Two novel adenovirus vector systems permitting regulated protein expression in gene transfer experiments

Methods for gene transfer to the central nervous system

Gene transfer is an increasingly utilized approach for research and clinical applications involving the central nervous system (CNS). Vectors for gene transfer can be as simple as an unmodified plasmid, but more commonly involve complex modifications to viruses to make them suitable gene delivery vehicles. This chapter will explain how tools for CNS gene transfer have been derived from naturally occurring viruses. The current capabilities of plasmid, retroviral, adeno-associated virus, adenovirus, and herpes simplex virus vectors for CNS gene delivery will be described. These include both focal and global CNS gene transfer strategies, with short- or long-term gene expression. As is described in this chapter, an important aspect of any vector is the cis-acting regulatory elements incorporated into the vector genome that control when, where, and how the transgene is expressed.

Gene regulation systems for gene therapy applications in the central nervous system

Substantial progress has been made in the development of novel gene therapy strategies for central nervous system (CNS) disorders in recent years. However, unregulated transgene expression is a significant issue limiting human applications due to the potential side effects from excessive levels of transgenic protein that indiscriminately affect both diseased and nondiseased cells. Gene regulation systems are a tool by which tight tissue-specific and temporal regulation of transgene expression may be achieved. This review covers the features of ideal regulatory systems and summarises the mechanics of current exogenous and endogenous gene regulation systems and their utility in the CNS.

A mouse model of inducible liver injury caused by tet-on regulated urokinase for studies of hepatocyte transplantation

Mouse models of liver injury provide useful tools for studying hepatocyte engraftment and proliferation. A representative model of liver injury is the albumin-urokinase (Alb-uPA) transgenic model, but neonatal lethality hampers its widespread application. To overcome this problem, we generated a transgenic mouse in which transcription of the reverse tetracycline transactivator was (rtTA) driven by the mouse albumin promoter, and backcrossed the rtTA mice onto severe combined immunodeficient (SCID)/bg mice to generate immunodeficient rtTA/SCID mice. We then produced recombinant adenoviruses Ad.TRE-uPA, in which the urokinase was located downstream of the tetracycline response element (TRE). The rtTA/SCID mouse hepatocytes were then infected with Ad.TRE-uPA to establish an inducible liver injury mouse model. In the presence of doxycycline, uPA was exclusively expressed in endogenous hepatocytes and caused extensive liver injury. Enhanced green fluorescent protein-labeled mouse hepatocytes selectively repopulated the rtTA/SCID mouse liver and replaced over 80% of the recipient liver mass after repeated administration of Ad.TRE-uPA. Compared with the original uPA mice, rtTA/SCID mice did not exhibit problems regarding breeding efficiency, and the time window for transplantation was flexible. In addition, we could control the extent of liver injury to facilitate transplantation surgery by regulating the dose of Ad.TRE-uPA. Our inducible mouse model will be convenient for studies of hepatocyte transplantation and hepatic regeneration, and this system will facilitate screening for potential genetic factors critical for engraftment and proliferation of hepatocytes in vivo.

Glucolipotoxicity in INS-1E cells is counteracted by carnitine palmitoyltransferase 1 over-expression

Effects of non-esterified fatty acids (FAs) are accentuated when applied together with elevated glucose through preferential use of glucose as fuel, which leads to decreased oxidation of FAs. We examined how over-expression of the mitochondrial FA transporter carnitine palmitoyltransferase 1 (CPT1) affects glucose-stimulated insulin secretion (GSIS), apoptosis and ER stress in INS-1E cells cultured in the presence of elevated levels of glucose and palmitate. INS-1E cells were infected with Tet-ON regulated adenovirus containing CPT1 and cultured for 48h in the presence of 0.5mM palmitate and 20mM glucose. Over-expressing CPT1 lowered basal insulin secretion in a dose-dependent manner thereby improving GSIS from INS-1E cells. Also, apoptosis was alleviated and ER-stress markers p-eIF2alpha and CHOP were decreased in cells over-expressing CPT1. We conclude that regulated over-expression of CPT1 is beneficial for glucolipotoxic beta-cells.

Attenuation of leakiness in doxycycline-inducible expression via incorporation of 3′ AU-rich mRNA destabilizing elements

Tetracycline-regulated expression systems have been widely used for inducible protein expression in cultured mammalian cells. With these systems, however, leakiness in expression of the target gene in the absence of the inducing agent is a frequent problem. Here we describe a novel approach to overcome this problem that involves the incorporation of AU-rich mRNA destabilizing elements (AREs) into the 3′ untranslated regions of the tetracycline-inducible constructs. Using the inducible expression of sphingosine kinase 1 and 2 in HEK293 cells as model systems, we found this ARE approach to be remarkably successful in ablating expression of these proteins in the absence of doxycycline through decreasing stability of their mRNAs. We show that this undemanding and flexible process results in a substantial decrease in the leakiness of the tetracycline-inducible expression system while maintaining a high level of target protein expression following induction.

Glucose-induced cyclic AMP oscillations regulate pulsatile insulin secretion

Cyclic AMP (cAMP) and Ca(2+) are key regulators of exocytosis in many cells, including insulin-secreting beta cells. Glucose-stimulated insulin secretion from beta cells is pulsatile and involves oscillations of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), but little is known about the detailed kinetics of cAMP signaling. Using evanescent-wave fluorescence imaging we found that glucose induces pronounced oscillations of cAMP in the submembrane space of single MIN6 cells and primary mouse beta cells. These oscillations were preceded and enhanced by elevations of [Ca(2+)](i). However, conditions raising cytoplasmic ATP could trigger cAMP elevations without accompanying [Ca(2+)](i) rise, indicating that adenylyl cyclase activity may be controlled also by the substrate concentration. The cAMP oscillations correlated with pulsatile insulin release. Whereas elevation of cAMP enhanced secretion, inhibition of adenylyl cyclases suppressed both cAMP oscillations and pulsatile insulin release. We conclude that cell metabolism directly controls cAMP and that glucose-induced cAMP oscillations regulate the magnitude and kinetics of insulin exocytosis.

Regulated expression of adenoviral vectors-based gene therapies: therapeutic expression of toxins and immune-modulators

Regulatable promoter systems allow gene expression to be tightly controlled in vivo. This is highly desirable for the development of safe, efficacious adenoviral vectors that can be used to treat human diseases in the clinic. Ideally, regulatable cassettes should have minimal gene expression in the "OFF" state, and expression should quickly reach therapeutic levels in the "ON" state. In addition, the components of regulatable cassettes should be non-toxic at physiological concentrations and should not be immunogenic, especially when treating chronic illness that requires long-lasting gene expression. In this chapter, we will describe in detail protocols to develop and validate first generation (Ad) and high-capacity adenoviral (HC-Ad) vectors that express therapeutic genes under the control of the TetON regulatable system. Our laboratory has successfully used these protocols to regulate the expression of marker genes, immune stimulatory genes, and toxins for cancer gene therapeutics, i.e., glioma that is a deadly form of brain cancer. We have shown that this third generation TetON regulatable system, incorporating a doxycycline (DOX)-sensitive rtTA(2)S-M2 inducer and tTS(Kid) silencer, is non-toxic, relatively non-immunogenic, and can tightly regulate reporter transgene expression downstream of a TRE promoter from adenoviral vectors in vitro and also in vivo.

Rescue and production of vaccine and therapeutic adenovirus vectors expressing inhibitory transgenes

Expression of certain transgenes from an adenovirus vector can be deleterious to its own replication. This can result in the inhibition of virus rescue, reduced viral yields, or, in the worst case, make it impossible to construct a vector expressing the inhibiting transgene product. A gene regulation system based on the tet operon was used to allow the rescue and efficient growth of adenovectors that express transgenes to high levels. A key advantage to this system is that repression of transgene expression is mediated by the packaging cell line, thus, expression of regulatory products from the adenovector are not required. This provides a simple, broadly applicable system wherein transgene repression is constitutive during vector rescue and growth and there is no effect on adenovector-mediated expression of gene products in transduced cells. Several high-level expression vectors based on first- and second-generation adenovectors were rescued and produced to high titer that otherwise could not be grown. Yields of adenovectors expressing inhibitory transgene products were increased, and the overgrowth of cultures by adenovectors with nonfunctional expression cassettes was prevented. The gene regulation system is a significant advancement for the development of adenovirus vectors for vaccine and other gene transfer applications.

Synthetic tetracycline-inducible regulatory networks: computer-aided design of dynamic phenotypes

BACKGROUND

Tightly regulated gene networks, precisely controlling the expression of protein molecules, have received considerable interest by the biomedical community due to their promising applications. Among the most well studied inducible transcription systems are the tetracycline regulatory expression systems based on the tetracycline resistance operon of Escherichia coli, Tet-Off (tTA) and Tet-On (rtTA). Despite their initial success and improved designs, limitations still persist, such as low inducer sensitivity. Instead of looking at these networks statically, and simply changing or mutating the promoter and operator regions with trial and error, a systematic investigation of the dynamic behavior of the network can result in rational design of regulatory gene expression systems. Sophisticated algorithms can accurately capture the dynamical behavior of gene networks. With computer aided design, we aim to improve the synthesis of regulatory networks and propose new designs that enable tighter control of expression.

RESULTS

In this paper we engineer novel networks by recombining existing genes or part of genes. We synthesize four novel regulatory networks based on the Tet-Off and Tet-On systems. We model all the known individual biomolecular interactions involved in transcription, translation, regulation and induction. With multiple time-scale stochastic-discrete and stochastic-continuous models we accurately capture the transient and steady state dynamics of these networks. Important biomolecular interactions are identified and the strength of the interactions engineered to satisfy design criteria. A set of clear design rules is developed and appropriate mutants of regulatory proteins and operator sites are proposed.

CONCLUSION

The complexity of biomolecular interactions is accurately captured through computer simulations. Computer simulations allow us to look into the molecular level, portray the dynamic behavior of gene regulatory networks and rationally engineer novel ones with useful applications. We are able to propose, test and accept or reject design principles for each network. Guided by simulations, we develop a set of design principles for novel tetracycline-inducible networks.

Regulatable gene expression systems for gene therapy

It is feasible to restrict transgene expression to a tissue or region in need of therapy by using promoters that respond to focusable physical stimuli. The most extensively investigated promoters of this type are radiation-inducible promoters and heat shock protein gene promoters that can be activated by directed, transient heat. Temporal regulation of transgenes can be achieved by various two- or three-component gene switches that are triggered by an appropriate small molecule inducer. The most commonly considered gene switches that are reviewed herein are based on small molecule-responsive transactivators derived from bacterial tetracycline repressor, insect or mammalian steroid receptors, or mammalian FKBP12/FRAP. A new generation of gene switches combines a heat shock protein gene promoter and a small molecule-responsive gene switch and can provide for both spatial and temporal regulation of transgene activity.

Inhibition of Apoptosis Reduces Immunogeneic Potential of Adenoviral-Treated Syngeneic Liver Grafts

Effects of adenoviral therapy and reduced apoptosis on immune response were investigated in a rat liver transplantation model after prolonged ischemia-reperfusion. Liver donors were treated i.v. either with an adenoviral construct, expressing bcl-2, green-fluorescent-protein, or doxycyclin. Intrahepatic apoptosis was assessed by terminal transferase dUTP nick end labeling assay. The intrahepatic presence of CD4, CD8a, CD163, immunoglobulin (Ig)beta, tumor necrosis factor (TNF)-alpha and myeloperoxidase (MPO) was quantified by realtime polymerase chain reaction at 24 hours and seven days after transplantation. Bcl-2 expression abrogated the TNF-alpha elevation and reduced apoptosis of hepatocytes and sinusoidal endothelial cells as compared to advCMV green fluorescent protein. No effects on CD4, CD8a, CD163 and MPO expression were noticed in bcl-2 pretreated livers, whereas Igbeta was slightly enhanced compared to controls. Adenoviral infected liver grafts trigger an immune response but reduced apoptosis resulted in down-regulation of TNF-alpha. Thus, bcl-2 transfer might simultaneously reduce graft ischemia reperfusion injury and immunogenicity.

Conditional expression of interferon-γ to enhance host responses to pulmonary bacterial infection

Strategies to augment host defense against pulmonary infection run the risk of inducing excess pulmonary inflammation and tissue injury. To address this problem, we investigated conditional expression in lung tissue of the murine interferon-gamma (IFN-gamma) transgene. A recombinant adenoviral vector (AdTetIFN) was constructed by placing a murine IFN-gamma cDNA downstream of a tetracycline (Tet)-responsive promoter, inserted into a replication-defective adenoviral vector. Co-infection of target cells with AdTetIFN and a second vector encoding a reverse tetracycline controlled transactivator allowed doxycycline (Dox)-regulated IFN-gamma production. We then administered 10(8) plaque-forming units (PFU) of AdTetIFN to mice by intratracheal injection. When the mice were provided with Dox in drinking water (0.5mg/ml in 5% sucrose), there was significant release of IFN-gamma in lavage fluid by ELISA in comparison to mice on water/sucrose alone (399+/-74 pg/ml vs undetectable, p<0.01). IFN-gamma in lavage fluid was associated with upregulation of Class II Major histocompatibility complex markers on alveolar macrophages by flow cytometry, suggesting macrophage activation. We then injected AdTetIFN into mice three days prior to challenge with 10(4) CFU Klebsiella pneumoniae. Test mice were maintained on water+Dox and control mice on water+sucrose. Bacterial burden was assayed in lung tissue at serial intervals. At 24h after challenge, mice on doxycycline had significantly lower infection burden in comparison to mice on water/sucrose (0.77+/-0.05 colony forming units/lung for 10(8) PFU AdTetIFN plus Dox compared to 1.4+/-0.11 colony-forming units/lung for AdTetIFN without Dox, p<0.05). Survival of the vector treated mice given doxycycline in drinking water was also enhanced. Microscopic examination of lavaged cells showed a significant increase in pulmonary neutrophils in the AdTetIFN+Dox mice in comparison to AdTetIFN+sucrose mice (16+/-1.0 x 10(5) vs 10+0.8 cells/lung, p<0.05). We conclude that local release of IFN-gamma can be selectively activated to enhance neutrophil recruitment and host resistance to bacterial pneumonia.

Cellular splicing and transcription regulatory protein p32 represses adenovirus major late transcription and causes hyperphosphorylation of RNA polymerase II

The cellular protein p32 is a multifunctional protein, which has been shown to interact with a large number of cellular and viral proteins and to regulate several important activities like transcription and RNA splicing. We have previously shown that p32 regulates RNA splicing by binding and inhibiting the essential SR protein ASF/SF2. To determine whether p32 also functions as a regulator of splicing in virus-infected cells, we constructed a recombinant adenovirus expressing p32 under the transcriptional control of an inducible promoter. Much to our surprise the results showed that p32 overexpression effectively blocked mRNA and protein expression from the adenovirus major late transcription unit (MLTU). Interestingly, the p32-mediated inhibition of MLTU transcription was accompanied by an approximately 4.5-fold increase in Ser 5 phosphorylation and an approximately 2-fold increase in Ser 2 phosphorylation of the carboxy-terminal domain (CTD). Further, in p32-overexpressing cells the efficiency of RNA polymerase elongation was reduced approximately twofold, resulting in a decrease in the number of polymerase molecules that reached the end of the major late L1 transcription unit. We further show that p32 stimulates CTD phosphorylation in vitro. The inhibitory effect of p32 on MLTU transcription appears to require the CAAT box element in the major late promoter, suggesting that p32 may become tethered to the MLTU via an interaction with the CAAT box binding transcription factor.

Adenoviral bcl-2 transfer improves survival and early graft function after ischemia and reperfusion in rat liver transplantation

BACKGROUND

Primary graft dysfunction due to ischemia and reperfusion injury represents a major problem in liver transplantation. The related cell stress may induce apoptosis, which can be suppressed by bcl-2. The purpose of the study was to investigate the effect of adenoviral bcl-2 gene transfer on early graft function and survival in rat liver transplantation.

METHODS

An adenoviral construct that transfers bcl-2 under the control of a tetracycline inducible promoter was generated (advTetOn bcl-2) and used with a second adenovirus that transfers the repressor protein (advCMV Rep). Forty-eight hours before explantation, donor rats were treated with advTetOn bcl-2/ advCMV Rep (n=7) and doxycyclin, with the control adenoviral construct advCMV GFP (n=8) or with doxycyclin alone (n=8). Liver transplantation was performed following 16 hours of cold storage (UW). Bcl-2 expression and intrahepatic apoptosis was assessed. Bile flow was monitored 90 min posttransplantation. The endpoint for survival was 7 days.

RESULTS

Bcl-2 was expressed in hepatocytes and sinusoidal lining cells. This was associated with a significant reduction of apoptotic sinusoidal lining cells and hepatocytes after 24 hours and 7 days. Bile production was significantly higher following bcl-2 pretreatment. Furthermore, bcl-2 transfer resulted in significantly improved survival (100% vs. 50% both control groups).

CONCLUSIONS

Adenoviral bcl-2 transfer results in protein expression in hepatocytes and sinusoidal lining cells resulting in early graft function and survival enhancement after prolonged ischemia and reperfusion injury. The inhibition of apoptosis in the context of liver transplantation might be a reasonable approach in the treatment of graft dysfunction.

Regulatable gene expression systems for gene therapy applications: progress and future challenges

Gene therapy aims to revert diseased phenotypes by the use of both viral and nonviral gene delivery systems. Substantial progress has been made in making gene transfer vehicles more efficient, less toxic, and nonimmunogenic and in allowing long-term transgene expression. One of the key issues in successfully implementing gene therapies in the clinical setting is to be able to regulate gene expression very tightly and consistently as and when it is needed. The regulation ought to be achievable using a compound that should be nontoxic, be able to penetrate into the desired target tissue or organ, and have a half-life of a few hours (as opposed to minutes or days) so that when withdrawn or added (depending on the regulatable system used) gene expression can be turned "on" or "off" quickly and effectively. Also, the genetic switches employed should ideally be nonimmunogenic in the host. The ability to switch transgenes on and off would be of paramount importance not only when the therapy is no longer needed, but also in the case of the development of adverse side effects to the therapy. Many regulatable systems are currently under development and some, i.e., the tetracycline-dependent transcriptional switch, have been used successfully for in vivo preclinical applications. Despite this, there are no examples of switches that have been employed in a human clinical trial. In this review, we aim to highlight the main regulatable systems currently under development, the gene transfer systems employed for their expression, and also the preclinical models in which they have been used successfully. We also discuss the substantial challenges that still remain before these regulatable switches can be employed in the clinical setting.

Binary AdEasy vector systems designed for Tet-ON or Tet-OFF regulated control of transgene expression

Here, we describe the construction of a set of binary adenovirus vectors encoding for a tetracycline-regulatable expression cassette and the Tet-ON or the Tet-OFF transcriptional activator proteins from a single viral chromosome. The rabies virus glycoprotein was cloned into the E1 region and the tetracycline activator proteins were inserted in both orientation in place of the E3 region. To further restrict background transcription, we also introduced a Lac repressor protein based roadblock to transcription elongation. To make the system more versatile it has been engineered into the commonly used AdEasy system. We show that rabies virus glycoprotein expression is tightly regulated with an essentially undetectable basal expression and a several 100-fold induced expression. In our vector backbone, the Tet-ON and the Tet-OFF systems appears to work with essentially the same efficiency. Thus, the choice of principle can be based on whether a positive or negative regulation of reporter gene activity is desirable. Taken together our results suggest that the binary vectors described here should be a valuable addition to the repertoire of viral vectors used in basic and medical research.

Liver-specific expression of interferon gamma following adenoviral gene transfer controls hepatitis B virus replication in mice

Interferons control viral replication and the growth of some malignant tumors. Since systemic application may cause severe adverse effects, tissue-specific expression is an attractive alternative. Liver-directed interferon gene therapy offers promising applications such as chronic viral hepatitis B or C or hepatocellular carcinoma and thus needs testing in vivo in suitable animal models. We therefore used the Tet-On system to regulate gene expression in adenoviral vectors, and studied the effect of liver-specific and regulated interferon gamma expression in a mouse model of chronic hepatitis B virus (HBV) infection. In a first generation adenoviral vector, genes encoding for firefly luciferase and interferons alpha, beta or gamma, respectively, were coexpressed under control of the bidirectional tetracycline-regulated promoter P(tet)bi. Liver-specific promoters driving expression of the reverse tetracycline controlled transactivator ensured local expression in the livers of HBV transgenic mice. Following gene transfer, we demonstrated low background, tight regulation and a 1000-fold induction of gene expression by doxycycline. Both genes within the bidirectional transcription unit were expressed simultaneously, and in a liver-specific fashion in cell culture and in living mice. Doxycycline-dependent interferon gamma expression effectively controlled HBV replication in mice, but did not eliminate HBV transcripts. This system will help to study the effects of local cytokine expression in mouse disease models in detail.

Adenoviral-mediated transduction of human pancreatic islets: importance of adenoviral genome for cell viability and association with a deficient antiviral response

As adenoviral vectors are extensively used for genetic manipulation of insulin-producing cells in vitro, there is an increasing need to evaluate their effects on the function, morphology, and viability of transduced pancreatic islets. In the present study we observed that specific adenoviral genotypes, carrying E4 and E1/E3 deletions, correlate with differential induction of necrosis in pancreatic islet cells. In particular, the adenovirus death protein encoded from the E3 region of the adenoviral genome was able to modulate the changes induced in the morphology and viability of the transduced cells. We also propose a putative role for the transcriptional regulator pIX. Although human islet cells showed an increased resistance in terms of viral concentrations required for the induction of cell toxicity, our results showed that they were unable to build up an efficient antiviral response after transduction and that their survival was dependent on the exogenous addition of alpha-interferon. An intact and fully functional beta-cell is crucial for the successful application of gene therapy approaches in type 1 diabetes, and therefore, the implications of our findings need to be considered when designing vectors for gene transfer into pancreatic beta-cells.

A novel binary adenovirus-based dual-regulated expression system for independent transcription control of two different transgenes

BACKGROUND

Stringent multitransgene control is a prerequisite for future gene-therapy and tissue-engineering scenarios and requires constant improvements in design to achieve optimal conditional transcription profiles.

METHODS

We have pioneered a variety of recombinant adenoviruses which (i) enable streptogramin-responsive transgene transduction in a compact autoregulated one-virus format, (ii) manage independent streptogramin- and tetracycline-responsive control of two different transgenes from a single divergent expression unit, and (iii) control sense and antisense expression of the human cyclin-dependent kinase inhibitor p27(Kip1) to engineer conditional positive (enforced S-phase entry, p27(Kip1)-antisense expression) or negative (G1-phase-specific growth arrest, p27(Kip1)-sense expression) growth control in mammalian cell lines and human primary cells.

RESULTS

The transgene control performance of all adenoviral expression configurations has been rigorously optimized for tight, balanced and maximum expression levels and was validated for intracellular as well as for secreted product in a variety of biotechnologically relevant cell lines (Chinese hamster ovary cells [CHO-K1], baby hamster kidney cells [BHK-21]) as well as in human cell lines (human fibrosarcoma cells [HT-1080]) and primary cells (human aortic fibroblasts [HAFs]).

CONCLUSIONS

We believe that multiregulated multigene-controlled adenoviruses are important assets for successful therapeutic reprogramming of mammalian cells in clinically relevant scenarios.

Expression and analysis of the Epstein-Barr virus BARF1-encoded protein from a tetracycline-regulatable adenovirus system

Epstein-Barr virus (EBV) has been associated with human cancers of lymphocytic or epithelial origin. Potential functions of the BARF1 early gene in EBV oncogenesis emerged from our observations showing expression of BARF1-encoded protein in nasopharyngeal carcinoma biopsies, and induction of either malignant transformation (in rodent fibroblast and human B cell lines) or immortalization (in monkey primary epithelial cells) following BARF1 transfection. We previously reported expression of the BARF1 product as a cytoplasm/membrane-associated protein from 293-tTA cells infected with a BARF1-recombinant adenovirus. Since constitutive expression of BARF1 from this heterologous system became inefficient, we developed a tetracycline-regulatable recombinant vector expressing BARF1 and green fluorescent protein from a dicistronic message. As here reported, stable and efficient expression of BARF1 from this vector in either permissive or non-permissive cell lines, allowed the first sequencing identification and further molecular characterization of BARF1-encoded protein.

Gene delivery in bone tissue engineering: progress and prospects using viral and nonviral strategies

Bone tissue loss as a consequence of the natural aging process or as a result of trauma and degenerative disease has led to the need for procedures to generate cartilage and bone for a variety of orthopedic applications. The ability to transfer genes into multipotential mesenchymal stem cells, while still in its infancy, offers considerable therapeutic hope in a variety of musculoskeletal disorders. However, the choice of gene delivery method is key. This review examines the various techniques and methods currently available to enable gene transfer into a target population from viral methods (transduction) to nonviral (transfection) methods and the limitations associated with each method. The potential applications and current understanding of each method are presented. Given the demographic challenge of an aging population, the ultimate goal remains the development of simple, safe, and reproducible strategies for gene delivery that will address the pressing orthopedic clinical imperatives of many.

Stable and strictly controlled expression of LTR-flanked autoregulated expression cassettes upon adenoviral transfer

An autoregulatory bidirectional expression cassette encoding all components necessary for regulated gene expression in a one-step gene transfer was evaluated for use in adenoviral vectors. Adenoviral vectors transducing this cassette provide about 1000-fold regulation. Regulation could be further improved by integrating the cassette as a retroviral vector into the adenoviral backbone. Moreover, with these adeno/retroviral hybrid vectors, the frequency of chromosomal integration is enhanced and about 1% of infected cells show stable chromosomal integration of the autoregulated cassette. In these stably transduced cells high regulation capacity is maintained. To elucidate the molecular mechanism underlying this unexpected observation we investigated the regulation capacity of these cassettes in a viral and non-viral vector background after stable integration into the host's DNA. While naked cassettes show regulated expression that is strongly influenced by the chromosomal surrounding sequences the regulatory capacity of LTR flanked cassettes is highly comparable amongst different cell clones. This strict regulation with little influence from the flanking sequences is obtained when LTR-flanked cassettes are transduced as DNA, by retroviral or by adenoviral infection.

A tetracycline-regulated adenoviral expression system for in vivo delivery of transgenes to lung and liver

BACKGROUND

Recombinant adenoviruses are an established tool for somatic gene transfer to multiple cell types in animals as well as in tissue culture. However, generation of adenoviruses expressing transgenes that are potentially toxic to the host cell line represents a practical problem. The aim of this study was to construct an adenoviral expression system that prevents transgene expression during the generation and propagation of the virus, and allows efficient gene transfer to lung and liver, major target organs of gene therapy.

METHODS

Using the tet-off system we constructed tetracycline (tet) regulatable recombinant adenoviruses expressing the marker gene LacZ (Adtet-off.LacZ) as well as a secretory protein, human group IIA secretory phospholipase A(2) (Adtet-off.hsPLA(2)). Expression (Western blot, activity assay) was tested in vitro (HeLa cells), and in vivo by gene transfer to lung and liver.

RESULTS

Without addition of tetracycline we demonstrated expression of LacZ (Adtet-off.LacZ) and sPLA(2) (Adtet-off.hsPLA(2)) in HeLa cells. Providing additional tet-transactivator (tTA) protein either by stable transfection or coinfection with a tTA-expressing adenovirus resulted in a further increase of LacZ and sPLA(2) expression. Transgene expression in vitro was eliminated by the addition of tetracycline to the culture medium. Adtet-off.LacZ and Adtet-off.hsPLA(2) allowed successful gene transfer in vivo to lung and liver. While the expression was highly efficient within the lungs, however, additional tTA was necessary to achieve high-level expression within liver.

CONCLUSIONS

Tet-regulatable adenoviral expression systems may facilitate the construction of recombinant adenoviruses encoding potentially toxic transgenes and permit regulated transgene expression.

gamma-Tubulin overexpression in Sertoli cells in vivo: I. Localization to sites of spermatid head attachment and alterations in Sertoli cell microtubule distribution

Sertoli cells play a number of roles in supporting spermatogenesis, including structural organization, physical and paracrine support of germ cells, and secretion of factors necessary for germ cell development. Studies with microtubule disrupting compounds indicate that intact microtubule networks are crucial for normal spermatogenesis. However, treatment with toxicants and pharmacologic agents that target microtubules lack cell-type selectivity and may therefore elicit direct effects on germ cells, which also require microtubule-mediated activities for division and morphological transformation. To evaluate the importance of Sertoli cell microtubule-based activities for spermatogenesis, an adenoviral vector that overexpresses the microtubule nucleating protein, gamma-tubulin, was used to selectively disrupt microtubule networks in Sertoli cells in vivo. gamma-Tubulin overexpression was observed to cause redistribution of Sertoli cell microtubule networks, and overexpression of a gamma-tubulin-enhanced green fluorescent protein fusion protein was observed to localize to the site of elongate spermatid head attachment to the seminiferous epithelium.

"Blue heart": characterization of a mifepristone-dependent system for conditional gene expression in genetically modified animals

A detailed characterization of a cardiac muscle-specific, ligand-regulated gene expression system was performed in transgenic mice using the inducing ligand mifepristone (MFP). Several lines of double transgenic mice were created that expressed a bacterial lacZ reporter gene in the heart, under the control of a MFP-activated transcription factor constitutively expressed in cardiac muscle. The transgenic mice, which were administered MFP at a dose of 1 micromol/l in the drinking water, responded to the ligand within 24 h. Induction of beta-galactosidase enzyme activity in the heart continued for up to 21 days and resulted in an average 17-fold increase in enzyme activity. The highest individual animal response measured was a 94-fold increase in enzyme activity. The EC(50) for MFP induction of beta-galactosidase activity in the heart was 0.7 micromol/l when MFP was administered in the drinking water. Pharmacokinetic analysis of MFP dosing in wild-type FVB/N mice showed that absorption was very rapid (T(max) 1-10 min), bioavailability was modest ( approximately 10%) and the t(1/2) of MFP in mouse plasma was determined to be approximately 5 h. Thus, the system functions effectively in transgenic mouse heart where induction of gene expression is sensitive and can be accomplished by a simple and broadly applicable drinking water protocol.

Highly suppressible expression of single-chain interleukin-12 by doxycycline following adenoviral infection with a single-vector Tet-regulatory system

BACKGROUND

Adenoviral vectors have been shown to efficiently transfer DNA into a wide variety of eukaryotic cells in vitro and in vivo. However, the therapeutic benefit of this approach is limited by severe side effects as a result of uncontrolled transgene expression.

METHODS

A bi-directional promoter that controls the desired transgene as well as a tetracycline-suppressible transactivator (tTA) was cloned into the E1-region of E1-deleted recombinant adenoviral vectors. Autoregulation within this construct was obtained by tTA expression under control of the operator, to which tTA binds in the absence of tetracycline. Consequently, binding of tetracycline to tTA results in downregulation of tTA as well as the co-expressed transgene in the infected cell.

RESULTS

We were able to suppress luciferase-reporter gene expression by up to 16 000-fold in the presence of doxycycline (dox, 2 micro g/ml). Under control of this tetracycline-regulated system, single-chain interleukin-12 (scIL12) was expressed. Adenovirally mediated expression of this potentially lethal cytokine with strong activation of antitumoral immune response was downregulated by up to 6000-fold in the presence of dox. Subsequently, this downregulation also resulted in a highly significant reduction of interferon-gamma secretion by stimulated splenocytes. These mainly contribute to the toxicity of this immunotherapeutic approach.

CONCLUSIONS

With expression levels exceeding those of the cytomegalovirus (CMV) promoter in almost all cell lines tested, these new vectors will also contribute to the safety of adenoviral approaches by controlled expression without compromising on maximum expression levels.

Regulation of gene expression in adeno-associated virus vectors in the brain

Regulated adeno-associated virus (AAV) vectors have broad utility in both experimental and applied gene therapy, and to date, several regulation systems have exhibited a capability to control gene expression from viral vectors over two orders of magnitude. The tetracycline responsive system has been the most used in AAV, although other regulation systems such as RU486- and rapamycin-responsive systems are reasonable options. AAV vectors influence how regulation systems function by several mechanisms, leading to increased background gene expression and restricted induction. Methods to reduce background expression continue to be explored and systems not yet tried in AAV may prove quite functional. Although regulated promoters are often assumed to exhibit ubiquitous expression, the tropism of different neuronal subtypes can be altered dramatically by changing promoters in recombinant AAV vectors. Differences in promoter-directed tropism have significant consequences for proper expression of gene products as well as the utility of dual vector regulation. Thus regulated vector systems must be carefully optimized for each application.

The tet-off system is more effective than the tet-on system for regulating transgene expression in a single adenovirus vector

BACKGROUND

Control of transgene expression in mammalian cells is desirable for gene therapy and the study of gene function in basic research. This study evaluates the functionality of single adenovirus (Ad) vectors containing a tetracycline-controllable expression system (tet-off or tet-on system).

METHODS

An Ad-mediated binary transgene expression system was generated containing a tet-off or a tet-on system, which introduced the gene of interest with a tetracycline-regulatable promoter and the tetracycline-responsive transcriptional activator gene into the E1 and E3 deletion regions, respectively. The functionality of the Ad-mediated tet-off and tet-on systems was compared under various conditions in vitro and in vivo.

RESULTS

The Ad vector containing the tet-off system provided negative control of gene expression ranging from 20- to 500-fold, depending on the cell type and condition. In contrast, the Ad vector containing the tet-on system increased gene expression by only 2- to 28-fold and required about two-log orders higher concentration of an inducer (doxycycline) to switch on the gene expression, compared with the Ad vector containing the tet-off system.

CONCLUSION

Ad vectors containing the tet-off system are a better choice for regulated gene expression than Ad vectors containing the tet-on system. Single Ad vectors containing the tetracycline-controllable expression system will greatly facilitate in vitro and in vivo analyses of gene function and may be useful for gene therapy.

Ligand-inducible transgene regulation for gene therapy

A synthetic ligand regulable system for gene transfer and expression has been developed in our laboratory based on mechanistic studies of steriod hormone receptor and transcriptional regulation. This gene switch system possesses most of the important features that are required for application of the system in biological research and clinical gene therapy in the future. As the primary ligand tested in this system, mifepristone can effectively turn on the regulatory circuit at doses much lower than those used in the clinic. By modification of the chimeric regulator and its feedback regulatory mode, this system has been optimized to produce very low basal activity with high inducibility in the presence of mifepristone. Also, improvements in regulator composition have been made to minimize immunogenicity and make the system more amenable to human gene therapy. Moreover, incorporation of this gene switch system into the HC-Ad vector system has further enhanced the efficiency of gene transfer and the long-term inducible expression of transgenes. However, for each application within a different biological system, the gene switch needs to be optimized to achieve appropriate inductions. In particular, the method used to deliver the transgenes and adjustment of ligand dosage are critical for in vivo gene expression.

Establishment of an artificial β-cell line expressing insulin under the control of doxycycline

AIM: Artificial β-cell lines may offer an abundant source of cells for the treatment of type I diabetes, but insulin secretion in β-cells is tightly regulated in physiological conditions. The Tet-On system is a “gene switch” system, which can induce gene expression by administration of tetracycline (Tet) derivatives such as doxcycline (Dox). Using this system, we established 293 cells to an artificial cell line secreting insulin in response to stimulation by Dox.

METHODS: The mutated proinsulin cDNA was obtained from plasmid pcDNA3.1/C-mINS by the polymerase chain reaction (PCR), and was inserted downstream from the promoter on the expression vector pTRE2, to construct a recombined expression vector pTRE2mINS. The promoter on pTRE2 consists of the tetracycline-response element and the CMV minimal promoter and is thus activated by the reverse tetracycline-controlled transactivator (rtTA) when Dox is administrated. pTRE2mINS and plasmid pTK-Hyg encoding hygromycin were co-transfected in the tet293 cells, which express rtTA stably. Following hygromycin screening, the survived cells expressing insulin were selected and enriched. Dox was used to control the expression of insulin in these cells. At the levels of mRNA and protein, the regulating effect of Dox in culture medium on the expression of proinsulin gene was estimated respectively with Northern blot, RT-PCR, and radioimmunoassay.

RESULTS: From the 28 hygromycin-resistant cell strains, we selected one cell strain (tet293/Ins6) secreting insulin not only automatically, but in response to stimulation by Dox. The amount on insulin secretion was dependent on the Dox dose (0, 10, 100, 200, 400, 800 and 1000 μg•L⁻¹), the level of insulin secreted by the cells treated with Dox (1000 μg·L^-1) was 241.0 pU·d^-1× cell^-1, which was 25-fold that of 9.7 pU·d^-1× cell^-1 without Dox treatment. Northern blot analyses and RT-PCR further confirmed that the transcription of insulin gene had already been up-regulated after exposing tet293/Ins6 cells to Dox for 15 min, and was also induced in a dose-dependent manner. However, the concentration of insulin in the media did not increase significantly until 5 h following the addition of Dox.

CONCLUSION: Human proinsulin gene was transfected successfully and expressed efficiently in 293 cells, and the expression was modulated by tetracycline and its derivatives, improving the accuracy, safety, and reliability of gene therapy, suggesting that conditional establishment of artificial β-cells may be a useful approach to develop cellular therapy for diabetes mellitus.

Splicing inhibition at the level of spliceosome assembly in the presence of herpes simplex virus protein ICP27

Herpes simplex virus (HSV) immediate-early protein ICP27 is a multifunctional regulator of viral and cellular gene expression. It has previously been shown that ICP27 directly or indirectly modulates several posttranscriptional processes, such as pre-mRNA splicing and polyadenylation. We show here that pre-mRNA splicing is inhibited in nuclear extracts prepared from cells in which ICP27 has been transiently expressed. Our results show that splicing inhibition in ICP27 extracts is manifested at early stages of the splicing process. Furthermore, our results suggest that an enzymatic activity in ICP27-containing extracts causes the splicing inhibition.

Unscheduled expression of capsid protein IIIa results in defects in adenovirus major late mRNA and protein expression

Adenovirus gene expression is to a large extent regulated at the level of alternative RNA splicing. For example, in the major late region 1 (L1) unit, a common 5' splice site can be joined to two alternative 3' splice sites, resulting in the formation of the so-called 52,55K (proximal 3' splice site) or the IIIa (distal 3' splice site) mRNAs. Whereas, the 52,55K mRNA is expressed both early and late during infection, the IIIa mRNA is strictly confined to the late phase of the infectious cycle. We have previously shown that IIIa mRNA splicing is subjected to a tight viral control of IIIa 3 splice site usage. In an attempt to determine why adenovirus uses elaborate mechanisms to confine IIIa mRNA production to the late phase of infection, we characterized the phenotype of a recombinant adenovirus expressing the IIIa protein from an inducible tetracycline regulated gene cassette. The results show that expression of the IIIa protein during the early phase of infection results in a significant reduction in late viral protein synthesis and a moderate block to viral DNA replication. Interestingly, unscheduled IIIa protein expression resulted in a perturbation of the accumulation of alternatively spliced L1 mRNAs. Thus, 52,55K mRNA accumulation was inhibited while no effects on endogenous IIIa mRNA expression was detected.

Stringent control of gene expression in vivo by using novel doxycycline-dependent trans-activators

The tetracycline (Tet)-dependent regulatory system has been widely used for controlling gene expression. The Tet-on version of the system, in which the reverse Tet-responsive transcriptional activator (rtTA) is positively regulated by Tet or its analogs, such as doxycycline (Dox), is of potential utility for gene therapy applications in humans. However, rtTA may display a high basal activity, especially when delivered in vivo by using episomal vectors such as plasmids. Two novel Dox-inducible activators, called rtTA2(S)-S2 and rtTA2(S)-M2, which have a significantly lower basal activity than rtTA in stably transfected cell lines, have been described. In this study we tested the capability of these trans-activators to control expression of mouse erythropoietin (mEpo) and to modulate hematocrit (Hct) increase in vivo on delivery of plasmids into quadriceps muscles of adult mice by DNA electroinjection. Both rtTA2(S)-M2 and rtTA2(S)-S2 displayed a considerably lower background activity and higher window of induction than rtTA in vivo. Moreover, a stringent control of mEpo gene expression and Hct levels in the absence of any background activity was maintained over a 10-month period by injecting as little as 1 microg of a single plasmid containing the rtTA2(S)-S2 expression cassette and the Tet-responsive mEpo cDNA. This constitutes the first report of a stringent ligand-dependent control of gene expression in vivo obtained by delivering a single plasmid encoding both the trans-activator and the regulated gene. Notably, the rtTA2(S)-S2-based system was induced by oral doses of doxycycline comparable to those normally used in clinical practice in humans.

Characteristics of adenovirus-mediated tetracycline-controllable expression system

The combination of recombinant adenovirus (Ad) vectors and the tetracycline-controllable expression system is clearly an advantage in gene therapy and gene transfer experiment. In this study, we examined the characteristics of Ad vectors containing the tet-off or tet-on system. The Ad vector containing the tet-off system showed tightly regulatable transgene expression even at low MOI (multiplicity of infection). In contrast, regulation of gene expression by the Ad vector containing the tet-on system was not tight at low MOI, while it showed moderate regulation at high MOI (MOI=100). The Ad vector-mediated tet-on system showed lower inducible and higher background (basal) luciferase production than that of the Ad vector-mediated tet-off system. Moreover, the former system required a concentration of doxycycline, a derivative of tetracycline, approx. 2-3 log orders higher than that of the latter system to switch the luciferase expression. A combination of the vector containing the tet-on system and the vector containing the tetracycline-controlled transcriptional silencer (tTS) gene reduced the background luciferase production and improved regulation. These results suggest that the Ad vector containing the tet-off system is considered to be functionally superior to the vector containing the tet-on system. Care should be taken regarding regulation (especially lower inducibility and higher background), which is decreased in the Ad vector-mediated tet-on system in comparison with the tet-off system. The Ad vector containing the tetracycline-controllable expression system should offer a powerful tool for gene therapy and gene transfer experimentation.

Adenovirus vector designed for expression of toxic proteins

To construct recombinant adenoviruses expressing biologically active proteins may be impossible, or result in a significant reduction in virus yield, if the protein expressed has an inhibitory effect on virus replication or cellular growth. To overcome this problem, we previously designed adenovirus vectors expressing foreign proteins from inducible promoters. However, during our work with a replication-deficient virus expressing the ASF/SF2 splicing factor from a progesterone antagonist-inducible gene cassette, we discovered that ASF/SF2 was expressed at a significant level in the 293 producer cell line, even in the absence of inducer. 293 cells code for adenovirus E1A and E1B proteins and thus support the growth of E1-deficient adenoviruses. Here we show that this background ASF/SF2 expression results from a low level of E1A-mediated transactivation of the basal promoter driving transgene expression. To overcome the problem of leaky expression, we reconstructed a novel gene cassette that combines an inducible promoter and a Lac repressor protein-based block to reduce transcriptional elongation. We show that this novel vector system dramatically reduced background transgene expression and therefore should be useful for the rescue and propagation of high-titer stocks of recombinant adenoviruses expressing toxic proteins.

ICP0, ICP4, or VP16 expressed from adenovirus vectors induces reactivation of latent herpes simplex virus type 1 in primary cultures of latently infected trigeminal ganglion cells

In a previous study, we demonstrated that infected-cell polypeptide 0 (ICP0) is necessary for the efficient reactivation of herpes simplex virus type 1 (HSV-1) in primary cultures of latently infected trigeminal ganglion (TG) cells (W. P. Halford and P. A. Schaffer, J. Virol. 75:3240-3249, 2001). The present study was undertaken to determine whether ICP0 is sufficient to trigger HSV-1 reactivation in latently infected TG cells. To test this hypothesis, replication-defective adenovirus vectors that express wild-type and mutant forms of ICP0 under the control of a tetracycline response element (TRE) promoter were constructed. Similar adenovirus vectors encoding wild-type ICP4, wild-type and mutant forms of the HSV-1 origin-binding protein (OBP), and wild-type and mutant forms of VP16 were also constructed. The TRE promoter was induced by coinfection of Vero cells with the test vector and an adenovirus vector that expresses the reverse tetracycline-regulated transactivator in the presence of doxycycline. Northern blot analysis demonstrated that transcription of the OBP gene in the adenovirus expression vector increased as a function of doxycycline concentration over a range of 0.1 to 10 microM. Likewise, Western blot analysis demonstrated that addition of 3 microM doxycycline to adenovirus vector-infected Vero cells resulted in a 100-fold increase in OBP expression. Wild-type forms of ICP0, ICP4, OBP, and VP16 expressed from adenovirus vectors were functional based on their ability to complement plaque formation in Vero cells by replication-defective HSV-1 strains with mutations in these genes. Adenovirus vectors that express wild-type forms of ICP0, ICP4, or VP16 induced reactivation of HSV-1 in 86% +/- 5%, 86% +/- 5%, and 97% +/- 5% of TG cell cultures, respectively (means +/- standard deviations). In contrast, vectors that express wild-type OBP or mutant forms of ICP0, OBP, or VP16 induced reactivation in 5% +/- 5%, 8% +/- 0%, 0% +/- 0%, and 13% +/- 6% of TG cell cultures, respectively. In control infections, an adenovirus vector expressed green fluorescent protein efficiently in TG neurons but did not induce HSV-1 reactivation. Therefore, expression of ICP0, ICP4, or VP16 is sufficient to induce HSV-1 reactivation in latently infected TG cell cultures. We conclude that this system provides a powerful tool for determining which cellular and viral proteins are sufficient to induce HSV-1 reactivation from neuronal latency.

Cisplatin induces the proapoptotic conformation of Bak in a deltaMEKK1-dependent manner

In a panel of four human melanoma cell lines, equitoxic doses of cisplatin induced the proapoptotic conformation of the Bcl-2 family protein Bak prior to the execution phase of apoptosis. Because cisplatin-induced modulation of the related Bax protein was seen in only one cell line, a degree of specificity in the signal to Bak is indicated. Little is known about upstream regulation of Bak activity. In this study, we examined whether the apoptosis-specific pathway mediated by a kinase fragment of MEKK1 (DeltaMEKK1) is involved in the observed Bak modulation. We report that expression of a kinase-inactive fragment of MEKK1 (dominant negative MEKK [dnMEKK]) efficiently blocked cisplatin-induced modulation of Bak and cytochrome c release and consequently also reduced DEVDase activation and nuclear fragmentation. Accordingly, expression of a kinase-active MEKK1 fragment (dominant positive MEKK) was sufficient to induce modulation of Bak in three cell lines and to induce apoptosis in two of these. dnMEKK did not block cisplatin-induced c-Jun N-terminal kinase (JNK) activation, in agreement with a specifically proapoptotic role for the DeltaMEKK1 pathway. Finally, we show that reduction of Bak expression by antisense Bak reduced cisplatin-induced loss of mitochondrial integrity and caspase cleavage activity in breast cancer cell lines. In summary, we have identified Bak as a cisplatin-regulated component downstream in a proapoptotic, JNK-independent DeltaMEKK1 pathway.

Tetracycline-regulatable adenovirus vectors: pharmacologic properties and clinical potential

Stringent control of gene expression in human gene therapy strategies is important for both therapeutic and safety reasons. Replication-defective vectors derived from adenoviruses have been shown to be capable of highly efficient in vivo gene delivery to a wide variety of dividing and nondividing human cells. Here, we review the progress in the development of regulatable adenovirus vectors that allow gene expression to be tightly controlled by low concentrations of tetracyclines. As an example of the potential clinical utility of this technology, we highlight our results obtained in an immunotherapy model for prostate cancer with a tetracycline-regulatable adenovirus vector expressing the cytokine interleukin-12. Recombinant adenovirus vectors with tetracycline-regulatable gene expression provide new opportunities and improved safety for gene therapy applications in humans.

The adenovirus E4-ORF4 splicing enhancer protein interacts with a subset of phosphorylated SR proteins

SR proteins purified from uninfected HeLa cells inhibit adenovirus IIIa pre-mRNA splicing by binding to the intronic IIIa repressor element (3RE). In contrast, SR proteins purified from late adenovirus-infected cells are functionally inactivated as splicing repressor proteins by a virus-induced dephosphorylation. We have shown that the adenovirus E4-ORF4 protein, which binds the cellular protein phos phatase 2A (PP2A) and activates IIIa splicing in vitro and in vivo, induces SR protein dephosphorylation. Here we show that E4-ORF4 interacts with only a subset of SR proteins present in HeLa cells. Thus, E4-ORF4 interacts efficiently with SF2/ASF and SRp30c, but not with other SR proteins. Interestingly, E4-ORF4 interacts with SF2/ASF through the latter's RNA recognition motifs. Furthermore, E4-ORF4 interacts preferentially with the hyperphosphorylated form of SR proteins found in uninfected HeLa cells. E4-ORF4 mutant proteins that fail to bind strongly to PP2A or SF2/ASF do not relieve the repressive effect of HeLa SR proteins on IIIa pre-mRNA splicing in transient transfection experiments, suggesting that an interaction between all three proteins is required for E4-ORF4-induced SR protein dephosphorylation.

Overexpression of essential splicing factor ASF/SF2 blocks the temporal shift in adenovirus pre-mRNA splicing and reduces virus progeny formation

Expression of cytoplasmic mRNA from most adenovirus transcription units is subjected to a temporal regulation at the level of alternative pre-mRNA splicing. The general tendency is that splice site selection changes from proximal to distal late after infection. Interestingly, ASF/SF2, which is a prototypical member of the SR family of splicing factors, has the opposite effect on splice site selection, inducing an increase in proximal splice site usage. We have previously shown that SR proteins late during an adenovirus infection become partially inactivated as splicing regulatory proteins. A prediction from these results is that overexpression of an SR protein, such as ASF/SF2, during virus growth will interfere with virus replication by disturbing the balance of functional and nonfunctional ASF/SF2 in the infected cell. To test this hypothesis, we reconstructed a recombinant adenovirus expressing ASF/SF2 under the transcriptional control of a regulated promoter. The results show that, as predicted, induction of ASF/SF2 during lytic virus growth prevents the early to late shift in mRNA expression from both early (E1A and E1B) and late (L1) transcription units. Furthermore, ASF/SF2 overexpression blocks viral DNA replication and reduces selectively cytoplasmic accumulation of major late mRNA, resulting in a lower virus yield. Collectively, our results provide additional support for the hypothesis that viral control of SR protein function is important for the proper expression of viral proteins during lytic virus growth.

ERK signalling in metastatic human MDA-MB-231 breast carcinoma cells is adapted to obtain high urokinase expression and rapid cell proliferation

Increased urokinase plasminogen activator (u-PA) production is associated with tumor invasion and metastasis in several malignancies, including breast cancer. The mechanisms underlying constitutive u-PA expression are not well understood. We examined the relationship between the signal strength of the ERK pathway and the level of u-PA expression in the metastatic human breast cancer cell line MDA-MB-231. Treatment with the MEK1 inhibitor PD98059 resulted in decreased ERK1/2 phosphorylation and decreased u-PA mRNA and protein expression. Inhibition of ERK1/2 activity also led to decreased cell proliferation and to decreased cyclin D1 expression. Less than 5% of total ERK1/2 was phosphorylated in exponentially growing MDA-MB-231 cells, and ERK1/2 activity could be stimulated by okadaic acid. Okadaic acid did not stimulate u-PA expression, but induced strong expression of the cdk-inhibitor p21Cip1. These findings suggest that ERK1/2 signaling is tuned to a level which results in high u-PA expression and rapid cell proliferation.

Expression of wild-type and noncleavable Fas ligand by tetracycline-regulated adenoviral vectors to limit intimal hyperplasia in vascular lesions

Proliferation of vascular smooth muscle cells (VSMCs) and the infiltration of T cells and macrophages into vessel wall are considered to be important for intimal lesion formation after balloon angioplasty. Previous studies have shown that Fas ligand (FasL) gene transfer to balloon-injured vessels inhibits lesion formation by killing both proliferating VSMCs and infiltrating inflammatory cells. Here, we describe the construction and utility of a binary, tetracycline-regulated adenovirus system that provides controlled transgene expression in vitro and in vivo. In this system, optimal transgene expression required cotransfection with an adenovirus encoding the tetracycline-dependent trans-activator (rtTA) and induction with doxycycline hydrochloride (DOX), an analog of tetracycline. Using this system, adenovirus constructs were designed that allow regulated expression of wild-type FasL and a noncleavable mutant of FasL (FasL-NC). Transduction of FasL and FasL-NC induced similar extents of apoptosis in proliferating VSMCs in vitro in a manner that was dependent on the doses of the rtTA adenovirus and the presence of DOX in the medium. Furthermore, inhibition of intimal hyperplasia in injured carotid arteries by FasL or FasL-NC transduction was also dependent on cotransfection with the rtTA adenovirus and administration of DOX by subcutaneous injection. In contrast to wild-type FasL, transduction of FasL-NC did not result in the production of soluble (cleaved) FasL in the medium of infected cells in vitro, or in the serum of rats after local gene transfer to carotid arteries. In conclusion, this binary tetracycline-inducible adenovirus system may allow for safer delivery of cytotoxic genes for therapeutic purposes.

Adenovirus vectors for gene delivery

Recent endeavors in the development of adenovirus as a gene vector have focused on the modification of virus tropism, the accommodation of larger genes, and the increase in stability and control of transgene expression. Whereas partial or total deletions of viral genes increase the cloning capacity and partly reduce the cellular immune response, control of the humoral response, which often precludes efficient readministration, remains a challenge.