or PPM1D) is a member of the protein phosphatase 2C (PP2C) family and functions as a negative regulator of the p38 MAP kinase-p53 signaling pathway. PPM1D is amplified or Wip1 is overexpressed in several human cancers, and it acts as a weak oncogene. Although inhibition of Wip1 may have therapeutic value, no specific inhibitors are available. In this study, we designed phosphopeptide inhibitors for Wip1 on the basis of its optimal substrate sequence. We found that phosphoserine-containing diphosphorylated peptides with the sequence pSXpY inhibited Wip1 phosphatase activity, whereas phosphothreonine-containing peptides with the sequence pTXpY were physiological substrates. Moreover, the X residue in the pSXpY sequence modulated inhibitor activity, and beta-branched amino acid-substituted (Ile or Val) phosphopeptides showed high inhibitory potencies. A thioether cyclic phosphopeptide c(MpSIpYVA) had a K(i) <1.0 microM. Two serine/threonine phosphatases, PP2Calpha and PP2A, were not significantly inhibited by the cyclic phosphopeptide with a nonhydrolyzable phosphoserine mimetic. A homology model of Wip1 bound to a cyclic phosphopeptide and site-directed mutagenesis helped to identify residues important for Wip1 inhibitor selectivity among the PP2C family. These results provide the first proof of concept of a specific inhibitor of the catalytic site of Wip1 and should be useful for developing potential anti-cancer drugs.     

Wip1 contributes to cell homeostasis maintained by the steady-state level of Wtp53

Wip1, a human protein Ser/Thr phosphatase also called PPM1D, stands for wild-type p53 induced phosphatase 1. Emerging evidences indicate that Wip1 can act as an oncogene largely by turning off DNA damage checkpoint responses. Here we report an unrecognized role of Wipl in normally growing cells. Wip1 can be induced by wild-type p53 under not only stressed but also non-stressed conditions. It can trigger G₂/M arrest in wild-type p53 containing cells, which was attributed to the decreased Cdc2 kinase activity resulting at least partly from a high level of inhibitory tyrosine phosphorylation on Cdc2 protein at Tyr-15. Furthermore, we also found that Wip1 not only causes G₂/M arrest but also decreases cell death triggered by microtubule assembly inhibitor in mouse fibroblasts when wild-type p53 function was restored. These results indicate that Wip1 can provide ample time for wild-type p53-containing cells to prepare entry into mitosis and avoid encountering mitotic catastrophe. Therefore, Wipl may play important roles in cell/tissue homeostasis maintained by wild-type p53 under normal conditions, enhancing our understanding of how p53 makes cell-fate decisions.Key words: p53, Wip1, cell homeostasis, cell arrest, cell death     

Wip1 contributes to cell homeostasis maintained by the steady-state level of Wtp53

Wip1, a human protein Ser/Thr phosphatase also called PPM1D, stands for wild type p53 induced phosphatase 1. Emerging evidences indicate that Wip1 can act as an oncogene largely by turning off DNA damage checkpoint responses. Here we report an unrecognized role of Wipl in normally growing cells. Wip1 can be induced by wild type p53 under not only stressed but also non-stressed conditions. It can trigger G₂/M arrest in wild type p53 containing cells, which was attributed to the decreased Cdc2 kinase activity resulting at least partly from a high level of inhibitory tyrosine phosphorylation on Cdc2 protein at Tyr-15. Furthermore, we also found that Wip1 not only causes G₂/M arrest but also decreases cell death triggered by microtubule assembly inhibitor in mouse fibroblasts when wild type p53 function was restored. These results indicate that Wip1 can provide ample time for wild type p53-containing cells to prepare entry into mitosis and avoid encountering mitotic catastrophe. Therefore, Wipl may play important roles in cell/tissue homeostasis maintained by wild type p53 under normal conditions, enhancing our understanding of how p53 makes cell-fate decisions.     

The Wip1 phosphatase PPM1D dephosphorylates SQ/TQ motifs in checkpoint substrates phosphorylated by PI3K-like kinases

The wild-type p53-induced phosphatase Wip1 (PP2Cdelta or PPM1D) is a member of the protein phosphatase 2C (PP2C) family and controls cell cycle checkpoints in response to DNA damage. p38 MAPK and ATM were identified as physiological substrates of Wip1, and we previously reported a substrate motif that was defined using variants of the p38(180pT 182pY) diphosphorylated peptide, TDDEMpTGpYVAT. However, the substrate recognition motifs for Wip1 have not been fully defined as the sequences surrounding the targeted residues in ATM and p38 MAPK appear to be unrelated. Using a recombinant human Wip1 catalytic domain (rWip1), in this study we measured the kinetic parameters for variants of the ATM(1981pS) phosphopeptide, AFEEGpSQSTTI. We found that rWip1 dephosphorylates phosphoserine and phosphothreonine in the p(S/T)Q motif, which is an essential requirement for substrate recognition. In addition, acidic, hydrophobic, or aromatic amino acids surrounding the p(S/T)Q sequence have a positive influence, while basic amino acids have a negative influence on substrate dephosphorylation. The kinetic constants allow discrimination between true substrates and nonsubstrates of Wip1, and we identified several new putative substrates that include HDM2, SMC1A, ATR, and Wip1 itself. A three-dimensional molecular model of Wip1 with a bound substrate peptide and site-directed mutagenesis analyses suggested that the important residues for ATM(1981pS) substrate recognition are similar but not identical to those for the p38(180pT 182pY) substrate. Results from this study should be useful for predicting new physiological substrates that may be regulated by Wip1 and for developing selective anticancer drugs.     

Mice Deficient for the Wild-Type p53-Induced Phosphatase Gene (Wip1) Exhibit Defects in Reproductive Organs, Immune Function, and Cell Cycle Control

The Wip1 gene is a serine/threonine phosphatase that is induced in a p53-dependent manner by DNA-damaging agents. We show here that Wip1 message is expressed in moderate levels in all organs, but is present at very high levels in the testes, particularly in the postmeiotic round spermatid compartment of the seminiferous tubules. We have confirmed that Wip1 mRNA is induced by ionizing radiation in mouse tissues in a p53-dependent manner. To further determine the normal biological function of Wip1 in mammalian organisms, we have generated Wip1-deficient mice. Wip1 null mice are viable but show a variety of postnatal abnormalities, including variable male runting, male reproductive organ atrophy, reduced male fertility, and reduced male longevity. Mice lacking Wip1 show increased susceptibility to pathogens and diminished T- and B-cell function. Fibroblasts derived from Wip1 null embryos have decreased proliferation rates and appear to be compromised in entering mitosis. The data are consistent with an important role for Wip1 in spermatogenesis, lymphoid cell function, and cell cycle regulation.     

Wip1 suppresses apoptotic cell death through direct dephosphorylation of BAX in response to γ-radiation

Wild-type p53-induced phosphatase 1 (Wip1) is a p53-inducible serine/threonine phosphatase that switches off DNA damage checkpoint responses by the dephosphorylation of certain proteins (i.e. p38 mitogen-activated protein kinase, p53, checkpoint kinase 1, checkpoint kinase 2, and uracil DNA glycosylase) involved in DNA repair and the cell cycle checkpoint. Emerging data indicate that Wip1 is amplified or overexpressed in various human tumors, and its detection implies a poor prognosis. In this study, we show that Wip1 interacts with and dephosphorylates BAX to suppress BAX-mediated apoptosis in response to γ-irradiation in prostate cancer cells. Radiation-resistant LNCaP cells showed dramatic increases in Wip1 levels and impaired BAX movement to the mitochondria after γ-irradiation, and these effects were reverted by a Wip1 inhibitor. These results show that Wip1 directly interacts with and dephosphorylates BAX. Dephosphorylation occurs at threonines 172, 174 and 186, and BAX proteins with mutations at these sites fail to translocate efficiently to the mitochondria following cellular γ-irradiation. Overexpression of Wip1 and BAX, but not phosphatase-dead Wip1, in BAX-deficient cells strongly reduces apoptosis. Our results suggest that BAX dephosphorylation of Wip1 phosphatase is an important regulator of resistance to anticancer therapy. This study is the first to report the downregulation of BAX activity by a protein phosphatase.     

Expression of a Homeostatic Regulator, Wip1 (Wild-type p53-induced Phosphatase), Is Temporally Induced by c-Jun and p53 in Response to UV Irradiation

Wild-type p53-induced phosphatase (Wip1) is induced by p53 in response to stress, which results in the dephosphorylation of proteins (i.e. p38 MAPK, p53, and uracil DNA glycosylase) involved in DNA repair and cell cycle checkpoint pathways. p38 MAPK-p53 signaling is a unique way to induce Wip1 in response to stress. Here, we show that c-Jun directly binds to and activates the Wip1 promoter in response to UV irradiation. The binding of p53 to the promoter occurs earlier than that of c-Jun. In experiments, mutation of the p53 response element (p53RE) or c-Jun consensus sites reduced promoter activity in both non-stressed and stressed A549 cells. Overexpression of p53 significantly decreased Wip1 expression in HCT116 p53^+/+ cells but increased it in HCT116 p53^−/− cells. Adenovirus-mediated p53 overexpression greatly decreased JNK activity. Up-regulation of Wip1 via the p38 MAPK-p53 and JNK-c-Jun pathways is specific, as demonstrated by our findings that p38 MAPK and JNK inhibitors affected the expression of the Wip1 protein, whereas an ERK inhibitor did not. c-Jun activation occurred much more quickly, and to a greater extent, in A549-E6 cells than in A549 cells, with delayed but fully induced Wip1 expression. These data indicate that Wip1 is activated via both the JNK-c-Jun and p38 MAPK-p53 signaling pathways and that temporal induction of Wip1 depends largely on the balance between c-Jun and p53, which compete for JNK binding. Moreover, our results suggest that JNK-c-Jun-mediated Wip1 induction could serve as a major signaling pathway in human tumors in response to frequent p53 mutation.     

Wip1 phosphatase-deficient mice exhibit defective T cell maturation due to sustained p53 activation

The PP2C phosphatase Wip1 dephosphorylates p38 and blocks UV-induced p53 activation in cultured human cells. Although the level of TCR-induced p38 MAPK activity is initially comparable between Wip1-/- and wild-type thymocytes, phosphatase-deficient cells failed to down-regulate p38 MAPK activity after 6 h. Analysis of young Wip1-deficient mice showed that they had fewer splenic T cells. Their thymi were smaller, contained significantly fewer cells, and failed to undergo age-dependent involution compared with wild-type animals. Analysis of thymocyte subset numbers by flow cytometry suggested that cell numbers starting at the double-negative (DN)4 stage are significantly reduced in Wip1-deficient mice, and p53 activity is elevated in cell-sorted DN4 and double-positive subpopulations. Although apoptosis and proliferation was normal in Wip1-/- DN4 cells, they appeared to be in cell cycle arrest. In contrast, a significantly higher percentage of apoptotic cells were found in the double-positive population, and down-regulation of thymocyte p38 MAPK activation by anti-CD3 was delayed. To examine the role of p38 MAPK in early thymic subpopulations, fetal thymic organ cultures cultured in the presence/absence of a p38 MAPK inhibitor did not correct the thymic phenotype. In contrast, the abnormal thymic phenotype of Wip1-deficient mice was reversed in the absence of p53. These data suggest that Wip1 down-regulates p53 activation in the thymus and is required for normal alphabeta T cell development.     

Wip1 cooperates with KPNA2 to modulate the cell proliferation and migration of colorectal cancer via a p53-dependent manner

Due to the increasing incidence and mortality, the early diagnosis, specific targeted therapies, and prognosis for colorectal cancer (CRC) attract more and more attention. Wild-type p53-induced phosphatase 1 (Wip1) and karyopherin α2 (KPNA2) have been regarded as oncogenes in many cancers, including CRC. Wip1 dephosphorylates p53 to inactivate it. TP53 activator and Wip1 inhibitor downregulate KPNA2 expression. Therefore, we speculate that Wip1 may co-operate with KPNA2 to modulate CRC progression in a p53-dependent manner. Here, Wip1 and KPNA2 messenger RNA expression and protein levels are significantly increased in CRC tissues and cell lines and are positively correlated with each other. Wip1 silence increases p53 phosphorylation while decreases KPNA2 protein. Wip1 knockdown remarkably suppresses CRC cell proliferation and migration while KPNA2 overexpression exerts an opposing effect. KPNA2 overexpression could partially rescue Wip1 silence-inhibited CRC cell proliferation and migration. Finally, Wip1 interacts with KPNA2 to modulate the activation of AKT/GSK-3β signaling and metastasis-related factors. In summary, Wip1 could co-operate with KPNA2 to modulate CRC cell proliferation and migration, possibly via a p53-dependent manner, through downstream AKT/GSK-3β pathway. We provided a novel mechanism of Wip1 interacting with KPNA2, therefore modulating CRC cell proliferation and migration.     

Regulation of the antioncogenic Chk2 kinase by the oncogenic Wip1 phosphatase

The antioncogenic Chk2 kinase plays a crucial role in DNA damage-induced cell-cycle checkpoint regulation. Here we show that Chk2 associates with the oncogenic protein Wip1 (wild-type p53-inducible phosphatase 1) (PPM1D), a p53-inducible protein phosphatase. Phosphorylation of Chk2 at threonine68 (Thr68), a critical event for Chk2 activation, which is normally induced by DNA damage or overexpression of Chk2, is inhibited by expression of wild-type (WT), but not a phosphatase-deficient mutant (D314A) of Wip1 in cultured cells. Furthermore, an in vitro phosphatase assay revealed that Wip1 (WT), but not Wip1 (D314A), dephosphorylates Thr68 on phosphorylated Chk2 in vitro, resulting in the inhibition of Chk2 kinase activity toward glutathione S-transferase-Cdc25C. Moreover, inhibition of Wip1 expression by RNA interference results in abnormally sustained Thr68 phosphorylation of Chk2 and increased susceptibility of cells in response to DNA damage, indicating that Wip1 acts as a negative regulator of Chk2 in response to DNA damage.     

Down-regulation of Wild-type p53-induced Phosphatase 1 (Wip1) Plays a Critical Role in Regulating Several p53-dependent Functions in Premature Senescent Tumor Cells

Premature or drug-induced senescence is a major cellular response to chemotherapy in solid tumors. The senescent phenotype develops slowly and is associated with chronic DNA damage response. We found that expression of wild-type p53-induced phosphatase 1 (Wip1) is markedly down-regulated during persistent DNA damage and after drug release during the acquisition of the senescent phenotype in carcinoma cells. We demonstrate that down-regulation of Wip1 is required for maintenance of permanent G₂ arrest. In fact, we show that forced expression of Wip1 in premature senescent tumor cells induces inappropriate re-initiation of mitosis, uncontrolled polyploid progression, and cell death by mitotic failure. Most of the effects of Wip1 may be attributed to its ability to dephosphorylate p53 at Ser¹⁵ and to inhibit DNA damage response. However, we also uncover a regulatory pathway whereby suppression of p53 Ser¹⁵ phosphorylation is associated with enhanced phosphorylation at Ser⁴⁶, increased p53 protein levels, and induction of Noxa expression. On the whole, our data indicate that down-regulation of Wip1 expression during premature senescence plays a pivotal role in regulating several p53-dependent aspects of the senescent phenotype.     

Substrate specificity of the human protein phosphatase 2Cdelta, Wip1

Wip1, the wild-type p53-induced phosphatase, selectively dephosphorylates a threonine residue on p38 MAPK and mediates a negative feedback loop of the p38 MAPK-p53 signaling pathway. To identify the substrate specificity of Wip1, we prepared a recombinant human Wip1 catalytic domain (rWip1) and measured kinetic parameters for phosphopeptides containing the dephosphorylation sites in p38alpha and in a new substrate, UNG2. rWip1 showed properties that were comparable to those of PP2Calpha or full-length Wip1 in terms of affinity for Mg(2+), insensitivity to okadaic acid, and threonine dephosphorylation. The substrate specificity constant k(cat)/K(m) for a diphosphorylated peptide with a pTXpY sequence was 6-8-fold higher than that of a monophosphorylated peptide with a pTXY sequence, while PP2Calpha showed a preference for monophosphorylated peptides. Although individual side chains before and after the pTXpY sequence of the substrate did not have a significant effect on rWip1 activity, a chain length of at least five residues, including the pTXpY sequence, was important for substrate recognition by rWip1. Moreover, the X residue in the pTXpY sequence affected affinity for rWip1 and correlated with selectivity for MAPKs. These findings suggest that substrate recognition by Wip1 is centered toward a very narrow region around the pTXpY sequence. Three-dimension homology models of Wip1 with bound substrate peptides were constructed, and site-directed mutagenesis was performed to confirm the importance of specific residues for substrate recognition. The results of our study should be useful for predicting new physiological substrates and for designing specific Wip1 inhibitors.     

Role of Gadd45a in Wip1-dependent regulation of intestinal tumorigenesis

Conversion of intestinal stem cells into tumor-initiating cells is an early step in Apc^Min-induced polyposis. Wild-type p53-induced phosphatase 1 (Wip1)-dependent activation of a DNA damage response and p53 has a permanent role in suppression of stem cell conversion, and deletion of Wip1 lowers the tumor burden in Apc^Min mice. Here we show that cyclin-dependent kinase inhibitor 2a, checkpoint kinase 2, and growth arrest and DNA damage gene 45a (Gadd45a) exert critical functions in the tumor-resistant phenotype of Wip1-deficient mice. We further identified Gadd45a as a haploinsufficient gene in the regulation of Wip1-dependent tumor resistance in mice. Gadd45a appears to function through its ability to activate the Jnk-dependent signaling pathway that in turn is a necessary mediator of the proapoptotic functions of p53 that respond to activation of the β-catenin signaling pathway. We propose that silencing of Gadd45a is sufficient to override p53 activation in the presence of active β-catenin under conditions of an enhanced DNA damage response.