Cyclin-dependent Kinase Subunit Research Articles

The Expression and Prognostic Significance of Cks1 in Salivary Cancer

Cks1 is an essential factor in facilitating Skp2-dependent degradation of p27, but its role in salivary malignancies is unknown. Expression of cyclin-dependent kinase subunit 1 (Cks1) was examined in 64 salivary malignancies, compared with p27, S-phase kinase protein 2 (Skp2), Ki-67, p53, and TDT-mediated dutp-biotin nick end labeling (TUNEL) expression, and with THE patient's clinical and pathological parameters. Cks1 expression was markedly increased in 30 patients (47%) and strongly correlated with increased expression of Skp2, Ki-67, p53, and TUNEL, but inversely with p27 levels. High expression of Cks1 WAS strongly associated with lymph node metastases and poor prognosis and survival. Cks1 alterations may have a significant biological role in the pathogenesis of salivary cancer.

Targeting cyclin B1 inhibits proliferation and sensitizes breast cancer cells to taxol

BackgroundCyclin B1, the regulatory subunit of cyclin-dependent kinase 1 (Cdk1), is essential for the transition from G2 phase to mitosis. Cyclin B1 is very often found to be overexpressed in primary breast and cervical cancer cells as well as in cancer cell lines. Its expression is correlated with the malignancy of gynecological cancers.MethodsIn order to explore cyclin B1 as a potential target for gynecological cancer therapy, we studied the effect of small interfering RNA (siRNA) on different gynecological cancer cell lines by monitoring their proliferation rate, cell cycle profile, protein expression and activity, apoptosis induction and colony formation. Tumor formation in vivo was examined using mouse xenograft models.ResultsDownregulation of cyclin B1 inhibited proliferation of several breast and cervical cancer cell lines including MCF-7, BT-474, SK-BR-3, MDA-MB-231 and HeLa. After combining cyclin B1 siRNA with taxol, we observed an increased apoptotic rate accompanied by an enhanced antiproliferative effect in breast cancer cells. Furthermore, control HeLa cells were progressively growing, whereas the tumor growth of HeLa cells pre-treated with cyclin B1 siRNA was strongly inhibited in nude mice, indicating that cyclin B1 is indispensable for tumor growth in vivo.ConclusionOur data support the notion of cyclin B1 being essential for survival and proliferation of gynecological cancer cells. Concordantly, knockdown of cyclin B1 inhibits proliferation in vitro as well as in vivo. Moreover, targeting cyclin B1 sensitizes breast cancer cells to taxol, suggesting that specific cyclin B1 targeting is an attractive strategy for the combination with conventionally used agents in gynecological cancer therapy.

Prognostic implication of p27Kip1, Skp2 and Cks1 expression in renal cell carcinoma: a tissue microarray study

Backgroundp27Kip1 plays a major role as a negative regulator of the cell cycle. The regulation of p27Kip1 degradation is mediated by its specific ubiquitin ligase subunits S-phase kinase protein (Skp) 2 and cyclin-dependent kinase subunit (Cks) 1. However, little is known regarding the prognostic utility of p27Kip1, Skp2 and Cks1 expression in renal cell carcinoma.MethodsImmunohistochemistry was performed for p27Kip1, Skp2 and Cks1 in tissue microarrays of 482 renal cell carcinomas with follow-up. The data were correlated with clinicopathological features. The univariate and multivariate survival analyses were also performed to determine their prognostic significance.ResultsImmunoreactivity of p27Kip1, Skp2 and Cks1 was noted in 357, 71 and 82 patients, respectively. Skp2 and Cks1 expression were not noted in chromophobe cancers. A strong correlation was found between Skp2 and Cks1 expression (P < 0.001), both of which were inversely related to p27Kip1 levels (P = 0.006 and P < 0.001), especially in primary and clear-cell cancers. Low p27Kip1 expression and Skp2 expression were correlated with larger tumor size and higher stage, as well as tumor necrosis. Cks1 expression was only correlated with tumor size. In univariate analysis, low p27Kip1 expression, Skp2 and Cks1 expression were all associated with a poor prognosis, while in multivariate analysis, only low p27Kip1 expression were independent prognostic factors for both cancer specific survival and recurrence-free survival in patients with RCC.ConclusionOur results suggest that immunohistochemical expression levels of p27Kip1, Skp2 and Cks1 may serve as markers with prognostic value in renal cell carcinoma.

Arabidopsis cyclin D2 expressed in rice forms a functional cyclin-dependent kinase complex that enhances seedling growth

D-class cyclins play important roles in controlling the cell cycle in development and in response to external signals by forming the regulatory subunit of cyclin-dependent kinase (CDK) complexes. To evaluate the effects of D-class cyclins in transgenic rice plants, Arabidopsis cyclin D2 gene (CycD2) was linked to the maize ubiquitin1 promoter (Ubi1) and introduced into rice by the Agrobacterium-mediated transformation method. Genomic deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and Western blot hybridizations of the Ubi1:CycD2 plants revealed copy number of transgene and its increased expression in leaf and callus cells at messenger RNA (mRNA) and/or protein levels. The H1 kinase assay using the immunoprecipitates of protein extracts from the Ubi1:CycD2 plants and nontransgenic controls demonstrated that the introduced Arabidopsis CycD2 forms a functional CycD2/CDK complex with an unidentified CDK of rice. Shoot and root growth was enhanced in the Ubi1:CycD2 seedlings compared with nontransgenic controls, together, suggesting that Arabidopsis cyclin D2 interacts with a rice cyclin-dependent kinase, consequently enhancing seedling growth.

A Role for the Cyclin Box in the Ubiquitin-Mediated Degradation of Cyclin G1

Cyclin G1 was identified as a transcriptional target of p53 that encodes a protein with strong homology to the cyclin family of cell cycle regulators. We show that either ectopically expressed or endogenous cyclin G1 protein is very unstable, undergoes modification with ubiquitin, and is likely degraded by the proteasome. Ectopic cyclin G1 protein stability is increased by cyclin box mutation or by association with inactive cyclin-dependent kinase (CDK) subunits, suggesting that a function of cyclin G1 as a CDK regulator may be required for its rapid turnover. Furthermore, cyclin G1 and the cyclin box mutant interact with and are ubiquitinated by MDM2, another transcriptional target of p53 that acts as a negative regulator of p53 stability. These data suggest that the cyclin box has a role in the proteasome-mediated degradation of cyclin G1 and thus suggest a putative role for a CDK in cyclin G1 metabolism and function.

WAVE1 controls neuronal activity-induced mitochondrial distribution in dendritic spines

Mitochondrial fission and trafficking to dendritic protrusions have been implicated in dendritic spine development. Here, we show that Wiskott-Aldrich syndrome protein (WASP)-family verprolin homologous protein 1 (WAVE1) controls depolarization-induced mitochondrial movement into dendritic spines and filopodia and regulates spine morphogenesis. Depolarization-induced degradation of the p35 regulatory subunit of cyclin-dependent kinase 5 (Cdk5), with the resultant decreased inhibitory phosphorylation on WAVE1, depend on NMDA receptor activation. Thus, WAVE1 dephosphorylation and activation are likely associated with mitochondrial redistribution and spine morphogenesis.

Fluoxetine mediates G0/G1 arrest by inducing functional inhibition of cyclin dependent kinase subunit (CKS)1

Fluoxetine, a well-known antidepressant used clinically for mental depression has gained attention in cancer research owing to its chemosensitizing potential in drug resistant cell lines. Some preliminary reports, however, suggested its independent cytotoxic potential which is not yet well characterized. Our aim in this study was to characterize its antiproliferative activity in tumor cells and to further elucidate the mechanism. We found that fluoxetine sensitized the effect of cyclophosphamide even in drug sensitive MDA MB 231 and SiHa cells. IC 50 values of 28 and 32 μM were obtained for fluoxetine mediated antiproliferative response in these cells. Further, PARP and caspase 3 cleavage analyses confirmed fluoxetine mediated apoptosis at molecular level. Cell cycle analysis showed that fluoxetine arrested cells at G0/G1 phase in a time dependent manner. The application of bioinformatics tools at this juncture predicted CKS1 as one of the possible targets of fluoxetine, which is of relevance to cell cycle biology. Fluoxetine showed the potential to disrupt skp2–CKS1 assembly required for ubiquitination and proteasomal degradation of p27 and p21. Our in vitro results were in agreement with the predictions made in silico. We found that fluoxetine treatment could accumulate p27 and p21, an immediate outcome characteristic of functional inhibition of CKS1. This was accompanied by the accumulation of cyclin E, another possible target of CKS1. We observed CKS1 downregulation also upon prolonged fluoxetine treatment. Fluoxetine had downregulated cyclin A which confirmed G0/G1 arrest at the molecular level. We conclude that fluoxetine induced cell cycle arrest is CKS1 dependent.

Regaining the Lost Past

People with Alzheimer’s disease or related neurodegenerative disorders may progress from absentmindedness and difficulty learning new things to a heartbreaking state in which they forget their own past and no longer recognize the faces of people they love. Expression of p25 (a truncated form of the p35 regulatory subunit of cyclin-dependent kinase 5) is associated with various neurodegenerative disorders, and the CK-p25 Tg mouse, in which p25 expression--and thereby neuronal loss--can be conditionally regulated, is a model for these disorders. Fischer et al . found that exposing CK-p25 Tg mice to an enriched environment (after neuronal loss had occurred and learning was impaired) improved their associative and spatial learning. Moreover, environmental enrichment led to the recovery of long-term memories that had apparently been lost. Experiments in wild-type mice revealed that exposure to an enriched environment stimulated histone acetylation in the hippocampus and cortex. The histone deacetylase (HDAC) inhibitor sodium butyrate (SB) improved associative and spatial learning in wild-type and CK-p25 Tg mice (after p25 induction) and, like environmental enrichment, enhanced dendritic and synaptic growth in CK-p25 Tg mice without affecting brain atrophy or neuronal loss. Furthermore, SB enabled the recovery of lost memories. The authors conclude that their data raise the possibility that HDAC inhibitors may provide a means to enable people with neurodegenerative disorders associated with memory loss to recover lost memories. Sweatt provides thoughtful commentary, noting (as do the authors) that HDAC inhibitors may affect the acetylation status of proteins other than histones. A. Fischer, F. Sananbenesi, X. Wang, M. Dobbin, L. H. Tsai, Recovery of learning and memory is associated with chromatin remodelling. Nature 447 , 178-182 (2007). [PubMed] J. D. Sweatt, Down memory lane. Nature 447 , 151-152 (2007). [PubMed]

Skp2 is an independent prognosticator of gallbladder carcinoma among p27Kip1-interacting cell cycle regulators: an immunohistochemical study of 62 cases by tissue microarray

AbstractDespite improvement in surgical techniques, prognosis of gallbladder carcinoma remains poor. It is desirable to identify prognostic biomarkers to aid in the development of targeted therapeutic strategies. Two SCFSkp2 ubiquitin ligase-related proteins, Skp2 and cyclin-dependent kinase subunit 1 (Cks1), are involved in post-transcriptional degradation of p27Kip1 tumor suppressor, which inhibits both cdk2/cyclin E and cdk2/cyclin A complexes and thus prevents transition to the S phase. However, the prognostic utility of p27Kip1-interacting cell cycle regulators has not been systematically assessed in gallbladder carcinoma. Immunohistochemistry was performed for p27Kip1, Skp2, Cks1, cyclin E, cyclin A, and Ki-67 in tissue microarrays of 62 gallbladder carcinomas with follow-up. The data were correlated with clinicopathological features and overall survival (OS). The cumulative OS rate for all 62 cases was 42.9% at 3 years. Aberrant labeling indices (LIs) of p27Kip1 (<20%), cyclin E (≥5%), cyclin A (≥5%), Cks1 (≥40%), and Skp2 (≥10%) were identified in 29, 58, 66, 21, and 57% of gallbladder carcinomas, respectively. By log-rank tests, downregulation of p27Kip1 (P=0.0319) and high LIs of Skp2 (P=0.0006), Cks1 (P=0.0460), cyclin E (P=0.0070), and Ki-67 (P=0.0037) were predictive of inferior OS. Furthermore, the combined expression status of Skp2 and Ki-67 robustly defined three prognostically different groups (P=0.0001). In multivariate comparison, Skp2 overexpression represented the strongest independent adverse prognosticator (P=0.004, risk ratio (RR): 5.538), followed by Ki-67 LI ≥50% (P=0.016, RR: 3.254) and American Joint Committee on Cancer stages II–IV (P=0.013, RR: 3.163). In conclusion, aberrations of p27Kip1-interacting cell cycle regulators are common in gallbladder carcinomas. Skp2 overexpression is highly representative of biological aggressiveness and independently associated with poor OS, suggesting that it is a promising novel target for therapeutic intervention in aggressive cases. The combined assessment of Skp2 and Ki-67 LIs effectively risk-stratifies gallbladder carcinomas with different prognosis, which is worth being prospectively validated in future study.

Expression of Cyclin-Dependent Kinase Subunit 1 (Cks1) is Regulated During the Cell Cycle by a CDE/CHR Tandem Element and is Downregulated by p53 but Not by p63 or p73

Cks1 is a member of the Cyclin-dependent kinase subunit family. These proteins are essential components of cyclin/cyclin-dependent kinase (cdk) complexes contributing to cell cycle control in all eukaryotes. Cks1 protein is found overexpressed in a number of tumors. Expression of Cks1 mRNA starts in late G1 reaching a peak in S/G2-phases of the cell cycle. We find that this expression pattern depends on transcriptional regulation and is controlled by a combination of a cell cycle-dependent element (CDE) together with a cell cycle genes homology region (CHR) in the Cks1 promoter. Furthermore, we observe Cks1 mRNA and protein to be downregulated after induced expression of the tumor suppressor p53. This repression is due to p53 downregulating transcription from the Cks1 promoter. p53-dependent repression is seen in a dose-dependent manner and in several cell types of different origin. In contrast to p53, its homologues p63 and p73 do not significantly repress transcription from the Cks1 promoter. The Cks1 promoter does not contain a p53 binding site. For some promoters the CCAAT box-binding transcription factor NF-Y had been implicated in p53-dependent repression. NF-Y is the main activator for Cks1 transcription but does not influence p53-dependent repression from the Cks1 promoter. Generally, the observation that the potential oncogene Cks1 is downregulated by the tumor suppressor p53 corresponds well with the idea that p53 employs multiple ways in order to halt the cell cycle.

Gene expression of cyclin-dependent kinase subunit Cks2 is repressed by the tumor suppressor p53 but not by the related proteins p63 or p73

Cks2 proteins are essential components of cyclin/cyclin-dependent kinase complexes and contribute to cell cycle control. We identify Cks2 as a transcriptional target downregulated by the tumor suppressor p53. Cks2 expression was found to be repressed by p53 both at the mRNA and the protein levels. p53 downregulates transcription from the Cks2 promoter in a dose-dependent manner and in all cell types tested. This repression appears to be independent of p53 binding to the Cks2 promoter. In contrast to p53, neither p63 nor p73 proteins can repress Cks2 transcription. Thus p53, rather than its homologues p63 and p73, may contribute to control of the first metaphase/anaphase transition of mammalian meiosis by downregulation of Cks2 expression.

The Alpha Helix of Ubiquitin Interacts with Yeast Cyclin-Dependent Kinase Subunit CKS1

Ubiquitin serves as a molecular zipcode to direct and sort ubiquitinylated proteins into distinct biological pathways. Although novel modes of ubiquitin interaction have recently been characterized, conventional ubiquitin-binding domains (UBDs) recognize ubiquitin through a hydrophobic pocket centered around isoleucine 44 and lined by residues in beta sheets 3 and 4. In this study, we report a novel mode of interaction between ubiquitin and the cyclin-dependent kinase subunit of Saccharomyces cerevisiae, Cks1p, an adaptor protein involved in transcriptional regulation through recruitment of proteasomal subunits to gene promoters. Cks1p interacts specifically with monoubiquitin and tetraubiquitin with an affinity several orders of magnitude greater than that of other ubiquitin-binding domains and in an unconventional fashion, which differs from interactions documented so far between ubiquitin and conventional UBDs. The loop between helices alpha 1 and alpha 2, and to a minor extent the N-terminal alpha-helix of Cks1p, are involved in the interaction with the alpha-helix of ubiquitin, instead of its I44-centered hydrophobic pocket. Not only is this the first time the alpha-helix of ubiquitin is implicated in a protein/protein interaction, thereby shedding new light on the mechanisms of ubiquitin recognition, but also the first report of a direct physical interaction between ubiquitin and Cks1p, inferring a role for ubiquitin binding in the transcriptional function of Cks1p.

Expression of the Ubiquitin Ligase Subunit Cyclin Kinase Subunit 1 and its Relationship to S-Phase Kinase Protein 2 and p27 Kip1 in Prostate Cancer

Loss of the cell cycle inhibitory protein p27Kip1 in cancer is associated with tumor aggressiveness and poor prognosis in the prostate. The decrease in p27(Kip1) results from increased proteasome dependent degradation, which is mediated by its specific ubiquitin ligase subunits S-phase kinase protein 2 and cyclin dependent kinase subunit 1. S-phase kinase protein 2 was found to be over expressed in aggressive prostate cancers but to our knowledge the role of cyclin dependent kinase subunit 1 in these cancers is unknown. The expression of cyclin dependent kinase subunit 1, S-phase kinase protein 2 and p27Kip1 was examined by immunohistochemistry in tissue sections from 45 patients with prostate cancer. The expression of cyclin dependent kinase subunit 1 was compared to that of S-phase kinase protein 2 and p27Kip1, and patient clinical and histological characteristics. Cyclin dependent kinase subunit 1 expression was strongly associated with S-phase kinase protein 2 expression (r = 0.666, p = 0.001) and inversely with p27Kip1 expression (r = -0.737, p < 0.001). Cyclin dependent kinase subunit 1 over expression was associated with loss of tumor differentiation (r = 0.631, p = 0.001), high serum prostate specific antigen (r = 0.627, p < 0.001) and metastatic disease (p < 0.001). These results suggest that cyclin dependent kinase subunit 1 is involved in p27Kip1 down-regulation and it may have an important causative role in the development of aggressive tumor behavior in prostate cancer.

Intraneuronal β-Amyloid Aggregates, Neurodegeneration, and Neuron Loss in Transgenic Mice with Five Familial Alzheimer's Disease Mutations: Potential Factors in Amyloid Plaque Formation

Mutations in the genes for amyloid precursor protein (APP) and presenilins (PS1, PS2) increase production of beta-amyloid 42 (Abeta42) and cause familial Alzheimer's disease (FAD). Transgenic mice that express FAD mutant APP and PS1 overproduce Abeta42 and exhibit amyloid plaque pathology similar to that found in AD, but most transgenic models develop plaques slowly. To accelerate plaque development and investigate the effects of very high cerebral Abeta42 levels, we generated APP/PS1 double transgenic mice that coexpress five FAD mutations (5XFAD mice) and additively increase Abeta42 production. 5XFAD mice generate Abeta42 almost exclusively and rapidly accumulate massive cerebral Abeta42 levels. Amyloid deposition (and gliosis) begins at 2 months and reaches a very large burden, especially in subiculum and deep cortical layers. Intraneuronal Abeta42 accumulates in 5XFAD brain starting at 1.5 months of age (before plaques form), is aggregated (as determined by thioflavin S staining), and occurs within neuron soma and neurites. Some amyloid deposits originate within morphologically abnormal neuron soma that contain intraneuronal Abeta. Synaptic markers synaptophysin, syntaxin, and postsynaptic density-95 decrease with age in 5XFAD brain, and large pyramidal neurons in cortical layer 5 and subiculum are lost. In addition, levels of the activation subunit of cyclin-dependent kinase 5, p25, are elevated significantly at 9 months in 5XFAD brain, although an upward trend is observed by 3 months of age, before significant neurodegeneration or neuron loss. Finally, 5XFAD mice have impaired memory in the Y-maze. Thus, 5XFAD mice rapidly recapitulate major features of AD amyloid pathology and may be useful models of intraneuronal Abeta42-induced neurodegeneration and amyloid plaque formation.

Retinoic acid induces leukemia cell G1arrest and transition into differentiation by inhibiting cyclin‐dependent kinase‐activating kinase binding and phosphorylation of PML/RAR

Acute promyelocytic leukemia (APL) cells express promyelocytic leukemia/retinoic acid receptor alpha (PML/RARalpha) fusion protein, which leads to the blocking of APL cell differentiation. Treatment of APL with all-trans-retinoic acid (ATRA) induces disease remission by in vivo differentiation of APL cells. Differentiation requires cell cycle exit; yet how ATRA couples cell cycle exit to differentiation of APL remains largely unknown. We previously found that ATRA-induced cell differentiation accompanies ubiquitination-proteolysis of ménage à trois 1 (MAT1), an assembly factor and targeting subunit of cyclin-dependent kinase (CDK)-activating kinase (CAK) that regulates G1 exit. We report here that CAK binds to and phosphorylates PML/RARalpha in actively proliferating APL cells. In response to ATRA, PML/RARalpha is dissociated from CAK, leading to MAT1 degradation, G1 arrest, and decreased CAK phosphorylation of PML/RARalpha. CAK phosphorylation of PML/RARalpha is inhibited when MAT1 levels are reduced. Both MAT1 degradation and PML/RARalpha hypophosphorylation occur in ATRA-induced G1-arresting cells undergoing differentiation but not in the synchronized G1 cells that do not differentiate. These findings reveal a novel ATRA signaling on APL cell differentiation, in which ATRA coordinates G1 arrest and transition into differentiation by inducing MAT1 degradation and PML/RARalpha hypophosphorylation through disrupting PML/RARalpha binding and phosphorylation by CAK.

Mitotic Checkpoint Slippage in Humans Occurs via Cyclin B Destruction in the Presence of an Active Checkpoint

In the presence of unattached/weakly attached kinetochores, the spindle assembly checkpoint (SAC) delays exit from mitosis by preventing the anaphase-promoting complex (APC)-mediated proteolysis of cyclin B, a regulatory subunit of cyclin-dependent kinase 1 (Cdk1). Like all checkpoints, the SAC does not arrest cells permanently, and escape from mitosis in the presence of an unsatisfied SAC requires that cyclin B/Cdk1 activity be inhibited. In yeast , and likely Drosophila, this occurs through an "adaptation" process involving an inhibitory phosphorylation on Cdk1 and/or activation of a cyclin-dependent kinase inhibitor (Cdki). The mechanism that allows vertebrate cells to escape mitosis when the SAC cannot be satisfied is unknown. To explore this issue, we conducted fluorescence microscopy studies on rat kangaroo (PtK) and human (RPE1) cells dividing in the presence of nocodazole. We find that in the absence of microtubules (MTs), escape from mitosis occurs in the presence of an active SAC and requires cyclin B destruction. We also find that cyclin B is progressively destroyed during the block by a proteasome-dependent mechanism. Thus, vertebrate cells do not adapt to the SAC. Rather, our data suggest that in normal cells, the SAC cannot prevent a slow but continuous degradation of cyclin B that ultimately drives the cell out of mitosis.

Peroxisome Proliferator-Activated Receptor γ Recruits the Positive Transcription Elongation Factor b Complex to Activate Transcription and Promote Adipogenesis

Positive transcription elongation factor b (P-TEFb) phosphorylates the C-terminal domain of RNA polymerase II, facilitating transcriptional elongation. In addition to its participation in general transcription, P-TEFb is recruited to specific promoters by some transcription factors such as c-Myc or MyoD. The P-TEFb complex is composed of a cyclin-dependent kinase (cdk9) subunit and a regulatory partner (cyclin T1, cyclin T2, or cyclin K). Because cdk9 has been shown to participate in differentiation processes, such as muscle cell differentiation, we studied a possible role of cdk9 in adipogenesis. In this study we show that the expression of the cdk9 p55 isoform is highly regulated during 3T3-L1 adipocyte differentiation at RNA and protein levels. Furthermore, cdk9, as well as cyclin T1 and cyclin T2, shows differences in nuclear localization at distinct stages of adipogenesis. Overexpression of cdk9 increases the adipogenic potential of 3T3-L1 cells, whereas inhibition of cdk9 by specific cdk inhibitors, and dominant-negative cdk9 mutant impairs adipogenesis. We show that the positive effects of cdk9 on the differentiation of 3T3-L1 cells are mediated by a direct interaction with and phosphorylation of peroxisome proliferator-activated receptor gamma (PPARgamma), which is the master regulator of this process, on the promoter of PPARgamma target genes. PPARgamma-cdk9 interaction results in increased transcriptional activity of PPARgamma and therefore increased adipogenesis.

Skp2 Overexpression Is Highly Representative of Intrinsic Biological Aggressiveness and Independently Associated with Poor Prognosis in Primary Localized Myxofibrosarcomas

Two SCF(Skp2) ubiquitin ligase-related proteins, Skp2 and cyclin-dependent kinase subunit 1 (Cks1), are involved in posttranscriptional degradation of p27(Kip1) tumor suppressor. We analyzed the prognostic utility of p27(Kip1) and its interacting cell cycle regulators in myxofibrosarcomas. Clinicopathologic features and tissue microarray-based immunohistochemical expression of p27(Kip1), Skp2, Cks1, cyclin E, cyclin A, Ki-67, and minichromosome maintenance protein 2 (Mcm2) were assessed in 70 primary myxofibrosarcomas and correlated with clinical outcomes. Skp2 mRNA expression and the relationship between Skp2 and p27(Kip1) proteins were examined in six cases by semiquantitative reverse transcription-PCR and Western blotting, respectively. High indices of Skp2 (> or =10%), cyclin A (> or =10%), and Mcm2 (> or =50%) were adverse prognosticators at the univariate level. Furthermore, co-overexpression of Skp2 and cyclin A identified highly lethal cases in the entire cohort [P < 0.0001 for disease-specific survival (DSS), P = 0.0004 for overall survival (OS)] and the lower-grade subset (Fédération Nationale des Centres de Lutte Contre le Cancer grade 1 and 2; P = 0.0006 for DSS, P = 0.0093 for OS). In multivariate analyses, Skp2 overexpression overshadowed most intrinsic clinicopathologic factors and independently correlated with worse metastasis-free survival (P = 0.0012), DSS (P = 0.0234), and OS (P = 0.0056). Notably, positive margins independently predicted inferior local recurrence-free survival (P = 0.0012) and also negatively affected metastasis-free survival (P = 0.0471), DSS (P = 0.0152), and OS (P = 0.0173). Reverse transcription-PCR showed up-regulation of Skp2 mRNA in four cases and Western blotting displayed a matched expression pattern of Skp2. Margin status and intrinsic property of myxofibrosarcomas both affect patient survival. Skp2 overexpression is highly representative of the biological aggressiveness of myxofibrosarcomas and plays an important prognostic role.

Stable gene silencing of cyclin B1 in tumor cells increases susceptibility to taxol and leads to growth arrest in vivo

Cyclin B1 is the regulatory subunit of cyclin-dependent kinase 1 (Cdk1) and is critical for the initiation of mitosis. Accumulating data indicate that the deregulation of cyclin B1 is tightly linked to neoplastic transformation. To study the phenotype and the potential preclinical relevance, we generated HeLa cell lines stably transfected with the plasmids encompassing short hairpin RNA (shRNA) targeting cyclin B1. We demonstrate that the reduction of cyclin B1 caused inhibition of proliferation by arresting cells in G2 phase and by inducing apoptosis. Cells, entering mitosis, were impaired in chromosome condensation and alignment. Importantly, HeLa cells with reduced cyclin B1 were more susceptible to the treatment of small interfering RNA targeting Polo-like kinase 1 (Plk1) and to the administration of the chemotherapeutic agent taxol. Finally, HeLa cells with reduced cyclin B1 showed inhibited tumor growth in nude mice compared to that of control cells. In summary, our data indicate that cyclin B1 is an essential molecule for tumor cell survival and aggressive proliferation, suggesting that the downregulation of cyclin B1, especially in combination with other molecular targets, might become an interesting strategy for antitumor intervention.

Analysis of the contribution of changes in mRNA stability to the changes in steady‐state levels of cyclin mRNA in the mammalian cell cycle

Cyclins are the essential regulatory subunits of cyclin-dependent protein kinases. They accumulate and disappear periodically at specific phases of the cell cycle. Here we investigated whether variations in cyclin mRNA levels in exponentially growing cells can be attributed to changes in mRNA stability. Mouse EL4 lymphoma cells and 3T3 fibroblasts were synchronized by elutriation or cell sorting. Steady-state levels and degradation of cyclin mRNAs and some other cell cycle related mRNAs were measured at early G1, late G1, S and G2/M phases. In both cell lines mRNAs of cyclins C, D1 and D3 remained unchanged throughout the cell cycle. In contrast, cyclin A2 and B1 mRNAs accumulated 3.1- and 5.7-fold between early G1 and G2/M phase, whereas cyclin E1 mRNA decreased 1.7-fold. Mouse cyclin A2 and B1 genes, by alternative polyadenylation, gave rise to more than one transcript. In both cases, the longer transcripts were the minor species but accumulated more strongly in G2/M phase. All mRNAs were rather stable with half-lives of 1.5-2 h for cyclin E1 mRNA and 3-4 h for the others. Changes in mRNA stability accounted for the accumulation in G2/M phase of the short cyclin A2 and B1 mRNAs, but contributed only partially to changes in levels of the other mRNAs.