Novel Sp family-like transcription factors are present in adult insect cells and are involved in transcription from the polyhedrin gene initiator promoter

Spodoptera frugiperda FKBP-46 is a consensus p53 motif binding protein

p53 protein, the central molecule of the apoptosis pathway, is mutated in 50% of the human cancers. Of late, p53 homologues have been identified from different invertebrates including Drosophila melanogaster, Caenorhabditis elegans, Squid, and Clams. We report the identification of a p53-like protein in Spodoptera frugiperda (Sf9) insect cells, which is activated during oxidative stress, caused by exposure to UV-B or H(2) O(2) , and binds to p53 consensus DNA binding motifs as well as other p53 cognate motifs. Sf9 p53 motif-binding protein is similar to murine and Drosophila p53 in terms of molecular size, which is around 50-60 kDa, as evident from UV cross-linking, and displays DNA binding characteristics similar to both insect and vertebrate p53 as seen from electrophoretic mobility shift assays. The N-terminal sequencing of the purified Sf9 p53 motif-binding protein reveals extensive homology to the pro-apoptotic FK-506 binding protein (FKBP-46), earlier identified in Sf9 cells as a factor which interacts with murine casein kinase. FKBP, an evolutionarily conserved protein of mammalian origin functions as a pro-apoptotic factor. Identification of FKBP-46 as a novel p53 motif-binding protein in insect cells adds a new facet to our understanding of the mechanisms of apoptosis under oxidative stress in the absence of a typical p53 homologue.

Baculovirus p35 gene is oppositely regulated by P53 and AP-1 like factors in Spodoptera frugiperda

Baculovirus p35 belongs to the early class of genes of AcMNPV and requires viral factors like Immediate Early protein-1 for its transcription. To investigate the role of host factors in regulating p35 gene expression, the putative transcription factor binding sites were examined in silico and the role of these factors in influencing the transcription of p35 gene was assessed. We focused our studies on AP-1 and P53-like factors, which are activated under oxidative stress conditions. The AP-1 motif is located at -1401 while P53 motif is at -1912 relative to p35 translation start site. The predicted AP-1 and P53 elements formed specific complexes with Spodoptera frugiperda nuclear extracts. Both AP-1 and P53 motif binding proteins were down regulated as a function of AcMNPV infection in Spodoptera cells. To address the question whether during an oxidative outburst, the p35 transcription is enhanced; we investigated the role of these oxidative stress induced host transcription factors in influencing p35 gene transcription. Reporter assays revealed that AP-1 element enhances the transcription of p35 by a factor of two. Interestingly, P53 element appears to repress the transcription of p35 gene.

Transcription of human resistin gene involves an interaction of Sp1 with peroxisome proliferator-activating receptor gamma (PPARgamma)

BACKGROUND

Resistin is a cysteine rich protein, mainly expressed and secreted by circulating human mononuclear cells. While several factors responsible for transcription of mouse resistin gene have been identified, not much is known about the factors responsible for the differential expression of human resistin.

METHODOLOGY/PRINCIPAL FINDING

We show that the minimal promoter of human resistin lies within approximately 80 bp sequence upstream of the transcriptional start site (-240) whereas binding sites for cRel, CCAAT enhancer binding protein alpha (C/EBP-alpha), activating transcription factor 2 (ATF-2) and activator protein 1 (AP-1) transcription factors, important for induced expression, are present within sequences up to -619. Specificity Protein 1(Sp1) binding site (-276 to -295) is also present and an interaction of Sp1 with peroxisome proliferator activating receptor gamma (PPARgamma) is necessary for constitutive expression in U937 cells. Indeed co-immunoprecipitation assay demonstrated a direct physical interaction of Sp1 with PPARgamma in whole cell extracts of U937 cells. Phorbol myristate acetate (PMA) upregulated the expression of resistin mRNA in U937 cells by increasing the recruitment of Sp1, ATF-2 and PPARgamma on the resistin gene promoter. Furthermore, PMA stimulation of U937 cells resulted in the disruption of Sp1 and PPARgamma interaction. Chromatin immunoprecipitation (ChIP) assay confirmed the recruitment of transcription factors phospho ATF-2, Sp1, Sp3, PPARgamma, chromatin modifier histone deacetylase 1 (HDAC1) and the acetylated form of histone H3 but not cRel, C/EBP-alpha and phospho c-Jun during resistin gene transcription.

CONCLUSION

Our findings suggest a complex interplay of Sp1 and PPARgamma along with other transcription factors that drives the expression of resistin in human monocytic U937 cells.

Trans-complementation of polyhedrin by a stably transformed Sf9 insect cell line allows occ- baculovirus occlusion and larval per os infectivity

Oral infection of insect larvae with baculovirus is an advantageous methodology for producing high levels of recombinant proteins and for achieving plague control. However, many recombinant baculoviruses express a foreign protein in lieu of the polyhedrin and hence do not form occlusion bodies (occ-), resulting in extremely reduced per os infectivity in larvae. To overcome this limitation, stably transformed insect cell lines expressing polyhedrin capable of occluding occ- recombinant baculovirus by trans-complementation were developed to obtain oral inoculum for insect larvae infection. First, the optimum regulatory region of polyhedrin promoter was determined utilizing chloramphenicol acetyl transferase (CAT) as the reporter gene. After infection with occ- baculovirus, the higher expression levels of CAT were achieved when a region of 2735bp that contained sequences known to have transcriptional enhancer functions were present upstream the polyhedrin coding sequence. This regulatory region was selected to drive polyhedrin expression in insect cell lines. Transfection of Sf9 cells with plasmid carrying polyhedrin gene and stable cell lines established by selection with blasticidin showed polyhedrin expression and, moreover, crystalline polyhedron-like structures were visualized by optic microscopy. Oral infectivity was demonstrated by fluorescence detection in Rachiplusia nu larvae infected with occluded AcGFPpolh- baculovirus obtained using the system presented here.

Characterization of LEF4 ligand binding property and its role as part of baculoviral transcription machinery

Late expression factor 4 (LEF4) is one of the four identified subunits of Autographa californica nucleopolyhedrosis virus (AcNPV) encoded RNA polymerase that carries out transcription from viral late and very late promoters. This 464-amino acid baculovirus-encoded protein also harbors 5' mRNA capping activity that includes RNA 5' triphosphatase, nucleoside triphosphatase, and guanylyltransferase activities. Hydrolysis of 5' triphosphate RNA and free NTPs is metal ion dependent property of the protein. In the present communication, we describe the structural changes in the recombinant LEF4 protein following ligand binding. Metal ion binding causes some alteration in the conformation around aromatic amino acids whereas there is no effect on tryptophan fluorescence on GTP binding in absence and presence of metal ion. It is found that GTP and divalent cation cofactor produce some prominent changes in the secondary structure of the protein. Electrophoretic mobility shift assay (EMSA) shows that LEF4 is the probable factor that acts as anchor to dock the viral RNA polymerase on the very late polyhedrin promoter (Ppolh) facilitated by other factors.

An inducible system for highly efficient production of recombinant adeno-associated virus (rAAV) vectors in insect Sf9 cells

Production of clinical-grade gene therapy vectors for human trials remains a major hurdle in advancing cures for a number of otherwise incurable diseases. We describe a system based on a stably transformed insect cell lines harboring helper genes required for vector production. Integrated genes remain silent until the cell is infected with a single baculovirus expression vector (BEV). The induction of expression results from a combination of the amplification of integrated resident genes (up to 1,200 copies per cell) and the enhancement of the expression mediated by the immediate-early trans-regulator 1 (IE-1) encoded by BEV. The integration cassette incorporates an IE-1 binding target sequence from wild-type Autographa californica multiple nuclear polyhedrosis virus, a homologous region 2 (hr2). A feed-forward loop is initiated by one of the induced proteins, Rep78, boosting the amplification of the integrated genes. The system was tested for the coordinated expression of 7 proteins required to package recombinant adeno-associated virus (rAAV)2 and rAAV1. The described arrangement provided high levels of Rep and Cap proteins, thus improving rAAV yield by 10-fold as compared with the previously described baculovirus/rAAV production system.

Functional and structural characterisation of AgMNPV ie1

We have located and cloned the Anticarsia gemmatalis multicapsid nucleopolyhedrovirus isolate 2D (AgMNPV-2D) genomic DNA fragment containing the immediate early 1 ORF and its flanking regions. Computer assisted analysis of the complete ie1 locus nucleotide sequence information was used to locate regulatory signals in the upstream region and conserved nucleotide and amino acid sequences. Comparative studies led to the identification of several characteristic protein motifs and to the conclusion that AgMNPV-2D is more closely related to Choristoneura fumiferana defective NPV than to other Group I nucleopolyhedrovirus. We have also shown that the AgMNPV IE1 protein was able to transactivate an early Autographa californica MNPV promoter and its own promoter in transient expression assays. In order to investigate the biological functionality of the ie1 promoter, the ie1 upstream activating region (UAR) was molecularly dissected and cloned upstream of the E. coli lacZ ORF. The results obtained, after transfection of UFL-AG-286 insect cells, leading us to find that the -492 and -357 versions contains sequence motifs important for the level of the lacZ reporter gene expression.

Biochemical characterization of Sf9 Sp-family-like protein factors reveals interesting features

We earlier documented the involvement of novel Sp-family-like protein factors in transcription from the Autographa californica nucleopolyhedrovirus (AcNPV) polyhedrin (polh) gene promoter [Ramachandran et al. (2001) J. Biol. Chem. 276: 23440-23449]. These zinc-dependent Sp-like factors bind to two putative Sp-factor-binding motifs, present within the AcSp sequence upstream of the polh promoter, with very high affinity (K(d) = 2.1 x 10(-12) M). Like other polh-promoter-associated host transcription factors, these Sp-like factors display tolerance to high ion concentrations up to even 3 M NaCl. An electrophoretic mobility shift assay demonstrated a probable cross-talk between the Spodoptera frugiperda (Sf9) Sp-family-like proteins and the TFIID complex. In complementary experiments, specific replacements of the Sp-factor-binding motifs with TATA-like elements resulted in expression of a luciferase reporter gene to almost the same level as that obtained with a wild-type native construct. Our results point to the possibility of the involvement of TFIID and Sf9 Sp protein interaction in transcription from the baculovirus polyhedrin promoter.

A comparative analysis of relative occurrence of transcription factor binding sites in vertebrate genomes and gene promoter areas

MOTIVATION

The detection of transcription factor binding sites (TFBS) in genomic sequences is a basic task for elucidating the transcriptional aspects of gene regulation. Evaluation procedures applicable to the TFBS prediction outputs need improvement. Predicted TFBS located outside of the transcription associated areas are often neglected from the functional and the evolutionary points of view, therefore deserving a systematic overview.

RESULTS

We calculated theoretical occurrences of 184 TFBS according to their position weight matrices and the dinucleotide statistics of the completed vertebrate genomes, then performed a TFBS prediction in the corresponding complete genomic sequences and their repeat-free, repetitive and regulatory fractions. Repeat-free fractions of the closely related mammalian genomes were characterized by strong similarities in TFBS occurrences. A significant over-representation of multiple TFBS was found in both repetitive and non-repetitive genome fractions.

AVAILABILITY

F-values and real TFBS occurrences calculated for human, chimp, mouse, rat, zebrafish and fugu genomes are available for free download at http://www.gmu.edu/departments/mmb/baranova/pages/bioinformatics

An additional copy of the homologous region (hr1) sequence in the Autographa californica multinucleocapsid polyhedrosis virus genome promotes hyperexpression of foreign genes

The Autographa californica multinucleocapsid nuclear polyhedrosis virus genome contains nine homologous region (hr1, hr1a, hr2, hr2a, hr3, hr4a, hr4b, hr4c, and hr5) sequences that are thought to be involved in viral replication and activation of transcription. Our results show that the 750 bp hr1 sequence is capable of functioning as an enhancer of transcription of foreign genes from the homologous late polyhderin gene promoter and the heterologous Drosophila heat shock protein (hsp70) promoter in insect cells. Introduction of an additional copy of the complete hr1 element downstream to the polyhedrin locus in the viral genome, while not affecting the stability of the recombinant virus for at least 30 serial passages, led to hyperexpression of reporter genes. The enhancement in the expression levels of foreign genes varied from 40 to 90-fold depending on the promoter used.

Characterization of emerging European-like porcine reproductive and respiratory syndrome virus isolates in the United States

European-like field isolates of porcine reproductive and respiratory syndrome virus (PRRSV) have recently emerged in North America. The full-length genomic sequence of an index isolate characterized in 1999, strain EuroPRRSV, served as the reference strain for further studies of the evolution and epidemiology of European-like isolates (type 1) in the United States. Strain EuroPRRSV shared 90.1 to 100% amino acid identity with the prototype European strain, Lelystad, within the structural and nonstructural open reading frames (ORFs) and 95.3% overall nucleotide identity. The 5' untranslated region and two nonstructural regions within ORF 1 were closely examined due to significant divergence from strain Lelystad. A 51-bp deletion in a region within ORF 1a, coding for nonstructural protein 2 (NSP2), was observed. Sequence analysis of the structural ORFs 2 to 7 of additional European-like isolates indicated that these isolates share 93% nucleotide identity with one another and 95 to 96% identity with the Lelystad strain but only 70% identity with the North American reference strain VR-2332. Phylogenetic analysis with published PRRSV ORF 3, 5, and 7 nucleotide sequences indicated that these newly emerging isolates form a clade with the Lelystad and United Kingdom PRRSV isolates. Detailed analysis of four of these isolates with a panel of 60 monoclonal antibodies directed against the structural proteins confirmed a recognition pattern that was more consistent with strain Lelystad than with other North American isolates.

The homologous region sequence (hr1) of Autographa californica multinucleocapsid polyhedrosis virus can enhance transcription from non-baculoviral promoters in mammalian cells

The Autographa californica multinucleocapsid polyhedrosis virus homologous region sequence hr1 enhances transcription from the viral polyhedrin promoter in Spodoptera frugiperda insect cells and independently functions as an origin of replication (ori) sequence. The binding of the host nuclear protein, hr1-binding protein (hr1-BP), is crucial for the enhancer activity (Habib, S., Pandey, S., Chatterji, U., Burma, S., Ahmad, R., Jain, A., and Hasnain, S. E. (1996) DNA Cell Biol. 15, 737-747 and Habib, S., and Hasnain, S. E. (1996) J. Biol. Chem. 271, 28250-28258). We demonstrate that hr1 can also enhance transcription from non-baculoviral promoters like cytomegalovirus and hsp70 in mammalian cells but does not support ori activity in these cells. Unlike insect cells, hr1 can also function in mammalian cells as an enhancer when present in trans. hr1 DNA sequence binds with high affinity and specificity to nuclear factors in the mammalian cells. The insect hr1-BP- and the hr1-BP-like proteins from mammalian cells (mhr1-BP) have different properties with respect to ion requirements, DNA groove binding, and molecular size. When mammalian cells are infected with a recombinant baculovirus containing two promoters, the baculovirus polyhedrin and Drosophila hsp70 gene promoter, the hsp70 gene promoter alone is active in these cells, and this activity is further enhanced by the presence of an additional hr1 in the recombinant virus. hr1 may thus also have a role in baculovirus-mediated gene delivery in mammalian cells.

The genomic organization of mouse resistin reveals major differences from the human resistin: functional implications

The resistin gene is a potential candidate for the etiology of insulin resistance and type 2 diabetes and has been implicated as the molecular link between type 2 diabetes and obesity. Unlike the mouse resistin, expression of the human resistin appears to be regulated differently. We report comparative analyses of the mouse and human genomic fragments encoding the resistin gene. At the amino acid level the two proteins exhibit 59% identity. While at the mRNA level the human resistin shows 64.4% sequence identity with its mouse counterpart, the mouse resistin genomic sequence displays only 46.7% sequence identity with the human resistin and is almost three times bigger than the human resistin. The intronic sequences per se displayed the least identities (28.7%), however the intron boundaries were highly conserved between human and mouse. The mouse resistin carries a very large intron in the 3' UTR, which has a number of regulatory sequences possibly involved in differential gene expression. Of particular significance is the presence of a PPAR/RXR heterodimer binding site within intron X (IntX-PPRE) which may possibly confer TZD responsiveness. Oligonucleotides carrying the authentic PPAR/RXR binding element (Aco-PPRE) as well as IntX-PPRE specifically bound factors (PPAR/RXR heterodimers) present in differentiated 3T3-L1 adipocyte cells in an electrophoretic mobility shift assay. IntX-PPRE oligonucleotide modulated the expression of the luciferase reporter gene in transient transfection assays using 3T3-L1 cells.

Differential transcriptional regulation by the alpha- and gamma-catalytic subunit isoforms of cAMP-dependent protein kinase

The C gamma and C alpha isoforms of the cAMP-dependent protein kinase (PKA) share 83% identity including all critical catalytic and substrate-binding residues defined to date. Compared to C alpha, C gamma has a different substrate specificity and a selective pseudosubstrate specificity, exhibiting inhibition by regulatory subunits, but not by the protein kinase inhibitor. In these studies, C gamma-mediated gene transcription regulation was compared with that of C alpha in four cell lines using transient transfection/dual luciferase assays. As compared to C gamma, C alpha more efficiently activated a cAMP-response element (CRE)-regulated fragment of the human alpha-glycoprotein hormone promoter which was coupled to a firefly luciferase reporter gene (pGH alpha-fluc). This occurred in Cos7, Y1, and Kin8 adrenal cells by 23-, 6.5-, and 1.4-fold, respectively. In contrast, C gamma, but not C alpha, activated the Sp1RE-regulated herpes simplex virus thymidine kinase promoter which was coupled to a Renilla luciferase reporter (pTK-rluc). In Sp1-deficient Sf9 cells, pGH alpha-fluc expression was maintained for both isoforms, but cotransfection with an Sp1 expression plasmid was necessary and sufficient for activation of pTK-rluc expression by C gamma. In all cell lines, cotransfection with a PDK1 expression plasmid enhanced the transcriptional activation of both C alpha and C gamma (1.5- to 3-fold), while a catalytically inactive PDK1 mutant (PDK.KD) did not. These results suggest that both C alpha and C gamma can activate CRE-responsive genes; however, C alpha does so with better efficiency than C gamma. In contrast to C alpha, C gamma activates transcription of genes containing pTK-like Sp1RE sites. Activation of different C subunit isoforms can provide a means to diversify cAMP-mediated transcription, possibly affecting cell phenotype.

Baculovirus as mammalian cell expression vector for gene therapy: an emerging strategy

The monopoly of insect cells to host baculovirus Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) as a eukaryotic gene expression system has been shattered with the growing evidence that it also infects mammalian cells in culture. Although AcMNPV fails to replicate in vertebrate cells, it does express alien genes with levels of expression that are dependent on the strength of the promoter used to drive transcription of the foreign gene. It also has been reported that the recombinant AcMNPV enters human hepatic cells in culture preferentially and specifically in comparison with the other mammalian cells of different origin and sources. This has resulted in the use of AcMNPV as a potent mammalian cell delivery system as a xenovector for gene therapy, more precisely liver-specific gene delivery in vitro and in vivo.

Novel baculovirus DNA elements strongly stimulate activities of exogenous and endogenous promoters

A DNA sequence upstream from the polyhedrin gene of baculovirus Autographa californica nucleopolyhedrovirus (AcMNPV) was found to activate strongly the expression of full or minimal promoters derived from AcMNPV and other sources. Promoters tested included the minimal CMV (CMVm) promoter from human cytomegalovirus, the full heat shock 70 promoter from Drosophila, and the minimal p35 promoter from baculovirus. Deletion and mutagenesis analyses showed that this functional polyhedrin upstream (pu) activator sequence contains three open reading frames (ORFs), ORF4, ORF5, and lef2. In plasmid transfection assays, the pu sequence was able to confer high level luciferase expression driven by all of these full or minimal promoters in insect Sf21 cells. A known baculovirus enhancer, the homologous region (hr) of AcMNPV, further enhanced the expression of these promoters. Experiments showed that although multiple hr sequences function in an additive manner, pu and hr together function synergistically, resulting in as much as 18,000-fold promoter activation. Furthermore, a modified CMVm promoter containing pu and/or hr was inserted into the baculovirus genome to drive the luciferase coding region. The CMVm promoter expressed luciferase much earlier, and although it expressed a bit less than did the p10 promoter, the CMVm promoter gave rise to greater luciferase activity. Therefore, we have uncovered a cryptic viral sequence capable of activating a diverse group of promoters. Finally, these experiments demonstrate that synthetic sequences containing pu, hr, and different full or minimal promoters can generate a set of essentially unlimited novel promoters for weak to very strong expression of foreign proteins using baculovirus.