Transactivation of BKV and SV40 early promoters by BKV and SV40 T-antigens

A potential gene delivery strategy using BK virus

Recombinant BK virus (rBKV) is able to express polypeptides under control of its native BKV late promoter. This ability helps to use this construct as a good reporter since it can infect human cells. In this study, we generate a BKV construct containing Renilla luciferase (Rluc) sequences under control of the BKV late promoter. The activity of Rluc was strongly detected in Vero-76 and Cos-1 cells transfected with rBKV-Rluc-myc-2A-VP2 construct, indicating the production of a functional enzyme driven by the native late promoter. Furthermore, a construct made of rBKV-IL2SP-Rluc-myc-2A-VP2 by introducing human IL2 secretion peptide (IL2 SP) caused secretion of IL2SP-Rluc-myc into the culture medium. As a concluding remark, a potential infectious rBKV that can express foreign antigens such as Rluc was generated successfully. The proposed strategy would be useful to engineer recombinant forms of rBKV with many potential applications including development of antiviral assay for new drugs, human vaccines and gene delivery systems for immunotherapeutic or cell transduction.

Development of infectious recombinant BK virus

The polyomavirus, BK virus (BKV) infects the majority of humans early in life, establishing persistent asymptomatic infections in immunocompetent individuals. The small size and non-redundant nature of the viral genome presents a challenge in developing recombinant BKV (rBKV). A strategy is described for engineering rBKV by fusing sequences coding for foreign polypeptides via the self-processing 2A peptide in frame to the BKV agnoprotein or VP2 capsid protein genes. This novel approach aims to minimize alterations to native BKV polypeptide sequences and expression, potentially allowing maintenance of viral viability. To test this concept, a panel of rBKV was constructed that express either enhanced green fluorescent protein (EGFP), or different forms of the HIV-1 Gag polypeptide under control of the native BKV late transcriptional unit, and with appropriate self-processing. Although most of these rBKV proved to have stability issues, such approaches may have utility as reporter viruses or as gene delivery vectors.

A suppressive effect of cyclosporine A on replication and noncoding control region activation of polyomavirus BK virus

BACKGROUND

The effect of cyclosporine A (CsA) on polyomavirus BK virus (BKV) replication remains unclear. The aim of this in vitro study was to investigate the effect of CsA on BKV replication in human uroepithelial cells.

METHODS

After infection of a human renal proximal tubular cell line, HK-2 with BKV, BKV viral load in the presence of CsA was assessed by real-time polymerase chain reaction. The BKV large T-antigen (LTag) expression was measured by Western blot analysis. The BKV early promoter activity was determined by measuring luciferase activity of the BKV noncoding control region luciferase reporter. The BKV LTag expression in a human bladder carcinoma cell line, T24, was assessed by immunofluorescence.

RESULTS

The results demonstrated that the increased levels of BKV LTag and viral protein 1 transcripts measured by real-time polymerase chain reaction were suppressed by CsA in a dose-dependent manner (0.5-4 microg/mL). Western blot analysis also showed that CsA inhibited BKV LTag expression. In addition, the activity of the BKV early promoter, which was enhanced by BKV LTag overexpression, was abrogated by CsA. Finally, the suppressive effect of CsA on BKV replication was also shown in T24 cells as CsA reduced immunofluorescent staining of BKV LTag in these cells.

CONCLUSION

This in vitro study indicates that CsA suppresses BKV replication in human proximal renal tubular cells and uroepithelial cells of the urinary bladder and inhibits the BKV-LTag-regulated increase in early promoter activity.

Inhibition of Simian Virus 40 Large T Antigen Helicase Activity by Fluoroquinolones

Background

Fluoroquinolones represent a potent group of antibiotics that inhibit bacterial DNA replication by targeting the essential bacterial enzymes gyrase and topoisomerase IV. Inhibition of gyrase activity by quinolones involves the interaction of these drugs with the helicase component of bacterial gyrase. DNA tumour viruses also encode helicases that are essential for their DNA replication in the host.

Methods

In this study we have evaluated the effect of fluoroquinolones on viral DNA replication using the DNA tumour virus Simian virus 40 (SV40) as our model. Four different fluoroquinolones, namely, levofloxacin, trovafloxacin, ciprofloxacin and ofloxacin, were tested for their ability to inhibit viral DNA replication.

Results

We show here that all four quinolones tested were effective in the inhibition of SV40 plaque formation and DNA replication in CV1-P cells. In addition, we found that each of these quinolones was inhibitory to the helicase activity of SV40 large tumour antigen.

Conclusions

Fluoroquinolones and their derivates may therefore be useful in the treatment and/or prevention of infection by SV40-homologous human DNA viruses that encode helicase activity for their survival.

Polyomavirus-associated nephropathy: update on antiviral strategies

Polyomavirus-associated nephropathy (PVAN) is a major complication of kidney transplantation. Many centers respond to PVAN by reducing immunosuppression. Concern over precipitating rejection, as well as situations in which some PVAN-afflicted individuals have multi-organ transplants, can make reduction of immunosuppression undesirable. In these cases, effective antiviral strategies would be useful. This article describes clinical observations and experiences with 3 different antiviral protocols. Two protocols address antiviral treatment of nephropathy (cidofovir in one, and leflunomide in the other). The third protocol examines fluoroquinolone control of polyoma urinary excretion. Patients responded to all 3 strategies. These promising approaches deserve further evaluation with prospective controlled studies.

Episomal vectors for gene expression in mammalian cells

An important reason for preferring mammalian cells for heterologous gene expression is their ability to make authentic proteins containing post-translational modifications similar to those of the native protein. The development of expression systems for mammalian cells has been ongoing for several years, resulting in a wide variety of effective expression vectors. The aim of this review is to highlight episomal expression vectors. Such episomal plasmids are usually based on sequences from DNA viruses, such as BK virus, bovine papilloma virus 1 and Epstein-Barr virus. In this review we will mainly focus on the improvements made towards the usefulness of these systems for gene expression studies and gene therapy.

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.

Replication dependent and cell specific activation of the polyomavirus early promoter

The relationship of viral DNA replication to the activation of viral gene expression is usually considered with respect to late genes. In this report we examine the replication activation of the polyomavirus early promoter. Using origin active and inactive mutants to drive luciferase gene expression from the polyomavirus early promoter, we show that the early promoter is also subjected to a replication dependent activation. The degree of activation can be up to a hundred fold greater than that seen without replication and is about 13 fold on a per template basis. This replication based activation is, however, cell type dependent and was seen in FOP cells but not in 3T6 cells. Analysis of the requirements of cis acting DNA show that these enhancer elements affect early transcription predominantly through the activation of replication, although some replication independent stimulation can also be seen. The implications of this result for the regulation of polyomavirus early gene regulation are considered.

Activity and enhancer binding factors for JC virus regulatory elements in differentiating embryonal carcinoma cells

We have studied cell-type-specific expression by JC virus (JCV) DNA regulatory sequences using embryonal carcinoma (EC) cells as a model system. In transient transfection assays, JCV enhancer demonstrated activity in retinoic acid-differentiated neuronal type cells but not in undifferentiated or DMSO-differentiated muscle type cells. To correlate in vivo activity with the binding of transcription factors, we performed DNasel footprinting experiments. Retinoic acid-treated EC cell extracts provided three completely protected regions, each containing sequences with homology to nuclear factor 1 (NF1) binding motifs and the partially protected TATA box. Oligonucleotide competition studies suggest that all three NF1 binding motifs are bound by the same factors but with different affinities and that there are cooperative interactions between NF1 proteins binding to adjacent regions. No protected region other than the partially protected TATA box was detected in undifferentiated and DMSO-differentiated EC cells in which JC regulatory sequences were not expressed.

An element of the BK virus enhancer required for DNA replication

The human papovavirus BK virus contains three 68-base-pair (bp) repeats that act as transcriptional enhancers. An analysis of plasmids containing the BK virus origin revealed that sequences within the 68-bp enhancer are required for DNA replication as well as transcription of the early promoter in COS-1 cells. Origins with a single 68-bp repeat replicated as efficiently as did those with three repeats when transfected into COS-1 cells. Replication did not occur in the absence of enhancer sequences and could not be restored by distal placement of enhancers to enhancerless origins. However, as with simian virus 40, replication in vitro was not dependent on the presence of any enhancer sequences. Deletion analysis showed that replication of BK virus origins was dependent on the presence of the first 21 bp of the enhancer contiguous with the A-T-rich stretch of the origin. This 21-bp element is referred to as the rep element. Although in combination with rep the remaining 47 bp of the enhancer appear to increase replication by two- to fivefold, they alone are not sufficient to support replication. Deletions or insertions in the enhancer which did not alter the rep element had no major effect on replication. Site-directed mutagenesis of the Sp1-like site within the rep element, the NF1 site present in the enhancer, or the NF1 site in adjacent late-side sequences each reduced transcription by two- to fivefold, but had no effect on replication, suggesting that replication and transcription can be uncoupled.

Absence of BK virus sequences in transformed hamster cells transfected by human tumor DNA

In an attempt to gain insight into the mechanism of oncogenic transformation by BK virus (BKV), a human papovavirus, we have probed for BKV sequences in transformed hamster cells in which oncogenic transformation had occurred as a result of transfection by human tumor DNA positive for BKV sequences. Even though the sources of the transfecting DNA contained BKV sequences, the transformed hamster cells which arose from the transfection for the most part did not retain BKV sequences. In only one barely detectable case was BKV-specific DNA found associated with chromosomal DNA, and in only a small minority of the transformed cells was BKV DNA detected in the Hirt supernatant, indicating an episomal configuration. Even in these few cases where BKV sequences were present in an episomal form, altered migration on gels of some BKV-positive bands (compared to bands derived from cloned viral DNA) suggested deletions and rearrangements of BKV DNA. We employed several different probe methodologies for these studies, including nick-translation, random primer and a non-isotopic biotinylated probe which gave a sensitivity that could detect better than 0.01 copy of viral genome per diploid cell. We conclude that transformation by transfection with human tumor DNA does not require persistence of the BKV viral genome, suggesting that either BKV virus was irrelevant to original oncogenesis, in analogy with models proposed by others for herpesvirus oncogenesis.

Analysis of origin of DNA replication of human papovavirus BK

We have analyzed the cis-acting sequence elements and properties of the origin of DNA replication of human papovavirus BK (BKV). The precise boundaries of the origin varied, depending on the cell type and the viral T antigen used for assay. The BKV minimal origin of replication consisted of an inverted repeat, T-antigen-binding site II, and a 20-base-pair AT block when assayed in monkey kidney CV1 and HeLa cells by using the BKV T antigen. This 76-base-pair minimal origin did not replicate in COS cells in the presence of the simian virus 40 (SV40) T antigen. Unlike that from the SV40 minimal origin, replication from the BKV minimal origin was not enhanced by BKV ori-flanking sequences in CV1 or HeLa cells, using the BKV T antigen. BKV ori-flanking sequences did activate the SV40 minimal origin of replication in COS cells and relieved the orientation-dependent property of this origin. Finally, the BKV T antigen was found to autoregulate activity of the BKV early transcriptional regulatory region. The BKV origin of replication shows similarities to and differences from those of the related viruses SV40 and polyomavirus, suggesting that the proteins involved in the initiation of replication interact with origin sequences differently in these viruses.