Ca2+ triggers premature inactivation of the cdc2 protein kinase in permeabilized sea urchin embryos

Influence of DL-beta-hydroxybutyric acid on cell proliferation and calcium influx

Poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBHHx), a member of the polyhydroxyalkanoate family of biopolyesters, has superior mechanical properties and biocompatibilities that enable it to meet diverse biomedical requirements. The main component of PHBHHx is DL-beta-hydroxybutyric acid (HB), a ketone body that is also produced in vivo. The effects of HB treatment on murine fibroblast L929 cells, human umbilical vein endothelial cells, and rabbit articular cartilages were investigated. HB (0.005-0.10 g/L) promoted cell proliferation for each cell line. Cell cycle analysis indicated that HB had a stimulatory effect on DNA synthesis. Flow cytometric analysis of L929 cells revealed changes in the [Ca2+]i for different stages of the cell cycle. In L929 cells, HB (0.02 g/L) stimulated a rapid increase in the concentration of cytosolic calcium that was blocked by verapamil and diltiazem, inhibitors of L-type Ca2+ channels. Finally, verapamil inhibited HB-induced L929 cell proliferation. Collectively, these results indicated that HB had a stimulatory effect on cell cycle progression that is mediated by a signaling pathway dependent upon increases in [Ca2+]i. This trophic effect may underlie the good biocompatibility observed for PHBHHx.

Signal transduction pathways leading to cell cycle arrest and apoptosis induction in cancer cells by Allium vegetable-derived organosulfur compounds: a review

Epidemiological studies continue to support the premise that dietary intake of Allium vegetables (e.g., garlic, onions and so forth) may lower the risk of various types of cancer. Anticarcinogenic effect of Allium vegetables is attributed to organosulfur compounds (OSCs) that are generated upon processing of these vegetables. Preclinical studies have provided convincing evidence to indicate that Allium vegetable-derived OSCs including diallyl sulfide, diallyl disulfide and diallyl trisulfide are highly effective in affording protection against cancer in laboratory animals induced by a variety of chemical carcinogens. Inhibition of carcinogen activation through modulation of cytochrome P450-dependent monooxygenases and/or acceleration of carcinogen detoxification via induction of phase II enzymes (glutathione transferases, quinone reductase, etc.) are believed to be responsible for protective effects of OSCs against chemically induced cancers. More recent studies have indicated that some naturally occurring OSC analogues can suppress proliferation of cancer cells in culture and inhibit growth of transplanted tumor xenografts in vivo by inducing apoptosis and/or by perturbing cell cycle progression. This review summarizes current knowledge on signal transduction pathways leading to perturbations in cell cycle progression and apoptosis induction by OSCs.

Activation of protein kinase C alters p34(cdc2) phosphorylation state and kinase activity in early sea urchin embryos by abolishing intracellular Ca2+ transients

The p34(cdc2) protein kinase, a universal regulator of mitosis, is controlled positively and negatively by phosphorylation, and by association with B-type mitotic cyclins. In addition, activation and inactivation of p34(cdc2) are induced by Ca(2+) and prevented by Ca(2+) chelators in permeabilized cells and cell-free systems. This suggests that intracellular Ca(2+) transients may play an important physiological role in the control of p34(cdc2) kinase activity. We have found that activators of protein kinase C can be used to block cell cycle-related alterations in intracellular Ca(2+) concentration ([Ca(2+)](i)) in early sea urchin embryos without altering the normal resting level of Ca(2+). We have used this finding to investigate whether [Ca(2+)](i) transients control p34(cdc2) kinase activity in living cells via a mechanism that involves cyclin B or the phosphorylation state of p34(cdc2). In the present study we show that the elimination of [Ca(2+)](i) transients during interphase blocks p34(cdc2) activation and entry into mitosis, while the elimination of mitotic [Ca(2+)](i) transients prevents p34(cdc2) inactivation and exit from mitosis. Moreover, we find that [Ca(2+)](i) transients are not required for the synthesis of cyclin B, its binding to p34(cdc2) or its destruction during anaphase. However, in the absence of interphase [Ca(2+)](i) transients p34(cdc2) does not undergo the tyrosine dephosphorylation that is required for activation, and in the absence of mitotic [Ca(2+)](i) transients p34(cdc2) does not undergo threonine dephosphorylation that is normally associated with inactivation. These results provide evidence that intracellular [Ca(2+)](i) transients trigger the dephosphorylation of p34(cdc2) at key regulatory sites, thereby controlling the timing of mitosis entry and exit.

Depressed p34cdc2 kinase activity and G2/M phase arrest induced by diallyl disulfide in HCT-15 cells

The present studies reveal the antiproliferative property of diallyl disulfide (DADS) in cultured human colon tumor cells (HCT-15) relative to its ability to decrease the proportion of cells in the G1 phase and increase the proportion of cells in the G2/M phase. The shift in the proportion of cells blocked in the G2/M phase increased as the concentration and duration of DADS exposure increased. Refeeding DADS-treated cells (50 microM) with complete medium without DADS resulted in a return to normal proliferation rates. Consistent with the G2/M phase arrest, DADS exposure inhibited p34cdc2 kinase activity within four hours of treatment. The maximum depression in p34cdc2 kinase activity (53%) occurred when 25 microM DADS was added to the medium. The present studies suggest that depressed p34cdc2 kinase activity is likely one of the early cellular events that may account for the antiproliferative property of DADS.

The coordination of centrosome reproduction with nuclear events of the cell cycle in the sea urchin zygote

Centrosomes repeatedly reproduce in sea urchin zygotes arrested in S phase, whether cyclin-dependent kinase 1-cyclin B (Cdk1-B) activity remains at prefertilization levels or rises to mitotic values. In contrast, when zygotes are arrested in mitosis using cyclin B Delta-90, anaphase occurs at the normal time, yet centrosomes do not reproduce. Together, these results reveal the cell cycle stage specificity for centrosome reproduction and demonstrate that neither the level nor the cycling of Cdk1-B activity coordinate centrosome reproduction with nuclear events. In addition, the proteolytic events of the metaphase-anaphase transition do not control when centrosomes duplicate. When we block protein synthesis at first prophase, the zygotes divide and arrest before second S phase. Both blastomeres contain just two complete centrosomes, which indicates that the cytoplasmic conditions between mitosis and S phase support centrosome reproduction. However, the fact that these daughter centrosomes do not reproduce again under such supportive conditions suggests that they are lacking a component required for reproduction. The repeated reproduction of centrosomes during S phase arrest points to the existence of a necessary "licensing" event that restores this component to daughter centrosomes during S phase, preparing them to reproduce in the next cell cycle.

An anaphase calcium signal controls chromosome disjunction in early sea urchin embryos

A transient increase in intracellular calcium concentration [Ca2+]i occurs throughout the cell as sea urchin embryos enter anaphase of the first cell cycle. The transient just precedes chromatid disjunction and spindle elongation. Microinjection of calcium chelators or heparin, an InsP3 receptor antagonist, blocks chromosome separation. Photorelease of calcium or InsP3 can reverse the block. Nuclear reformation is merely delayed by calcium antagonists at concentrations that block chromatid separation. Thus, the calcium signal triggers the separation of chromatids, while calcium-independent pathways can bring about the alterations in microtubule dynamics and nuclear events associated with anaphase progression. That calcium triggers chromosome disjunction alone is unexpected. It helps explain previous conflicting results and allows the prediction that calcium plays a similar role at anaphase in other cell types.

Cyclopiazonic acid induces accelerated progress of meiosis in pig oocytes

In mammalian oocytes, calcium plays an important role in the regulation of meiotic maturation. In our study, we used the mycotoxin cyclopiazonic acid (CPA), an inhibitor of calcium-dependent ATPases, to mobilise intracellular calcium deposits during in vitro maturation of pig oocytes. The CPA treatment of maturing oocytes significantly accelerated the progress of their maturation. Oocytes entered the CPA-sensitive period after 21 h of in vitro culture. A very short (5 min) exposure to CPA (100 mM) is sufficient to accelerate maturation and it seems that accelerated maturation can be triggered by a transient elevation of intracellular calcium levels. The effect of CPA is not mediated through the cumulus cells, because maturation is accelerated by CPA treatment even in oocytes devoid of cumulus cells. Culture of oocytes with the calcium channel blocker verapamil (concentrations ranging from 0.01 to 0.04 mM) blocked the progress of oocyte maturation beyond the stage of metaphase I. This block can be overcome by the mobilisation of intracellular calcium deposits after CPA treatment (100 nM). The microinjection of heparin (20 pl, 0.1 mg/ml), the inhibitor of inositol triphosphate receptors, before CPA treatment prevented the acceleration of oocyte maturation. This indicates that CPA mobilises the release of calcium deposits through inositol trisphosphate receptors. On the other hand, the microinjection of procaine (20 pl, 200 nM) or the microinjection of ruthenium red (20 pl, 50 mM), both inhibitors of ryanodine receptors, did not prevent accelerated maturation in CPA-treated oocytes. If present in pig oocytes, ryanodine receptors evidently play no part in the liberation of calcium from intracellular stores after CPA treatment.

Characterization of N-ethylmaleimide-sensitive thiol groups required for the GTP-dependent fusion of endoplasmic reticulum membranes

The GTP-dependent fusion activity of endoplasmic reticulum membranes is thought to be required for the structural maintenance and post-mitotic regeneration of the endoplasmic reticulum. This fusion is sensitive to the thiol-alkylating agent N-ethylmaleimide. In many intracellular fusion events N-ethylmaleimide-sensitivity is associated with a homotrimeric ATPase called N-ethylmaleimide-sensitive fusion protein or NSF. The addition of cytosol containing NSF is known to restore fusion activity to N-ethylmaleimide-treated membranes. We found that the inhibition of fusion of rat liver endoplasmic reticulum membranes (microsomes) by N-ethylmaleimide was not reversed by the addition of untreated cytosol. Fusion was also unaffected by treatment with a buffer known to remove NSF from membranes. Accordingly, no membrane-associated NSF was detected by immunoblot analysis. These data suggest that microsome fusion requires an N-ethylmaleimide-sensitive component distinct from NSF. This component was tightly associated with the membranes, so we used a number of chemical probes to characterize it in situ. Its thiol groups did not appear to be part of a GTP-binding site. They showed relatively low reactivity with sodium periodate, which induces the formation of disulphide bonds between proximate thiol groups. The thiols were not protected against N-ethylmaleimide by Zn2+, a potent inhibitor of fusion which is known to efficiently co-ordinate thiol groups. To characterize the topology of the fusion-related thiol groups we used bulky thiol-specific reagents prepared by conjugating BSA or 10 kDa aminodextran to the bifunctional reagent N-succinimidyl 3-(2-pyridyldithio)propionate. The inhibition of fusion by these reagents indicated that these thiols are highly exposed on the membranes. This exposure might be important for the function of these groups during GTP-triggered fusion.

Nuclear envelope breakdown is under nuclear not cytoplasmic control in sea urchin zygotes

Nuclear envelope breakdown (NEB) and entry into mitosis are though to be driven by the activation of the p34cdc2-cyclin B kinase complex or mitosis promoting factor (MPF). Checkpoint control mechanisms that monitor essential preparatory events for mitosis, such as DNA replication, are thought to prevent entry into mitosis by downregulating MPF activation until these events are completed. Thus, we were surprised to find that when pronuclear fusion in sea urchin zygotes is blocked with Colcemid, the female pronucleus consistently breaks down before the male pronucleus. This is not due to regional differences in the time of MPF activation, because pronuclei touching each other break down asynchronously to the same extent. To test whether NEB is controlled at the nuclear or cytoplasmic level, we activated the checkpoint for the completion of DNA synthesis separately in female and male pronuclei by treating either eggs or sperm before fertilization with psoralen to covalently cross-link base-paired strands of DNA. When only the maternal DNA is cross-linked, the male pronucleus breaks down first. When the sperm DNA is cross-linked, male pronuclear breakdown is substantially delayed relative to female pronuclear breakdown and sometimes does not occur. Inactivation of the Colcemid after female NEB in such zygotes with touching pronuclei yields a functional spindle composed of maternal chromosomes and paternal centrosomes. The intact male pronucleus remains located at one aster throughout mitosis. In other experiments, when psoralen-treated sperm nuclei, over 90% of the zygote nuclei do not break down for at least 2 h after the controls even though H1 histone kinase activity gradually rises close to, or higher than, control mitotic levels. The same is true for normal zygotes treated with aphidicolin to block DNA synthesis. From these results, we conclude that NEB in sea urchin zygotes is controlled at the nuclear, not cytoplasmic, level, and that mitotic levels of cytoplasmic MPF activity are not sufficient to drive NEB for a nucleus that is under checkpoint control. Our results also demonstrate that the checkpoint for the completion of DNA synthesis inhibits NEB by acting primarily within the nucleus, not by downregulating the activity of cytoplasmic MPF.