E1B functions of type C adenoviruses play a role in the complementation of blocked adenovirus type 12 DNA replication and late gene transcription in hamster cells

Epigenetic mechanisms in human adenovirus type 12 oncogenesis

For the past 30 years, my laboratory has concentrated its work on demonstrating that the epigenetic consequences of foreign DNA insertion into established mammalian genomes – de novo DNA methylation of the integrate and alterations of methylation patterns across the recipient genome – are essential elements in setting the stage towards oncogenic transformation. We have primarily studied human adenovirus type 12 (Ad12) which induces undifferentiated tumors in Syrian hamsters (Mesocricetus auratus) either at the site of subcutaneous Ad12 injection or intraperitoneally upon intramuscular injection. Up to 90% of the hamsters injected with Ad12 develop tumors within 3–6 weeks. Integration of foreign DNA, its de novo methylation, and the consequences of insertion on the cellular methylation and transcription profiles have been studied in detail. While viral infections are a frequent source of foreign genomes entering mammalian and other hosts and often their genomes, we have also pursued the fate of food-ingested foreign DNA in the mouse organism. The persistence of this DNA in the animals is transient and there is no evidence for the expression or germ line fixation of foreign DNA. Nevertheless, the occasional cell that carries integrated genomes from that foreign source deserves the oncologist's sustained interest.

Transcriptional blocks limit adenoviral replication in primary ovarian tumor

PURPOSE

Despite the success of conditionally replicating adenoviruses in tumor models, clinical success has been limited when they are used as a single modality agent. Overcoming the disparity in efficacy between in vivo animal models and human use is a key hurdle for better conditionally replicating adenovirus therapy in humans. We endeavored to identify biological blocks to adenoviral infection and replication in tumor cells.

EXPERIMENTAL DESIGN

We hypothesized that the differences in adenoviral replication between ovarian cancer cell lines and patient tumor samples are the result of a block in viral RNA transcription. To test this hypothesis, established ovarian cancer cell lines and purified patient ovarian cancer cells were infected with wild-type adenovirus. RNA for early adenoviral genes E1A and E1B as well as the late transcripts for fiber and hexon were measured using real-time PCR.

RESULTS

Established ovarian cancer cell lines treated with wild-type virus had a lower E1A:E1B ratio than the patient samples. Additionally, the levels of fiber and hexon relative to E1A were also decreased in the patient samples compared with the established cell lines. These findings were consistent with an early- to late-phase block in the adenovirus replication cycle.

CONCLUSIONS

These data suggest that the biology of abortive infection in the patient samples may be linked to a defect in the production of early and late viral transcripts. Identification of factors leading to abortive infection will be crucial to understanding the low viral replication in patient samples.

The abortive infection of Syrian hamster cells with human adenovirus type 12

Human adenovirus type 12 (Ad12) induces undifferentiated tumors in newborn Syrian hamsters, and this tumor model has been investigated in detail in our laboratory. One of the characteristics of the Ad12-hamster cell system is a strictly abortive infection cycle. In this chapter, we summarize previous and more recent results of studies on the interaction of Ad12 with the nonpermissive BHK21 hamster cell line. The block of Ad12 replication lies before viral DNA replication and late gene transcription which cannot be detected with the most sensitive techniques. Ad12 adsorption, cellular uptake and transport of the viral DNA to the nucleus are less efficient in the nonpermissive hamster cells than in permissive human cells. However, most of the early functions of the Ad12 genome are expressed in BHK21 cells, though at a low level. In the downstream region, the first exon, of the major late promoter (MLP) of Ad12 DNA, a mitigator element of 33 nucleotide pairs in length has been identified which contributes to the inactivity of the MLP in hamster cells and its markedly decreased activity in human cells. The E1 functions of Ad2 or Ad5 are capable of partly complementing the Ad12 deficiencies in hamster cells in that Ad12 viral DNA replication and late gene transcription can proceed, e.g. in a BHK hamster cell line, BHK297-C131,which carries in an integrated form and constitutively expresses the E1 region of Ad5 DNA. Nevertheless, the late Ad12 mRNAs, which are synthesized in this system with the authentic nucleotide sequence, fail to be translated to structural viral proteins. Hence, infectious virions are not produced in the partly complementing system. Probably there is also a translational block for late Ad12 mRNAs in hamster cells. We have recently shown that the overexpression of the Ad12 preterminal protein (pTP) gene or of the E1A gene facilitates the synthesis of full-length, authentic Ad12 DNA in BHK21 cells infected with Ad12. Apparently the pTP has a hitherto unknown function in eliciting full cycles of Ad12 DNA replication even in nonpermissive BHK21 cells when sufficient levels of Ad12 pTP are produced. We pursue the possibility that the completely abortive infection cycle of Ad12 in hamster cells ensures the survival of Ad12-induced hamster tumor cells which all carry, integrated in their genomes, multiple copies of Ad12 DNA. In this way, the viral genomes are immortalized and expanded in a huge number of tumor cells.

Overexpression of the adenovirus type 12 (Ad12) pTP or E1A gene facilitates Ad12 DNA replication in nonpermissive BHK21 hamster cells

In the adenovirus type 12 (Ad12) hamster cell system, abortive virus infection is one of the factors associated with the highly efficient oncogenesis in newborn Syrian hamsters. We have shown earlier that the replication and efficient late transcription of the Ad12 genome are blocked in Syrian hamster cells. Some of the early Ad12 functions are transcribed in these cells, although at a minimal rate. In the present study, we demonstrate that low expression levels of the E1A and precursor to terminal protein (pTP) genes of Ad12 seem to be responsible for the lack of Ad12 DNA replication in hamster cells. The Ad12 genes for the E1A functions or for pTP were tethered to the strong early promoter of the human cytomegalovirus and transfected into BHK21 cells. Subsequently, these cells were infected with Ad12 virions. In Ad12-infected BHK21 cells, which overexpressed pTP or E1A, full-length Ad12 DNA was de novo synthesized, as documented by metabolic labeling with [3H]thymidine and by zone velocity sedimentation in alkaline sucrose gradients followed by gel electrophoresis of the 3H-labeled DNA and Southern blot hybridization to 32P-labeled Ad12 DNA. Transfection of the cloned E1A region of Ad2 yielded similar results. The newly synthesized Ad12 DNA was covalently linked to pTP. The Ad12 DNA binding protein (DBP) and DNA polymerase (pol) genes were transcribed at levels similar to those in merely Ad12-infected cells. In pTP or E1A gene-transfected and Ad12-infected BHK21 cells, marginal levels of late Ad12 mRNA were detectable. Late Ad12 proteins were, however, not synthesized. Apparently, Ad12 DNA replication in hamster cells is rendered impossible due to insufficient threshold levels of the viral E1A and/or pTP.

A new concept in (adenoviral) oncogenesis: integration of foreign DNA and its consequences

A new concept for viral oncogenesis is presented which is based on experimental work on the chromosomal integration of adenovirus DNA into mammalian genomes. The mechanism of adenovirus DNA integration is akin to non-sequence-specific insertional recombination in which patch homologies between the recombination partners are frequently observed. This reaction has been imitated in a cell-free system by using nuclear extracts from hamster cells and partly purified fractions derived from them. As a consequence of foreign DNA insertion into the mammalian genome, the foreign DNA is extensively de novo methylated in specific patterns, presumably as part of a mammalian host cell defense mechanism against inserted foreign DNA which can be permanently silenced in this way. A further corollary of foreign (adenovirus or bacteriophage lambda) DNA integration is seen in extensive changes in cellular DNA methylation patterns at sites far remote from the locus of insertional recombination. Repetitive cellular, retrotransposon-like sequences are particularly, but not exclusively, prone to these increases in DNA methylation. It is conceivable that these changes in DNA methylation are a reflection of a profound overall reorganization process in the affected genomes. Could these alterations significantly contribute to the transformation events during viral or other types of oncogenesis? These sequelae of foreign DNA integration into established mammalian genomes will have to be critically considered when interpreting results obtained with transgenic, knock-out, and knock-in animals and when devising schemes for human somatic gene therapy. The interpretation of de novo methylation as a cellular defense mechanism has prompted investigations on the fate of food-ingested foreign DNA. The gastrointestinal (GI) tract provides a large surface for the entry of foreign DNA into any organism. As a tracer molecule, bacteriophage M13 DNA has been fed to mice. Fragments of this DNA can be found in small amounts (about 1% of the administered DNA) in all parts of the intestinal tract and in the feces. Furthermore, M13 DNA can be traced in the columnar epithelia of the intestine, in Peyer's plaque leukocytes, in peripheral white blood cells, in spleen, and liver. Authentic M13 DNA has been recloned from total spleen DNA. If integrated, this DNA might elicit some of the described consequences of foreign DNA insertion into the mammalian genome. Food-ingested DNA will likely infiltrate the organism more frequently than viral DNA.

Late transcripts of adenovirus type 12 DNA are not translated in hamster cells expressing the E1 region of adenovirus type 5

Hamster cells are completely nonpermissive for the replication of human adenovirus type 12 (Ad12), whereas types 2 and 5 can replicate in hamster cells. The Ad5-transformed hamster cell line BHK297-C131, which carries the left terminal 18.7% of the Ad5 genome and expresses at least the viral E1A region, can somehow complement Ad12 DNA replication and the transcription of the late Ad12 genes. Since the interaction of Ad12 with hamster cells must constitute a significant factor in the induction of Ad12 tumors in neonatal hamsters, we have continued to examine details of this abortive virus infection. The late Ad12 mRNAs in BHK297-C131 cells are polyadenylated but are synthesized in reduced amounts compared with the Ad12 products in Ad12-infected human cells, which are permissive for viral replication. The late mRNA derived from the Ad12 fiber gene has been assessed for its structural properties. By cloning cDNA transcripts from this region and determining their nucleotide sequences, the authenticity of the complete Ad12 fiber sequence and the completeness of the Ad12-typical tripartite leader have been confirmed. Moreover, in Ad12-infected BHK297-C131 cells the Ad12 virus-associated RNA, a virus-encoded translational activator with the correct nucleotide sequence, is synthesized. Nevertheless, the synthesis of detectable amounts of Ad12 virion-specific proteins, and in particular that of the main viral antigens, hexons and fibers, cannot be documented. Cellular factors needed to promote late mRNA translation might be missing, or inhibitory factors might exist in Ad12-infected BHK297-C131 cells.

A unique mitigator sequence determines the species specificity of the major late promoter in adenovirus type 12 DNA

Human adenovirus type 12 (Ad12) cannot replicate in hamster cells, whereas human cells are permissive for Ad12. Ad12 DNA replication and late-gene and virus-associated RNA expression are blocked in hamster cells. Early Ad12 genes are transcribed, and the viral DNA can be integrated into the host genome. Ad12 DNA replication and late-gene transcription can be complemented in hamster cells by E1 functions of Ad2 or Ad5, for which hamster cells are fully permissive (for a review, see W. Doerfler, Adv. Virus Res. 39:89-128, 1991). We have previously demonstrated that a 33-nucleotide mitigator sequence, which is located in the downstream region of the major late promoter (MLP) of Ad12 DNA, is responsible for the inactivity of the Ad12 MLP in hamster cells (C. Zock and W. Doerfler, EMBO J. 9:1615-1623, 1990). A similar negative regulator has not been found in the MLP of Ad2 DNA. We have now studied the mechanism of action of this mitigator element. The results of nuclear run-on experiments document the absence of MLP transcripts in the nuclei of Ad12-infected BHK21 hamster cells. Surprisingly, the mitigator element cannot elicit its function in in vitro transcription experiments with nuclear extracts from both hamster BHK21 and human HeLa cells. Intact nuclear topology and/or tightly bound nuclear elements that cannot be eluted in nuclear extracts are somehow required for recognition of the Ad12 mitigator. Electrophoretic mobility shift assays have not revealed significant differences in the binding of proteins from human HeLa or hamster BHK21 cells to the mitigator sequence in the MLP of Ad12 DNA or to the corresponding sequence in Ad2 DNA. We have converted the sequence of the mitigator in the MLP of Ad12 DNA to the equivalent sequence in the MLP of Ad2 DNA by site-directed mutagenesis. This construct was not active in hamster cells. When the Ad12 mitigator, on the other hand, was inserted into the Ad2 MLP, the latter's function in hamster cells was not compromised. Deletions in the 5' upstream region of the Ad12 MLP have provided evidence for the existence of additional sequences that codetermine the deficiency of the Ad12 MLP in hamster cells. The amphifunctional YY1 protein from HeLa cells can bind specifically to the mitigator and to upstream elements of the MLP of Ad12 DNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Altered expression of adenovirus 12 DNA-binding protein but not DNA polymerase during abortive infection of hamster cells

Replication of human adenovirus type 12 DNA is blocked in abortively infected baby hamster kidney cells. The activity and accumulation of adenovirus 12 DNA polymerase is equivalent in infected hamster and human cell extracts. However, the accumulation of adenovirus type 12 DNA-binding protein is approximately 120-fold lower in extracts from infected hamster cells when compared to infected permissive human cells. This difference in accumulation is not due to replication of viral DNA during productive infection, since this difference is observed in the presence of hydroxyurea. The DNA-binding protein from infected hamster cells retains the ability to bind denatured DNA-cellulose. An adenovirus 5 early region 1 transformed hamster cell line competent to complement the adenovirus 12 DNA replication defect also stimulates accumulation of the DNA-binding protein even when the cells are treated with hydroxyurea. Thus, the reduced expression of the viral DNA-binding protein may play a role in the mechanism of abortive infection of hamster cells by adenovirus 12.

Enteric adenovirus 41 (Tak) requires low serum for growth in human primary cells

It had been postulated that due to lack of growth of enteric adenovirus 41 (Ad41) on human primary cells and its growth on Graham-293 cells there was a defect in the Ad41 E1A region. However, we found as a result of careful evaluation of Ad41 growth on several primary cell lines (HEK, WI-38, or Detroit 551) that efficient viral multiplication is possible if the serum concentration in the medium used postinfection (p.i.) is kept between 0.2 and 1%. In contrast, only slight growth of Ad41 occurs in infected cells maintained in 5% serum and virtually no viral replication is found in infected cells cultivated in medium with 10% serum. The serum inhibitory effect appears limited to primary cells because no difference in Ad41 replication, as assayed by accumulation of Ad41 DNA, was found in infected continuous cell lines (HEp-2, 293) cultivated p.i. in either 1 or 10% FBS. Also, this effect appears specific for enteric adenoviruses, such as Ad41, since conventional adenoviruses, such as Ad5, grow well in both 1 and 10% FBS. The above results show that Ad41 can grow in a variety of primary cell lines, under specific culture conditions. In addition, we found that Ad41-infected primary cells grown in medium containing 0.2% serum had an increase in synthesis of the 70-kDa heat shock protein (HSP70) at about 6 hr p.i. and also Ad41 was able to complement the Ad5 E1A deletion mutant dl312. These results show that the E1A function of Ad41 is not impaired in infected cells.

Inhibitory effect of interferon-gamma on adenovirus replication and late transcription

We have previously shown that human interferon-gamma inhibited adenovirus multiplication in vitro in a dose-dependent fashion. This action was previous to capsid proteins synthesis and did not involve virus adsorption nor penetration. In this report we have analysed viral mRNA levels at early (7 hr post infection (p.i.)) or late (20 hr p.i.) times, as well as DNA replication in Wish cells pretreated with interferon-gamma and infected with adenovirus 5. Controls included untreated cells as well as cells treated with interferon-alpha, to which adenovirus are reported to be resistant. Transcription of adenovirus regions E1, E4, L1 and L2 has been analysed by Northern blot. Adenovirus DNA replication was determined by DNA-DNA hybridization with total adenovirus 2 DNA. We have also searched for adenovirus E1A proteins by immunoblot with a specific monoclonal antibody. Although pretreatment of cells with either interferon-alpha or interferon-gamma resulted in reduced amounts of E1 and E4 mRNA in the early phase of infection (7 hr p.i.), the near complete inhibition of viral DNA and late transcription was only achieved by interferon-gamma. Immunoblot has shown the absence of the 48-kD E1A protein in cells pretreated with interferon-gamma. The lack of this regulatory adenovirus protein may be involved in the inhibitory mechanism of interferon-gamma on adenovirus.