Identification of a functional link for the p53 tumor suppressor protein in dexamethasone-induced growth suppression

Prunetin Protects Against Dexamethasone-Induced Pancreatic Β-Cell Apoptosis via Modulation of p53 Signaling Pathway

Long-term administration of dexamethasone results in insulin resistance and pancreatic β-cell apoptosis. Prunetin (an O-methylated isoflavone, a type of flavonoid) is demonstrated to protect diabetes, but the molecular mechanism of this protection is still unclear. This study thus aims to investigate how prunetin protects against dexamethasone-induced pancreatic β-cell apoptosis. Rat insulinoma (INS-1) cells were cultured in medium with or without dexamethasone in the presence or absence of prunetin or pifithrin-α, a p53 inhibitor. Cell apoptosis was measured by Annexin V/propidium iodide staining. Dexamethasone significantly induced INS-1 apoptosis but dexamethasone plus prunetin significantly reduced INS-1 apoptosis. Dexamethasone-treated INS-1 upregulated p53 protein expression; the induction of p53 was also reduced in the presence of RU486, a glucocorticoid receptor (GR) inhibitor. This suggested that dexamethasone induced P53 via GR. Dexamethasone-treated INS-1 significantly increased p53, Bax, and Rb protein expressions, whereas treatments of dexamethasone plus prunetin or pifithrin-α significantly decreased these protein expressions. In addition, dexamethasone significantly decreased B-cell lymphoma 2 (Bcl2), while dexamethasone plus prunetin or pifithrin-α significantly increased Bcl2. Dexamethasone significantly increased caspase-3 activity while co-treatment of dexamethasone plus prunetin or pifithrin-α significantly decreased caspase-3 activity to the control level. Taken together, our results revealed that prunetin protected against dexamethasone-induced pancreatic β-cells apoptosis via modulation of the p53 signaling pathway.

An Ultra-High-Throughput Screen for Catalytic Inhibitors of Serine/Threonine Protein Phosphatases Types 1 and 5 (PP1C and PP5C)

Although there has been substantial success in the development of specific inhibitors for protein kinases, little progress has been made in the identification of specific inhibitors for their protein phosphatase counterparts. Inhibitors of PP1 and PP5 are desired as probes for research and to test their potential for drug development. We developed and miniaturized (1536-well plate format) nearly identical homogeneous, fluorescence intensity (FLINT) enzymatic assays to detect inhibitors of PP1 or PP5. The assays were used in an ultra-high-throughput screening (uHTS) campaign, testing >315,000 small-molecule compounds. Both assays demonstrated robust performance, with a Z' of 0.92 ± 0.03 and 0.95 ± 0.01 for the PP1 and PP5 assays, respectively. Screening the same library with both assays aided the identification of class inhibitors and assay artifacts. Confirmation screening and hit prioritization assays used [32P/33P]-radiolabel protein substrates, revealing excellent agreement between the FLINT and radiolabel assays. This screening campaign led to the discovery of four novel unrelated small-molecule inhibitors of PP1 and ~30 related small-molecule inhibitors of PP5. The results suggest that this uHTS approach is suitable for identifying selective chemical probes that inhibit PP1 or PP5 activity, and it is likely that similar assays can be developed for other PPP-family phosphatases.

The role of p53 in myelodysplastic syndromes and acute myeloid leukemia: molecular aspects and clinical implications

TP53 gene mutations occurring in patients with myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) are associated with high-risk karyotypes including 17p abnormalities, monosomal and complex cytogenetics. TP53 mutations in these disorders portend rapid disease progression and resistance to conventional therapeutics. Notably, the size of the TP53 mutant clone as measured by mutation allele burden is directly linked to overall survival (OS) confirming the importance of p53 as a negative prognostic variable. In nucleolar stress-induced ribosomopathies, such as del(5q) MDS, disassociation of MDM2 and p53 results in p53 accumulation in erythroid precursors manifested as erythroid hypoplasia. P53 antagonism by lenalidomide or other therapeutics such as antisense oligonucleotides, repopulates erythroid precursors and enhances effective erythropoiesis. These findings demonstrate that p53 is an intriguing therapeutic target that is currently under investigation in MDS and AML. This study reviews molecular advances in understanding the role of p53 in MDS and AML, and explores potential therapeutic strategies in this era of personalized medicine.

Knockdown of protein phosphatase 5 inhibits ovarian cancer growth in vitro

Ovarian cancer is the most common cause of gynecological cancer-related mortality. Serine/threonine protein phosphatase 5 (PP5, PPP5C) has been recognized to be involved in the regulation of multiple cellular signaling cascades that control diverse cellular processes, including cell growth, differentiation, proliferation, motility and apoptosis. In this study, to evaluate the functional role of PP5 in ovarian cancer cells, lentivirus-mediated RNA interference (RNAi) was applied to silence PPP5C in the human ovarian cancer cell line CAOV-3. Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Cell colony forming ability was measured by colony formation. Cell cycle progression was determined by propidium iodide staining and flow cytometry. The results demonstrated that lentivirus-mediated RNAi specifically suppressed the expression of PPP5C at the mRNA and protein levels in CAOV-3 cells. Further investigations revealed that PP5 knockdown significantly inhibited the proliferation and colony formation of CAOV-3 cells. Moreover, the cell cycle of CAOV-3 cells was arrested at the G0/G1 phase following PP5 knockdown. This study highlights the crucial role of PP5 in promoting ovarian cancer cell proliferation, and provides a foundation for further study into the clinical potential of lentiviral-mediated delivery of PP5 RNAi therapy for the treatment of ovarian cancer.

Inhibition of protein phosphatase 5 (PP5) suppresses survival and growth of colorectal cancer cells

Protein phosphatase 5 (PP5) is a unique member of the protein phosphatases family that functions in multiple signaling pathways involved in DNA damage, cell cycle control, cell growth, and apoptosis. Recent evidence indicated that PP5 may play a role in cancer progression. In this study, we aimed to examine the biological effect of PP5 on cell growth and apoptosis in human colorectal cancer (CRC). We first knocked down PP5 expression in RKO cells via a short hairpin RNA containing lentivirus system. Then, methylthiazoletetrazolium assay, colony formation assay, and flow cytometry analysis were performed. The proliferation and colony formation ability of RKO cells were remarkably suppressed in PP5-silenced groups, as compared with control groups. Moreover, downregulation of PP5 resulted in a significant G0/G1 phase cell cycle arrest and an induction of apoptosis. In all, these results demonstrated the importance of PP5 in CRC cell growth, and it might be used as a potential therapeutic target for the treatment of CRC.

Serine/Threonine Protein Phosphatase-5 Accelerates Cell Growth and Migration in Human Glioma

Glioma is the most common type of primary central nervous system tumor. Ser/Thr protein phosphatase 5 (PP5) has been shown to regulate multiple signaling cascades that suppress growth and facilitate apoptosis in several human cancer cells. However, the role of PP5 in human gliomas remains unclear. Herein, the relationship between PP5 expression and glioma cell growth was investigated, and the therapeutic value of PP5 in glioma was further evaluated. We employed a short hairpin RNA targeting PPP5C gene to knock down PP5 expression in human glioma cell lines U251 and U373. Depletion of PPP5C via RNAi remarkably inhibited glioma cell proliferation and colony formation, and arrested cell cycle in the G0/G1 phase. Moreover, knockdown of PP5 markedly suppressed glioma cell migration, as determined by Transwell assay. Our findings suggest that PPP5C could be essential for glioma cell growth and serve as a promising therapeutic target in human gliomas.

Knockdown of PPP5C inhibits growth of hepatocellular carcinoma cells in vitro

Ser/Thr protein phosphatase 5 (PPP5C) has been reported to participate in tumor progression. However, its functional role in hepatocellular carcinoma (HCC) remains unknown yet. In this study, we firstly evaluated the expression levels of PPP5C in six HCC cell lines by real-time PCR and found that PPP5C was widely expressed in HCC cells. To explore the role of PPP5C in HCC cell growth, lentivirus-mediated short hairpin RNA (shRNA) was employed to silence PPP5C expression in HepG2 and Bel-7404 cells. The expression of PPP5C was significantly downregulated in PPP5C knockdown cells. Knockdown of PPP5C markedly suppressed the proliferation and colony formation ability of HCC cells. Moreover, cell cycle analysis showed that PPP5C depletion in HepG2 cells led to G0/G1 phase and G2/M phase arrest. We demonstrate for the first time that PPP5C is essential for growth of HCC cells, which suggests that inhibition of PPP5C by RNAi may be a potential therapeutic strategy for the treatment of HCC.

Development and validation of a robust and sensitive assay for the discovery of selective inhibitors for serine/threonine protein phosphatases PP1α (PPP1C) and PP5 (PPP5C)

Protein phosphatase types 1 α (PP1α/PPP1C) and 5 (PP5/PPP5C) are members of the PPP family of serine/threonine protein phosphatases. PP1 and PP5 share a common catalytic mechanism, and several natural compounds, including okadaic acid, microcystin, and cantharidin, act as strong inhibitors of both enzymes. However, to date there have been no reports of compounds that can selectively inhibit PP1 or PP5, and specific or highly selective inhibitors for either PP1 or PP5 are greatly desired by both the research and pharmaceutical communities. Here we describe the development and optimization of a sensitive and robust (representative PP5C assay data: Z'=0.93; representative PP1Cα assay data: Z'=0.90) fluorescent phosphatase assay that can be used to simultaneously screen chemical libraries and natural product extracts for the presence of catalytic inhibitors of PP1 and PP5.

P53 dependent mitochondrial permeability transition pore opening is required for dexamethasone-induced death of osteoblasts

Prolonged or overdose glucocorticoids (GCs) usage is the common cause of osteoporosis. In the present study, we studied the cellular mechanism of dexamethasone (Dex)-induce osteoblast cell death by focusing on the role of mitochondrial permeability transition pore (mPTP). In cultured osteoblastic MC3T3-E1 cells, Dex-induced mPTP opening, which was demonstrated by mitochondrial membrane potential (MPP) decrease, cyclophilin-D (CyPD)-adenine nucleotide translocator 1 (ANT-1) mitochondrial complexation and cytochrome C (cyto-C) release. The mPTP inhibitor sanglifehrin A (SfA) dramatically inhibited Dex-induced MPP loss, cyto-C release and MC3T3-E1 cell death. Dex-induced cell death requires mPTP composing protein CyPD, as CyPD inhibitor cyclosporin A (CsA) and CyPD siRNA knockdown inhibited Dex-induced MC3T3-E1 cell death, while CyPD overexpression aggravated Dex's cytotoxic effect. We found that Dex induced P53 phosphorylation and translocation to mitochondria, where it formed a complex with CyPD. Glucocorticoid receptor (GR) siRNA knockdown, or P53 inhibition (by its inhibitor pifithrin-α or shRNA silencing) suppressed Dex-induced CyPD-P53 mitochondrial association and subsequent MC3T3-E1 cell death. Finally, in primary cultured osteoblasts, Dex-induced cell death was inhibited by CsA, SfA or pifithrin-α. Together, our data suggest that Dex-induced osteoblast cell death is associated with GR-P53-regulated mPTP opening.

L-GILZ binds p53 and MDM2 and suppresses tumor growth through p53 activation in human cancer cells

The transcription factor p53 regulates the expression of genes crucial for biological processes such as cell proliferation, metabolism, cell repair, senescence and apoptosis. Activation of p53 also suppresses neoplastic transformations, thereby inhibiting the growth of mutated and/or damaged cells. p53-binding proteins, such as mouse double minute 2 homolog (MDM2), inhibit p53 activation and thus regulate p53-mediated stress responses. Here, we found that long glucocorticoid-induced leucine zipper (L-GILZ), a recently identified isoform of GILZ, activates p53 and that the overexpression of L-GILZ in p53(+/+) HCT116 human colorectal carcinoma cells suppresses the growth of xenografts in mice. In the presence of both p53 and MDM2, L-GILZ binds preferentially to MDM2 and interferes with p53/MDM2 complex formation, making p53 available for downstream gene activation. Consistent with this finding, L-GILZ induced p21 and p53 upregulated modulator of apoptosis (PUMA) expression only in p53(+/+) cells, while L-GILZ silencing reversed the anti-proliferative activity of dexamethasone as well as expression of p53, p21 and PUMA. Furthermore, L-GILZ stabilizes p53 proteins by decreasing p53 ubiquitination and increasing MDM2 ubiquitination. These findings reveal L-GILZ as a regulator of p53 and a candidate for new therapeutic anti-cancer strategies for tumors associated with p53 deregulation.

Suppression of Ser/Thr phosphatase 4 (PP4C/PPP4C) mimics a novel post-mitotic action of fostriecin, producing mitotic slippage followed by tetraploid cell death

Fostriecin is a natural product purified from Sterptomyces extracts with antitumor activity sufficient to warrant human clinical trials. Unfortunately, difficulties associated with supply and stable drug formulation stalled further development. At a molecular level, fostriecin is known to act as a catalytic inhibitor of four PPP-family phosphatases, and reports describing the design of molecules in this class suggest derivatives targeting enzymes within the fostriecin-sensitive subfamily can be successful. However, it is not clear if the tumor-selective cytotoxicity of fostriecin results from the inhibition of a specific phosphatase, multiple phosphatases, or a limited subset of fostriecin sensitive phosphatases. How the inhibition of sensitive phosphatases contributes to tumor-selective cytotoxicity is also not clear. Here, high-content time-lapse imaging of live cells revealed novel insight into the cellular actions of fostriecin, showing that fostriecin-induced apoptosis is not simply induced following a sustained mitotic arrest. Rather, apoptosis occurred in an apparent second interphase produced when tetraploid cells undergo mitotic slippage. Comparison of the actions of fostriecin and antisense-oligonucleotides specifically targeting human fostriecin-sensitive phosphatases revealed that the suppression PP4C alone is sufficient to mimic many actions of fostriecin. Importantly, targeted suppression of PP4C induced apoptosis, with death occurring in tetraploid cells following mitotic slippage. This effect was not observed following the suppression of PP1C, PP2AC, or PP5C. These data clarify PP4C as a fostriecin-sensitive phosphatase and demonstrate that the suppression of PP4C triggers mitotic slippage/apoptosis.

IMPLICATIONS

Future development of fostriecin class inhibitors should consider PP4C as a potentially important target. Mol Cancer Res; 11(8); 845-55. ©2013 AACR.

Glucocorticoid receptor and sequential P53 activation by dexamethasone mediates apoptosis and cell cycle arrest of osteoblastic MC3T3-E1 cells

Glucocorticoids play a pivotal role in the proliferation of osteoblasts, but the underlying mechanism has not been successfully elucidated. In this report, we have investigated the molecular mechanism which elucidates the inhibitory effects of dexamethasone on murine osteoblastic MC3T3-E1 cells. It was found that the inhibitory effects were largely attributed to apoptosis and G1 phase arrest. Both the cell cycle arrest and apoptosis were dependent on glucocorticoid receptor (GR), as they were abolished by GR blocker RU486 pre-treatment and GR interference. G1 phase arrest and apoptosis were accompanied with a p53-dependent up-regulation of p21 and pro-apoptotic genes NOXA and PUMA. We also proved that dexamethasone can’t induce apoptosis and cell cycle arrest when p53 was inhibited by p53 RNA interference. These data demonstrate that proliferation of MC3T3-E1 cell was significantly and directly inhibited by dexamethasone treatment via aberrant GR activation and subsequently P53 activation.

Therapeutic strategies for head and neck cancer based on p53 status

Squamous cell carcinomas of the head and neck (HNSCC) are one of the most common types of cancers worldwide, and despite advances in treatment, they still represent a clinical challenge. Inactivation of one or more components in the p53 signaling pathway is an extremely common event in human neoplasia, including HNSCC. The loss of p53 function is responsible for increased aggressiveness in cancers, while tumor chemoresistance and radioresistance can depend on deleted p53 expression, or on the expression of mutated-p53 proteins. Thus, consideration and manipulation of the p53 status during HNSCC cancer therapy should be considered. This review discusses the p53 signaling pathways activated by various cellular stresses, including exposure to cancer therapies. The recognition of the p53 status in cancer cells is a significant factor and could provide valuable assistance during the selection of an effective therapeutic approach.

Chaperoning steroidal physiology: lessons from mouse genetic models of Hsp90 and its cochaperones

The molecular chaperone Hsp90 is abundant, ubiquitous, and catholic to biological processes in eukaryotes, controlling phosphorylation cascades, protein stability and turnover, client localization and trafficking, and ligand-receptor interactions. Not surprisingly, Hsp90 does not accomplish these activities alone. Instead, an ever-growing number of cochaperones have been identified, leading to an explosion of reports on their molecular and cellular effects on Hsp90 chaperoning of client substrates. Notable among these clients are many members of the steroid receptor family, such as glucocorticoid, androgen, estrogen and progesterone receptors. Cochaperones typically associated with the mature, hormone-competent states of these receptors include p23, the FK506-binding protein 52 (FKBP52), FKBP51, protein phosphatase 5 (PP5) and cyclophilin 40 (Cyp40). The ultimate relevance of these cochaperones to steroid receptor action depends on their physiological effects. In recent years, the first mouse genetic models of these cochaperones have been developed. This work will review the complex and intriguing phenotypes so far obtained in genetically-altered mice and compare them to the known molecular and cellular impacts of cochaperones on steroid receptors. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Tumor suppressor protein (p)53, is a regulator of NF-kappaB repression by the glucocorticoid receptor

Glucocorticoids can inhibit inflammation by abrogating the activity of NF-κB, a family of transcription factors that regulates the production of proinflammatory cytokines. To understand the molecular mechanism of repression of NF-κB activity by glucocorticoids, we performed a high-throughput siRNA oligo screen to identify novel genes involved in this process. Here, we report that loss of p53, a tumor suppressor protein, impaired repression of NF-κB target gene transcription by glucocorticoids. Additionally, loss of p53 also impaired transcription of glucocorticoid receptor (GR) target genes, whereas upstream NF-κB and glucocorticoid receptor signaling cascades remained intact. We further demonstrate that p53 loss severely impaired glucocorticoid rescue of death in a mouse model of LPS shock. Our findings unveil a new role for p53 in the repression of NF-κB by glucocorticoids and suggest important implications for treatment of the proinflammatory microenvironments found in tumors with aberrant p53 activity.

Glucocorticoid receptor and breast cancer

Stress enhances glucocorticoid (GC) synthesis, which alters inflammation and immune responses, as well as cellular proliferation and apoptosis in a number of tissues. Increasingly, stress has been associated with cancer progression, and in particular in breast cancer. Consequently, an operational glucocorticoid receptor system in breast tissue influences breast cancer development. In this review, we summarize the data on the GC/GR system in normal and tumoral breast tissue. We also review the molecular mechanisms by which GCs control apoptosis and proliferation in breast cancer models and how GCs alter the chemotherapy of breast cancer treatment when used in combination. Finally, we discuss the participation of GR in breast tumorigenesis under hormone replacement therapy.

The phosphoprotein phosphatase family of Ser/Thr phosphatases as principal targets of naturally occurring toxins

Phosphoprotein phosphatases (PPPs) constitute one of three otherwise unrelated families of enzymes that specialize in removing the phosphate group from phosphorylated serine and threonine residues. The involvement of PPP enzymes in the regulation of processes such as gene expression, DNA replication, morphogenesis, synaptic transmission, glycogen metabolism, and apoptosis has underscored their potential as targets for the treatment of a variety of conditions such as cancer, diabetes, or Alzheimer's disease. Interestingly, PPP enzymes also constitute the physiological target of multiple naturally occurring toxins, including microcystins from cyanobacteria and cantharidin from beetles. This review is devoted to the PPP family of enzymes--with a focus on the human PPPs--and the naturally occurring toxins that are known to potently impair their activity. The interaction of the toxins with the enzymes is evaluated in atomic detail to obtain insight on two complementary aspects: (1) which specific structural differences within the similarly folded catalytic core of the PPP enzymes explain their diverse sensitivities to toxin inhibition and (2) which structural features presented by the various toxins account for the differential inhibitory potency towards each PPP. These analyses take advantage of numerous site-directed mutagenesis studies, structure-activity evaluations, and recent crystallographic structures of PPPs bound to different toxins.

Label-free quantitative proteomics and SAINT analysis enable interactome mapping for the human Ser/Thr protein phosphatase 5

Affinity purification coupled to mass spectrometry (AP-MS) represents a powerful and proven approach for the analysis of protein-protein interactions. However, the detection of true interactions for proteins that are commonly considered background contaminants is currently a limitation of AP-MS. Here using spectral counts and the new statistical tool, Significance Analysis of INTeractome (SAINT), true interaction between the serine/threonine protein phosphatase 5 (PP5) and a chaperonin, heat shock protein 90 (Hsp90), is discerned. Furthermore, we report and validate a new interaction between PP5 and an Hsp90 adaptor protein, stress-induced phosphoprotein 1 (STIP1; HOP). Mutation of PP5, replacing key basic amino acids (K97A and R101A) in the tetratricopeptide repeat (TPR) region known to be necessary for the interactions with Hsp90, abolished both the known interaction of PP5 with cell division cycle 37 homolog and the novel interaction of PP5 with stress-induced phosphoprotein 1. Taken together, the results presented demonstrate the usefulness of label-free quantitative proteomics and statistical tools to discriminate between noise and true interactions, even for proteins normally considered as background contaminants.

The promoter for intestinal cell kinase is head-to-head with F-Box 9 and contains functional sites for TCF7L2 and FOXA factors

Background

Intestinal cell kinase (ICK; GeneID 22858) is a conserved MAPK and CDK-like kinase that is widely expressed in human tissues. Data from the Cancer Genome Anatomy Project indicated ICK mRNA is increased in cancer, and that its expression correlated with expression of mRNA for an uncharacterized F-box protein, FBX9 (GeneID: 26268). ICK and FBX9 genes are arranged head-to-head on opposite strands, with start sites for transcription separated by ~3.3 kb. We hypothesized ICK and FBX9 are potentially important genes in cancer controlled by a bidirectional promoter.

Results

We assessed promoter activity of the intergenic region in both orientations in cancer cell lines derived from breast (AU565, SKBR3), colon (HCT-15, KM12), and stomach (AGS) cancers, as well as in embryonic human kidney (HEK293T) cells. The intergenic segment was active in both orientations in all of these lines, and ICK promoter activity was greater than FBX9 promoter activity. Results from deletions and truncations defined a minimal promoter for ICK, and revealed that repressors and enhancers differentially regulate ICK versus FBX9 promoter activity. The ICK promoter contains consensus motifs for several FOX-family transcription factors that align when mouse and human are compared using EMBOSS. FOXA1 and FOXA2 increase luciferase activity of a minimal promoter 10-20 fold in HEK293T cells. Consensus sites for TCF7L2 (TCF4) (Gene Id: 6934) are also present in both mouse and human. The expression of β-catenin increased activity of the minimal promoter ~10 fold. ICK reference mRNAs (NM_014920.3, NM_016513) are expressed in low copy number and increased in some breast cancers, using a ten base tag 5'-TCAACCTTAT-3' specific for both ICK transcripts.

Conclusion

ICK and FBX9 are divergently transcribed from a bidirectional promoter that is GC-rich and contains a CpG island. A minimal promoter for ICK contains functional sites for β-cateinin/TCF7L2 and FOXA. These data are consistent with functions that have been proposed for ICK in development and in proliferation or survival of some breast and colon cancers.

Anxiety and the aging brain: stressed out over p53?

We propose a model in which cell loss in the aging brain is seen as a root cause of behavioral changes that compromise quality of life, including the onset of generalized anxiety disorder, in elderly individuals. According to this model, as stem cells in neurogenic regions of the adult brain lose regenerative capacity, worn-out, dead, or damaged neurons fail to be replaced, leaving gaps in function. As most replacement involves inhibitory interneurons, either directly or indirectly, the net result is the acquisition over time of a hyper-excitable state. The stress axis is subserved by all three neurogenic regions in the adult brain, making it particularly susceptible to these age-dependent changes. We outline a molecular mechanism by which hyper-excitation of the stress axis in turn activates the tumor suppressor p53. This reinforces the loss of stem cell proliferative capacity and interferes with the feedback mechanism by which the glucocorticoid receptor turns off neuroendocrine pathways and resets the axis.

Estrogen inhibits glucocorticoid action via protein phosphatase 5 (PP5)-mediated glucocorticoid receptor dephosphorylation

Although glucocorticoids suppress proliferation of many cell types and are used in the treatment of certain cancers, trials of glucocorticoid therapy in breast cancer have been a disappointment. Another suggestion that estrogens may affect glucocorticoid action is that the course of some inflammatory diseases tends to be more severe and less responsive to corticosteroid treatment in females. To date, the molecular mechanism of cross-talk between estrogens and glucocorticoids is poorly understood. Here we show that, in both MCF-7 and T47D breast cancer cells, estrogen inhibits glucocorticoid induction of the MKP-1 (mitogen-activated protein kinase phosphatase-1) and serum/glucocorticoid-regulated kinase genes. Estrogen did not affect glucocorticoid-induced glucocorticoid receptor (GR) nuclear translocation but reduced ligand-induced GR phosphorylation at Ser-211, which is associated with the active form of GR. We show that estrogen increases expression of protein phosphatase 5 (PP5), which mediates the dephosphorylation of GR at Ser-211. Gene knockdown of PP5 abolished the estrogen-mediated suppression of GR phosphorylation and induction of MKP-1 and serum/glucocorticoid-regulated kinase. More importantly, after PP5 knockdown estrogen-promoted cell proliferation was significantly suppressed by glucocorticoids. This study demonstrates cross-talk between estrogen-induced PP5 and GR action. It also reveals that PP5 inhibition may antagonize estrogen-promoted events in response to corticosteroid therapy.

Targeting protein serine/threonine phosphatases for drug development

With the recent clinical success of drugs targeting protein kinase activity, drug discovery efforts are focusing on the role of reversible protein phosphorylation in disease states. The activity of protein phosphatases, enzymes that oppose protein kinases, can also be manipulated to alter cellular signaling for therapeutic benefits. In this review, we present protein serine/threonine phosphatases as viable therapeutic targets, discussing past successes, current challenges, and future strategies for modulating phosphatase activity.

HDAC4 represses p21(WAF1/Cip1) expression in human cancer cells through a Sp1-dependent, p53-independent mechanism

Cancer cells have complex, unique characteristics that distinguish them from normal cells, such as increased growth rates and evasion of anti-proliferative signals. Global inhibition of class I and II histone deacetylases (HDACs) stops cancer cell proliferation in vitro and has proven effective against cancer in clinical trials, at least in part, through transcriptional reactivation of the p21(WAF1/Cip1)gene. The HDACs that regulate p21(WAF1/Cip1) are not fully identified. Using small interfering RNAs, we found that HDAC4 participates in the repression of p21(WAF1/Cip1) through Sp1/Sp3-, but not p53-binding sites. HDAC4 interacts with Sp1, binds and reduces histone H3 acetylation at the Sp1/Sp3 binding site-rich p21(WAF1/Cip1) proximal promoter, suggesting a key role for Sp1 in HDAC4-mediated repression of p21(WAF1/Cip1). Induction of p21(WAF1/Cip1) mediated by silencing of HDAC4 arrested cancer cell growth in vitro and inhibited tumor growth in an in vivo human glioblastoma model. Thus, HDAC4 could be a useful target for new anti-cancer therapies based on selective inhibition of specific HDACs.

The role of serine/threonine protein phosphatase type 5 (PP5) in the regulation of stress-induced signaling networks and cancer

Although the aberrant actions of protein kinases have long been known to contribute to tumor promotion and carcinogenesis, roles for protein phosphatases in the development of human cancer have only emerged in the last decade. In this review, we discuss the data obtained from studies examining the biological and pathological roles of a serine/threonine protein phosphatase, PP5, which suggest that PP5 is a potentially important regulator of both hormone- and stress-induced signaling networks that enable a cell to respond appropriately to genomic stress.

MDM2 antagonist nutlin-3 is a potent inducer of apoptosis in pediatric acute lymphoblastic leukemia cells with wild-type p53 and overexpression of MDM2

In pediatric acute lymphoblastic leukemia (ALL), overexpression of murine double minute 2 (MDM2) protein by leukemic cells is typically associated with a wild-type (wt)-p53 phenotype and chemoresistance. A recently developed small-molecule antagonist of MDM2, nutlin-3, inhibits the MDM2-p53 interaction, resulting in induction of p53 activity and apoptosis. In this study, we evaluated the cytotoxic effect of nutlin-3 on ALL cells with different p53 status and MDM2 expression, using 18 cell lines and 30 primary leukemia samples. We found that both ALL cell lines and primary ALL samples with wt-p53 are sensitive to nutlin-3. No cytotoxic effect of nutlin-3 was detected in ALL cells with either p53-mutant or -null phenotype. In wt-p53 ALL cells, there was a significant positive correlation between MDM2 expression levels and sensitivity to nutlin-3. Nutlin-3-induced cell death was mediated by p53-induced activation of proapoptotic proteins and by p53-induced repression of the anti-apoptotic protein survivin. As p53 function is inhibited by MDM2 in chemoresistant, MDM2-overexpressing ALL cells, potent killing of these cells by nutlin-3 suggests that this agent may be a novel therapeutic for refractory ALL.

Elevated levels of Ser/Thr protein phosphatase 5 (PP5) in human breast cancer

Ser/Thr protein phosphatase 5 (PP5) regulates several signaling-cascades that suppress growth and/or facilitate apoptosis in response to genomic stress. The expression of PP5 is responsive to hypoxia inducible factor-1 (HIF-1) and estrogen, which have both been linked to the progression of human breast cancer. Still, it is not clear if PP5 plays a role in the development of human cancer. Here, immunostaining of breast cancer tissue-microarrays (TMAs) revealed a positive correlation between PP5 over-expression and ductal carcinoma in situ (DCIS; P value 0.0028), invasive ductal carcinoma (IDC; P value 0.012) and IDC with metastases at the time of diagnosis (P value 0.0001). In a mouse xenograft model, the constitutive over-expression of PP5 was associated with an increase in the rate of tumor growth. In a MCF-7 cell culture model over-expression correlated with both an increase in the rate of proliferation and protection from cell death induced by oxidative stress, UVC-irradiation, adriamycin, and vinblastine. PP5 over-expression had no apparent effect on the sensitivity of MCF-7 cells to taxol or rapamycin. Western analysis of extracts from cells over-expressing PP5 revealed a decrease in the phosphorylation of known substrates for PP5. Together, these studies indicate that elevated levels of PP5 protein occur in human breast cancer and suggest that PP5 over-expression may aid tumor progression.

High yield expression of serine/threonine protein phosphatase type 5, and a fluorescent assay suitable for use in the detection of catalytic inhibitors

Protein phosphatase type 5 (PP5) belongs to the PPP family of serine/threonine protein phosphatases and is expressed in most, if not all, human tissues. Although the physiological roles played by PP5 are not yet clear, PP5 is found in association with several proteins that influence intracellular signaling networks initiated by hormones (i.e., glucocorticoids) or cellular stress (i.e., hypoxia, oxidative stress). Recently, studies conducted with short interfering RNA and antisense oligonucleotides indicate that PP5 plays an important role in the regulation of stress-induced signaling cascades that influence both cell growth and the onset of apoptosis. Therefore, the identification of small molecule inhibitors of PP5 is desired for use in studies to further define the biological/pathological roles of PP5. Such inhibitors may also prove useful for development into novel antitumor agents. Here we describe methods to express and purify large amounts of biologically active PP5c, an inhibitor titration-based assay to determine the amount of PP5 in solution, and a fluorescent phosphatase assay that can be used to screen chemical libraries and natural extracts for the presence of catalytic inhibitors.

The glucocorticoid receptor signalling in breast cancer

Glucocorticoids (GCs) are provided as co-medication with chemotherapy in breast cancer, albeit several lines of evidence indicate that their use may have diverse effects and in fact may inhibit chemosensitivity. The molecular basis of GC-induced resistance to chemotherapy in breast cancer remains poorly defined. Recent researchers, in an attempt to clarify some aspects of the underlying pathways, provide convincing evidence that GCs induce effects that are dependent upon the glucocorticoid-receptor (GR)-mediated transcriptional regulation of specific genes known to play key roles in cellular/tissue functions, including growth, apoptosis, differentiation, metastasis and cell survival. In this review, we focus on how GC-induced chemoresistance in breast cancer is mediated by the GR, unravelling the molecular interplay of GR signalling with other signalling cascades prevalent in breast cancer. We also include a detailed description of GR structure and function, summarizing data gained during recent years into the mechanism(s) of the cross-talk between the GR and other signalling cascades and secondary messengers, via which GCs exert their pleiotropic effects.

Cantharidin-induced mitotic arrest is associated with the formation of aberrant mitotic spindles and lagging chromosomes resulting, in part, from the suppression of PP2Aalpha

Cantharidin, a natural vesicant, inhibits the activity of several PPP family phosphatases, displays antitumor activity, and induces apoptosis in many types of tumor cells. However, the molecular mechanisms underlying the antitumor activity of cantharidin are not clear. Here, dose-response studies confirm a strong correlation between the suppression of phosphatase activity and cell death. Flow cytometry analysis indicates that before apoptosis, cantharidin delays cell cycle progression following DNA replication with no apparent effect on G(1)-S or S-G(2) phase progression. In contrast, studies with double thymidine-synchronized populations of cells indicate that cantharidin can rapidly arrest growth when added during G(2) or early M phase. Immunostaining indicates that cell cycle arrest occurs before the completion of mitosis and is associated with the appearance of aberrant mitotic spindles. Live cell imaging with time-lapse microscopy shows that cantharidin disrupts the metaphase alignment of chromosomes and produces a prolonged mitotic arrest, with the onset of apoptosis occurring before the onset of anaphase. To explore the contribution of individual phosphatases, antisense oligonucleotides and small interfering RNA were developed to suppress the expression of cantharidin-sensitive phosphatases. The suppression of PP2Aalpha, but not PP2Abeta, is sufficient to induce metaphase arrest, during which time lagging chromosomes are observed moving between the spindle poles and the metaphase plate. Immunostaining revealed slightly abnormal, yet predominately bipolar, mitotic spindles. Nonetheless, after a 10- to 15-hour delay, the cells enter anaphase, suggesting that an additional cantharidin-sensitive phosphatase is involved in the progression from metaphase into anaphase or to prevent the onset of apoptosis in cells arrested during mitosis.

Differential proteomic analysis of the anti-proliferative effect of glucocorticoid hormones in ST1 rat glioma cells

Glucocorticoid hormones (GCs) exert a potent anti-proliferative activity on several cell types. The classic molecular mechanism of GCs involves modulation of the activity of the glucocorticoids receptor, a transcriptional regulator. However, the anti-proliferative effect of GCs may also involve modulation of processes such as translation, subcellular localization and post-translational modifications, which are not reflected at the mRNA level. To investigate these potential effects of GCs, we employed the proteomic approach (two-dimensional electrophoresis and mass spectrometry) and the ST1 cells, obtained from the C6 rat glioma cell line, as a model. GC treatment leads ST1 cells to a complete transformed-to-normal phenotypic reversion and loss of their tumorigenic potential. By comparing sets of 2D nuclear protein profiles of ST1 cells treated (or not) with hydrocortisone (Hy), 13 polypeptides displaying >or=two-fold difference in abundance upon Hy treatment were found. Five of these polypeptides were identified by peptide mass fingerprinting, including Annexin 2 (ANX2), hnRNP A3 and Ubiquitin. Evidence obtained by Western blot analysis indicates that ANX2 is present in the nucleus and has its subcellular localization modulated by GC-treatment of ST1 cells. Our findings indicate complementary mechanisms contributing to the regulation of gene expression associated with ST1 cells' response to GCs.

Posttranslational regulation of the mammalian circadian clock by cryptochrome and protein phosphatase 5

The molecular oscillator that drives circadian rhythmicity in mammals obtains its near 24-h periodicity from posttranslational regulation of clock proteins. Activity of the major clock kinase casein kinase I (CKI) epsilon is regulated by inhibitory autophosphorylation. Here we show that protein phosphatase (PP) 5 regulates the kinase activity of CKIepsilon. We demonstrate that cryptochrome regulates clock protein phosphorylation by modulating the effect of PP5 on CKIepsilon. Like CKIepsilon, PP5 is expressed both in the master circadian clock in the suprachiasmatic nuclei and in peripheral tissues independent of the clock. Expression of a dominant-negative PP5 mutant reduces PER phosphorylation by CKIepsilon in vivo, and down-regulation of PP5 significantly reduces the amplitude of circadian cycling in cultured human fibroblasts. Collectively, these findings indicate that PP5, CKIepsilon, and cryptochrome dynamically regulate the mammalian circadian clock.

Inhibition of PP2A, but not PP5, mediates p53 activation by low levels of okadaic acid in rat liver epithelial cells

The microbial toxin okadaic acid (OA) specifically inhibits PPP-type ser/thr protein phosphatases. OA is an established tumor promoter with numerous cellular effects that include p53-mediated cell cycle arrest. In T51B rat liver epithelial cells, a model useful for tumor promotion studies, p53 activation is induced by tumor-promoting (low nanomolar) concentrations of OA. Two phosphatases sensitive to these concentrations of OA, PP2A and protein phosphatase 5 (PP5), have been implicated as negative regulators of p53. In this study we examined the respective roles of these phosphatases in p53 activation in non-neoplastic T51B cells. Increases in p53 activity were deduced from levels of p21 (cip1) and/or the rat orthologue of mdm2, two p53-regulated gene products whose induction was blocked by siRNA-mediated knockdown of p53. As observed with 10 nM OA, both phospho-ser15-p53 levels and p53 activity were increased by 10 microM fostriecin or SV40 small t-antigen. Both of these treatments selectively inhibit PP2A but not PP5. siRNA-mediated knockdown of PP2A, but not PP5, also increased p53 activity. Finally, adenoviral-mediated over-expression of an OA-resistant form of PP5 did not prevent increased phospho-ser15-p53, p53 protein, or p53 activity caused by 10 nM OA. Together these results indicate that PP5 blockade is not responsible for OA-induced p53 activation and G1 arrest in T51B cells. In contrast, specific blockade of PP2A mimics p53-related responses to OA in T51B cells, suggesting that PP2A is the target for this response to OA.

Human protein phosphatase 5 dissociates from heat-shock proteins and is proteolytically activated in response to arachidonic acid and the microtubule-depolymerizing drug nocodazole

Ppp5 (protein phosphatase 5) is a serine/threonine protein phosphatase that has been conserved throughout eukaryotic evolution. In mammalian cells, FLAG-tagged Ppp5 and endogenous Ppp5 are found to interact with endogenous Hsp (heat-shock protein) 70, as well as Hsp90. Incubation of cells with arachidonic acid or the microtubule-depolymerizing agent, nocodazole, causes loss of interaction of Hsp70 and Hsp90 with FLAG-tagged Ppp5 and increase of Ppp5 activity. In response to the same treatments, endogenous Ppp5 undergoes proteolytic cleavage of the N- and C-termini, with the subsequent appearance of high-molecular-mass species. The results indicate that Ppp5 is activated by proteolysis on dissociation from Hsps, and is destroyed via the proteasome after ubiquitination. Cleavage at the C-terminus removes a nuclear localization sequence, allowing these active cleaved forms of Ppp5 to translocate to the cytoplasm. The response of Ppp5 to arachidonic acid and nocodazole suggests that Ppp5 may be required for stress-related processes that can sometimes cause cell-cycle arrest, and leads to the first description for in vivo regulation of Ppp5 activity.

Regulation of apoptosis signal-regulating kinase 1 (ASK1) by polyamine levels via protein phosphatase 5

Recent evidence has implicated the protein phosphatase PP5 in a variety of signaling pathways. Whereas several proteins have been identified that interact with PP5 and regulate its activity, a possibility of its regulation by second messengers remains speculative. Activation of PP5 in vitro by polyunsaturated fatty acids (e.g. arachidonic acid) and fatty acyl-CoA esters (e.g. arachidonoyl-CoA) has been reported. We report here that PP5 is strongly inhibited by micromolar concentrations of a natural polyamine spermine. This inhibition was observed both in assays with a low molecular weight substrate p-nitrophenyl phosphate as well as phosphocasein and apoptosis signal-regulating kinase 1 (ASK1), thought to be a physiological substrate of PP5. Furthermore, a decrease in polyamine levels in COS-7 cells induced by alpha-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, led to accelerated dephosphorylation of oxidative stress-activated ASK1. This effect was suppressed by okadaic acid and by siRNA-mediated PP5 depletion, indicating that the effect of polyamine levels on ASK1 dephosphorylation was mediated by PP5. In line with the decreased ASK1 activation, polyamine depletion in COS-7 cells abrogated oxidative stress-induced activation of caspase-3, which executes ASK1-induced apoptosis, as well as caspase-3 activation induced by ASK1 overexpression, but had no effect on basal caspase-3 activity. These results implicate polyamines, emerging intracellular signaling molecules, as potential physiological regulators of PP5. Our findings also suggest a novel mechanism of the anti-apoptotic action of a decrease in polyamine levels via de-inhibition of PP5 and accelerated dephosphorylation and deactivation of ASK1.

Distinctive gene expression pattern in VH3-21 utilizing B-cell chronic lymphocytic leukemia

The usage of the immunoglobulin (Ig) V(H)3-21 gene is associated with poor prognosis in B-cell chronic lymphocytic leukemia (B-CLL) despite V(H) gene mutation status. Many V(H)3-21+ patients also display restricted heavy- and light-chain Ig gene rearrangements, implying a role of antigen selection in disease development. To explore the specific phenotypic/genotypic features among V(H)3-21+ B-CLLs, we compared gene expression patterns in 15 V(H)3-21+ and 24 non-V(H)3-21 patients (11 with unmutated and 13 with mutated V(H) genes) using Affymetrix microarray analysis (approximately 12,500 genes). A distinct expression profile was identified for V(H)3-21+ patients in contrast to the Ig-unmutated and -mutated groups. By applying different algorithms, the data enabled an efficient class discrimination of the V(H)3-21+ subset based on 27 or 57 genes. A set of genes was sorted out which, using different analytical methods, consistently gave a distinction between V(H)3-21+ and non-V(H)3-21 samples. Several of these genes are involved in regulation of DNA replication/cell-cycle control, transcription and protein kinase activity, which may render the V(H)3-21+ cells with a higher proliferative drive. However, no clear evidence of increased B-cell receptor signaling was found in the V(H)3-21+ group. Altogether, our identification of a specific V(H)3-21 profile may provide insights into the pathogenesis of the V(H)3-21+ subgroup.

Molecular basis for TPR domain-mediated regulation of protein phosphatase 5

Protein phosphatase 5 (Ppp5) is a serine/threonine protein phosphatase comprising a regulatory tetratricopeptide repeat (TPR) domain N-terminal to its phosphatase domain. Ppp5 functions in signalling pathways that control cellular responses to stress, glucocorticoids and DNA damage. Its phosphatase activity is suppressed by an autoinhibited conformation maintained by the TPR domain and a C-terminal subdomain. By interacting with the TPR domain, heat shock protein 90 (Hsp90) and fatty acids including arachidonic acid stimulate phosphatase activity. Here, we describe the structure of the autoinhibited state of Ppp5, revealing mechanisms of TPR-mediated phosphatase inhibition and Hsp90- and arachidonic acid-induced stimulation of phosphatase activity. The TPR domain engages with the catalytic channel of the phosphatase domain, restricting access to the catalytic site. This autoinhibited conformation of Ppp5 is stabilised by the C-terminal alphaJ helix that contacts a region of the Hsp90-binding groove on the TPR domain. Hsp90 activates Ppp5 by disrupting TPR-phosphatase domain interactions, permitting substrate access to the constitutively active phosphatase domain, whereas arachidonic acid prompts an alternate conformation of the TPR domain, destabilising the TPR-phosphatase domain interface.

Constitutive over expression of serine/threonine protein phosphatase 5 (PP5) augments estrogen-dependent tumor growth in mice

Serine/threonine protein phosphatase 5 (PP5) appears to play an underappreciated role in the regulation of cellular proliferation. In estrogen-responsive cells, PP5 expression is stimulated by 17 beta-estradiol, and in a variety of p53 wild-type tumor cells the suppression of PP5 expression with ISIS 15534 inhibits growth. To further explore the relationship between PP5 and the development of human cancer, here we tested the effect of elevated PP5 expression on tumor growth using a mouse xenograph model and a stable MCF-7 cell line in which the expression of wild-type PP5 was placed under the control of tetracycline-off regulated transactivator and operator plasmids. In the xenograph model a modest two fold increase in PP5 protein levels significantly enhanced the growth rate of estrogen-dependent tumors, suggesting PP5 plays a positive role in tumor development.

Modulation of nuclear steroid receptors by ursodeoxycholic acid inhibits TGF-beta1-induced E2F-1/p53-mediated apoptosis of rat hepatocytes

We have recently shown that both ursodeoxycholic acid (UDCA) and tauroursodeoxycholic acid (TUDCA) prevent transforming growth factor beta1 (TGF-beta1)-induced hepatocyte apoptosis by modulating the E2F-1/p53/Bax pathway. In addition, activation of glucocorticoid (GR) and mineralocorticoid receptors (MR) inhibits apoptosis in various systems. UDCA induces a ligand-independent activation of the GR, thus potentially regulating a number of targets. In this study, we investigated the role of GR and MR during TGF-beta1-induced hepatocyte apoptosis, and identified additional antiapoptotic targets for UDCA. Our results showed that in primary hepatocytes, TGF-beta1 induced 40-50% decreases in gr and mr mRNA expression (p < 0.01), together with up to 10-fold reductions in their protein levels (p < 0.01). Notably, pretreatment with UDCA resulted in a significant upregulation of nuclear steroid receptors (p < 0.05), which coincided with 2- and 3-fold increases in the level of GR and MR nuclear translocation, respectively, when compared with that of TGF-beta1 alone (p < 0.05). Similarly, TUDCA induced GR and MR nuclear translocations (p < 0.05) and markedly prevented MR protein changes associated with TGF-beta1 (p < 0.05) without affecting GR protein levels. Moreover, when interference RNA was used to inhibit GR and MR, UDCA no longer protected hepatocytes against TGF-beta1-induced apoptosis. In fact, the protective effect of UDCA in TGF-beta1-associated caspase activation decreased from 65 to <10% when GR or MR function was blocked. Finally, the TGF-beta1-induced E2F-1/Mdm-2/p53 apoptotic pathway, normally inhibited by UDCA, was not regulated by the bile acid after GR or MR silencing. These results demonstrate that UDCA protects against apoptosis through an additional pathway that involves nuclear receptors GR and MR as key factors. Further, the E2F-1/Mdm-2/p53 apoptotic pathway appears to be a prime target for UDCA-induced steroid receptor activation.

Ser/Thr protein phosphatase 5 inactivates hypoxia-induced activation of an apoptosis signal-regulating kinase 1/MKK-4/JNK signaling cascade

Mitogen-activated protein kinase (MAPK) signaling cascades are multifunctional signaling networks that influence cell growth, differentiation, apoptosis, and cellular responses to stress. Since the activation/propagation of MAPK signaling requires the sequential phosphorylation of many downstream proteins, the phosphatases that dephosphorylate MAPKs represent critical elements in the control of MAPK-signaling networks. Here we show that hypoxia induces a transient increase in the activity of apoptosis signal-regulating kinase 1 (ASK-1), a MAPKKK that responds to oxidative stress by triggering cascades leading to the phosphorylation/activation of c-Jun N-terminal kinases (JNK) and p38-MAPK. Hypoxia-induced ASK-1/MKK-4/JNK signaling is suppressed by serine/threonine protein phosphatase type 5 (PP5), which acts to turn off ASK-1/MKK-4/JNK signaling via two mechanisms. First, in a rapid response hypoxia facilitates the association of endogenous PP5 with ASK-1. PP5 binds to the C-terminal domain of ASK-1, and studies with siRNA targeting PP5 indicate that PP5 acts to suppress the phosphorylation of MKK4 (Thr-261), JNK (Thr-183/Tyr-185), and c-Jun (Ser-63) without affecting the activating phosphorylation of p38 MAPK (Thr-180/Tyr-182), p44/p42-MAPK/ERK1/2 (Thr-202/Tyr-204), or c-Jun protein levels. If hypoxia is prolonged, the expression of PP5 is increased due to the activation of a transcriptional activator, which was identified as hypoxia-inducible factor-1. Together, these studies indicate that PP5 plays an important role in the survival of cells in a low oxygen environment by suppressing a hypoxia-induced ASK-1/MKK4/JNK signaling cascade that promotes an apoptotic response.

Physiological and pathological consequences of the interactions of the p53 tumor suppressor with the glucocorticoid, androgen, and estrogen receptors

The p53 tumor suppressor plays a key role in protection from the effects of different physiological stresses (DNA damage, hypoxia, transcriptional defects, etc.), and loss of its activity has dire consequences, such as cancer. Its activity is finely tuned through interactions with other important regulatory circuits in the cell. Recently, striking evidence has emerged for crosstalk with another class of important regulators, the steroid hormone receptors, and in particular the glucocorticoid (GR), androgen (AR), and estrogen (ER) receptors. These receptors are important in maintaining homeostasis in response to internal and external stresses (GR) and in the development, growth, and maintenance of the male and female reproductive systems (AR and ER, respectively). We review how p53 interacts closely with these receptors, to the extent that they share the same E3 ubiquitin ligase, the MDM2 oncoprotein. We discuss the different physiological contexts in which such interactions occur, and also how these interactions have been undermined in various pathological situations. We will describe future areas for research, with special emphasis on GR, and how certain common features, such as cytoplasmic anchoring of p53 by the receptors, may become targets for the development of therapeutic interventions. Given the importance of GR in inflammation, erythropoiesis, and autoimmune diseases, and the importance of AR and ER in prostate and breast cancer (respectively), the studies on p53 interactions with the steroid receptors will be an important domain in the near future.

Structural Basis for the Catalytic Activity of Human Serine/Threonine Protein Phosphatase-5

Serine/threonine protein phosphatase-5 (PP5) affects many signaling networks that regulate cell growth and cellular responses to stress. Here we report the crystal structure of the PP5 catalytic domain (PP5c) at a resolution of 1.6 A. From this structure we propose a mechanism for PP5-mediated hydrolysis of phosphoprotein substrates, which requires the precise positioning of two metal ions within a conserved Asp271-M1:M2-W1-His427-His304-Asp274 catalytic motif (where M1 and M2 are metals and W1 is a water molecule). The structure of PP5c provides a structural basis for explaining the exceptional catalytic proficiency of protein phosphatases, which are among the most powerful known catalysts. Resolution of the entire C terminus revealed a novel subdomain, and the structure of the PP5c should also aid development of type-specific inhibitors.