Distinct and Overlapping Roles for E2F Family Members in Transcription, Proliferation and Apoptosis

Since the discovery almost fifteen years ago that E2F transcription factors are key targets of the retinoblastoma protein (RB), studies of the E2F family have uncovered critical roles in the control of transcription, cell cycle and apoptosis. E2F proteins are encoded by at least eight genes, E2F1 through E2F8. While specific roles for individual E2Fs in mediating the effects of RB loss are emerging, it is also becoming clear that there are no simple divisions of labor among the E2F family. Instead, an individual E2F can function to activate or repress transcription, promote or impede cell cycle progression and enhance or inhibit cell death, dependent on the cellular context. While functional redundancy among E2Fs and the striking influences of cellular context on the effects of E2F loss or gain of function have prevented a simple delineation of unique functions within the E2F family, these complexities undoubtedly reflect the extensive regulation and importance of this transcription factor family.

Putting the Oncogenic and Tumor Suppressive Activities of E2F into Context

Deregulation of E2F transcriptional activity as a result of alterations in the p16(INK4a)-cyclin D1-Rb pathway is a hallmark of human cancer. E2F is a family of related factors that controls the expression of genes important for cell cycle progression as well as other processes such as apoptosis, DNA repair, and differentiation. Some E2F family members are associated with the activation of transcription and the promotion of proliferation while others are implicated in repressing transcription and inhibiting cell growth. It is now becoming clear however, that this view of the E2F family is overly simplistic and that the role of a given E2F in regulating transcription and cell growth is highly dependent on context. This complexity is also evident when analyzing how perturbations in E2F modulate tumor development. As expected, some E2F family members are found to be critical for mediating the oncogenic effects of Rb loss. On the other hand, several E2Fs have tumor suppressive properties in mouse models and this appears to be reflected in some human cancers with decreased E2F expression. Surprisingly, tumor suppressive activity is not associated with the repressor E2Fs but instead is associated with the same E2Fs shown to have oncogenic activities. For example, deregulated E2F1 expression can either promote or inhibit tumorigenesis depending on the nature of the other oncogenic mutations that are present. Thus, the ability of some E2F family members to behave as both oncogene and tumor suppressor gene can be reconciled by putting E2F into context.

Gram-positive rod surveillance for early anthrax detection

Connecticut established telephone-based gram-positive rod (GPR) reporting primarily to detect inhalational anthrax cases more quickly. From March to December 2003, annualized incidence of blood isolates was 21.3/100,000 persons; reports included 293 Corynebacterium spp., 193 Bacillus spp., 73 Clostridium spp., 26 Lactobacillus spp., and 49 other genera. Around-the-clock GPR reporting has described GPR epidemiology and enhanced rapid communication with clinical laboratories.

Oncogenes and the DNA Damage Response: Myc and E2F1 Engage the ATM Signaling Pathway to Activate p53 and Induce Apoptosis

Activation of the ATM DNA damage response pathway is commonly observed in a variety of early-stage neoplasias. It has been proposed that this checkpoint response functions to suppress the development of cancer. A recent report from our laboratory demonstrates that ATM does indeed function to suppress tumorigenesis by responding to at least some oncogenic stresses. Transgenic expression of Myc is found to cause DNA damage in vivo and ATM is shown to respond to this damage by inducing the accumulation and phosphorylation of p53. In the absence of ATM, p53-dependent apoptosis is reduced and epithelial tumorigenesis is accelerated in Myc transgenic mice. Deregulated expression of the E2F1 transcription factor also elicits an ATM-dependent checkpoint response that activates p53 and promotes apoptosis, although the mechanism by which E2F1 and Myc stimulate ATM may differ. These findings have relevance for understanding why the ATM pathway is activated in many human cancers, what generates the selective pressure for p53 inactivation during tumorigenesis, and why AT patients and carriers are predisposed to developing cancer.

E2F1 suppresses skin carcinogenesis via the ARF-p53 pathway

The E2F1 transcription factor, which is deregulated in most human cancers by mutations in the p16-cyclin D-Rb pathway, has both oncogenic and tumor-suppressive properties. This is dramatically illustrated by the phenotype of an E2F1 transgenic mouse model that spontaneously develops tumors in the skin and other epithelial tissues but is resistant to papilloma formation when subjected to a two-stage carcinogenesis protocol. Here, this E2F1 transgenic model was used to further explore the tumor-suppressive property of E2F1. Transgenic expression of E2F1 was found to inhibit ras-driven skin carcinogenesis at the promotion stage independent of the type of promoting agent used. E2F1 transgenic epidermis displayed increased expression of p19(ARF), p53, and p21(Cip1). Inactivation of either p53 or Arf in E2F1 transgenic mice restored sensitivity to two-stage skin carcinogenesis. While Arf inactivation impaired tumor suppression and p21 induction by E2F1, it did not reduce the level of apoptosis observed in E2F1 transgenic mice. Based on these findings, we propose that E2F1 suppresses ras-driven skin carcinogenesis through a nonapoptotic mechanism involving ARF and p53.

ATM promotes apoptosis and suppresses tumorigenesis in response to Myc

Overexpression of the c-myc oncogene contributes to the development of a significant number of human cancers. In response to deregulated Myc activity, the p53 tumor suppressor is activated to promote apoptosis and inhibit tumor formation. Here we demonstrate that p53 induction in response to Myc overexpression requires the ataxia-telangiectasia mutated (ATM) kinase, a major regulator of the cellular response to DNA double-strand breaks. In a transgenic mouse model overexpressing Myc in squamous epithelial tissues, inactivation of Atm suppresses apoptosis and accelerates tumorigenesis. Deregulated Myc expression induces DNA damage in primary transgenic keratinocytes and the formation of gammaH2AX and phospho-SMC1 foci in transgenic tissue. These findings suggest that Myc overexpression causes DNA damage in vivo and that the ATM-dependent response to this damage is critical for p53 activation, apoptosis, and the suppression of tumor development.

E2F3a stimulates proliferation, p53-independent apoptosis and carcinogenesis in a transgenic mouse model

Mutation or inactivation of the retinoblastoma (Rb) tumor suppressor occurs in most human tumors and results in the deregulation of several members of the E2F family of transcription factors. Among the E2F family, E2F3 has been implicated as a key regulator of cell proliferation and E2F3 gene amplification and overexpression is detected in some human tumors. To study the role of E2F3 in tumor development, we established a transgenic mouse model expressing E2F3a in a number of epithelial tissues via a keratin 5 (K5) promoter. Transgenic expression of E2F3a leads to hyperproliferation, hyperplasia and increased levels of p53-independent apoptosis in transgenic epidermis. Consistent with data from human cancers, the E2F3a transgene is found to have a weak oncogenic activity on its own and to significantly enhance the response to a skin carcinogenesis protocol. The phenotype of K5 E2F3a transgenic mice is distinct from similar transgenic mice expressing E2F1 or E2F4. In particular, E2F3a has a unique apoptotic activity and lacks the tumor suppressive property of E2F1 in this model system.

Expression of transcription factor E2F1 and telomerase in glioblastomas: mechanistic linkage and prognostic significance

BACKGROUND

Several tumor suppressor pathways have been identified as modulators of telomerase function. We examined the functional role of the retinoblastoma-E2F1 pathway in regulating telomerase activity in malignant gliomas.

METHODS

Adenovirus vectors were used to transfer cDNAs into human glioblastoma and sarcoma cells. Telomerase activity was assessed with a telomere repeat amplification protocol. Promoter activity in cancer cells was assessed with promoter-luciferase reporter constructs. Promoter binding was assessed with the chromatin immunoprecipitation (ChIP) assay. We isolated astrocytes from E2F1 transgenic mice and normal mice for in vivo studies. We evaluated the expression of E2F1 and hTERT (the catalytic subunit of human telomerase) mRNAs by reverse transcriptase-polymerase chain reaction and proteins in human glioblastoma samples by immunoblot analysis. Associations between survival among 61 glioblastoma multiforme patients and expression of E2F1 and hTERT mRNA and protein were examined with Kaplan-Meier analysis, the log-rank test, and Cox proportional hazards regression models. All statistical tests were two-sided.

RESULTS

Ectopic E2F1 expression increased hTERT promoter activity in cancer cells. We detected an interaction between E2F1 protein and the hTERT promoter. Transgenic E2F1 astrocytes contained functional telomerase protein. E2F1 mRNA expression and hTERT mRNA expression were statistically significantly correlated in human glioblastoma specimens (R = .8; P < .001). Longer median survival was statistically significantly associated with lower E2F1 mRNA expression in tumors (103.6 weeks) rather than with higher expression (46.1 weeks) (difference = 57.5 weeks; 95% confidence interval [CI] = 14.7 to 159.7; log-rank P = .002). E2F1 mRNA was the only factor that was statistically significantly associated with overall survival in a multivariable model (P = .04). Among 27 patients with glioblastoma multiforme samples, the expression of E2F1 protein was statistically significantly associated with survival (log-rank P < .001).

CONCLUSIONS

E2F1 may participate in telomerase activity regulation in malignant glioma cells. Its expression appears to be strongly associated with the survival of patients with malignant brain tumors.

Regulation of epidermal apoptosis and DNA repair by E2F1 in response to ultraviolet B radiation

The E2F1 transcription factor regulates the expression of genes involved in cell proliferation, apoptosis and DNA repair. Following DNA damage, E2F1 is phosphorylated and stabilized, but the physiological role of E2F1 in the response to DNA damage is unclear. We find that mice lacking E2F1 have increased levels of epidermal apoptosis compared to wild-type mice following exposure to ultraviolet B (UVB) radiation. Moreover, transgenic overexpression of E2F1 in basal layer keratinocytes suppresses apoptosis induced by UVB. Inhibition of UVB-induced apoptosis by E2F1 is unexpected given that most studies have demonstrated a proapoptotic function for E2F1. E2F1-mediated suppression of apoptosis does not involve alterations in mitogen-activated protein kinase activation or Bcl-2 downregulation in response to UVB and is independent of p53. Instead, inhibition of UVB-induced apoptosis by E2F1 correlates with a stimulation of DNA repair. Mice lacking E2F1 are impaired for the removal of DNA photoproducts, while E2F1 transgenic mice repair UVB-induced DNA damage at an accelerated rate compared to wild-type mice. These findings suggest that E2F1 participates in the response to UVB by promoting DNA repair and suppressing apoptosis.

Ruptured “giant” supratentorial dermoid cyst

Intracranial dermoid and epidermoid cysts are rare lesions formed from the inclusion of ectodermal elements during neural tube closure. Although not entirely consistent, imaging characteristics on CT and MRI can aid differentiation of dermoids and epidermoids, as can age at presentation, location and tendency to rupture. The distinction between dermoid and epidermoid lesions is important prognostically and may impact on surgical management as a subtotally resected dermoid is less likely to recur than its epidermoid counterpart. The distinction of large dermoid lesions as "giant" adds little to information regarding the natural history or prognosis of these lesions and should be abandoned.

SAGE profiling of UV-induced mouse skin squamous cell carcinomas, comparison with acute UV irradiation effects

Ultraviolet (UV) irradiation is the primary environmental insult responsible for the development of most common skin cancers. To better understand the multiple molecular events that contribute to the development of UV-induced skin cancer, in a first study, serial analysis of gene expression (SAGE) was used to compare the global gene expression profiles of normal SKH-1 mice epidermis with that of UV-induced squamous cell carcinomas (SCCs) from SKH-1 mice. More than 200 genes were found to be differentially expressed in SCCs compared to normal skin (P < 0.0005 level of significance). As expected, genes related to epidermal proliferation and differentiation were deregulated in SCCs relative to normal skin. However, various novel genes, not previously associated with skin carcinogenesis, were also identified as deregulated in SCCs. Northern blot analyses on various selected genes validated the SAGE findings: caspase-14 (reduced 8.5-fold in SCCs); cathepsins D and S (reduced 3-fold and increased 11.3-fold, respectively, in SCCs); decorin, glutathione S-transferase omega-1, hypoxia-inducible factor 1 alpha, insulin-like growth factor binding protein-7, and matrix metalloproteinase-13 (increased 18-, 12-, 12-, 18.3-, and 11-folds, respectively, in SCCs). Chemokine (C-C motif), ligand 27 (CCL27), which was found downregulated 12.7-fold in SCCs by SAGE, was also observed to be strongly downregulated 6-24 h after a single and multiple UV treatments. In a second independent study we compared the expression profile of UV-irradiated versus sham-treated SKH-1 epidermis. Interestingly, numerous genes determined to be deregulated 8 h after a single UV dose were also deregulated in SCCs. For instance, genes whose expression was upregulated both after acute UV-treated skin and SCCs included keratins 6 and 16, small proline-rich proteins, and S100 calcium binding protein A9. Studies like those described here do not only provide insights into genes and pathways involved in skin carcinogenesis but also allow us to identify early UV irradiation deregulated surrogate biomarkers of potential use in chemoprevention studies.

The American Jobs Creation Act and its impact on deferred compensation: reassessment from a business perspective

The American Jobs Creation Act (AJCA), which was signed into law in October 2004, will have an impact on almost every deferred compensation program in the United States. This article argues that as companies continue to evaluate the transition alternatives under AJCA and contemplate the necessary changes to the plan program, companies also should consider simultaneously addressing broader issues surrounding nonqualified deferred compensation arrangements. These include ongoing business purpose, financial planning considerations, education of participants, corporate governance considerations and the potential implications to international assignees.

Role of sympathetic neural activation in age- and habitual exercise-related differences in the thermic effect of food

The thermic effect of food (TEF) declines with advancing age in adult humans but is enhanced in the habitually exercising state. The responsiveness of the sympathetic nervous system (SNS) has been implicated in these differences in TEF. We tested the hypotheses that 1) the reduction in TEF with aging is associated with an attenuated SNS response to acute energy intake; and 2) the greater TEF observed in endurance exercise-trained adults is associated with an augmented SNS response. Four groups of healthy men were studied: 16 young and 11 older sedentary men and nine young and 10 older habitually exercising men. Metabolic rate (indirect calorimetry, ventilated hood), skeletal muscle sympathetic nerve activity (MSNA; peroneal microneurography), and plasma norepinephrine and plasma epinephrine concentrations were measured before and for up to 4 h after ingestion of a carbohydrate drink (2.5 g/kg fat-free mass). TEF was approximately 50% greater in young compared with older men (P < 0.05) and approximately 25% greater in exercising compared with sedentary men (P < 0.05). In contrast, the MSNA, plasma norepinephrine, and plasma epinephrine responses were not different among the four groups. Covarying for MSNA did not significantly alter the observed differences in TEF. Habitual exercise status did not affect the age-associated decline in TEF. These findings demonstrate that altered postprandial whole-body and skeletal muscle SNS activation is not an important mechanism mediating either the reduction in TEF with aging or the augmented TEF associated with the exercise-trained state in healthy men. Differences in beta-adrenergic responsiveness to postprandial sympathoadrenal stimulation and/or nonsympathetic adrenergic influences likely explain the age- and habitual exercise-related differences in TEF.

Lack of cyclin-dependent kinase 4 inhibits c-myc tumorigenic activities in epithelial tissues

The proto-oncogene c-myc encodes a transcription factor that is implicated in the regulation of cellular proliferation, differentiation, and apoptosis and that has also been found to be deregulated in several forms of human and experimental tumors. We have shown that forced expression of c-myc in epithelial tissues of transgenic mice (K5-Myc) resulted in keratinocyte hyperproliferation and the development of spontaneous tumors in the skin and oral cavity. Although a number of genes involved in cancer development are regulated by c-myc, the actual mechanisms leading to Myc-induced neoplasia are not known. Among the genes regulated by Myc is the cyclin-dependent kinase 4 (CDK4) gene. Interestingly, previous studies from our laboratory showed that the overexpression of CDK4 led to keratinocyte hyperproliferation, although no spontaneous tumor development was observed. Thus, we tested the hypothesis that CDK4 may be one of the critical downstream genes involved in Myc carcinogenesis. Our results showed that CDK4 inhibition in K5-Myc transgenic mice resulted in the complete inhibition of tumor development, suggesting that CDK4 is a critical mediator of tumor formation induced by deregulated Myc. Furthermore, a lack of CDK4 expression resulted in marked decreases in epidermal thickness and keratinocyte proliferation compared to the results obtained for K5-Myc littermates. Biochemical analysis of the K5-Myc epidermis showed that CDK4 mediates the proliferative activities of Myc by sequestering p21Cip1 and p27Kip1 and thereby indirectly activating CDK2 kinase activity. These results show that CDK4 mediates the proliferative and oncogenic activities of Myc in vivo through a mechanism that involves the sequestration of specific CDK inhibitors.

E2F1 Uses the ATM Signaling Pathway to Induce p53 and Chk2 Phosphorylation and Apoptosis

The p53 tumor suppressor protein is phosphorylated and activated by several DNA damage-inducible kinases, such as ATM, and is a key effector of the DNA damage response by promoting cell cycle arrest or apoptosis. Deregulation of the Rb-E2F1 pathway also results in the activation of p53 and the promotion of apoptosis, and this contributes to the suppression of tumor development. Here, we describe a novel connection between E2F1 and the ATM DNA damage response pathway. In primary human fibroblasts lacking functional ATM, the ability of E2F1 to induce the phosphorylation of p53 and apoptosis is impaired. In contrast, ATM status has no effect on transcriptional activation of target genes or the stimulation of DNA synthesis by E2F1. Cells containing mutant Nijmegen breakage syndrome protein (NBS1), a component of the Mre11-Rad50 DNA repair complex, also have attenuated p53 phosphorylation and apoptosis in response to E2F1 expression. Moreover, E2F1 induces ATM- and NBS1-dependent phosphorylation of the checkpoint kinase Chk2 at Thr68, a phosphorylation site that stimulates Chk2 activity. Delayed γH2AX phosphorylation and absence of ATM autophosphorylation at Ser1981 suggest that E2F1 stimulates ATM through a unique mechanism that is distinct from agents that cause DNA double-strand breaks. These findings identify new roles for several DNA damage response factors by demonstrating that they also participate in the oncogenic stress signaling pathway between E2F1 and p53.

E2F1 uses the ATM signaling pathway to induce p53 and Chk2 phosphorylation and apoptosis

The p53 tumor suppressor protein is phosphorylated and activated by several DNA damage-inducible kinases, such as ATM, and is a key effector of the DNA damage response by promoting cell cycle arrest or apoptosis. Deregulation of the Rb-E2F1 pathway also results in the activation of p53 and the promotion of apoptosis, and this contributes to the suppression of tumor development. Here, we describe a novel connection between E2F1 and the ATM DNA damage response pathway. In primary human fibroblasts lacking functional ATM, the ability of E2F1 to induce the phosphorylation of p53 and apoptosis is impaired. In contrast, ATM status has no effect on transcriptional activation of target genes or the stimulation of DNA synthesis by E2F1. Cells containing mutant Nijmegen breakage syndrome protein (NBS1), a component of the Mre11-Rad50 DNA repair complex, also have attenuated p53 phosphorylation and apoptosis in response to E2F1 expression. Moreover, E2F1 induces ATM- and NBS1-dependent phosphorylation of the checkpoint kinase Chk2 at Thr68, a phosphorylation site that stimulates Chk2 activity. Delayed gammaH2AX phosphorylation and absence of ATM autophosphorylation at Ser1981 suggest that E2F1 stimulates ATM through a unique mechanism that is distinct from agents that cause DNA double-strand breaks. These findings identify new roles for several DNA damage response factors by demonstrating that they also participate in the oncogenic stress signaling pathway between E2F1 and p53.

Executive pay trends and golden parachute tax: a collision on the horizon

Ironically, many corporations will likely discover that tying equity-based executive compensation more closely to performance will cost millions of dollars when there is a merger or acquisition. The reason: Internal Revenue Code Section 280G, which is designed to discourage "excess" parachute payments, often assesses a significantly higher toll on performance-based compensation than on time-vested equity payments. There is no magic remedy, but advance planning can often help mitigate the impact. This article describes the dilemma and suggests several approaches to the challenge.

Comparison of feeding corn silages from leafy or conventional corn hybrids to lactating dairy cows

Three corn hybrids (Pioneer 36F30, Mycogen TMF2450, and Mycogen TMF2404) were compared for yield and quality traits, and lactation performance and apparent digestibility by Holstein cows. The three corn silages were harvested at a target of 33 to 35% dry matter. Before harvest, six corn plants were randomly selected for plant fractionation. Grain-to-stover ratios were 0.92, 0.70, and 0.95 for the 36F30, TMF2450, and TMF2404 corn plants, respectively. Fifty-two multiparous Holstein cows were placed on a 120-d lactation trial after a 21-d covariate diet. Cows were blocked by calving date and randomly assigned within block to one of three dietary treatments, containing approximately 40% (dry matter basis) corn silage. Milk yield, milk components, and dry matter intake did not differ among dietary treatments. In vitro true and neutral detergent fiber digestibilities were numerically higher for TMF2404 than the other corn silage hybrids. Apparent total-tract crude protein and neutral detergent fiber digestibilities, as measured by acid insoluble ash, were higher for TMF2450 than the other two hybrids, but starch digestibility was not different between the corn silage dietary treatments. Although small differences in nutrient content and digestibility existed among corn silage hybrids, inclusion of these leafy hybrids in lactating cow diets at 40% of the dietary dry matter did not have a significant impact on lactation performance of dairy cattle.

Tumor formation in mice with conditional inactivation of Brca1 in epithelial tissues

The BRCA1 tumor-suppressor protein has been implicated in the regulation of transcription, DNA repair, proliferation, and apoptosis. BRCA1 is expressed in many proliferative tissues and this is at least in part due to E2F-dependent transcriptional control. In this study, inactivation of a conditional murine Brca1 allele was achieved in a variety of epithelial tissues via expression of the Cre recombinase under the control of a keratin 5 (K5) promoter. The K5 Cre:Brca1 conditional knockout mice exhibited modest epidermal hyperproliferation, increased apoptosis, and were predisposed to developing tumors in the skin, the inner ear canal, and the oral epithelium after 1 year of age. Overexpression of the E2F1 transcription factor in K5 Cre:Brca1 conditional knockout mice dramatically accelerated tumor development. In addition, Brca1 heterozygous female mice that had elevated E2F1 expression developed tumors of the reproductive tract at high incidence. These findings demonstrate that in mice Brca1 functions as a tumor suppressor in other epithelial tissues in addition to the mammary gland. Moreover, inactivation of Brca1 is shown to cooperate with deregulation of the Rb-E2F1 pathway to promote tumorigenesis.

Inactivating E2f1 reverts apoptosis resistance and cancer sensitivity in Trp53-deficient mice

The E2f1 transcription factor, which regulates genes required for S-phase entry, also induces apoptosis by transcriptional and post-translational mechanisms. As E2f1 is inducible by DNA damage we investigated its importance in vivo in ultraviolet (UV)-induced apoptosis, a protective mechanism that prevents the epidermis from accumulating UV-induced mutations. Contrary to expectation, E2f1-/- mice demonstrated enhanced keratinocyte apoptosis after UVB exposure, whereas apoptosis was suppressed by epidermis-specific overexpression of human E2F1. Apoptosis induced by -radiation was also repressed by E2f1. E2f1-/-;Trp53-/- double knockout mice exhibited the elevated UVB-induced apoptosis of E2f1-/- alone, rather than the profound apoptosis defect seen in Trp53-/- mice, indicating that Trp53 (p53) lies functionally upstream of E2f1. Transfecting E2F1 into E2f1-/-;Trp53-/- primary fibroblasts suppressed UVB-induced apoptosis and this suppression was relieved by Trp53. The double knockout also reverted the abnormal sex ratio and early-onset tumours of Trp53-/- mice. These results imply that E2f1 functions as a suppressor of an apoptosis pathway that is initiated by DNA photoproducts and perhaps genetic abnormalities; p53 relieves this suppression.