Dual-specificity phosphatases as targets for antineoplastic agents

Induction of the hepatic aryl hydrocarbon receptor by alcohol dysregulates autophagy and phospholipid metabolism via PPP2R2D

Disturbed lipid metabolism precedes alcoholic liver injury. Whether and how AhR alters degradation of lipids, particularly phospho-/sphingo-lipids during alcohol exposure, was not explored. Here, we show that alcohol consumption in mice results in induction and activation of aryl hydrocarbon receptor (AhR) in the liver, and changes the hepatic phospho-/sphingo-lipids content. The levels of kynurenine, an endogenous AhR ligand, are elevated with increased hepatic tryptophan metabolic enzymes in alcohol-fed mice. Either alcohol or kynurenine treatment promotes AhR activation with autophagy dysregulation via AMPK. Protein Phosphatase 2 Regulatory Subunit-Bdelta (Ppp2r2d) is identified as a transcriptional target of AhR. Consequently, PPP2R2D-dependent AMPKα dephosphorylation causes autophagy inhibition and mitochondrial dysfunction. Hepatocyte-specific AhR ablation attenuates steatosis, which is associated with recovery of phospho-/sphingo-lipids content. Changes of AhR targets are corroborated using patient specimens. Overall, AhR induction by alcohol inhibits autophagy in hepatocytes through AMPKα, which is mediated by Ppp2r2d gene transactivation, revealing an AhR-dependent metabolism of phospho-/sphingo-lipids.

Exposure of the cytoplasm to low-dose X-rays modifies ataxia telangiectasia mutated-mediated DNA damage responses

We recently showed that when a low X-ray dose is used, cell death is enhanced in nucleus-irradiated compared with whole-cell-irradiated cells; however, the role of the cytoplasm remains unclear. Here, we show changes in the DNA damage responses with or without X-ray microbeam irradiation of the cytoplasm. Phosphorylated histone H2AX foci, a surrogate marker for DNA double-strand breaks, in V79 and WI-38 cells are not observed in nucleus irradiations at ≤ 2 Gy, whereas they are observed in whole-cell irradiations. Addition of an ataxia telangiectasia mutated (ATM) kinase inhibitor to whole-cell irradiations suppresses foci formation at ≤ 2 Gy. ABL1 and p73 expression is upregulated following nucleus irradiation, suggesting the induction of p73-dependent cell death. Furthermore, CDKN1A (p21) is upregulated following whole-cell irradiation, indicating the induction of cell cycle arrest. These data reveal that cytoplasmic radioresponses modify ATM-mediated DNA damage responses and determine the fate of cells irradiated at low doses.

Specificity protein 1: Its role in colorectal cancer progression and metastasis

Specificity protein 1 (Sp1) is a widely expressed transcription factor that plays an important role in the promotion of oncogenes required for tumor survival, progression and metastasis. Sp1 is highly expressed in several cancers including colorectal cancer (CRC) and is related to poor prognosis. Therefore, targeting Sp1 is a rational for CRC therapy. In this review, we will recapitulate the current understanding of Sp1 signaling, its molecular mechanisms, and its potential involvement in CRC growth, progression and metastasis. We will also discuss the current therapeutic drugs for CRC and their mechanism of action via Sp1.

Comparative genomics reveals high biological diversity and specific adaptations in the industrially and medically important fungal genus Aspergillus

BACKGROUND

The fungal genus Aspergillus is of critical importance to humankind. Species include those with industrial applications, important pathogens of humans, animals and crops, a source of potent carcinogenic contaminants of food, and an important genetic model. The genome sequences of eight aspergilli have already been explored to investigate aspects of fungal biology, raising questions about evolution and specialization within this genus.

RESULTS

We have generated genome sequences for ten novel, highly diverse Aspergillus species and compared these in detail to sister and more distant genera. Comparative studies of key aspects of fungal biology, including primary and secondary metabolism, stress response, biomass degradation, and signal transduction, revealed both conservation and diversity among the species. Observed genomic differences were validated with experimental studies. This revealed several highlights, such as the potential for sex in asexual species, organic acid production genes being a key feature of black aspergilli, alternative approaches for degrading plant biomass, and indications for the genetic basis of stress response. A genome-wide phylogenetic analysis demonstrated in detail the relationship of the newly genome sequenced species with other aspergilli.

CONCLUSIONS

Many aspects of biological differences between fungal species cannot be explained by current knowledge obtained from genome sequences. The comparative genomics and experimental study, presented here, allows for the first time a genus-wide view of the biological diversity of the aspergilli and in many, but not all, cases linked genome differences to phenotype. Insights gained could be exploited for biotechnological and medical applications of fungi.

Deregulation of the protein phosphatase 2A, PP2A in cancer: complexity and therapeutic options

The complexity of the phosphatase, PP2A, is being unravelled and current research is increasingly providing information on the association of deregulated PP2A function with cancer initiation and progression. It has been reported that decreased activity of PP2A is a recurrent observation in many types of cancer, including colorectal and breast cancer (Baldacchino et al. EPMA J. 5:3, 2014; Cristobal et al. Mol Cancer Ther. 13:938-947, 2014). Since deregulation of PP2A and its regulatory subunits is a common event in cancer, PP2A is a potential target for therapy (Baldacchino et al. EPMA J. 5:3, 2014). In this review, the structural components of the PP2A complex are described, giving an in depth overview of the diversity of regulatory subunits. Regulation of the active PP2A trimeric complex, through phosphorylation and methylation, can be targeted using known compounds, to reactivate the complex. The endogenous inhibitors of the PP2A complex are highly deregulated in cancer, representing cases that are eligible to PP2A-activating drugs. Pharmacological opportunities to target low PP2A activity are available and preclinical data support the efficacy of these drugs, but clinical trials are lacking. We highlight the importance of PP2A deregulation in cancer and the current trends in targeting the phosphatase.

Focused library with a core structure extracted from natural products and modified: application to phosphatase inhibitors and several biochemical findings

Synthesis of a focused library is an important strategy to create novel modulators of specific classes of proteins. Compounds in a focused library are composed of a common core structure and different diversity structures. In this Account, we describe our design and synthesis of libraries focused on selective inhibitors of protein phosphatases (PPases). We considered that core structures having structural and electronic features similar to those of PPase substrates, phosphate esters, would be a reasonable choice. Therefore, we extracted core structures from natural products already identified as PPase inhibitors. Since many PPases share similar active-site structures, such phosphate-mimicking core structures should interact with many enzymes in the same family, and therefore the choice of diversity structures is pivotal both to increase the binding affinity and to achieve specificity for individual enzymes. Here we present case studies of application of focused libraries to obtain PPase inhibitors, covering the overall process from selection of core structures to identification and evaluation of candidates in the focused libraries. To synthesize a library focused on protein serine-threonine phosphatases (PPs), we chose norcantharidin as a core structure, because norcantharidin dicarboxylate shows a broad inhibition profile toward several PPs. From the resulting focused library, we identified a highly selective PP2B inhibitor, NCA-01. On the other hand, to find inhibitors of dual-specificity protein phosphatases (DSPs), we chose 3-acyltetronic acid extracted from natural product RK-682 as a core structure, because its structure resembles the transition state in the dephosphorylation reaction of DSPs. However, a highly selective inhibitor was not found in the resulting focused library. Furthermore, an inherent drawback of compounds having the highly acidic 3-acyltetronic acid as a core structure is very weak potency in cellulo, probably due to poor cell membrane permeability. Therefore, we next modified the core structure from acidic to neutral by transformation to the enamine derivative and constructed a second-generation focused library (RE derivatives). The resulting compounds showed dramatically improved cell membrane permeability and inhibitory selectivity and included VHR (vaccinia VH1-related)-selective RE12 and CDC25A/B (cell division cycle 25A/B)-selective RE44. These inhibitors act on target enzymes in cellulo and do not generate reactive oxygen species, which is a potential problem with quinoid-type inhibitors of CDC25s. The cellular activity of RE12 was further improved by replacement of the side chain to afford RE176, which showed more potent antiproliferative activity than RE12 against HeLa cells. The dramatic change of inhibitory selectivity obtained by core structure modification from 3-acyltetronic acid to its enamine derivative was associated with a change in the mode of action. Namely, RE derivatives were found to be noncompetitive inhibitors with respect to a small-molecular substrate of CDC25A/B, whereas RK-682 was a competitive inhibitor of VHR. We identified the binding site of RE derivatives on the CDC25A as a pocket adjacent to the active site; this appears to be a promising target site for development of further novel inhibitors of CDC25s.

Inhibition of CDC25B phosphatase through disruption of protein-protein interaction

CDC25 phosphatases are key cell cycle regulators and represent very attractive but challenging targets for anticancer drug discovery. Here, we explored whether fragment-based screening represents a valid approach to identify inhibitors of CDC25B. This resulted in identification of 2-fluoro-4-hydroxybenzonitrile, which directly binds to the catalytic domain of CDC25B. Interestingly, NMR data and the crystal structure demonstrate that this compound binds to the pocket distant from the active site and adjacent to the protein-protein interaction interface with CDK2/Cyclin A substrate. Furthermore, we developed a more potent analogue that disrupts CDC25B interaction with CDK2/Cyclin A and inhibits dephosphorylation of CDK2. Based on these studies, we provide a proof of concept that targeting CDC25 phosphatases by inhibiting their protein-protein interactions with CDK2/Cyclin A substrate represents a novel, viable opportunity to target this important class of enzymes.

Mimicking/extracting structure and functions of natural products: synthetic approaches that address unexplored needs in chemical biology

Natural products are often attractive and challenging targets for synthetic chemists, and many have interesting biological activities. However, synthetic chemists need to be more than simply suppliers of compounds to biologists. Therefore, we have been seeking ways to actively apply organic synthetic methods to chemical biology studies of natural products and their activities. In this personal review, I would like to introduce our work on the development of new biologically active compounds inspired by, or extracted from, the structures of natural products, focusing on enhancement of functional activity and specificity and overcoming various drawbacks of the parent natural products.

Protein tyrosine phosphatases as potential therapeutic targets

Protein tyrosine phosphorylation is a key regulatory process in virtually all aspects of cellular functions. Dysregulation of protein tyrosine phosphorylation is a major cause of human diseases, such as cancers, diabetes, autoimmune disorders, and neurological diseases. Indeed, protein tyrosine phosphorylation-mediated signaling events offer ample therapeutic targets, and drug discovery efforts to date have brought over two dozen kinase inhibitors to the clinic. Accordingly, protein tyrosine phosphatases (PTPs) are considered next-generation drug targets. For instance, PTP1B is a well-known targets of type 2 diabetes and obesity, and recent studies indicate that it is also a promising target for breast cancer. SHP2 is a bona-fide oncoprotein, mutations of which cause juvenile myelomonocytic leukemia, acute myeloid leukemia, and solid tumors. In addition, LYP is strongly associated with type 1 diabetes and many other autoimmune diseases. This review summarizes recent findings on several highly recognized PTP family drug targets, including PTP1B, Src homology phosphotyrosyl phosphatase 2(SHP2), lymphoid-specific tyrosine phosphatase (LYP), CD45, Fas associated phosphatase-1 (FAP-1), striatal enriched tyrosine phosphatases (STEP), mitogen-activated protein kinase/dual-specificity phosphatase 1 (MKP-1), phosphatases of regenerating liver-1 (PRL), low molecular weight PTPs (LMWPTP), and CDC25. Given that there are over 100 family members, we hope this review will serve as a road map for innovative drug discovery targeting PTPs.

Targeting cell division cycle 25 homolog B to regulate influenza virus replication

Influenza virus is a worldwide global health concern causing seasonal morbidity mortality and economic burden. Chemotherapeutics is available; however, rapid emergence of drug-resistant influenza virus strains has reduced its efficacy. Thus, there is a need to discover novel antiviral agents. In this study, RNA interference (RNAi) was used to screen host genes required for influenza virus replication. One pro-influenza virus host gene identified was dual-specificity phosphatase cell division cycle 25 B (CDC25B). RNAi screening of CDC25B resulted in reduced influenza A virus replication, and a CDC25B small-molecule inhibitor (NSC95397) inhibited influenza A virus replication in a dose-dependent fashion. Viral RNA synthesis was reduced by NSC95397 in favor of increased beta interferon (IFN-β) expression, and NSC95397 was found to interfere with nuclear localization and chromatin association of NS1, an influenza virus protein. As NS1 has been shown to be chromatin associated and to suppress host transcription, it is likely that CDC25B supports NS1 nuclear function to hijack host transcription machinery in favor of viral RNA synthesis, a process that is blocked by NSC95397. Importantly, NSC95397 treatment protects mice against lethal influenza virus challenge. The findings establish CDC25B as a pro-influenza A virus host factor that may be targeted as a novel influenza A therapeutic strategy.

Specific targeting of caspase-9/PP2A interaction as potential new anti-cancer therapy

PURPOSE

PP2A is a serine/threonine phosphatase critical to physiological processes, including apoptosis. Cell penetrating peptides are molecules that can translocate into cells without causing membrane damage. Our goal was to develop cell-penetrating fusion peptides specifically designed to disrupt the caspase-9/PP2A interaction and evaluate their therapeutic potential in vitro and in vivo.

EXPERIMENTAL DESIGN

We generated a peptide containing a penetrating sequence associated to the interaction motif between human caspase-9 and PP2A (DPT-C9h), in order to target their association. Using tumour cell lines, primary human cells and primary human breast cancer (BC) xenografts, we investigated the capacity of DPT-C9h to provoke apoptosis in vitro and inhibition of tumour growth (TGI) in vivo. DPT-C9h was intraperitoneally administered at doses from 1 to 25 mg/kg/day for 5 weeks. Relative Tumour Volume (RTV) was calculated.

RESULTS

We demonstrated that DPT-C9h specifically target caspase-9/PP2A interaction in vitro and in vivo and induced caspase-9-dependent apoptosis in cancer cell lines. DPT-C9h also induced significant TGI in BC xenografts models. The mouse-specific peptide DPT-C9 also induced TGI in lung (K-Ras model) and breast cancer (PyMT) models. DPT-C9h has a specific effect on transformed B cells isolated from chronic lymphocytic leukemia patients without any effect on primary healthy cells. Finally, neither toxicity nor immunogenic responses were observed.

CONCLUSION

Using the cell-penetrating peptides blocking caspase-9/PP2A interactions, we have demonstrated that DPT-C9h had a strong therapeutic effect in vitro and in vivo in mouse models of tumour progression.

Deregulated Cdk5 triggers aberrant activation of cell cycle kinases and phosphatases inducing neuronal death

Aberrant activation of cell cycle proteins is believed to play a critical role in Alzheimer's disease (AD) pathogenesis; although, the molecular mechanisms leading to their activation in diseased neurons remain elusive. The goal of this study was to investigate the mechanistic link between Cdk5 deregulation and cell cycle re-activation in β-amyloid(1-42) (Aβ(1-42))-induced neurotoxicity. Using a chemical genetic approach, we identified Cdc25A, Cdc25B and Cdc25C as direct Cdk5 substrates in mouse brain lysates. We show that deregulated Cdk5 directly phosphorylates Cdc25A, Cdc25B and Cdc25C at multiple sites, which not only increases their phosphatase activities but also facilitates their release from 14-3-3 inhibitory binding. Cdc25A, Cdc25B and Cdc25C in turn activate Cdk1, Cdk2 and Cdk4 kinases causing neuronal death. Selective inhibition of Cdk5 abrogates Cdc25 and Cdk activations in Aβ(1-42)-treated neurons. Similarly, phosphorylation-resistant mutants of Cdc25 isoforms at Cdk5 sites are defective in activating Cdk1, Cdk2 and Cdk4 in Aβ(1-42)-treated primary cortical neurons, emphasizing a major role of Cdk5 in the activation of Cdc25 isoforms and Cdks in AD pathogenesis. These results were further confirmed in human AD clinical samples, which had higher Cdc25A, Cdc25B and Cdc25C activities that were coincident with increased Cdk5 activity, as compared to age-matched controls. Inhibition of Cdk5 confers the highest neuroprotection against Aβ(1-42) toxicity, whereas inhibition of Cdc25 isoforms was partially neuroprotective, further emphasizing a decisive role of Cdk5 deregulation in cell-cycle-driven AD neuronal death.

Prognostic significance of high phosphatase of regenerating liver-3 expression in patients with gastric cancer who underwent curative gastrectomy

BACKGROUND

Although a close correlation between PRL-3 overexpression and lymph node metastasis has been reported in gastric cancer, its clinical utility as a useful prognostic molecular marker remains unclear.

METHODS

Prognostic significance of PRL-3 expression was analyzed immunohistochemically in 110 patients with gastric cancer who had undergone curative gastrectomy.

RESULTS

There was a significant correlation between gender, histology, lymph node metastasis, the presence of recurrence, and the level of PRL-3 expression. Recurrence in patients with high PRL-3 expression was significantly higher than that for patients with low PRL-3 expression (p < 0.001). The median overall survival (OS) time and 2-year OS rate for patients with high or moderate PRL-3 expressed tumors were worse than those of patients with low PRL-3 expressed tumor (p = 0.001). In addition, patients with low PRL-3 expression had a higher DFS rate and the median DFS interval than those of moderate or high PRL-3 expressed patients (p < 0.001). Multivariate analysis indicated that the rate of PRL-3 expression was an independent prognostic factor, in addition to the already-known important clinicopathological prognostic indicator for both DFS and OS.

CONCLUSIONS

The potential value of PRL-3 expression as a useful molecular marker in gastric cancer progression should be evaluated comprehensively; it may predict recurrence and poor prognosis in patients with gastric cancer after curative resection.

Dual-Specificity Phosphatase CDC25A/B Inhibitor Identified from a Focused Library with Nonelectrophilic Core Structure

Focused libraries of enamine derivatives with a nonacidic, nonelectrophilic core structure were screened for inhibitors of dual-specificity protein phosphatases, and an o-hydroxybenzyl derivative RE44 (10d) was identified as a selective inhibitor of CDC25A/B. This inhibitor induced cell-cycle arrest of tsFT210 cells at the G2/M phase and inhibited dephosphorylation of the CDC25B substrate CDK1. Unlike most quinone-based inhibitors, 10d does not generate reactive oxygen species.

Mutational analysis of mononucleotide repeats in dual specificity tyrosine phosphatase genes in gastric and colon carcinomas with microsatellite instability

Coordinated protein phosphorylation and dephosphorylation are crucial in the regulation of cell signaling, and disruption of the coordination is known to play important roles in cancer development. Recent reports revealed that classical protein tyrosine phosphatase (PTP)-encoded genes are somatically mutated in human colorectal cancer. However, data on dual specificity phosphatase (DPTP) gene mutations in human cancers are lacking. By analyzing a public genomic database, we found that five DPTP genes, CDC14A, MTM1, MTMR3, SSH1, and SSH2, have mononucleotide repeats in their coding DNA sequences. To see whether these genes are mutated in cancers with microsatellite instability (MSI), we analyzed the mononucleotide repeats in 26 gastric cancers (GC) with MSI (MSI-H), 12 GC with low MSI (MSI-L), 45 GC with stable MSI (MSS), 33 colorectal cancers (CRC) with MSI-H, 14 CRC with MSI-L, and 45 CRC with MSS by single-strand conformation polymorphism (SSCP). We found CDC14A and MTMR3 mutations in five and one cancer (s), respectively. These mutations were detected in MSI-H cancers, but not in MSI-L or MSS cancers. The GC and CRC with MSI-H harbored the mutations in 15% and 6%, respectively. The CDC14A and MTMR3 mutations detected in the GC and CRC were deletion or duplication mutations of one base in the nucleotide repeats that would result in premature stops of the amino acid syntheses. Our data show that frameshift mutations of DPTP genes in MSI-H cancers occur at moderate frequencies. The data suggested that alterations in the CDC14A and MTMR3 genes may play a role in the development of GC and CRC with MSI-H by deregulating phosphatase functions possibly together with mutations of classical PTP genes.

Inhibitors of Cdc25 phosphatases as anticancer agents: a patent review

IMPORTANCE OF THE FIELD

The cell division cycle 25 (Cdc25) family of proteins are highly conserved dual specificity phosphatases that regulate cyclin-dependent kinases, the main gatekeepers of the eukaryotic cell division cycle. The three isoforms of Cdc25, including Cdc25A, Cdc25B and Cdc25C, appear to act on different cyclin-dependent kinase/cyclin complexes at different stages of the cell cycle. Overexpression of Cdc25A and/or Cdc25B, but not Cdc25C, has been detected in numerous cancers and is often correlated with a poor clinical prognosis. Thus, inhibition of these phosphatases may represent a promising therapeutic approach in oncology.

AREAS COVERED IN THIS REVIEW

The main focus of the present review is to describe the development of Cdc25 inhibitors over the years. We describe different compounds according to the decade of discovery and focus attention on molecules that were published in patents.

WHAT THE READER WILL GAIN

Insight into the most clinically relevant therapeutic Cdc25 analogues that have been published in over 40 patents over the past 19 years.

TAKE HOME MESSAGE

Some Cdc25 inhibitors have suppressed in vivo the growth of human tumor xenografts in animals; this confirmed the validity of using Cdc25 phosphatase inhibition as an anticancer strategy, but side effects and toxicity remain to be investigated.

Development of antiproliferative phenylmaleimides that activate the unfolded protein response

The current paper presents the synthesis and evaluation of a series of maleimides that were designed to inhibit the Cdc25 phosphatase by alkylation of catalytically essential cysteine residues. Although in HepB3 cell culture assays the analogues did exhibit antiproliferative IC(50) values ranging from sub-micromolar to greater than 100 microM, inhibition of Cdc25 through cysteine alkylation could not be demonstrated. It was also found that analysis using fluorescence activated cell sorting (FACS) following treatment with the most potent analogue (1t) did not provide data consistent with inhibition at one specific point in the cell cycle, as would be expected if Cdc25A were inhibited. Further studies with a subset of analogues resulted in a correlation of antiproliferative potencies with activation of the unfolded protein response (UPR). The UPR is a regulatory pathway that temporarily suspends protein production when misfolding of proteins occurs within the endoplastic reticulum (ER). In addition, ER chaperones that promote proper refolding become up-regulated. If cellular damage cannot be resolved by these mechanisms, then the UPR can initiate apoptosis. The current study indicates that these maleimide analogues lead to UPR activation, which is predictive of the selective antiproliferative activity of the series.

Synthetic strategies toward carbocyclic purine-pyrimidine hybrid nucleosides

The blending of key structural features from the purine and pyrimidine nucleobase scaffolds gives rise to a new class of hybrid nucleosides. The purine-pyrimidine hybrid nucleosides can be viewed as either N-3 ribosylated purines or 5,6-disubstituted pyrimidines, thus recognition by both purine- and pyrimidine-metabolizing enzymes is possible. Given the increasing reports of the development of resistance in many enzymatic systems, a drug that could be recognized by more than one enzyme could prove highly advantageous in overcoming resistance mechanisms related to binding site mutations. In that regard, the design, synthesis and results of preliminary biological activity for a series of carbocyclic uracil derivatives with either a fused imidazole or thiazole ring are presented herein.

Mechanism of Cdc25B phosphatase with the small molecule substrate p-nitrophenyl phosphate from QM/MM-MFEP calculations

Cdc25B is a dual-specificity phosphatase that catalyzes the dephosphorylation of the Cdk2/CycA protein complex. This enzyme is an important regulator of the human cell cycle and has been identified as a potential anticancer target. In general, protein tyrosine phosphatases are thought to bind the dianionic form of the phosphate and employ general acid catalysis via the Asp residue in the highly conserved WPD-loop. However, the Cdc25 phosphatases form a special subfamily based on their distinct differences from other protein tyrosine phosphatases. Although Cdc25B contains the (H/V)CX(5)R catalytic motif present in all other protein tyrosine phosphatases, it lacks an analogous catalytic acid residue. No crystallographic data currently exist for the complex of Cdc25B with Cdk2/CycA, so in addition to its natural protein substrate, experimental and theoretical studies are often carried out with small molecule substrates. In an effort to gain understanding of the dephosphorylation mechanism of Cdc25B with a commonly used small molecule substrate, we have performed simulations of the rate-limiting step of the reaction catalyzed by Cdc25B with the substrate p-nitrophenyl phosphate using the recently developed QM/MM Minimum Free Energy Path method (Hu et al. J. Chem. Phys. 2008, 034105). We have simulated the first step of the reaction with both the monoanionic and the dianionic forms of the substrate, and our calculations favor a mechanism involving the monoanionic form. Thus, Cdc25 may employ a unique dephosphorylation mechanism among protein tyrosine phosphatases, at least in the case of the small molecule substrate p-nitrophenyl phosphate.

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.

Prognostic significance of phosphatase of regenerating liver-3 expression in ovarian cancer

Phosphatase of regenerating liver-3 (PRL-3) is overexpressed in several human cancers and associated with tumor progression, invasion and metastasis. However, the correlation between PRL-3 expression and clinical outcome in ovarian cancer has not been studied. In the present study, we investigated the expression of PRL-3 in 119 ovarian cancers and 30 normal ovarian tissues by immunohistochemistry with an anti-PRL-3 mouse monoclonal antibody 3B6, and analyzed its relationship with clinicopathologic factors and survival. The results demonstrated that PRL-3 expression was significantly higher in ovarian cancers compared to normal ovarian tissues (P < 0.001). PRL-3 expression is not correlated with patient age, menstruation, tumor size, histological type, residual tumor, or other clinical findings. The patients with PRL-3-positive tumors had a significant poor prognosis than those with PRL-3-negative tumors. Univariate analysis identified PRL-3 expression as a poor outcome predictor (HR 1.925, 95% CI, 1.046-3.544, P = 0.035). Multivariate analysis indicated that PRL-3 expression was an independent prognostic factor of borderline significance (HR 1.695, 95% CI, 0.914-3.145, P = 0.094). Our results suggest that PRL-3 may serve as a valuable marker for diagnosis of ovarian cancer and as a potential independent prognostic factor for ovarian cancer.

Toward a molecular understanding of the interaction of dual specificity phosphatases with substrates: insights from structure-based modeling and high throughput screening

Dual-specificity phosphatases (DSPs) are important, but poorly understood, cell signaling enzymes that remove phosphate groups from tyrosine and serine/threonine residues on their substrate. Deregulation of DSPs has been implicated in cancer, obesity, diabetes, inflammation, and Alzheimer's disease. Due to their biological and biomedical significance, DSPs have increasingly become the subject of drug discovery high-throughput screening (HTS) and focused compound library development efforts. Progress in identifying selective and potent DSP inhibitors has, however, been restricted by the lack of sufficient structural data on inhibitor-bound DSPs. The shallow, almost flat, substrate binding sites in DSPs have been a major factor in hampering the rational design and the experimental development of active site inhibitors. Recent experimental and virtual HTS studies, as well as advances in molecular modeling, provide new insights into the potential mechanisms for substrate recognition and binding by this important class of enzymes. We present herein an overview of the progress, along with a brief description of applications to two types of DSPs: Cdc25 and MAP kinase phosphatase (MKP) family members. In particular, we focus on combined computational and experimental efforts for designing Cdc25B and MKP-1 inhibitors and understanding their mechanisms of interactions with their target proteins. These studies emphasize the utility of developing computational models and methods that meet the two major challenges currently faced in structure-based in silico design of lead compounds: the conformational flexibility of the target protein and the entropic contribution to the selection and stabilization of particular bound conformers.

Is Cdc25 a druggable target?

Proper control of cell cycle progression requires the functionality of a small family of activating phosphatases termed Cdc25, which have been implicated in cancer and Alzheimer's disease. These protein tyrosine phosphatases are therefore recognized as attractive molecular targets for small molecules. We review the rationale, approaches, progress and challenges for developing small molecule inhibitors of the Cdc25 family. A number of potential chemical probes are discussed and their characteristics are summarized.

Bioactivities of simplified adociaquinone B and naphthoquinone derivatives against Cdc25B, MKP-1, and MKP-3 phosphatases

Some simplified adociaquinone B analogs and a series of 1,4-naphthoquinone derivatives were synthesized and tested against the three enzymes Cdc25B, MKP-1, and MKP-3. Cdc25B and MKP-1 in particular are enzymes overexpressed in human cancer cells, and they represent potential molecular targets for novel cancer chemotherapeutic treatments. A number of analogs exhibited significant inhibitory activity against these enzymes, and the bioassay data in addition to structure-activity relationships of these compounds will be discussed.

Discovery and characterization of a novel inhibitor of CDC25B, LGH00045

AIM

Cell division cycle 25 (CDC25) phosphatases have recently been considered as potential targets for the development of new cancer therapeutic agents. We aimed to discover novel CDC25B inhibitors in the present study.

METHODS

A molecular level high-throughput screening (HTS) assay was set up to screen a set of 48000 pure compounds.

RESULTS

HTS, whose average Z' factor is 0.55, was finished and LGH00045, a mixed-type CDC25B inhibitor with a novel structure and relative selectivity for protein tyrosine phosphatases, was identified. Furthermore, LGH00045 impaired the proliferation of tumor cells and increased cyclin-dependent kinase 1 inhibitory tyrosine phosphorylation. In synchronized HeLa cells, LGH00045 delayed cell cycle progression at the G2-M transition.

CONCLUSION

LGH00045, a novel CDC25B inhibitor identified through HTS, showed good inhibition on the proliferation of tumor cells and affected the cell cycle progression, which makes it a good hit for further structure modification.

Expression of phosphatase of regenerating liver 1 and 3 mRNA in esophageal squamous cell carcinoma

CONTEXT

Phosphatase of regenerating liver (PRL) 3 messenger RNA (mRNA) was reported to express in human colorectal, gastric, ovarian, breast, and hepatic cancers.

OBJECTIVE

To examine the expression of PRL-1 and PRL-3 mRNAs in human esophageal squamous cell carcinoma (ESCC).

DESIGN

Expression of PRL-1 and PRL-3 mRNA was examined with reverse transcriptase-polymerase chain reaction in fresh tissue collected from 40 cases of ESCC with matched lymph node metastasis in 21 cases. The association of expression of PRL-1 and PRL-3 mRNAs with clinicopathologic parameters was analyzed.

RESULTS

The frequencies of PRL-1 and PRL-3 mRNA expression were significantly higher in ESCC than in normal esophageal tissue (P = .001; P = .01) and also significantly higher in ESCC with lymph node metastasis than in those without lymph node metastasis (P = .01; P = .03). The levels of PRL-1 and PRL-3 mRNA expression were significantly higher in ESCC with lymph node metastasis than in those without lymph node metastasis (P = .04; P = .04). The frequencies and levels of PRL-1 and PRL-3 mRNA expression were correlated with the later stages but not with tumor differentiation, tumor location in the esophagus, patient's sex, and age.

CONCLUSIONS

PRL-1 and PRL-3 mRNAs may be involved in and used to predict the metastasis of ESCC. The possibility of using PRL-1 and PRL-3 as the therapeutical target is also discussed.

Targeting PTPs with small molecule inhibitors in cancer treatment

Protein tyrosine phosphorylation plays a major role in cellular signaling. The level of tyrosine phosphorylation is controlled by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Disturbance of the normal balance between PTK and PTP activity results in aberrant tyrosine phosphorylation, which has been linked to the etiology of several human diseases, including cancer. A number of PTPs have been implicated in oncogenesis and tumor progression and therefore are potential drug targets for cancer chemotherapy. These include PTP1B, which may augment signaling downstream of HER2/Neu; SHP2, which is the first oncogene in the PTP superfamily and is essential for growth factor-mediated signaling; the Cdc25 phosphatases, which are positive regulators of cell cycle progression; and the phosphatase of regenerating liver (PRL) phosphatases, which promote tumor metastases. As PTPs have emerged as drug targets for cancer, a number of strategies are currently been explored for the identification of various classes of PTP inhibitors. These efforts have resulted many potent, and in some cases selective, inhibitors for PTP1B, SHP2, Cdc25 and PRL phosphatases. Structural information derived from these compounds serves as a solid foundation upon which novel anti-cancer agents targeted to these PTPs can be developed.

Computational design, synthesis and biological evaluation of para-quinone-based inhibitors for redox regulation of the dual-specificity phosphatase Cdc25B

Quinoid inhibitors of Cdc25B were designed based on the Linear Combination of Atomic Potentials (LCAP) methodology. In contrast to a published hypothesis, the biological activities and hydrogen peroxide generation in reducing media of three synthetic models did not correlate with the quinone half-wave potential, E(1/2).

A natural love of natural products

Recent research on the chemistry of natural products from the author’s group that led to the receipt of the ACS Ernest Guenther Award in the Chemistry of Natural Products is reviewed. REDOR NMR and synthetic studies established the T-taxol conformation as the bioactive tubulin-binding conformation, and these results were confirmed by the synthesis of compounds which clearly owed their activity or lack of activity to whether or not they could adopt the T-taxol conformation. Similar studies with the epothilones suggest that the current tubulin-binding model needs to be modified. Examples of natural products discovery and biodiversity conservation in Suriname and Madagascar are also presented, and it is concluded that natural products chemistry will continue to make significant contributions to drug discovery.

Design and synthesis of N-alkyl oxindolylidene acetic acids as a new class of potent Cdc25A inhibitors

The oxindolylidene acetic acids having long N-alkyl chains exhibited strong inhibitory activity toward dual specificity phosphatase Cdc25A.

MPL1, a novel phosphatase with leucine-rich repeats, is essential for proper ERK2 phosphorylation and cell motility

The novel Dictyostelium phosphatase MPL1 contains six leucine-rich repeats at the amino-terminal end and a phosphatase domain at the carboxyl end. Similarly architectured phosphatases exist among other protozoa, such as Entamoeba histolytica, Leishmania major, and Trypanosoma cruzi. MPL1 was strongly induced after 5 h of development; ablation by homologous recombination led to defective streaming and aggregation during development. In addition, cyclic AMP (cAMP)-pulsed mpl1(-) cells showed reduced random and directional motility. At the molecular level, mpl1(-) cells displayed higher prestimulus and persistent poststimulus ERK2 phosphorylation in response to cAMP stimulation. Consistent with their phenotype of persistent ERK2 phosphorylation, mpl1(-) cells also displayed an aberrant pattern of cAMP production, resembling that of the regA(-) cells. Reintroduction of a full-length MPL1 into mpl1(-) cells restored aggregation, ERK2 regulation, random and directional motility, and cAMP production similar to wild-type cells. We propose that MPL1 is a novel phosphatase essential for proper regulation of ERK2 phosphorylation and optimal motility during development.

Role of type 2C protein phosphatases in growth regulation and in cellular stress signaling

A number of interesting features, phenotypes, and potential clinical applications have recently been ascribed to the type 2C family of protein phosphatases. Thus far, 16 different PP2C genes have been identified in the human genome, encoding (by means of alternative splicing) for at least 22 different isozymes. Virtually ever since their discovery, type 2C phosphatases have been predominantly linked to cell growth and to cellular stress signaling. Here, we provide an overview of the involvement of type 2C phosphatases in these two processes, and we show that four of them (PP2Calpha, PP2Cbeta, ILKAP, and PHLPP) can be expected to function as tumor suppressor proteins, and one as an oncoprotein (PP2Cdelta /Wip1). In addition, we demonstrate that in virtually all cases in which they have been linked to the stress response, PP2Cs act as inhibitors of cellular stress signaling. Based on the vast amount of experimental evidence obtained thus far, it therefore seems justified to conclude that type 2C protein phosphatases are important physiological regulators of cell growth and of cellular stress signaling.

A comparative study of gene expression profiles in rat liver after administration of alpha-hexachlorocyclohexane and lindane

Alpha-hexachlorocyclohexane (alpha-HCH) is a stereoisomer of gamma-HCH, the active ingredient of lindane (> 99% gamma-HCH). In the present study, cDNA microarray technology was employed to identify changes in gene expression associated with toxicity in livers of male Fischer 344 rats after treatment with alpha-HCH (2, 20 mg/kg/day) and lindane (1, 10 mg/kg/day) by daily oral gavage for up to 28 days. Liver samples were obtained after 1, 3, 7, 14 and 28 days and compared for gene expression profiles. The dose of the alpha-HCH was higher than that of lindane and toxicity was greater, but the numbers of probe sets with differences in expression were fewer for the alpha-HCH-treated group except on Day 3. Only very few probe sets with differences in expression overlapped between alpha-HCH and lindane at each time point and the gene expression profiles were very dissimilar. Important liver-based differences in expression between alpha-HCH and lindane might possibly account for hepato-carcinogenicity of alpha-HCH.

Anthraquinones, Cdc25B phosphatase inhibitors, isolated from the roots of Polygonum multiflorum Thunb

Three anthraquinones, Cdc25B phosphatase inhibitors, were isolated from the methanolic extract of the roots of Polygonum multiflorum Thunb. (Polygonaceae). Anthraquinones, physcion (1), emodin (2), and questin (3), inhibited the enzymatic activity of Cdc25B phosphatase with IC(50) values of 62.5, 30, and 34 microg mL(-1), respectively. Emodin (2) and questin (3) strongly inhibited the growth of human colon cancer cells, SW620 with GI(50) values of 6.1 and 0.9 microg mL(-1), respectively. Commercially available anthraquinones, chrysophanol (4), and rhein (5) also inhibited Cdc25B phosphatase with IC(50) values of 10.7 and 22.1 microg mL(-1), respectively.

Association of tyrosine PRL-3 phosphatase protein expression with peritoneal metastasis of gastric carcinoma and prognosis

PURPOSE

In gastric carcinoma, high expression of PRL-3, a protein tyrosine phosphatase, is associated with lymph node metastasis. We studied the relationship between PRL-3 expression and peritoneal metastasis in gastric carcinoma.

METHODS

Immunohistochemical analysis using the anti-PRL-3 antibody was done in 639 patients with gastric carcinoma including 89 with peritoneal metastases. We then compared the clinicopathologic characteristics of the PRL-3-positive and PRL-3-negative carcinomas.

RESULTS

PRL-3 was expressed in 70.4% of the primary gastric carcinomas overall; in 80.9% of the cancers with peritoneal metastasis and in 68.7% of those without peritoneal metastasis (P = 0.020). PRL-3 expression was higher in peritoneal metastasis than in the corresponding primary gastric cancers (P = 0.028). PRL-3 expression was correlated with tumor stage (coefficient = 0.343, P = 0.01) and cancer progression, including lymphatic invasion (coefficient = 0.325, P = 0.02), extent of lymph node metastasis (coefficient = 0.322, P = 0.01), and peritoneal metastasis (coefficient = 0.316, P = 0.03). Patients who were PRL-3-negative had a better survival rate than those who were PRL-3-positive at all stages (stage I: log-rank P = 0.046, Wilcoxon P = 0.048; stage II: log-rank P = 0.035, Wilcoxon P = 0.041; stage III: log-rank P = 0.027, Wilcoxon P = 0.033; stage IV: log-rank P = 0.032, Wilcoxon P = 0.030).

CONCLUSIONS

Peritoneal metastasis appears to be correlated with PRL-3 expression, tumor stage, lymphatic invasion, and extent of lymph node metastasis. PRL-3 expression was negatively correlated with prognosis in patients with gastric cancer.

Targeting dual-specificity phosphatases: manipulating MAP kinase signalling and immune responses

Dual-specificity phosphatases (DUSPs) are a subset of protein tyrosine phosphatases, many of which dephosphorylate threonine and tyrosine residues on mitogen-activated protein kinases (MAPKs), and hence are also referred to as MAPK phosphatases (MKPs). The regulated expression and activity of DUSP family members in different cells and tissues controls MAPK intensity and duration to determine the type of physiological response. For immune cells, DUSPs regulate responses in both positive and negative ways, and DUSP-deficient mice have been used to identify individual DUSPs as key regulators of immune responses. From a drug discovery perspective, DUSP family members are promising drug targets for manipulating MAPK-dependent immune responses in a cell-type and disease-context-dependent manner, to either boost or subdue immune responses in cancers, infectious diseases or inflammatory disorders.

Development and optimization of high-throughput in vitro protein phosphatase screening assays

We describe here detailed protocols to design, optimize and validate in vitro phosphatase assays that we have utilized to conduct high-throughput screens for inhibitors of dual-specificity phosphatases: CDC25B, mitogen-activated protein kinase phosphatase (MKP)-1 and MKP-3. We provide details of the critical steps that are needed to effectively miniaturize the assay into a 384-well, high-throughput format that is both reproducible and cost effective. In vitro phosphatase assays that are optimized according to these protocols should satisfy the assay performance criteria required for a robust high-throughput assay with Z-factors >0.5, and with low intra-plate, inter-plate and day-to-day variability (CV <20%). Assuming the availability of sufficient active phosphatase enzyme and access to appropriate liquid handling automation and detection instruments, a single investigator should be able to develop a 384-well format high-throughput assay in a period of 3-4 weeks.

Synthesis and biological activity of a focused library of mitogen-activated protein kinase phosphatase inhibitors

Mitogen-activated protein kinase phosphatase 1 is a tyrosine phosphatase superfamily member that dephosphorylates and inactivates mitogen-activated protein kinase substrates, such as p38, c-Jun-N-terminal kinase, and extracellular signal-related kinase. These mitogen-activated protein kinase substrates regulate many cellular processes associated with human diseases. In spite of this potential as a molecular target for chemotherapy, however, pharmacologically tractable inhibitors of mitogen-activated protein kinase phosphatase-1 have yet to be developed. Based on the results from a high-throughput screen for small molecule inhibitors of mitogen-activated protein kinase phosphatase-1, we designed, synthesized, and evaluated a focused library in an effort to further understand the structural requirements for mitogen-activated protein kinase phosphatase-1 inhibitory activity.