Cellular promoters incorporated into the adenovirus genome: effects of viral regulatory elements on transcription rates and cell specificity of albumin and beta-globin promoters

Targeted expression of herpes simplex virus thymidine kinase in adenovirus-infected cells reduces virus titers upon treatment with ganciclovir in vitro

BACKGROUND

Adenoviruses are a frequent cause of life-threatening infections in immunocompromised patients. Available therapeutics still cannot completely prevent fatal outcomes. By contrast, herpes viruses are well treatable with prodrugs such as ganciclovir (GCV), which are selectively activated in virus-infected cells by virus-encoded thymidine kinases. This effective group of prodrugs is not applicable to adenoviruses and other DNA viruses because they lack those kinases.

METHODS

To render adenoviruses amenable to GCV treatment, we generated an adenoviral vector-based delivery system for targeted expression of herpes simplex virus thymidine kinase (HSV-TK) in wild-type adenovirus 5 (wt Ad5)-infected cells. HSV-TK expression was largely restricted to wt virus-infected cells by transcription of the gene from the Ad5 E4 promoter. Its activity is dependent on the adenoviral E1A gene product which is not produced by the vector but is only provided in cells infected with wt adenovirus. The anti-adenoviral effect of HSV-TK expression and concomitant treatment with GCV was assessed in vitro in four different cell lines or primary cells.

RESULTS

E4 promoter-mediated HSV-TK background expression was sufficiently low to prevent cytotoxicity in the presence of low-levels GCV in cells not infected with wt Ad5. However, expression was several-fold increased in wt Ad5-infected cells and treatment with low levels of GCV efficiently inhibited wt Ad5 DNA replication. Genome copy numbers and output of infectious particles were reduced by up to > 99.99% and cell viability was greatly increased.

CONCLUSIONS

We extended the concept of enzyme/prodrug therapy to adenovirus infections by selectively sensitizing adenovirus-infected cells to treatment with GCV.

miR-122a-regulated expression of a suicide gene prevents hepatotoxicity without altering antitumor effects in suicide gene therapy

The combined use of adenovirus (Ad) vectors expressing herpes simplex virus thymidine kinase (HSVtk) and ganciclovir (GCV) offers a potential therapeutic strategy against cancer. However, intratumorally injected Ad vectors are disseminated into the systemic circulation and efficiently transduce the liver, resulting in severe hepatotoxicity. In order to overcome this problem, an Ad vector carrying a microRNA (miRNA)-regulated expression system was developed by inserting into the 3'-untranslated region (3'-UTR) of the expression cassette four tandem copies of sequences with perfect complementarity to miR-122a, which exhibits liver-specific expression. Transgene expression from the Ad vector carrying the miR-122a target sequences was 7- to 70-fold lower in cells with high miR-122a expression as compared to expression from a conventional Ad vector. Intratumoral injection of the Ad vector containing the miR-122a target sequences resulted in a 130- to 1,500-fold reduction in hepatic transgene products (without affecting the transgene expression in the tumor) when compared with those from a conventional Ad vector. In suicide gene therapy, the inclusion of the miR-122a target sequences in the HSVtk expression cassette achieved not only significant antitumor effects, but also a dramatic reduction in HSVtk/GCV-induced hepatotoxicity. These results indicate that Ad vectors that mediate miR-122a-regulated HSVtk expression provide a safe and efficient suicide gene therapy strategy.

A small chimeric promoter for high prostate-specific transgene expression from adenoviral vectors

BACKGROUND

Specificity of transgene expression is important for safety during gene therapeutical applications. For prostate cancer, transcriptional targeting has been applied but was hampered by loss of specificity and low activity. We constructed a small chimeric promoter for high and prostate-specific transgene expression from adenoviral vectors.

METHODS

A chimeric promoter, composed of the prostate-specific antigen (PSA) enhancer and the rat probasin promoter, was cloned into an adenoviral vector and its activity was compared to vectors containing conventional prostate-specific promoters and the constitutive Cytomegalovirus (CMV) promoter in in vitro and in vivo prostate cancer models.

RESULTS

The chimeric PSA-probasin promoter was the most active prostate-specific promoter reaching up to 20% of CMV promoter activity while maintaining prostate-specificity.

CONCLUSIONS

The chimeric PSA-probasin promoter is a small promoter that can be utilized in viral vectors for high prostate-specific transgene expression.

Effects of the Ad5 upstream E1 region and gene products on heterologous promoters

BACKGROUND

All recombinant adenovirus vectors contain the upstream region of the E1A gene comprising the viral origin of replication, encapsidation signal, and cis-acting regulatory elements for transcription of the E1A and other early genes. Using different reporter genes, some previous studies demonstrated the maintenance of heterologous promoter specificity in the adenoviral context, while others reported that adenoviral sequences interfere with promoter activity.

METHODS

Plasmid DNA-based luciferase reporter gene assays and adenovirus type 5 (Ad5) infection were combined to examine the effect of the Ad5 (nt 1-353) element and/or adenoviral gene products on tissue-specific (Midkine (MK) and COX-2), cell cycle associated (Ki-67 and E2F1) and viral promoters (Ad5 E1, Ad5 E4 and SV40). As a proof of concept, data were verified in the setting of recombinant replication-defective and replication-competent adenoviral vectors.

RESULTS

Viral and E2F1 promoter activities were enhanced by the Ad5 (nt 1-353) segment by approximately 100% and 145%, respectively, regardless of its position. A polyadenylation sequence (polyA) upstream of the promoter had no effect, confirming an enhancer element within the Ad5 (nt 1-353) segment. Ad5 (nt 1-353) increased COX-2 promoter activity by 146% but was blocked by an upstream polyA, indicating a cryptic transcription start site. When placing the reporter gene cassette in a replication-defective adenovirus, similar data were obtained. In the plasmid vector-based system, adenoviral gene products transactivated the E2F1 and viral promoters by 194%, 19%, 67%, and 16%, respectively. Tissue-specific promoter activities were not significantly affected by the Ad5 (nt 1-353) segment, nor adenoviral gene products. In concert with these data, we were able to target replication-competent adenoviral vectors with the COX-2 promoter, but not with the cell cycle associated promotor.

CONCLUSIONS

The adenovirus E1A upstream regulatory region and gene products interact with some but not all heterologous promoters. Often, the basal promoter activity can be reduced with an upstream polyA. Since the data obtained in our plasmid vector-based assay with internal control and infection with adenovirus could be confirmed in the adenoviral setting, our system might be suitable to speed up the identification of promoters which maintain their specificity in the adenoviral context and circumvent the problems associated with determining infectious adenovirus titers.

Restoration of a functional open reading frame by homologous recombination between two adenoviral vectors

In this study, we examined the ability of adenoviral (Ad) vectors to undergo homologous recombination. The lacZ gene was divided between two parental, first-generation vectors such that neither encoded a functional product but both shared 494 bp in common. The open reading frame could only be restored by homologous recombination. We observed beta-galactosidase activity only upon co-infection of both parental vectors and after the onset of viral DNA replication, creating a delay in expression of 24-36 hours in HeLa cells. At peak efficiency, this recombination vector system resulted in beta-galactosidase activity levels 100x above background and just 18x less than a conventional, first-generation vector in HeLa cells. After recombination, the resultant progeny vector genomes containing reconstituted expression cassettes were devoid of all viral genes and contained two packaging signals. These progeny genomes were efficiently packaged, could be separated from their parental vectors based on their lighter buoyant densities in CsCl gradients, and were subsequently used as functional gene transfer vectors. This novel recombination vector system should be useful for transferring large transgenes (because the carrying capacity of two Ad vectors can be exploited) or expressing any cytotoxic or Ad replication inhibitory protein (because the parental vectors exhibit no background expression).

An adenoviral expression system for AAV rep78 using homologous recombination

The construction and amplification of adenoviral (Ad) vectors expressing biologically active transgenes that are cytotoxic or inhibit Ad replication can be extremely difficult, if not impossible. In this study, we harnessed the ability of Ad genomes to undergo efficient homologous recombination to reconstitute the adeno-associated virus (AAV) rep78 gene, a cytotoxic gene that strongly inhibits Ad replication, which was divided between two parental, first-generation Ad vectors. A functional open reading frame was generated by recombination only upon co-infection of both parental vectors and after the onset of viral DNA replication. We were able to amplify both parental rep78 vectors to normal titers without any signs of inhibition or toxicity and could use them to generate progeny vectors containing a functional rep78 gene without any Ad genes. Using this vector recombination system in AAV rescue assays demonstrated that no Ad protein was essential for Rep78 mediated rescue of AAV ITR flanked DNA from plasmid or Ad backbones; the amount of rescue product generated was substantially greater in the presence of Ad infection; neither cellular nor viral DNA replication was necessary for rescue to occur; and progeny vector genomes were efficiently co-replicated along with conventional, first-generation Ad vectors.

Superior tissue-specific expression from tyrosinase and prostate-specific antigen promoters/enhancers in helper-dependent compared with first-generation adenoviral vectors

The ability to target specific tissues is important in many applications of gene therapy. In this respect, a disadvantage of adenoviral vectors is the relative lack of specificity with which they transduce cells. One approach to overcome this is to express the therapeutic gene under the control of a tissue-specific promoter. However, the specificity and activity of these promoters may be altered by adenoviral sequences in the vector backbone. In contrast, helper-dependent adenoviral (HDAd) vectors [Parks, R.J., Chen, L., Anton, M., Sankar, U., Rudnicki, M.A., and Graham, F.L. (1996). Proc. Natl. Acad. Sci. U.S.A. 93, 13565-13570] are almost completely devoid of adenovirus sequences, and this may preserve the specificity of these heterologous promoters. We have compared HDAd and first-generation adenoviral (FGAd) vectors with respect to tissue-specific expression from prostate-specific antigen (PSA) or tyrosinase promoters/enhancers. A PSA-positive cell line (LNCaP) and a panel of PSA-negative cell lines were infected with HDAd vectors expressing luciferase under the control of three different kinds of PSA promoter/enhancer constructs. The results showed that these PSA promoter/enhancer cassettes in HDAd vectors maintained strict tissue-specific expression, but lost specificity when expressed from FGAd vectors. Similar results were observed with tyrosinase promoter-carrying vectors, except that the tyrosinase promoter retained a small degree of tissue specificity in FGAd vectors. Insertion of a murine cytomegalovirus immediate-early gene promoter-beta-galactosidase (MCMV-lacZ)-expressing cassette into a second site in the HDAd vector backbone significantly impaired the tissue specificity of the PSA and tyrosinase promoters. These results indicate that HDAd vectors are superior to FGAd vectors in their ability to maintain high levels of tissue-specific expression from PSA and tyrosinase promoters/enhancers. They also suggest that tissue-specific expression can be influenced not only by Ad sequences, but also by other viral and/or strong constitutive promoter/enhancers (such as the MCMV promoter) in the vector backbone.

High-level, beta-catenin/TCF-dependent transgene expression in secondary colorectal cancer tissue

There is an urgent need for improved therapies for inoperable metastatic colon cancer. Gene-directed enzyme prodrug therapy (GDEPT) using adenovirus vectors works well in preclinical models of this disease, but successful clinical application is hampered by an inability to construct vectors that express at high levels in infected tumor cells but not in infected normal cells. Constitutive activation of beta-catenin-dependent gene expression is almost certainly a key causative event in the genesis of colon and some other cancers. Here we have exploited this oncogenic defect to design a synthetic promoter, CTP1, that, in contrast to currently available tumor-selective promoters, is both highly active in cancer cells and highly cancer-cell-specific. CTP1 directs high-level beta-galactosidase expression in freshly isolated biopsies of secondary colon cancer, but is not detectably active in associated normal liver tissue. We also demonstrate that CTP1 can direct high-level, tumor-specific therapeutic gene expression in vivo. Intratumoral injection of an adenovirus vector encoding Escherichia coli nitroreductase driven by CTP1 efficiently sensitized SW480 xenografts to the prodrug CB1954, whereas systemic vector and prodrug administration produced no apparent signs of toxicity. CTP1 may form the basis for effective, targeted gene therapy of metastatic colon cancer and other tumors with deregulated beta-catenin/T cell factor.

Creation of a new transgene cloning site near the right ITR of Ad5 results in reduced enhancer interference with tissue-specific and regulatable promoters

Tissue-specific transgene expression is a valuable research tool and is of great importance in delivering toxic gene products with adenovirus vectors to tumors. Limiting cytotoxic gene expression to the target cells is highly desirable. While a number of successful applications of tissue- and tumor-specific gene expression using Ad vectors has been reported, cloning of some promoters into Ad vectors resulted in modulation or loss of tissue specificity. This phenomenon is likely the result of the interaction of E1A enhancer (and possibly other Ad sequences) with the promoter cloned in the E1 region. We have compared performance parameters of prostate-specific and tet-regulatable promoters in plasmids containing the terminal repeat sequences of Ad5 with or without the E1A enhancer. Subsequently, adenoviral vectors were constructed containing identical expression units either in the E1 region or near the right ITR, and tested in several cell lines. Here, we report that promoters placed near the right ITR of Ad5 retain higher selectivity and lower background expression in both plasmid and adenovirus vectors. We confirm that the E1A enhancer can interfere with the desired activity of nearby promoters, and describe an alternative transgene insertion site for construction of Ad vectors.

Tumor-specific gene expression in hepatic metastases by a replication-activated adenovirus vector

Clinical applications of tumor gene therapy require tumor-specific delivery or expression of therapeutic genes in order to maximize the oncolytic index and minimize side effects. This study demonstrates activation of transgene expression exclusively in hepatic metastases after systemic application of a modified first-generation (E1A/E1B-deleted) adenovirus vector (AdE1-) in mouse tumor models. The discrimination between tumors and normal liver tissue is based on selective DNA replication of AdE1- vectors in tumor cells. This new AdE1- based vector system uses homologous recombination between inverted repeats to mediate precise rearrangements within the viral genome. As a result of these rearrangements, a promoter is brought into conjunction with a reporter gene creating a functional expression cassette. Genomic rearrangements are dependent upon viral DNA replication, which in turn occurs specifically in tumor cells. In a mouse tumor model with liver metastases derived from human tumor cells, a single systemic administration of replication activated AdE1- vectors achieved transgene expression in every metastasis, whereas no extra-tumoral transgene induction was observed. Here we provide a new concept for tumor-specific gene expression that is also applicable for other conditionally replicating adenovirus vectors.

Insulation from viral transcriptional regulatory elements improves inducible transgene expression from adenovirus vectors in vitro and in vivo

Recombinant adenoviruses (Ad) are attractive vectors for gene transfer in vitro and in vivo. However, the widely used E1-deleted vectors as well as newer generation vectors contain viral sequences, including transcriptional elements for viral gene expression. These viral regulatory elements can interfere with heterologous promoters used to drive transgene expression and may impair tissue-specific or inducible transgene expression. This study demonstrates that the activity of a metal-inducible promoter is affected by Ad sequences both upstream and downstream of the transgene cassette in both orientations. Interference with expression from the heterologous promoter was particularly strong by viral regulatory elements located within Ad sequences nucleotides 1-341. This region is present in all recombinant Ad vectors, including helper-dependent vectors. An insulator element derived from the chicken gamma-globin locus (HS-4) was employed to shield the inducible promoter from viral enhancers as tested after gene transfer with first-generation Ad vectors in vitro and in vivo. Optimal shielding was obtained when the transgene expression cassette was flanked on both sides by HS-4 elements, except for when the HS-4 element was placed in 3'-->5' orientation in front of the promoter. The insulators reduced basal expression to barely detectable levels in the non-induced stage, and allowed for induction factors of approximately 40 and approximately 230 in vitro and in vivo, respectively. Induction ratios from Ad vectors without insulators were approximately 40-fold lower in vitro and approximately 15-fold lower in vivo. This study proves the potential of insulators to improve inducible or tissue-specific gene expression from adenovirus vectors, which is important for studying gene functions as well as for gene therapy approaches. Furthermore, our data show that insulators exert enhancer-blocking effects in episomal DNA.

Efficient gene transfer and long-term expression in neurons using a recombinant adenovirus with a neuron-specific promoter

Adenoviruses are highly efficient vectors for gene transfer into brain cells. Restricting transgene expression to specific cell types and maintaining long-term expression are major goals for gene therapy in the central nervous system. We targeted gene expression to neurons by constructing an adenoviral vector that expressed the E. coli LacZ reporter gene under the control of the rat neuron-specific enolase promoter (Ad-NSE). Expression from Ad-NSE was compared with that from an adenoviral vector encoding the same reporter gene under the control of the Rous sarcoma virus LTR promoter (Ad-RSV). Both recombinant adenoviruses were injected stereotactically into rat hippocampus, cerebellum and striatum. Anatomical and immunohistochemical analyses of the Ad-NSE-stained cells showed that neurons were preferentially transduced. More neurons were stained in the hippocampus following infection with Ad-NSE than with Ad-RSV. Cytotoxicity from Ad-NSE was lower than from Ad-RSV. beta-Galactosidase gene expression after Ad-NSE infection remained stable for 3(1/2) months, and was detectable for 6 months. Thus, the NSE-adenoviral vector can be used to transfer potentially therapeutic genes into neuronal cells. The use of a cell-specific promoter also resulted in high in vivo efficiency and long-term transgene expression.

Efficacy of replication-defective adenovirus-vectored vaccines: protection following intramuscular injection is linked to promoter efficiency in muscle representative cells

To investigate the respective role of transduced cells in the induction of immune response following intramuscular inoculation of adenovirus-based vaccines, we generated several replication-defective adenoviruses expressing the glycoprotein D gene of pseudorabies virus under the control of four different promoters: major late promoter of adenovirus type 2, human cytomegalovirus immediate-early promoter/enhancer (CMV), Rous sarcoma virus-long terminal repeat promoter, and human desmin gene 5' regulatory region (DES). All the adenovirus constructs were able to fully protect mice, in the contrary of direct DNA inoculation of plasmids harboring the same transcription units. The far most effective adenovirus constructs, on the criterion of protective doses and specific antibody response induction, were those in which the foreign gene was driven by the DES or CMV promoter. Wide variations in promoter strength in vitro were evidenced in several cell culture types representative of putative target cells following muscular inoculation (myoblasts, myotubes, fibroblasts, macrophages, and endothelial cells). The level of efficacy in vivo, was not correlated with the level of expression in vitro in myotubes, but paralleled the level of expression in endothelial cells and in myoblasts. Together with previously published data, these results suggest that, following adenovirus injection, locally produced cytokines may induce myoblasts to act as local antigen presenting cells.

Modulation of the specificity and activity of a cellular promoter in an adenoviral vector

Most gene therapy studies with recombinant adenoviruses employ viral promoters and lack tissue specificity. To determine whether a tissue-specific cellular promoter inserted into the adenoviral genome can direct the expression of a reporter gene in a tissue-specific manner, recombinant adenoviruses containing a nuclear lacZ gene driven by a human ventricular/slow muscle myosin light chain 1 promoter with and without a muscle creatine kinase enhancer were constructed. The ability of these viruses to express the reporter genes in infected myogenic and nonmyogenic cell lines was studied. Intramuscular injection of these viruses into mice showed that little or no reporter gene expression occurred in muscle fibers, although a relatively high level of lacZ gene expression was observed in surrounding connective tissue. Insertion of adenovirus sequences from the 5' inverted terminal repeat (ITR) region and/or the protein IX region into plasmids resulted in decreased reporter gene expression from myosin light chain 1 promoter in transfected C2C12 myotubes and 293 cells, as well as in injected muscles. These results suggested that negative elements are present in the adenoviral genome. This negative effect seems neither tissue nor species specific. Adenovirus cis-elements that may affect the specificity and activity of a cellular promoter are discussed.

High-level tissue-specific expression of functional human factor VIII in mice

Hemophilia A results from subnormal levels of blood coagulation factor VIII (FVIII) and is an attractive target for gene therapy. However, progress has been impeded by features of FVIII biology such as low mRNA accumulation and the instability of the protein. We have shown previously that a FVIII adenoviral vector, Av1ALH81, allowed high-level expression of human FVIII in mice sustained for several weeks. Here, we have generated a second FVIII adenoviral vector, Av1ALAPH81, in which an intron was introduced into the FVIII expression cassette. Administration of Av1ALAPH81 to mice resulted in significantly increased FVIII plasma levels, 1,046 +/- 163 ng/ml compared to 307 +/- 93 ng/ml of FVIII detected in mice that received Av1ALH81. Normal FVIII levels in humans are 100-200 ng/ml and therapeutic levels are as low as 10 ng/ml. Therapeutic levels are defined as the amount of FVIII necessary to convert severe hemophilia to a moderate or mild hemophiliac condition. The increased potency of the second FVIII adenoviral vector allowed the administration of significantly lower, less toxic vector doses, while retaining the potential for high FVIII expression. Furthermore, we demonstrate that adenoviral-mediated expression of human FVIII can be limited to the liver by inclusion of a liver-specific promoter, thereby achieving the first step in regulated expression of human FVIII in vivo.

Evaluation of the transfer and expression in mice of an enzyme-encoding gene using a human adenovirus vector

Mutant mice of the Spf-ash strain have an inherited defect in ornithine transcarbamylase (OTC) protein synthesis, and were used to ascertain the potential of recombinant adenoviruses for introducing and expressing the normal gene lacking in these mice. These OTC mutant mice are characterized by a reduction in the amount of OTC activity, resulting in hyperammonemia, pronounced orotic aciduria, growth retardation, and sparse fur until weaning. A recombinant adenovirus that harbors the rat OTC cDNA under the control of the viral major late promoter (MLP) was constructed and injected into such newborn mice. The effect of the virus was analyzed by monitoring the hepatic OTC enzyme during several months after the injection. An increase in OTC activity was detected and was accompanied by a diminution of orotic acid in the urine. The observation of MLP-OTC mRNA transcripts over 1 year following the injection attests to the relatively long-term presence of the transferred gene. In those mice showing the greatest OTC activity, a normalization of the fur was also observed. The experiments reported here document the feasibility of using adenovirus for the direct delivery in vivo of a gene to restore an impaired metabolism.

Tissue-specific activity of heterologous viral promoters in primary rat hepatocytes and Hep G2 cells

In preparation for studies using gene transfer, we have identified transcriptional control elements which are active in primary rat hepatocytes. We used plasmids which were constructed so that the promoter or enhancer of interest initiated transcription of the bacterial chloramphenicol acetyltransferase (CAT) gene. Plasmids were introduced into primary rat hepatocytes in culture, into Hep G2 cells and other human and animal cell lines and into bone marrow stromal cells, and CAT activity was assayed after 48 hr. In primary rat hepatocytes, the highest CAT activity was obtained with plasmids carrying the Rous sarcoma virus long terminal repeat (pRSVCAT), or the SV40 early region promoter and enhancer (pSV2CAT). Hepatocytes carrying the murine cytomegalovirus immediate early promoter (pUCRNmCMVX/HCAT) also had appreciable CAT activity. No CAT activity was detected in rat hepatocytes carrying pSVOCAT (a promoterless construct), pUCRNtKCAT (herpes simplex thymidine kinase gene promoter), pLPVCAT (lymphocytotrophic papovavirus promoter) and pHBV1CAT (hepatitis B virus enhancer and core gene promoter). Therefore, for future studies of gene transfer in primary rat hepatocytes, the Rous sarcoma virus long terminal repeat or the SV40 early region promoter and enhancer can be effectively used to drive gene expression. Hep G2 cells carrying pHBV1CAT had high CAT activity. Hep G2 cells carrying pHBV2CAT (similar to pHBV1CAT, but with the hepatitis B virus sequences in reverse orientation with respect to the CAT sequences) and pHBV3CAT (similar to pHBV2CAT, but hepatitis B virus sequences are separated from the CAT sequences by about 700 bases) also expressed CAT activity, but not as strongly as with pHBV1CAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Transcription elements and factors of RNA polymerase B promoters of higher eukaryotes

The promoter for eukaryotic genes transcribed by RNA polymerase B can be divided into the TATA box (located at -30) and startsite (+1), the upstream element (situated between -40 and about -110), and the enhancer (no fixed position relative to the startsite). Trans-acting factors, which bind to these elements, have been identified and at least partially purified. The role of the TATA box is to bind factors which focus the transcription machinery to initiate at the startsite. The upstream element and the enhancer somehow modulate this interaction, possibly through direct protein-protein interactions. Another class of transcription factors, typified by viral proteins such as the adenovirus EIA products, do not appear to require binding to a particular DNA sequence to regulate transcription. The latest findings in these various subjects are discussed.

Cellular promoters incorporated into the adenovirus genome. Effect of viral DNA replication on endogenous and exogenous gene transcription

Cell-specific utilization of the albumin and immunoglobulin promoters within non-replicating adenovirus genomes has been established. With such viruses plus helper viruses we now consider the impact of infection by replicating viruses on liver-specific gene expression in hepatoma cells and of the effect of replication on the previously established limits of cell-specific expression. We find that replication of any adenovirus, whether or not it contains the albumin promoter, decreases albumin and apolipoprotein A transcription in hepatoma cells but does not affect transcription of at least four other genes expressed mainly (or only) in the liver. Thus, it may be that some hepatoma-specific genes and late adenoviral genes require a factor(s) in common, and these factors become limiting during replication. In hepatoma cells, the increased copy number of the exogenous promoters resulting from viral DNA replication did not influence the cell-specific expression noted previously; only albumin and not globin or immunoglobulin promoters in the virus were active. In contrast, replication by the same viruses in HeLa cells resulted in high levels of expression from all the exogenous promoters.