Human Cdc25 A inactivation in response to S phase inhibition and its role in preventing premature mitosis

Absence of polo-like kinase 3 in mice stabilizes Cdc25A after DNA damage but is not sufficient to produce tumors

The polo-like kinases (Plks1-5) are emerging as an important class of proteins involved in many facets of cell cycle regulation and response to DNA damage and stress. Here we show that Plk3 phosphorylates the key cell cycle protein phosphatase Cdc25A on two serine residues in its cyclinB/cdk1 docking domain and regulates its stability in response to DNA damage. We generated a Plk3 knock-out mouse and show that Cdc25A protein from Plk3-deficient cells is less susceptible to DNA damage-mediated degradation than cells with functional Plk3. We also show that absence of Plk3 correlates with loss of the G1/S cell cycle checkpoint. However, neither this compromised DNA damage checkpoint nor reduced susceptibility to proteasome-mediated degradation after DNA damage translated into a significant increase in tumor incidence in the Plk3-deficient mice.

A human cancer-predisposing polymorphism in Cdc25A is embryonic lethal in the mouse and promotes ASK-1 mediated apoptosis

Background

Failure to regulate the levels of Cdc25A phosphatase during the cell cycle or during a checkpoint response causes bypass of DNA damage and replication checkpoints resulting in genomic instability and cancer. During G1 and S and in cellular response to DNA damage, Cdc25A is targeted for degradation through the Skp1-cullin-β-TrCP (SCF^β-TrCP) complex. This complex binds to the Cdc25A DSG motif which contains serine residues at positions 82 and 88. Phosphorylation of one or both residues is necessary for the binding and degradation to occur.

Results

We now show that mutation of serine 88 to phenylalanine, which is a cancer-predisposing polymorphic variant in humans, leads to early embryonic lethality in mice. The mutant protein retains its phosphatase activity both in vitro and in cultured cells. It fails to interact with the apoptosis signal-regulating kinase 1 (ASK1), however, and therefore does not suppress ASK1-mediated apoptosis.

Conclusions

These data suggest that the DSG motif, in addition to its function in Cdc25A-mediated degradation, plays a role in cell survival during early embyogenesis through suppression of ASK1-mediated apoptosis.

Dual mode of regulation of cell division cycle 25 A protein by TRB3

We have recently demonstrated that TRB3, a novel stress-inducible protein, is an unstable protein regulated by the ubiquitin-proteasome system. The expression level of TRB3 protein is down-regulated by anaphase-promoting complex/cyclosome-cell division cycle division 20 homolog 1 (APC/C(Cdh1)) through its D-box motif. Here we demonstrate that TRB3 regulates the stability of cell division cycle 25 A (Cdc25A), an essential activator of cyclin dependent kinases (CDKs). The expression level of Cdc25A protein is suppressed by over-expression of TRB3, while knockdown of TRB3 enhances the endogenous Cdc25A expression level. On the other hand, Cdc25A degradation induced by DNA damage is significantly rescued by TRB3. When serine residues in the DSG motif, which is the critical sequences for the degradation of Cdc25A induced by DNA damage, is mutated to alanine (Cdc25A(DSG2X)), both stimulatory and protective effects of TRB3 on the Cdc25A degradation is disappeared. TRB3 protein interacts with both wild Cdc25A and mutant Cdc25A(DSG2X). Expression level of the endogenous TRB3 protein is down-regulated in a genotoxic condition. These results suggest TRB3 is a regulator for adjusting the expression level of Cdc25A both in a normal and a genotoxic conditions.

The ubiquitous role of ubiquitin in the DNA damage response

Protein ubiquitylation has emerged as an important regulatory mechanism that impacts almost every aspect of the DNA damage response. In this review, we discuss how DNA repair and checkpoint pathways utilize the diversity offered by the ubiquitin conjugation system to modulate the response to genotoxic lesions in space and time. In particular, we will highlight recent work done on the regulation of DNA double-strand breaks signalling and repair by the RNF8/RNF168 E3 ubiquitin ligases, the Fanconi anemia pathway and the role of protein degradation in the enforcement and termination of checkpoint signalling. We also discuss the various functions of deubiquitylating enzymes in these processes along with potential avenues for exploiting the ubiquitin conjugation/deconjugation system for therapeutic purposes.

Cell-cycle regulation in embryonic stem cells: centrosomal decisions on self-renewal

Embryonic stem cells seem to have the intriguing capacity to divide indefinitely while retaining their pluripotency. This self-renewal is accomplished by specialized mechanisms of cell-cycle control. In the last few years, several studies have provided evidence for a direct link between cell-cycle regulation and cell-fate decisions in stem cells. In this review, we discuss the peculiarities of embryonic stem cell-cycle control mechanisms, implicate their involvement in cell-fate decisions, and distinguish centrosomes as important players in the self-renewal versus differentiation roulette.

Phyllanthus spp. induces selective growth inhibition of PC-3 and MeWo human cancer cells through modulation of cell cycle and induction of apoptosis

BACKGROUND

Phyllanthus is a traditional medicinal plant that has been used in the treatment of many diseases including hepatitis and diabetes. The main aim of the present work was to investigate the potential cytotoxic effects of aqueous and methanolic extracts of four Phyllanthus species (P.amarus, P.niruri, P.urinaria and P.watsonii) against skin melanoma and prostate cancer cells.

METHODOLOGY/PRINCIPAL FINDINGS

Phyllanthus plant appears to possess cytotoxic properties with half-maximal inhibitory concentration (IC(50)) values of 150-300 µg/ml for aqueous extract and 50-150 µg/ml for methanolic extract that were determined using the MTS reduction assay. In comparison, the plant extracts did not show any significant cytotoxicity on normal human skin (CCD-1127Sk) and prostate (RWPE-1) cells. The extracts appeared to act by causing the formation of a clear "ladder" fragmentation of apoptotic DNA on agarose gel, displayed TUNEL-positive cells with an elevation of caspase-3 and -7 activities. The Lactate Dehydrogenase (LDH) level was lower than 15% in Phyllanthus treated-cancer cells. These indicate that Phyllanthus extracts have the ability to induce apoptosis with minimal necrotic effects. Furthermore, cell cycle analysis revealed that Phyllanthus induced a Go/G1-phase arrest on PC-3 cells and a S-phase arrest on MeWo cells and these were accompanied by accumulation of cells in the Sub-G1 (apoptosis) phase. The cytotoxic properties may be due to the presence of polyphenol compounds such as ellagitannins, gallotannins, flavonoids and phenolic acids found both in the water and methanol extract of the plants.

CONCLUSIONS/SIGNIFICANCE

Phyllanthus plant exerts its growth inhibition effect in a selective manner towards cancer cells through the modulation of cell cycle and induction of apoptosis via caspases activation in melanoma and prostate cancer cells. Hence, Phyllanthus may be sourced for the development of a potent apoptosis-inducing anticancer agent.

Synthesis and biological evaluation of novel coumarin-based inhibitors of Cdc25 phosphatases

The cell division cycle 25 (Cdc25) family of proteins are dual specificity phosphatases that activate cyclin-dependent kinase (CDK) complexes, which in turn regulate progression through the cell division cycle. Overexpression of Cdc25 proteins has been reported in a wide variety of cancers; their inhibition may thus represent a novel approach for the development of anticancer therapeutics. Herein we report new coumarin-based scaffolds endowed with a selective inhibition against Cdc25A and Cdc25C, being 6a and 6d the most efficient inhibitors and worthy of further investigation as anticancer agents.

Impaired tissue growth is mediated by checkpoint kinase 1 (CHK1) in the integrated stress response

The integrated stress response (ISR) protects cells from numerous forms of stress and is involved in the growth of solid tumours; however, it is unclear how the ISR acts on cellular proliferation. We have developed a model of ISR signalling with which to study its effects on tissue growth. Overexpression of the ISR kinase PERK resulted in a striking atrophic eye phenotype in Drosophila melanogaster that could be rescued by co-expressing the eIF2alpha phosphatase GADD34. A genetic screen of 3000 transposon insertions identified grapes, the gene that encodes the Drosophila orthologue of checkpoint kinase 1 (CHK1). Knockdown of grapes by RNAi rescued eye development despite ongoing PERK activation. In mammalian cells, CHK1 was activated by agents that induce ER stress, which resulted in a G2 cell cycle delay. PERK was both necessary and sufficient for CHK1 activation. These findings indicate that non-genotoxic misfolded protein stress accesses DNA-damage-induced cell cycle checkpoints to couple the ISR to cell cycle arrest.

Cell cycle G2/M arrest through an S phase-dependent mechanism by HIV-1 viral protein R

Background

Cell cycle G2 arrest induced by HIV-1 Vpr is thought to benefit viral proliferation by providing an optimized cellular environment for viral replication and by skipping host immune responses. Even though Vpr-induced G2 arrest has been studied extensively, how Vpr triggers G2 arrest remains elusive.

Results

To examine this initiation event, we measured the Vpr effect over a single cell cycle. We found that even though Vpr stops the cell cycle at the G2/M phase, but the initiation event actually occurs in the S phase of the cell cycle. Specifically, Vpr triggers activation of Chk1 through Ser³⁴⁵ phosphorylation in an S phase-dependent manner. The S phase-dependent requirement of Chk1-Ser³⁴⁵ phosphorylation by Vpr was confirmed by siRNA gene silencing and site-directed mutagenesis. Moreover, downregulation of DNA replication licensing factors Cdt1 by siRNA significantly reduced Vpr-induced Chk1-Ser³⁴⁵ phosphorylation and G2 arrest. Even though hydroxyurea (HU) and ultraviolet light (UV) also induce Chk1-Ser³⁴⁵ phosphorylation in S phase under the same conditions, neither HU nor UV-treated cells were able to pass through S phase, whereas vpr-expressing cells completed S phase and stopped at the G2/M boundary. Furthermore, unlike HU/UV, Vpr promotes Chk1- and proteasome-mediated protein degradations of Cdc25B/C for G2 induction; in contrast, Vpr had little or no effect on Cdc25A protein degradation normally mediated by HU/UV.

Conclusions

These data suggest that Vpr induces cell cycle G2 arrest through a unique molecular mechanism that regulates host cell cycle regulation in an S-phase dependent fashion.

Casein kinase 1 functions as both penultimate and ultimate kinase in regulating Cdc25A destruction

The Cdc25A protein phosphatase drives cell-cycle transitions by activating cyclin-dependent protein kinases. Failure to regulate Cdc25A leads to deregulated cell-cycle progression, bypass of cell-cycle checkpoints and genome instability. Ubiquitin-mediated proteolysis has an important role in balancing Cdc25A levels. Cdc25A contains a DS(82)G motif whose phosphorylation is targeted by beta-TrCP E3 ligase during interphase. Targeting beta-TrCP to Cdc25A requires phosphorylation of serines 79 (S79) and 82 (S82). Here, we report that casein kinase 1 alpha (CK1alpha) phosphorylates Cdc25A on both S79 and S82 in a hierarchical manner requiring prior phosphorylation of S76 by Chk1 or GSK-3beta. This facilitates beta-TrCP binding and ubiquitin-mediated proteolysis of Cdc25A throughout interphase and after exposure to genotoxic stress. The priming of Cdc25A by at least three kinases (Chk1, GSK-3beta, CK1alpha), some of which also require priming, ensures diverse extra- and intracellular signals interface with Cdc25A to precisely control cell division.

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.

Pro-apoptotic role of Cdc25A: activation of cyclin B1/Cdc2 by the Cdc25A C-terminal domain

Cdc25A is a dual specificity protein phosphatase that activates cyclin/cyclin-dependent protein kinase (Cdk) complexes by removing inhibitory phosphates from conserved threonine and tyrosine in Cdks. To address how Cdc25A promotes apoptosis, Jurkat cells were treated with staurosporine, an apoptosis inducer. Upon staurosporine treatment, a Cdc25A C-terminal 37-kDa fragment, designated C37, was generated by caspase cleavage at Asp-223. Thr-507 in C37 became dephosphorylated, which prevented 14-3-3 binding, as shown previously. C37 exhibited higher phosphatase activity than full-length Cdc25A. C37 with alanine substitution for Thr-507 (C37/T507A) that imitated the cleavage product during staurosporine treatment interacted with Cdc2, Cdk2, cyclin A, and cyclin B1 and markedly activated cyclin B1/Cdc2. The dephosphorylation of Thr-507 might expose the Cdc2/Cdk2-docking site in C37. C37/T507A also induced apoptosis in Jurkat and K562 cells, resulting from activating cyclin B1/Cdc2 but not Cdk2. Thus, this study reveals that Cdc25A is a pro-apoptotic protein that amplifies staurosporine-induced apoptosis through the activation of cyclin B1/Cdc2 by its C-terminal domain.

Ubiquitin hydrolase Dub3 promotes oncogenic transformation by stabilizing Cdc25A

The dual specificity (Tyr/Thr) phosphatase Cdc25A activates cyclin-dependent kinases (Cdks) to promote cell-cycle progression and has significant oncogenic potential. Cdc25A protein levels are regulated tightly in normal tissues, but many human cancers overexpress Cdc25A. The underlying mechanism for overexpression has been enigmatic. Here we show that Cdc25A is stabilized by the ubiquitin hydrolase Dub3. Upon binding Cdc25A, Dub3 removes the polyubiquitin modifications that mark Cdc25A for proteasomal degradation. Dub3 knockdown in cells increased Cdc25A ubiquitylation and degradation, resulting in reduced Cdk/Cyclin activity and arrest at G1/S and G2/M phases of the cell cycle. In contrast, acute Dub3 overexpression produced a signature response to oncogene induction: cells accumulated in S and G2 because of replication stress, and activated a DNA damage response. Dub3 also transformed NIH-3T3 cells and cooperated with activated H-Ras to promote growth in soft agar. Importantly, we show that Dub3 overexpression is responsible for an abnormally high level of Cdc25A in a subset of human breast cancers. Moreover, Dub3 knockdown significantly retarded the growth of breast tumour xenografts in nude mice. As a major regulator of Cdc25A, Dub3 is an example of a transforming ubiquitin hydrolase that subverts a key component of the cell cycle machinery.

Cdc25 phosphatases are required for timely assembly of CDK1-cyclin B at the G2/M transition

Progression through mitosis requires the coordinated regulation of Cdk1 kinase activity. Activation of Cdk1 is a multistep process comprising binding of Cdk1 to cyclin B, relocation of cyclin-kinase complexes to the nucleus, activating phosphorylation of Cdk1 on Thr(161) by the Cdk-activating kinase (CAK; Cdk7 in metazoans), and removal of inhibitory Thr(14) and Tyr(15) phosphorylations. This dephosphorylation is catalyzed by the dual specific Cdc25 phosphatases, which occur in three isoforms in mammalian cells, Cdc25A, -B, and -C. We find that expression of Cdc25A leads to an accelerated G(2)/M phase transition. In Cdc25A-overexpressing cells, Cdk1 exhibits high kinase activity despite being phosphorylated on Tyr(15). In addition, Tyr(15)-phosphorylated Cdk1 binds more cyclin B in Cdc25A-overexpressing cells compared with control cells. Consistent with this observation, we demonstrate that in human transformed cells, Cdc25A and Cdc25B, but not Cdc25C phosphatases have an effect on timing and efficiency of cyclin-kinase complex formation. Overexpression of Cdc25A or Cdc25B promotes earlier assembly and activation of Cdk1-cyclin B complexes, whereas repression of these phosphatases by short hairpin RNA has a reverse effect, leading to a substantial decrease in amounts of cyclin B-bound Cdk1 in G(2) and mitosis. Importantly, we find that Cdc25A overexpression leads to an activation of Cdk7 and increase in Thr(161) phosphorylation of Cdk1. In conclusion, our data suggest that complex assembly and dephosphorylation of Cdk1 at G(2)/M is tightly coupled and regulated by Cdc25 phosphatases.

Stepping up melanocytes to the challenge of UV exposure

Exposure to solar ultraviolet radiation (UV) is the main etiological factor for skin cancer, including melanoma. Cutaneous pigmentation, particularly eumelanin, afforded by melanocytes is the main photoprotective mechanism, as it prevents UV-induced DNA damage in the epidermis. Therefore, maintaining genomic stability of melanocytes is crucial for prevention of melanoma, as well as keratinocyte-derived basal and squamous cell carcinoma. A critical independent factor for preventing melanoma is DNA repair capacity. The response of melanocytes to UV is mediated mainly by a network of paracrine factors that not only activate melanogenesis, but also DNA repair, anti-oxidant, and survival pathways that are pivotal for maintenance of genomic stability and prevention of malignant transformation or apoptosis. However, little is known about the stress response of melanocytes to UV and the regulation of DNA repair pathways in melanocytes. Unraveling these mechanisms might lead to strategies to prevent melanoma, as well as non-melanoma skin cancer.

NEK11 regulates CDC25A degradation and the IR-induced G2/M checkpoint

DNA damage-induced cell-cycle checkpoints have a critical role in maintaining genomic stability. A key target of the checkpoints is the CDC25A (cell division cycle 25 homologue A) phosphatase, which is essential for the activation of cyclin-dependent kinases and cell-cycle progression. To identify new genes involved in the G2/M checkpoint we performed a large-scale short hairpin RNA (shRNA) library screen. We show that NIMA (never in mitosis gene A)-related kinase 11 (NEK11) is required for DNA damage-induced G2/M arrest. Depletion of NEK11 prevents proteasome-dependent degradation of CDC25A, both in unperturbed and DNA-damaged cells. We show that NEK11 directly phosphorylates CDC25A on residues whose phosphorylation is required for beta-TrCP (beta-transducin repeat-containing protein)-mediated polyubiquitylation and degradation of CDC25A. Furthermore, we demonstrate that CHK1 (checkpoint kinase 1) directly activates NEK11 by phosphorylating it on Ser 273, indicating that CHK1 and NEK11 operate in a single pathway that controls proteolysis of CDC25A. Taken together, these results demonstrate that NEK11 is an important component of the pathway enforcing the G2/M checkpoint, suggesting that genetic mutations in NEK11 may contribute to the development of human cancer.

Development of novel thiazolopyrimidines as CDC25B phosphatase inhibitors

The development of CDC25 phosphatase inhibitors is an interesting approach toward new antitumor agents, as CDC25 play key roles in cell-cycle regulation and are overexpressed in numerous cancers. We previously reported a novel compound belonging to the thiazolopyrimidine family that inhibits CDC25 activity with an IC(50) value of 13 microM and displays cytotoxic properties against HeLa cells. Structural modifications were subsequently conducted on this new pharmacophore which led to a library of 45 thiazolopyrimidines. Regarding the in vitro effects, 14 compounds inhibit CDC25B with IC(50)<20 microM, with the most efficient inhibitor 44 improving the potency to 4.5 microM. Steady-state kinetics were performed and showed a mixed inhibition pattern for all tested compounds. Furthermore, 44 was able to revert the bypass of genotoxicity-induced G(2) arrest upon CDC25B overexpression, indicating that this compound targets the dual-specificity phosphatase in cultured cells. Finally, the cytotoxic activities of the compounds were determined against two human cancer cell lines. The results indicate that the prostatic LNCaP cell line is more sensitive to these derivatives than the pancreatic adenocarcinoma MiaPaCa-2 line. With its interesting enzymatic and cellular properties, compound 44 appears to be a promising CDC25B inhibitor for further development.

The extracellular signal-regulated kinase-mitogen-activated protein kinase pathway phosphorylates and targets Cdc25A for SCF beta-TrCP-dependent degradation for cell cycle arrest

The extracellular signal-regulated kinase (ERK) pathway is generally mitogenic, but, upon strong activation, it causes cell cycle arrest by a not-yet fully understood mechanism. In response to genotoxic stress, Chk1 hyperphosphorylates Cdc25A, a positive cell cycle regulator, and targets it for Skp1/Cullin1/F-box protein (SCF)(beta-TrCP) ubiquitin ligase-dependent degradation, thereby leading to cell cycle arrest. Here, we show that strong ERK activation can also phosphorylate and target Cdc25A for SCF(beta-TrCP)-dependent degradation. When strongly activated in Xenopus eggs, the ERK pathway induces prominent phosphorylation and SCF(beta-TrCP)-dependent degradation of Cdc25A. p90rsk, the kinase downstream of ERK, directly phosphorylates Cdc25A on multiple sites, which, interestingly, overlap with Chk1 phosphorylation sites. Furthermore, ERK itself phosphorylates Cdc25A on multiple sites, a major site of which apparently is phosphorylated by cyclin-dependent kinase (Cdk) in Chk1-induced degradation. p90rsk phosphorylation and ERK phosphorylation contribute, roughly equally and additively, to the degradation of Cdc25A, and such Cdc25A degradation occurs during oocyte maturation in which the endogenous ERK pathway is fully activated. Finally, and importantly, ERK-induced Cdc25A degradation can elicit cell cycle arrest in early embryos. These results suggest that strong ERK activation can target Cdc25A for degradation in a manner similar to, but independent of, Chk1 for cell cycle arrest.

Human Cdc25A phosphatase has a non-redundant function in G2 phase by activating Cyclin A-dependent kinases

Cdc25 phosphatases activate Cdk/Cyclin complexes by dephosphorylation and thus promote cell cycle progression. We observed that the peak activity of Cdc25A precedes the one of Cdc25B in prophase and the maximum of Cyclin/Cdk kinase activity. Furthermore, Cdc25A activates both Cdk1-2/Cyclin A and Cdk1/Cyclin B complexes while Cdc25B seems to be involved only in activation of Cdk1/Cyclin B. Concomitantly, repression of Cdc25A led to a decrease in Cyclin A-associated kinase activity and attenuated Cdk1 activation. Our results indicate that Cdc25A acts before Cdc25B - at least in cancer cells, and has non-redundant functions in late G2/early M-phase as a major regulator of Cyclin A/kinase complexes.

Cdc25A serine 123 phosphorylation couples centrosome duplication with DNA replication and regulates tumorigenesis

The cell division cycle 25A (Cdc25A) phosphatase is a critical regulator of cell cycle progression under normal conditions and after stress. Stress-induced degradation of Cdc25A has been proposed as a major way of delaying cell cycle progression. In vitro studies pointed toward serine 123 as a key site in regulation of Cdc25A stability after exposure to ionizing radiation (IR). To address the role of this phosphorylation site in vivo, we generated a knock-in mouse in which alanine was substituted for serine 123. The Cdc25 S123A knock-in mice appeared normal, and, unexpectedly, cells derived from them exhibited unperturbed cell cycle and DNA damage responses. In turn, we found that Cdc25A was present in centrosomes and that Cdc25A levels were not reduced after IR in knock-in cells. This resulted in centrosome amplification due to lack of induction of Cdk2 inhibitory phosphorylation after IR specifically in centrosomes. Further, Cdc25A knock-in animals appeared sensitive to IR-induced carcinogenesis. Our findings indicate that Cdc25A S123 phosphorylation is crucial for coupling centrosome duplication to DNA replication cycles after DNA damage and therefore is likely to play a role in the regulation of tumorigenesis.

The regulatory beta-subunit of protein kinase CK2 regulates cell-cycle progression at the onset of mitosis

Cell-cycle transition from the G(2) phase into mitosis is regulated by the cyclin-dependent protein kinase 1 (CDK1) in complex with cyclin B. CDK1 activity is controlled by both inhibitory phosphorylation, catalysed by the Myt1 and Wee1 kinases, and activating dephosphorylation, mediated by the CDC25 dual-specificity phosphatase family members. In somatic cells, Wee1 is downregulated by phosphorylation and ubiquitin-mediated degradation to ensure rapid activation of CDK1 at the beginning of M phase. Here, we show that downregulation of the regulatory beta-subunit of protein kinase CK2 by RNA interference results in delayed cell-cycle progression at the onset of mitosis. Knockdown of CK2beta causes stabilization of Wee1 and increased phosphorylation of CDK1 at the inhibitory Tyr15. PLK1-Wee1 association is an essential event in the degradation of Wee1 in unperturbed cell cycle. We have found that CK2beta participates in PLK1-Wee1 complex formation whereas its cellular depletion leads to disruption of PLK1-Wee1 interaction and reduced Wee1 phosphorylation at Ser53 and 121. The data reported here reinforce the notion that CK2beta has functions that are independent of its role as the CK2 regulatory subunit, identifying it as a new component of signaling pathways that regulate cell-cycle progression at the entry of mitosis.

Novel naphthoquinone and quinolinedione inhibitors of CDC25 phosphatase activity with antiproliferative properties

CDC25 phosphatases are considered as attractive targets for anti-cancer therapy. To date, quinone derivatives are among the most potent inhibitors of CDC25 phosphatase activity. We present in this paper the synthesis and the biological evaluation of new quinolinedione and naphthoquinone derivatives, containing carboxylic or malonic acids groups introduced to mimic the role of the phosphate moieties of Cyclin-Dependent Kinase complexes. The most efficient compounds show inhibitory activity against CDC25B with IC(50) values in the 10 microM range, and are cytotoxic against HeLa cells.

CDC25A phosphatase controls meiosis I progression in mouse oocytes

CDK1 is a pivotal regulator of resumption of meiosis and meiotic maturation of oocytes. CDC25A/B/C are dual-specificity phosphatases and activate cyclin-dependent kinases (CDKs). Although CDC25C is not essential for either mitotic or meiotic cell cycle regulation, CDC25B is essential for CDK1 activation during resumption of meiosis. Cdc25a -/- mice are embryonic lethal and therefore a role for CDC25A in meiosis is unknown. We report that activation of CDK1 results in a maturation-associated decrease in the amount of CDC25A protein, but not Cdc25a mRNA, such that little CDC25A is present by metaphase I. In addition, expression of exogenous CDC25A overcomes cAMP-mediated maintenance of meiotic arrest. Microinjection of Gfp-Cdc25a and Gpf-Cdc25b mRNAs constructs reveals that CDC25A is exclusively localized to the nucleus prior to nuclear envelope breakdown (NEBD). In contrast, CDC25B localizes to cytoplasm in GV-intact oocytes and translocates to the nucleus shortly before NEBD. Over-expressing GFP-CDC25A, which compensates for the normal maturation-associated decrease in CDC25A, blocks meiotic maturation at MI. This MI block is characterized by defects in chromosome congression and spindle formation and a transient reduction in both CDK1 and MAPK activities. Lastly, RNAi-mediated reduction of CDC25A results in fewer oocytes resuming meiosis and reaching MII. These data demonstrate that CDC25A behaves differently during female meiosis than during mitosis, and moreover, that CDC25A has a function in resumption of meiosis, MI spindle formation and the MI-MII transition. Thus, both CDC25A and CDC25B are critical for meiotic maturation of oocytes.

2-Methoxyestradiol inhibits hepatocellular carcinoma cell growth by inhibiting Cdc25 and inducing cell cycle arrest and apoptosis

PURPOSE

2-Methoxyestradiol (2-ME) is a physiological metabolite of estrogen, which can inhibit growth of many types of tumor cells, including hepatocellular carcinoma, both in vitro and in vivo. The exact mechanisms of its action are still unclear. We have studied the mechanisms of growth inhibition of several of human and rat hepatoma and normal liver cells by 2-ME.

METHODS

Human (Hep3B, HepG2, PLC/PRF5) and rat (McA-RH7777, JM-1) hepatoma and normal rat (CRL-1439) and human (CRL-11233) liver cell lines were cultured in vitro, in presence of 2-ME, and its IC50s were determined. Cell cycle arrest, Cdc25 phosphatase inhibition and apoptosis induction were studied. Finally, the effect of 2-ME on the growth of JM-1 rat hepatoma cells in rat liver was determined in vivo.

RESULTS

The IC50 range for growth inhibition of hepatoma cells was found to be between 0.5 and 3 microM. In contrast, normal rat hepatocytes and liver cell lines were resistant to 2-ME up to 20 microM. JM-1 cells were arrested in the G2/M phase of the cell cycle. Cdc25A and Cdc25B, cell cycle controlling phosphatases, activities were inhibited in vitro and 2-ME was found to likely bind to their catalytic site cysteines. As a consequence, their cellular substrates Cdk1 and Cdk2 were tyrosine phosphorylated. Caspase-3 was cleaved suggesting apoptotic cell death. Moreover, growth of JM-1 tumors, which were transplanted into rat liver, was also inhibited by treatment with 2-ME in vivo.

CONCLUSIONS

2-Methoxyestradiol is a selective, potent and relatively non-toxic hepatoma growth inhibitor both in vitro and in vivo. Cell cycle arrest of hepatoma cells was likely mediated by binding and inactivation of the Cdc25 phosphatases and induction of apoptosis.

GSK-3 beta targets Cdc25A for ubiquitin-mediated proteolysis, and GSK-3 beta inactivation correlates with Cdc25A overproduction in human cancers

The Cdc25A phosphatase positively regulates cell-cycle transitions, is degraded by the proteosome throughout interphase and in response to stress, and is overproduced in human cancers. The kinases targeting Cdc25A for proteolysis during early cell-cycle phases have not been identified, and mechanistic insight into the cause of Cdc25A overproduction in human cancers is lacking. Here, we demonstrate that glycogen synthase kinase-3beta (GSK-3beta) phosphorylates Cdc25A to promote its proteolysis in early cell-cycle phases. Phosphorylation by GSK-3beta requires priming of Cdc25A, and this can be catalyzed by polo-like kinase 3 (Plk-3). Importantly, a strong correlation between Cdc25A overproduction and GSK-3beta inactivation was observed in human tumor tissues, indicating that GSK-3beta inactivation may account for Cdc25A overproduction in a subset of human tumors.

Hemizygous disruption of Cdc25A inhibits cellular transformation and mammary tumorigenesis in mice

CDC25A phosphatase activates multiple cyclin-dependent kinases (CDK) during cell cycle progression. Inactivation of CDC25A by ubiquitin-mediated degradation is a major mechanism of DNA damage-induced S-G(2) checkpoint. Although increased CDC25A expression has been reported in various human cancer tissues, it remains unclear whether CDC25A activation is a critical rate-limiting step of carcinogenesis. To assess the role for CDC25A in cell cycle control and carcinogenesis, we used a Cdc25A-null mouse strain we recently generated. Whereas Cdc25A(-/-) mice exhibit early embryonic lethality, Cdc25A(+/-) mice show no appreciable developmental defect. Cdc25A(+/-) mouse embryonic fibroblasts (MEF) exhibit normal kinetics of cell cycle progression at early passages, modestly enhanced G(2) checkpoint response to DNA damage, and shortened proliferative life span, compared with wild-type MEFs. Importantly, Cdc25A(+/-) MEFs are significantly resistant to malignant transformation induced by coexpression of H-ras(V12) and a dominant negative p53 mutant. The rate-limiting role for CDC25A in transformation is further supported by decreased transformation efficiency in MCF-10A human mammary epithelial cells stably expressing CDC25A small interfering RNA. Consistently, Cdc25A(+/-) mice show substantially prolonged latency in mammary tumorigenesis induced by MMTV-H-ras or MMTV-neu transgene, whereas MMTV-myc-induced tumorigenesis is not significantly affected by Cdc25A heterozygosity. Mammary tissues of Cdc25A(+/-);MMTV-neu mice before tumor development display less proliferative response to the oncogene with increased tyrosine phosphorylation of CDK1/2, but show no significant change in apoptosis. These results suggest that Cdc25A plays a rate-limiting role in transformation and tumor initiation mediated by ras activation.

CDC25 phosphatases in cancer cells: key players? Good targets?

Cell division cycle 25 (CDC25) phosphatases regulate key transitions between cell cycle phases during normal cell division, and in the event of DNA damage they are key targets of the checkpoint machinery that ensures genetic stability. Taking only this into consideration, it is not surprising that CDC25 overexpression has been reported in a significant number of human cancers. However, in light of the significant body of evidence detailing the stringent complexity with which CDC25 activities are regulated, the significance of CDC25 overexpression in a subset of cancers and its association with poor prognosis are proving difficult to assess. We will focus on the roles of CDC25 phosphatases in both normal and abnormal cell proliferation, provide a critical assessment of the current data on CDC25 overexpression in cancer, and discuss both current and future therapeutic strategies for targeting CDC25 activity in cancer treatment.

The Cdc25A is involved in S-phase checkpoint induced by benzo(a)pyrene

Environmental carcinogen benzo(a)pyrene (BaP) generates electrophilic products BaP diolepoxide (BPDE) that react covalently with genomic DNA. Cells that acquire BaP/BPDE-induced DNA damage undergo S-phase arrest in a p53-independent manner. However, the role of Cdc25A in the BaP/BPDE-induced checkpoint is not clear. In the present study, we investigated the change of checkpoint kinase 1 (Chk1) and Cdc25A in S-phase arrest elicited by BaP. The results indicated that BaP (10microM, with S9 mixture) treatment induced S-phase arrest in both human lung carcinoma A549 cells and human bronchial epithelial cells line 16HBE cells, increasing the proportions of cells in S-phase 19.0% and 21.1%, respectively, at 12h after treatment, compared with DMSO control (p<0.01). Then, the S-phase arrest was weakened after 24h. The level of phorsphorylated Chk1 obviously increased and Cdc25A protein level decreased in both two cell lines after treatment with BaP. The results of RT-PCR indicate Cdc25A mRNA in both A549 cells and 16HBE cells was not changed after BaP treatment 12h, and 24h. The treatment of the proteasome inhibitor MG132 greatly increased Cdc25A protein in abundance. Over all, our results indicated Chk1-Cdc25A checkpoint pathway is involved in BaP-induced S-phase arrest. Moreover, transcription of Cdc25A did not change in BaP induced S-phase arrest, the decrease of Cdc25A level was due to increased degradation through the ubiqutin-proteasome pathway.

Cdc7-Dbf4 and the human S checkpoint response to UVC

The S checkpoint response to ultraviolet radiation (UVC) that inhibits replicon initiation is dependent on the ATR and Chk1 kinases. Downstream effectors of this response, however, are not well characterized. Data reported here eliminated Cdc25A degradation and inhibition of Cdk2-cyclin E as intrinsic components of the UVC-induced pathway of inhibition of replicon initiation in human cells. A sublethal dose of UVC (1 J/m(2)), which selectively inhibits replicon initiation by 50%, failed to reduce the amount of Cdc25A protein or decrease Cdk2-cyclin E kinase activity. Cdc25A degradation was observed after irradiation with cytotoxic fluences of UVC, suggesting that severe inhibition of DNA chain elongation and activation of the replication checkpoint might be responsible for the UVC-induced degradation of Cdc25A. Another proposed effector of the S checkpoint is the Cdc7-Dbf4 complex. Dbf4 interacted weakly with Chk1 in vivo but was recognized as a substrate for Chk1-dependent phosphorylation in vitro. FLAG-Dbf4 formed complexes with endogenous Cdc7, and this interaction was stable in UVC-irradiated HeLa cells. Overexpression of FLAG- or Myc-tagged Dbf4 abrogated the S checkpoint response to UVC but not ionizing radiation. These findings implicate a Dbf4-dependent kinase as a possible target of the ATR- and Chk1-dependent S checkpoint response to UVC.

Deregulated CDC25A expression promotes mammary tumorigenesis with genomic instability

Checkpoint pathways help cells maintain genomic integrity, delaying cell cycle progression in response to various risks of fidelity, such as genotoxic stresses, compromised DNA replication, and impaired spindle control. Cancer cells frequently exhibit genomic instability, and recent studies showed that checkpoint pathways are likely to serve as a tumor-suppressive barrier in vivo. The cell cycle-promoting phosphatase CDC25A is an activator of cyclin-dependent kinases and one of the downstream targets for the CHK1-mediated checkpoint pathway. Whereas CDC25A overexpression is observed in various human cancer tissues, it has not been determined whether deregulated CDC25A expression triggers or promotes tumorigenesis in vivo. Here, we show that transgenic expression of CDC25A cooperates markedly with oncogenic ras or neu in murine mammary tumorigenesis. MMTV-CDC25A transgenic mice exhibit alveolar hyperplasia in the mammary tissue but do not develop spontaneous mammary tumors. The MMTV-CDC25A transgene markedly shortens latency of tumorigenesis in MMTV-ras mice. The MMTV-CDC25A transgene also accelerates tumor growth in MMTV-neu mice with apparent cell cycle miscoordination. CDC25A-overexpressing tumors, which invade more aggressively, exhibit various chromosomal aberrations on fragile regions, including the mouse counterpart of human 1p31-36, according to array-based comparative genomic hybridization and karyotyping. The chromosomal aberrations account for substantial changes in gene expression profile rendered by transgenic expression of CDC25A, including down-regulation of Trp73. These data indicate that deregulated control of cellular CDC25A levels leads to in vivo genomic instability, which cooperates with the neu-ras oncogenic pathway in mammary tumorigenesis.

Inhibiting transient protein-protein interactions: lessons from the Cdc25 protein tyrosine phosphatases

Transient protein-protein interactions have key regulatory functions in many of the cellular processes that are implicated in cancerous growth, particularly the cell cycle. Targeting these transient interactions as therapeutic targets for anticancer drug development seems like a good idea, but it is not a trivial task. This Review discusses the issues and difficulties that are encountered when considering these transient interactions as drug targets, using the example of the cell division cycle 25 (Cdc25) phosphatases and their cyclin-dependent kinase (CDK)-cyclin protein substrates.

DNA damage checkpoints: from initiation to recovery or adaptation

In response to diverse genotoxic stresses, cells activate DNA damage checkpoint pathways to protect genomic integrity and promote survival of the organism. Depending on DNA lesions and context, damaged cells with alarmed checkpoints can be eliminated by apoptosis or silenced by cellular senescence, or can survive and resume cell cycle progression upon checkpoint termination. Over the past two years a plethora of mechanistic studies have provided exciting insights into the biology and pathology of checkpoint initiation and signal propagation, and have revealed the various ways in which the response can be terminated: through recovery, adaptation or cancer-prone subversion. Such studies highlight the dynamic nature of these processes and help us to better understand the molecular basis, spatiotemporal orchestration and biological significance of the DNA damage response in normal and cancerous cells.

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.

High-risk human papillomavirus type 16 E7 oncogene associates with Cdc25A over-expression in oral squamous cell carcinoma

Cells expressing high-risk human papillomavirus (HPV) E7 protein display impaired checkpoint control after DNA damage and exhibit elevated rates of mutagenesis. Repression of HPV E7 expression results in the subsequent accumulation of hypophosphorylated retinoblastoma protein and repression of the Cdc25A genes. No study has been conducted to elucidate the role of Cdc25A in the development and progression of human oral carcinomas. To confirm Cdc25A protein expression together with HPV, immunohistochemistry, Western blotting, polymerase chain reaction (PCR), and reverse transcriptase (RT)-PCR were performed using various histological subtypes of oral carcinomas. Cdc25A protein was localized predominantly in the cell nuclei in carcinomas, and high expression was found in 54% of primary tumors. HPV-16 E7 was not found in non-neoplastic oral tissues, whereas it was observed in eight (36%) of 22 oral carcinomas. We found a significant correlation between Cdc25A over-expression and HPV-16 E7 positive carcinomas. There was a strong positive correlation between Cdc25A over-expression and tumor size and TNM stage. This study suggests that Cdc25A is likely to be an important mediator in the progression of oral tumors, and HPV-16 E7 may be a sensitive indicator of the involvement of viral oncogenes in oral carcinogenesis.

Fluorinated Cpd 5, a pure arylating K-vitamin derivative, inhibits human hepatoma cell growth by inhibiting Cdc25 and activating MAPK

We previously synthesized several K-vitamin derivatives, which are potent growth inhibitors of human tumor cells, including Hep3B human hepatoma cells. Among these, Cpd 5 was the most potent. However, being a quinone derivative, Cpd 5 has the potential for generating toxic reactive oxygen species (ROS). We therefore synthesized a fluorinated derivative of Cpd 5, F-Cpd 5. The calculated reduction potential of F-Cpd 5 was much higher than that for Cpd 5 and it was not predicted to generate ROS. This was supported by our observation that F-Cpd 5 generated significantly lower ROS than Cpd 5. F-Cpd 5 was three times more potent than Cpd 5 in inhibiting Hep3B cell growth. Interestingly, under identical culture conditions, F-Cpd 5 inhibited mitogen-induced DNA synthesis in normal rat hepatocytes 12-fold less potently than Hep3B cells. F-Cpd 5 was found to induce caspase-3 cleavage and nuclear DNA laddering, evidences for apoptosis. It preferentially inhibited the activities of the cell cycle controlling phosphatases Cdc25A and Cdc25B, by binding to their catalytic cysteines. Consequently, inhibitory tyrosine phosphorylation of the Cdc25 substrate kinases Cdk2 and Cdk4 were induced. F-Cpd 5 also induced phosphorylation of the MAPK proteins ERK1/2, JNK1/2 and p38 in Hep3B cells and the MAPK inhibitors (U0126, JNKI-II, and SB 203580) antagonized its growth inhibition. F-Cpd 5 inhibited the action of cytosolic ERK phosphatase activity, which likely caused the ERK phosphorylation. F-Cpd 5 thus differentially inhibited growth of normal and tumor cells by preferentially inhibiting the actions of Cdc25A and Cdc25B phosphatases and inducing MAPK phosphorylation.

Anandamide inhibits Cdk2 and activates Chk1 leading to cell cycle arrest in human breast cancer cells

This study was designed to determine the molecular mechanisms underlying the anti-proliferative effect of the endocannabinoid anandamide on highly invasive human breast cancer cells, MDA-MB-231. We show that a metabolically stable analogue of anandamide, Met-F-AEA, induces an S phase growth arrest correlated with Chk1 activation, Cdc25A degradation and suppression of Cdk2 activity. These findings demonstrate that Met-F-AEA induced cell cycle blockade relies on modulated expression and activity of key S phase regulatory proteins. The observed mechanism of action, already reported for well-known chemotherapeutic drugs, provides strong evidence for a direct role of anandamide related compounds in the activation of cell cycle checkpoints.

CHK1 phosphorylates CDC25B during the cell cycle in the absence of DNA damage

CDC25B is one of the three human phosphatases that activate the CDK-cyclin complexes, thereby triggering cell-cycle progression and division. Commitment to early mitotic events depends on the activation of a centrosomal pool of CDK1-cyclin-B1, and CDC25B is thought to be involved in initiating this centrosomal CDK1-cyclin-B1 activity. Centrosome-associated checkpoint kinase 1 (CHK1) has been proposed to contribute to the proper timing of a normal cell division cycle by inhibiting the activation of the centrosomal pool of CDK1. Here, we show that CDC25B is phosphorylated by CHK1 in vitro on multiple residues, including S230 and S563. We demonstrate these phosphorylations occur in vivo and that they are dependent on CHK1 activity. S230 CHK1-mediated phosphorylation is detected in cell extracts during S phase and G2 phase in the absence of DNA damage. We show that the S230-phosphorylated form of CDC25B is located at the centrosome from early S phase until mitosis. Furthermore, mutation of S230 to alanine increases the mitotic-inducing activity of CDC25B. Our results support a model in which, under normal cell cycle conditions and in the absence of DNA damage, CHK1 constitutively phosphorylates CDC25B during interphase and thus prevents the premature initiation of mitosis by negatively regulating the activity of CDC25B at the centrosome.

Regulation of mitotic entry by microcephalin and its overlap with ATR signalling

Ataxia-telangiectasia mutated and Rad3 related (ATR)-Seckel syndrome and autosomal recessive primary microcephaly (MCPH) syndrome share clinical features. RNA interference (RNAi) of MCPH1 have implicated the protein it encodes as a DNA-damage response protein that regulates the transcription of Chk1 and BRCA1, two genes involved in the response to DNA damage. Here, we report that truncating mutations observed in MCPH-syndrome patients do not impact on Chk1 or BRCA1 expression or early ATR-dependent damage-induced phosphorylation events. However, like ATR-Seckel syndrome cells, MCPH1-mutant cell lines show defective G2-M checkpoint arrest and nuclear fragmentation after DNA damage, and contain supernumerary mitotic centrosomes. MCPH1-mutant and ATR-Seckel cells also show impaired degradation of Cdc25A and fail to inhibit Cdc45 loading onto chromatin after replication arrest. Additionally, microcephalin interacts with Chk1. We conclude that MCPH1 has a function downstream of Chk1 in the ATR-signalling pathway. In contrast with ATR-Seckel syndrome cells, MCPH1-mutant cells have low levels of Tyr 15-phosphorylated Cdk1 (pY15-Cdk1) in S and G2 phases, which correlates with an elevated frequency of G2-like cells displaying premature chromosome condensation (PCC). Thus, MCPH1 also has an ATR-independent role in maintaining inhibitory Cdk1 phosphorylation, which prevents premature entry into mitosis.

The when and wheres of CDC25 phosphatases

The CDC25 phosphatases are key regulators of normal cell division and the cell's response to DNA damage. Earlier studies suggested non-overlapping roles for each isoform during a specific cell cycle phase. However, recent data suggest that multiple CDC25 isoforms cooperate to regulate each cell cycle transition. For instance, although CDC25A was initially thought to exclusively regulate the G(1)-S transition, recent data demonstrate a significant role for CDC25A in the G(2)-M transition. Further evidence demonstrates that in addition to the ATM/ATR-CHK pathway, a p38-MAPKAP pathway is also involved in controlling CDC25 activity during G(2)/M checkpoint activation. Together with the fact that CDC25 overexpression is reported in many cancers, these data highlight the significance of developing specific CDC25 inhibitors for cancer therapy.

Cdc25B cooperates with Cdc25A to induce mitosis but has a unique role in activating cyclin B1-Cdk1 at the centrosome

Cdc25 phosphatases are essential for the activation of mitotic cyclin–Cdks, but the precise roles of the three mammalian isoforms (A, B, and C) are unclear. Using RNA interference to reduce the expression of each Cdc25 isoform in HeLa and HEK293 cells, we observed that Cdc25A and -B are both needed for mitotic entry, whereas Cdc25C alone cannot induce mitosis. We found that the G2 delay caused by small interfering RNA to Cdc25A or -B was accompanied by reduced activities of both cyclin B1–Cdk1 and cyclin A–Cdk2 complexes and a delayed accumulation of cyclin B1 protein. Further, three-dimensional time-lapse microscopy and quantification of Cdk1 phosphorylation versus cyclin B1 levels in individual cells revealed that Cdc25A and -B exert specific functions in the initiation of mitosis: Cdc25A may play a role in chromatin condensation, whereas Cdc25B specifically activates cyclin B1–Cdk1 on centrosomes.

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.

Diallyl trisulfide-induced G(2)-M phase cell cycle arrest in human prostate cancer cells is caused by reactive oxygen species-dependent destruction and hyperphosphorylation of Cdc 25 C

Molecular mechanism of cell cycle arrest caused by diallyl trisulfide (DATS), a garlic-derived cancer chemopreventive agent, has been investigated using PC-3 and DU 145 human prostate cancer cells as a model. Treatment of PC-3 and DU 145 cells, but not a normal prostate epithelial cell line (PrEC), with growth suppressive concentrations of DATS caused enrichment of the G(2)-M fraction. The DATS-induced cell cycle arrest in PC-3 cells was associated with increased Tyr(15) phosphorylation of cyclin-dependent kinase 1 (Cdk 1) and inhibition of Cdk 1/cyclinB 1 kinase activity. The DATS-treated PC-3 and DU 145 cells also exhibited a decrease in the protein level of Cdc 25 C and an increase in its Ser(216) phosphorylation. The DATS-mediated decrease in protein level and Ser(216) phosphorylation of Cdc 25 C as well as G(2)-M phase cell cycle arrest were significantly attenuated in the presence of N-acetylcysteine implicating reactive oxygen species (ROS) in cell cycle arrest caused by DATS. ROS generation was observed in DATS-treated PC-3 and DU 145 cells. DATS treatment also caused an increase in the protein level of Cdk inhibitor p21, but DATS-induced G(2)-M phase arrest was not affected by antisense-mediated suppression of p21 protein level. In conclusion, the results of the present study indicate that DATS-induced G(2)-M phase cell cycle arrest in human prostate cancer cells is caused by ROS-mediated destruction and hyperphosphorylation of Cdc 25 C.

Role of checkpoint kinase 1 in preventing premature mitosis in response to gemcitabine

The deoxycytidine analogue 2',2'-difluoro-2'-deoxycytidine (gemcitabine) is a potent radiation sensitizer in a variety of solid tumors and tumor cell lines. Previous studies have shown that radiosensitization by gemcitabine is accompanied by simultaneous depletion of dATP pools (through ribonucleotide reductase inhibition) and accumulation in the S-phase of the cell cycle. Because of the importance of cell cycle redistribution in gemcitabine-mediated radiosensitization, we investigated the role of checkpoint kinase (Chk) 1 and Chk2 in gemcitabine-induced cell cycle arrest. We hypothesized that gemcitabine might induce Chk1 or Chk2 signal transduction pathways that mediate S-phase arrest. We found that radiosensitizing concentrations of gemcitabine induced accumulation of phosphorylated Chk1 and Chk2 and down-regulation of Cdc25A in BxPC-3 (10 nmol/L), Panc-1 (100 nmol/L), A549 (30 nmol/L), RKO (30 nmol/L), and SW620 (30 nmol/L) cells. Depletion of Chk1 from Panc-1 cells prevented the down-regulation of Cdc25A in response to gemcitabine. Furthermore, Chk1 depletion permitted Panc-1 and SW620 cells treated with gemcitabine to enter mitosis despite incomplete DNA synthesis. However, depletion of neither Chk1 nor Chk2 abrogated the inhibition of DNA synthesis in response to gemcitabine. These results provide evidence that Chk1 negatively regulates entry into mitosis in response to gemcitabine. Furthermore, these data imply that Chk1 acts to coordinate the cell cycle with DNA synthesis, thus preventing premature mitotic entry in gemcitabine-treated cells.

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.

Design, synthesis, and biological evaluation of novel naphthoquinone derivatives with CDC25 phosphatase inhibitory activity

CDC25 dual-specificity phosphatases are essential key regulators of eukaryotic cell cycle progression and the CDC25A and B isoforms are over-expressed in different tumors and related cancer cell lines. CDC25s are now considered to be interesting targets in the search for novel anticancer agents. We describe new compounds derived from vitamin K3 that inhibit CDC25B activity with IC50 values in the low micromolar range. These naphthoquinone derivatives also display antiproliferative activity on HeLa cells as expected for CDC25 inhibitors and inhibit cell growth in a clonogenic assay at submicromolar concentrations. They increase inhibitory tyrosine 15 phosphorylation of CDK and induce the cleavage of PARP, a hallmark of apoptosis.

Characterisation of Cdc25B localisation and nuclear export during the cell cycle and in response to stress

Cdc25 phosphatases are essential regulators of the cell cycle. In mammalian cells, the Cdc25B isoform activates cyclin A- and cyclin B1-containing complexes and is necessary for entry into mitosis. In this report, we characterise the subcellular localisation of Cdc25B by immunofluorescence in combination with RNA interference to identify specific antibody staining. We find that endogenous Cdc25B is mainly nuclear, but a fraction resides in the cytoplasm during the G2 phase of the cell cycle. Cdc25B starts to appear in S-phase cells and accumulates until prophase, after which the protein disappears. We characterise a nuclear export sequence in the N-terminus of Cdc25B (amino acids 54-67) that, when mutated, greatly reduces the ability of Cdc25B to shuttle in a fluorescence loss in photobleaching assay. Mutation of the nuclear export sequence makes Cdc25B less efficient in inducing mitosis, suggesting that an important mitotic function of Cdc25B occurs in the cytoplasm. Furthermore, we find that when cells are exposed to cycloheximide or ultraviolet irradiation, Cdc25B partially translocates to the cytoplasm. The dependence of this translocation event on a functional nuclear export sequence, an intact serine 323 residue (a 14-3-3 binding site) and p38 mitogen-activated protein kinase activity indicates that the p38 pathway regulates Cdc25B localisation in different situations of cellular stress.