Accele-ration of the G1/S phase transition by expression of cyclins D1 and E with an inducible system. Mol Cell Biol. 1994;14:1669-1679. [PMID: 8114703 DOI: 10.1128/MCB.14.3.1669]

Production of vesicular stomatitis virus G glycoprotein (VSV-G) pseudotyped retroviral vectors

Retrovirus pseudotype is defined as the genome of one retrovirus encapsidated by the envelope protein of a second virus. The host range of the pseudotype is that of the virus donating the envelope protein. Two procedures that use 293GP cells, which are derived from human kidney 293 cells, are described here. The first is based on the high transient transfection efficiency of 293 cells. The retroviral construct and an expression plasmid for VSV-G are co-transfected into 293GP cells that stably express MLV gag and pol proteins. Transiently generated virus is then harvested during consecutive days following DNA transfection. The second procedure involves stable 293GP cell lines containing the VSV-G gene under the control of a promoter whose activity is regulated by tetracycline. Cell lines containing the retroviral vector of interest are established under noninduced conditions. Infectious virus can be harvested following the induction of VSV-G expression in these cell lines.

Truncation in CCND1 mRNA alters miR-16-1 regulation in mantle cell lymphoma

Cyclin D1 (CCND1) is a well-known regulator of cell-cycle progression. It is overexpressed in several types of cancer including breast, lung, squamous, neuroblastoma, and lymphomas. The most well-known mechanism of overexpression is the t(11;14)(q13;q32) translocation found in mantle cell lymphoma (MCL). It has previously been shown that truncated CCND1 mRNA in MCL correlates with poor prognosis. We hypothesized that truncations of the CCND1 mRNA alter its ability to be down-regulated by microRNAs in MCL. MicroRNAs are a new class of abundant small RNAs that play important regulatory roles at the posttranscriptional level by binding to the 3' untranslated region (UTR) of mRNAs blocking either their translation or initiating their degradation. In this study, we have identified the truncation in CCND1 mRNA in MCL cell lines. We also found that truncated CCND1 mRNA leads to increased CCND1 protein expression and increased S-phase cell fraction. Furthermore, we demonstrated that this truncation alters miR-16-1 binding sites, and through the use of reporter constructs, we were able to show that miR-16-1 regulates CCND1 mRNA expression. This study introduces the role of miR-16-1 in the regulation of CCND1 in MCL.

Airway smooth muscle in asthma

Airway smooth muscle plays a multifaceted role in the pathogenesis of asthma. We review the current understanding of the contribution of airway myocytes to airway inflammation, airway wall remodeling, and airflow obstruction in this prevalent disease syndrome. Together, these roles make airway smooth muscle an attractive target for asthma therapy.

Synergistic function of E2F7 and E2F8 is essential for cell survival and embryonic development

The E2f7 and E2f8 family members are thought to function as transcriptional repressors important for the control of cell proliferation. Here, we have analyzed the consequences of inactivating E2f7 and E2f8 in mice and show that their individual loss had no significant effect on development. Their combined ablation, however, resulted in massive apoptosis and dilation of blood vessels, culminating in lethality by embryonic day E11.5. A deficiency in E2f7 and E2f8 led to an increase in E2f1 and p53, as well as in many stress-related genes. Homo- and heterodimers of E2F7 and E2F8 were found on target promoters, including E2f1. Importantly, loss of either E2f1 or p53 suppressed the massive apoptosis in double-mutant embryos. These results identify E2F7 and E2F8 as a unique repressive arm of the E2F transcriptional network that is critical for embryonic development and control of the E2F1-p53 apoptotic axis.

Nerve Growth factor regulation of cyclin D1 in PC12 cells through a p21RAS extracellular signal-regulated kinase pathway requires cooperative interactions between Sp1 and nuclear factor-kappaB

The PC12 pheochromocytoma cell line responds to nerve growth factor (NGF) by exiting from the cell cycle and differentiating to induce extending neurites. Cyclin D1 is an important regulator of G1/S phase cell cycle progression, and it is known to play a role in myocyte differentiation in cultured cells. Herein, NGF induced cyclin D1 promoter, mRNA, and protein expression via the p21(RAS) pathway. Antisense- or small interfering RNA to cyclin D1 abolished NGF-mediated neurite outgrowth, demonstrating the essential role of cyclin D1 in NGF-mediated differentiation. Expression vectors encoding mutants of the Ras/mitogen-activated protein kinase pathway, and chemical inhibitors, demonstrated NGF induction of cyclin D1 involved cooperative interactions of extracellular signal-regulated kinase, p38, and phosphatidylinositol 3-kinase pathways downstream of p21(RAS). NGF induced the cyclin D1 promoter via Sp1, nuclear factor-kappaB, and cAMP-response element/activated transcription factor sites. NGF induction via Sp1 involved the formation of a Sp1/p50/p107 complex. Cyclin D1 induction by NGF governs differentiation and neurite outgrowth in PC12 cells.

Endothelin 3 Induces Skin Pigmentation in a Keratin-Driven Inducible Mouse Model

Endothelin 3 (Edn3) encodes a ligand important to developing neural crest cells and is allelic to the spontaneous mouse mutation occurring at the lethal spotting (ls) locus. Edn3(ls/ls) mutants exhibit a spotted phenotype due to reduced numbers of neural crest-derived melanocyte precursors in the skin. In this study, we show that when Edn3 is driven by the keratin 5 promoter and thereby placed proximal to melanocyte lineage cells, adult mice manifest pigmented skin harboring dermal melanocytes. Using a tetracycline inducible system, we show that the postnatal expression of Edn3 is required to maintain these dermal melanocytes, and that early expression of the Edn3 transgene is important to the onset of the hyperpigmentation phenotype. Crosses into Edn3(ls/ls) mutants demonstrate that the Edn3 transgene expression does not fully compensate for the endogenous expression pattern. Crosses into tyrosine kinase receptor Kit(Wv) mutants indicate that Edn3 can partially compensate for Kit's role in early development. Crosses into A(y) mutant mice considerably darkened their yellow coat color suggesting a previously unreported role for endothelin signaling in pigment switching. These results demonstrate that exogenous Edn3 affects both precursors and differentiated melanocytes, leading to a phenotype with characteristics similar to the human skin condition dermal melanocytosis.

Cyclin E overexpression impairs progression through mitosis by inhibiting APC(Cdh1)

Overexpression of cyclin E, an activator of cyclin-dependent kinase 2, has been linked to human cancer. In cell culture models, the forced expression of cyclin E leads to aneuploidy and polyploidy, which is consistent with a direct role of cyclin E overexpression in tumorigenesis. In this study, we show that the overexpression of cyclin E has a direct effect on progression through the latter stages of mitotic prometaphase before the complete alignment of chromosomes at the metaphase plate. In some cases, such cells fail to divide chromosomes, resulting in polyploidy. In others, cells proceed to anaphase without the complete alignment of chromosomes. These phenotypes can be explained by an ability of overexpressed cyclin E to inhibit residual anaphase-promoting complex (APC^Cdh1) activity that persists as cells progress up to and through the early stages of mitosis, resulting in the abnormal accumulation of APC^Cdh1 substrates as cells enter mitosis. We further show that the accumulation of securin and cyclin B1 can account for the cyclin E–mediated mitotic phenotype.

Osteonectin downregulates E-cadherin, induces osteopontin and focal adhesion kinase activity stimulating an invasive melanoma phenotype

Osteonectin is recognised as a marker of metastasis progression in melanoma and has been implicated in the transition from radial to vertical growth phase. A Tetracycline-inducible system was used to regulate Osteonectin protein levels in melanoma cell lines to examine the morphological, biochemical and invasive changes that accompany its altered expression. Assay of protein and phosphorylation changes showed a downregulation of E-cadherin, upregulation of Osteopontin and a corresponding increase in phosphorylation of Focal Adhesion Kinase on Tyr(397) and Tyr(576) concomitant with Osteonectin induction. Melanoma cells overexpressing Osteonectin displayed increased invasive potential, whereas ablation of Osteonectin gene transcription using siRNA suppressed the invasive potential of these cells and resulted in the upregulation of E-cadherin. The recently described interaction of Osteonectin with Integrin Linked Kinase leading to modulation of its activity suggests a mechanism relevant to the loss of E-cadherin and cell adhesion that occurs during melanoma progression. These results indicate a central role for Osteonectin in the regulation of gene expression changes driving the progression of melanoma toward metastasis.

Histone deacetylase degradation and MEF2 activation promote the formation of slow-twitch myofibers

Skeletal muscle is composed of heterogeneous myofibers with distinctive rates of contraction, metabolic properties, and susceptibility to fatigue. We show that class II histone deacetylase (HDAC) proteins, which function as transcriptional repressors of the myocyte enhancer factor 2 (MEF2) transcription factor, fail to accumulate in the soleus, a slow muscle, compared with fast muscles (e.g., white vastus lateralis). Accordingly, pharmacological blockade of proteasome function specifically increases expression of class II HDAC proteins in the soleus in vivo. Using gain- and loss-of-function approaches in mice, we discovered that class II HDAC proteins suppress the formation of slow twitch, oxidative myofibers through the repression of MEF2 activity. Conversely, expression of a hyperactive form of MEF2 in skeletal muscle of transgenic mice promotes the formation of slow fibers and enhances running endurance, enabling mice to run almost twice the distance of WT littermates. Thus, the selective degradation of class II HDACs in slow skeletal muscle provides a mechanism for enhancing physical performance and resistance to fatigue by augmenting the transcriptional activity of MEF2. These findings provide what we believe are new insights into the molecular basis of skeletal muscle function and have important implications for possible therapeutic interventions into muscular diseases.

Cyclin-dependent kinase 2/cyclin E complex is involved in p120 catenin (p120ctn)-dependent cell growth control: a new role for p120ctn in cancer

Depending on its cellular localization, p120 catenin (p120ctn) can participate in various processes, such as cadherin-dependent cell-cell adhesion, actin cytoskeleton remodeling, and intracellular trafficking. Recent studies also indicate that p120ctn could regulate cell proliferation and contact inhibition. This report describes a new function of p120ctn in the regulation of cell cycle progression. Overexpression of the p120ctn isoform 3A in human colon adenocarcinoma cells (HT-29) results in cytoplasmic accumulation of the protein, as observed in many tumors. This cytoplasmic increase is correlated with a reduction in proliferation and inhibition of DNA synthesis. Under these conditions, experiments on synchronized cells revealed a prolonged S phase associated with cyclin E stabilization. Both confocal microscopy and biochemical analysis showed that cyclin E and cyclin-dependent kinase 2 colocalized with p120ctn in centrosomes during mitosis. These proteins are associated in a functional complex evidenced by coimmunoprecipitation experiments and the emergence of Thr199-phosphorylated nucleophosmin/B23. Such post-translational modification of this centrosomal target has been shown to trigger the initiation of centrosome duplication. Therefore, p120ctn-mediated accumulation of cyclin E in centrosomes may participate in abnormal amplification of centrosomes and the inhibition of DNA replication, thus leading to aberrant mitosis and polyploidy. Because these modifications are often observed in cancer, p120ctn may represent a new therapeutic target for future therapy.

Alternative cyclin D1 forms a and b have different biological functions in the cell cycle of B lymphocytes

Cyclin D1 is an important regulator of the early phase of the cell cycle and the transcriptional machinery. It is often deregulated in human tumors of various origins and is considered to be an oncogene. The CCND1 gene encoding cyclin D1 generates two mRNAs by alternative splicing, leading to the production of two alternative proteins: a long form a (36 kDa) and a short form b (30-31 kDa) from which the C-terminal moiety required for protein stability and sub-cellular localization has been deleted. Both forms of RNA and protein have been detected in B-cell hemopathies, but their respective roles are unclear. We investigated the function of cyclin D1b in cell cycle regulation, by generating B-cell lines displaying conditional expression of isoform b. Comparisons of these cell lines (BD1b series) with previously obtained cell lines expressing cyclin D1a demonstrated that cyclin D1b had no cell cycle regulatory properties.

Expression of genomic AtCYCD2;1 in Arabidopsis induces cell division at smaller cell sizes: implications for the control of plant growth

The Arabidopsis (Arabidopsis thaliana) CYCD2;1 gene introduced in genomic form increased cell formation in the Arabidopsis root apex and leaf, while generating full-length mRNA, raised CDK/CYCLIN enzyme activity, reduced G1-phase duration, and reduced size of cells at S phase and division. Other cell cycle genes, CDKA;1, CYCLIN B;1, and the cDNA form of CYCD2;1 that produced an aberrantly spliced mRNA, produced smaller or zero increases in CDK/CYCLIN activity and did not increase the number of cells formed. Plants with a homozygous single insert of genomic CYCD2;1 grew with normal morphology and without accelerated growth of root or shoot, not providing evidence that cell formation or CYCLIN D2 controls growth of postembryonic vegetative tissues. At the root apex, cells progressed normally from meristem to elongation, but their smaller size enclosed less growth and a 40% reduction in final size of epidermal and cortical cells was seen. Smaller elongated cell size inhibited endoreduplication, indicating a cell size requirement. Leaf cells were also smaller and more numerous during proliferation and epidermal pavement and palisade cells attained 59% and 69% of controls, whereas laminas reached normal size. Autonomous control of expansion was therefore not evident in abundant cell types that formed tissues of root or leaf. Cell size was reduced by a greater number formed in a tissue prior to cell and tissue expansion. Initiation and termination of expansion did not correlate with cell dimension or number and may be determined by tissue-wide signals acting across cellular boundaries.

Cyclin D1 Gene G870A Polymorphism Predicts Response to Neoadjuvant Radiotherapy and Prognosis in Rectal Cancer

PURPOSE

To investigate whether CCND1 genetic variations associated with a constitutive nuclear protein may influence either the pathologic response to preoperative RT or the prognosis in a series of rectal cancer patients.

METHODS AND MATERIALS

Seventy rectal cancer patients treated by neoadjuvant radiotherapy were included in the study. CCND1 exon 5 mutations were screened, and the G870A polymorphism was assessed for correlation with clinical variables, tumor response, and patient outcome.

RESULTS

No exon 5 mutation was found. Concerning the G870A polymorphism, the A/A variant was significantly associated with radiosensitivity (p = 0.022). Moreover, patients harboring the A allele were correlated with a lower risk of local failure (p = 0.017). Also, combination of the G870A polymorphism with the post-therapeutic lymph node status allowed the elaboration of a prognostic index, which accurately distinguished subgroups of patients with predictable recurrence-free (p = 0.003) and overall (p = 0.044) survival.

CONCLUSIONS

Although CCND1 exon 5 mutations are rare in rectal cancer, G870A polymorphism is a frequent variation that may predict radiosensitivity and prognosis.

Synergistic effect of cyclin D1 and c-Myc leads to more aggressive and invasive mammary tumors in severe combined immunodeficient mice

Cyclin D1 is one of the most commonly overexpressed oncogenes in breast cancer; yet, it is not clear whether cyclin D1 alone is capable of causing malignant transformation of mammary epithelial cells. Here, we show that ectopic expression of cyclin D1 in benign mouse mammary epithelial cells promotes cell proliferation, anchorage-independent growth in soft agar, and tumorigenesis in severe combined immunodeficient mice. To address the possible interaction of cyclin D1 and c-myc in malignant transformation, we used cyclin D1/c-myc dual-expressing clones, which displayed more aggressive and invasive phenotype than cyclin D1-expressing clones. These data provide evidence that overexpression of cyclin D1 or coexpression with c-myc could cause invasive malignant transformation of benign mouse mammary epithelial cells. Furthermore, microarray analysis of cyclin D1 and cyclin D1/c-myc clones showed that these two tumor-producing clones might use distinct invasive pathways. In summary, overexpression of cyclin D1 may commit mammary epithelia to a tumor-prone phenotype in which cooperation with other genes, such as synergy with c-myc, may lead to a more aggressive phenotype.

Functions of cyclin D1 as an oncogene and regulation of cyclin D1 expression

Cyclin D1 binds to the Cdk4 and Cdk6 to form a pRB kinase. Upon phosphorylation, pRB loses its repressive activity for the E2F transcription factor, which then activates transcription of several genes required for the transition from the G1- to S-phase and for DNA replication. The cyclin D1 gene is rearranged and overexpressed in centrocytic lymphomas and parathyroid tumors and it is amplified and/or overexpressed in a major fraction of human tumors of various types of cancer. Ectopic overexpression of cyclin D1 in fibroblast cultures shortens the G1 phase of the cell cycle. Furthermore, it has been demonstrated that introduction of an antisense cyclin D1 into a human carcinoma cell line, in which the cyclin D1 gene is amplified and overexpressed, causes reversion of the malignant phenotype. Thus, increased expression of cyclin D1 can play a critical role in tumor development and in maintenance of the malignant phenotype. However, it is insufficient to confer transformed properties on primary or established fibroblasts. In this review, we summarize the role of cyclin D1 on tumor development and malignant transformation. In addition, our chemical biology study to understand the regulatory mechanism of cyclin D1 transcription is also reviewed.

TGFbeta regulates the expression and activities of G2 checkpoint kinases in human myeloid leukemia cells

Transforming Growth Factor-beta (TGFbeta) is known to be a negative regulator of G1 cyclin/cdk activity. It is not clear whether TGFbeta has any effect on G2 checkpoint kinases. We have found that TGFbeta downregulated the expression of several G2 checkpoint kinases including cdc2, cyclin B1, and cdc25c without causing cell accumulation in G2/M phases in two human leukemia cell lines. The inhibition was time-dependent with a maximal inhibition being observed by 24h for cyclin B1 and cdc2 and by 48h for cdc25c. The inhibition was not a result of G1 arrest but a direct effect of TGFbeta which downregulates their expression at mRNA level. In proliferating cells, there was a significant formation of cdc2-pRb complexes, which was decreased to 30% of control levels by 48h after initiating TGFbeta treatment. Cdc2 showed a marked kinase activity on GST-Rb protein in proliferating cells detected by in vitro kinase assay, which was downregulated in response to TGFbeta. In addition, TGFbeta caused a rapid and transient dephosphorylation of cdc2 (Tyr15) and cdc25c (Ser216) for about 2-3h before a dramatic decrease of both molecules by 48h. Taken together, our data suggest that TGFbeta has a direct inhibitory effect on G2 checkpoint kinases, which is regulated at mRNA level. The transient activation of cdc2 and cdc25c and subsequent inhibition of cdc2, cyclin B1, and cdc25c could amplify TGFbeta-induced G1 arrest and growth inhibition.

Conditional RNA interference achieved by Oct-1 POU/rtTA fusion protein activator and a modified TRE-mouse U6 promoter

RNA interference (RNAi) is a powerful technique and is widely used to down-regulate expression of specific genes in cultured cells and in vivo. In this paper, we report our development of a new tetracycline-inducible RNAi expression using a modified TRE-mouse U6 promoter in which the distal sequence element (DSE) was replaced by the tetracycline-responsive element (TRE). The modified TRE-mouse U6 promoter can be activated by a Tet-on version tetracycline-regulated artificial activator rTetOct which was constructed by fusing the rtTA DNA binding domain with the Oct-1 POU activation domain. This rTetOct/TRE-U6 system was successfully applied to conditionally and reversibly down-regulate the expression of endogenous p53 gene in MCF7 cells, and the expression of beta-defensin gene (mBin1b) either transiently expressed in COS7 cells or stably expressed in CHO cells.

RhoE interferes with Rb inactivation and regulates the proliferation and survival of the U87 human glioblastoma cell line

Rho GTPases are important regulators of actin cytoskeleton, but they are also involved in cell proliferation, transformation and oncogenesis. One of this proteins, RhoE, inhibits cell proliferation, however the mechanism that regulates this effect remains poorly understood. Therefore, we undertook the present study to determine the role of RhoE in the regulation of cell proliferation. For this purpose we generated an adenovirus system to overexpress RhoE in U87 glioblastoma cells. Our results show that RhoE disrupts actin cytoskeleton organization and inhibits U87 glioblastoma cell proliferation. Importantly, RhoE expressing cells show a reduction in Rb phosphorylation and in cyclin D1 expression. Furthermore, RhoE inhibits ERK activation following serum stimulation of quiescent cells. Based in these findings, we propose that RhoE inhibits ERK activation, thereby decreasing cyclin D1 expression and leading to a reduction in Rb inactivation, and that this mechanism is involved in the RhoE-induced cell growth inhibition. Moreover, we also demonstrate that RhoE induces apoptosis in U87 cells and also in colon carcinoma and melanoma cells. These results indicate that RhoE plays an important role in the regulation of cell proliferation and survival, and suggest that this protein may be considered as an oncosupressor since it is capable to induce apoptosis in several tumor cell lines.

Dexamethasone inhibits epidermal growth factor-stimulated gastric epithelial cell proliferation

Epidermal growth factor (EGF) is essential to heal gastric ulcers, whereas glucocorticoid delays rat gastric ulcer healing. We found that dexamethasone inhibited EGF-stimulated rat gastric epithelial cell (RGM-1) proliferation by cell count and DNA synthesis analysis of flow cytometry and attempted to elucidate the possible mechanistic pathway via Western blot analysis. EGF (10 ng/ml) treatment for 24 h significantly increased RGM-1 cell proliferation, and dexamethasone (10(-8) and 10(-6) M) markedly suppressed EGF-stimulated cell proliferation. Western blotting results demonstrated that the phosphorylated extracellular signal-regulated kinase (pERK) (pERK1/pERK2) significantly increased at 10 min after EGF treatment. This was followed by increase of cyclooxygenase (COX)-2 expression at 3 h after EGF treatment. The continued increase of COX-2 (up to 18 h) resulted in increased intracellular prostaglandin E(2) and cyclin D1 expression significantly after 8 and 12 h of EGF treatment. Dexamethasone substantially reduced EGF-stimulated COX-2 expression at 3 and 6 h and cyclin D1 expression at 8 and 12 h. Pretreatment of RGM-1 cells with dexamethasone or 2'-amino-3'-methoxyflavone (PD98059)-mitogen-activated protein kinase kinase inhibitor (5 x 10(-5) M) significantly reduced EGF-stimulated pERK1/pERK2 expression. Simultaneous treatment of RGM-1 cells with PD98059 and EGF also markedly decreased EGF-stimulated COX-2 expression at 6 h. These findings indicate that dexamethasone significantly suppresses EGF-stimulated gastric epithelial cell proliferation, and one of the pathways involved is via inhibiting activation of ERK1/ERK2, followed by inhibition of COX-2, cyclin D1 expression, and finally DNA synthesis.

Energy-dependent nucleolar localization of p53 in vitro requires two discrete regions within the p53 carboxyl terminus

The p53 tumor suppressor is a nucleocytoplasmic shuttling protein that is found predominantly in the nucleus of cells. In addition to mutation, abnormal p53 cellular localization is one of the mechanisms that inactivate p53 function. To further understand features of p53 that contribute to the regulation of its trafficking within the cell, we analysed the subnuclear localization of wild-type and mutant p53 in human cells that were either permeabilized with detergent or treated with the proteasome inhibitor MG132. We, here, show that either endogenously expressed or exogenously added p53 protein localizes to the nucleolus in detergent-permeabilized cells in a concentration- and ATP hydrolysis-dependent manner. Two discrete regions within the carboxyl terminus of p53 are essential for nucleolar localization in permeabilized cells. Similarly, localization of p53 to the nucleolus after proteasome inhibition in unpermeabilized cells requires sequences within the carboxyl terminus of p53. Interestingly, genotoxic stress markedly decreases the association of p53 with the nucleolus, and phosphorylation of p53 at S392, a site that is modified by such stress, partially impairs its nucleolar localization. The possible significance of these findings is discussed.

Refined spatial manipulation of neuronal function by combinatorial restriction of transgene expression

Selective genetic manipulation of neuronal function in vivo requires techniques for targeting gene expression to specific cells. Existing systems accomplish this using the promoters of endogenous genes to drive expression of transgenes directly in cells of interest or, in "binary" systems, to drive expression of a transcription factor or recombinase that subsequently activates the expression of other transgenes. All such techniques are constrained by the limited specificity of the available promoters. We introduce here a combinatorial system in which the DNA-binding (DBD) and transcription-activation (AD) domains of a transcription factor are independently targeted using two different promoters. The domains heterodimerize to become transcriptionally competent and thus drive transgene expression only at the intersection of the expression patterns of the two promoters. We use this system to dissect a neuronal network in Drosophila by selectively targeting expression of the cell death gene reaper to subsets of neurons within the network.

Iron chelation regulates cyclin D1 expression via the proteasome: a link to iron deficiency-mediated growth suppression

Iron (Fe) plays an important role in proliferation, and Fe deficiency results in G(1)/S arrest. Despite this, the precise role of Fe in cell-cycle control remains unclear. Cyclin D1 plays a critical function in G(1) progression by interacting with cyclin-dependent kinases. Previously, we examined the effect of Fe depletion on the expression of cell-cycle control molecules and identified a marked decrease in cyclin D1 protein, although the mechanism involved was unknown. In this study, we showed that cyclin D1 was regulated posttranscriptionally by Fe depletion. Iron chelation of cells in culture using desferrioxamine (DFO) or 2-hydroxy-1-naphthylaldehyde isonicotinoyl hydrazone (311) decreased cyclin D1 protein levels after 14 hours and was rescued by the addition of Fe. Cyclin D1 half-life in control cells was 80 +/- 15 minutes (n = 5), while in chelator-treated cells it was significantly (P < .008) decreased to 38 +/- 3 minutes (n = 5). Proteasomal inhibitors rescued the Fe chelator-mediated decrease in cyclin D1 protein, suggesting the role of the proteasome. In Fe-replete cells, cyclin D1 was degraded in an ubiquitin-dependent manner, while Fe depletion induced a ubiquitin-independent pathway. This is the first report linking Fe depletion-mediated growth suppression at G(1)/S to a mechanism inducing cyclin D1 proteolysis.

Impact of cyclins E, neutrophil elastase and proteinase 3 expression levels on clinical outcome in primary breast cancer patients

Uncontrolled cell proliferation is one of the hallmarks of cancer and the transition from the G1 to S phase is the most commonly reported cell cycle abnormality in tumors. It has been shown that the oncogenic activity of G1 cyclin E (CCNE) can be amplified by generating hyperactive low molecular weight forms (LMW) through elastase-mediated proteolytic processing. Neutrophil elastase (NE) and proteinase 3 (PR3) are 2 proteases that are aberrantly expressed in breast cancer cells and seem to be involved in cell proliferation. In this study, we evaluated the effect of the expression of these 2 proteases in addition to 2 potential intracellular targets of NE (CCNE1 and CCNE2) on clinical outcome in a population of 205 primary breast cancer patients. By univariate analysis, CCNE1, CCNE2, estrogen receptor and grade significantly predicted relapse free interval (RFI). NE and PR3 did not achieve statistical significance. In a multivariate analysis, elevated CCNE2 [hazard ratio (HR) 2.10, p = 0.008] predicted shorter RFI. In subgroup analyses of the tamoxifen-only treated patients, high CCNE1 levels predicted treatment resistance, while high levels of CCNE2 were associated with poor RFI in untreated patients. Investigation of the relationship between CCNE1, CCNE2 and NE did not show any impact on RFI. To conclude, this study was the first to evaluate these markers at the mRNA level by RT-PCR in a series of primary breast cancer patients, and our results confirmed the impact of high CCNE levels on clinical outcome in systemically untreated and of CCNE1 in tamoxifen-only treated early breast cancer patients.

Stable HepG2- and Huh7-based human hepatoma cell lines for efficient regulated expression of infectious hepatitis B virus

BACKGROUND/AIMS

Hepatitis B virus (HBV) cannot be propagated in cultured cells but two human hepatoma cell lines, HepG2 and Huh7, support virus replication when transfected with HBV DNA. If standardization is required stably transfected cell lines provide distinct advantages. One such line, HepG2.2.15, is widely used in antiviral research but HBV production is limited and difficult to control. Our aim was to establish stable, inducibly HBV producing HepG2 and Huh7 cell lines that overcome these limitations.

METHODS

Based on the tetracycline (Tet)-regulated TetOFF system, a Tet-responsive promoter-controlled HBV genome was introduced into separately established, well-regulatable HepG2 and Huh7 lines expressing Tet-responsive trans-activators (tTAs). Stable clones were analyzed for regulatability and levels of HBV expression, quality of the virus produced, and responsiveness towards antivirals.

RESULTS

HepG2- and Huh7-based cell lines were established which, Tet-controllably, produce more HBV than HepG2.2.15 cells. The secreted virions were infectious for primary tupaia hepatocytes, and the cell lines responded as well as HepG2.215 cells to different antivirals.

CONCLUSIONS

The new HBV cell lines should be valuable tools for academic and pharmaceutical HBV research. The parental tTA-cells will facilitate the generation of additional lines, producing HBV variants, or other genes, in an identical host cell background.

High glucose increase cell cycle regulatory proteins level of mouse embryonic stem cells via PI3-K/Akt and MAPKs signal pathways

This study examined the effects of high glucose on cell proliferation and its related signal pathways using mouse embryonic stem (ES) cells. Here, we showed that high glucose level significantly increased [3H]thymidine incorporation, BrdU incorporation, the number of cells, [3H]leucine, and [3H]proline incorporation in a time-( >3 hr) and dose-(> 25 mM) dependent manner. Moreover, high glucose level increased the cellular reactive oxygen species (ROS), Akt, and mitogen-activated protein kinases (MAPKs) phosphorylation. Subsequently, these signaling molecules involved in high glucose-induced increase of [3H]thymidine incorporation. High glucose level also increased cyclin D1, cyclin E, cyclin-dependent kinase (CDK) 2, and CDK 4 protein levels, which is cell cycle regulatory proteins acting in G1-S phase of cell cycle. Inhibition of phosphatidylinositol 3-kinase (PI3-K) (LY 294002: PI3-kinase inhibitor, 10(-6) M), Akt (Akt inhibitor, 10(-5) M), and p44/42 MAPKs (PD 98059: MEK inhibitor, 10(-5) M) decreased these proteins. High glucose level phosphorylated the RB protein, which was decreased by inhibition of PI3-K and Akt. In conclusion, high glucose level stimulates mouse ES cell proliferation via the PI3-K/Akt and MAPKs pathways.

A versatile system for site-specific enzymatic biotinylation and regulated expression of proteins in cultured mammalian cells

We have developed a system for producing biotinylated recombinant proteins in mammalian cells. The expression construct consists of an inducible tetracycline response element (TRE) that drives expression of a bicistronic cassette comprising a biotin acceptor peptide (BioTag) fused to either terminus of the target protein, the gene for Escherichia coli biotin ligase (BirA), and an intervening internal ribosome entry site (IRES). By either transient or stable transfection of Chinese hamster ovary (CHO) Tet-On cells, we successfully expressed, detected, and immobilized biotinylated human Itch, a pleiotropic multi-domain ubiquitin-protein ligase, as well as Gla-RTK, a putative vitamin K-dependent receptor tyrosine kinase. The biotinylation of recombinant Itch in transiently transfected CHO Tet-On cells required biotin supplementation and coexpression of BirA, occurred quantitatively and specifically on the lysine residue of the BioTag, and enabled detection of Itch by Western blot in as little as 10ng of total lysate protein. Stably selected clones were rapidly pre-screened for doxycycline (dox)-inducible BirA expression by ELISA, and subsequently screened for dox-inducible expression of biotinylated Itch. Biotinylated Gla-RTK was detectable in as little as 5ng of total lysate protein from transiently transfected CHO Tet-On cells, and exhibited pronounced tyrosine phosphorylation. In stable clones however, constitutive phosphorylation was prevented by reducing the expression level of Gla-RTK through the titration of dox. These results demonstrate the utility of this system for the expression of 'difficult' proteins, particularly those that are cytotoxic or those that may require lower expression levels to ensure appropriate post-translational modification.

RB and cell cycle progression

The Rb protein is a tumor suppressor, which plays a pivotal role in the negative control of the cell cycle and in tumor progression. It has been shown that Rb protein (pRb) is responsible for a major G1 checkpoint, blocking S-phase entry and cell growth. The retinoblastoma family includes three members, Rb/p105, p107 and Rb2/p130, collectively referred to as 'pocket proteins'. The pRb protein represses gene transcription, required for transition from G1 to S phase, by directly binding to the transactivation domain of E2F and by binding to the promoter of these genes as a complex with E2F. pRb represses transcription also by remodeling chromatin structure through interaction with proteins such as hBRM, BRG1, HDAC1 and SUV39H1, which are involved in nucleosome remodeling, histone acetylation/deacetylation and methylation, respectively. Loss of pRb functions may induce cell cycle deregulation and so lead to a malignant phenotype. Gene inactivation of pRB through chromosomal mutations is one of the principal reasons for retinoblastoma tumor development. Functional inactivation of pRb by viral oncoprotein binding is also shown in many neoplasias such as cervical cancer, mesothelioma and AIDS-related Burkitt's lymphoma.

The kinase-inhibitory domain of p21-activated kinase 1 (PAK1) inhibits cell cycle progression independent of PAK1 kinase activity

p21-activated kinase 1 (PAK1) is a mediator of downstream signaling from the small GTPases Rac and Cdc42. In its inactive state, PAK1 forms a homodimer where two kinases inhibit each other in trans. The kinase inhibitory domain (KID) of one molecule of PAK1 binds to the kinase domain of its counterpart and keeps it inactive. Therefore, the isolated KID of PAK1 has been widely used to specifically inhibit and study PAK function. Here, we show that the isolated KID induced a cell cycle arrest with accumulation of cells in the G1 phase of the cell cycle with an inhibition of cyclin D1 and D2 expression. This cell cycle arrest required the intact KID and was also induced by a mutated KID unable to block PAK1 kinase activity. Furthermore, the KID-induced cell cycle arrest could not be rescued by the expression of a constitutively active PAK1-T423E mutant, concluding that this arrest occurs independently of PAK1 kinase activity. Our results suggest that PAK1 through its KID inhibits cyclin D expression and thereby enforces a cell cycle arrest. Our results also call for serious precaution in the use of KID to study PAK function.

The tumor-suppressor gene fat controls tissue growth upstream of expanded in the hippo signaling pathway

BACKGROUND

The tight control of cell proliferation and cell death is essential to normal tissue development, and the loss of this control is a hallmark of cancers. Cell growth and cell death are coordinately regulated during development by the Hippo signaling pathway. The Hippo pathway consists of the Ste20 family kinase Hippo, the WW adaptor protein Salvador, and the NDR kinase Warts. Loss of Hippo signaling in Drosophila leads to enhanced cell proliferation and decreased apoptosis, resulting in massive tissue overgrowth through increased expression of targets such as Cyclin E and Diap1. The cytoskeletal proteins Merlin and Expanded colocalize at apical junctions and function redundantly upstream of Hippo. It is not clear how they regulate growth or how they are localized to apical junctions.

RESULTS

We find that another Drosophila tumor-suppressor gene, the atypical cadherin fat, regulates both cell proliferation and cell death in developing imaginal discs. Loss of fat leads to increased Cyclin E and Diap1 expression, phenocopying loss of Hippo signaling. Ft can regulate Hippo phosphorylation, a measure of its activation, in tissue culture. Importantly, fat is needed for normal localization of Expanded at apical junctions in vivo. Genetic-epistasis experiments place fat with expanded in the Hippo pathway.

CONCLUSIONS

Together, these data suggest that Fat functions as a cell-surface receptor for the Expanded branch of the conserved Hippo growth control pathway.

Cyclin E and p16 Immunoreactivity in Epithelioid Trophoblastic Tumor???An Aid in Differential Diagnosis

Epithelioid trophoblastic tumor (ETT) is a relatively uncommon trophoblastic tumor that can be confused with several trophoblastic and nontrophoblastic lesions, notably the placental site nodule and invasive squamous carcinoma of the cervix. In this report, we analyzed the immunoreactivity of two cell cycle-regulated proteins, cyclin E and p16, in ETTs, placental site nodules and cervical squamous carcinomas to determine whether they are useful in their differential diagnosis. Other trophoblastic lesions were also evaluated. Using an H-score based on both percentage of positively stained cells and immunointensity, we found that ETTs demonstrated a much higher cyclin E staining score than placental site nodules (P<0.0001) permitting distinction of ETTs and placental site nodules with a sensitivity of 94.7% at a specificity of 91.7% using a cutoff H-score value of >40. Only two placental site nodules had scores above the cutoff and both showed morphologic features that placed them in an intermediate position between a typical placental site nodule and an ETT, so-called "atypical PSN." p16 immunoreactivity, was not detected in any of the ETTs and placental site nodules, whereas it was strongly and diffusely positive in the vast majority of cervical squamous carcinomas examined (83/87 cases) (P<0.001). Therefore, cyclin E expression is useful in distinguishing an ETT from a placental site nodule and p16 expression is useful in distinguishing an ETT from a cervical squamous carcinoma. The majority of other types of trophoblastic lesions showed diffuse and intense nuclear immunoreactivity for cyclin E whereas none were positive for p16.

Ubiquitylation of cyclin E requires the sequential function of SCF complexes containing distinct hCdc4 isoforms

Cyclin E, an activator of cyclin-dependent kinase 2 (Cdk2), is targeted for proteasomal degradation by phosphorylation-dependent multiubiquitylation via the ubiquitin ligase SCF(hCdc4). SCF ubiquitin ligases are composed of a core of conserved subunits and one variable subunit (an F box protein) involved in substrate recognition. We show here that multiubiquitylation of cyclin E requires the sequential function of two distinct splice variant isoforms of the F box protein hCdc4 known as alpha and gamma. SCF(hCdc4alpha) binds a complex containing cyclin E, Cdk2, and the prolyl cis/trans isomerase Pin1 and promotes the activity of Pin1 without directly ubiquitylating cyclin E. However, due to the action of this SCF(hCdc4alpha)-Pin1 complex, cyclin E becomes an efficient ubiquitylation substrate of SCF(hCdc4gamma). Furthermore, in the context of Cdc4alpha and cyclin E, mutational data suggest that Pin1 isomerizes a noncanonical proline-proline bond, with the possibility that Cdc4alpha may serve as a cofactor for altering the specificity of Pin1.

Inhibition of smooth muscle proliferation by urea-based alkanoic acids via peroxisome proliferator-activated receptor alpha-dependent repression of cyclin D1

OBJECTIVE

Proliferation of smooth muscle cells is implicated in cardiovascular complications. Previously, a urea-based soluble epoxide hydrolase inhibitor was shown to attenuate smooth muscle cell proliferation. We examined the possibility that urea-based alkanoic acids activate the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha) and the role of PPARalpha in smooth muscle cell proliferation.

METHODS AND RESULTS

Alkanoic acids transactivated PPARalpha, induced binding of PPARalpha to its response element, and significantly induced the expression of PPARalpha-responsive genes, showing their function as PPARalpha agonists. Furthermore, the alkanoic acids attenuated platelet-derived growth factor-induced smooth muscle cell proliferation via repression of cyclin D1 expression. Using small interfering RNA to decrease endogenous PPARalpha expression, it was determined that PPARalpha was partially involved in the cyclin D1 repression. The antiproliferative effects of alkanoic acids may also be attributed to their inhibitory effects on soluble epoxide hydrolase, because epoxyeicosatrienoic acids alone inhibited smooth muscle cell proliferation.

CONCLUSIONS

These results show that attenuation of smooth muscle cell proliferation by urea-based alkanoic acids is mediated, in part, by the activation of PPARalpha. These acids may be useful for designing therapeutics to treat diseases characterized by excessive smooth muscle cell proliferation.

On the mechanisms of 12-O-tetradecanoylphorbol-13-acetate-induced growth arrest in pancreatic cancer cells

OBJECTIVES

Protein kinase C (PKC) is involved in cell growth, differentiation, and apoptosis. We investigated the effects of the PKC activator, the tetradecanylphorbol acetate (TPA), in human pancreatic cancer cells.

METHODS

Cell proliferation was measured by thymidine incorporation. Expression of cell cycle proteins was investigated by Western blot. Real-time reverse transcriptase-polymerase chain reaction was used to measure p21 messenger RNA expression, whereas knockdown of its expression was accomplished with a specific small interferring RNA. Cell cycle phases were determined by flow cytometry.

RESULTS

TPA time and concentration dependently inhibited thymidine incorporation in Panc-1 and CD18 cells and induced G2/M cell cycle arrest. The TPA decreased cyclin A and B expression, increased cyclin E, and markedly increased the expression of p21 at both the messenger RNA and protein levels. TPA-induced p21 expression and growth inhibition were blocked by the PKC inhibitor, bisindoylmaleimide. TPA induced extracellular signal-regulated kinase1/2 phosphorylation, whereas the MEK inhibitor, PD98059, blocked the TPA-induced p21 expression. Small interferring RNA targeted to p21 blocked TPA-induced p21 protein expression but not TPA-induced cell growth arrest.

CONCLUSIONS

TPA-induced p21 expression is mediated by the MEK/ERK pathway but is not involved in TPA-induced growth inhibition. In contrast, cyclin A and cyclin B are likely involved in TPA-induced G2/M arrest because both proteins are involved in S phase and G2/M transition during cell proliferation.

The Fat1 cadherin integrates vascular smooth muscle cell growth and migration signals

The significance of cadherin superfamily proteins in vascular smooth muscle cell (VSMC) biology is undefined. Here we describe recent studies of the Fat1 protocadherin. Fat1 expression in VSMCs increases significantly after arterial injury or growth factor stimulation. Fat1 knockdown decreases VSMC migration in vitro, but surprisingly, enhances cyclin D1 expression and proliferation. Despite limited similarity to classical cadherins, the Fat1 intracellular domain (Fat1(IC)) interacts with beta-catenin, inhibiting both its nuclear localization and transcriptional activity. Fat1 undergoes cleavage and Fat1(IC) species localize to the nucleus; however, inhibition of the cyclin D1 promoter by truncated Fat1(IC) proteins corresponds to their presence outside the nucleus, which argues against repression of beta-catenin-dependent transcription by nuclear Fat1(IC). These findings extend recent observations about Fat1 and migration in other cell types, and demonstrate for the first time its anti-proliferative activity and interaction with beta-catenin. Because it is induced after arterial injury, Fat1 may control VSMC functions central to vascular remodeling by facilitating migration and limiting proliferation.

The roles of Chk 1 and Chk 2 in hypoxia and reoxygenation

Both Chk 1 and Chk 2 are critically important checkpoint kinases. Chk 1 is an essential gene that is required for normal cell division and Chk 2 has been found to be mutated in an ever-growing list of human malignancies. Our recent studies indicate that both Chk 1 and Chk 2 have roles to play in the physiological stress of hypoxia/reoxygenation. Loss or inhibition of either kinase sensitizes cells to hypoxia/reoxygenation indicating that either or both could represent significant therapeutic targets.

Expression of a cyclin E1 isoform in mice is correlated with the quiescent cell cycle status of hepatocytes in vivo

Cyclin E1 controls G1/S phase transition of the eukaryotic cell cycle. We report the impact of alternative spliced cyclin E1 isoforms on cell cycle regulation in hepatocytes. We show that expression of new cyclin E1 mRNA variants IN3, Delta4, and Delta5 is associated with retarded proliferation in murine hepatocellular carcinoma. Additionally, we demonstrate that a new cyclin E1 isoform Delta3/8 lacking the central part of wild-type mRNA is expressed predominantly in nonproliferating murine hepatocytes. Following partial hepatectomy, Delta3/8 is downregulated when hepatocytes enter the cell cycle from quiescence. The Delta3/8 protein does not exhibit any cyclin box motif but binds cyclin-dependent kinase 2 without stimulating kinase activity. We demonstrate that Delta3/8 lacks any nuclear localization signal and is exclusively located in the cytoplasm. Overexpression of Delta3/8 in cultured cells leads to a delayed G0-G1 transition, indicating that this splice variant helps to maintain a quiescent state of hepatocytes. In conclusion, we identified an isoform of cyclin E1 involved in G0 maintenance and suggest an additional mechanism for cell cycle control.

Deregulated cyclin E promotes p53 loss of heterozygosity and tumorigenesis in the mouse mammary gland

Deregulation of cyclin E expression and/or high levels have been reported in a variety of tumors and have been used as indicators of poor prognosis. Although the role that cyclin E plays in tumorigenesis remains unclear, there is evidence that it confers genomic instability when deregulated in cultured cells. Here we show that deregulated expression of a hyperstable allele of cyclin E in mice heterozygous for p53 synergistically increases mammary tumorigenesis more than that in mice carrying either of these markers individually. Most tumors and tumor-derived cell lines demonstrated loss of p53 heterozygosity. Furthermore, this tumor susceptibility is related to the number of times the transgene is induced indicating that it is directly attributable to the expression of the cyclin E transgene. An indirect assay indicates that loss of p53 function is an early event occurring in the mammary epithelia of midlactation mammary glands in which cyclin E is deregulated long before evidence of malignancy. These data support the hypothesis that deregulated expression of cyclin E stimulates p53 loss of heterozygosity by promoting genomic instability and provides specific evidence for this in vivo. Cyclin E deregulation and p53 loss are characteristics often observed in human breast carcinoma.

Benzo[a]pyrene-induced cell cycle progression is through ERKs/cyclin D1 pathway and requires the activation of JNKs and p38 mapk in human diploid lung fibroblasts

Treatment of cells with carcinogen Benzo[a]pyrene (B[a]P) allows cells to evade G1 arrest and induces cells abnormal proliferation. However, the mechanisms of its action at cellular level are not well understood. To address this question, normal human embryo lung diploid fibroblasts (HELF) were selected in the present study. We found that exposure of cells with 2.5 microM of B[a]P for 24 h resulted in a decrease of G1 population by 11.9% (P < 0.05) and a increase of S population by 17.2% (P < 0.05). Treatment of cells with B[a]P also caused dose-related activation of MAPK and induction of cyclin D1 protein expression, whereas the CDK4 protein levels were not significantly affected by B[a]P. Overexpression of cyclin D1 protein stimulated by B[a]P was significantly inhibited by 50 microM AG126 (an inhibitor of ERK1/2), but not by 25 microM SP600125 (an inhibitor of JNK1/2) or 5 microM SB203580 (an inhibitor of p38 mapk), suggesting that B[a]P-induced cyclin D1 expression was only regulated by ERK1/2 pathway. However, AG126, SP600125 or SB203580 led to cell cycle significantly arrested in G1 phase, indicating that ERK1/2, JNK1/2 and p38 mapk pathways are all required for B[a]P-induced G1/S transition. In addition, HELF cells transfecting with antisense cyclin D1 cDNA or antisense CDK4 cDNA showed significantly G1 arrest after B[a]P stimulation. These results suggested that B[a]P exposure accelerated the G1-->S transition by activation of MAPK signaling pathways. Cyclin D1 and CDK4 are rate-limiting regulators of the G1-->S transition and expression of cyclin D1 is predominantly regulated by ERK1/2 pathway in HELF cells.

Expression and significances of P27 and Cyclin E protein in gastric carcinoma and precancerous lesion

AIM: To investigate the expression of P27 and Cyclin E protein in gastric carcinoma (GC) and precancerous lesion and their correlations with the clinicopathological parameters.

METHODS: Immunohistochemical staining was used to evaluate the expression of P27 and Cyclin E protein in the samples of normal gastric mucosa (NGM, n = 20), chronic superficial gastritis (CSG, n = 20), chronic atrophic gastritis (CAG) with metaplasia (n = 20), CAG with dysplasia (n = 20) and GC (n = 60), and their correlations with the clinicopathological parameters were analyzed.

RESULTS: The positive rates of P27 and Cyclin E protein expression were 100% and 5% in NGM, 85% and 10% in CSG, 70% and 20% in CAG with metaplasia, 45% and 30% in CAG with dysplasia, and 38.3%(23/60) and 40%(24/60) in GC, respectively. The positive rate of P27 expression in GC and CAG with dysplasia were lower than that in NGM, CSG, and CAG with metaplasia (P < 0.05), and the positive rate of Cyclin E expression in GC and CAG with dysplasia were higher than that in NGM, CSG, and CAG with metaplasia (P < 0.05). The expression of P27 and Cyclin E were significantly correlated with tumor differentiation, infiltration and clinical stages, and P27 expression was also correlated with lymph nodes metastasis. Furthermore, the expression of P27 was inversely correlated with Cyclin E expression in GC (r = -0.768, P < 0.05).

CONCLUSION: It is helpful to detect of P27 and Cyclin E protein expression in the judgment of GC progression, and combined analysis of them is of added prognostic value.

The D-type cyclin CYCD3;1 is limiting for the G1-to-S-phase transition in Arabidopsis

The G1-to-S-phase transition is a key regulatory point in the cell cycle, but the rate-limiting component in plants is unknown. Overexpression of CYCLIN D3;1 (CYCD3;1) in transgenic plants increases mitotic cycles and reduces endocycles, but its effects on cell cycle progression cannot be unambiguously determined. To analyze the cell cycle roles of plant D-type cyclins, we overexpressed CYCD3;1 in Arabidopsis thaliana cell suspension cultures. Changes in cell number and doubling time were insignificant, but cultures exhibited an increased proportion of G2- over G1-phase cells, as well as increased G2 arrest in response to stationary phase and sucrose starvation. Synchronized cultures confirm that CYCD3;1-expressing (but not CYCD2;1-expressing) cells show increased G2-phase length and delayed activation of mitotic genes such as B-type cyclins, suggesting that CYCD3;1 has a specific G1/S role. Analysis of putative cyclin-dependent kinase phosphorylation sites within CYCD3;1 shows that mutating Ser-343 to Ala enhances CYCD3;1 potency without affecting its rate of turnover and results in a fivefold increase in the level of cell death in response to sucrose removal. We conclude that CYCD3;1 dominantly drives the G1/S transition, and in sucrose-depleted cells the decline in CYCD3;1 levels leads to G1 arrest, which is overcome by ectopic CYCD3;1 expression. Ser-343 is likely a key residue in modulating CYCD3;1 activity in response to sucrose depletion.

Relationship between gastric carcinoma traditional Chinese medicine classifications of syndrome and tumor proliferation

AIM: To study the relationship between gastric carcinoma traditional Chinese medicine (TCM) classifications of syndrome and tumor proliferation.

METHODS: Ninety-one patients with gastric carcinoma were assigned into 6 groups according to TCM classification of syndrome criteria before operation. Specimens prepared from operation were detected for Ki67 and Cyclin E protein expression by EnVision method of immunohistochemistry.

RESULTS: The positive rates of Ki67 and Cyclin E expression were 98.90% (90/91) and 72.53% (66/91), respectively, in gastric carcinoma. The expression of Cyclin E protein was correlated with the histological types (P = 0.039 4) and distant metastasis (P= 0.009 6). There was significantly positive relationship between Cyclin E protein expression and distant metastasis in female patients (P = 0.019 3). The expression of Ki67 protein had significant correlations with gastric carcinoma TCM classifications of syndromes (P = 0.037 7), but the expression of Cyclin E protein did not (P = 0.125 4). The expression of Ki67 in the syndrome of "Incoordination between the Liver and Stomach" was significantly higher than that of "Phlegm-damp Coagulation" (P < 0.05) or "Deficiency of both Qi and Blood" (P < 0.01), and it was more significant in the syndrome of "Stomach-heat and Deficiency of Stomach-yin" than that of "Phlegm-damp Coagulation" (P < 0.05).

CONCLUSION: Cyclin E can be used as an important marker in the judgment of gastric carcinoma prognosis. Tumor proliferation is partly different among different gastric carcinoma TCM classifications of syndromes, with which Ki67 protein expression is correlated.

Location, location, location: the role of cyclin D1 nuclear localization in cancer

The control of cell proliferation is crucial in maintaining cellular homeostasis and loss of this mechanism is a principle hallmark of cancer cells. A primary target of growth factor signaling is the cyclin D1-dependent kinase (D1-CDK4/6) whose activity promotes G1 phase progression by phosphorylating the retinoblastoma protein (Rb) along with related pocket proteins 107 and p130, relieving inhibition of E2F family transcription factors. Cyclin D1 accumulation is regulated at multiple levels including transcription, post-translational activation and cellular localization throughout the cell cycle. While overexpression of cyclin D1 has been observed in a number of human cancers, mouse cancer models overexpressing D1 have fallen short of establishing a role for cyclin D1 in the initiation of malignant phenotypes suggesting an additional regulatory mechanism exists that prevents cyclin D1-driven cancer. This article will present an overview of current data investigating the regulation of cyclin D1 nuclear localization and the prevalence of these aberrations in cancer. Finally, future avenues of research involving cyclin D1 cellular localization and its regulation in cancer will be addressed.

Linking cell division to cell growth in a spatiotemporal model of the cell cycle

Cell division must be tightly coupled to cell growth in order to maintain cell size, yet the mechanisms linking these two processes are unclear. It is known that almost all proteins involved in cell division shuttle between cytoplasm and nucleus during the cell cycle; however, the implications of this process for cell cycle dynamics and its coupling to cell growth remains to be elucidated. We developed mathematical models of the cell cycle which incorporate protein translocation between cytoplasm and nucleus. We show that protein translocation between cytoplasm and nucleus not only modulates temporal cell cycle dynamics, but also provides a natural mechanism coupling cell division to cell growth. This coupling is mediated by the effect of cytoplasmic-to-nuclear size ratio on the activation threshold of critical cell cycle proteins, leading to the size-sensing checkpoint (sizer) and the size-independent clock (timer) observed in many cell cycle experiments.

Validation of cyclin D1/CDK4 as an anticancer drug target in MCF-7 breast cancer cells: Effect of regulated overexpression of cyclin D1 and siRNA-mediated inhibition of endogenous cyclin D1 and CDK4 expression

We have examined the role of cyclin D1 and cyclin-dependent kinase-4 (CDK4) in the cell cycle progression and proliferation of MCF-7 breast cancer cells. Forced expression of cyclin D1 using a tetracycline-regulated expression system, and suppression of endogenous cyclin D1 and CDK4 using small interfering RNA (siRNA) were used to validate this protein complex as a drug target in cancer drug discovery. Overexpression of cyclin D1 increased both phosphorylation of the retinoblastoma gene product (RB) and passage through the G1-S phase transition, resulting in increased proliferation of cells. When cyclin D1 expression was shut off, growth rates fell below those seen in control cell lines transfected with the vector, indicating an increased dependence on this protein for proliferation. Inhibition of endogenous cyclin D1 or CDK4 expression by RNA interference resulted in hypophosphorylation of RB and accumulation of cells in G1. These results support the prevailing view that pharmacological inhibition of cyclin D1/CDK4 complexes is a useful strategy to inhibit the growth of tumors. Furthermore, since MCF-7 cells appear to be dependent on this pathway for their continued proliferation, it is a suitable cell line to test novel cyclin D1/CDK4 inhibitors.

G1 phase regulation, area-specific cell cycle control, and cytoarchitectonics in the primate cortex

We have investigated the cell cycle-related mechanisms that lead to the emergence of primate areas 17 and 18. These areas are characterized by striking differences in cytoarchitectonics and neuron number. We show in vivo that (1) area 17 precursors of supragranular neurons exhibit a shorter cell cycle duration, a reduced G1 phase, and a higher rate of cell cycle reentry than area 18 precursors; (2) area 17 and area 18 precursors show contrasting and specific levels of expression of cyclin E (high in area 17, low in area 18) and p27Kip1 (low in area 17, high in area 18); (3) ex vivo up- and downmodulation of cyclin E and p27Kip1 show that both regulators influence cell cycle kinetics by modifying rates of cell cycle progression and cell cycle reentry; (4) modeling the areal differences in cell cycle parameters suggests that they contribute to areal differences in numbers of precursors and neuron production.

Smad7 induces G0/G1 cell cycle arrest in mesenchymal cells by inhibiting the expression of G1 cyclins

The major Smad pathways serve in regulating the expression of genes downstream of TGFbeta signals. In this study, we examined the effects of sustained Smad7 expression in cultured cells. Interestingly, Smad7 caused various mesenchymal cells, including NIH3T3 fibroblast and ST2 bone-marrow stromal cells, to undergo a marked morphological alteration into a flattened cell shape, but kept them alive for as long as 60 days. Furthermore, Smad7 arrested the proliferation of the cells even before they reached confluence. These cells became quiescent in G0/G1 phase and accumulated a hypophosphorylated form of retinoblastoma. The cytostatic effect of Smad7 was closely associated with a preceding decrease in the levels of G1 cyclins, such as cyclin D1 and cyclin E. Accordingly, ectopic cyclin E was able to overcome the Smad7-induced arrest of proliferation. These results indicate that Smad7 functions upstream of G1 cyclins and suggest a novel role for Smad7 as an antiproliferative factor. In contrast to the growth of mesenchymal cells, that of epithelial cells was little susceptible to Smad7. The present findings raise the possibility that a link between Smad7 and the G1 to S phase transition may also contribute to the cell cycle control by certain Smad7-inducing stimuli in a cell-type-dependent fashion.

The HTLV-1 Tax protein binding domain of cyclin-dependent kinase 4 (CDK4) includes the regulatory PSTAIRE helix

Background

The Tax oncoprotein of human T-cell leukemia virus type 1 (HTLV-1) is leukemogenic in transgenic mice and induces permanent T-cell growth in vitro. It is found in active CDK holoenzyme complexes from adult T-cell leukemia-derived cultures and stimulates the G1- to-S phase transition by activating the cyclin-dependent kinase (CDK) CDK4. The Tax protein directly and specifically interacts with CDK4 and cyclin D2 and binding is required for enhanced CDK4 kinase activity. The protein-protein contact between Tax and the components of the cyclin D/CDK complexes increases the association of CDK4 and its positive regulatory subunit cyclin D and renders the complex resistant to p21^CIP inhibition. Tax mutants affecting the N-terminus cannot bind cyclin D and CDK4.

Results

To analyze, whether the N-terminus of Tax is capable of CDK4-binding, in vitro binding -, pull down -, and mammalian two-hybrid analyses were performed. These experiments revealed that a segment of 40 amino acids is sufficient to interact with CDK4 and cyclin D2. To define a Tax-binding domain and analyze how Tax influences the kinase activity, a series of CDK4 deletion mutants was tested. Different assays revealed two regions which upon deletion consistently result in reduced binding activity. These were isolated and subjected to mammalian two-hybrid analysis to test their potential to interact with the Tax N-terminus. These experiments concurrently revealed binding at the N- and C-terminus of CDK4. The N-terminal segment contains the PSTAIRE helix, which is known to control the access of substrate to the active cleft of CDK4 and thus the kinase activity.

Conclusion

Since the N- and C-terminus of CDK4 are neighboring in the predicted three-dimensional protein structure, it is conceivable that they comprise a single binding domain, which interacts with the Tax N-terminus.

Cell cycle regulation and its aberrations in human lung carcinoma

Cell cycle is strictly regulated by complex and redundant mechanisms. Basically, cell cycle transition is promoted by accelerator molecules termed 'cyclin' and 'cyclin-dependent kinase' (cdk), and inhibited by brake molecules termed 'cdk-inhibitor' (CKI). Although based on the results of early experimental studies and of clinicopathological analyses, there was much speculation that gene aberration of those molecules would be common; this has not turned out to be the case. One reason may be that activation or inactivation of a single molecule by itself usually does not lead to cell transformation, but rather to apoptosis. Successful transformation and unchecked cell proliferation appears to require the coordinated up-regulation of cyclin/cdk and/or suppression of CKI. In this article, I focus on the precise regulation of the cell cycle and describe abnormalities found in these proteins in lung carcinoma. Notable findings in lung carcinoma include: (i) cyclin A/cdk2 plays a key role in cell proliferation, while protein amount of cyclin E does not necessarily reflect cellular proliferative activity, depending on the tumor type; (ii) CKI function not only as suppressors, but also as activators of cdk, depending on expression levels; and (iii) aberrant expression of cyclin/cdk can lead to apoptosis in vivo in humans. Another key point is that as lung carcinoma is composed of a mixture of heterogeneous histological subtypes, the growth control of carcinoma cells is diversely regulated, depending on each histological subtype. This diversity is also described with our experimental results.