Identification of the p53 protein domain involved in formation of the simian virus 40 large T-antigen-p53 protein complex

Polyomavirus Wakes Up and Chooses Neurovirulence

JC polyomavirus (JCPyV) is a human-specific polyomavirus that establishes a silent lifelong infection in multiple peripheral organs, predominantly those of the urinary tract, of immunocompetent individuals. In immunocompromised settings, however, JCPyV can infiltrate the central nervous system (CNS), where it causes several encephalopathies of high morbidity and mortality. JCPyV-induced progressive multifocal leukoencephalopathy (PML), a devastating demyelinating brain disease, was an AIDS-defining illness before antiretroviral therapy that has "reemerged" as a complication of immunomodulating and chemotherapeutic agents. No effective anti-polyomavirus therapeutics are currently available. How depressed immune status sets the stage for JCPyV resurgence in the urinary tract, how the virus evades pre-existing antiviral antibodies to become viremic, and where/how it enters the CNS are incompletely understood. Addressing these questions requires a tractable animal model of JCPyV CNS infection. Although no animal model can replicate all aspects of any human disease, mouse polyomavirus (MuPyV) in mice and JCPyV in humans share key features of peripheral and CNS infection and antiviral immunity. In this review, we discuss the evidence suggesting how JCPyV migrates from the periphery to the CNS, innate and adaptive immune responses to polyomavirus infection, and how the MuPyV-mouse model provides insights into the pathogenesis of JCPyV CNS disease.

Impact of a haplotype (composed of the APC, KRAS, and TP53 genes) on colorectal adenocarcinoma differentiation and patient prognosis

BACKGROUND

Many types of gene mutation are associated with the drug resistance of cancer cells. XELOX is a new and efficient surgical adjuvant chemotherapy for colorectal adenocarcinoma. However, drug-resistant related genetic mutations associated with this treatment remain unknown.

METHODS

Next-generation sequencing (NGS) was performed on 36 colorectal cancer patients to identify mutations among patients with residual tumors following preoperative chemotherapy. Enrichment and prognosis of these mutations were evaluated in a TCGA cohort. The pathology of cases with poor prognosis-related mutations was also determined.

RESULTS

A sequence of SNPs associated with the APC, KRAS, and TP53 genes in 13 of 19 subjects with residual tumors after preoperative chemotherapy was identified. Using survival analysis data from 317 cases in the TCGA database, a prognosis-related haplotype composed of SNPs from APC, KRAS, and TP53 was assembled. Colorectal cancer patients with these mutations had a lower 5-year tumor-specific survival rate than those without (p < 0.05). Most patients with these mutations were at a higher clinical stage (III-IV) of disease. Enrolled subjects with the identified haplotype tended to have poor cancer cell differentiation.

CONCLUSIONS

The prognosis-related haplotype can be used as a marker of drug resistance and prognosis in colorectal cancer patients after preoperative chemotherapy.

DAB2IP suppresses tumor malignancy by inhibiting GRP75-driven p53 ubiquitination in colon cancer

Increasing evidence has shown that DAB2IP acts as a tumor suppressor and plays an inhibitory role in many signals associated with tumorigenesis. However, the underlying mechanism of this function remains unclear. Our study shows that DAB2IP was positively associated with a good prognosis in patients with colorectal cancer and wild-type p53 expression. An in vitro assay showed that DAB2IP elicited potent tumor-suppressive effects by inhibiting cell invasiveness and colony formation and promoting cell apoptosis in wild-type p53 colon cancer cells. In addition, DAB2IP improved the stability of wild-type p53 by inhibiting its degradation in a ubiquitin-proteasome-dependent manner. Using mass spectrometry profiling, we revealed that DAB2IP and p53 interacted with the ubiquitin ligase-related protein GRP75. Mechanistically, DAB2IP is competitively bound to GRP75, thus reducing GRP75-driven p53 ubiquitination and degradation. Moreover, the Ras-GAP domain was required for the DAB2IP-GRP75 interaction and DAB2IP-mediated p53 ubiquitination. Finally, animal experiments revealed that DAB2IP inhibited tumor progression in vivo. In conclusion, our study presents a novel function of DAB2IP in GRP75-driven wild-type p53 degradation, providing new insight into DAB2IP-induced tumor suppression and a novel molecular interpretation of the p53 pathway.

A Candidate Secreted Effector Protein of Rubber Tree Powdery Mildew Fungus Contributes to Infection by Regulating Plant ABA Biosynthesis

Powdery mildew infects a wide range of crops and economic plants, causing substantial losses. Rubber trees (Hevea brasiliensis) are the primary source of natural rubber, and powdery mildew infection causes significant losses to natural rubber yields. How the causal agent, Erysiphe quercicola, establishes successful infection in rubber trees is largely unknown. Previously, 133 candidate secreted effector proteins (CSEPs) were identified in powdery mildew fungus. In this study, we characterize a CSEP named EqCSEP01276 for its function in suppressing host plant defense responses. We show that EqCSEP01276 is a secreted protein and is able to disturb the localization of 9-cis-epoxycarotenoid dioxygenase 5 (HbNCED5), a key enzyme in abscisic acid (ABA) biosynthesis in plant cell chloroplasts of H. brasiliensis. We also show that this effector inhibits ABA biosynthesis, and that in H. brasiliensis ABA is a positive regulator of the plant immune response against powdery mildew. Our study reveals a strategy by which powdery mildew fungus manipulates plant ABA-mediated defense for a successful infection.

The Role of the JC Virus in Central Nervous System Tumorigenesis

Cancer is the second leading cause of mortality worldwide. The study of DNA tumor-inducing viruses and their oncoproteins as a causative agent in cancer initiation and tumor progression has greatly enhanced our understanding of cancer cell biology. The initiation of oncogenesis is a complex process. Specific gene mutations cause functional changes in the cell that ultimately result in the inability to regulate cell differentiation and proliferation effectively. The human neurotropic Polyomavirus JC (JCV) belongs to the family Polyomaviridae and it is the causative agent of progressive multifocal leukoencephalopathy (PML), which is a fatal neurodegenerative disease in an immunosuppressed state. Sero-epidemiological studies have indicated JCV infection is prevalent in the population (85%) and that initial infection usually occurs during childhood. The JC virus has small circular, double-stranded DNA that includes coding sequences for viral early and late proteins. Persistence of the virus in the brain and other tissues, as well as its potential to transform cells, has made it a subject of study for its role in brain tumor development. Earlier observation of malignant astrocytes and oligodendrocytes in PML, as well as glioblastoma formation in non-human primates inoculated with JCV, led to the hypothesis that JCV plays a role in central nervous system (CNS) tumorigenesis. Some studies have reported the presence of both JC viral DNA and its proteins in several primary brain tumor specimens. The discovery of new Polyomaviruses such as the Merkel cell Polyomavirus, which is associated with Merkel cell carcinomas in humans, ignited our interest in the role of the JC virus in CNS tumors. The current evidence known about JCV and its effects, which are sufficient to produce tumors in animal models, suggest it can be a causative factor in central nervous system tumorigenesis. However, there is no clear association between JCV presence in CNS and its ability to initiate CNS cancer and tumor formation in humans. In this review, we will discuss the correlation between JCV and tumorigenesis of CNS in animal models, and we will give an overview of the current evidence for the JC virus’s role in brain tumor formation.

Human Polyomavirus JCPyV and Its Role in Progressive Multifocal Leukoencephalopathy and Oncogenesis

The human neurotropic virus JCPyV, a member of the Polyomaviridiae family, is the opportunistic infectious agent of Progressive Multifocal Leukoencephalopathy (PML), a fatal disease seen in severe immunosuppressive conditions and, during the last decade, in patients undergoing immunotherapy. JCPyV is a ubiquitous pathogen with up to 85% of the adult population word-wide exhibiting antibodies against it. Early experiments demonstrated that direct inoculation of JCPyV into the brain of different species resulted in the development of brain tumors and other neuroectodermal-derived neoplasias. Later, several reports showed the detection of viral sequences in medulloblastomas and glial tumors, as well as expression of the viral protein T-Antigen. Few oncogenic viruses, however, have caused so much controversy regarding their role in the pathogenesis of brain tumors, but the discovery of new Polyomaviruses that cause Merkel cell carcinomas in humans and brain tumors in racoons, in addition to the role of JCPyV in colon cancer and multiple mechanistic studies have shed much needed light on the role of JCPyV in cancer. The pathways affected by the viral protein T-Antigen include cell cycle regulators, like p53 and pRb, and transcription factors that activate pro-proliferative genes, like c-Myc. In addition, infection with JCPyV causes chromosomal damage and T-Antigen inhibits homologous recombination, and activates anti-apoptotic proteins, such as Survivin. Here we review the different aspects of the biology and physiopathology of JCPyV.

Breaking Bad: How Viruses Subvert the Cell Cycle

Interactions between the host and viruses during the course of their co-evolution have not only shaped cellular function and the immune system, but also the counter measures employed by viruses. Relatively small genomes and high replication rates allow viruses to accumulate mutations and continuously present the host with new challenges. It is therefore, no surprise that they either escape detection or modulate host physiology, often by redirecting normal cellular pathways to their own advantage. Viruses utilize a diverse array of strategies and molecular targets to subvert host cellular processes, while evading detection. These include cell-cycle regulation, major histocompatibility complex-restricted antigen presentation, intracellular protein transport, apoptosis, cytokine-mediated signaling, and humoral immune responses. Moreover, viruses routinely manipulate the host cell cycle to create a favorable environment for replication, largely by deregulating cell cycle checkpoints. This review focuses on our current understanding of the molecular aspects of cell cycle regulation that are often targeted by viruses. Further study of their interactions should provide fundamental insights into cell cycle regulation and improve our ability to exploit these viruses.

Autoantigens as Partners in Initiation and Propagation of Autoimmune Rheumatic Diseases

Systemic autoimmune diseases are characterized by specific targeting of a limited group of ubiquitously expressed autoantigens by the immune system. This review examines the mechanisms underlying their selection as immune targets. Initiation of autoimmune responses likely reflects the presentation of antigens with a distinct structure not previously encountered by the immune system, in a proimmune context (injury, malignancy, or infection). Causes of modified structure include somatic mutation and posttranslational modifications (including citrullination and proteolysis). Many autoantigens are components of multimolecular complexes, and some of the other components may provide adjuvant activity. Propagation of autoimmune responses appears to reflect a bidirectional interaction between the immune response and the target tissues in a mutually reinforcing cycle: Immune effector pathways generate additional autoantigen, which feeds further immune response. We propose that this resonance may be a critical principle underlying disease propagation, with specific autoantigens functioning as the hubs around which amplification occurs.

Detection of human polyomavirus proteins, T-antigen and agnoprotein, in human tumor tissue arrays

Expression of the human polyomavirus JCV genome in several experimental animals induces a variety of neural origin tumors. The viral proteins, T-antigen and Agnoprotein, contribute to the oncogenesis of JCV by associating with several tumor suppressor proteins and dysregulating signaling pathways, which results in uncontrolled cell proliferation. In addition, T-antigen and Agnoprotein have been associated with DNA damage and interfering with DNA repair mechanisms. In this study, we have utilized commercially available tissue arrays of human tumors of various origins and demonstrated the expression of both T-antigen and Agnoprotein in some, but not all, tumors of neural and non-neural origin. Most notably, more than 40% of human glioblastomas and greater than 30% of colon adenocarcinomas express viral proteins. The detection of viral transforming proteins, T-antigen and Agnoprotein in the absence of viral capsid proteins suggests a role for JCV in the development and/or progression of human tumors. These results invite further large-scale investigation on the role of polyomaviruses, particularly JCV in the pathogenesis of human cancer.

Frequent hypermethylation and loss of heterozygosity of the testis derived transcript gene in ovarian cancer

Testis derived transcript (TES) is a candidate tumor suppressor gene located at the human chromosome 7q31, and its function in ovarian cancer is still unknown. Using ovarian cancer cell lines and tissue samples, we demonstrated that both loss of heterozygosity and hypermethylation of the TES gene occurred in ovarian cancer at high frequencies, and there were significant correlations between TES expression and hypermethylation or loss of heterozygosity. We also detected methylation in ovarian cancer cell line A2780 after treatment with 5-aza-2-deoxycytidine. The expression level of TES was enormously up-regulated, then caused changes to the biological behaviors of A2780 cells: cell growth properties were greatly impaired, colony formatting abilities were suppressed to very low levels, and the apoptosis rate was highly raised compared to the control group. Our findings suggest that the TES gene functions as a tumor suppressor gene and is frequently silenced by hypermethylation and loss of heterozygosity in ovarian cancers.

IGF-IR-dependent expression of Survivin is required for T-antigen-mediated protection from apoptosis and proliferation of neural progenitors

The Insulin-like Growth Factor-1 Receptor (IGF-IR) and the human polyomavirus JCV protein, T-Antigen cooperate in the transformation of neuronal precursors in the cerebellum, which may be a contributing factor in the development of brain tumors. Since it is not clear why T-Antigen requires IGF-IR for transformation, we investigated this process in neural progenitors from IGF-IR knockout embryos (ko-IGF-IR) and from their wild type non-transgenic littermates (wt-IGF-IR). In contrast to wt-IGF-IR, the brain and dorsal root ganglia of ko-IGF-IR embryos showed low levels of the anti-apoptotic protein Survivin, accompanied by elevated numbers of apoptotic neurons and an earlier differentiation phenotype. In wt-IGF-IR neural progenitors in vitro, induction of T-Antigen expression tripled the expression of Survivin, and accelerated cell proliferation. In ko-IGF-IR progenitors induction of T-Antigen failed to increase Survivin, resulting in massive apoptosis. Importantly, ectopic expression of Survivin protected ko-IGF-IR progenitor cells from apoptosis and siRNA inhibition of Survivin activated apoptosis in wt-IGF-IR progenitors expressing T-Antigen. Our results indicate that reactivation of the anti-apoptotic Survivin may be a critical step in JCV T-Antigen induced transformation, which in neural progenitors requires IGF-IR.

Mutation of p53 in head and neck squamous cell carcinoma correlates with Bcl-2 expression and increased susceptibility to cisplatin-induced apoptosis

BACKGROUND

The p53 protein, a well-known tumor suppressor that functions primarily as a transcription factor, initiates cell cycle arrest and apoptosis after genotoxic stress. The antiapoptotic regulator Bcl-2 is a downstream modulator of p53-induced apoptosis. Loss of function of the p53 tumor suppressor through mutation is an important event that contributes to cellular transformation. Mutation of p53 is one of the most common genetic alterations in squamous cell carcinomas of the head and neck (SCCHN). We hypothesized that p53 mutation is associated with Bcl-2 expression and susceptibility to apoptosis in SCCHN.

METHODS

Exons 5 to 8 of the p53 gene were sequenced in 22 SCCHN tumor samples and correlated with the Bcl-2 expression and apoptosis rates in these tumors. In addition, a Bcl-2-expressing SCCHN cell line, UMSCC74B, was stably transfected with a temperature-sensitive mutant p53 construct, and Bcl-2 expression levels were examined at the mutant and the wild-type temperatures.

RESULTS

Bcl-2 expression was inversely correlated with wild-type p53 status in SCCHN tumors (p = .05). Furthermore, there was a modest increase (1.7-fold) in apoptosis in the wild-type p53 tumors compared with mutant p53 SCCHN. Immunoblotting of UMSCC74B cells stably transfected with the temperature-sensitive mutant p53 construct demonstrated that shifting these cells to the mutant p53 temperature (39.5 degrees C) resulted in decreased expression of Bcl-2 compared with levels in cells grown at the wild-type p53 temperature (32.5 degrees C). Further investigation showed that SCCHN cells expressing predominantly mutant p53 and decreased Bcl-2 were more susceptible to cisplatin-induced apoptosis than vector-transfected controls (p < .0001).

CONCLUSIONS

These results suggest that p53 mutation directly modulates Bcl-2 expression and therefore susceptibility to chemotherapy-induced apoptosis in SCCHN cells in vitro.

Polo-like kinase 1 (Plk1) inhibits p53 function by physical interaction and phosphorylation

Polo-like kinase 1 (Plk1) has an important role in the regulation of M phase of the cell cycle. In addition to its cell cycle-regulatory function, Plk1 has a potential role in tumorigenesis. Here we found for the first time that Plk1 physically binds to the tumor suppressor p53 in mammalian cultured cells, and inhibits its transactivation activity as well as its pro-apoptotic function. During the cisplatin-induced apoptosis in human neuroblastoma SH-SY5Y cells, the expression level of Plk1 was significantly decreased both at mRNA and protein levels, whereas cisplatin treatment caused a remarkable stabilization of p53. Systematic immunoprecipitation analyses using a series of deletion mutants of p53 revealed that a sequence-specific DNA-binding region of p53 is required and sufficient for the physical interaction with Plk1. The ectopically overexpressed Plk1 was co-localized with the endogenous p53 in mammalian cell nucleus, as shown by confocal laser microscopy. Expression of exogenous Plk1 and p53 in p53-deficient lung carcinoma H1299 cells greatly decreased the p53-mediated transcription from the p53-responsive p21(WAF1), MDM2, and BAX promoters, whereas the kinase-deficient mutant form of Plk1 failed to reduce the transcriptional activity of p53. Consistent with the luciferase reporter analysis, Plk1 had an ability to block the p53-dependent induction of the endogenous p21(WAF1). In addition, Plk1 inhibited the pro-apoptotic function of p53 in H1299 cells. Intriguingly, Plk1-mediated repression of p53 was attenuated with ATM. Thus, our present findings strongly suggest that p53 is a critical target of Plk1, and its function is abrogated through the physical interaction with Plk1.

Cul7/p185/p193 binding to simian virus 40 large T antigen has a role in cellular transformation

Simian virus 40 large T antigen (TAg) is a viral oncoprotein that can promote cellular transformation. TAg's transforming activity results in part by binding and inactivating key tumor suppressors, including p53 and the retinoblastoma protein (pRb). We have identified a TAg-associated 185-kDa protein that has significant homology to the cullin family of E3 ubiquitin ligases. TAg binds to an SCF-like complex that contains p185/Cul7, Rbx1, and the F box protein Fbw6. This SCF-like complex binds to an N-terminal region of TAg. Several p185/Cul7-binding-deficient mutants of TAg were generated that retained binding to pRb and p53 and were capable of overcoming Rb-mediated repression of E2F transcription. Despite binding to pRb and p53, these p185/Cul7-binding-defective mutants of TAg were unable to transform primary mouse embryo fibroblasts. Cells expressing p185/Cul7-binding-defective mutants of TAg were unable to grow to high density or grow in an anchorage-independent manner as determined by growth in soft agar. Considering the significance of other TAg-interacting proteins in regulation of the cell cycle, p185/Cul7 may also regulate an important growth control pathway.

ARF tumor suppressor induces mitochondria-dependent apoptosis by modulation of mitochondrial Bcl-2 family proteins

A tumor suppressor gene product, ARF, sensitizes cells to apoptosis in the presence of appropriate collateral signals. In this study, we analyzed the mechanism of ARF-dependent apoptosis and demonstrated that ARF induces mitochondria-dependent apoptosis in p53 wild-type, ARF/p16-null cells. We also found that ARF evokes cytochrome c release from mitochondria, decreases mitochondrial membrane potential, and activates pro-caspase-9 to induce apoptosis. Our findings suggest that this apoptotic cellular modulation is brought about by up-regulation of the proapoptotic Bcl-2 family proteins Bax and Bim and down-regulation of antiapoptotic Bcl-2 in mitochondrial fractions. Additionally, ARF seems to down-regulate Bcl-2 in a p53-dependent manner while up-regulating Bax/Bim via a p53-independent pathway.

Pre-clinical applications of transgenic mouse mammary cancer models

Breast cancer is a leading cause of cancer morbidity and mortality. Given that the majority of human breast cancers appear to be due to non-genetic factors, identifying agents and mechanisms of prevention is key to lowering the incidence of cancer. Genetically engineered mouse models of mammary cancer have been important in elucidating molecular pathways and signaling events associated with the initiation, promotion, and the progression of cancer. Since several transgenic mammary models of human breast cancer progress through well-defined cancer stages, they are useful pre-clinical systems to test the efficacy of chemopreventive and chemotherapeutic agents. This review outlines several oncogenic pathways through which mammary cancer can be induced in transgenic models and describes several types of preventive and therapeutic agents that have been tested in transgenic models of mammary cancer. The effectiveness of farnesyl inhibitors, aromatase inhibitors, differentiating agents, polyamine inhibitors, anti-angiogenic inhibitors, and immunotherapeutic compounds including vaccines have been evaluated in reducing mammary cancer and tumor progression in transgenic models.

Induction and bypass of p53 during productive infection by polyomavirus

Lytic infection by polyomavirus leads to elevated levels of p53 and induction of p53 target genes p21Cip1/WAF1 (p21) and BAX. This is seen both in polyomavirus-infected primary mouse cell cultures and in kidney tissue of infected mice. Stabilization of p53 and induction of a p53 response are accompanied by phosphorylation of p53 on serine 18, mimicking a DNA damage response. Stabilization of p53 does not depend on p19Arf interaction with mdm2. Cells infected by a mutant virus defective in binding pRb and in inducing G(1)-to-S progression show a greatly diminished p53 response. However, cells infected by wild-type virus and blocked from entering S phase by addition of mimosine still show a p53 response. These results suggest a role of E2F target genes in inducing a p53 response. Polyomavirus large T antigen coprecipitates with p53 phosphorylated on serine 18 and also with p21Cip1/WAF1. Implications of these and other findings on possible mechanisms of induction and override of p53 functions during productive infection by polyomavirus are discussed.

Analysis of molecular interactions of the p53-family p51(p63) gene products in a yeast two-hybrid system: homotypic and heterotypic interactions and association with p53-regulatory factors

p51 in the p53 tumor suppressor family, also referred to as p63, encodes multiple isoforms including p51A (TAp63gamma) and p51B (TAp63alpha). The p53 protein forms a tetramer, and its stability and activity are regulated by molecular association with viral and cellular proteins and by biochemical modifications. Using a yeast two-hybrid system, the p51A and p51B isoforms were examined for homotypic and heterotypic interactions in the p53 family proteins and for their affinity to the p53-regulatory factors. Results indicate a homotypic interaction dependent on the presumed oligomerization domain of the p51 proteins. The possibility of a weak heterotypic interaction between p51 and p73 proteins was suggested, while association between p51 and p53 appeared improbable. Furthermore, unlike p53, the p51 proteins failed to display an affinity to SV40 large T antigen or MDM2-family proteins. Having several features in common with p53, the p51 proteins may function in biological processes apart from p53.

Role of T antigen interactions with p53 in tumorigenesis

SV40 induces neoplastic transformation by disabling several key cellular growth regulatory circuits. Among these are the Rb- and p53-families of tumor suppressors. The multifunctional, virus-encoded large T antigen blocks the function of both Rb and p53. Large T antigen uses multiple mechanisms to block p53 activity, and this action contributes to tumorigenesis, in part, by blocking p53-mediated growth suppression and apoptosis. Since the p53 pathway is inactivated in most human tumors, T antigen/p53 interactions offer a possible mechanism by which SV40 contributes to human cancer.

Loss of p19(ARF) eliminates the requirement for the pRB-binding motif in simian virus 40 large T antigen-mediated transformation

At least three domains of simian virus 40 large T antigen (TAg) participate in cellular transformation. The LXCXE motif of TAg binds to all members of the retinoblastoma protein (pRB) family of tumor suppressors. The N-terminal 70 residues of TAg have significant homology to the J domain of Hsp40/DnaJ and cooperate with the LXCXE motif to inactivate the pRB family. A bipartite C-terminal domain of TAg binds to p53 and thereby disrupts the ability of p53 to act as a sequence-specific transcription factor. The contribution of these three domains of TAg to cellular transformation was evaluated in cells that contained inactivating mutations in the pRB and p53 pathways. Cells that stably expressed wild-type or selected mutant forms of TAg were generated in mouse embryo fibroblasts (MEFs) containing homozygous deletions in the RB, INK4a, and ARF loci. It was determined that the J domain, the LXCXE motif, and the p53-binding domain of TAg were required for full transformation of wild-type and RB(-/-) MEFs. In contrast, INK4a(-/-) MEFs that lacked expression of p16(INK4a) and p19(ARF) and ARF(-/-) MEFs that lacked p19(ARF) but expressed p16(INK4a) acquired anchorage-independent growth when expressing wild-type TAg or mutant derivatives that disrupted either the pRB-binding or p53-binding domain. The expression and function of the pRB family members were not overly disrupted in ARF(-/-) MEFs expressing LXCXE mutants of TAg. These results suggest that inactivating mutations of p19(ARF) can relieve the requirement for the LXCXE motif in TAg-mediated transformation and that TAg may have additional functions in transformation.

Adenovirus E1B 55-kilodalton oncoprotein inhibits p53 acetylation by PCAF

The adenovirus E1B 55-kDa protein binds to cellular tumor suppressor p53 and inactivates its transcriptional transactivation function. p53 transactivation activity is dependent upon its ability to bind to specific DNA sequences near the promoters of its target genes. It was shown recently that p53 is acetylated by transcriptional coactivators p300, CREB bidning protein (CBP), and PCAF and that acetylation of p53 by these proteins enhances p53 sequence-specific DNA binding. Here we show that the E1B 55-kDa protein specifically inhibits p53 acetylation by PCAF in vivo and in vitro, while acetylation of histones and PCAF autoacetylation is not affected. Furthermore, the DNA-binding activity of p53 is diminished in cells expressing the E1B 55-kDa protein. PCAF binds to the E1B 55-kDa protein and to a region near the C terminus of p53 encompassing Lys-320, the specific PCAF acetylation site. We further show that the E1B 55-kDa protein interferes with the physical interaction between PCAF and p53, suggesting that the E1B 55-kDa protein inhibits PCAF acetylase function on p53 by preventing enzyme-substrate interaction. These results underscore the importance of p53 acetylation for its function and suggest that inhibition of p53 acetylation by viral oncoproteins prevent its activation, thereby contributing to viral transformation.

Cellular response to oncogenic ras involves induction of the Cdk4 and Cdk6 inhibitor p15(INK4b)

The cell cycle inhibitor p15(INK4b) is frequently inactivated by homozygous deletion together with p16(INK4a) and p19(ARF) in some types of tumors. Although the tumor suppressor capability of p15(INK4b) is still questioned, it has been found to be specifically inactivated by hypermethylation in hematopoietic malignancies in the absence of p16(INK4a) alterations. Here we show that, in vitro, p15(INK4b) is a strong inhibitor of cellular transformation by Ras. Surprisingly, p15(INK4b) is induced in cultured cells by oncogenic Ras to an extent similar to that of p16(INK4a), and their expression is associated with premature G(1) arrest and senescence. Ras-dependent induction of these two INK4 genes is mediated mainly by the Raf-Mek-Erk pathway. Studies with activated and dominant negative forms of Ras effectors indicate that the Raf-Mek-Erk pathway is essential for induction of both the p15(INK4b) and p16(INK4a) promoters, although other Ras effector pathways can collaborate, giving rise to a stronger response. Our results indicate that p15(INK4b), by itself, is able to stop cell transformation by Ras and other oncogenes such as Rgr (a new oncogene member of the Ral-GDS family, whose action is mediated through Ras). In fact, embryonic fibroblasts isolated from p15(INK4b) knockout mice are susceptible to transformation by the Ras or Rgr oncogene whereas wild-type embryonic fibroblasts are not. Similarly, p15(INK4b)-deficient mouse embryo fibroblasts are more sensitive than wild-type cells to transformation by a combination of the Rgr and E1A oncogenes. The cell cycle inhibitor p15(INK4b) is therefore involved, at least in some cell types, in the tumor suppressor activity triggered after inappropriate oncogenic Ras activation in the cell.

Insulin-like growth factor II signaling in neoplastic proliferation is blocked by transgenic expression of the metalloproteinase inhibitor TIMP-1

Insulin-like growth factor (IGF) II is overexpressed in many human cancers and is reactivated by, and crucial for viral oncogene (SV40 T antigen, [TAg])-induced tumorigenesis in several tumor models. Using a double transgenic murine hepatic tumor model, we demonstrate that tissue inhibitor of metalloproteinase 1 (TIMP-1) blocks liver hyperplasia during tumor development, despite TAg-mediated reactivation of IGF-II. Because the activity of IGFs is controlled by IGF-binding proteins (IGFBPs), we investigated whether TIMP-1 overexpression altered the IGFBP status in the transgenic liver. Ligand blotting showed that IGFBP-3 protein levels were increased in TIMP-1-overexpressing double transgenic littermates, whereas IGFBP-3 mRNA levels were not different, suggesting that TIMP-1 affects IGFBP-3 at a posttranscriptional level. IGFBP-3 proteolysis assays demonstrated that IGFBP-3 degradation was lower in TIMP-1-overexpressing livers, and zymography showed that matrix metalloproteinases (MMPs) were present in the liver homogenates and were capable of degrading IGFBP-3. As a consequence of reduced IGFBP-3 proteolysis and elevated IGFBP-3 protein levels, dissociable IGF-II levels were significantly lower in TIMP-1-overexpressing animals. This decrease in bioavailable IGF-II ultimately resulted in diminished IGF-I receptor signaling in vivo as evidenced by diminished receptor kinase activity and decreased tyrosine phosphorylation of the IGF-I receptor downstream effectors, insulin receptor substrate 1 (IRS-1), extracellular signal regulatory kinase (Erk)-1, and Erk-2. Together, these results provide evidence that TIMP-1 inhibits liver hyperplasia, an early event in TAg-mediated tumorigenesis, by reducing the activity of the tumor-inducing mitogen, IGF-II. These data implicate the control of MMP-mediated degradation of IGFBPs as a novel therapy for controlling IGF bioavailability in cancer.

Epstein-Barr virus infection and P53 expression in HIV-related oral large B cell lymphoma

BACKGROUND

Head and neck non-Hodgkin's lymphomas in HIV positive patients are highly related with Epstein-Barr virus (EBV) infection. In general, viral agents can alter p53 protein levels by enhancing degradation of cellular p53 or by increasing its half-life by viral protein-p53 interaction. Moreover, it has been reported that modifications of p53 gene can modulate tumor cells' response to radio- and chemotherapy.

METHODS

To assess a possible role of EBV infection, p53 protein deregulation, and p53 gene alterations in exons 5 to 8, we have studied six cases of HIV-related primary oral large B-cell lymphoma. We used in situ hybridization (ISH) for EBV-DNA and EBV-encoded nuclear RNA-1 (EBER-1), immunohistochemistry (IHC) for EBV latent membrane protein -1 (LMP-1) and p53 proteins expression, and single strand conformational polymorphism (SSCP) analysis to screen p53 gene mutations in exons 5 to 8.

RESULTS

The EBV-DNA was present in all specimens, according to conventional DNA-ISH. No evidence for EBER-1 was found by ISH. The presence of EBV-DNA was correlated with the LMP-1 expression in all but one case. Moreover, p53 protein expression was negative in three cases and strongly positive in the others. However, mutational analysis of p53 gene in exons 5-8 showed no alteration.

CONCLUSIONS

Our data may suggest that both EBV infection and LMP-1 expression may cause p53 loss of function even in the absence of p53 gene mutations, as assessed by SSCP. We speculate that the presence of EBV-infection and p53 protein deregulation may be responsible for radio- and chemotherapy resistance, by influencing apoptosis of cancer cells.

p53-mediated repression of alpha-fetoprotein gene expression by specific DNA binding

Aberrant expression of the alpha-fetoprotein (AFP) gene is characteristic of a majority of hepatocellular carcinoma cases and serves as a diagnostic tumor-specific marker. By dissecting regulatory mechanisms through electromobility gel shift, transient-transfection, Western blot, and in vitro transcription analyses, we find that AFP gene expression is controlled in part by mutually exclusive binding of two trans-acting factors, p53 and hepatic nuclear factor 3 (HNF-3). HNF-3 protein activates while p53 represses AFP transcription through sequence-specific binding within the previously identified AFP developmental repressor domain. A single mutation within the DNA binding domain of p53 protein or a mutation of the p53 DNA binding element within the AFP developmental repressor eliminates p53-repressive effects in both transient-transfection and cell-free expression systems. Coexpression of p300 histone acetyltransferase, which has been shown to acetylate p53 and increase specific DNA binding, amplifies the p53-mediated repression. Western blot analysis of proteins present in developmentally staged, liver nuclear extracts reveal a one-to-one correlation between activation of p53 protein and repression of AFP during hepatic development. Induction of p53 in response to actinomycin D or hypoxic stress decreases AFP expression. Studies in fibroblast cells lacking HNF-3 further support a model for p53-mediated repression that is both passive through displacement of a tissue-specific activating factor and active in the presence of tissue-specific corepressors. This mechanism for p53-mediated repression of AFP gene expression may be active during hepatic differentiation and lost in the process of tumorigenesis.

Expression of bcl-2-, p53, and Ki-67 and outcome of patients with primary nasopharyngeal carcinomas following DNA-damaging treatment

BACKGROUND

Recent studies suggest that apoptosis is important in the cell death induced by treatment that damages deoxyribonucleic acid (DNA). We assessed the correlation between the expression of the apoptosis-related proteins, p53 and bcl-2, and the clinical outcome of patients with nasopharyngeal carcinomas (NPCs) who were treated with both DNA-damaging treatments. We also assessed the level of Ki-67, a marker of cell proliferation, in these tumors.

METHODS

We evaluated statistically the relationships among the expression of p53, bcl-2, and Ki-67 and clinicopathologic factors, the sensitivity to radiation, the incidence of distant metastases, and survival.

RESULTS

The group that was positive for p53 tended to be resistant to radiotherapy and to have a significantly poorer prognosis (p = .05).

CONCLUSIONS

The enhanced expression of p53 may be a prognostic factor in patients with NPCs whose tumor is resistant to therapy that damages DNA.

Functional and physical interactions of the ARF tumor suppressor with p53 and Mdm2

The INK4a-ARF locus encodes two proteins, p16(INK4a) and p19(ARF), that restrain cell growth by affecting the functions of the retinoblastoma protein and p53, respectively. Disruption of this locus by deletions or point mutations is a common event in human cancer, perhaps second only to the loss of p53. Using insect cells infected with baculovirus vectors and NIH 3T3 fibroblasts infected with ARF retrovirus, we determined that mouse p19(ARF) can interact directly with p53, as well as with the p53 regulator mdm2. ARF can bind p53-DNA complexes, and it depends upon functional p53 to transcriptionally induce mdm2 and the cyclin-dependent kinase inhibitor p21(Cip1), and to arrest cell proliferation. Binding of p19(ARF) to p53 requires the ARF N-terminal domain (amino acids 1-62) that is necessary and sufficient to induce cell cycle arrest. Overexpression of p19(ARF) in wild type or ARF-null mouse embryo fibroblasts increases the half-life of p53 from 15 to approximately 75 min, correlating with an increased p53-dependent transcriptional response and growth arrest. Surprisingly, when overexpressed at supra-physiologic levels after introduction into ARF-null NIH 3T3 cells or mouse embryo fibroblasts, the p53 protein is handicapped in inducing this checkpoint response. In this setting, reintroduction of p19(ARF) restores p53's ability to induce p21(Cip1) and mdm2, implying that, in addition to stabilizing p53, ARF modulates p53-dependent function through an additional mechanism.

Repair of cyclobutane pyrimidine dimers in the O6-methylguanine-DNA methyltransferase (MGMT) gene of MGMT proficient and deficient human cell lines and comparison with the repair of other genes and a repressed X-chromosomal locus

We studied the repair of cyclobutane pyrimidine dimers (CPDs) in the 5' terminal part of the transcriptionally inactive O6-methylguanine-DNA methyltransferase (MGMT) gene of MGMT-deficient human cell lines (A172, A-253 and WI-38 VA13) and in a proficient cell line (HaCaT), in which the MGMT gene was transcribed. Repair rates in the MGMT gene were compared with those in the active uracil-DNA glycosylase (UNG) and c-myc genes, and those in the repressed X-linked 754 locus and the RNA polymerase I-transcribed ribosomal gene cluster. In the active MGMT gene, there was a distinct strand specificity with more repair in the template (transcribed) strand (TS) than in the non-template strand (NTS). In contrast, no apparent strand bias in the repair of CPDs was observed in the inactive MGMT gene in the MGMT deficient cell lines, although the rates of repair varied between different cell lines. Repair in the inactive MGMT gene was consistently lower than repair in the NTSs of the expressed genes, and approached the generally poor repair of the repressed 754 locus. Whereas repair in the UNG gene was strand-specific in HaCaT, A-172 and WI-38 VA13 cells, no clear strand bias in repair of this gene was evident in A253 cells and repair was relatively inefficient. Although the repair kinetics was essentially similar in the two strands of the c-myc gene in all cell lines examined, the rate and extent of repair were in general significant, probably due to an observed transcription of both strands in the c-myc region. In conclusion, our results indicate that the relative rates of repair in inactive MGMT genes are comparable to those of repressed loci and are lower than repair rates in the NTSs of active genes, but the absolute rate of repair varies between different transformed cells.

Cooperative effects of INK4a and ras in melanoma susceptibility in vivo

The familial melanoma gene (INK4a/MTS1/CDKN2) encodes potent tumor suppressor activity. Although mice null for the ink4a homolog develop a cancer-prone condition, a pathogenetic link to melanoma susceptibility has yet to be established. Here we report that mice with melanocyte-specific expression of activated H-rasG12V on an ink4a-deficient background develop spontaneous cutaneous melanomas after a short latency and with high penetrance. Consistent loss of the wild-type ink4a allele was observed in tumors arising in ink4a heterozygous transgenic mice. No homozygous deletion of the neighboring ink4b gene was detected. Moreover, as in human melanomas, the p53 gene remained in a wild-type configuration with no observed mutation or allelic loss. These results show that loss of ink4a and activation of Ras can cooperate to accelerate the development of melanoma and provide the first in vivo experimental evidence for a causal relationship between ink4a deficiency and the pathogenesis of melanoma. In addition, this mouse model affords a system in which to identify and analyze pathways involved in tumor progression against the backdrop of genetic alterations encountered in human melanomas.

p53-dependent regulation of MDR1 gene expression causes selective resistance to chemotherapeutic agents

Loss of functional p53 paradoxically results in either increased or decreased resistance to chemotherapeutic drugs. The inconsistent relationship between p53 status and drug sensitivity may reflect p53's selective regulation of genes important to cytotoxic response of chemotherapeutic agents. We reasoned that the discrepant effects of p53 on chemotherapeutic cytotoxicity is due to p53-dependent regulation of the multidrug resistance gene (MDR1) expression in tumors that normally express MDR1. To test the hypothesis that wild-type p53 regulates the endogenous mdr1 gene we stably introduced a trans-dominant negative (TDN) p53 into rodent H35 hepatoma cells that express P-glycoprotein (Pgp) and have wild-type p53. Levels of Pgp and mdr1a mRNA were markedly elevated in cells expressing TDN p53 and were linked to impaired p53 function (both transactivation and transrepression) in these cells. Enhanced mdr1a gene expression in the TDN p53 cells was not secondary to mdr1 gene amplification and Pgp was functional as demonstrated by the decreased uptake of vinblastine. Cytotoxicity assays revealed that the TDN p53 cell lines were selectively insensitive to Pgp substrates. Sensitivity was restored by the Pgp inhibitor reserpine, demonstrating that only drug retention was the basis for loss of drug sensitivity. Similar findings were evident in human LS180 colon carcinoma cells engineered to overexpress TDN p53. Therefore, the p53 inactivation seen in cancers likely leads to selective resistance to chemotherapeutic agents because of up-regulation of MDR1 expression.

p53 and K-ras mutational genotyping in pulmonary carcinosarcoma, spindle cell carcinoma, and pulmonary blastoma: implications for histogenesis

In an attempt to understand the molecular pathogenesis of biphasic pulmonary neoplasms, the authors studied 25 cases of carcinosarcoma, spindle cell carcinoma, and pulmonary blastoma using a combined immunohistochemical and topographic genotyping approach for the presence of p53 abnormalities within the different epithelial and mesenchymal components of these tumors. Genotyping involved a search for point mutational damage in p53 exons 5-8, which was correlated with p53 immunoreactivity. This analytical approach demonstrated p53 missense point mutations in four of nine cases of spindle cell carcinoma with a 100% concordance rate between p53 immunopositivity and the presence of DNA mutational damage. One of six carcinosarcomas, heterologous in type, exhibited a p53 mutation. The concordance rate among carcinosarcomas was also 100%. However, the concordance rate among classic biphasic pulmonary blastomas was only 43%, with one of seven cases demonstrating a p53 mutation by DNA genotyping. The lack of concordance in pulmonary blastomas was possibly due to the existence of genotypically distinct subsets of tumor cells likely bearing mutations among largely nonmutated cells. In a similar fashion, among three well-differentiated fetal type adenocarcinomas, no p53 mutations were detected despite the presence of focal p53 immunopositivity in one of the cases. No K-ras mutations were detected in any of the 25 tumors examined. Monoclonal histogenesis from a single totipotential cell in a subset of these neoplasms (six of 22 cases) was supported by the finding of p53 overexpression and identical p53 mutational genotype in both the epithelial and spindle elements of the tumors. Furthermore, the finding of a small percentage of p53-positive tumor cells within one or both components suggests late acquisition of p53 mutational change in a subset of pulmonary blastomas.

p53 and the hypoxia-induced apoptosis of cultured neonatal rat cardiac myocytes

Myocyte cell loss is a prominent and important pathogenic feature of cardiac ischemia. We have used cultured neonatal rat cardiac myocytes exposed to prolonged hypoxia as an experimental system to identify critical factors involved in cardiomyocyte death. Exposure of myocytes to hypoxia for 48 h resulted in intranucleosomal cleavage of genomic DNA characteristic of apoptosis and was accompanied by increased p53 transactivating activity and protein accumulation. Expression of p21/WAF-1/CIP-1, a well-characterized target of p53 transactivation, also increased in response to hypoxia. Hypoxia did not cause DNA laddering or cell loss in cardiac fibroblasts. To determine whether the increase in p53 expression in myocytes was sufficient to induce apoptosis, normoxic cultures were infected with a replication-defective adenovirus expressing wild-type human p53 (AdCMV.p53). Infected cells expressed high intracellular levels of p53 protein and exhibited the morphological changes and genomic DNA fragmentation characteristic of apoptosis. In contrast, no genomic DNA fragmentation was observed in myocytes infected with the control virus lacking an insert (AdCMV.null) or in cardiac fibroblasts infected with AdCMV.p53. These results suggest that the intracellular signaling pathways activated by p53 might play a critical role in the regulation of hypoxia-induced apoptosis of cardiomyocytes.

Cytoplasmic p53 polypeptide is associated with ribosomes

Our previous finding that the tumor suppressor p53 is covalently linked to 5.8S rRNA suggested functional association of p53 polypeptide with ribosomes. p53 polypeptide is expressed at low basal levels in the cytoplasm of normal growing cells in the G1 phase of the cell cycle. We report here that cytoplasmic wild-type p53 polypeptide from both rat embryo fibroblasts and MCF7 cells and the A135V transforming mutant p53 polypeptide were found associated with ribosomes to various extents. Treatment of cytoplasmic extracts with RNase or puromycin in the presence of high salt, both of which are known to disrupt ribosomal function, dissociated p53 polypeptide from the ribosomes. In immunoprecipitates of p53 polypeptide-associated ribosomes, 5.8S rRNA was detectable only after proteinase K treatment, indicating all of the 5.8S rRNA in p53-associated ribosomes is covalently linked to protein. While 5.8S rRNA linked to protein was found in the immunoprecipitates of either wild-type or A135V mutant p53 polypeptide associated with ribosomes, little 5.8S rRNA was found in the immunoprecipitates of the slowly sedimenting p53 polypeptide, which was not associated with ribosomes. In contrast, 5.8S rRNA was liberated from bulk ribosomes by 1% sodium dodecyl sulfate, without digestion with proteinase K, indicating that these ribosomes contain 5.8S rRNA, which is not linked to protein. Immunoprecipitation of p53 polypeptide coprecipitated a small fraction of ribosomes. p53 mRNA immunoprecipitated with cytoplasmic p53 polypeptide, while GAPDH mRNA did not. These results show that cytoplasmic p53 polypeptide is associated with a subset of ribosomes, having covalently modified 5.8S rRNA.

Expression of p53, Ki-67 and cytokeratin-4 (CK4) in oral papillomas

P53 is overexpressed in more than 50% of all human cancers. A previous study suggested that p53 was also overexpressed in oral papillomas. This study was carried out to investigate whether p53 expression was correlated with expression of the cellular proliferation marker Ki-67 and the epithelial differentiation marker cytokeratin-4 (CK4) in oral papillomas. Formalin-fixed paraffin-embedded sections of 30 oral papilloma specimens and 30 unmatched normal oral mucosal specimens were processed for immunohistochemistry, using an avidin-biotin-peroxidase procedure and monoclonal antibodies. A semiquantification analysis on p53 and Ki-67 labeling indices was performed. Twenty-eight of 30 (93%) papilloma specimens were positive for p53. The percentage of p53-positive cells in the basal layer was 60.4 +/- 14.8 (mean +/- SD, n = 28), and that of Ki-67-positive cells was 26.7 +/- 14.4. There was no correlation between expression of p53 and that of Ki-67. Expression of CK4 was inversely correlated with the expression of Ki-67 but not correlated with the expression of p53.

p53 and tumour viruses: catching the guardian off-guard

Oncogenic viruses have evolved direct and indirect mechanisms to overcome the tumour suppressor p53. Fortunately, tumour development is limited by the narrow cell tropisms of the viruses concerned and the host immune response. However, such viruses are helping to elucidate the p53 response pathway and may play a future role as novel cancer therapeutic agents.

Established cell lines transformed by E2F-1 overexpression contain wild-type p53

Overexpression of genes encoding E2F transcription factors can transform some cultured cell lines and cause apoptosis of others. Apoptosis due to E2F overexpression requires the presence of wild-type p53. Cell lines in which stable E2F overexpression is possible might be expected, therefore, to contain mutant p53. In this report, it was asked whether endogenous p53 was mutant or wild-type in four established fibroblast cell lines that this laboratory previously showed stably overexpressed and were transformed by exogenous E2F-1. Unexpectedly, it was found that the p53 in these cells was wild-type by the criteria of immunoprecipitation with conformation-specific, p53 monoclonal antibodies and by transactivation of a p53-dependent reporter gene construct in transient transfection assays. These data indicate that stable overexpression of E2F-1 is possible in the presence of wild-type p53 and may result in cell transformation.

Tumor suppressor p53 gene forms multiple isoforms: evidence for single locus origin and cytoplasmic complex formation with heat shock proteins

The tumor suppressor protein p53 is a major cell cycle control factor, and mutations in p53 are the most common genetic lesion found in human tumors, resulting in loss of function and contributing to malignant transformation. This report reviews several studies which show that p53 protein appears as at least eleven isoforms having the same amino acid backbone but varying in charge by level of phosphorylation. All isoforms are derived from a single locus, which indicates that p53 activity is modulated by post-translational modification. In addition, mutant p53 forms hetero-oligomers with two families of proteins: HSP70 and a 90 kDa group similar to HSP90. Cytoplasmic complexes are most likely formed to protect p53 from proteolysis and are probably involved in translocation of activated p53 from the cytoplasm to the nucleus for transactivation of other cell cycle control genes.

A polymorphism but no mutations in the GADD45 gene in breast cancers

The p53 gene product is part of a pathway regulating growth arrest at the G1 checkpoint of the cell cycle. Mutation of other components of this pathway, including the products of the ataxia telangiectasia (AT), GADD45, mdm2, and p21WAF1/CIP1 genes may have effects comparable to mutations in the p53 gene. The GADD45 gene is induced by ionizing radiation and several DNA-damaging xenobiotics. Induction requires the binding of wild-type p53 to an evoulutionarily highly conserved putative intronic p53 binding site in intron 3 of GADD45. We recently analyzed the entire coding region of the p53 gene in primary breast cancers of Midwestern white women and found 21 mutations among 53 tumors (39.6%). We now have shown by direct sequencing that there are no mutations in the intronic p53 binding site of the GADD45 gene in any of the 53 primary breast cancers and no mutations in the entire coding region of the GADD45 gene in a subset of 26 consecutive tumors (12 with p53 mutation and 14 without p53 mutation). The only sequence variation detected was a common polymorphism in intron 3. The absence of mutations in the GADD45 gene, including the putative p53-binding intronic site, suggests that this gene is not a frequent target of mutations in breast cancer. Although mutations of the p53 gene have been studied in a wide spectrum of human cancers, GADD45 has not been examined in any tumor or cell line to the best of our knowledge. Our results raise the possibility that mutation of the GADD45 gene alone is not functionally equivalent to loss of wild-type p53 activity.

p53 tumour suppressor gene expression in pancreatic neuroendocrine tumour cells

Neuroendocrine pancreatic tumours grow slower and metastasise later than ductal and acinar carcinomas. The expression of the p53 tumour suppressor gene in pancreatic neuroendocrine tumour cells is unknown. Pancreatic neuroendocrine cell lines (n = 5) and human tumour tissues (n = 19) were studied for changed p53 coding sequence, transcription, and translation. Proliferative activity of tumour cells was determined analysing Ki-67 expression. No mutation in the p53 nucleotide sequence of neuroendocrine tumour cell was found. However, an overexpression of p53 could be detected in neuroendocrine pancreatic tumour cell lines at a protein level. As no p53 mutations were seen, it is suggested that post-translational events can also lead to an overexpression of p53.

Transforming growth factor beta induces the cyclin-dependent kinase inhibitor p21 through a p53-independent mechanism

The transforming growth factor beta s (TGF-beta s) are a group of multifunctional growth factors which inhibit cell cycle progression in many cell types. The TGF-beta-induced cell cycle arrest has been partially attributed to the regulatory effects of TGF-beta on both the levels and the activities of the G1 cyclins and their kinase partners. The activities of these kinases are negatively regulated by a number of small proteins, p21 (WAF1, Cip1), p27Kip1, p16, and p15INK4B, that physically associate with cyclins, cyclin-dependent kinases, or cyclin-Cdk complexes. p21 has been previously shown to be transcriptionally induced by DNA damage through p53 as a mediator. We demonstrate that TGF-beta also causes a rapid transcriptional induction of p21, suggesting that p21 can respond to both intracellular and extracellular signals for cell cycle arrest. In contrast to DNA damage, however, induction of p21 by TGF-beta is not dependent on wild-type p53. The cell line studied in these experiments, HaCaT, contains two mutant alleles of p53, which are unable to activate transcription from the p21 promoter when overexpressed. In addition, TGF-beta and p53 act through distinct elements in the p21 promoter. Taken together, these findings suggest that TGF-beta can induce p21 through a p53-independent pathway. Previous findings have implicated p27Kip1 and p15INK2B as effectors mediating the TGF-beta growth inhibitory effect. These results demonstrate that a single extracellular antiproliferative signal, TGF-beta, can act through multiple signaling pathways to elicit a growth arrest response.

Characterization of a murine p53ser246 mutant equivalent to the human p53ser249 associated with hepatocellular carcinoma and aflatoxin exposure

A mutation in the tumor suppressor p53 gene resulting in an Arg-->Ser substitution in position 249 is found frequently in human hepatocellular carcinomas associated with hepatitis B infection and with aflatoxin exposure. To determine the significance of this mutation in an in vivo experimental model using transgenic mice, we introduced a two-nucleotide change in the mouse p53 gene at amino-acid position 246, which is equivalent to position 249 in human p53, by the recombinant polymerase chain reaction mismatched primer method. This p53 mutation resulted in the same change, an Arg-->Ser substitution, as in the human p53 gene at position 249. We now report that the protein product of this mutant mouse p53ser246 had properties similar to those of the wild-type protein when tested by binding to (i) monoclonal antibodies PAb246 and PAb240, ii) simian virus 40 large T antigen, and (iii) heat-shock protein. However, it had mutant-type transforming properties when tested for colony formation with an osteosarcoma cell line. It was not active, as is wild-type p53, in transcription activation of the muscle creatine kinase promoter. These properties are the same as those found in the p53trp248 product of the p53 mutation associated with the Li-Fraumeni syndrome. Although less is known about the human p53ser249 product associated with hepatocellular carcinoma, the mutant murine p53ser246 protein shares the known properties of the human gene product.

Assessment of sensitivity and specificity of immunohistochemical staining of p53 in lung and head and neck cancers

Thirty-two primary carcinomas of the lung and 17 carcinomas of the head and neck (HN) were systematically analyzed for p53 mutations in the highly conserved regions of the gene (exons 5-8). Frozen sections of the same tumors were stained immunohistochemically to assess the sensitivity and specificity of p53 expression as determined by the presence or absence of the protein. On the basis of histology, the lung tumors studied were divided into adenocarcinomas (AC; n = 15), squamous-cell carcinomas (SCC; n = 12), and large-cell carcinomas (LCC; n = 5). All the HN cancers were SCC. Mutations in the p53 gene were detected by direct sequencing of amplified polymerase chain reaction products in six AC of the lungs (40%), three SCC of the lungs (25%), and one LCC (20%), with an overall mutation frequency of 31%. Nine AC (60%) of the lungs, five SCC (42%), and four LCC (80%) were p53-positive by immunohistochemistry. Among HN cancers, p53 mutations were detected in seven tumors (41%). Nine HN tumors (53%) were positive for p53. Negative staining, despite the presence of p53 mutations, was confined to nonsense mutations with truncated p53 and to single-base mutations not causing any change in the amino acid. Although immunohistochemical staining for mutated p53 is sensitive and simple to perform as a screening method, it is not as specific for evaluation of p53 mutations in lung and HN cancers.

Modulation of p53-mediated transcriptional repression and apoptosis by the adenovirus E1B 19K protein

BRK cell lines that stably express adenovirus E1A and a murine temperature-sensitive p53 undergo apoptosis when p53 assumes the wild-type conformation. Expression of the E1B 19,000-molecular-weight (19K) protein rescues cells from this p53-mediated apoptosis and diverts cells to a growth-arrested state. As p53 likely functions as a tumor suppressor by regulating transcription, the ability of the E1B 19K protein to regulate p53-mediated transactivation and transcriptional repression was investigated. In promoter-reporter assays the E1B 19K did not block p53-mediated transactivation but did alleviate p53-mediated transcriptional repression. E1B 19K expression permitted efficient transcriptional activation of the p21/WAF-1/cip-1 mRNA by p53, consistent with maintenance of the growth arrest function of p53. The E1B 19K protein is thereby unique among DNA virus-transforming proteins that target p53 for inactivation in that it selectively modulates the transcriptional properties of p53. The E1B 19K protein also rescued cells from apoptosis induced by inhibitors of transcription and protein synthesis. This suggests that cell death may result from the inhibition of expression of survival factors which function to maintain cell viability. p53 may induce apoptosis through generalized transcriptional repression. In turn, the E1B 19K protein may prevent p53-mediated apoptosis by alleviating p53-mediated transcriptional repression.

Mouse p53 represses the rat brain creatine kinase gene but activates the rat muscle creatine kinase gene

The creatine kinases (CK) regenerate ATP for cellular reactions with a high energy expenditure. While muscle CK (CKM) is expressed almost exclusively in adult skeletal and cardiac muscle, brain CK (CKB) expression is more widespread and is highest in brain glial cells. CKB expression is also high in human lung tumor cells, many of which contain mutations in p53 alleles. We have recently detected high levels of CKB mRNA in HeLa cells and, in this study, have tested whether this may be due to the extremely low amounts of p53 protein present in HeLa cells. Transient transfection experiments showed that wild-type mouse p53 severely repressed the rat CKB promoter in HeLa but not CV-1 monkey kidney cells, suggesting that, in HeLa but not CV-1 cells, p53 either associates with a required corepressor or undergoes a posttranslational modification necessary for CKB repression. Conversely, mouse wild-type p53 strongly activated the rat CKM promoter in CV-1 cells but not in HeLa cells, suggesting that, in CV-1 cells, p53 may associate with a required coactivator or is modified in a manner necessary for CKM activation. The DNA sequences required for p53-mediated modulations were found to be within bp -195 to +5 of the CKB promoter and within bp -168 to -97 of the CKM promoter. Moreover, a 112-bp fragment from the proximal rat CKM promoter (bp -168 to -57), which contained five degenerate p53-binding elements, was capable of conferring p53-mediated activation on a heterologous promoter in CV-1 cells. Also, this novel p53 sequence, when situated in the native 168-bp rat CKM promoter, conferred p53-mediated activation equal to or greater than that of the originally characterized far-upstream (bp -3160) mouse CKM p53 element. Therefore, CKB and CKM may be among the few cellular genes which could be targets of p53 in vivo. In addition, we analyzed a series of missense mutants with alterations in conserved region II of p53. Mutations affected p53 transrepression and transactivation activities differently, indicating that these activities in p53 are separable. The ability of p53 mutants to transactivate correlated well with their ability to inhibit transformation of rat embryonic fibroblasts by adenovirus E1a and activated Ras.

Mouse p53 represses the rat brain creatine kinase gene but activates the rat muscle creatine kinase gene

The creatine kinases (CK) regenerate ATP for cellular reactions with a high energy expenditure. While muscle CK (CKM) is expressed almost exclusively in adult skeletal and cardiac muscle, brain CK (CKB) expression is more widespread and is highest in brain glial cells. CKB expression is also high in human lung tumor cells, many of which contain mutations in p53 alleles. We have recently detected high levels of CKB mRNA in HeLa cells and, in this study, have tested whether this may be due to the extremely low amounts of p53 protein present in HeLa cells. Transient transfection experiments showed that wild-type mouse p53 severely repressed the rat CKB promoter in HeLa but not CV-1 monkey kidney cells, suggesting that, in HeLa but not CV-1 cells, p53 either associates with a required corepressor or undergoes a posttranslational modification necessary for CKB repression. Conversely, mouse wild-type p53 strongly activated the rat CKM promoter in CV-1 cells but not in HeLa cells, suggesting that, in CV-1 cells, p53 may associate with a required coactivator or is modified in a manner necessary for CKM activation. The DNA sequences required for p53-mediated modulations were found to be within bp -195 to +5 of the CKB promoter and within bp -168 to -97 of the CKM promoter. Moreover, a 112-bp fragment from the proximal rat CKM promoter (bp -168 to -57), which contained five degenerate p53-binding elements, was capable of conferring p53-mediated activation on a heterologous promoter in CV-1 cells. Also, this novel p53 sequence, when situated in the native 168-bp rat CKM promoter, conferred p53-mediated activation equal to or greater than that of the originally characterized far-upstream (bp -3160) mouse CKM p53 element. Therefore, CKB and CKM may be among the few cellular genes which could be targets of p53 in vivo. In addition, we analyzed a series of missense mutants with alterations in conserved region II of p53. Mutations affected p53 transrepression and transactivation activities differently, indicating that these activities in p53 are separable. The ability of p53 mutants to transactivate correlated well with their ability to inhibit transformation of rat embryonic fibroblasts by adenovirus E1a and activated Ras.

Myc-mediated apoptosis requires wild-type p53 in a manner independent of cell cycle arrest and the ability of p53 to induce p21waf1/cip1

Deregulated expression of the c-myc proto-oncogene can lead to apoptosis under certain physiological conditions. By introducing a conditionally active Myc allele into primary embryo fibroblasts null for p53, and into fibroblasts without endogenous p53 expression but ectopically expressing a temperature-sensitive p53 allele, we show that expression of wild-type p53 is required for susceptibility to Myc-mediated apoptosis. Although ectopic expression of wild-type p53 blocked cells in the G1 phase of the cell cycle, G1 arrest by isoleucine starvation, in a manner independent of p53, did not confer susceptibility to apoptosis. Thus, growth arrest per se is not sufficient to induce Myc-mediated apoptosis; instead, a property intrinsic to p53 is specifically required. Moreover, apoptosis did not require induction of p53 target proteins, including the cyclin-dependent kinase inhibitor p21waf1/cip1. Therefore, the role of p53 in apoptosis may be distinct from its role in cell cycle arrest.