Inhibition of Cdc25 phosphatases by indolyldihydroxyquinones

Chemical Characterization, DNA-Damage Protection, Antiproliferative Activity and in Silico Studies of the Essential Oils from Perralderia coronopifolia Coss

In this study, for the first time, we analyzed the chemical composition of essential oils (EOs) steam-distilled from the flowers and leaves of Perralderia coronopifolia by GC-FID/MS. The objective was to explore new anticancer and antioxidant bioactive substances and understand their mechanisms of action through the use of plant-derived natural products. The major chemical components characterizing the EOs were cis-chrysanthenyl acetate 1, 6-oxocyclonerolidol 2, cis-8-acetoxychrysanthenyl acetate 3, and 6α-hydroxycyclonerolidol 4, respectively. Furthermore, the EOs inhibited cell proliferation in HeLa (human cervix carcinoma) and PC3 (human prostate cancer) cells and protected plasmid DNA from oxidative damage caused by UV-photolyzed H2 O2 . Employing a molecular docking study, we elucidated the main compounds' inhibition mechanisms. Consequently, the antitumor activity could be related to the inhibitory property of compound 3 against CDC25B phosphatase. The evaluation of ADMET (absorption, distribution, metabolism, excretion, and toxicity) properties and the density functional theory (DFT) calculations of the major compounds, especially compound 3, offer potential insights for designing and developing new cancer drug candidates. In conclusion, our study provides a framework for future research and development in the field by establishing a scientific foundation for the use of Perralderia coronopifolia essential oils as a prospective source of antioxidant and anticancer agents.

Novel Reversible Inhibitors of Xanthine Oxidase Targeting the Active Site of the Enzyme

Xanthine oxidase (XO) is a flavoprotein catalysing the oxidation of hypoxanthine to xanthine and then to uric acid, while simultaneously producing reactive oxygen species. Altered functions of XO may lead to severe pathological diseases, including gout-causing hyperuricemia and oxidative damage of tissues. These findings prompted research studies aimed at targeting the activity of this crucial enzyme. During the course of a virtual screening study aimed at the discovery of novel inhibitors targeting another oxidoreductase, superoxide dismutase, we identified four compounds with non-purine-like structures, namely ALS-1, -8, -15 and -28, that were capable of causing direct inhibition of XO. The kinetic studies of their inhibition mechanism allowed a definition of these compounds as competitive inhibitors of XO. The most potent molecule was ALS-28 (Ki 2.7 ± 1.5 µM), followed by ALS-8 (Ki 4.5 ± 1.5 µM) and by the less potent ALS-15 (Ki 23 ± 9 µM) and ALS-1 (Ki 41 ± 14 µM). Docking studies shed light on the molecular basis of the inhibitory activity of ALS-28, which hinders the enzyme cavity channel for substrate entry consistently with the competitive mechanism observed in kinetic studies. Moreover, the structural features emerging from the docked poses of ALS-8, -15 and -1 may explain the lower inhibition power with respect to ALS-28. All these structurally unrelated compounds represent valuable candidates for further elaboration into promising lead compounds.

A Comprehensive Overview of the Developments of Cdc25 Phosphatase Inhibitors

Cdc25 phosphatases have been considered promising targets for anticancer development due to the correlation of their overexpression with a wide variety of cancers. In the last two decades, the interest in this subject has considerably increased and many publications have been launched concerning this issue. An overview is constructed based on data analysis of the results of the previous publications covering the years from 1992 to 2021. Thus, the main objective of the current review is to report the chemical structures of Cdc25s inhibitors and answer the question, how to design an inhibitor with better efficacy and lower toxicity?

Discovery of Novel Naphthylphenylketone and Naphthylphenylamine Derivatives as Cell Division Cycle 25B (CDC25B) Phosphatase Inhibitors: Design, Synthesis, Inhibition Mechanism, and in Vitro Efficacy against Melanoma Cell Lines

CDC25 phosphatases play a critical role in the regulation of the cell cycle and thus represent attractive cancer therapeutic targets. We previously discovered the 4-(2-carboxybenzoyl)phthalic acid (NSC28620) as a new CDC25 inhibitor endowed with promising anticancer activity in breast, prostate, and leukemia cells. Herein, we report a structure-based optimization of NSC28620, leading to the identification of a series of novel naphthylphenylketone and naphthylphenylamine derivatives as CDC25B inhibitors. Compounds 7j, 7i, 6e, 7f, and 3 showed higher inhibitory activity than the initial lead, with K_i values in the low micromolar range. Kinetic analysis, intrinsic fluorescence studies, and induced fit docking simulations provided a mechanistic understanding of the activity of these derivatives. All compounds were tested in the highly aggressive human melanoma cell lines A2058 and A375. Compound 4a potently inhibited cell proliferation and colony formation, causing an increase of the G2/M phase and a reduction of the G0/G1 phase of the cell cycle in both cell lines.

3D QSAR Pharmacophore Based Virtual Screening for Identification of Potential Inhibitors for CDC25B

Owing to its fundamental roles in cell cycle phases, the cell division cycle 25B (CDC25B) was broadly considered as potent clinical drug target for cancers. In this study, 3D QSAR pharmacophore models for CDC25B inhibitors were developed by the module of Hypogen. Three methods (cost analysis, test set prediction, and Fisher's test) were applied to validate that the models could be used to predict the biological activities of compounds. Subsequently, 26 compounds satisfied Lipinski's rule of five were obtained by the virtual screening of the Hypo-1-CDC25B against ZINC databases. It was then discovered that 9 identified molecules had better binding affinity than a known CDC25B inhibitors-compound 1 using docking studies. The molecular dynamics simulations showed that the compound had favorable conformations for binding to the CDC25B. Thus, our findings here would be helpful to discover potent lead compounds for the treatment of cancers.

2,4-Di-chloro-1-iodo-6-nitro-benzene

In the crystal structure of the title compound, C6H2Cl2INO2, there are weak C-H⋯Cl inter-actions and I⋯O [3.387 (4) Å] close contacts. These inter-actions form sheets in the ac plane, with the closest contact between adjacent planes occurring between inversion-related nitro O atoms [3.025 (8) Å]. The molecule possesses mirror symmetry, with the halogen, N and C atoms all lying in the mirror plane. Hence, the dihedral angle between the benzene ring and the nitro group is 90°.

A novel coumarin-quinone derivative SV37 inhibits CDC25 phosphatases, impairs proliferation, and induces cell death

Cell division cycle (CDC) 25 proteins are key phosphatases regulating cell cycle transition and proliferation by regulating CDK/cyclin complexes. Overexpression of these enzymes is frequently observed in cancer and is related to aggressiveness, high-grade tumors and poor prognosis. Thus, targeting CDC25 by compounds, able to inhibit their activity, appears a good therapeutic approach. Here, we describe the synthesis of a new inhibitor (SV37) whose structure is based on both coumarin and quinone moieties. An analytical in vitro approach shows that this compound efficiently inhibits all three purified human CDC25 isoforms (IC50 1-9 µM) in a mixed-type mode. Moreover, SV37 inhibits growth of breast cancer cell lines. In MDA-MB-231 cells, reactive oxygen species generation is followed by pCDK accumulation, a mark of CDC25 dysfunction. Eventually, SV37 treatment leads to activation of apoptosis and DNA cleavage, underlining the potential of this new type of coumarin-quinone structure.

Discovery of new inhibitors of Cdc25B dual specificity phosphatases by structure-based virtual screening

Cell division cycle 25 (Cdc25) proteins are highly conserved dual specificity phosphatases that regulate cyclin-dependent kinases and represent attractive drug targets for anticancer therapies. To discover more potent and diverse inhibitors of Cdc25 biological activity, virtual screening was performed by docking 2.1 million compounds into the Cdc25B active site. An initial subset of top-ranked compounds was selected and assayed, and 15 were found to have enzyme inhibition activity at micromolar concentration. Among these, four structurally diverse inhibitors with a different inhibition profile were found to inhibit human MCF-7, PC-3, and K562 cancer cell proliferation and significantly affect the cell cycle progression. A subsequent hierarchical similarity search with the most active reversible Cdc25B inhibitor found led to the identification of an additional set of 19 ligands, three of which were confirmed as Cdc25B inhibitors with IC(50) values of 7.9, 4.2, and 9.9 μM, respectively.

Flexibility and inhibitor binding in cdc25 phosphatases

Cdc25 phosphatases involved in cell cycle checkpoints are now active targets for the development of anti-cancer therapies. Rational drug design would certainly benefit from detailed structural information for Cdc25s. However, only apo- or sulfate-bound crystal structures of the Cdc25 catalytic domain have been described so far. Together with previously available crystalographic data, results from molecular dynamics simulations, bioinformatic analysis, and computer-generated conformational ensembles shown here indicate that the last 30-40 residues in the C-terminus of Cdc25B are partially unfolded or disordered in solution. The effect of C-terminal flexibility upon binding of two potent small molecule inhibitors to Cdc25B is then analyzed by using three structural models with variable levels of flexibility, including an equilibrium distributed ensemble of Cdc25B backbone conformations. The three Cdc25B structural models are used in combination with flexible docking, clustering, and calculation of binding free energies by the linear interaction energy approximation to construct and validate Cdc25B-inhibitor complexes. Two binding sites are identified on top and beside the Cdc25B active site. The diversity of interaction modes found increases with receptor flexibility. Backbone flexibility allows the formation of transient cavities or compact hydrophobic units on the surface of the stable, folded protein core that are unexposed or unavailable for ligand binding in rigid and densely packed crystal structures. The present results may help to speculate on the mechanisms of small molecule complexation to partially unfolded or locally disordered proteins.

Structure-based virtual screening approach to identify novel classes of Cdc25B phosphatase inhibitors

Discovery of Cdc25B phosphatase inhibitors has been actively pursued with the aim to develop anticancer agents. We have been able to identify eight novel Cdc25B inhibitors by means of a computer-aided drug design protocol involving the virtual screening with docking simulations under consideration of the effects of ligand solvation in the binding free energy function. Structural features relevant to the interactions of the newly identified inhibitors with the active-site residues of Cdc25B are also discussed in detail.

IRC-083864, a novel bis quinone inhibitor of CDC25 phosphatases active against human cancer cells

CDC25 phosphatases are key actors in cyclin-dependent kinases activation whose role is essential at various stages of the cell cycle. CDC25 expression is upregulated in a number of human cancers. CDC25 phosphatases are therefore thought to represent promising novel targets in cancer therapy. Here, we report the identification and the characterization of IRC-083864, an original bis-quinone moiety that is a potent and selective inhibitor of CDC25 phosphatases in the low nanomolar range. IRC-083864 inhibits cell proliferation of a number of cell lines, regardless of their resistance to other drugs. It irreversibly inhibits cell proliferation and cell cycle progression and prevents entry into mitosis. In addition, it inhibits the growth of HCT-116 tumor spheroids with induction of p21 and apoptosis. Finally, IRC-083864 reduced tumor growth in mice with established human prostatic and pancreatic tumor xenografts. This study describes a novel compound, which merits further study as a potential anticancer agent.

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.

Discovery of novel Cdc25 phosphatase inhibitors with micromolar activity based on the structure-based virtual screening

Cdc25 phosphatases have been considered as attractive drug targets for anticancer therapy because of the correlation of their overexpression with a wide variety of cancers. We have been able to identify five novel Cdc25 phosphatase inhibitors with micromolar activity by means of a computer-aided drug design protocol involving the homology modeling of Cdc25A and the virtual screening with the automated AutoDock program implementing the effects of ligand solvation in the scoring function. Because the newly discovered inhibitors are structurally diverse and reveal a significant potency with IC 50 values lower than 10 microM, they can be considered for further development by structure-activity relationship studies or de novo design methods. The differences in binding modes of the identified inhibitors in the active sites of Cdc25A and B are discussed in detail.

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).

Toward the virtual screening of Cdc25A phosphatase inhibitors with the homology modeled protein structure

Cdc25 phosphatases have been considered as attractive drug targets for anticancer therapy due to the correlation of their overexpression with a wide variety of cancers. As a method for the discovery of novel inhibitors of Cdc25 phosphatases, we have evaluated the computer-aided drug design protocol involving the homology modeling of Cdc25A and virtual screening with the two docking tools: FlexX and the modified AutoDock program implementing the effects of ligand solvation in the scoring function. The homology modeling with the X-ray crystal structure of Cdc25B as a template provides a high-quality structure of Cdc25A that enables the structure-based inhibitor design. Of the two docking programs under consideration, AutoDock is found to be more accurate than FlexX in terms of scoring putative ligands. A detailed binding mode analysis of the known inhibitors shows that they can be stabilized in the active site of Cdc25A through the simultaneous establishment of the multiple hydrogen bonds and the hydrophobic interactions. The present study demonstrates the usefulness of the modified AutoDock program as a docking tool for virtual screening of new Cdc25 phosphatase inhibitors as well as for binding mode analysis to elucidate the activities of known inhibitors.

Parallel synthesis of indolylquinones and their cell-based insulin mimicry

A synthetic route to bis-indolyldihydroxybenzoquinones was adapted for parallel organic synthesis. The route involves selective conjugate addition of an indole to dichlorobenzoquinone promoted by Brønsted acid, followed by a Lewis acid-promoted conjugate addition of a second indole and a final hydrolysis. Methods for high-throughput purification of the products of this synthesis were also developed. Using these methods, we prepared a library whose structures are based on asterriquinone natural products, which have a wide range of biological activities. In this report, the activities of the library members in activation of the insulin receptor on mammalian cells were examined. Novel compounds were discovered that fall outside earlier developed structure-activity relationships for insulin mimics, supporting the value of systematic investigation (inspired by Nature) for the discovery of novel biologically active molecules.

What's new on CDC25 phosphatase inhibitors

The CDC25 phosphatases are key regulators of cell cycle progression and play a central role in the checkpoint response to DNA damage. Their inhibition may therefore represent a promising therapeutic approach in oncology, and small molecule design strategies are currently leading to the identification of various classes of CDC25 inhibitors. Most structures developed so far are quinonoid-based compounds, but also phosphate surrogates or electrophilic entities. Considering the characteristics of the highly conserved active sites of the enzymes, many mechanisms of action have been proposed for these inhibitors. Quinonoid compounds may oxidize the catalytic site cysteine, but can also be considered as Michaël acceptors capable of reacting with the activated thiolate or other electrophilic entities. Phosphate surrogates are thought to interfere with the arginine residue, leading to reversible enzyme inhibition. But some inhibitors can combine in the same molecule several of these mechanisms, thus by fitting into the active site of the enzyme through one part of the molecule and bringing the reactive moiety in close proximity to the catalytic cysteine. This review summarizes novel classes of inhibitors that show specificity for the CDC25s over other phosphatases, cause cell proliferation inhibition and cell cycle arrest in vitro but also, for several of them, inhibition of xenografted tumoral cell growth in vivo. These promising results confirm the interest of the inhibition of CDC25 phosphatases as an anticancer therapeutic strategy.

Modeling of Cdc25B dual specifity protein phosphatase inhibitors: docking of ligands and enzymatic inhibition mechanism

The Cdc25 dual specificity phosphatases have central roles in coordinating cellular signalling processes and cell proliferation. It has been reported that an improper amplification or activation of these enzymes is a distinctive feature of a number of human cancers, including breast cancers. Thus, the inhibition of Cdc25 phosphatases might provide a novel approach for the discovery of new and selective antitumor agents. By using the crystal structure of the catalytic domain of Cdc25B, structural models for the interaction of various Cdc25B inhibitors (1-13) with the enzyme were generated by computational docking. The parallel use of two efficient and predictive docking programs, AutoDock and GOLD, allowed mutual validation of the predicted binding poses. To evaluate their quality, the models were validated with known structure-activity relationships and site-directed mutagenesis data. The results provide an improved basis for structure-based ligand design and suggest a possible explanation for the inhibition mechanism of the examined Cdc25B ligands. We suggest that the recurring motif of a tight interaction between the inhibitor and the two arginine residues, 482 and 544, is of prime importance for reversible enzyme inhibition. In contrast, the irreversible inhibition mechanism of 1-4 seems to be associated with the close vicinity of the quinone ring and the Cys473 catalytic thiolate. We believe that this extensive study might provide useful hints to guide the development of new potent Cdc25B inhibitors as novel anticancer drugs.

Independent Mechanistic Inhibition of Cdc25 Phosphatases by a Natural Product Caulibugulone

Caulibugulones are novel but poorly characterized cytotoxic isoquinoline quinones and iminoquinones identified in extracts from the marine bryozoan Caulibugula intermis. We now report that the caulibugulones are selective in vitro inhibitors of the Cdc25 family of cell cycle-controlling protein phosphatases compared with either human vaccinia H1-related phosphatase (VHR) or tyrosine phosphatase 1B (PTP1B). The in vitro inhibition of Cdc25B by caulibugulone A was irreversible and attenuated by reducing agents or catalase, consistent with direct oxidation of the enzyme by reactive oxygen species. Mechanistically, caulibugulone A directly inhibited cellular Cdc25B activity, generated intracellular reactive oxygen species and arrested cells in both G1 and G2/M phases of the cell cycle. Caulibugulone A also caused the selective degradation of Cdc25A protein by a process that was independent of reactive oxygen species production, proteasome activity, and the Chk1 signaling pathway. Instead, caulibugulone A stimulated the phosphorylation and subsequent activation of p38 stress kinase, leading to Cdc25A degradation. Thus, caulibugulone inhibition of cellular Cdc25A and B phosphatases occurred through at least two different mechanisms, leading to pronounced cell cycle arrest.

Inhibitors of protein tyrosine phosphatases: next-generation drugs?

The protein tyrosine phosphatases (PTPs) constitute a family of closely related key regulatory enzymes that dephosphorylate phosphotyrosine residues in their protein substrates. Malfunctions in PTP activity are linked to various diseases, ranging from cancer to neurological disorders and diabetes. Consequently, PTPs have emerged as promising targets for therapeutic intervention in recent years. In this review, general aspects of PTPs and the development of small-molecule inhibitors of PTPs by both academic research groups and pharmaceutical companies are discussed. Different strategies have been successfully applied to identify potent and selective inhibitors. These studies constitute the basis for the future development of PTP inhibitors as drugs.

Targeting the PTPome in human disease

Protein tyrosine phosphatases (PTPs) play vital roles in numerous cellular processes and are implicated in a growing number of human diseases, ranging from cancer to cardiovascular, immunological, infectious, neurological and metabolic diseases. There are at least 107 genes in the human genome, collectively referred to as the human 'PTPome'. Here the authors review the involvement of PTPs in human disease, discuss their potential as drug targets, and current efforts to develop PTP inhibitors for the treatment of human disease. Finally, the authors present their view of the future for PTPs as drug targets.

Synthesis of miltirone analogues as inhibitors of Cdc25 phosphatases

Miltirone analogues were synthesized and evaluated for inhibitory activity against Cdc25 and PTP1B. Most of the compounds demonstrated potent Cdc25 inhibitory activity, and several exhibited higher selectivity for Cdc25 than for PTP1B. In a cytotoxic assay, most of the compounds displayed cytotoxicity against the tumor cell lines A549 and HCT-116, producing IC(50) values in the micromolar range.

Inhibition of human tumor cell growth in vivo by an orally bioavailable inhibitor of CDC25 phosphatases

Cell cycle regulators, such as the CDC25 phosphatases, are potential targets for the development of new anticancer drugs. Here we report the identification and the characterization of BN82685, a quinone-based CDC25 inhibitor that is active in vitro and in vivo. BN82685 inhibits recombinant CDC25A, B, and C phosphatases in vitro. It inhibits the growth of human tumor cell lines with an IC(50) in the submicromolar range, independently of their resistance to chemotherapeutic agents. This inhibitory effect is irreversible on both the purified CDC25 enzyme in vitro and on tumor cell proliferation. The specificity of BN82685 towards the CDC25 phosphatases is shown by an increase in cyclin-dependent kinase 1 tyrosine 15 phosphorylation, by the reversion of the mitosis-inducing effect of CDC25B overexpression in HeLa cells, and by the lack of a growth inhibitory effect in an assay based on the use of a CDC25-independent fission yeast model. Finally, when administered p.o., BN82685 is shown to inhibit the growth of the human pancreatic tumor Mia PaCa-2 xenografted in athymic nude mice. BN82685 is therefore a promising new compound targeting CDC25, which confirms the interest of the inhibition of these enzymes as an anticancer therapeutic strategy.

EK-6136 (3-methyl-4-(O-methyl-oximino)-1-phenylpyrazolin-5-one): A novel Cdc25B inhibitor with antiproliferative activity

Cdc25B is a dual specific phosphatase, which plays a pivotal role in the activation of cell-cycle-dependent kinase 1 (Cdk1). A novel Cdc25B inhibitor, EK-6136, was identified by high throughput screening (HTS) using compounds from Korea Chemical Bank and examined for its biological effects. EK-6136 inhibited Cdc25B with an IC50 of 6.4+/-1.5 microM. EK-6136 showed selectivity against several phosphatases including PTP-1B, CD45, Cdc25A, PP1, VHR and Yop. In the inhibition kinetic study, EK-6136 displayed a mixed inhibition pattern with a Ki value of 7.8+/-1.2 microM. Consistent with in vitro results, EK-6136 inhibited the proliferation of MCF-7 (human breast carcinoma), HT-29 (human colorectal adenocarcinoma) and A549 (lung carcinoma) cells with increased Cdk-1 phosphorylation. Herein, we propose that EK-6136 is an active HTS hit as a Cdc25B inhibitor with antiproliferative activity, and can be used for the design of more potent and selective antiproliferative agents.

Screening the active constituents of Chinese medicinal herbs as potent inhibitors of Cdc25 tyrosine phosphatase, an activator of the mitosis-inducing p34cdc2 kinase

OBJECTIVE

To screen and evaluate the active constituents of Chinese medicinal herbs as potent inhibitors of Cdc25 phosphatase.

METHODS

The affinity chromatography purified glutashione-S-transferase/Cdc25A phosphatase fusion protein and Cdc2/cyclin B from the extracts of starfish M phase oocytes are used as the cell cycle-specific targets for screening the antimitotic constituents. We tested 9 extracts isolated from the Chinese medicinal herbs and vegetables including the agents currently used in cancer treatment by measuring the inhibition of Cdc25A phosphatase and Cdc2 kinase activity. The antitumor activity of the extracts was also evaluated by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay and flow cytometry.

RESULTS

Cdc25A inhibitory activity and antitumor activity are detected in the extracts isolated from three Chinese medicinal herbs Agrimona pilosa; Herba solani lyrati; Galla chinesis.

CONCLUSION

We found three extracts isolated from Chinese medicinal herbs have potential inhibitory activity of Cdc25 phosphatase using a highly specific mechanism-based screen assay for antimitotic drug discovery.

DUAL SPECIFICITY PROTEIN PHOSPHATASES: Therapeutic Targets for Cancer and Alzheimer's Disease

The complete sequencing of the human genome is generating many novel targets for drug discovery. Understanding the pathophysiological roles of these putative targets and assessing their suitability for therapeutic intervention has become the major hurdle for drug discovery efforts. The dual-specificity phosphatases (DSPases), which dephosphorylate serine, threonine, and tyrosine residues in the same protein substrate, have important roles in multiple signaling pathways and appear to be deregulated in cancer and Alzheimer's disease. We examine the potential of DSPases as new molecular therapeutic targets for the treatment of human disease.

Redox regulation of Cdc25B by cell-active quinolinediones

Intracellular reduction and oxidation pathways regulate protein functionality through both reversible and irreversible mechanisms. The Cdc25 phosphatases, which control cell cycle progression, are potential subjects of oxidative regulation. Many of the more potent Cdc25 phosphatase inhibitors reported to date are quinones, which are capable of redox cycling. Therefore, we used the previously characterized quinolinedione Cdc25 inhibitor DA3003-1 [NSC 663284 or 6-chloro-7-(2-morpholin-4-yl-ethylamino)-quinoline-5,8-dione] and a newly synthesized congener JUN1111 [7-(2-morpholin-4-yl-ethylamino)-quinoline-5,8-dione] to test the hypothesis that quinone inhibitors of Cdc25 regulate phosphatase activity through redox mechanisms. Like DA3003-1, JUN1111 selectively inhibited Cdc25 phosphatases in vitro in an irreversible, time-dependent manner and arrested cells in the G1 and G2/M phases of the cell cycle. It is noteworthy that both DA3003-1 and JUN1111 directly inhibited Cdc25B activity in cells. Depletion of glutathione increased cellular sensitivity to DA3003-1 and JUN1111, and in vitro Cdc25B inhibition by these compounds was sensitive to pH, catalase, and reductants (dithiothreitol and glutathione), consistent with oxidative inactivation. In addition, both DA3003-1 and JUN1111 rapidly generated intracellular reactive oxygen species. Analysis of Cdc25B by mass spectrometry revealed sulfonic acid formation on the catalytic cysteine of Cdc25B after in vitro treatment with DA3003-1. These results indicate that irreversible oxidation of the catalytic cysteine of Cdc25B is indeed a mechanism by which these quinolinediones inactivate this protein phosphatase.

A novel cinnamic acid derivative that inhibits Cdc25 dual-specificity phosphatase activity

The Cdc25 dual-specificity phosphatases are key regulators of cell cycle progression through activation of cyclin-dependent kinases (Cdk). Three homologs exist in humans: Cdc25A, Cdc25B, and Cdc25C. Cdc25A and Cdc25B have oncogenic properties and are overexpressed in some types of tumors. Compounds that inhibit Cdc25 dual-specificity phosphatase activity might thus be potent anticancer agents. We screened several hundred compounds in a library using an in vitro phosphatase assay, with colorimetric measurement of the conversion of p-nitrophenyl phosphate (pNPP) to p-nitrophenol by the catalytic domain of recombinant human Cdc25, and discovered TPY-835, which inhibits Cdc25A and Cdc25B activity (IC50 = 5.1 and 5.7 microM, respectively). TPY-835 had mixed inhibition kinetics for Cdc25A and Cdc25B. TPY-835 caused cell cycle arrest in the G1 phase in human lung cancer cells (A549 and SBC-5) but not cell cycle arrest in the G2/M phase. After treatment with TPY-835, the activation of Cdk2 was suppressed and phosphorylation of the retinoblastoma (Rb) protein was decreased in SBC-5 cells. In addition, TPY-835 induced an increase of the sub-G1 phase cell population after 48-72 h treatment. The growth inhibitory effects of TPY-835 against cisplatin (CDDP)-, camptothecin- and 5-FU-resistant cell lines are comparable to the growth inhibitory effect on their parental lines, thus indicating that TPY-835 did not show cross-resistance to these cell lines. These results suggest that TPY-835 is a promising candidate for constructing a novel class of antitumor agents that can control the cell cycle progression of cancer cells.

Methyl scanning: total synthesis of demethylasterriquinone B1 and derivatives for identification of sites of interaction with and isolation of its receptor(s)

The principle of methyl scanning is proposed for determination of the sites of interaction between biologically active small molecules and their macromolecular target(s). It involves the systematic preparation of a family of methylated derivatives of a compound and their biological testing. As a functional assay, the method can identify the regions of a molecule that are important (and unimportant) for biological activity against even unknown targets, and thus provides an excellent complement to structural biology. Methyl scanning was applied to demethylasterriquinone B1, a small-molecule mimetic of insulin. A new, optimal total synthesis of this natural product was developed that enables the family of methyl scan derivatives to be concisely prepared for evaluation in a cellular assay. The results of this experiment were used to design a biotin-demethylasterriquinone conjugate for use as an affinity reagent. This compound was prepared in tens of milligram quantities in a four-step synthesis.

Redox regulation of the Cdc25 phosphatases

The Cdc25 phosphatases are essential for cell-cycle control in eukaryotes under normal conditions and in response to DNA damage via checkpoint controls. Recent evidence indicates direct control of the Cdc25s, and therefore the cell cycle, in response to changes in cellular redox status. These redox changes may originate intracellularly from mitochondrial leakage or in response to specific external triggers leading to production of reactive oxygen species (ROS). This review shows that the known chemistry and biology of the Cdc25s favor a direct role for these phosphatases in temporarily blocking cell-cycle progression until favorable reducing conditions are restored. First, the Cdc25s contain a highly reactive cysteine at the active site that can react directly with ROS, leading to enzyme inactivation. Second, the ROS-inactivated form of Cdc25 is expected to prevent cell-cycle progression based on precedent from cellular responses to DNA damage. Third, ROS-mediated oxidation of the Cdc25s leads to an intramolecular disulfide that is readily reversible by the cellular reductant thioredoxin. Finally, in vivo data supporting a direct role for the Cdc25s in redox regulation are considered.

Fluorinated quinoid inhibitor: possible "pure" arylator predicted by the simple theoretical calculation

We report on the fluorinated form of Cpd 5 as a cell growth inhibitor. This compound is 3-fold more potent than the parent Cpd 5 and is predicted, using the semi-empirical AM1 method to be only an arylator of cysteine-containing proteins, without generating reactive oxygen species.

Synthesis of the naphthalene-derived inhibitors against Cdc25A dual-specificity protein phosphatase and their biological activity

The novel naphthalene-type analogues 14 and 18 and the naphthoquinone-type analogues, 8, 9, 15, 16, 19, 21, 22, and 23-28 have been synthesized, and their in vitro Cdc25A phosphatase-inhibitory activity was examined. In assessment of the inhibitory activity, it was revealed that the naphthoquinone core is contributed to the activity, rather than the alkyl side chain.