Adenovirus E1A activates cyclin A gene transcription in the absence of growth factors through interaction with p107

Attempts on producing lymphoid cell line from Penaeus monodon by induction with SV40-T and 12S EIA oncogenes

In an attempt of in vitro transformation, transfection mediated expression of Simian virus-40 (T) antigen (SV40-T) and transduction mediated expression of Adenovirus type 12 early region 1A (12S E1A) oncogene were performed in Penaeus monodon lymphoid cells. pSV3-neo vector encoding SV40-T oncogene and a recombinant baculovirus BacP2-12S E1A-GFP encoding 12S E1A oncogene under the control of hybrid promoters were used. Electroporation and lipofection mediated transformation of SV40-T in lymphoid cells confirmed the transgene expression by phenotypic variation and the expression of GFP in co-transfection experiment. The cells transfected by lipofection (≥ 5%) survived for 14 days with lower toxicity (30%), whilst on electroporation, most of the cells succumbed to death (60%) and survived cells lived up to 7 days. Transduction efficiency in primary lymphoid cells was more than 80% within 14 days of post-transduction, however, an incubation period of 7 days post-transduction was observed without detectable expression of 12S E1A. High level of oncogenic 12S E1A expression were observed after 14 day post-transduction and the proliferating cells survived for more than 90 days with GFP expression, however, without in vitro transformation and immortalization. The study put forth the requirement of transduction mediated 'specific' oncogene expression along with telomerase activation and epigenetic induction for the immortalization and establishment of shrimp cell line.

Regulation of cyclin B2 expression and cell cycle G2/m transition by menin

Multiple endocrine neoplasia type 1 (MEN1) results from mutations in tumor suppressor gene Men1, which encodes nuclear protein menin. Menin up-regulates certain cyclin-dependent kinase inhibitors through increasing histone H3 lysine 4 (H3K4) methylation and inhibits G(0)/G(1) to S phase transition. However, little is known as to whether menin controls G(2)/M-phase transition, another important cell cycle checkpoint. Here, we show that menin expression delays G(2)/M phase transition and reduces expression of Ccnb2 (encoding cyclin B2). Menin associates with the promoter of Ccnb2 and reduces histone H3 acetylation, a positive chromatin marker for gene transcription, at the Ccnb2 locus. Moreover, Men1 ablation leads to an increase in cyclin B2 expression, histone H3 acetylation at the Ccnb2 locus, and G(2)/M transition. In contrast, knockdown of cyclin B2 diminishes the number of cells at M phase and reduces cell proliferation. Furthermore, menin interferes with binding of certain positive transcriptional regulators, such as nuclear factor Y (NF-Y), E2 factors (E2Fs), and histone acetyltransferase CREB (cAMP-response element-binding protein)-binding protein (CBP) to the Ccnb2 locus. Notably, MEN1 disease-related mutations, A242V and L22R, abrogate the ability of menin to repress cyclin B2 expression and G(2)/M transition. Both of the mutants fail to reduce the acetylated level of the Ccnb2 locus. Together, these results suggest that menin-mediated repression of cyclin B2 is crucial for inhibiting G(2)/M transition and cell proliferation through a previously unrecognized molecular mechanism for menin-induced suppression of MEN1 tumorigenesis.

Bevacizumab increases viral distribution in human anaplastic thyroid carcinoma xenografts and enhances the effects of E1A-defective adenovirus dl922-947

PURPOSE

Anaplastic thyroid carcinoma is a prime target for innovative therapy because it represents one of the most lethal human neoplasms and is refractory to conventional treatments such as chemotherapy and radiotherapy. We have evaluated a novel therapeutic approach based on the oncolytic replication-selective adenovirus dl922-947.

EXPERIMENTAL DESIGN

The antitumor efficacies of the E1ADeltaCR2 (dl922-947) and DeltaE1B55K (dl1520) mutants were compared in human thyroid anaplastic carcinoma cells in culture and in xenografts in vivo. To enhance the effects of dl922-947, anaplastic thyroid carcinoma tumor xenografts were treated with dl922-947 in combination with bevacizumab.

RESULTS

We showed that the efficacy of dl922-947 exceeded that of dl1520 in all tested anaplastic thyroid carcinoma cells in vitro and in vivo. Furthermore, bevacizumab in combination with dl922-947 significantly reduced tumor growth compared with single treatments alone. Bevacizumab treatment significantly improved viral distribution in neoplastic tissues.

CONCLUSIONS

Our data showed that dl922-947 had a higher oncolytic activity compared with dl1520 in anaplastic thyroid carcinoma cell lines and might represent a better option for virotherapy of anaplastic thyroid carcinoma. Moreover, bevacizumab increased the oncolytic effects of dl922-947 by enhancing viral distribution in tumors. The results described herein encourage the use of the dl922-947 virus in combination with bevacizumab.

Transactivation of human cdc2 promoter by adenovirus E1A

Expression of the adenovirus oncoprotein E1A 12S induces the heterotrimeric transcription factor, NF-Y. NF-Y binds to the two CCAAT motifs upstream of the transcriptional start site of the human cdc2 promoter and is required for activation of the promoter by E1A 12S in cycling cells. The observations that a number of eukaryotic cell cycle regulatory genes also contain the CCAAT motifs and NF-Y binds to them support the notion that E1A 12S could play an important role in deregulated expression of these genes through activation of NF-Y gene in cycling cells.

Transactivation of murine cyclin A by polyomavirus large and small T antigens

Polyomavirus large and small T antigens cooperate in the induction of S phase in serum-deprived Swiss 3T3 cells. While the large T antigen is able to induce S phase-specific enzymes, we have recently shown that both T antigens contribute to the production of the cyclins E and A and that the small T antigen is essential for the induction of cyclin A-dependent cdk2 activity (S. Schüchner and E. Wintersberger, J. Virol. 73:9266-9273, 1999). Here we present our attempts to elucidate the mechanisms by which the large and the small T antigens transactivate the murine cyclin A gene. Using Swiss 3T3 cells carrying the T antigens and various mutants thereof under the hormone-inducible mouse mammary tumor virus promoter, as well as transient-cotransfection experiments with the T antigens and cyclin A promoter-luciferase reporter constructs, we found the following. The large T antigen activates the cyclin A promoter via two transcription factor binding sites, a cyclic AMP responsive element (CRE), and the major negative regulatory site called CDE-CHR. While an intact binding site for pocket proteins is required for the function of this T antigen at the CDE-CHR, its activity at the CRE is largely independent thereof. In contrast, an intact J domain and an intact zinc finger are required at both sites. The small T antigen also appears to have an influence on the cyclin A promoter through the CRE as well as the CDE-CHR. For this an interaction with protein phosphatase 2A is essential; mutation of the J domain does not totally eliminate but greatly reduces the transactivating ability.

Distinct roles of Jun : Fos and Jun : ATF dimers in oncogenesis

Jun : Fos and Jun : ATF complexes represent two classes of AP-1 dimers that (1) preferentially bind to either heptameric or octameric AP-1 binding sites, and (2) are differently regulated by cellular signaling pathways and oncogene products. To discriminate between the functions of Jun : Fos, Jun : ATF and Jun : Jun, mutants were developed that restrict the ability of Jun to dimerize either to itself, or to Fos(-like) or ATF(-like) partners. Introduction of these mutants in chicken embryo fibroblasts shows that Jun : Fra2 and Jun : ATF2 dimers play distinct, complementary roles in in vitro oncogenesis by inducing either anchorage independence or growth factor independence, respectively. v-Jun : ATF2 rather than v-Jun : Fra2 triggers the development of primary fibrosarcomas in the chicken wing. Genes encoding extracellular matrix components seem to constitute an important subset of v-Jun : ATF2-target genes. Repression of the matrix component SPARC by Jun is essential for the induction of fibrosarcomas. Avian primary cells transformed by either Jun : Fra2 or Jun : ATF2 thus provide powerful tools for the investigation of the downstream pathways involved in oncogenesis. Further genetic studies with Jun dimerization mutants will be required to be precise and extend the specific roles of the Jun : Fos and Jun : ATF dimers during cancer progression in avian and mammalian systems.

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.

Defect in the p53-Mdm2 autoregulatory loop resulting from inactivation of TAF(II)250 in cell cycle mutant tsBN462 cells

The cell cycle arrest and proapoptotic functions of p53 are under tight control by Mdm2. After stress activation of p53 by nontranscriptional mechanisms, transcription of the mdm2 gene results in increased synthesis of Mdm2 and down-regulation of p53. Disruption of this autoregulatory loop has profound effects on cell survival and tumorigenesis. We show that a defective p53-Mdm2 autoregulatory loop results from inactivation of a basal transcription factor, TAF(II)250, in tsBN462 cells. We found that Mdm2 expression rescues the temperature-sensitive phenotype of tsBN462 cells, as shown by activation of cell cycle-regulated gene promoters (B-myb, cyclin A, and cdc25C), increased cell growth and DNA synthesis, and inhibition of apoptosis. These effects of Mdm2 are mediated by p53. Exogenous Mdm2 expression apparently complements endogenous Mdm2 synthesis in tsBN462 cells, which is reduced compared to that in the equivalent parental cells with wild-type TAF(II)250, BHK21. Expression of wild-type TAF(II)250 in tsBN462 stimulates and prolongs the synthesis of Mdm2 and rescues the temperature-sensitive phenotype. The TAF(II)250 rescue is blocked by inhibition of Mdm2-p53 interactions. We also show that Mdm2 promoter activation, after transfer to the nonpermissive temperature, is attenuated in cells with mutant TAF(II)250. The temperature-sensitive phenotype apparently results from inefficient inhibition of heat-induced p53 by reduced Mdm2 synthesis due to low mdm2 promoter activity. These results raise the possibility that the p53-Mdm2 autoregulatory loop could guard against transcriptional defects in cells.

A CDE/CHR-like element mediates repression of transcription of the mouse RB2 (p130) gene

The bipartite repressor elements, termed cell cycle-dependent element (CDE)/cell cycle regulatory element (CCRE)-cell cycle homology region (CHR) control the growth-dependent transcription of the cyclin A, cdc25C, cdc2 genes. Here, we have identified a functional element displaying the signature of the CDE-CHR in the promoter of the mouse RB2 (p130) gene, encoding the retinoblastoma protein family (pRB)-related protein p130. This element locates close to the major transcription start site where it makes major groove contacts with proteins that can be detected in a cellular context using in vivo genomic footprinting techniques. Inactivation of either the CDE or CHR sequence strongly up-regulates the p130 promoter activity in exponentially growing cells, a situation where endogenous p130 gene expression is almost undetectable. Electrophoretic mobility shift assays suggest that two different protein complexes bind independently to the p130 CDE and CHR elements, and that the protein(s) bound to the CDE might be related to those bound on cyclin A and cdc2 promoters.

Activation of the cyclin D1 gene by the E1A-associated protein p300 through AP-1 inhibits cellular apoptosis

The adenovirus E1A protein interferes with regulators of apoptosis and growth by physically interacting with cell cycle regulatory proteins including the retinoblastoma tumor suppressor protein and the coactivator proteins p300/CBP (where CBP is the CREB-binding protein). The p300/CBP proteins occupy a pivotal role in regulating mitogenic signaling and apoptosis. The mechanisms by which cell cycle control genes are directly regulated by p300 remain to be determined. The cyclin D1 gene, which is overexpressed in many different tumor types, encodes a regulatory subunit of a holoenzyme that phosphorylates and inactivates PRB. In the present study E1A12S inhibited the cyclin D1 promoter via the amino-terminal p300/CBP binding domain in human choriocarcinoma JEG-3 cells. p300 induced cyclin D1 protein abundance, and p300, but not CBP, induced the cyclin D1 promoter. cyclin D1 or p300 overexpression inhibited apoptosis in JEG-3 cells. The CH3 region of p300, which was required for induction of cyclin D1, was also required for the inhibition of apoptosis. p300 activated the cyclin D1 promoter through an activator protein-1 (AP-1) site at -954 and was identified within a DNA-bound complex with c-Jun at the AP-1 site. Apoptosis rates of embryonic fibroblasts derived from mice homozygously deleted of the cyclin D1 gene (cyclin D1(-/-)) were increased compared with wild type control on several distinct matrices. p300 inhibited apoptosis in cyclin D1(+/+) fibroblasts but increased apoptosis in cyclin D1(-/-) cells. The anti-apoptotic function of cyclin D1, demonstrated by sub-G(1) analysis and annexin V staining, may contribute to its cellular transforming and cooperative oncogenic properties.

Binding of polyomavirus small T antigen to protein phosphatase 2A is required for elimination of p27 and support of S-phase induction in concert with large T antigen

Although polyomavirus large T antigen readily transactivates S-phase-specific enzymes in serum-starved Swiss 3T3 mouse fibroblasts, it is incapable by itself to efficiently drive such cells into S phase. We describe here that this inability correlates with a weak proficiency of the viral protein to induce the synthesis of cyclin A and cyclin E and to stimulate the respective cyclin/cdk activities. Polyomavirus small T antigen, which together with the large T protein supports S-phase induction, strongly contributes to the synthesis of cyclin A. In addition, small T antigen causes a dramatic induction of cyclin A- and, together with large T antigen, of cyclin E-specific protein kinase activity. This latter function of polyomavirus small T antigen correlates with its competence to provoke the elimination of the kinase inhibitor p27(Kip1). An interaction of the small T antigen with the protein phosphatase 2A is essential for this activity. Hence, the ability to drive quiescent Swiss 3T3 cells into S phase results from the capacity of large T antigen to transactivate DNA synthesis enzymes by its interaction with retinoblastoma-type proteins and from the potential of the large and the small T antigens together to stimulate cyclin A synthesis and cyclin A- and cyclin E-dependent protein kinase activity.

Deregulation of p53/p21Cip1/Waf1 pathway contributes to polyploidy and apoptosis of E1A+cHa-ras transformed cells after gamma-irradiation

The p53/p21Cip1/Waf1-dependent checkpoint control of G1/S and G2/M phases of the cell cycle in response to DNA damage is an important mechanism of genome stability maintenance in normal cells. In many tumor cells, due to frequent point mutations and deletions of p53, the stringent control of the cell cycle and apoptosis is compromised. We have examined the cell cycle control and cell death of the rat embryo fibroblast cells (REF) transformed by E1A+cHa-ras oncogenes and expressing wild type p53. Gamma-irradiation at a dosage of 6 Gy has been used to analyse the p53-dependent trans-activation of the target p21cip1/waf1 gene and the levels of activity of cyclin-dependent kinases. Our results show that the cell cycle inhibitors p21Cip1/Waf1 and p27KIP accumulate in response to irradiation both in REF and E1A+cHa-ras cells. In contrast to normal REF cells, the accumulation of p21Cip1/Waf1 and p27KIP inhibitors, however, does not lead to inhibition of Cdk2 and cyclins E, A-associated kinase activities and to a G1/S block in E1A+cHa-ras cells. It is unlikely that the lack of inhibitory function of p21Cip1/Waf1 can be explained by its inability to bind Cdk2 and Cdk4 kinases or PCNA. Moreover, the p21Cip1/Waf1-associated kinase activity is increased upon gamma-irradiation of E1A+cHa-ras cells. We suggest that inactivation of p21Cip1/Waf1 may be accounted for by its interaction with E1A oncoproducts as the inhibitor is detected in immunoprecipitates using E1A-specific antibodies. During a temporary G2/M delay induced by gamma-irradiation, E1A+cHa-ras transformants continue DNA replication, which leads to accumulation of polyploid cells with lobulated nuclei and micronuclei. Thus, DNA damage of E1A+cHa-ras transformed cells, with a combination of functionally active wild type p53 and inactive p21Cip1/Waf1, contributes to formation of polyploid cells which then die due to apoptosis.

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.

Transactivation of the human cdc2 promoter by adenovirus E1A. E1A induces the expression and assembly of a heteromeric complex consisting of the CCAAT box binding factor, CBF/NF-Y, and a 110-kDa DNA-binding protein

Cyclin-dependent kinases (CDKs) play an important role in the eukaryotic cell cycle progression. Cdc2 (CDK1) is expressed in late G(1)/S phase and required for G(2) to M phase transition in higher eukaryotes. The oncoproteins, SV40 large T antigen and adenovirus E1A, induce a 110-kDa protein which specifically recognizes the two inverted CCAAT motifs of the cdc2 promoter in cycling cells and plays an essential role in transactivation of the human cdc2 promoter. Since these CCAAT motifs also conform to the consensus binding sites for the ubiquitous heterotrimeric transcription factor, CBF/NF-Y, the role of CBF/NF-Y in the transactivation of the cdc2 promoter was examined in this study. Our results indicate that CBF/NF-Y and the 110-kDa protein interact with the CCAAT box motif to form a heteromeric complex. However, mutagenesis of the pentanucleotide CCAAT motif or in the presence of urea greater than 2.5 M, no heteromeric complex was formed. In contrast, the 110-kDa protein could still bind the mutant CCAAT motif or with the wild type motif in the presence of 2.5 M urea. Furthermore, E1A.12S induced the gene expression of all three subunits of CBF/NF-Y. Coexpression of E1A and a dominant negative mutant NF-YA subunit significantly reduced the E1A-mediated transactivation of the cdc2 promoter in a dose-dependent manner. These results support the conclusion that E1A protein mediates optimal transactivation of the human cdc2 promoter by inducing the expression and assembly of a heteromeric complex consisting of the 110-kDa protein and the CBF/NF-Y which interacts with the two CCAAT motifs of the cdc2 promoter.

Critical role played by cyclin D3 in the MyoD-mediated arrest of cell cycle during myoblast differentiation

During the terminal differentiation of skeletal myoblasts, the activities of myogenic factors regulate not only tissue-specific gene expressions but also the exit from the cell cycle. The induction of cell cycle inhibitors such as p21 and pRb has been shown to play a prominent role in the growth arrest of differentiating myoblasts. Here we report that, at the onset of differentiation, activation by MyoD of the Rb, p21, and cyclin D3 genes occurs in the absence of new protein synthesis and with the requirement of the p300 transcriptional coactivator. In differentiated myocytes, cyclin D3 also becomes stabilized and is found nearly totally complexed with unphosphorylated pRb. The detection of complexes containing cyclin D3, cdk4, p21, and PCNA suggests that cdk4, along with PCNA, may get sequestered into high-order structures held together by pRb and cyclin D3. Cyclin D3 up-regulation and stabilization is inhibited by adenovirus E1A, and this correlates with the ability of E1A to promote pRb phosphorylation; conversely, the overexpression of cyclin D3 in differentiated myotubes counteracts the E1A-mediated reactivation of DNA synthesis. These results indicate that cyclin D3 critically contributes to the irreversible exit of differentiating myoblasts from the cell cycle.

Timing of cyclin E gene expression depends on the regulated association of a bipartite repressor element with a novel E2F complex

Transient induction of the cyclin E gene in late G1 gates progression into S. We show that this event is controlled via a cyclin E repressor module (CERM), a novel bipartite repressor element located near the cyclin E transcription start site. CERM consists of a variant E2F-binding site and a contiguous upstream AT-rich sequence which cooperate during G0/G1 to delay cyclin E expression until late G1. CERM binds the protein complex CERC, which disappears upon progression through G0-G1 and reappears upon entry into the following G1. CERC disappearance correlates kinetically with the liberation of the CERM module in vivo and cyclin E transcriptional induction. CERC contains E2F4/DP1 and a pocket protein, and sediments faster than classical E2F complexes in a glycerol gradient, suggesting the presence of additional components in a novel high molecular weight complex. Affinity purified CERC binds to CERM but not to canonical E2F sites, thus displaying behavior different from known E2F complexes. In cells nullizygous for members of the Rb family, CERC is still detectable and CERM-dependent repression is functional. Thus p130, p107 and pRb function interchangeably in CERC. Notably, the CERC-CERM complex dissociates prematurely in pRb-/- cells in correspondence with the premature expression of cyclin E. Thus, we identify a new regulatory module that controls repression of G1-specific genes in G0/G1.

The three members of the pocket proteins family share the ability to repress E2F activity through recruitment of a histone deacetylase

The transcription factor E2F plays a major role in cell cycle control in mammalian cells. E2F binding sites, which are present in the promoters of a variety of genes required for S phase, shift from a negative to a positive role in transcription at the commitment point, a crucial point in G1 that precedes the G1/S transition. Before the commitment point, E2F activity is repressed by members of the pocket proteins family. This repression is believed to be crucial for the proper control of cell growth. We have previously shown that Rb, the founding member of the pocket proteins family, represses E2F1 activity by recruiting the histone deacetylase HDAC1. Here, we show that the two other members of the pocket proteins family, p107 and p130, also are able to interact physically with HDAC1 in live cells. HDAC1 interacts with p107 and Rb through an "LXCXE"-like motif, similar to that used by viral transforming proteins to bind and inactivate pocket proteins. Indeed, we find that the viral transforming protein E1A competes with HDAC1 for p107 interaction. We also demonstrate that p107 is able to interact simultaneously with HDAC1 and E2F4, suggesting a model in which p107 recruits HDAC1 to repress E2F sites. Indeed, we demonstrate that histone deacetylase activity is involved in the p107- or p130-induced repression of E2F4. Taken together, our data suggest that all members of the E2F family are regulated in early G1 by similar complexes, containing a pocket protein and the histone deacetylase HDAC1.

The MDM2 C-terminal region binds to TAFII250 and is required for MDM2 regulation of the cyclin A promoter

MDM2 proto-oncogene expression is aberrant in many human tumors. Its normal role is to modulate the functions of p53. The N terminus of MDM2 interacts with p53, whereas the properties of the rest of the molecule are poorly understood. We show that MDM2 binds to the general transcription factor TFIID in vivo. The C-terminal Ring finger interacts with TAFII250/CCG1, and the central acidic domain interacts with TBP. Expression of MDM2 activates the cyclin A gene promoter but not c-fos, showing that the effects of MDM2 are specific. Deletion of the C-terminal region of MDM2 abolishes activation, showing that the C-terminal domain of MDM2 is functionally important. We found that increasing MDM2 expression to higher levels inhibits the cyclin A promoter. Inhibition appears to result from titration of general transcription factors because MDM2 overexpression inhibits c-fos as well as other promoters in vivo and basal transcription in vitro. The mechanisms of repression of the cyclin A and fos promoters appear to be different. Cyclin A repression is lost by deleting the C terminus, whereas that of c-fos is lost by removal of the acidic domain. These results reinforce the conclusion that the C terminus of MDM2 mediates effects on the cyclin A promoter. MDM2 transformed cells contain elevated levels of cyclin A mRNA, showing that activation occurs under physiological conditions. There is a positive correlation between MDM2 binding to TAFII250 and MDM2 activation of the cyclin A promoter. The C-terminal region of MDM2, which contains the Ring finger, interacts with TAFII250 and is required for regulation of the cyclin A promoter by MDM2. Our results link the activity of MDM2, a transforming protein implicated in many human tumors, with cyclin A, a regulator of the cell cycle.

p27KIP1 blocks cyclin E-dependent transactivation of cyclin A gene expression

Cyclin E is necessary and rate limiting for the passage of mammalian cells through the G1 phase of the cell cycle. Control of cell cycle progression by cyclin E involves cdk2 kinase, which requires cyclin E for catalytic activity. Expression of cyclin E/cdk2 leads to an activation of cyclin A gene expression, as monitored by reporter gene constructs derived from the human cyclin A promoter. Promoter activation by cyclin E/cdk2 requires an E2F binding site in the cyclin A promoter. We show here that cyclin E/cdk2 kinase can directly bind to E2F/p107 complexes formed on the cyclin A promoter-derived E2F binding site, and this association is controlled by p27KIP1, most likely through direct protein-protein interaction. These observation suggest that cyclin E/cdk2 associates with E2F/p107 complexes in late G1 phase, once p27KIP1 has decreased below a critical threshold level. Since a kinase-negative mutant of cdk2 prevents promoter activation, it appears that transcriptional activation of the cyclin A gene requires an active cdk2 kinase tethered to its promoter region.

How viral oncogenes make the cell cycle

Oncogenic viruses provide their host cells with additional growth stimuli, thereby extending their proliferative capacity. This implies that viral oncogenes can override growth-suppressive signals, which control cell-cycle progression in untransformed cells. Viral oncoproteins deregulate cell-cycle control by interfering with receptor-mediated signal transduction pathways and the function of nuclear cell-cycle regulatory proteins. As a consequence of these regulatory interactions, many viral oncogenes induce the expression of cellular genes required for cell-cycle progression, including genes encoding G1 cyclins. Apparently, different oncogenic viruses target different subsets of these cell-cycle regulatory pathways to transform cells.