Novel link between E2F and p53: proapoptotic cofactors of p53 are transcriptionally upregulated by E2F

Apoptosis-stimulating protein of p53 (ASPP2) heterozygous mice are tumor-prone and have attenuated cellular damage-response thresholds

The expression of ASPP2 (53BP2L), a proapoptotic member of a family of p53-binding proteins, is frequently suppressed in many human cancers. Accumulating evidence suggests that ASPP2 inhibits tumor growth; however, the mechanisms by which ASPP2 suppresses tumor formation remain to be clarified. To study this, we targeted the ASPP2 allele in a mouse by replacing exons 10-17 with a neoR gene. ASPP2(-/-) mice were not viable because of an early embryonic lethal event. Although ASPP2(+/-) mice appeared developmentally normal, they displayed an increased incidence of a variety of spontaneous tumors as they aged. Moreover, gamma-irradiated 6-week-old ASPP2(+/-) mice developed an increased incidence of high-grade T cell lymphomas of thymic origin compared with ASPP2(+/+) mice. Primary thymocytes derived from ASPP2(+/-) mice exhibited an attenuated apoptotic response to gamma-irradiation compared with ASPP2(+/+) thymocytes. Additionally, ASPP2(+/-) primary mouse embryonic fibroblasts demonstrated a defective G(0)/G(1) cell cycle checkpoint after gamma-irradiation. Our results demonstrate that ASPP2 is a haploinsufficient tumor suppressor and, importantly, open new avenues for investigation into the mechanisms by which disruption of ASPP2 pathways could play a role in tumorigenesis and response to therapy.

Tumor protein 53-induced nuclear protein 1 is a major mediator of p53 antioxidant function

p53 exerts its tumor suppressor function mainly through transcriptional induction of target genes involved in several processes, including cell cycle checkpoints, apoptosis, and regulation of cell redox status. p53 antioxidant function is dependent on its transcriptional activity and proceeds by sequential induction of antioxidant and proapoptotic targets. However, none of the thus far renowned p53 targets have proved able to abolish on their own the intracellular reactive oxygen species (ROS) accumulation caused by p53 deficiency, therefore pointing to the existence of other prominent and yet unknown p53 antioxidant targets. Here, we show that TP53INP1 represents such a target. Indeed, TP53INP1 transcript induction on oxidative stress is strictly dependent on p53. Mouse embryonic fibroblasts (MEF) and splenocytes derived from TP53INP1-deficient (inp1(-/-)) mice accumulate intracellular ROS, whereas overexpression of TP53INP1 in p53-deficient MEFs rescues ROS levels to those of p53-proficient cells, indicating that TP53INP1 antioxidant function is p53 independent. Furthermore, accumulation of ROS in inp1(-/-) cells on oxidant challenge is associated with decreased expression of p53 targets p21/Cdkn1a, Sesn2, TAp73, Puma, and Bax. Mutation of p53 Ser(58) (equivalent to human p53 Ser(46)) abrogates transcription of these genes, indicating that TP53INP1-mediated p53 Ser(58) phosphorylation is implicated in this process. In addition, TP53INP1 deficiency results in an antioxidant (N-acetylcysteine)-sensitive acceleration of cell proliferation. Finally, TP53INP1 deficiency increases oxidative stress-related lymphoma incidence and decreases survival of p53(+/-) mice. In conclusion, our data show that TP53INP1 is a major actor of p53-driven oxidative stress response that possesses both a p53-independent intracellular ROS regulatory function and a p53-dependent transcription regulatory function.

Roles for microRNAs, miR-93 and miR-130b, and tumor protein 53-induced nuclear protein 1 tumor suppressor in cell growth dysregulation by human T-cell lymphotrophic virus 1

A role for microRNAs (miRNA) in human T-cell leukemia virus 1 (HTLV-1)-mediated cellular transformation has not been described. Here, we profiled miRNA expression in HTLV-1-transformed human T-cell lines and primary peripheral blood mononuclear cells from adult T-cell leukemia patients. Analyses of 11 different profiles revealed six miRNAs that were consistently up-regulated. Two of the up-regulated miRNAs (miR-93 and miR-130b) target the 3' untranslated region (3'UTR) of the mRNA for a tumor suppressor protein, tumor protein 53-induced nuclear protein 1 (TP53INP1). A low expression level of TP53INP1 protein was found in HTLV-1-transformed cells. Additionally, when antagomirs were used to knock down miR-93 and miR-130b in these cells, the expression of TP53INP1 was increased, suggesting that the latter is regulated inside cells by the former. A role for TP53INP1 in regulating cell growth was established by experiments that showed that enhanced TP53INP1 expression increased apoptosis. Collectively, the findings implicate a miR-93/miR-130b-TP53INP1 axis that affects the proliferation and survival of HTLV-1-infected/transformed cells.

The TP53INP2 protein is required for autophagy in mammalian cells

Using a bioinformatic approach, we identified a TP53INP1-related gene encoding a protein with 30% identity with tumor protein 53-induced nuclear protein 1 (TP53INP1), which was named TP53INP2. TP53INP1 and TP53INP2 sequences were found in several species ranging from Homo sapiens to Drosophila melanogaster, but orthologues were found neither in earlier eukaryotes nor in prokaryotes. To gain insight into the function of the TP53INP2 protein, we carried out a yeast two-hybrid screening that showed that TP53INP2 binds to the LC3-related proteins GABARAP and GABARAP-like2, and then we demonstrated by coimmunoprecipitation that TP53INP2 interacts with these proteins, as well as with LC3 and with the autophagosome transmembrane protein VMP1. TP53INP2 translocates from the nucleus to the autophagosome structures after activation of autophagy by rapamycin or starvation. Also, we showed that TP53INP2 expression is necessary for autophagosome development because its small interfering RNA-mediated knockdown strongly decreases sensitivity of mammalian cells to autophagy. Finally, we found that interactions between TP53INP2 and LC3 or the LC3-related proteins GABARAP and GABARAP-like2 require autophagy and are modulated by wortmannin as judged by bioluminescence resonance energy transfer assays. We suggest that TP53INP2 is a scaffold protein that recruits LC3 and/or LC3-related proteins to the autophagosome membrane by interacting with the transmembrane protein VMP1. It is concluded that TP53INP2 is a novel gene involved in the autophagy of mammalian cells.

E2F - at the crossroads of life and death

The retinoblastoma tumor suppressor, pRb, restricts cell-cycle progression mainly by regulating members of the E2F-transcription-factor family. The Rb pathway is often inactivated in human tumors, resulting in deregulated-E2F activity that promotes proliferation or cell death, depending on the cellular context. Specifically, the outcome of deregulated-E2F activity is determined by integration of signals coming from the cellular DNA and the external environment. Alterations in cell proliferation and cell-death pathways are key features of transformed cells and, therefore, an understanding of the variables that determine the outcome of E2F activation is pivotal for cancer research and treatment. In this review, we discuss recent studies that have elucidated some of the signals affecting E2F activity and that have revealed additional E2F targets and functions, thereby enriching the understanding of this versatile transcription-factor family.

Identification of E1AF as a target gene of E2F1-induced apoptosis in response to DNA damage

Transcription factor E1AF plays critical roles in neuronal development and tumour metastasis and is regulated by a number of signalling cascades, including the mitogen-activated protein kinase pathways. Accumulated evidence indicted that E1AF might contribute to cell survival in response to environment factors. Here, we provided evidence the cell cycle and apoptosis regulator E2F1 induces E1AF expression at the transcriptional level. DNA damage by etoposide causes E2F1-dependent induction of E1AF expression at transcriptional level. Furthermore, disruption of E1AF expression by E1AF RNAi decreased E2F1-induced apoptosis in response to etoposide. Thus, we conclude that activation of E1AF provides a means for E2F1 to induce cell apoptosis in response to DNA damage.

Hepatitis B virus X protein via the p38MAPK pathway induces E2F1 release and ATR kinase activation mediating p53 apoptosis

Hepatitis B virus (HBV) X protein (pX) is implicated in hepatocellular carcinoma (HCC) pathogenesis by an unknown mechanism. Deletions or mutations of genes involved in the p53 pathway are often associated with HBV-mediated HCC, indicating rescue from p53 apoptosis is a likely mechanism in HBV-HCC pathogenesis. Herein, we determined the mechanism by which pX sensitizes hepatocytes to p53-mediated apoptosis. Although it is well established that the Rb/E2F/ARF pathway stabilizes p53, and the DNA damage-activated ATM/ATR kinases activate p53, the mechanism that coordinates these two pathways has not been determined. We demonstrate that the p38MAPK pathway activated by pX serves this role in p53 apoptosis. Specifically, the activated p38MAPK pathway stabilizes p53 via E2F1-mediated ARF expression, and also activates the transcriptional function of p53 by activating ATR. Knockdown of p53, E2F1, ATR, or p38MAPKalpha abrogates pX-mediated apoptosis, demonstrating that E2F1, ATR, and p38MAPKalpha are all essential in p53 apoptosis in response to pX. Specifically, in response to pX expression, the p38MAPK pathway activates Cdk4 and Cdk2, leading to phosphorylation of Rb, release of E2F1, and transcription of ARF. The p38MAPK pathway also activates ATR, leading to phosphorylation of p53 on Ser-18 and Ser-23, transcription of pro-apoptotic genes Bax, Fas, and Noxa, and apoptosis. In conclusion, pX sensitizes hepatocytes to p53 apoptosis via activation of the p38MAPK pathway, which couples p53 stabilization and p53 activation, by E2F1 induction and ATR activation, respectively.

Life and death decisions by the E2F transcription factors

The E2F transcription factors are critical regulators of genes required for appropriate progression through the cell cycle, and in special circumstances they can also promote the expression of another class of genes that function in the apoptotic program. Since E2Fs can initiate both cell proliferation and cell death, it is not surprising that the pro-apoptotic capacity of these proteins is subject to complex regulation. Recent study has expanded our knowledge of the factors influencing E2F-induced apoptosis as well as downstream targets of E2F in this process.

Tumor protein 53-induced nuclear protein 1 expression is repressed by miR-155, and its restoration inhibits pancreatic tumor development

Pancreatic cancer is a disease with an extremely poor prognosis. Tumor protein 53-induced nuclear protein 1 (TP53INP1) is a proapoptotic stress-induced p53 target gene. In this article, we show by immunohistochemical analysis that TP53INP1 expression is dramatically reduced in pancreatic ductal adenocarcinoma (PDAC) and this decrease occurs early during pancreatic cancer development. TP53INP1 reexpression in the pancreatic cancer-derived cell line MiaPaCa2 strongly reduced its capacity to form s.c., i.p., and intrapancreatic tumors in nude mice. This anti-tumoral capacity is, at least in part, due to the induction of caspase 3-mediated apoptosis. In addition, TP53INP1(-/-) mouse embryonic fibroblasts (MEFs) transformed with a retrovirus expressing E1A/ras(V12) oncoproteins developed bigger tumors than TP53INP1(+/+) transformed MEFs or TP53INP1(-/-) transformed MEFs with restored TP53INP1 expression. Finally, TP53INP1 expression is repressed by the oncogenic micro RNA miR-155, which is overexpressed in PDAC cells. TP53INP1 is a previously unknown miR-155 target presenting anti-tumoral activity.

TFDP3 inhibits E2F1-induced, p53-mediated apoptosis

By dimerizing with E2F proteins, TFDP has profound influence on cellular E2F activities. While TFDP1 and 2 enhance the DNA binding and the transcriptional activity of E2F, the newly identified member of the DP family, TFDP3 primarily functions as a negative regulator. To further characterize the inhibitory property of TFDP3, the present study specifically examined the modulatory role of TFDP3 on E2F1-induced cell death. HEK-293 cells underwent apoptosis following ectopic expression of E2F1. This effect was virtually abolished by co-transfection with TFDP3. In the meantime, the accumulation of p53 proteins and the increased expression of the pro-apoptotic molecules, including Bax, Puma, Noxa, and Bid were found to be suppressed. These data suggest a new mechanism for the regulation of E2F1-induced apoptosis and provide further evidence for the general involvement of TFDP3 in the regulation of E2F functions.

The coding region of TP53INP2, a gene expressed in the developing nervous system, is not altered in a family with autosomal recessive non-progressive infantile ataxia on chromosome 20q11-q13

The locus for autosomal recessive infantile cerebellar ataxia (CLA3 or SCAR6) has been mapped to chromosome 20q11-q13 in a single Norwegian pedigree. We identified a relatively uncharacterised mouse gene Tp53inp2, and showed that its human orthologue mapped within this candidate interval. Tp53inp2 appears to encode a mammalian-specific protein with homology to the two Tp53inp1 isoforms that respond to cellular stress and interact with p53. We show that Tp53inp2 expression is highly restricted during mouse embryogenesis, with strong expression in the developing brain and spinal cord, as well as in the sensory and motor neuron tracts of the peripheral nervous system. Given this expression pattern, the neurological phenotype of CLA3 and the chromosomal localisation of TP53INP2, we searched the coding region for mutations in samples from individuals from the CLA3 pedigree. Our failure to detect causative mutations suggests that alterations in the coding region of TP53INP2 are not responsible for ataxia in this family, although we cannot rule out changes in non-coding elements of this gene.

E2F1 death pathways as targets for cancer therapy

Defects in apoptotic programs contribute to a number of human diseases, ranging from neurodegenerative disorders to malignancy, and treatment failure. The genetic basis for apoptosis implies that cell death can be disrupted by mutations, raising the intriguing possibility that cell numbers can be regulated by factors that influence cell survival. It is well documented that the E2F1 transcription factor is a key regulator of apoptotic programs. E2F1-induced cell death occurs via multiple pathways, some of which involve the tumour suppressor p53, and autonomous of p53. This has led to the opinion that E2F1 functions as a tumour surveillance factor, detecting aberrant proliferation and engaging apoptotic pathways to protect the organism from developing tumours. Frequently, novel players are discovered that expand the interpretation of apoptosis control by E2F1. This information will help to produce new strategies to exploit E2F1-induced apoptosis for therapeutic benefit.

Loss of robustness and addiction to IGF1 during early keratinocyte transformation by human Papilloma virus 16

Infection of keratinocytes with high risk human Papilloma virus causes immortalization, and when followed by further mutations, leads to cervical cancer and other anogenital tumors. Here we monitor the progressive loss of robustness in an in vitro model of the early stages of transformation that comprises normal keratinocytes and progressive passages of HPV16 immortalized cells. As transformation progresses, the cells acquire higher proliferation rates and gain the ability to grow in soft agar. Concurrently, the cells lose robustness, becoming more sensitive to serum starvation and DNA damage by Cisplatin. Loss of robustness in the course of transformation correlates with significant reductions in the activities of the anti-apoptotic proteins PKB/Akt, Erk, Jnk and p38 both under normal growth conditions and upon stress. In parallel, loss of robustness is manifested by the shrinkage of the number of growth factors that can rescue starving cells from apoptosis, with the emergence of dependence solely on IGF1. Treatment with IGF1 activates PKB/Akt and Jnk and through them inhibits p53, rescuing the cells from starvation. We conclude that transformation in this model induces higher susceptibility of cells to stress due to reduced anti-apoptotic signaling and hyper-activation of p53 upon stress.

Differential effects of c-Ras upon transformation, adipocytic differentiation, and apoptosis mediated by the simian virus 40 large tumor antigen

To investigate the functional relationship between the ability of the simian virus 40 large tumor antigen (TAg) to transform and its ability to block adipocytic differentiation and induce apoptosis, we expressed TAg in C3H10T1/2 (10T1/2)-derived preadipocytes. The results demonstrated that differentiation could be suppressed at lower TAg levels than at the levels required for full neoplastic conversion. Progressively higher TAg levels were accompanied by apoptosis induction in this system. To further examine the role of the cellular Ras protooncogene product (Ras) in TAg function, TAg was expressed in 10T1/2-derived preadipocytes rendered deficient in Ras activity by transfection with inducible or constitutive antisense ras gene constructs. The results indicated that Ras is required for TAg-mediated transformation and for suppression of adipocytic differentiation, while TAg-mediated apoptosis following serum starvation was independent from Ras action. Unexpectedly, our results further demonstrated a dramatic reduction in the levels of the TAg protein itself as differentiation progressed in Ras-knockdown cells, with a concomitant reduction in TAg's ability to induce apoptosis as a result. These findings suggest that Ras, although cytoplasmic, is an integral component of the pathway whereby TAg, an oncoprotein believed to have primarily nuclear targets, suppresses differentiation or induces neoplastic conversion of murine preadipocytes.

Colitis and colitis-associated cancer are exacerbated in mice deficient for tumor protein 53-induced nuclear protein 1

Tumor protein 53-induced nuclear protein 1 (TP53INP1) is an antiproliferative and proapoptotic protein involved in cell stress response. To address its physiological roles in colorectal cancer and colitis, we generated and tested the susceptibility of Trp53inp1-deficient mice to the development of colorectal tumors induced by injection of the carcinogen azoxymethane followed by dextran sulfate sodium (DSS)-induced chronic colitis. Trp53inp1-deficient mice showed an increased incidence and multiplicity of tumors compared to those of wild-type (WT) mice. Furthermore, acute colitis induced by DSS treatment was more severe in Trp53inp1-deficient mice than in WT mice. Treatment with the antioxidant N-acetylcysteine prevented colitis and colitis-associated tumorigenesis more efficiently in WT mice than in Trp53inp1-deficient mice, suggesting a higher oxidative load in the latter. Consistently, we demonstrated by electron spin resonance and spin trapping that colons derived from deficient mice produced more free radicals than those of the WT during colitis and that the basal blood level of the antioxidant ascorbate was decreased in Trp53inp1-deficient mice. Collectively, these results indicate that the oxidative load is higher in Trp53inp1-deficient mice than in WT mice, generating a more-severe DSS-induced colitis, which favors development of colorectal tumors in Trp53inp1-deficient mice. Therefore, TP53INP1 is a potential target for the prevention of colorectal cancer in patients with inflammatory bowel disease.

ASPP: a new family of oncogenes and tumour suppressor genes

The apoptosis stimulating proteins of p53 (ASPP) family consists of three members, ASPP1, ASPP2 and iASPP. They bind to proteins that are key players in controlling apoptosis (p53, Bcl-2 and RelA/p65) and cell growth (APCL, PP1). So far, the best-known function of the ASPP family members is their ability to regulate the apoptotic function of p53 and its family members, p63 and p73. Biochemical and genetic evidence has shown that ASPP1 and ASPP2 activate, whereas iASPP inhibits, the apoptotic but not the cell-cycle arrest function of p53. The p53 tumour suppressor gene, one of the most frequently mutated genes in human cancer, is capable of suppressing tumour growth through its ability to induce apoptosis or cell-cycle arrest. Thus, the ASPP family of proteins helps to determine how cells choose to die and may therefore be a novel target for cancer therapy.

E2F1 modulates p38 MAPK phosphorylation via transcriptional regulation of ASK1 and Wip1

The E2F family of transcription factors regulates a diverse array of cellular functions, including cell proliferation, cell differentiation, and apoptosis. Recent studies indicate that E2F can also regulate transcription of upstream components of signal transduction pathways. We show here that E2F1 modulates the activity of the p38 MAPK pathway via E2F1-induced transient up-regulation of p38 MAPK phosphorylation. The mechanism by which E2F1 modulates p38 MAPK phosphorylation involves transcriptional induction of the kinase ASK1, a member of the MAPKKK family that phosphorylates p38 MKKs. Subsequent E2F-dependent down-regulation of the p38 signaling pathway is achieved through E2F-induced up-regulation of Wip1, a phosphatase that dephosphorylates and inactivates p38. Both ASK1 and Wip1 are essential mediators of the E2F-p38 connection: knock down of ASK1 inhibits E2F1-induced phosphorylation of p38, whereas knock down of Wip1 prolongs E2F1-induced p38 phosphorylation. Furthermore, Wip1 knock down enhances E2F1-induced apoptosis. Therefore, our data reveal a novel link between a central signaling pathway and the transcription factor E2F and identify Wip1 as a modulator of E2F1-induced apoptosis.

Adenovirus-mediated p53 tumor suppressor gene therapy of osteosarcoma

The clinical outcome for osteosarcoma (OS) remains discouraging despite efforts to optimize treatment using conventional modalities including surgery, radiotherapy and chemotherapy. Novel therapeutic approaches based on our expanding understanding of the mechanisms of tumor cell killing have the potential to alter this situation. Tumor suppressor gene therapy aims to restore the function of a tumor suppressor gene lost or functionally inactivated in cancer cells. One such molecule, the p53 tumor suppressor gene plays a critical role in safeguarding the integrity of the genome and preventing tumorigenesis. Introduction of wild-type (wt) p53 into transformed cells has been shown to be lethal for most cancer cells in vitro, but clinical trials of p53 gene replacement have had limited success. Analysis of these clinical trials highlighted the insufficient efficacy of current vectors and low proapoptotic activity of wt p53 as a single agent in vivo. In this review, a contemporary summarization of the current status of adenovirus-mediated p53 gene therapy of OS is presented. Advancement in our understanding of p53 tumor suppressor activity, the molecular biology of chemoresistant OS, and recent advances in tumor targeting with adenoviral vectors are also addressed. Based on these parameters, prospects for future investigations are proposed.

RETRACTED: The E3 SUMO Ligase PIASy Is a Regulator of Cellular Senescence and Apoptosis

Cellular senescence and apoptosis have evolved to restrain unwarranted proliferation of potentially tumorigenic cells. Here we show that overexpression of the E3 SUMO ligase PIASy in normal human fibroblasts recruits the p53 and Rb tumor suppressor pathways to provoke a senescence arrest. By contrast, in Rb-deficient fibroblasts, expression of PIASy leads to p53-dependent apoptosis. Induction of senescence requires PIASy E3 activity and is specific for this member of the PIAS ligase family. PIASy stimulates sumoylation and transcriptional activity of p53 and increases Rb-dependent corepression through recruitment to E2F-responsive promoters. Viral oncoprotein E6 suppresses both PIASy-induced senescence and sumoylation of PIASy substrates. Finally, we show that fibroblasts lacking PIASy exhibit a highly reduced propensity to undergo senescence in response to a prosenescence stimulus. Altogether, these data provide the first evidence for a direct role of an E3 SUMO ligase, and by implication of the SUMO pathway, in cellular senescence and apoptosis.

DNA-binding independent cell death from a minimal proapoptotic region of E2F-1

The ability to induce cell cycle progression while evading cell death is a defining characteristic of cancer. Deregulation of E2F is a common event in most human cancers. Paradoxically, this can lead to both cell cycle progression and apoptosis. Although the way in which E2F causes cell cycle progression is well characterized, the pathways by which E2F induces cell death are less well defined. Many of the known mechanisms through which E2F induces apoptosis occur through regulation of E2F target genes. However, mutants of E2F-1 that lack the transactivation domain are still able to induce cell death. To further investigate this activity, we refined a transactivation independent mutant to identify a minimal apoptotic domain. This revealed that only 75 amino acids from within the DNA-binding domain of E2F-1 is sufficient for cell death and that this activity is also present in the DNA-binding domains of E2F-2 and E2F-3. However, analysis of this domain from E2F-1 revealed it does not bind DNA and is consequently unable to transactivate, repress or de-repress E2F target genes. This provocative observation therefore defines a potential new mechanism of death from E2F and opens up new opportunities for inducing cell death in tumours for therapeutic gain.

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.

ASPP [corrected] and cancer

One of the most frequently mutated genes in human cancers, tumour suppressor p53 (TP53), can induce cell-cycle arrest and apoptosis. The apoptotic function of p53 is tightly linked to its tumour-suppression function and the efficacy of many cancer therapies depends on this. The identification of a new family of proteins, known as ASPPs (ankyrin-repeat-, SH3-domain- and proline-rich-region-containing proteins), has led to the discovery of a novel mechanism that selectively regulates the apoptotic function, but not the cell-cycle-arrest function, of p53, and gives an insight into how p53 responds to different stress signals. ASPPs might be new molecular targets for cancer therapy.

Involvement of E2F transcription factor family in cancer

The E2F family of transcription factors is a central modulator of important cellular events, including cell cycle progression, apoptosis and DNA damage response. The role of E2F family members in various human malignancies is yet unclear and may provide vital clues to the diagnosis, prognosis and therapy of cancer patients. In this review we provide a brief but concise overview of E2F function and its putative role in the most common human tumour types.

Control of ASPP2/(53BP2L) protein levels by proteasomal degradation modulates p53 apoptotic function

The p53 pathway is a central mediator of the apoptotic response. ASPP2/(53BP2L) (apoptosis-stimulating protein of p53 2, also known as 53BP2L) enhances apoptosis through selective stimulation of p53 transactivation of proapoptotic target genes. Although the Rb/E2F pathway regulates ASPP2/(53BP2L) transcription, the complex mechanisms controlling ASPP2/(53BP2L) levels and function remain unknown. We now report that proteasomal degradation modulates ASPP2/(53BP2L) protein levels and apoptotic function. Treatment of cells with proteasomal inhibitors, including the clinically utilized proteasomal inhibitor bortezomib, increases ASPP2/(53BP2L) protein but not RNA levels. Likewise, anthracycline-based chemotherapy, which has multiple mechanisms of action, including proteasomal inhibition, increases ASPP2/(53BP2L) protein but not RNA levels. Proteasomal inhibition or anthracycline treatment increases ASPP2/(53BP2L) protein stability and half-life. Furthermore, the central region of the ASPP2/(53BP2L) protein is ubiquitinated as would be expected for a proteasomal substrate. More importantly, small interfering RNA knockdown of ASPP2/(53BP2L) levels attenuated bortezomib-induced apoptosis, and this effect was greater in wild-type p53 cells. Because elevated levels of ASPP2/(53BP2L) are proapoptotic, these results described an important new molecular mechanism that modulates the p53-ASPP2/(53BP2L) apoptotic pathway.