Expression of early adenovirus genes requires a viral encoded acidic polypeptide

Adenovirus type 5 exerts genome-wide control over cellular programs governing proliferation, quiescence, and survival

The effects of the adenovirus Ad5 on basic host cell programs, such as cell-cycle regulation, were studied in a microarray analysis of human fibroblasts. About 2,000 genes were up- or down-regulated after Ad5 infection and Ad5 infection was shown to induce reversal of the quiescence program and recapitulation of the core serum response.

Background

Human adenoviruses, such as serotype 5 (Ad5), encode several proteins that can perturb cellular mechanisms that regulate cell cycle progression and apoptosis, as well as those that mediate mRNA production and translation. However, a global view of the effects of Ad5 infection on such programs in normal human cells is not available, despite widespread efforts to develop adenoviruses for therapeutic applications.

Results

We used two-color hybridization and oligonucleotide microarrays to monitor changes in cellular RNA concentrations as a function of time after Ad5 infection of quiescent, normal human fibroblasts. We observed that the expression of some 2,000 genes, about 10% of those examined, increased or decreased by a factor of two or greater following Ad5 infection, but were not altered in mock-infected cells. Consensus k-means clustering established that the temporal patterns of these changes were unexpectedly complex. Gene Ontology terms associated with cell proliferation were significantly over-represented in several clusters. The results of comparative analyses demonstrate that Ad5 infection induces reversal of the quiescence program and recapitulation of the core serum response, and that only a small subset of the observed changes in cellular gene expression can be ascribed to well characterized functions of the viral E1A and E1B proteins.

Conclusion

These findings establish that the impact of adenovirus infection on host cell programs is far greater than appreciated hitherto. Furthermore, they provide a new framework for investigating the molecular functions of viral early proteins and information relevant to the design of conditionally replicating adenoviral vectors.

Default assembly of early adenovirus chromatin

In adenovirus particles, the viral nucleoprotein is organized into a highly compacted core structure. Upon delivery to the nucleus, the viral nucleoprotein is very likely to be remodeled to a form accessible to the transcription and replication machinery. Viral protein VII binds to intra-nuclear viral DNA, as do at least two cellular proteins, SET/TAF-Ibeta and pp32, components of a chromatin assembly complex that is implicated in template remodeling. We showed previously that viral DNA-protein complexes released from infecting particles were sensitive to shearing after cross-linking with formaldehyde, presumably after transport of the genome into the nucleus. We report here the application of equilibrium-density gradient centrifugation to the analysis of the fate of these complexes. Most of the incoming protein VII was recovered in a form that was not cross-linked to viral DNA. This release of protein VII, as well as the binding of SET/TAF-Ibeta and cellular transcription factors to the viral chromatin, did not require de novo viral gene expression. The distinct density profiles of viral DNA complexes containing protein VII, compared to those containing SET/TAF-Ibeta or transcription factors, were consistent with the notion that the assembly of early viral chromatin requires both the association of SET/TAF-1beta and the release of protein VII.

Identification and characterization of a major early-transcribed gene of Trichoplusia ni single nucleocapsid nucleopolyhedrovirus using the baculovirus expression system

An early transcribed gene (me-53) of a South Africa strain of Trichoplusia ni single nucleocapsid nucleopolyhedrovirus (TnSNPV) was sequenced and identified. It has an open reading frame of 1146 nucleotides that encodes a protein of 382 amino acids with a molecular mass of 45.2 kDa. The deduced protein sequence alignment of 13 baculovirus ME-53s indicated that the TnSNPV ME-53 shares the highest homologies with NPV subgroup II-A Spodoptera exigua multiple and Mamestra configurata (Maco) nucleopolyhedrovirus ME-53s. The zinc finger-like motifs at the C-termini of ME-53s are highly conserved with similar patterns of cysteine positions. Upon introduction of the gene and a green fluorescent protein reporter gene into the baculovirus expression vector system, the transcriptional analysis of me-53 in two cell lines infected with the Autographa californica nucleopolyhedrovirus (AcMNPV) recombinant revealed that an early TnSNPV me-53 transcript can be detected by 1 h postinfection (hpi) until 12 hpi and a late one from 18 hpi up to 48 hpi, while early and late transcripts of the AcMNPV me-53 of the recombinant can be detected at 3 and 24 hpi, respectively. This suggested that the early and late promoters of both AcMNPV and TnSNPV me-53s were recognized in recombinant virus-infected cells. The regulatory elements of the TnSNPV me-53 promoter were also analyzed.

The E1b promoter of Ad12 in mouse L tk- cells is activated by adenovirus region E1a

We have investigated the effect of the E1a region of adenovirus on the expression of the E1b transcriptional unit in mouse L tk- cells. To that end we have fused the promoter region of the E1b gene of Ad12 to the coding sequence of the thymidine kinase gene of herpes simplex virus type 1. We found that expression of this fusion gene is completely dependent on the presence of the E1a region. Sequences involved in this regulation were mapped between positions -135 and +11 from the E1b cap site. In addition, we found that the largest E1a protein is essential in this regulation process.

Readthrough activation of early adenovirus E1b gene transcription

In cells productively infected with adenovirus type 5, transcription is not terminated between the E1a gene and the adjacent downstream E1b gene. Insertion of the mouse beta(maj)-globin transcription termination sequence (GGT) into the E1a coding region dramatically reduces early, but not late, E1b expression (E. Falck-Pedersen, J. Logan, T. Shenk, and J. E. Darnell, Jr., Cell 40:897-905, 1985). In the study described herein, we showed that base substitution mutations in the globin DNA that specifically relieved transcription termination also restored early E1b promoter activity in cis, establishing that maximal early E1b expression requires readthrough transcription originating from the adjacent upstream gene. To identify potential targets of readthrough activation, a series of recombinant viruses with double mutations was constructed. Each double-mutant virus strain had the transcription termination sequences in the first exon of E1a and a deletion within the transcription control region of E1b. Early E1b expression from the double-mutant strains was more defective than that from strains containing either mutation alone, indicating that the deleted regions (positions -362 to -35) are not the target for readthrough activation. Two findings suggested that a cis-dominant property of early viral templates is important for readthrough activation. First, the early E1b defect caused by the GGT insertion was not complemented in trans by factors present in late-infected cells. Second, restoration of E1b transcription at late times occurred concurrently with viral DNA replication. Readthrough activation may help convert virion DNA into a transcriptionally competent template prior to DNA replication and late transcription.

p300 binding by E1A cosegregates with p53 induction but is dispensable for apoptosis

E1A expression during adenovirus infection induces apoptosis. E1A expression causes accumulation of the p53 tumor suppressor protein, and E1A-induced apoptosis is p53 mediated in primary rodent cells, implying that p53 induction may be linked to apoptosis induction by E1A. Adenoviruses containing mutations in the E1A gene were tested for the ability to trigger both p53 accumulation and the appearance of enhanced cytopathy (cyt phenotype) and degradation of DNA (deg phenotype), indicative of apoptosis in infected HeLa cells. The adenoviruses had mutations which disrupted the pRb- and/or p300-binding activities of E1A so that the relationship between p53 induction and apoptosis and binding to these cellular proteins by E1A could be determined. An E1A mutation that specifically disrupted the p300-binding activity failed to induce p53 accumulation, whereas mutations in E1A which affected pRb binding induced p53 accumulation. Thus, p300 binding was required and pRb binding was dispensable for E1A-mediated accumulation of p53 in HeLa cells. All the E1A mutant viruses, regardless of the ability to induce p53 accumulation, induced the cyt and deg phenotypes, suggesting that p53 induction in infected HeLa cells was not essential for apoptosis, nor was binding of E1A to the pRb and/or p300 protein. The possibility that E1A induced a p53-independent apoptosis pathway was tested by analyzing the appearance of the cyt and deg phenotypes in Saos-2 cells, which were null for both alleles of p53, upon adenovirus infection. An adenovirus expressing wild-type 12S E1A induced both the cyt and deg phenotypes in Saos-2 cells, as did all the E1A mutant viruses. Thus, E1A expression during infection of human cells may trigger redundant p53-independent and -dependent apoptotic pathways.

Phosphorylation within the transactivation domain of adenovirus E1A protein by mitogen-activated protein kinase regulates expression of early region 4

A critical role of the 289-residue (289R) E1A protein of human adenovirus type 5 during productive infection is to transactivate expression of all early viral transcription. Sequences within and proximal to conserved region 3 (CR3) promote expression of these viral genes through interactions with a variety of transcription factors requiring the zinc binding motif in CR3 and in some cases a region at the carboxy-terminal end of CR3, including residues 183 to 188. It is known that 3',5' cyclic AMP (cAMP) reduces the level of phosphorylation of the 289R E1A protein through the activation of protein phosphatase 2A by the E4orf4 protein. This study was designed to identify the E1A phosphorylation sites affected by E4orf4 expression and to determine their importance in regulation of E1A activity. We report here that two previously unidentified sites at Ser-185 and Ser-188 are the targets for decreased phosphorylation in response to cAMP. At least one of these sites, presumably Ser-185, is phosphorylated in vitro by purified mitogen-activated protein kinase (MAPK), and both are hyperphosphorylated in cells which express a constitutively active form of MAPK kinase. Analysis of E1A-mediated transactivation activity indicated that elevated phosphorylation at these sites increased expression of the E4 promoter but not that of E3. We have recently shown that one or more E4 products induce cell death due to p53-independent apoptosis, and thus it seems likely that one role of the E4orf4 protein is to limit production of toxic E4 products by limiting expression of the E4 promoter.

Importance of the Ser-132 phosphorylation site in cell transformation and apoptosis induced by the adenovirus type 5 E1A protein

The 289-residue (289R) and 243R early region 1A (E1A) proteins of human adenovirus type 5 induce cell transformation in cooperation with either E1B or activated ras. Here we report that Ser-132 in both E1A products is a site of phosphorylation in vivo and is the only site phosphorylated in vitro by purified casein kinase II. Ser-132 is located in conserved region 2 near the primary binding site for the pRB tumor suppressor and, in 289R, just upstream of the conserved region 3 transactivation domain involved in regulation of early viral gene expression. Mutants containing alanine or glycine in place of Ser-132 interacted with pRB-related proteins at somewhat reduced efficiency; however, all Ser-132 mutants transformed primary rat cells in cooperation with E1B as well as or better than the wild type when both major E1A proteins were expressed. Such was not the case with mutants expressing only 289R. In cooperation with E1B, the Asp-132 and Gly-132 mutants yielded reduced numbers of smaller transformed foci. With activated ras, all Ser-132 mutants were significantly defective for transformation and the rare foci produced were small and contained extensive areas populated by low densities of flat cells. In the absence of E1B, all Ser-132 mutants induced p53-independent cell death more readily than virus expressing wild-type 289R. These results suggested that phosphorylation at Ser-132 may enhance the binding of pRB and related proteins and also reduce the toxicity of E1A 289R, thus increasing transforming activity.

The adenovirus E1A-associated kinase consists of cyclin E-p33cdk2 and cyclin A-p33cdk2

The adenovirus E1A oncoproteins form stable complexes with several cellular proteins. Association of E1A with these proteins has been shown to be important for the oncogenic potential of E1A. Several of these proteins have been identified and include the product of the retinoblastoma gene and a key cell cycle regulatory protein, cyclin A. E1A also associates with a potent histone H1 kinase. The two major components of this activity are the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes. Both the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes have been implicated in regulatory events controlling entry into or passage through DNA synthesis. Although the architecture of such interactions remains unclear, it is likely that by targeting such complexes, adenovirus is affecting some aspect of cell cycle control.

Genetic analysis of carcinogen enhancement of type 5 adenovirus transformation of cloned Fischer rat embryo fibroblast cells

Pretreatment of CREF cells with methyl methanesulfonate (MMS) before infection with the host-range cold-sensitive type 5 adenovirus (Ad5) mutant H5hr1 results in a dose-dependent carcinogen enhancement of viral transformation (CET). The properties of CET observed with H5hr1, which include both an MMS dose-dependent enhancement in the number of transformed foci and an increase in transformation frequency after correction for cell toxicity, are not observed in carcinogen-pretreated wild-type (wt) Ad5 (H5wt)-infected CREF cells. This study was conducted to determine the role of the viral E1A and E1B transforming genes of H5hr1 in mediating the unique CET phenotype of H5hr1. Coinfection of MMS-pretreated CREF cells with H5wt or H5sub309 (which displays a wt Ad5 phenotype) and H5hr1 resulted in a suppression of the unique CET phenotype that was directly related to the multiplicity of infection with wt Ad5. Suppression of the unique H5hr1 CET phenotype was also apparent in MMS-pretreated CREF cells coinfected with H5hr1 and an Ad5 mutant expressing either a wt 13S E1A-encoded 289 amino-acid (aa) protein and an intact wt E1B gene or a wt 13S E1A-encoded 289-aa protein and a 22S E1B-encoded 495-aa protein. In contrast, the unique H5hr1 CET phenotype was not suppressed in MMS-pretreated CREF cells coinfected with H5hr1 and Ad5 or Ad2 mutants expressing either a wt 12S E1A-encoded 243-aa protein and both wt E1B gene products or an intact wt E1A gene and a wt E1B 13S-encoded 175-aa protein. That genetic changes in both the E1A and E1B viral regions of H5hr1 were required to induce the unique CET phenotype was also indicated by the inability of a recombinant Ad5 containing the 0-4.5 map-unit region of H5hr1 and the 4.5-100 map-unit region of H5sub309 to display the H5hr1 unique CET phenotype. Direct confirmation of the requirement for both gene regions of H5hr1 to mediate its unique CET was obtained by generating CREF cells stably expressing a wt Ad5 E1A 13S-encoded 289-aa protein and a wt E1B 22S-encoded 495-aa protein. In these CREF transformants (which displayed a CREF-like morphology), transformation by H5hr1 was not reduced, but the unique CET phenotype after MMS pretreatment was eliminated. These results suggest that alterations in both the 13S-encoded E1A and 22S-encoded E1B gene products of H5hr1 contribute to its unique CET.

Promoter-specific trans-activation by the adenovirus E1A12S product involves separate E1A domains

Recent studies have shown that the adenovirus E1A12S product can trans-activate transcription by activating the transcription factor E2F. However, E2F cannot be the only target for the E1A12S product, since several cellular promoters have been found to be activated by the E1A12S protein even though they lack E2F sites. Indeed, we now show that activation of the hsp70 promoter by the E1A12S product requires the TATAA sequence. Moreover, activation of the hsp70 promoter requires the N-terminal domain of the E1A protein and does not require the conserved region 2 sequences which are required for the E2F-dependent activation of transcription. We conclude that the targeting of distinct transcription factors, leading to trans-activation of transcription of multiple promoters, involves distinct domains of the E1A proteins that are also required for oncogenic activity.

Promoter-specific trans-activation by the adenovirus E1A12S product involves separate E1A domains

Recent studies have shown that the adenovirus E1A12S product can trans-activate transcription by activating the transcription factor E2F. However, E2F cannot be the only target for the E1A12S product, since several cellular promoters have been found to be activated by the E1A12S protein even though they lack E2F sites. Indeed, we now show that activation of the hsp70 promoter by the E1A12S product requires the TATAA sequence. Moreover, activation of the hsp70 promoter requires the N-terminal domain of the E1A protein and does not require the conserved region 2 sequences which are required for the E2F-dependent activation of transcription. We conclude that the targeting of distinct transcription factors, leading to trans-activation of transcription of multiple promoters, involves distinct domains of the E1A proteins that are also required for oncogenic activity.

Analysis of trans activation by human papillomavirus type 16 E7 and adenovirus 12S E1A suggests a common mechanism

The human papillomavirus E7 gene product is an oncoprotein with properties similar to those of the adenovirus E1A proteins. The human papillomavirus E7 proteins possess substantial amino acid sequence similarity to portions of conserved regions 1 and 2 of E1A, and the human papillomavirus type 16 E7 protein trans-activates the adenovirus E2 early promoter. Analysis of point mutations in the E2 promoter indicated that the E2F recognition sites were critical to E7 stimulation. In contrast to the activation of the E2 promoter, E7 could not trans-activate various other E1A-inducible promoters. Although the promoter specificity for E7 differs from that of 13S E1A trans activation, it is very similar to activation by the E1A 12S product. Moreover, analysis of the E7 protein has suggested that amino acid sequences critical for trans activation include those shared with E1A within conserved region 2. Biochemical studies demonstrate that the E7 protein, like the 12S E1A product, can alter the interaction of cellular factors with the E2F transcription factor. We therefore conclude that E7 trans activation is functionally related to that mediated by the 12S E1A product.

Conversion of the E1A Cys4 zinc finger to a nonfunctional His2,Cys2 zinc finger by a single point mutation

Trans-activation by the adenovirus E1A 289R protein requires a zinc finger defined by Cys-154, Cys-157, Cys-171, and Cys-174. Whereas individually replacing the four cysteine residues with serines resulted in a loss of transactivation, only three of the Cys----Ser mutants (C157S, C171S, and C174S) lost the ability to bind Zn(II). X-ray absorption fine structure analysis revealed that, in the wild-type protein, Zn(II) is coordinated by four cysteine residues whereas in the C154S mutant, Zn(II) is coordinated by two histidines and two cysteines. The mutant protein probably retains, as ligands, two cysteines on the right side of the zinc finger (Cys-171 and Cys-174) and recruits two of the four histidines on the left side (His-149, His-152, His-158, and His-160), despite the presence of Cys-157. This finding may shed light on the general structural requirements of zinc fingers.

Characterization of a potent varicella-zoster virus-encoded trans-repressor

Using a transient expression assay in Vero cells, we have shown that the protein product from gene 61 of varicella-zoster virus (VZV) can repress the function of the VZV encoded trans-activators on putative viral immediate-early, early, and late gene promoters. The repression is exerted at the transcriptional level and requires functional gene 61 protein. This trans-repressor is the herpes simplex type 1 ICP0 (a trans-activator) homolog, as defined by gene location, the sharing of a cysteine-rich putative zinc-binding finger in the amino-terminal region, and limited amino acid homology. Open reading frame 61 (ORF61)-mediated trans-repression appears to be specific for VZV-encoded trans-activators in that it has no effect on simian virus 40 and Rous sarcoma virus promoters. Moreover, it does not inhibit trans-activation of the human T-lymphotropic virus type I and human immunodeficiency virus long terminal repeats by tax and tat genes, respectively. We constructed plasmids with mutations in ORF61 and tested them for their ability to inhibit trans-activator (VZV genes 4 and 62)-mediated activation of the viral thymidine kinase promoter-chloramphenicol acetyltransferase construct. Mutants containing interruptions in ORF61 lost their trans-repressing ability, as demonstrated at both the protein and steady-state RNA levels. These results suggest that the ORF61 protein product can mediate down-regulation of VZV gene expression.

trans-dominant mutants of E1A provide genetic evidence that the zinc finger of the trans-activating domain binds a transcription factor

The 289R E1A protein of adenovirus stimulates transcription of early viral and certain cellular genes. trans-Activation requires residues 140 to 188, which encompass a zinc finger. Several studies have indicated that trans-activation by E1A is mediated through cellular transcription factors. In particular, the ability of the trans-dominant E1A point mutant hr5 (Ser-185 to Asn) to inhibit wild-type E1A trans-activation was proposed to result from the sequestration of a cellular factor. Using site-directed mutagenesis, we individually replaced every residue within and flanking the trans-activating domain with a conservative amino acid, revealing 16 critical residues. Six of the individual substitutions lying in a contiguous stretch C terminal to the zinc finger (carboxyl region183-188) imparted a trans-dominant phenotype. trans-Dominance was even produced by deletion of the entire carboxyl region183-188. Conversely, an intact finger region147-177 was absolutely required for trans-dominance, since second-site substitution of every critical residue in this region abrogated the trans-dominant phenotype of the hr5 protein. These data indicate that the finger region147-177 bind a limiting cellular transcription factor and that the carboxyl region183-188 provides a separate and essential function. In addition, we show that four negatively charged residues within the trans-activating domain do not comprise a distinct acidic activating region. We present a model in which the trans-activating domain of E1A binds to two different cellular protein targets through the finger and carboxyl regions.

trans-dominant mutants of E1A provide genetic evidence that the zinc finger of the trans-activating domain binds a transcription factor

The 289R E1A protein of adenovirus stimulates transcription of early viral and certain cellular genes. trans-Activation requires residues 140 to 188, which encompass a zinc finger. Several studies have indicated that trans-activation by E1A is mediated through cellular transcription factors. In particular, the ability of the trans-dominant E1A point mutant hr5 (Ser-185 to Asn) to inhibit wild-type E1A trans-activation was proposed to result from the sequestration of a cellular factor. Using site-directed mutagenesis, we individually replaced every residue within and flanking the trans-activating domain with a conservative amino acid, revealing 16 critical residues. Six of the individual substitutions lying in a contiguous stretch C terminal to the zinc finger (carboxyl region183-188) imparted a trans-dominant phenotype. trans-Dominance was even produced by deletion of the entire carboxyl region183-188. Conversely, an intact finger region147-177 was absolutely required for trans-dominance, since second-site substitution of every critical residue in this region abrogated the trans-dominant phenotype of the hr5 protein. These data indicate that the finger region147-177 bind a limiting cellular transcription factor and that the carboxyl region183-188 provides a separate and essential function. In addition, we show that four negatively charged residues within the trans-activating domain do not comprise a distinct acidic activating region. We present a model in which the trans-activating domain of E1A binds to two different cellular protein targets through the finger and carboxyl regions.

Domains of the adenovirus E1A protein required for oncogenic activity are also required for dissociation of E2F transcription factor complexes

Recent experiments have shown that the cellular E2F transcription factor is found in complexes with cellular proteins and that one such complex contains the cyclin-A protein. Isolation of a cellular activity, which we term E2F-BF, can reconstitute the E2F-cyclin-A complex and has permitted a more detailed analysis of the mechanism of E1A dissociation. Through the analysis of a series of E1A mutants, we find that sequences in conserved region 1 (CR1) and conserved region 2 (CR2) are important for dissociation of the E2F complex, whereas amino-terminal sequences are not required. In contrast to the requirements for dissociation, only the CR1 sequences are required to block formation of the complex if E1A is added when the components are combined. We have also identified an activity, termed E2F-I, that inhibits E2F binding to DNA, again apparently through the formation of a complex with E2F. This inhibitory activity is also blocked by E1A, dependent on the same elements of the E1A protein that disrupt the interaction with E2F-BF. Because the E1A sequences that are important for releasing E2F from these interactions are also sequences necessary for oncogenesis, we suggest that this activity may be a critical component of the transforming activity of E1A.

Suppression of adenovirus type 5 E1A-mediated transformation and expression of the transformed phenotype by caffeic acid phenethyl ester (CAPE)

Viral transformation and DNA-transfection assays were employed to investigate the differential toxic effect of caffeic acid phenethyl ester (CAPE), an extract of the honeybee hive product propolis, on adenovirus type 5 (Ad5)-transformed cloned rat embryo fibroblast (CREF) cells. CAPE inhibited, in a dose-dependent manner, both de novo and carcinogen-enhanced transformation of CREF cells by H5hr1, the cold-sensitive (cs) host-range mutant of Ad5. CAPE had a selective inhibitory effect on Ad5-induced transformation when a wild-type (wt) Ad5 E1A gene or a cs Ad5 E1A gene (at 37 degrees C, but not at 32 degrees C) was cotransfected into CREF cells with a dominant-acting bacterial hygromycin-resistance gene. A requirement for the expression of Ad5 E1A-encoded mRNAs and transforming proteins and sensitivity to CAPE was demonstrated using CREF cells stably transformed by a cs Ad5 E1A gene and an Ad5 E1A gene under the transcriptional control of a mouse mammary tumor virus promoter. To distinguish between the effects of the two Ad5 E1A-encoded proteins of 289 amino acids (aa) and 243 aa, CREF cells were stably transformed with cDNAs encoding either the 13S or the 12S E1A mRNA. CREF cells expressing the 13S E1A-encoded 289-aa protein were more sensitive to the growth-suppressing effect of CAPE than cells producing only the 12S E1A-encoded 243-aa protein. However, the growth-suppressing and toxic effects of CAPE were greatest in cells expressing both E1A-encoded transforming proteins. Analysis of the effect of CAPE on E1A and beta-actin gene expression in wt and cs E1A and H5hr1-transformed CREF cells indicated that low levels of CAPE, which were growth suppressive, did not selectively suppress E1A expression. These results demonstrated that cellular changes induced in CREF cells by the 13S E1A-encoded 289-aa protein of Ad5, when expressed alone or in combination with the 12S E1A-encoded 243-aa protein, rendered transformed cells sensitive to the growth-suppressing and toxic effects of CAPE.

Adenovirus infection of differentiated F9 cells results in a global shut-off of differentiation-induced gene expression

Previous experiments have demonstrated a link between transcriptional regulatory mechanisms acting during F9 cell differentiation and transcription control by the adenovirus E1A gene. We have isolated a number of differentiation-specific genes by cDNA cloning to determine if E1A exerts a coordinated control over differentiation specific gene expression. The mRNAs encoded by these cDNAs were undetectable or only barely detectable in undifferentiated cells but then rose in concentration upon differentiation. Analysis of transcription rates in isolated nuclei revealed that all but one of the genes was transcriptionally regulated during differentiation. Interestingly, alpha 2-type IV collagen expression was activated by a post-transcriptional mechanism since the gene was transcribed in both undifferentiated and differentiated cells whereas the cytoplasmic mRNA was undetectable in undifferentiated cells but rose in abundance in parallel with other regulated transcripts. Adenovirus infection of differentiated F9 cells reduced the cytoplasmic mRNA levels of each of the differentiation specific genes to near that found in the undifferentiated cell. Of those genes that were transcriptionally activated by differentiation, adenovirus infection specifically inhibited transcription. In contrast, although the alpha 2 collagen mRNA levels were reduced by adenovirus infection similar to the other mRNAs, the control was post-transcriptional since transcription of the gene was unaffected. Thus, the mechanism for loss of gene expression mediated by E1A reflects the mechanism by which the gene was activated during differentiation. Based on these results we suggest that E1A controls the expression of the F9 cell phenotype by targeting a regulatory activity acting early in the differentiation program.

Differential requirement for adenovirus type 12 E1A gene products in oncogenic transformation

During the early period of infection, adenovirus type 12 E1A gene is expressed as overlapping, spliced mRNAs of 12 and 13S, which encode in-frame proteins of 235 and 266 amino acid residues (235R and 266R), respectively. To define the functions of these related products in the infection of human cells and transformation of rodent cells, we created single T-to-C transitions at the second base of each mRNA intron which specifically prevent splicing of the respective mRNAs. Mutant pm712 expresses only the 13S mRNA and 266R protein, while pm713 expresses only the 12S mRNA and 235R protein. By using these mutants, we showed that only the larger product is required for growth in human cells, including growth-arrested W138 cells, that the capacity to activate other viral genes (in human cells, at least) lies primarily with that protein, and that the 266R product is not required for autoregulation of its own transcription. In the presence of the 266R protein the 235R product was not required for complete and efficient transformation of a variety of rodent cells or for direct induction of tumors in rats, whereas in its absence the smaller product was insufficient for transformation or tumor induction. Finally, we showed that transformants resulting from infection of rodent cells with pm712 possess a fully-transformed phenotype and are tumorigenic. Previous studies with group C adenoviruses led to the conclusion that both E1A products are required for complete transformation; we conclude that with oncogenic serotype 12, only the 266R product is required for this process.

The efficiency of adenovirus transformation of rodent cells is inversely related to the rate of viral E1A gene expression

While the products of the type 5 adenovirus E1A and E1B genes can initiate pathways leading to a transformed rodent cell, little is known about how the rate of viral early gene expression influences the efficiency of this process. An adenovirus mutant [E1a(r) virus] that expresses its viral E1A and E1B genes at as much as a 100-fold-reduced rate relative to wild-type virus in infected CREF or HeLa cells transforms CREF cells at an 8-fold-higher efficiency than wild-type virus. Additional studies show that the reduction in viral E1A gene expression is solely responsible for this transformation phenotype, and at this low rate of viral E1A gene expression both E1A gene products must be expressed. Unlike previously characterized viruses which transform CREF cells at frequencies greater than wild-type virus, the foci obtained following E1a(r) virus infection were indistinguishable from those arising from wild-type virus by several criteria (morphological characteristics and anchorage-independent growth). Surprisingly, an analysis of viral early gene expression from a panel of wild-type- and E1a(r) virus-transformed CREF cell lines showed similar average rates of both viral E1A and E1B gene expression. By using an adenovirus-transformed cell line that is cold-sensitive for maintenance of the transformed cell phenotype, we show that both wild-type and the E1a(r) viruses can transform these cells at equally high efficiencies at the nonpermissive temperature of 32 degrees C. Our findings suggest that the process leading to a fully transformed cell involves multiple stages, with an early stage being facilitated by a reduced rate of viral E1A gene expression.

cis-dominant defect in activation of adenovirus type 5 E1b early RNA synthesis

A cis-dominant mutation in the adjacent E1a gene disrupted the accumulation of adenovirus type 5 E1b mRNA during the early phase of infection. Steady-state levels of cytoplasmic and nuclear E1b RNAs in cells infected with dl312, a strain that lacks the E1a TATA box, cap site, and much of the coding sequence, were reduced 5- to 10-fold even when the E1a activator was provided in trans. The strain was defective for early E1b RNA synthesis but not for E1b RNA made late or during prolonged incubation in the presence of an inhibitor of DNA replication. The defect in E1b RNA synthesis could not be attributed to the E1a promoter sequences missing in dl312 DNA. If the E1a protein-coding region contains cis-acting regulatory sequences, they are not part of the previously mapped E1b transcriptional control region and may represent additional regulatory elements that ensure prompt and efficient E1b expression during the early phase of infection.

Novel phenotype of C3H 10T1/2 fibroblasts cotransfected with the c-Ha-ras and adenovirus 5 E1A oncogenes

C3H 10T1/2 fibroblasts are converted to fully transformed phenotype following coexpression of an activated c-Ha-ras gene and either a constitutively expressed viral or cellular myc gene. In this report, we examined whether the early region 1A (E1A) of adenovirus 5, which synergizes with ras to convert primary embryonic cells to a transformed phenotype, can synergize with ras to transform the established mouse embryonic cell line, C3H 10T1/2. We demonstrate that coexpression of ras and E1A generated a transformed phenotype that could be scored by colony assays and by soft agarose assays but not by standard focus assays. The ras-E1A-transformed phenotype relies on sequences present in conserved regions 1 and 2 of the E1A proteins and, in part, on information encoded by the extreme carboxy terminus of E1A. The contrast between the transformed phenotypes generated following the transfection of C3H 10T1/2 cells with either ras and myc or ras and E1A suggests that myc and E1A cooperate with ras to transform C3H 10T1/2 cells by mechanisms that can be distinguished using this established cell line as a model system.

Fusion of adenovirus E1A to the glucocorticoid receptor by high-resolution deletion cloning creates a hormonally inducible viral transactivator

The 289-amino-acid E1A protein of adenovirus type 2 stimulates transcription from early viral and certain cellular promoters. Its mechanism is not known, and there exist no temperature-sensitive mutants of E1A that could help to elucidate the details of E1A transcriptional activation. To create for E1A such a conditional phenotype, we fused portions of E1A to the human glucocorticoid receptor (GR) to make transactivation by E1A dependent on the presence of dexamethasone. Nested subsets of the E1A coding region, centered around the 46-amino-acid transactivating domain, were substituted for the DNA-binding domain of the GR. One of the resulting chimeric proteins (GR/E1A-99), which included the entire E1A transactivating domain, stimulated expression from a viral early promoter (E3) exclusively in the presence of hormone. GR/E1A-99 did not transactivate a GR-responsive promoter. It therefore exhibited the promoter specificity of E1A while possessing the hormone inducibility of the GR. Two smaller chimeras that contained only portions of the E1A transactivating domain failed to transactivate E3. These three chimeras were constructed by a novel strategy, high-resolution deletion cloning. In this procedure, series of unidirectional deletions were made with exonuclease III on each side of the E1A coding region at a resolution of 1 to 2 nucleotides. The large number of in-frame fragments present in the collection of deleted clones facilitated the construction of the GR/E1A chimeras and can be used to create many additional fusions.

Fusion of adenovirus E1A to the glucocorticoid receptor by high-resolution deletion cloning creates a hormonally inducible viral transactivator

The 289-amino-acid E1A protein of adenovirus type 2 stimulates transcription from early viral and certain cellular promoters. Its mechanism is not known, and there exist no temperature-sensitive mutants of E1A that could help to elucidate the details of E1A transcriptional activation. To create for E1A such a conditional phenotype, we fused portions of E1A to the human glucocorticoid receptor (GR) to make transactivation by E1A dependent on the presence of dexamethasone. Nested subsets of the E1A coding region, centered around the 46-amino-acid transactivating domain, were substituted for the DNA-binding domain of the GR. One of the resulting chimeric proteins (GR/E1A-99), which included the entire E1A transactivating domain, stimulated expression from a viral early promoter (E3) exclusively in the presence of hormone. GR/E1A-99 did not transactivate a GR-responsive promoter. It therefore exhibited the promoter specificity of E1A while possessing the hormone inducibility of the GR. Two smaller chimeras that contained only portions of the E1A transactivating domain failed to transactivate E3. These three chimeras were constructed by a novel strategy, high-resolution deletion cloning. In this procedure, series of unidirectional deletions were made with exonuclease III on each side of the E1A coding region at a resolution of 1 to 2 nucleotides. The large number of in-frame fragments present in the collection of deleted clones facilitated the construction of the GR/E1A chimeras and can be used to create many additional fusions.

Phosphorylation-dependent activation of the adenovirus-inducible E2F transcription factor in a cell-free system

Adenovirus infection induces a large increase in the DNA binding activity of a cellular transcription factor that is utilized by the viral E2 promoter and termed E2F. Using cell-free extracts, we have developed an assay for the in vitro activation of DNA binding activity of E2F. E2F activity is undetectable in HeLa extracts but upon incubation with a fraction from adenovirus-infected cells, there is an ATP-dependent increase in E2F DNA binding activity. This increase does not occur using an equivalent fraction from dl312 (E1A-)-infected cells. Incubation of E2F with phosphatase inactivates E2F binding activity. Incubation of the phosphatase-inactivated E2F with an infected cell fraction restores E2F activity as does incubation with a known protein kinase. In contrast, incubation with an extract from mock-infected cells does not restore activity. We conclude that the DNA binding activity of E2F is regulated by phosphorylation in an E1A-dependent manner.

The cellular transcription factor E2f requires viral E1A and E4 gene products for increased DNA-binding activity and functions to stimulate adenovirus E2A gene expression

Whereas a wide variety of cellular proteins interact with the cis-regulatory elements of the adenovirus E1A and E2A genes, only the DNA-binding activity of the cellular E2f factor is modulated by viral early-gene expression. An analysis of cellular E2f protein levels and adenovirus early-gene expression in a panel of independently cloned virus-transformed rodent cell lines and in virus-infected rodent cells has established that both the E1A 289-amino-acid (289R) protein and a yet-to-be-defined E4 gene product are required for maximal E2f DNA-binding activity. To distinguish between the multiple roles the E1A protein could serve in this process, the E2f DNA-binding activity was determined in a virus-transformed cell line which contains a conditional-lethal mutation affecting the 289R protein. Since E4 gene expression was not altered by the incubation conditions, the observation of reduced cellular E2f activity at the nonpermissive temperature suggests a direct role for the E1A 289R protein in E2f activation. When a virus containing a deletion in the E4 gene was introduced into cell lines which can complement the E4 gene defect, a correlation between high cellular E2f levels and increased rates of E2A gene transcription was observed. A time course analysis of the viral infection revealed that E2f functions catalytically to stimulate viral E2A gene transcription. These observations have led to several hypotheses concerning possible mechanisms by which elevated E2A gene expression, which leads to cytotoxicity, might be avoided in the transformed cell.

Limited temperature-sensitive transactivation by mutant adenovirus type 2 E1a proteins

A series of linker-scanning and deletion mutations was generated in the transactivating domain of the larger, 289-amino-acid-residue E1a protein of adenovirus type 2. Mutant genes were recombined into virus to assay the ability of the variant E1a proteins to activate expression of an E1a-dependent viral gene during infection. Results of assays performed at 32, 37, and 40 degrees C indicated that at least 2 of the 10 mutants tested showed limited temperature sensitivity for transactivation.

The amino-terminal region of the adenovirus serotype 5 E1a protein performs two separate functions when expressed in primary baby rat kidney cells

Adenovirus serotype 5 E1a proteins immortalize primary cells and in cooperation with products of a second oncogene, such as adenovirus serotype 5 E1b or EJ ras, produce full transformation. E1a also activates transcription of specific viral and cellular promoters, represses enhancer-dependent genes, and induces cellular DNA synthesis in quiescent cells. Comparison of different adenovirus serotypes has identified three conserved regions in the E1a protein sequence. We have analyzed E1a mutants with deletions-linker insertions in or preceding the first conserved region, region 1 (amino acids 40 through 77 of adenovirus serotype 5 E1a). E1a mutants which have in-frame deletions-substitutions in region 1 or pre-region 1 sequences were reconstructed into adenovirus to yield a total of 14 mutant viruses. All the mutant viruses showed wild-type growth in HeLa cells, confirming that region 1 is nonessential in these cells. However, we show that region 1 provides two distinct functions in infected primary rodent cells. One function is essential for induction of cell DNA synthesis, and the other is essential for focus formation. In addition, our results are consistent with a requirement for the DNA induction function in focus formation.

Adenovirus early region 4 is essential for normal stability of late nuclear RNAs

H2dl808 is a deletion mutant of adenovirus type 2 lacking most of transcriptional early region E4. In most normal adenovirus host cells this virus displayed a complex mutant phenotype that included a dramatic reduction in the level of cytoplasmic late RNA, a corresponding defect in late protein synthesis, and a 5- to 10-fold defect in viral DNA accumulation. H5dl1004 is a deletion mutant of adenovirus type 5 that also lacks a portion of E4. It exhibited a reduction in levels of cytoplasmic late RNAs that was somewhat less severe than that of H2dl808 and a corresponding late protein synthetic defect but no defect in the production of viral DNA. In addition to the defect in the accumulation of late cytoplasmic mRNAs, HeLa cells infected by either H2dl808 or H5dl1004 showed substantially reduced levels of viral RNAs in their nuclei at late times after infection. Both mature mRNAs and apparent mRNA precursors were affected. The late transcription rates of the deletion mutant viruses were similar to that of wild-type virus. These results suggest that the underaccumulation of RNA in H2dl808- and H5dl1004-infected cells is caused by a reduction in the stability of viral RNA in the nucleus, and they implicate E4 products in a novel aspect of the regulation of viral gene expression.

Early events in methyl methanesulfonate enhancement of adenovirus transformation of cloned rat embryo fibroblast cells

Pretreatment of a cloned rat embryo fibroblast (CREF) cell line with methyl methanesulfonate (MMS) prior to infection with a specific host-range and cold-sensitive type 5 adenovirus mutant (H5hr1), results in a unique carcinogen enhancement of transformation (CET) phenotype (Carcinogenesis 8:967, 1987). By using low-density clonal assays and in situ hybridization techniques with 32P-labeled type 5 adenovirus (Ad5) probes, we demonstrated that 5-10 d following infection the proportion of CREF colonies containing H5hr1 DNA and RNA is increased two- to threefold as a result of pretreatment with MMS. Twenty-five days following infection of CREF cells, Ad5 DNA assays showed that both solvent and MMS-pretreated CREF colonies no longer contained detectable levels of viral DNA or RNA. Analysis of free viral DNA by the Hirt procedure suggested that more free viral DNA persisted in MMS-pretreated H5hr1-infected CREF cells than in solvent-pretreated H5hr1-infected CREF cells. The relative amount of free viral DNA in both types of cultures was directly related to the multiplicity of H5hr1 infection and decreased with time following infection. As observed using in situ hybridization techniques, by 25 d after infection no free viral DNA was detected in either MMS- or solvent-pretreated H5hr1-infected CREF cells. By using a protein synthesis inhibitor (cycloheximide) and an RNA transcription inhibitor (actinomycin D), it was further demonstrated that the ability of MMS to induce a unique CET in CREF cells following infection with H5hr1 was dependent on the synthesis of new protein and RNA. In contrast, inhibition of protein and RNA synthesis did not alter the de novo rate of H5hr1 transformation of CREF cells. Temporal kinetic studies indicated that the ability of MMS to enhance H5hr1 transformation of CREF cells and to increase the percentage of CREF colonies containing Ad5 genetic information is regulated in a strict temporal manner. The results of the present investigation suggest that the ability of MMS to enhance H5hr1 transformation of CREF cells is dependent on the induction of new protein(s) in CREF cells, and enhancement is associated with an increase in the proportion of cells in the infected CREF cell population that initially contain Ad5 DNA/RNA.

Regulation of adenovirus and cellular gene expression and of cellular transformation by the E1B-encoded 175-amino-acid protein

Mutants of type 5 adenovirus that fail to express the E1B-gene-encoded 175-amino-acid (175R) protein are unable to morphologically transform primary or continuous cultures of rat embryo fibroblast cells. This phenotype could result from a direct effect of this E1B polypeptide (along with E1A polypeptides) on cellular gene expression resulting in a pathway leading to altered cell growth or from an indirect role of the 175R protein made possible by its ability to modulate viral early-gene (most likely E1A) expression. To distinguish between these two models, viruses were constructed that expressed the individual E1A 13S and 12S genes in the presence of either the E1B 175R or 495R protein. Regardless of the E1A gene product that was expressed, viruses that failed to express the E1B 175R protein were transformation defective. Additional studies suggest that the E1A 289R protein and E1B 495R protein function in a common pathway leading to the establishment of the transformed cell. We also observe that E3 gene expression by viruses that fail to express the E1A 289R protein affects the efficiency of focus formation. When tested in both nonpermissive CREF cells and permissive HeLa cells, the lack of 175R protein expression appeared to have no effect (a transient twofold decrease in E1A mRNA accumulation was observed in CREF cells) on viral early-gene expression. These results suggest that the initiation of the transformed cell phenotype occurs because of some interaction in a common pathway between the viral E1A proteins and E1B 175R protein. Furthermore, we have shown that the E1B 175R protein does not enhance the rate of transcription initiation from the mouse immunoglobulin heavy chain gene promoter when these sequences are localized on a viral genome, and it does not diminish the ability of the E1A proteins to decrease the rate of enhancer-dependent transcription.

The amino-terminal region of the adenovirus serotype 5 E1a protein performs two separate functions when expressed in primary baby rat kidney cells

Adenovirus serotype 5 E1a proteins immortalize primary cells and in cooperation with products of a second oncogene, such as adenovirus serotype 5 E1b or EJ ras, produce full transformation. E1a also activates transcription of specific viral and cellular promoters, represses enhancer-dependent genes, and induces cellular DNA synthesis in quiescent cells. Comparison of different adenovirus serotypes has identified three conserved regions in the E1a protein sequence. We have analyzed E1a mutants with deletions-linker insertions in or preceding the first conserved region, region 1 (amino acids 40 through 77 of adenovirus serotype 5 E1a). E1a mutants which have in-frame deletions-substitutions in region 1 or pre-region 1 sequences were reconstructed into adenovirus to yield a total of 14 mutant viruses. All the mutant viruses showed wild-type growth in HeLa cells, confirming that region 1 is nonessential in these cells. However, we show that region 1 provides two distinct functions in infected primary rodent cells. One function is essential for induction of cell DNA synthesis, and the other is essential for focus formation. In addition, our results are consistent with a requirement for the DNA induction function in focus formation.

Adenovirus transcriptional complexes contain EIa encoded tumour antigens physically bound to cellular proteins

Adenovirus type 12 transcriptional complexes were isolated from cells during the early phase of infection. Sedimentation analysis identified a fast sedimenting complex type I and a slow sedimenting complex type II. Both complexes made virus specific RNA complementary to all the early genes and both contained viral DNA, which in type II but not in type I had nucleosome like configuration. Analysis of the proteins of the complexes with antiserum against Ad 12 EIa-beta-galactosidase fusion protein expressed in E. coli demonstrated the following: (a) type I complex contained EIa 45 K protein, which co-precipitated with cellular proteins of mol. wt. 42, 58, and 60 K, (b) type II complex contained EIa 47 K protein, which co-precipitated with major cellular proteins of 35, 40-46 K and minor proteins of 58, 60, 68, 76, 86, and 120-150 K. Association of EIa specific and cellular proteins to transcriptional complexes was sensitive to both 1 M NaCl and DNAse I indicating the DNA binding nature of these proteins. Treatment of transcriptional complexes with 1 M NaCl or DNase I released EIa proteins, which still remained strongly bound to cellular proteins. These findings suggested that EIa proteins bind to viral DNA and that this binding is probably mediated by cellular proteins.

The 289-amino acid E1A protein of adenovirus binds zinc in a region that is important for trans-activation

The E1A gene of adenovirus type 5 encodes two major proteins of 289 and 243 amino acid residues, which are identical except that the larger protein has an internal stretch of 46 amino acids required for efficient trans-activation of early viral promoters. This domain contains a consensus zinc finger motif (Cys-Xaa2-Cys-Xaa13-Cys-Xaa2-Cys) in which the cysteine residues serve as postulated ligands. Atomic absorption spectrophotometry applied to bacterially expressed E1A proteins revealed that the 289-amino acid protein binds one zinc ion, whereas the 243-amino acid protein binds no zinc. Replacing individual cysteine residues of the finger with other amino acids destroyed the trans-activating ability of the 289-amino acid protein, even when structurally or functionally conserved amino acids were substituted. These results strongly suggest that the zinc finger of the 46-amino acid domain is intimately linked to the ability of the large E1A protein to stimulate transcription of E1A-inducible promoters. Furthermore, zinc binding to one of the mutant finger proteins suggests either that only a precise finger structure formed by the tetrahedral coordination of zinc to the four consensus ligands is required for trans-activation or, possibly, that one of several neighboring histidine residues in various combinations with three of the consensus cysteine residues normally coordinates zinc. How the zinc finger in E1A might interact with DNA or protein to bring about trans-activation is discussed.

Use of recombinant retroviruses to study the regulation of integrated adenovirus early promoters

Adenovirus E1A gene products are capable of modulating the expression of a variety of integrated genes. To study the mechanisms by which this regulation occurs, recombinant retroviruses have been utilized to establish cell lines containing an integrated copy of either the adenovirus E2 or E3 promoter adjacent to the bacterial guanine phosphoribosyl transferase (GPT) gene. These cell lines have been characterized with respect to both basal and E1A-induced levels of GPT gene expression. Cell lines with low levels of GPT gene expression showed increased expression in the presence of E1A, whereas cell lines with high basal levels of GPT gene expression had decreased GPT RNA levels in the presence of E1A. Further characterization of these cell lines revealed E1A modulation of the accumulation of RNA initiating at a retrovirus promoter adjacent to the E2 or E3 promoter. The use of the GPT gene as a marker of E2 or E3 promoter activity has allowed the isolation of cell lines which have spontaneously increased their levels of GPT RNA. A preliminary characterization of four of these cell lines has indicated that GPT gene expression is increased as a result of cis activation of the E2 promoter.

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The DNA sequence of the early E3 transcription unit of adenovirus 2 (Ad2) (J. Hérissé et al., Nucleic Acids Res. 8:2173-2192, 1980), indicates that an open reading frame exists between nucleotides 1860 and 2163 that could encode a protein of Mr 11,600 (11.6K). We have determined the DNA sequence of the corresponding region in Ad5 (closely related to Ad2) and have established that this putative gene is conserved in Ad5 (a 10.5K protein). To determine whether this protein is expressed, we prepared an antiserum in rabbits against a synthetic peptide corresponding to amino acids 66 to 74 in the 11.6K protein of Ad2. The peptide antiserum immunoprecipitated a ca. 13K-14K protein doublet, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, from [35S]methionine-labeled Ad2- or Ad5-early-infected KB cells. The antiserum also immunoprecipitated a 13K-14K protein doublet translated in vitro from Ad2 or Ad5 early E3-specific mRNA purified by hybridization to Ad2 EcoRI-D (nucleotides -236 to 2437). The synthetic peptide successfully competed with the 13K-14K protein doublet in immunoprecipitation experiments, thereby confirming the specificity of the antiserum. As deduced from the DNA sequence, the 11.6K protein (and the corresponding 10.5K Ad5 protein) has a conserved 22-amino-acid hydrophobic domain, suggesting that the protein may be associated with membranes. We conclude that a gene located at nucleotides 1860 to 2143 in the Ad2 E3 transcription unit (nucleotides 1924 to 2203) in the Ad5 E3 transcription unit) encodes an 11.6K protein (10.5K in Ad5).

Activation of a preexisting cellular factor as a basis for adenovirus E1A-mediated transcription control

Transcription of the set of early adenovirus genes is subject to positive control by the viral E1A gene. For one early viral gene, the E2 gene, this induction involves an increase in a cellular promoter-specific factor termed E2F. We have analyzed the kinetics for this induction and find that E2F is present at only very low levels in extracts of uninfected cells or cells infected for up to 3 hr with adenovirus type 5. The factor increases rapidly at 5 hr and reaches a maximal level at 7-8 hr. The kinetics of induction of the factor are thus coincident with the induction of E2 transcription. The 13S E1A gene product (289-amino acid protein), which is required for the efficient activation of E2 transcription in a productive infection, is also responsible for the activation of E2F, because infection with mutant strain pm975 (13S+, 12S-) induces the factor, whereas no increase of E2F occurs in cells infected by mutant strain dl1500 (13S-, 12S+). Finally, increase in the factor does not involve synthesis of any new protein, because extracts prepared from cells infected with adenovirus type 5 and treated with cycloheximide from 1 hr after infection contain approximately the same level of E2F as extracts from infected but untreated cells. From these results, we conclude that activation of E2F, as a posttranslational event, is responsible for the stimulation of E2 transcription by E1A.

Two regions of the adenovirus early region 1A proteins are required for transformation

Regions of the adenovirus type 5 early region 1A (E1A) proteins that are required for transformation were defined by using a series of deletion mutants. Deletion mutations collectively spanning the entire protein-coding region of E1A were constructed and assayed for their ability to cooperate with an activated ras oncogene to induce transformation in primary baby rat kidney cells. Two regions of E1A (amino acids 1 to 85 and 121 to 127) were found to be essential for transformation. Deletion of all or part of the region from amino acids 121 to 127 resulted in a total loss of transforming ability. An adjacent stretch of amino acids (residues 128 to 139), largely consisting of acidic residues, was found to be dispensable for transformation but appeared to influence the efficiency of transformation. Amino acids 1 to 85 made up a second region of the E1A protein that was essential for transformation. Deletion of all or part of this region resulted in a loss of the transforming activity. Even a mutation resulting in a single amino acid change at position 2 of the polypeptide chain was sufficient to eliminate transformation. Deletion of amino acids 86 to 120 or 128 to 289 did not eliminate transformation, although some mutations in these regions had lowered efficiencies of transformation. Foci induced by transformation-competent mutants could be expanded into cell lines that retained their transformed morphology and constitutively expressed the mutant E1A proteins.

Selective induction of human heat shock gene transcription by the adenovirus E1A gene products, including the 12S E1A product

We have previously shown that the human 70-kilodalton heat shock protein gene (hsp70) is induced by the adenovirus E1A gene product and during the S-G2 phase of the cell cycle. In this study, we investigated the effect of E1A on the expression of other human hsp genes. A gene encoding one form of the hsp89 protein (hsp89 alpha) was activated during an adenovirus infection with kinetics similar to those of activation of hsp70. The induction required a functional E1A gene. However, the hsp89 transcript was not cell cycle regulated. Genes encoding another form of hsp89 and the hsp27 protein were not induced by E1A or during the cell cycle. Further examination of hsp70 expression revealed a greater complexity than previously seen. S1 nuclease analysis using an hsp70 cDNA as well as a distinct hsp70 genomic clone demonstrated three related hsp70 transcripts; two were induced by E1A, and one was not. Both of the E1A-inducible genes were regulated during the cell cycle. All three were induced by heat shock. These results suggest common aspects of control among certain members of this family of cellular genes distinct from heat shock control. Finally, using viruses that express the individual E1A proteins, we found that the hsp70 gene is induced by the 12S and the 13S E1A products. The efficiency of induction by the 12S product was somewhat less than that by the 13S product but only by a factor of less than 2. This is in contrast to the induction of early viral genes, for which the 13S product is considerably more efficient than the 12S product.

Regulation of thyroidal inducibility of Na,K-ATPase and binding of epidermal growth factor in wild-type and cold-sensitive E1a mutant type 5 adenovirus-transformed CREF cells

We have analyzed the relationship between expression of the transformed phenotype and thyroid hormone (triiodothyronine, T3) inducibility of Na,K-ATPase and binding of 125I-epidermal growth factor (EGF) to cell membrane receptors in wild-type (wt) and mutant type 5 adenovirus (Ad5)-transformed CREF cells displaying a cold-sensitive (cs) expression of the transformed phenotype. CREF cells respond to thyroid hormone treatment with increased Na,K-ATPase activity and bind similar levels of 125I-EGF at 32 degrees C, 37 degrees C and 39.5 degrees C. In contrast, CREF cells transformed by wt Ad5 or the E1a plus E1b-transforming genes of wt Ad5 are refractile to T3 treatment and bind lower levels of 125I-EGF than CREF cells at all three temperatures. By employing a series of cloned CREF cell lines transformed by a host-range cold-sensitive mutant virus, H5hr1 or H5dl101, or the E1a or E1a plus E1b genes from these viruses, we have investigated expression of the transformed state and its relationship with hormone inducibility and EGF binding. When cs virus, cs E1a- or cs E1a plus E1b-transformed CREF clones were grown at 32 degrees C, a nonpermissive transforming temperature in which cs-transformed cells exhibit properties similar to untransformed CREF cells, T3 induced Na,K-ATPase activity and these cells bound similar levels of 125I-EGF as CREF cells. However, when cs virus- and cs Ela plus E1b-transformed CREF clones were incubated at 37 degrees C or 39.5 degrees C, temperatures at which cs-transformed cells exhibit properties similar to wt Ad5-transformed CREF cells, they did not respond to T3 and bound lower levels of 125I-EGF than CREF cells. In the case of cs E1a-transformed CREF clones, thyroid hormone responsiveness was observed at both 32 degrees C and 37 degrees C, but not at 39.5 degrees C. By performing temperature shift experiments--i.e. 32 degrees C to 37 degrees C, 32 degrees C to 39.5 degrees C, 37 degrees C to 32 degrees C, and 39.5 degrees C to 32 degrees C, it was demonstrated that after a shift from lower to higher temperature a 24-hr lag period was required for cs-transformed CREF cells to lose T3 inducibility and exhibit reduced EGF binding, whereas 96 hr after a shift from higher to lower temperature a 96-hr lag period was required for cs-transformed cells to regain T3 inducibility and increased 125I-EGF binding.(ABSTRACT TRUNCATED AT 400 WORDS)

Upstream regulatory regions required to stabilize binding to the TATA sequence in an adenovirus early promoter

Of the five early adenovirus promoters, the early region 3 (E3) promoter is one of the most strongly induced by the E1A protein. To identify cellular proteins involved in both the basal and E1A-induced transcriptional regulation of the E3 promoter, DNase I footprinting using partially purified Hela cell extracts was performed. Four regions of the E3 promoter serve as binding domains for cellular proteins. These regions are found between -156 to -179 (site IV), -83 to -103 (site III), -47 to -67 (site II), and -16 to -37 (site I), relative to the start of transcription. Examination of the DNA sequences in each binding domain suggests that site III likely serves as a binding site for activator protein 1 (AP-1), site II for the cyclic AMP regulatory element binding protein (CREB), and site I for a TATA binding factor. The factors binding to either site II or III were sufficient to stabilize binding to the TATA sequence (site I). Mutagenesis studies indicated that both sites II and III, in addition to site I, are needed for complete basal and E1A-induced transcription. These results suggest that multiple cellular factors are involved in both the basal and E1A-induced transcriptional regulation of the E3 promoter, and that either of two upstream regions are capable of stabilizing factor binding to the TATA sequence.

Selective induction of human heat shock gene transcription by the adenovirus E1A gene products, including the 12S E1A product

We have previously shown that the human 70-kilodalton heat shock protein gene (hsp70) is induced by the adenovirus E1A gene product and during the S-G2 phase of the cell cycle. In this study, we investigated the effect of E1A on the expression of other human hsp genes. A gene encoding one form of the hsp89 protein (hsp89 alpha) was activated during an adenovirus infection with kinetics similar to those of activation of hsp70. The induction required a functional E1A gene. However, the hsp89 transcript was not cell cycle regulated. Genes encoding another form of hsp89 and the hsp27 protein were not induced by E1A or during the cell cycle. Further examination of hsp70 expression revealed a greater complexity than previously seen. S1 nuclease analysis using an hsp70 cDNA as well as a distinct hsp70 genomic clone demonstrated three related hsp70 transcripts; two were induced by E1A, and one was not. Both of the E1A-inducible genes were regulated during the cell cycle. All three were induced by heat shock. These results suggest common aspects of control among certain members of this family of cellular genes distinct from heat shock control. Finally, using viruses that express the individual E1A proteins, we found that the hsp70 gene is induced by the 12S and the 13S E1A products. The efficiency of induction by the 12S product was somewhat less than that by the 13S product but only by a factor of less than 2. This is in contrast to the induction of early viral genes, for which the 13S product is considerably more efficient than the 12S product.