Construction and properties of an Epstein-Barr-virus-derived cDNA expression vector for human cells

ROS quenching potential of the epidermal cornified cell envelope

The cornified cell envelope (CE) is a specialized structure assembled beneath the plasma membrane of keratinocytes in the outermost layers of the epidermis. It is essential for the physical and permeability properties of the barrier function of the skin. Our skin is continuously exposed to atmospheric oxygen and threatened by reactive oxygen species (ROS). Here, we identify the CE as a first line of antioxidant defense and show that the small proline-rich (SPRR) family of CE precursor proteins have a major role in ROS detoxification. Cysteine residues within these proteins are responsible for ROS quenching, resulting in inter- and intramolecular S-S bond formation, both in isolated proteins and purified CEs. The related keratinocyte proline-rich protein is also oxidized on several cysteine residues within the CE. Differences in antioxidant potential between various SPRR family members are likely determined by structural differences rather than by the amount of cysteine residues per protein. Loricrin, a major component of the CE with a higher cysteine content than SPRRs, is a weak ROS quencher and oxidized on a single cysteine residue within the CE. It is inferred that SPRR proteins provide the outermost layer of our skin with a highly adaptive and protective antioxidant shield.

Global proteome analysis of hepatitis B virus expressing human hepatoblastoma cell line HepG2

In countries where hepatitis B virus (HBV) is endemic, a high incidence of hepatocellular carcinoma (HCC) occur in HBV carriers and the prolonged replication and expression of HBV proteins in the liver is considered an important risk factor for progression to malignancy. However, the mechanism of pathogenesis of HBV-associated carcinoma remains elusive. In this study, the human hepatoblastoma HepG2 cell line harboring 1.2 x unit-length of the HBV genome was generated and subjected to a proteomic approach analyzing the global protein expression profiles of HepG2 cells with and without HBV replication and protein expression. By using fluorescence two-dimensional difference gel electrophoresis (2D-DIGE), followed by MALDI-TOF-MS and database searching, a total of 50 differentially expressed proteins were identified, including some cell cycle-related proteins. These cycle-related proteins may lead to accumulation of HepG2-HBV cells in the G2/M phase, and an increase in the proportion of HepG2 cells with tripolar or multipolar spindles. This study described the proteomic alterations in HepG2 cells HBV-harboring, which may provide new insights into the underlying molecular mechanisms involved in HBV replication and pathogenesis.

Transcriptional signature and memory retention of human-induced pluripotent stem cells

Genetic reprogramming of somatic cells to a pluripotent state (induced pluripotent stem cells or iPSCs) by over-expression of specific genes has been accomplished using mouse and human cells. However, it is still unclear how similar human iPSCs are to human Embryonic Stem Cells (hESCs). Here, we describe the transcriptional profile of human iPSCs generated without viral vectors or genomic insertions, revealing that these cells are in general similar to hESCs but with significant differences. For the generation of human iPSCs without viral vectors or genomic insertions, pluripotent factors Oct4 and Nanog were cloned in episomal vectors and transfected into human fetal neural progenitor cells. The transient expression of these two factors, or from Oct4 alone, resulted in efficient generation of human iPSCs. The reprogramming strategy described here revealed a potential transcriptional signature for human iPSCs yet retaining the gene expression of donor cells in human reprogrammed cells free of viral and transgene interference. Moreover, the episomal reprogramming strategy represents a safe way to generate human iPSCs for clinical purposes and basic research.

A single EBV-based vector for stable episomal maintenance and expression of GFP in human embryonic stem cells

Aim: Stable expression of transgenes in stem cells has been a challenge due to the nonavailability of efficient transfection methods and the inability of transgenes to support sustained gene expression. Several methods have been reported to stably modify both embryonic and adult stem cells. These methods rely on integration of the transgene into the genome of the host cell, which could result in an expression pattern dependent on the number of integrations and the genomic locus of integration. To overcome this issue, site-specific integration methods mediated by integrase, adeno-associated virus or via homologous recombination have been used to generate stable human embryonic stem cell (hESC) lines. In this study, we describe a vector that is maintained episomally in hESCs. Methods: The vector used in this study is based on components derived from the Epstein–Barr virus, containing the Epstein–Barr virus nuclear antigen 1 expression cassette and the OriP origin of replication. The vector also expresses the drug-resistance marker gene hygromycin, which allows for selection and long-term maintenance of cells harboring the plasmid. Results: Using this vector system, we show sustained expression of green fluorescent protein in undifferentiated hESCs and their differentiating embryoid bodies. In addition, the stable hESC clones show comparable expression with and without drug selection. Consistent with this observation, bulk-transfected adipose tissue-derived mesenchymal stem cells showed persistent marker gene expression as they differentiate into adipocytes, osteoblasts and chondroblasts. Conclusions: Episomal vectors offer a fast and efficient method to create hESC reporter lines, which in turn allows one to test the effect of overexpression of various genes on stem cell growth, proliferation and differentiation.

A method for isolating prosurvival targets of NF-kappaB/Rel transcription factors

NF-KappaB/Rel transcription factors are critical regulators of immunity, inflammation, development, and cell survival. Activation of NF-KB inhibits programmed cell death (PCD) triggered by tumor necrosis factor alpha (TNFalpha) and several other stimuli. The prosurvival activity of NF-KB is also crucial to lymphopoiesis, neuroprotection, tumorigenesis, and cancer chemoresistance. The characterization of the downstream targets that mediate the prosurvival activity of NF-KB is therefore a topic of intense investigation. Early screens aimed at identifying these genes were mainly based on expression criteria and so were poised to only isolate genes already known to have protective effects. Here, we describe a new method for the identification of these genes, whereby expression libraries are screened for their ability to halt PCD in NF-KB-deficient cells. This complementation approach provides substantial advantages over other approaches, as it enables functional assessment of isolated genes without any preconceived notion about their sequence or presumed role. Expression libraries are generated from cells that are resistant to TNFalpha-induced cytotoxicity and are then enriched in prosurvival genes upon selection with TNFa in NF-kappaB/RelA-null cells, which are highly susceptible instead to this cytotoxicity. Upon enrichment, libraries are screened through a randomized two-step approach, whereby cDNAs are first tested for cytoprotective function and then for differential expression in NF-kappaB-proficient and NF-KappaB-deficient cells.

Assessing the functional characteristics of synonymous and nonsynonymous mutation candidates by use of large DNA constructs

As we identify more and more genetic changes, either through mutation studies or population screens, we need powerful tools to study their potential molecular effects. With these tools, we can begin to understand the contributions of genetic variations to the wide range of human phenotypes. We used our catalogue of molecular changes in patients with carbamyl phosphate synthetase I (CPSI) deficiency to develop such a system for use in eukaryotic cells. We developed the tools and methods for rapidly modifying bacterial artificial chromosomes (BACs) for eukaryotic episomal replication, marker expression, and selection and then applied this protocol to a BAC containing the entire CPSI gene. Although this CPSI BAC construct was suitable for studying nonsynonymous mutations, potential splicing defects, and promoter variations, our focus was on studying potential splicing and RNA-processing defects to validate this system. In this article, we describe the construction of this system and subsequently examine the mechanism of four putative splicing mutations in patients deficient in CPSI. Using this model, we also demonstrate the reversible role of nonsense-mediated decay in all four mutations, using small interfering RNA knockdown of hUPF2. Furthermore, we were able to locate cryptic splicing sites for the two intronic mutations. This BAC-based system permits expression studies in the absence of patient RNA or tissues with relevant gene expression and provides experimental flexibility not available in genomic DNA or plasmid constructs. Our splicing and RNA degradation data demonstrate the advantages of using whole-gene constructs to study the effects of sequence variation on gene expression and function.

Radiation response and cell cycle regulation of p53 rescued malignant keratinocytes

Mutations in the tumor suppressor gene p53 were found in more than 90% of all human squamous cell carcinomas (SCC). To study the function of p53 in a keratinocyte background, a tetracycline-controlled p53 transgene was introduced into a human SCC cell line (SCC15), lacking endogenous p53. Conditional expression of wild-type p53 protein upon withdrawal of tetracycline was accompanied with increased expression of p21(WAF1/Cip1) resulting in reduced cell proliferation. Flow-cytometric analysis revealed that these cells were transiently arrested in the G1/S phase of the cell cycle. However, when SCC15 cells expressing p53 were exposed to ionizing radiation (IR), a clear shift from a G1/S to a G2/M cell cycle arrest was observed. This effect was greatly depending on the presence of wild-type p53, as it was not observed to the same extent in SCC15 cells lacking p53. Unexpectedly, the p53- and IR-dependent G2/M cell cycle arrest in the keratinocyte background was not depending on increased expression or stabilization of 14-3-3sigma, a p53-regulated effector of G2/M progression in colorectal cancer cells. In keratinocytes, 14-3-3sigma (stratifin) is involved in terminal differentiation and its cell cycle function in this cell type might diverge from the one it fulfills in other cellular backgrounds.

Repair characteristics and differentiation propensity of long-term cultures of epidermal keratinocytes derived from normal and NER-deficient mice

Epidermal keratinocytes constitute the most relevant cellular system in terms of DNA damage because of their continuous exposure to UV light and genotoxic chemicals from the environment. Here, we describe the establishment of long-term keratinocyte cultures from the skin of wild-type and nucleotide excision repair (NER) deficient mouse mutants. The use of media with a lowered calcium concentration and the inclusion of keratinocyte growth factor (KGF) permitted repeated passaging of the cultures and resulted in the generation of stable cell lines that proliferated efficiently. The cells retained their normal ability to engage into terminal differentiation when triggered with high calcium concentrations or after suspension in semi-solid medium. The cultures reflected the cellular characteristics (i.e. repair and transcription profiles) of the Xpa(-/-), Xpc(-/-), Csb(-/-) and Xpd(TTD) mouse models from which they were derived. For instance, in line with earlier in vivo results, Xpd(TTD) keratinocytes were disturbed in their ability to terminally differentiate in vitro. This was concluded from a delay in calcium-induced stratification and by reduced transcription of both early (keratin 10) and late (loricrin) terminal differentiation marker genes. UDS measurements in wild-type cells committed to terminal differentiation did not reveal any reduction in global DNA repair that could be indicative of differentiation associated repair (DAR) as found in neurons. UV sensitivity data revealed that in keratinocytes global genome repair contributes more to cell survival than previously concluded from fibroblast studies. It is inferred that these fully controllable in vitro cultures will be a valuable tool to assess critical parameters of genome care-taking systems in cell proliferation and differentiation.

New perspectives on the assembly process of mitochondrial respiratory chain complex cytochrome c oxidase

The assembly of cytochrome c oxidase (COX) is a complicated process and requires a number of assembly factors to put all the necessary subunits in the correct position. Defects in COX assembly lead in particular to serious neuromuscular disorders. We demonstrated that COX-deficient patients can be associated with different assembly patterns. To obtain more insight in the biogenesis of COX in a living cell, we used yeast as a model organism to design a way to pulse label holo-COX with green fluorescent protein (GFP). Using blue native electrophoresis, we showed that the GFP-tagged subunit is incorporated into fully assembled COX and this GFP tagged complex still has enzymatic activity. This allows us to correlate the GFP fluorescence signal detected in vivo by microscopy with the synthesis, turnover and assembly of COX.

Structural organization and regulation of the small proline-rich family of cornified envelope precursors suggest a role in adaptive barrier function

The protective barrier provided by stratified squamous epithelia relies on the cornified cell envelope (CE), a structure synthesized at late stages of keratinocyte differentiation. It is composed of structural proteins, including involucrin, loricrin, and the small proline-rich (SPRR) proteins, all encoded by genes localized at human chromosome 1q21. The genetic characterization of the SPRR locus reveals that the various members of this multigene family can be classified into two distinct groups with separate evolutionary histories. Whereas group 1 genes have diverged in protein structure and are composed of three different classes (SPRR1 (2x), SPRR3, and SPRR4), an active process of gene conversion has counteracted diversification of the protein sequences of group 2 genes (SPRR2 class, seven genes). Contrasting with this homogenization process, all individual members of the SPRR gene family show specific in vivo and in vitro expression patterns and react selectively to UV irradiation. Apparently, creation of regulatory rather than structural diversity has been the driving force behind the evolution of the SPRR gene family. Differential regulation of highly homologous genes underlines the importance of SPRR protein dosage in providing optimal barrier function to different epithelia, while allowing adaptation to diverse external insults.

Microtubule-associated protein 4 (MAP4) regulates assembly, protomer-polymer partitioning and synthesis of tubulin in cultured cells

We depleted MAP4, a ubiquitously expressed microtubule (MT)-associated protein previously shown to be capable of stabilizing MTs, from HeLa cells by stably expressing antisense RNA. These HeLa-AS cells, in which the MAP4 level was decreased to 33% of the wild-type level, displayed decreased content of total tubulin (65% of the wild-type level). The partitioning of cellular tubulin into protomer and polymer was altered in HeLa-AS cells: polymeric tubulin was decreased to 46% of the level in control cells, while protomeric tubulin was increased to 226% of the level in control cells. Tubulin protein synthesis was decreased, consistent with the tubulin autoregulation model, which proposes that tubulin protomer inhibits its own synthesis. Following release from drug-induced depolymerization, MTs in HeLa-AS cells reformed more slowly, and showed an increased focus on the centrosome, as compared to control cells. HeLa-AS cells also appeared to be less bipolar in shape and flatter than control cells. Our data suggest that MAP4 regulates assembly level of MTs and, perhaps through this mechanism, is involved in controlling spreading and shape of cells.

The muscle-specific laminin receptor alpha7 beta1 integrin negatively regulates alpha5 beta1 fibronectin receptor function

alpha7 beta1 is the major integrin complex expressed in differentiated muscle cells where it functions as a laminin receptor. In this work we have expressed the alpha7 integrin subunit in CHO cells to investigate the functional properties of this receptor. After transfection with alpha7 CHO cells acquired the ability to adhere and spread on laminin 1 consistent with the laminin receptor activity of the alpha7 beta1. alpha7 transfectants, however, showed a 70% reduction in the ability to adhere to fibronectin and were unable to assemble a fibronectin matrix. The degree of reduction was inversely related to the level of alpha7 expression. To define the mechanisms underlying this adhesive defect we analyzed surface expression and functional properties of the alpha5 beta1 fibronectin receptor. Although cell surface expression of alpha5 beta1 was reduced by a factor of 20-25% in alpha7 transfectants compared to control untransfected cells, this slight reduction was not sufficient to explain the dramatic reduction in cell adhesion (70%) and matrix assembly (close to 100%). Binding studies showed that the affinity of 125I-fibronectin for its surface receptor was decreased by 50% in alpha7 transfectants, indicating that the alpha5 beta1 integrin is partially inactivated in these cells. Inactivation can be reversed by Mn2+, a cation known to increase integrin affinity for their ligands. In fact, incubation of cells with Mn2+ restored fibronectin binding affinity, adhesion to fibronectin, and assembly of fibronectin matrix in alpha7 transfectants. These data indicate that alpha7 expression leads to the functional down regulation of alpha5beta1 integrin by decreasing ligand binding affinity and surface expression. In conclusion, the data reported establish the existence of a negative cooperativity between alpha7 and alpha5 integrins that may be important in determining functional regulation of integrins during myogenic differentiation.

Importance of poly(ADP-ribose) polymerase and its cleavage in apoptosis. Lesson from an uncleavable mutant

We have studied the apoptotic response of poly(ADP-ribose) polymerase (PARP)-/- cells to different inducers and the consequences of the expression of an uncleavable mutant of PARP on the apoptotic process. The absence of PARP drastically increases the sensitivity of primary bone marrow PARP-/- cells to apoptosis induced by an alkylating agent but not by a topoisomerase I inhibitor CPT-11 or by interleukin-3 removal. cDNA of wild type or of an uncleavable PARP mutant (D214A-PARP) has been introduced into PARP-/- fibroblasts, which were exposed to anti-CD95 or an alkylating agent to induce apoptosis. The expression of D214A-PARP results in a significant delay of cell death upon CD95 stimulation. Morphological analysis shows a retarded cell shrinkage and nuclear condensation. Upon treatment with an alkylating agent, expression of wild-type PARP cDNA into PARP-deficient mouse embryonic fibroblasts results in the restoration of the cell viability, and the D214A-PARP mutant had no further effect on cell recovery. In conclusion, PARP-/- cells are extremely sensitive to apoptosis induced by triggers (like alkylating agents), which activates the base excision repair pathway of DNA, and the cleavage of PARP during apoptosis facilitates cellular disassembly and ensures the completion and irreversibility of the process.

DNA repair defect in poly(ADP-ribose) polymerase-deficient cell lines

To investigate the physiological function of poly(ADP-ribose) polymerase (PARP), we used a gene targeting strategy to generate mice lacking a functional PARP gene. These PARP -/- mice were exquisitely sensitive to the monofunctional-alkylating agent N -methyl- N -nitrosourea (MNU) and gamma-irradiation. In this report, we have analysed the cause of this increased lethality using primary and/or spontaneously immortalized mouse embryonic fibroblasts (MEFs) derived from PARP -/- mice. We found that the lack of PARP renders cells significantly more sensitive to methylmethanesulfonate (MMS), causing cell growth retardation, G2/M accumulation and chromosome instability. An important delay in DNA strand-break resealing was observed following treatment with MMS. This severe DNA repair defect appears to be the primary cause for the observed cytoxicity of monofunctional-alkylating agents, leading to cell death occurring after G2/M arrest. Cell viability following MMS treatment could be fully restored after transient expression of the PARP gene. Altogether, these results unequivocally demonstrate that PARP is required for efficient base excision repair in vivo and strengthens the role of PARP as a survival factor following genotoxic stress.

beta1-integrin cytoplasmic subdomains involved in dominant negative function

The beta1-integrin cytoplasmic domain consists of a membrane proximal subdomain common to the four known isoforms ("common" region) and a distal subdomain specific for each isoform ("variable" region). To investigate in detail the role of these subdomains in integrin-dependent cellular functions, we used beta1A and beta1B isoforms as well as four mutants lacking the entire cytoplasmic domain (beta1TR), the variable region (beta1COM), or the common region (beta1 deltaCOM-B and beta1 deltaCOM-A). By expressing these constructs in Chinese hamster ovary and beta1 integrin-deficient GD25 cells (Wennerberg et al., J Cell Biol 132, 227-238, 1996), we show that beta1B, beta1COM, beta1 deltaCOM-B, and beta1 deltaCOM-A molecules are unable to support efficient cell adhesion to matrix proteins. On exposure to Mn++ ions, however, beta1B, but none of the mutants, can mediate cell adhesion, indicating specific functional properties of this isoform. Analysis of adhesive functions of transfected cells shows that beta1B interferes in a dominant negative manner with beta1A and beta3/beta5 integrins in cell spreading, focal adhesion formation, focal adhesion kinase tyrosine phosphorylation, and fibronectin matrix assembly. None of the beta1 mutants tested shows this property, indicating that the dominant negative effect depends on the specific combination of common and B subdomains, rather than from the absence of the A subdomain in the beta1B isoform.

The design of a new mutation model for active genes: expression of the Escherichia coli lac operon in mammalian cells

The design of a novel transgenic mouse model is described that should allow analysis of mutations at a single cell level in all tissues of a model animal. The model is based on the correct regulation of the Escherichia coli lac operon in mammalian cells. Induction of a mutation in the lacI gene will result in the loss of transcriptional repression of the lacZ gene in mutated cells. Expression of beta-galactosidase can subsequently be detected at the single cell level. The model was first tested in vitro using transfection of mouse LTK- cells. LacZ expression was very heterogeneous in most of the stable transfectants and seemed to be subject to epigenetic inactivation. One clone (IIB1) was isolated that stably expressed lacZ in more than 99% of its cells. Subsequent introduction of the lacI gene into IIB1 cells resulted in correct transcriptional repression of the lacZ gene that could be alleviated by IPTG, an allosteric inducer of lacI repression. However, in time the extent of beta-galactosidase induction gradually declined suggesting that the prolonged repressed transcriptional state triggers epigenetic inactivation. Variegated expression of the lacZ gene was not confined to cultured cells since several transgenic lines also did not express the lacZ transgene. This study shows that while the susceptibility of the lacZ gene to inactivation processes poses a fundamental problem, correct regulation of the expression of a reporter gene by the lacI repressor protein is feasible in mammalian cells when assayed at the single cell level. Thus, the model can in principle be used for the detection of mutagenic events at the lacI locus. Targeting of the lacZ gene to an endogenous housekeeping gene might prevent epigenetic inactivation. Alternatively, with the use of another reporter gene in the mutation detection system the proposed transgenic mouse model could be realized.

Scrapie susceptibility-linked polymorphisms modulate the in vitro conversion of sheep prion protein to protease-resistant forms

Prion diseases are natural transmissible neurodegenerative disorders in humans and animals. They are characterized by the accumulation of a protease-resistant scrapie-associated prion protein (PrPSc) of the host-encoded cellular prion protein (PrPC) mainly in the central nervous system. Polymorphisms in the PrP gene are linked to differences in susceptibility for prion diseases. The mechanisms underlying these effects are still unknown. Here we describe studies of the influence of sheep PrP polymorphisms on the conversion of PrPC into protease-resistant forms. In a cell-free system, sheep PrPSc induced the conversion of sheep PrPC into protease-resistant PrP (PrP-res) similar or identical to PrPSc. Polymorphisms present in either PrPC or PrPSc had dramatic effects on the cell-free conversion efficiencies. The PrP variant associated with a high susceptibility to scrapie and short survival times of scrapie-affected sheep was efficiently converted into PrP-res. The wild-type PrP variant associated with a neutral effect on susceptibility and intermediate survival times was converted with intermediate efficiency. The PrP variant associated with scrapie resistance and long survival times was poorly converted. Thus the in vitro conversion characteristics of the sheep PrP variants reflect their linkage with scrapie susceptibility and survival times of scrapie-affected sheep. The modulating effect of the polymorphisms in PrPC and PrPSc on the cell-free conversion characteristics suggests that, besides the species barrier, polymorphism barriers play a significant role in the transmissibility of prion diseases.

Expression of the SPRR cornification genes is differentially affected by carcinogenic transformation

The small proline rich protein (SPRR) genes constitute a family of conserved genes which are part of the human epidermal differentiation complex (EDC) on chromosome 1q21 and code for precursor proteins of the cornified cell envelope. The expression of these genes is strictly linked to keratinocyte terminal differentiation both in vivo and in vitro. Here we show that cultured cell lines derived from squamous cell carcinoma (SCC) show significantly lower levels of SPRR expression than normal human keratinocytes. However, the residual SPRR expression in SCC lines appears to be both gene and cell line specific. Expression of SPRR2 appears to correlate well with the residual ability of these cells to differentiate. However, the kinetics of SPRR2 expression, following treatment with calcium, an inducer of keratinocyte differentiation, are typical for each cell line and differ substantially from the ones found in normal cells. In most cell lines a rapid transient expression of SPRR2 contrasts with a slow induction leading to a high sustained level of expression in normal cells. This pattern of expression is typical for SPRR2 and not observed for the other SPRR genes or involucrin. Our analysis indicates that the expression of various keratinocyte terminal differentiation markers, even when involved in the same biological process (cornification), can be differentially affected by carcinogenic transformation.

A shuttle system for transfer of YACs between yeast and mammalian cells

The development of a system for shuttling DNA cloned as yeast artificial chromosomes (YACs) between yeast and mammalian cells requires that the DNA is maintained as extrachromosomal elements in both cell types. We have recently shown that circular YACs carrying the Epstein-Barr virus origin of plasmid replication (oriP) are maintained as stable, episomal elements in a human kidney cell line constitutively expressing the viral transactivator protein EBNA-1. Here, we demonstrate that a 90-kb episomal YAC can be isolated intact from human cells by a simple alkaline lysis procedure and shuttled back into Saccharomyces cerevisiae by spheroplast transformation. In addition, we demonstrate that the 90-kb YAC can be isolated intact from yeast cells. The ability to shuttle large, intact fragments of DNA between yeast and human cells should provide a powerful tool in the manipulation and analysis of functional regions of mammalian DNA.

Regulation of keratinocyte proliferation and differentiation by all-trans-retinoic acid, 9-cis-retinoic acid and 1,25-dihydroxy vitamin D3

We studied the effect of all-trans retinoic acid (all-trans-RA), 9-cis-retinoic acid (9-cis-RA) and 1,25-dihydroxy vitamin D3 (1,25(OH)2D3) on proliferation and differentiation of human keratinocytes cultured in a submerged culture system for up to 5 weeks and evaluated changes in cell morphology and in the expression of proliferation- and terminal differentiation-related genes on both the mRNA and the protein levels. Under control culture conditions, the expression of small proline-rich proteins (SPRR1 and SPRR2), involucrin, Ki67 and c-jun reached a maximum after 2 weeks in culture (1 week postconfluence) and then decreased as the tissue architecture of the cultures deteriorated. Upon simultaneous treatment with both retinoids and 1,25(OH)2D3 a culture was generated that remained stable for 4 weeks with at least eight living cell layers. Furthermore, this culture showed a pattern of SPRR2 and involucrin expression which closely resembled that of native epidermis, a maintained Ki67 expression and a strongly induced c-jun expression. Treatment with 1,25(OH)2D3 alone inhibited cell proliferation and stimulated cell differentiation resulting in acceleration of the differentiated phenotype and was accompanied by inhibition of c-jun and Ki67 expression and also, surprisingly by inhibition of SPRR1, SPRR2 and involucrin expression. In contrast, treatment with all-trans-RA and/or 9-cis-RA induced a more proliferative phenotype with a prolonged lifespan as compared to control cultures. SPRR1 was weakly repressed, SPRR2 was strongly repressed, a delayed induction of involucrin occurred, and c-jun and Ki67 expression were maintained. These results show that modulation of the composition of the medium by the addition of various vitamins results in changes in the balance between keratinocyte proliferation and differentiation which correspond to changes in the expression of proliferation and differentiation markers and prolongation of the culture lifespan.

Interdependent transcription control elements regulate the expression of the SPRR2A gene during keratinocyte terminal differentiation

Expression of the SPRR2A gene, a member of the small proline-rich family of cornified cell envelope precursor proteins, is strictly linked to keratinocyte terminal differentiation both in vivo and in vitro. In this study, we explored the molecular mechanisms underlying this regulation in transiently transfected primary keratinocytes induced to differentiate in vitro. Deletion mapping and site-directed mutagenesis of SPRR2A promoter-chloramphenicol acetyltransferase constructs indicate that four transcription control elements are essential and sufficient for promoter activity. These elements were further characterized by electrophoretic mobility shift and identified as (i) an inverted octamer doublet, bound by the POU domain factor Oct-11 (Skn-1a/i, Epoc-1), (ii) an interferon-stimulated response element recognized by interferon regulatory factors 1 and 2, (iii) an Ets binding site partially overlapping the interferon-stimulated response element, and (iv) a TG box recognized by the Sp1 family of zinc finger transcription factors. Destruction of a single terminal differentiation element is sufficient to completely abolish transcription from the SPRR2A promoter, indicating that these transcription control elements function in concert in an interdependent manner. Apparently, integration of signals transmitted by the above-mentioned transcription factors is necessary and sufficient to promote gene expression during keratinocyte terminal differentiation.

Efficient retroviral infection of mammalian cells is blocked by inhibition of poly(ADP-ribose) polymerase activity

Integration of proviral DNA into the host cell genome is a characteristic feature of the retroviral life cycle. This process involves coordinate DNA strand break formation and rejoining reactions. The full details of the integration process are not yet fully understood. However, the endonuclease and DNA strand-joining activities of the virus-encoded integrase protein (IN) are thought to act in concert with other, as-yet-unidentified, endogenous nuclear components which are involved in the DNA repair process. The nuclear enzyme poly(ADP-ribose) polymerase (PARP), which is dependent on DNA strand breaks for its activity, is involved in the efficient repair of DNA strand breaks, and maintenance of genomic integrity, in nucleated eukaryotic cells. In the present work, we examine the possible involvement of PARP in the retroviral life cycle and demonstrate that inhibition of PARP activity, by any one of three independent mechanisms, blocks the infection of mammalian cells by recombinant retroviral vectors. This requirement for PARP activity appears to be restricted to processes involved in the integration of provirus into the host cell DNA. PARP inhibition does not affect viral entry into the host cell, reverse transcription of the viral RNA genome, postintegration synthesis of viral gene products, synthesis of the viral RNA genome, or the generation of infective virions. Therefore, efficient retroviral infection of mammalian cells is blocked by inhibition or PARP activity.

Quantification of cytokine messenger RNA in transfected human T cells by RT-PCR and an automated electrochemiluminescence-based post-PCR detection system

A fast method is reported for the precise and accurate quantification of cytokine mRNA, based on a quantitative polymerase chain reaction (PCR) assay. Post-PCR detection of the amplification products is achieved using an automated electrochemiluminescent (ECL) detection system. The target is amplified using a biotinylated forward and a tris(2,2'-bipyridine)ruthenium (II) (TBR)-labeled reverse primer. The amplification products are then captured on streptavidin coated paramagnetic beads and quantified by measuring the ECL signal of the TBR label. The results obtained are reproducible and accurate over a wide range (3 orders of magnitude) of concentrations. Quantitative results can be obtained using a standard curve which is generated with a synthetic external standard. This technique was applied to quantify tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma) and interleukin-2(IL-2) mRNA levels in human T cells transfected with the corresponding genes.

Genetic complementation of radiation response by 3' untranslated regions (UTR) of RNA

The molecular basis of radiosensitivity was studied using a cDNA complementation approach to correct radiosensitivity in cells. Four cDNAs of sizes 1.6, 2.0, 2.2 and 2.5 kb were isolated that corrected several aspects of the phenotype of cells from patients with the human genetic disorder ataxia-telangiectasia, characterized by hypersensitivity to ionizing radiation. The criteria used to assess correction included cell viability, induced chromosome aberrations, G2 phase delay and induction of p53 after exposure to radiation. One cDNA (2.5 kb) was identified as the complete sequence of the RNA helicase p68, which was capable of correcting radiosensitivity based on two of the above four criteria, with p53 induction post irradiation being partially corrected. The 2.2 kb cDNA was shown to correspond to the complete sequence of arginyl tRNA synthetase and the other two cDNAs were identical to the 3' untranslated regions (UTR) of the transcription factor TFIIS (1.6 kb) and phospholipase A2 (2.0 kb) respectively. Additional transfections with the 3'UTR (198 nucleotides) of p68 RNA helicase and its inverse sequence revealed that the 3'UTR had the same complementation capacity as the full-length cDNA, whereas the inverse construct failed to complement radiosensitivity. These data provide additional support for a novel role for 3'UTRs in the regulation of gene expression.

Epstein-Barr virus replication studies and their application to vector design

Vectors containing elements of the Epstein-Barr virus genome are used primarily to maintain cloned DNA inserts as plasmids in mammalian cells. However, Epstein-Bar-virus-based vectors have also been valuable tools in the hands of those studying the life cycle of Epstein-Barr virus. In this article, we discuss those characteristics of Epstein-Barr virus and its life cycle that have been used in vector construction and describe methods that are particularly applicable to the use of Epstein-Barr-virus-based vectors.

The small proline-rich proteins constitute a multigene family of differentially regulated cornified cell envelope precursor proteins

Loricrin, involucrin, small proline-rich protein (SPRR)1, SPRR2, and SPRR3 genes are located within a cluster of 1.5 Mbp on chromosome 1q21 and most likely evolved from a common ancestor. Monospecific polyclonal antibodies and cDNA probes were produced to investigate SPRR transcripts and proteins. SPRR expression was restricted to terminally differentiating squamous cells, preferentially located at the cell periphery, and immunoreactivity was greatly reduced in cells with a mature cornified cell envelope. Furthermore, detectable SPRR2 and SPRR3 levels were strongly increased in differentiating keratinocyte cultures after addition of LTB-2, a specific inhibitor of transglutaminases, suggesting that they are precursor proteins of the cornified cell envelope. In normal epidermis, SPRR1 was restricted to appendageal areas, SPRR2 was expressed coherently, and SPRR3 was completely absent. In the upper digestive tract, SPRR1 was expressed in sublingual and tongue epithelium, SPRR2 was mostly restricted to lingual papillae, and SPRR3 was abundant in oral and esophageal epithelium. In psoriatic epidermis, SPRR1 and SPRR2 were expressed at much higher levels than in normal epidermis. Addition of 10(-7) M retinoic acid to cultured differentiating keratinocytes significantly down-regulated the expression of SPRR2 and SPRR3 transcripts and slightly decreased that of SPRR1. Thus, SPRR1, SPRR2, and SPRR3 are differentially expressed in vivo and in vitro, suggesting that the SPRR multigene family evolved to serve as highly specialized cornified cell envelope precursor proteins in stratified epithelia.

A dominant-negative mutant of human poly(ADP-ribose) polymerase affects cell recovery, apoptosis, and sister chromatid exchange following DNA damage

Poly(ADP-ribose) polymerase [PARP; NAD+ ADP-ribosyltransferase; NAD+:poly(adenosine-diphosphate-D-ribosyl)-acceptor ADP-D-ribosyltransferase, EC 2.4.2.30] is a zinc-dependent eukaryotic DNA-binding protein that specifically recognizes DNA strand breaks produced by various genotoxic agents. To study the biological function of this enzyme, we have established stable HeLa cell lines that constitutively produce the 46-kDa DNA-binding domain of human PARP (PARP-DBD), leading to the trans-dominant inhibition of resident PARP activity. As a control, a cell line was constructed, producing a point-mutated version of the DBD, which has no affinity for DNA in vitro. Expression of the PARP-DBD had only a slight effect on undamaged cells but had drastic consequences for cells treated with genotoxic agents. Exposure of cell lines expressing the wild-type (wt) or the mutated PARP-DBD, with low doses of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) resulted in an increase in their doubling time, a G2 + M accumulation, and a marked reduction in cell survival. However, UVC irradiation had no preferential effect on the cell growth or viability of cell lines expressing the PARP-DBD. These PARP-DBD-expressing cells treated with MNNG presented the characteristic nucleosomal DNA ladder, one of the hallmarks of cell death by apoptosis. Moreover, these cells exhibited chromosomal instability as demonstrated by higher frequencies of both spontaneous and MNNG-induced sister chromatid exchanges. Surprisingly, the line producing the mutated DBD had the same behavior as those producing the wt DBD, indicating that the mechanism of action of the dominant-negative mutant involves more than its DNA-binding function. Altogether, these results strongly suggest that PARP is an element of the G2 checkpoint in mammalian cells.

Human cDNA clones that modify radiomimetic sensitivity of ataxia-telangiectasia (group A) cells

Genes responsible for genetic diseases with increased sensitivity to DNA-damaging agents can be identified using complementation cloning. This strategy is based on in vitro complementation of the cellular sensitivity by gene transfer. Ataxia-telangiectasia (A-T) is a multisystem autosomal recessive disorder involving cellular sensitivity to ionizing radiation and radiomimetic drugs. A-T is genetically heterogeneous, with four complementation groups. We attempted to identify cDNA clones that modify the radiomimetic sensitivity of A-T cells assigned to complementation group [A-T(A)]. The cells were transfected with human cDNA libraries cloned in episomal vectors, and various protocols of radiomimetic selection were applied. Thirteen cDNAs rescued from survivor cells were found to confer various degrees of radiomimetic resistance to A-T(A) cells upon repeated introduction, and one of them also partially influenced another feature of the A-T phenotype, radioresistant DNA synthesis. None of the clones mapped to the A-T locus on chromosome 11q22-23. Nine of the clones were derived from known genes, some of which are involved in cellular stress responses. We concluded that a number of different genes, not necessarily associated with A-T, can influence the response of A-T cells to radiomimetic drugs, and hence the complementation cloning approach may be less applicable to A-T than to other diseases involving abnormal processing of DNA damage.

Epstein-Barr virus-based vectors that replicate in rodent cells

Vectors based on Epstein-Barr virus (EBV) have been useful for cloning and gene expression studies in primate cells. However, these vectors do not replicate in rodent cells. We demonstrate here that the addition of large fragments of mammalian DNA to vectors containing the EBNA-1 gene (encoding the Epstein-Barr nuclear antigen-1) and the family of repeats from EBV generates autonomously replicating vectors which are stably maintained as extrachromosomal plasmids in hamster cells. By using a density-shift assay in Cs2SO4 density gradients, we demonstrate that the plasmids replicate once per cell cycle at high efficiency. These plasmids represent, for rodent cells, the only available class of stable, autonomous vectors replicating once per cell cycle. It is likely that this type of EBV-based autonomous replication system can also be extended to other organisms for which stable, extrachromosomal plasmid systems are not available.

Efficient episomal expression vector for human transitional carcinoma cells

To develop an efficient expression vector for human transitional carcinoma cells, we evaluated the replication activity of episomal vectors derived from the BK virus (BKV) and the Epstein-Barr virus (EBV) in HT-1376 bladder carcinoma cells. Southern blot analysis of transient transfectants indicated that the BKV-derived episome replicated extrachromosomally whereas the EBV replicon did not appear to be functional in these cells. HT-1376 cells were stably transfected with BKV-derived episomes containing the neomycin resistance gene as a selectable marker. Southern analysis demonstrated that these stable transfectants contained approximately 150 copies of the BKV episome per cell. There was no evidence of integration of the BKV episome into genomic DNA following selection with G418 for 10 weeks. These stable episomal transfectants had approximately 20-fold higher levels of expression of neomycin resistance gene mRNA than clones of HT-1376 cells transfected with pSV2NEO which contained five integrated copies of this gene. In soft agar cloning experiments, BKV episomes were efficiently transferred to the progeny of these transfectants during cell division over multiple generations. Additionally, BKV episomal copy number is maintained in stable transfectants after withdrawal of selection pressure for over 2 months. These data demonstrate that BKV-derived episomes replicate efficiently in bladder carcinoma cells, yielding stable transfectants having a high episomal copy number and expressing encoded genes at high levels. BKV-derived episomes may be useful in gene therapy strategies to modulate the growth of bladder carcinoma cells.

Human genetic instability syndromes: single gene defects with increased risk of cancer

The experimental findings of the last 5 years are reviewed for the genetic instability syndromes: Xeroderma pigmentosum, Fanconi's anaemia, Ataxia telangiectasia and Bloom's syndrome. In these autosomal recessive genetic diseases, single gene defects lead to genetic instability, increased mutation rates and cancer. Deficiencies in the ability to effectively repair DNA lesions have been suggested for all of these syndromes. The status of characterization of these DNA repair defects is presented and the possible mechanisms of lesion fixation as mutation are discussed. The four known human genes whose mutation leads to inherited genetic instability are described.

Targeting of the creatine kinase M gene in embryonic stem cells using isogenic and nonisogenic vectors

Replacement vectors with genomic DNA originating from different mouse strains were used to introduce site-specific mutations into the creatine kinase M (CKM) gene of mouse embryonic stem (ES) cells. Here we demonstrate that in mouse strain 129-derived ES cells, the gene is at least 25-fold more efficiently targeted with an isogenic, 129-derived vector (129-pRV8.3) than with a nonisogenic, BALB/c-specific vector (BALB/c-pRV8.3). The two targeting constructs were identical except for allelic differences which were typed by partial sequencing. These included base pair mismatches (2%) and a polymorphic [GTC]-repeat length variation. Both in separate transfections as well as in cotransfections with mixed vectors, homologous disruption of the CKM gene resulted uniquely from the 129-isogenic DNA. Our data confirm earlier observations on requirements for homologous recombination in pro- and eukaryotic systems and indicate that targeting of the CKM locus is highly sensitive to small sequence differences between cognate segments in the endogenous and incoming DNA.

Cloning of cDNAs for Fanconi's anaemia by functional complementation

Fanconi's anaemia is a rare autosomal recessive disorder characterized by progressive pancytopaenia and a cellular hypersensitivity to DNA crosslinking agents. Four genetic complementation groups have been identified so far, and here we use a functional complementation method to clone complementary DNAs that correct the defect of group C cells. The cDNAs encode alternatively processed transcripts of a new gene, designated FACC, which is mutated in group C patients. The predicted FACC polypeptide does not contain any motifs common to other proteins and so represents a new gene involved in the cellular response to DNA damage.

High-frequency transformation of human repair-deficient cell lines by an Epstein-Barr virus-based cDNA expression vector

We constructed a human cDNA expression vector by combining an episomal Epstein-Barr virus (EBV) vector with the expression cassette from the transient-expression vector, pCDM8. This new vector, designated pEBS7, exhibited high-level expression of reporter genes in normal and repair-deficient xeroderma pigmentosum cell lines. Reconstruction experiments indicated that marker genes diluted to a frequency of 10(-5) can be rescued on a single transfection dish. Moreover, derivative cell lines that constitutively express the gene encoding EBV nuclear antigen 1 exhibited a tenfold enhancement in the frequency of rescue of marker genes. The feasibility of preparing large-scale directional or nondirectional cDNA libraries in pEBS7 was demonstrated and reconstruction experiments indicated that marker genes could be rescued from either library with equal efficiency. These results establish a high-efficiency system for the isolation of genes by direct phenotypic selection in human mutant cell lines.

Induction of a mutant phenotype in human repair proficient cells after overexpression of a mutated human DNA repair gene

Antisense and mutated cDNA of the human excision repair gene ERCC-1 were overexpressed in repair proficient HeLa cells by means of an Epstein-Barr-virus derived cDNA expression vector. Whereas antisense RNA did not influence the survival of the transfected cells, a mutated cDNA generating an ERCC-1 protein with two extra amino acids in a conserved region of its C-terminal part resulted in a significant sensitization of the HeLa transfectants to mitomycin C-induced damage. These results suggest that overexpression of the mutated ERCC-1 protein interferes with proper functioning of the excision repair pathway in repair proficient cells and is compatible with a model in which the mutated ERCC-1 protein competes with the wild-type polypeptide for a specific step in the repair process or for occupation of a site in a repair complex. Apparently, this effect is more pronounced for mitomycin C induced crosslink repair than for UV-induced DNA damage.

Efficient cDNA cloning by direct phenotypic correction of a mutant human cell line (HPRT-) using an Epstein-Barr virus-derived cDNA expression vector

Human cells are, in general, poor recipients of foreign DNA, which has severely hampered the cloning of genes by direct phenotypic correction of deficient human cell lines after DNA mediated gene transfer. In this communication a methodology is presented which largely circumvents this problems. The method relies on the use of a recently developed episomal Epstein-Barr-virus-derived cDNA expression vector (Belt et al. (1989) Gene 84, 407-417). The cloning of hypoxanthine phosphoribosyltransferase (HPRT) cDNA, corresponding to a low abundant mRNA in wild type cells is used as a model system. Size fractionated poly (A)+ RNA from wild type cells, which resulted in an approximately 10 fold enrichment in HPRT mRNA, was used to construct a cDNA library of 25,000 independent clones in the pECV25 vector. An HPRT deficient human cell line was transfected and subsequently selected with hygromycin B for DNA uptake. In a small scale experiment only 7000 hygromycin BR transfectants were sufficient to isolate 2 independent HATR clones which were shown to replicate episomes harbouring HPRT cDNA. The first insert had a 5' untranslated region (UTR) and a 3' UTR perfectly in agreement with published data. The second cDNA clone harboured an unusually long 5' UTR and a shorter 3' UTR due to alternative polyadenylation of the HPRT transcript which has not been previously recognized.

Modulation of gene activity by consecutive gene targeting of one creatine kinase M allele in mouse embryonic stem cells

The cytosolic creatine kinases (CK's; EC 2.7.3.2) BB, BM and MM are dimeric isoenzymes which have an important role in energy metabolism and display characteristic tissue- and stage-specific patterns of expression in mammals. To study the functional role of the distribution of the CK isoenzymes we have focussed on the modulation of expression of the genes encoding the individual B and M subunits, starting at the muscle creatine kinase (CKM) gene which is transcriptionally inactive during early embryogenesis. Using repeated rounds of gene targeting in mouse embryonic stem (ES) cells, two types of mutant cell lines were obtained. First, we generated a cell line in which insertion of a neomycin resistance (neor) gene had disrupted one of the CKM alleles. Subsequently, from this cell line, following introduction of an insertion type vector designed for replacement of the muscle specific CKM-enhancer by the constitutively acting polyoma virus enhancer PyF441, several independent doubly targeted clones were isolated which all had insertions in the previously neo-disrupted CKM allele. In some of these ES clones, the targeted enhancer replacement resulted in gene correction and functional activation of the silent CKM gene. Dimerisation between the ectopically expressed CKM subunits and CKB subunits which are normally present at high levels in ES cells, led to the formation of the BM isoform of CK in these clones.

Characterization of the human spr2 promoter: induction after UV irradiation or TPA treatment and regulation during differentiation of cultured primary keratinocytes

We have isolated genomic clones from several members of the UV and TPA inducible human spr2 gene-family in order to analyse the regulation of these genes at a molecular level. From one of these members, the spr2-1 gene, we have identified and sequenced the regulatory region. By using CAT fusion plasmids and a liposome mediated transfection procedure we show that the isolated promoter region contains all the cis-elements necessary for induced expression after UV irradiation or phorbolester treatment of cultured human keratinocytes. Additionally the spr2-1 promoter is shown to be regulated aswell during the normal process of keratinocyte differentiation. This makes the spr2-1 promoter sequence an ideal tool to study the molecular mechanisms by which environmental agents such as UV radiation and chemical tumor promoters interfere with normal gene expression during cell proliferation and differentiation.