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

p53, p63 and p73--solos, alliances and feuds among family members

p53 controls crucial stress responses that play a major role in preventing malignant transformation. Hence, inactivation of p53 is the single most common genetic defect in human cancer. With the recent discovery of two close structural homologs, p63 en p73, we are getting a broader view of a fascinating gene family that links developmental biology with tumor biology. While unique roles are apparent for each of these genes, intimate biochemical cross-talk among family members suggests a functional network that might influence many different aspects of individual gene action. The most interesting part of this family network derives from the fact that the p63 and p73 genes are based on the "two-genes-in-one" idea, encoding both agonist and antagonist in the same open reading frame. In this review, we attempt to present an overview of the current status of this fast moving field.

Adenovirus early E4 genes in viral oncogenesis

Previous investigations into potential transforming activities of adenovirus (Ad) early genes were largely overshadowed by the more obvious roles of E1A and E1B products. One exception was an Ad9 E4 protein (ORF1) shown to enhance transformation of cultured cells and promote mammary tumors in female rats. Recently, significant advances in understanding Ad E4 gene products at the molecular level have revealed that these proteins possess an unexpectedly diverse collection of functions, which not only orchestrate many viral processes, but overlap with oncogenic transformation of primary mammalian cells. Operating through a complex network of protein interactions with key viral and cellular regulatory components, Ad E4 products are apparently involved in transcription, apoptosis, cell cycle control, DNA repair, cell signaling, posttranslational modifications and the integrity of nuclear multiprotein complexes known as PML oncogenic domains (PODs). Some of these functions directly relate to known transforming and oncogenic processes, or implicate mechanisms such as modulating the function and subcellular localization of cellular PDZ domain-containing proteins, POD reorganization, targeted proteolytic degradation, inhibition of DNA double-strand break repair and 'hit-and-run' mutagenesis. Here, we summarize the recent data and discuss how E4 gene product interactions may contribute to viral oncogenesis.

Molecular regulation and biological function of adenovirus early genes: the E4 ORFs

Over the past few years there have been a number of interesting advances in our understanding of the functions encoded by the adenovirus early transcription unit 4 (Ad E4). A large body of recent data demonstrates that E4 proteins encompass an unexpectedly diverse collection of functions required for efficient viral replication. E4 gene products operate through a complex network of protein interactions with key viral and cellular regulatory components involved in transcription, apoptosis, cell cycle control and DNA repair, as well as host cell factors that regulate cell signaling, posttranslational modifications and the integrity of nuclear multiprotein complexes known as nuclear bodies (NBs) or PML oncogenic domains (PODs). As understood at present, some of the lytic functions overlap with roles in oncogenic transformation of primary mammalian cells. These observations, together with findings that E4 proteins substantially affect cell toxicity and the immune response of the host have profound implications for the development of Ad vectors for gene therapy. In this article we will summarize recent findings regarding the diverse functions of E4 gene products in the context of earlier work. We will emphasize the interaction of E4 proteins with cellular and viral interaction partners, the role of these interactions for lytic virus growth and how these interactions may contribute to viral oncogenesis. Finally, we will discuss their role in Ad vector and adeno-associated virus infections.

Amphiphysin IIb-1, a novel splicing variant of amphiphysin II, regulates p73beta function through protein-protein interactions

p73 is a nuclear protein that is similar in structure and function to p53. Notably, the C-terminal region of p73 has a regulatory function, through interactions with a positive or negative regulator. In this study, we use the yeast two-hybrid technique to identify a novel p73beta binding protein, designated amphiphysin IIb-1. Amphiphysin IIb-1 is one of the splicing variants of amphiphysin II, and has a shorter protein product than amphiphysin IIb, which has been previously reported. We confirmed that amphiphysin IIb-1 binds full-length p73beta, both in vitro and in vivo. This association is mediated via the SH3 domain of amphiphysin IIb-1 and C-terminal amino acids 321-376 of p73beta. Double immunofluorescence patterns revealed that p73beta is relocalized to the cytoplasm in the presence of amphiphysin IIb-1. Overexpression of amphiphysin IIb-1 was found to significantly inhibit the transcriptional activity of p73beta in a dose-dependent manner. In addition, the cell death function of p73beta was inhibited by amphiphysin IIb-1. These findings offer a new insight into the regulation mechanism of p73beta, and suggest that amphiphysin IIb-1 modulates p73beta function by direct binding.

p53 and its homologues, p63 and p73, induce a replicative senescence through inactivation of NF-Y transcription factor

Recent studies have identified two p53 homologues, p63 and p73. They activate p53-responsive promoters and induce apoptosis when overexpressed in certain human tumors. Here, we report that p63, like p53 and p73, induces replicative senescence when expressed in a tetracycline-regulated manner in EJ cells lacking a functional p53. In addition to transcription activation of p53-responsive genes, we found that p63 and p73 repress transcription of the cdk1 and cyclin B genes, both of which are irreversibly repressed in senescent human fibroblast. In transient transfection assay, p63 and p73 repress the cdk1 promoter regardless of the presence of a dominant negative mutant form of p53. Furthermore, we found that DNA binding activity of NF-Y transcription factor, which is essential for transcription of the cdk1 and cyclin B genes and inactivated in senescent fibroblast, is significantly decreased by expression of either of p53, p63, or p73. Since NF-Y binds to many promoters besides the cdk1 and cyclin B promoters, inactivation of NF-Y by p53 family genes may be a general mechanism for transcription repression in replicative senescence.

Human T-cell leukemia virus type I tax repression of p73beta is mediated through competition for the C/H1 domain of CBP

The Tax protein, encoded by the human T-cell leukemia virus type I (HTLV-I), is required for high level viral transcription and HTLV-I-associated malignant transformation. Although the precise mechanism of malignant transformation by Tax is unclear, it is well established that Tax represses the transcription function of the tumor suppressor p53, possibly accelerating the accumulation of genetic mutations that are critical in HTLV-I-mediated malignant transformation. Tax repression of p53 transcription function appears to occur, at least in part, through competition for the cellular coactivator CBP/p300. In this study, we characterize the effect of Tax on the p53 family member, p73. We demonstrate that Tax also represses the transcription function of p73beta and that the repression is reciprocal in vivo, consistent with the idea that both transcription factors may compete for CBP/p300 in vivo. We provide evidence showing that both Tax and p73 interact strongly with the C/H1 domain of CBP and that their binding to this region is mutually exclusive in vitro. This finding provides evidence supporting the idea that reciprocal transcriptional repression between Tax and p73 is mediated through coactivator competition.

Analyses of single-amino-acid substitution mutants of adenovirus type 5 E1B-55K protein

The E1B-55K protein plays an important role during human adenovirus type 5 productive infection. In the early phase of the viral infection, E1B-55K binds to and inactivates the tumor suppressor protein p53, allowing efficient replication of the virus. During the late phase of infection, E1B-55K is required for efficient nucleocytoplasmic transport and translation of late viral mRNAs, as well as for host cell shutoff. In an effort to separate the p53 binding and inactivation function and the late functions of the E1B-55K protein, we have generated 26 single-amino-acid mutations in the E1B-55K protein. These mutants were characterized for their ability to modulate the p53 level, interact with the E4orf6 protein, mediate viral late-gene expression, and support virus replication in human cancer cells. Of the 26 mutants, 24 can mediate p53 degradation as efficiently as the wild-type protein. Two mutants, R240A (ONYX-051) and H260A (ONYX-053), failed to degrade p53 in the infected cells. In vitro binding assays indicated that R240A and H260A bound p53 poorly compared to the wild-type protein. When interaction with another viral protein, E4orf6, was examined, H260A significantly lost its ability to bind E4orf6, while R240A was fully functional in this interaction. Another mutant, T255A, lost the ability to bind E4orf6, but unexpectedly, viral late-gene expression was not affected. This raised the possibility that the interaction between E1B-55K and E4orf6 was not required for efficient viral mRNA transport. Both R240A and H260A have retained, at least partially, the late functions of wild-type E1B-55K, as determined by the expression of viral late proteins, host cell shutoff, and lack of a cold-sensitive phenotype. Virus expressing R240A (ONYX-051) replicated very efficiently in human cancer cells, while virus expressing H260A (ONYX-053) was attenuated compared to wild-type virus dl309 but was more active than ONYX-015. The ability to separate the p53-inactivation activity and the late functions of E1B-55K raises the possibility of generating adenovirus variants that retain the tumor selectivity of ONYX-015 but can replicate more efficiently than ONYX-015 in a broad spectrum of cell types.

Tissue-specific, tumor-selective, replication-competent adenovirus vector for cancer gene therapy

We have previously described two replication-competent adenovirus vectors, named KD1 and KD3, for potential use in cancer gene therapy. KD1 and KD3 have two small deletions in the E1A gene that restrict efficient replication of these vectors to human cancer cell lines. These vectors also have increased capacity to lyse cells and spread from cell to cell because they overexpress the adenovirus death protein, an adenovirus protein required for efficient cell lysis and release of adenovirus from the cell. We now describe a new vector, named KD1-SPB, which is the KD1 vector with the E4 promoter replaced by the promoter for surfactant protein B (SPB). SPB promoter activity is restricted in the adult to type II alveolar epithelial cells and bronchial epithelial cells. Because KD1-SPB has the E1A mutations, it should replicate within and destroy only alveolar and bronchial cancer cells. We show that KD1-SPB replicates, lyses cells, and spreads from cell to cell as well as does KD1 in H441 cells, a human cancer cell line where the SPB promoter is active. KD1-SPB replicates, lyses cells, and spreads only poorly in Hep3B liver cancer cells. Replication was determined by expression of the E4ORF3 protein, viral DNA accumulation, fiber synthesis, and virus yield. Cell lysis and vector spread were measured by lactate dehydrogenase release and a "vector spread" assay. In addition to Hep3B cells, KD1-SPB also did not express E4ORF3 in HT29.14S (colon), HeLa (cervix), KB (nasopharynx), or LNCaP (prostate) cancer cell lines, in which the SPB promoter is not expected to be active. Following injection into H441 or Hep3B tumors growing in nude mice, KD1-SPB caused a three- to fourfold suppression of growth of H441 tumors, similar to that seen with KD1. KD1-SPB had only a minimal effect on the growth of Hep3B tumors, whereas KD1 again caused a three- to fourfold suppression. These results establish that the adenovirus E4 promoter can be replaced by a tissue-specific promoter in a replication-competent vector. The vector has three engineered safety features: the tissue-specific promoter, the mutations in E1A that preclude efficient replication in nondividing cells, and a deletion of the E3 genes which shield the virus from attack by the immune system. KD1-SPB may have use in treating human lung cancers in which the SPB promoter is active.

Identification of three functions of the adenovirus e4orf6 protein that mediate p53 degradation by the E4orf6-E1B55K complex

Complexes containing adenovirus E4orf6 and E1B55K proteins play critical roles in productive infection. Both proteins interact directly with the cellular tumor suppressor p53, and in combination they promote its rapid degradation. To examine the mechanism of this process, degradation of exogenously expressed p53 was analyzed in p53-null human cells infected with adenovirus vectors encoding E4orf6 and/or E1B55K. Coexpression of E4orf6 and E1B55K greatly reduced both the level and the half-life of wild-type p53. No effect was observed with the p53-related p73 proteins, which did not appear to interact with E4orf6 or E1B55K. Mutant forms of p53 were not degraded if they could not efficiently bind E1B55K, suggesting that direct interaction between p53 and E1B55K may be required. Degradation of p53 was independent of both MDM2 and p19ARF, regulators of p53 stability in mammalian cells, but required an extended region of E4orf6 from residues 44 to 274, which appeared to possess three separate biological functions. First, residues 39 to 107 were necessary to interact with E1B55K. Second, an overlapping region from about residues 44 to 218 corresponded to the ability of E4orf6 to form complexes with cellular proteins of 19 and 14 kDa. Third, the nuclear retention signal/amphipathic arginine-rich alpha-helical region from residues 239 to 253 was required. Interestingly, neither the E4orf6 nuclear localization signal nor the nuclear export signal was essential. These results suggested that if nuclear-cytoplasmic shuttling is involved in this process, it must involve another export signal. Degradation was significantly blocked by the 26S proteasome inhibitor MG132, but unlike the HPV E6 protein, E4orf6 and E1B55K were unable to induce p53 degradation in vitro in reticulocyte lysates. Thus, this study implies that the E4orf6-E1B55K complex may direct p53 for degradation by a novel mechanism.

Interactions between adenovirus proteins and the p53 pathway: the development of ONYX-015

dl1520 (ONYX-015) is an adenovirus mutant that lacks the E1b 55K gene. As a result it cannot neutralize p53. Therefore dl1520 should only grow in cells that lack p53, and should replicate selectively in cancer cells. However, there is no correlation between replication and p53 status, in cancer cells. This is for two reasons: (1) E1B 55K has additional functions, that are necessary in some tumor cells and not in others. (2) p53 function can be lost by alternative mechanisms such as loss of p14ARF. In normal cells, dl1520 induces p53, and is generally strongly attenuated for replication. ONYX-015 is currently being tested in clinical trials, and is a promising new therapeutic agent in cancer.

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.

The adenovirus-2 E1B-55K protein interacts with a mSin3A/histone deacetylase 1 complex

The adenovirus E1B-55K protein is a multifunctional phosphoprotein that regulates nuclear to cytoplasmic export of host cell and viral mRNAs during lytic viral growth. E1B-55K also blocks apoptosis by binding and functionally inactivating the human tumor suppressor protein p53. Here, we show that E1B-55K interacts with histone deacetylase 1 (HDAC1) and the transcriptional corepressor protein mSin3A, both in the adenovirus-transformed 293 cell line and during a lytic adenovirus infection. Furthermore, we show that the central amino acids 156-261 in E1B-55K are necessary for efficient HDAC1 interaction. Importantly, the E1B-55K/mSin3A/HDAC1 complex is also enzymatically active, catalyzing deacetylation of a histone substrate peptide. Collectively, our results suggest that E1B-55K interaction with mSin3A/HDAC1 containing complexes may be significant for one or several of the multiple activities ascribed to this protein.