Connexin43 phosphorylation at S368 is acute during S and G2/M and in response to protein kinase C activation

Phorbol esters such as 12-O-tetradeconylphorbol-13-acetate (TPA) activate protein kinase C, increase Connexin43 (Cx43) phosphorylation, and decrease cell-cell communication via gap junctions in many cell types. Previous work has implicated protein kinase C (PKC) in the direct phosphorylation of Cx43 at S368, which results in a change in single channel behavior that contributes to a decrease in intercellular communication. We have examined Cx43 phosphorylation in several cell lines with an antibody specific for phosphorylated S368. We show that this antibody detects Cx43 only when it is phosphorylated at S368 and, consistent with previous results, TPA treatment causes a dramatic increase in phosphorylation at S368. However, in some cell types, the increased phosphorylation at S368 did not cause a detectable shift in migration as compared with the nonphosphorylated Cx43. Immunofluorescence showed increased S368 immunolabeling in cytoplasmic and plasma membrane structures in response to TPA. Immunoblot analysis of synchronized cells showed increased phosphorylation at S368 during S and G2/M phases of the cell cycle. S-phase cells contained more total Cx43 but assembled fewer functional gap junctional channels than G0-phase cells. Since M-phase cells also communicate poorly and contain few assembled gap junctions, phosphorylation at S368 appears to be negatively correlated with gap junction assembly. Thus, both gap junctional communication and S368 phosphorylation change during S phase and G2/M, implying that phosphorylation at S368 might play a role in key cell-cycle events.

Colocalization of multiple DNA double-strand breaks at a single Rad52 repair centre

DNA double-strand break repair (DSBR) is an essential process for preserving genomic integrity in all organisms. To investigate this process at the cellular level, we engineered a system of fluorescently marked DNA double-strand breaks (DSBs) in the yeast Saccharomyces cerevisiae to visualize in vivo DSBR in single cells. Using this system, we demonstrate for the first time that Rad52 DNA repair foci and DSBs colocalize. Time-lapse microscopy reveals that the relocalization of Rad52 protein into a focal assembly is a rapid and reversible process. In addition, analysis of DNA damage checkpoint-deficient cells provides direct evidence for coordination between DNA repair and subsequent release from checkpoint arrest. Finally, analyses of cells experiencing multiple DSBs demonstrate that Rad52 foci are centres of DNA repair capable of simultaneously recruiting more than one DSB.

Centrosome number is controlled by a centrosome-intrinsic block to reduplication

The centrosome duplicates once in S phase. To determine whether there is a block in centrosome reduplication, we used a cell fusion assay to compare the duplication potential of unduplicated G1 centrosomes and recently duplicated G2 centrosomes. By fusing cells in different cell cycle stages, we found that G2 centrosomes were unable to reduplicate in a cellular environment that supports centrosome duplication. Furthermore, G2 cytoplasm did not inhibit centrosome duplication in fused cells, indicating that the block to reduplication is intrinsic to the centrosomes rather than the cytoplasm. To test the underlying mechanism, we created mononucleate G1 cells with two centrosomes by fusing cells with enucleated cytoplasts. Both centrosomes duplicated, indicating that the block is not controlled by centrosome:nucleus ratio. We also found that human primary cells have tight control over centrosome number during prolonged S-phase arrest and that this control is partially abrogated in transformed cells. This suggests a link between the control of centrosome duplication and maintenance of genomic stability.

Dual phosphorylation controls Cdc25 phosphatases and mitotic entry

Negative regulation of the Cdc25C protein phosphatase by phosphorylation on Ser 216, the 14-3-3-binding site, is an important regulatory mechanism used by cells to block mitotic entry under normal conditions and after DNA damage. During mitosis, Cdc25C is not phosphorylated on Ser 216 and ionizing radiation (IR) does not induce either phosphorylation of Ser 216, or binding to 14-3-3. Here, we show that Cdc25C is phosphorylated on Ser 214 during mitosis, which in turn prevents phosphorylation of Ser 216. Mutation of Ser 214 to Ala reconstitutes Ser 216 phosphorylation and 14-3-3 binding during mitosis. Introduction of exogenous Cdc25C(S214A) into HeLa cells depleted of endogenous Cdc25C results in a substantial delay to mitotic entry. This effect was fully reversed in a S214A/S216A double-mutant, implying that the inhibitory effect of S214A mutant was entirely dependent on Ser 216 phosphorylation. A similar regulatory mechanism may also apply to another mitotic phosphatase, Cdc25B, as well as mitotic phosphatases of other species, including Xenopus laevis. We propose that this pathway ensures that Cdc2 remains active once mitosis is initiated and is a key control mechanism for maintaining the proper order of cell-cycle transitions.

Mitotic entry: tipping the balance

In the eukaryotic cell cycle, the G2-M transition is regulated by a counteracting kinase/phosphatase mechanism. A novel player, Tome-1, has now been found to trigger entry into mitosis by interfering with this kinase/phosphatase 'switch'.

Cell adhesion to the extracellular matrix protein fibronectin modulates radiation-dependent G2 phase arrest involving integrin-linked kinase (ILK) and glycogen synthase kinase-3beta (GSK-3beta) in vitro

Cell adhesion to extracellular matrix (ECM) is thought to confer resistance against cell-damaging agents, that is, drugs and radiation, in tumour and normal cells in vitro. The dependence of cell survival on beta1-integrin-linked kinase (ILK), protein kinase Balpha/Akt (PKBalpha/Akt) and glycogen synthase kinase-3beta (GSK-3beta) activity, which participate in beta1-integrin signalling and cell cycle progression was investigated as a function of radiation exposure. Colony-formation assays on polystyrene, fibronectin (FN), laminin (LA), bovine serum albumin (BSA) or poly-L-lysine (poly-L) (0-8 Gy), kinase assays, flow cytometric DNA and annexin-V analysis and immunoblotting were performed in nonirradiated and irradiated (2 or 6 Gy) A549 human lung cancer cells and CCD32 normal human lung fibroblasts. Cell contact to FN in contrast to polystyrene elevated basal ILK, PKBalpha/Akt and GSK-3beta kinase activities in A549 and CCD32 cells, as well as the basal amount of A549 G2 phase cells. Irradiation on FN or LA as compared to polystyrene, BSA or poly-L significantly improved cell survival. Following irradiation, kinase activities were stimulated strongly on polystyrene but showed to be less prominent on FN, which was because of the FN-related basal induction. Following irradiation, FN compared to polystyrene enlarged and prolonged G2 arrest in both the cell lines. For the analysis of phosphatidylinositol-3 kinase (PI3-K) dependence of protein kinases and cell cycle transition, the PI3-K inhibitors LY294002 and wortmannin were used showing decreased kinase activities, antiproliferative and radiation-dependent G2 accumulation-abrogating effects accompanied by downregulation of cyclin D1 and phospho-pRb in cells attached to polystyrene. Fibronectin partly abrogated these effects PI3-K-independently. These findings suggest a novel pathway that makes direct phosphorylation of GSK-3beta by ILK feasible after irradiation. Conclusively, the data indicate that ILK, PKBalpha/Akt and GSK-3beta are involved in modulations of the cell cycle after irradiation. These interactions are strictly dependent on ECM components in a cell line-specific manner. Our findings provide molecular insights into mechanisms likely to be important for ECM-dependent cell survival and cellular radioresistance as well as tumour growth.

Dependence of pre-mRNA introns on PRP17, a non-essential splicing factor: implications for efficient progression through cell cycle transitions

Saccharomyces cerevisiae PRP17 (CDC40) encodes a second-step pre-mRNA splicing factor with a role in cell division. The functions of Prp17 in specific cell cycle transitions were examined using temperature-sensitive alleles in arrest/release experiments. We find that G(1)/S and G(2)/M transitions depend on Prp17. G(1)-synchronized prp17::LEU2 cells arrest at non-permissive temperatures as unbudded haploid cells with low levels of CLN1, CLB5 and RNR1 transcripts. This indicates a Prp17 execution point at or prior to Start. Reduced levels of alpha-tubulin protein, a mitotic spindle component, underlie the benomyl sensitivity of prp17 mutants and possibly their G(2)/M arrest. Splicing of TUB1 and TUB3 transcripts, which encode alpha-tubulin, was analyzed in prp17 and other second-step factor mutants. TUB1 splicing is inefficient in prp17, prp16 and prp22, and marginally affected in prp18, slu7-1 and psf1-1. TUB3 splicing is similarly affected. In vitro splicing with TUB3 pre-mRNA demonstrates a compromised second step in prp17::LEU2 extracts, implicating a direct role for Prp17 in its efficient splicing. Genomic replacement of an intronless TUB1 gene relieves the benomyl sensitivity of prp17 mutants; however, they remain temperature sensitive, implying multiple limiting factors for mitosis. The data suggest that integration of splicing with the cell cycle is important for G(1)/S and G(2)/M transitions.

Specific G2 arrest of caprine cells infected with a caprine arthritis encephalitis virus expressing vpr and vpx genes from simian immunodeficiency virus

Primate lentivirus (HIV and SIV) vpr accessory genes encode 12- to 14-kDa proteins which induce cell cycle arrest at the G2 phase of infected cells, preventing them from going through mitosis. Members of the HIV-2/SIVmac/SIVsmm group also encode a second closely related accessory protein called Vpx. Vpx and HIV Vpr are critical for virus replication in nondividing cells due to their participation in nuclear import of the preintegration complex. Caprine arthritis encephalitis virus (CAEV) and maedi visna virus are the natural lentiviruses of domestic goat and sheep, respectively, and their genomes do not carry vpr and vpx genes. In this study, we generated chimeric CAEV-based genomes carrying vpr and vpx genes from SIVmac239 and tested their ability to induce G2 cell cycle arrest in infected caprine cells. CAEV-pBSCAvpxvpr is the chimeric genome that was shown to be infectious and replication competent. Our data demonstrated that CAEV-pBSCAvpxvpr-infected goat synovial membrane cell monolayer developed more cytopathic effects and a high proportion of cells remained in the G2 phase of cell cycle. This G2 arrest was observed both at the early and at the late stages of infection, while minimal effect was observed with the parental CAEV-pBSCA. These results, described for the first time in mammalian cells other than those of primates, indicate that Vpr-induced G2 cell cycle arrest is not restricted to only primate cells. Thus, conservation of Vpx/Vpr protein functions in caprine cells suggests a possible role for these proteins in the virus life cycle and its ability to adapt to new hosts. The data presented here thus raise a pertinent question about the biological significance of the conservation of Vpr and Vpx functions in caprine cells despite the high phylogenic distance between primates and small ruminants.

HCaRG is a novel regulator of renal epithelial cell growth and differentiation causing G2M arrest

We recently identified a novel calcium-regulated gene, HCaRG, that is highly expressed in the kidney and maps to a chromosomal locus determining kidney weight in rats. The mRNA levels of HCaRG negatively correlate with the proliferative status of the kidney cells. To investigate its role in renal epithelial cellular growth directly, we studied the human embryonic kidney cell line (HEK-293) stably transfected with either plasmid alone or plasmid containing rat HCaRG. [(3)H]thymidine incorporation was significantly lower in HCaRG clones. Although HCaRG clones exhibited some enhanced susceptibility to cell death, this was not the primary mechanism of reduced proliferation. Cell cycle analysis revealed a G(2)M phase accumulation in HCaRG clones that was associated with upregulation of p21(Cip1/WAF1) and downregulation of p27(Kip1). HCaRG clones had a greater protein content, larger cell size, and released 4.5- to 8-fold more of an atrial natriuretic peptide-like immunoreactivity compared with controls. In addition, HCaRG clones demonstrated the presence of differentiated junctions and a lower incidence of mitotic figures. Genistein treatment of wild-type HEK-293 cells mimicked several phenotypic characteristics associated with HCaRG overexpresssion, including increased cell size and increased release of atrial natriuretic peptide. Taken together, our results suggest that HCaRG is a regulator of renal epithelial cell growth and differentiation causing G(2)M cell cycle arrest.

Cyclin D1 serves as a cell cycle regulatory switch in actively proliferating cells

Much of our current understanding of the cell cycle involves analyses of its induction in quiescent cells. To better understand the control of cell cycle propagation and termination, studies have been performed in actively cycling cultures using time-lapse photography and quantitative image analysis. These studies reveal a highly ordered sequence of events required for promotion of continued proliferation. The decision to continue cell cycle progression takes place in G2 phase, when cellular Ras induces the elevation of cyclin D1 levels. These levels are maintained through G1 phase and are required for the initiation of S phase, at which time cyclin D1 levels are automatically reduced to low levels. The reduction of cyclin D1 to low levels during S phase is required for DNA synthesis, and forces the cell to induce high cyclin D1 levels once again when it enters G2 phase. In this way, cyclin D1 is proposed to serve as an active switch in the regulation of continued cell cycle progression.

p14ARF induces G2 arrest and apoptosis independently of p53 leading to regression of tumours established in nude mice

Until recently, the ability of ARF (human p14(ARF), murine p19(ARF)) tumour-suppressor protein, encoded by the INK4A/ARF locus, to inhibit cell growth in response to various stimuli was related to its ability to stabilize p53 through the so-called ARF/MDM2/p53 pathway. However, recent data have demonstrated that ARF is not implicated in this unique p53-dependent pathway. By use of transient and stable expression, we show here that human p14(ARF) inhibits the growth of human tumoral cells lacking functional p53 by inducing a transient G(2) arrest and subsequently apoptosis. This p14(ARF)-induced G(2) arrest was correlated with inhibition of CDC2 activity, inactivation of CDC25C phosphatase and induction of the CDK inhibitor p21(WAFI). Apoptosis was demonstrated using Hoechst 33352 staining, proteolytic activation of caspase-3 and PARP cleavage. Similar results were obtained in experiments with cells synchronized by hydroxyurea block. Importantly, we were able to reproduce these effects 'in vivo' by showing that p14(ARF) inhibits the growth of p53 nullizygous human tumours in nude mice and induces the regression of p53 -/- established tumours. In these experiments, tumoral regression was associated with inhibition of cell proliferation as well as induction of apoptosis confirming the data obtained in cell lines.

The ATM-related Tel1 protein of Saccharomyces cerevisiae controls a checkpoint response following phleomycin treatment

MEC1 and TEL1 encode ATR- and ATM-related proteins in the budding yeast Saccharomyces cerevisiae, respectively. Phleomycin is an agent that catalyzes double-strand breaks in DNA. We show here that both Mec1 and Tel1 regulate the checkpoint response following phleomycin treatment. MEC1 is required for Rad53 phosphorylation and cell-cycle progression delay following phleomycin treatment in G1, S or G2/M phases. The tel1Delta mutation confers a defect in the checkpoint responses to phleomycin treatment in S phase. In addition, the tel1Delta mutation enhances the mec1 defect in activation of the phleomycin-induced checkpoint pathway in S phase. In contrast, the tel1Delta mutation confers only a minor defect in the checkpoint responses in G1 phase and no apparent defect in G2/M phase. Methyl methanesulfonate (MMS) treatment also activates checkpoints, inducing Rad53 phosphorylation in S phase. MMS-induced Rad53 phosphorylation is not detected in mec1Delta mutants during S phase, but occurs in tel1Delta mutants similar to wild-type cells. Finally, Xrs2 is phosphorylated after phleomycin treatment in a TEL1-dependent manner during S phase, whereas no significant Xrs2 phosphorylation is detected after MMS treatment. Together, our results support a model in which Tel1 contributes to checkpoint control in response to phleomycin-induced DNA damage in S phase.

A novel RING finger protein, human enhancer of invasion 10, alters mitotic progression through regulation of cyclin B levels

The process of cellular morphogenesis is highly conserved in eukaryotes and is dependent upon the function of proteins that are centrally involved in specification of the cell cycle. The human enhancer of invasion clone 10 (HEI10) protein was identified from a HeLa cell library based on its ability to promote yeast agar invasion and filamentation. Through two-hybrid screening, the mitotic cyclin B1 and an E2 ubiquitin-conjugating enzyme were isolated as HEI10-interacting proteins. Mutation of the HEI10 divergent RING finger motif (characteristic of E3 ubiquitin ligases) and Cdc2/cyclin binding and phosphorylation sites alter HEI10-dependent yeast phenotypes, including delay in G(2)/M transition. In vertebrates, the addition of HEI10 inhibits nuclear envelope breakdown and mitotic entry in Xenopus egg extracts. Mechanistically, HEI10 expression reduces cyclin B levels in cycling Xenopus eggs and reduces levels of the cyclin B ortholog Clb2p in yeast. HEI10 is itself a specific in vitro substrate of purified cyclin B/cdc2, with a TPVR motif as primary phosphorylation site. Finally, HEI10 is itself ubiquitinated in egg extracts and is also autoubiquitinated in vitro. These and other points lead to a model in which HEI10 defines a divergent class of E3 ubiquitin ligase, functioning in progression through G(2)/M.

Fanconi anemia protein complex is a novel target of the IKK signalsome

Fanconi anemia (FA), a genetic disorder predisposing to aplastic anemia and cancer, is characterized by hypersensitivity to DNA-damaging agents and oxidative stress. Five of the cloned FA proteins (FANCA, FANCC, FANCE, FANCF, FANCG) appear to be involved in a common functional pathway that is required for the monoubiquitination of a sixth gene product, FANCD2. Here, we report that FANCA associates with the IkappaB kinase (IKK) signalsome via interaction with IKK2. Components of the FANCA complex undergo rapid, stimulus-dependent changes in phosphorylation, which are blocked by kinase-inactive IKK2 (IKK2 K > M). When exposed to mitomycin C, cells expressing IKK2 K > M develop a cell cycle abnormality characteristic of FA. Thus, FANCA may function to recruit IKK2, thus providing the cell a means of rapidly responding to stress.

Direct interaction with and activation of p53 by SMAR1 retards cell-cycle progression at G2/M phase and delays tumor growth in mice

The tumor-suppressor p53 is a multifunctional protein mainly responsible for maintaining genomic integrity. p53 induces its tumor-suppressor activity by either causing cell-cycle arrest (G(1)/S or G(2)/M) or inducing cells to undergo apoptosis. This function of wild-type p53 as "guardian of the genome" is presumably achieved by forming molecular complexes with different DNA targets as well as by interacting with a number of cellular proteins, e.g., Mdm2, Gadd45, p21, 14-3-3sigma, Bax and Apaf-1. Upon activation, p53 activates p21, which in turn controls the cell cycle by regulating G(1) or G(2) checkpoints. Here, we report SMAR1 as one such p53-interacting protein that is involved in delaying tumor progression in vivo as well as in regulating the cell cycle. SMAR1 is a newly identified MARBP involved in chromatin-mediated gene regulation. The SMAR1 gene encodes at least 2 alternatively spliced variants: SMAR1(L) (the full-length form) and SMAR1(S) (the shorter form). We report that expression of SMAR1(S), but not of SMAR1(L), mRNA was decreased in most of the human cell lines examined, suggesting selective silencing of SMAR1(S). Overexpression of SMAR1(S) in mouse melanoma cells (B16F1) and their subsequent injection in C57BL/6 mice delays tumor growth. Exogenous SMAR1(S) causes significant retardation of B16F1 cells in the G(2)/M phase of the cell cycle compared to SMAR1(L). SMAR1(S) activates p53-mediated reporter gene expression in mouse melanoma cells, breast cancer cells (MCF-7) and p53 null cells (K562), followed by activation of its downstream effector, p21. We further demonstrate that SMAR1 physically interacts and colocalizes with p53. These data together suggest that SMAR1 is the only known MARBP that delays tumor progression via direct activation and interaction with tumor-suppressor p53.

Tip60 acetyltransferase activity is controlled by phosphorylation

Here we show that the phosphorylation of histone acetyltransferase Tip60, a target of human immunodeficiency virus, type 1-encoded transactivator Tat, plays a crucial role in the control of its catalytic activity. Baculovirus-based expression and purification of Tip60 combined with mass spectrometry allowed the identification of serines 86 and 90 as two major sites of phosphorylation in vivo. The phosphorylation of Tip60 was found to modulate its histone acetyltransferase activity. One of the identified phosphorylated serines, Ser-90, was within a consensus cyclin B/Cdc2 site. Ser-90 was specifically phosphorylated in vitro by the cyclin B/Cdc2 complex. Accordingly, the phosphorylation of Tip60 was enhanced after drug-induced arrest of cells in G(2)/M. This G(2)/M-dependent phosphorylation of Tip60 was abolished by treating cells with a specific inhibitor of the cyclin-dependent kinase, roscovitin. All together, these results strongly suggest a G(2)/M-dependent control of Tip60 activity.

Helicobacter pylori inhibits activity of cdc2 kinase and delays G2/M to G1 progression in gastric adenocarcinoma cell line

BACKGROUND

Helicobacter pylori is a bacterial pathogen strongly associated with ulcer diseases and gastric cancer. The bacterial-induced alteration of cell-cycle control in host cells may play a role in the pathogenetic mechanisms. The aims of this study were to define the effect of H. pylori on the G2/M to G1 transition in a gastric cell line.

METHODS

Cultured gastric cells, AGS, were synchronized in the S/early G2 phase and treated with intact H. pylori. The cell-cycle distribution of AGS cells was determined by flow cytometry. The activity of cdc2 kinase, as well as of some parameters that affect the kinase activity, was also examined.

RESULTS

H. pylori delays cell-cycle progression at the G2/M phase in AGS cells. The G2/M delay was associated with reduced activity of cdc2 kinase. Both down-regulation of cell-cycle regulators (p34cdc2, cyclin B1 and cdc25C) and decreased association between p34cdc2 and cyclin B1 were found to be associated with the activity of cdc2 kinase abated after the H. pylori infection. In addition, the H. pylori-induced G2/M delay required direct contact between the bacteria and host cells.

CONCLUSIONS

H. pylori inhibits G2/M to G1 progression and causes a reduction of cell division in gastric epithelial cells.

Activation of the G2 cell cycle checkpoint enhances survival of epithelial cells exposed to hyperoxia

Reactive oxygen species produced during hyperoxia damage DNA, inhibit proliferation in G1- through p53-dependent activation of p21(Cip1/WAF1/Sdi1), and kill cells. Because checkpoint activation protects cells from genotoxic stress, we investigated cell proliferation and survival of the murine type II epithelial cell line MLE15 during hyperoxia. These cells were chosen for study because they express Simian large and small-T antigens, which transform cells in part by disrupting the p53-dependent G1 checkpoint. Cell counts, 5-bromo-2'-deoxyuridine labeling, and flow cytometry revealed that hyperoxia slowed cell cycle progression after one replication, resulting in a pronounced G2 arrest by 72 h. Addition of caffeine, which inactivates the G2 checkpoint, diminished the percentage of hyperoxic cells in G2 and increased the percentage in sub-G1 and G1. Abrogation of the G2 checkpoint was associated with enhanced oxygen-induced DNA strand breaks and cell death. Caffeine did not affect DNA integrity or viability of cells exposed to room air. Similarly, caffeine abrogated the G2 checkpoint in hyperoxic A549 epithelial cells and enhanced oxygen-induced toxicity. These data indicate that hyperoxia rapidly inhibits proliferation after one cell cycle and that the G2 checkpoint is critical for limiting DNA damage and cell death.

Critical role for mouse Hus1 in an S-phase DNA damage cell cycle checkpoint

Mouse Hus1 encodes an evolutionarily conserved DNA damage response protein. In this study we examined how targeted deletion of Hus1 affects cell cycle checkpoint responses to genotoxic stress. Unlike hus1(-) fission yeast (Schizosaccharomyces pombe) cells, which are defective for the G(2)/M DNA damage checkpoint, Hus1-null mouse cells did not inappropriately enter mitosis following genotoxin treatment. However, Hus1-deficient cells displayed a striking S-phase DNA damage checkpoint defect. Whereas wild-type cells transiently repressed DNA replication in response to benzo(a)pyrene dihydrodiol epoxide (BPDE), a genotoxin that causes bulky DNA adducts, Hus1-null cells maintained relatively high levels of DNA synthesis following treatment with this agent. However, when treated with DNA strand break-inducing agents such as ionizing radiation (IR), Hus1-deficient cells showed intact S-phase checkpoint responses. Conversely, checkpoint-mediated inhibition of DNA synthesis in response to BPDE did not require NBS1, a component of the IR-responsive S-phase checkpoint pathway. Taken together, these results demonstrate that Hus1 is required specifically for one of two separable mammalian checkpoint pathways that respond to distinct forms of genome damage during S phase.

Downregulation of uPA inhibits migration and PI3k/Akt signaling in glioblastoma cells

The ability of glioma cells to migrate great distances from a primary tumor mass is the primary cause of tumor recurrence. The urokinase-type plasminogen activator (uPA) is a serine protease that can initiate proteolytic cascades, which result in remodeling of extracellular matrix and basement membrane, allowing cells to move across and through these barriers. The binding between uPA and its receptor uPAR also mediates several signaling events that seem to contribute to the evolution of a migratory phenotype. In this study, we determined how the downregulation of uPA affects the signaling pathways leading to cell migration. Stably transfecting human glioblastoma cells with antisense uPA decreased the amount of cell-bound uPA and disrupted actin cytoskeleton formation and cell migration. The phosphatidylinositol 3-kinase (PI3k) and Akt signaling pathway has been suggested to mediate migration in various cancer cells. The antisense-uPA clones also had less phosphorylated PI3k and Akt than control cells, a finding associated with decreased cell migration, G2/M-phase arrest, and decreased clonogenic survival. Decreased activation of PI3k and the antiapoptotic factor Akt was not sufficient to induce apoptosis in the antisense-uPA clones, but staurosporine sensitized them to apoptosis to a greater extent than control cells. These results indicate that PI3k/Akt pathway is involved in the signaling cascade required to induce cell migration and that uPA has a direct role in regulating migration.

E2F mediates sustained G2 arrest and down-regulation of Stathmin and AIM-1 expression in response to genotoxic stress

Exposure of cells to genotoxic agents results in activation of checkpoint pathways leading to cell cycle arrest. These arrest pathways allow repair of damaged DNA before its replication and segregation, thus preventing accumulation of mutations. The tumor suppressor retinoblastoma (RB) is required for the G(1)/S checkpoint function. In addition, regulation of the G(2) checkpoint by the tumor suppressor p53 is RB-dependent. However, the molecular mechanism underlying the involvement of RB and its related proteins p107 and p130 in the G(2) checkpoint is not fully understood. We show here that sustained G(2)/M arrest induced by the genotoxic agent doxorubicin is E2F-dependent and involves a decrease in expression of two mitotic regulators, Stathmin and AIM-1. Abrogation of E2F function by dominant negative E2F abolishes the doxorubicin-induced down-regulation of Stathmin and AIM-1 and leads to premature exit from G(2). Expression of the E7 papilloma virus protein, which dissociates complexes containing E2F and RB family members, also prevents the down-regulation of these mitotic genes and leads to premature exit from G(2) after genotoxic stress. Furthermore, genotoxic stress increases the levels of nuclear E2F-4 and p130 as well as their in vivo binding to the Stathmin promoter. Thus, functional complexes containing E2F and RB family members appear to be essential for repressing expression of critical mitotic regulators and maintaining the G(2)/M checkpoint.

Complexity of the mechanisms of initiation and maintenance of DNA damage-induced G2-phase arrest and subsequent G1-phase arrest: TP53-dependent and TP53-independent roles

Through a detailed study of cell cycle progression, protein expression, and kinase activity in gamma-irradiated synchronized cultures of human skin fibroblasts, distinct mechanisms of initiation and maintenance of G2-phase and subsequent G1-phase arrests have been elucidated. Normal and E6-expressing fibroblasts were used to examine the role of TP53 in these processes. While G2 arrest is correlated with decreased cyclin B1/CDC2 kinase activity, the mechanisms associated with initiation and maintenance of the arrest are quite different. Initiation of the transient arrest is TP53-independent and is due to inhibitory phosphorylation of CDC2 at Tyr15. Maintenance of the G2 arrest is dependent on TP53 and is due to decreased levels of cyclin B1 mRNA and a corresponding decline in cyclin B1 protein level. After transiently arresting in G2 phase, normal cells chronically arrest in the subsequent G1 phase while E6-expressing cells continue to cycle. The initiation of this TP53-dependent G1-phase arrest occurs despite the presence of substantial levels of cyclin D1/CDK4 and cyclin E/CDK2 kinase activities, hyperphosphoryated RB, and active E2F1. CDKN1A (also known as p21(WAF1/CIP1)) levels remain elevated during this period. Furthermore, CDKN1A-dependent inhibition of PCNA activity does not appear to be the mechanism for this early G1 arrest. Thus the inhibition of entry of irradiated cells into S phase does not appear to be related to DNA-bound PCNA complexed to CDKN1A. The mechanism of chronic G1 arrest involves the down-regulation of specific proteins with a resultant loss of cyclin E/CDK2 kinase activity.

Cdc2 Expression in Primary Thyroid Lymphoma: Its Relationship with Biological Aggressiveness and G2 Cyclins

Previous studies have demonstrated that the G2-M modulators contribute to the progression of human neoplasms. In this study, we investigated the expression of these modulators, cyclin-dependent kinase 1 (cdc2), and G2 cyclins, cyclin A and cyclin B1, in primary thyroid lymphoma. Cdc2 immunoexpression was observed in 51.0% of the 49 cases examined and was related to grade of malignancy, high Ki-67 labeling index, and aberrant p53 expression. The incidences of immunoexpression of cyclin A and cyclin B1 were 63.3% and 40.9%, respectively, and they were also related to the above three parameters. Furthermore, a correlation was found between the immunoexpression of cdc2 and G2 cyclins. These findings suggest that the cdc2 and G2 cyclins play an important role in the progression of thyroid malignant lymphoma.

Distinct Domains of Human CDC5 Direct Its Nuclear Import and Association with the Spliceosome

Genetic studies have shown that CDC5 proteins are essential for G2 progression and mitotic entry. CDC5 homologs in yeast and mammals are essential for pre-messenger ribonucleic acid (mRNA) processing. Other gene products also have been shown to play roles in both pre-mRNA splicing and cell cycle regulation, prompting the description of several models to explain the mechanism(s) linking these two processes. In this study, we demonstrate that the amino-terminus of human CDC5 directs nuclear import independent of consensus nuclear localization signals or phosphorylation, and that the carboxyl-terminus of human CDC5 preferentially associates with spliceosomal complexes in proximity of RNA transcription during interphase. hCDC5 colocalizes with Sm proteins in a cell cycle- and domain-dependent manner, and several proteins in the human CDC5-associated complex are identified that suggest potential roles for the complex in coupling pre-mRNA splicing to transcriptional activation and protein translation. These results indicate that human CDC5 may function in pre-mRNA processing and cell cycle progression through more than one mechanism.

Ectopic expression of cell cycle markers in models of induced programmed cell death in dopamine neurons of the rat substantia nigra pars compacta

There is increasing evidence that proteins normally involved in the cell cycle can regulate neuronal programmed cell death (PCD). However, it remains unknown whether cell cycle markers are expressed in normal, postmitotic, postmigratory neurons undergoing PCD in vivo. We have previously shown that natural cell death occurs postnatally in dopamine neurons of the substantia nigra pars compacta (SNpc). PCD can be induced postnatally in these neurons either by intrastriatal injection of the neurotoxin 6-hydroxydopamine (6-OHDA) or by medial forebrain bundle (MFB) axotomy. At the time of induction of death in these models, these neurons are long postmitotic and postmigratory. We have studied three cell cycle markers in these models: 5-bromo-2'-deoxyuridine (BrdU) incorporation (a marker of S phase), cdc2 protein expression (a marker of G2 phase), and expression of MPM2 (a marker of M phase), an epitope phosphorylated by cdc2. We report here that postmitotic dopaminergic neurons undergoing PCD in the SNpc following 6-OHDA and axotomy lesions incorporate BrdU and overexpress cdc2, but do not express MPM2. This is the first in vivo evidence that postmitotic dopamine neurons of the SNpc undergoing apoptosis express markers for S phase and G2 phase. These results raise the possibility that cell cycle regulatory proteins may play a role in the demise of dopaminergic neurons in Parkinson's disease, in which PCD has been postulated to play a role.