Function and regulation of cdc25 protein phosphate through mitosis and meiosis

Lectins CGL and MTL, representatives of mytilectin family, exhibit different antiproliferative activity in Burkitt's lymphoma cells

Lectins are carbohydrate-binding proteins, whose biological effects are exerted via binding to glycoconjugates expressed on the surface of cells. Exposure to lectins can lead not only to a change in the structure and properties of cells but also to their death. Here, we studied the biological activity of lectins from the mussels Crenomytilus graynus (CGL) and Mytilus trossulus (MTL) and showed that these proteins can affect the proliferation of human lymphoma cells. Both lectins suppressed the formation of colonies as well as cell cycle progression. The mechanism of action of these lectins was not mediated by reactive oxygen species but included damaging of mitochondria, inhibition of key cell cycle points, and activation of MAPK signaling pathway in tumor cells. Computer modeling suggested that various effects of CGL and MTL on lymphoma cells may be due to the difference in the energy of binding of these lectins to carbohydrate ligands on the cell surface. Thus, molecular recognition of residues of terminal carbohydrates on the surface of tumor cells is a key factor in the manifestation of the biological action of lectins.

PIM Kinase Inhibitors as Novel Promising Therapeutic Scaffolds in Cancer Therapy

Cancer involves the uncontrolled, abnormal growth of cells and affects other tissues. Kinase has an impact on proliferating the cells and causing cancer. For the purpose of treating cancer, PIM kinase is a potential target. The pro-viral Integration site for moloney murine leukaemia virus (PIM) kinases is responsible for the tumorigenesis, by phosphorylating the proteins that control the cell cycle and cell proliferation. PIM-1, PIM-2, and PIM-3 are the three distinct isoforms of PIM kinases. The JAK/STAT pathway is essential for controlling how PIM genes are expressed. PIM kinase is also linked withPI3K/AKT/mTOR pathway in various types of cancers. The overexpression of PIM kinase will cause cancer. Currently, there are significant efforts being made in medication design and development to target its inhibition. A few small chemical inhibitors (E.g., SGI-1776, AZD1208, LGH447) that specifically target the PIM proteins' adenosine triphosphate (ATP)-binding domain have been identified. PIM kinase antagonists have a remarkable effect on different types of cancer. Despite conducting clinical trials on SGI-1776, the first PIM inhibitory agent, was prematurely withdrawn, making it unable to generate concept evidence. On the other hand, in recent years, it has aided in hastening the identification of multiple new PIM inhibitors. Cyanopyridines and Pyrazolo[1,5-a]pyrimidinecan act as potent PIM kinase inhibitors for cancer therapy. We explore the involvement of oncogenic transcription factor c-Mycandmi-RNA in relation to PIM kinase. In this article, we highlight the oncogenic effects, and structural insights into PIM kinase inhibitors for the treatment of cancer.

Design, synthesis and cytotoxic evaluation of novel bis-thiazole derivatives as preferential Pim1 kinase inhibitors with in vivo and in silico study

Bis-thiazole derivatives were synthesised conforming to the Pim1 pharmacophore model following Hantzsch condensation. Pim1 has a major role in regulating the G1/S phase which upon inhibition the cell cycle stops at its early stages. Derivatives 3b and 8b showed the best Pim1 IC₅₀ 0.32 and 0.24 µM, respectively relative to staurosporine IC₅₀ 0.36 µM. Further confirmation of 3b and 8b Pim1 inhibition was implemented by hindering the T47D cell cycle at G0/G1 and S phases where 3b showed 66.5% cells accumulation at G0/G1 phase while 8b demonstrated 26.5% cells accumulation at the S phase compared to 53.9% and 14.9% of a control group for both phases, respectively. Additional in vivo cytotoxic evaluation of 3b and 8b revealed strong antitumor activity with up-regulation of caspase-3 and down-regulation of VEGF and TNF α immune expression with concomitant elevation of malondialdehyde levels in case of 8b.

Anti-hepatoma Effect of DC2.4 Cells Transfected with Tumor-Associated Antigen Cdc25C In Vitro

OBJECTIVE

Cell division cyclin 25 homolog C (Cdc25C) is a tumor-associated antigen candidate gene, and this may be used as an effective target in cancer treatment. The present study aims to evaluate the lysis effect of cytotoxic T lymphocytes (CTLs) induced by dendritic cell line DC2.4 overexpressing Cdc25C, and the feasibility of Cdc25C as a component in hepatoma immunotherapy.

METHODS

The mouse Cdc25C gene was ligated into a lentiviral vector, and transfected into DC2.4 cells. The DC2.4 cell phenotype and cytokine secretion were determined by flow cytometry and ELISA, respectively. CD8⁺ T cells were sorted from the spleens of C57BL/6 mice using a magnetic bead sorting kit obtained from Miltenyi Biotech, Germany, and co-cultured with DC2.4 cells for one week as effector cells. Then, IL-2, granzyme B and perforin were detected in the CTL culture medium by ELISA. Next, time-resolved fluorescence immunoassay was used to detect the immune killing effect of Cdc25C-specific CTLs on target cells. Meanwhile, the effect of blocking MHC-I sites on target cells with a monoclonal anti-MHC-I antibody was evaluated.

RESULTS

The results revealed that Cdc25C could be stably overexpressed in DC2.4 cells by LV-Cdc25C infection. DC2.4 cells transfected with LV-Cdc25C secreted more IL-6, IL-12, TNF-α and IFN-γ, and had higher expression levels of CD40, CD86, CCR7 and MHC-II than unaltered DC2.4 cells. The elevated Cdc25C in dendritic cells also further increased the secretion of IL-2, granzyme B and perforin to elicit Cdc25C-specific CTLs, and induced the higher cytotoxicity in Hepa1-6 cell lines (P<0.05), but this had no effect on the target cells when MHC-I monoclonal antibodies were blocked.

CONCLUSION

DC2.4 cells transfected with LV-Cdc25C can induce specific CTLs, and result in a strong cellular immune response. The dendritic cells that overexpress Cdc25C may be useful for hepatoma immunotherapy.

Therapeutic targeting of PIM KINASE signaling in cancer therapy: Structural and clinical prospects

BACKGROUND

PIM kinases are well-studied drug targets for cancer, belonging to Serine/Threonine kinases family. They are the downstream target of various signaling pathways, and their up/down-regulation affects various physiological processes. PIM family comprises three isoforms, namely, PIM-1, PIM-2, and PIM-3, on alternative initiation of translation and they have different levels of expression in different types of cancers. Its structure shows a unique ATP-binding site in the hinge region which makes it unique among other kinases.

SCOPE OF REVIEW

PIM kinases are widely reported in hematological malignancies along with prostate and breast cancers. Currently, many drugs are used as inhibitors of PIM kinases. In this review, we highlighted the physiological significance of PIM kinases in the context of disease progression and therapeutic targeting. We comprehensively reviewed the PIM kinases in terms of their expression and regulation of different physiological roles. We further predicted functional partners of PIM kinases to elucidate their role in the cellular physiology of different cancer and mapped their interaction network.

MAJOR CONCLUSIONS

A deeper mechanistic insight into the PIM signaling involved in regulating different cellular processes, including transcription, apoptosis, cell cycle regulation, cell proliferation, cell migration and senescence, is provided. Furthermore, structural features of PIM have been dissected to understand the mechanism of inhibition and subsequent implication of designed inhibitors towards therapeutic management of prostate, breast and other cancers.

GENERAL SIGNIFICANCE

Being a potential drug target for cancer therapy, available drugs and PIM inhibitors at different stages of clinical trials are discussed in detail.

Ouabain Exhibited Strong Anticancer Effects in Melanoma Cells via Induction of Apoptosis, G2/M Phase Arrest, and Migration Inhibition

Background

Malignant melanoma was characterized by insensitive chemotherapy, drug resistance, and high metastatic ability, which resulted in the main reason for the mortality among skin-related cancers. The current agents were not sufficient to improve the treatment status of melanoma patients, and it was still needed to develop new chemotherapeutic drugs for melanoma. Our study aimed to study the anticancer effects and potential mechanisms of ouabain on melanoma cells.

Methods

The inhibitory effects of ouabain were determined by CCK8 and colony formation assays, and the morphological changes of melanoma cells were observed by inverted microscope. The apoptosis induction and cell cycle distribution were detected by annexin V/PI double staining and PI staining, respectively. The expression of the biomarker proteins in apoptosis and G2/M phase were determined by Western blotting analysis. The effects of ouabain on the migration of melanoma cells were measured by transwell migration assay and wound closure analysis. The potential mechanisms of ouabain in melanoma cells were analyzed by transcriptome sequencing.

Results

Our present study demonstrated that ouabain exhibited strong inhibitory effects on cell proliferation and triggered dramatical morphological changes of melanoma cells. Moreover, ouabain induced significant apoptosis in A375 rather than SK-Mel-28 cells via upregulation of bax expression and downregulation of bcl-2 expression. Consistently, ouabain treatment induced cell cycle arrest at G2/M phase in both A375 and SK-Mel-28 cells via upregulation of cyclin B1 and downregulation of cdc2 and cdc25c. Importantly, ouabain suppressed the migration of A375 and SK-Mel-28 cells. Furthermore, the transcriptome sequencing demonstrated that p53 and MAPK signaling pathway might play important roles in the inhibitory effects of ouabain.

Conclusion

Our study revealed that ouabain exhibited dramatical anticancer effects, which provided a novel application for cardiac glycoside drugs in the clinical treatment of melanoma.

The Antiproliferative Activity of Oxypeucedanin via Induction of G2/M Phase Cell Cycle Arrest and p53-Dependent MDM2/p21 Expression in Human Hepatoma Cells

Oxypeucedanin (OPD), a furocoumarin compound from Angelica dahurica (Umbelliferae), exhibits potential antiproliferative activities in human cancer cells. However, the underlying molecular mechanisms of OPD as an anticancer agent in human hepatocellular cancer cells have not been fully elucidated. Therefore, the present study investigated the antiproliferative effect of OPD in SK-Hep-1 human hepatoma cells. OPD effectively inhibited the growth of SK-Hep-1 cells. Flow cytometric analysis revealed that OPD was able to induce G₂/M phase cell cycle arrest in cells. The G₂/M phase cell cycle arrest by OPD was associated with the downregulation of the checkpoint proteins cyclin B1, cyclin E, cdc2, and cdc25c, and the up-regulation of p-chk1 (Ser345) expression. The growth-inhibitory activity of OPD against hepatoma cells was found to be p53-dependent. The p53-expressing cells (SK-Hep-1 and HepG2) were sensitive, but p53-null cells (Hep3B) were insensitive to the antiproliferative activity of OPD. OPD also activated the expression of p53, and thus leading to the induction of MDM2 and p21, which indicates that the antiproliferative activity of OPD is in part correlated with the modulation of p53 in cancer cells. In addition, the combination of OPD with gemcitabine showed synergistic growth-inhibitory activity in SK-Hep-1 cells. These findings suggest that the anti-proliferative activity of OPD may be highly associated with the induction of G₂/M phase cell cycle arrest and upregulation of the p53/MDM2/p21 axis in SK-HEP-1 hepatoma cells.

PIM Kinase as an Executional Target in Cancer

PIM (proviral integration site for moloney murine leukemia virus) kinase plays a key role as an oncogene in various cancers including myeloma, leukemia, prostate and breast cancers. The aberrant expression and/or activation of PIM kinases in various cancers follow an isoform-specific pattern. While PIM1 is predominantly expressed in hematological and solid tumors, PIM2 and PIM3 are largely expressed in leukemia and solid tumors, respectively. All of PIM kinases cause transcriptional activation of genes involved in cell survival and cell cycle progression in cancer. A variety of pro-tumorigenic signaling molecules, such as MYC, p21^Cip1/Waf1/p27^kip1, CDC25, Notch1 and BAD have been identified as the downstream targets of PIM kinases. So far, three kinds of adenosine triphosphate-competitive PIM inhibitors, SGI-1776, AZD1208, and LGH447 have been in clinical trials for the treatment of acute myelogenous leukemia, prostate cancer, lymphoma, or multiple myeloma. This review sheds light on the signaling pathways involved in the PIM kinase regulation and current status of developing PIM kinase inhibitors as clinical success in combating human cancer.

Nitric oxide (NO) inhibition of meiotic G2-M1 transition in Anabas testudineus oocytes: Participation of cAMP-dependent protein kinase (PKA) in regulation of intra-oocyte signaling events

Nitric oxide (NO) regulation of ovarian function in mammals has been studied extensively. However, relatively less information is available on NO action on meiotic G2-M1 transition in teleost oocytes. In the present study using follicle-enclosed oocytes of Anabas testudineus, NO regulation of intra-oocyte signaling events during meiotic G2-M1 transition were examined. Priming with NO donor, sodium nitroprusside (SNP) prevented 17α,20β-dihydroxy-4-pregenen-3-one (17,20β-P)-induced germinal vesicle break down (GVBD) in dose- and duration-dependent manner. Impaired GVBD response in SNP-treated groups corroborated well with reduced p34Cdc2 (Thr161) phosphorylation. Immunoblot analysis revealed that congruent with elevated cAMP-dependent protein kinase (PKA) phosphorylation (activation), NO inhibition of meiotic maturation involves down regulation of Cdc25 activation, Mos synthesis and MAPK3/1 (ERK1/2) phosphorylation. However, priming with PKA inhibitor (H89) could reverse SNP attenuation of oocyte GVBD significantly. Collectively our results indicate that negative influence of NO on meiotic G2-M1 transition in perch oocytes might involve PKA activation.

High Mobility Group A2 protects cancer cells against telomere dysfunction

The non-histone chromatin binding protein High Mobility Group AT-hook protein 2 (HMGA2) plays important roles in the repair and protection of genomic DNA in embryonic stem cells and cancer cells. Here we show that HMGA2 localizes to mammalian telomeres and enhances telomere stability in cancer cells. We present a novel interaction of HMGA2 with the key shelterin protein TRF2. We found that the linker (L1) region of HMGA2 contributes to this interaction but the ATI-L1-ATII molecular region of HMGA2 is required for strong interaction with TRF2. This interaction was independent of HMGA2 DNA-binding and did not require the TRF2 interacting partner RAP1 but involved the homodimerization and hinge regions of TRF2. HMGA2 retained TRF2 at telomeres and reduced telomere-dysfunction despite induced telomere stress. Silencing of HMGA2 resulted in (i) reduced binding of TRF2 to telomere DNA as observed by ChIP, (ii) increased telomere instability and (iii) the formation of telomere dysfunction-induced foci (TIF). This resulted in increased telomere aggregation, anaphase bridges and micronuclei. HMGA2 prevented ATM-dependent pTRF2T188 phosphorylation and attenuated signaling via the telomere specific ATM-CHK2-CDC25C DNA damage signaling axis. In summary, our data demonstrate a unique and novel role of HMGA2 in telomere protection and promoting telomere stability in cancer cells. This identifies HMGA2 as a new therapeutic target for the destabilization of telomeres in HMGA2+ cancer cells.

Desmethylanhydroicaritin isolated from Sophora flavescens, shows antitumor activities in U87MG cells via inhibiting the proliferation, migration and invasion

This study is the first report of the antitumor activities of desmethylanhydroicaritin (DMAI) isolated from Sophora flavescens on U87MG cells. Human glioblastoma is one of the most aggressive malignant type of brain tumors and highly diffuses to around normal brain tissues. DMAI showed anti-proliferation effects on U87MG cells at the concentration of 30μM, however did not affect to HEK-293 cells. DMAI induced anti-proliferation effects via ERK/MAPK, PI3K/Akt/mTOR signal pathway and G2/M phase cell cycle arrest. DMAI led to morphological change and inhibition of filapodia formation through regulation of Rac 1 and Cdc 42. In addition, migration and invasion of U87MG cells were inhibited by DMAI via down-regulation of matrix metalloproteinase (MMP) -2 and MMP -9 expressions and activities. Our results suggest that DMAI has a potential as a therapeutic agent against glioblastoma cells.

Novel Pactamycin Analogs Induce p53 Dependent Cell-Cycle Arrest at S-Phase in Human Head and Neck Squamous Cell Carcinoma (HNSCC) Cells

Pactamycin, although putatively touted as a potent antitumor agent, has never been used as an anticancer drug due to its high cytotoxicity. In this study, we characterized the effects of two novel biosynthetically engineered analogs of pactamycin, de-6MSA-7-demethyl-7-deoxypactamycin (TM-025) and 7-demethyl-7-deoxypactamycin (TM-026), in head and neck squamous cell carcinoma (HNSCC) cell lines SCC25 and SCC104. Both TM-025 and TM-026 exert growth inhibitory effects on HNSCC cells by inhibiting cell proliferation. Interestingly, unlike their parent compound pactamycin, the analogs do not inhibit synthesis of nascent protein in a cell-based assay. Furthermore, they do not induce apoptosis or autophagy in a dose- or a time-dependent manner, but induce mild senescence in the tested cell lines. Cell cycle analysis demonstrated that both analogs significantly induce cell cycle arrest of the HNSCC cells at S-phase resulting in reduced accumulation of G₂/M-phase cells. The pactamycin analogs induce expression of cell cycle regulatory proteins including master regulator p53, its downstream target p21^Cip1/WAF1, p27^kip21, p19, cyclin E, total and phospho Cdc2 (Tyr15) and Cdc25C. Besides, the analogs mildly reduce cyclin D1 expression without affecting expression of cyclin B, Cdk2 and Cdk4. Specific inhibition of p53 by pifithrin-α reduces the percentage of cells accumulated in S-phase, suggesting contribution of p53 to S-phase increase. Altogether, our results demonstrate that Pactamycin analogs TM-025 and TM-026 induce senescence and inhibit proliferation of HNSCC cells via accumulation in S-phase through possible contribution of p53. The two PCT analogs can be widely used as research tools for cell cycle inhibition studies in proliferating cancer cells with specific mechanisms of action.

Gossypin induces G2/M arrest in human malignant glioma U251 cells by the activation of Chk1/Cdc25C pathway

Gossypin is a flavone that was originally isolated from Hibiscus vitifolius and has traditionally been used for the treatment of diabetes, jaundice, and inflammation. Recently, gossypin was found to have potent anticancer properties; however, its effect on human gliomas still remain unknown. To investigate the potential anticancer effects of gossypin on malignant gliomas and analyze the associated molecular mechanisms, we treated human glioma U251 cells with gossypin. Our study showed that the treatment of U251 cells with gossypin inhibited cell proliferation in a dose- and time-dependent manner and was observed to be minimally toxic to normal human astrocytes. Gossypin's effect on cell cycle distribution was observed, and we found that it induced G2/M-phase arrest in U251 cells. An analysis of cell-cycle regulatory proteins indicated that the arresting effect of gossypin on the cell cycle at G2/M phase was involved in the phosphorylation of cell division cycle 25C (Cdc25C) tyrosine phosphatase via the activation of checkpoint kinase 1 (Chk1). These findings indicate that gossypin is a potential treatment of gliomas because of gossypin's potential to regulate the proliferation of U251 cells via the cell-cycle regulatory proteins Chk1 and Cdc25C.

Rapid alterations of cell cycle control proteins in human T lymphocytes in microgravity

In our study we aimed to identify rapidly reacting gravity-responsive mechanisms in mammalian cells in order to understand if and how altered gravity is translated into a cellular response. In a combination of experiments using "functional weightlessness" provided by 2D-clinostats and real microgravity provided by several parabolic flight campaigns and compared to in-flight-1g-controls, we identified rapid gravity-responsive reactions inside the cell cycle regulatory machinery of human T lymphocytes. In response to 2D clinorotation, we detected an enhanced expression of p21 ^Waf1/Cip1 protein within minutes, less cdc25C protein expression and enhanced Ser147-phosphorylation of cyclinB1 after CD3/CD28 stimulation. Additionally, during 2D clinorotation, Tyr-15-phosphorylation occurred later and was shorter than in the 1 g controls. In CD3/CD28-stimulated primary human T cells, mRNA expression of the cell cycle arrest protein p21 increased 4.1-fold after 20s real microgravity in primary CD4⁺ T cells and 2.9-fold in Jurkat T cells, compared to 1 g in-flight controls after CD3/CD28 stimulation. The histone acetyltransferase (HAT) inhibitor curcumin was able to abrogate microgravity-induced p21 mRNA expression, whereas expression was enhanced by a histone deacetylase (HDAC) inhibitor. Therefore, we suppose that cell cycle progression in human T lymphocytes requires Earth gravity and that the disturbed expression of cell cycle regulatory proteins could contribute to the breakdown of the human immune system in space.

Protein kinase C (PKC) role in bovine oocyte maturation and early embryo development

The aims of the present study were to determine the role of protein kinase C (PKC) on meiotic resumption and its effects on pronuclear formation and cleavage in the bovine. Oocytes were matured in the presence of 0, 1, 10 and 100 nM of phorbol 12-myristate 13-acetate (PMA), to evaluate the percentage of germinal vesicle breakdown. To study pronuclear formation and cleavage, oocytes were randomly distributed in four groups and matured in modified TCM-199 with LH and FSH (negative control); 10% of estrous cow serum (positive control); 100 nM of PMA (treatment); 100 nM of 4alpha-PDD (phorbol ester control). Oocytes were also matured in positive control medium, fertilized and transferred to KSOM with increasing concentrations of a PKC inhibitor. The protein profile and the presence of PKC at the end of maturation period were determined by SDS-PAGE followed by Silver Stain and Western blot, respectively. PMA stimulated meiotic resumption in a concentration-dependent manner. PKC stimulation during oocyte maturation caused an increase in pronuclear formation and did not cause parthenogenetic activation. Inhibitor of PKC (MyrPKC) inhibited cleavage in a dose-dependent and irreversible manner. A protein band around 74 kDa was not detected in PMA-treated oocytes and PKC was not detected by Western blot at the end of the maturation period. In conclusion, meiotic resumption was accelerated and the rate of oocytes with two pronuclei was increased when PKC was activated during oocyte maturation. Moreover, cleavage was inhibited in the presence of PMA.

Effects of tetrandrine on apoptosis and radiosensitivity of nasopharyngeal carcinoma cell line CNE

Tetrandrine is known to exert antitumor effect, however, little is known about its effect on nasopharyngeal carcinoma cells. In this study, we tested tetrandrine-induced apoptosis and radiosensitivity in nasopharyngeal carcinoma cell line CNE and investigated the possible mechanisms. Using flow cytometry and DNA electrophoresis, we found that tetrandrine could induce cell apoptosis. Further, it was shown that the level of Bcl-2 mRNA decreased and Bax mRNA increased after addition of tetrandrine by using reverse transcription-polymerase chain reaction. X-ray-induced G2 arrest was abrogated by treatment with tetrandrine, as detected by flow cytometry and mitotic index. The accumulation of cyclinB1 protein and the suppression of Cdc2 tyrosine-15 and Cdc25C serine-216 phosphorylation were detected in irradiated cells treated with tetrandrine using Western blot analysis. Taken together, these results show that tetrandrine can induce apoptosis and abrogate radiation-induced G2 arrest in CNE cells.

Human Raf-1 proteins associate with Rad24 and Cdc25 in cell-cycle checkpoint pathway of fission yeast, Schizosaccharomyces pombe

Raf-1 is a serine/threonine protein kinase that connects cell surface receptor signals to nuclear transcription factors. By screening Schizosaccharomyces pombe (S. pombe) cDNA library, we isolated Rad24, which is a 14-3-3 homolog that is important in the DNA damage checkpoint in S. pombe, as a Raf-1 interacting protein. The interaction found in yeast was confirmed by co-immunoprecipitation. Furthermore, Cdc25, which has been known to bind to Rad24, also associated with Raf-1 and was phosphorylated in vitro by catalytically active Raf-1. However, in the presence of Raf-1, an interaction between Rad24 and Cdc25 was inhibited in triple hybrid assay, indicating that Raf-1 inhibits the interaction between Rad24 and Cdc25. An in vitro competition assay showed that the binding of Cdc25 and of Rad24 to Raf-1 is mutually exclusive. Western blots of whole cell lysates probed with polyclonal antibodies specific for tyrosine-15-phosphorylated Cdc2 showed that overproduction of Rad24 led to the dephosphorylation of tyrosine residue on Cdc2, which is known to be activated through dephosphorylation by Cdc25 phosphatase. Unexpectedly, overexpression of catalytically inactive mutant protein of Raf-1, S624A, also caused tyrosine dephosphorylation of Cdc2. Thus, these data suggest that Raf-1 may interfere with the role of Rad24 by competing with Rad24 for binding to Cdc25 or a direct phosphorylation of Cdc25, bypassing the checkpoint pathway in DNA repair through Cdc25 activation.

Epidermal growth factor receptor pathway mitigates UVA-induced G2/M arrest in keratinocyte cells

UVA irradiation contributes largely to photocarcinogenesis. In the process of keratinocyte transformation, the activation of EGFR by UV is now considered as a critical event. However, the mechanism that links the EGFR pathway and photocarcinogenesis is not totally understood. In this study, we report that the EGFR/Akt pathway mitigated G2/M arrest in human HaCaT keratinocytes and normal human keratinocytes treated with low doses of UVA irradiation. EGFR-mediated Akt activation resulted in increased level of checkpoint 1 kinase (Chk1) inhibitory phosphorylation (Ser280). In contrast, EGFR/Akt pathway inhibition resulted in the abrogation of Ser280 Chk1 phosphorylation, increased level of Chk1 stimulatory phosphorylation (Ser345), and restoration of G2/M arrest. Altogether, these results suggest that, after UVA exposure, the EGFR/Akt pathway subverts the G2/M checkpoint. This effect may have serious implications in photocarcinogenesis by allowing damaged cells to transit through the cell cycle.

Activation of maturation promoting factor in Bufo arenarum oocytes: injection of mature cytoplasm and germinal vesicle contents

Although progesterone is the established maturation inducer in amphibians, Bufo arenarum oocytes obtained during the reproductive period (spring-summer) resume meiosis with no need of an exogenous hormonal stimulus if deprived of their enveloping follicle cells, a phenomenon called spontaneous maturation. In this species it is possible to obtain oocytes competent and incompetent to undergo spontaneous maturation according to the seasonal period in which animals are captured. Reinitiation of meiosis is regulated by maturation promoting factor (MPF), a complex of the cyclin-dependent kinase p34cdc2 and cyclin B. Although the function and molecule of MPF are common among species, the formation and activation mechanisms of MPF differ according to species. This study was undertaken to evaluate the presence of pre-MPF in Bufo arenarum oocytes incompetent to mature spontaneously and the effect of the injection of mature cytoplasm or germinal vesicle contents on the resumption of meiosis. The results of our treatment of Bufo arenarum immature oocytes incompetent to mature spontaneously with sodium metavanadate (NaVO3) and dexamethasone (DEX) indicates that these oocytes have a pre-MPF, which activates and induces germinal vesicle breakdown (GVBD) by dephosphorylation on Thr-14/Tyr-15 by cdc25 phosphatase and without cyclin B synthesis. The injection of cytoplasm containing active MPF is sufficient to activate an amplification loop that requires the activation of cdc25 and protein kinase C, the decrease in cAMP levels, and is independent of protein synthesis. However, the injection of germinal vesicle content also induces GVBD in the immature receptor oocyte, a process dependent on protein synthesis but not on cdc25 phosphatase or PKC activity.

The Pyst2-L phosphatase is involved in cell-crowding

The dual-specificity phosphatase Pyst2-L was found to be over expressed in leukocytes derived from AML and ALL patients as well as in certain other solid tumors and lymphoblastoid cell lines. Pyst2-L, binds and dephosphorylates both pERKs and pJNKs proteins, and thus, plays a role in regulating the MAP kinase signaling pathway. In the present study, a comparative genomic application was used and sequence analysis of multi-organisms databases were searched in order to identify genes homologous to Pyst2-L. The Xenopus laevis MAP kinase phosphatase X17c gene and the Yeast nitrogen starvation-induced protein phosphatase Yvh1p gene were revealed to be highly homologous with Pyst2-L. Both X17c and Yvh1p genes play a role in cell cycle regulation. A down regulated expression of the Yvh1p gene occurred in Saccharomyces cerevisiae that were synchronized to the G2-phase of the cell cycle by alpha-factor. In conformity with this result, a reduction in Pyst2-L expression levels was observed in G2-phase-synchronized Human K562 cells. Finally, we were able to show that cells in highly crowded cultures express high levels of the Pyst2-L phosphatase. These observations may indicate that low levels of the Pyst2-L phosphatase are essential for the G2-phase of the cell cycle and that this phosphatase might play a role in signaling cascades induced by cellular crowding.

Activation of Cdc2 kinase during meiotic maturation of axolotl oocyte

Activity of Cdc2, the universal inducer of mitosis, is regulated by phosphorylation and binding to cyclin B. Comparative studies using oocytes from several amphibian species have shown that different mechanisms allow Cdc2 activation and entry into first meiotic division. In Xenopus, immature oocytes stockpile pre-M-phase promoting factor (MPF) composed of Cdc2-cyclin B complexes maintained inactive by Thr14 and Tyr15 phosphorylation of Cdc2. Activation of MPF relies on the conversion of pre-MPF into MPF by Cdc2 dephosphorylation, implying a positive feedback loop known as MPF auto-amplification. On the contrary, it has been proposed that pre-MPF is absent in immature oocyte and that MPF activation depends on cyclin synthesis in some fishes and other amphibians. We demonstrate here that MPF activation in the axolotl oocyte, an urodele amphibian, is achieved through mechanisms resembling partly those found in Xenopus oocyte. Pre-MPF is present in axolotl immature oocyte and is activated during meiotic maturation. However, monomeric Cdc2 is expressed in large excess over pre-MPF, and pre-MPF activation by Cdc2 dephosphorylation takes place progressively and not abruptly as in Xenopus oocyte. The intracellular compartmentalization as well as the low level of pre-MPF in axolotl oocyte could account for the differences in oocyte MPF activation in both species.

Cdc2-cyclin B triggers H3 kinase activation of Aurora-A in Xenopus oocytes

Xenopus oocytes are arrested in meiotic prophase I and resume meiotic divisions in response to progesterone. Progesterone triggers activation of M-phase promoting factor (MPF) or Cdc2-cyclin B complex and neosynthesis of Mos kinase, responsible for MAPK activation. Both Cdc2 and MAPK activities are required for the success of meiotic maturation. However, the signaling pathway induced by progesterone and leading to MPF activation is poorly understood, and most of the targets of both Cdc2 and MAPK in the oocyte remain to be determined. Aurora-A is a Ser/Thr kinase involved in separation of centrosomes and in spindle assembly during mitosis. It has been proposed that in Xenopus oocytes Aurora-A could be an early component of the progesterone-transduction pathway, acting through the regulation of Mos synthesis upstream Cdc2 activation. We addressed here the question of Aurora-A regulation during meiotic maturation by using new in vitro and in vivo experimental approaches. We demonstrate that Cdc2 kinase activity is necessary and sufficient to trigger both Aurora-A phosphorylation and kinase activation in Xenopus oocyte. In contrast, these events are independent of the Mos/MAPK pathway. Aurora-A is phosphorylated in vivo at least on three residues that regulate differentially its kinase activity. Therefore, Aurora-A is under the control of Cdc2 in the Xenopus oocyte and could be involved in meiotic spindle establishment.

Zebrafish DAZ-like protein controls translation via the sequence ‘GUUC’

BACKGROUND

In many species, DAZ homologous genes encode RNA-binding proteins containing two conserved motifs, namely the RNA-recognition motif (RRM) and the DAZ motif. Genetic analysis and gene disruption studies have demonstrated that DAZ family proteins play important roles in gametogenesis. However, little is known about the biochemical functions of DAZ family proteins.

RESULTS

Using in vitro selection and UV-crosslinking experiments, we identified the sequence 'GUUC' as the target RNA sequence of zebrafish DAZ-like protein (zDAZL). In transfection experiments, zDAZL protein activated translation in a manner dependent on the binding sequence in the 3'UTR of the Drosophila twine gene or zDazl gene. Moreover, it is highly likely that the zDAZL protein associates with polysomes through the DAZ motif in vivo, and that the association with polysomes is indispensable for translational activation.

CONCLUSIONS

This is the first report that the DAZ family protein directly promotes the translation of the target mRNAs in vertebrates. This study provides important insights into the molecular mechanisms underlying the post-transcriptional regulation of DAZ family proteins in gametogenesis.

Cdc25b phosphatase is required for resumption of meiosis during oocyte maturation

In a wide variety of animal species, oocyte maturation is arrested temporarily at prophase of meiosis I (ref. 1). Resumption of meiosis requires activation of cyclin-dependent kinase-1 (CDK1, p34cdc2), one component of maturation-promoting factor (MPF). The dual specificity phosphatases Cdc25a, Cdc25b and Cdc25c are activators of cyclin-dependent kinases; consequently, they are postulated to regulate cell-cycle progression in meiosis and mitosis as well as the DNA-damage response. We generated Cdc25b-deficient (Cdc25b-/-) mice and found that they are viable. As compared with wildtype cells, fibroblasts from Cdc25b-/- mice grew vigorously in culture and arrested normally in response to DNA damage. Female Cdc25b-/- mice were sterile, and Cdc25b-/- oocytes remained arrested at prophase with low MPF activity. Microinjection of wildtype Cdc25b mRNA into Cdc25b-/- oocytes caused activation of MPF and resumption of meiosis. Thus, Cdc25b-/- female mice are sterile because of permanent meiotic arrest resulting from the inability to activate MPF. Cdc25b is therefore essential for meiotic resumption in female mice. Mice lacking Cdc25b provide the first genetic model for studying the mechanisms regulating prophase arrest in vertebrates.

Allyl sulfides modify cell growth

Extensive evidence points to the ability of allyl sulfides from garlic to suppress tumor proliferation both in vitro and in vivo. This antineoplastic effect is generally greater for lipid-soluble than water-soluble allyl sulfides. Both concentration and duration of exposure can increase the antiproliferative effects of lipid- and water-soluble allyl sulfides. Part of their antiproliferative effects may relate to an increase in membrane fluidity and a suppression of integrin glycoprotein IIb-IIIa mediated adhesion. Alterations in cholesterol, arachidonic acid, phospholipids and/or thiols may account for these changes in membrane function. Allyl sulfides are also recognized for their ability to suppress cellular proliferation by blocking cells in the G2/M phase and by the induction of apoptosis. This increase in the G2/M and apoptotic cell populations correlates with depressed p34cdc2 kinase activity, increased histone acetylation, increased intracellular calcium and elevated cellular peroxide production. While impressive pre-clinical data exist about the antineoplastic effects of allyl sulfur compounds, considerably more attention needs to be given to their effects in humans. The composition of the entire diet and a host of genetic/epigenetic factors will likely determine the true benefits that might arise from allyl sulfur compounds from garlic and other Allium foods.

Regulation of CDC25B phosphatases subcellular localization

The CDC25B dual specificity phosphatase is involved in the control of the G2/M transition of the cell cycle. Subcellular localization might represent an important aspect of the regulation of its activity. We have examined in transiently transfected asynchronous HeLa cells the localization of HA-tagged CDC25B proteins and found that they are nuclear or cytoplasmic suggesting the existence of an active shuttling. Accordingly, localization analysis of deletion and truncation proteins indicates that CDC25B contains a putative nuclear localization signal located between residues 335 and 354. We also demonstrated that a short 58 residues deletion of the amino-terminus end of CDC25B is sufficient to retain it to the nucleus. Mutational analysis indicates that a nuclear export sequence is located between residues 28 and 40. In addition, treatment of the cells with the exportin inhibitor, Leptomycin B, has the same effect. The mutation of Ser-323, a residue that is essential for the interaction with 14-3-3 proteins, also abolishes cytoplasmic staining. The subcellular localization of CDC25B is therefore dependent on the combined effects of a nuclear localization signal, a nuclear export signal and on the interaction with 14-3-3 proteins.

Actinobacillus actinomycetemcomitans Immunosuppressive Protein Is a Member of the Family of Cytolethal Distending Toxins Capable of Causing a G2 Arrest in Human T Cells

We have previously shown that Actinobacillus actinomycetecomitans produces an immunosuppressive factor (ISF) capable of impairing human lymphocyte function by perturbing cell cycle progression. We now report that ISF is the product of the cdtB gene, one of three genes encoding the family of cytolethal distending toxins (Cdt). The ISF polypeptide exhibits ≥95% identity with Hemophilus ducreyi CdtB protein and ≤60% homology with Escherichia coli or Campylobacter jejuni CdtB. Pretreatment of PHA-activated lymphocytes with 5–25 ng ISF results in G2 arrest of CD4+ and CD8+ T cells. Similarly, treatment of HeLa cells results in G2 arrest and cell elongation and distension. However, lymphocytes are at least 5 times more sensitive to ISF than HeLa cells and do not undergo the elongation and distension that characterizes interactions of Cdts with cell lines. ISF-treated lymphocytes express normal cyclin A and B1 levels, but contain reduced levels of cell cycle-dependent kinase-1 (Cdk1). Additionally, the majority of Cdk1 is in the hyperphosphorylated, inactive, form. In contrast, PHA-induced G2 cells contain elevated levels of the hypophosphorylated, active Cdk1. Failure of ISF-treated cells to dephosphorylate Cdk1 is not associated with decreased availability of Cdc25. These studies suggest that the CdtB protein alone is capable of inducing G2 arrest in lymphocytes and cell cycle arrest, elongation, and distension of HeLa cells. Our studies also suggest that lymphocytes may be primary targets for A. actinomycetemcomitans CdtB (ISF) and possibly for other Cdt family members as well. Thus, Cdts may function to impair host immunity and contribute to the pathogenesis of disease associated with Cdt-producing organisms.

In vitro binding of free cdc2 and raf kinase to membrane vesicles: a possible new regulatory mechanism for cdc2 kinase activation in Xenopus oocyte

The G2-M transition of the cell cycle is under the control of the M-phase promoting factor (MPF) formed of cdc2 kinase and cyclin B. The Xenopus prophase-blocked oocyte contains a stockpile of cyclin B2-cdc2 complexes that are maintained inactive by a double inhibitory phosphorylation on Thr-14 and Tyr-15 of cdc2. Free cdc2 molecules that are not associated with cyclin, are present in excess as compared to cyclin B2-associated cdc2. This pool of free cdc2 is permanently recruited to associate with neosynthetized cyclin B2 in the resting prophase oocyte, to feed up the pre-MPF stockpile. During re-entry into meiosis, free cdc2 could generate with newly synthesized cyclin B a small level of active MPF, that could serve as starter to initiate the conversion of pre-MPF into MPF. It was, therefore, of high interest to investigate whether free cdc2 interacts with other proteins and what could be its intracellular localization. To address these questions, we developed an in vitro system of membrane vesicles. We demonstrate here that free cdc2 is recovered in association with the external layer of membrane vesicles, whereas cyclin B2-associated cdc2 is not. Cyclin is able to associate in vitro with cdc2-containing membrane vesicles. This association does not induce the inhibitory cdc2 phosphorylations. However, it does not lead to active complexes, suggesting that membrane vesicles prevent cdc2 activation. C-Raf1, another kinase activated during reentry into meiosis, is also totally recovered in association with the membrane vesicles.

Regulation of Cdc28 cyclin-dependent protein kinase activity during the cell cycle of the yeast Saccharomyces cerevisiae

The cyclin-dependent protein kinase (CDK) encoded by CDC28 is the master regulator of cell division in the budding yeast Saccharomyces cerevisiae. By mechanisms that, for the most part, remain to be delineated, Cdc28 activity controls the timing of mitotic commitment, bud initiation, DNA replication, spindle formation, and chromosome separation. Environmental stimuli and progress through the cell cycle are monitored through checkpoint mechanisms that influence Cdc28 activity at key cell cycle stages. A vast body of information concerning how Cdc28 activity is timed and coordinated with various mitotic events has accrued. This article reviews that literature. Following an introduction to the properties of CDKs common to many eukaryotic species, the key influences on Cdc28 activity-cyclin-CKI binding and phosphorylation-dephosphorylation events-are examined. The processes controlling the abundance and activity of key Cdc28 regulators, especially transcriptional and proteolytic mechanisms, are then discussed in detail. Finally, the mechanisms by which environmental stimuli influence Cdc28 activity are summarized.

MPF amplification in Xenopus oocyte extracts depends on a two-step activation of cdc25 phosphatase

The activation of Cdc2 kinase induces the entry into M-phase of all eukaryotic cells. We have developed a cell-free system prepared from prophase-arrested Xenopus oocytes to analyze the mechanism initiating the all-or-none activation of Cdc2 kinase. Inhibition of phosphatase 2A, the major okadaic acid-sensitive Ser/Thr phosphatase, in these extracts, provokes Cdc2 kinase amplification and concomitant hyperphosphorylation of Cdc25 phosphatase, with a lag of about 1 h. Polo-like kinase (Plx1 kinase) is activated slightly after Cdc2. All these events are totally inhibited by the cdk inhibitor p21(Cip1), demonstrating that Plx1 kinase activation depends on Cdc2 kinase activity. Addition of a threshold level of recombinant Cdc25 induces a linear activation of Cdc2 and Plx1 kinases and a partial phosphorylation of Cdc25. We propose that the Cdc2 positive feedback loop involves two successive phosphorylation steps of Cdc25 phosphatase: the first one is catalyzed by Cdc2 kinase and/or Plx1 kinase but it does not modify Cdc25 enzymatic activity, the second one requires a new kinase counteracted by phosphatase 2A. Furthermore we demonstrate that, under our conditions, Cdc2 amplification and MAP kinase activation are two independent events.

The plant cell cycle in context

Biological scientists are eagerly confronting the challenge of understanding the regulatory mechanisms that control the cell division cycle in eukaryotes. New information will have major implications for the treatment of growth-related diseases and cancer in animals. In plants, cell division has a key role in root and shoot growth as well as in the development of vegetative storage organs and reproductive tissues such as flowers and seeds. Many of the strategies for crop improvement, especially those aimed at increasing yield, involve the manipulation of cell division. This review describes, in some detail, the current status of our understanding of the regulation of cell division in eukaryotes and especially in plants. It also features an outline of some preliminary attempts to exploit transgenesis for manipulation of plant cell division.

Human retinoblastoma protein (Rb) is phosphorylated by cdc2 kinase and MAP kinase in Xenopus maturing oocytes

Xenopus oocyte meiotic maturation combines features of G0/G1 and G2/M transitions of the cell cycle. To study the in ovo Rb kinase activity, we have microinjected human Rb into oocytes. Microinjected human Rb localizes into the nucleus, is hypophosphorylated in prophase oocytes, becomes hyperphosphorylated during meiotic maturation and is dephosphorylated as the cell reenters interphase. Inactivation or overexpression of the cyclin D-cdk4/6 complex in an oocyte extract does not affect the Rb kinase activity. This kinase activity could be attributed to both cdc2-cyclin B and MAP kinase, opening new perspectives of investigation in somatic cells.

Inhibition of glycosphingolipid synthesis induces p34cdc2 activation in Xenopus oocyte

In Xenopus prophase-blocked oocytes, it is assumed that progesterone interacts with the plasma membrane to initiate a signalling cascade that ultimately leads to MPF activation. Progesterone regulates negatively the cAMP pathway through an inhibition of adenylate cyclase. However, the mechanisms linking the initial action of the hormone with adenylate cyclase activity remain to be elucidated. Here, we demonstrate that PDMP, an inhibitor of glucosphingolipid synthesis, triggers oocyte meiotic maturation in a cAMP- and cycloheximide-dependent manner, whereas exogenous ceramide is unefficient. We propose that sphingolipid metabolism and targeting represent an important regulatory process of oocyte meiosis.