Adenovirus type 12 early region 1B 54K protein significantly extends the life span of normal mammalian cells in culture

The Promotion of Genomic Instability in Human Fibroblasts by Adenovirus 12 Early Region 1B 55K Protein in the Absence of Viral Infection

The adenovirus 12 early region 1B55K (Ad12E1B55K) protein has long been known to cause non-random damage to chromosomes 1 and 17 in human cells. These sites, referred to as Ad12 modification sites, have marked similarities to classic fragile sites. In the present report we have investigated the effects of Ad12E1B55K on the cellular DNA damage response and on DNA replication, considering our increased understanding of the pathways involved. We have compared human skin fibroblasts expressing Ad12E1B55K (55K+HSF), but no other viral proteins, with the parental cells. Appreciable chromosomal damage was observed in 55K+HSFs compared to parental cells. Similarly, an increased number of micronuclei was observed in 55K+HSFs, both in cycling cells and after DNA damage. We compared DNA replication in the two cell populations; 55K+HSFs showed increased fork stalling and a decrease in fork speed. When replication stress was introduced with hydroxyurea the percentage of stalled forks and replication speeds were broadly similar, but efficiency of fork restart was significantly reduced in 55K+HSFs. After DNA damage, appreciably more foci were formed in 55K+HSFs up to 48 h post treatment. In addition, phosphorylation of ATM substrates was greater in Ad12E1B55K-expressing cells following DNA damage. Following DNA damage, 55K+HSFs showed an inability to arrest in cell cycle, probably due to the association of Ad12E1B55K with p53. To confirm that Ad12E1B55K was targeting components of the double-strand break repair pathways, co-immunoprecipitation experiments were performed which showed an association of the viral protein with ATM, MRE11, NBS1, DNA-PK, BLM, TOPBP1 and p53, as well as with components of the replisome, MCM3, MCM7, ORC1, DNA polymerase δ, TICRR and cdc45, which may account for some of the observed effects on DNA replication. We conclude that Ad12E1B55K impacts the cellular DNA damage response pathways and the replisome at multiple points through protein-protein interactions, causing genomic instability.

Nek4 regulates entry into replicative senescence and the response to DNA damage in human fibroblasts

When explanted into culture, normal human cells exhibit a finite number of cell divisions before entering a proliferative arrest termed replicative senescence. To identify genes essential for entry into replicative senescence, we performed an RNA interference (RNAi)-based loss-of-function screen and found that suppression of the Never in Mitosis Gene A (NIMA)-related protein kinase gene NEK4 disrupted timely entry into senescence. NEK4 suppression extended the number of population doublings required to reach replicative senescence in several human fibroblast strains and resulted in decreased transcription of the cyclin-dependent kinase inhibitor p21. NEK4-suppressed cells displayed impaired cell cycle arrest in response to double-stranded DNA damage, and mass spectrometric analysis of Nek4 immune complexes identified a complex containing DNA-dependent protein kinase catalytic subunit [DNA-PK(cs)], Ku70, and Ku80. NEK4 suppression causes defects in the recruitment of DNA-PK(cs) to DNA upon induction of double-stranded DNA damage, resulting in reduced p53 activation and H2AX phosphorylation. Together, these observations implicate Nek4 as a novel regulator of replicative senescence and the response to double-stranded DNA damage.

Adenovirus type 5 early region 1B 55-kDa oncoprotein can promote cell transformation by a mechanism independent from blocking p53-activated transcription

Inhibition of p53-activated transcription is an integral part of the mechanism by which early region 1B 55K oncoprotein (E1B-55K) from adenovirus type 5 (Ad5) contributes to complete cell transformation in combination with Ad E1A. In addition, more recent data suggest that the mode of action of the Ad protein during transformation may involve additional functions and other protein interactions. In the present study, we performed a comprehensive mutational analysis to assign further transforming functions of Ad5 E1B-55K to distinct domains within the viral polypeptide. Results from these studies show that the functions required for transformation are encoded within several patches of the 55K primary sequence, including several clustered cysteine and histidine residues, some of which match the consensus for zinc fingers. In addition, two amino-acid substitutions (C454S/C456S) created a 55K mutant protein, which had substantially reduced transforming activity. Interestingly, the same mutations neither affected binding to p53 nor inhibition of p53-mediated transactivation. Therefore, an activity necessary for efficient transformation of primary rat cells can be separated from functions required for inhibition of p53-stimulated transcription. Our data indicate that this activity is linked to the ability of the Ad5 protein to bind to components of the Mre11/Rad50/NBS1 DNA double-strand break repair complex, and/or its ability to assemble multiprotein aggregates in the cytoplasm and nucleus of transformed rat cells. These results introduce a new function for Ad5 E1B-55K and suggest that the viral protein contributes to cell transformation through p53 transcription-dependent and -independent pathways.

Enhanced efficacy of Escherichia coli nitroreductase/CB1954 prodrug activation gene therapy using an E1B-55K-deleted oncolytic adenovirus vector

Viruses that replicate selectively in cancer cells constitute an exciting new class of anticancer agent. The conditionally replicating adenovirus (CRAd) dl1520, which lacks the E1B-55K gene, has elicited significant clinical responses in humans when used in combination with chemotherapy. A convergent development has been to use replication-defective viruses to express prodrug-activating enzymes in cancer cells. This can sensitize the cancer to prodrug, but depends upon achieving sufficient level, distribution and specificity of enzyme expression within the tumour. In this study, we have expressed the prodrug-activating enzyme nitroreductase (NTR) in the context of an E1B-55K-deleted adenovirus, CRAd-NTR(PS1217H6). We show that CRAd-NTR(PS1217H6) retains oncolytic growth properties, and expresses substantially more NTR than a comparable, replication-defective adenovirus. The combination of viral oncolysis and NTR expression results in significantly greater sensitization of SW480 and WiDr colorectal cancer cells to the prodrug CB1954 in vitro. In vivo, CRAd-NTR(PS1217H6) was shown to replicate in subcutaneous SW480 tumour xenografts in immunodeficient mice, resulting in more NTR expression and greater sensitization to CB1954 than with replication-defective virus. Combination therapy of CRAd-NTR(PS1217H6) with CB1954 reduced tumour growth from 13.5- to 2.8-fold over 5 weeks, and extended median survival from 42 to 81 days, compared with no treatment.

Cell transformation by human adenoviruses

The last 40 years of molecular biological investigations into human adenoviruses have contributed enormously to our understanding of the basic principles of normal and malignant cell growth. Much of this knowledge stems from analyses of their productive infection cycle in permissive host cells. Also, initial observations concerning the carcinogenic potential of human adenoviruses subsequently revealed decisive insights into the molecular mechanisms of the origins of cancer, and established adenoviruses as a model system for explaining virus-mediated transformation processes. Today it is well established that cell transformation by human adenoviruses is a multistep process involving several gene products encoded in early transcription units 1A (E1A) and 1B (E1B). Moreover, a large body of evidence now indicates that alternative or additional mechanisms are engaged in adenovirus-mediated oncogenic transformation involving gene products encoded in early region 4 (E4) as well as epigenetic changes resulting from viral DNA integration. In particular, detailed studies on the tumorigenic potential of subgroup D adenovirus type 9 (Ad9) E4 have now revealed a new pathway that points to a novel, general mechanism of virus-mediated oncogenesis. In this chapter, we summarize the current state of knowledge about the oncogenes and oncogene products of human adenoviruses, focusing particularly on recent findings concerning the transforming and oncogenic properties of viral proteins encoded in the E1B and E4 transcription units.

A role for p53 in the frequency and mechanism of mutation

The tumor suppressor protein, p53, is often referred to as the guardian of the genome. When p53 function is impaired, its ability to preserve genomic integrity is compromised. This may result in an increase in mutation on both a molecular and chromosomal level and contribute to the progression to a malignant phenotype. In order to study the effect of p53 function on the acquisition of mutation, in vitro and in vivo models have been developed in which both the frequency and mechanism of mutation can be analyzed. In human lymphoblastoid cells in which p53 function was impaired, both the spontaneous and induced mutant frequency increased at the autosomal thymidine kinase (TK) locus. The mutant frequency increased to a greater extent in cell lines in which p53 harbored a point mutation than in those lines in which a "null" mutation had been introduced by molecular targeting or by viral degradation indicating a possible "gain-of-function" associated with the mutant protein. Further, molecular analysis revealed that the loss of p53 function was associated with a greater tendency towards loss-of-heterozygosity (LOH) within the TK gene that was due to non-homologous recombination than that found in wild-type cells. Most data obtained from the in vivo models uses the LacI reporter gene that does not efficiently detect mutation that results in LOH. However, studies that have examined the effect of p53 status on mutation in the adenine phosphoribosyl transferase (APRT) gene in transgenic mice also suggest that loss of p53 function results in an increase in mutation resulting from non-homologous recombination. The results of these studies provide clear and convincing evidence that p53 plays a role in modulating the mutant frequency and the mechanism of mutation. In addition, the types of mutation that occur within the p53 gene are also of importance in determining the mutant frequency and the pathways leading to mutation.

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.

Consequences of disruption of the interaction between p53 and the larger adenovirus early region 1B protein in adenovirus E1 transformed human cells

The adenovirus early region 1B (Ad E1B) genes have no transforming capability of their own but markedly increase the transformation frequency of Ad E1A following co-transfection into mammalian cells. The larger E1B proteins of both Ad2/5 and Ad12 bind to p53 and inhibit its ability to transcriptionally activate other genes. We have previously demonstrated that synthetic peptides identical to the binding sites for p53 on both the Ad2 and Ad12 E1B proteins will disrupt the interaction in vivo and in vitro. In the work presented here we have examined the effects of complex dissociation on Ad E1-transformed human cells. It has been shown, using confocal microscopy, that when the peptide identical to the p53 binding site was added to Ad5 E1-transformed cells it initally located in the cytoplasmic dense bodies where it caused disruption of the p53/E1B complex. Peptide and p53 then translocated to the nucleus. In Ad12 E1-transformed cells the peptide localized in the nucleus directly and there caused a reorganization of p53 staining from a highly organized, 'flecked' distribution to one in which nuclear staining was homogeneous and diffuse. Peptides added to either Ad5 E1 or Ad12 E1 transformed cells resulted in the release of transcriptionally active p53. Interestingly, the level of p53 then fell presumably as a result of proteasomal action - this was probably a reflection of the short half-life of 'free' (i.e. dissociated) p53 compared to that of the bound protein. Free p53 did not cause apoptosis in target cells probably due to the presence of the smaller (19K) E1B proteins. However, addition of peptide leads to a significant reduction in cell growth rate. We have further demonstrated that a significant proportion of those cells which had taken up peptide had ceased DNA synthesis, probably due to a p53-induced cell cycle arrest. The role of the larger EIB protein during transformation is considered in view of these data.

E1B 55-kilodalton oncoproteins of adenovirus types 5 and 12 inactivate and relocalize p53, but not p51 or p73, and cooperate with E4orf6 proteins to destabilize p53

The p53 tumor suppressor protein represents a target for viral and cellular oncoproteins, including adenovirus gene products. Recently, it was discovered that several proteins with structural and functional homologies to p53 exist in human cells. Two of them were termed p51 and p73. We have shown previously that the E1B 55-kDa protein (E1B-55 kDa) of adenovirus type 5 (Ad5) binds and inactivates p53 but not p73. Further, p53 is rapidly degraded in the presence of E1B-55 kDa and the E4orf6 protein of this virus. Here, it is demonstrated that p51 does not detectably associate with E1B-55 kDa. While p53 is relocalized to the cytoplasm by E1B-55 kDa, p51's location is unaffected. Finally, p51 retains its full transcriptional activity in the presence of E1B-55 kDa. Apparently, p51 does not represent a target of Ad5 E1B-55 kDa, suggesting that the functions of p51 are distinct from p53-like tumor suppression. E1B-55 kDa from highly oncogenic adenovirus type 12 (Ad12) was previously shown to surpass the oncogenic activity of Ad5 E1B-55 kDa in various assay systems, raising the possibility that Ad12 E1B-55 kDa might target a broader range of p53-like proteins. However, we show here that Ad12 E1B-55 kDa also inhibits p53's transcriptional activity without measurably affecting p73 or p51. Moderate inhibition of p51's transcriptional activity was observed in the presence of the E4orf6 proteins from Ad5 and Ad12. p53 and Ad12-E1B-55 kDa colocalize in the nucleus and also in cytoplasmic clusters when transiently coexpressed. Finally, E1B-55 kDa and E4orf6 of Ad12 mediate rapid degradation of p53 with an efficiency comparable to that of the Ad5 proteins in human and rodent cells. Our results suggest that E1B-55 kDa of either virus type has similar effects on p53 but does not affect p73 and p51.

Murine fibroblasts lacking p21 undergo senescence and are resistant to transformation by oncogenic Ras

The cell-cycle inhibitor p21 is upregulated during senescence and upon induction of senescence-like arrest by oncogenic Ras. We have used primary fibroblasts derived from p21-null mice to evaluate the role of p21 in these processes. We find that primary p21-/- cells enter senescence and have a lifespan similar to wild-type cells. Upon immortalization, most wild-type and p21-/- cultures acquire alterations in either p53 or p16INK4a, further indicating that p21-deficiency is not sufficient by itself to allow immortalization. Primary p21-/- cells, like wild-type cells, respond to oncogenic Ras by accumulating p53 and p16INK4a, and by decreasing their proliferation rate. In agreement with this, p21-/- cells are refractory to neoplasic transformation by oncogenic Ras when compared to p53-/- cells. We conclude that, in murine fibroblasts, p21 is not essential neither for senescence nor for preventing neoplasic transformation by oncogenic Ras.

Does the immune system of a mouse age faster than the immune system of a human?

One of the characteristics of all somatic cells is a finite life span. Cells may proliferate until they reach a point after which, although they are metabolically active, they can no longer produce daughter cells. This observation is central to the clonal exhaustion hypothesis, a mechanism cited to explain age-associated immune dysfunction. In this hypothesis, repeated division of lymphocytes leads to a replicative limit, after which they enter the senescent phase but are not lost from the pool of T cells. Advancing age would then be associated with an increase in the number of T cells that are unable to proliferate to a stimulus which induces a proliferative response in T cells from younger individuals. This hypothesis seems both logical and reasonable and is supported by data from both humans and mice with the demonstration of an age-related accumulation of senescent T cells in both species. However, there is an apparent paradox. The paradox arises because the onset of immunosenescence appears to be more closely linked to the life span of the animal rather than the life span of the lymphocyte.

Telomeric instability and reduced proliferative potential in ovarian surface epithelial cells from women with a family history of ovarian cancer

OBJECTIVE

Increased telomeric instability in normal ovarian surface epithelium may contribute to ovarian carcinogenesis in women from families with a high frequency of breast/ovarian cancer. To test this hypothesis, we compared proliferative potential, mean telomeric length, and telomerase activity in SV-40 large T-antigen transfected cell lines derived from normal ovarian surface epithelium of women with and without a familial history of breast/ovarian cancer.

METHODS

Telomeric instability was examined in SV-40 large T-antigen transfected cell lines of normal ovarian surface epithelium from patients with (FHIOSE, N = 5) and without (NFHIOSE, N = 11) a history of familial breast/ovarian cancer. The duration and total attainable number of population doublings, mean telomeric length, rate of telomeric loss, and telomerase activity were determined by cell counts, Southern blot analysis, and PCR ELISA.

RESULTS

FHIOSE cells attained fewer population doublings than NFHIOSE cells and doubled at approximately half the rate of NFHIOSE cells, indicating a reduced proliferative capacity in FHIOSE cells. While telomerase activity was not detected in FHIOSE or NFHIOSE cell lines, mean telomeric lengths in FHIOSE were generally 1 kb shorter than in NFHIOSE cells and the rate of telomeric loss as a function of population doublings was up to threefold greater in FHIOSE cells.

CONCLUSIONS

Increased telomeric instability and reduced growth potential suggest greater proximity to replicative senescence in ovarian surface epithelium from women with a familial history of breast/ovarian cancer. Consequently, an accumulation of genetic aberrations due to accelerated cellular aging may contribute to the enhanced susceptibility for malignant transformation and earlier onset in heritable ovarian cancer.

The replicative capacities of large E1B-null group A and group C adenoviruses are independent of host cell p53 status

Recent reports suggest that an early region 1B (E1B) 55, 000-molecular-weight polypeptide (55K)-null adenovirus type 5 (Ad5) mutant (dl1520) can replicate to the same extent as wild-type (wt) Ad5 in cells either deficient or mutated in p53, implicating p53 in limiting viral replication in vivo. In contrast, we show here that the replicative capacity of Ad5 dl1520 is wholly independent of host cell p53 status, as is the replicative capacity of comparable Ad12 E1B 54K-null adenoviruses (Ad12 dl620 and Ad12 hr703). Furthermore, we show that there is no requirement for complex formation between p53 and Ad5 E1B 55K or Ad12 E1B 54K for a productive infection, such that wt Ad5 and wt Ad12 will both replicate in cells which are null for p53. In addition, we find that these Ad5 and Ad12 mutant viruses induce S phase irrespective of the p53 status of the cell and that, therefore, S-phase induction does not correlate with the replicative capacity of the virus. Interestingly, the replicative capacities of the large E1B-null adenoviruses correlated positively with the ability to express E1B 19K and were related to the ability to repress premature adenovirus-induced apoptosis. Infection of primary human cells indicated that Ad5 dl1520, wt Ad5, and wt Ad12 replicated better in cycling normal human skin fibroblasts (HSFs) than in quiescent HSFs. Thus, the cell cycle status of the host cell, upon infection, also influences viral yield.

Definition of a major p53 binding site on Ad2E1B58K protein and a possible nuclear localization signal on the Ad12E1B54K protein

Previous studies have established that adenovirus 2/5 early region 1B (Ad E1B) 58K protein binds p53 strongly and co-localizes with it to cytoplasmic dense bodies whilst the homologous Ad12E1B54K protein binds only weakly and co-localizes primarily to the nucleus in Ad12E1 transformed cells. We have used these properties of the E1B proteins from different viral serotypes to map the p53 binding site on the Ad2/5 protein. A set of chimaeric genes was constructed containing different proportions of the Ad12 and Ad2E1B DNA. These, together with Ad12E1A and E1B19K DNA, were transfected into baby rat kidney cells and transformed lines isolated. From an examination of the properties of these Ad12/Ad2E1B fusion proteins in co-immunoprecipitation and subcellular localization experiments it has been concluded that the p53 binding site on Ad2E1B58K protein lies between amino acids 216 and 235 and that the homologous region on Ad12E1B54K protein also binds p53. In addition, a unique nuclear localization signal is located on Ad12E1B54K between residues 228 and 239. We suggest that primary structure differences in these regions of the Ad2 and Ad12E1B proteins are responsible for the different subcellular localizations in AdE1 transformants.

A reassessment of the telomere hypothesis of senescence

According to the telomere hypothesis of senescence, the telomeric shortening that accompanies the replication of normal somatic cells acts as the mitotic clock that eventually results in their permanent exit from the cell cycle. Although evidence consistent with the telomere hypothesis continues to accumulate, on the basis of recent findings it is suggested that instead of a single clock mechanism there are multiple inducers of senescence.