CG&D
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Cancer Research Clinical Cancer Research
Cancer Epidemiology Biomarkers & Prevention Molecular Cancer Therapeutics
Molecular Cancer Research Cell Growth & Differentiation

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Pruschy, M.
Right arrow Articles by Krek, W.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Pruschy, M.
Right arrow Articles by Krek, W.
Cell Growth & Differentiation Vol. 10, 141-146, March 1999
© 1999 American Association for Cancer Research

E2F-1 Has Properties of a Radiosensitizer and Its Regulation by Cyclin A Kinase Is Required for Cell Survival of Fibrosarcoma Cells Lacking p531

Martin Pruschy, Christiane Wirbelauer, Christoph Glanzmann, Stephan Bodis and Wilhelm Krek2

Department of Radiation Oncology, University Hospital Zürich, CH-8091 Zürich [M. P., C. G., S. B.]; and Friedrich Miescher Institut, CH-4058 Basel [C. W., W. K.], Switzerland


    Abstract
 TOP
 Abstract
 Introduction
 Results and Discussion
 Materials and Methods
 References
 
Negative regulation of E2F-1 DNA binding function by cyclin A kinase represents part of an S-phase checkpoint control system that, when activated, leads to apoptosis. In this study, we examined the cellular sensitivity and resistance of isogenic mouse fibrosarcoma cell lines, differing primarily in their p53 status, to ectopic expression of wild-type (wt) E2F-1 and cyclin A kinase binding-defective mutants of it. We found that E2F-1(wt) potently affected the survival of p53+/+ tumor cells but not that of p53-/- cells. In contrast, expression of cyclin A kinase binding-defective E2F-1 species interfered with cell survival of fibrosarcoma cells irrespective of their p53 status. Finally, expression of E2F-1(wt) in p53-/- fibrosarcoma cells enhanced the cytotoxic effect of ionizing radiation in vitro and in vivo in a mouse tumor model. These results suggest that E2F-1-dependent activation of an S-phase checkpoint is p53 independent and that E2F-1 possesses radiosensitizing properties in the absence of p53.


    Introduction
 TOP
 Abstract
 Introduction
 Results and Discussion
 Materials and Methods
 References
 
The transcription factor E2F-1, the first cloned member of a family of transcription factors generically referred to as E2F, participates in aspects of cell proliferation and cell survival (1) . It directs the expression of genes with products that are required for G1 and S-phase progression, and its function as a transcription factor is tightly regulated by multiple mechanisms (2) . In G1, the retinoblastoma gene product (pRB) binds, in its hypophosphorylated form, to E2F-1, thereby suppressing the transactivation potential of the transcription factor and converting it into an active transcriptional repressor (3 , 4) . In late G1, these transcriptionally repressive complexes are lost as a result of phosphorylation of pRB by G1 cyclin-dependent protein kinases, which, in turn, results in activation of S-phase genes (5) . At the end of S phase, cyclin A kinase associates directly with E2F-1 and inhibits E2F-1 DNA binding function (6, 7, 8) . In this manner, cyclin A kinase contributes to the shutoff of E2F-1-dependent gene activity in the S/G2 phases of the cell cycle. Recent evidence suggests that reversal of E2F-1 function in the S/G2 phases may also involve the action of SCFSKP2 E3 ligase, which it thought to limit the levels of free E2F-1 beyond G1/S (9) .

In keeping with the notion that E2F-1 plays a crucial role in the regulation of cell proliferation and apoptosis, deregulated expression of E2F-1 has been shown to lead to neoplastic transformation and tumor formation by the resultant transformed cells (10, 11, 12) . Moreover, its induction in serum-deprived quiescent cells has been reported to promote S-phase entry followed by p53-dependent apoptosis (13, 14, 15) . Recent evidence suggest that the apoptosis-inducing activity of deregulated E2F-1 may not simply be a consequence of unscheduled cell cycle progression triggered by E2F-1 but rather a result of the capacity of E2F-1 to directly activate the expression of p19ARF, which, in turn, leads to the stabilization of p53 followed by apoptosis (16, 17, 18, 19) . Although induction of apoptosis by E2F-1 requires p53 in several systems (14 , 15 , 20 , 21) , E2F-1 can also drive cells to undergo p53-independent apoptosis (22, 23, 24) .

Recently, E2F-1 has also been implicated in an S-phase checkpoint control system (25) . Specifically, failure of phosphorylation and inactivation of E2F-1 DNA binding function by cyclin A kinase has been shown to result in S-phase arrest/delay followed by apoptosis. It has been suggested that, in this case, cell death induced by disrupted cyclin A kinase/E2F-1 linkages is a result of uninterrupted E2F-1 activity beyond G1/S. Thus, the mechanism underlying this process may be different to E2F-1-dependent apoptosis seen in growth-factor deprived cells because, in the report cited above, cells were cultivated in high serum-containing medium. Because disruption of cyclin A-kinase/E2F-1 linkages results in apoptosis, their complex formation represents a possible target for the development of antitumor therapeutic strategies. In principle, compounds that would interfere selectively with the cyclin A kinase-E2F-1 interaction may lead to death of certain replicating cells.

A crucial role for p53 in the execution of apoptosis in response to cell cycle checkpoint activation is well established (26) . However, no information exists regarding a participation of p53 in E2F-1-dependent activation of an S-phase checkpoint and the resultant execution of apoptosis. In this study, we examined the cytotoxic effects of E2F-1 wt3 and mutant derivatives, defective in cyclin A-kinase binding on E1A/ras-transformed isogenic fibrosarcoma cells that differ primarily in the p53 status, alone or in combination with ionizing radiation.


    Results and Discussion
 TOP
 Abstract
 Introduction
 Results and Discussion
 Materials and Methods
 References
 
Cyclin A Kinase-E2F-1 Complex Formation Is Required for Cell Survival of Fibrosarcoma Cells Lacking p53.
In this study, we used a cellular model system that had been described previously (27 , 28) , namely, MEFSs and MEFs derived from p53 knockout animals, both transformed with the adenovirus E1A oncoprotein and T24 H-ras. These otherwise isogenically identical fibrosarcoma cell lines differ primarily in their p53 status (27) . For simplicity, these E1A/ras-transformed cells are referred here to as p53-/- or p53+/+ cells to indicate their p53 status.

To determine whether apoptosis induced by failure of cyclin A kinase to down-regulate E2F-1 function in S phase, is dependent on p53 status, we infected exponentially growing cultures of p53+/+ or p53-/- cells with high-titer retroviruses encoding HA-tagged E2F-1(wt), a cyclin A kinase binding-defective mutant, HA-E2F-1({Delta}24) or ({Delta}7), a DNA binding-defective mutant HA-E2F-1(E177), and the double-mutant derivative of it, HA-E2F-1(E177/{Delta}24), defective also in cyclin A-kinase binding (see Ref. 25 and "Materials and Methods"). In all experiments, the production of the relevant HA-tagged E2F-1 products was determined by immunoblotting of whole-cell extracts prepared from infected cultures 24 hpi with an anti-HA-antibody. A representative Western blot is shown in Fig. 1CCitation and displays that all E2F-1 species under investigation were synthesized (Lanes 2–4). We note that comparable quantities of each pair of E2F-1 proteins (the single mutant and the cognate {Delta}24 double mutant) were present (e.g., compare Fig. 1CCitation , Lanes 2 and 4). Thus, one can compare the behaviour of a given E2F-1 species with its cyclin A-kinase binding-defective counterpart in biological assays and any difference in the cellular sensitivity of E2F-1(wt) or E2F-1({Delta}24)-infected cell populations cannot be attributed to differences in the quantity of E2F-1 proteins present.



View larger version (52K):
[in this window]
[in a new window]
 
Fig. 1. Synthesis of cyclin A kinase binding-deficient mutants of E2F-1 leads to cell death of p53-/- fibrosarcoma cells. A and B, p53+/+ (A) and p53-/- (B) fibrosarcoma cells were infected with identical volumes (4 ml) of tissue culture supernatant containing retroviruses encoding the various E2F-1 species, as indicated. After selection in puromycin, the number of puromycin-resistant colonies was determined as described in "Materials and Methods." The number of puromycin-resistant colonies obtained with vector virus served as a standard and was set to 1. C, aliquots of p53+/+ cells infected with the retroviruses described in A were lysed 24 hpi, equalized for protein content, and then processed for Western blotting using an anti-HA antibody. Note, that equivalent quantities of tissue culture supernatant contain similar amounts of E2F-1 protein-inducing virus.

 
As shown in Fig. 1Citation , ACitation and BCitation , when singular cells of previously infected p53+/+ or p53-/- cultures were seeded for clonogenic growth in the presence of puromycin (to select for successfully infected cells), the expression of the E2F-1 allele that escapes cyclin A kinase control, greatly decreased the reproductive integrity of both p53+/+ and p53-/- tumor cells. We also observed consistently that the cytotoxic effect of E2F-1({Delta}24) was significantly greater in p53+/+ cells than in cells lacking p53 (Fig. 1Citation , compare ACitation and BCitation ). Similar results were obtained with E2F-1({Delta}7), another cyclin A kinase binding-defective mutant (data not shown and below). In contrast, E2F-1(wt) reduced the surviving fraction of p53+/+ cells only but had little effect on clonogenic growth of p53-/- cells (Fig. 1Citation , compare A and B). As expected, in cells infected with those mutants that can no longer bind to DNA, HA-E2F-1(E177) and HA-E2F-1(E177/{Delta}24), no cytotoxic effects were measurable, suggesting that the observed effects require at a minimum E2F-1 to bind to DNA (Fig. 1, A and B)Citation .

To further support these findings, we analyzed and compared the growth over time of p53+/+ and p53-/- cells that had been infected with either vector, E2F-1(wt) or ({Delta}7) virus. Puromycin selection was initiated 24 hpi. Virtually all vector-infected p53+/+ cells were puromycin resistant, suggesting that the efficiency of infection was high. These cells grew efficiently, and their cell number increased over time (Table 1)Citation . In contrast, cell growth of E2F-1(wt)-infected p53+/+ cells was significantly impaired (Table 1)Citation , and as one would have predicted, only a small fraction of E2F-1({Delta}7)-infected p53+/+ cell populations survived the puromycin selection (Table 1)Citation . Consistent with the results obtained with the clonogenic assay cited above, infection of p53-/- cell population with the E2F-1({Delta}7) virus inhibited the growth of these cells. In comparison, E2F-1(wt) virus was rather inefficient in this regard when analyzed in parallel (Table 1)Citation .


View this table:
[in this window]
[in a new window]
 
Table 1 Growth over time of E2F-1-producing p53+/+ or p53-/- fibrosarcoma cells

 
Taken together, these results suggest (a) that suppression of an E2F-1-dependent S-phase checkpoint is cyclin A kinase dependent but p53-independent and (b) that, in this experimental system, efficient induction of cell death by E2F-1(wt) depends on functional p53. That the absence of p53 and the presence of the cooperating oncogenes E1A and ras in these cells is insufficient to suppress the responsiveness of these fibrosarcoma cells to E2F-1({Delta}24)- or ({Delta}7)-promoted cell death further suggests that the molecular mechanism underlying the activation of E2F-1({Delta}24)- or ({Delta}7)-linked cell death program is likely dependent upon different factors. It is well established that p53 is required for apoptosis in response to a variety of different stresses (26) and unscheduled activation of E2F-1 in late G1, either due to loss of pRB or to overexpression, has been shown to result in stabilization of p53 and activation of p53-dependent apoptosis (21) . In light of the results reported here, one might argue that unscheduled E2F-1 activity as a result of prolonged E2F-1 function during S/G2 is monitored by the cell as well. However, this monitoring system may be activated as a result of overactivation and/or too-long activation of one or more genes under the immediate control of E2F-1 during the S/G2 phases of the cell cycle.

Expression of E2F-1(wt) in p53-deficient Fibrosarcoma Cells Heightens Their Radioresponsiveness in Vitro and in Vivo.
Previously, it has been shown that exposure of p53+/+ E1A/ras-transformed cells to ionizing radiation causes a decrease in cell viability at doses as low as 2 Gy, whereas the same dose had only a minimal effect on p53-/- cells (27 , 28) . In addition, several lines of evidence suggest that overexpression of oncogenes can enhance apoptosis induced by anticancer agents (29 , 30) . Although deregulated expression of E2F-1 displayed little effect on the clonogenic growth of p53-/- cells, we asked whether its presence alters the sensitivity of these cells to ionizing radiation.

To this end, exponentially growing cultures of p53+/+ or p53-/- cells were infected with E2F-1(wt) or ({Delta}7) virus. Twenty-four hpi, singular seeded cells were either irradiated with different doses of ionizing radiation (2 or 5 Gy) or left untreated and cell survival was analyzed in multiple independent experiments in the clonogenic assay described before. The major conclusion that can be drawn from these results is that expression of E2F-1(wt) in radioresistant p53-/- cells enhances dramatically the cytotoxicity of ionizing radiation (Fig. 2, C and D)Citation . Interestingly, expression of E2F-1(wt) in combination with 5 Gy of irradiation decreased the reproductive integrity of these otherwise treatment-resistant cells to a level comparable to that seen for p53-/- cells infected with E2F-1({Delta}7) virus alone (Fig. 2Citation , compare ACitation and BCitation to CCitation and DCitation ). Again, infected cell cultures contained similar amounts of E2F-1 protein, as determined by Western blotting with an anti-HA antibodies (data not shown). These data suggest that E2F-1 has properties of a radiosensitizing element in cells deficient for wt p53 function. Moreover, E2F-1 and ionizing radiation can cooperate to lessen cell viability in a p53-independent manner. Whether disruption of the cyclin A kinase/E2F-1 linkage by an DNA damage-activated signal transduction pathway underlies this cooperation is not known. However, it is certainly a possibility, given that the magnitude of the cytotoxic effect in response to E2F-1({Delta}7) expression in p53-/- cells is similar to the effect achieved by E2F-1(wt) and ionizing radiation together.



View larger version (42K):
[in this window]
[in a new window]
 
Fig. 2. Ectopic expression of E2F-1(wt) enhances the radiosensitivity of p53-/- fibrosarcoma cells. A and C, p53+/+ (A) and p53-/- (C) fibrosarcoma cells were infected with indicated recombinant retroviruses. Subsequently, cells were either treated or not treated with the indicated doses of ionizing radiation prior to puromycin selection. The cytotoxic effects of these treatments were determined as described in Fig. 1Citation , ACitation and BCitation . B and D, mathematical transformation of the different survival fractions obtained in A and C to illustrate the radiosensitizing effect of E2F-1(wt) or ({Delta}7) on p53+/+ or p53-/- cells.

 
Next, we determined whether ectopic expression of E2F-1(wt) in p53-/- fibrosarcoma cells influences tumor responsiveness to ionizing radiation in vivo. p53-/- cells were infected with either vector virus or E2F-1(wt) virus. Infected cell cultures were selected as pools of puromycin-resistant cells (uncloned mass cultures) and injected into nude mice (see "Materials and Methods"). E2F-1(wt) protein expression was confirmed by Western blotting using anti-HA-antibody (data not shown). As expected, the different cell lines formed palpable tumors at all injected sites and grew at similar rates (data not shown). Upon achievement of the indicated tumor volume, tumors were irradiated with a single dose of 5 Gy, and tumor response was monitored by daily measurements of tumor volume (Fig. 3)Citation . Consistent with earlier data (28) , no significant remission of tumor size was observed in the tumors derived from p53-/- cells following application of 5 Gy (Fig. 3Citation , open squares). In contrast, all tumors derived from p53-/- cells expression E2F-1(wt) protein showed enhanced sensitivity to 5 Gy of irradiation. More specifically, tumor growth was significantly delayed for up to 5 days (Fig. 3Citation , closed squares). Tumor growth, thereafter, increased at a rate similar to that observed for tumors derived from control virus-infected cells (data not shown). Thus, similar to the results obtained in vitro, E2F-1(wt) augments the cytotoxic effects of ionizing radiation in p53-deficient cells in vivo as well.



View larger version (13K):
[in this window]
[in a new window]
 
Fig. 3. Enhanced sensitivity of tumors derived from E2F-1(wt)-producing p53-/- cells in response to high doses of ionizing radiation. Stably infected, puromycin-resistant cell pools derived from p53-/- fibrosarcoma cells infected with either vector control virus or with virus encoding E2F-1(wt) were injected into nude mice and tumors were allowed to expand to a volume of at least 0.175 cm3, at which time mice were given a whole-body irradiation of 5 Gy. Thereafter, tumor volumes were measured with a caliper at days 3, 4, and 5 after treatment. Data points, averages of four tumors with the volume of 175 mm3 on the day of irradiation; bars, ±25% of the value.

 
p53 is the most frequently mutated gene in human cancers, and p53 mutation is linked to the development of drug resistance (27) . The ability to restore p53 function may lead to successful treatment of certain tumors. Therefore, an alternative way to treat such tumors may be to induce cell death by p53-independent means, e.g., through expression of E2F-1 or the activation of the S-phase checkpoint studied here. Further understanding of the mechanism underlying the activation of this E2F-1-dependent S-phase checkpoint could lead to alternative therapies for tumors lacking p53.


    Materials and Methods
 TOP
 Abstract
 Introduction
 Results and Discussion
 Materials and Methods
 References
 
Cell Lines and Cell Culture.
The retroviral packaging cell line BOSC23 (31) was maintained in DMEM containing 10% FCS at 37°C in a 10% CO2 atmosphere. Cell lines of p53+/+ and p53-/- MEFs expressing E1A and T24 H-ras (28) were used at low passage numbers and cultured in DMEM containing 10% FCS and 10% bovine calf serum (HyClone Laboratories, Logan, UT) in a 5% CO2 atmosphere.

DNA Transfection, Retroviral Transfection, and Irradiation.
BOSC23 cells were transfected with a total of 30 µg of pBabe(puro) plasmid DNA or its derivatives (containing the indicated HA-tagged E2F-1 alleles) by calcium phosphate coprecipitation as described (31) . Medium containing the retrovirus was harvested 24–30 h following removal of the precipitate and used to infect the E1A/ras-transformed MEFs at 40% confluency (11 , 25) . Irradiation of cells with different doses was performed with a 6-MV linear accelerator at a dose rate of 2 Gy/min.

Clonogenic and Proliferation Assays.
For analysis of clonogenic survival after retroviral infection alone or in combination with irradiation, different numbers of singular cells were seeded 24 h after retroviral infection. Puromycin selection of infected cells (2 µg/ml medium) was initiated 6 h after seeding. Irradiation of the cells was performed 48 h after retroviral infection. Cells were then allowed to grow for 8–10 days before they were fixed in a solution of methanol/acetic acid (75%/25%, v/v) and stained with crystal violet. The number of seeded cells was adjusted to allow 50–100 colonies to grow after each treatment. Only colonies with more than 50 cells/colony were counted. The surviving fraction of treated cells was corrected for the plating efficiency after puromycin selection. For cell number counting, 24 hpi, 1 x 106 cells per 78-cm2 dish were plated, and cell numbers were determined with a hemocytometer after a 5-day proliferation-interval in presence of puromycin. All cultures of the different cell lines were still subconfluent at this time.

Whole-Cell Extracts, Western Blotting, and Antibodies.
The procedure for preparing whole cell extracts has been described (32) . Immunoblotting experiments were performed as described (33) . Mouse monoclonal antibody 16B12 recognizing the HA epitope was purchased from BAbCo (Richmond, CA). Cellular proteins were resolved by SDS-PAGE. Antibody detection was achieved by ECL enhanced chemiluminescence (Amersham, Switzerland), according to the manufacturer’s protocol.

Growth of Tumor Xenografts in Nude Mice.
Stably transfected p53-/- fibrosarcoma cells (2 x 106 cells) were injected into both flanks of 4–8-week-old athymic mice. Tumor volumes were determined from caliper measurements of tumor length (L) and (l), according to the formula (L x l2)/2. Tumors were allowed to expand to a volume of at least 0.175 cm3 (±25%) before irradiation. Mice were given a total body irradiation of 5 Gy using a Pantak Therapax 300 kV X-ray unit at 0.7 Gy/min.


    Acknowledgments
 
We thank the members of our laboratories for many helpful discussions and Hildegard Resch and Margarete Arras for technical support.


    Footnotes
 
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1 W. K. is a START fellow and is supported by the Swiss National Science Foundation. This research was supported in part by the Friedrich Miescher Institut and by a grant from the Swiss Cancer League (to S. B.). Back

2 To whom requests for reprints should be addressed, at Friedrich Miescher Institut, Maulbeerstrasse 66, CH-4058, Basel, Switzerland. Phone: 41 61 697 8620; Fax: 41 61 697 3976; E-mail: wilhelm.krek{at}fmi.ch Back

3 The abbreviations used are: wt, wild-type; MEF, mouse embryo fibroblast; HA, hemagglutinin; hpi, hours postinfection. Back

Received for publication 9/15/98. Revision received 1/ 5/99. Accepted for publication 1/11/99.


    References
 TOP
 Abstract
 Introduction
 Results and Discussion
 Materials and Methods
 References
 

  1. Dyson N. The regulation of E2F by pRB-family members. Genes Dev., 12: 2245-2262, 1998.[Free Full Text]
  2. Helin K. Regulation of cell proliferation by the E2F transcription factors. Curr. Opin. Genet. Dev., 8: 28-35, 1998.[Medline]
  3. Sellers W. R., Rodgers J. W., Kaelin W. G. A potent transrepression domain in the retinoblastoma protein induces a cell cycle arrest when bound to E2F sites. Proc. Natl. Acad. Sci. USA, 92: 11544-11548, 1995.[Abstract/Free Full Text]
  4. Weintraub S. J., Chow K. N. B., Luo R. X., Zhang S. H., He S., Dean S. C. Mechanism of active transcriptional repression by the retinoblastoma protein. Nature (Lond.), 375: 812-815, 1995.[Medline]
  5. Weinberg R. A. The retinoblastoma protein and cell cycle control. Cell, 81: 323-330, 1995.[Medline]
  6. Xu M., Sheppard K. A., Peng C. Y., Yee A. S., Piwnica-Worms H. Cyclin A/CDK2 binds directly to E2F-1 and inhibits the DNA-binding activity of E2F-1/DP-1 by phosphorylation. Mol. Cell. Biol., 14: 8420-8431, 1994.[Abstract/Free Full Text]
  7. Krek W., Ewen M. E., Shirodkar S., Arany Z., Kaelin W. G., Jr., Livingston D. M. Negative regulation of the growth-promoting transcription factor E2F-1 by a stably bound cyclin A-dependent protein kinase. Cell, 78: 161-172, 1994.[Medline]
  8. Dynlacht B. D., Flores O., Lees J. A., Harlow E. Differential regulation of E2F transactivation by cyclin/cdk2 complexes. Genes Dev., 8: 1772-1786, 1994.[Abstract/Free Full Text]
  9. Marti, A., Wirbelauer, C., Scheffner, M., and Krek, W. Interaction between SCFSKP2 ubiquitin protein ligase and E2F-1 underlies regulation of E2F-1 degradation. Nat. Cell Biol., in press, 1999.
  10. Singh P., Wong S., Hong W. Overexpression of E2F-1 in rat embryo fibroblasts leads to neoplastic transformation. EMBO J., 13: 3329-3338, 1994.[Medline]
  11. Xu G., Livingston D. M., Krek W. Multiple members of the E2F transcription factor family are the products of oncogenes. Proc. Natl. Acad. Sci. USA, 92: 1357-1361, 1995.[Abstract/Free Full Text]
  12. Johnson D. G., Ohtani K., Nevins J. R. Oncogenic capacity of the E2F-1 gene. Proc. Natl. Acad. Sci. USA, 91: 12823-12827, 1994.[Abstract/Free Full Text]
  13. Johnson D. G., Schwarz J. K., Cress W. D., Nevins J. R. Expression of transcription factor E2F-1 induced quiescent cells to enter S phase. Nature (Lond.), 365: 349-352, 1993.[Medline]
  14. Qin X-Q., Livingston D. M., Kaelin W. G., Adams P. Deregulated transcription factor E2F-1 expression leads to S phase entry and p53-mediated apoptosis. Proc. Natl. Acad. Sci. USA, 91: 10918-10922, 1994.[Abstract/Free Full Text]
  15. Shan B., Lee W-H. Deregulated expression of E2F-1 induces S phase entry and leads to apoptosis. Mol. Cell. Biol., 14: 8166-8173, 1994.[Abstract/Free Full Text]
  16. DeGregori J. L. G., Miron A., Jakoi L., Nevins J. R. Distinct roles for E2F proteins in cell growth control and apoptosis. Proc. Natl. Acad. Sci. USA, 94: 7245-7250, 1997.[Abstract/Free Full Text]
  17. Zindy F., Eischen C. M., Randle D. H., Kamijo T., Cleveland J. L., Sherr C. J., Roussel M. F. Myc signaling via the ARF tumor suppressor regulates p53-dependent apoptosis and immortalization. Genes Dev., 12: 2424-2433, 1998.[Abstract/Free Full Text]
  18. Bates S., Phillips A. C., Clark P. A., Stott F., Peters G., Ludwig R. L., Vousden K. H. p14ARF links the tumour suppressors RB and p53. Nature (Lond.), 395: 124-125, 1998.[Medline]
  19. Kowalik T. F., DeGregori J., Leone G., Jakoi L., Nevins J. R. E2F1-specific induction of apoptosis and p53 accumulation, which is blocked by Mdm2. Cell Growth Differ., 9: 113-118, 1998.[Abstract]
  20. Kowalik T. F., DeGregori J., Schwarz J. K., Nevins J. R. E2F-1 overexpression in quiescent fibroblasts leads to induction of DNA synthesis and apoptosis. J. Virol., 69: 2491-2500, 1995.[Abstract/Free Full Text]
  21. Wu X., Levine A. J. p53 and E2F-1 cooperate to mediate apoptosis. Proc. Natl. Acad. Sci. USA, 91: 3602-3606, 1994.[Abstract/Free Full Text]
  22. Phillips A. C., Bates S., Ryan K. M., Helin K., Vousden K. H. Induction of DNA synthesis and apoptosis are separable functions of E2F-1. Genes Dev., 11: 1853-1863, 1997.[Abstract/Free Full Text]
  23. Nip J., Strom D. K., Fee B. E., Zambetti G., Cleveland J., Hiebert S. W. E2F-1 cooperates with topoisomerase II inhibition and DNA damage to selectively augment p53-independent apoptosis. Mol. Cell. Biol., 17: 1049-1056, 1997.[Abstract/Free Full Text]
  24. Hsieh J-K., Fredersdorf S., Kouzarides T., Martin K., Lu X. E2F-1-induced apoptosis requires DNA binding but not transactivation and is inhibited by the retinoblastoma protein through direct interaction. Genes Dev., 11: 1840-1852, 1997.[Abstract/Free Full Text]
  25. Krek W., Xu G., Livingston D. M. Cyclin A-kinase regulation of E2F-1 DNA binding function underlies suppression of an S phase checkpoint. Cell, 83: 1149-1158, 1995.[Medline]
  26. Levine A. p53, the cellular gatekeeper for growth and division. Cell, 88: 323-331, 1997.[Medline]
  27. Lowe S. L., Ruley H. E., Jacks T., Housman D. E. p53-dependent apoptosis modulates the cytotoxicity of anticancer agents. Cell, 74: 957-976, 1993.[Medline]
  28. Lowe S. W., Bodis S., McClattchey A., Remington L., Ruley H. E., Fisher D. E., Housman D. E., Jacks T. p53 status and the efficacy of cancer therapy in vivo. Science (Washington DC), 266: 806-810, 1994.
  29. Fisher D. E. Apoptosis in cancer therapy: crossing the threshold. Cell, 78: 539-542, 1994.[Medline]
  30. Samuelson A. V., Lowe S. W. Selective induction of p53 and chemosensitivity in RB-deficient cells by E1A mutants unable to bind the RB-related proteins. Proc. Natl. Acad. Sci. USA, 94: 12094-12099, 1997.[Abstract/Free Full Text]
  31. Pear W. S., Nolen G. P., Scott M. L., Baltimore D. Production of high-titer helper-free retroviruses by transient transfection. Proc. Natl. Acad. Sci. USA, 90: 8392-8396, 1993.[Abstract/Free Full Text]
  32. Krek W., Livingston D. M., Shirodkar S. Binding to DNA and the retinoblastoma gene product promoted by complex formation of different E2F family members. Science (Washington DC), 262: 1557-1560, 1993.[Abstract/Free Full Text]
  33. Lisztwan J., Marti A., Sutterluety H., Gstaiger M., Wirbelauer C., Krek W. Association of human CUL-1 and ubiquitin-conjugating enzyme CDC34 with the F-box protein p45SKP2: evidence for evolutionary conservation in the subunit composition of the CDC34-SCF pathway. EMBO J., 17: 368-383, 1998.[Medline]



This article has been cited by other articles:


Home page
J Biol ChemHome page
J. C. Paik, B. Wang, K. Liu, J. K. Lue, and W.-C. Lin
Regulation of E2F1-induced Apoptosis by the Nucleolar Protein RRP1B
J. Biol. Chem., February 26, 2010; 285(9): 6348 - 6363.
[Abstract] [Full Text] [PDF]


Home page
Mol Cancer ResHome page
T. S. Udayakumar, P. Hachem, M. M. Ahmed, S. Agrawal, and A. Pollack
Antisense MDM2 Enhances E2F1-Induced Apoptosis and the Combination Sensitizes Androgen-Dependent and Androgen-Independent Prostate Cancer Cells to Radiation
Mol. Cancer Res., November 1, 2008; 6(11): 1742 - 1754.
[Abstract] [Full Text] [PDF]


Home page
Clin. Cancer Res.Home page
J. Lee, C. K. Park, J. O. Park, T. Lim, Y. S. Park, H. Y. Lim, I. Lee, T. S. Sohn, J. H. Noh, J. S. Heo, et al.
Impact of E2F-1 Expression on Clinical Outcome of Gastric Adenocarcinoma Patients with Adjuvant Chemoradiation Therapy
Clin. Cancer Res., January 1, 2008; 14(1): 82 - 88.
[Abstract] [Full Text] [PDF]


Home page
J Biol ChemHome page
C. Wang, F. J. Rauscher III, W. D. Cress, and J. Chen
Regulation of E2F1 Function by the Nuclear Corepressor KAP1
J. Biol. Chem., October 12, 2007; 282(41): 29902 - 29909.
[Abstract] [Full Text] [PDF]


Home page
Clin. Cancer Res.Home page
B. Hofstetter, V. Vuong, A. Broggini-Tenzer, S. Bodis, I. F. Ciernik, D. Fabbro, M. Wartmann, G. Folkers, and M. Pruschy
Patupilone Acts as Radiosensitizing Agent in Multidrug-Resistant Cancer Cells In vitro and In vivo
Clin. Cancer Res., February 15, 2005; 11(4): 1588 - 1596.
[Abstract] [Full Text] [PDF]


Home page
Cancer Res.Home page
J. Jiang, C. B. Matranga, D. Cai, V. M. Latham Jr., X. Zhang, A. M. Lowell, F. Martelli, and G. I. Shapiro
Flavopiridol-Induced Apoptosis during S Phase Requires E2F-1 and Inhibition of Cyclin A-Dependent Kinase Activity
Cancer Res., November 1, 2003; 63(21): 7410 - 7422.
[Abstract] [Full Text] [PDF]


Home page
EMBO J.Home page
P. F. de Borja, N. K. Collins, P. Du, J. Azizkhan-Clifford, and M. Mudryj
Cyclin A-CDK phosphorylates Sp1 and enhances Sp1-mediated transcription
EMBO J., October 15, 2001; 20(20): 5737 - 5747.
[Abstract] [Full Text] [PDF]


Home page
Cancer Res.Home page
K. Zaugg, S. Rocha, H. Resch, I. Hegyi, C. Oehler, C. Glanzmann, D. Fabbro, S. Bodis, and M. Pruschy
Differential p53-dependent Mechanism of Radiosensitization in Vitro and in Vivo by the Protein Kinase C-specific Inhibitor PKC412
Cancer Res., January 1, 2001; 61(2): 732 - 738.
[Abstract] [Full Text]


This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Pruschy, M.
Right arrow Articles by Krek, W.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Pruschy, M.
Right arrow Articles by Krek, W.


HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Cancer Research Clinical Cancer Research
Cancer Epidemiology Biomarkers & Prevention Molecular Cancer Therapeutics
Molecular Cancer Research Cell Growth & Differentiation