Distinct effects of mitogens and the actin cytoskeleton on CREB and pocket protein phosphorylation control the extent and timing of cyclin A promoter activity

Human cytomegalovirus riding the cell cycle

Human cytomegalovirus (HCMV) infection modulates the host cell cycle to create an environment that is optimal for viral gene expression, DNA replication, and production of infectious virus. The virus mostly infects quiescent cells and thus must push the cell into G1 phase of the cell cycle to co-opt the cellular mechanisms that could be used for DNA synthesis. However, at the same time, cellular functions must be subverted such that synthesis of viral DNA is favored over that of the host. The molecular mechanisms by which this is accomplished include altered RNA transcription, changes in the levels and activity of cyclin-dependent kinases, and other proteins involved in cell cycle control, posttranslational modifications of proteins, modulation of protein stability through targeted effects on the ubiquitin-proteasome degradation pathway, and movement of proteins to different cellular locations. When the cell is in the optimal G0/G1 phase, multiple signaling pathways are altered to allow rapid induction of viral gene expression once negative factors have been eliminated. For the most part, the cell cycle will stop prior to initiation of host cell DNA synthesis (S phase), although many cell cycle proteins characteristic of the S/G2/M phase accumulate. The environment of a cell progressing through the cell cycle and dividing is not favorable for viral replication, and HCMV has evolved ways to sense whether cells are in S/G2 phase, and if so, to prevent initiation of viral gene expression until the cells cycle back to G1. A major target of HCMV is the anaphase-promoting complex E3 ubiquitin ligase, which is responsible for the ubiquitination and subsequent degradation of cyclins A and B and other cell cycle proteins at specific phases in the cell cycle. This review will discuss the effects of HCMV infection on cell cycle regulatory pathways, with the focus on selected viral proteins that are responsible for these effects.

MAPK uncouples cell cycle progression from cell spreading and cytoskeletal organization in cycling cells

Integrin-mediated cytoskeletal tension supports growth-factor-induced proliferation, and disruption of the actin cytoskeleton in growth factor-stimulated cells prevents the re-expression of cyclin D and cell cycle re-entry from quiescence. In contrast to cells that enter the cell cycle from G0, cycling cells continuously express cyclin D, and are subject to major cell shape changes during the cell cycle. Here, we investigated the cell cycle requirements for cytoskeletal tension and cell spreading in cycling mammalian cells that enter G1-phase from mitosis. Disruption of the actin cytoskeleton at progressive time-points in G1-phase induced cell rounding, FA disassembly, and attenuated both integrin signaling and growth factor-induced p44/p42 mitogen-activated protein kinase activation. Although cyclin D expression was reduced, the expression of cyclin A and entry into S-phase were not affected. Moreover, expression of cyclin B1, progression through G2- and M-phase, and commitment to a new cell cycle occurred normally. In contrast, cell cycle progression was strongly prevented by inhibition of MAPK activity in G1-phase, whereas cell spreading, cytoskeletal organization, and integrin signaling were not impaired. MAPK inhibition also prevented cytoskeleton-independent cell cycle progression. Thus, these results uncouple the requirements for cell spreading and cytoskeletal organization from MAPK signaling, and show that cycling mammalian cells can proliferate independently of actin stress fibers, focal adhesions, or cell spreading, as long as a threshold level of MAPK activity is sustained.

Attachment of HeLa cells during early G1 phase

Both growth factor directed and integrin dependent signal transduction were shown to take place directly after completion of mitosis. The local activation of these signal transduction cascades was investigated in early G1 cells. Interestingly, various key signal transduction proteins were found in blebs at the cell membrane within 30 min after mitosis. These membrane blebs appeared in round, mitotic-like cells and disappeared rapidly during spreading of the cells in G1 phase. In addition to tyrosine-phosphorylated proteins, the blebs contained also phosphorylated FAK and phosphorylated MAP kinase. The formation of membrane blebs in round, mitotic cells before cell spreading is not specific for mitotic cells, because similar features were observed in trypsinized cells. Just before cell spreading also these cells exhibited membrane blebs containing active signal transduction proteins. Inhibition of signal transduction did not affect membrane bleb formation, suggesting that the membrane blebs were formed independent of signal transduction.

The cell cycle is a redox cycle: linking phase-specific targets to cell fate

Reactive oxygen species (ROS) regulate the strength and duration of signaling through redox-dependent signal transduction pathways via the cyclic oxidation/reduction of cysteine residues in kinases, phosphatases, and other regulatory factors. Signaling circuits may be segregated in organelles or other subcellular domains with distinct redox states, permitting them to respond independently to changes in the oxidation state of two major thiol reductants, glutathione and thioredoxin. Studies in yeast, and in complex eukaryotes, show that oscillations in oxygen consumption, energy metabolism, and redox state are intimately integrated with cell cycle progression. Because signaling pathways play specific roles in different phases of the cell cycle and the hierarchy of redox-dependent regulatory checkpoints changes during cell cycle progression, the effects of ROS on cell fate vary during the cell cycle. In G1, ROS stimulate mitogenic pathways that control the activity of cyclin-dependent kinases (CDKs) and phosphorylation of the retinoblastoma protein (pRB), thereby regulating S-phase entry. In response to oxidative stress, Nrf2 and Foxo3a promote cell survival by inducing the expression of antioxidant enzymes and factors involved in cell cycle withdrawal, such as the cyclin-dependent kinase inhibitor (CKI) p27. In S phase, ROS induce S-phase arrest via PP2A-dependent dephosphorylation of pRB. In precancerous cells, unconstrained mitogenic signaling by activated oncogenes induces replication stress in S phase, which activates the DNA-damage response and induces cell senescence. A number of studies suggest that interactions of ROS with the G1 CDK/CKI network play a fundamental role in senescence, which is considered a barrier to tumorigenesis. Adaptive responses and loss of checkpoint proteins such as p53 and p16(INK4a) allow tumor cells to tolerate constitutive mitogenic signaling and enhanced production of ROS, leading to altered redox status in many fully transformed cells. Alterations in oxidant and energy metabolism of cancer cells have emerged as fertile ground for new therapeutic targets. The present challenge is to identify redox-dependent targets relevant to each cell cycle phase, to understand how these targets control fate decisions, and to describe the mechanisms that link metabolism to cell cycle progression.

Tumor necrosis factor-alpha upregulates 11beta-hydroxysteroid dehydrogenase type 1 expression by CCAAT/enhancer binding protein-beta in HepG2 cells

The enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) catalyzes the conversion of inactive to active glucocorticoids. 11beta-HSD1 plays a crucial role in the pathogenesis of obesity and controls glucocorticoid actions in inflammation. Several studies have demonstrated that TNF-alpha increases 11beta-HSD1 mRNA and activity in various cell models. Here, we demonstrate that mRNA and activity of 11beta-HSD1 is increased in liver tissue from transgenic mice overexpressing TNF-alpha, indicating that this effect also occurs in vivo. To dissect the molecular mechanism of this increase, we investigated basal and TNF-alpha-induced transcription of the 11beta-HSD1 gene (HSD11B1) in HepG2 cells. We found that TNF-alpha acts via p38 MAPK pathway. Transient transfections with variable lengths of human HSD11B1 promoter revealed highest activity with or without TNF-alpha in the proximal promoter region (-180 to +74). Cotransfection with human CCAAT/enhancer binding protein-alpha (C/EBPalpha) and C/EBPbeta-LAP expression vectors activated the HSD11B1 promoter with the strongest effect within the same region. Gel shift and RNA interference assays revealed the involvement of mainly C/EBPalpha, but also C/EBPbeta, in basal and only of C/EBPbeta in the TNF-alpha-induced HSD11B1 expression. Chromatin immunoprecipitation assay confirmed in vivo the increased abundance of C/EBPbeta on the proximal HSD11B1 promoter upon TNF-alpha treatment. In conclusion, C/EBPalpha and C/EBPbeta control basal transcription, and TNF-alpha upregulates 11beta-HSD1, most likely by p38 MAPK-mediated increased binding of C/EBPbeta to the human HSD11B1 promoter. To our knowledge, this is the first study showing involvement of p38 MAPK in the TNF-alpha-mediated 11beta-HSD1 regulation, and that TNF-alpha stimulates enzyme activity in vivo.

Subversion of cell cycle regulatory pathways

Human cytomegalovirus (HCMV) has evolved numerous strategies to commandeer the host cell for producing viral progeny. The virus manipulates host cell cycle pathways from the early stages of infection to stimulate viral DNA replication at the expense of cellular DNA synthesis. At the same time, cell cycle checkpoints are by-passed, preventing apoptosis and allowing sufficient time for the assembly of infectious virus.

CREB is a critical regulator of normal hematopoiesis and leukemogenesis

The cAMP-responsive element binding protein (CREB) is a 43-kDa nuclear transcription factor that regulates cell growth, memory, and glucose homeostasis. We showed previously that CREB is amplified in myeloid leukemia blasts and expressed at higher levels in leukemia stem cells from patients with myeloid leukemia. CREB transgenic mice develop myeloproliferative disease after 1 year, but not leukemia, suggesting that CREB contributes to but is not sufficient for leukemogenesis. Here, we show that CREB is most highly expressed in lineage negative hematopoietic stem cells (HSCs). To understand the role of CREB in hematopoietic progenitors and leukemia cells, we examined the effects of RNA interference (RNAi) to knock down CREB expression in vitro and in vivo. Transduction of primary HSCs or myeloid leukemia cells with lentiviral CREB shRNAs resulted in decreased proliferation of stem cells, cell- cycle abnormalities, and inhibition of CREB transcription. Mice that received transplants of bone marrow transduced with CREB shRNA had decreased committed progenitors compared with control mice. Mice injected with Ba/F3 cells expressing either Bcr-Abl wild-type or T315I mutation with CREB shRNA had delayed leukemic infiltration by bioluminescence imaging and prolonged median survival. Our results suggest that CREB is critical for normal myelopoiesis and leukemia cell proliferation.

Focal adhesion signaling and actin stress fibers are dispensable for progression through the ongoing cell cycle

Prevention of cell spreading or disruption of actin filaments inhibits growth factor stimulated cell cycle re-entry from quiescence, mainly because of a failure to induce cyclin D expression. Ectopic cyclin D expression overrules anchorage-dependency, suggesting that cell spreading per se is not required as long as cyclin D is otherwise induced. We investigated whether cyclin D expression in cells exiting mitosis is sufficient to drive morphology-independent cell cycle progression in continuously cycling (i.e. not quiescent) cells. Disruption of post-mitotic actin reorganization did not affect substratum reattachment but abolished the formation of filopodia, lamellipodia and ruffles, as well as stress fiber organization, focal adhesion assembly and cell spreading. Furthermore, integrin-mediated focal adhesion kinase (FAK) autophosphorylation and growth factor stimulated p42/p44 mitogen activated protein kinase (MAPK) activation were inhibited. Despite a progressive loss of cyclin D expression in late G1, cyclin E and cyclin A were normally induced. In addition, cells committed to DNA synthesis and completed their entire cycle. Our results demonstrate that post-mitotic disruption of the actin cytoskeleton allows cell cycle progression independent of focal adhesion signaling, cytoskeletal organization and cell shape, presumably because pre-existing cyclin D levels are sufficient to drive cell cycle progression at the M-G1 border.

The mitogenic function of hepatitis B virus X protein resides within amino acids 51 to 140 and is modulated by N- and C-terminal regulatory regions

The hepatitis B virus (HBV) X protein (pX) is implicated in hepatocarcinogenesis by an unknown mechanism. pX variants encoded by HBV genomes found integrated in genomic DNA from liver tumors of patients with hepatocellular carcinoma (HCC) generally lack amino acids 134 to 154. Since deregulation of mitogenic pathways is linked to oncogenic transformation, herein we define the pX region required for mitogenic pathway activation. A series of pX deletions was used to construct tetracycline-regulated pX-expressing cell lines. The activation of the mitogenic pathways by these pX deletions expressed in the constructed cell lines was measured by transient transreporter assays, effects on endogenous cyclin A expression, and apoptosis. Conditional expression of pX51-140 in AML12 clone 4 cell line activates the mitogenic pathways, induces endogenous cyclin A expression, and sensitizes cells to apoptosis, similar to wild-type (WT) pX. By contrast, pX1-115 is inactive, supporting the idea that amino acids 116 to 140 are required for mitogenic pathway activation. Moreover, this pX deletion analysis demonstrates that WT pX function is modulated by two regions spanning amino acids 1 to 78 and 141 to 154. The N-terminal X1-78, expressed via a retroviral vector in WT pX-expressing 4pX-1 cells, coimmunoprecipitates with WT pX, indicating this pX region participates in protein-protein interactions leading to pX oligomerization. Interestingly, pX1-78 interferes with WT pX in mediating mitogenic pathway activation, endogenous gene expression, and apoptosis. The C-terminal pX region spanning amino acids 141 to 154 decreases pX stability, determined by pulse-chase studies of WT pX and pX1-140, suggesting that increased stability of naturally occurring pX variants lacking amino acids 134 to 154 may play a role in HCC development.

The Rho GTPase effector ROCK regulates cyclin A, cyclin D1, and p27Kip1 levels by distinct mechanisms

The members of the Rho GTPase family are well known for their regulation of actin cytoskeletal structures. In addition, they influence progression through the cell cycle. The RhoA and RhoC proteins regulate numerous effector proteins, with a central and vital signaling role mediated by the ROCK I and ROCK II serine/threonine kinases. The requirement for ROCK function in the proliferation of numerous cell types has been revealed by studies utilizing ROCK-selective inhibitors such as Y-27632. However, the mechanisms by which ROCK signaling promotes cell cycle progression have not been thoroughly characterized. Using a conditionally activated ROCK-estrogen receptor fusion protein, we found that ROCK activation is sufficient to stimulate G1/S cell cycle progression in NIH 3T3 mouse fibroblasts. Further analysis revealed that ROCK acts via independent pathways to alter the levels of cell cycle regulatory proteins: cyclin D1 and p21(Cip1) elevation via Ras and the mitogen-activated protein kinase pathway, increased cyclin A via LIM kinase 2, and reduction of p27(Kip1) protein levels. Therefore, the influence of ROCK on cell cycle regulatory proteins occurs by multiple independent mechanisms.

Differential involvement of the actin cytoskeleton in differentiation and mitogenesis of thyroid cells: inactivation of Rho proteins contributes to cyclic adenosine monophosphate-dependent gene expression but prevents mitogenesis

In thyroid epithelial cells, TSH via cAMP induces a rounding up of the cells associated with actin stress fiber disruption, expression of differentiation genes and cell cycle progression. Here we have evaluated the role of small G proteins of the Rho family and their impact on the actin cytoskeleton in these different processes in primary cultures of canine thyrocytes. TSH and forskolin, but not growth factors, rapidly inactivated RhoA, Rac1, and Cdc42, as assayed by detection of GTP-bound forms. Using toxins that inactivate Rho proteins (toxin B, C3 exoenzyme) or activate them [cytotoxic necrotizing factor 1 (CNF1)], in comparison with disruption of the actin cytoskeleton by dihydrocytochalasin B (DCB) or latrunculin, two unexpected conclusions were reached: 1) inactivation of Rho proteins by cAMP, by disorganizing actin microfilaments and inducing cell retraction, could be necessary and sufficient to mediate at least part of the cAMP-dependent induction of thyroglobulin and thyroid oxidases, but only partly necessary for the induction of Na(+)/I(-) symporter and thyroperoxidase; 2) as indicated by the effect of their inhibition by toxin B and C3, some residual activity of Rho proteins could be required for the induction by cAMP-dependent or -independent mitogenic cascades of DNA synthesis and retinoblastoma protein (pRb) phosphorylation, through mechanisms targeting the activity, but not the stimulated assembly, of cyclin D3-cyclin-dependent kinase 4 complexes. However, at variance with current concepts mostly derived from fibroblast models, DNA synthesis induction and cyclin D3-cyclin-dependent kinase 4 activation were resistant to actin depolymerization by dihydrocytochalasin B in canine thyrocytes, which provides a first such example in a normal adherent cell.

Type I collagen structure regulates cell morphology and EGF signaling in primary rat hepatocytes through cAMP-dependent protein kinase A

Adhesion to type 1 collagen elicits different responses dependent on whether the collagen is in fibrillar (gel) or monomeric form (film). Hepatocytes adherent to collagen film spread and proliferate, whereas those adherent to collagen gel remain rounded and growth arrested. To explore the role of potential intracellular inhibitory signals responsible for collagen gel-mediated growth arrest, cAMP-dependent protein kinase A (PKA) was examined in hepatocytes adherent to collagen film or gel. PKA activity was higher in hepatocytes on collagen gel than on film during G1 of the hepatocyte cell cycle. Inhibition of PKA using H89 increased cell spreading on collagen gel in an EGF-dependent manner, whereas activation of PKA using 8-Br-cAMP decreased cell spreading on collagen film. PKA inhibition also restored ERK activation, cyclin D1 expression and G1-S progression on collagen gel, but had no effect on cells adherent to collagen film. Analysis of EGF receptor phosphorylation revealed that adhesion to collagen gel alters tyrosine phosphorylation of the EGF receptor, leading to reduced phosphorylation of tyrosine residue 845, which was increased by inhibition of PKA. These results demonstrate that fibrillar type 1 collagen can actively disrupt cell cycle progression by inhibiting specific signals from the EGF receptor through a PKA-dependent pathway.

Transactivation of the cyclin A promoter by bovine papillomavirus type 4 E5 protein

Bovine papillomavirus type 4 (BPV-4) E5 (formerly E8) is a 42-residue hydrophobic, membrane-localised protein that can transform NIH-3T3 cells by a poorly defined mechanism. In E5-expressing cells, the observed up-regulation of cyclin A is underpinned by transactivation of the cyclin A promoter. Here we show that E5 transactivates the minimal cell cycle-regulated cyclin A promoter in cells both stably and acutely expressing the viral protein. There are no detectable differences between control and E5 cells in protein complexes binding the E2F-like cell cycle-dependent element (CDE)/cell cycle-regulated element (CCRE) of the cyclin A promoter and E5 does not transactivate E2F reporter plasmids in an E2F-dependent manner in vivo. CCAAT box integrity and functional NF-Y complexes are required for E5-mediated transactivation and a Mr approximately 110 K CCAAT-box binding factor (p110 CBF) associates with NF-YA only in E5 cells. This suggests that E5 sets the extent of cyclin A promoter activation by a mechanism similar to other, structurally unrelated, DNA tumour virus oncoproteins but distinct from the action of serum factors and so is inconsistent with E5 acting through constitutive activation of tyrosine kinase growth factor receptors.

Myosin light chain kinase and acto-myosin contractility modulate activation of the ERK cascade downstream of oncogenic Ras

The actin cytoskeleton is recognized as an important component of both adhesion- and growth factor-dependent signaling, but its role in oncogene-dependent signaling has received much less attention. In this study, we investigated the role played by the acto-myosin cytoskeleton and its main regulators, i.e., myosin light chain kinase and Rho kinase, in oncogenic Ki-Ras-induced signaling. We found that activation of the ERK cascade by Ras is dependent on acto-myosin contractility, under the regulation of myosin light chain kinase but not Rho kinase. Inhibition of myosin II or myosin light chain kinase caused a complete loss of ERK phosphorylation in a time- and dose-dependent manner, but proved dispensable for activation of the PI3K pathway. We also provide evidence that the target of myosin light chain kinase lays at the level of Raf activation. Since myosin light chain kinase is a target of ERK, these results suggest a previously uncharacterized signaling pathway involving Ras-mediated alterations of the actin cytoskeleton, which might play a critical role in ERK activation by the Ras oncogene and contribute to aberrant signaling and enhanced cell motility. In addition, restoration of stress fibers following ectopic expression of tropomyosin 2 resulted in reduced levels of ERK phosphorylation. Finally, these studies suggest that myosin light chain kinase but not Rho kinase plays an essential role in the generation of ERK signaling in transformed cells and indicate distinct cellular roles for Rho-kinase and myosin light chain kinase-dependent functions involving the regulation of acto-myosin contractility.

Epidermal growth factor receptor-dependent regulation of integrin-mediated signaling and cell cycle entry in epithelial cells

Integrin-mediated adhesion of epithelial cells to extracellular matrix (ECM) proteins induces prolonged tyrosine phosphorylation and partial activation of epidermal growth factor receptor (EGFR) in an integrin-dependent and EGFR ligand-independent manner. Integrin-mediated activation of EGFR in epithelial cells is required for multiple signal transduction events previously shown to be induced by cell adhesion to matrix proteins, including tyrosine phosphorylation of Shc, Cbl, and phospholipase Cgamma, and activation of the Ras/Erk and phosphatidylinositol 3'-kinase/Akt signaling pathways. In contrast, activation of focal adhesion kinase, Src, and protein kinase C, adhesion to matrix proteins, cell spreading, migration, and actin cytoskeletal rearrangements are induced independently of EGFR kinase activity. The ability of integrins to induce the activation of EGFR and its subsequent regulation of Erk and Akt activation permitted adhesion-dependent induction of cyclin D1 and p21, Rb phosphorylation, and activation of cdk4 in epithelial cells in the absence of exogenous growth factors. Adhesion of epithelial cells to the ECM failed to efficiently induce degradation of p27, to induce cdk2 activity, or to induce Myc and cyclin A synthesis; subsequently, cells did not progress into S phase. Treatment of ECM-adherent cells with EGF, or overexpression of EGFR or Myc, resulted in restoration of late-G(1) cell cycle events and progression into S phase. These results indicate that partial activation of EGFR by integrin receptors plays an important role in mediating events triggered by epithelial cell attachment to ECM; EGFR is necessary for activation of multiple integrin-induced signaling enzymes and sufficient for early events in G(1) cell cycle progression. Furthermore, these findings suggest that EGFR or Myc overexpression may provoke ligand-independent proliferation in matrix-attached cells in vivo and could contribute to carcinoma development.

Dual and opposing roles of ERK in regulating G1 and S-G2/M delays in A549 cells caused by hyperoxia

This study explores the role of ERK activation in regulating G(1) and S-G(2)/M delays during hyperoxia. We demonstrate here that exposing A549 human alveolar type 2 adenocarcinoma cells to hyperoxia (95% O(2)) for 0.5-24 h time-dependently increases phospho-ERK, phospho-p53(Ser15), p53, and p21(CIP1) protein levels. Decreasing phospho-ERK with the pharmacological inhibitors, PD98059 and U0126, markedly suppresses hyperoxia-stimulated phospho-p53(Ser15), p53, and p21(CIP1), and also restores the hyperoxia-reduced kinase activities of cyclin D1/E1-Cdks. Our results suggest that ERK activation during hyperoxia contributes to the p53/p21-mediated G(1) checkpoint. However, inhibition of ERK signaling during hyperoxia further delays S-phase entry and progression. Hyperoxia induces significant expression of cyclin A/B1 and translocation of cyclin A into nuclei while marginally decreasing cyclin A/B1-Cdks kinase activities, which may be related to nuclear association with p21. Interestingly, inhibition of ERK signaling markedly suppresses the elevation of cyclin A/B1 proteins and cyclin A/B1-Cdks kinase activities during hyperoxia. Taken together, the results presented here suggest that hyperoxia-activated ERK acts upstream of p53 and p21 to suppress G(1)-Cdk activities; however, it is also required for induction of cyclin A/B1 and maintenance of cyclin A/B1-Cdk activities that oppose delays in S-phase entry and progression.

The role of cyclin D2 and p21/waf1 in human T-cell leukemia virus type 1 infected cells

Background

The human T-cell leukemia virus type 1 (HTLV-1) Tax protein indirectly influences transcriptional activation, signal transduction, cell cycle control, and apoptosis. The function of Tax primarily relies on protein-protein interactions. We have previously shown that Tax upregulates the cell cycle checkpoint proteins p21/waf1 and cyclin D2. Here we describe the consequences of upregulating these G₁/S checkpoint regulators in HTLV-1 infected cells.

Results

To further decipher any physical and functional interactions between cyclin D2 and p21/waf1, we used a series of biochemical assays from HTLV-1 infected and uninfected cells. Immunoprecipitations from HTLV-1 infected cells showed p21/waf1 in a stable complex with cyclin D2/cdk4. This complex is active as it phosphorylates the Rb protein in kinase assays. Confocal fluorescent microscopy indicated that p21/waf1 and cyclin D2 colocalize in HTLV-1 infected, but not in uninfected cells. Furthermore, in vitro kinase assays using purified proteins demonstrated that the addition of p21/waf1 to cyclin D2/cdk4 increased the kinase activity of cdk4.

Conclusion

These data suggest that the p21/cyclin D2/cdk4 complex is not an inhibitory complex and that p21/waf1 could potentially function as an assembly factor for the cyclin D2/cdk4 complex in HTLV-1 infected cells. A by-product of this assembly with cyclin D2/cdk4 is the sequestration of p21/waf1 away from the cyclin E/cdk2 complex, allowing this active cyclin-cdk complex to phosphorylate Rb pocket proteins efficiently and push cells through the G₁/S checkpoint. These two distinct functional and physical activities of p21/waf1 suggest that RNA tumor viruses manipulate the G₁/S checkpoint by deregulating cyclin and cdk complexes.

Antimitogenic effects of HDL and APOE mediated by Cox-2-dependent IP activation

HDL and its associated apo, APOE, inhibit S-phase entry of murine aortic smooth muscle cells. We report here that the antimitogenic effect of APOE maps to the N-terminal receptor-binding domain, that APOE and its N-terminal domain inhibit activation of the cyclin A promoter, and that these effects involve both pocket protein-dependent and independent pathways. These antimitogenic effects closely resemble those seen in response to activation of the prostacyclin receptor IP. Indeed, we found that HDL and APOE suppress aortic smooth muscle cell cycle progression by stimulating Cox-2 expression, leading to prostacyclin synthesis and an IP-dependent inhibition of the cyclin A gene. Similar results were detected in human aortic smooth muscle cells and in vivo using mice overexpressing APOE. Our results identify the Cox-2 gene as a target of APOE signaling, link HDL and APOE to IP action, and describe a potential new basis for the cardioprotective effect of HDL and APOE.

Involvement of neuron-derived orphan receptor-1 (NOR-1) in LDL-induced mitogenic stimulus in vascular smooth muscle cells: role of CREB

OBJECTIVE

Low density lipoproteins (LDLs) modulate the expression of key genes involved in atherogenesis. Recently, we have shown that the transcription factor neuron-derived orphan receptor-1 (NOR-1) is involved in vascular smooth muscle cell (VSMC) proliferation. Our aim was to analyze whether NOR-1 is involved in LDL-induced mitogenic effects in VSMC.

METHODS AND RESULTS

LDL induced NOR-1 expression in a time- and dose-dependent manner. Antisense oligonucleotides against NOR-1 inhibit DNA synthesis induced by LDL in VSMCs as efficiently as antisense against the protooncogene c-fos. The upregulation of NOR-1 mRNA levels by LDL involves pertusis-sensitive G protein-coupled receptors, Ca2+ mobilization, protein kinases A (PKA) and C (PKC) activation, and mitogen-activated protein kinase pathways (MAPK) (p44/p42 and p38). LDL promotes cAMP response element binding protein (CREB) activation (phosphorylation in Ser133). In transfection assays a dominant-negative of CREB inhibits NOR-1 promoter activity, while mutation of specific (cAMP response element) CRE sites in the NOR-1 promoter abolishes LDL-induced NOR-1 promoter activity.

CONCLUSIONS

In VSMCs, LDL-induced mitogenesis involves NOR-1 upregulation through a CREB-dependent mechanism. CREB could play a role in the modulation by LDL of key genes (containing CRE sites) involved in atherogenesis.

Annexin 1 regulates cell proliferation by disruption of cell morphology and inhibition of cyclin D1 expression through sustained activation of the ERK1/2 MAPK signal

Cellular proliferation is controlled by the integration and coordination of extracellular signals. This study explores the role of the protein annexin 1 (ANXA1) in the regulation of such events. We show that ANXA1 has a cell-type independent, anti-proliferative function through sustained activation of the ERK signaling cascade. Moreover, ANXA1 reduces proliferation by ERK-mediated disruption of the actin cytoskeleton and ablation of cyclin D1 protein expression and not by ERK-mediated induction of the cyclin-dependent kinase, CDK2, inhibitor p21(cip/waf). Finally, ANXA1 regulates the ERK pathway at a proximal location, by SH2 domain-independent association with the adapter protein Grb-2. In summary, overexpression of ANXA1 mediates the disruption of normal cell morphology and inhibits cyclin D1 expression, therefore reducing cell proliferation through proximal modulation of the ERK signal transduction pathway.

RhoA influences the nuclear localization of extracellular signal-regulated kinases to modulate p21Waf/Cip1 expression

BACKGROUND

The 42/44-kD mitogen-activated protein kinases (extracellular signal-regulated kinases, ERKs) regulate smooth muscle cell (SMC) cell-cycle progression and can either promote or inhibit proliferation depending on the activation status of the small GTPase RhoA. RhoA is involved in the regulation of the actin cytoskeleton and converges on multiple signaling pathways. However, the mechanism by which RhoA modulates ERK signaling is not well defined. The purpose of this investigation was to examine whether RhoA regulates ERK downstream signaling and cellular proliferation through its effects on the cytoskeleton and the nuclear localization of ERK.

METHODS AND RESULTS

Treatment of SMCs with Clostridia botulinum C3 exoenzyme, which inhibits RhoA activation, decreased SMC proliferation to 24+/-7% of that of controls and increased p21Waf1/Cip1 transcription and protein levels. These effects of RhoA were reversed by inhibition of ERK phosphorylation. However, inactivation of RhoA did not alter levels of ERK phosphorylation but did increase nuclear localization of phosphorylated ERK. In addition, immunostaining demonstrated that phosphorylated ERK associated with the actin cytoskeleton, which was disrupted by C3 exoenzyme. Leptomycin B, an inhibitor of Crm1 that results in ERK nuclear accumulation, similarly increased p21Waf1/Cip1.

CONCLUSIONS

RhoA inhibition increased levels of phosphorylated ERK in the cell nucleus. Inhibition of RhoA or pharmacological inhibition of nuclear export resulted in increased p21Waf1/Cip1 expression and decreased SMC proliferation, effects that were partially dependent on ERK. RhoA regulation of the actin cytoskeleton may determine ERK subcellular localization and its subsequent effects on SMC proliferation.

NOX5 NAD(P)H oxidase regulates growth and apoptosis in DU 145 prostate cancer cells

Reactive oxygen species (ROS) appear to play an important role in regulating growth and survival of prostate cancer. However, the sources for ROS production in prostate cancer cells have not been determined. We report that ROS are generated by intact American Type Culture Collection DU 145 cells and by their membranes through a mechanism blocked by NAD(P)H oxidase inhibitors. ROS are critical for growth in these cells, because NAD(P)H oxidase inhibitors and antioxidants blocked proliferation. Components of the human phagocyte NAD(P)H oxidase, p22phox and gp91phox, as well as the Ca2+ concentration-responsive gp91phox homolog NOX5 were demonstrated in DU 145 cells by RT-PCR and sequencing. Although the protein product for p22phox was not detectable, both gp91phox and NOX5 were identified throughout the cell by immunostaining and confocal microscopy and NOX5 immunostaining was enhanced in a perinuclear location, corresponding to enhanced ROS production adjacent to the nuclear membrane imaged by 2',7'-dichlorofluorescin diacetate oxidation. The calcium ionophore ionomycin dramatically stimulated ferricytochrome c reduction in cell media, further supporting the importance of NOX5 for ROS production. Antisense oligonucleotides for NOX5 inhibited ROS production and cell proliferation in DU 145 cells. In contrast, antisense oligonucleotides to p22phox or gp91phox did not impair cell growth. Inhibition of ROS generation with antioxidants or NAD(P)H oxidase inhibitors increased apoptosis in cells. These results indicate that ROS generated by the newly described NOX5 oxidase are essential for prostate cancer growth, possibly by providing trophic intracellular oxidant tone that retards programmed cell death.

Prostacylin receptor activation inhibits proliferation of aortic smooth muscle cells by regulating cAMP response element-binding protein- and pocket protein-dependent cyclin a gene expression

The prostanoid prostacyclin (PGI2) inhibits aortic smooth muscle cell proliferation by blocking cell cycle progression from G1-to S-phase. However, the mechanism of this inhibition is poorly understood. We report here that the PGI2 mimetic, cicaprost, inhibits the induction of cyclin A and activation of the cyclin A promoter in primary and established rodent aortic smooth muscle cells. The inhibition of cyclin A gene expression is associated with a block in cyclin E-cdk2 activity and phosphorylation of both the retinoblastoma protein and p107. Inactivation of pocket proteins with human papilloma virus protein E7 partially, but not completely, restored cyclin A promoter activity in cicaprost-treated cells. Complementary studies showed that occupancy of the cAMP response element (CRE) is required for efficient activation of the cyclin A promoter in aortic smooth muscle cells, that the CRE is primarily occupied by the CRE-binding protein (CREB) and phospho-CREB, and that cicaprost blocks the binding of CREB and phospho-CREB to the cyclin A promoter CRE. Treatment with pertussis toxin reversed the inhibitory effects of cicaprost on CRE occupancy, cyclin E-cdk2 activity, and S phase entry, suggesting the involvement of Gi signaling in cicaprost action. We conclude that PGI2 inhibits proliferation of aortic smooth muscle cells by coordinately blocking CRE- and pocket protein-dependent cyclin A gene expression.

Effects of rho kinase and actin stress fibers on sustained extracellular signal-regulated kinase activity and activation of G(1) phase cyclin-dependent kinases

We recently reported that Rho kinase is required for sustained ERK signaling and the consequent mid-G(1) phase induction of cyclin D1 in fibroblasts. The results presented here indicate that these Rho kinase effects are mediated by the formation of stress fibers and the consequent clustering of alpha5beta1 integrin. Mechanistically, alpha5beta1 signaling and stress fiber formation allowed for the sustained activation of MEK, and this effect was mediated upstream of Ras-GTP loading. Interestingly, disruption of stress fibers with ML-7 led to G(1) phase arrest while comparable disruption of stress fibers with Y27632 (an inhibitor of Rho kinase) or dominant-negative Rho kinase led to a more rapid progression through G(1) phase. Inhibition of either MLCK or Rho kinase blocked sustained ERK signaling, but only Rho kinase inhibition allowed for the induction of cyclin D1 and activation of cdk4 via Rac/Cdc42. The levels of cyclin E, cdk2, and their major inhibitors, p21(cip1) and p27(kip1), were not affected by inhibition of MLCK or Rho kinase. Overall, our results indicate that Rho kinase-dependent stress fiber formation is required for sustained activation of the MEK/ERK pathway and the mid-G(1) phase induction of cyclin D1, but not for other aspects of cdk4 or cdk2 activation. They also emphasize that G(1) phase cell cycle progression in fibroblasts does not require stress fibers if Rac/Cdc42 signaling is allowed to induce cyclin D1.