An improved Tet-On regulatable FasL-adenovirus vector system for lung cancer therapy

Oncolytic viruses in lung cancer treatment: a review article

Lung cancer has a high morbidity rate worldwide due to its resistance to therapy. So new treatment options are needed to improve the outcomes of lung cancer treatment. This study aimed to evaluate the effectiveness of oncolytic viruses (OVs) as a new type of cancer treatment. In this study, 158 articles from PubMed and Scopus from 1994 to 2022 were reviewed on the effectiveness of OVs in the treatment of lung cancer. The oncolytic properties of eight categories of OVs and their interactions with treatment options were investigated. OVs can be applied as a promising immunotherapy option, as they are reproduced selectively in different types of cancer cells, cause tumor cell lysis and trigger efficient immune responses.

Immortalization Reversibility in the Context of Cell Therapy Biosafety

Immortalization (genetically induced prevention of replicative senescence) is a promising approach to obtain cellular material for cell therapy or for bio-artificial organs aimed at overcoming the problem of donor material shortage. Immortalization is reversed before cells are used in vivo to allow cell differentiation into the mature phenotype and avoid tumorigenic effects of unlimited cell proliferation. However, there is no certainty that the process of de-immortalization is 100% effective and that it does not cause unwanted changes in the cell. In this review, we discuss various approaches to reversible immortalization, emphasizing their advantages and disadvantages in terms of biosafety. We describe the most promising approaches in improving the biosafety of reversibly immortalized cells: CRISPR/Cas9-mediated immortogene insertion, tamoxifen-mediated self-recombination, tools for selection of successfully immortalized cells, using a decellularized extracellular matrix, and ensuring post-transplant safety with the use of suicide genes. The last process may be used as an add-on for previously existing reversible immortalized cell lines.

Cell Immortalization: In Vivo Molecular Bases and In Vitro Techniques for Obtention

Somatic human cells can divide a finite number of times, a phenomenon known as the Hayflick limit. It is based on the progressive erosion of the telomeric ends each time the cell completes a replicative cycle. Given this problem, researchers need cell lines that do not enter the senescence phase after a certain number of divisions. In this way, more lasting studies can be carried out over time and avoid the tedious work involved in performing cell passes to fresh media. However, some cells have a high replicative potential, such as embryonic stem cells and cancer cells. To accomplish this, these cells express the enzyme telomerase or activate the mechanisms of alternative telomere elongation, which favors the maintenance of the length of their stable telomeres. Researchers have been able to develop cell immortalization technology by studying the cellular and molecular bases of both mechanisms and the genes involved in the control of the cell cycle. Through it, cells with infinite replicative capacity are obtained. To obtain them, viral oncogenes/oncoproteins, myc genes, ectopic expression of telomerase, and the manipulation of genes that regulate the cell cycle, such as p53 and Rb, have been used.

A prospect of cell immortalization combined with matrix microenvironmental optimization strategy for tissue engineering and regeneration

Cellular senescence is a major hurdle for primary cell-based tissue engineering and regenerative medicine. Telomere erosion, oxidative stress, the expression of oncogenes and the loss of tumor suppressor genes all may account for the cellular senescence process with the involvement of various signaling pathways. To establish immortalized cell lines for research and clinical use, strategies have been applied including internal genomic or external matrix microenvironment modification. Considering the potential risks of malignant transformation and tumorigenesis of genetic manipulation, environmental modification methods, especially the decellularized cell-deposited extracellular matrix (dECM)-based preconditioning strategy, appear to be promising for tissue engineering-aimed cell immortalization. Due to few review articles focusing on this topic, this review provides a summary of cell senescence and immortalization and discusses advantages and limitations of tissue engineering and regeneration with the use of immortalized cells as well as a potential rejuvenation strategy through combination with the dECM approach.

Construction and characterization of regulated cycle inhibiting factors induced upon Tet-On system in human colon cancer cell lines

A previous study has proven that cycle inhibiting factors (Cifs) inhibit Cullin E3 ubiquitin ligases, resulting in cell cycle arrest. More importantly, Cifs are also involved in cancer progression by deamidating Nedd8. Here we aimed to explore a novel insight into the treatment implications of Cifs in colon cancers by Tet-on system. The anticancer activity of Cif by doxycycline induction was investigated in the colon cell lines based upon Tet-On system. The expression of Cif in the colon cancer cells was determined by western blot. Furthermore, the cell viability and flow cytometry analysis were respectively performed to evaluate the cell proliferation and survival of colon cells. More importantly, the p21 and p27 levels were also evaluated after the induction of Cif with Tet-On system. Multiple clones of colon cancer cells for doxycycline-regulated Cif expression were constructed for maintenance purposes including HCT116 and SW480 cell lines. The result of western blot displayed good inducibility of expressing Cif in the cell lines. The clones with Cif preserved their transformed phenotype compared with the control group (clones with GFP or with Cif), in terms of the inhibition of cancer cell proliferation and survival. Furthermore, western blot analysis showed that p27 and p21 were accumulated in the clones with Cif, compared with the colon cancer cell lines with GFP or with Cif. Using the Tet-On system, we developed an efficient approach toward generation of colon cancer cells induced with Cif. These engineered colons tightly controlled Cif expression in vitro, which is a good inducible model system for cancer treatment.

Inducible immortality in hTERT-human mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) are attractive candidates for tissue engineering and cell-based therapy because of their multipotentiality and availability in adult donors. However, in vitro expansion and differentiation of these cells is limited by replicative senescence. The proliferative capacity of hMSCs can be enhanced by ectopic expression of telomerase, allowing for long-term culture. However, hMSCs with constitutive telomerase expression demonstrate unregulated growth and even tumor formation. To address this problem, we used an inducible Tet-On gene expression system to create hMSCs in which ectopic telomerase expression can be induced selectively by the addition of doxycycline (i-hTERT hMSCs). i-hTERT hMSCs have inducible hTERT expression and telomerase activity, and are able to proliferate significantly longer than wild type hMSCs when hTERT expression is induced. They stop proliferating when hTERT expression is turned off and can be rescued when expression is re-induced. They retain multipotentiality in vitro even at an advanced age. We also used a selective inhibitor of telomere elongation to show that the mechanism driving immortalization of hMSCs by hTERT is dependent upon maintenance of telomere length. Thanks to their extended lifespan, preserved multipotentiality and controlled growth, i-hTERT hMSCs may prove to be a useful tool for the development and testing of novel stem cell therapies.

Targeting the Fas/FasL signaling pathway in cancer therapy

INTRODUCTION

The Fas/FasL system plays a significant role in tumorigenesis. Research has shown that its impairment in cancer cells may lead to apoptosis resistance and contribute to tumor progression. Thus, the development of effective therapies targeting the Fas/FasL system may play an important role in the fight against cancer.

AREAS COVERED

In this review the recent literature on targeting the Fas/FasL system for therapeutic exploitation at different levels is reviewed. Promising pre-clinical approaches and various exceptions are highlighted. The potential of combined therapies is also explored, whereby tumor sensitivity to Fas-mediated apoptosis is restored, before an effective targeted therapy is employed.

EXPERT OPINION

The success of the Fas/FasL system targeting for therapeutics will require a better understanding of the alterations conferring resistance, in order to use the most appropriate sensitizing chemotherapeutic or radiotherapeutic agents in combination with effective targeted therapies.

Genetic aspects of cell line development from a synthetic biology perspective

Animal cells can be regarded as factories for the production of relevant proteins. The advances described in this chapter towards the development of cell lines with higher productivity capacities, certain metabolic and proliferation properties, reduced apoptosis and other features must be regarded in an integrative perspective. The systematic application of systems biology approaches in combination with a synthetic arsenal for targeted modification of endogenous networks are proposed to lead towards the achievement of a predictable and technologically advanced cell system with high biotechnological impact.

Functional identification of optimized RNAi triggers using a massively parallel sensor assay

Short hairpin RNAs (shRNAs) provide powerful experimental tools by enabling stable and regulated gene silencing through programming of endogenous microRNA pathways. Since requirements for efficient shRNA biogenesis and target suppression are largely unknown, many predicted shRNAs fail to efficiently suppress their target. To overcome this barrier, we developed a "Sensor assay" that enables the biological identification of effective shRNAs at large scale. By constructing and evaluating 20,000 RNAi reporters covering every possible target site in nine mammalian transcripts, we show that our assay reliably identifies potent shRNAs that are surprisingly rare and predominantly missed by existing algorithms. Our unbiased analyses reveal that potent shRNAs share various predicted and previously unknown features associated with specific microRNA processing steps, and suggest a model for competitive strand selection. Together, our study establishes a powerful tool for large-scale identification of highly potent shRNAs and provides insights into sequence requirements of effective RNAi.

Efficient melanoma cell killing and reduced melanoma growth in mice by a selective replicating adenovirus armed with tumor necrosis factor-related apoptosis-inducing ligand

High mortality and therapy resistance of melanoma demand the development of new strategies, and overcoming apoptosis deficiency appears as particularly promising. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has shown high potential for apoptosis induction in melanoma cells and may be applicable for gene therapy because of its selective impact on tumor cells. We have constructed a conditional replication-competent adenoviral vector with TRAIL controlled by a tetracycline-inducible promoter (AdV-TRAIL). A variant E1A protein and the lack of E1B aimed at the restriction of viral replication to tumor cells. In particular, the replication gene E1A is controlled by a tyrosinase promoter with high selectivity for melanoma cells. AdV-TRAIL mediated strong expression of E1A and doxycycline-dependent induction of TRAIL selectively in melanoma cells, which resulted in tumor cell lysis and induction of apoptosis. In contrast, non-melanoma cells and normal human melanocytes appeared to be protected. Comparison of the AdV-TRAIL approach with a comparable CD95L vector revealed similar efficacy in vitro. In mouse xenotransplantation models, AdV-TRAIL demonstrated its activity by significant melanoma growth reduction. Melanoma cell killing by AdV-TRAIL was further improved in vitro by combinations with chemotherapeutics. We demonstrate that melanoma cells may be efficiently targeted by TRAIL-based gene therapy, and resistance may be overcome by combined chemotherapy.

The role of FasL and Fas in health and disease

The FS7-associated cell surface antigen (Fas, also named CD95, APO-1 or TNFRSF6) attracted considerable interest in the field of apoptosis research since its discovery in 1989. The groups of Shin Yonehara and Peter Krammer were the first reporting extensive apoptotic cell death induction upon treating cells with Fas-specific monoclonal antibodies.1,2 Cloning of Fas3 and its ligand,4,5 FasL (also known as CD178, CD95L or TNFSF6), laid the cornerstone in establishing this receptor-ligand system as a central regulator of apoptosis in mammals. Therapeutic exploitation of FasL-Fas-mediated cytotoxicity was soon an ambitous goal and during the last decade numerous strategies have been developed for its realization. In this chapter, we will briefly introduce essential general aspects of the FasL-Fas system before reviewing its physiological and pathophysiological relevance. Finally, FasL-Fas-related therapeutic tools and concepts will be addressed.

Promoter control over foreign antigen expression in a murine cytomegalovirus vaccine vector

Previous studies have reported on the development of a recombinant murine cytomegalovirus (rMCMV) containing the mouse zona pellucida 3 (mZP3) gene for use as a virally vectored immunocontraceptive (VVIC). This study aimed to alter promoter control over foreign antigen expression and cellular localisation of the antigen expressed in order to overcome virus attenuation previously encountered. Early studies reported on the mZP3 gene expressed by a strong constitutive human cytomegalovirus immediate-early 1 promoter (pHCMV IE1). This virus was able to induce >90% infertility in BALB/c mice despite being heavily attenuated in vivo. In this study the mZP3 was placed under the control of the MCMV early 1 (pMCMV E1) promoter and the inducible tetracycline promoter (Tet-On). In both instances the recombinant virus was able to induce infertility in directly infected mice. However, the viruses remained attenuated. This study demonstrated the capacity to manipulate the nature of the immune response by altering promoter control over foreign antigen expression and cellular localisation of the expressed antigen. We were able to demonstrate that by using the MCMV E1 promoter it was still possible to sterilize female BALB/c mice with an MCMV vector expressing mZP3. The use of the MCMV E1 promoter provides an added level of safety to any MCMV based VVIC approach as it only allows for transgene expression in MCMV permissive cells.

Tuning and controlling gene expression noise in synthetic gene networks

Synthetic gene networks can be used to control gene expression and cellular phenotypes in a variety of applications. In many instances, however, such networks can behave unreliably due to gene expression noise. Accordingly, there is a need to develop systematic means to tune gene expression noise, so that it can be suppressed in some cases and harnessed in others, e.g. in cellular differentiation to create population-wide heterogeneity. Here, we present a method for controlling noise in synthetic eukaryotic gene expression systems, utilizing reduction of noise levels by TATA box mutations and noise propagation in transcriptional cascades. Specifically, we introduce TATA box mutations into promoters driving TetR expression and show that these mutations can be used to effectively tune the noise of a target gene while decoupling it from the mean, with negligible effects on the dynamic range and basal expression. We apply mathematical and computational modeling to explain the experimentally observed effects of TATA box mutations. This work, which highlights some important aspects of noise propagation in gene regulatory cascades, has practical implications for implementing gene expression control in synthetic gene networks.

Prevention of cardiac dysfunction in acute coxsackievirus B3 cardiomyopathy by inducible expression of a soluble coxsackievirus-adenovirus receptor

BACKGROUND

Group B coxsackieviruses (CVBs) are the prototypical agents of acute myocarditis and chronic dilated cardiomyopathy, but an effective targeted therapy is still not available. Here, we analyze the therapeutic potential of a soluble (s) virus receptor molecule against CVB3 myocarditis using a gene therapy approach.

METHODS AND RESULTS

We generated an inducible adenoviral vector (AdG12) for strict drug-dependent delivery of sCAR-Fc, a fusion protein composed of the coxsackievirus-adenovirus receptor (CAR) extracellular domains and the carboxyl terminus of human IgG1-Fc. Decoy receptor expression was strictly doxycycline dependent, with no expression in the absence of an inducer. CVB3 infection of HeLa cells was efficiently blocked by supernatant from AdG12-transduced cells, but only in the presence of doxycycline. After liver-specific transfer, AdG12 (plus doxycycline) significantly improved cardiac contractility and diastolic relaxation compared with a control vector in CVB3-infected mice if sCAR-Fc was induced before infection (left ventricular pressure 59+/-3.8 versus 45.4+/-2.7 mm Hg, median 59 versus 45.8 mm Hg, P<0.01; dP/dt(max) 3645.1+/-443.6 versus 2057.9+/-490.2 mm Hg/s, median 3526.6 versus 2072 mm Hg/s, P<0.01; and dP/dt(min) -2125.5+/-330.5 versus -1310.2+/-330.3 mm Hg/s, median -2083.7 versus -1295.9 mm Hg/s, P<0.01) and improved contractility if induced concomitantly with infection (left ventricular pressure 76.4+/-19.2 versus 56.8+/-10.3 mm Hg, median 74.8 versus 54.4 mm Hg, P<0.05; dP/dt(max) 5214.2+/-1786.2 versus 3011.6+/-918.3 mm Hg/s, median 5182.1 versus 3106.6 mm Hg/s, P<0.05), respectively. Importantly, hemodynamics of animals treated with AdG12 (plus doxycycline) were similar to uninfected controls. Preinfection induction of sCAR-Fc completely blocked and concomitant induction strongly reduced cardiac CVB3 infection, myocardial injury, and inflammation.

CONCLUSIONS

AdG12-mediated sCAR-Fc delivery prevents cardiac dysfunction in CVB3 myocarditis under prophylactic and therapeutic conditions.

Intronically encoded siRNAs improve dynamic range of mammalian gene regulation systems and toggle switch

Applications of conditional gene expression, whether for therapeutic or basic research purposes, are increasingly requiring mammalian gene control systems that exhibit far tighter control properties. While numerous approaches have been used to improve the widely used Tet-regulatory system, many applications, particularly with respect to the engineering of synthetic gene networks, will require a broader range of tightly performing gene control systems. Here, a generically applicable approach is described that utilizes intronically encoded siRNA on the relevant transregulator construct, and siRNA sequence-specific tags on the reporter construct, to minimize basal gene activity in the off-state of a range of common gene control systems. To demonstrate tight control of residual expression the approach was successfully used to conditionally express the toxic proteins RipDD and Linamarase. The intronic siRNA concept was also extended to create a new generation of compact, single-vector, autoinducible siRNA vectors. Finally, using improved regulation systems a mammalian epigenetic toggle switch was engineered that exhibited superior in vitro and in vivo induction characteristics in mice compared to the equivalent non-intronic system.

Turning the gene tap off; implications of regulating gene expression for cancer therapeutics

Cancer poses a tremendous therapeutic challenge worldwide, highlighting the critical need for developing novel therapeutics. A promising cancer treatment modality is gene therapy, which is a form of molecular medicine designed to introduce into target cells genetic material with therapeutic intent. Anticancer gene therapy strategies currently used in preclinical models, and in some cases in the clinic, include proapoptotic genes, oncolytic/replicative vectors, conditional cytotoxic approaches, inhibition of angiogenesis, inhibition of growth factor signaling, inactivation of oncogenes, inhibition of tumor invasion and stimulation of the immune system. The translation of these novel therapeutic modalities from the preclinical setting to the clinic has been driven by encouraging preclinical efficacy data and advances in gene delivery technologies. One area of intense research involves the ability to accurately regulate the levels of therapeutic gene expression to achieve enhanced efficacy and provide the capability to switch gene expression off completely if adverse side effects should arise. This feature could also be implemented to switch gene expression off when a successful therapeutic outcome ensues. Here, we will review recent developments related to the engineering of transcriptional switches within gene delivery systems, which could be implemented in clinical gene therapy applications directed at the treatment of cancer.

Apoptosis pathways and oncolytic adenoviral vectors: promising targets and tools to overcome therapy resistance of malignant melanoma

In the last decades melanoma incidence has been increasing worldwide, while mortality remained on a high level. Until now, there is no suitable therapy for metastasized melanoma, which could lead to a significant increase in overall survival. Apoptosis deficiency is supposed to be a critical factor for therapy resistance, and previous work has characterized the basic mechanisms of apoptosis regulation in melanoma. Genes and strategies suitable for efficient induction of apoptosis in melanoma cells were identified, which are based on proapoptotic Bcl-2 proteins (Bcl-x(S), Bcl-x(AK), Bik/Nbk and Bax) as well as on tumor necrosis factor (TNF)-related death ligands (CD95L/Fas ligand and TNF-related apoptosis-inducing ligand, TRAIL). Proapoptotic genes may be employed in improved gene therapeutic strategies, based on conditional oncolytic adenoviral vectors.

Novel strategies in tailoring human adenoviruses into therapeutic cancer gene therapy vectors

Gene therapy is a novel approach for the treatment of cancer that has so far not been realized. The scope of this review is to try to define the remaining barriers to the successful use of adenovirus vectors for gene and viral therapy of human tumors and to suggest solutions whereby these barriers can be bypassed. It is the conviction of the authors that too many studies have been performed in animal models that are not sufficiently comprehensive to allow conclusions to be drawn for application in humans. For example, in the case of the murine experimental model, in which most studies have been performed, mice are devoid of circulating antibodies to adenovirus type 5 and adenovirus cannot replicate in mouse cells. While the problems are real enough, as witnessed by the quite limited success in human trials, some of the solutions that will be suggested here are hypothetical and have not as yet been tried, even in animals. The review has no ambition to be exhaustive but is intended as a contribution in order to forward the field of gene therapy vectors for systemic clinical application.

Anticancer activity of oncolytic adenovirus vector armed with IFN-alpha and ADP is enhanced by pharmacologically controlled expression of TRAIL

We have previously described oncolytic adenovirus (Ad) vectors KD3 and KD3-interferon (IFN) that were rendered cancer-specific by mutations in the E1A region of Ad; these mutations abolish binding of E1A proteins to p300/CBP and pRB. The antitumor activity of the vectors was enhanced by overexpression of the Adenovirus Death Protein (ADP, E3-11.6K) and by replication-linked expression of IFN-alpha. We hypothesized that the anticancer efficacy of the KD3-IFN vector could be further improved by expression of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). E1-deleted Ad vectors were constructed carrying reporter genes for enhanced green fluorescent protein or secreted placental alkaline phosphatase (SEAP) and a therapeutic gene for TRAIL under control of the TetON system. Expression of the genes was increased in the presence of a helper virus and the inducer doxycycline such that up to 231-fold activation of expression for the TetON-SEAP vector was obtained. Coinfection with TetON-TRAIL augmented oncolytic activity of KD3 and KD3-IFN in vitro. Induction of TRAIL expression did not reduce the yield of progeny virus. Combination of TetON-TRAIL and KD3-IFN produced superior antitumor activity in vivo as compared with either vector alone demonstrating the efficacy of a four-pronged cancer gene therapy approach, which includes Ad oncolysis, ADP overexpression, IFN-alpha-mediated immunotherapy, and pharmacologically controlled TRAIL activity.

Inducibility of doxycycline-regulated gene in neural and neuroendocrine cells strongly depends on the appropriate choice of a tetracycline-responsive promoter

Elucidation of the mechanisms underlying specific receptor activation of neural and neuroendocrine cells will require the establishment of cellular systems that permit the regulation of the expression of the protein of interest. In a tetracycline (Tet)-regulated system, the gene encoding the protein of interest is under the control of a Tet promoter and its transcription is activated in the presence of doxycycline (Dox) by the Tet transactivator rtTA. Acceptable inducibility of the gene's expression requires a high level of its expression in the presence of Dox and a minimal basal expression in the absence of Dox. Two Tet promoters are compared here, the original PhCMV*-1 and the second-generation Ptight, with respect to the inducibility of the gene of interest in neuroendocrine and neural cells genetically engineered to express rtTA, namely PC12-Tet-On cells and MB-G-18 cells (mouse brain-derived cells with the phenotype of neuron-restricted precursors). This study demonstrates that the use of Ptight provided a much higher Dox-induced maximal expression in both cell lines, while the basal activities of the two Tet promoters were at similar levels. The additional use of the Tet-controlled silencer (tTS) caused almost complete abrogation of the leakiness of the Ptight promoter and an increase in the inducibility of the regulated gene, but the maximal levels of gene expression driven in the presence of Dox were also markedly reduced.

Coxsackievirus B3 and adenovirus infections of cardiac cells are efficiently inhibited by vector-mediated RNA interference targeting their common receptor

As coxsackievirus B3 (CoxB3) and adenoviruses may cause acute myocarditis and inflammatory cardiomyopathy, isolation of the common coxsackievirus–adenovirus-receptor (CAR) has provided an interesting new target for molecular antiviral therapy. Whereas many viruses show high mutation rates enabling them to develop escape mutants, mutations of their cellular virus receptors are far less likely. We report on antiviral efficacies of CAR gene silencing by short hairpin (sh)RNAs in the cardiac-derived HL-1 cell line and in primary neonatal rat cardiomyocytes (PNCMs). Treatment with shRNA vectors mediating RNA interference against the CAR resulted in almost complete silencing of receptor expression both in HL-1 cells and PNCMs. Whereas CAR was silenced in HL-1 cells as early as 24 h after vector treatment, its downregulation in PNCMs did not become significant before day 6. CAR knockout resulted in inhibition of CoxB3 infections by up to 97% in HL-1 cells and up to 90% in PNCMs. Adenovirus was inhibited by only 75% in HL-1 cells, but up to 92% in PNCMs. We conclude that CAR knockout by shRNA vectors is efficient against CoxB3 and adenovirus in primary cardiac cells, but the efficacy of this approach in vivo may be influenced by cell type-specific silencing kinetics in different tissues.

A bidirectional Tet-dependent promotor construct regulating the expression of E1A for tight control of oncolytic adenovirus replication

Tight regulation of oncolytic adenoviruses (oAdV) represents an important requirement for their safe application. Here we describe a new doxycycline (Dox)-dependent oAdV with a bidirectional expression cassette, which drives the expression of the reverse tetracycline-controlled transactivator (rtTA(s)-M2) from a lung tumor-specific promoter and, in the opposite direction, the expression of the adenoviral E1A gene from a second generation TetO(7) sequence linked to an isolated TATA box. In H441 lung cancer cells, this oAdV showed a strictly Dox-dependent E1A expression, adenoviral replication, cell killing activity and a 450-fold induction of progeny virus production. The virus could be shut off again by withdrawal of Dox and, in contrast to a control oAdV expressing E1A directly from the SP-B promoter, did not replicate in non-target cells. However, the absolute values of virus production and the cell killing activity in the presence of the inducer were still reduced as compared to the control oAdV. The results demonstrate, for the first time, Dox-dependent oAdV replication from a single adenoviral vector genome. Future improvement of the Dox-dependent E1A regulation cassette should lead to the generation of an oAdV well suited to meet the demands for a highly regulated and efficient oncolytic virus for in vivo applications.

Secretory expression of synthetic human Fas ligand extracellular domain gene in Pichia pastoris: influences of tag addition and N-glycosylation site deletion, and development of a purification method

Human Fas ligand is a medically important membrane glycoprotein that induces the apoptosis of harmful cells. A new secretory expression and purification method was devised for the production of a large amount of recombinant human Fas ligand extracellular domain (hFasLECD) by Pichia pastoris. The expression plasmid containing a synthetic hFasLECD gene designed using yeast optimal codons was constructed for the secretion of hFasLECD. The secreted product exhibited the specific binding activity toward soluble human Fas receptor extracellular domain-human IgG(1)-Fc domain fusion protein, and the receptor-ligand complex was immunoprecipitated by Protein A conjugated agarose-gel beads. The influences of the N- and C- terminal addition of FLAG/(His)(6) tag spaced by pentaglycine sequence and the sequentially accumulative deletions of N-glycosylation sites within hFasLECD were investigated. The secretion of functional hFasLECD was retained after the N-terminal tagging and the deletion of either single or double N-glycosylation sites. As judged from SDS-PAGE analysis of the culture supernatant, the N-terminal addition of FLAG-(Gly)(5) tag and the deletion of single N-glycosylation site via N184Q mutation increased the secretion level of the product. In contrast, the C-terminal tagged genes and all N-glycosylation sites deleted gene failed to direct the secretion of functional hFasLECD. The secreted products in the culture medium were purified using a cation-exchange chromatography and a gel-filtration chromatography. The purified hFasLECDs existed as trimers composed of a mixture of monomer species in different glycosylation states. Approximately five milligram of functional N-terminal FLAG-(Gly)(5) tagged hFasLECD N184Q mutant was obtained from one liter culture supernatant.