Skp2 induction and phosphorylation is associated with the late G1 phase of proliferating rat hepatocytes

Regulation of modular Cyclin and CDK feedback loops by an E2F transcription oscillator in the mammalian cell cycle

The cell cycle is regulated by a large number of enzymes and transcription factors. We have developed a modular description of the cell cycle, based on a set of interleaved modular feedback loops, each leading to a cyclic behavior. The slowest loop is the E2F transcription and ubiquitination, which determines the cycling frequency of the entire cell cycle. Faster feedback loops describe the dynamics of each Cyclin by itself. Our model shows that the cell cycle progression as well as the checkpoints of the cell cycle can be understood through the interactions between the main E2F feedback loop and the driven Cyclin feedback loops. Multiple models were proposed for the cell cycle dynamics; each with differing basic mechanisms. We here propose a new generic formalism. In contrast with existing models, the proposed formalism allows a straightforward analysis and understanding of the dynamics, neglecting the details of each interaction. This model is not sensitive to small changes in the parameters used and it reproduces the observed behavior of the transcription factor E2F and different Cyclins in continuous or regulated cycling conditions. The modular description of the cell cycle resolves the gap between cyclic models, solely based on protein-protein reactions and transcription reactions based models. Beyond the explanation of existing observations, this model suggests the existence of unknown interactions, such as the need for a functional interaction between Cyclin B and retinoblastoma protein (Rb) de-phosphorylation.

Ubiquitin Ligases in Malignant Lymphoma

The highly controlled degradation of proteins via the ubiquitin-proteasome pathway represents a key mechanism for cell regulation and homeostasis. Ubiquitin-dependent proteolysis, carried out in large part by the E3 ubiquitin ligases, is a critical mode of post-translational modification that is important in regulation of cell cycle progression, signal transduction, gene transcription, antigen receptor signaling, immune response and cell differentiation. Recent studies demonstrate that increasing numbers of proteins with ubiquitin ligase activity are being characterized. Identification and characterization of their substrates indicate that they regulate the turnover of key cell cycle proteins (p27Kip1, p21Cip1, p57Kip2, cyclin E), tumor suppressor proteins (p53, RB), signaling kinases (Src, Zap70, PI-3 kinase), apoptosis regulators (Bcl-2, Bax, Bik) and transcription factors (Myc, NF-kappaB, E1F1), all of which have been implicated in the pathogenesis of malignant lymphoma. Studies to determine the functional role of ubiquitin ligases in the pathogenesis of malignant lymphoma represent potential areas of investigation.

Histologic-type specific role of cell cycle regulators in non-small cell lung carcinoma

BACKGROUND

Lung cancer is the most lethal type of cancer in humans. Cell cycle alterations have commonly been encountered in lung cancer and may have prognostic value.

MATERIALS AND METHODS

This study investigates the immunohistochemical expression of the important cell cycle regulators phosphatase and tensin homolog deleted on chromosome 10 (PTEN), p27, Cks1, and Skp2 in 128 non-small cell lung carcinomas (64 adenocarcinomas, 46 squamous cell carcinomas, and 18 large cell undifferentiated carcinomas) and adjacent non-neoplastic lung tissue.

RESULTS

PTEN and p27 were always highly expressed in non-neoplastic lung whereas Cks1 and Skp2 were not expressed in normal tissue. Decreased PTEN expression was noted in 19/64 adenocarcinomas, 15/46 squamous cell carcinomas, and 7/18 undifferentiated large cell carcinomas. Reduced expression of p27 was noted in 28/64, 19/46, and 6/18 of the tumors, respectively. Increased expression of Cks1 was seen in 38/64, 26/46, and 11/18 and increased expression of Skp2 in 29/64, 30/46, and 14/18 of the tumors, respectively. An inverse relationship between p27 and Skp2 levels was found in adenocarcinomas and between p27 and Cks1 levels in squamous cell carcinomas. Decreased PTEN and p27 expression were associated with advanced tumor stage in squamous cell carcinomas. Univariate analysis showed that high p27 and PTEN and low Cks1 expression correlated with increased survival in patients with squamous cell carcinoma independently of tumor stage.

CONCLUSIONS

Aberrant expression of PTEN, p27, Cks1, and Skp2 is a common feature of all three major types of non-small cell lung cancer NSCLC, but seems to be involved in the progression of squamous cell carcinoma alone.

Phosphorylation-dependent regulation of cytosolic localization and oncogenic function of Skp2 by Akt/PKB

Skp2 is an F-box protein that forms the SCF complex with Skp1 and Cullin-1 to constitute an E3 ligase for ubiquitylation. Ubiquitylation and degradation of the p27 are critical for Skp2-mediated entry to the cell cycle, and overexpression and cytosolic accumulation of Skp2 have been clearly associated with tumorigenesis, although the functional significance of the latter is still unknown. Here we show that Akt/protein kinase B (PKB) interacts with and directly phosphorylates Skp2. We find that Skp2 phosphorylation by Akt triggers SCF complex formation and E3 ligase activity. A phosphorylation-defective Skp2 mutant is drastically impaired in its ability to promote cell proliferation and tumorigenesis. Furthermore, we show that Akt-mediated phosphorylation triggers 14-3-3beta-dependent Skp2 relocalization to the cytosol, and we attribute a specific role to cytosolic Skp2 in the positive regulation of cell migration. Finally, we demonstrate that high levels of activation of Akt correlate with the cytosolic accumulation of Skp2 in human cancer specimens. Our results therefore define a novel proto-oncogenic Akt/PKB-dependent signalling pathway.

ERK2 but not ERK1 plays a key role in hepatocyte replication: an RNAi-mediated ERK2 knockdown approach in wild-type and ERK1 null hepatocytes

The mitogen-activated protein kinases (MAPKs) ERK1 and ERK2 have been implicated in various physiological events, and specific targeting of these MAPKs could affect cell proliferation in many cell types. First, to evaluate the potential specific roles of these two MAPKs, we analyzed the mitogenic response in regenerating liver after partial hepatectomy (PH) and in primary culture of hepatocytes isolated from ERK1-deficient mice. We show that ERK1 knockout and wild-type (wt) cells replicate with the same kinetics after PH in liver, in vivo, and in primary cultures of hepatocytes, in vitro. Indeed, Cyclin D1 and Cdk1 appear to be expressed concomitantly in knockout and wt cells, highlighting that hepatocytes progress in the cell cycle independently of the presence of ERK1. Second, we specifically abolished ERK2 expression by RNA interference in mouse and rat hepatocytes. We investigated whether small interfering RNA (siRNA) targeting ERK2 could specifically inhibit its expression and interfere with the process of replication. In ERK1-deficient hepatocytes, silencing ERK2 expression by RNA interference and ERK2 activation by U0126 clearly demonstrate that DNA replication is regulated by an ERK2-dependent mechanism. Furthermore, in rat wt hepatocytes, whereas ERK2 targeting inhibits late G(1) and S phase progression, ERK1 silencing is devoid of any effect on cell proliferation, indicating that ERK1 cannot rescue ERK2 deficiency.

CONCLUSION

Our results emphasize the importance of the MAPK cascade in hepatocyte replication and allow us to conclude that ERK2 is the key form involved in this regulation, in vivo and in vitro.

PC3, but not DU145, human prostate cancer cells retain the coregulators required for tumor suppressor ability of androgen receptor

BACKGROUND

Androgen receptor (AR) functions in normal prostate epithelium as a tumor suppressor to inhibit continuous proliferation of these cells. Such tumor suppressor function of AR is lost in androgen depletion independent (ADI) prostate cancers. In type-I ADI cancers AR is not expressed, while in type-II ADI cancers AR is recaptured as an oncogene. The PC3 and DU145 human prostate cancer cell lines are representative of the earlier type-I ADI prostate cancers. While these cells do not express AR, it is unclear whether they retained the coactivators necessary for AR-dependent tumor suppression. To answer this question the response to AR protein expression by PC3 and DU145 cells was evaluated.

METHODS

To do this, a lentiviral AR (Lenti-AR) expression system was engineered to encode an AR transcript which includes appropriate 5' and 3' untranslated regions (UTRs) containing all previously identified post-transcriptional regulatory sequences. AR expression and transcriptional activity were evaluated in Lenti-AR transduced cells by Western blot and luciferase assay, respectively. Cell growth in culture and in mouse xenografts was evaluated in correlation to expression changes in p21, p27, and p45(SKP2) proteins.

RESULTS

Lenti-AR transduced PC3 and DU145 lines expressed transcriptionally functional AR protein at appropriate physiological levels. Expression and engagement of AR protein in PC3-Lenti-AR cells resulted in transactivation of p21 and subsequent growth inhibition of these cells in culture and in mouse xenografts. Such inhibition was due to induced G1 arrest of these cells as documented by expression changes in p27 and p45(SKP2) proteins. Such growth inhibition was not observed in DU145-Lenti-AR cells.

CONCLUSIONS

These results document that PC3, but not DU145 cells retain the coregulators needed for AR tumor suppressor ability.

Skp2 and p27kip1 expression in melanocytic nevi and melanoma: an inverse relationship

BACKGROUND

S-phase kinase associated protein-2 (Skp2) ubiquitin ligase p45(SKP2) is important in the degradation of p27kip1 (a cyclin dependent kinase inhibitor) and progression through the G1-S cell-cycle checkpoint. Low levels of p27 and high levels of Skp2 are related to poor prognosis in some cancers.

METHODS

Clinicopathologic features and immunohistochemical expression of Skp2 and p27kip1 were investigated in 198 melanocytic proliferations: 21 melanocytic nevi, 23 melanoma in situ, 119 primary melanoma, and 35 metastatic melanoma samples. Comparative and survival analyses were performed.

RESULTS

Progressive and significant increases and decreases in the nuclear expression of Skp2 and p27kip1, respectively, was identified moving from melanocytic nevi (0.05 +/- 0.2/85 +/- 15) to melanoma in situ (3 +/- 2/45 +/- 20) to primary cutaneous melanoma (12 +/- 9/30 +/- 25) to metastatic melanoma (25 +/- 15/15 +/- 20) (p < or = 0.006). Expression of these proteins also significantly correlated with increasing American Joint Committee on Cancer (AJCC) T (tumor) classification and AJCC stage (p < or = 0.01). Moreover, the level of these two proteins exhibited a significant inverse relationship (r = -0.4, p = 0.0001). Skp2 cytoplasmic labeling index of >20% predicted worse 10-year overall survival (38% vs. 86%, p = 0.04) in primary melanoma. Neither p27 nor Skp2 nuclear expression impacted significantly on prognosis.

CONCLUSIONS

Gain of Skp2 and loss of p27kip1 protein expression are implicated in melanoma progression where the level of p27kip1 may be regulated by targeted proteolysis via Skp2. Cytoplasmic expression of Skp2 defines a subset of aggressive melanomas and could represent another pathway of deregulation of the cell cycle.

Activation of RB/E2F signaling pathway is required for the modulation of hepatitis C virus core protein-induced cell growth in liver and non-liver cells

Hepatitis C virus (HCV) core protein is a multifunctional protein that affects transcription and cell growth in vitro and in vivo. Here, we confirm the proliferative activities of core protein in liver and non-liver cells and delineate part of the mechanism whereby core protein promotes cell growth. We show that core protein suppresses the expression of tumor suppressor protein p53 and cyclin-dependent kinase (CDK) inhibitor p21 and enhances the activation of cyclin-dependent kinase 2 (CDK2), the phosphorylation of retinoblastoma (Rb), the activation of the transcription factor E2F-1, and the expression of E2F-1 and S phase kinase-interacting protein 2 (SKP2) genes. Pretreatment of core protein-expressing cells with the inhibitor of CDK2, Butyrolactone I, abolished the phosphorylation of Rb, the activation of E2F-1, and inhibited the expression of E2F-1 gene and cell growth induced. Consistent with these findings, we define a new signaling pathway whereby the HCV core protein mediates cell growth in infected cells.

Multisite phosphorylation and network dynamics of cyclin-dependent kinase signaling in the eukaryotic cell cycle

Multisite phosphorylation of regulatory proteins has been proposed to underlie ultrasensitive responses required to generate nontrivial dynamics in complex biological signaling networks. We used a random search strategy to analyze the role of multisite phosphorylation of key proteins regulating cyclin-dependent kinase (CDK) activity in a model of the eukaryotic cell cycle. We show that multisite phosphorylation of either CDK, CDC25, wee1, or CDK-activating kinase is sufficient to generate dynamical behaviors including bistability and limit cycles. Moreover, combining multiple feedback loops based on multisite phosphorylation do not destabilize the cell cycle network by inducing complex behavior, but rather increase the overall robustness of the network. In this model we find that bistability is the major dynamical behavior of the CDK signaling network, and that negative feedback converts bistability into limit cycle behavior. We also compare the dynamical behavior of several simplified models of CDK regulation to the fully detailed model. In summary, our findings suggest that multisite phosphorylation of proteins is a critical biological mechanism in generating the essential dynamics and ensuring robust behavior of the cell cycle.

Elevated S-Phase Kinase-Associated Protein 2 Protein Expression in Acute Myelogenous Leukemia

PURPOSE

The F-box protein S-phase kinase-associated protein 2 (Skp2) positively regulates the G(1)-S phase transition by controlling the stability of several G(1) regulators, such as p27Kip1. However, the clinical significance of Skp2 in patients with acute myelogenous leukemia (AML) remains unknown.

EXPERIMENTAL DESIGN

We examined the clinical and biological significance of Skp2 expression in AML and evaluated the relationship between Skp2 and p27Kip1 expression and phosphatase and tensin homologue (PTEN) phosphorylation.

RESULTS

Western blot analysis showed that high Skp2 expression was observed in 57 (57.6%) cases and significantly correlated with unfavorable cytogenetics (P = 0.035) but not with age, white blood cell count, serum lactic dehydrogenase level, and the French-American-British subtype. An inverse correlation was not observed between Skp2 and p27Kip1 expression. However, p27Kip1 protein was preferentially localized to cytoplasm in the high-Skp2-expression group. The cytoplasmic to nuclear ratio of p27Kip1 expression was significantly correlated with the levels of Skp2 expression (P < 0.001). The frequency of PTEN phosphorylation was significantly higher in the high-Skp2-expression group compared with the low- Skp2-expression group (P = 0.035). The Skp2 overexpression was significantly associated with shorter disease-free survival and overall survival (P = 0.0386 and P = 0.0369, respectively). Multivariate analysis showed that Skp2 expression was an independent prognostic factor both in the disease-free survival and overall survival.

CONCLUSION

These findings suggest that Skp2 expression is an independent marker for a poor prognosis in AML. The presence of a positive correlation between Skp2 and phosphorylated PTEN suggests that an aberration in the PTEN/Skp2 signaling pathway might be operating in AML.

Dynamics of the cell cycle: checkpoints, sizers, and timers

We have developed a generic mathematical model of a cell cycle signaling network in higher eukaryotes that can be used to simulate both the G1/S and G2/M transitions. In our model, the positive feedback facilitated by CDC25 and wee1 causes bistability in cyclin-dependent kinase activity, whereas the negative feedback facilitated by SKP2 or anaphase-promoting-complex turns this bistable behavior into limit cycle behavior. The cell cycle checkpoint is a Hopf bifurcation point. These behaviors are coordinated by growth and division to maintain normal cell cycle and size homeostasis. This model successfully reproduces sizer, timer, and the restriction point features of the eukaryotic cell cycle, in addition to other experimental findings.

Estrogens down-regulate p27Kip1 in breast cancer cells through Skp2 and through nuclear export mediated by the ERK pathway

The cyclin-dependent kinase (CDK) inhibitor p27Kip1 plays a key role in growth and development of the mammary epithelium and in breast cancer. p27Kip1 levels are regulated through ubiquitin/proteasome-mediated proteolysis, promoted by CDK2 and the F box protein Skp2 at the G1/S transition, and independent of Skp2 in mid-G1. We investigated the respective roles of Skp2 and subcellular localization of p27Kip1 in down-regulation of p27Kip1 induced in MCF-7 cells by estrogens. 17beta-Estradiol treatment increased Skp2 expression in MCF-7 cells; however, this increase was prevented by G1 blockade mediated by p16Ink4a or the CDK inhibitor roscovitine, whereas down-regulation of p27Kip1 was maintained. Exogenous Skp2 prevented growth arrest of MCF-7 cells by antiestrogen, coinciding with decreased p27Kip1 expression. Under conditions of G1 blockade, p27Kip1 was stabilized by inhibition of CRM1-dependent nuclear export with leptomycin B or by mutation of p27Kip1 (Ser10 --> Ala; S10A) interfering with CRM1/p27Kip1 interaction. Antisense Skp2 oligonucleotides and a dominant-interfering Cul-1(1-452) mutant prevented down-regulation of p27Kip1S10A, whereas Skp2 overexpression elicited its destruction in mitogen-deprived cells. Active mediators of the extracellular signal-regulated kinase (ERK) pathway including Raf-1caax induced cytoplasmic localization of p27Kip1 in antiestrogen-treated cells and prevented accumulation of p27Kip1 in these cells independent of Skp2 expression and coinciding with ERK activation. Genetic or chemical blockade of the ERK pathway prevented down-regulation and cytoplasmic localization of p27Kip1 in response to estrogen. Our studies indicate that estrogens elicit down-regulation of p27Kip1 in MCF-7 cells through Skp2-dependent and -independent mechanisms that depend upon subcellular localization of p27Kip1 and require the participation of mediators of the Ras/Raf-1/ERK signaling pathway.

Expression of Skp2, a p27(Kip1) ubiquitin ligase, in malignant lymphoma: correlation with p27(Kip1) and proliferation index

Reduced levels of p27(Kip1) are frequent in human cancers and have been associated with poor prognosis. Skp2, a component of the Skp1-Cul1-F-box protein (SCF) ubiquitin ligase complex, has been implicated in p27(Kip1) degradation. Increased Skp2 levels are seen in some solid tumors and are associated with reduced p27(Kip1). We examined the expression of these proteins using single and double immunolabeling in a large series of lymphomas to determine if alterations in their relative levels are associated with changes in cell proliferation and lymphoma subgroups. We studied the expression of Skp2 in low-grade and aggressive B-cell lymphomas (n = 86) and compared them with p27(Kip1) and the proliferation index (PI). Fifteen hematopoietic cell lines and peripheral blood lymphocytes were studied by Western blot analysis. In reactive tonsils, Skp2 expression was limited to proliferating germinal center and interfollicular cells. Skp2 expression in small lymphocytic lymphomas (SLLs) and follicular lymphomas (FCLs) was low (mean percentage of positive tumor cells, less than 20%) and was inversely correlated (r = -0.67; P <.0001) with p27(Kip1) and positively correlated with the PI (r = 0.82; P <.005). By contrast, whereas most mantle cell lymphomas (MCLs) demonstrated low expression of p27(Kip1) and Skp2, a subset (n = 6) expressed high Skp2 (exceeding 20%) with a high PI (exceeding 50%). Skp2 expression was highest in diffuse large B-cell lymphomas (DLBCLs) (mean, 22%) and correlated with Ki-67 (r = 0.55; P <.005) but not with p27(Kip1). Cytoplasmic Skp2 was seen in a subset of aggressive lymphomas. Our data provide evidence for p27(Kip1) degradative function of Skp2 in low-grade lymphomas. The absence of this relationship in aggressive lymphomas suggests that other factors contribute to deregulation of p27(Kip1) expression in these tumors.

Paradigms of growth control: relation to Cdk activation

The cyclin-dependent kinases (CDKs) play a key role in cell cycle control, and in this review, we focus on the events that regulate their activities. Emphasis is placed on the CDKs that function during the G(1) phase of the cell cycle and on the CDK inhibitor p27(Kip1). We discuss how CDK activation relates to two basic concepts of cell cycle regulation: (i) the need for multiple mitogens for the proliferation of nontransformed cells and (ii) the inhibitory effect of high culture density on proliferative capacity. We also describe how Cdk2 modulates the expression of the alpha subunit of the interleukin-2 receptor in T cells, and address the question of whether p27(Kip1) functions as an activator or inhibitor of the CDKs associated with the D cyclins.

Induction of hepatocyte proliferation and liver hyperplasia by the targeted expression of cyclin E and skp2

Cells in culture become competent to replicate in the absence of growth factor after progressing beyond the late G1 restriction point, suggesting that a set of genes expressed during G1 phase is sufficient to trigger completion of the cell cycle. However, this has not been demonstrated in an in vivo system. In this study, we examined whether transfection of genes associated with the G1/S transition could trigger hepatocyte replication. Co-transfection of cyclin E and skp2 synergistically promoted cell cycle progression in cultured primary hepatocytes in the absence of mitogen or in the presence of growth inhibitors. Furthermore, transfection of hepatocytes in vivo with cyclin E and skp2 promoted abundant hepatocyte replication and hyperplasia of the liver. These studies confirm that transfection with a small number of genes can trigger proliferation of quiescent hepatocytes in vivo, and suggest that therapies to enhance liver regeneration by targeting cell cycle control genes may be feasible.

Identification of a novel Skp2-like mammalian protein containing F-box and leucine-rich repeats

The F-box protein Skp2 is important for S phase entry and binds to Skp1 and the cyclin A-Cdk2 complex. Here we report the cloning, analysis of genomic organization and characterization of a novel gene product related to Skp2 named FBL2. The human FBL2 gene was found to be a highly interrupted gene of at least 126.6 kb located on chromosome 17 in close proximity to the TRAP220 gene in a head-to-tail orientation. The predicted protein contains an F-box and six perfect C-terminal leucine-rich repeats. Similar to Skp2, this protein interacts with Skp1 and deletion of the F-box inhibits this association. However, in contrast to Skp2, FBL2 was detected in non-proliferating hepatocytes and its expression increased in growth-arrested liver epithelial cells. In addition, FBL2 was localized primarily in the cytoplasm concentrated around the nucleus. Overall, our data indicate that although FBL2 shares strong structural homology with Skp2 as well as having a similar ability to associate with Skp1, these proteins likely play distinct roles and target different substrates to the SCF complex.