Cyclin A2 maintains colon homeostasis and is a prognostic factor in colorectal cancer

To clarify the function of cyclin A2 in colon homeostasis and colorectal cancer (CRC), we generated mice deficient for cyclin A2 in colonic epithelial cells (CECs). Colons of these mice displayed architectural changes in the mucosa and signs of inflammation, as well as increased proliferation of CECs associated with the appearance of low- and high-grade dysplasias. The main initial events triggering those alterations in cyclin A2-deficient CECs appeared to be abnormal mitoses and DNA damage. Cyclin A2 deletion in CECs promoted the development of dysplasia and adenocarcinomas in a murine colitis-associated cancer model. We next explored the status of cyclin A2 expression in clinical CRC samples at the mRNA and protein levels and found higher expression in tumors of patients with stage 1 or 2 CRC compared with those of patients with stage 3 or 4 CRC. A meta-analysis of 11 transcriptome data sets comprising 2239 primary CRC tumors revealed different expression levels of CCNA2 (the mRNA coding for cyclin A2) among the CRC tumor subtypes, with the highest expression detected in consensus molecular subtype 1 (CMS1) and the lowest in CMS4 tumors. Moreover, we found high expression of CCNA2 to be a new, independent prognosis factor for CRC tumors.

Cyclin A2 modulates EMT via β-catenin and phospholipase C pathways

We have previously demonstrated that Cyclin A2 is involved in cytoskeletal dynamics, epithelial-mesenchymal transition (EMT) and metastasis. This phenotype was potentiated by activated oncogenic H-Ras. However, the mechanisms governing EMT in these cells have not yet been elucidated. Here, we dissected the pathways that are responsible for EMT in cells deficient for Cyclin A2. In Cyclin A2-depleted normal murine mammary gland (NMuMG) cells expressing RasV12, we found that β-catenin was liberated from the cell membrane and cell-cell junctions and underwent nuclear translocation and activation. Components of the canonical wingless (WNT) pathway, including WNT8b, WNT10a, WNT10b, frizzled 1 and 2 and TCF4 were upregulated at the messenger RNA and protein levels following Cyclin A2 depletion. However, suppression of the WNT pathway using the acetyltransferase porcupine inhibitor C59 did not reverse EMT whereas a dominant negative form of TCF4 as well as inhibition of phospholipase C using U73122 were able to do so. This suggests that a WNT-independent mechanism of β-catenin activation via phospholipase C is involved in the EMT induced by Cyclin A2 depletion. Our findings will broaden our knowledge on how Cyclin A2 contributes to EMT and metastasis.

High-resolution live-cell imaging reveals novel cyclin A2 degradation foci involving autophagy

Cyclin A2 is a key player in the regulation of the cell cycle. Its degradation in mid-mitosis relies on the ubiquitin-proteasome system (UPS). Using high-resolution microscopic imaging, we find that cyclin A2 persists beyond metaphase. Indeed, we identify a novel cyclin-A2-containing compartment that forms dynamic foci. Förster (or fluorescence) resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM) analyses show that cyclin A2 ubiquitylation takes place predominantly in these foci before spreading throughout the cell. Moreover, inhibition of autophagy in proliferating cells induces the stabilisation of a subset of cyclin A2, whereas induction of autophagy accelerates the degradation of cyclin A2, thus showing that autophagy is a novel regulator of cyclin A2 degradation.

Kizuna is a novel mitotic substrate for CDC25B phosphatase

CDC25 dual-specificity phosphatases play a central role in cell cycle control through the activation of Cyclin-Dependent Kinases (CDKs). Expression during mitosis of a stabilized CDC25B mutant (CDC25B-DDA), which cannot interact with the F-box protein βTrCP for proteasome-dependent degradation, causes mitotic defects and chromosome segregation errors in mammalian cells. We found, using the same CDC25B mutant, that stabilization and failure to degrade CDC25B during mitosis lead to the appearance of multipolar spindle cells resulting from a fragmentation of pericentriolar material (PCM) and abolish mitotic Plk1-dependent phosphorylation of Kizuna (Kiz), which is essential for the function of Kiz in maintaining spindle pole integrity. Thus, in mitosis Kiz is a new substrate of CDC25B whose dephosphorylation following CDC25B stabilization leads to the formation of multipolar spindles. Furthermore, endogenous Kiz and CDC25B interact only in mitosis, suggesting that Kiz phosphorylation depends on a balance between CDC25B and Plk1 activities. Our data identify a novel mitotic substrate of CDC25B phosphatase that plays a key role in mitosis control.

To be or not to be a proliferation marker?

Whether it is nobler in the mind to suffer the slings and arrows of outrageous proliferation, or to take arms against stroma, and favor metastasis… This pastiche of Hamlet's famous monologue illustrates recent reports on the paradoxical functions of well-established proliferation markers such as c-Myc or cyclin A2 that have revealed their ambiguous roles in the control of proliferation and metastasis. On the one hand, overexpression of c-Myc, while stimulating local proliferation, inhibits invasiveness of cancer cells, whereas on the other, downregulation of cyclin A2 leads to increased motility of transformed cells.

Cyclin E drives human keratinocyte growth into differentiation

Human epidermis is continuously exposed to environmental mutagenic hazard and is the most frequent target of human cancer. How the epidermis coordinates proliferation with differentiation to maintain homeostasis, even in hyperproliferative conditions, is unclear. For instance, overactivation of the proto-oncogene MYC in keratinocytes stimulates differentiation. Here we explore the cell cycle regulation as proliferating human keratinocytes commit to terminal differentiation upon loss of anchorage or overactivation of MYC. The S-phase of the cell cycle is deregulated as mitotic regulators are inhibited in the onset of differentiation. Experimental inhibition of mitotic kinase cdk1 or kinases of the mitosis spindle checkpoint Aurora B or Polo-like Kinase, triggered keratinocyte terminal differentiation. Furthermore, hyperactivation of the cell cycle by overexpressing the DNA replication regulator Cyclin E induced mitosis failure and differentiation. Inhibition of Cyclin E by shRNAs attenuated the induction of differentiation by MYC. In addition, we present evidence that Cyclin E induces DNA damage and the p53 pathway. The results provide novel clues for the mechanisms committing proliferative keratinocytes to differentiate, with implications for tissue homeostasis maintenance, HPV amplification and tumorigenesis.

Cyclin A2 mutagenesis analysis: a new insight into CDK activation and cellular localization requirements

Cyclin A2 is essential at two critical points in the somatic cell cycle: during S phase, when it activates CDK2, and during the G2 to M transition when it activates CDK1. Based on the crystal structure of Cyclin A2 in association with CDKs, we generated a panel of mutants to characterize the specific amino acids required for partner binding, CDK activation and subcellular localization. We find that CDK1, CDK2, p21, p27 and p107 have overlapping but distinct requirements for association with this protein. Our data highlight the crucial importance of the N-terminal α helix, in conjunction with the α3 helix within the cyclin box, in activating CDK. Several Cyclin A2 mutants selectively bind to either CDK1 or CDK2. We demonstrate that association of Cyclin A2 to proteins such as CDK2 that was previously suggested as crucial is not a prerequisite for its nuclear localization, and we propose that the whole protein structure is involved.

A novel mouse c-fos intronic promoter that responds to CREB and AP-1 is developmentally regulated in vivo

Background

The c-fos proto-oncogene is an archetype for rapid and integrative transcriptional activation. Innumerable studies have focused on the canonical promoter, located upstream from the transcriptional start site. However, several regulatory sequences have been found in the first intron.

Methodology/Principal Findings

Here we describe an extremely conserved region in c-fos first intron that contains a putative TATA box, and functional TRE and CRE sites. This fragment drives reporter gene activation in fibroblasts, which is enhanced by increasing intracellular calcium and cAMP and by cotransfection of CREB or c-Fos/c-Jun expression vectors. We produced transgenic mice expressing a lacZ reporter controlled by the intronic promoter. Lac Z expression of this promoter is restricted to the developing central nervous system (CNS) and the mesenchyme of developing mammary buds in embryos 12.5 days post-conception, and to brain tissue in adults. RT-QPCR analysis of tissue mRNA, including the anlage of the mammary gland and the CNS, confirms the existence of a novel, nested mRNA initiated in the first intron.

Conclusions/Significance

Our results provide evidence for a novel, developmentally regulated promoter in the first intron of the c-fos gene.

Role for Brm in Cell Growth Control

Recently, we have shown implication of Brm, the catalytic subunit of the SWI/SNF chromatin remodeling complex, in repression of cyclin A expression in quiescent cells. Here, we have examined the fate of cells lacking Brm throughout the cycle. We find that despite elevated levels of cyclins A and E, these cells can respond to serum starvation, however, without reaching a canonical G(0) phase as they continue to express high levels of c-Myc and have an abnormally large average size. The response to serum starvation can be correlated with increased levels of Rb proteins p130 and p107 as well as increased association of p27 with the cyclin-dependent kinases, possibly compensating for the higher levels of G(1) cyclins by reducing their associated kinase activity. After serum stimulation, reentry into the cycle occurs normally, but the S phase is delayed and shorter. In addition, the M phase has an increased duration, and we observed frequent faulty chromosome segregation events in anaphase. Altogether, our data suggest that cells can partially overcome the absence of Brm by activating several compensatory mechanisms to control the cell cycle. However, they remain profoundly affected, unable to enter a canonical quiescent state, presenting a shorter S phase, and finally unable to perform correct chromosome segregation.

Cell type-dependent control of NF-Y activity by TGF-beta

Transforming growth factor beta (TGF-beta) is a pluripotent cytokine that regulates cell growth and differentiation in a cell type-dependent fashion. TGF-beta exerts its effects through the activation of several signaling pathways. One involves membrane proximal events that lead to nuclear translocation of members of the Smad family of transcriptional regulators. TGF-beta can also activate MAPK cascades. Here, we show that TGF-beta induces nuclear translocation of the NF-YA subunit of the transcription factor NF-Y by a process that requires activation of the ERK cascade. This results in increased binding of endogenous NF-Y to chromatin and TGF-beta-dependent transcriptional regulation of the NF-Y target gene cyclin A2. Interestingly, the kinetics of NF-YA relocalization differs between epithelial cells and fibroblasts. NIH3T3 fibroblasts show an elevated basal level of phosphorylated p38 and delayed nuclear accumulation of NF-YA after TGF-beta treatment. In contrast, MDCK cells show low basal p38 activation, higher basal ERK phosphorylation and more rapid localization of NF-YA after induction. Thus, NF-Y activation by TGF-beta1 involves ERK1/2 and potentially an interplay between MAPK pathways, thereby opening the possibility for finely tuned transcriptional regulation.

Cyclin A repression in quiescent cells is associated with chromatin remodeling of its promoter and requires Brahma/SNF2alpha

Cell cycle-dependent expression of cyclin A is controlled by transcriptional repression in early phase of the cell cycle. In this study, we directly examine the chromatin structure of the mouse cyclin A promoter through in vivo micrococcal nuclease footprinting. We describe here that cyclin A repression is associated with two positioned nucleosomes and that histones progressively lose DNA contact synchronously with gene activation. This particular nucleosomal organization is disrupted by mutations of the cyclin A bipartite repressor sequence. Moreover, the same sequence recruits the chromatin remodeling factor Brahma/SNF2alpha (Brm) onto the cyclin A promoter. Accordingly, cyclin A proximal promoter is not wrapped around nucleosomes and not repressed in quiescent cells lacking Brm. These results provide molecular explanations for the transcriptional repression state of cyclin A, as well as insights into the action of Brm chromatin remodeling factor as cell cycle regulator.

Toxicological effects of disinfections using sodium hypochlorite on aquatic organisms and its contribution to AOX formation in hospital wastewater

Sodium hypochlorite (NaOCl) is often used for disinfecting hospital wastewater in order to prevent the spread of pathogenic microorganisms, causal agents of nosocomial infectious diseases. Chlorine disinfectants in wastewater react with organic matters, giving rise to organic chlorine compounds such as AOX (halogenated organic compounds adsorbable on activated carbon), which are toxic for aquatic organisms and are persistent environmental contaminants. The aim of this study was to evaluate the toxicity on aquatic organisms of hospital wastewater from services using NaOCl in pre-chlorination. Wastewater samples from the infectious and tropical diseases department of a hospital of a large city in southeast of France were collected. Three samples per day were collected in the connecting well department at 9 a.m., 1 p.m. and 5 p.m. during 8 days from 13 March to 22 March 2001, and a mixture was made at 6 p.m. with the three samples in order to obtain a representative sample for the day. The toxicity test comprised the 24-h EC50 on Daphnia magna and a bioluminescence assay using Vibrio fischeri photobacteria. Fecal coliforms and physicochemical analyses such as total organic carbon (TOC), chloride, AOX, total suspended solids (TSS) and chemical oxygen demand (COD) were carried out. Wastewater samples highlighted considerable acute toxicity on D. magna and V. fischeri photobacteria. However, low most probable numbers (MPN), ranging from <3 to 2400 for 100 ml, were detected for fecal coliforms. Statistical analysis, with a confidence interval of 95%, gave a strong linear regression assessed with r=0.98 between AOX concentrations and EC50 (TU) on daphnia. The identification of an ideal concentration of NaOCl in disinfecting hospital wastewater, i.e. its non-observed effect concentration (NOEC) on algae and D. magna, seems to be a research issue that could facilitate the control of AOX toxicity effects on aquatic organisms. Therefore, it would be necessary to monitor the biocide properties of NaOCl on fecal coliforms at various doses and its toxicity effects on aquatic organisms.

Retroviral genomic RNAs are transported to the plasma membrane by endosomal vesicles

The viral genomes of alpha- and gamma-retroviruses follow an outbound route through the cytoplasm before assembling with the budding particle at the plasma membrane. We show here that murine leukemia virus (MLV) RNAs are transported on lysosomes and transferrin-positive endosomes. Transport on transferrin-positive vesicles requires both Gag and Env polyproteins. In the presence of Env, Gag is rerouted from lysosomes to transferrin-positive endosomes, and virion production becomes highly sensitive to drugs poisoning vesicular and endosomal traffic. Vesicular transport of the RNA does not require prior endocytosis, indicating that it is recruited directly from the cytosol. Viral prebudding complexes containing Env, Gag, and retroviral RNAs are thus formed on endosomes, and subsequently routed to the plasma membrane. This may allow retroviruses to hijack the endosomal machinery as part of their biosynthetic pathway. More generally, tethering to vesicles may provide an efficient mechanism for directed RNA transport.

[Oncogenes and mitotic regulators: a change in perspective of our view of neoplastic processes]

Our vision of the cancer cell has dramatically changed since the discovery of proto-oncogenes, whose deregulation was proposed to mimic normal growth signalling. This notion, linking cancer to cell signalling pathways, has progressively led the way to the concept of the mutator phenotype, in which genetic instability plays an essential role in the onset of cancer. This then transformed cancer into a DNA repair disease. However, as foreseen decades ago by cytogeneticists, point mutations are not sufficient to give a full picture of the whole process. As a result, aneuploidy, rather than gene mutation, has been proposed as the explanation for the complex changes observed in cancer cells. The culprits were found among genes involved in the control of the cell division cycle, and work aimed at understanding the regulation of S phase and mitosis have yielded new insights into our understanding of cancer.

Single mRNA molecules demonstrate probabilistic movement in living mammalian cells

Cytoplasmic mRNA movements ultimately determine the spatial distribution of protein synthesis. Although some mRNAs are compartmentalized in cytoplasmic regions, most mRNAs, such as housekeeping mRNAs or the poly-adenylated mRNA population, are believed to be distributed throughout the cytoplasm. The general mechanism by which all mRNAs may move, and how this may be related to localization, is unknown. Here, we report a method to visualize single mRNA molecules in living mammalian cells, and we report that, regardless of any specific cytoplasmic distribution, individual mRNA molecules exhibit rapid and directional movements on microtubules. Importantly, the beta-actin mRNA zipcode increased both the frequency and length of these movements, providing a common mechanistic basis for both localized and nonlocalized mRNAs. Disruption of the cytoskeleton with drugs showed that microtubules and microfilaments are involved in the types of mRNA movements we have observed, which included complete immobility and corralled and nonrestricted diffusion. Individual mRNA molecules switched frequently among these movements, suggesting that mRNAs undergo continuous cycles of anchoring, diffusion, and active transport.

Oct-1 potentiates CREB-driven cyclin D1 promoter activation via a phospho-CREB- and CREB binding protein-independent mechanism

Cyclin D1, the regulatory subunit for mid-G(1) cyclin-dependent kinases, controls the expression of numerous cell cycle genes. A cyclic AMP-responsive element (CRE), located upstream of the cyclin D1 mRNA start site, integrates mitogenic signals that target the CRE-binding factor CREB, which can recruit the transcriptional coactivator CREB-binding protein (CBP). We describe an alternative mechanism for CREB-driven cyclin D1 induction that involves the ubiquitous POU domain protein Oct-1. In the breast cancer cell line MCF-7, overexpression of Oct-1 or its POU domain strongly increases transcriptional activation of cyclin D1 and GAL4 reporter genes that is specifically dependent upon CREB but independent of Oct-1 DNA binding. Gel retardation and chromatin immunoprecipitation assays confirm that POU forms a complex with CREB bound to the cyclin D1 CRE. In solution, CREB interaction with POU requires the CREB Q2 domain and, notably, occurs with CREB that is not phosphorylated on Ser 133. Accordingly, Oct-1 also potently enhances transcriptional activation mediated by a Ser133Ala CREB mutant. Oct-1/CREB synergy is not diminished by the adenovirus E1A 12S protein, a repressor of CBP coactivator function. In contrast, E1A strongly represses CBP-enhanced transactivation by CREB phosphorylated on Ser 133. Our observation that Oct-1 potentiates CREB-dependent cyclin D1 transcriptional activity independently of Ser 133 phosphorylation and E1A-sensitive coactivator function offers a new paradigm for the regulation of cyclin D1 induction by proliferative signals.

Mammalian and yeast U3 snoRNPs are matured in specific and related nuclear compartments

Nucleolar localization of vertebrate box C/D snoRNA involves transit through Cajal bodies, but the significance of this event is unknown. To define better the function of this compartment, we analyzed here the maturation pathway of mammalian U3. We show that 3'-extended U3 precursors possess a mono-methylated cap, and are not associated with fibrillarin and hNop58. Importantly, these precursors are detected at both their transcription sites and in Cajal bodies. In addition, mature U3, the core box C/D proteins and the human homolog of the methyltransferase responsible for U3 cap tri-methylation, hTgs1, are all present in Cajal bodies. In yeast, U3 follows a similar maturation pathway, and equivalent 3'-extended precursors are enriched in the nucleolus and in the nucleolar body, a nucleolar domain that concentrates Tgs1p under certain growth conditions. Thus, spatial organization of U3 maturation appears to be conserved across evolution, and involves specialized and related nuclear compartments, the nucleolus/nucleolar body in yeast and Cajal bodies in higher eukaryotes. These are likely places for snoRNP assembly, 3' end maturation and cap modification.

[Molecular mechanisms of oncogenic transformation: what's new?]

During the past two years, new molecular targets have been discovered which link cell cycle, cell proliferation and cellular growth. It has become more and more evident that whereas gain-of-function mutations in specific genes can lead to cancer, genomic instability plays also an important role in tumour progression. With examples taken from the recent literature, we describe in this short review crucial findings on the molecular mechanisms controlling cell cycle and proliferation. We illustrate how specific combinations of proto-oncogenes alterations can result in tissue-specific tumours. Finally, impairment of the interactions of a cancer cell with its surrounding neighbours is also shown to participate in the progression toward aggressive phenotypes.

Box C/D small nucleolar RNA trafficking involves small nucleolar RNP proteins, nucleolar factors and a novel nuclear domain

Nucleolar localization of box C/D small nucleolar (sno) RNAs requires the box C/D motif and, in vertebrates, involves transit through Cajal bodies (CB). We report that in yeast, overexpression of a box C/D reporter leads to a block in the localization pathway with snoRNA accumulation in a specific sub-nucleolar structure, the nucleolar body (NB). The human survival of motor neuron protein (SMN), a marker of gems/CB, specifically localizes to the NB when expressed in yeast, supporting similarities between these structures. Box C/D snoRNA accumulation in the NB was decreased by mutation of Srp40 and increased by mutation of Nsr1p, two related nucleolar proteins that are homologous to human Nopp140 and nucleolin, respectively. Box C/D snoRNAs also failed to accumulate in the NB, and became delocalized to the nucleoplasm, upon depletion of any of the core snoRNP proteins, Nop1p/fibrillarin, Snu13p, Nop56p and Nop5p/Nop58p. We conclude that snoRNP assembly occurs either in the nucleoplasm, or during transit of snoRNAs through the NB, followed by routing of the complete snoRNP to functional sites of ribosome synthesis.

Terminal minihelix, a novel RNA motif that directs polymerase III transcripts to the cell cytoplasm. Terminal minihelix and RNA export

Determining the cis-acting elements controlling nuclear export of RNA is critical, because they specify which RNA will be selected for transport. We have characterized the nuclear export motif of the adenoviral VA1 RNA, a small cytoplasmic RNA transcribed by RNA polymerase III. Using a large panel of VA1 mutants in both transfected COS cells and injected Xenopus oocytes, we showed that the terminal stem of VA1 is necessary and sufficient for its export. Surprisingly, we found that the nucleotide sequence within the terminal stem is not important. Rather, the salient features of this motif are its length and its relative position within the RNA. Such stems thus define a novel and degenerate cytoplasmic localization motif that we termed the minihelix. This motif is found in a variety of polymerase III transcripts, and cross-competition analysis in Xenopus oocytes revealed that export of one such RNA, like hY1 RNA, is specifically competed by VA1 or artificial minihelix. Taken together these results show that the minihelix defines a new cis-acting export element and that this motif could be exported via a novel and specific nuclear export pathway.

mRNA localization by a 145-nucleotide region of the c-fos 3'--untranslated region. Links to translation but not stability

The presence of a localization signal in the 3'-untranslated region of c-fos mRNA was investigated by in situ hybridization and cell fractionation techniques. Cells were transfected with chimeric gene constructs in which the beta-globin coding region was used as a reporter and linked to either its own 3'-untranslated region, the c-fos 3'-untranslated region, or the c-fos 3'-untranslated region containing different deletions. Replacement of the endogenous beta-globin 3'-untranslated region by that from c-fos caused a redistribution of the transcripts so that they were recovered in cytoskeletal-bound polysomes and seen localized in the perinuclear cytoplasm. Deletion of the AU-rich instability region did not affect transcript localization, but removal of a distinct 145-nucleotide region of the 3'-untranslated region abolished it. The prevention of transcript translation by desferrioxamine led to a marked loss of transcript localization, independent of mRNA instability. The data show that the 3'-untranslated region of c-fos mRNA, as c-myc, contains a localization signal, which targets the mRNA to the perinuclear cytoskeleton. We propose that this is important to ensure efficient nuclear import of these key regulatory proteins. mRNA localization by the fos 3'-untranslated region is independent of mRNA instability, and the two are determined by different regulatory elements.

Cyclin A is a mediator of p120E4F-dependent cell cycle arrest in G1

E4F is a ubiquitously expressed GLI-Krüppel-related transcription factor which has been identified for its capacity to regulate transcription of the adenovirus E4 gene in response to E1A. However, cellular genes regulated by E4F are still unknown. Some of these genes are likely to be involved in cell cycle progression since ectopic p120E4F expression induces cell cycle arrest in G1. Although p21WAF1 stabilization was proposed to mediate E4F-dependent cell cycle arrest, we found that p120E4F can induce a G1 block in p21(-/-) cells, suggesting that other proteins are essential for the p120E4F-dependent block in G1. We show here that cyclin A promoter activity can be repressed by p120E4F and that this repression correlates with p120E4F binding to the cyclic AMP-responsive element site of the cyclin A promoter. In addition, enforced expression of cyclin A releases p120E4F-arrested cells from the G1 block. These data identify the cyclin A gene as a cellular target for p120E4F and suggest a mechanism for p120E4F-dependent cell cycle regulation.

Cyclin A2 transcriptional regulation: modulation of cell cycle control at the G1/S transition by peripheral cues

Several types of cyclins have been identified and among these, cyclin A2 is synthesized in somatic cells at the onset of DNA synthesis as well as during the G2/M transition associated with cyclin-dependent protein kinases 1 and 2. Modulation of cyclin A transcription is due to the interplay between a cell cycle-dependent periodic relief of a transcriptional repression and signals transduced through adenosine 3',5'-cyclic monophosphate, transforming growth factor-beta, and the integrin-mediated pathways. Using primary mouse embryonic fibroblasts from embryos where the genes coding for the protein responsible for susceptibility to retinoblastoma (pRB) and the related p107 and p130 proteins had been individually inactivated, we showed that cyclin A is a functional target of pRB-mediated cell cycle arrest. The factors involved are discussed.

Switch from p53 to MDM2 as differentiating human keratinocytes lose their proliferative potential and increase in cellular size

p53 transcription factor is mutated in most skin cell carcinomas and in more than 50% of all human malignancies. One of its transcriptional targets is MDM2, which in turn down-regulates p53. The role of the p53/MDM2 regulatory loop upon genotoxic stress is well documented, but less is known about its role in normal tissue homeostasis. We have explored this pathway during the different transitions of the human epidermal differentiation programme and after isolating stem cells, transit amplifying cells or differentiating cells from epidermis. Maximum expression of p53 was found in proliferating keratinocytes. A striking and transient induction of MDM2 and a down-modulation of p53 characterized the transition from proliferation to differentiation in primary human keratinocytes. These changes were delayed in late differentiating carcinoma cells, and were clearly different in suspended primary fibroblasts. Interestingly, these changes correlated with an increase in cell size, at the time of irreversible commitment to differentiation. Induction of MDM2 was also associated with suppression of proliferation in normal, or hyperproliferative, psoriatic epidermis. Moreover, both proteins were induced as keratinocytes were driven to leave the stem cell compartment by c-Myc activation. Overall, our results show a critical regulation of the p53/MDM2 pathway at the epidermal transition from proliferation to differentiation.

Normal and c-Myc-promoted human keratinocyte differentiation both occur via a novel cell cycle involving cellular growth and endoreplication

The relationship between cell cycle and differentiation in human keratinocytes is poorly understood. It is believed that keratinocytes suppress DNA replication and cell cycle arrest in G0 before they initiate terminal differentiation. However, a temporal separation between both events has not been established. Moreover, c-Myc promotes keratinocyte differentiation without causing cell cycle arrest. To address these paradoxes we have analysed cell cycle control during normal and c-Myc-promoted differentiation. Continuous activation of c-Myc or initiation of terminal differentiation results in a block of G2/M, cellular growth, endoreplication and polyploidy. Keratinocytes abandon G1, continue replicating DNA as they differentiate terminally and become polyploid. In fact, simply blocking mitosis with nocodazole resulted in increased cell size, terminal differentiation and endoreplication. This indicates that terminal differentiation associates with defective cell cycle progression and provides a novel insight into c-Myc biology.

Organ donor or graft pretreatment to prolong allograft survival: lessons learned in the murine model

Permanent donor-specific tolerance to allografts is the goal of transplantation research. Currently, morbid immunosuppressive therapy is used to mitigate rejection initiated in part by Ia-bearing interstitial graft dendritic or antigen-presenting cells (APCs) that are thought to migrate into the host after transplantation. We hypothesized that donor or organ immune modulation directed against graft APCs might influence graft immunogenicity and promote prolonged graft acceptance in histoincompatible hosts in the absence of immunosuppressive therapy. Haplotype-specific monoclonal antibodies (mAb), mAb specific to graft APC, adhesion or costimulatory molecules and anti-LFA-1-Ricin and anti-Iak-Ricin immunoconjugates (IC) were prepared and administered in varying doses and time intervals to donor C3H/HeJ (H-2k) mice. Thereafter, their spleens and hearts were removed at varying time intervals and used either as stimulator cells in one-way mixed lymphocyte reaction or transplanted into naive Balb/c (H-2d) recipients, respectively. Explanted C3H hearts were pretreated with anti-Iak mAb on the Langendorf apparatus. Hearts were also used from major histocompatibility complex (MHC) class I, MHC class II, and MHC class I- and II-deficient "knockout" mice. Splenocytes exposed to at least 500 microg of anti-Iak mAb in vivo for more than 4 hr were able to inhibit the mixed lymphocyte reaction to almost background levels, but only after incubation with rabbit complement in vitro. Similarly pretreated cardiac allografts (both in vivo or explanted and pretreated on the Langendorf apparatus) did not experience prolonged survival in nonimmunosuppressed Balb/C recipients when compared with control solutions, regardless of the concomitant use of complement. Splenocytes from immunoconjugate pretreated donors inhibited the mixed lymphocyte reaction completely without the use of complement; however, hearts from these donors also did not experience prolonged survival nor donor hearts exposed to mAb specific for graft APC, adhesion or costimulatory molecules. Only hearts from MHC class I and class II "knockout" mice survived significantly longer than controls. We conclude that donor or graft pretreatment with haplotype-specific anti-Ia mAb, haplotype-specific immunoconjugates, or mAb directed against graft APC, adhesion or costimulation molecules have little efficacy in promoting acceptance of cardiac allografts in nonimmunosuppressed recipients. The enhanced survival of hearts from MHC class I- and class II-deficient donors suggest that novel methods to effect the immunogenicity of the graft will be required if long-term allograft acceptance is to be achieved in the absence of host immunosuppression.

Small GTPases, adhesion, cell cycle control and proliferation

In many cells, proliferation is under the coordinated control of growth factors and the extracellular matrix (ECM). Autocriny and anchorage-independent growth are observed in many transformed cells where this balance is often altered. The outside-in and inside-out exchanges are mediated by integrins, acting as molecular bridges between the ECM and the cytoskeleton. Integrins, as the major receptors for components of the ECM, are important not only for the physical aspects of cell adhesion, but also for two key aspects of cell fate: cell cycle progression, and apoptosis. The latter aspect will not be covered in the present analysis, which will rather focus on the data accumulated on Ras and Ras-related small GTPases such as Rho, Rac and Cdc42.

Differential effect of Rac and Cdc42 on p38 kinase activity and cell cycle progression of nonadherent primary mouse fibroblasts

The Rho GTPases play an important role in transducing signals linking plasma membrane receptors to the organization of the cytoskeleton and also regulate gene transcription. Here, we show that expression of constitutively active Ras or Cdc42, but not RhoA, RhoG, and Rac1, is sufficient to cause anchorage-independent cell cycle progression of mouse embryonic fibroblasts. However, in anchorage free conditions, whereas activation of either Cdc42 or Ras results in cyclin A transcription and cell cycle progression, Cdc42 is not required for Ras-mediated cyclin A induction, and the two proteins act in a synergistic manner in this process. Surprisingly, the ability of Cdc42 to induce p38 MAPK activity in suspended mouse embryonic fibroblast was impaired. Moreover, inhibition of p38 activity allowed Rac1 to induce anchorage-independent cyclin A transcription, indicating that p38 MAPK has an inhibitory function on cell cycle progression of primary fibroblasts. Finally, a Rac mutant, which is unable to induce lamellipodia and focal complex formation, promoted cyclin A transcription in the presence of SB203580, suggesting that the organization of the cytoskeleton is not required for anchorage-independent proliferation. This demonstrates a novel function for Cdc42, distinct from that of Rac1, in the control of cell proliferation.

Localisation of a reporter transcript by the c-myc 3'-UTR is linked to translation

The 3'-untranslated region of c-myc mRNA contains a perinuclear localisation signal which is sufficient to target beta-globin coding sequences. The link between perinuclear mRNA localisation and translation has been investigated using cells transfected with chimeric gene constructs in which globin reporter sequences were linked to the c-myc 3'-untranslated region and the iron-responsive element from ferritin mRNA. Iron supplementation of the medium promoted translation of the chimeric mRNA as assessed by its presence in polysomes; in situ hybridisation showed that the mRNA was localised around the nucleus. Treatment with the iron chelator desferrioxamine for 16 h prevented both translation and mRNA localisation. In controls where the expressed mRNA lacked the iron-responsive element desferrioxamine had no effect upon localisation. In contrast, arrest of on-going global translation by puromycin treatment had no effect on mRNA localisation. The data suggest that if initiation of translation of a mRNA containing the c-myc localisation signal is prevented in some way then localisation does not occur, whereas once the mRNA has been localised further translation is not required to maintain mRNA localisation.

CHF: a novel factor binding to cyclin A CHR corepressor element

Cell cycle modulation of cyclin A expression is due to the periodic relief of a transcriptional repression mediated by a bipartite negative DNA regulatory region. The 5' element (Cell Cycle Responsive Element: CCRE; cell Cycle Dependent Element: CDE) is clearly occupied in a cyclic manner in vivo, whereas the 3' element, whose sequence is shared by B-myb, cdc25C and cdc2 genes (cell Cycle gene Homology Region: CHR), is involved in more subtle interactions. Mutation of either element results in complete deregulation of cyclin A promoter activity. Whereas some reports claim that E2F/DP can bind to the CCRE/CDE, the nature of the protein(s) interacting with the CHR is unknown. In the present work we have characterized an activity present in quiescent cells and absent in cells blocked in S phase, which binds specifically to cyclin A CHR, but not to B-myb, or to cdc25C, or to cdc2 CHRs. A 90 kD protein, named CHF (cyclin A CHR binding factor), has been identified through preparative electrophoresis and UV crosslinking experiments. In order to address in more functional terms the binding of CHF to cyclin A CHR, we developed in vitro and in vivo oligonucleotide competition assays. Both in vitro transcription and in vivo microinjection experiments demonstrate that a functional difference exists between the composite CCRE/CDE-CHR repressor regions of cell cycle regulated genes such as cyclin A and cdc25C.

A novel calcium signaling pathway targets the c-fos intragenic transcriptional pausing site

In many cell types, increased intracellular calcium gives rise to a robust induction of c-fos gene expression. Here we show that in mouse Ltk(-) fibroblasts, calcium ionophore acts in synergy with either cAMP or PMA to strongly induce the endogenous c-fos gene. Run-on analysis shows that this corresponds to a substantial increase in active polymerases on downstream gene sequences, i.e. relief of an elongation block by calcium. Correspondingly a chimeric gene, in which the human metallothionein promoter is fused to the fos gene, is strongly induced by ionophore alone, unlike a c-fos promoter/beta-globin coding unit chimeric construct. Internal deletions in the hMT-fos reporter localize the intragenic calcium regulatory element to the 5' portion of intron 1, thereby confirming and extending previous in vitro mapping data. Ionophore induced cAMP response element-binding protein phosphorylation on Ser(133) without affecting the extracellular signal-regulated kinase cascade. Surprisingly, induction involved neither CaM-Ks nor calcineurin, while the calmodulin antagonist W7 activated c-fos transcription on its own. These data suggest that a novel calcium signaling pathway mediates intragenic regulation of c-fos expression via suppression of a transcriptional pause site.

Cyclin A is a functional target of retinoblastoma tumor suppressor protein-mediated cell cycle arrest

Although RB inhibits the G(1)-S transition, the mechanism through which RB prevents cell cycle advancement remains unidentified. To delineate the mechanism(s) utilized by RB to exert its anti-proliferative activity, constitutively active RB proteins (which cannot be inactivated by phosphorylation) or p16ink4a (which prevents RB inactivation) were utilized. Both proteins inhibited the G(1)-S transition, whereas wild-type RB did not. We show that active RB acts to attenuate cyclin A promoter activity, and that overexpression of cyclin E reverses RB-mediated repression of the cyclin A promoter. Although cyclin A is an E2F-regulated gene, and it has been long hypothesized that RB mediates cell cycle advancement through binding to E2F and attenuating its transactivation potential, cyclin E does not reverse dominant negative E2F-mediated repression of the cyclin A promoter. Although active RB repressed both cyclin A and two other paradigm E2F-regulated promoters, only cyclin A transcription was restored upon co-expression of cyclin E. Additionally, we show that RB but not dominant negative E2F regulates the cyclin A promoter through the CCRE element. These data identify cyclin A as a downstream target of RB-mediated arrest. Consistent with this idea, ectopic expression of cyclin A reversed RB-mediated G(1) arrest. The findings presented suggest a pathway wherein cyclin A is a downstream target of RB, and cyclin E functions to antagonize this aspect of RB-mediated G(1)-S inhibition.

Anchorage-dependent expression of cyclin A in primary cells requires a negative DNA regulatory element and a functional Rb

Many cells, when cultured in suspension, fail to express cyclin A, a regulatory component of cell cycle kinases cdc2 and cdk2 and as a consequence, do not enter S phase. However, many cell type-specific differences are disclosed between not only normal and transformed cells, but also between cell lines whose proliferation is strictly anchorage-dependent. These apparent discrepancies are seen in established cell lines most probably because of adaptative events that have occurred during cell culture. We have therefore used primary cells to understand how cyclin A transcription is controlled by cell anchorage properties. To this aim, we have used embryonic fibroblasts from either wild type, Rb(-/-) or p107(-/-)/p130(-/-) mice and tested the effect of an ectopic expression of Rb mutants. In the experiments reported here, we show that anchorage-dependent expression of cyclin A (i) is reflected by the in vivo occupancy of a negative DNA regulatory element previously shown to be instrumental in the down regulation of cyclin A transcription in quiescent cells (Cell Cycle Responsive Element: CCRE) (ii) requires a functional Rb but neither p107 nor p130 (iii) mutation of the CCRE abolishes both adhesion-dependent regulation and response to Rb.

[Cyclin A: a good markers for the study of cell cycle control and tumor progression?]

Cyclin A is a positive regulatory component of kinases required for the progression through S phase and for the transition between the G2 and M phases of the cell division cycle. Previous studies conducted in established cell lines and in primary human T lymphocytes, have demonstrated that the promoter of its gene is under negative transcriptional control in quiescent cells. The DNA sequences mediating this repression have been delineated through in vitro mutagenesis as well as in vivo genomic footprinting experiments. Indirect observations suggest the involvement of proteins related to the retinoblastoma tumor suppressor protein (pRb). Using primary fibroblasts from either pRb(-/-), p107(-/-), p130(-/-) or p107(-/-)/p130(-/-) mice, we show in this work that mutation of the pRb gene has the more profound effect on cyclin A transcription. Finally, normal fibroblasts cultured in suspension fail to express cyclin A and can no longer enter S phase and proliferate, revealing thus a dependence of cyclin A expression on cell anchorage. Our work suggests the existence of at least two sets of regulators controlling cell cycle progression. On the one hand, proteins like cyclin D1, whose expression is a direct consequence of the activation of the ras signalling pathway and on the other hand, proteins like cyclin A which are secondary response effectors. As a result, growth factor stimulation leads to a transcriptional activation of the former set, while the transcription of the latter set is under the control of a repressor whose effect is alleviated after triggering the ras cascade. The status of pRb thus dictates whether cells continue their progression through the cell cycle when ras is mutated, probably by allowing the uncontrolled expression of critical genes like cyclin A.

Hypersensitivity of Ku-deficient cells toward the DNA topoisomerase II inhibitor ICRF-193 suggests a novel role for Ku antigen during the G2 and M phases of the cell cycle

Ku antigen is a heterodimer, comprised of 86- and 70-kDa subunits, which binds preferentially to free DNA ends. Ku is associated with a catalytic subunit of 450 kDa in the DNA-dependent protein kinase (DNA-PK), which plays a crucial role in DNA double-strand break (DSB) repair and V(D)J recombination of immunoglobulin and T-cell receptor genes. We now demonstrate that Ku86 (86-kDa subunit)-deficient Chinese hamster cell lines are hypersensitive to ICRF-193, a DNA topoisomerase II inhibitor that does not produce DSB in DNA. Mutant cells were blocked in G2 at drug doses which had no effect on wild-type cells. Moreover, bypass of this G2 block by caffeine revealed defective chromosome condensation in Ku86-deficient cells. The hypersensitivity of Ku86-deficient cells toward ICRF-193 was not due to impaired in vitro decatenation activity or altered levels of DNA topoisomerase IIalpha or -beta. Rather, wild-type sensitivity was restored by transfection of a Ku86 expression plasmid into mutant cells. In contrast to cells deficient in the Ku86 subunit of DNA-PK, cells deficient in the catalytic subunit of the enzyme neither accumulated in G2/M nor displayed defective chromosome condensation at lower doses of ICRF-193 compared to wild-type cells. Our data suggests a novel role for Ku antigen in the G2 and M phases of the cell cycle, a role that is not related to its role in DNA-PK-dependent DNA repair.

The retinoblastoma protein is essential for cyclin A repression in quiescent cells

Cyclin A is a positive regulatory component of kinases required for the progression through S phase and for the transition between the G2 and M phases of the cell division cycle. Previous studies have demonstrated that the promoter of its gene is under transcriptional repression in quiescent cells. Whereas the DNA sequences mediating this effect have been clearly delineated, the nature of the proteins acting in trans is still debated. Indirect observations suggest the involvement of proteins related to the retinoblastoma tumor suppressor protein (pRb). However, the precise role of these proteins has been difficult to assess, since most experiments designed to analyse their function have been carried out in transformed cell lines. Nevertheless, a current model has emerged whereby the role of the p130 protein would be restricted to resting and early G1 cells and p107, absent in quiescent cells, would be involved later in the control of the G1/S transition, whilst pRb would be effective throughout the cell cycle. We show here that cyclin A transcriptional inhibition is relieved in primary fibroblasts from pRb(-/-) embryos and not in fibroblasts from p13O(-/-), p107(-/-) or even p130(-/-)/p107(-/-) double mutant embryos. This suggests a unique role for pRb in controlling the extinction of specific genes in G0, providing thus the first example of non-overlapping functions achieved by the different pocket proteins.

Relief of cyclin A gene transcriptional inhibition during activation of human primary T lymphocytes via CD2 and CD28 adhesion molecules

Cyclin A transcription is cell cycle regulated and induced by cell proliferative signals. To understand the mechanisms underlined in this regulation in normal human cells, we have analysed in vivo protein-DNA interactions at the Cyclin A locus in primary T lymphocytes. Stimulation of purified T lymphocytes by a combination of monoclonal antibodies directed at CD2 and CD28 adhesion molecules gives rise to a long lasting proliferation in the absence of accessory cells. Cyclin A was observed after 4 days of costimulation with anti CD2 + CD28 whereas stimulation by anti CD2 or anti CD28 alone was not effective. In vivo genomic DMS footprinting revealed upstream of the major transcription initiation sites, the presence of at least three protein binding sites, two of which were constitutively occupied. They bind in vitro respectively ATF-1 and NF-Y proteins. The third site was occupied in quiescent cells or in cells stimulated by anti CD2 or anti CD28 alone. The mitogenic combination of anti CD2 + anti CD28 released the footprint as cells were committed to proliferation. Consistent with theses results, nuclear extracts prepared from quiescent cells formed a specific complex with this element, whereas extracts prepared from cells treated with anti CD2 + anti CD28 failed to do so after cells entered a proliferative state.

Cyclin A expression is under negative transcriptional control during the cell cycle

Transcription of the gene coding for cyclin A, a protein required for S-phase transit, is cell cycle regulated and is restricted to proliferating cells. To further explore transcriptional regulation linked to cell division cycle control, a genomic clone containing 5' flanking sequences of the murine cyclin A gene was isolated. When it was fused to a luciferase reporter gene, it was shown to function as a proliferation-regulated promoter in NIH 3T3 cells. Transcription of the mouse cyclin A gene is negatively regulated by arrest of cell proliferation. A mutation of a GC-rich sequence conserved between mice and humans is sufficient to relieve transcriptional repression, resulting in a promoter with constitutively high activity. In agreement with this result, in vivo footprinting reveals a protection of the cell cycle-responsive element in G0/early G1 cells which is not observed at later stages of the cell cycle. Moreover, the footprint is present in dimethyl sulfoxide-induced differentiating and not in proliferating Friend erythroleukemia cells. Conversely, two other sites, which in vitro bind ATF-1 and NF-Y, respectively, are constitutively occupied throughout cell cycle progression.

A localisation signal in the 3′ untranslated region of c-myc mRNA targets c-myc mRNA and beta-globin reporter sequences to the perinuclear cytoplasm and cytoskeletal-bound polysomes

There is increasing evidence that in mammalian cells some mRNAs are localised to specific parts of the cytoplasm and a proportion of mRNAs and polyribosomes are associated with the cytoskeleton. It has been shown previously that c-myc mRNA is present in the perinuclear cytoplasm and associated with the cytoskeleton, and that this localisation is dependent upon the 3′ untranslated region of the mRNA. The present studies show that in transfected fibroblasts the c-myc 3′ untranslated region is able to localise beta-globin reporter sequences to the perinuclear cytoplasm. Studies with constructs containing deletions within the 3′ untranslated region identify the region between bases 194 and 280 as critical for localisation. Transfection of cells with constructs in which this region is linked to beta-globin sequences showed that it was sufficient to localise the chimaeric transcripts to the perinuclear cytoplasm and to cytoskeletal-bound polyribosomes. Transfection with constructs containing a mutated AUUUA sequence within the 194–280 base region showed that this conserved AUUUA is required for targeting of both c-myc mRNA and a chimaeric transcript of beta-globin transcripts linked to the c-myc 3′ untranslated region. The region between bases 194 and 280 did not induce instability of beta-globin transcripts and the AUUUA mutation had little effect upon mRNA stability. We propose that this 86 nt region of the 3′ untranslated region contains a localisation signal to target c-myc mRNA so that it is retained on cytoskeletal-bound polysomes in the perinuclear cytoplasm; a conserved AUUUA sequence appears to be a critical part of this signal.

Photic regulation of c-fos gene expression in the suprachiasmatic nucleus and the circadian rhythm of photosensitivity in the mink

The relationship between the photic stimulation of the c-fos gene product in cells of the suprachiasmatic nuclei and the photoperiodic control of testicular activity were examined in mink. Mink were kept in a short photoperiod (control, LD4:20), or in 'asymmetric skeleton photoperiods' (groups A and B). The light period for groups A and B was divided into two fractions (3 h 30 min and 30 min); the shorter fraction occurred in the night, 4 h (group A) or 8 h (group B) after the end of the main light period. There was no photic activation of the proto-oncogene c-fos on the control or group A, and 4 weeks on this photoperiodic treatment produced marked testicular development. In contrast, in group B, c-fos mRNA was induced 30 min after the end of the secondary photofraction, was maximal 30 min later and then decreased. Fos-like immunoreactivity was detected 2 h after the end of the secondary photofraction, with activity peaking 1 h later. The animals of this group remained sexually quiescent. These results suggest that photo-induction of the proto-oncogene c-fos is implicated in the gonadal inhibition induced in this species when the light period, extends into the photosensitive phase of the circadian rhythm of photosensitivity.