Abstract P3-06-03: Copy number analysis identifies ESR1 and MDM4 as drivers of progression in invasive lobular breast carcinoma

Background: Invasive lobular carcinoma (ILC) is the second most common histological subtype of breast cancer after invasive ductal carcinoma (IDC). While specific clinical and pathological features differ between ILC and IDC, both histologies are treated the same, due to a lack of knowledge of targetable pathways underlying the observed differences. To identify potential genetic drivers of ILC progression, we set out to identify genes with copy number (CN) alterations, comparing tumors with good outcome to those with poor outcome.

Method: We designed probes for a total of 67 genes known to be frequently altered in breast cancer and used sensitive nanoString technology to comprehensively investigate CN alterations of these genes in 70 well-curated primary ILCs. ILC cell lines MDA-MB-134-VI, SUM44PE, and BCK4 were used for functional studies including proliferation, apoptosis, colony formation, and analysis of gene expression.

Results: Our studies reveal that ESR1 is frequently amplified in primary ILC (14% gains and 10% amplification), and that tumors with amplified ESR1 are more likely to recur compared to those with normal CN. Our analysis also identified a subset of ILCs with HER2 amplification (19%) despite a negative clinical IHC score, and these tumors expressed high HER2 mRNA, protein, and demonstrated enrichment of a molecular HER2 signature. The other most frequently amplified genes included CCND1 (33%), MDM4 (17%), and MYC (17%), and most frequently lost genes were NCOR2 (7%), FGFR4 (6%) and TP53 (6%). MDM4, a negative regulator of p53, has previously been reported to play a role in breast cancer, though little is known about its role in ILC. We demonstrate that decreasing MDM4 levels in p53 wild type ILC cell lines results in increased apoptosis, decreased proliferation associated with cell cycle arrest, and activation of p53 target genes. Intriguingly, a similar induction of G0/G1 cell cycle arrest and increase in apoptosis was observed in p53 mutant ILC cells after MDM4 downregulation, suggesting a p53-independent function of MDM4.

Conclusion: Sensitive detection of CN changes identified amplifications of ESR1 and MDM4 as potential drivers of ILC. Functional studies demonstrate that MDM4 has both p53 dependent and independent functions that warrant further study.

Citation Format: Atkinson JM, Cao L, Basudan A, Sikora MJ, Bahreini A, Tasdemir N, Jankowitz RC, McAuliffe PF, Dabbs D, Haupt S, Haupt Y, Peter Lucas PC, Lee AV, Oesterreich S. Copy number analysis identifies ESR1 and MDM4 as drivers of progression in invasive lobular breast carcinoma [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-06-03.

Abstract P4-04-12: Therapeutic targeting of CDK4/6 inhibitor resistant breast cancer

Background:

Selective inhibitors of CDK4/6 kinases (CDK4/6i) were recently FDA approved for use in combination with endocrine therapy (ET), and represent the new standard of care. There are however patients who do not respond or develop resistance to these treatments, and therapies are required in this setting. While there is emerging data on the mechanisms of intrinsic insensitivity to CDK4/6i as monotherapies, which include cyclin E1 amplification, CDK6 amplification and Rb deletion, there is little data on mechanisms of resistance to combined ET and CDK4/6i.

Methods:

We established MCF7 cell line and patient-derived xenograft (PDX) models that are resistant to combined ET and Palbociclib (CDK4/6i) through long-term culture, allowing us to better understand mechanisms underlying CDK4/6i resistance and to model therapeutic strategies in this setting. We also evaluated our therapeutic strategy in vitro and in vivo using MCF cell lines that are resistant to ET, and in an ER+ PDX model derived from a patient who progressed on ET.

Results:

Cells resistant to CDK4/6i alone and in combination with ET show disrupted senescent pathways, and insensitivity to the induction of senescence. MDM2 inhibitors induce cells to enter into senescence, and consequently we are investigating the use of a new generation MDM2 inhibitor (CGM097, Novartis) either in combination with CDK4/6i treatment, or following acquisition of CDK4/6i resistance to prevent exit from senescence. We evaluated a CGM097 either in combination with CDK4/6i treatment, or in combination with fulvestrant following acquisition of CDK4/6i resistance to prevent exit from senescence. CGM097 was effective alone or in combination with fulvestrant in CDK4/6i resistant cells in vitro and in vivo, and resulted in a loss of G1 cells, and a reduction in B galactosidase, a senescence marker.

Another mechanisms of CDK4/6i resistance that has been identified is CDK2 activation, which can occur through Cyclin E amplification. As a second therapeutic strategy, we screened a panel of pan-CDK inhibitors with CDK2 activity in our resistant lines, and identified that CYC065 (Cyclacel), a highly selective CDK2/9 inhibitor, had the most durable response and highest synergy with ET in long-term culture. The combined resistant models were sensitive to CYC065 in vitro and in vivo. CYC065 was mechanistically distinct to CDK4/6i's as it caused arrest in a different phase of the cell cycle and affected expression of different cell cycle proteins.

Conclusion:

An underlying mechanism of combined ET and CDK4/6i resistance is senescent escape, which allows for normal proliferation upon removal of the drug. Using our in vitro and in vivo models of combined ET and CDK4/6i resistance, we have identified two novel therapeutic strategies for this disease, which represents the next clinical challenge in ER+ breast cancer as the natural history of disease is changed with the increasing use of CDK4/6i.

Citation Format: Lim E, Portman N, Alexandrou S, Haupt S, Haupt Y, Caldon E. Therapeutic targeting of CDK4/6 inhibitor resistant breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-04-12.

Uncovering a novel pathway for p16 silencing: Therapeutic implications for lung cancer

A key step during onset of most cases of non-small cell lung cancer (NSCLC) is the loss of the tumor suppressor p16^INK4a (best known as p16), commonly due to promoter hypermethylation. We recently reported a novel regulatory pathway involving E6-associated protein and cell division control protein 6, which provides a methylation-independent mechanism for p16 silencing in patients with a particularly aggressive form of NSCLC.

Restoration of tumor suppression in prostate cancer by targeting the E3 ligase E6AP

Restoration of tumor suppression is an attractive onco-therapeutic approach. It is particularly relevant when a tumor suppressor is excessively degraded by an overactive oncogenic E3 ligase. We previously discovered that the E6-associated protein (E6AP; as classified in the human papilloma virus context) is an E3 ligase that has an important role in the cellular stress response, and it directly targets the tumor-suppressor promyelocytic leukemia protein (PML) for proteasomal degradation. In this study, we have examined the role of the E6AP-PML axis in prostate cancer (PC). We show that knockdown (KD) of E6AP expression attenuates growth of PC cell lines in vitro. We validated this finding in vivo using cell line xenografts, patient-derived xenografts and mouse genetics. We found that KD of E6AP attenuates cancer cell growth by promoting cellular senescence in vivo, which correlates with restoration of tumor suppression by PML. In addition, we show that KD of E6AP sensitizes cells to radiation-induced death. Overall, our findings demonstrate a role for E6AP in the promotion of PC and support E6AP targeting as a novel approach for PC treatment, either alone or in combination with radiation.

Targeting Mdmx to treat breast cancers with wild-type p53

The function of the tumor suppressor p53 is universally compromised in cancers. It is the most frequently mutated gene in human cancers (reviewed). In cases where p53 is not mutated, alternative regulatory pathways inactivate its tumor suppressive functions. This is primarily achieved through elevation in the expression of the key inhibitors of p53: Mdm2 or Mdmx (also called Mdm4) (reviewed). In breast cancer (BrCa), the frequency of p53 mutations varies markedly between the different subtypes, with basal-like BrCas bearing a high frequency of p53 mutations, whereas luminal BrCas generally express wild-type (wt) p53. Here we show that Mdmx is unexpectedly highly expressed in normal breast epithelial cells and its expression is further elevated in most luminal BrCas, whereas p53 expression is generally low, consistent with wt p53 status. Inducible knockdown (KD) of Mdmx in luminal BrCa MCF-7 cells impedes the growth of these cells in culture, in a p53-dependent manner. Importantly, KD of Mdmx in orthotopic xenograft transplants resulted in growth inhibition associated with prolonged survival, both in a preventative model and also in a treatment model. Growth impediment in response to Mdmx KD was associated with cellular senescence. The growth inhibitory capacity of Mdmx KD was recapitulated in an additional luminal BrCa cell line MPE600, which expresses wt p53. Further, the growth inhibitory capacity of Mdmx KD was also demonstrated in the wt p53 basal-like cell line SKBR7 line. These results identify Mdmx growth dependency in wt p53 expressing BrCas, across a range of subtypes. Based on our findings, we propose that Mdmx targeting is an attractive strategy for treating BrCas harboring wt p53.

Expression of E6AP and PML predicts for prostate cancer progression and cancer-specific death

BACKGROUND

The promyelocytic leukemia (PML) tumor suppressor plays an important role in the response to a variety of cellular stressors and its expression is downregulated or lost in a range of human tumors. We have previously shown that the E3 ligase E6-associated protein (E6AP) is an important regulator of PML protein stability but the relationship and clinical impact of PML and E6AP expression in prostatic carcinoma is unknown.

METHODS

E6AP and PML expression was assessed in tissue microarrays from a phase I discovery cohort of 170 patients treated by radical prostatectomy for localized prostate cancer (PC). Correlation analysis was carried out between PML and E6AP expression and clinicopathological variates including PSA as a surrogate of disease recurrence. The results were confirmed in a phase II validation cohort of 318 patients with associated clinical recurrence and survival data.

RESULTS

Survival analysis of the phase I cohort revealed that patients whose tumors showed reduced PML and high E6AP expression had reduced time to PSA relapse (P = 0.012). This was confirmed in the phase II validation cohort where the expression profile of high E6AP/low PML was significantly associated with reduced time to PSA relapse (P < 0.001), clinical relapse (P = 0.016) and PC-specific death (P = 0.014). In multivariate analysis, this expression profile was an independent prognostic indicator of PSA relapse and clinical relapse independent of clinicopathologic factors predicting recurrence.

CONCLUSION

This study identifies E6AP and PML as potential prognostic markers in localized prostate carcinoma and supports a role for E6AP in driving the downregulation or loss of PML expression in prostate carcinomas.

AKT induces senescence in human cells via mTORC1 and p53 in the absence of DNA damage: implications for targeting mTOR during malignancy

The phosphatidylinositol 3-kinase (PI3K)/AKT and RAS oncogenic signalling modules are frequently mutated in sporadic human cancer. Although each of these pathways has been shown to play critical roles in driving tumour growth and proliferation, their activation in normal human cells can also promote cell senescence. Although the mechanisms mediating RAS-induced senescence have been well characterised, those controlling PI3K/AKT-induced senescence are poorly understood. Here we show that PI3K/AKT pathway activation in response to phosphatase and tensin homolog (PTEN) knockdown, mutant PI3K, catalytic, α polypeptide (PIK3CA) or activated AKT expression, promotes accumulation of p53 and p21, increases cell size and induces senescence-associated β-galactosidase activity. We demonstrate that AKT-induced senescence is p53-dependent and is characterised by mTORC1-dependent regulation of p53 translation and stabilisation of p53 protein following nucleolar localisation and inactivation of MDM2. The underlying mechanisms of RAS and AKT-induced senescence appear to be distinct, demonstrating that different mediators of senescence may be deregulated during transformation by specific oncogenes. Unlike RAS, AKT promotes rapid proliferative arrest in the absence of a hyperproliferative phase or DNA damage, indicating that inactivation of the senescence response is critical at the early stages of PI3K/AKT-driven tumourigenesis. Furthermore, our data imply that chronic activation of AKT signalling provides selective pressure for the loss of p53 function, consistent with observations that PTEN or PIK3CA mutations are significantly associated with p53 mutation in a number of human tumour types. Importantly, the demonstration that mTORC1 is an essential mediator of AKT-induced senescence raises the possibility that targeting mTORC1 in tumours with activated PI3K/AKT signalling may exert unexpected detrimental effects due to inactivation of a senescence brake on potential cancer-initiating cells.

Tumor suppressor p53 regulates heparanase gene expression

Mammalian heparanase degrades heparan sulfate, the most prominent polysaccharide of the extracellular matrix. Causal involvement of heparanase in tumor progression is well documented. Little is known, however, about mechanisms that regulate heparanase gene expression. Mutational inactivation of tumor suppressor p53 is the most frequent genetic alteration in human tumors. p53 is a transcription factor that regulates a wide variety of cellular promoters. In this study, we demonstrate that wild-type (wt) p53 binds to heparanase promoter and inhibits its activity, whereas mutant p53 variants failed to exert an inhibitory effect. Moreover, p53-H175R mutant even activated heparanase promoter activity. Elimination or inhibition of p53 in several cell types resulted in a significant increase in heparanase gene expression and enzymatic activity. Trichostatin A abolished the inhibitory effect of wt p53, suggesting the involvement of histone deacetylation in negative regulation of the heparanase promoter. Altogether, our results indicate that the heparanase gene is regulated by p53 under normal conditions, while mutational inactivation of p53 during cancer development leads to induction of heparanase expression, providing a possible explanation for the frequent increase of heparanase levels observed in the course of tumorigenesis.

c-Abl regulates p53 levels under normal and stress conditions by preventing its nuclear export and ubiquitination

The p53 protein is subject to Mdm2-mediated degradation by the ubiquitin-proteasome pathway. This degradation requires interaction between p53 and Mdm2 and the subsequent ubiquitination and nuclear export of p53. Exposure of cells to DNA damage results in the stabilization of the p53 protein in the nucleus. However, the underlying mechanism of this effect is poorly defined. Here we demonstrate a key role for c-Abl in the nuclear accumulation of endogenous p53 in cells exposed to DNA damage. This effect of c-Abl is achieved by preventing the ubiquitination and nuclear export of p53 by Mdm2, or by human papillomavirus E6. c-Abl null cells fail to accumulate p53 efficiently following DNA damage. Reconstitution of these cells with physiological levels of c-Abl is sufficient to promote the normal response of p53 to DNA damage via nuclear retention. Our results help to explain how p53 is accumulated in the nucleus in response to DNA damage.

A role for the polyproline domain of p53 in its regulation by Mdm2

The p53 protein plays a key role in the cellular response to stress by inducing cell growth arrest or apoptosis. The polyproline region of p53 has been shown to be important for its growth suppression activity. p53 protein lacking the polyproline region has impaired apoptotic activity and altered specificity for certain apoptotic target genes. Here we describe the role of this region in the regulation of p53 by its inhibitor Mdm2. p53 lacking the polyproline region was identified to be more susceptible to inhibition by Mdm2. Furthermore, the absence of this region renders p53 more accessible to ubiquitination, nuclear export, and Mdm2-mediated degradation. This increased sensitivity to Mdm2 results from an enhanced affinity of Mdm2 toward p53 lacking the polyproline region. Our results provide a new explanation for the impaired growth suppression activity of p53 lacking this region. The polyproline region is proposed to be important in the modulation of the inhibitory effects of Mdm2 on p53 activities and stability.

Transactivation-deficient p73alpha (p73Deltaexon2) inhibits apoptosis and competes with p53

p73 has recently been identified as a structural and functional homolog of the tumor suppressor protein p53. Overexpression of p53 activates transcription of p53 effector genes, causes growth inhibition and induced apoptosis. We describe here the effects of a tumor-derived truncated transcript of p73alpha (p73Deltaexon2) on p53 function and on cell death. This transcript, which lacks the acidic N-terminus corresponding to the transactivation domain of p53, was initially detected in a neuroblastoma cell line. Overexpression of p73Deltaexon2 partially protects lymphoblastoid cells against apoptosis induced by anti-Fas antibody or cisplatin. By cotransfecting p73Deltaexon2 with wild-type p53 in the p53 null line Saos 2, we found that this truncated transcript reduces the ability of wild-type p53 to promote apoptosis. This anti-apoptotic effect was also observed when p73Deltaexon2 was co-transfected with full-length p73 (p73alpha). This was further substantiated by suppression of p53 transactivation of the effector gene p21/Waf1 in p73Deltaexon2 transfected cells and by inhibition of expression of a reporter gene under the control of the p53 promoter. Thus, this truncated form of p73 can act as a dominant-negative agent towards transactivation by p53 and p73alpha, highlighting the potential implications of these findings for p53 signaling pathway. Furthermore, we demonstrate the existence of a p73Deltaexon2 transcript in a very significant proportion (46%) of breast cancer cell lines. However, a large spectrum of normal and malignant tissues need to be surveyed to determine whether this transdominant p73 variant occurs in a tumor-specific manner.

c-Jun and p53 activity is modulated by SUMO-1 modification

The ubiquitin-related SUMO-1 molecule has been shown recently to modify covalently a number of cellular proteins including IkappaBalpha. SUMO-1 modification was found to antagonize IkappaBalpha ubiquitination and protect it from degradation. Here we identify the transcription factors c-Jun and p53, two well known targets of ubiquitin, as new substrates for SUMO-1 both in vitro and in vivo. In contrast to ubiquitin, SUMO-1 preferentially targets a single lysine residue in c-Jun (Lys-229), and the abrogation of SUMO-1 modification does not compromise its ubiquitination. Activation of Jun NH(2)-terminal kinases, which induces a reduction in c-Jun ubiquitination, similarly decreases SUMO-1 modification. Accordingly, loss of the two major Jun NH(2)-terminal kinase phosphorylation sites in c-Jun, Ser-63 and Ser-73, greatly enhances conjugation by SUMO-1. A SUMO-1- deficient c-JunK229R mutant shows an increased transactivation potential on an AP-1-containing promoter compared with wild-type c-Jun, suggesting that SUMO-1 negatively regulates c-Jun activity. As with c-Jun, SUMO-1 modification of p53 is abrogated by phosphorylation but remains unaltered upon chemical damage to DNA or Mdm2-mediated ubiquitination. The SUMO-1 attachment site in p53 (Lys-386) resides within a region known to regulate the DNA binding activity of the protein. A p53 mutant, defective for SUMO-1 conjugation, shows unaltered ubiquitination but has a slightly impaired apoptotic activity, indicating that modification by SUMO-1 might be important for the full biological activity of p53. Taken together, these data provide a first link between the SUMO-1 conjugation pathway and the regulation of transcription factors.

p53 and thymic 'death by neglect': thymic epithelial cell-induced apoptosis of CD4+8+ thymocytes is p53-independent

The involvement of the tumor suppressor protein, p53, in thymic epithelial cell-induced apoptosis of CD4+8+ (double positive) thymocytes, was studied in an in vitro model consisting of a thymic epithelial cell line (TEC) and thymocytes. p53 expression was not augmented in double positive (DP) thymocytes upon co-culturing with TEC, although extensive apoptosis was observed. In the same cells, p53 expression was upregulated in response to low ionizing irradiation, which was accompanied with massive apoptosis. Moreover, TEC induced apoptosis in two DP thymomas, derived from p53(-/-) mice, and in a double positive thymoma clone expressing mutant p53. The extent and kinetics of TEC-induced apoptosis was not affected by the status of p53 in the thymocytes tested. We conclude that thymic epithelial cell-induced apoptosis of immature DP thymocytes is p53-independent and apparently, involves a different apoptotic pathway than that triggered by DNA damage.

The cellular response to p53: the decision between life and death

The p53 tumor suppressor protein plays a crucial role in regulating cell growth following exposure to various stress stimuli. p53 induces either growth arrest, which prevents the replication of damaged DNA, or programmed cell death (apoptosis), which is important for eliminating defective cells. Whether the cell enters growth arrest or undergoes apoptosis, depends on the final integration of incoming signals with antagonistic effects on cell growth. Many factors affect the cellular response to activated p53. These include the cell type, the oncogenic status of the cell with emphasis on the Rb/E2F balance, the extracellular growth and survival stimuli, the intensity of the stress signals, the level of p53 expression and the interaction of p53 with specific proteins. p53 is regulated both at the levels of protein stability and biochemical activities. This complex regulation is mediated by a range of viral and cellular proteins. This review discusses this intriguing complexity which affects the cell response to p53 activation.

Mutations in serines 15 and 20 of human p53 impair its apoptotic activity

Phosphorylation of the p53 tumor suppressor protein is likely to play an important role in regulating its activity. To study the regulatory role of potential phosphorylation sites within the N-terminal transactivation domain of human p53 (hp53), a series of p53 serine mutants were evaluated for transcriptional transactivation and sequence specific DNA binding. The role of these mutations in regulating p53-mediated growth suppression and programmed cell death was examined. This mutational analysis comprised serine residues located at positions 6, 9, 15, 20, 33 and 37 of human p53. Substitution of serine for alanine, either at individual residues or at all six residues together, did not affect the suppression of cell growth and cell transformation, or the ability to bind DNA specifically and to transactivate different promoters, nor did it alter p53 expression. However, the ability of p53 to induce apoptosis was impaired by specific serine substitutions. Mutations in all six N-terminal serines together reduced the apoptotic activity of p53 in H1299 cells by 50%. Analysis of individual mutants revealed that mutations in serine 15 and 20 are primarily responsible for this impairment. Our results suggest that these serines play a role in the regulation of p53-mediated apoptosis.

Critical role for Ser20 of human p53 in the negative regulation of p53 by Mdm2

In response to environmental stress, the p53 phosphoprotein is stabilized and activated to inhibit cell growth. p53 stability and activity are negatively regulated by the murine double minute (Mdm2) oncoprotein in an autoregulatory feedback loop. The inhibitory effect of Mdm2 on p53 has to be tightly regulated for proper p53 activity. Phosphorylation is an important level of p53 regulation. In response to DNA damage, p53 is phosphorylated at several N-terminal serines. In this study we examined the role of Ser20, a potential phosphorylation site in human p53, in the regulation of p53 stability and function. Substitution of Ser20 by Ala (p53-Ala20) significantly increases the susceptibility of human p53 to negative regulation by Mdm2 in vivo, as measured by apoptosis and transcription activation assays. Mutation of Ser20 to Ala renders p53 less stable and more prone to Mdm2-mediated degradation. While the in vitro binding of p53 to Mdm2 is not increased by the Ala20 mutation, the same mutation results in a markedly enhanced binding in vivo. This is consistent with the conclusion that phosphorylation of Ser20 in vivo attenuates the binding of wild-type p53 to Mdm2. Peptides bearing non-phosphorylated Ser20 or Ala20 compete with p53 for Mdm2 binding, while a similar peptide with phosphorylated Ser20 does not. This implies a critical role for Ser20 in modulating the negative regulation of p53 by Mdm2, probably through phosphorylation-dependent inhibition of p53-Mdm2 interaction.

c-Abl neutralizes the inhibitory effect of Mdm2 on p53

Upon exposure to stress signals, the p53 tumor suppressor protein is stabilized and induces growth suppression. p53 activities are efficiently inhibited by the Mdm2 oncoprotein through an autoregulatory feedback loop. In addition, Mdm2 promotes p53 degradation, thereby terminating its growth inhibitory signal. Hence, p53 exerts its effects during the interval between p53 activation and the subsequent inhibition by Mdm2. Modulation of this interval by regulatory proteins may determine the extent and duration of p53 activity. Recent studies have shown that the c-Abl protein-tyrosine kinase binds p53 and enhances its transcriptional activity. Here we provide an explanation for the cooperation between these proteins. We demonstrate that c-Abl increases the expression level of the p53 protein. The enhanced expression is achieved by inhibiting Mdm2-mediated degradation of p53. This provides a likely mechanistic explanation for the findings that c-Abl overcomes the inhibitory effects of Mdm2 on p53-mediated transcriptional activation and apoptosis. These results suggest that c-Abl modulates the time window within which p53 remains active. The ability of c-Abl to neutralize the inhibitory effects of Mdm2 on p53 may be important for its growth inhibitory function.

The Mdm2 oncoprotein interacts with the cell fate regulator Numb

The Mdm2 oncoprotein is a well-known inhibitor of the p53 tumor suppressor, but it may also possess p53-independent activities. In search of such p53-independent activities, the yeast two-hybrid screen was employed to identify Mdm2-binding proteins. We report that in vitro and in transfected cells, Mdm2 can associate with Numb, a protein involved in the determination of cell fate. This association causes translocation of overexpressed Numb into the nucleus and leads to a reduction in overall cellular Numb levels. Through its interaction with Numb, Mdm2 may influence processes such as differentiation and survival. This could potentially contribute to the altered properties of tumor cells which overexpress Mdm2.

A novel p53-inducible gene, PAG608, encodes a nuclear zinc finger protein whose overexpression promotes apoptosis

The biological effects of the p53 tumor suppressor protein are elicited, at least in part, through sequence-specific transactivation of a battery of target genes. The differential display method was employed towards identifying additional p53 target genes, with emphasis on genes whose induction may contribute to p53-mediated apoptosis. We report here the cloning of a novel p53-inducible gene, designated PAG608. PAG608 transcripts are induced by DNA damage in a p53-dependent manner. PAG608 encodes a nuclear zinc finger protein, which appears to localize preferentially to nucleoli when expressed at moderate levels in transfected cells. Transient overexpression of PAG608 in human tumor-derived cells leads to distinctive changes in nuclear morphology, and can promote apoptosis. Together with additional p53 target genes, PAG608 may therefore play a role in mediating the biological activities of p53.

Mdm2 promotes the rapid degradation of p53

The p53 tumour-suppressor protein exerts antiproliferative effects, including growth arrest and apoptosis, in response to various types of stress. The activity of p53 is abrogated by mutations that occur frequently in tumours, as well as by several viral and cellular proteins. The Mdm2 oncoprotein is a potent inhibitor of p53. Mdm2 binds the transcriptional activation domain of p53 and blocks its ability to regulate target genes and to exert antiproliferative effects. On the other hand, p53 activates the expression of the mdm2 gene in an autoregulatory feedback loop. The interval between p53 activation and consequent Mdm2 accumulation defines a time window during which p53 exerts its effects. We now report that Mdm2 also promotes the rapid degradation of p53 under conditions in which p53 is otherwise stabilized. This effect of Mdm2 requires binding of p53; moreover, a small domain of p53, encompassing the Mdm2-binding site, confers Mdm2-dependent detstabilization upon heterologous proteins. Raised amounts of Mdm2 strongly repress mutant p53 accumulation in tumour-derived cells. During recovery from DNA damage, maximal Mdm2 induction coincides with rapid p53 loss. We propose that the Mdm2-promoted degradation of p53 provides a new mechanism to ensure effective termination of the p53 signal.

p53 mediated apoptosis in HeLa cells: transcription dependent and independent mechanisms

The most frequent target for genetic alterations in human cancers is the p53 tumor suppressor gene. Mutations in p53 abrogate its ability to inhibit cell growth and to suppress tumor progression. The anti-proliferative activity of p53 can be mediated by the induction of growth arrest and/or programmed cell death (apoptosis). Recent in vivo studies support the involvement of apoptosis in tumor suppression by p53. To gain further insight into the mechanisms by which p53 induces apoptosis, the activity of p53 was studied in HeLa cells using a transient transfection assay. To define the functional domains of p53 required for apoptosis a C-terminal deletion mutant of p53 was used. This mutant, p53d1214, lacks the oligomerization domain, the nuclear localization signal and a large part of the core DNA binding domain. This mutant was shown to be deficient in sequence specific transactivation activity. Overexpression of wt p53 induced an efficient apoptosis in transiently transfected HeLa cells. Surprisingly p53d1214, containing only the first 214 N-terminal residues induced extensive apoptosis. The induction of apoptosis by p53d1214 is slower than that induced by wt p53. Furthermore, p53d1214 suppressed the transformation of rat embryo fibroblasts by several oncogene combinations, such as myc plus ras. In view of the fact that p53d1214 lacks measurable transactivation potential, our findings suggest the existence of two p53 dependent-apoptotic pathways--one involves activation of specific target genes, and the other is independent of it. Transactivation independent apoptosis may play a central role in tumor suppression by p53.

p53-mediated apoptosis: mechanisms and regulation

The p53 tumor suppressor gene is a key target for inactivation in human cancer. One of the main biological functions of the p53 protein is the positive regulation of apoptosis in response to signals such as genomic damage and the aberrant activation of certain oncogenes. A transient transfection assay was utilized in order to study the mechanism and regulation of p53-mediated apoptosis in human cancer cells. It was found that the sequence specific transcriptional activation (SST) function of p53 is essential for apoptosis in certain cell types, but not in others. This implies the existence of at least two distinct mechanisms for p53-mediated apoptosis, one requiring the activation of specific target genes, and the other being SST-independent. Typically, both mechanisms may be triggered simultaneously, and their cooperation may be required for maximal apoptotic effects. In addition, in cells lacking the function of the Rb tumor suppressor, the apoptotic activity of p53 could be inhibited by reconstitution of active Rb. p53-mediated apoptosis could also be inhibited by the protein encoded by the mdm2 oncogene. The latter inhibition required the formation of complexes between the Mdm2 protein and p53, and operated only on SST-dependent apoptosis but not SST-independent apoptosis. Together, the data imply that p53 induces apoptosis through the activation of multiple biochemical pathways, and that the efficiency of the process is dictated by the cellular context.

A mutant p53 that discriminates between p53-responsive genes cannot induce apoptosis

Human wild-type (wt) p53 can induce apoptosis in transiently transfected H1299 cells maintained at 37 degrees C, whereas tumor-derived mutant forms of p53 (with the mutation Ala-143, His-175, or Trp-248) fail to do so. At 37 degrees C, p53 with a mutation to Ala at amino acid 143 (p53Ala143) was transcriptionally inactive. However, at 32 degrees C, p53Ala143 strongly activated transcription from several physiologically relevant p53-responsive promoters, to extents similar or greater than that of wt p53. Unexpectedly, p53Ala143 was defective in inducing apoptosis in H1299 cells at 32 degrees C. Concomitantly with the loss of apoptotic activity, p53Ala143 was found to be deficient in its ability to activate transcription from the p53-responsive portions of the Bax and insulin-like growth factor-binding protein 3 gene promoters. It is proposed that there may exist distinct classes of p53-responsive promoters, whose ability to be activated by p53 can be regulated differentially. Such differential regulation may have functional consequences for the effects of p53 on cell fate.

Cell type-specific inhibition of p53-mediated apoptosis by mdm2

The effect of excess mdm2 on p53-mediated apoptosis was investigated in two human-derived cell lines, H1299 and HeLa. In H1299 cells, overexpression of mdm2 resulted in effective protection from apoptosis. This protective effect was seen only under conditions allowing the formation of p53-Mdm2 complexes. In contrast, excess mdm2 failed to abolish p53-mediated apoptosis in HeLa cells, despite a complete abrogation of p53-dependent sequence-specific transcriptional activation (SST). These data strongly support the contention that SST is dispensable for at least some types of p53-mediated apoptosis. Further, they suggest that one of the roles of mdm2 may be to modulate the apoptotic activity of p53, in a manner which is dictated by the pathway through which p53 induced apoptosis in a given cell type

Specific loss of apoptotic but not cell-cycle arrest function in a human tumor derived p53 mutant

The p53 tumor-suppressor gene product is frequently inactivated in malignancies by point mutation. Although most tumor-derived p53 mutants show loss of sequence specific transcriptional activation, some mutants have been identified which retain this activity. One such mutant, p53175P, is defective for the suppression of transformation in rodent cells, despite retaining the ability to suppress the growth of p53-null human cells. We now demonstrate that p53175P can induce a cell-cycle arrest in appropriate cell types but shows loss of apoptotic function. Our results therefore support a direct role of p53 transcriptional activation in mediating a cell-cycle arrest and demonstrate that such activity is not sufficient for the full apoptotic response. These data suggest that either p53 can induce apoptosis through a transcriptionally independent mechanism, a function lost by p53175P, or that this mutant has specifically lost the ability to activate genes which contribute to cell death, despite activation of genes responsible for the G1 arrest. This dissociation of the cell-cycle arrest and apoptotic activities of p53 indicates that inactivation of p53 apoptotic function without concomitant loss of growth inhibition can suffice to relieve p53-dependent tumor-suppression in vivo and thereby contribute to tumor development.

Induction of apoptosis in HeLa cells by trans-activation-deficient p53

The p53 tumor suppressor protein is a transcriptional activator, which can mediate apoptotic cell death in a variety of cell types. To determine whether sequence-specific trans-activation is a prerequisite for the induction of apoptosis by p53, the apoptotic effects of various p53 deletion mutants were monitored in an assay based on the transient transfection of HeLa cells. A truncated protein (p53dl214), containing only the first 214 amino-terminal residues of murine p53, induced extensive apoptosis, albeit at a slower rate than trans-activation-competent wild-type p53. p53dl214 also suppressed the transformation of rat fibroblasts by several oncogene combinations and particularly by myc plus ras and HPV E7 plus ras. p53dl214 lacks a major portion of the DNA-binding domain and cannot activate p53-responsive promoters. Moreover, a human p53 protein carrying mutations in residues 22 and 23 also triggered HeLa cell apoptosis, despite failing to induce significant activation of relevant p53 target promoters. These data suggest the existence of two p53-dependent apoptotic pathways--one requiring activation of specific target genes, and the other independent of sequence-specific trans-activation. The latter pathway may actually be totally uncoupled from the binding of p53 to its consensus DNA sites. The relative contribution of trans-activation-independent apoptosis to tumor suppression by p53 may be dictated by the specific genetic lesions present in the particular tumor.

A functional p53-responsive intronic promoter is contained within the human mdm2 gene

The mdm2 gene is a target for transcriptional activation by the p53 tumor suppressor gene product. Previous work has revealed that the mouse mdm2 gene contains two promoters: one is located upstream to the gene and is active in the absence of p53, the other resides within the first intron and requires p53 for transcriptional activity. To determine whether this unique promoter activation pattern is biologically important, we investigated the structure and function of the corresponding region of the human mdm2 (hmdm2) gene. We report here that the hmdm2 gene also contains an intronic, p53-dependent promoter. The structural features of this promoter are highly conserved between mouse and man, as opposed to the lack of conservation of the first exon. This promoter is triggered in vivo in the presence of activated wild type p53, leading to the production of novel mRNA species. The intronic hmdm2 promoter contains two tandem p53 binding elements. Deletion analysis suggests that optimal promoter activity requires the simultaneous binding of p53 to both elements; this may serve to prevent premature triggering of the promoter by p53.

p53-mediated apoptosis in HeLa cells can be overcome by excess pRB

Studies on DNA tumor viruses have suggested a link between p53 and pRB in the control of cell growth and apoptosis. We examined the role of pRB in the control of p53-mediated apoptosis in HeLa cells, in which the activities of p53 and members of the pRB family are very low. Transient overexpression of wild type (wt) p53 in HeLa cells induced apoptotic cell death. Importantly, coexpression of functional pRB resulted in significant protection of HeLa cells from p53-mediated apoptosis, without interfering with the transcriptional activity of wt p53. These results suggest that pRB, and possibly other pRB-related proteins, play a major role in the decision of whether cells respond to activated p53 by undergoing growth arrest or apoptosis. Our findings demonstrate a direct link between these two tumor suppressors in the control of cell growth and cell death.

bmi-1 transgene induces lymphomas and collaborates with myc in tumorigenesis

The bmi-1 gene was discovered as a frequent target of Moloney virus insertion in virally accelerated B-lymphoid tumors of E mu-myc transgenic mice and hence is thought to collaborate with the myc gene in lymphomagenesis, but its oncogenic potential has not previously been tested directly. To determine whether bmi-1 overexpression can contribute to hematopoietic neoplasia in vivo, strains of transgenic mice were generated in which bmi-1 expression was directed to the lymphoid compartment by a coupled immunoglobulin heavy chain enhancer (E mu). Although the E mu-bmi-1 transgene was expressed in both B and T cells, lymphoid development was not perturbed. Nevertheless, 14% of the mice in the strain with highest expression have developed lymphoma. Unexpectedly, most tumors were of the T-cell lineage, although one case of B lymphoma was observed. Furthermore, cross breeding E mu-bmi-1 and E mu-myc mice established that the bmi-1 transgene markedly accelerated the onset of pre-B and B lymphomas. These results demonstrate directly that bmi-1 can contribute to lymphomagenesis in the T and B cell lineages and collaborate with the myc gene in tumor development.

Moloney virus induction of T-cell lymphomas in a plasmacytomagenic strain of E mu-v-abl transgenic mice

Although the v-abl gene can provoke several types of lymphoid neoplasm, mice of a transgenic strain (E mu-v-abl 40) in which lymphocytes are targeted for expression of v-abl by a linked immunoglobulin enhancer (E mu) spontaneously develop only plasmacytomas. To determine whether other lymphocytes of this strain were susceptible to transformation, and to identify genes that can collaborate with v-abl in tumorigenesis, E mu-v-abl 40 mice were subjected to insertional mutagenesis by neonatal infection with Moloney murine leukemia virus. Tumorigenesis was accelerated moderately, but nearly all the tumors were T lymphomas. The altered tumor type may reflect both the T-cell tropism of Moloney virus and the higher level of E mu-v-abl 40 expression found in T lymphocytes than in B lymphocytes. Insertion near the c-myc, N-myc or pim-I gene was observed in 42% of the induced tumors, indicating that each of these genes may collaborate with v-abl in lymphomagenesis. Most of the accelerated tumors had a surprisingly low level of transgene expression. Thus, high expression of v-abl may not be required for Moloney-induced T lymphomagenesis.

Nucleotide sequence of bup, an upstream gene in the bmi-1 proviral insertion locus

The ability of Moloney murine leukemia virus to accelerate lymphomagenesis in E mu-myc transgenic mice is frequently associated with proviral integration within a locus denoted bmi-1. This locus contains not only the bmi-1 gene implicated as a collaborator with myc in lymphomagenesis but also just upstream an unknown gene denoted bup. The nucleotide sequence reported here for bup cDNA and flanking genomic sequences reveals that this widely expressed gene comprises at least 7 exons and potentially encodes a polypeptide of 195 amino acid residues. Computer searches with this polypeptide sequence revealed no close homolog in the databases, nor any conserved motifs, and it is unrelated to the product of the mel-13 gene, which lies just upstream from the bmi-1 homolog mel-18.

Retroviral infection accelerates T lymphomagenesis in E mu-N-ras transgenic mice by activating c-myc or N-myc

Transgenic mice bearing a mutant, activated N-ras oncogene directed to express within hematopoietic cells by an immunoglobulin enhancer (E mu) sporadically develop T-cell lymphomas and non-lymphoid tumors that may be of macrophage origin. To identify genes that can collaborate with N-ras in hematopoietic neoplasia, Moloney murine leukemia virus was used as an insertional mutagen. Infection of newborn E mu-N-ras mice with the virus greatly accelerated tumorigenesis, and nearly all the tumors proved to be T-cell lymphomas. Their variable surface phenotype (CD4+CD8-, CD4+CD8+ and CD4-CD8-) suggested that cells at several stages of T-cell development were susceptible to tumorigenesis. Southern blot analysis revealed that 68% of the tumors bore a proviral insert 5' to the c-myc gene, while 13% had an insert within the 3' untranslated region of the N-myc gene. Insertion was associated with elevated expression of these genes. Hence, activation of a myc gene appears to be the dominant pathway to tumorigenesis by insertional mutagenesis in lymphoid cells expressing a mutant ras gene. However, since many of the tumors were not transplantable, even the partnership of myc and ras may not suffice for full lymphoid malignancy.

Novel zinc finger gene implicated as myc collaborator by retrovirally accelerated lymphomagenesis in E mu-myc transgenic mice

To search for genes that can collaborate with myc in lymphomagenesis, we exploited retroviral insertional mutagenesis in E mu-myc transgenic mice. Moloney murine leukemia virus accelerated development of B lymphoid tumors. Three quarters contained a provirus within the known pim-1 or pim-2 loci, new loci bmi-1 and emi-1, or combinations of these. bmi-1 insertions predominated, occurring in half the tumors, and resulted in elevated bmi-1 mRNA levels. Significantly, the bmi-1 gene, which is expressed in diverse normal cells, encodes a Cys/His metal-binding motif (C3HC4) that resembles those in several DNA-binding proteins and defines a new category of zinc finger gene. Thus, myc-induced lymphomagenesis can entail the concerted action of several genes, including the presumptive nuclear regulator bmi-1.