Regulation of the G2 to M transition

Intrinsic S phase checkpoint enforced by an antiproliferative oncosuppressor cytokine

The cell cycle is strictly programmed with control mechanisms that dictate order in cell cycle progression to ensure faithful DNA replication, whose deviance may lead to cancer. Checkpoint control at the G1/S, S/G2 and G2/M portals have been defined but no statutory time-programmed control for securing orderly transition through S phase has so far been identified. Here we report that in normal cells DNA synthesis is controlled by a checkpoint sited within the early part of S phase, enforced by the βGBP cytokine an antiproliferative molecule otherwise known for its oncosuppressor properties that normal cells constitutively produce for self-regulation. Suppression of active Ras and active MAPK, block of cyclin A gene expression and suppression of CDK2-cyclin A activity are events which while specific to the control of a cell cycle phase in normal cells are part of the apoptotic network in cancer cells.

Phosphorylation of p27(KIP1) in the Mitotic Cells of the Corneal Epithelium

PURPOSE

The mechanism in regulation of the cell cycle and proliferation of corneal epithelium in the homeostatic ocular surface remains unclear. The aim of this study is to examine the expression of p27(KIP1) and its phosphorylation in corneal epithelium.

METHODS

The eyes of C57BL/6 mice (7 weeks old) were enucleated. Formalin-fixed and paraffin-embedded tissue sections were examined using immunohistochemistry with anti-p27(KIP1), threonine 187 phosphorylated p27(KIP1) (T187-phospho-p27), and phosphorylated Histon H3 (pHiston H3) antibodies. Anti-T187-phospho-p27 and anti-pHiston H3 polyclonal antibodies were used for parallel immunofluorescent staining.

RESULTS

pHiston H3-immunopositive cells were noted in basal cells of the corneal epithelium. At high magnification of DAPI nuclear staining, mitotic and non-mitotic cells were observed in corneal basal layer. p27(KIP1)-positive nuclei were detected in corneal basal cells, where non-mitotic basal cells were located. In contrast, mitotic cells showed under detectable level on p27(KIP1) immunoreactivity. Immunoreactivity for T187-phospho-p27 was detected in basal cells of the corneal epithelium. At high magnification, it was confirmed that the immunopositive cells were mitotic cells. Immunoreactivity of T187-phospho-p27 as well as pHiston H3 was localized in the same corneal basal cells using double-staining immunohistochemistry.

CONCLUSIONS

These results suggested that degradation of p27(KIP1) regulates progression into mitosis in corneal basal cells.

Crocetin inhibits pancreatic cancer cell proliferation and tumor progression in a xenograft mouse model

Crocetin, a carotenoid compound derived from saffron, has long been used as a traditional ancient medicine against different human diseases including cancer. The aim of the series of experiments was to systematically determine whether crocetin significantly affects pancreatic cancer growth both in vitro and/or in vivo. For the in vitro studies, first, MIA-PaCa-2 cells were treated with crocetin and in these sets of experiments, a proliferation assay using H(3)-thymidine incorporation and flow cytometric analysis suggested that crocetin inhibited proliferation. Next, cell cycle proteins were investigated. Cdc-2, Cdc-25C, Cyclin-B1, and epidermal growth factor receptor were altered significantly by crocetin. To further confirm the findings of inhibition of proliferation, H(3)-thymidine incorporation in BxPC-3, Capan-1, and ASPC-1 pancreatic cancer cells was also significantly inhibited by crocetin treatment. For the in vivo studies, MIA-PaCa-2 as highly aggressive cells than other pancreatic cancer cells used in this study were injected into the right hind leg of the athymic nude mice and crocetin was given orally after the development of a palpable tumor. The in vivo results showed significant regression in tumor growth with inhibition of proliferation as determined by proliferating cell nuclear antigen and epidermal growth factor receptor expression in the crocetin-treated animals compared with the controls. Both the in vitro pancreatic cancer cells and in vivo athymic nude mice tumor, apoptosis was significantly stimulated as indicated by Bax/Bcl-2 ratio. This study indicates that crocetin has a significant antitumorigenic effect in both in vitro and in vivo on pancreatic cancer.

O2/3 exposure inhibits cell progression affecting cyclin B1/cdk1 activity in SK-N-SH while induces apoptosis in SK-N-DZ neuroblastoma cells

In search for innovative therapeutic agents for children neuroblastoma, the oxygen therapy could be considered an alternative anti-tumoral treatment. Given the physiochemical properties of O(2/3) gas mixture including fairly low aqueous solubility and spreading, and the interesting perspective of hyperoxia, we analyzed the inhibitory effect of O(2/3) treatment on two human neuroblastoma cell lines (SK-N-SH and SK-N-DZ). In this study, we demonstrated that O(2/3) treatment was able to induce cell growth inhibition and cell cycle perturbation in both cell lines. We observed an arrest at G(2) phase, accompanied by an alteration in the expression and localization of cyclin B1/cdk1 complex and a reduction in its activity in SK-N-SH cells. This reduction was consistent with the increase in both Wee1 and chk1 protein levels. On the contrary, O(2/3) induced apoptosis in SK-N-DZ cells via caspase 3 activation and Poly ADP-ribose polymerase-1 (PARP) cleavage, associated with an increase in the pro-apoptotic Bax protein. Consequently, we considered the possibility of improving the responsiveness to chemotherapeutic agents such as Cisplatin, Etoposide, and Gemcitabine in combination with O(2/3) treatment. The combined treatments produced a stronger cell inhibitory effect than Cisplatin and Etoposide used alone in SK-N-SH cells. On the contrary, the combination data were not significantly different from O(2/3) treatment alone in SK-N-DZ cells, thus suggesting that the obtained changes in cell growth inhibition were due to the effect of O(2/3) alone.

Heliocoverpa armigera single nucleocapsid nucleopolyhedrovirus induces Hz-AM1 cell cycle arrest at the G2 phase with accumulation of cyclin B1

The cell cycle phase distributions of Hz-AM1 cells grown in monolayer culture were G1 = 49.7 +/- 3.3%, S = 22.7 +/- 3.8% and G2/M = 27.8 +/- 4.2% without >4N DNA content. The culture doubling time was about 40 h and the duration of the G1, S and G2/M phases was estimated to be 10, 14 and 16 h, respectively. HaSNPV infection of Hz-AM1 cells resulted in both unsynchronized and synchronized G1 phase arrest at G2/M phase. HaSNPV infection also resulted in the appearance of more than 4N DNA content, which accumulated to the highest levels 72 h post-infection. We also found that the expression level of cyclin B1 increased significantly after 16 h post-infection, while cyclin A did not show any change. This observation supports the Hz-AM1-infected arrest at the G2/M phage. Cytoplasm location of cyclin B1 indicated that the Hz-AM1 cell cycle arrest was at the G2 phase.

The human decatenation checkpoint

Chromatid catenation is actively monitored in human cells, with progression from G(2) to mitosis being inhibited when chromatids are insufficiently decatenated. Mitotic delay was quantified in normal and checkpoint-deficient human cells during treatment with ICRF-193, a topoisomerase II catalytic inhibitor that prevents chromatid decatenation without producing topoisomerase-associated DNA strand breaks. Ataxia telangiectasia (A-T) cells, defective in DNA damage checkpoints, showed normal mitotic delay when treated with ICRF-193. The mitotic delay in response to ICRF-193 was ablated in human fibroblasts expressing an ataxia telangiectasia mutated- and rad3-related (ATR) kinase-inactive ATR allele (ATR(ki)). BRCA1-mutant HCC1937 cells also displayed a defect in ICRF-193-induced mitotic delay, which was corrected by expression of wild-type BRCA1. Phosphorylations of hCds1 or Chk1 and inhibition of Cdk1 kinase activity, which are elements of checkpoints associated with DNA damage or replication, did not occur during ICRF-193-induced mitotic delay. Over-expression of cyclin B1 containing a dominant nuclear localization signal, and inhibition of Crm1-mediated nuclear export, reversed ICRF-193-induced mitotic delay. In combination, these results imply that ATR and BRCA1 enforce the decatenation G(2) checkpoint, which may act to exclude cyclin B1/Cdk1 complexes from the nucleus. Moreover, induction of ATR(ki) produced a 10-fold increase in chromosomal aberrations, further emphasizing the vital role for ATR in genetic stability.

Depletion of a Cks homolog in C. elegans embryos uncovers a post-metaphase role in both meiosis and mitosis

In eukaryotic cells, the key regulators of cell-cycle transitions are the cyclin-dependent kinases (CDKs). The best studied CDK is a component of the M-phase promoting factor (MPF), which promotes entry into and progression through meiosis and mitosis. One of the enduring mysteries of the MPF complex has been the role of Cks/Suc1, a highly conserved member of the cell-cycle machinery in eukaryotes [1,2]. Cks has been proposed to be involved in activation of MPF [3], general interactions of MPF with its mitotic substrates [4] and/or inactivation of MPF [5,6]. We identified two Cks homologs in the genome of Caenorhabditis elegans and used RNA-mediated interference (RNAi) to investigate their roles in development. Whereas cks-2(RNAi) embryos display no apparent defects, cks-1(RNAi) embryos display defects in both meiosis and mitosis. Specifically, cks-1(RNAi) embryos fail to exit M phase properly. We propose that CKS-1 has an essential role in the inactivation of MPF during early C. elegans embryogenesis.

Roughex mediates G(1) arrest through a physical association with cyclin A

Differentiation in the developing Drosophila eye requires synchronization of cells in the G(1) phase of the cell cycle. The roughex gene product plays a key role in this synchronization by negatively regulating cyclin A protein levels in G(1). We show here that coexpressed Roughex and cyclin A physically interact in vivo. Roughex is a nuclear protein, while cyclin A was previously shown to be exclusively cytoplasmic during interphase in the embryo. In contrast, we demonstrate that in interphase cells in the eye imaginal disk cyclin A is present in both the nucleus and the cytoplasm. In the presence of ectopic Roughex, cyclin A becomes strictly nuclear and is later degraded. Nuclear targeting of both Roughex and cyclin A under these conditions is dependent on a C-terminal nuclear localization signal in Roughex. Disruption of this signal results in cytoplasmic localization of both Roughex and cyclin A, confirming a physical interaction between these molecules. Cyclin A interacts with both Cdc2 and Cdc2c, the Drosophila Cdk2 homolog, and Roughex inhibits the histone H1 kinase activities of both cyclin A-Cdc2 and cyclin A-Cdc2c complexes in whole-cell extracts. Two-hybrid experiments suggested that the inhibition of kinase activity by Roughex results from competition with the cyclin-dependent kinase subunit for binding to cyclin A. These findings suggest that Roughex can influence the intracellular distribution of cyclin A and define Roughex as a distinct and specialized cell cycle inhibitor for cyclin A-dependent kinase activity.

Cyclin-dependent kinase inhibitors: novel anticancer agents

In current models of cell cycle control, the transition between different cell cycle states is regulated at checkpoints. Transition through the cell-cycle is induced by a family of protein kinase holoenzymes, the cyclin-dependent kinases (CDKs) and their heterodimeric cyclin partner. Orderly progression through the cell-cycle involves co-ordinated activation of the CDKs, which in the presence of an associated CDK-activating kinase, phosphorylate target substrates including members of the 'pocket protein' family. This family includes the product of the retinoblastoma susceptibility gene (the pRb protein) and the related p107 and p130 proteins. Activity of these holoenzymes is regulated by post-translational modification. Phosphorylation of inhibitory sites on a conserved threonine residue within the activation segment is regulated by CDK7/cyclin H, referred to as CDK-activating kinase [1]. In addition, the cdc25 phosphatases activate the CDKs by dephosphorylating their inhibitory tyrosine and threonine phosphorylated residues [2,3]. Among the many roles for endogenous inhibitors (CDKIs), including members of the p21(CIP1/Waf1) family and the p16 family, one role is to regulate cyclin activity. Cellular neoplastic transformation is accompanied by loss of regulation of cell cycle checkpoints in conjunction with aberrant expression of CDKs and/or cyclins and the loss or mutation of the negative regulators (the CDKIs or the pocket protein pRb). One strategy to inhibit malignant cellular proliferation involves inhibiting CDK activity or enhancing function of the CDKI. Novel inhibitors of CDKs showing promise in the clinic include flavopiridol and UCN-01, which show early evidence of human tolerability in clinical trials. This review examines pertinent advances in the field of CDK inhibitors.

Tribbles coordinates mitosis and morphogenesis in Drosophila by regulating string/CDC25 proteolysis

Morphogenesis and cell differentiation in multicellular organisms often require accurate control of cell divisions. We show that a novel cell cycle regulator, tribbles, is critical for this control during Drosophila development. During oogenesis, the level of tribbles affects the number of germ cell divisions as well as oocyte determination. The mesoderm anlage enters mitosis prematurely in tribbles mutant embryos, leading to gastrulation defects. We show that Tribbles acts by specifically inducing degradation of the CDC25 mitotic activators String and Twine via the proteosome pathway. By regulating CDC25, Tribbles serves to coordinate entry into mitosis with morphogenesis and cell fate determination.

Cell cycle control of PDGF-induced Ca(2+) signaling through modulation of sphingolipid metabolism

The effects of growth factors have been shown to depend on the position of a cell in the cell cycle. However, the physiological basis for this phenomenon remains unclear. Here we show that the majority of both CEINGE clone3 (cl3) and human embryonic kidney 293 cells, when arrested in a quiescent phase (G(0)), responded to platelet-derived growth factor BB (PDGF-BB) with non-oscillatory Ca(2+) signals. Furthermore, the same type of Ca(2+) response was also observed in CEINGE cl3 cells (and to a lesser extent in HEK 293 cells) blocked at the G(1)/S boundary. In contrast, CEINGE cl3 cells synchronized in early G(1) or released from G(1)/S arrest responded in an oscillatory fashion. This cell cycle-dependent modulation of Ca(2+) signaling was not observed on epidermal growth factor and G-protein-coupled receptor stimulation and was not due to differences in the expression of PDGF receptors (PDGFRs) during the cell cycle. We demonstrate that inhibition of sphingosine-kinase, which converts sphingosine to sphingosine-1-phosphate, caused G(0) as well as G(1)/S synchronized cells to restore the oscillatory Ca(2+) response to PDGF-BB. In addition, we show that the synthesis of sphingosine and sphingosine-1-phosphate is regulated by the cell cycle and may underlie the differences in Ca(2+) signaling after PDGFR stimulation.