Tetracycline-regulatable factors with distinct dimerization domains allow reversible growth inhibition by p16

A Dual-Function "TRE-Lox" System for Genetic Deletion or Reversible, Titratable, and Near-Complete Downregulation of Cathepsin D

Commonly employed methods for reversibly disrupting gene expression, such as those based on RNAi or CRISPRi, are rarely capable of achieving >80-90% downregulation, making them unsuitable for targeting genes that require more complete disruption to elicit a phenotype. Genetic deletion, on the other hand, while enabling complete disruption of target genes, often produces undesirable irreversible consequences such as cytotoxicity or cell death. Here we describe the design, development, and detailed characterization of a dual-function "TRE-Lox" system for effecting either (a) doxycycline (Dox)-mediated downregulation or (b) genetic deletion of a target gene-the lysosomal aspartyl protease cathepsin D (CatD)-based on targeted insertion of a tetracycline-response element (TRE) and two LoxP sites into the 5' end of the endogenous CatD gene (CTSD). Using an optimized reverse-tetracycline transrepressor (rtTR) variant fused with the Krüppel-associated box (KRAB) domain, we show that CatD expression can be disrupted by as much as 98% in mouse embryonic fibroblasts (MEFs). This system is highly sensitive to Dox (IC₅₀ = 1.46 ng/mL) and results in rapid (t_1/2 = 0.57 d) and titratable downregulation of CatD. Notably, even near-total disruption of CatD expression was completely reversed by withdrawal of Dox. As expected, transient expression of Cre recombinase results in complete deletion of the CTSD gene. The dual functionality of this novel system will facilitate future studies of the involvement of CatD in various diseases, particularly those attributable to partial loss of CatD function. In addition, the TRE-Lox approach should be applicable to the regulation of other target genes requiring more complete disruption than can be achieved by traditional methods.

New use for CETSA: monitoring innate immune receptor stability via post-translational modification by OGT

O-GlcNAcylation is a dynamic and functionally diverse post-translational modification shown to affect thousands of proteins, including the innate immune receptor nucleotide-binding oligomerization domain-containing protein 2 (Nod2). Mutations of Nod2 (R702W, G908R and 1007 fs) are associated with Crohn's disease and have lower stabilities compared to wild type. Cycloheximide (CHX)-chase half-life assays have been used to show that O-GlcNAcylation increases the stability and response of both wild type and Crohn's variant Nod2, R702W. A more rapid method to assess stability afforded by post-translational modifications is necessary to fully comprehend the correlation between NLR stability and O-GlcNAcylation. Here, a recently developed cellular thermal shift assay (CETSA) that is typically used to demonstrate protein-ligand binding was adapted to detect shifts in protein stabilization upon increasing O-GlcNAcylation levels in Nod2. This assay was used as a method to predict if other Crohn's associated Nod2 variants were O-GlcNAcylated, and also identified the modification on another NLR, Nod1. Classical immunoprecipitations and NF-κB transcriptional assays were used to confirm the presence and effect of this modification on these proteins. The results presented here demonstrate that CETSA is a convenient method that can be used to detect the stability effect of O-GlcNAcylation on O-GlcNAc-transferase (OGT) client proteins and will be a powerful tool in studying post-translational modification.

The molecular chaperone HSP70 binds to and stabilizes NOD2, an important protein involved in Crohn disease

Microbes are detected by the pathogen-associated molecular patterns through specific host pattern recognition receptors. Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) is an intracellular pattern recognition receptor that recognizes fragments of the bacterial cell wall. NOD2 is important to human biology; when it is mutated it loses the ability to respond properly to bacterial cell wall fragments. To determine the mechanisms of misactivation in the NOD2 Crohn mutants, we developed a cell-based system to screen for protein-protein interactors of NOD2. We identified heat shock protein 70 (HSP70) as a protein interactor of both wild type and Crohn mutant NOD2. HSP70 has previously been linked to inflammation, especially in the regulation of anti-inflammatory molecules. Induced HSP70 expression in cells increased the response of NOD2 to bacterial cell wall fragments. In addition, an HSP70 inhibitor, KNK437, was capable of decreasing NOD2-mediated NF-κB activation in response to bacterial cell wall stimulation. We found HSP70 to regulate the half-life of NOD2, as increasing the HSP70 level in cells increased the half-life of NOD2, and down-regulating HSP70 decreased the half-life of NOD2. The expression levels of the Crohn-associated NOD2 variants were less compared with wild type. The overexpression of HSP70 significantly increased NOD2 levels as well as the signaling capacity of the mutants. Thus, our study shows that restoring the stability of the NOD2 Crohn mutants is sufficient for rescuing the ability of these mutations to signal the presence of a bacterial cell wall ligand.

MAR-mediated integration of plasmid vectors for in vivo gene transfer and regulation

BACKGROUND

The in vivo transfer of naked plasmid DNA into organs such as muscles is commonly used to assess the expression of prophylactic or therapeutic genes in animal disease models.

RESULTS

In this study, we devised vectors allowing a tight regulation of transgene expression in mice from such non-viral vectors using a doxycycline-controlled network of activator and repressor proteins. Using these vectors, we demonstrate proper physiological response as consequence of the induced expression of two therapeutically relevant proteins, namely erythropoietin and utrophin. Kinetic studies showed that the induction of transgene expression was only transient, unless epigenetic regulatory elements termed Matrix Attachment Regions, or MAR, were inserted upstream of the regulated promoters. Using episomal plasmid rescue and quantitative PCR assays, we observed that similar amounts of plasmids remained in muscles after electrotransfer with or without MAR elements, but that a significant portion had integrated into the muscle fiber chromosomes. Interestingly, the MAR elements were found to promote plasmid genomic integration but to oppose silencing effects in vivo, thereby mediating long-term expression.

CONCLUSIONS

This study thus elucidates some of the determinants of transient or sustained expression from the use of non-viral regulated vectors in vivo.

Development of single-vector Tet-on inducible systems with high sensitivity to doxycycline

Single-vector Tet-on systems were developed to enable the tight regulation of transgenes in mammalian cells with a low dosage of doxycycline. Both the regulatory and the responsive units were integrated in a single vector and separated by a short DNA segment (214 bp). In the developed single-vector Tet-on systems, a high level of expression of the transgene can be induced by doxycycline at a concentration of as low as 1 ng/ml, which is 500-1,000 times lower than that usually utilized in other Tet-on systems. The single-vector Tet-on system developed here exhibited 3.5-10.8 times greater inducibility of the transgene in response to doxycycline than did a dual-vector system from a commercial source. Further studies indicate that the basal activity of Tet-on systems depends greatly on the strength of the promoter that controls the transactivator. The basal activity of Tet-on systems was high when the transactivator that was directed by the human cytomegalovirus promoter, and it was almost undetectable when the transactivator was placed under the control of a moderate strength mouse mammary tumor virus promoter. Moreover, the introduction of selectable markers allows the developed single-vector Tet-on systems to facilitate the generation of conditional transgenic cells and animals with high inducibility, low basal activity and detrimental effects of the long-term administration of doxycycline.

Tetracycline-regulated expression implemented through transcriptional activation combined with proximal and distal repression

Tetracycline-regulated expression systems are widely used to control ectopic gene expression in mammalian cells. However, background or "leaky" expression in the "off" state can limit applications that require control of expression at low levels. In this work we have engineered a tetracycline-regulated expression system with an improved range of control and lower background expression. To lower background expression without diminishing the controllable expression range, we designed a feed-forward scheme that repressed both expression of the gene of interest and the transcriptional activator. By using a tetracycline-responsive repressor that can modify chromatin and repress transcription over short and long distances, we were able to repress these two expression targets using a single tetracycline-responsive genetic element. This dual-targeting repressor/activation system demonstrated decreased background expression in its "off" state and a 25-fold range of expression in response to doxycycline. This study demonstrates that genetic circuits can be improved by leveraging trans-acting factors with long-range capabilities.

Adjusting transgene expression levels in lymphocytes with a set of inducible promoters

BACKGROUND

Inducible gene expression systems are powerful research tools and could be of clinical value in the future, with lymphocytes being likely prime application targets. However, currently available regulatable promoters exhibit variation in their efficiency in a cell line-dependent-manner and are notorious for basal leakiness or poor inducibility. Data concerning the regulatory properties of different inducible promoters are scarce for lymphocytes. In the present study, we report a comprehensive analysis of how various inducible promoters perform and how their combination with a transsilencer and a reverse transactivator can result in optimally controlled gene expression in T-cells.

METHODS

The performance of the tetracycline-regulated (Tet)-inducible promoters Tet-responsive element (TRE), mouse mammary tumor virus (MMTV)/TRE, TREtight and second generation TRE (SG/TRE) was compared in several B-cell lines and in Jurkat T-cells using transient transfections in combination with Tet-On. To monitor transgene expression in a Jurkat cell line containing a transsilencer and a reverse transactivator, expression cassettes encoding enhanced green fluorescent protein, CD123 or a constitutively active, cytotoxic caspase-3 were flanked with insulators and stably integrated. The performance of TREtight and SG/TRE was furthermore analysed in transiently transfected primary CD4(+) human T-cells.

RESULTS

The promoters exhibit greatly diverging characteristics. MMTV/TRE permits moderate, TRE and TREtight permits intermediate and SG/TRE permits very high expression levels. TRE and SG/TRE are leaky, whereas MMTV/TRE and TREtight provide stringent expression control. Tetracycline derivatives add flexibility to transgene expression by introducing intermediate expression levels.

CONCLUSIONS

The different expression profiles of the promoters increase the flexibility to adjust transgene expression levels. The promoters provide an additional option to optimize system performance for many applications.

Recent advances in the photochemical control of protein function

Biological processes are regulated with a high level of spatial and temporal resolution. To understand and manipulate these processes, scientists need to be able to regulate them with Nature's level of precision. In this context, light is a unique regulatory element because it can be precisely controlled in terms of location, timing and amplitude. Moreover, most biological laboratories have a wide range of light sources as standard equipment. This review article summarizes the most recent advances in light-mediated regulation of protein function and its application in a cellular context. Specifically, the photocaging of small-molecule modulators of protein function and of specific amino acid residues in proteins is discussed. In addition, examples of the photochemical control of protein function through the application of genetically engineered natural-light receptors are presented.

Light-activated gene expression directs segregation of co-cultured cells in vitro

Light-directed gene patterning methods have been described as a means to regulate gene expression in a spatially and temporally controlled manner. Several methods have been reported that use photocaged forms of small molecule effectors to control ligand-dependent transcription factors. Whereas these methods offer many advantages including high specificity and transient light-sensitivity, the free diffusion of the uncaged effector can limit both the magnitude and resolution of localized gene induction. Methods to date have been limited by the small fraction of irradiated cells that have expression levels significantly above uninduced background and have not been shown to affect a defined biological response. The tetracycline-dependent transactivator/transrepressor system, RetroTET-ART, combined with a photocaged form of doxycycline (NvOC-Dox) can be used to form photolithographic patterns of induced expression wherein up to 85% of the patterned cells show expression levels above uninduced regions. The efficiency and inducibility of the RetroTET-ART system allows one to quantitatively measure the limits of resolution and the relative induction levels mediated by a small molecule photocaged effector for the first time. Well-defined patterns of reporter genes were reproducibly formed within 6-36 h with feature sizes as small as 300 microm. After photo-patterning, NvOC-Dox can be rapidly removed, rendering cells photoinsensitive and allowing one to monitor GFP product formation in real time. Patterned co-expression of the cell surface ligand ephrin A5 on cell monolayers creates well-defined patterns that are sufficient to direct and segregate co-cultured cells via either attractive or repulsive signaling cues. The ability to direct the arrangement of cells on living cell monolayers through the action of light may serve as a model system for engineering artificial tissues.

Coherent activation of a synthetic mammalian gene network

A quantitative analysis of naturally-occurring regulatory networks, especially those present in mammalian cells, is difficult due to their high complexity. Much simpler gene networks can be engineered in model organisms and analyzed as isolated regulatory modules. Recently, several synthetic networks have been constructed in mammalian systems. However, most of these engineered mammalian networks have been characterized using steady-state population level measurements. Here, we use an integrated experimental-computational approach to analyze the dynamical response of a synthetic positive feedback network in individual mammalian cells. We observe a switch-like activation of the network with variable delay times in individual cells. In agreement with a stochastic model of the network, we find that increasing the strength of the positive feedback results in a decrease in the mean delay time and a more coherent activation of individual cells. Our results are important for gaining insight into biological processes which rely on positive feedback regulation.

Conditional protein stabilization via the small molecules Shld-1 and rapamycin increases the signal-to-noise ratio with tet-inducible gene expression

Cellular mechanisms control one or more of the three basic levels of regulation (transcription, translation, and protein activity/locality), allowing for finely tuned spatial and temporal regulation of protein expression patterns. Gene regulation constructs in wide use today often employ a constitutively expressed transcription factor whose activity is determined by the presence or absence of a small molecule. A case in point is the tet transcription system, wherein transcription is regulated by doxycycline (Dox), allowing the researcher to turn protein expression on or off depending on the presence/absence of Dox. However in many applications of that system, there is basal transcription from the promoter element that is independent of Dox. Moreover, in vivo, heterogeneous distribution of Dox leads to concurrent differences in gene expression. We addressed these limitations by introducing conditional destabilizing elements to the system. First, we created a transactivator protein fusion regulated at the additional level of protein stability. This modification enabled a system that demonstrated an off state that is less sensitive to variations in Dox concentrations. We also regulated the stability of the protein expressed from the tet operator cassette, observing greatly improved signal-to-noise ratios. The results underscore how investigator-defined regulation at multiple levels of protein expression can attain afiner degree of control over the final expression of introduced genes.

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.

Therapeutic stem-cells for cancer treatment: hopes and hurdles in tactical warfare

As our understanding of stem-cell behaviour rapidly increases, more and more reports suggest that use of stem-cell therapy will extend well beyond regenerative medicine in the near future. Due to their inherent tumoritropic migratory properties, stem cells can serve as vehicles for the delivery of effective, targeted treatment to isolated tumours and to metastatic disease. In vitro, stem cells can readily be engineered by inserting specifically tailored transgenes with antitumour effects to create tumour-seeking therapeutic vehicles. Transgene effects include direct tumour-cell killing, promotion of local immune responses, oncolytic virus production, and prodrug activation schemes. Many of these strategies have been validated in a wide range of studies assessing treatment feasibility or efficacy and establishing methods for real-time monitoring of stem-cell migration and fate in vivo. New insights into avenues for stem-cell sourcing have shortened the probable time to realisation of such treatments for patients. In this Review, we provide an outline of the rationale and status of stem-cell-based treatments for tumours, and we discuss prospects for clinical implementation and the factors crucial for maintaining momentum towards this goal.

Visualization of protein interactions in living cells using bimolecular fluorescence complementation (BiFC) analysis

Protein interactions integrate stimuli from different signaling pathways and developmental programs. Bimolecular fluorescence complementation (BiFC) analysis has been developed for visualization of protein interactions in living cells. This approach is based on complementation between two fragments of a fluorescent protein when they are brought together by an interaction between proteins fused to the fragments, and it enables visualization of the subcellular locations of protein interactions in the normal cellular environment. It can be used for the analysis of many protein interactions and does not require information about the structures of the interaction partners. A multicolor BiFC approach has been developed for simultaneous visualization of interactions with multiple alternative partners in the same cell, based on complementation between fragments of engineered fluorescent proteins that produce bimolecular fluorescent complexes with distinct spectral characteristics. This enables comparison of subcellular distributions of different protein complexes in the same cell and allows analysis of competition between mutually exclusive interaction partners.

Hypoxic response elements control expression of human vascular endothelial growth factor(165) genes transferred to ischemia myocardium in vivo and in vitro

BACKGROUND

Vascular endothelial growth factor (VEGF) gene transfer with recombinant adeno-associated viral (rAAV) vector for ischemia heart disease therapy is being increasingly studied. However, uncontrolled long-term expression of VEGF may cause some side effects. Therefore, an attempt to develop an effective gene control system for safeguarding against such side effects should be made. Pathphysiologically, an ideal control system for VEGF gene expression is letting it respond to hypoxia. We used nine copies of hypoxic response element (HRE) to regulate expression of hVEGF(165) in the myocardium, and tried to elucidate the feasibility and safety of the application of the HIF-1-HRE system.

METHODS

Cardiomyocytes of neonatal Sprague Dawley rats were cultured and incubated with rAAV-9HRE-hVEGF(165), and pig ischemic heart models were established and rAAV-9HRE-hVEGF(165) was injected into ischemia myocardium. RT-PCR, Western blot, ELISA, and immunohistochemistry were used to determine hVEGF(165) expressions of cultured cardiomyocytes and myocardium under hypoxic and reoxygenation conditions.

RESULTS

The results of RT-PCR and ELISA determinations revealed that, in cultured cardiomyocytes, expressions of hVEGF(165)mRNA and protein were up-regulated under hypoxic conditions. After 4 h of reoxygenation, hVEGF(165)mNRA expression was decreased, and disappeared following 8 to 12 h of reoxygenation (P < 0.01). RT-PCR and Western blot also showed that, under myocardial ischemia, hVEGF(165) expression was increased significantly (P < 0.01). Following myocardial reperfusion, both hVEGF(165)mRNA and protein expressions were inhibited (P < 0.01). The new vessels in the reperfusion condition were decreased.

CONCLUSIONS

This study suggested that 9HRE can effectively control hVEGF(165) gene expression in vivo and in vitro. It has feasibility for using the HIF-1-HRE system for regulation of angiogenic factor expression in ischemia heart.

Adeno-associated viral vector-mediated hypoxia-regulated VEGF gene transfer promotes angiogenesis following focal cerebral ischemia in mice

Uncontrolled expression of vascular endothelial growth factor (VEGF) in vivo may cause unexpected side effects, such as brain hemangioma or tumor growth. Because hypoxia-inducible factor-1 (HIF-1) is upregulated during cerebral ischemia and regulates gene expression by binding to a cis-acting hypoxia-responsive element (HRE), we therefore used a novel HRE, originating in the 3'-end of the erythropoietin (Epo) gene, to control gene expression in the ischemic brain. A concatemer of nine copies (H9) of the consensus sequence of HRE was used to mediate hypoxia induction. Three groups of adult CD-1 mice received AAVH9-VEGF, AAVH9-lacZ or saline injection, and then underwent 45 min of transient middle cerebral artery occlusion (tMCAO). Results show that HIF-1 was persistently expressed in the ischemic brain. VEGF was overexpressed in the ischemic perifocal region in AAVH9-VEGF-transduced mice. Double-labeled immunostaining showed that VEGF expressed in neurons and astrocytes but not endothelial cells, suggesting that adeno-associated virus (AAV) vectors transduced neurons and astrocytes predominantly. The total number of microvessels/enlarged microvessels was greatly increased in the AAVH9-VEGF-transduced mice (180+/-29/27+/-4) compared to the AAVH9-lacZ (118+/-19/14+/-3) or saline-treated (119+/-20/14+/-2) mice after tMCAO (P<0.05). Cell proliferation examination demonstrated that these microvessels were newly formed. Regional cerebral blood flow recovery in the AAVH9-VEGF-transduced mice was also better than in AAVH9-lacZ or saline-treated mice (P<0.05). Our data indicated that HRE is a novel trigger for the control of VEGF expression in the ischemic brain. VEGF overexpression through AAVH9-VEGF gene transfer showed stable focal angiogenic effects in post-ischemic repair process, providing an opportunity to rebuild injured brain tissue.

Alien inhibits E2F1 gene expression and cell proliferation

Recently, using a proteomic approach we have identified the corepressor Alien as a novel interacting factor of the cell cycle regulator E2F1. Unclear was whether this interaction influences cell proliferation and endogenous E2F1 target gene expression. Here, we show by chromatin immunoprecipitation (ChIP) that Alien is recruited in vivo to the E2F binding sites present in the E2F1 gene promoter, inhibits the transactivation of E2F1 and represses endogenous E2F1 gene expression. Interestingly, using synchronized cells to assess the expression of Alien profile during cell cycle the levels of endogenous Alien are increased during G1, G1/S and G2 phase. Furthermore, stable transfection of Alien leads to reduction of cell proliferation. Thus, the data suggest that Alien acts as a corepressor for E2F1 and is involved in cell cycle regulation.

Molecular mechanisms of thrombin-induced interleukin-8 (IL-8/CXCL8) expression in THP-1-derived and primary human macrophages

Under normal conditions, macrophages provide essential innate immune surveillance in tissues. These cells also play key functions during wound healing and in pathological conditions. When macrophages are exposed to thrombin, an enzyme released from leaky blood vessels, they are stimulated to produce inflammatory cytokines, which are critical for wound healing and can also facilitate tumor growth and invasion. Using antibody cytokine arrays, we identified IL-8/CXCL8, a chemokine that plays important functions in inflammation and angiogenesis and consequently in healing and tumor development, as one of the cytokines that is highly stimulated in macrophages by thrombin. Here, we investigated the signal transduction mechanism by which thrombin stimulates IL-8/CXCL8 expression in THP-1-derived and primary human macrophages. We show that JNK is a crucial mediator of the thrombin signaling pathways in macrophages, and the activation of JNK is dependent on stimulation of the Rho small GTPase. The thrombin-induced Rho/JNK cascade is a novel signaling cascade for IL-8/CXCL8 transcription activation. Understanding the molecular mechanism by which thrombin controls the expression of inflammatory cytokines in macrophages can lead to therapeutic interventions, which can provide better management of healing, inflammation, and tumorigenesis.

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.

Stringent and reproducible tetracycline-regulated transgene expression by site-specific insertion at chromosomal loci with pre-characterised induction characteristics

Background

The ability to regulate transgene expression has many applications, mostly concerning the analysis of gene function. Desirable induction characteristics, such as low un-induced expression, high induced expression and limited cellular heterogeneity, can be seriously impaired by chromosomal position effects at the site of transgene integration. Many clones may therefore need to be screened before one with optimal induction characteristics is identified. Furthermore, such screens must be repeated for each new transgene investigated, and comparisons between clones with different transgenes is complicated by their different integration sites.

Results

To circumvent these problems we have developed a "screen and insert" strategy in which clones carrying a transgene for a fluorescent reporter are first screened for those with optimal induction characteristics. Site-specific recombination (SSR) is then be used repeatedly to insert any new transgene at the reporter transgene locus of such clones so that optimal induction characteristics are conferred upon it. Here we have tested in a human fibrosarcoma cell line (HT1080) two of many possible implementations of this approach. Clones (e.g. Rht14-10) in which a GFP reporter gene is very stringently regulated by the tetracycline (tet) transactivator (tTA) protein were first identified flow-cytometrically. Transgenes encoding luciferase, I-SceI endonuclease or Rad52 were then inserted by SSR at a LoxP site adjacent to the GFP gene resulting stringent tet-regulated transgene expression. In clone Rht14-10, increases in expression from essentially background levels (+tet) to more than 10⁴-fold above background (-tet) were reproducibly detected after Cre-mediated insertion of either the luciferase or the I-SceI transgenes.

Conclusion

Although previous methods have made use of SSR to integrate transgenes at defined sites, none has effectively combined this with a pre-selection step to identify integration sites that support optimal regulatory characteristics. Rht14-10 and similar HT1080-derived clones can now be used in conjunction with a convenient delivery vector (pIN2-neoMCS), in a simple 3-step protocol leading to stringent and reproducible transgene regulation. This approach will be particularly useful for transgenes whose products are very active at low concentrations and/or for comparisons of multiple related transgenes.

Regulatable gutless adenovirus vectors sustain inducible transgene expression in the brain in the presence of an immune response against adenoviruses

In view of recent serious adverse events and advances in gene therapy technologies, the use of regulatable expression systems is becoming recognized as indispensable adjuncts to successful clinical gene therapy. In the present work we optimized high-capacity adenoviral (HC-Ad) vectors encoding the novel tetracycline-dependent (TetOn)-regulatory elements for efficient and regulatable gene expression in the rat brain in vivo. We constructed two HC-Ad vectors encoding beta-galactosidase (beta-gal) driven by a TetOn system containing the rtTAS(s)M2 transactivator and the tTS(Kid) repressor under the control of the murine cytomegalovirus (mCMV) (HC-Ad-mTetON-beta-Gal) or the human CMV (hCMV) promoter (HC-Ad-hTetON-beta-Gal). Expression was tightly regulatable by doxycycline (Dox), reaching maximum expression in vivo at 6 days and returning to basal levels at 10 days following the addition or removal of Dox, respectively. Both vectors achieved higher transgene expression levels compared to the expression from vectors encoding the constitutive mCMV or hCMV promoter. HC-Ad-mTetON-beta-Gal yielded the highest transgene expression levels and expressed in both neurons and astrocytes. Antivector immune responses continue to limit the clinical use of vectors. We thus tested the inducibility and longevity of HC-Ad-mediated transgene expression in the brain of rats immunized against adenovirus by prior intradermal injections of RAds. Regulated transgene expression from HC-Ad-mTetON-beta-Gal remained active even in the presence of a significant systemic immune response. Therefore, these vectors display two coveted characteristics of clinically useful vectors, namely their regulation and effectiveness even in the presence of prior immunization against adenovirus.

Improved single-chain transactivators of the Tet-On gene expression system

Background

The Tet-Off (tTA) and Tet-On (rtTA) regulatory systems are widely applied to control gene expression in eukaryotes. Both systems are based on the Tet repressor (TetR) from transposon Tn10, a dimeric DNA-binding protein that binds to specific operator sequences (tetO). To allow the independent regulation of multiple genes, novel Tet systems are being developed that respond to different effectors and bind to different tetO sites. To prevent heterodimerization when multiple Tet systems are expressed in the same cell, single-chain variants of the transactivators have been constructed. Unfortunately, the activity of the single-chain rtTA (sc-rtTA) is reduced when compared with the regular rtTA, which might limit its application.

Results

We recently identified amino acid substitutions in rtTA that greatly improved the transcriptional activity and doxycycline-sensitivity of the protein. To test whether we can similarly improve other TetR-based gene regulation systems, we introduced these mutations into tTA and sc-rtTA. Whereas none of the tested mutations improved tTA activity, they did significantly enhance sc-rtTA activity. We thus generated a novel sc-rtTA variant that is almost as active and dox-sensitive as the regular dimeric rtTA. This variant was also less sensitive to interference by co-expressed TetR-based tTS repressor protein and may therefore be more suitable for applications where multiple TetR-based regulatory systems are used.

Conclusion

We developed an improved sc-rtTA variant that may replace regular rtTA in applications where multiple TetR-based regulatory systems are used.

Adeno-associated viral vector-delivered hypoxia-inducible gene expression in ischemic hearts

This chapter describes a system using adeno-associated viral (AAV) vector to deliver hypoxia-inducible gene expression to ischemic hearts. The hypoxia induction of gene expression in this system is based on the accumulation of hypoxia-inducible factor-1 (HIF-1) in ischemic hearts and the use of hypoxia-response element (HRE) identified from the enhancers of genes, the expression of which can be induced by hypoxia. The methods of plasmid and AAV vector construction for hypoxia-inducible gene expression, viral vector production and purification, and viral titer determination are described. This chapter also illustrates the methods that can be used to test hypoxia-inducible gene expression in vitro and in vivo, including hypoxia treatment of cultured cells, generation of murine ischemic heart models, and analysis of gene expression.

Inducible and Reversible Transgene Expression in Human Stem Cells After Efficient and Stable Gene Transfer

We report here a lentiviral vector system for regulated transgene expression. We used the tetracycline repressor fused with a transcriptional suppression domain (tTS) to specifically suppress transgene expression. Human cells were first transduced with a tTS-expressing vector and subsequently transduced with a second lentiviral vector-containing transgene controlled by a regular promoter adjacent to a high-affinity tTS-binding site (tetO). After optimizing the location of the tetO site in the latter vector, we achieved a better inducible transgene expression than the previous lentiviral vectors using the tetracycline repressor systems. In this new system, the transgene transcription from a cellular promoter such as EF1alpha or ubiquitin-C promoter is suppressed by the tTS bound to the nearby tetO site. In the presence of the tetracycline analog doxycycline (Dox), however, the tTS binding is released from the transgene vector and transcription from the promoter is restored. Thus, this system simply adds an extra level of regulation, suitable for any types of promoters (ubiquitous or cell-specific). We tested this tTS-suppressive, Dox-inducible system in 293T cells, human multipotent hematopoietic progenitor cells, and three human embryonic stem cell lines, using a dual-gene vector containing the green fluorescent protein reporter or a cellular gene. We observed a tight suppression in the uninduced state. However, the suppression is reversible, and transgene expression was restored at 5 ng/ml Dox. The lentiviral vectors containing the tTS-suppressive, Dox-inducible system offer a universal, inducible, and reversible transgene expression system in essentially any mammalian cell types, including human embryonic stem cells.

A retroviral expression system based on tetracycline-regulated tricistronic transactivator/repressor vectors for functional analyses of antiproliferative and toxic genes

Establishment of stably transfected mammalian cells with conditional expression of antiproliferative or proapoptotic proteins is often hampered by varying expression within bulk-selected cells and high background in the absence of the inducing drug. To overcome such limitations, we designed a gene expression system that transcribes the tetracycline-dependent rtTA2-M2-activator, TRSID-silencer, and selection marker as a tricistronic mRNA from a single retroviral vector. More than 92% of bulk-selected cells expressed enhanced green fluorescent protein or luciferase over more than three orders of magnitude in an almost linear, dose-dependent manner. To functionally test this system, we studied how dose-dependent expression of p27(Kip1) affects proliferation and viability of SH-EP neuroblastoma cells. Low to moderate p27(Kip1) expression caused transient G(0)-G(1) accumulation without reduced viability, whereas high p27(Kip1) levels induced significant apoptosis after 72 hours. This proves that this expression system allows concentration-dependent analysis of gene function and implicates p27(Kip1) as a critical regulator of both proliferation and apoptosis in SH-EP neuroblastoma cells.

Gene-modified tissue-engineered skin: the next generation of skin substitutes

Tissue engineering combines the principles of cell biology, engineering and materials science to develop three-dimensional tissues to replace or restore tissue function. Tissue engineered skin is one of most advanced tissue constructs, yet it lacks several important functions including those provided by hair follicles, sebaceous glands, sweat glands and dendritic cells. Although the complexity of skin may be difficult to recapitulate entirely, new or improved functions can be provided by genetic modification of the cells that make up the tissues. Gene therapy can also be used in wound healing to promote tissue regeneration or prevent healing abnormalities such as formation of scars and keloids. Finally, gene-enhanced skin substitutes have great potential as cell-based devices to deliver therapeutics locally or systemically. Although significant progress has been made in the development of gene transfer technologies, several challenges have to be met before clinical application of genetically modified skin tissue. Engineering challenges include methods for improved efficiency and targeted gene delivery; efficient gene transfer to the stem cells that constantly regenerate the dynamic epidermal tissue; and development of novel biomaterials for controlled gene delivery. In addition, advances in regulatable vectors to achieve spatially and temporally controlled gene expression by physiological or exogenous signals may facilitate pharmacological administration of therapeutics through genetically engineered skin. Gene modified skin substitutes are also employed as biological models to understand tissue development or disease progression in a realistic three-dimensional context. In summary, gene therapy has the potential to generate the next generation of skin substitutes with enhanced capacity for treatment of burns, chronic wounds and even systemic diseases.

Human Langerhans-cell activation triggered in vitro by conditionally expressed MKK6 is counterregulated by the downstream effector RelB

Environmentally exposed epithelial Langerhans cells (LCs) encounter diverse innate stress signals, which lead to the activation of complex intracellular signaling cascades. Among these, p38 MAPK is consistently phosphorylated. For which aspects of LC activation triggering of p38 signaling is sufficient remains to be elucidated. We show that conditional induction of a dominant active form of MAPK kinase 6 (d.a.MKK6), a direct upstream kinase of p38, in LCs efficiently induces the up-regulation of costimulatory molecules and enhances their T-cell stimulatory capacity. These immediate effects showed no or only a minor requirement for classical NF-κB signaling. Concomitant with LC activation, d.a.MKK6 induced the alternative NF-κB member RelB, whose nuclear localization marks mature DCs. Specific inhibition of nuclear RelB during d.a.MKK6-induced LC activation further enhanced their maturation state. This observation was validated using the p38 activator anisomycin, thus suggesting a novel LC intrinsic control mechanism regulated by RelB.

Current status of transcriptional regulation systems

Many attempts have been undertaken to control transgene activity in mammalian cells. This is of importance for both applied biotechnology and basic research activities. State of the art regulatory systems use elements for transgene regulation which are unrelated to host regulatory networks and thus do not interfere with endogenous activities. Most of these regulation systems consist of transregulators and transregulator responding promoter elements that are derived from non mammalian origin. Apart from the tetracycline (Tet) regulated system which is most widely used for conditional gene expression at the moment, a number of new systems were created. These systems have been significantly refined and their performance makes them suitable for regulating transgenes not only in cellular systems but also in transgenic animals and for human therapeutic use.

Tight regulation of transgene expression by tetracycline-dependent activator and repressor in brain

Methods to temporally and spatially regulate gene mutations will provide a powerful strategy to investigate gene function in the brain. To develop these methods, we have established a tightly regulated system for transgene expression in the forebrain using both a tetracycline (Tc)-dependent transcription activator (rtTA) and a repressor (TetR-Kruppel-associated box). In this system, the repressor binds to the Tc-responsive element (TRE) in the absence of doxycycline (Dox), leading to the repression of leaky activation of TRE-mediated transcription caused by weak binding of rtTA to TRE. Upon Dox administration, only the activator binds to TRE and activates transcription. We tested this system in cultured cells by bicistronically expressing both the regulators using an internal ribosome entry site (IRES). In COS-1, HeLa and SHSY5Y cells, leaky transcription activation led by rtTA in the absence of Dox was repressed without decreasing the level of activated transcription in the presence of Dox. Using this system, transgenic mice were produced that express both the regulators using IRES in the forebrain under the control of the alphaCaMKII promoter and were bred with transgenic mice carrying the TRE-dependent reporter transgene. In reverse transcription-polymerase chain reaction and in situ hybridization analyses of the forebrain in adult double transgenic mice, the treatment of Dox induces reporter mRNA expression, which was not detected before the treatment and after the withdraw of Dox following the treatment. These results indicate that this system allows the tight regulation of transgene expression in a Dox-dependent fashion in the forebrain and will be useful in investigating gene function in the brain.

A versatile tool for conditional gene expression and knockdown

Drug-inducible systems allowing the control of gene expression in mammalian cells are invaluable tools for genetic research, and could also fulfill essential roles in gene- and cell-based therapy. Currently available systems, however, often have limited in vivo functionality because of leakiness, insufficient levels of induction, lack of tissue specificity or prohibitively complicated designs. Here we describe a lentiviral vector-based, conditional gene expression system for drug-controllable expression of polymerase (Pol) II promoter-driven transgenes or Pol III promoter-controlled sequences encoding small inhibitory hairpin RNAs (shRNAs). This system has great robustness and versatility, governing tightly controlled gene expression in cell lines, in embryonic or hematopoietic stem cells, in human tumors xenotransplanted into nude mice, in the brain of rats injected intraparenchymally with the vector, and in transgenic mice generated by infection of fertilized oocytes. These results open up promising perspectives for basic or translational research and for the development of gene-based therapeutics.

Bloom's syndrome gene-deficient phenotype in mouse primary cells induced by a modified tetracycline-controlled trans-silencer

We recently reported genome-wide bi-allelic mutagenesis and phenotype-based genetic screening by tetracycline-regulated disruption of the Bloom's syndrome gene (Blm) in mouse embryonic stem (ES) cells. However, the same approach was hampered in mouse tissues owing to leaky expression of the Blm gene, which is the major obstacle in the tetracycline regulatory system. Here we describe a single-chain reverse tetracycline-controlled trans-silencer (sc rtTS) which reduces leaky expression in the tet-off system. The sc rtTS consists of two silencer moieties linked by a 36 amino acid linker. Although the silencer moiety contained a dimerization domain compatible with the tetracycline-controlled transactivator (tTA), heterodimerization with tTA was prevented because intramolecular self-assembly between linked silencer moieties was preferred. The system was applied to mouse splenic lymphocytes and elevation of sister chromatid exchange, the hallmark of Blm dysfunction, was observed in the presence of doxycycline. A cassette containing both sc rtTS and tTA was introduced into the Blm allele in ES cells and reduction of basal activity was observed upon doxycycline treatment. Our data demonstrate effectiveness of sc rtTS in the tet-off system. Application of sc rtTS in mice may allow us to implement bi-allelic mutagenesis in vivo.

The 3' region of human papillomavirus type 16 early mRNAs decrease expression

Background

High risk human papillomavirus (HR-HPV) infects mucosal surfaces and HR-HPV infection is required for development of cervical cancer. Accordingly, enforced expression of the early HR-HPV proteins can induce immortalisation of human cells. In most cervical cancers and cervical cancer cell lines the HR-HPV double stranded DNA genome has been integrated into the host cell genome.

Methods

We have used a retroviral GUS reporter system to generate pools of stably transfected HaCaT and SiHa cells. The HPV-16 early sequences that are deleted upon integration of the HPV-16 genome was inserted into the 3' UTR of the reporter mRNA. Pools containing thousands of independent integrations were tested for the steady state levels of the reporter mRNA by Real Time PCR and reporter protein by a GUS enzymatic activity assays. In addition, we tested the cellular distribution and half lives of the reporter mRNAs. The integrity of the reporter mRNAs were tested by northern blotting.

Results

We show that the 3' region of the HPV-16 early mRNAs (HPV-16 nucleotide (nt.) 2582–4214) act in cis to decrease both mRNA and protein levels. This region seems to affect transcription from the exogenous minimal CMV promoter or processing of the reporter mRNA. The observed repression was most pronounced at the protein level, suggesting that this sequence may also affect translation. For the HPV types: 2, 6, 11, 13, 18, 30, 31, and 35 we have investigated the regulatory effect of the regions corresponding to the HPV-16 nt. 3358–4214. For all types, except HPV-18, the region was found to repress expression by posttranscriptional mechanisms.

Conclusion

We find that the 3' region of HPV-16 early mRNAs interfere with gene expression. It is therefore possible that the deletion of the 3' part of early HPV-16 mRNAs occurring during cervical oncogenesis could contribute to transformation of cells through deregulation of the viral oncogene synthesis. Moreover, we find that the corresponding region from several other HPV types also repress expression, suggesting that the repression by this region may be a general feature of the HPV life cycle.

Construction and characterization of multiple human colon cancer cell lines for inducibly regulated gene expression

Validation of targets for cancer drug discovery requires robust experimental models. Systems based on inducible gene expression are well suited to this purpose but are difficult to establish in several epithelial cell types. Using the recently discovered transcriptional transactivator (rtTA2S-M2), we developed a strategy for fast and efficient generation of Tet On cells. Multiple clones of HCT116, SW480, and HT29 human colon cancer cells for doxycycline-regulated gene expression were constructed that constitutively express green fluorescent protein (GFP) for selection/maintenance purposes. The cell lines displayed good fold inducibility (49-124xHCT116; 178-621xSW480; 261-787xHT29) and minimal leakiness after transient transfection with a luciferase reporter or with vectors driving inducible expression of red fluorescent protein (dsRed2), constitutively active c-Src or dominant negative K-Ras4B. The clones preserved their transformed phenotype as demonstrated by comparing their properties to respective wild type cells, in terms of growth in vitro and in vivo (as tumor xenografts), cell cycle traverse, and sensitivity to drugs used in chemotherapy. These engineered cell lines enabled tightly controlled inducible gene expression both in vitro and in vivo, and proved well suited for construction of double-stable cell lines inducibly expressing a protein of interest. As such they represent a useful research tool for example, to dissect oncogene function(s) in colon cancer. Supplementary material for this article be found at http://www.mrw.interscience.wiley.com/suppmat/0730-2312/suppmat/94/suppmat_welman.doc.

Visualization of protein interactions in living cells using bimolecular fluorescence complementation (BiFC) analysis

Protein interactions integrate stimuli from different signaling pathways and developmental programs. Bimolecular fluorescence complementation (BiFC) analysis has been developed for visualization of protein interactions in living cells. This approach is based on complementation between two fragments of a fluorescent protein when they are brought together by an interaction between proteins fused to the fragments, and it enables visualization of the subcellular locations of protein interactions in the normal cellular environment. It can be used for the analysis of many protein interactions and does not require information about the structures of the interaction partners. A multicolor BiFC approach has been developed for simultaneous visualization of interactions with multiple alternative partners in the same cell, based on complementation between fragments of engineered fluorescent proteins that produce bimolecular fluorescent complexes with distinct spectral characteristics. This enables comparison of subcellular distributions of different protein complexes in the same cell and allows analysis of competition between mutually exclusive interaction partners.

THY-1 induction is associated with up-regulation of fibronectin and thrombospondin-1 in human ovarian cancer

We recently identified THY-1 as a putative tumor suppressor gene for human ovarian cancer. To understand the carcinogenic role of THY-1, and its downstream effects on cancer cells, a THY-1 inducible system was established in the human ovarian cancer cell line SKOV-3 based on the tetracycline (tet) regulating system. To establish an inducible system for Thy-1 expression, two plasmids, pUHD172-1neo and pTEP4mThy-1, which are neomycin and hygromycin resistant were co-transfected into the ovarian carcinoma cell line, SKOV-3. The inducibility of Thy-1 expression in the SKOV-3 cell line by doxycycline (dox) was determined by northern blot analysis, immunocytochemistry, and flow cytometry. A time course study revealed that Thy-1 expression is induced 3 hours post dox exposure. Expression was reversible such that 12 hours post the removal of dox almost no Thy-1 could be detected. Furthermore, 2 genes, Fibronectin (FN) and Thrombospondin (TSP-1) involved in cellular differentiation and the regulation of tumor angiogenesis, respectively, were found to be up-regulated upon THY-1 induction. In contrast, the gene SPARC was found to be independent of Thy-1 expression. This study supports the hypothesis that THY-1 plays a critical role in regulating downstream genes associated with the regulation of ovarian tumor growth and cellular differentiation.

Optimization of tetracycline‐responsive recombinant protein production and effect on cell growth and ER stress in mammalian cells

The inducible T-REx system and other inducible expression systems have been developed in order to control the expression levels of recombinant protein in mammalian cells. In order to study the effects of heterologous protein expression on mammalian host behavior, the gene for recombinant Human transferrin (hTf) was integrated into HEK-293 cells and expressed under the control of the T-REx inducible technology (293-TetR-Hyg-hTf) or using a constitutive promoter (293-CMV-hTf). A number of inducible clones with variable expression levels were identified for the T-REx system with levels of hTf for the high expressing clones nearly double those obtained using the constitutive cytomegalovirus (CMV) promoter. The level of transferrin produced was found to increase proportionately with tetracycline concentration between 0 and 1 mug/mL with no significant increases in transferrin production above 1 mug/mL. As a result, the optimal induction time and tetracycline concentrations were determined to be the day of plating and 1 mug/mL, respectively. Interestingly, the cells induced to express transferrin, 293-TetR-Hyg-hTf, exhibited lower viable cell densities and percent viabilities than the uninduced cultures for multiple clonal isolates. In addition, the induction of transferrin expression was found to cause an increase in the expression of the ER-stress gene, BiP, that was not observed in the uninduced cells. However, both uninduced and induced cell lines containing the hTf gene exhibited longer survival in culture than the control cells, possibly as a result of the positive effects of hTf on cell survival. Taken together, these results suggest that the high level expression of complex proteins in mammalian cells can limit the viable cell densities of cells in culture as a result of cellular stresses caused by generating proteins that may be difficult to fold or are otherwise toxic to cells. The application of inducible systems such as the T-REx technology will allow us to optimize protein production while limiting the negative effects that result from these cellular stresses.

hCMV and Tet promoters for inducible gene expression in rat neurons in vitro and in vivo

To advance our understanding of the central nervous system, there is a need for refined approaches to control gene expression in neuronal culture as well as in the brain in vivo. In this study, we have applied a doxycycline-responsive Tet system to obtain a tightly controlled gene expression in neurons. In the absence of doxycycline, the Tet promoter-driven transgene expression was blocked by Tet transrepressor (tTR). Expression was doxycycline activated with the aid of a reverse Tet transactivator (rtTA). Application of both tTR and rtTA resulted in a much greater inducibility, as compared to rtTA alone, mainly due to a decreased basal level of expression. Such effects were observed when tTR and rtTA were driven in cultured neurons by the alpha CaMKII promoter. However, introduction of the human CMV major immediate-early promoter resulted only in a mediocre neuronal gene expression, unless the cells were treated, either in culture or in vivo, with depolarizing concentrations of KCl. Thus, in the present report, we have examined hCMV and Tet promoter inducibility in neurons to produce an important improvement in the functioning of the Tet system.

Doxycycline- and tetracycline-regulated transcriptional silencer enhance the expression level and transactivating performance of rtTA

BACKGROUND

The tetracycline-regulated transcriptional silencer (tTS) has been demonstrated to mitigate leaky expression of the tetracycline-inducible promoter under uninduced condition, and, when conjugated with reverse-type tetracycline-controlled transactivator (rtTA), shows great promise for gene therapy. This effect was attributed to the effectiveness of tTS as a repressor of transcription at the tetracycline-regulated promoter. However, we observed an unexpected increase in transactivational activity by rtTA in the presence of tTS under inducible condition.

METHODS

To explore the nature of this co-activational effect of tTS on rtTA, we examined the expression patterns of rtTA by Western blotting analysis of total cellular lysates or an enriched ubiquitinated pool of proteins under various conditions, including the one when proteasomal degradation is inhibited.

RESULTS

We demonstrate tTS, in addition to its established role as a transcriptional silencer, can enhance rtTA expression level by salvaging rtTA from the ubiquitin-dependent proteasomal degradation pathway. Along with this finding, we also demonstrate that doxycycline, a commonly used tetracycline analogue, inhibits the susceptibility of rtTA to ubiquitin/proteasome-mediated degradation and enhances the expression level of rtTA.

CONCLUSIONS

Taken together, our data establish an unappreciated role of doxycycline and tTS in tetracycline-regulated gene expression and the functionality of rtTA, and should shed light on the design of gene therapy vectors based on tetracycline-controlled transcriptional regulation systems.

Strategies of Conditional Gene Expression in Myocardium

The use of specialized reporter genes to monitor real-time, tissue-specific transgene expression in animal models offers an opportunity to circumvent current limitations associated with the establishment of transgenic mouse models. The Cre-loxP and the tetracycline (Tet)-inducible systems are useful methods of conditional gene expression that allow spatial (cell-type-specific) and temporal (inducer-dependent) control. Most often, the alpha-myosin heavy chain (alpha-MHC) promoter is used in these inducible systems to restrict expression of reporter genes and transgenes to the myocardium. An overview of each inducible system is described, along with suggested reporter genes for real-time, noninvasive imaging in the myocardium. Effective gene delivery of the inducible gene expression system is carried out by lentiviral vectors, which offer high transduction efficiency, long-term transgene expression, and low immunogenicity. This chapter outlines the packaging of myocardium-specific inducible expression systems into lentiviral vectors, in which a transgene and a reporter gene are transduced into cardiomyocytes. In doing so, transgene and reporter expression can be monitored/tracked with bioluminescence imaging (BLI) and positron emission tomography (PET).

cCXCR1 is a receptor for cIL-8 (9E3/cCAF) and its N- and C-terminal peptides and is also activated by hIL-8 (CXCL8)

Chemokines are chemotactic cytokines that play important roles in immune responses and wound healing, as well as in pathological conditions such as chronic inflammation and tumorigenesis. The chemokines and their receptors are highly conserved and maintain similar functions in different species. One noteworthy exception is the chemokine interleukin (IL)8/CXC ligand 8 and its specific receptor CXCR1, which are found in humans but are not found in the traditional model organisms, mice and rats. As a consequence, we are using model organisms other than mice to study the functions of IL-8 and CXCR1, as well as the mechanisms involved in receptor activation by IL-8. Toward this goal, we have isolated and characterized a new receptor that is highly homologous to human (h)CXCR1, which we named chicken (c)CXCR1. To determine whether this receptor is activated by cIL-8 and its N- and C-terminal peptides and whether it responds to hIL-8, we expressed cCXCR1 in NIH3T3 cells, which naturally lack this receptor, and used single-cell Ca(2)(+) imaging to detect increases in intracellular Ca(2)(+) and immunoblot analysis to detect extracellular signal-regulated kinase 1/2 phosphorylation. We show that cIL-8, its N and C peptides, and hIL-8 activate cCXCR1. We further show that cIL-8 and hIL-8 stimulate chemotaxis of chicken embryonic fibroblasts, cells that express cCXCR1, and that this effect is specific for each chemokine and this receptor. These results strongly suggest that cCXCR1 is the ortholog for hCXCR1 and that chickens can be used as an effective model system to study the functions of IL-8, its terminal peptides, and its specific receptor CXCR1.

The N- and C-terminal peptides of hIL8/CXCL8 are ligands for hCXCR1 and hCXCR2

Chemokines are small cytokines that function in immune responses, wound healing, and pathological conditions such as chronic inflammation and tumorigenesis. This multifunctionality has been attributed primarily to ligand interaction with multiple or dimerized receptors. However, multifunctionality could also result from interactions of the receptors with small peptides produced by processing of the chemokines. Chemokine peptides are functional in vivo, but it is not yet known whether they can interact with and activate their receptors. The work presented here examines the interactions between the two forms of human interleukin 8 (hIL-8), and its N- and C-peptides, with the chemokine receptors hCXCR1 and hCXCR2. We used a Tet-on retroviral system to introduce CXCR1 into mouse NIH 3T3 cells (that lack endogenous CXCR1) and monitored activation of this receptor by the ligands by using quantitative Ca2+ imaging and mitogen-activated protein kinase (MAPK) activation. We found that the N and C termini of the chemokine can stimulate the respective CXCR1 to induce intracellular Ca2+ release and MAPK activation independent of the other regions of the molecules. Furthermore, we showed that these peptides can also stimulate chemotaxis of several cell types, including primary human microvascular endothelial cells, and that this function is specific and mediated by hCXCR1 and/or hCXCR2. These findings advance understanding of the multifunctionality exhibited by chemokines, reveal a new mode of functional regulation, and may serve as the basis for therapeutic targeting.

Prolonged and inducible transgene expression in the liver using gutless adenovirus: a potential therapy for liver cancer

BACKGROUND AND AIMS

Gene therapy of liver diseases would benefit from systems allowing prolonged, regulable, and tissue-specific transgene expression. We attempted to produce a vector fulfilling these requirements.

METHODS

We generated gutless adenoviral vectors containing a mifepristone (RU486)-inducible system for controlled and liver-specific expression of human interleukin-12 (hIL-12) (GL-Ad/RUhIL-12) and mouse IL-12 (mIL-12) (GL-Ad/RUmIL-12). The properties of these vectors were tested both in vitro and in vivo.

RESULTS

Infection of cells with GL-Ad/RUhIL-12 resulted in high level of hIL-12 expression in the presence of RU486 only in hepatocytic cells. In animals injected with GL-Ad/RUhIL-12, the administration of RU486 induced a transient rise of serum hIL-12 that peaked at 10 hours and completely disappeared by 72 hours. The peak value of hIL-12 was dependent on the doses of the vector and the inducer. High and sustained serum levels of hIL-12 could be attained by continuing administration of RU486 every 12 or 24 hours. Repetitive induction of hIL-12 could be obtained over, at least, a period of 48 weeks after a single injection of GL-Ad/RUhIL-12. Although the vector was detected in many tissues after systemic injection, transcription of the transgene was only found in the liver. Treatment of liver metastases with 5 x 10(8) infectious units of GL-Ad/RUmIL-12 plus RU846 resulted in complete tumor regression in all animals.

CONCLUSION

Gutless adenoviral vectors allow liver-specific and regulable transgene expression for prolonged periods of time. These vectors are promising tools for gene therapy of liver cancer and could also be useful for other forms of hepatic disease.

Effects of substrate stiffness on cell morphology, cytoskeletal structure, and adhesion

The morphology and cytoskeletal structure of fibroblasts, endothelial cells, and neutrophils are documented for cells cultured on surfaces with stiffness ranging from 2 to 55,000 Pa that have been laminated with fibronectin or collagen as adhesive ligand. When grown in sparse culture with no cell-cell contacts, fibroblasts and endothelial cells show an abrupt change in spread area that occurs at a stiffness range around 3,000 Pa. No actin stress fibers are seen in fibroblasts on soft surfaces, and the appearance of stress fibers is abrupt and complete at a stiffness range coincident with that at which they spread. Upregulation of alpha5 integrin also occurs in the same stiffness range, but exogenous expression of alpha5 integrin is not sufficient to cause cell spreading on soft surfaces. Neutrophils, in contrast, show no dependence of either resting shape or ability to spread after activation when cultured on surfaces as soft as 2 Pa compared to glass. The shape and cytoskeletal differences evident in single cells on soft compared to hard substrates are eliminated when fibroblasts or endothelial cells make cell-cell contact. These results support the hypothesis that mechanical factors impact different cell types in fundamentally different ways, and can trigger specific changes similar to those stimulated by soluble ligands.

Regulation of HIF prolyl hydroxylases by hypoxia-inducible factors

Hypoxia and induction of hypoxia-inducible factors (HIF-1alpha and HIF-2alpha) is a hallmark of many tumors. Under normal oxygen tension HIF-alpha subunits are rapidly degraded through prolyl hydroxylase dependent interaction with the von Hippel-Lindau (VHL) tumor suppressor protein, a component of E3 ubuiquitin ligase complex. Using microarray analysis of VHL mutated and re-introduced cells, we found that one of the prolyl hydroxylases (PHD3) is coordinately expressed with known HIF target genes, while the other two family members (PHD1 and 2) did not respond to VHL. We further tested the regulation of these genes by HIF-1 and HIF-2 and found that siRNA targeted degradation of HIF-1alpha and HIF-2alpha results in decreased hypoxia-induced PHD3 expression. Ectopic overexpression of HIF-2alpha in two different cell lines provided a much better induction of PHD3 gene than HIF-1alpha. In contrast, we demonstrate that PHD2 is not affected by overexpression or downregulation of HIF-2alpha. However, induction of PHD2 by hypoxia has HIF-1-independent and -dependent components. Short-term hypoxia (4 h) results in induction of PHD2 independent of HIF-1, while PHD2 accumulation by prolonged hypoxia (16 h) was decreased by siRNA-mediated degradation of HIF-1alpha subunit. These data further advance our understanding of the differential role of HIF factors and putative feedback loop in HIF regulation.

Gene regulation by tetracyclines

Gene regulation by tetracyclines has become a widely-used tool to study gene functions in pro- and eukaryotes. This regulatory system originates from Gram-negative bacteria, in which it fine-tunes expression of a tetracycline-specific export protein mediating resistance against this antibiotic. This review attempts to describe briefly the selective pressures governing the evolution of tetracycline regulation, which have led to the unique regulatory properties underlying its success in manifold applications. After discussing the basic mechanisms we will present the large variety of designed alterations of activities which have contributed to the still growing tool-box of components available for adjusting the regulatory properties to study gene functions in different organisms or tissues. Finally, we provide an overview of the various experimental setups available for pro- and eukaryotes, and touch upon some highlights discovered by the use of tetracycline-dependent gene regulation.

A novel tetracycline-controlled transactivator-transrepressor system enables external control of oncolytic adenovirus replication

The use of restricted replication-competent adenoviruses (RRCAs) inducing tumor cell-specific lysis is a promising approach in cancer gene therapy. However, the use of RRCAs in humans carries considerable risk, since after injection into the patient, further regulation or inhibition of virus replication from the outside is impossible. Therefore, we have developed a novel system allowing external pharmacological control of RRCA replication. We show here that a tumor-selective E1B-deleted RRCA can be tightly regulated by use of doxycycline (dox)-controlled adenoviral E1A gene expression, which in turn determines vector replication. RRCA replication is switched on by addition and switched off by withdrawal of dox. The system results in efficient tumor cell killing after induction by dox, whereas cells are unaffected by the uninduced system. It was also employed for efficient external control of transgene expression from cotransfected replication-deficient adenovectors. Furthermore, the use of a liver cell-specific human alpha1-antitrypsin (hAAT)-promoter driving a tetracycline-controlled transcriptional silencer allowed specific protection of cells with hAAT-promoter activity in the absence of dox in vitro and in vivo, delineating a new principle of 'tissue protective' gene therapy. The concept of external control of RRCAs may help to improve the safety of cancer gene therapy.

An ecdysone and tetracycline dual regulatory expression system for studies on Rac1 small GTPase-mediated signaling

Regulated expression systems are invaluable for studying gene function, offer advantages of dosage-dependent and temporally defined gene expression, and limit possible clonal variation when toxic or pleiotropic genes are overexpressed. Previously, establishment of inducible expression systems, such as tetracycline- and ecdysone-inducible systems, required assessment of the inducible characteristics of individual clones by tedious luciferase assays. Taking advantage of a green fluorescent protein (GFP) reporter controlled by tetracycline- or ecdysone-responsive element and fluorescence-activated cell sorting, we propose a simple and efficient strategy to select highly inducible cell lines according to their fluorescence profiles after transiently transfecting the candidate cell pools with a surrogate GFP reporter. We have demonstrated that tetracycline- and ecdysone-inducible systems could be set up in Madin-Darby canine kidney and HEK-293 cells by employing this selection scheme. Importantly, this dual regulatory expression system is applied in studying the complex interplay between two Ras-related small GTPases, Cdc42 and Rac1, on detachment-induced apoptosis. Furthermore, establishment of two tightly regulated expression systems in one target cell line could be of great advantage for dissecting small GTPase Rac1-transduced signaling pathways by using global gene expression approaches such as proteomic assays.

Gene regulation by tetracyclines. Constraints of resistance regulation in bacteria shape TetR for application in eukaryotes

The Tet repressor protein (TetR) regulates transcription of a family of tetracycline (tc) resistance determinants in Gram-negative bacteria. The resistance protein TetA, a membrane-spanning H+-[tc.M]+ antiporter, must be sensitively regulated because its expression is harmful in the absence of tc, yet it has to be expressed before the drugs' concentration reaches cytoplasmic levels inhibitory for protein synthesis. Consequently, TetR shows highly specific tetO binding to reduce basal expression and high affinity to tc to ensure sensitive induction. Tc can cross biological membranes by diffusion enabling this inducer to penetrate the majority of cells. These regulatory and pharmacological properties are the basis for application of TetR to selectively control the expression of single genes in lower and higher eukaryotes. TetR can be used for that purpose in some organisms without further modifications. In mammals and in a large variety of other organisms, however, eukaryotic transcriptional activator or repressor domains are fused to TetR to turn it into an efficient regulator. Mechanistic understanding and the ability to engineer and screen for mutants with specific properties allow tailoring of the DNA recognition specificity, the response to inducer tc and the dimerization specificity of TetR-based eukaryotic regulators. This review provides an overview of the TetR properties as they evolved in bacteria, the functional modifications necessary to transform it into a convenient, specific and efficient regulator for use in eukaryotes and how the interplay between structure--function studies in bacteria and specific requirements of particular applications in eukaryotes have made it a versatile and highly adaptable regulatory system.