Regulation of Ras signaling by the cell cycle.

It is well known that upregulation of Ras activity can promote cell-cycle progression. Now recent studies indicate that a reciprocal relationship also exists; that is, the consequences of Ras signaling are dependent upon cell-cycle position. In quiescent cells stimulated with growth factors, one Ras effector, phosphatidylinositol-3-kinase, is activated twice as cells transition from G(0) into G(1) phase, and then later in G(1) phase. It is only during the later stages of G(1) phase that PI3K activity promotes entry into S-phase. In cycling cells, Ras activity is enhanced throughout the cell cycle, but is able to stimulate cyclin D1 elevation only during G(2) phase.     

Mechanism of mitosis-specific activation of MEK1

Activation of cyclin B-Cdc2 is an absolute requirement for entry into mitosis, but other protein kinase pathways that also have mitotic functions are activated during G(2)/M progression. The MAPK cascade has well established roles in entry and exit from mitosis in Xenopus, but relatively little is known about the regulation and function of this pathway in mammalian mitosis. Here we report a detailed analysis of the activity of all components of the Ras/Raf/MEK/ERK pathway in HeLa cells during normal G(2)/M. The focus of this pathway is the dramatic activation of an endomembrane-associated MEK1 without the corresponding activation of the MEK substrate ERK. This is because of the uncoupling of MEK1 activation from ERK activation. The mechanism of this uncoupling involves the cyclin B-Cdc2-dependent proteolytic cleavage of the N-terminal ERK-binding domain of MEK1 and the phosphorylation of Thr(286). These results demonstrate that cyclin B-Cdc2 activity regulates signaling through the MAPK pathway in mitosis.     

Continuous ERK activation downregulates antiproliferative genes throughout G1 phase to allow cell-cycle progression

BACKGROUND: The ERK family of MAP kinase plays a critical role in growth factor-stimulated cell-cycle progression from G0/G1 to S phase. It has been suggested that sustained activation, but not transient activation, of ERK is necessary for inducing S phase entry. Although the essential role of ERK MAP kinase in growth factor-stimulated gene expression, especially expression of immediate-early genes, is well established, it has remained unclear how ERK activity duration affects the promotion of G1 phase progression to S phase. RESULTS: We have found that inhibition of ERK activation by the MEK inhibitor or dominant-negative MEK1 even immediately before the onset of S phase leads to the cessation of S phase entry. Our analyses reveal that there are ERK-dependent downregulated genes, whose expression levels return to their original levels rapidly after ERK inactivation, and that their downregulation mostly requires AP-1 activity. Remarkably, microinjection experiments demonstrate that many of the downregulated genes act as antiproliferative genes during G1 phase and that their forced expression to the levels before growth factor stimulation even in late G1 phase blocks S phase entry. CONCLUSIONS: Thus, continuous ERK activation downregulates antiproliferative genes until the onset of S phase to allow successful G1 phase progression. This mechanism may also work as a fail-safe mechanism, which prevents inappropriate stimuli that induce transient ERK activation from causing S phase entry.     

Retinoblastoma susceptibility gene product (pRb) and p107 functionally separate the requirements for serum and anchorage in the cell cycle G1-phase

Growth factors and cell anchorage are both required for cell cycle G(1)-phase progression, but it is unclear whether their function is mediated through the same set of cell cycle components and whether they are both required during the same periods of time. We separately analyzed the requirements of serum and anchorage during G(1)-phase progression and found that human dermal fibroblasts as well as wild type, pRb(-/-), and p107(-/-) mouse embryonic fibroblasts needed serum (growth factors) until mid-G(1)-phase but required cell anchorage until late G(1)-phase to be competent for S-phase entry. Importantly, however, pRb/p107 double-null mouse embryonic fibroblasts lacked serum requirement in mid-G(1)-phase but still required cell anchorage until late G(1)-phase to enter S-phase. Our results indicate that pRb and p107 do not constitute the last control point for extracellular factors during G(1)-phase progression, and they functionally separate the requirements for serum and cell anchorage in terms of involved cell cycle components.     

Sanguinarine-induced G1-phase arrest of the cell cycle results from increased p27KIP1 expression mediated via activation of the Ras/ERK signaling pathway in vascular smooth muscle cells

The present study identified a novel mechanism for the effects of sanguinarine in vascular smooth muscle cells (VSMC). Sanguinarine treatment of VSMC resulted in significant growth inhibition as a result of G₁-phase cell-cycle arrest mediated by induction of p27KIP1 expression, and resulted in a down-regulation of the expression of cyclins and CDKs in VSMC. Moreover, sanguinarine-induced inhibition of cell growth appeared to be linked to activation of Ras/ERK through p27KIP1-mediated G₁-phase cell-cycle arrest. Overall, the unexpected effects of sanguinarine treatment in VSMC provide a theoretical basis for clinical use of therapeutic agents in the treatment of atherosclerosis.     

Coordinate signaling by integrins and receptor tyrosine kinases in the regulation of G1 phase cell-cycle progression

Cell proliferation is dependent upon the activation of receptor tyrosine kinases and integrins by soluble growth factors and extracellular matrix proteins, respectively. It is now apparent that concerted, rather than individual, signaling by these receptors is the critical feature responsible for cell-cycle progression through G1 phase. ERK (extracellular signal-regulated kinase), Rho GTPases and G1-phase cyclin-dependent kinases are all regulated jointly by growth-factor receptors and integrins. Recent studies have begun to reveal how this regulated signaling in the cytoplasm is linked to activation of the G1-phase cyclin-dependent kinases in the nucleus.     

Joint requirement for Rac and ERK activities underlies the mid-G1 phase induction of cyclin D1 and S phase entry in both epithelial and mesenchymal cells

Cyclin D1 gene induction is a key event in G1 phase progression. Our previous studies indicated that signaling to cyclin D1 is cell type-dependent because the timing of cyclin D1 gene expression in MCF10A mammary epithelial cells and mesenchymal cells such as fibroblasts and vascular smooth muscle cells is very different, with epithelial cells first expressing cyclin D1 in early rather than mid-G1 phase. In this report, we induced a mesenchymal phenotype in MCF10A cells by long-term exposure to TGF-beta and used the control and transitioned cells to examine cell type specificity of the signaling pathways that regulate cyclin D1 gene expression. We show that early-G1 phase cyclin D1 gene expression in MCF10A cells is under the control of Rac, whereas mid-G1 phase cyclin D1 induction requires parallel signaling from Rac and ERK, both in the control and transitioned cells. This combined requirement for Rac and ERK signaling is associated with an increased requirement for intracellular tension, Rb phosphorylation, and S phase entry. A similar co-regulation of cyclin D1 mRNA by Rac and ERK is seen in primary mesenchymal cells. Overall, our results reveal two mechanistically distinct phases of Rac-dependent cyclin D1 expression and emphasize that the acquisition of Rac/ERK co-dependence is required for the mid-G1 phase induction of cyclin D1 associated with S phase entry.     

Ras signals principally via Erk in G1 but cooperates with PI3K/Akt for Cyclin D induction and S-phase entry

Numerous studies exploring oncogenic Ras or manipulating physiological Ras signalling have established an irrefutable role for Ras as driver of cell cycle progression. Despite this wealth of information the precise signalling timeline and effectors engaged by Ras, particularly during G1, remain obscure as approaches for Ras inhibition are slow-acting and ill-suited for charting discrete Ras signalling episodes along the cell cycle. We have developed an approach based on the inducible recruitment of a Ras-GAP that enforces endogenous Ras inhibition within minutes. Applying this strategy to inhibit Ras stepwise in synchronous cell populations revealed that Ras signaling was required well into G1 for Cyclin D induction, pocket protein phosphorylation and S-phase entry, irrespective of whether cells emerged from quiescence or G2/M. Unexpectedly, Erk, and not PI3K/Akt or Ral was activated by Ras at mid-G1, albeit PI3K/Akt signalling was a necessary companion of Ras/Erk for sustaining cyclin-D levels and G1/S transition. Our findings chart mitogenic signaling by endogenous Ras during G1 and identify limited effector engagement restricted to Raf/MEK/Erk as a cogent distinction from oncogenic Ras signalling.     

Expression of c-Myc in response to colony-stimulating factor-1 requires mitogen-activated protein kinase kinase-1

The mitogen-inducible gene c-myc is a key regulator of cell proliferation and transformation. Yet, the signaling pathway(s) that regulate its expression have remained largely unresolved. Using the mitogen-activated protein kinase kinase (MEK1/2) inhibitor PD98059 and dominant negative forms of Ras (N17) and ERK1 (K71R), we found that activation of Ras and extracellular signal-regulated kinase (ERK) is necessary for colony-stimulating factor-1 (CSF-1)-mediated c-Myc expression and DNA synthetic (S) phase entry. Quiescent NIH-3T3 cells expressing a partially defective CSF-1 receptor, CSF-1R (Y809F), exhibited impaired ERK1 activation and c-Myc expression and failed to enter the S phase of the cell division cycle in response to CSF-1 stimulation. Ectopic expression of a constitutively active form of MEK1 in cells expressing CSF-1R (Y809F) rescued c-Myc expression and S phase entry, but only in the presence of CSF-1-induced cooperating signals. Therefore, MEK1 participates in an obligate signaling pathway linking CSF-1R to c-Myc expression, but other signals from CSF-1R must cooperate with the MEK/ERK pathway to induce c-Myc expression and S phase entry in response to CSF-1 stimulation.     

Capacity for resolution of Ras-MAPK-initiated early pathogenic myocardial hypertrophy modeled in mice

Activation of Ras signaling in cardiomyocytes has been linked to pathogenic myocardial hypertrophy progression and subsequent heart failure. Whether cardiomyopathy can regress once initiated needs to be established more fully. A 'tet-off' system was used to regulate expression of H-Ras-G12V in myocardium to examine whether Ras-induced pathogenic myocardial hypertrophy could resolve after removal of Ras signaling in vivo. Ras activation at weaning for 2 wk caused hypertrophy, whereas activation for 4 to 8 wk led to cardiomyopathy and heart failure. Discontinuing H-Ras-G12V transgene expression after cardiomyopathy onset led to improved survival and cardiomyopathy lesion scores, with reduced heart:body weight ratios, demonstrating the reversibility of early pathogenic hypertrophy. Activation of Ras and downstream ERK 1/2 was associated with elevated expression of proliferating cell nuclear antigen and cyclins B1 and D1, indicating cell-cycle activation and reentry. Coordinate elevation of broad-spectrum cyclin-dependent kinase inhibitors (p21, p27, and p57) and Tyr15 phosphorylation of cdc2 signified the activation of cell-cycle checkpoints; absence of cell-cycle completion and cardiomyocyte replication were documented by using immunohistochemistry for mitosis and cytokinesis markers. After resolution of cardiomyopathy, cell-cycle activators and inhibitors examined returned to basal levels, a change that we interpreted as exit from the cell cycle. Cardiac cell-cycle regulation plays a role in recovery from pathogenic hypertrophy. The model we present provides a means to further explore the underlying mechanisms governing cell-cycle capacity in cardiomyocytes, as well as progression and regression of pathogenic cardiomyocyte hypertrophy.     

Ⅰ型磷酸酶抑制亚基-1对人宫颈癌HeLa细胞增殖的抑制作用

PPI1(Inhibitor-1 ofprotein phosphatase 1)是I型磷酸酶的抑制亚基之一,其活化依赖于35位苏氨酸蛋白激酶(PKA)的磷酸化而发挥抑制作用.本研究目的在于探讨PPIl持续活化型突变体的表达对人宫颈癌细胞株增殖的影响及其可能的作用机制.利用PPI1野生型和活化型突变体表达质粒分别转染HeLa细胞,首先通过H~3TdR掺入实验、迁移实验观察PPI1基因对HeLa细胞增殖能力的影响,研究结果表明:在活化型突变体表达的HeLa细胞株中,细胞~3H掺入量和迁移能力明显受抑.其次通过流式细胞术、Giemsa染色法分析PPIl对HeLa细胞的细胞周期的影响;FACS分析表明HeLa细胞G2/M期比例明显升高;经脱氧胸苷同步化后,该组细胞进入有丝分裂期明显滞后.最后利用Western blot分析PPI1对MAPK信号转导通路的影响,Western blot分析显示该组细胞的ERK磷酸化水平明显下降.研究表明PPI1活化型突变体的表达可抑制人宫颈癌细胞株的增殖,这与其诱导HeLa细胞G2/M期停滞、有丝分裂的进入延缓有关,其中涉及到MAPK信号转导通路的活化受抑制.     

Activation of extracellular signal-regulated kinase (ERK) in G2 phase delays mitotic entry through p21CIP1

Extracellular signal-regulated kinase activity is essential for mediating cell cycle progression from G(1) phase to S phase (DNA synthesis). In contrast, the role of extracellular signal-regulated kinase during G(2) phase and mitosis (M phase) is largely undefined. Previous studies have suggested that inhibition of basal extracellular signal-regulated kinase activity delays G(2)- and M-phase progression. In the current investigation, we have examined the consequence of activating the extracellular signal-regulated kinase pathway during G(2) phase on subsequent progression through mitosis. Using synchronized HeLa cells, we show that activation of the extracellular signal-regulated kinase pathway with phorbol 12-myristate 13-acetate or epidermal growth factor during G(2) phase causes a rapid cell cycle arrest in G(2) as measured by flow cytometry, mitotic indices and cyclin B1 expression. This G(2)-phase arrest was reversed by pre-treatment with bisindolylmaleimide or U0126, which are selective inhibitors of protein kinase C proteins or the extracellular signal-regulated kinase activators, MEK1/2, respectively. The extracellular signal-regulated kinase-mediated delay in M-phase entry appeared to involve de novo synthesis of the cyclin-dependent kinase inhibitor, p21(CIP1), during G(2) through a p53-independent mechanism. To establish a function for the increased expression of p21(CIP1) and delayed cell cycle progression, we show that extracellular signal-regulated kinase activation in G(2)-phase cells results in an increased number of cells containing chromosome aberrations characteristic of genomic instability. The presence of chromosome aberrations following extracellular signal-regulated kinase activation during G(2)-phase was further augmented in cells lacking p21(CIP1). These findings suggest that p21(CIP1) mediated inhibition of cell cycle progression during G(2)/M phase protects against inappropriate activation of signalling pathways, which may cause excessive chromosome damage and be detrimental to cell survival.     

The Retinoblastoma Protein Is Linked to the Activation of Ras

The inner membrane-bound protein Ras integrates various extracellular signals that are subsequently communicated from the cytoplasm to the nucleus via the Raf/MEK/MAPK cascade. Here we show that the retinoblastoma protein pRb, previously reported to be a nuclear target of this pathway, can in turn influence the activation state of Ras. Rb-deficient fibroblasts display elevated levels (up to 30-fold) of activated Ras during G(1). Expression of wild-type pRb or a number of pRb mutants defective in E2F regulation reverses this effect. We provide evidence that the mid-G(1) activation of Ras in Rb-deficient cells, which occurs at the level of guanine nucleotide binding, differs from that of epidermal growth factor-induced stimulation of Ras, being dependent on protein synthesis. The aberrant levels of Ras activity associated with loss of pRb may be responsible for the differentiation defects in Rb-deficient cells, because suppression of Ras activity in Rb(-/-) fibroblasts restores the transactivation function of MyoD and the expression of a late marker of skeletal muscle differentiation. These data suggest that nuclear-cytoplasmic communication between pRb and Ras is bidirectional.     

Cholecystokinin stimulates extracellular signal-regulated kinase through activation of the epidermal growth factor receptor,Yes, and protein kinase C. Signal amplification at the level of Raf by activation of protein kinase Cepsilon

Cholecystokinin (CCK) and related peptides are potent growth factors in the gastrointestinal tract and may be important for human cancer. CCK exerts its growth modulatory effects through G(q)-coupled receptors (CCK(A) and CCK(B)) and activation of extracellular signal-regulated protein kinase 1/2 (ERK1/2). In the present study, we investigated the different mechanisms participating in CCK-induced activation of ERK1/2 in pancreatic AR42J cells expressing both CCK(A) and CCK(B). CCK activated ERK1/2 and Raf-1 to a similar extent as epidermal growth factor (EGF). Inhibition of EGF receptor (EGFR) tyrosine kinase or expression of dominant-negative Ras reduced CCK-induced ERK1/2 activation, indicating participation of the EGFR and Ras in CCK-induced ERK1/2 activation. However, compared with EGF, CCK caused only small increases in tyrosine phosphorylation of the EGFR and Shc, Shc-Grb2 complex formation, and Ras activation. Signal amplification between Ras and Raf in a CCK-induced ERK cascade appears to be mediated by activation of protein kinase Cepsilon (PKCepsilon), because 1) down-modulation of phorbol ester-sensitive PKCs inhibited CCK-induced activation of Ras, Raf, and ERK1/2 without influencing Shc-Grb2 complex formation; 2) PKCepsilon, but not PKCalpha or PKCdelta, was detectable in Raf-1 immunoprecipitates, although CCK activated all three PKC isoenzymes. In addition, the present study provides evidence that the Src family tyrosine kinase Yes is activated by CCK and mediates CCK-induced tyrosine phosphorylation of Shc. Furthermore, we show that CCK-induced activation of the EGFR and Yes is achieved through the CCK(B) receptor. Together, our data show that different signals emanating from the CCK receptors mediate ERK1/2 activation; activation of Yes and the EGFR mediate Shc-Grb2 recruitment, and activation of PKC, most likely PKCepsilon, augments CCK-stimulated ERK1/2 activation at the Ras/Raf level.     

Ras-Signaling Pathways: Positive and Negative Regulation of Tau Expression in PC12 Cells

Abstract: Tau is a microtubule-associated protein whose promoter is activated during the first phase of nerve growth factor-induced PC12 cell differentiation, whereas levels of its mRNA are accumulating throughout differentiation. In this study, we have followed the signal transduction cascades regulating tau induction. Using dominant negative Ras-expressing PC12 cells, we show that ras regulates tau expression during the first phase of PC12 cell differentiation. The ERK and JNK cascades, which are downstream of Ras; have opposing effects on tau promoter activity: ERK induces tau promoter activity, JNK inhibits it. Tau promoter activity in PC12 cells is correlated with a short-term activation of ERK, which declines after a few hours and is followed by an activation of the inhibitory JNK cascade 76 h later. These observations suggest that the induction and inhibition of tau promoter are mediated by alternate ERK and JNK activities, which may underlie a mechanism to turn on and off genes during PC12 cell differentiation.     

Distinct cell cycle timing requirements for extracellular signal-regulated kinase and phosphoinositide 3-kinase signaling pathways in somatic cell mitosis

Mitogen-activated protein (MAP) kinase and phosphoinositide 3-kinase (PI3K) pathways are necessary for cell cycle progression into S phase; however the importance of these pathways after the restriction point is poorly understood. In this study, we examined the regulation and function of extracellular signal-regulated kinase (ERK) and PI3K during G(2)/M in synchronized HeLa and NIH 3T3 cells. Phosphorylation and activation of both the MAP kinase kinase/ERK and PI3K/Akt pathways occur in late S and persist until the end of mitosis. Signaling was rapidly reversed by cell-permeable inhibitors, indicating that both pathways are continuously activated and rapidly cycle between active and inactive states during G(2)/M. The serum-dependent behavior of PI3K/Akt versus ERK pathway activation indicates that their mechanisms of regulation differ during G(2)/M. Effects of cell-permeable inhibitors and dominant-negative mutants show that both pathways are needed for mitotic progression. However, inhibiting the PI3K pathway interferes with cdc2 activation, cyclin B1 expression, and mitotic entry, whereas inhibiting the ERK pathway interferes with mitotic entry but has little effect on cdc2 activation and cyclin B1 and retards progression from metaphase to anaphase. Thus, our study provides novel evidence that ERK and PI3K pathways both promote cell cycle progression during G(2)/M but have different regulatory mechanisms and function at distinct times.     

Role of the ERK pathway in the activation of store-mediated calcium entry in human platelets

Extracellular signal-regulated kinases (ERKs), are common participants in a broad variety of signal transduction pathways. Several studies have demonstrated the presence of ERKs in human platelets and their activation by the physiological agonist thrombin. Here we report the involvement of the ERK cascade in store-mediated Ca(2+) entry in human platelets. Treatment of dimethyl-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid-loaded platelets with thapsigargin to deplete the intracellular Ca(2+) stores resulted in a time- and concentration-dependent activation of ERK1 and ERK2. Incubation with either U0126 or PD 184352, specific inhibitors of mitogen-activated protein kinase kinase (MEK), prevented thapsigargin-induced ERK activation. Furthermore, U0126 and PD 184352 reduced Ca(2+) entry stimulated by thapsigargin or thrombin, in a concentration-dependent manner. The role of ERK in store-mediated Ca(2+) entry was found to be independent of phosphatidylinositol 3- and 4-kinases, the tyrosine kinase pathway, and actin polymerization but sensitive to treatment with inhibitors of Ras, suggesting that the ERK pathway might be a downstream effector of Ras in mediating store-mediated Ca(2+) entry in human platelets. In addition, we have found that store depletion stimulated ERK activation does not require PKC activity. This study demonstrates for the first time a novel mechanism for regulation of store-mediated Ca(2+) entry in human platelets involving the ERK cascade.