E2F-1 induces the stabilization of p53 but blocks p53-mediated transactivation

TP53/mTORC1-mediated bidirectional regulation of PD-L1 modulates immune evasion in hepatocellular carcinoma

BACKGROUND

Immunotherapy has facilitated great breakthroughs in the treatment of hepatocellular carcinoma (HCC). However, the efficacy and response rate of immunotherapy are limited and vary among different patients with HCC. TP53 mutation substantially affects the expression of immune checkpoint molecules in multiple cancers. However, the regulatory relationship between programmed death ligand 1 (PD-L1) and TP53 is poorly studied in HCC. We aimed to elucidate the regulatory mechanism of PD-L1 in HCC with different TP53 statuses and to assess its role in modulating immune evasion in HCC.

METHODS

HCC mouse models and cell lines with different TP53 statuses were constructed. PD-L1 levels were detected by PCR, western blotting and flow cytometry. RNA-seqencing, immunoprecipitation, chromatin immunoprecipitation and transmission electron microscopy were used to elucidate the regulatory mechanism in HCC with different TP53 status. HCC mouse models and patient with HCC samples were analyzed to demonstrate the preclinical and clinical significance of the findings.

RESULTS

We report that loss of p53 promoted PD-L1 expression and reduced CD8+ T-cell infiltration in patient with HCC samples and mouse models. Mammalian target of rapamycin (mTOR) pathway was activated in p53-loss-of-function HCC or after knocking down TP53. The transcription factor E2F1 was found to bind to the p53 protein in TP53 wild-type HCC cells, and inhibiting mammalian target of rapamycin complex 1 (mTORC1) disrupted this binding and enhanced E2F1 translocation to the nucleus, where it bound to the PD-L1 promoter and transcriptionally upregulated PD-L1. In p53-loss-of-function HCC cells, autophagosomes were activated after mTORC1 suppression, promoting the degradation of PD-L1 protein. The combination of mTOR inhibitor and anti-PD-L1 antibody enhanced CD8+ T-cell infiltration and tumor suppression in TP53 wild-type HCC mouse models, but no benefit was observed in p53-loss-of-function HCC mouse models. In patients with TP53 wild-type HCC, PD-L1 levels were significantly higher in the high E2F1 group than in the low E2F1 group, and the low E2F1 level group had significantly superior survival.

CONCLUSION

We revealed the bidirectional regulatory mechanism of PD-L1 mediated by TP53/mTORC1 in HCC. The combination of mTOR inhibitor and anti-PD-L1 antibody could be a novel precise immunotherapy scheme for TP53 wild-type HCC.

Free radical biology in neurological manifestations: mechanisms to therapeutics interventions

Recent advancements and growing attention about free radicals (ROS) and redox signaling enable the scientific fraternity to consider their involvement in the pathophysiology of inflammatory diseases, metabolic disorders, and neurological defects. Free radicals increase the concentration of reactive oxygen and nitrogen species in the biological system through different endogenous sources and thus increased the overall oxidative stress. An increase in oxidative stress causes cell death through different signaling mechanisms such as mitochondrial impairment, cell-cycle arrest, DNA damage response, inflammation, negative regulation of protein, and lipid peroxidation. Thus, an appropriate balance between free radicals and antioxidants becomes crucial to maintain physiological function. Since the 1brain requires high oxygen for its functioning, it is highly vulnerable to free radical generation and enhanced ROS in the brain adversely affects axonal regeneration and synaptic plasticity, which results in neuronal cell death. In addition, increased ROS in the brain alters various signaling pathways such as apoptosis, autophagy, inflammation and microglial activation, DNA damage response, and cell-cycle arrest, leading to memory and learning defects. Mounting evidence suggests the potential involvement of micro-RNAs, circular-RNAs, natural and dietary compounds, synthetic inhibitors, and heat-shock proteins as therapeutic agents to combat neurological diseases. Herein, we explain the mechanism of free radical generation and its role in mitochondrial, protein, and lipid peroxidation biology. Further, we discuss the negative role of free radicals in synaptic plasticity and axonal regeneration through the modulation of various signaling molecules and also in the involvement of free radicals in various neurological diseases and their potential therapeutic approaches. The primary cause of free radical generation is drug overdosing, industrial air pollution, toxic heavy metals, ionizing radiation, smoking, alcohol, pesticides, and ultraviolet radiation. Excessive generation of free radicals inside the cell R1Q1 increases reactive oxygen and nitrogen species, which causes oxidative damage. An increase in oxidative damage alters different cellular pathways and processes such as mitochondrial impairment, DNA damage response, cell cycle arrest, and inflammatory response, leading to pathogenesis and progression of neurodegenerative disease other neurological defects.

Cell cycle re-entry of neurons and reactive neuroblastosis in Huntington's disease: Possibilities for neural-glial transition in the brain

Huntington's disease (HD) is an autosomal dominant pathogenic condition that causes progressive degeneration of GABAergic neurons in the brain. The abnormal expansion of the CAG repeats in the exon 1 of the Huntingtin gene (HTT gene) has been associated with the onset and progression of movement disorders, psychiatric disturbance and cognitive decline in HD. Microglial activation and reactive astrogliosis have been recognized as the key pathogenic cellular events in the brains of HD subjects. Besides, HD has been characterized by induced quiescence of neural stem cells (NSCs), reactive neuroblastosis and reduced survival of newborn neurons in the brain. Strikingly, the expression of the mutant HTT gene has been reported to induce the cell cycle re-entry of neurons in HD brains. However, the underlying basis for the induction of cell cycle in neurons and the fate of dedifferentiating neurons in the pathological brain remain largely unknown. Thus, this review article revisits the reports on the regulation of key signaling pathways responsible for altered cell cycle events in diseased brains, with special reference to HD and postulates the occurrence of reactive neuroblastosis as a consequential cellular event of dedifferentiation of neurons. Meanwhile, a substantial number of studies indicate that many neuropathogenic events are associated with the expression of potential glial cell markers by neuroblasts. Taken together, this article represents a hypothesis that transdifferentiation of neurons into glial cells might be highly possible through the transient generation of reactive neuroblasts in the brain upon certain pathological conditions.

MiR-1258 promotes the apoptosis of cervical cancer cells by regulating the E2F1/P53 signaling pathway

OBJECTIVE

Cervical cancer is the most common malignant tumor in gynaecology with high mortality. MiRNA has been reported to regulate cell biological processes in cervical cancer. This study aimed to explore the expression of miR-1258 and role of miR-1258 by targeting E2F1 in cervical cancer cells.

METHODS

The expression of miR-1258 and E2F1 in cervical cancer cells and transfection effects was determined by RT-qPCR analysis. The expression of E2F1, MMP2, MMP7, MMP9, Bcl2, Bax, cleaved caspase3, caspase3, KI67, p-AKT, cyclinD1, CDK2, P53 and AKT in cervical cancer cells was detected by western blot analysis. The proliferation, invasion, migration and apoptosis were respectively analyzed by CCK-8 assay, transwell assay, wound healing assay and flow cytometry analysis. E2F1 was a potential target of miR-1258, which demonstrated by a dual-luciferase reporter assay.

RESULTS

miR-1258 expression was decreased while E2F1 expression was increased in cervical cancer cells. MiR-1258 overexpression could down-regulate the E2F1 expression. Overexpression of miR-1258 inhibited the proliferation, invasion and migration and promoted the apoptosis of cervical cancer cells by AKT and P53 signal pathway. And, Overexpression of miR-1258 also suppressed the tumor growth by AKT and P53 signal pathway. Overexpression of E2F1 reduced the inhibition effects of miR-1258 in cervical cancer.

CONCLUSION

Taken together, miR-1258 overexpression exerts its inhibition effects on the proliferation, invasion and migration and promotion effects on the apoptosis of cervical cancer cells by targeting the E2F1, which might provide new ideas for clinical treatment of cervical cancer.

Crosstalk between E2F1 and P53 transcription factors in doxorubicin-induced DNA damage: evidence for preventive/protective effects of silymarin

Objectives

To study the effects of silymarin in various forms of applications on the molecular mechanism(s) of doxorubicin-induced testicular toxicity in male rats.

Methods

Following DOX administration with or without SMN in male rats, sperm quality assays were conducted. Moreover, total antioxidant capacity and nitric oxide content of testis were determined. Expression profile of p53 and E2F1 was analysed by PCR technique. Ultimately, the rate of DNA fragmentation in the testes was quantitatively measured.

Key findings

P53 and E2F1 expression in DOX-received animals at mRNA level showed a revers profile of an up- and down-regulation, respectively. Administration of SMN in preventive and protective forms resulted in a significant (P < 0.05) reduction in DOX-induced sperm abnormalities, DNA fragmentation, nitric oxide concentration and a marked increase in total antioxidant power, rate of sperm motility and viability. SMN lowered the DOX-up-regulated expression of p53 at mRNA level.

Conclusions

DOX-induced testicular toxicity was characterized by lowering sperm quality values, induction of oxidative and nitrosative stress and DNA fragmentation. Preventive and protective effects of SMN on DOX-induced testicular toxicity may attribute to its antioxidant property. DOX-induced testicular damages and SMN preventive/protective effects might be mediated via up- and down-regulation of p53 and E2F1 transcription factors.

Transcription factors that interact with p53 and Mdm2

The tumor suppressor p53 is activated upon cellular stresses such as DNA damage, oncogene activation, hypoxia, which transactivates sets of genes that induce DNA repair, cell cycle arrest, apoptosis, or autophagy, playing crucial roles in the prevention of tumor formation. The central regulator of the p53 pathway is Mdm2 which inhibits transcriptional activity, nuclear localization and protein stability. More than 30 cellular p53-binding proteins have been isolated and characterized including Mdm2, Mdm4, p300, BRCA1/2, ATM, ABL and 53BP-1/2. Most of them are nuclear proteins; however, not much is known about p53-binding transcription factors. In this review, we focus on transcription factors that directly interact with p53/Mdm2 through direct binding including Dmp1, E2F1, YB-1 and YY1. Dmp1 and YB-1 bind only to p53 while E2F1 and YY1 bind to both p53 and Mdm2. Dmp1 has been shown to bind to p53 and block all the known functions for Mdm2 on p53 inhibition, providing a secondary mechanism for tumor suppression in Arf-null cells. Although E2F1-p53 binding provides a checkpoint mechanism to silence hyperactive E2F1, YB-1 or YY1 interaction with p53 subverts the activity of p53, contributing to cell cycle progression and tumorigenesis. Thus, the modes and consequences for each protein-protein interaction vary from the viewpoint of tumor development and suppression.

Isg15 controls p53 stability and functions

Degradation of p53 is a cornerstone in the control of its functions as a tumor suppressor. This process is attributed to ubiquitin-dependent modification of p53. In addition to polyubiquitination, we found that p53 is targeted for degradation through ISGylation. Isg15, a ubiquitin-like protein, covalently modifies p53 at 2 sites in the N and C terminus, and ISGylated p53 can be degraded by the 20S proteasome. ISGylation primarily targets a misfolded, dominant-negative p53, and Isg15 deletion in normal cells results in suppression of p53 activity and functions. We propose that Isg15-dependent degradation of p53 represents an alternative mechanism of controlling p53 protein levels, and, thus, it is an attractive pathway for drug discovery.

ATM-dependent E2F1 accumulation in the nucleolus is an indicator of ribosomal stress in early response to DNA damage

The nucleolus plays a major role in ribosome biogenesis. Most genotoxic agents disrupt nucleolar structure and function, which results in the stabilization/activation of p53, inducing cell cycle arrest or apoptosis. Likewise, transcription factor E2F1 as a DNA damage responsive protein also plays roles in cell cycle arrest, DNA repair, or apoptosis in response to DNA damage through transcriptional response and protein-protein interaction. Furthermore, E2F1 is known to be involved in regulating rRNA transcription. However, how E2F1 displays in coordinating DNA damage and nucleolar stress is unclear. In this study, we demonstrate that ATM-dependent E2F1 accumulation in the nucleolus is a characteristic feature of nucleolar stress in early response to DNA damage. We found that at the early stage of DNA damage, E2F1 accumulation in the nucleolus was an ATM-dependent and a common event in p53-suficient and -deficient cells. Increased nucleolar E2F1 was sequestered by the nucleolar protein p14ARF, which repressed E2F1-dependent rRNA transcription initiation, and was coupled with S phase. Our data indicate that early accumulation of E2F1 in the nucleolus is an indicator for nucleolar stress and a component of ATM pathway, which presumably buffers elevation of E2F1 in the nucleoplasm and coordinates the diversifying mechanisms of E2F1 acts in cell cycle progression and apoptosis in early response to DNA damage.

p53 Suppresses E2F1-dependent PLK1 expression upon DNA damage by forming p53-E2F1-DNA complex

E2F1 is implicated in transcriptional activation of polo-like kinase-1 (PLK1), but yet the mechanism is not fully understood. PLK1 suppression plays an important checkpoint role in response to DNA damage. Suppression of the PLK1 gene by binding of p53 to upstream p53RE2 element in the promoter has been recently revealed. Here we report another mechanism, in which p53 interacts with E2F1 to form p53-E2F1-DNA complex repressing E2F1-dependent PLK1 expression. PLK1 was downregulated in cisplatin exposed HCT116p53(+/+) but not HCT116p53(-/-) cells, indicating p53-suppressed PLK1 upon DNA damage. Co-transfection and reporter enzyme assays revealed that p53 suppressed but E2F1 promoted PLK1 gene activation. 5'-Deletion and substitution mutations showed multiple positive cis-elements residing in the PLK1 promoter, of which at least two E2F1 sites at positions -75/-68 and -40/-32 were required for the full activity of the promoter. Combination of 5'-deletion and substitution mutations with over-expression of p53 showed that suppression of the PLK1 gene by p53 was E2F1-dependent: mutation of the E2F1 site at position -75/-68 partially abrogated suppression activity of p53; mutation of E2F1 site at position -40/-32 released from p53 suppression of PLK1 gene completely. Co-immunoprecipitation and electrophoretic mobility shift assay showed that DNA damage promoted p53-E2F1 interaction, thereby creating a p53-E2F1 complex assembly on the PLK1 promoter in vitro. The in vivo formation of p53-E2F1-PLK1 promoter complex upon DNA damage was further evidenced by chromatin immunoprecipitation (ChIP) and re-ChIP. In addition, we showed that suppression of PLK1 by p53 promoted apoptosis. Our data suggest that p53 may interact with E2F1 to form p53-E2F1-DNA complex suppressing E2F1-dependent PLK1 expression. The model of p53 action on E2F1-activated PLK1 gene may explain at least partly how p53 as a suppressor regulates the downstream effects of E2F1 in cellular stresses including DNA damage stress.

A novel E2F-like protein involved in transcriptional activation of cyst wall protein genes in Giardia lamblia

Giardia lamblia differentiates into resistant walled cysts for survival outside the host and transmission. During encystation, synthesis of cyst wall proteins is coordinately induced. The E2F family of transcription factors in higher eukaryotes is involved in cell cycle progression and cell differentiation. We asked whether Giardia has E2F-like genes and whether they influence gene expression during Giardia encystation. Blast searches of the Giardia genome database identified one gene (e2f1) encoding a putative E2F protein with two putative DNA-binding domains. We found that the e2f1 gene expression levels increased significantly during encystation. Epitope-tagged E2F1 was found to localize to nuclei. Recombinant E2F1 specifically bound to the thymidine kinase and cwp1-3 gene promoters. E2F1 contains several key residues for DNA binding, and mutation analysis revealed that its binding sequence is similar to those of the known E2F family proteins. The E2F1-binding sequences were positive cis-acting elements of the thymidine kinase and cwp1 promoters. We also found that E2F1 transactivated the thymidine kinase and cwp1 promoters through its binding sequences in vivo. Interestingly, E2F1 overexpression resulted in a significant increase of the levels of CWP1 protein, cwp1-3 gene mRNA, and cyst formation. We also found E2F1 can interact with Myb2, a transcription factor that coordinate up-regulates the cwp1-3 genes during encystation. Our results suggest that E2F family has been conserved during evolution and that E2F1 is an important transcription factor in regulation of the Giardia cwp genes, which are key to Giardia differentiation into cysts.

MEF/ELF4 transactivation by E2F1 is inhibited by p53

Myeloid elf-1-like factor (MEF) or Elf4 is an E-twenty-six (ETS)-related transcription factor with strong transcriptional activity that influences cellular senescence by affecting tumor suppressor p53. MEF downregulates p53 expression and inhibits p53-mediated cellular senescence by transcriptionally activating MDM2. However, whether p53 reciprocally opposes MEF remains unex-plored. Here, we show that MEF is modulated by p53 in human cells and mice tissues. MEF expression and promoter activity were suppressed by p53. While we found that MEF promoter does not contain p53 response elements, intriguingly, it contains E2F consensus sites. Subsequently, we determined that E2F1 specifically binds to MEF promoter and transactivates MEF. Nevertheless, E2F1 DNA binding and transactivation of MEF promoter was inhibited by p53 through the association between p53 and E2F1. Furthermore, we showed that activation of p53 in doxorubicin-induced senescent cells increased E2F1 and p53 interaction, diminished E2F1 recruitment to MEF promoter and reduced MEF expression. These observations suggest that p53 downregulates MEF by associating with and inhibiting the binding activity of E2F1, a novel transcriptional activator of MEF. Together with previous findings, our present results indicate that a negative regulatory mechanism exists between p53 and MEF.

Deregulated expression of E2F1 promotes proteolytic degradation of tumor suppressor p73 and inhibits its transcriptional activity

The expression of tumor suppressor p73 is regulated at mRNA and protein levels. It has been shown that E2F1 acts as a transcriptional activator for p73. In this study, we have found that deregulated expression of E2F1 increases the mRNA level of p73, however, E2F1 promotes the degradation of p73. Immunoprecipitation experiments demonstrated that E2F1 forms a complex with p73 and inhibits the transcriptional activity of p73. Enforced expression of E2F1 induces degradation of p73 in a proteasome-independent manner. Additionally, the deletion analysis showed that E2F1(1-117) has an undetectable effect on p73, whereas E2F1(1-285) and E2F1(1-414) have an ability to promote degradation of p73 and inhibition of p73 transcriptional activity, suggesting that the region of E2F1 between amino acid residues 118 and 285 has a critical role in the regulation of p73. Taken together, our present study indicates that E2F1 has a dual role in the regulation of p73.

Differential effect of silibinin on E2F transcription factors and associated biological events in chronically UVB-exposed skin versus tumors in SKH-1 hairless mice

UVB radiation-induced DNA damage in skin activates cellular pathways involved in DNA repair, cell cycle regulation, and apoptosis, important events that prevent conversion of damaged skin cells into cancer. We reported recently the efficacy of silibinin against photocarcinogenesis along with altered molecular events in tumors (Cancer Research, 64:6349-56, 2004). The molecular and biological events modulated by silibinin in chronically UVB-irradiated skin leading to cancer prevention, however, are not known. Herein, we describe effect of silibinin on skin 15 and 25 weeks after UVB exposure and compared them with molecular alterations in skin tumors. UVB decreased E2F1 but increased E2F2 and E2F3 protein levels in skin, and these were reversed by silibinin treatment. Silibinin-induced E2F1 was accompanied by an inhibition of apoptosis and decreases in p53 and cyclin-dependent kinase inhibitors. Silibinin-caused decrease in E2F2 and E2F3 was accompanied by reduced levels of cyclin-dependent kinases, cyclins, CDC25C, and mitogen-activated protein kinases and Akt signaling and inhibition of cell proliferation. In tumorigenesis protocols, topical or dietary silibinin significantly inhibited tumor appearance and growth. As opposed to UVB-exposed skin, UVB-induced tumors showed elevated levels of E2F1, but these were reduced in silibinin-treated tumors without any effect on E2F2 and E2F3. Contrary to the inhibition of apoptosis and p53 expression in UVB-exposed skin cells, silibinin increased these variables in tumors. These differential effects of silibinin on E2F1 versus E2F2 and E2F3 and their associated molecular alterations and biological effects in chronic UVB-exposed skin suggest their role in silibinin interference with photocarcinogenesis.

Structural assessment of single amino acid mutations: application to TP53 function

Single amino acid substitution is the type of protein alteration most related to human diseases. Current studies seek primarily to distinguish neutral mutations from harmful ones. Very few methods offer an explanation of the final prediction result in terms of the probable structural or functional effect on the protein. In this study, we describe the use of three novel parameters to identify experimentally-verified critical residues of the TP53 protein (p53). The first two parameters make use of a surface clustering method to calculate the protein surface area of highly conserved regions or regions with high nonlocal atomic interaction energy (ANOLEA) score. These parameters help identify important functional regions on the surface of a protein. The last parameter involves the use of a new method for pseudobinding free-energy estimation to specifically probe the importance of residue side-chains to the stability of protein fold. A decision tree was designed to optimally combine these three parameters. The result was compared to the functional data stored in the International Agency for Research on Cancer (IARC) TP53 mutation database. The final prediction achieved a prediction accuracy of 70% and a Matthews correlation coefficient of 0.45. It also showed a high specificity of 91.8%. Mutations in the 85 correctly identified important residues represented 81.7% of the total mutations recorded in the database. In addition, the method was able to correctly assign a probable functional or structural role to the residues. Such information could be critical for the interpretation and prediction of the effect of missense mutations, as it not only provided the fundamental explanation of the observed effect, but also helped design the most appropriate laboratory experiment to verify the prediction results.

Cell cycle regulator E2F1 modulates angiogenesis via p53-dependent transcriptional control of VEGF

The transcription factor E2F1 is known to regulate cell proliferation and has been thought to modulate tumorigenesis via this mechanism alone. Here we show that mice deficient in E2F1 exhibit enhanced angiogenesis. The proangiogenic phenotype in E2F1 deficiency is the result of overproduction of vascular endothelial growth factor (VEGF) and is prevented by VEGF blockade. Under hypoxic conditions, E2F1 down-regulates the expression of VEGF promoter activity by associating with p53 and specifically down-regulating expression of VEGF but not other hypoxia-inducible genes, suggesting a promoter structure context-dependent regulation mechanism. We found that the minimum VEGF promoter mediating transcriptional repression by E2F1 features an E2F1- binding site with four Sp-1 sites in close proximity. These data disclose an unexpected function of endogenous E2F1: regulation of angiogenic activity via p53-dependent transcriptional control of VEGF expression.

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.

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.

Evaluation of cadmium-induced transcriptome alterations by three color cDNA labeling microarray analysis on a T-cell line

Beside heavy metals, cadmium (Cd(2+)) is a ubiquitous toxic metal with a well established apoptotic and genotoxic effect, chronic exposure of which has been involved in a variety of pathological conditions. In the present study, we investigated by 1455 genes cDNA microarrays the toxic and apoptotic effect of Cd(2+), on the T-cell line CCRF-CEM, applying a three laser differential analysis, on the same microarray slide. The cells were cultured for 6 and 24 h in the absence (control) or presence of Cd(2+) (10 or 20 microM), RNAs were extracted and the produced cDNAs were labeled with rhodamine derivatives fluorescent dyes. A microarray slide was simultaneously hybridized by the labeled cDNAs and analyzed. We found that, in relation to control, treatment of the cells for 6 h with 10 and 20 microM Cd(2+), induces up-regulation in 20 and 34 genes, respectively. Treatment for 24 h with 10 and 20 microM Cd(2+) induces up-regulation in 22 and 84 genes, respectively. Twenty-eight genes were found down-regulated only after treatment for 24 h with Cd(2+) 10 microM. These data suggest that Cd(2+) produces a time- and dose-dependent molecular cascade, induces disturbances in different subcellular compartments, influencing thereafter the normal cellular functions, the differentiation process, the malignant transformation and the cell death.

Expression of cell cycle proteins in head and neck cancer correlates with tumor site rather than tobacco use

Head and neck squamous cell carcinomas of non-smoking patients may result from specific defects in cell cycle control. Expression of p53, pRb, p16(INK4a) and Cyclin D1 was examined by immunohistochemistry of biopsies obtained from 24 non-smoking and 25 smoking patients, both groups representing similar clinical features (tumor site, stage of disease, gender). Expression of p16(INK4a) was restricted to carcinomas of the tonsils (8/24), P=0.0069. In 6/8 p16(INK4a)-positive cases, expression of pRb was absent or reduced. p16 was the only marker showing a significant correlation with a negative smoking history (P=0.0208). Overexpression of Cyclin D1 was frequent in carcinomas of the tongue (6/14) but rare in tonsillar carcinomas (2/24), P=0.0303. Expression of p53 was independent of the smoking history and the tumor site. Our results implicate that there are factors other than tobacco consumption which may influence the development of head and neck cancers at distinct tumor sites.

E2F1 induces phosphorylation of p53 that is coincident with p53 accumulation and apoptosis

It has been proposed that the E2F1 transcription factor serves as a link between the Rb/E2F proliferation pathway and the p53 apoptosis pathway by inducing the expression of p19ARF, a protein that regulates p53 stability. We find that although p19ARF contributes to p53 accumulation in response to E2F expression, p19ARF is not required for E2F1-mediated apoptosis. E2F1 can signal p53 phosphorylation in the absence of p19ARF, similar to the observed modifications to p53 in response to DNA damage. These modifications are not observed in the absence of p19ARF following expression of E2F2, an E2F family member that does not induce apoptosis in mouse embryo fibroblasts but can induce p19ARF and p53 protein expression. p53 modification is found to be crucial for E2F1-mediated apoptosis, and this apoptosis is compromised when E2F1 is coexpressed with a p53 mutant lacking many N- and C-terminal phosphorylation sites. Additionally, E2F1-mediated apoptosis is abolished in the presence of caffeine, an inhibitor of phosphatidylinositol 3-kinase-related kinases that phosphorylate p53. These findings suggest that p53 phosphorylation is a key step in E2F1-mediated apoptosis and that this modification can occur in the absence of p19ARF.

Loss of neurofibromatosis-1 and p19(ARF) cooperate to induce a multiple tumor phenotype

Inactivation of the neurofibromatosis-1 (NF1) gene de-regulates RAS and cooperates with mutation or loss of the p53 tumor suppressor to induce tumorigenesis. p19(ARF) acts upstream of p53 in an oncogene checkpoint to induce apoptosis in response to activated RAS and other factors that stimulate proliferation. Therefore, we bred p19(ARF-/-) to NF1(+/-) mice to determine if loss of these genes collaborates in tumorigenesis. As expected from the embryonic lethality of NF1 null mice, no mice lacking both p19(ARF) and NF1 were born. Unexpectedly, the loss of one allele of NF1 did not greatly shorten the time to tumor formation in a p19(ARF) null background. The tumor types observed were characteristic of p19(ARF) null animals, not those associated with neurofibromatosis or those observed with NF1(+/-)/p53(+/-) mice. However, seven out of 12 animals developed multiple tumors, some with metastases. This multiple tumor phenotype was not previously observed with p19(ARF)-null mice and suggests a distinct form of cooperation between the loss of these tumor suppressors.

Mdm2 inhibition of p53 induces E2F1 transactivation via p21

The transcription factor E2F1 functions as a key regulator for both cell-cycle progression and apoptosis. Mdm2, a major cellular regulator of the p53 tumor suppressor protein, is also closely involved in cell cycle and apoptosis. In addition to regulation of p53, Mdm2 has been reported to stimulate E2F1 transactivation by a mechanism that remains unclear. Here we examined how overexpression of Mdm2 alters E2F1/DP1 transactivation. Using a set of cell lines with differing p53 and Rb status we determined that Mdm2 induction of E2F1 transactivation was p53-dependent, resulting from release of repression by p53. While Mdm2 association with p53 was required to increase E2F1 transactivation, Mdm2 mediated degradation of p53 was not. p53 repression of E2F1 transactivation required a functional DNA binding and transactivation domain. Consistent with Mdm2 activation of E2F1 via an inhibition of p53 transactivation we demonstrate a concomitant reduction in p21 protein levels with Mdm2 overexpression. Furthermore, E2F1 repression by an Rb-phosphorylation mutant could not be reversed by Mdm2 overexpression. Mdm2 was also unable to enhance E2F1 transactivation in Mouse embryo fibroblasts lacking p21. Taken together, these results suggest that Mdm2 activation of E2F1 occurs through the repression of p53-dependent transcription of p21, a p53-target gene and cyclin dependent kinase inhibitor.

E2F1 and c-Myc potentiate apoptosis through inhibition of NF-kappaB activity that facilitates MnSOD-mediated ROS elimination

Overexpression of c-Myc or E2F1 sensitizes host cells to various types of apoptosis. Here, we found that overexpressed c-Myc or E2F1 induces accumulation of reactive oxygen species (ROS) and thereby enhances serum-deprived apoptosis in NIH3T3 and Saos-2. During serum deprivation, MnSOD mRNA was induced by NF-kappaB in mock-transfected NIH3T3, while this induction was inhibited in NIH3T3 overexpressing c-Myc or E2F1. In these clones, E2F1 inhibited NF-kappaB activity by binding to its subunit p65 in competition with a heterodimeric partner p50. In addition to overexpressed E2F1, endogenous E2F1 released from Rb was also found to inhibit NF-kappaB activity in a cell cycle-dependent manner by using E2F1(+/+) and E2F1(-/-) murine embryonic fibroblasts. These results indicate that E2F1 promotes apoptosis by inhibiting NF-kappaB activity.

Autoinhibitory regulation of p73 by Delta Np73 to modulate cell survival and death through a p73-specific target element within the Delta Np73 promoter

p73 is a p53-related tumor suppressor but is also induced by oncogene products such as E2F-1, raising a question as to whether p73 is a tumor suppressor gene or oncogene. Unlike p53, p73 has several variants, including Delta Np73, which lacks the NH(2)-terminal transactivation domain. Although, in developing neurons, Delta Np73 is expressed abundantly and seems to inhibit the proapoptotic function of p53, the role of p73 and Delta Np73 and their regulatory mechanism in cell growth and differentiation are poorly understood. Here we report that p73, but not p53, directly activates the transcription of endogenous Delta Np73 by binding to the p73-specific target element located at positions -76 to -57 within the Delta Np73 promoter region. The activation of Delta Np73 promoter by p63 was marginal. Delta Np73 was associated with p73 alpha, p73 beta, and p53, as demonstrated by immunoprecipitation assays, and inhibited their transactivation activities when we used reporters of Mdm2, Bax, or Delta Np73 itself in SAOS-2 cells. Furthermore, induction or overexpression of Delta Np73 promoted cell survival by competing with p53 and p73 itself. Thus, our results suggest that the negative feedback regulation of p73 by its target Delta Np73 is a novel autoregulatory system for modulating cell survival and death.

Novel function of the cyclin A binding site of E2F in regulating p53-induced apoptosis in response to DNA damage

We demonstrate here that the E2F1 induced by DNA damage can bind to and promote the apoptotic function of p53 via the cyclin A binding site of E2F1. This function of E2F1 does not require its DP-1 binding, DNA binding, or transcriptional activity and is independent of mdm2. All the cyclin A binding E2F family members can interact and cooperate with p53 to induce apoptosis. This suggests a novel role for E2F in regulating apoptosis in response to DNA damage. Cyclin A, but not cyclin E, prevents E2F1 from interacting and cooperating with p53 to induce apoptosis. However, in response to DNA damage, cyclin A levels decrease, with a concomitant increase in E2F1-p53 complex formation. These results suggest that the binding of E2F1 to p53 can specifically stimulate the apoptotic function of p53 in response to DNA damage.

Neuronal survival and cell death signaling pathways

Neuronal viability is maintained through a complex interacting network of signaling pathways that can be perturbed in response to a multitude of cellular stresses. A shift in the balance of signaling pathways after stress or in response to pathology can have drastic consequences for the function or the fate of a neuron. There is significant evidence that acutely injured and degenerating neurons may die by an active mechanism of cell death. This process involves the activation of discrete signaling pathways that ultimately compromise mitochondrial structure, energy metabolism and nuclear integrity. In this review we examine recent evidence pertaining to the presence and activation of anti- and pro-cell death regulatory pathways in nervous system injury and degeneration.

Bcl-2 is an apoptotic target suppressed by both c-Myc and E2F-1

Malignant transformation occurs in cells that overexpress c-Myc or that inappropriately activate E2F-1. Transformation occurs after the selection of cells that have acquired resistance to apoptosis that is triggered by these oncogenes, and a key mediator of this cell death process is the p53 tumor suppressor. In IL-3-dependent immortal 32D.3 myeloid cells the ARF/p53 apoptotic pathway is inactivated, as these cells fail to express ARF. Nonetheless, both c-Myc and E2F-1 overexpression accelerated apoptosis when these cells were deprived of IL-3. Here we report that c-Myc or E2F-1 overexpression suppresses Bcl-2 protein and RNA levels, and that restoration of Bcl-2 protein effectively blocks the accelerated apoptosis that occurs when c-Myc- or E2F-1-overexpressing cells are deprived of IL-3. Blocking p53 activity with mutant p53 did not abrogate E2F-1-induced suppression of Bcl-2. Analysis of immortal myeloid cells engineered to overexpress c-Myc and E2F-1 DNA binding mutants revealed that DNA binding activity of these oncoproteins is required to suppress Bcl-2 expression. These results suggest that the targeting of Bcl-2 family members is an important mechanism of oncogene-induced apoptosis, and that this occurs independent of the ARF/p53 pathway.

c-Myc-mediated regulation of telomerase activity is disabled in immortalized cells

Myc overexpression is a hallmark of human cancer and promotes transformation by facilitating immortalization. This function has been linked to the ability of c-Myc to induce the expression of the catalytic subunit of telomerase, telomerase reverse transcriptase (TERT), as ectopic expression of TERT immortalizes some primary human cell types. c-Myc up-regulates telomerase activity in primary mouse embryonic fibroblasts (MEFs) and myeloid cells. Paradoxically, Myc overexpression also triggers the ARF-p53 apoptotic program, which is activated when MEFs undergo replicative crises following culture ex vivo. The rare immortal variants that arise from these cultures generally suffer mutations in p53 or delete Ink4a/ARF, and Myc greatly increases the frequency of these events. Alternative reading frame (ARF)- and p53-null MEFs have increased telomerase activity, as do variant immortal clones that bypass replicative crisis. Similarly, immortal murine NIH-3T3 fibroblasts and myeloid 32D.3 and FDC-P1.2 cells do not express ARF and have robust telomerase activity. However, Myc overexpression in these immortal cells results in remarkably discordant regulation of TERT and telomerase activity. Furthermore, in MEFs and 32D.3 cells TERT expression and telomerase activity are regulated independently of endogenous c-Myc. Thus, the regulation of TERT and telomerase activity is complex and is also regulated by factors other than Myc, ARF, or p53.