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.

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.

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.

c-fos mRNA instability determinants present within both the coding and the 3' non coding region link the degradation of this mRNA to its translation

The instability of oncogenic mRNA such as c-fos mRNA is controlled in cis by sequences present in both the coding and the 3' untranslated regions (3'UTR). The latter contains AU-rich elements (ARE) which, depending on the cellular context, mediate either their rapid degradation or inhibit their translation. These observations, along with the known increase of the life spans of many unstable mRNA promoted by inhibitors of protein synthesis, raise the possibility that both processes are linked. To investigate further the putative involvement of translation in both coding region and ARE-mediated rapid decay of c-fos mRNA, we designed an expression vector based on the use of the ferritin mRNA iron regulatory element (IRE). The latter structure links translation to intracellular iron concentration when inserted at the proper location within the 5'UTR. Rapid degradation of a beta-globin/c-fos 3'UTR construct was prevented by Desferrioxamine, an iron chelator, and facilitated by ferric ammonium citrate or hemin, while stability of other mRNAs not containing the IRE or the ARE were unchanged. The same conclusion was reached when the stability of a c-fos mRNA devoid of ARE was assessed in function of iron availability.

TGF-beta 1 and cAMP attenuate cyclin A gene transcription via a cAMP responsive element through independent pathways

Transforming growth factor beta (TGF-beta) is a potent inhibitor of the proliferation of many cell lines. The expression of Cyclin A is down-regulated by TGF-beta 1 in Chinese hamster lung fibroblasts and most of this effect is mediated at the transcriptional level through a cAMP-responsive element (CRE), but does not require a functional cAMP-dependent protein kinase. However, activation of the cAMP pathway in these cells gives rise to a strong inhibition of proliferation, paralleled by a down-regulation of Cyclin A promoter activity. This effect requires the integrity of the CRE, suggesting a role for CRE-binding proteins in late G1/S controls.

Elongation and premature termination of transcripts initiated from c-fos and c-myc promoters show dissimilar patterns

RNA polymerase II seems to be prone to stop at intrinsic pause sites, thus introducing a further potential level of regulation. It was recently shown that RNA polymerase II was held at the P2 promoter of c-myc gene. We confirmed the presence of engaged polymerases in the murine fibroblastic Ltk- and pre-B lymphoid 70Z3 cell lines. High resolution run-on analysis and in vivo permanganate-dependent footprinting showed that this holds true for the c-fos gene in unstimulated cells where a strong block to transcription elongation was evidenced. In contrast to what was observed in the c-myc gene, an even more intense signal was observed in run-on experiments downstream to the promoter, on a c-fos oligonucleotide including position +385 where an in vitro transcription arrest site was previously mapped. Genomic footprinting of DNA from intact cells and isolated nuclei confirmed the involvement of several thymidines belonging to a T-rich stretch in a melted region which was not detected upon polymerase release. In order to observe a short abortive c-fos transcript accumulating in vivo we resorted to microinjection of c-fos templates in Xenopus oocytes where transcripts were stable.

Nuclear import of serum response factor (SRF) requires a short amino-terminal nuclear localization sequence and is independent of the casein kinase II phosphorylation site

Serum stimulation of resting cells is mediated at least in part at the transcriptional level by the activation of numerous genes among which c-fos constitutes a model. Serum response factor (SRF) forms a ternary complex at the c-fos serum response element (SRE) with an accessory protein p62TCF/Elk-1. Both proteins are the targets of multiple phosphorylation events and their role is still unknown in the amino terminus of SRF. While the transcriptional activation domain has been mapped between amino acids 339 and 508, the DNA-binding and the dimerization domains have been mapped to between amino acids 133–235 and 168–235, respectively, no role has been proposed for the amino-terminal portion of the molecule. We demonstrate in the present work that amino acids 95 to 100 contain a stretch of basic amino acids that are sufficient to target a reporter protein to the nucleus. Moreover, this sequence appears to be the only nuclear localization signal operating in SRF. Finally, whereas the global structure around this putative nuclear location signal is reminiscent of what is found in the SV40 T antigen, the casein kinase II phosphorylation site does not determine the rate of cyto-nuclear protein transport of this protein.

Cloning, expression and subcellular localization of the human homolog of p40MO15 catalytic subunit of cdk-activating kinase

Transitions of the cell cycle are controlled by cyclin-dependent protein kinases (cdks) whose phosphorylation on the Thr residue included in the conserved sequence YTHEVV dramatically increases the activity. A kinase responsible for this specific phosphorylation, called CAK for cdk-activating kinase, has been recently purified from starfish and Xenopus oocytes and shown to contain the MO15 gene product as a catalytic subunit. In the present paper, we have cloned the human homolog of Xenopus p40MO15 by probing a HeLa cell cDNA library with degenerate oligonucleotides deduced from Xenopus and starfish MO15 sequences. Human and Xenopus MO15 displayed a strong homology showing 86% identity with regard to amino acid sequences. Northern blot analysis of RNA extracts from a series of human tissues as well as from cultured rodent fibroblasts revealed a unique 1.4 kb MO15 mRNA. No variation in the amount of MO15 transcript or protein was found along the entire course of the fibroblast cell cycle. Fluorescence in situ hybridization on human lymphocyte metaphases showed two distinct chromosomal locations of human MO15 gene at 5q12-q13 and 2q22-q24. By using gene tagging and mammalian cell transfection, we demonstrate that the KRKR motif located at the carboxy terminal end of MO15 is required for nuclear targeting of the protein. Mutation of KRKR to NGER retains MO15 in the cytoplasmic compartment, whilst the wild-type protein is detected exclusively in the nucleus. Interestingly, we demonstrate that the nuclear targeting of MO15 is necessary to confer the protein its CAK activity. In contrast to the wild-type, the NLS-mutated MO15 expressed in Xenopus oocytes is unable to generate CAK as long as the nuclear envelope is not broken. The nuclear localization of both the MO15 gene product and CAK activity may imply that cdks activation primarily occurs in the cell nucleus.

Comparative analysis of cellular and tissular expression of c-fos in human keratinocytes: evidence of its role in cell differentiation

Recent studies on normal and pathological skin have suggested a role of the c-fos proto-oncogene in keratinocyte differentiation. To further elucidate this question we have used keratinocyte and skin culture models to study in vitro regulation of c-fos expression and attempted to correlate it with the keratinocyte maturation process. Our results show that c-fos expression is prolonged in keratinocyte monolayers both at the mRNA and protein level. Extracellular calcium which stimulate keratinocyte differentiation is able to induce c-fos expression in the presence of growth factors. However this c-fos expression cannot be maintained by these factors as seen in normal human skin in vivo. Conversely, spontaneous expression of c-fos can be seen in reconstituted skin when the neo-epidermis has completed its differentiation. All these data strongly support a role of c-fos as a switch between the early and late phases of keratinocyte differentiation allowing them to be definitively committed to their elimination process. Additionally, a differential regulation of c-fos seems to exist between keratinocyte culture and reconstituted epidermis, suggesting that tissular and serum factors are involved in the prolonged c-fos expression observed in human epidermis.

Targeting of c-myc and beta-globin coding sequences to cytoskeletal-bound polysomes by c-myc 3' untranslated region

The influence of the 3' untranslated region on mRNA localization was investigated by measuring the distribution of myc, beta-globin and hybrid myc-globin mRNAs between free, cytoskeletal-bound and membrane-bound polysomes in cells transfected with either control or chimeric gene constructs. c-myc sequences and beta-globin-coding sequences linked to the myc 3' untranslated region were present at greatest enrichment in cytoskeletal-bound polysomes. beta-Globin mRNA and myc-coding sequences linked to the beta-globin 3' untranslated region were recovered largely in the free polysomes. In situ hybridization confirmed that replacement of the c-myc 3' untranslated region by that of globin caused a relocalization of the mRNA. The results suggest that mRNA localization in differentiated eukaryotic cells depends on a mechanism that involves directional information in the 3' untranslated region of mRNAs.

Chromosomal location and expression of the genes coding for Ku p70 and p80 in human cell lines and normal tissues

Ku protein is a relatively abundant DNA-binding nuclear protein complex composed of two polypeptide subunits, p70 and p80. Ku has been recently identified as the regulatory component of the DNA-dependent protein kinase that phosphorylates RNA polymerase II. To further characterize in vivo regulation of Ku protein, we studied the expression of the transcripts coding for the Ku p70 and p80 subunits in different human cell lines and normal tissues by Northern blot hybridization, using specific cDNA probes. The expression level of both genes was approximately 10-fold higher in established cell lines than in normal tissues. However, mRNA expression levels in permanent cell lines correlated more strongly with their proliferative state than with their level of malignant transformation. In purified T lymphocytes induced to proliferate by the combined action of monoclonal antibodies directed against the CD2 and CD28 adhesion molecules, Ku p70 and p80 mRNA steady-state levels increased as soon as 6 h after activation and lasted at least 72 h. The human genes coding for the Ku p70 and p80 subunits were localized by cytogenetic mapping, using fluorescence in situ hybridization, to 22q13 and 2q33-->q35, respectively.

Expression and purification of the DNA-binding domain of SRF: SRF-DB, a part of a DNA-binding protein which can act as a dominant negative mutant in vivo

We have developed an approach which allows functional in vivo examination of DNA-binding proteins through microinjection of polypeptides containing the DNA-binding domain into living fibroblasts. The present analysis utilizes serum response factor (SRF), a transcription factor that binds to the serum response element. We have expressed in bacteria a 30-kDa portion of this protein (amino acids 113 to 265) containing the DNA-binding domain of SRF (SRF-DB) and purified it to homogeneity by a single DNA affinity chromatography step using the high-affinity SRF-binding site (ACT.L). We have tested the efficiency of SRF-DB to prevent endogenous SRF function through analysis of c-fos expression and DNA synthesis stimulated by fetal calf serum, two events known to require SRF. Injection of purified SRF-DB into rat embryo fibroblasts inhibits c-fos induction by growth factors. Moreover, DNA synthesis, induced after serum addition, is also suppressed by SRF-DB injection. This implies that overproduction of SRF-DB makes the cell deficient in the function of wild-type SRF and that SRF-DB acts as a dominant negative mutant. These data show that, for the study of DNA-binding proteins, expressing and using portions of the protein that corresponds to the DNA-binding domain present a useful method for generating dominant negative mutants and illustrate the potential application of the DNA-binding region to facilitate the study of events at the DNA/protein level.

Loss of the G1-S control of cyclin A expression during tumoral progression of Chinese hamster lung fibroblasts

The expression of cyclin A, one of the key regulators of cell cycle progression in association with cdc2/cdk2 protein kinases and which undergoes cyclic accumulation during the cell cycle, has been investigated in CCL39 Chinese hamster lung fibroblasts and in two transformed variants, A71 and 39Py. Whereas A71 (selected after tumor induction in nude mice) is subject to growth arrest (less than 5% of labeled nuclei after 24 h of serum starvation), 39Py (obtained after transformation by polyoma virus) is not (more than 50% of labeled nuclei). In both cells, cyclin A expression was correlated with establishment of S phase, with a progressive deregulation of its G1 controls. This deregulation was not detected with the two early response genes c-fos and c-myc. The kinetics of accumulation of cyclin A lagged behind that of [3H]thymidine incorporation, thereby questioning a direct role for cyclin A in S phase triggering. Moreover, transforming growth factor beta 1, which is known to inhibit alpha-thrombin or fibroblast growth factor-induced mitogenicity in G0-arrested CCL39 cells, is shown here to down-regulate cyclin A expression in both CCL39 and A71 cells but has no effect on 39Py cells. These data establish cyclin A as a sensitive marker for the loss of growth factor requirement.

c-fos mRNA constitutive expression by mature human megakaryocytes

By in situ hybridization with a c-fos probe, we have shown that human bone marrow megakaryocytes cultured in the presence of 20% aplastic anemia plasma constitutively express c-fos mRNA. At day 0, megakaryocytes are mostly immature and only 3% of them are labeled. The number of labeled cells reached 23% after 12 days of culture. Interleukin 3 (IL-3) and IL-6 added together at day 10 further increased this number to 31% 2 days later. Mature labeled megakaryocytes were more numerous and more strongly labeled than immature ones. These results suggest that c-fos could play a role in megakaryocytic terminal differentiation, either in the polyploidization or in the thrombopoietic function unique to these cells.

In vivo footprints between the murine c-myc P1 and P2 promoters

Assuming that when transcription starts at the P2 promoter of the c-myc gene sites located immediately upstream from P2 are occupied whereas in the absence of initiation they are not, the polymerase chain reaction (PCR)-based method of Mueller & Wold [(1989). Science, 246, 780-786] was used to map in vivo footprints upstream from the P2 promoter in various mouse cell lines. In cultured Friend erythroleukemic cells induced to differentiate with dimethysulfoxide (DMSO), a clear protection corresponding to ME1a2 and E2F sites was observed, consistent with in vitro band-shift and footprint data. However, in cell lines in which the gene was either silent or truncated the footprints were no longer visible. Friend c-myc transcripts decreased to a barely detectable level after 3 h of DMSO treatment. Transcription, as measured by in vitro run-on, was turned off at the level of RNA polymerase elongation rather than initiation [Mechti N., Piechaczyk, M. Blanchard, J.-M., Marty, L., Bonnieu, A., Jeanteur, Ph. & Lebler, B. (1986). Nucleic Acids Res., 24, 9653-9666]. The state of occupancy of the sites did not vary from the first hours up to 9 days of DMSO treatment, suggesting that DNA occupancy per se cannot explain premature termination, which rather would involve a more complex phenomenon.

Casein kinase II induces c-fos expression via the serum response element pathway and p67SRF phosphorylation in living fibroblasts

Elevation of intracellular casein kinase II (CKII) levels through microinjection of purified CKII results in the rapid and transient induction of c-fos in quiescent rat embryo fibroblasts, and activation of quiescent cells by serum is accompanied by the nuclear relocation of endogenous CKII. The induction of c-fos by CKII is inhibited by coinjection of oligonucleotides corresponding to the sequence of the serum response element (SRE) present in the c-fos promoter, indicating that competitive displacement of positive factors from the endogenous c-fos SRE prevents c-fos induction by CKII. Furthermore, the expression of c-fos induced by either CKII injection or serum activation is also inhibited by microinjection of antibodies against the 67 kDa serum response factor (p67SRF) indicating the absolute requirement of p67SRF in this process. Finally, we show the specific phosphorylation of p67SRF in vivo following microinjection of CKII into quiescent cells. Together, these data strongly support that CKII induces c-fos expression through binding/activation of the phosphorylated p67SRF at the SRE sequence.

C-fos and c-jun proto-oncogene expression is decreased in psoriasis: an in situ quantitative analysis

Psoriasis is a common, sometimes severe, non-malignant skin disease characterized by hyperproliferation and abnormal differentiation of keratinocytes. Because proto-oncogenes are implicated in both cell proliferation and differentiation, their expression could be modified in skin diseases such as psoriasis. The c-fos and c-jun proto-oncogenes, whose products associate to form a heterodimeric transcription factor, are among the first genes to be expressed when certain cells are stimulated to either proliferate or differentiate. Recent studies in our laboratory have shown that the c-fos proto-oncogene is highly expressed in normal human adult skin. In the present study, we used in situ hybridization with RNA to compare the expression and localization of c-fos and c-jun transcripts in 15 lesional and non-lesional psoriatic skin samples. Two clinical variants of psoriasis were studied: the most severe and chronic form or plaque-type psoriasis (N = 10) and rapidly resolutive guttate-type psoriasis (N = 5). Quantitative analysis was performed using a semi-automatic image analyzer and the "Starwise grain" software program. Our control samples included 10 normal skins and eight specimens from other benign hyperproliferative non-psoriatic skin diseases, consisting of three with inflammation (seborrheic dermatitis and atopic dermatitis), and 5 without inflammation (seborrheic keratoses). Control genes we used for in situ hybridization and RNA integrity were keratin 14, which is expressed in the epidermis and was normally expressed in all tissue analyzed, and ribosomal RNA. Our data showed that c-fos and c-jun were expressed to an equivalent extent, both spatially and quantitatively, in all specimens tested. Expression was significantly decreased in plaque-type but not in guttate-type psoriasis. It was also decreased in the three other benign inflammatory cutaneous hyperproliferative disorders, but not in the five non-inflammatory cases. These results were surprising because hyperproliferation was here associated with a decrease in proto-oncogene expression, thus suggesting that c-fos and c-jun do not play a crucial role in the control of keratinocyte proliferation in vivo. However, their reduced expression in some abnormally differentiated skins indicates that both c-fos and c-jun proto-oncogenes may play a key role in keratinocyte differentiation. Their altered expression correlated with severity of the disease and the presence of an inflammatory infiltrate. These data offer a new insight into the role and regulation of these proto-oncogenes in vivo in humans.