Cyclin A: function and expression during cell proliferation

Mechanisms of Microglia Proliferation in a Rat Model of Facial Nerve Anatomy

Although microglia exist as a minor glial cell type in the normal state of the brain, they increase in number in response to various disorders and insults. However, it remains unclear whether microglia proliferate in the affected area, and the mechanism of the proliferation has long attracted the attention of researchers. We analyzed microglial mitosis using a facial nerve transection model in which the blood-brain barrier is left unimpaired when the nerves are axotomized. Our results showed that the levels of macrophage colony-stimulating factor (M-CSF), cFms (the receptor for M-CSF), cyclin A/D, and proliferating cell nuclear antigen (PCNA) were increased in microglia in the axotomized facial nucleus (axotFN). In vitro experiments revealed that M-CSF induced cFms, cyclin A/D, and PCNA in microglia, suggesting that microglia proliferate in response to M-CSF in vivo. In addition, M-CSF caused the activation of c-Jun N-terminal kinase (JNK) and p38, and the specific inhibitors of JNK and p38 arrested the microglial mitosis. JNK and p38 were shown to play roles in the induction of cyclins/PCNA and cFms, respectively. cFms was suggested to be induced through a signaling cascade of p38-mitogen- and stress-activated kinase-1 (MSK1)-cAMP-responsive element binding protein (CREB) and/or p38-activating transcription factor 2 (ATF2). Microglia proliferating in the axotFN are anticipated to serve as neuroprotective cells by supplying neurotrophic factors and/or scavenging excite toxins and reactive oxygen radicals.

Inhibition of Suv39h1/2 expression improves the early development of Debao porcine somatic cell nuclear transfer embryos

Suppressor of variegation 3-9 homolog (Suv39h)1 and 2, Histone H3 lysine 9 trimethylation (H3K9me3)-specific methyltransferases, are mainly involved in regulating the dynamic changes of H3K9me3. Regulating Suv39h expression influences the early development of mice somatic cell nuclear transfer (SCNT) embryos, there are few reports concerning their features in domestic animals. The aim of the present study was to characterize the Suv39h function in early development of Debao porcine SCNT embryos. The global level of H3K9me3 and the expression profiles of Suv39h1/2 in porcine early embryos were analysed by immunohistochemistry and qRT-PCR methods, respectively. Their roles in cell proliferation and histone modification of Debao porcine foetal fibroblast cells (PFFs), and developmental competence of porcine SCNT embryos were investigated by shRNA technology. The methylation levels of H3K9me3 and the expression patterns of Suv39h1 and Suv39h2 were similar (p < .05), and both of them displayed higher levels in Debao porcine SCNT embryos compared with that in PA embryos. The global levels of H3K9me3 and the expressions of G9a, HDAC1 and DNMT1 were decreased by combined inhibition of Suv39h1 and Suv39h2 (p < .05), while the expression of HAT1 was increased (p < .05). Downregulation of Suv39h1/2 also promoted cell proliferation and resulted in a significant increase in the expression of CyclinA2, CyclinB and PCNA in PFFs (p < .05). Furthermore, the use of donor somatic nuclei which depleted H3K9me3 by inhibiting Suv39h1/2 expression markedly increased the cleavage rate, the blastocyst rate and the total cell number of blastocysts of Debao porcine SCNT embryos (p < .05). Altogether, the above results indicate that H3K9me3 levels and Suv39h1/2 expressions display similar patterns in porcine early embryo, and low levels of them are critical to cell proliferation of PFFs and early development of SCNT embryos.

Standardized Saponin Extract from Baiye No.1 Tea (Camellia sinensis) Flowers Induced S Phase Cell Cycle Arrest and Apoptosis via AKT-MDM2-p53 Signaling Pathway in Ovarian Cancer Cells

Ovarian cancer is considered to be one of the most serious malignant tumors in women. Natural compounds have been considered as important sources in the search for new anti-cancer agents. Saponins are characteristic components of tea (Camellia sinensis) flower and have various biological activities, including anti-tumor effects. In this study, a high purity standardized saponin extract, namely Baiye No.1 tea flower saponin (BTFS), which contained Floratheasaponin A and Floratheasaponin D, were isolated from tea (Camellia sinensis cv. Baiye 1) flowers by macroporous resin and preparative liquid chromatography. Then, the component and purity were detected by UPLC-Q-TOF/MS/MS. This high purity BTFS inhibited the proliferation of A2780/CP70 cancer cells dose-dependently, which is evidenced by the inhibition of cell viability, reduction of colony formation ability, and suppression of PCNA protein expression. Further research found BTFS induced S phase cell cycle arrest by up-regulating p21 proteins expression and down-regulating Cyclin A2, CDK2, and Cdc25A protein expression. Furthermore, BTFS caused DNA damage and activated the ATM-Chk2 signaling pathway to block cell cycle progression. Moreover, BTFS trigged both extrinsic and intrinsic apoptosis—BTFS up-regulated the expression of death receptor pathway-related proteins DR5, Fas, and FADD and increased the ratio of pro-apoptotic/anti-apoptotic proteins of the Bcl-2 family. BTFS-induced apoptosis seems to be related to the AKT-MDM2-p53 signaling pathway. In summary, our results demonstrate that BTFS has the potential to be used as a nutraceutical for the prevention and treatment of ovarian cancer.

Cyclin A is a reliable proliferation marker in endometrial cancer cell lines

Cyclin A, a cell cycle regulatory protein, promotes cell proliferation and has been observed to be highly expressed in cancer and to promote tumor growth; however, its value as a marker for endometrial carcinoma has not yet been established. Accordingly, the aim of the present study was to clarify whether cyclin A can be used as a cell proliferation marker using the endometrial carcinoma cell lines Ishikawa and HEC-50B, derived from patients with low-grade and high-grade cancer, respectively. The expression of cyclin A was determined by flow cytometry using double staining with FITC and 7-AAD, and immunocytochemical staining. The results were compared to those of Ki-67, the widely used cell proliferation marker that is considered to be a prognostic marker in endometrial cancer. The flow cytometry results revealed that cyclin A expression was significantly higher in HEC-50B than in Ishikawa cells during the logarithmic growth phase. In addition, cyclin A expression was consistently higher than Ki-67 expression in the examined cell lines. Immunocytochemical staining confirmed cyclin A expression in HEC-50B and Ishikawa cells, demonstrating significantly higher expression during the logarithmic growth phase than during the stationary phase. By contrast, Ki-67 was expressed in almost 90% of the cells, irrespective of their growth state. These results indicate that cyclin A expression is significantly increased in cells with higher proliferative ability and is specifically expressed in cells that have passed the G1-S checkpoint. Therefore, cyclin A may be a reliable proliferation biomarker for endometrioid carcinoma.

The Crosstalk between HDPSCs and HUCMSCs on Proliferation and Osteogenic Genes Expression in Coculture System

Objectives: The present study established a non-contact coculture system in vitro, aiming to investigate the crosstalk between human dental pulp stem cells (hDPSCs) and human umbilical cord mesenchymal stem cells (hUCMSCs) on proliferation activity and osteogenic genes expression through paracrine. Materials and methods: The stemness of hDPSCs and hUCMSCs were identified by flow cytometric analysis and multipotential differentiation assays. With the help of transwell inserts, the non-contact coculture system in vitro was established between hDPSCs and hUCMSCs. EdU labeling analysis and Western Blot were used to detect the proliferation activity. The mRNA and protein levels of osteogenic genes were evaluated by RT-PCR and Western Blot. The expression of elements in Akt/mTOR signaling pathway were detected by Western Blot. Results: Both hDPSCs and hUCMSCs were positive to MSCs specific surface markers and had multi-differentiation potential. The proportion of EdU-positive cells increased and the expression of CDK6 and CYCLIN A were up-regulated in cocultured hDPSCs. Both prior coculture and persistent coculture improved mRNA and protein levels of osteogenic genes in hDPSCs. While in cocultured hUCMSCs, no statistical differences were observed on proliferation and osteogenesis. The phosphorylation of Akt and mTOR was up-regulated in cocultured hDPSCs. Conclusions: The crosstalk between hDPSCs and hUCMSCs in coculture system increased the proliferation activity and enhanced osteogenic genes expression in hDPSCs. Akt/mTOR signaling pathway might take part in the enhancing effects in both cell proliferation and gene expression.

Moderate (2%, v/v) Ethanol Feeding Alters Hepatic Wound Healing after Acute Carbon Tetrachloride Exposure in Mice

Wound healing consists of three overlapping phases: inflammation, proliferation, and matrix synthesis and remodeling. Prolonged alcohol abuse can cause liver fibrosis due to deregulated matrix remodeling. Previous studies demonstrated that moderate ethanol feeding enhances liver fibrogenic markers and frank fibrosis independent of differences in CCl₄-induced liver injury. Our objective was to determine whether or not other phases of the hepatic wound healing response were affected by moderate ethanol after CCl₄ exposure. Mice were fed moderate ethanol (2% v/v) for two days and then were exposed to CCl₄ and euthanized 24–96 h later. Liver injury was not different between pair- and ethanol-fed mice; however, removal of necrotic tissue was delayed after CCl₄-induced liver injury in ethanol-fed mice. Inflammation, measured by TNFα mRNA and protein and hepatic Ly6c transcript accumulation, was reduced and associated with enhanced hepatocyte apoptosis after ethanol feeding. Hepatocytes entered the cell cycle equivalently in pair- and ethanol-fed mice after CCl₄ exposure, but hepatocyte proliferation was prolonged in livers from ethanol-fed mice. CCl₄-induced hepatic stellate cell activation was increased and matrix remodeling was prolonged in ethanol-fed mice compared to controls. Taken together, moderate ethanol affected each phase of the wound healing response to CCl₄. These data highlight previously unknown effects of moderate ethanol exposure on hepatic wound healing after acute hepatotoxicant exposure.

TR1 promotes cell proliferation and inhibits apoptosis through cyclin A and CTGF regulation in non-small cell lung cancer

The Hippo pathway plays a major role in development and organ size control, and its dysregulation contributes to tumorigenesis. WWTR1 is a transcription coactivator acting downstream of the Hippo pathway. Recently, WWTR1 has been reported to be overexpressed in several human cancers including lung cancer. However, the molecular mechanism of WWTR1 regulating lung cancer aggressiveness remains ambiguous. In the present study, we analyzed the expression of WWTR1 in NSCLC cell lines and found that WWTR1 was overexpressed at both the mRNA and protein levels. Knockdown of WWTR1 by siRNA interference in A549 cells significantly inhibited cell proliferation and increased paclitaxel-induced apoptosis. On the other side, WWTR1 overexpression in HBE cell line promoted cell proliferation and inhibited apoptosis. In addition, we found that the decreased proliferation after siRNA treatment was due to cell cycle arrest. Further analysis showed that WWTR1 could induce cyclin A, connective tissue growth factor (CTGF) expression, and inhibit caspase3 cleavage. In conclusion, WWTR1 promotes malignant cell growth and inhibits apoptosis by cyclin A and CTGF regulation.

The transcription factor FOXM1 (Forkhead box M1): proliferation-specific expression, transcription factor function, target genes, mouse models, and normal biological roles

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor, which stimulates cell proliferation and exhibits a proliferation-specific expression pattern. Accordingly, both the expression and the transcriptional activity of FOXM1 are increased by proliferation signals, but decreased by antiproliferation signals, including the positive and negative regulation by protooncoproteins or tumor suppressors, respectively. FOXM1 stimulates cell cycle progression by promoting the entry into S-phase and M-phase. Moreover, FOXM1 is required for proper execution of mitosis. Accordingly, FOXM1 regulates the expression of genes, whose products control G1/S-transition, S-phase progression, G2/M-transition, and M-phase progression. Additionally, FOXM1 target genes encode proteins with functions in the execution of DNA replication and mitosis. FOXM1 is a transcriptional activator with a forkhead domain as DNA binding domain and with a very strong acidic transactivation domain. However, wild-type FOXM1 is (almost) inactive because the transactivation domain is repressed by three inhibitory domains. Inactive FOXM1 can be converted into a very potent transactivator by activating signals, which release the transactivation domain from its inhibition by the inhibitory domains. FOXM1 is essential for embryonic development and the foxm1 knockout is embryonically lethal. In adults, FOXM1 is important for tissue repair after injury. FOXM1 prevents premature senescence and interferes with contact inhibition. FOXM1 plays a role for maintenance of stem cell pluripotency and for self-renewal capacity of stem cells. The functions of FOXM1 in prevention of polyploidy and aneuploidy and in homologous recombination repair of DNA-double-strand breaks suggest an importance of FOXM1 for the maintenance of genomic stability and chromosomal integrity.

Effect of the HIV nucleoside reverse transcriptase inhibitor zidovudine on the growth and differentiation of primary gingival epithelium

OBJECTIVES

Oral complications associated with HIV infection and with the antiretroviral drugs used to treat it are of increasing concern in HIV-infected patients. Protease inhibitors have been shown to change the proliferation and differentiation state of oral tissues but the effect of nucleoside reverse transcriptase inhibitors is currently unknown. This study examined the effect of zidovudine on the growth and differentiation of the gingival epithelium.

METHODS

Gingival keratinocyte organotypic (raft) cultures were established. The raft cultures were treated with a range of zidovudine concentrations. Haematoxylin and eosin staining was performed to examine the effect of zidovudine on gingival epithelium growth and stratification. Raft cultures were immunohistochemically analysed to determine the effect of this drug on the expression of key differentiation and proliferation markers, including cytokeratins and proliferating cell nuclear antigen (PCNA).

RESULTS

Zidovudine dramatically changed the proliferation and differentiation state of gingival tissues both when it was present throughout the growth period of the tissue and when it was added to established tissue at day 8. Zidovudine treatment increased the expression of cytokeratin 10, PCNA and cyclin A. Conversely, cytokeratin 5, involucrin and cytokeratin 6 expression was decreased. The tissue exhibited characteristics of increased proliferation in the suprabasal layers as well as an increased fragility and an inability to heal itself.

CONCLUSIONS

Zidovudine treatment, even when applied at low concentrations for short periods of time, deregulated the cell cycle/proliferation and differentiation pathways, resulting in abnormal epithelial repair and proliferation. Our system could potentially be developed as a model for studying the effects of HIV and highly active antiretroviral therapy in vitro.

Antibody-mediated allograft rejection: the emerging role of endothelial cell signalling and transcription factors

The presence of antibodies against class I human leukocyte antigens (HLA) can cause the development of chronic allograft rejection. Although endothelial cell activation has been identified as a main effector, the mechanisms mediating this process are not fully understood. Exposure of endothelium to antibodies against HLA antigens induces cell activation which promotes rejection. This activation process can be divided into two phases: an early response in which intracellular signalling proteins and transcription factors are activated, and a later change in protein expression and cell function. In this review, antibody-mediated endothelial signalling and the role of transcription factors in organ transplantation will be described with a particular focus on their potential to mediate some of the graft-damaging effects of anti-HLA class I antibodies.

Cell Cycle Deregulation in Ewing's Sarcoma Pathogenesis

Ewing's sarcoma is a highly aggressive pediatric tumor of bone that usually contains the characteristic chromosomal translocation t(11;22)(q24;q12). This translocation encodes the oncogenic fusion protein EWS/FLI, which acts as an aberrant transcription factor to deregulate target genes necessary for oncogenesis. One key feature of oncogenic transformation is dysregulation of cell cycle control. It is therefore likely that EWS/FLI and other cooperating mutations in Ewing's sarcoma modulate the cell cycle to facilitate tumorigenesis. This paper will summarize current published data associated with deregulation of the cell cycle in Ewing's sarcoma and highlight important questions that remain to be answered.

Effect of the HIV protease inhibitor amprenavir on the growth and differentiation of primary gingival epithelium

BACKGROUND

HIV-positive patients taking antiretroviral drugs, including protease inhibitors, have shown a significant increase in the development of oral complications; these complications are a major health issue for affected patients. The effect of these drugs on oral epithelium growth and differentiation is presently unknown. In this study, we explore for the first time the effect of the HIV protease inhibitor amprenavir on gingival epithelium growth and differentiation.

METHODS

Organotypic (raft) cultures of gingival keratinocytes were established and the raft cultures treated with a range of amprenavir concentrations. Haematoxylin and eosin staining was performed to examine the effect of amprenavir on gingival epithelium growth and stratification. The raft cultures were also immunohistochemically analysed to determine the effect of amprenavir on the expression of key differentiation and proliferation markers, including cytokeratins, proliferating cell nuclear antigen (PCNA) and cyclin A.

RESULTS

Amprenavir severely inhibited the growth of gingival epithelium when the drug was present throughout the growth period of the tissue. When the drug was added at day 8, amprenavir treatment altered the proliferation and differentiation of gingival keratinocytes. Expression of the cytokeratins 5, 14, 6 and 10, PCNA and cyclin A was increased; their expression pattern was also altered over time in treated rafts. Biochemically, the tissue exhibited characteristics of increased proliferation in the suprabasal layers of amprenavir-treated tissue.

CONCLUSIONS

Our results suggest that amprenavir treatment deregulates the cell cycle/proliferation and differentiation pathways, resulting in abnormal epithelial repair and proliferation. Our system could be developed as a potential model for studying the effects of highly active antiretroviral therapy in vitro.

Zinc depletion efficiently inhibits pancreatic cancer cell growth by increasing the ratio of antiproliferative/proliferative genes

We investigated the ability of the zinc chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) to reduce pancreatic cancer cell viability. TPEN was much more efficient to inhibit pancreatic adenocarcinoma cell growth than a panel of anti-cancer drugs, including 5-fluorouracil, irinotecan, cisplatin, edelfosine, trichostatin A, mitomycin C, and gemcitabine, the gold standard chemotherapeutic agent for pancreatic cancer. Moreover, TPEN showed a dose- and time-dependent anti-proliferative effect significantly higher on pancreatic cancer cells than on normal primary fibroblasts. This effect may be explained by a significantly higher zinc depletion by TPEN in pancreatic cancer cells as compared to fibroblasts. Cell viability reduction by TPEN was associated to both G1-phase cell cycle arrest and apoptosis, and to the increased ratio of the expression level of cyclin-Cdk inhibitor versus cyclin genes and apoptotic versus anti-apoptotic genes. Finally, we show that apoptotic cell death induced by TPEN involved mitochondrial injury and caspase 3 and caspase 8 activation. In this study, we suggest that zinc depletion may be an efficient strategy in the treatment of pancreatic cancer because of its reduced antiproliferative effect on normal cells.

Usefulness of automatic quantification of immunochemical staining on whole tumor sections for correlation with oncological small animal PET studies: an example with cell proliferation, glucose transporter 1 and FDG

AIM

To highlight the use of automatic quantification of immunochemical staining on digitized images of whole tumor sections in preclinical positron emission tomography (PET) studies.

MATERIALS AND METHODS

Xenografted human testicular tumors (36) were imaged with 2-deoxy-2[F-18]fluoro-D: -glucose (FDG) small animal PET (SA-PET). Tumor cell proliferation and glucose transportation were assessed with cyclin A and Glut-1 immunostaining. Tumor slides were digitized and processed with PixCyt software enabling whole slide quantification, then compared with junior and senior pathologist manual scoring. Manual and automatic quantification results were correlated to FDG uptake.

RESULTS

For cyclin A, inter- and intra-observer agreement for manual scoring was 0.52 and 0.72 and concordance between senior pathologist and automatic quantification was 0.84. Correlations between Tumor/Background ratio and tumor cell proliferation assessed by automatic quantification, junior and senior pathologists were 0.75, 0.55, and 0.61, respectively. Correlation between Tumor/Background ratio and Glut-1 assessed by automatic quantification was 0.74.

CONCLUSION

Automatic quantification of immunostaining is a valuable tool to overcome inter- and intra-observer variability for correlation of cell proliferation or other markers with tumor tracer uptake.

Roles of cyclins A and E in induction of centrosome amplification in p53-compromised cells

Abnormal amplification of centrosomes, which occurs frequently in cancers, leads to high frequencies of mitotic defect and chromosome segregation error, profoundly affecting the rate of tumor progression. Centrosome amplification results primarily from overduplication of centrosomes, and p53 is involved in the regulation of centrosome duplication partly through controlling the activity of cyclin-dependent kinase (CDK) 2-cyclin E, a kinase complex critical for the initiation of centrosome duplication. Thus, loss or mutational inactivation of p53 leads to an increased frequency of centrosome amplification. Moreover, the status of cyclin E greatly influences the frequency of centrosome amplification in cells lacking functional p53. Here, we dissected the roles of CDK2-associating cyclins, namely cyclins E and A, in centrosome amplification in the p53-negative cells. We found that loss of cyclin E was readily compensated by cyclin A for triggering the initiation of centrosome duplication, and thus the centrosome duplication kinetics was not significantly altered in cyclin E-deficient cells. It has been shown that cells lacking functional p53, when arrested in either early S-phase or late G(2) phase, continue to reduplicate centrosomes, resulting in centrosome amplification. In cells arrested in early S phase, cyclin E, but not cyclin A, is important in centrosome amplification, whereas in the absence of cyclin E, cyclin A is important for centrosome amplification. In late G(2)-arrested cells, cyclin A is important in centrosome amplification irrespective of the cyclin E status. These findings advance our understandings of the mechanisms underlying the numeral abnormality of centrosomes and consequential genomic instability associated with loss of p53 function and aberrant expression of cyclins E and A in cancer cells.

Cyclin A expression and its diagnostic value in pleomorphic adenoma and carcinoma expleomorphic adenoma of the parotid gland

AIMS

To investigate cyclin A expression in pleomorphic adenoma (PA) and carcinoma expleomorphic adenoma (CXPA) of the parotid gland with a view to assessing its potential value as a diagnostic marker for CXPA.

METHODS AND RESULTS

Cyclin A expression in PA and CXPA was studied using semiquantitative immunohistochemistry. The epithelial component of the tumours expressed cyclin A in a statistically significantly (P < 0.005) higher number of CXPA cases (86%) compared with the PA cases (39%). Cyclin A was not expressed in normal salivary tissues of PA and CXPA.

CONCLUSIONS

High cyclin A expression is a useful marker for the pathological diagnosis of CXPA.

Syndecan-4 dependent FGF stimulation of mouse vibrissae growth

The development, maintenance and regeneration of epithelial appendages such as hairs or vibrissae depend on reciprocal interactions between the epidermal and the dermal components of the integument. Growth factors are among a number of signaling molecules that have been identified during these developmental events. Growth factors such as fibroblast growth factors (FGFs) bind cell surface heparan sulfate proteoglycans (HSPGs) on their heparan sulfate side chains and as such these proteoglycans act as co-receptors for FGF receptors (FGFRs) by forming a ternary signaling complex of HSPG, FGFR and FGF. The syndecans make up a family (syndecan-1-4) of transmembrane HSPGs. In the present study we examined the growth response of mouse vibrissae to HSPG-binding growth factors as a function of the presence or absence of syndecan-4 in an organ culture system. Syndecan-4 is expressed on keratinocytes that make up the inner root sheath of the vibrissa. Vibrissae from wild-type mice, but not from syndecan-4 null mice, displayed a statistically significant and dose-dependent growth response to FGF-1, FGF-2 and FGF-7. In contrast, a statistically significant growth response is seen in vibrissae from both wild-type and syndecan-4 null mice when the culture medium is supplemented with either hepatocyte growth factor (HGF) that binds to HSPG, insulin that does not bind to HSPG or 5% fetal bovine serum. The syndecan-4 dependent effect of FGF-1, -2 and -7 on the transcriptional activity of IRS expressed genes and of genes involved in cell proliferation reveals a number of different response patterns. In vivo, the vibrissae of syndecan-4 null mice are shorter and have a smaller diameter than those of wild-type mice and this phenotype may result from a suboptimal response to growth factors. Syndecan-1, which is expressed in the outer root sheath of the vibrissae shaft, does not influence the response of the vibrissae to FGF-1, -2 and -7 and the length and diameter of vibrissae of syndecan-1 null mice do not differ from those of wild-type mice.

Neutral lipid isolated from endosperm of Job's tears inhibits the growth of pancreatic cancer cells via apoptosis, G2/M arrest, and regulation of gene expression

BACKGROUND

The neutral lipid isolated from the endosperm of Job's tears (NLEJ) has been known to possess an anticancer activity with relatively low toxicity. The present study was designed to examine its antiproliferative effects in the PaTu-8988 and SW1990 human pancreatic cancer cells and to investigate its potential mechanism(s).

METHODS

Pancreatic cancer cells were treated with NLEJ to evaluate cell viability, cell cycle progression, nuclear morphology, DNA fragmentation and annexin V binding analysis. Regulation of gene expression was determined by using cDNA microarrays and western immunoblotting.

RESULTS

The NLEJ induced a dose- and time-dependent inhibition of proliferation in both PaTu-8988 and SW1990 cell lines. Further studies were carried out with only the PaTu-8988 cells. Flow cytometry analysis showed that NLEJ blocked cell cycle progression at the G(2)/M phase. There was also an increase in annexin V binding and DNA fragmentation, indicative of apoptosis. The cDNA microarray analysis with cell cycle- and apoptosis-targeted arrays showed that the expression signals of 24 genes were found to be significantly altered at 24 h of NLEJ treatment. These genes are involved in cell cycle control (e.g. p21, p27, CDK2, and cyclins), apoptosis regulation (e.g. bcl-2 and bax), and signal transduction (e.g. ATM, RAD50, and p53). Some of these results were confirmed by western blot analysis.

CONCLUSIONS

These data show that NLEJ inhibits pancreatic cancer cell growth through induction of apoptosis and cell cycle arrest as well as regulation of gene expression in vitro. Therefore, NLEJ might be a chemotherapeutic agent against pancreatic cancer.

Regulation of vascular smooth muscle proliferation by heparin: inhibition of cyclin-dependent kinase 2 activity by p27(kip1)

Uncontrolled proliferation of vascular smooth muscle cells (VSMCs) contribute to intimal hyperplasia during atherosclerosis and restenosis. Heparin is an antiproliferative agent for VSMCs and has been shown to block VSMC proliferation both in tissue culture systems and in animals. Despite the well documented antiproliferative actions of heparin, its cellular targets largely remain unknown. In an effort to characterize the mechanism of the antiproliferative property of heparin, we have analyzed the effect of heparin on cell cycle in VSMC. Our results indicate that the heparin-induced block in G(1) to S phase transition is imposed by p27(kip1)-mediated inhibition of cyclin-dependent kinase 2 activity. Further analysis of p27(kip1) mRNA levels showed that the increase in p27(kip1) protein levels in heparin-treated VSMC occurs at posttranscriptional levels. We present evidence that heparin causes stabilization of p27(kip1) protein during G(1) phase and thereby prevents activation of cyclin-dependent kinase 2.

Phenotypic persistence after reoxygenation of hypoxic neuroblastoma cells

Fast-growing solid tumors are usually insufficiently vascularized, leading to areas with necrosis and/or poorly oxygenated cells. Tumor cells adapt to acute hypoxic stress. Central to this adaptation are the hypoxia-inducible transcription factors (HIFs), which are degraded at normoxic but become stabilized at hypoxic conditions. Hypoxic (1% O2) neuroblastoma cells downregulate sympathetic nervous system marker genes, whereas neural crest cell markers are upregulated, suggesting that hypoxic tumor cells adopt a less mature phenotype, which in the clinical setting would translate to more aggressive tumors with increased metastatic potential. Here, we compared gene expression patterns in neuroblastoma cells grown at 1%, 5% (a physiologic oxygen level) and 21% O2. At 5% O2, cells developed a weak hypoxic phenotype and HIF-2 alpha, but not HIF-1 alpha, was acutely stabilized. At 1% O2, HIF-2 alpha protein remained present in long-term cultures, while HIF-1 alpha was present only transiently. The stability of the hypoxia-induced dedifferentiated phenotype in cells acutely reoxygenated at either 21% or 5% O2 persisted for at least 24 hr. Genes associated with a differentiated state, like NPY, ChrA and ChrB, were still downregulated and hypoxia-induced genes, like TH and Id2, remained upregulated. Thus, if these culture conditions are viewed as models for acute reoxygenation of metastasizing hypoxic tumor cells, our data suggest that an aggressive hypoxic phenotype persists for 24 hr or more, which might be long enough for the cells to be able to home to secondary sites, in part as a consequence of their immature hypoxic characteristics.

Correlation between clinical characteristics and proliferative activity in patients with craniopharyngioma

OBJECTIVES

The aim of the study was to correlate the Ki-67 and cyclin A labelling index (LI) with clinical characteristics and risk of recurrence of craniopharyngiomas.

METHODS

47 consecutive patients were studied, 21 female and 26 male, aged 34.3 (2.8) years. Immunohistochemical analysis was performed on paraffin wax embedded material using monoclonal antibodies directed against the proliferation associated nuclear antigen Ki-67 and cyclin A.

RESULTS

The median Ki-67 LI was 8.6% (interquartile range, 4.4%-14.0%). Ki-67 LI was significantly higher in tumours with a heavy inflammatory reaction and diabetes insipidus at presentation, whereas other clinical and histological features were not associated with the proliferation index. There was a strong linear correlation between Ki-67 LI and cyclin A LI (r = 0.77; p<0.0001); therefore, cyclin A LI showed the same clinical and histological relations described for Ki-67 LI. Recurrence of craniopharyngioma occurred in 13 of 46 patients (28.3%). The median Ki-67 LI in the 13 recurrent craniopharyngiomas (9.0%) was not significantly different from that of non-recurring tumours (7.9%). Cyclin A LI was also not associated with the risk of relapse.

CONCLUSIONS

This study confirms the great variability of proliferative activity in craniopharyngiomas. Ki-67 and cyclin A LIs were associated with the presence of a heavy inflammatory reaction and diabetes insipidus, but did not correlate with the long term risk of tumour regrowth.

Identification of interaction partners and substrates of the cyclin A1-CDK2 complex

The CDK2-associated cyclin A1 is essential for spermatogenesis and contributes to leukemogenesis. The detailed molecular functions of cyclin A1 remain unclear, since the molecular networks involving cyclin A1-CDK2 have not been elucidated. Here, we identified novel cyclin A1/CDK2 interaction partners in a yeast triple-hybrid approach. Several novel proteins (INCA1, KARCA1, and PROCA1) as well as the known proteins GPS2 (G-protein pathway suppressor 2), Ku70, receptor for activated protein kinase C1/guanine nucleotide-binding protein beta-2-like-1, and mRNA-binding motif protein 4 were identified as interaction partners. These proteins link the cyclin A1-CDK2 complex to diverse cellular processes such as DNA repair, signaling, and splicing. Interactions were confirmed by GST pull-down assays and co-immunoprecipitation. We cloned and characterized the most frequently isolated unknown gene, which we named INCA1 (inhibitor of CDK interacting with cyclin A1). The nuclear INCA1 protein is evolutionarily conserved and lacks homology to any known gene. This novel protein and two other interacting partners served as substrates for the cyclin A1-CDK2 kinase complex. Cyclin A1 and all interaction partners were highly expressed in testis with varying degrees of tissue specificity. The highest expression levels were observed at different time points during testis maturation, whereas expression levels in germ cell cancers and infertile testes decreased. Taken together, we identified testicular interaction partners of the cyclin A1-CDK2 complex and studied their expression pattern in normal organs, testis development, and testicular malignancies. Thereby, we establish a new basis for future functional analyses of cyclin A1. We provide evidence that the cyclin A1-CDK2 complex plays a role in several signaling pathways important for cell cycle control and meiosis.

The evaluation of low dose hyper-radiosensitivity in normal human skin

BACKGROUND AND PURPOSE

The laboratory phenomenon of low dose hyper-radiosensitivity (LDHRS) describes an excess of cell kill at doses below 1Gy relative to that predicted by the linear quadratic model. These data have stimulated clinical investigation into LDHRS in vivo.

PATIENTS AND METHODS

Skin was used as a model of normal human tissue. Two studies were initiated investigating the response to low doses of radiation. Study 1 compared once daily skin doses of approximately 0.5 and >1.0Gy in 24 patients receiving pelvic radiotherapy. Skin biopsies before and during radiotherapy were analysed histologically to assess the basal cell density (BCD). Study 2 compared two regimens of equal dose/time intensity--an ultrafractionated regimen (0.5Gy TDS x 12 days) with a conventional regimen (1.5Gy OD x 12 days). Skin biopsies taken during treatment assessed BCD and proliferative index. In both studies the changes in BCD were compared using non-linear regression analysis.

RESULTS

Study 1. The results show a significantly greater reduction in BCD in the low dose group when BCD is plotted against dose. This effect is lost when BCD is plotted against time Study 2. The results demonstrate a significantly greater reduction in BCD in the higher dose/fraction arm. The proliferative response was similar in both treatment groups.

CONCLUSIONS

These data suggest that LDHRS does not occur in skin following doses of approximately 0.5Gy/fraction when regimens of equal dose/time intensity are compared. As only small volumes of normal tissue were irradiated it is difficult to predict the biological relevance of this with respect to larger field low dose per fraction irradiation regimens or risk of cancer induction. Equally we cannot extrapolate to effects resulting from exposure to doses <0.5Gy or to the effects of low doses on other endpoints.

Concomitant differentiation and partial proteasome inhibition trigger apoptosis in neuroblastoma cells

Proteasome activity is essential during cAMP-induced terminal differentiation of a murine neuroblastoma cell line (NBP2). However, the mechanisms through which proteasome affects NBP2 differentiation have not been characterized. We hypothesized that proteasome is required to implement the differentiation-mediated effects on cell cycle, and its partial inhibition during differentiation may have adverse consequences. Here we show that partial inhibition of proteasome during cAMP-induced differentiation of NBP2 cells causes apoptosis. Whereas differentiation induced growth arrest at G1 phase, partial proteasome inhibition during differentiation resulted in the accumulation of cells at G2M phase. Cell cycle data correlated with the level of cyclin-dependent kinase inhibitors p21WAF and p27Kip1, and cyclin A. While the level of p21 and p27 increased, the level of cyclin A decreased upon differentiation. In contrast, cells treated with proteasome inhibitor in the presence of cAMP-inducing agents showed increased levels of p21 and cyclin A early in the course of differentiation. However, the level of p21 and p27, but not cyclin A, decreased later during concomitant differentiation and partial proteasome inhibition when cells were undergoing apoptosis. Our data suggest that differentiation-mediated growth arrest is dependent on the temporal activity of cell cycle proteins. Partial inhibition of proteasome interferes with differentiation events partly by stabilizing cell cycle proteins and this triggers apoptosis. Thus, differentiating drugs combined with partial proteasome inhibition may impart higher therapeutic efficacy than differentiating agents alone for the treatment of neuroblastoma tumors.

Cyclin A correlates with carcinogenesis and metastasis, and p27(kip1) correlates with lymphatic invasion, in colorectal neoplasms

Cyclin A binds to CDK2 and plays critical roles when cells proliferate; staining for Ki67 can monitor the proliferation. The cyclin A expression pattern remains unclear in colorectal carcinogenesis and remote metastasis, however, and no one has reported on the association of its expression with key clinicopathologic factors in primary cancer. p27(kip1) protein-an extremely important inhibitor of CDK2-seems unchanged as colorectal cancers metastasize to the lymph nodes, a result contrary to that seen in gastric and prostatic cancers. To clarify the role of cyclin A in multistage colorectal neoplasms, cyclin A, CDK2, and Ki67 were immunohistochemically stained in 22 normal mucosa, 9 hyperplastic polyps, 61 adenomas, 197 primary carcinomas, 21 lymph node metastases, and 10 hepatic metastases. To clarify the alteration of p27(kip1) during lymphatic invasion, p27(kip1) was also stained in 21 primary cancers and paired lymph node foci. Situated in nuclei, cyclin A expression gradually increased from mild through moderate to severe dysplasia in adenomas and from normal tissue through hyperplasia to adenoma to early carcinoma. Expression was significantly decreased in the hepatic metastases and in the primary cancers showing venous invasion, deep infiltration, lymph node metastasis, mucinous type, advanced stage, or short postoperative survival time. Elevated cyclin A not only was linked with elevated CDK2 in primary cancers, but also was associated with increased Ki67 in both adenomas and primary carcinomas. Lymph node metastases lost more p27(kip1) than primary foci and hepatic lesions. Thus, dysregulation of cyclin A and its control mechanisms may contribute to colorectal carcinogenesis; abatement of overexpression of cyclin A is associated with hepatic metastasis and cancerous invasion. Loss of p27(kip1) may promote lymph node metastasis.

Cell cycle promoting activity of JunB through cyclin A activation

JunB, a major component of the AP-1 transcription factor, is known to act antagonistically to c-Jun in transcriptional regulation and is proposed to be a negative regulator of cell proliferation. Employing fibroblasts derived from E9.5 junB(-/-) mouse embryos we provide evidence for a novel cell cycle promoting role of JunB. Despite a normal proliferation rate, primary and immortalized junB(-/-) fibroblasts exhibited an altered cell cycle profile, which was characterized by an increase in the population of S-phase cells, while that of cells in G(2)/M-phase was diminished. This delay in G(2)/M-transition is caused by impaired cyclin A-CDK2 and cyclin B-CDC2 kinase activities and counteracts the accelerated S-phase entry. Cells lacking JunB show severely delayed kinetics of cyclin A mRNA expression due to the loss of proper transcriptional activation mediated via binding of JunB to the CRE element in the cyclin A promoter. Upon reintroduction of an inducible JunB-ER(TM) expression vector the cell cycle distribution and the cell cycle-associated cyclin A-CDK2 kinase activity could be restored. Thus, cyclin A is a direct transcriptional target of JunB driving cell proliferation.

Cdc6 requires anchorage for its expression

Fibroblasts need anchorage to extracellular matrix to transit from G1 to S phase, but no longer after oncogenic transformation. Here we report that Cdc6 protein essential for the activation of replication origins requires anchorage or oncogenic stimulation for its execution. Upon anchorage loss, Cdc6 expression is shut off both transcriptionally and post-transcriptionally in a rat fibroblast despite enforced activation of E2F-dependent promoters. However, stimulation of this cell with oncogenic growth factors suppresses this shutoff and concurrently activates Cdk2 and Cdk6/4, thereby overriding the anchorage requirement for the G1-S transition and consequently enabling cells to perform anchorage-independent S phase entry. Analysis with enforced expression of Cdc6 indicates that the G1 cyclin-dependent kinases and Cdc6 constitute major cell cycle targets for the restriction of the G1-S transition by anchorage loss.

Effect of phosphatidic acid on human breast cancer cells exposed to doxorubicin

We previously demonstrated that phosphatidic acid (PA) induces chemotactic migration of highly metastatic breast cancer cells MDA-MB-231. The widely used anticancer drug doxorubicin was reported to induce apoptosis of cancer cells. Growth factors such as epidermal growth factor (EGF) and bioactive lipids such as lysophosphatidic acid (LPA) and sphingosine 1-phosphate (SPP) have been shown to enhance viability and to protect cancer cells against apoptosis. In this study, we investigated the effect of PA on MDA-MB-231 cells exposed to the anticancer drug doxorubicin. Cell migration toward PA was partially inhibited by doxorubicin treatment, and PA moderately diminished cell cycle arrest of cells exposed to doxorubicin. Although PA itself was not able to induce apoptosis of MDA-MB-231 cells, apoptosis of cells exposed to doxorubicin was markedly enhanced by PA treatment. Thus, PA is able to increase the apoptotic potential of doxorubicin, and may regulate the effects of doxorubicin used for chemotherapy.

Protease inhibitor-induced stabilization of p21(waf1/cip1) and cell-cycle arrest in chemical carcinogen-exposed mammary and lung cells

In previous studies, we have shown that human breast and lung carcinoma cells and mouse nontransformed type II lung cells fail to undergo cell-cycle arrest in G(1) phase in response to treatment with hydrocarbon carcinogens but rather accumulate in the S phase with damaged DNA. This situation may lead to replication of DNA on a damaged template and enhance frequency of mutations. The mechanism of this G(1) arrest failure was examined. Western immunoblot analyses of MCF7 human mammary cancer cells exposed to actinomycin D (used as a positive control for G(1) cell-cycle arrest) or hydrocarbon carcinogens revealed that while all of these chemicals caused an increase in p53, only trace levels of p21(waf1/cip1) protein were observed in the hydrocarbon carcinogen-treated samples. Similarly, in murine lung E10 type II cells, p53 but not p21(waf1/cip1) protein increased in response to benzo[a]pyrene dihydrodiol epoxide. Treatment of either MCF7 mammary or E10 lung cells with the protease inhibitor calpain I resulted in increased levels of p21(waf1/cip1) protein and enhancement of arrest of the cells in early phases of the cell cycle (G(1) and early S phase). The results suggest that failure of cell-cycle arrest in carcinogen-treated mammary and lung cells is related to increased protease-mediated degradation of p21(waf1/cip1) and/or related regulatory proteins.

Characterization of an N-terminally truncated cyclin A isoform in mammalian cells

Cyclin A is essential for regulating key transitions in the eukaryotic cell cycle including initiation of DNA replication and mitosis. This paper describes the characterization of a truncated cyclin A isoform (cyclin A(t)) in vitro in cultured mammalian cells and in mouse tissues. The presence of cyclin A(t) in specific cell types correlates with the ability of cell extracts to cleave in vitro translated cyclin A. In CHO-K1 cells, cyclin A processing to cyclin A(t) occurs at the N terminus; it does not involve the 26 S proteasome, nor could it be induced by conditional overexpression of the cyclin-dependent kinase inhibitor p27(Kip1). However, high cell densities lead to increased cyclin A(t) levels. Unlike full-length cyclin A, cyclin A(t) localizes to the cytoplasm, where it binds Cdk2. The data suggest that cyclin A processing occurs in vivo to yield an N-terminally truncated isoform by an unknown mechanism that is regulated by cell density. Differential subcellular localization may provide the first insights into the physiological role of cyclin A(t).

Molecular bases for the development of hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC)

Hepatocellular carcinoma (HCC) is the most common histological form of primary liver cancer; the tumor cells having retained features of hepatocytic differentiation. It is important to emphasize the heterogeneity of the histological background on which the tumor develops. Most HCCs complicate the evolution of an active or inactive cirrhosis. However, some tumors occur on livers with minimal histological changes; the prevalence of such cases varies from one geographical region to the other; being much higher in the southern half of Africa (around 40% of HCCs) than in Asia, America and Europe, where at least 90% of HCCs are associated in the cirrhosis. This heterogeneity is probably a reflection of different environmental and genetic factors. A large number of epidemiological and molecular studies have indeed clearly demonstrated the prime importance of environmental factors to the development of primary liver cancers in humans. Chronic hepatitis B (HBV) and C (HCV) infections are major risk factors. This review will mainly analyse the impact of chronic HBV infection but it is important to emphasize the potential synergistic effects between HBV and HCV, as well as between viral infections and other environmental factors, such as alcohol, chemical carcinogens (see review by Dr Wogan) and other, still poorly defined, hormonal factors which may account for the higher incidence of the tumor in man. Finally the review by Dr Buendia highlights the emerging issue of liver-cancer genetics.

Evaluation of cyclin expression in testicular germ cell tumors: cyclin E correlates with tumor type, advanced clinical stage, and pulmonary metastasis

The measurement of proliferative index has yielded promising yet conflicting results in the evaluation of testicular tumors. We have examined the role of Ki-67, along with the cyclins A and E in testicular tumorigenesis. We compared the immunoreactivity of 20 pure seminomas with 20 mixed germ cell tumors composed predominantly of embryonal carcinoma with a variety of proliferation markers, including Ki-67, cyclin A, and cyclin E. All 40 tumors stained for Ki-67, and 19 of 20 (95%) seminomas and 18 of 20 (90%) embryonal carcinomas stained positively for cyclin A. Cyclin E stained 14 of 19 (74%) of the embryonal carcinomas and only 4 of 20 (20%) of the seminomas (Fisher's exact two-tailed test, P = .0012). There was a trend toward larger tumor size for cyclin E-positive seminomas (median, 5.92 cm versus 3.96 cm; P = .08), although the same correlation was not significant in embryonal carcinomas. For both seminomas and embryonal carcinomas, staining with cyclin E did not correlate with the presence of lymphovascular invasion or capsular invasion. However, patients who had cyclin E-positive tumors presented with higher clinical stage (P = .0015). In addition, pulmonary spread in embryonal carcinomas (four patients) and seminomas (one patient) occurred only in patients whose tumors were cyclin E positive (P = .014). Although Ki-67 and cyclin A offer little prognostic information in testicular germ cell tumors, cyclin E immunoreactivity correlates with tumor type and is strongly predictive of distant tumor spread.

Cell cycle basis for the onset and progression of c-Myc-induced, TGFalpha-enhanced mouse mammary gland carcinogenesis

Using single and double transgenic mouse models, we investigated how c-Myc modulates the mammary epithelial cell cycle to induce cancer and how TGFalpha enhanced the process. In c-myc transgenic mice, c-myc expression was high in the hyperplastic mammary epithelium and in the majority of tumor areas. However, the tumors displayed focal areas of low expression of c-myc but high rates of proliferation. In contrast to E2F1 and cyclin A2, which were induced and co-localized with c-myc expression, induction of cyclins D1 and E occurred only in these tumor foci. Overexpression of cyclin D1 also occurred in the hyperplastic epithelium of tgfalpha-single and tgfalpha/c-myc-double transgenic mice. In tgfalpha/c-myc tumors, cells positive for cyclins D1 and E were randomly spread, without showing a reciprocal relationship to c-myc expression. In contrast to c-myc tumors, most tgfalpha/c-myc tumors showed undetectable levels of retinoblastoma protein (pRB), and the loss of pRB occurred in some cases at the mRNA level. These results suggest that E2F1 and cyclin A2 may be induced by c-Myc to mediate the onset of mammary cancer, whereas overexpression of cyclins D1 and E may occur later to facilitate tumor progression. TGFalpha may play its synergistic role, at least in part, by inducing cyclin D1 and facilitating the loss of pRB.

Herpes simplex virus type 1 infection imposes a G(1)/S block in asynchronously growing cells and prevents G(1) entry in quiescent cells

Herpes simplex virus type 1 (HSV-1) infection disrupted cell cycle regulation in at least two ways. First, infection of quiescent human embryonic lung cells simultaneously with readdition of serum caused inhibition of cyclin D/cyclin-dependent kinase (CDK) 4,6-specific and cyclin E/CDK2-specific phosphorylation of the retinoblastoma protein pRb. The inhibition of cyclin D/CDK4,6 kinase activity corresponded to a loss of cyclin D1 protein and a failure of CDK4 and CDK6 to translocate to the nucleus. Failure to detect cyclin E/CDK2 kinase activity was accompanied by a loss of cyclin E protein and a failure of CDK2 to translocate to the nucleus. Levels of pocket protein p130 persisted, whereas p107 did not accumulate. As a result of these effects on cyclin kinase, G(0)-infected cells failed to reenter the cell cycle. The second type of HSV-induced cell cycle dysregulation was observed in asynchronously dividing cell cultures. A rapid inhibition of preexisting cyclin E/CDK2 and cyclin A/CDK2 activities was observed in human embryonic lung cells, as well as two other human cell lines: C33 and U2OS. HSV-1 immediate-early gene expression was necessary for the inhibition of CDK2 kinase activity. Cyclin and CDK subunit protein levels, intracellular localization, and complex stability were unaffected by infection. In addition, levels of cyclin-dependent kinase inhibitors, p27 and p21, were not affected by HSV-1. Previous experiments demonstrated that in asynchronous infected cells, hypophosphorylated pRb and pocket protein-E2F complexes accumulated, and cellular DNA synthesis was rapidly inhibited. Coupled with the present results, this indicates that HSV-1 has evolved mechanisms for preventing cells in G(1) from proceeding through the restriction point and for cells in S from completing a round of DNA replication.

cAMP-dependent positive control of cyclin A2 expression during G1/S transition in primary hepatocytes

cAMP positively and negatively regulates hepatocyte proliferation but its molecular targets are still unknown. Cyclin A2 is a major regulator of the cell cycle progression and its synthesis is required for progression to S phase. We have investigated whether cyclin A2 and cyclin A2-associated kinase might be one of the targets for the cAMP transduction pathway during progression of hepatocytes through G1 and G1/S. We show that stimulation of primary cultured hepatocytes by glucagon differentially modulated the expression of G1/S cyclins. Glucagon indeed upregulated cyclin A2 and cyclin A2-associated kinase while cyclin E-associated kinase was unmodified. In conclusion, our study identifies cyclin A2 as an important effector of the cAMP transduction network during hepatocyte proliferation.

New concepts on the role of human papillomavirus in cell cycle regulation

Human papillomaviruses (HPVs) are strictly host-specific and also show a distinct tropism to squamous epithelial cells. Upon HPV infection, only a portion of the virus reaching the nucleus seems to undergo replication, suggesting that HPV replication remains confined to a small number of cells. HPVs critically depend on the cellular machinery for the replication of their genome. Viral replication is restricted to differentiated keratinocytes that are normally growth arrested. Hence, HPVs have developed strategies to subvert cellular growth regulatory pathways and are able to uncouple cellular proliferation and differentiation. Endogenous growth factors and cellular oncogenes modify HPV E (early) and L (late) gene expression and influence on the pathogenesis of HPV infections. HPV oncoproteins (E5, E6, E7) are important proteins not only in cell transformation but also in the regulation of the mitotic cycle of the cell, thus allowing the continuous proliferation of the host cells. Cyclins are important regulators of cell cycle transitions through their ability to bind cyclin-dependent kinases (cdks). Cdks have no kinase activity unless they are associated with a cyclin. Several classes of cyclins exist which are thought to coordinate the timing of different events necessary for cell cycle progression. Each cdk catalytic subunit can associate with different cyclins, and the associated cyclin determines which proteins are phosphorylated by the cdk-cyclin complex. The effects of HPVs on the cell cycle are mediated through the inhibition of antioncogens (mostly p53 and retinoblastoma) and through interference with the cyclins and cdks, resulting in target cell proliferation, their delayed differentiation, and as a side-effect, in malignant transformation.

Analysis of a Drosophila cyclin E hypomorphic mutation suggests a novel role for cyclin E in cell proliferation control during eye imaginal disc development

We have generated and characterized a Drosophila cyclin E hypomorphic mutation, DmcycEJP, that is homozygous viable and fertile, but results in adults with rough eyes. The mutation arose from an internal deletion of an existing P[w+lacZ] element inserted 14 kb upstream of the transcription start site of the DmcycE zygotic mRNA. The presence of this deleted P element, but not the P[w+lacZ] element from which it was derived, leads to a decreased level of DmcycE expression during eye imaginal disc development. Eye imaginal discs from DmcycEJP larvae contain fewer S phase cells, both anterior and posterior to the morphogenetic furrow. This results in adults with small rough eyes, largely due to insufficient numbers of pigment cells. Altering the dosage of the Drosophila cdk2 homolog, cdc2c, retinoblastoma, or p21(CIP1) homolog dacapo, which encode proteins known to physically interact with Cyclin E, modified the DmcycEJP rough eye phenotype as expected. Decreasing the dosage of the S phase transcription factor gene, dE2F, enhanced the DmcycEJP rough eye phenotype. Surprisingly, mutations in G2/M phase regulators cyclin A and string (cdc25), but not cyclin B1, B3, or cdc2, enhanced the DmcycEJP phenotype without affecting the number of cells entering S phase, but by decreasing the number of cells entering mitosis. Our analysis establishes the DmcycEJP allele as an excellent resource for searching for novel cyclin E genetic interactors. In addition, this analysis has identified cyclin A and string as DmcycEJP interactors, suggesting a novel role for cyclin E in the regulation of Cyclin A and String function during eye development.