Role for the p53 homologue p73 in E2F-1-induced apoptosis

p73 cooperates with Ras in the activation of MAP kinase signaling cascade

The p73 gene is capable of inducing cell cycle arrest, apoptosis, senescence, differentiation and to cooperate with oncogenic Ras in cellular transformation. Ras can be considered as a branch point in signal transduction, where diverse extracellular stimuli converge. The intensity of the mitogen-activated protein kinase (MAPK) cascade activation influences the cellular response to Ras. Despite the fundamental role of p53 in Ras-induced growth arrest and senescence, it remains unclear how the Ras/MEK/ERK pathway induces growth arrest in the absence of p53. We report here that oncogenic Ras stabilizes p73 resulting in p73 accumulation and enhancement of its activity. p73, in turn, induces a sustained activation of the MAP kinase cascade synergizing with oncogenic Ras. We also found that inhibition of p73 function modifies the cellular outcome to Ras activation inhibiting Ras-dependent differentiation. Here, we show for the first time that there is a signaling loop between Ras-dependent MAPK cascade activation and p73 function.

Increased DNA damage sensitivity and apoptosis in cells lacking the Snf5/Ini1 subunit of the SWI/SNF chromatin remodeling complex

The gene encoding the SNF5/Ini1 core subunit of the SWI/SNF chromatin remodeling complex is a tumor suppressor in humans and mice, with an essential role in early embryonic development. To investigate further the function of this gene, we have generated a Cre/lox-conditional mouse line. We demonstrate that Snf5 deletion in primary fibroblasts impairs cell proliferation and survival without the expected derepression of most retinoblastoma protein-controlled, E2F-responsive genes. Furthermore, Snf5-deficient cells are hypersensitive to genotoxic stress, display increased aberrant mitotic features, and accumulate phosphorylated p53, leading to elevated expression of a specific subset of p53 target genes, suggesting a role for Snf5 in the DNA damage response. p53 inactivation does not rescue the proliferation defect caused by Snf5 deficiency but reduces apoptosis and strongly accelerates tumor formation in Snf5-heterozygous mice.

E2F1-induced apoptosis: turning killers into therapeutics

The cellular transcription factor E2F1 is part of an anti-tumor safeguard mechanism: it engages cell-death pathways either alone or in cooperation with p53 to protect organisms from the development of tumors. E2F1 activates downstream factors, which in turn produce secondary changes in gene expression that trigger apoptosis. Although the mechanisms are incompletely understood, several studies have demonstrated that E2F1 is involved in many different aspects of programmed cell death depending on the cellular background. Here, these findings are highlighted in the context of the most recent follow-up studies that have used apoptotic E2F1 genes as new therapeutics or drug targets, thereby providing insight into the basic mechanisms of E2F1-induced apoptosis and its possible clinical implications.

E2F1 induces apoptosis and sensitizes human lung adenocarcinoma cells to death-receptor-mediated apoptosis through specific downregulation of c-FLIP(short)

E2F1 is a transcription factor that plays a well-documented role during S phase progression and apoptosis. We had previously postulated that the low level of E2F1 in primary lung adenocarcinoma contributes to their carcinogenesis. Here, we show that E2F1 triggers apoptosis in various lung adenocarcinoma cell lines by a mechanism involving the specific downregulation of the cellular FLICE-inhibitory protein short, leading to caspase-8 activation at the death-inducing signaling complex. Importantly, we also provide evidence that E2F1 sensitizes tumor as well as primary cells to apoptosis mediated by FAS ligand or tumor necrosis factor-related apoptosis-inducing ligand, and enhances the cytotoxic effect of T lymphocytes against tumor cells. Finally, we describe the specific overexpression of c-FLIP(S) in human lung adenocarcinomas with low level of E2F1. Overall, our data identify E2F1 as a critical determinant of the cellular response to death-receptor-mediated apoptosis, and suggest that its downregulation contributes to the immune escape of lung adenocarcinoma tumor cells.

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.

Regulation of telomerase activity by the p53 family member p73

The terminal ends of eukaryotic chromosomes, termed telomeres, progressively shorten during each round of cell division eventually leading cells into senescence. Tumor cells typically overcome this barrier to unlimited proliferation by activation of the human telomerase reverse transcriptase (hTERT) gene. In contrast, in most human somatic cells hTERT expression is tightly repressed by multiple tumor suppressors. Here, we studied the regulation of hTERT by the p53 family member p73. We show that forced expression of p73 or activation of endogenous p73 by E2F1 results in the downregulation of telomerase activity. Vice versa, siRNA-mediated knockdown of p73 induces hTERT expression. Responsiveness to p73 is conferred by Sp1 binding sites within the hTERT core promoter. In tumor cells, p73 isoforms lacking the transactivation domain (DeltaNp73) are frequently overexpressed and believed to function as oncogenes. We show that DeltaNp73 antagonizes the repressive effect of the proapoptotic p53 family members on hTERT expression and, in addition, induces hTERT expression in telomerase-negative cells by interfering with E2F-RB-mediated repression of the hTERT core promoter. These data provide evidence that the p73 gene functions as an important regulator of telomerase activity with implications for embryonic development, cellular differentiation and tumorigenesis.

DNA-damage response pathways triggered by viral replication

Many viruses, with distinct replication strategies, activate DNA-damage response pathways, including the lentivirus human immunodeficiency virus (HIV) and the DNA viruses Epstein-Barr virus (EBV), herpes simplex virus 1, adenovirus and SV40. DNA-damage response pathways involving DNA-dependent protein kinase, ataxia-telengiectasia mutated (ATM) and 'ataxia-telengiectasia and Rad3-related' (ATR) have all been implicated. This review focuses on the effects of HIV and EBV replication on DNA repair pathways. It has been suggested that activation of cellular DNA repair and recombination enzymes is beneficial for viral replication, as illustrated by the ability of suppressors of the ATM and ATR family to inhibit HIV replication. However, activation of DNA-damage response pathways can also promote apoptosis. Viruses can tailor the cellular response by suppressing downstream signalling from DNA-damage sensors, as exemplified by EBV. New small-molecule inhibitors of the DNA-damage response pathways could therefore be of value to treat viral infections.

Jab1 is a specificity factor for E2F1-induced apoptosis

The members of the E2F family of transcription factors are key regulators of genes involved in cell cycle progression, cell fate determination, DNA damage repair, and apoptosis. Many cell-based experiments suggest that E2F1 is a stronger inducer of apoptosis than the other E2Fs. Our previous work identified the E2F1 marked box and flanking region as critical for the specificity in E2F1 apoptosis induction. We have now used a yeast two-hybrid screen to identify proteins that bind the E2F1 marked box and flanking regions, with a potential role in E2F1 apoptosis induction. We identified Jab1 as an E2F1-specific binding protein and showed that Jab1 and E2F1 coexpression synergistically induce apoptosis, coincident with an induction of p53 protein accumulation. In contrast, Jab1 does not synergize with E2F1 to promote cell cycle entry. Cells depleted of Jab1 are deficient for both E2F1-induced apoptosis and induction of p53 accumulation. We suggest that Jab1 is an essential cofactor for the apoptotic function of E2F1.

Apoptosis signal-regulating kinase 1 is a direct target of E2F1 and contributes to histone deacetylase inhibitor-induced apoptosis through positive feedback regulation of E2F1 apoptotic activity

The oncogenic retinoblastoma protein (Rb)/E2F pathway links cellular proliferation control to apoptosis as a fail-safe mechanism to protect aberrant oncogenic transformation. We have previously shown that histone deacetylase inhibitors (HDACIs) activate the E2F1-Bim apoptotic pathway, leading to efficient cell killing in cancer cells with deregulated E2F1 activity. To identify additional gene cassettes that might contribute HDACI-induced apoptosis upon E2F1 activation, we investigated the apoptotic transcriptional network affected by HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) in cancer cells with inducible E2F1. Data analysis focusing on 220 apoptosis-related genes identified apoptosis signal-regulating kinase 1 (ASK1) as one of a few genes in addition to Bim that are substantially up-regulated by SAHA upon E2F1 activation. We show that ASK1 is directly regulated by E2F1 and that prevention of ASK1 induction by RNA interference decreases SAHA-induced apoptosis. We further show that the role of ASK1 in the SAHA apoptotic response is not associated with its downstream effectors p38 or JNK. Instead, ASK1 knockdown results in reduced E2F1 transcriptional activity, leading to decreased Bim induction by SAHA. Moreover, ASK1 expression reverses the negative effect of Rb on E2F1 activity. These results indicate that ASK1 induction by E2F1 provides positive feedback regulation of E2F1 activity via Rb inhibition, which allows an efficient E2F1-Bim activation. Thus, the concomitant induction of E2F1 targets ASK1 and Bim by HDACIs warrants an effective activation of E2F1-dependent apoptosis in response to SAHA.

p57Kip2 is Induced by MyoD Through a p73-dependent Pathway

The cyclin-dependent-kinase inhibitors p21 and p57 are highly expressed in skeletal muscle where they redundantly control cell cycle arrest during differentiation. We have previously shown that p57 is a target of the myogenic factor MyoD in cells lacking p21. Here we show that MyoD induces p57 at the transcriptional level through a mechanism different from that involved in p21 regulation, since it is E-box-independent and requires new synthesized protein(s). We have identified p73 family members as the factors that mediate the activation of p57 through a 165bp promoter region. The levels of p73 alpha, beta and delta isoforms increase during muscle differentiation both in MyoD-expressing fibroblasts and in spontaneously differentiating C2 myoblasts. Moreover, the expression of a p73 dominant negative mutant interferes with the induction of p57. Finally, each of the isoforms up-regulated by MyoD, even when over-expressed alone, is capable of inducing p57 in p21-lacking fibroblasts. In contrast, the same p73 isoforms, either induced by MyoD or exogenously over-expressed, are unable to activate the expression of p57 in p21-expressing fibroblasts. Our finding that a transfected p57 promoter-reporter construct, unlike the endogenous gene, is responsive to both MyoD and p73 even in these cells, suggests that a cis-acting mechanism, probably involving a repressive chromatin structure, prevents the induction of p57 in p21-expressing fibroblasts.

Gene therapy with E2F-1 up-regulates the protein kinase PKR and inhibits growth of leiomyosarcoma in vivo

Overexpression of the transcription factor E2F-1 induces apoptosis in a variety of carcinoma cells and inactivates murine double minute protein 2, a factor associated with poor prognosis in soft tissue sarcomas. We have shown previously that the double-stranded RNA-activated protein kinase PKR plays an important role in mediating this apoptotic response in carcinoma cells to E2F-1. We sought to evaluate the potential of E2F-1 gene therapy in soft tissue sarcomas and to study the involvement of PKR in the response to E2F-1 overexpression in mesenchymal cells. A replication-deficient adenovirus carrying the E2F-1 gene (Ad5E2F) was used to induce E2F-1 overexpression in the p53 mutated leiomyosarcoma cell line, SKLMS-1. Western blot analysis confirmed E2F-1 overexpression and up-regulation of the antiapoptotic factor Bcl-2 48 hours following infection with Ad5E2F. Apoptosis in Ad5E2F-treated cells was confirmed by fluorescence-activated cell sorting analysis and by poly(ADP-ribose) polymerase cleavage and DNA fragmentation assays. Vector-dependent up-regulation of PKR correlated with the amount of Ad5E2F-induced apoptosis. In vivo treatment of SKLMS-1 tumor-bearing BALB/c mice with intratumoral injections of Ad5E2F at a dose of 2 x 10(10) viral particles resulted in significant inhibition in tumor growth compared with control-treated animals (P < 0.016). Complete disappearance of all tumors was seen in two of seven mice in the Ad5E2F-treated animals. Immunohistochemical analysis of tumor specimens showed overexpression of E2F-1 and up-regulation of PKR in Ad5E2F-treated tumors. These findings show that adenovirus-mediated overexpression of E2F-1 results in up-regulation of PKR and significant growth suppression of leiomyosarcomas in vivo. Taken together, these data suggest that E2F-1 gene therapy and PKR modulation might be a promising treatment strategy for these tumors that are highly resistant to conventional therapies.

p73, the “Assistant” Guardian of the Genome?

Although p53 is clearly involved in the salvage pathway to DNA damage, its frequent mutations do not explain the efficacy of radiotherapy and chemotherapy. Indeed, around 50% of all human cancers show mutations in p53, and a further fraction show a functional inactivation of the protein. Nevertheless, patients seem to respond to therapy that would otherwise require a functional p53. At least in part, these responses could be explained by the pathway mediated by p73. This mechanism is parallel to, but independent of the p53 pathway. Several pieces of evidence show a significant interaction between these two proteins. Therefore, while p53 can be rightly defined as the guardian of the genome, we could think of p73 as the "assistant" guardian of the genome!

AZ703, an Imidazo[1,2-a]Pyridine Inhibitor of Cyclin-Dependent Kinases 1 and 2, Induces E2F-1-Dependent Apoptosis Enhanced by Depletion of Cyclin-Dependent Kinase 9

Preclinical studies were performed of a novel selective imidazopyridine cyclin-dependent kinase (cdk) inhibitor, AZ703. In vitro kinase assays showed that IC50 values for AZ703 against purified cyclin E/cdk2 and cyclin B/cdk1 were 34 and 29 nmol/L, respectively. In contrast, the IC50 against cdk4 was 10 micromol/L. AZ703 also inhibited cdk7 and cdk9 with IC50 values of 2.1 micromol/L and 521 nmol/L, respectively. Treatment of U2OS, NCI-H1299, and A549 cells for 24 hours resulted in growth arrest involving multiple cell cycle phases. At low drug concentrations (< 2 micromol/L), G2 arrest predominated, whereas at higher concentrations (> or = 2 micromol/L), S-G2 arrest was observed. When cells were synchronized in G1 by starvation and released into AZ703, a block in G1 occurred that was not evident in exponentially growing cells. Cell cycle arrest was associated with reduced phosphorylation of the retinoblastoma protein and p27(Kip1) at cdk2 phospho-sites. Following longer exposures, apoptosis was evident. Cells were further sensitized to AZ703 following recruitment to S phase by synchronization. Consistent with the inhibition of cdks during S and G2 that modulate the activity and stability of E2F-1, AZ703 treatment induced E2F-1 expression. In U2OS and NCI-H1299 cells engineered to inducibly express the dominant-negative mutant E2F-1 (1-374), expression of the mutant decreased AZ703-mediated apoptosis, indicating dependence on E2F-1 transcriptional targets. AZ703-induced apoptosis in NCI-H1299 cells was enhanced by small interfering RNA-mediated depletion of cdk9, which caused reduced levels of Mcl-1 and XIAP, suggesting that cdk2, cdk1, and cdk9 represent a rational subset of family members for drug targeting.

The retinoblastoma gene family in cell cycle regulation and suppression of tumorigenesis

Since its discovery in 1986, as the first tumor suppressor gene, the retinoblastoma gene (Rb) has been extensively studied. Numerous biochemical and genetic studies have elucidated in great detail the function of the Rb gene and placed it at the heart of the molecular machinery controlling the cell cycle. As more insight was gained into the genetic events required for oncogenic transformation, it became clear that the retinoblastoma gene is connected to biochemical pathways that are dysfunctional in virtually all tumor types. Besides regulating the E2F transcription factors, pRb is involved in numerous biological processes such as apoptosis, DNA repair, chromatin modification, and differentiation. Further complexity was added to the system with the discovery of p107 and p130, two close homologs of Rb. Although the three family members share similar functions, it is becoming clear that these proteins also have unique functions in differentiation and regulation of transcription. In contrast to Rb, p107 and p130 are rarely found inactivated in human tumors. Yet, evidence is accumulating that these proteins are part of a "tumor-surveillance" mechanism and can suppress tumorigenesis. Here we provide an overview of the knowledge obtained from studies involving the retinoblastoma gene family with particular focus on its role in suppressing tumorigenesis.

Over-expression of p73β results in apoptotic death of post-mitotic hNT neurons

The p53-related p73 protein is an important mediator of apoptosis, development and tumorigenesis. Previously, we showed that over-expression of the p73beta isoform induced apoptosis in proliferating neuronal cells; however, the study did not address the effect of p73 in post-mitotic neurons. To address this question, we used post-mitotic hNT neurons, which have been used as a model of human central nervous system neurons. We found that over-expression of p73beta in hNT neurons resulted in apoptosis and an increase in the expression of p57Kip2 and Bax, but no increase in p53 expression. These results suggest that apoptosis of post-mitotic neurons induced by p73beta may involve these mediators. Understanding the regulation of p73 expression will be important for understanding the development of the nervous system and may have implications for the treatment of neurological diseases.

Apoptosis induced by ultraviolet B in HPV-immortalized human keratinocytes requires caspase-9 and is death receptor independent

Ultraviolet B (UVB) induces both apoptosis and skin cancer. We found that human keratinocytes (KC) immortalized by Human Papillomavirus (HPV)16 E6/E7 were sensitized to UVB-induced apoptosis, possibly representing a transient regression-prone precancerous stage equivalent to actinic keratosis. To further examine which caspases are apical and essential, we utilized retroviral constructs expressing dominant-negative caspase-9 (caspase-9-DN) or Fas-associated protein with death domain (FADD)-DN as well as caspase inhibitor peptides. Caspase-9-DN and zLEHD-fmk both suppressed caspase-9, -3, and -8 activity after UVB exposure, as well as proteolytic processing of procaspase-3 into its active form, DNA fragmentation factor 45 cleavage, and internucleosomal DNA fragmentation. By contrast, stable expression of FADD-DN in HPV-immortalized KC did not inhibit UVB-induced activation of caspases-9, -3, and -8 nor downstream apoptotic events, although inhibition of caspase-8 with zIETD-fmk attenuated apoptosis. This study indicates that caspase-9 activation is upstream of caspases-3 and -8 and that UVB-induced apoptosis in HPV-immortalized human KC is death receptor (DR) independent and requires both caspase-9 upstream and caspase-8 downstream for maximal apoptosis. These studies further indicate that cell type as well as transformation state determine the sensitivity and mode of cell death (DR vs. mitochondrial apoptotic pathways) in response to UVB and explain the high regression rates of premalignant lesions.

E2F1 expression in LNCaP prostate cancer cells deregulates androgen dependent growth, suppresses differentiation, and enhances apoptosis

INTRODUCTION AND OBJECTIVES

To investigate the role of E2F/RB in androgen independent proliferation, differentiation, and sensitivity to apoptotic stimuli of LNCaP prostate cancer cells.

METHODS

The effects of E2F1 overexpression on androgen independent proliferation, differentiation, and apoptotic responses was assessed by flow cytometry, Western blot analysis and staining of nuclei.

RESULTS

Overexpression of E2F1 in LNCaP cells confers resistance to an androgen withdrawal-mediated growth arrest, prevents differentiation, and modifies apoptotic responses. Androgen independent proliferation is associated with a dose dependent elevation of cyclin E. Cells expressing high levels of E2F1 continue to express androgen receptor and have a diminished expression of neuronal specific enolase when cultured in androgen-depleted media. Additionally, E2F1-expressing cells are more sensitive to etoposide-induced apoptosis. Western blot analysis revealed that LNCaP-E2F1 cells have elevated expression of p73, Apaf-1, caspase-3, caspase-7, but expression of caspase-8 and -9, p14(ARF), and Mcl-1, is unaltered.

CONCLUSION

This is the first study that describes E2F1-dependent modifications of androgen dependence, differentiation, and sensitivity to apoptotic stimuli in LNCaP cells. Our analysis also identifies a subset of E2F1 targets that are instrumental in altering proliferative, differentiation, and apoptotic properties. Deregulation of the E2F/RB pathway and subsequent modification of key regulatory proteins may promote the development of hormone-refractory prostate tumors.

Drosophila E2F1 has context-specific pro- and antiapoptotic properties during development

E2F transcription factors are generally believed to be positive regulators of apoptosis. In this study, we show that dE2F1 and dDP are important for the normal pattern of DNA damage-induced apoptosis in Drosophila wing discs. Unexpectedly, the role that E2F plays varies depending on the position of the cells within the disc. In irradiated wild-type discs, intervein cells show a high level of DNA damage-induced apoptosis, while cells within the D/V boundary are protected. In irradiated discs lacking E2F regulation, intervein cells are largely protected, but apoptotic cells are found at the D/V boundary. The protective effect of E2F at the D/V boundary is due to a spatially restricted role in the repression of hid. These loss-of-function experiments demonstrate that E2F cannot be classified simply as a pro- or antiapoptotic factor. Instead, the overall role of E2F in the damage response varies greatly and depends on the cellular context.

Role of p73 in Regulating Human Caspase-1 Gene Transcription Induced by Interferon-γ and Cisplatin

Caspase-1, a cysteine protease is primarily involved in proteolytic activation of proinflammatory cytokines such as interleukin-1beta. It is also involved in some forms of apoptosis. Here we have analyzed the role of p73, a homolog of tumor suppressor p53, in regulating human caspase-1 gene transcription. The caspase-1 promoter was strongly activated by p73alpha and p73beta primarily through a p53/p73 responsive site. Overexpression of p73 by transient transfection increased the caspase-1 mRNA level. Treatment of cells with cisplatin (which increases p73 protein level) resulted in increased caspase-1 promoter activity and its mRNA level. Blocking of p73 function by a dominant negative mutant reduced basal as well as cisplatin-induced caspase-1 promoter activity. Mutation of the p73 responsive site abolished cisplatin-induced activation of the promoter. Interferon-gamma induced caspase-1 promoter activity and this was reduced by p73-directed small hairpin RNA and also by a dominant negative mutant of p73. Abrogation of the p73 responsive site partially inhibited interferon-gamma-induced activation of the caspase-1 promoter. Treatment of HeLa cells with interferon-gamma resulted in an increase in p73 protein as well as its activity. Mutation of the IRF-1 binding site abolished interferon-gamma-induced caspase-1 promoter activity but p73-induced activation was only marginally reduced. IRF-1 cooperated with p73 and cisplatin cooperated with interferon-gamma in the activation of the caspase-1 promoter. Our results show that p73 is a regulator of caspase-1 gene transcription, and is required for optimal activation of the caspase-1 promoter by interferon-gamma.

p73, a sophisticated p53 family member in the cancer world

p73 belongs to a family of p53-related nuclear transcription factors that includes p53, p73 and p63. The overall structure and sequence homology indicates that a p63/p73-like protogene is the ancestral gene, whereas p53 evolved later in higher organisms. In accordance with their structural similarity, p73 functions in a manner analogous to p53 by inducing tumor cell apoptosis and participating in the cell cycle checkpoint control through transactivating an overlapping set of p53/p73-target genes. In sharp contrast to p53, however, p73 is expressed as two NH(2)-terminally distinct isoforms including transcriptionally active (TA) and transcriptionally inactive (DeltaN) forms. DeltaNp73, which has oncogenic potential, acts in a dominant negative manner against TAp73 as well as p53. p73 is induced to be stabilized in response to a subset of DNA-damaging agents in a way that is distinct from that of p53, and exerts its pro-apoptotic activity. Several lines of evidence suggest that p73 can induce tumor cell apoptosis in a p53-dependent and p53-independent manner. Some tumors exhibit resistance to the p53-dependent apoptotic program, therefore p73, which can induce apoptotic cell death by p53-independent mechanisms, is particularly useful. In this review, we discuss the regulatory mechanisms of p73 activity, and also the functional significance of p73 in the regulation of cellular processes including tumorigenesis, apoptosis and neurogenesis.

Inhibitors of histone deacetylases target the Rb-E2F1 pathway for apoptosis induction through activation of proapoptotic protein Bim

Inhibitors of histone deacetylases (HDACIs) are a new generation of anticancer agents that selectively kill tumor cells. However, the molecular basis for their tumor selectivity is not well understood. We investigated the effects of HDACIs on the oncogenic Rb-E2F1 pathway, which is frequently deregulated in human cancers. Here, we report that cancer cells with elevated E2F1 activity, caused either by enforced E2F1 expression, or by E1A oncogene expression, are highly susceptible to HDACI-induced cell death. This E2F1-mediated apoptosis is neither p53- nor p73-dependent but proceeds through selective induction of proapoptotic BH3-only protein Bim. We show that Bim is a direct target of E2F1 and that HDAC inhibition promotes the recruitment of E2F1 to the Bim promoter. Moreover, silencing of Bim by specific small interfering RNA (siRNA) effectively abolishes the E2F1-mediated cell death sensitization to HDACIs. These findings suggest that the oncogenic E2F1 pathway participates in HDACIs-induced apoptosis in cancer cells and underscore the importance of Bim as a key mediator of oncogene-induced apoptosis. Our study provides an important insight into the molecular mechanism of tumor selectivity of HDACIs and predicts that, clinically, HDACIs will be more effective in tumors with high E2F1 activity.

Tumor promotion by caspase-resistant retinoblastoma protein

The retinoblastoma (RB) protein regulates cell proliferation and cell death. RB is cleaved by caspase during apoptosis. A mutation of the caspase-cleavage site in the RB C terminus has been made in the mouse Rb-1 locus; the resulting Rb-MI mice are resistant to endotoxin-induced apoptosis in the intestine. The Rb-MI mice do not exhibit increased tumor incidence, because the MI mutation does not disrupt the Rb tumor suppressor function. In this study, we show that Rb-MI can promote the formation of colonic adenomas in the p53-null genetic background. Consistent with this tumor phenotype, Rb-MI reduces colorectal epithelial apoptosis and ulceration caused by dextran sulfate sodium. By contrast, Rb-MI does not affect the lymphoma phenotype of p53-null mice, in keeping with its inability to protect thymocytes and splenocytes from apoptosis. The Rb-MI protein is expressed and phosphorylated in the tumors, thereby inactivating its growth suppression function. These results suggest that RB tumor suppressor function, i.e., inhibition of proliferation, is inactivated by phosphorylation, whereas RB tumor promoting function, i.e., inhibition of apoptosis, is inactivated by caspase cleavage.

Genetic and epigenetic alterations of RB2/p130 tumor suppressor gene in human sporadic retinoblastoma: implications for pathogenesis and therapeutic approach

Human retinoblastoma occurs in two forms (familial and sporadic) both due to biallelic mutation of the RB1/p105 gene even if its loss is insufficient for malignancy. We have recently reported that loss of expression of the retinoblastoma-related protein pRb2/p130 correlates with low apoptotic index, suggesting that RB2/p130 gene could be involved in retinoblastoma. Mutational analysis of RB2/p130 in primary tumors showed a tight correlation between Exon 1 mutations and pRb2/p130 expression level in sporadic retinoblastoma. These mutations are located within a CpG-enriched region prone to de novo methylation. Analysis of RB2/p130 methylation status revealed that epigenetic events, most probably consequent to the Exon 1 mutations, determined the observed phenotype. Treatment of Weri-Rb1 cell line by 5-Aza-dC induced an increase in expression level of pRb2/p130, E2F1, p73 and p53. Overall, our results highlight a crucial role of epigenetic events in sporadic retinoblastoma, which opens a perspective for new therapeutic approaches.

Rb/E2F: a two-edged sword in the melanocytic system

Rb is a tumor suppressor that represses the expression of E2F regulated genes required for cell cycle progression. It is inactivated in melanomas and other cancer cells by phosphorylation catalyzed by persistent cyclin dependent kinase (CDK) activity. CDK activity is sustained in melanoma cells mostly by the elimination of the CDK inhibitor p16INK4A and by high levels of cyclins whose expression is maintained by stimuli emanating from activated cell surface receptors and/or mutated intracellular intermediates, such as N-Ras and B-Raf. However, Rb also suppresses the expression of apoptosis genes, and its presence protects normal melanocytes from cell death. Its high expression in human melanoma cells and tumors suggests a similar role in malignant cells as well. The differential release and suppression of E2F transcriptional activity is likely to depend on promoter-specific E2F/Rb interaction. Phosphorylated Rb is displaced from cell cycle genes but not from others. In addition, Rb gene repression is dependent on the nature of Rb-E2F interaction and the activity of the Rb-bound proteins recruited to the promoter. Deciphering the differences in Rb/E2F complex formation in normal and malignant melanocytes is likely to shed light on the mechanism by which Rb can exert tumor suppressing and promoting activities in this cellular system. The Rb/E2F pathway provides opportunities for efficient therapy at multiple levels. Novel drugs can reactivate Rb potential to suppress growth cycle promoting genes. In addition, the high E2F transcriptional activity in melanoma cells can be exploited to deliver cytotoxic molecules specifically to tumors, sparing the normal tissues.

A novel mitochondrial protein DIP mediates E2F1-induced apoptosis independently of p53

The transcription factor E2F1 does not only induce cell proliferation but also shows the strongest proapoptotic effect of all E2F family members as part of an antitumor safeguard mechanism. We have recently identified KIAA0767 as a novel p53-independent target of E2F1. Here, we investigated the biological function of interaction. Overexpression studies of KIAA0767, termed D(eath)-I(nducing)-P(rotein), revealed its strong proapoptotic effect. DIP greatly reduced cell viability in several in vitro systems accompanied by typical apoptotic features such as caspase-3 activation and cleavage of poly(ADP-ribose)-polymerase. Endogenous DIP levels increased following E2F1 activation. Yet, inhibition of endogenous DIP function by small interfering RNA rescued p53-negative cells from E2F1-induced apoptosis, indicating that DIP is an essential mediator of the p53-independent E2F1 death pathway. Localization studies showed that DIP localizes to the mitochondria, where endogenous DIP is upregulated following E2F1 induction. These results provide new insights to the incompletely understood regulatory mechanisms of E2F1-induced apoptosis.

ASPP1 and ASPP2 are new transcriptional targets of E2F

The E2F family of transcription factors regulates the expression of a number of genes whose products are involved in cell cycle control, DNA replication and apoptosis. We show here that E2F-1 binds in vivo the promoters of ASPP1 and ASPP2 genes, two activators of p53-mediated apoptosis, E2F-1, E2F-2 and E2F-3 all activate the isolated ASPP1 and ASPP2 promoters. Overexpression or deregulation of E2F-1 increased the expression levels of ASPP1 and ASPP2 mRNA and proteins. The identification of ASPP1 and ASPP2 genes as transcriptional targets of E2F provides another mechanism by which E2F cooperates with p53 to induce apoptosis.

Testicular germ cell tumours: the paradigm of chemo-sensitive solid tumours

Testicular germ cell tumours (TGCTs) are the most frequent solid malignant tumour in men 20-40 years of age and the most frequent cause of death from solid tumours in this age group. Up to 50% of the patients suffer from metastatic disease at diagnosis. The majority of metastatic testicular cancer patients, in contrast to most other metastatic solid tumours, can be cured with highly effective cisplatin-based chemotherapy. From a genetic point of view, almost all TGCTs in contrast to solid tumours are characterised by the presence of wild type p53. High p53 expression levels are associated with elevated Mdm2 levels and a loss of p21(Waf1/Cip1) expression suggesting a changed functionality of p53. Expression levels of other proteins involved in the regulation of cell cycle progression indicate a deregulated G1-S phase checkpoint in TGCTs. After cisplatin-induced DNA damage, the increasing levels of p53 lead to the trans-activation of a number of genes but not of p21(Waf1/Cip1), preferentially directing TGCT cells into apoptosis or programmed cell death, both via the mitochondrial and the death receptor apoptosis pathways. The sensitivity of TGCTs to chemotherapeutic drugs may lay in the susceptibility of germ cells to apoptosis. Taken together, this provides TGCT as a tumour type model to investigate and understand the molecular determinants of chemotherapy sensitivity of solid tumours. This review aims to summarise the current knowledge on the biological basis of cisplatin-induced apoptosis and response to chemotherapy in TGCTs.

Significance of the Y-box proteins in human cancers

Y-box proteins belong to the cold shock domain family of proteins that are known to be involved in both transcriptional and translational control. Here, we give a brief overview of the structure, regulation and physiological functions of the Y-box proteins. This is followed by examining the role of Y-box protein 1 (YB-1), the most extensively studied of the Y-box protein in tumorigenesis, and its clinicopathological significance. YB-1 has the potential to be a prognostic marker and predictor of chemoresistance in human cancers.

Distinct capacities of individual E2Fs to induce cell cycle re-entry in postmitotic lens fiber cells of transgenic mice

PURPOSE

Inactivation of the retinoblastoma gene in human retinoblasts or mouse lens fiber cells causes inappropriate cell cycle entry, presumably as a consequence of elevated activity of the E2F transcription factors. Although E2Fs are known to be critical regulators of the cell cycle, it is still unclear whether family members E2F3a, E2F4 or E2F5 are individually capable of inducing cell cycle entry in vivo. In this study, we designed experiments to test whether lens-specific expression of these E2F family members would induce postmitotic fiber cells to re-enter the cell cycle.

METHODS

Transgenic mice were generated by microinjection of constructs that contained E2F cDNAs (E2F3a, E2F4 or E2F5) linked to the mouse lens-specific alphaA-crystallin promoter. The mice were characterized by histology, in situ hybridization, immunohistochemistry, BrdU incorporation, TUNEL assay and Western blots.

RESULTS

E2F3a expression was sufficient to induce cell cycle entry in lens fiber cells. Cell cycle re-entry was accompanied by apoptotic cell death resulting in microphthalmia. E2F4 expression stimulated a modest level of cell cycle re-entry, but the transgenic lenses remained normal in size and did not show significant apoptosis. Transgenic mice expressing E2F5 did not show lens defects. In both the E2F3a and E2F4 transgenic lenses, cyclin A2 and cyclin B1 expression were upregulated. Phosphorylated histone H3, a marker for mitosis, was detected in the E2F3a fiber cells. Western blots showed that both p53 and p73alpha were upregulated in the E2F3a lenses. However, expression of p21, a well-known p53 target gene, was not activated, suggesting that p73alpha might be responsible for inducing apoptosis and blocking unregulated proliferation in lens cells overexpressing E2F3a.

CONCLUSIONS

E2F3a and E2F4, but not E2F5, function to induce cell cycle entry, although E2F4 has more modest activity. E2F3a may induce cell death primarily through activation of p73alpha.

E2F1 is crucial for E2F-dependent apoptosis

Loss of the retinoblastoma protein, pRB, leads to apoptosis, and several results have suggested that this is dependent on the E2F transcription factors. However, so far, the ability of the different E2F family members to contribute to apoptosis is controversial. Here, we show that ectopic expression of E2F3 results in apoptosis in both primary mouse fibroblasts and transgenic mice. Apoptosis induced by E2F3 is associated with the accumulation of E2F1 and, strikingly, we found that E2F3-induced apoptosis is dependent on E2F1. On the basis of these results, we propose that the accumulation of crucial levels of E2F1 activity, and not total E2F activity, is essential for the induction of apoptosis in response to a deregulated pRB pathway. These results are consistent with previous findings that E2F1, but not other E2Fs, can have tumour-suppressing activities.

A role of p73 in mitotic exit

The p53-related p73 proteins regulate developmental processes, cell growth, and DNA damage response. p73 function is regulated by post-translational modifications and protein-protein interactions. At the G2/M transition, p73 is phosphorylated at Thr-86 by the p34cdc2/cyclin B complex; this is associated with its exclusion from condensed chromosomes and loss of DNA binding and transcriptional activation ability. Here we showed that p73 hypo-phosphorylated species reappear during mitotic exit, concomitant with p73 relocalization to telophase nuclei and recovered ability to activate transcription. Functional knock-out of p73 gene expression by small interfering RNAs (siRNAs) alters mitotic progression, yielding an increase of ana-telophase cells, the accumulation of aberrant late mitotic figures, and the appearance of abnormalities in the subsequent interphase. This p73 activity at the M-to-G1 transition is mediated by its transactivating function because expression of the transcription dominant negative mutant p73DD induces the same mitotic exit phenotype. We also found that the cyclin-dependent kinase inhibitor Kip2/p57 gene is a specific target of p73 regulation during mitotic exit and re-entry into G1. Both knock-out of p73 gene expression by siRNAs and abrogation of p73-dependent transcription by the p73DD mutant abrogate Kip2/p57 increase at the M-to-G1 transition. Moreover, similar abnormalities (e.g. delay in late mitotic stages with the accumulation of aberrant ana-telophase figures, and abnormalities in the following interphase) are observed in cultures in which the expression of Kip2/p57 is abrogated by siRNAs. These results identify a novel p73-Kip2/p57 pathway that coordinates mitotic exit and transition to G1.

E2F1−/−C57BL/6 Mice Overexpressing a Human Bcl-2 Transgene in B Cells Develop a Mild Autoimmune Syndrome

Bcl-2, besides having an anti-apoptotic function, delays cell cycle progression at G1 to S. Overexpression of Bcl-2 in B cells induces an autoimmune syndrome (AIS) in pro-autoimmune genetic backgrounds. E2F1, a member of the E2F transcription factors, controls cell cycle, but it also induces cell death. E2F1(-/-) mice show an altered negative thymic selection but a conserved peripheral tolerance. As a consequence, these mice do not develop autoimmunity. Our aim was to evaluate whether deregulation of both apoptosis and cell cycle alters the mechanisms of tolerance and induces an AIS. C57BL/6 E2F1(-/-) mice were crossed with C57BL/6 mice overexpressing a human Bcl-2 transgene in B cells to obtain E2F1(-/-) hbcl-2 Tg mice. These mice were followed for up to 15 months of age with bleedings every three months to obtain serum and whole blood. The production of an AIS was assessed by quantitation of serum anti-DNA antibodies, renal light microscopy, and direct immunofluorescence in search of immunoglobulin deposits. E2F1(-/-) hbcl-2 Tg mice developed an AIS characterized by anti-DNA autoantibody production with renal damage observed after the 9th month of age. The lesions consisted mainly on cellular proliferation and mesangial deposits, compatible with a mesangial glomerulonephritis. The composition of deposits was predominantly of IgA, followed by IgM and IgG. Despite the development of renal damage, the AIS observed did not induce an accelerated mortality. The coexistence of an altered B cell apoptosis, together with the lack of E2F1, induces a mild AIS in the non-autoimmune background of C57BL/6 mice.

p73-induced apoptosis: A question of compartments and cooperation

The transcriptionally active forms of p73 are capable of inducing apoptosis, and the isoforms termed TAp73 are important players when E2F and its oncogenic activators induce programmed cell death. However, the conditions under that TAp73 can kill a cell remain to be clarified. Recently, it has been found that p73 proteins are not merely floating in the nucleoplasm but rather can associate with specific compartments in the cell. Examples of intranuclear compartments associated with p73 proteins include the PML oncogenic domains and the nuclear matrix. In addition, p73 is found in the cytoplasm. It remains to be seen whether p73 might also associate with mitochondria, in analogy with p53. The relocalization of p73 is expected to be mediated by specific binding partners, mostly other proteins. Here, we discuss the possibility that the compartmentalization of p73, and the cooperation with the corresponding binding partners, might decide about its apoptosis-inducing activity.

RNA silencing of checkpoint regulators sensitizes p53-defective prostate cancer cells to chemotherapy while sparing normal cells

p53 is frequently mutated in patients with prostate cancer, especially in those with advanced disease. Therefore, the selective elimination of p53 mutant cells will likely have an impact in the treatment of prostate cancer. Because p53 has important roles in cell cycle checkpoints, it has been anticipated that modulation of checkpoint pathways should sensitize p53-defective cells to chemotherapy while sparing normal cells. To test this idea, we knocked down ataxia telangiectasia mutated (ATM) gene by RNA interference in prostate cancer cell lines and in normal human diploid fibroblasts IMR90. ATM knockdown in p53-defective PC3 prostate cancer cells accelerated their cell cycle transition, increased both E2F activity and proliferating cell nuclear antigen expression, and compromised cell cycle checkpoints, which are normally induced by DNA damage. Consequently, PC3 cells were sensitized to the killing effects of the DNA-damaging drug doxorubicin. Combining ATM knockdown with the Chk1 inhibitor UCN-01 further increased doxorubicin sensitivity in these cells. In contrast, the same strategy did not sensitize either IMR90 or LNCaP prostate cancer cells, both of which have normal p53. However, IMR90 and LNCaP cells became more sensitive to doxorubicin or doxorubicin plus UCN-01 when both p53 and ATM functions were suppressed. In addition, knockdown of the G(2) checkpoint regulators ATR and Chk1 also sensitized PC3 cells to doxorubicin and increased the expression of the E2F target gene PCNA. Together, our data support the concept of selective elimination of p53 mutant cells by combining DNA damage with checkpoint inhibitors and suggest a novel mechanistic insight into how such treatment may selectively kill tumor cells.

The E2F transcriptional network: old acquaintances with new faces

The E2 factor (E2F) family of transcription factors are downstream targets of the retinoblastoma protein. E2F factors have been known for several years to be important regulators of S-phase entry. Recent studies have improved our understanding of the molecular mechanisms of action used by this transcriptional network. In addition, they have given us an appreciation of the fact that E2F has functions that reach beyond G1/S control and impact cell proliferation in several different ways. The discovery of new family members with unusual properties, the unexpected phenotypes of mutant animals, a diverse collection of biological activities, a large number of new putative target genes and the new modes of transcriptional regulation have all contributed to an increasingly complex view of E2F function. In this review, we will discuss these recent developments and describe how they are beginning to shape a new and revised picture of the E2F transcriptional program.

Selection of novel mediators of E2F1-induced apoptosis through retroviral expression of an antisense cDNA library

The E2F1 transcription factor is an essential mediator of p53-dependent and p53-independent apoptosis as part of an anti-tumour safeguard mechanism. In this study, a functional so-called technical knockout (TKO) approach was applied to Saos-2ERE2F1 cells that conditionally activate E2F1 by the addition of 4-hydroxytamoxifen to search for p53-independent pro-apoptotic E2F1 targets. The approach was based on random inactivation of genes after retroviral transfer of an antisense cDNA library enriched of E2F1-induced genes, followed by the selection of Saos-2ERE2F1 cells that survive in the presence of the apoptotic stimulus. We identified 13 novel E2F1 target genes encoding proteins of known cellular function, including apoptosis and RNA binding. FACS analysis revealed that E2F1-induced apoptosis was significantly attenuated in cell clones containing the antisense cDNA fragments of these genes, demonstrating their participation in E2F1 death pathways. Moreover, inactivation of the target genes resulted in a clear increase of cell viability (>80%) in response to E2F1 activation compared with controls (∼30%). Four genes showed an increase in expression intensity in the presence of cycloheximide, suggesting a direct effect of E2F1 on gene transcription, whereas one gene was identified as an indirect target. Our data provide new insight in the regulation of E2F1-induced apoptosis.

Expression and clinical significance of TAp73α, p53, PCNA and apoptosis in hepatocellular carcinoma

AIM: To study the prognostic role of TAp73α, p53, proliferating cell nuclear antigen (PCNA) and apoptosis in patients with hepatocellular carcinoma (HCC) after surgical tumor ablation.

METHODS: Forty-seven human resected HCC tissues and 42 adjacent non-cancerous tissues were studied with 10 normal liver tissues as control group. TAp73α, p53, and PCNA were detected with Elivision immunohistochemistry. Terminal deoxynucleotidyl transferase (TdT)-mediated d-UTP-biotin nick-end labeling (TUNEL) method was used to detect the apoptosis cells. All clinical and pathological materials were analyzed by SPSS10.0 statistical package.

RESULTS: TAp73α overexpressed in HCC tissues (36.2%) when compared with adjacent non-cancerous tissues (2.38%, P<0.005) and normal liver tissues (0, P<0.01). Mutant type p53 (mt-p53) overexpressed in HCC tissues (38.3%) when contracted with adjacent non-cancerous tissues (16.7%, P<0.05) and normal liver tissues (0, P<0.01). Proliferation index (PI) level in HCC tissues was significantly higher than that in adjacent non-cancerous tissues (30.34%±4.46% vs 27.88%±5.89%, t, P = 0.028). Apoptosis index (AI) level in HCC tissues was higher than that in adjacent non-cancerous tissues (8.62%±2.28% vs 7.38%±2.61%, t, P = 0.019). Expression of TAp73α was associated with lymph node metastasis and mt-p53, with r = 0.407 and 0.265, respectively. Expression of mt-p53 was associated with Edmondson’s stage and AFP, with r = 0.295 and -0.357, respectively. In Kaplan-Meier univariant analysis, TAp73α, AFP, TNM stage, portal vein invasion, liver membrane invasion and HBsAg correlated with prognosis (log rank, P = 0.039, 0.012, 0.002, 0.000, 0.014, 0.007, respectively). Multivariant Cox regression analysis showed that TAp73α, AFP, TNM stage, portal vein invasion, liver membrane invasion and age were independent factors of prognosis.

CONCLUSION: These results suggest that TAp73α can be used as a prognostic indicator of patients with HCC undergoing surgical tumor ablation. AFP, TNM, portal vein invasion, liver membrane invasion and age also have a potency of predicting the prognosis of HCC.

p73beta-Mediated apoptosis requires p57kip2 induction and IEX-1 inhibition

Similarly to p53, p73alpha and p73beta induce growth arrest and/or apoptosis in response to DNA damage or when exogenously expressed. However, how they trigger apoptosis remains unresolved. After stable transduction of either p73alpha or p73beta, a greater apoptotic response was observed for p73beta in both primary and tumor cells. Consistently, blocking ectopic and endogenous p73beta expression by specific shRNA significantly decreased apoptotic levels after DNA damage. We found that p73beta targets the apoptotic program at multiple levels: (i) facilitating caspase activation through p53-dependent signals and (ii) inducing p57KIP2, while down-regulating c-IPA1 and IEX1 through a p53-independent mechanism. p73beta-mediated apoptosis was considerably reduced after inhibition of p57(KIP2) by small interfering RNA, IEX-1 overexpression, and in mouse embryo fibroblasts derived from p57-/- mice. Data from this study offer evidence for the apoptotic activity exclusive of p73beta. In the clinical context, these results might have potential therapeutic implications, because p73beta could induce alternative apoptotic responses in tumors harboring p53 mutations.

Multiple stress signals induce p73beta accumulation

Although p73 is a structural and functional homologue of the tumor-suppressor gene p53, it is not mutated in many human cancers as p53. Besides, p73 was shown to be activated by only a subset of signals that activate p53, such as gamma-irradiation and cisplatin, but not by other common genotoxic stress-inducing agents such as ultraviolet (UV) irradiation, although many of these signals are also capable of inducing p53-independent cell death. Using a p73-specific antibody, we confirmed that c-Abl is required for cisplatin-induced p73 upregulation, and further demonstrate that the p73 protein is upregulated by UV irradiation and other stress stimuli including sorbitol, hydrogen peroxide, nocodazol, and taxol. These stress signals upregulate both p73 mRNA and increases the stability of p73, indicating that p73 is regulated transcriptionally and posttranslationally. Cells stably expressing the dominant-negative p73 inhibitor protein (p73DD) and p73(-/-) fibroblasts are more resistant than control cells to apoptosis induced by these stress signals, suggesting that p73 contributes to apoptosis induction. Together, the data demonstrate that several stress signals can signal to p73 in vivo, which raises the possibility of eradicating cancers with an unmutated p73 gene by activating them with stress-inducing agents or their mimetics.

Induction of human metallothionein 1G promoter by VEGF and heavy metals: differential involvement of E2F and metal transcription factors

The E2F transcription factors induce the expression of many genes in response to specific extracellular stimuli. Here, we show that human metallothionein 1G (hMT1G) promoter is upregulated by E2F1 upon VEGF stimulation of human aortic endothelial cells. Analysis of the hMT1G promoter showed the presence of many potential E2F-binding sites flanked by potential SP1 sites and metal response elements (MREs). hMT1G promoter could be induced by E2F1 in transient transfections; further, deletion analysis suggested that the region spanning the E2F-binding sites was necessary for VEGF-mediated induction. E2Fs 1-5 could bind to the hMT1G promoter in a chromatin immunoprecipitation assay. VEGF stimulation led to an increased binding of E2Fs 1-3 to the endogenous hMT1G promoter; at the same time, the binding of Rb, p107 and p130 to the promoter was abolished. VEGF stimulation also led to the increased acetylation E2F1 as well as the histones in the hMT1G promoter region. Stimulation with metals or VEGF led to dissociation of histone deacetylase 1 (HDAC1) from the promoter, leading to acetylation of histones. Induction of the hMT1G promoter upon exposure to heavy metals such as Zn and Cd is mediated by the MRE. Interestingly, mutation of MRE affected the metal response, but not the VEGF response of the hMT1G promoter. In contrast, deletion of the E2F-binding sites did not affect the metal response. Based on these findings, we conclude that induction of the hMT1G promoter by VEGF and heavy metals occurs through the utilization of different transcription factors.

Mutant p53 gain of function in two mouse models of Li-Fraumeni syndrome

The p53 tumor suppressor gene is commonly altered in human tumors, predominantly through missense mutations that result in accumulation of mutant p53 protein. These mutations may confer dominant-negative or gain-of-function properties to p53. To ascertain the physiological effects of p53 point mutation, the structural mutant p53R172H and the contact mutant p53R270H (codons 175 and 273 in humans) were engineered into the endogenous p53 locus in mice. p53R270H/+ and p53R172H/+ mice are models of Li-Fraumeni Syndrome; they developed allele-specific tumor spectra distinct from p53+/- mice. In addition, p53R270H/- and p53R172H/- mice developed novel tumors compared to p53-/- mice, including a variety of carcinomas and more frequent endothelial tumors. Dominant effects that varied by allele and function were observed in primary cells derived from p53R270H/+ and p53R172H/+ mice. These results demonstrate that point mutant p53 alleles expressed under physiological control have enhanced oncogenic potential beyond the simple loss of p53 function.

The C-terminal sterile alpha motif and the extreme C terminus regulate the transcriptional activity of the alpha isoform of p73

p73, a member of the p53 family, is expressed from two separate promoters, generating TA and DeltaN variants. Each variant potentially encodes at least seven alternatively spliced isoforms (alpha-eta). Interestingly, we and others have shown that the alpha isoform of p73 has a weaker transcriptional activity than the beta isoform. Because the alpha isoform has an extended C terminus consisting of a sterile alpha motif (SAM) and an extreme C terminus, it appears that the C terminus is inhibitory. However, how the C terminus inhibits the transcriptional activity of p73 has not been determined. Here, we found that both the SAM and the extreme C terminus exert their inhibitory activity by preventing the accessibility of p300/CBP to the activation domain in p73. Specifically, we showed that the SAM and the extreme C terminus together or individually are capable of repressing the function of p73 activation domain, but neither interacts directly with the activation domain, or suppresses the DNA-binding activity, of the p73 protein. We also showed that the intact state of the SAM and the extreme C terminus is essential for their inhibitory functions such that a small deletion of either the SAM or the extreme C terminus abolishes its inhibitory activity. Furthermore, we showed that both inhibitory domains in the C terminus are capable of suppressing the function of a cis heterologous activation domain from p53 or Gal4. Finally, we showed that both inhibitory domains suppress the ability of p73 to interact with the transcriptional coactivators p300/CBP that are necessary for the initiation of transcription.

Conditional E2F1 activation in transgenic mice causes testicular atrophy and dysplasia mimicking human CIS

E2F1 is a crucial downstream effector of the retinoblastoma protein (pRB) pathway. To address the consequences of short-term increase in E2F1 activity in adult tissues, we generated transgenic mice expressing the human E2F1 protein fused to the oestrogen receptor (ER) ligand-binding domain. The expression of the ER-E2F1 fusion protein, which is inactive in the absence of 4-hydroxy tamoxifen (OHT), was targeted to the testes. We show that short-term activation of E2F1 results in activation of E2F target genes and apoptosis of germ cells. Consistent with our previously published results, the apoptotic response was independent of p53. Persistent E2F1 activation for 3 weeks led to massive apoptosis and severe testicular atrophy with seminiferous tubules containing only Sertoli cells and clusters of undifferentiated spermatogonia. The latter showed high expression of ER-E2F1 and excessive mitotic activity, including atypical mitoses. In addition, gonocyte-like dysplastic germ cells, resembling carcinoma in situ (CIS) cells in humans, appeared. Our results show that a relatively short period of deregulated E2F1 activity in testicles can induce premalignant changes. Moreover, we demonstrate the feasibility of tissue-specific expression of conditional ER-E2F1 in transgenic mice.

p73 induction after DNA damage is regulated by checkpoint kinases Chk1 and Chk2

The checkpoint kinases Chk1 and Chk2 are central to the induction of cell cycle arrest, DNA repair, and apoptosis as elements in the DNA-damage checkpoint. We report here that in several human tumor cell lines, Chk1 and Chk2 control the induction of the p53 related transcription factor p73 in response to DNA damage. Multiple experimental systems were used to show that interference with or augmentation of Chk1 or Chk2 signaling strongly impacts p73 accumulation. Furthermore, Chk1 and Chk2 control p73 mRNA accumulation after DNA damage. We demonstrate as well that E2F1 directs p73 expression in the presence and absence of DNA damage. Chk1 and Chk2, in turn, are vital to E2F1 stabilization and activity after genotoxic stress. Thus, Chk1, Chk2, E2F1, and p73 function in a pathway mediating p53-independent cell death produced by cytotoxic drugs. Since p53 is often obviated through mutation as a cellular port for anticancer intervention, this pathway controlling p53 autonomous pro-apoptotic signaling is of potential therapeutic importance.

Relief of p53-mediated telomerase suppression by p73

Reactivation of telomerase is a feature in many cancer cells. Telomerase activation inhibits telomere shortening, thereby preventing cell cycle arrest and apoptosis activated by shortened telomeres or chromosomal rearrangements. The tumor-suppressor gene product, p53, was previously shown to transcriptionally suppress the activation of the catalytic subunit of telomerase (hTERT). Here we have evaluated the role of p73 in hTERT regulation. We found that ectoptic expression of p73beta, in contrast to p73alpha or p53, in p53 null H1299 cells does not lead to suppression of hTERT transcription. However co-expression of p73alpha or p73beta together with p53 abolished p53-mediated hTERT suppression. This phenomenon was found to be dependent on the DNA binding ability of p73. We also show that p53-mediated suppression of hTERT transcription requires a minimum threshold level of p53, and p73 abrogates p53-mediated suppression by reducing p53 levels through the activation of HDM2. Moreover, p53-mediated hTERT suppression was not relieved by p73beta in cells depleted of HDM2 through small interfering RNA-mediated gene silencing. In addition, knockdown of HDM2 in MCF7 cells, which express moderately high levels of p73 and p53, resulted in the reduction of endogenous hTERT levels. Finally, knockdown of p73 in MCF7 cells resulted in increased p53 protein levels and a concomitant decrease in hTERT levels. Together, our data indicate a plausible way by which p73, through HDM2, can oppose p53 tumor suppressor function, thereby possibly contributing to tumorigenesis.

The emerging role of E2F-1 in the DNA damage response and checkpoint control

Genotoxic stress triggers a myriad of cellular responses including cell cycle arrest, stimulation of DNA repair and apoptosis. A central role for the E2F-1 transcription factor in the DNA damage response pathway is gaining support. E2F-1 is phosphorylated by DNA damage responsive protein kinases, which leads to E2F-1 accumulation and the induction of apoptosis. In addition, emerging information suggests that E2F-1 may play a role in the detection and subsequent repair of damaged DNA.

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.

Ectopic Expression of p73α, but Not p73β, Suppresses Myogenic Differentiation

The TRP73 gene, a member of the p53 family, encodes several variants through differential splicing and use of alternative promoters. At the N terminus, two different promoters generate the full-length and the DeltaN isoforms, with or without the transactivating domain. At the C terminus, seven isoforms generated through alternative splicing have been cloned. Previous studies have demonstrated that DeltaN-p73 interferes with p73-induced apoptosis. However, there has been no evidence for functional diversity of the C-terminal p73 variants. In this study, we found that p73alpha and p73beta exerted differential effect on the differentiation of C2C12 myoblasts. Although p73beta lacked any detectable effect on differentiation, p73alpha caused a substantial delay in the expression of muscle-specific genes. In co-transfection experiments p73alpha, but not p73beta, attenuated the transcriptional activity of MyoD. Microarray-based gene profiling confirmed the protraction of MyoD-dependent gene expression in C2C12 cells stably expressing p73alpha. Notwithstanding the differential effect on differentiation, p73alpha and p73beta showed similar activity in sensitizing C2C12 myoblasts to cisplatin-induced cell death. These results demonstrated a functional diversity between the two C-terminal variants of p73 and suggested that p73alpha can regulate cellular differentiation in addition to its role in stimulating cell death.