Mangosteen Inhibits Growth and Survival of Cervical Cancer Cells.

BACKGROUND: Cervical cancer is the most common cancer of the female reproductive system. Late-stage cervical cancer treatment has been largely unsuccessful, and urgent anti-cancer therapy is needed. Mangosteen, a tropical fruit, has been studied and found to be rich in xanthones, known anti-cancer compounds. This study was designed to investigate the effect of mangosteen extract (ME) on SiHa cervical cancer cells and to explore the underlying molecular mechanisms. MATERIALS AND METHODS: Clonogenic survival assay, Quick Cell Proliferation Assay, terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) staining, and caspase-3 activity kits were used to investigate the in vitro role of ME treatment in SiHa cervical cancer cell growth. We further investigated the possible molecular mechanisms using RT-PCR. Statistical analysis was done with unpaired two-tailed Student's t-test and significance at p-value <0.05; each experiment was repeated three times. RESULTS: Our study found that the growth and proliferation of SiHa cervical cancer cells was inhibited by ME. ME also induced apoptosis in SiHa cervical cancer cells. The anti-proliferative effect of ME on cervical cancer cells was associated with statistically significant (p<0.05) down-regulation of the pro-proliferative molecules cyclin B, cyclin D and cyclin E. The pro-apoptotic effect of ME was associated with statistically significant (p<0.05) down-regulation of the anti-apoptotic molecules flice-like inhibitory protein (FLIP) and survivin. CONCLUSION: ME impedes the growth and survival of SiHa cervical cancer cells by down-regulating cyclin B, cyclin D, cyclin E as well as FLIP and survivin. ME may be a promising strategy for targeted cancer immunotherapy development.

Prenatal glucocorticoid administration accelerates the maturation of fetal rat hepatocytes.

BACKGROUND: Prenatal glucocorticoid (GC) is clinically administered to pregnant women who are at risk of preterm birth for the maturation of cardiopulmonary function. Preterm and low-birth-weight infants often experience liver dysfunction after birth because their livers are immature. However, the effects of prenatal GC administration on the liver remain unclear. We aimed to investigate the effects of prenatal GC administration on the maturation of liver hepatocytes in preterm rats. METHODS AND RESULTS: Dexamethasone (DEX) was administered to pregnant Wistar rats on gestational days 17 and 19 before cesarean section. Real-time reverse transcription-polymerase chain reaction (RT-PCR) was performed to determine the mRNA levels of albumin, hepatocyte nuclear factor-4 alpha (HNF4α), hepatocyte growth factor (HGF), thymus cell antigen 1 (Thy-1), cyclin B, and Cyclin-dependent kinase 1 (CDK1) in the liver samples. Immunohistochemical staining and enzyme-linked immunosorbent assay were performed to examine protein production. The hepatocytes enlarged because of growth and prenatal DEX administration. Albumin, HNF4α, and HGF levels increased secondary to growth and prenatal DEX administration. The levels of the cell cycle markers cyclin B and CDK1 gradually decreased during growth and with DEX administration. CONCLUSIONS: The results suggest that prenatal GC administration leads to hepatocyte maturation via expression of HNF4α and HGF in preterm fetuses.

PP1-mediated dephosphorylation of phosphoproteins at mitotic exit is controlled by inhibitor-1 and PP1 phosphorylation.

Loss of cell division cycle 2 (Cdc2, also known as Cdk1) activity after cyclin B degradation is necessary, but not sufficient, for mitotic exit. Proteins phosphorylated by Cdc2 and downstream mitotic kinases must be dephosphorylated. We report here that protein phosphatase-1 (PP1) is the main catalyst of mitotic phosphoprotein dephosphorylation. Suppression of PP1 during early mitosis is maintained through dual inhibition by Cdc2 phosphorylation and the binding of inhibitor-1. Protein kinase A (PKA) phosphorylates inhibitor-1, mediating binding to PP1. As Cdc2 levels drop after cyclin B degradation, auto-dephosphorylation of PP1 at its Cdc2 phosphorylation site (Thr 320) allows partial PP1 activation. This promotes PP1-regulated dephosphorylation at the activating site of inhibitor-1 (Thr 35) followed by dissociation of the inhibitor-1-PP1 complex and then full PP1 activation to promote mitotic exit. Thus, Cdc2 both phosphorylates multiple mitotic substrates and inhibits their PP1-mediated dephosphorylation.

Grapes(Chk1) prevents nuclear CDK1 activation by delaying cyclin B nuclear accumulation.

Entry into mitosis is characterized by a dramatic remodeling of nuclear and cytoplasmic compartments. These changes are driven by cyclin-dependent kinase 1 (CDK1) activity, yet how cytoplasmic and nuclear CDK1 activities are coordinated is unclear. We injected cyclin B (CycB) into Drosophila melanogaster embryos during interphase of syncytial cycles and monitored effects on cytoplasmic and nuclear mitotic events. In untreated embryos or embryos arrested in interphase with a protein synthesis inhibitor, injection of CycB accelerates nuclear envelope breakdown and mitotic remodeling of the cytoskeleton. Upon activation of the Grapes(checkpoint kinase 1) (Grp(Chk1))-dependent S-phase checkpoint, increased levels of CycB drives cytoplasmic but not nuclear mitotic events. Grp(Chk1) prevents nuclear CDK1 activation by delaying CycB nuclear accumulation through Wee1-dependent and independent mechanisms.

Pin1 stabilizes Emi1 during G2 phase by preventing its association with SCF(betatrcp).

The anaphase-promoting complex (APC) early mitotic inhibitor 1 (Emi1) is required to induce S- and M-phase entries by stimulating the accumulation of cyclin A and cyclin B through APC(Cdh1/cdc20) inhibition. In this report, we show that Emi1 proteolysis can be induced by cyclin A/cdk (cdk for cyclin-dependent kinase). Paradoxically, Emi1 is stable during G2 phase, when cyclin A/cdk, Plx1 and SCF(betatrcp) (SCF for Skp1-Cul1-Fbox protein)--which play a role in its degradation--are active. Here, we identify Pin1 as a new regulator of Emi1 that induces Emi1 stabilization by preventing its association with SCF(betatrcp). We show that Pin1 binds to Emi1 and prevents its association with betatrcp in an isomerization-dependent pathway. We also show that Emi1-Pin1 binding is present in vivo in XL2 cells during G2 phase and that this association protects Emi1 from being degraded during this phase of the cell cycle. We propose that S- and M-phase entries are mediated by the accumulation of cyclin A and cyclin B through a Pin1-dependent stabilization of Emi1 during G2.

[Effect of full-length cyclin B1 antisense cDNA on chemosensitivity of lewis lung carcinoma cells to gemicitabine in vitro and in vivo].

OBJECTIVE: To evaluate the effect of full-length cyclin B1 antisense cDNA (AS-CLB1) on chemosensitivity of Lewis lung carcinoma cells (LL/2) to gemicitabine (GEM) in vitro and in vivo and hence provide a therapeutic regiment for treating non-small cell lung (NSCL) cancer using AS-CLB1 combined with GEM. METHODS: Cell cycle phase distribution and apoptosis of LL/2 parent cells, LL/2/vect and LL/2/AS-CLB1 transfectants (LP, LV and LA cells) were determined by flow cytometry. In addition, the three kinds of cells were treated with GEM (20 nmol/L-20 micromol/L) in vitro for 1 h and 24 h respectively, and then cytotoxicity of GEM was measured by MTT assay. After inoculation with the three kinds of cells respectively, the C57BL/6 mice were treated with GEM [25 - 125 mg/(kg x day)] once every three days for four times when tumors developed; tumorigenicity and survival were observed and cell apoptosis in tumor tissues was determined by flow cytometry. RESULTS: LA cells displayed apparent apoptosis and G1 arrest compared with LP and LV cells (controls). Additionally, cytotoxicity of GEM to LA cells was more obvious than that to controls. Moreover, tumorigenicity was inhibited, cell apoptosis in tumor tissues was induced, and survival was evidently increased in LA cells group. CONCLUSION: AS-CLB1 slightly increased the sensitivity of LL/2 cells to GEM in vitro and in vivo. The function of AS-CLB1 may be associated with its ability to enhance the anti-tumor activity of GEM by inducing cell apoptosis and G1 arrest.

The clinical significance of Cyclin B1 and Wee1 expression in non-small-cell lung cancer.

BACKGROUND: Cyclin B1 has an important role in the G2-M phase transition of the cell cycle. Wee1 delays mitosis by suppressing the activity of the Cyclin B1/cdc2 complex. The objective of the present study was to elucidate the clinicopathological and prognostic significance of Cyclin B1 and Wee1 expression in non-small-cell lung cancer (NSCLC). PATIENTS AND METHODS: An immunohistochemical assessment of Cyclin B1 and Wee1 expression was performed in 79 patients with NSCLC. RESULTS: The expression of Cyclin B1 was correlated with differentiation (P = 0.0423) and vascular invasion (P = 0.001). Patients with overexpression of Cyclin B1 had higher mean values for both the Ki-67 proliferative index (Ki-Index) (P <0.0001) and proliferating cell nuclear antigen labeling index (PCNA-LI) (P <0.0001), and a poorer prognosis (P = 0.0068). Patients lacking expression of Wee1 had a higher recurrence rate (P = 0.0084) and a poorer prognosis (P = 0.0457), and tended to have higher Ki-Index and PCNA-LI values. Multivariate analysis suggested that both Cyclin B1 (P = 0.0244) and Wee1 (P = 0.0444) expression were significant prognostic factors. CONCLUSIONS: These findings suggest that Cyclin B1 expression could be a significant prognostic parameter in NSCLC. The loss of Wee1 expression may have a potential role in promoting tumor progression and may be a significant prognostic indicator in NSCLC.

[Triggering cell mitosis in higher eukaryotes]. / Le déclenchement de la mitose chez les eucaryotes supérieurs.

Dramatic changes of cell organisation occur at onset of mitosis. Genetic analysis of fission yeast and physiological studies of vertebrate and invertebrate oocytes showed that activation of cyclin B-cdc2 kinase triggers mitosis. Nevertheless, upstream mechanisms responsible for this activation remain largely unknown in somatic cells of higher eukaryotes. This review discusses possible pathways and mechanisms involved in triggering onset of mitosis in such cells, including inhibitory checkpoint mechanisms that detect defects in structural organisation of the cell.

Effect of 5-methoxypsoralen (5-MOP) on cell apoptosis and cell cycle in human hepatocellular carcinoma cell line.

The chemopreventive role of 5-methoxypsoralen (5-MOP) in the human hepatocellular carcinoma (HCC) cell line was investigated by studying the regulation of proliferation and apoptosis in HCC (J5) cells. Morphological analysis, cell viability assay, DNA analysis and cell-cycle analysis suggest that there are at least three modes of the suppressive effects shown by 5-MOP: (a) kills J5 cells directly; (b) induces apoptosis by arresting J5 cells at the G2/M phase in the cell cycle; (c) induces apoptosis through an independent pathway with cell-cycle arrest at 24-72 h of exposure. Further immunoblot analysis demonstrated that inhibition of cyclin B1 by 5-MOP may play an important role in G2/M arrest of J5 cells and provides an additional way to prevent cells from entering the M phase and undergoing apoptosis. 5-MOP therefore appears to exert its anticarcinogenic properties by cytotoxic effect, inducing apoptosis and inhibiting proliferation in the human hepatocellular carcinoma cell line.

Unmasking the S-phase-promoting potential of cyclin B1.

In higher eukaryotes, the S phase and M phase of the cell cycle are triggered by different cyclin-dependent kinases (CDKs). For example, in frog egg extracts, Cdk1-cyclin B catalyzes entry into mitosis but cannot trigger DNA replication. Two hypotheses can explain this observation: Either Cdk1-cyclin B fails to recognize the key substrates of its S-phase-promoting counterparts, or its activity is somehow regulated to prevent it from activating DNA synthesis. Here, we show that Cdk1-cyclin B1 has cryptic S-phase-promoting abilities that can be unmasked by relocating it from the cytoplasm to the nucleus and moderately stimulating its activity. Subcellular localization of vertebrate CDKs and the control of their activity are thus critical factors for determining their specificity.

The RRASK motif in Xenopus cyclin B2 is required for the substrate recognition of Cdc25C by the cyclin B-Cdc2 complex.

The FLRRXSK sequence is conserved in the second cyclin box fold of B-type cyclins. We show that this conserved sequence in Xenopus cyclin B2, termed the RRASK motif, is required for the substrate recognition by the cyclin B-Cdc2 complex of Cdc25C. Mutations to charged residues of the RRASK motif of cyclin B2 abolished its ability to activate Cdc2 kinase without affecting its capacity to bind to Cdc2. Cdc2 bound to the cyclin B2 RRASK mutant was not dephosphorylated by Cdc25C, and as a result, the complex was inactive. The cyclin B2 RRASK mutants can form a complex with the constitutively active Cdc2, but a resulting active complex did not phosphorylate a preferred substrate Cdc25C in vitro, although it can phosphorylate the non-specific substrate histone H1. The RRASK mutations prevented the interaction of Cdc25C with the cyclin B2-Cdc2 complex. Consistently, the RRASK mutants neither induced germinal vesicle breakdown in Xenopus oocyte maturation nor activated in vivo Cdc2 kinase during the cell cycle in mitotic extracts. These results suggest that the RRASK motif in Xenopus cyclin B2 plays an important role in defining the substrate specificity of the cyclin B-Cdc2 complex.

Transitions regulating the timing of cytokinesis in embryonic cells.

Anaphase, mitotic exit, and cytokinesis proceed in rapid succession, and while mitotic exit is a requirement for cytokinesis in yeast, it may not be a direct requirement for furrow initiation in animal cells. In this report, we physically manipulated the proximity of the mitotic apparatus (MA) to the cell cortex in combination with microinjection of effectors of the spindle checkpoint and CDK1 activity to determine how the initiation of cytokinesis is coupled to the onset of anaphase and mitotic exit. Whereas precocious contact between the MA and the cell surface advanced the onset of cytokinesis into early anaphase A, furrowing could not be advanced prior to the metaphase-anaphase transition. Additionally, while cells arrested in anaphase could be induced to initiate cleavage furrows, cells arrested in metaphase could not. Finally, activation of the mitotic checkpoint in one spindle of a binucleate cell failed to arrest cytokinesis induced by the control spindle but did inhibit the formation of furrows between the arrested MA and the control, nonarrested MA. Our experiments suggest that the competence of the mitotic apparatus to initiate cytokinesis is not dependent on cyclin degradation but does require anaphase-promoting complex (APC) activity and, thus, inactivation of the mitotic checkpoint.

Characterization of MPF and MAPK activities during meiotic maturation of Xenopus tropicalis oocytes.

Resumption of meiosis in oocytes of Xenopus tropicalis required translation but not transcription, and was marked by the appearance of a white spot and a dark ring, coincident with entry into metaphase I and the onset of anaphase I, respectively. Cyclin B(2)/p34(cdc2) activity increased prior to the first meiotic division, declined at the onset of anaphase I, and subsequently increased again. The capacity of egg cytoplasm to induce germinal vesicle breakdown (GVBD) was inhibited by cycloheximide, despite the fact that these oocytes contained cyclin B(2)/p34(cdc2) complexes. However, cycloheximide-treated oocytes underwent GVBD following injection of constitutively active mitogen-activated protein kinase (MAPK) kinase 2 (MEK2), p33(Ringo), or Delta 90 cyclin B. MAPK activity increased just prior to the first meiotic division and remained stable thereafter. Although injection of constitutively active MEK2 induced GVBD, treatment with the MEK inhibitors U0126 or anthrax lethal factor delayed GVBD and prevented spindle formation. Interestingly, the ability of egg cytoplasm to induce GVBD was unaffected by the inhibition of MEK activity. Our results indicate that the synthesis of a novel or short-lived protein(s) which acts in a MEK-independent fashion is required in order for egg cytoplasm to induce GVBD in X. tropicalis oocytes.

Induction of maturation-promoting factor during Xenopus oocyte maturation uncouples Ca(2+) store depletion from store-operated Ca(2+) entry.

During oocyte maturation, eggs acquire the ability to generate specialized Ca(2+) signals in response to sperm entry. Such Ca(2+) signals are crucial for egg activation and the initiation of embryonic development. We examined the regulation during Xenopus oocyte maturation of store-operated Ca(2+) entry (SOCE), an important Ca(2+) influx pathway in oocytes and other nonexcitable cells. We have previously shown that SOCE inactivates during Xenopus oocyte meiosis. SOCE inactivation may be important in preventing premature egg activation. In this study, we investigated the correlation between SOCE inactivation and the Mos-mitogen-activated protein kinase (MAPK)-maturation-promoting factor (MPF) kinase cascade, which drives Xenopus oocyte maturation. SOCE inactivation at germinal vesicle breakdown coincides with an increase in the levels of MAPK and MPF. By differentially inducing Mos, MAPK, and MPF, we demonstrate that the activation of MPF is necessary for SOCE inactivation during oocyte maturation. In contrast, sustained high levels of Mos kinase and the MAPK cascade have no effect on SOCE activation. We further show that preactivated SOCE is not inactivated by MPF, suggesting that MPF does not block Ca(2+) influx through SOCE channels, but rather inhibits coupling between store depletion and SOCE activation.

Rough deal and Zw10 are required for the metaphase checkpoint in Drosophila.

The metaphase-anaphase transition during mitosis is carefully regulated in order to assure high-fidelity transmission of genetic information to the daughter cells. A surveillance mechanism known as the metaphase checkpoint (or spindle-assembly checkpoint) monitors the attachment of kinetochores to the spindle microtubules, and inhibits anaphase onset until all chromosomes have achieved a proper bipolar orientation on the spindle. Defects in this checkpoint lead to premature anaphase onset, and consequently to greatly increased rates of aneuploidy. Here we show that the Drosophila kinetochore components Rough deal (Rod) and Zeste-White 10 (Zw10) are required for the proper functioning of the metaphase checkpoint in flies. Drosophila cells lacking either ROD or Zw10 exhibit a phenotype that is similar to that of bub1 mutants - they do not arrest in metaphase in response to spindle damage, but instead separate sister chromatids, degrade cyclin B and exit mitosis. These are the first checkpoint components to be identified that do not have obvious homologues in budding yeast.

Phosphorylation of the cyclosome is required for its stimulation by Fizzy/cdc20.

Exit from mitosis in eukaryotic cells is regulated by the cyclosome (also called anaphase promoting complex or APC), a multisubunit ubiquitin ligase that acts on mitotic cyclins. Previous studies in a cell-free system from clam oocytes have shown that the activation of the cyclosome at the end of mitosis involves its phosphorylation by protein kinase Cdk1/cyclin B. Genetic and biochemical studies have furthermore indicated that cyclosome activity also requires a WD-40 repeat containing protein called Fizzy (FZY) or Cdc20. It has been suggested [Fang et al. (1998) Mol. Cell 2, 163-171] that in the presence of FZY, the phosphorylation of the cyclosome is not critical for its activation. By contrast, we find that the activity of the interphase, non-phosphorylated form of the cyclosome from clam embryos is not stimulated by FZY to a significant extent. However, when interphase cyclosome is first incubated with protein kinase Cdk1/cyclin B, the subsequent supplementation of FZY greatly stimulates its cyclin-ubiquitin ligase activity. Furthermore, phosphatase treatment of purified mitotic cyclosome prevents its stimulation by FZY, a process that can be reversed by the action of protein kinase Cdk1/cyclin B. We conclude that in the early embryonic cell cycles, the primary event in the activation of the cyclosome at the end of mitosis is its Cdk1-dependent phosphorylation and activation by FZY takes place in a subsequent process.

A novel role for glycogen synthase kinase-3 in Xenopus development: maintenance of oocyte cell cycle arrest by a beta-catenin-independent mechanism.

We have examined the expression of glycogen synthase kinase-3beta in oocytes and early embryos of Xenopus and found that the protein is developmentally regulated. In resting oocytes, GSK-3beta is active and it is inactivated on maturation in response to progesterone. GSK-3beta inactivation is necessary and rate limiting for the cell cycle response to this hormone and the subsequent accumulation of beta-catenin. Overexpression of a dominant negative form of the kinase accelerates maturation, as does inactivation by expression of Xenopus Dishevelled or microinjection of an inactivating antibody. Cell cycle inhibition by GSK-3beta is not mediated by the level of beta-catenin or by a direct effect on either the MAP kinase pathway or translation of mos and cyclin B1. These data indicate a novel role for GSK-3beta in Xenopus development: in addition to controlling specification of the dorsoventral axis in embryos, it mediates cell cycle arrest in oocytes.

Activation of the p42 mitogen-activated protein kinase pathway inhibits Cdc2 activation and entry into M-phase in cycling Xenopus egg extracts.

We have added constitutively active MAP kinase/ERK kinase (MEK), an activator of the mitogen-activated protein kinase (MAPK) signaling pathway, to cycling Xenopus egg extracts at various times during the cell cycle. p42MAPK activation during entry into M-phase arrested the cell cycle in metaphase, as has been shown previously. Unexpectedly, p42MAPK activation during interphase inhibited entry into M-phase. In these interphase-arrested extracts, H1 kinase activity remained low, Cdc2 was tyrosine phosphorylated, and nuclei continued to enlarge. The interphase arrest was overcome by recombinant cyclin B. In other experiments, p42MAPK activation by MEK or by Mos inhibited Cdc2 activation by cyclin B. PD098059, a specific inhibitor of MEK, blocked the effects of MEK(QP) and Mos. Mos-induced activation of p42MAPK did not inhibit DNA replication. These results indicate that, in addition to the established role of p42MAPK activation in M-phase arrest, the inappropriate activation of p42MAPK during interphase prevents normal entry into M-phase.

Introduction of cyclin B induces activation of the maturation-promoting factor and breakdown of germinal vesicle in growing zebrafish oocytes unresponsive to the maturation-inducing hormone.

When treated with 17alpha,20beta-dihydroxy-4-pregnen-3-one (17alpha,20beta-DP), a natural maturation-inducing hormone in fishes, fully grown zebrafish oocytes are induced to mature via the activation of the maturation-promoting factor (MPF), which consists of cdc2 (a catalytic subunit) and cyclin B (a regulatory subunit). In contrast, 17alpha,20beta-DP is unable to induce growing (previtellogenic and vitellogenic) oocytes to mature. To know the reason growing oocytes fail to mature upon 17alpha,20beta-DP treatment, we investigated changes in the components of machinery responsible for MPF activation during zebrafish oogenesis. Immunoblotting experiments using monoclonal antibodies against cdc2, cyclin B, and cdk7 (an activator of cdc2) have revealed that the concentrations of cdc2 and cdk7 are almost constant during oogenesis. Cyclin B was present in mature oocytes but absent in growing and fully grown immature oocytes. These results, which are identical to those in goldfish, strongly suggest that cyclin B is synthesized from stored (masked) mRNA after 17alpha,20beta-DP stimulation and that its binding to the preexisting cdc2 allows cdk7 to activate MPF. Microinjection of cyclin B protein induced MPF activation and germinal vesicle breakdown in growing oocytes, as well as in fully grown oocytes, indicating that cdk7 present in growing oocytes is already active. Northern blot analysis revealed the presence of cyclin B mRNA in both previtellogenic and fully grown oocytes. These results indicate that, as in fully grown oocytes, growing oocytes are already equipped with the catalytic subunit of MPF (cdc2) and its activator (cdk7) and that the appearance of the regulatory subunit of MPF (cyclin B) is sufficient for initiating maturation. Therefore, the unresponsiveness of growing oocytes to 17alpha,20beta-DP is attributable to a deficiency in the processes leading to cyclin B synthesis, which include 17alpha,20beta-DP reception on the oocyte surface, subsequent signal transduction pathways, and unmasking the stored cyclin B mRNA.