Recycling the cell cycle: cyclins revisited

Aryl hydrocarbon receptor, cell cycle regulation, toxicity, and tumorigenesis

Most effects of exposure to halogenated and polycyclic aromatic hydrocarbons are mediated by the aryl hydrocarbon receptor (AHR). It has long been recognized that the AHR is a ligand-activated transcription factor that plays a central role in the induction of drug-metabolizing enzymes and hence in xenobiotic detoxification. Of late, it has become evident that outside this well-characterized role, the AHR also functions as a modulator of cellular signaling pathways. In this Prospect, we discuss the involvement of the AHR in pathways critical to cell cycle regulation, mitogen-activated protein kinase cascades, immediate-early gene induction, and the functions of the RB protein. Ultimately, the toxicity of AHR xenobiotic ligands may be intrinsically connected with the perturbation of these pathways and depend on the many critical signaling pathways and effectors with which the AHR itself interacts.

CIL-102 Interacts with Microtubule Polymerization and Causes Mitotic Arrest following Apoptosis in the Human Prostate Cancer PC-3 Cell Line

There have been no therapeutic agents that provide a survival advantage in hormone-refractory prostate cancer. Recently, the Food and Drug Administration approved docetaxel combined with prednisone for the treatment of patients with advanced metastatic prostate cancer, and it does show a survival benefit. Hence, anti-microtubule drugs might be of benefit in chemotherapy of hormone-refractory prostate cancer. We used metastatic hormone-refractory prostate cancer PC-3 cells to investigate potential molecular mechanisms for CIL-102, a semisynthetic alkaloid derivative. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenylte-trazolium bromide and sulforhodamine B assays indicated that CIL-102 inhibits cell growth dose-dependently. Immunofluorescence microscopy and in vitro tubulin assembly assays indicated that CIL-102 binds to tubulin and disrupts microtubule organization. Flow cytometry showed that CIL-102 causes cells to accumulate in G(2)/M phase and sub-G(0)/G(1) phase. CIL-102-induced apoptosis was also characterized by immunofluorescence microscopy. Western blotting and kinase assays showed that CIL-102 exposure induced up-regulation of cyclin B1 and p34(cdc2) kinase activity and olomoucine, a p34(cdc2) inhibitor, profoundly reduced the number of cells accumulated in mitotic phase. Moreover, Bcl-2 phosphorylation, Cdc25C phosphorylation, and survivin expression were increased. CIL-102-induced apoptosis was associated with activation of caspase-3, but a noncaspase pathway may also be involved, since benzyloxycarbonyl-VAD-fluoromethyl ketone, a pancaspase inhibitor, only partially inhibited the apoptosis, and apoptosis-inducing factor was translocated from mitochondria to cytosol. We conclude that CIL-102 induces mitotic arrest and apoptosis by binding to tubulin and inhibiting tubulin polymerization. CIL-102 causes mitotic arrest, at least partly, by modulating cyclin-dependent kinases and then apoptosis executed by caspase and noncaspase pathways.

Candidate nsSNPs that can affect the functions and interactions of cell cycle proteins

Nonsynonymous single nucleotide polymorphisms (nsSNPs) alter the encoded amino acid sequence, and are thus likely to affect the function of the proteins, and represent potential disease-modifiers. There is an enormous number of nsSNPs in the human population, and the major challenge lies in distinguishing the functionally significant and potentially disease-related ones from the rest. In this study, we analyzed the genetic variations that can alter the functions and the interactions of a group of cell cycle proteins (n = 60) and the proteins interacting with them (n = 26) using computational tools. As a result, we extracted 249 nsSNPs from 77 cell cycle proteins and their interaction partners from public SNP databases. Only 31 (12.4%) of the nsSNPs were validated. The majority (64.5%) of the validated SNPs were rare (minor allele frequencies < 5%). Evolutionary conservation analysis using the SIFT tool suggested that 16.1% of the validated nsSNPs may disrupt the protein function. In addition, 58% of the validated nsSNPs were located in functional protein domains/motifs, which together with the evolutionary conservation analysis enabled us to infer possible biological consequences of the nsSNPs in our set. Our study strongly suggests the presence of naturally occurring genetic variations in the cell cycle proteins that may affect their interactions and functions with possible roles in complex human diseases, such as cancer.

G1 cell-cycle control and cancer

Before replicating DNA during their reproductive cycle, our cells enter a phase called G1 during which they interpret a flood of signals that influence cell division and cell fate. Mistakes in this process lead to cancer. An increasingly complex and coherent view of G1 signalling networks, which coordinate cell growth, proliferation, stress management and survival, is helping to define the roots of malignancies and shows promise for the development of better cancer therapies.

Living with or without cyclins and cyclin-dependent kinases

Entry into, progression through, and exit from the G1 phase of the mammalian cell cycle in response to extracellular mitogenic cues are presumed to be governed by cyclin-dependent kinases (Cdks) regulated by the D-type and E-type cyclins. Studies performed over more than a decade have supported the view that these holoenzymes are important, if not required, for these processes. However, recent experiments in which the genes encoding all three D-type cyclins, the two E-type cyclins, cyclin D-dependent Cdk4 and Cdk6, or cyclin E-dependent Cdk2 have been disrupted in the mouse germ line have revealed that much of fetal development occurs normally in their absence. Thus, none of these genes is strictly essential for cell cycle progression. To what extent is the prevailing dogma incorrect, and how can the recent findings be reconciled with past work?

Disruption of the Rb--Raf-1 interaction inhibits tumor growth and angiogenesis

The retinoblastoma tumor suppressor protein (Rb) plays a vital role in regulating mammalian cell cycle progression and inactivation of Rb is necessary for entry into S phase. Rb is inactivated by phosphorylation upon growth factor stimulation of quiescent cells, facilitating the transition from G(1) phase to S phase. Although the signaling events after growth factor stimulation have been well characterized, it is not yet clear how these signals contact the cell cycle machinery. We had found previously that growth factor stimulation of quiescent cells lead to the direct binding of Raf-1 kinase to Rb, leading to its inactivation. Here we show that the Rb-Raf-1 interaction occurs prior to the activation of cyclin and/or cyclin-dependent kinases and facilitates normal cell cycle progression. Raf-1-mediated inactivation of Rb is independent of the mitogen-activated protein kinase cascade, as well as cyclin-dependent kinases. Binding of Raf-1 seemed to correlate with the dissociation of the chromatin remodeling protein Brg1 from Rb. Disruption of the Rb-Raf-1 interaction by a nine-amino-acid peptide inhibits Rb phosphorylation, cell proliferation, and vascular endothelial growth factor-mediated capillary tubule formation. Delivery of this peptide by a carrier molecule led to a 79% reduction in tumor volume and a 57% reduction in microvessel formation in nude mice. It appears that Raf-1 links mitogenic signaling to Rb and that disruption of this interaction could aid in controlling proliferative disorders.

Cholesterol is essential for mitosis progression and its deficiency induces polyploid cell formation

As an essential component of mammalian cell membranes, cells require cholesterol for proliferation, which is either obtained from plasma lipoproteins or synthesized intracellularly from acetyl-CoA. In addition to cholesterol, other non-sterol mevalonate derivatives are necessary for DNA synthesis, such as the phosphorylated forms of isopentane, farnesol, geranylgeraniol, and dolichol. The aim of the present study was to elucidate the role of cholesterol in mitosis. For this, human leukemia cells (HL-60) were incubated in a cholesterol-free medium and treated with SKF 104976, which inhibits cholesterol biosynthesis by blocking sterol 14alpha-demethylase, and the expression of relevant cyclins in the different phases of the cell cycle was analyzed by flow cytometry. Prolonged cholesterol starvation induced the inhibition of cytokinesis and the formation of polyploid cells, which were multinucleated and had mitotic aberrations. Supplementing the medium with cholesterol completely abolished these effects, demonstrating they were specifically due to cholesterol deficiency. This is the first evidence that cholesterol is essential for mitosis completion and that, in the absence of cholesterol, the cells fail to undergo cytokinesis, entered G1 phase at higher DNA ploidy (tetraploidy), and then progressed through S (rereplication) into G2, generating polyploid cells.

Plk1 regulates activation of the anaphase promoting complex by phosphorylating and triggering SCFbetaTrCP-dependent destruction of the APC Inhibitor Emi1

Progression through mitosis requires activation of cyclin B/Cdk1 and its downstream targets, including Polo-like kinase and the anaphase-promoting complex (APC), the ubiquitin ligase directing degradation of cyclins A and B. Recent evidence shows that APC activation requires destruction of the APC inhibitor Emi1. In prophase, phosphorylation of Emi1 generates a D-pS-G-X-X-pS degron to recruit the SCF(betaTrCP) ubiquitin ligase, causing Emi1 destruction and allowing progression beyond prometaphase, but the kinases directing this phosphorylation remain undefined. We show here that the polo-like kinase Plk1 is strictly required for Emi1 destruction and that overexpression of Plk1 is sufficient to trigger Emi1 destruction. Plk1 stimulates Emi1 phosphorylation, betaTrCP binding, and ubiquitination in vitro and cyclin B/Cdk1 enhances these effects. Plk1 binds to Emi1 in mitosis and the two proteins colocalize on the mitotic spindle poles, suggesting that Plk1 may spatially control Emi1 destruction. These data support the hypothesis that Plk1 activates the APC by directing the SCF-dependent destruction of Emi1 in prophase.

Principles of MAP Kinase Signaling Specificity inSaccharomyces cerevisiae

Cells respond to a plethora of signals using a limited set of intracellular signal transduction components. Surprisingly, pathways that transduce distinct signals can share protein components, yet avoid erroneous cross-talk. A highly tractable model system in which to study this paradox is the yeast Saccharomyces cerevisiae, which harbors three mitogen-activated protein kinase (MAPK) signal transduction cascades that share multiple signaling components. In this review we first describe potential mechanisms by which specificity could be achieved by signaling pathways that share components. Second, we summarize key features and components of the yeast MAPK pathways that control the mating pheromone response, filamentous growth, and the response to high osmolarity. Finally, we review biochemical analyses in yeast of mutations that cause cross-talk between these three MAPK pathways and their implications for the mechanistic bases for signaling specificity. Although much remains to be learned, current data indicate that scaffolding and cross pathway inhibition play key roles in the maintenance of fidelity.

Patterns of gene dysregulation in the frontal cortex of patients with HIV encephalitis

The neurodegenerative process in HIV encephalitis (HIVE) is associated with extensive damage to the dendritic and synaptic structure that often leads to cognitive impairment. Several mechanisms might be at play, including release of neurotoxins, oxidative stress and decreased activity of neurotrophic factors. Furthermore, HIV-mediated dysregulation of genes involved in neuronal maintenance might play an important role. For this purpose, cRNA was prepared from the brains of 17 AIDS patients for analysis with the Affymetrix Human U95Av2 GeneChip and analyzed with the GeneSpring Expression Analysis Software. Out of 12,625 genes analyzed, 74 were downregulated and 59 were upregulated compared to controls. Initial alternative analysis of RNA was performed by ribonuclease protection assay (RPA). In cases with HIVE, downregulated genes included neuronal molecules involved in synaptic plasticity and transmission (ion channels, synaptogyrin, synapsin II), cell cycle (p35, p39, CDC-L2, CDC42, PAK1) and signaling molecules (PI3K, Ras-Raf-MEK1), transcription factors and cytoskeletal components (MAP-1B, MAP-2, tubulin, adducin-2). Upregulated genes included those involved in neuroimmune (IgG, MHC, beta2microglobulin) and anti-viral responses (interferon-inducible molecules), transcription (STAT1, OLIG2, Pax-6) and signaling modulation (MEK3, EphB1) of the cytoskeleton (myosin, aduccin-3, radixin, dystrobrevin). Taken together, this study suggests that HIV proteins released from infected macrophages might not only induce a neuroinflammatory response, but also may promote neurodegeneration by interfering with neuronal transcription of genes involved in regulating signaling and cytoskeletal molecules important in maintaining synapto-dendritic functioning and integrity.

Progression through meiosis I and meiosis II in Arabidopsis anthers is regulated by an A-type cyclin predominately expressed in prophase I

Meiosis is often described as a special case of cell division since it differs from mitosis in having two nuclear divisions without an intervening S-phase. It will be of great interest to uncover what molecular mechanisms underlie these special features of meiosis. We previously reported that the tardy asynchronous meiosis (tam) mutant of Arabidopsis (Arabidopsis thaliana) is slower in cell cycle progression in male meiosis. Here we report that TAM encodes the A-type cyclin, CYCA1;2. The point mutation in tam replaced a conserved threonine with an isoleucine in the linker region between the alpha4 and alpha5 helices of the first cyclin fold. By studying the dynamics of a CYCA1;2-green fluorescent protein fusion protein under the control of the CYCA1;2 promoter, we found that the fusion protein was most abundant at pachytene, but was undetectable from late prophase I until telophase II. Nonetheless, cell cycle progression in tam was delayed in both pachytene and meiosis II. We conclude either that the CYCA1;2 produced in prophase I indirectly regulates meiosis II progression, or that a very low level of CYCA1;2 directly regulates meiosis II progression. Either of these scenarios is a deviation from the typical mode of action of mitotic cyclins in mitosis and meiosis I, in which each nuclear division is coupled with a peak of expression of mitotic cyclins.

A Tel1/MRX-dependent checkpoint inhibits the metaphase-to-anaphase transition after UV irradiation in the absence of Mec1

In Saccharomyces cerevisiae, Mec1/ATR plays a primary role in sensing and transducing checkpoint signals in response to different types of DNA lesions, while the role of the Tel1/ATM kinase in DNA damage checkpoints is not as well defined. We found that UV irradiation in G(1) in the absence of Mec1 activates a Tel1/MRX-dependent checkpoint, which specifically inhibits the metaphase-to-anaphase transition. Activation of this checkpoint leads to phosphorylation of the downstream checkpoint kinases Rad53 and Chk1, which are required for Tel1-dependent cell cycle arrest, and their adaptor Rad9. The spindle assembly checkpoint protein Mad2 also partially contributes to the G(2)/M arrest of UV-irradiated mec1Delta cells independently of Rad53 phosphorylation and activation. The inability of UV-irradiated mec1Delta cells to undergo anaphase can be relieved by eliminating the anaphase inhibitor Pds1, whose phosphorylation and stabilization in these cells depend on Tel1, suggesting that Pds1 persistence may be responsible for the inability to undergo anaphase. Moreover, while UV irradiation can trigger Mec1-dependent Rad53 phosphorylation and activation in G(1)- and G(2)-arrested cells, Tel1-dependent checkpoint activation requires entry into S phase independently of the cell cycle phase at which cells are UV irradiated, and it is decreased when single-stranded DNA signaling is affected by the rfa1-t11 allele. This indicates that UV-damaged DNA molecules need to undergo structural changes in order to activate the Tel1-dependent checkpoint. Active Clb-cyclin-dependent kinase 1 (CDK1) complexes also participate in triggering this checkpoint and are required to maintain both Mec1- and Tel1-dependent Rad53 phosphorylation, suggesting that they may provide critical phosphorylation events in the DNA damage checkpoint cascade.

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.

The death domain protein p84N5, but not the short isoform p84N5s, is cell cycle-regulated and shuttles between the nucleus and the cytoplasm

P84N5 is a death domain containing protein that interacts with the tumor suppressor retinoblastoma protein and induces apoptosis. We cloned and characterized two novel alternatively spliced versions of p84N5. The p84N5 short isoform (p84N5s) lacks the death domain and does not induce apoptosis. We showed that p84N5, but not p84N5s, is cell cycle regulated. We found that p84N5-GFP chimera can rapidly shuttle between the nucleus and the cytoplasm. Taken together, these observations suggest that p84N5 may transmit signals from the nucleus to cytoplasmic effectors.

A cell sizer network involving Cln3 and Far1 controls entrance into S phase in the mitotic cycle of budding yeast

Saccharomyces cerevisiae must reach a carbon source-modulated critical cell size, protein content per cell at the onset of DNA replication (Ps), in order to enter S phase. Cells grown in glucose are larger than cells grown in ethanol. Here, we show that an increased level of the cyclin-dependent inhibitor Far1 increases cell size, whereas far1Δ cells start bud emergence and DNA replication at a smaller size than wild type. Cln3Δ, far1Δ, and strains overexpressing Far1 do not delay budding during an ethanol glucose shift-up as wild type does. Together, these findings indicate that Cln3 has to overcome Far1 to trigger Cln–Cdc28 activation, which then turns on SBF- and MBF-dependent transcription. We show that a second threshold is required together with the Cln3/Far1 threshold for carbon source modulation of Ps. A new molecular network accounting for the setting of Ps is proposed.

Xenoestrogen-induced ERK-1 and ERK-2 activation via multiple membrane-initiated signaling pathways

Xenoestrogens can mimic or antagonize the activity of physiological estrogens, and the suggested mechanism of xenoestrogen action involves binding to estrogen receptors (ERs). However, the failure of various in vitro or in vivo assays to show strong genomic activity of xenoestrogens compared with estradiol (E2) makes it difficult to explain their ability to cause abnormalities in animal (and perhaps human) reproductive functions via this pathway of steroid action. E2 has also been shown to initiate rapid intracellular signaling, such as changes in levels of intracellular calcium, cAMP, and nitric oxide, and activations of a variety of kinases, via action at the membrane. In this study, we demonstrate that several xenoestrogens can rapidly activate extracellular-regulated kinases (ERKs) in the pituitary tumor cell line GH3/B6/F10, which expresses high levels of the membrane receptor for ER-alpha (mER). We tested a phytoestrogen (coumestrol), organochlorine pesticides or their metabolites (endosulfan, dieldrin, and DDE), and detergent by-products of plastics manufacturing (p-nonylphenol and bisphenol A). These xenoestrogens (except bisphenolA) produced rapid (3-30 min after application), concentration (10(-14)-10(-8) M)-dependent ERK-1/2 phosphorylation but with distinctly different activation patterns. To identify signaling pathways involved in ERK activation, we used specific inhibitors of ERs, epidermal growth factor receptors, Ca2+ signaling, Src and phosphoinositide-3 kinases, and a membrane structure disruption agent. Multiple inhibitors blocked ERK activation, suggesting simultaneous use of multiple pathways and complex signaling web interactions. However, inhibitors differentially affected each xenoestrogen response examined. These actions may help to explain the distinct abilities of xenoestrogens to disrupt reproductive functions at low concentrations.

Anillin binds nonmuscle myosin II and regulates the contractile ring

We demonstrate that the contractile ring protein anillin interacts directly with nonmuscle myosin II and that this interaction is regulated by myosin light chain phosphorylation. We show that despite their interaction, anillin and myosin II are independently targeted to the contractile ring. Depletion of anillin in Drosophila or human cultured cells results in cytokinesis failure. Human cells depleted for anillin fail to properly regulate contraction by myosin II late in cytokinesis and fail in abscission. We propose a role for anillin in spatially regulating the contractile activity of myosin II during cytokinesis.

The SCF ubiquitin ligase: insights into a molecular machine

Ubiquitin ligases are well suited to regulate molecular networks that operate on a post-translational timescale. The F-box family of proteins - which are the substrate-recognition components of the Skp1-Cul1-F-box-protein (SCF) ubiquitin ligase - are important players in many mammalian functions. Here we explore a unifying and structurally detailed view of SCF-mediated proteolytic control of cellular processes that has been revealed by recent studies.

Prokaryotic development: a new player on the cell cycle circuit

A genetic regulatory circuit recently described in the bacterium Caulobacter crescentus generates reciprocal oscillations in the abundance of two key transcription factors to control landmark events in the cell cycle.

Control of smooth muscle cell proliferation in vascular disease

PURPOSE OF REVIEW

Smooth muscle cell proliferation has previously been regarded as a central feature in vascular disease. The role of this process has recently been substantially re-evaluated, and we have reconsidered the functional importance of smooth muscle cell proliferation, the origin of proliferating smooth muscle cells in lesions, and the mechanisms whereby smooth muscle cell proliferation is controlled. In this review, we summarize recent progress in the understanding of smooth muscle cell proliferation, with a particular focus on how interactions between the extracellular matrix, smooth muscle cells, and mitogens control critical steps in this process.

RECENT FINDINGS

Irrespective of the origin of smooth muscle cells in vascular lesions, fundamental interactions between the extracellular matrix and cell surface integrins are necessary in order to initiate a proliferative response in a quiescent smooth muscle cell, in a similar manner to any non-malignant cell. These interactions trigger intracellular signaling and cell cycle entry, which facilitate cell cycle progression and proliferation by mitogens. In addition, extracellular matrix interactions may also control the availability and activity of growth factors such as heparin-binding mitogens, which can be sequestered by heparan sulfate containing extracellular matrix components and regulate smooth muscle cell proliferation.

SUMMARY

New insights into mechanisms whereby the extracellular matrix takes part in the control of smooth muscle cell proliferation suggest a number of putative targets for future therapies that can be applied to increase plaque stability, prevent the clinical consequences of atherosclerosis and improve outcomes after interventional procedures and organ transplantation.

Isolation of a dinoflagellate mitotic cyclin by functional complementation in yeast

Dinoflagellates are protists with permanently condensed chromosomes that lack histones and whose nuclear membrane remains intact during mitosis. These unusual nuclear characters have suggested that the typical cell cycle regulators might be slightly different than those in more typical eukaryotes. To test this, a cyclin has been isolated from the dinoflagellate Gonyaulax polyedra by functional complementation in cln123 mutant yeast. This GpCyc1 sequence contains two cyclin domains in its C-terminal region and a degradation box typical of mitotic cyclins. Similar to other dinoflagellate genes, GpCyc1 has a high copy number, with approximately 5000 copies found in the Gonyaulax genome. An antibody raised against the N-terminal region of the GpCYC1 reacts with a 68kDa protein on Western blots that is more abundant in cell cultures enriched for G2-phase cells than in those containing primarily G1-phase cells, indicating its cellular level follows a pattern expected for a mitotic cyclin. This is the first report of a cell cycle regulator cloned and sequenced from a dinoflagellate, and our results suggest control of the dinoflagellate cell cycle will be very similar to that of other organisms.

Systems Biology and the Molecular Circuits of Cancer

Proliferative disorders are a major challenge for human health. The understanding of the organization of cell-cycle events is of the utmost importance to devise effective therapeutic strategies for cancer. The awareness that cells and organisms are complex, modular, hierarchical systems and the availability of genome-wide gene expression and protein analyses, should make it feasible to elucidate human diseases in terms of dysfunctions of molecular systems. Here we review evidence in support of a systems model of the cell cycle, in which two sequential growth-sensitive thresholds control entry into S-phase. The putative molecular determinants that set the threshold for entry into S-phase are consistently altered in cancer cells. Such a framework could be useful in guiding both experimental investigation and data analysis by allowing wiring to other relevant cell modules thereby highlighting the differential responses, or lack of response of cancer cells to intra- and extracellular factors. Pharmacological approaches that take advantage of transformation-induced fragility to glucose shortage are discussed. Extension of this hierarchical, modular approach to tumors as a whole holds promise for the development of effective drug discovery approaches and more efficient therapeutic protocols.

The basic helix-loop-helix transcription factor neuroD is expressed in the rod lineage of the teleost retina

Persistent rod genesis in the retinas of teleost fish was first described over 2 decades ago, but little is known regarding the underlying genetic and molecular mechanisms that govern this phenomenon. Because of its function in the developing mammalian retina and persistently mitotic adult tissues, we sought to characterize the cellular expression of the basic helix-loop-helix (bHLH) transcription factor neuroD in the persistently neurogenic retina of adult teleosts. We show here that, in the adult retina of the goldfish, neuroD is expressed by putative amacrine cells, nascent cones, and the mitotically active cells of the rod lineage. neuroD is the first gene shown to be expressed by rod precursors, the immediate antecedents of rod photoreceptors. In contrast to the vertebrate classes described previously, neuroD is not expressed in multipotent progenitors in the teleost retina. Combining neuroD in situ hybridizations with cell-cycle-specific markers suggests that, in rod precursors, neuroD expression is cell cycle specific.

4-hydroxynonenal and regulation of cell cycle: effects on the pRb/E2F pathway

The hypothesis that 4-hydroxynonenal (HNE), a product of lipid peroxidation, might negatively affect cell proliferation, arose from the observation that lipid peroxidation is very low in tumors. In leukemic cells HNE inhibited cell growth and reduced c-myc and c-myb expression. HNE also induced differentiation in different leukemic cell lines. In HL-60 human leukemic cells, HNE induced the accumulation of cells in the G(0)/G(1) phase of the cell cycle accompanied by a decrease of cyclins D1, D2, and A. Moreover, HNE caused an increase in p21 expression. As cyclin D/CDK2 and cyclin A/CDK2 phosphorylate pRB, these findings suggested that pRb phosphorylation could be affected by HNE. Hypophosphorylated pRb binds and inactivates the E2F transcription factors. HNE induced the dephosphorylation of pRb and the increase in pRb/E2F1 complexes, whereas pRb/E2F4 complexes were reduced, because HNE downregulated E2F4 protein expression. The analysis of E2F binding to the P2 c-myc promoter revealed that HNE caused a decrease in "free" E2F, as well as an increase in pRb (and pRB family members) bound to E2F, with consequent repression of the transcription. In conclusion, HNE reduces E2F transcriptional activity by modifying a number of genes involved in regulation of the pRb/E2F pathway.

A comprehensive gene expression analysis toward the understanding of growth and differentiation of tobacco BY-2 cells

To understand how plant cell changes gene expression during cell division and after termination of cell division, we analyzed the change of gene expression during the growth of tobacco BY-2 cell lines using a cDNA microarray, which contained about 9,200 expression sequence tag fragments and corresponded to about 7,000 genes. We found that log phase cells predominantly expressed DNA/chromosome duplication gene homologs. In addition, many genes for basic transcription and translation machineries, as well as proteasomal genes, were up-regulated at the log phase. About half of the kinesin homolog genes, but not myosin homolog genes, were predominantly expressed at the dividing phase as well. In contrast, stationary phase cells expressed genes for many receptor kinases, signal transduction machineries and transcription factors. Several hundreds of genes showed differential expression after incubation of stationary phase cells with medium containing either salicylic acid or abscisic acid. These findings suggested that BY-2 cells at the stationary phase express genes for perceiving extracellular signals.

The in vitro effects of CRE-decoy oligonucleotides in combination with conventional chemotherapy in colorectal cancer cell lines

The cAMP response element consensus sequence directs the transcription of a wide range of genes. A 24-mer single-stranded cAMP response element decoy oligonucleotide (CDO) has been shown to compete with these sequences for binding transcription factors and therefore interferes with cAMP-induced gene transcription. We have examined the effect of this CDO alone and in combination with a range of common chemotherapeutic agents in colorectal cancer cell lines. CDO had a potent anti-proliferative effect in colorectal cell lines, yet, a similar enhancement of cell death was not observed. Simple drug-drug interaction studies showed that combining CDO with chemotherapy resulted in an enhancement of the antiproliferative effects. Furthermore, this cytostatic effect was protracted and associated with an increase in senescence-associated beta-galactosidase activity at pH 6. There is a possible role for p21(waf1) in mediating this effect, as the enhancement of cell growth inhibition was not observed in cells lacking the ability to correctly upregulate this protein. Additionally, significant decreases in cyclin-dependent kinase (CDK) 1 and CDK 4 function were seen in the responsive cells. These data provide a possible model of drug interaction in colorectal cell lines, which involves the complex interplay of the molecules regulating the cell cycle. Clinically, the cytostatic ability of CDO could improve and enhance the antiproliferative effects of conventional cytotoxic agents.

Repression of G0/G1 traverse in human fibroblasts exposed to low levels of ionizing radiation

Quiescent cultures of human fibroblasts were exposed to levels of ionizing radiation sufficient to induce a transient growth delay, while causing only small decreases in long term clonogenicity. Following the mitogenic stimulation of damaged cells, cyclin D-associated kinase activity was induced to levels equivalent to those seen in control cultures. In addition, late G0/G1 E2F-dependent transcriptional and translational activity was observed in restimulated irradiated cells. However, cells became arrested prior to entry into S phase in a manner that paralleled the repression of cdk2-associated kinase activity. Cyclin A/cdk2-associated kinase activity was repressed in a biphasic manner following the irradiation of logarithmically growing cells. The initial rapid decline in activity to levels approximately 50% of those observed in control cultures occurred prior to increases in cellular levels of p21Cip1 protein, was not blocked by the addition of cycloheximide, and was not accompanied by alterations in cdk2 phosphotyrosine content. The subsequent repression to undetectable levels was coincident with the induction of p21Cip1 and was dependent on de novo protein synthesis. Only a subpopulation of cyclin A complexes were associated with p21Cip1 regardless of the magnitude of the repression of catalytic activity, although all cyclin A-cdk2-p21Cip1 complexes were inactive. These data suggest that temporally and functionally distinct mechanisms mediate the repression of cyclin-cdk activity in damaged cells. In addition, we present evidence that irradiated cells are competent to traverse S phase and arrest in G2 in the complete absence of cdk2-associated kinase activity.

Down-regulation of p27kip1 increases proliferation of progenitor cells in adult rats

Cyclin-dependent kinases (CDK) mediate the cell division cycle during G1 phase and CDK inhibitors negatively regulate the cell cycle. We investigated expression of the CDK inhibitor p27Kip1 and the effects of PI-3K/Akt on proliferation of progenitors in the subventricular zone of adult rat. Western blots and immunostaining revealed that 4 days after stroke, p27kip1 levels decreased and were absent in nuclei of the ipsilateral subventricular zone cells 7 days after stroke. Reduction in p27kip1 levels was concurrent with significant increases in BrdU immunoreactive cells in the subventricular zone. Treatment of stroke neurospheres with LY294002 significantly (p<0.05) reduced the numbers of neurospheres from 32 +/-5 in the stroke group to 14 +/- 3 in the stroke and LY294002 group. Collectively, our data suggest that stroke induced neural progenitor proliferation is mediated by down-regulation of p27Kip1 and activation of Akt.

Inhibition of JNK2 disrupts anaphase and produces aneuploidy in mammalian cells

The JNK family members JNK1 and JNK2 regulate tumor growth and are essential for transformation by oncogenes such as constitutively activated Ras. The mechanisms downstream of JNK that regulate cell cycle progression and transformation are unclear. Here we show that inhibition of JNK2, but not JNK1, with either a dominant-negative mutant, a pharmacological inhibitor, or RNA interference caused an accumulation of mammalian cells with 4N DNA content. When observed by immunofluorescence, these cells progressed to metaphase without apparent defects in spindle formation or chromosome alignment to the metaphase plate, suggesting that the 4N accumulation is a result of postmetaphase defects. Consistent with this prediction, when JNK activity was suppressed, we observed defects in central spindle formation and chromosome segregation during anaphase. In contrast, cyclin-dependent kinase 1 activity, cyclin B1 protein, and Polo-like kinase 1 protein turnover remained intact when JNK was inhibited. In addition, continued inhibition of JNK activity did not block reentry into subsequent cell cycles but instead resulted in polyploidy. This evidence suggests that JNK2 functions in maintaining the genomic stability of mammalian cells by signaling that is independent of cyclin-dependent kinase 1/cyclin B1 down-regulation.

The Centrosome in Higher Organisms: Structure, Composition, and Duplication

The centrosome found in higher organisms is an organelle with a complex and dynamic architecture and composition. This organelle not only functions as a microtubule-organizing center, but also is integrated with or impacts a number of cellular processes. Defects associated with this organelle have been linked to a variety of human diseases including several forms of cancer. Here we review the emerging picture of how the structure, composition, duplication, and function of the centrosome found in higher organisms are interrelated.

Back to the Future with Ubiquitin

Two papers published in 1984 by the Varshavsky laboratory revealed that the ubiquitin/proteasome pathway is the principal system for degradation of short-lived proteins in mammalian cells, setting the stage for future demonstrations of this pathway's many regulatory roles. This perspective discusses the impact of those papers and highlights some of the subsequent insights that have led to our current appreciation of the breadth of ubiquitin-mediated signaling.