The adenovirus type 12 early-region 1B 58,000-Mr gene product is required for viral DNA synthesis and for initiation of cell transformation

Modification of the Rb-binding domain of replication-competent adenoviral vector enhances cytotoxicity against human esophageal cancers via NF-kappaB activity

A replication-competent adenoviral vector deficient for expression of the early E1B55K protein has been applied in clinical studies. The vector, however, was not fully effective for the treatment of human cancer. In this study, the E1A gene (which encodes an Rb-binding domain protein) of the adenoviral vector AxE1AdB was further engineered with a point mutation designed to abolish binding to Rb protein (pRb) and arrest the cell cycle (AxdAdB-3). The difference in the cytotoxicity of these vectors in two cancer cell lines was observed in association with differences in replication, infection efficiency, and expression levels of adenovirus receptors. Relative to the parent vector (AxE1AdB), which worked in a manner similar to ONYX-015, AxdAdB-3 with the mutated pRb-binding motif demonstrated increased cytotoxicity against p53-mutant human esophageal cancer cell lines EC-GI-10 and T.Tn. AxdAdB-3 showed a greater oncolytic effect than AxE1AdB in vivo despite almost the same replication efficiency in vitro. Unexpectedly, cell cycle arrest in AxdAdB-3-infected cells was less efficient than that in cell lines infected with AxE1AdB. However, AxdAdB-3 strongly reduced NF-kappaB activity and thereby enhanced apoptosis more than AxE1AdB did. These data demonstrate that the Rb-binding domain of E1A can regulate NF-kappaB activity and that modifications to this domain may lead to advances in gene therapies for the treatment of human cancers.

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.

Phosphorylation at the carboxy terminus of the 55-kilodalton adenovirus type 5 E1B protein regulates transforming activity

The 55-kDa product of early region 1B (E1B) of human adenoviruses is required for viral replication and participates in cell transformation through complex formation with and inactivation of the cellular tumor suppressor p53. We have used both biochemical and genetic approaches to show that this 496-residue (496R) protein of adenovirus type 5 is phosphorylated at serine and threonine residues near the carboxy terminus within sequences characteristic of substrates of casein kinase II. Mutations which converted serines 490 and 491 to alanine residues decreased viral replication and greatly reduced the efficiency of transformation of primary baby rat kidney cells. Such mutant 496R proteins interacted with p53 at efficiencies similar to those of wild-type 496R but only partially inhibited p53 transactivation activity. These results indicated that phosphorylation at these carboxy-terminal sites either regulates the inhibition of p53 or regulates some other 496R function required for cell transformation.

Adenovirus type 12 early region 1B proteins and metabolism of early viral mRNAs

Early region 1B (E1B) of human adenoviruses encodes two major proteins. The 19-kDa polypeptide appears to prevent E1A-induced cytolysis and DNA degradation. The larger E1B product of approximately 55 kDa, which is essential for viral replication, plays a role in the accumulation and stability of viral mRNAs and the late shutoff of host metabolism. For serotype 12 (Ad12), this 482-residue (482R) protein is essential for viral DNA replication. In the present report we have used a series of mutants to examine the roles of Ad12 482R and the 19-kDa, 163R protein in the metabolism of early viral mRNAs. No specific effects on the accumulation of early (or late) mRNAs were detected with any of the mutants affecting 163R. With mutant dl42, which encodes an altered 482R product that lacks residues 114-155, both viral DNA replication and late viral protein synthesis were defective. Accumulation of E1A transcripts in the nucleus and cytoplasm resembled wt. The levels of mRNAs from early regions E1B, E2A and E3 at later times during infection were somewhat lower than those of wt, but this decrease may have been due to the absence of progeny viral DNA in dl42-infected cells. However, the accumulation of both E2B and E4 mRNAs at all times was severely reduced. These data suggested that the requirement of 482R for Ad12 DNA replication may be related to its specific role in the metabolism of E2B and E4 mRNAs that encode products necessary for viral DNA synthesis.

Overexpression of the E1B 55-kilodalton (482R) protein of human adenovirus type 12 appears to permit efficient transformation of primary baby rat kidney cells in the absence of the E1B 19-kilodalton protein

To analyze the structure and function of the E1B 19,000-molecular-weight protein (19K protein) (163R) of human adenovirus type 12, mutants were produced at various positions across the 163R-coding sequence. Viruses bearing mutations within the first 100 or so amino acids yielded unstable 163R-related products, induced DNA degradation and enhanced cytopathic effect (cyt/deg phenotype) in KB cells, and transformed primary rodent cells at much lower efficiencies than wild-type (wt) virus. Deletion of the final 16 residues at the carboxy terminus had no phenotypic effect. Alteration of residue 105 reduced transforming efficiency significantly, suggesting that this region of 163R is functionally important. Disruption of the AUG initiation codon at nucleotide 1542 blocked production of 163R completely but resulted in higher levels of E1B 55K-482R protein synthesis and a transforming efficiency similar to that of wt virus. These data suggested that while 163R is of some importance, normal transforming efficiencies can be obtained in its absence if 482R is overexpressed.

Enteric adenovirus type 40: complementation of the E4 defect in Ad2 dl808

The enteric adenovirus type 40 cannot be passaged in HeLa cells, but will grow productively in cells that express the E1B region of adenovirus types 2 or 5. Even in such permissive cells, the lytic cycle is prolonged, there is an abnormal pattern of E1B early gene expression and a failure to switch off host cell functions, suggesting that other gene functions might be impaired in Ad40. For Ad2, E4 ORF 6 and ORF 3 proteins are known to have an essential role in progressing from the early to the late phase of lytic infection and the shutoff of host functions requires an interaction between the E4 ORF 6 34K protein and the E1B 55K protein. To test whether E4 functions of Ad40 are impaired, complementation tests have been made between Ad40 and the E4 deletion mutant Ad2 dl808, which lacks all but ORF 1 of the E4 region. In HeLa and Vero cells, Ad40 complements dl808 to levels equivalent to an Ad2 wild-type infection, as demonstrated by measuring virion packaged DNA, virus titration, and viral protein synthesis. Surprisingly, Ad2 dl808 fails to reciprocally complement Ad40. The results show that Ad40 produces functional E4 ORF 6 and/or ORF 3 activity, and that their expression precedes DNA replication.

Fastidious human adenovirus type 40 can propagate efficiently and produce plaques on a human cell line, A549, derived from lung carcinoma

Human adenovirus type 40 (Ad40) cannot propagate in conventional established human cell lines such as KB or HeLa cells. However, it has been shown that Ad40 DNA replicates in KB18 cells which express Ad2 E1B genes, suggesting that Ad40 is defective in the E1B gene function in KB or HeLa cells. We show here that Ad40 can propagate and produce plaques on A549 cells which do not contain Ad E1B genes. Our experiments show that the levels of replication of Ad40 DNA and production of infectious Ad40 virus in A549 cells are the same as or higher than those in 293 or KB18 cells. Dot blot analysis shows that the levels of Ad40 E1A and E1B mRNAs expressed in A549 cells at early to intermediate times postinfection are at least 10-fold higher than those in KB or KB18 cells. Northern (RNA) blot analysis shows that large E1B mRNA species (approximately 24S to 26S) are synthesized prior to the onset of DNA replication in A549 cells. No E1B mRNA species are synthesized in KB or KB18 cells at early times postinfection, and no differences in the expression of E1B mRNAs are seen between KB and KB18 cells. The experiment suggests that A549 cells have a cellular factor(s) which activates Ad40 E1B mRNA synthesis and that the E1B mRNA synthesis helps Ad40 propagation. In contrast, Ad40 can propagate in KB18 cells by using Ad2 E1B gene products that are constitutively expressed in this cell line. Furthermore, this result shows that Ad40 cannot propagate in KB cells because of the failure in the expression of E1B genes at early times postinfection.

Enteric adenovirus type 40: expression of E1B mRNA and proteins in permissive and nonpermissive cells

The enteric adenovirus type 40 (strain Dugan) grows well in tissue culture only when the E1B 55K protein of Ad5 or Ad12 is supplied in trans, either constitutively expressed in an established cell line or by coinfection with an appropriate helper virus (V. Mautner, N. Mackay, and V. Steinthorsdottir, 1989, Virology 171, 619-622). The synthesis of Ad40 E1B mRNAs and proteins has been examined under permissive and nonpermissive conditions: At late times postinfection in permissive cells, E1B-specific mRNA species of 22 and 13-14 S are made, as well as 15 and 9 S messages for the late IVa2 and ppIX proteins. None of these are detected before the onset of DNA replication and none of them accumulate in the presence of a cytosine arabinoside block to DNA replication. The failure to detect cytoplasmic mRNAs as early times cannot be attributed to a failure of mRNA transport from the nucleus as there is no accumulation of nuclear E1 RNA. In nonpermissive Hela cells only traces of E1B- and ppIX-specific mRNAs are detectable, at very late times postinfection. Antibodies raised to synthetic oligopeptides corresponding to the N- and C-terminal domains of the putative E1B 19K and 55K proteins show a high titer against the cognate peptide by ELISA, but only the E1B 19K C-terminus-specific sera have detected a unique polypeptide in Ad40-infected cells, at late times postinfection. There is no shut-off of host protein synthesis in permissive cells, despite the expression of Ad2 55K protein.

Separate regions of an adenovirus E1B protein critical for different biological functions

The E1B region of Ad12 encodes two major proteins, the 482R (55K) and 163R (19K). In this report we showed that the E1B 482R is multifunctional, in that its structure may somehow contribute to its own stability, in viral DNA and virus replication, in transformation of primary cells, and in tumorigenicity. Deletion of the first 24 amino acids and of aa residues 114-155 (dl42) results in an instability of the 55K protein. The N-terminal 24 aa residues (pm1852) or amino acids residues 80-96 (dl17) are not required for viral DNA or virus replication, whereas amino acid residues between 114 and 155 (dl42) are absolutely necessary for viral DNA synthesis. Deletion of amino acid residues 1-24, 80-96, and 114-155 (dl42) greatly reduces the transforming ability of both virus and plasmids containing any one of these deletions. One of the critical regions for tumorigenicity residues within amino acid residues 80-96, since cells transformed by this plasmid are nontumorigenic. On the other hand the region bounded by amino acid residues 114-155 (dl42) is not required for tumorigenicity in immunocompetent animals.

Chromosomal damage induced by human adenovirus type 12 requires expression of the E1B 55-kilodalton viral protein

Infection of human embryonic kidney cells with adenovirus type 12 results in the induction of damage at specific (17q21-22, 1p36, 1q21, and 1q42-43) and random sites in the cellular chromosomes. A previous study by Durnam et al. (D. M. Durnam, P. P. Smith, J. C. Menninger, and J. K. McDougall, Cancer Cells 4:349-354, 1986) indicated that the expression of viral early region 1 (E1) is sufficient for the induction of damage at band 17q21-22. In the present report we used an adenovirus type 12-adenovirus type 5 recombinant with E1A hybrid sequences as well as viruses with mutations in the adenovirus type 12 E1B genes to map adenovirus type 12 E1 functions involved in the induction of genetic damage. Our results show that the expression of the E1A proteins is not sufficient for this effect. On the other hand, mutations within the E1B 55-kilodalton protein but not the E1B 19-kilodalton protein affect the ability of the virus to induce both specific and random chromosomal damage.

Complementation of enteric adenovirus type 40 for lytic growth in tissue culture by E1B 55K function of adenovirus types 5 and 12

The enteric adenovirus type 40 strain Dugan (Ad40) cannot be passaged in HeLa cells, but will grow in 293 cells, which express Ad5 E1 functions. To determine the reason for this limited host range, KB cell lines expressing Ad2 E1A, E1B, or E1A + E1B (L. E. Babiss, C. S. H. Young, P. B. Fisher, and H. S. Ginsberg, 1983, J. Virol. 46, 454-465) have been tested for their ability to support Ad40 replication. Only cell lines which supply E1B functions, but not those expressing E1A alone, are permissive for Ad40, suggesting that Ad40 may require some function supplied by E1B or induced in E1B-containing cells. In coinfection assays Ad40 complements Ad5 dl312 (delta E1A) but not Ad5 dl313 (delta E1B) and is itself complemented by dl312 but not by dl313. Mutants of Ad2 and Ad12 with lesions in E1B 55K or 19K protein have been used to further delineate the requirements for Ad40 growth in HeLa cells. For mutants lacking 55K function there is minimal complementation in either direction, whereas those lacking only the 19K product are able to complement Ad40.

A cell cycle G0-ts mutant, tsJT60, becomes lethal at the nonpermissive temperature after transformation with adenovirus 12 E1B 19K mutant

tsJT60, a temperature-sensitive (ts) cell-cycle mutant of Fischer rats, is viable at both the permissive (34 degrees C) and nonpermissive (40 degrees C) temperatures. The cells grow normally in exponential growth phase at both temperatures, but when stimulated with serum from G0 phase they enter S phase at 34 degrees C but not at 40 degrees C. tsJT60 cells transformed with human adenovirus (Ad) 12 dl205, which lacks the E1B 19-kDa polypeptide gene, were lethal at 40 degrees C, whereas tsJT60 cells transformed with Ad12 wt, dl207, which lacks E1B 58-kDa protein gene, or in206B, which produces 19- to 58- kDa fused protein, were viable. Degradation of cell DNA occurred in dl205-transformed tsJT60 cultured at both 34 degrees C and 40 degrees C. Neither cytocidal phenotype nor degradation of DNA occurred in 3Y1 cells (a parental line of tsJT60) transformed with dl205. These results suggest that the lethal phenotype and degradation of DNA are related to the ts mutation in tsJT60 and also to the lack of Ad12 E1B 19kDa polypeptide.

Host range mutants of adenovirus type 12 E1 defective for lytic infection, transformation, and oncogenicity

The human adenovirus type 12 (H12) E1A region encodes two early proteins of 266 amino acid residues (266R) and 235R whilst the H12 E1B promoter directs the synthesis of two major proteins of 163R and 482R. To determine the functions of E1A and E1B in lytic infection and oncogenic transformation we have isolated and characterized a series of H12 E1 mutants. Mutant H12 hr 700 contains a point mutation in exon 1 that alters a single amino acid common to both the 266 and 235R proteins. This mutant synthesized reduced levels of E1 and structural proteins at delayed times in HEK cells, transformed BRK cells, and induced tumors in newborn rats at reduced efficiency compared to wild-type virus. The mutation in H12 in 600 truncates the 266R protein in its unique sequences but this mutant synthesized the 235R, E1B, and structural proteins at delayed times in HEK cells. H12 in 600 was nontransforming but induced rare tumors in newborn rats. A third E1A mutant H12 in 601 synthesized no E1A proteins, reduced levels of E1B and structural proteins at delayed times in lytic infections, and was not a transforming or oncogenic virus. Three E1B mutants were studied in detail. Both H12 hr 703 and H12 in 602 encode N-terminal truncated 482R proteins whereas H12 del 620 encodes an in-frame internally deleted 482R protein. All three synthesized reduced amounts of E1A proteins and the E1B 163R protein, identifying a regulatory function for the 482R protein. None of the E1B mutants could transform and only H12 del 620 could induce rare tumors in newborn rats. These results show that H12 oncogenesis requires the coordinated expression of the E1 proteins.

Activation of the human beta interferon gene by the adenovirus type 12 E1B gene

The transcription of endogenous beta interferon mRNA was activated in human embryo kidney (HEK) cells infected with adenovirus type 12 (Ad12) but was activated only inefficiently or not at all in HEK cells infected with Ad5 and rc-1 (Ad5 dl312 containing the Ad12 E1A region). The analysis with Ad12 mutants showed that Ad12 E1B products, especially the 19K protein, were important for the expression of the endogenous beta interferon gene and Ad12 E1A products were not involved in the expression. The expression of exogenously transfected pIFN-CAT (a hybrid plasmid having the human beta interferon promoter fused with the CAT gene) was activated in HEK and chicken embryo fibroblast (CEF) cells infected with either Ad12 or Ad5. The analysis of cotransfection of CEF cells with pIFN-CAT and plasmids containing fragments of Ad12 or Ad5 DNA showed that Ad12 or Ad5 E1B (possibly the 19K protein) was and E1A was not involved in the expression of the exogenous pIFN-CAT.

Regulation of adenovirus gene expression in human WI38 cells by an E1B-encoded tumor antigen

Adenovirus mutants carrying alterations in the gene encoding the E1B 19-kilodalton tumor antigen (19K protein) cause enhanced cytopathic effect (cyt phenotype) and the degradation of host-cell chromosomal DNA (deg phenotype) upon infection of human HeLa or KB cells. Furthermore, E1B 19K gene mutant viruses are defective for cellular transformation. We report that these mutant viruses possess a host-range phenotype for growth in human cells. In human HeLa cells the mutant viruses grew to the same levels as the wild-type virus, but they were severely defective for growth in KB cells. In human WI38 cells, the E1B 19K gene mutant viruses had a substantial growth advantage over the wild-type virus, yielding 500-fold-higher titers. Viral DNA synthesis was reduced 10- to 20-fold in WI38 cells infected with the wild-type virus relative to that synthesized by the E1B mutant viruses. Viral early and late protein synthesis was similarly reduced in wild type- relative to mutant-infected cells. These reduced levels of early gene expression in wild-type virus-infected cells were paralleled by comparably reduced levels of early cytoplasmic mRNA. The primary cause of this host-range phenotype appeared at the level of early gene transcription, since transcription of viral early genes in the mutant-infected cells was substantially greater than levels found in cells infected with the wild-type virus. These results implicate the E1B 19K tumor antigen in the regulation of adenovirus early gene expression. Specifically, the E1B 19K protein directly or indirectly exerts a negative effect on early gene transcription accounting for efficient gene expression from the E1B mutant viruses in WI38 cells. Based on these findings it is probable that the cyt and deg phenotypes observed in mutant-infected HeLa and KB cells are the result of the pleiotropic effect of this altered gene regulation.

Expression of adenovirus type 12 E1b 58-kDa protein in Escherichia coli and production of antibodies raised against a 58-kDa::beta-galactosidase fusion protein

DNA fragments coding for the N-terminal 185 amino acids (aa) and for the entire coding region of the adenovirus (Ad)12 E1b 58-kDa protein have been cloned in a prokaryotic expression vector. The N-terminal region of the 58-kDa viral protein (aa 21-205) is expressed as a beta-galactosidase (beta Gal) fusion protein encoded by plasmid pB58Ngal. Escherichia coli strains transformed with this plasmid synthesize a full-length fusion protein of 150-kDa and two truncated proteins: a 140-kDa protein containing aa 64-205 and a 120-kDa polypeptide containing aa 158-205 of the E1b 58-kDa protein. Antibodies raised against purified fusion proteins specifically immunoprecipitate the E1b 58-kDa protein from Ad12-infected and transformed cells. Bacteria transformed with plasmid pB58 carrying the entire E1b 58-kDa coding region (minus the first N-terminal 20 aa which are replaced by 4 aa of beta Gal) showed dramatically reduced growth properties after induction of 58K gene expression. We have not been able to detect substantial amounts of the 58-kDa protein in these cells. However, the viral 58-kDa polypeptide could be synthesized in vitro from plasmid pB58 in a DNA-dependent translation system from E. coli.

Regulation of adenovirus gene expression in human WI38 cells by an E1B-encoded tumor antigen

Adenovirus mutants carrying alterations in the gene encoding the E1B 19-kilodalton tumor antigen (19K protein) cause enhanced cytopathic effect (cyt phenotype) and the degradation of host-cell chromosomal DNA (deg phenotype) upon infection of human HeLa or KB cells. Furthermore, E1B 19K gene mutant viruses are defective for cellular transformation. We report that these mutant viruses possess a host-range phenotype for growth in human cells. In human HeLa cells the mutant viruses grew to the same levels as the wild-type virus, but they were severely defective for growth in KB cells. In human WI38 cells, the E1B 19K gene mutant viruses had a substantial growth advantage over the wild-type virus, yielding 500-fold-higher titers. Viral DNA synthesis was reduced 10- to 20-fold in WI38 cells infected with the wild-type virus relative to that synthesized by the E1B mutant viruses. Viral early and late protein synthesis was similarly reduced in wild type- relative to mutant-infected cells. These reduced levels of early gene expression in wild-type virus-infected cells were paralleled by comparably reduced levels of early cytoplasmic mRNA. The primary cause of this host-range phenotype appeared at the level of early gene transcription, since transcription of viral early genes in the mutant-infected cells was substantially greater than levels found in cells infected with the wild-type virus. These results implicate the E1B 19K tumor antigen in the regulation of adenovirus early gene expression. Specifically, the E1B 19K protein directly or indirectly exerts a negative effect on early gene transcription accounting for efficient gene expression from the E1B mutant viruses in WI38 cells. Based on these findings it is probable that the cyt and deg phenotypes observed in mutant-infected HeLa and KB cells are the result of the pleiotropic effect of this altered gene regulation.