Transcriptional analysis of human herpesvirus-8 open reading frames 71, 72, 73, K14, and 74 in a primary effusion lymphoma cell line

Herpesvirus saimiri episomal persistence is maintained via interaction between open reading frame 73 and the cellular chromosome-associated protein MeCP2

Herpesvirus saimiri (HVS) is the prototype gamma-2 herpesvirus, which naturally infects the squirrel monkey Saimiri sciureus, causing an asymptomatic but persistent infection. The latent phase of gamma-2 herpesviruses is characterized by their ability to persist in a dividing cell population while expressing a limited subset of latency-associated genes. In HVS only three genes, open reading frame 71 (ORF71), ORF72, and ORF73, are expressed from a polycistronic mRNA. ORF73 has been shown to be the only gene essential for HVS episomal maintenance and can therefore be functionally compared to the human gammaherpesvirus latency-associated proteins, EBNA-1 and Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA). HVS ORF73 is the positional homologue of KSHV LANA and, although it shares limited sequence homology, has significant structural and functional similarities. Investigation of KSHV LANA has demonstrated that it is able to mediate KSHV episomal persistence by tethering the KSHV episome to host mitotic chromosomes via interactions with cellular chromosome-associated proteins. These include associations with core and linker histones, several bromodomain proteins, and the chromosome-associated proteins methyl CpG binding protein 2 (MeCP2) and DEK. Here we show that HVS ORF73 associates with MeCP2 via a 72-amino-acid domain within the ORF73 C terminus. Furthermore, we have assessed the functional significance of this interaction, using a variety of techniques including small hairpin RNA knockdown, and show that association between ORF73 and MeCP2 is essential for HVS chromosomal attachment and episomal persistence.

Molecular biology of KSHV in relation to AIDS-associated oncogenesis

KSHV has been established as the causative agent of KS, PEL, and MCD, malignancies occurring more frequently in AIDS patients. The aggressive nature of KSHV in the context of HIV infection suggests that interactions between the two viruses enhance pathogenesis. KSHV latent infection and lytic reactivation are characterized by distinct gene expression profiles, and both latency and lytic reactivation seem to be required for malignant progression. As a sophisticated oncogenic virus, KSHV has evolved to possess a formidable repertoire of potent mechanisms that enable it to target and manipulate host cell pathways, leading to increased cell proliferation, increased cell survival, dysregulated angiogenesis, evasion of immunity, and malignant progression in the immunocompromised host. Worldwide, approximately 40.3 million people are currently living with HIV infection. Of these, a significant number are coinfected with KSHV. The complex interplay between the two viruses dramatically elevates the risk for development of KSHV-induced malignancies, KS, PEL, and MCD. Although HAART significantly reduces HIV viral load, the entire T-cell repertoire and immune function may not be completely restored. In fact, clinically significant immune deficiency is not necessary for the induction of KSHV-related malignancy. Because of variables such as lack of access to therapy noncompliance with prescribed treatment, failure to respond to treatment and the development of drug-resistant strains of HIV, KSHV-induced malignancies will continue to present as major health concerns.

Functional p53 signaling in Kaposi's sarcoma-associated herpesvirus lymphomas: implications for therapy

The Kaposi's sarcoma-associated herpesvirus (KSHV/HHV8) is associated with Kaposi's sarcoma (KS) as well as primary effusion lymphomas (PEL). The expression of viral proteins capable of inactivating the p53 tumor suppressor protein has been implicated in KSHV oncogenesis. However, DNA-damaging drugs such as doxorubicin are clinically efficacious against PEL and KS, suggesting that p53 signaling remains intact despite the presence of KSHV. To investigate the functionality of p53 in PEL, we examined the response of a large number of PEL cell lines to doxorubicin. Two out of seven (29%) PEL cell lines harbored a mutant p53 allele (BCBL-1 and BCP-1) which led to doxorubicin resistance. In contrast, all other PEL containing wild-type p53 showed DNA damage-induced cell cycle arrest, p53 phosphorylation, and p53 target gene activation. These data imply that p53-mediated DNA damage signaling was intact. Supporting this finding, chemical inhibition of p53 signaling in PEL led to doxorubicin resistance, and chemical activation of p53 by the Hdm2 antagonist Nutlin-3 led to unimpaired induction of p53 target genes as well as growth inhibition and apoptosis.

Targeted disruption of Kaposi's sarcoma-associated herpesvirus ORF57 in the viral genome is detrimental for the expression of ORF59, K8alpha, and K8.1 and the production of infectious virus

Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57 regulates viral gene expression at the posttranscriptional level during viral lytic infection. To study its function in the context of the viral genome, we disrupted KSHV ORF57 in the KSHV genome by transposon-based mutagenesis. The insertion of the transposon into the ORF57 exon 2 region also interrupted the 3' untranslated region of KSHV ORF56, which overlaps with the ORF57 coding region. The disrupted viral genome, Bac36-Delta57, did not express ORF57, ORF59, K8alpha, K8.1, or a higher level of polyadenylated nuclear RNA after butyrate induction and could not be induced to produce infectious viruses in the presence of valproic acid, a histone deacetylase inhibitor and a novel KSHV lytic cycle inducer. The ectopic expression of ORF57 partially complemented the replication deficiency of the disrupted KSHV genome and the expression of the lytic gene ORF59. The induced production of infectious virus particles from the disrupted KSHV genome was also substantially restored by the simultaneous expression of both ORF57 and ORF56; complementation by ORF57 alone only partially restored the production of virus, and expression of ORF56 alone showed no effect. Altogether, our data indicate that in the context of the viral genome, KSHV ORF57 is essential for ORF59, K8alpha, and K8.1 expression and infectious virus production.

Kaposi sarcoma-associated herpesvirus and other viruses in human lymphomagenesis

Kaposi sarcoma-associated herpesvirus (KSHV), also called human herpesvirus 8 (HHV-8), is associated with a specific subset of lymphoproliferative disorders. These include two main categories. The first is primary effusion lymphomas and related solid variants. The second is multicentric Castleman disease, from which KSHV-positive plasmablastic lymphomas can arise. KSHV contributes to lymphomagenesis by subverting the host cell molecular signaling machinery to deregulate cell growth and survival. KSHV expresses a selected set of genes in the lymphoma cells, encoding viral proteins that play important roles in KSHV lymphomagenesis. Deregulation of the NF-kappaB pathway is an important strategy used by KSHV to promote lymphoma cell survival, and the viral protein vFLIP is essential for this process. Two other viruses that are well documented to be causally associated with lymphoid neoplasia in humans are Epstein-Barr virus (EBV/HHV-4) and human T-cell lymphotropic virus (HTLV-1). Both of these are similar to KSHV in their use of viral proteins to promote cell survival by deregulating the NF-kappaB pathway. Here we review the basic information and recent developments that have contributed to our knowledge of lymphomas caused by KSHV and other viruses. The understanding of the mechanisms of viral lymphomagenesis should lead to the identification of novel therapeutic targets and to the development of rationally designed therapies.

Structure and function of latency-associated nuclear antigen

Latency-associated nuclear antigen (LANA) encoded by open reading frame 73 (ORF73) is the major latent protein expressed in all forms of KSHV-associated malignancies. LANA is a large (222-234 kDa) nuclear protein that interacts with various cellular as well as viral proteins. LANA has been classified as an oncogenic protein as it dysregulates various cellular pathways including tumor suppressor pathways associated with pRb and p53 and can transform primary rat embryo fibroblasts in cooperation with the cellular oncogene Hras. It associates with GSK-3beta, an important modulator of Wnt signaling pathway leading to the accumulation of cytoplasmic beta-catenin, which upregulates Tcf/Lef regulated genes after entering into the nucleus. LANA also blocks the expression of RTA, the reactivation transcriptional activator, which is critical for the latency to lytic switch, and thus helps in maintaining viral latency. LANA tethers the viral episomal DNA to the host chromosomes by directly binding to its cognate binding sequence within the TR region of the genome through its C terminus and to the nucleosomes through the N terminus of the molecule. Tethering to the host chromosomes helps in efficient partitioning of the viral episomes in the dividing cells. Disruptions of LANA expression led to reduction in the episomal copies of the viral DNA, supporting its role in persistence of the viral DNA. The functions known so far suggest that LANA is a key player in KSHV-mediated pathogenesis.

Temporal transcription program of recombinant Autographa californica multiple nucleopolyhedrosis virus

Baculoviruses, a family of large, rod-shaped viruses that mainly infect lepidopteran insects, have been widely used to transduce various cells for exogenous gene expression. Nonetheless, how a virus controls its transcription program in cells is poorly understood. With a custom-made baculovirus DNA microarray, we investigated the recombinant Autographa californica multiple nucleopolyhedrosis virus (AcMNPV) gene expression program in lepidopteran Sf21 cells over the time course of infection. Our analysis of transcription kinetics in the cells uncovered sequential viral gene expression patterns possibly regulated by different mechanisms during different phases of infection. To gain further insight into the regulatory network, we investigated the transcription program of a mutant virus deficient in an early transactivator (pe38) and uncovered several pe38-dependent and pe38-independent genes. This study of baculovirus dynamic transcription programs in different virus genetic backgrounds provides new molecular insights into how gene expression in viruses is regulated.

Gene structure and expression of Kaposi's sarcoma-associated herpesvirus ORF56, ORF57, ORF58, and ORF59

Though similar to those of herpesvirus saimiri and Epstein-Barr virus (EBV), the Kaposi's sarcoma-associated herpesvirus (KSHV) genome features more splice genes and encodes many genes with bicistronic or polycistronic transcripts. In the present study, the gene structure and expression of KSHV ORF56 (primase), ORF57 (MTA), ORF58 (EBV BMRF2 homologue), and ORF59 (DNA polymerase processivity factor) were analyzed in butyrate-activated KSHV(+) JSC-1 cells. ORF56 was expressed at low abundance as a bicistronic ORF56/57 transcript that utilized the same intron, with two alternative branch points, as ORF57 for its RNA splicing. ORF56 was transcribed from two transcription start sites, nucleotides (nt) 78994 (minor) and 79075 (major), but selected the same poly(A) signal as ORF57 for RNA polyadenylation. The majority of ORF56 and ORF57 transcripts were cleaved at nt 83628, although other nearby cleavage sites were selectable. On the opposite strand of the viral genome, colinear ORF58 and ORF59 were transcribed from different transcription start sites, nt 95821 (major) or 95824 (minor) for ORF58 and nt 96790 (minor) or 96794 (major) for ORF59, but shared overlapping poly(A) signals at nt 94492 and 94488. Two cleavage sites, at nt 94477 and nt 94469, could be equally selected for ORF59 polyadenylation, but only the cleavage site at nt 94469 could be selected for ORF58 polyadenylation without disrupting the ORF58 stop codon immediately upstream. ORF58 was expressed in low abundance as a monocistronic transcript, with a long 5' untranslated region (UTR) but a short 3' UTR, whereas ORF59 was expressed in high abundance as a bicistronic transcript, with a short 5' UTR and a long 3' UTR similar to those of polycistronic ORF60 and ORF62. Both ORF56 and ORF59 are targets of ORF57 and were up-regulated significantly in the presence of ORF57, a posttranscriptional regulator.

Genetic organization and hypoxic activation of the Kaposi's sarcoma-associated herpesvirus ORF34-37 gene cluster

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent for Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL). We previously reported that hypoxia activates KSHV lytic replication and that the promoter for open reading frame 34 (ORF34) contains a functional hypoxia-responsive element (HRE). ORF34 is part of a cluster of lytic genes (ORF34-37) that includes ORF36, a phosphotransferase, and ORF37, a shutoff exonuclease. Rapid amplification of cDNA ends analysis revealed that they share a common polyadenylation signal but have two start sites. Two transcripts were identified, one 3.4 kb encoding ORF35-37, and the other 4.2 kb encoding ORF34 and also having coding potential for ORF35-37. Exposure of PEL cell lines to hypoxia induced messages of lengths consistent with those of these transcripts. Reporter assays with Hep3B cells showed activation of both transcripts by hypoxia. The ORF34-37 promoter region has six consensus HREs. Sequential deletion, site-directed mutagenesis experiments, and Northern blot analysis of RNA produced by constructs indicated that the second HRE (HRE-2) plays a critical role in the hypoxic activation of both RNA transcripts. The ORF35-37 transcript was upregulated by cotransfected hypoxia-inducible factor (HIF). Electrophoretic mobility shift assays demonstrated that HRE-2 and ancillary sequences bind and compete for HIF with hypoxic Hep3B nuclear extract. The activation of this gene cluster by hypoxia may have implications for the pathogenesis of PEL and KS. Moreover, the activation of ORF36 by hypoxia might be exploited to develop targeted therapy for PEL, which arises in a hypoxic environment (pleural effusions).

Kaposi's sarcoma-associated herpesvirus immune modulation: an overview

Kaposi's sarcoma-associated herpesvirus (KSHV) is the most recently discovered human herpesvirus. It is the aetiological agent of Kaposi's sarcoma (KS), a tumour frequently affecting AIDS patients not receiving treatment. KSHV is also a likely cause of two lymphoproliferative diseases: multicentric Castleman's disease and primary effusion lymphoma. The study of KSHV offers exciting challenges for understanding the mechanisms of virus pathogenesis, including those involved in establishing infection and dissemination in the host. To facilitate these processes, approximately one-quarter of KSHV genes encode cellular homologues or unique proteins that have immunomodulatory roles in cytokine production, apoptosis, cell signalling and the immunological synapse. The activities of these molecules are considered in the present review and the positions of their genes are mapped from a complete KSHV genome sequence derived from a KS biopsy. The understanding gained enables the significance of different components of the immune response in protection against KSHV infection to be evaluated. It also helps to unravel the complexities of cellular and immunological pathways and offers the potential for exploiting viral immunomodulators and derivatives in disease therapy.

Inhibition of replication and transcription activator and latency-associated nuclear antigen of Kaposi's sarcoma-associated herpesvirus by morpholino oligomers

Kaposi's sarcoma-associated herpesvirus (KSHV) is associated with Kaposi's sarcoma and primary effusion lymphoma (PEL). The KSHV replication and transcription activator (RTA) and latency-associated nuclear antigen (LANA) play key roles in activating KSHV lytic replication and maintaining KSHV latency, respectively. Phosphorodiamidate morpholino oligomers (PMO) are similar to short single-stranded DNA oligomers, but possess a modified backbone that confers highly specific binding and resistance to nucleases. In this study, RTA and LANA mRNA in PEL cells were targeted by antisense peptide-conjugated PMO (P-PMO) in an effort to suppress KSHV replication. Highly efficient P-PMO uptake by PEL cells was observed. Treatment of PEL cells with a RTA P-PMO (RP1) reduced RTA expression in a dose-dependent and sequence-specific manner, and also caused a significant decrease in several KSHV early and late gene products, including vIL-6, vIRF-1, and ORF-K8.1A. KSHV viral DNA levels were reduced both in cells and culture supernatants of RP1 P-PMO-treated cells, indicating that KSHV lytic replication was suppressed. Treatment of BCBL-1 cells with P-PMO against LANA resulted in a reduction of LANA expression. Cell viability assays detected no cytotoxicity from P-PMO alone, within the concentration range used for the experiments in this study. These results suggest that RP1 P-PMO can specifically block KSHV replication, and further study is warranted.

Long-term-infected telomerase-immortalized endothelial cells: a model for Kaposi's sarcoma-associated herpesvirus latency in vitro and in vivo

Kaposi's sarcoma-associated herpesvirus (KSHV) is associated with Kaposi's sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman's disease. Most KS tumor cells are latently infected with KSHV and are of endothelial origin. While PEL-derived cell lines maintain KSHV indefinitely, all KS tumor-derived cells to date have lost viral genomes upon ex vivo cultivation. To study KSHV latency and tumorigenesis in endothelial cells, we generated telomerase-immortalized human umbilical vein endothelial (TIVE) cells. TIVE cells express all KSHV latent genes 48 h postinfection, and productive lytic replication could be induced by RTA/Orf50. Similar to prior models, infected cultures gradually lost viral episomes. However, we also obtained, for the first time, two endothelial cell lines in which KSHV episomes were maintained indefinitely in the absence of selection. Long-term KSHV maintenance correlated with loss of reactivation in response to RTA/Orf50 and complete oncogenic transformation. Long-term-infected TIVE cells (LTC) grew in soft agar and proliferated under reduced-serum conditions. LTC, but not parental TIVE cells, formed tumors in nude mice. These tumors expressed high levels of the latency-associated nuclear antigen (LANA) and expressed lymphatic endothelial specific antigens as found in KS (LYVE-1). Furthermore, host genes, like those encoding interleukin 6, vascular endothelial growth factor, and basic fibroblast growth factor, known to be highly expressed in KS lesions were also induced in LTC-derived tumors. KSHV-infected LTCs represent the first xenograft model for KS and should be of use to study KS pathogenesis and for the validation of anti-KS drug candidates.

Promoter switching allows simultaneous transcription of LANA and K14/vGPCR of Kaposi's sarcoma-associated herpesvirus

Latent transcription of the latency-associated nuclear antigen (LANA/ORF73) of Kaposi's sarcoma-associated herpesvirus is driven by the LANAp-c. Complexity arises during lytic reactivation, however, as the bicistronic K14/vGPCR transcript initiates 32 bp downstream of LANAp-c in the opposite orientation. We identify an Rta/ORF50-inducible LANA promoter (LANAp-i) that is distinct from the LANAp-c. LANAp-c is unaffected by Rta/ORF50. Utilization of the second, downstream LANAp-i explains how LANA and K14/vGPCR are simultaneously transcribed during de novo infection or lytic reactivation. Transactivation of LANAp-i and K14/vGPCRp requires the C-terminal activation domain of Rta/ORF50 and is mediated by DNA-binding-dependent and -independent Rta/ORF50 mechanisms. Transcriptional profiling following viral reactivation support promoter reporter phenotypes. In sum, cis-elements within the LANAp were selected to ensure faithful expression of LANA and other genes regulated by LANAp during all stages of the KSHV lifecycle despite potential interference from K14/vGPCRp activity.

Transcriptional origin of Kaposi's sarcoma-associated herpesvirus microRNAs

Kaposi's sarcoma-associated herpesvirus (KSHV) encodes 11 distinct microRNAs, all of which are found clustered within the major latency-associated region of the KSHV genome in the same transcriptional orientation. Because the KSHV microRNAs are all expressed in latently infected cells and are largely unaffected by induction of lytic replication, it appeared probable that they would be processed out of KSHV transcripts that are derived from a latent promoter(s) present in this region. Here, we define three latent transcripts, derived from two distinct KSHV latent promoters, that function as both KSHV primary microRNA precursors and as kaposin pre-mRNAs. These activities require the readthrough of a leaky viral polyadenylation signal located at nucleotide 122070 in the KSHV genome. In contrast, recognition of this polyadenylation signal gives rise to previously identified mRNAs that encode the KSHV open reading frames (ORFs) 71, 72 and 73 proteins as well as a novel unspliced KSHV mRNA that encodes only ORF72 and ORF71. Thus, transcripts initiating at the two latent promoters present in the KSHV latency-associated region can undergo two entirely distinct fates, i.e., processing to give a kaposin mRNA and viral microRNAs on the one hand or expression as KSHV ORF71, ORF72, or ORF73 mRNAs on the other, depending on whether the viral polyadenylation site located at position 122070 is ignored or recognized, respectively.

Identification of spliced gammaherpesvirus 68 LANA and v-cyclin transcripts and analysis of their expression in vivo during latent infection

Regulation of orf73 (LANA) gene expression is critical to the establishment and maintenance of latency following infection by members of the gamma-2 herpesvirus (rhadinovirus) family. Previous studies of murine gammaherpesvirus 68 (gammaHV68) have demonstrated that loss of LANA function results in a complete failure to establish virus latency in the spleens of laboratory mice. Here we report the characterization of alternatively spliced LANA and v-cyclin (orf72) transcripts encoded by gammaHV68. Similar to other rhadinoviruses, alternative splicing, coupled with alternative 3' processing, of a ca. 16-kb transcriptional unit can lead to expression of either LANA or v-cyclin during gammaHV68 infection. Spliced LANA and v-cyclin transcripts were initially identified from an analysis of the gammaHV68 latently infected B-cell lymphoma cell line S11E, but were also detected during lytic infection of NIH 3T12 fibroblasts. 5' Random amplification of cDNA ends (RACE) analyses identified two distinct promoters, p1 and p2, that drive expression of spliced LANA transcripts. Analysis of p1 and p2, using transiently transfected reporter constructs, mapped the minimal sequences required for promoter activity and demonstrated that both promoters are active in the absence of any viral antigens. Analysis of spliced LANA and v-cyclin transcripts in spleens recovered from latently infected mice at days 16 and 42 postinfection revealed that spliced v-cyclin transcripts can only be detected sporadically, suggesting that these may be associated with cells reactivating from latency. In contrast, spliced LANA transcripts were detected in ca. 1 in 4,000 splenocytes harvested at day 16 postinfection. Notably, based on the frequency of viral genome-positive splenocytes at day 16 postinfection (ca. 1 in 200), only 5 to 10% of viral genome-positive splenocytes express LANA. The failure of the majority of infected splenocytes at day 16 postinfection to express LANA may contribute to the contraction in the frequency of latently infected splenocytes as chronic infection is established, due to failure to maintain the viral episome in proliferating B cells.

Transcripts encoding K12, v-FLIP, v-cyclin, and the microRNA cluster of Kaposi's sarcoma-associated herpesvirus originate from a common promoter

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of three malignancies associated with AIDS and immunosuppression. Tumor cells harbor latent virus and express kaposin (open reading frame [ORF] K12), v-FLIP (ORF 71), v-Cyclin (ORF 72), and latency-associated nuclear antigen (LANA; ORF 73). ORFs 71 to 73 are transcribed as multicistronic RNAs initiating from adjacent constitutive and inducible promoters upstream of ORF 73. Here we characterize a third promoter embedded within the ORF 71-to-73 transcription unit specifying transcripts that encode ORF 71/72 or K12. These transcripts may also be the source of 11 microRNAs arranged as a cluster between K12 and ORF 71. Our studies reveal a complex arrangement of interlaced transcription units, incorporating four important protein-encoding genes required for latency and pathogenesis and the entire KSHV microRNA repertoire.

Open reading frame 73 is required for herpesvirus saimiri A11-S4 episomal persistence

Herpesvirus saimiri (HVS) establishes a latent infection in which the viral genome persists as a non-integrated episome. Analysis has shown that only open reading frames (ORFs) 71-73 are transcribed in an in vitro model of HVS latency. ORF73 also colocalizes with HVS genomic DNA on host mitotic chromosomes and maintains the stability of HVS terminal-repeat-containing plasmids. However, it is not known whether ORF73 is the only HVS-encoded protein required for episomal maintenance. In this study, the elements required for episomal maintenance in the context of a full-length HVS genome were examined by mutational analysis. A recombinant virus, HVS-BAC delta71-73, lacking the latency-associated genes was unable to persist in a dividing cell population. However, retrofitting an ORF73 expression cassette into the recombinant virus rescued episomal maintenance. This indicates that ORF73 is the key trans-acting factor for episomal persistence and efficient establishment of a latent infection.

KSHV G protein-coupled receptor inhibits lytic gene transcription in primary-effusion lymphoma cells via p21-mediated inhibition of Cdk2

Kaposi sarcoma (KS) remains the most common AIDS-associated malignancy worldwide. In sub-Saharan Africa especially, this aggressive endothelial-cell tumor is a cause of widespread morbidity and mortality. Infection with Kaposi sarcoma-associated herpesvirus (KSHV) is now known to be an etiologic force behind KS and primary-effusion lymphoma (PEL). Over time, KSHV has pirated many human genes whose products regulate angiogenesis, inflammation, and the cell cycle. One of these, the KSHV vGPCR, is a lytic product that is a constitutively active homolog of the IL-8 receptor. Although it is considered a viral oncogene and causes KS-like lesions in mice, vGPCR expression results in cell-cycle arrest of KSHV-infected PEL cells. In the present study, we show that this arrest is mediated by p21 in a p53-independent manner; the resulting Cdk2 inhibition decreases the efficiency of chemical induction of KSHV lytic transcripts ORF 50 and 26. Importantly, Cdk2 activity is also essential for replication in other human herpesviruses. The ability of vGPCR to delay or abort KSHV replication may explain how despite being a lytic product, this potent signaling molecule has a vital role in tumor formation via its induction of various KS-associated cytokines.

Activation of the Kaposi's sarcoma-associated herpesvirus major latency locus by the lytic switch protein RTA (ORF50)

Kaposi's sarcoma-associated herpesvirus (KSHV) maintains a latent infection in primary effusion lymphoma cells but can be induced to enter full lytic replication by exposure to a variety of chemical inducing agents or by expression of the KSHV-encoded replication and transcription activator (RTA) protein. During latency, only a few viral genes are expressed, and these include the three genes of the so-called latency transcript (LT) cluster: v-FLIP (open reading frame 71 [ORF71]), v-cyclin (ORF72), and latency-associated nuclear antigen (ORF73). During latency, all three open reading frames are transcribed from a common promoter as part of a multicistronic mRNA. Subsequent alternative mRNA splicing and internal ribosome entry allows for the expression of each protein. Here, we show that transcription of LT cassette mRNA can be induced by RTA through the activation of a second promoter (LT(i)) immediately downstream of the constitutively active promoter (LT(c)). We identified a minimal cis-regulatory region, which overlaps with the promoter for the bicistronic K14/v-GPCR delayed early gene that is transcribed in the opposite direction. In addition to a TATA box at -30 relative to the LT(i) mRNA start sites, we identified three separate RTA response elements that are also utilized by the K14/v-GPCR promoter. Interestingly, LT(i) is unresponsive to sodium butyrate, a potent inducer of lytic replication. This suggests there is a previously unrecognized class of RTA-responsive promoters that respond to direct, but not indirect, induction of RTA. These studies highlight the fact that induction method can influence the precise program of viral gene expression during early events in reactivation and also suggest a mechanism by which RTA contributes to establishment of latency during de novo infections.

Cloning and identification of a microRNA cluster within the latency-associated region of Kaposi's sarcoma-associated herpesvirus

MicroRNAs (miRNAs) are small, noncoding regulatory RNA molecules that bind to 3' untranslated regions (UTRs) of mRNAs to either prevent their translation or induce their degradation. Previously identified in a variety of organisms ranging from plants to mammals, miRNAs are also now known to be produced by viruses. The human gammaherpesvirus Epstein-Barr virus has been shown to encode miRNAs, which potentially regulate both viral and cellular genes. To determine whether Kaposi's sarcoma-associated herpesvirus (KSHV) encodes miRNAs, we cloned small RNAs from KSHV-positive primary effusion lymphoma-derived cells and endothelial cells. Sequence analysis revealed 11 isolated RNAs of 19 to 23 bases in length that perfectly align with KSHV. Surprisingly, all candidate miRNAs mapped to a single genomic locale within the latency-associated region of KSHV. These data suggest that viral and host cellular gene expression may be regulated by miRNAs during both latent and lytic KSHV replication.

Transcription of the murine gammaherpesvirus 68 ORF73 from promoters in the viral terminal repeats

Gammaherpesviruses persist as latent episomes in a dynamic lymphocyte pool. The regulated production of an episome maintenance protein is therefore crucial to their survival. The transcription initiation site of the murine gammaherpesvirus 68 episome maintenance protein, ORF73, was mapped to the viral terminal repeats, more than 10 kb distant from the open reading frame (ORF) itself. A 5' non-coding exon in the terminal repeats was spliced to the right end of the viral unique sequence, and then across ORFs 75a, 75b, 75c and 74 to ORF73. The right-hand portion of a single repeat unit was sufficient for constitutive promoter activity. The unique left end of the viral genome further enhanced ORF73 transcription. This, together with the large size of the predominant ORF73 mRNA, suggested that transcription initiates in distal repeat units and then splices between repeats to generate an extensive 5' untranslated region. A second promoter in the left-hand portion of the proximal terminal repeat unit generated a transcript which overlapped that of ORF73, but failed to splice to the ORF73 coding exon and so transcribed ORF75a. In distal repeat copies, however, transcription from this promoter would enter the next repeat unit to become an ORF73 mRNA. There was a third promoter just upstream of ORF73 itself. These data indicate that ORF73 transcription is highly complex, and support the idea that the terminal repeats of gamma-2-herpesviruses constitute a vital component of episomal persistence.

Splicing of rhesus rhadinovirus R15 and ORF74 bicistronic transcripts during lytic infection and analysis of effects on production of vCD200 and vGPCR

Rhesus macaque rhadinovirus (RRV) is the rhesus macaque homologue of human herpesvirus 8 (HHV-8). Here we examine expression of RRV R15 and ORF74, homologues of K14 and ORF74 of HHV-8, respectively. As in HHV-8, transcripts encoding RRV R15 and ORF74 are bicistronic. However, unlike what has been suggested for HHV-8, RRV R15- and ORF74-encoding transcripts are expressed late during lytic infection and undergo unique splicing events that result in the production of transcripts capable of encoding vGPCR, as well as membrane-associated and secreted forms of vCD200. The alternative splicing for vCD200 has implications for viral pathogenesis.

The latency-associated nuclear antigen of rhesus monkey rhadinovirus inhibits viral replication through repression of Orf50/Rta transcriptional activation

Rhesus monkey rhadinovirus (RRV) is a gamma-2-herpesvirus that is closely related to Kaposi's sarcoma-associated herpesvirus/human herpesvirus-8. We have previously reported that the transcript for RRV latency-associated nuclear antigen (R-LANA) is expressed during lytic replication in rhesus fibroblasts. In this article, we report the development of a latent culture system for RRV and show that mRNA specific for R-LANA is expressed during latency as well. We have characterized the R-LANA protein and demonstrate that it exhibits a nuclear speckled localization and possesses the ability to homodimerize. When expressed in rhesus fibroblasts, R-LANA can inhibit RRV lytic replication in vitro. We have investigated the mechanism behind this inhibition and find that, while R-LANA itself has very little effect on lytic promoters, it can dramatically decrease the transactivation function of RRV Orf50 (Rta), which is the major viral transcription factor. We further show that the mechanism for this repression involves the recruitment of histone deacetylase complexes (HDACs), because R-LANA's ability to repress Orf50 transactivation is completely reversed by the addition of the HDAC inhibitor trichostatin A (TSA). We also report that TSA alone can significantly reactivate RRV from latently infected cells. We propose that the repressive effects of R-LANA on RRV Orf50 transactivation serve to downregulate the transcription of early genes at late times during the lytic cycle and also help to maintain viral latency by preventing viral reactivation.

Multiplex detection and quantitation of latent and lytic transcripts of human herpesvirus-8 using RNase Protection Assay

Human herpesvirus-8 (HHV-8, also called Kaposi's sarcoma-associated herpesvirus) infectious cycle can be divided into latent and lytic phases. During the latent phase viral gene expression is reduced to a minimum, while during the lytic phase, numerous genes are expressed sequentially. The development of an RNase Protection Assay (RPA) is described that allows the detection and quantitation of three latent (ORFs 71-72-73) and three lytic (ORFs 74-K4-K2) HHV-8 genes as well as two cellular housekeeping gene transcripts (glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and cyclophilin) for normalization purposes is described. The RPA was validated using 293T cells transfected with corresponding HHV-8 expression vectors and using resting and phorbol ester-butyric acid-activated BC-3 and BCBL-1 cells. The results obtained indicate that this RPA is specific, sensitive and allows for the simultaneous monitoring of HHV-8 latent and lytic genes expression. HHV-8-RPA is therefore a useful technique to monitor the status of HHV-8 infection in infected cells (latent versus lytic) by comparing and quantitating multiple viral transcripts expression from a single RNA sample.

Inhibiting primary effusion lymphoma by lentiviral vectors encoding short hairpin RNA

We use lentiviral-delivered RNA interference (RNAi) to inhibit the growth of a model of primary effusion lymphoma (PEL) in vitro and in vivo. RNAi is a phenomenon allowing the sequence-specific targeting and silencing of exogenous and endogenous gene expression and is being applied to inhibit viral replication both in vitro and in vivo. We show that silencing of genes believed to be essential for the Kaposi sarcoma-associated herpesvirus (KSHV) latent life cycle (the oncogenic cluster) has a varied effect in PEL cell lines cultured in vitro, however, concomitant silencing of the viral cyclin (vcyclin) and viral FLICE (Fas-associating protein with death domain-like interleukin-1β-converting enzyme) inhibitory protein (vFLIP) caused efficient apoptosis in all PEL lines tested. We demonstrate that in a murine model of PEL, lentiviral-mediated RNA interference both inhibits development of ascites and can act as a treatment for established ascites. We also show that the administered lentiviral vectors are essentially limited to the peritoneal cavity, which has advantages for safety and dosage in a therapeutic setting. This shows the use of lentiviral-mediated RNA interference in vivo as a potential therapeutic against a virally driven human cancer.

Regulation of growth signalling and cell cycle by Kaposi's sarcoma-associated herpesvirus genes

Kaposi's sarcoma-associated herpesvirus (KSHV) is the primary aetiological agent of at least three malignancies associated with HIV infection and immunosuppression: Kaposi's sarcoma, primary effusion lymphoma and multicentric Castleman's disease. KSHV encodes proteins that deregulate key checkpoints in the signalling pathways governing cell proliferation, which may ultimately contribute to the virus' oncogenic potential. To alter cellular signalling associated with proliferation, these viral proteins function like growth factor ligands/receptors, signal transduction proteins, transcription factors and cell cycle regulators. This review focuses on the mechanisms by which some KSHV-encoded proteins activate signalling pathways and cell proliferation and their role in the pathogenesis of KSHV-driven mechanisms.

Human herpesvirus 8 K14 protein mimics CD200 in down-regulating macrophage activation through CD200 receptor

Many viral proteins limit host immune defenses, and their genes often originate from their hosts. CD200 (OX2) is a broadly distributed cell surface glycoprotein that interacts with a receptor on myeloid cells (CD200R) that is implicated in locally preventing macrophage activation. Distant, but recognizable, homologues of CD200 have been identified in many herpesviruses and poxviruses. Here, we show that the product of the K14 open reading frame from human herpesvirus 8 (Kaposi's sarcoma-associated herpesvirus) interacts with human CD200R and is expressed at the surfaces of infected cells solely during the lytic cycle. Despite sharing only 40% primary sequence identity, K14 and CD200 interacted with CD200R with an almost identical and low affinity (K(D) = 0.5 microM), in contrast to other characterized viral homologue interactions. Cells expressing CD200 or K14 on the cell surface were able to inhibit secretion by activated macrophages of proinflammatory cytokines such as tumor necrosis factor alpha, an effect that could be specifically relieved by addition of monoclonal antibodies and soluble monomeric CD200 protein. We conclude that CD200 delivers local down-modulatory signals to myeloid cells through direct cell-cell contact and that the K14 viral homologue closely mimics this.

A naturally occurring C-terminal truncated isoform of the latent nuclear antigen of Kaposi's sarcoma-associated herpesvirus does not associate with viral episomal DNA

The latency-associated nuclear antigen (LANA) encoded by orf73 of Kaposi's sarcoma-associated herpesvirus (KSHV) binds to viral episomal DNA and nuclear heterochromatin in infected cells. A 3.2 kb transcript in KSHV-positive primary effusion lymphoma (PEL) cells (BCP-1 and BC-3) encoding a C-terminal truncated form of LANA (LANA-Delta76) has been identified. This transcript has the addition of a poly(A) tail at nt 3264 of orf73 resulting in an in-frame stop codon (TAA) effectively truncating LANA by 76 aa ( approximately 8 kDa). Examination of the coding region revealed the presence of a non-canonical polyadenylation signal (AGTAAA) 17 nt upstream of the poly(A) tail. The protein expressed from this transcript is representative of the faster migration of the LANA doublet bands observed by SDS-PAGE and Western blot. Mutation of the poly(A) signal from AGTAAA to TGTACA produced a protein that co-migrated with the larger LANA isoform. A C-terminal LANA-Delta76 EGFP fusion protein localized to the nucleus but did not co-localize with endogenous LANA in BCP-1 cells, or heterochromatin in HEK293 cells. Using an electrophoretic mobility shift assay (EMSA), the authors were able to show that LANA-Delta76 does not bind to the KSHV terminal repeat motif known to interact with LANA. These data provide evidence for the presence of an isoform of LANA that may perform alternative functions in KSHV-infected cells.

A polypyrimidine tract facilitates the expression of Kaposi's sarcoma-associated herpesvirus vFLIP through an internal ribosome entry site

We have identified a novel internal ribosome entry site (IRES) within a latently expressed Kaposi's sarcoma-associated herpesvirus (KSHV) gene (vCyclin) that controls the expression of a downstream open reading frame encoding an inhibitor of apoptosis (vFLIP). This IRES is the first such element to be identified in a DNA virus and may represent a novel mechanism through which this virus controls gene expression. We have used a dual luciferase reporter assay to identify important sequence elements essential for the activity of the IRES. A sequence of 32 nucleotides incorporating a polypyrimidine tract (PPT) was found to be required for the proper functioning of the IRES. We also show, using an electrophoretic mobility shift assay (EMSA), that proteins specific to a KSHV-infected cell line (BCP-1) but not a KSHV-negative cell line (HEK293) were able to form complexes with the IRES. By using an in vitro RNA binding assay, the cellular polypyrimidine tract binding protein (PTB, hnRNP-I) was found to bind to the IRES RNA. These results suggest that the interaction of PTB with the PPT may contribute to the correct functioning of the KSHV IRES in infected cells.

Regulation and Autoregulation of the Promoter for the Latency-associated Nuclear Antigen of Kaposi's Sarcoma-associated Herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) or human herpesvirus 8 has been established as the etiological agent of Kaposi's sarcoma and certain AIDS-associated lymphomas. KSHV establishes latent infection in these tumors, invariably expressing high levels of the viral latency-associated nuclear antigen (LANA) protein. LANA is necessary and sufficient to maintain the KSHV episome. It also modulates viral and cellular transcription and has been implicated directly in oncogenesis because of its ability to bind to the p53 and pRb tumor suppressor proteins. Previously, we identified the LANA promoter (LANAp) and showed that it was positively regulated by LANA itself. Here, we present a detailed mutational analysis and define cis-acting elements and trans-acting factors for the core LANAp. We found that a downstream promoter element, TATA box, and GC box/Sp1 site at -29 are all individually required for activity. This architecture places LANAp into the small and unusual group of eukaryotic promoters that contain both the downstream promoter element and TATA element but lack a defined initiation site. Furthermore, we demonstrate that LANA regulates its own promoter via its C-terminal domain and does bind to a defined site within the core promoter.

Does dysregulated expression of a deregulated viral GPCR trigger Kaposi's sarcomagenesis?

In 1994, the Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8) was identified as the etiologic agent of Kaposi's sarcoma (KS). KSHV has since been associated with two additional AIDS-related malignancies: primary effusion lymphomas (PEL) and multicentric Castleman's disease (MCD). Although molecular characterization of the KSHV genome has revealed several candidate oncogenes, infection with KSHV alone is not sufficient to cause KS, suggestive of an accomplice in KS initiation. Recent experimental evidence supports a key role for a particular KSHV gene, a constitutively-active G-protein-coupled receptor (vGPCR), in the development of KS. However, it is unclear how a lytic gene expressed in cells destined to die can cause cancer. Here we propose that dysregulation of the viral gene program may lead to nonlytic vGPCR expression. Several candidate cofactors (e.g., HIV-1 Tat, inflammation, aborted lytic cycle progression) are identified that may trigger vGPCR dysregulation, enabling oncogenic signaling pathways up-regulated by vGPCR, combined with the paracrine secretions from vGPCR-expressing cells, to promote the initiation of KS. If KS is indeed dependent on vGPCR dysregulation, then the development of new therapeutic modalities specifically targeting this viral protein or its downstream targets may ultimately prove to be the most effective treatment strategy for this enigmatic disease.

ORF73 of herpesvirus Saimiri strain C488 tethers the viral genome to metaphase chromosomes and binds to cis-acting DNA sequences in the terminal repeats

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV), a human oncogenic gamma-2-herpesvirus, transforms human endothelial cells and establishes latent infection at a low efficiency in vitro. During latent infection, only a limited number of genes are expressed, and the circularized viral genome is maintained as a multicopy episome. Latency-associated nuclear antigen (LANA), exclusively expressed during latency, has been shown to have a multifunctional role in KS pathogenesis. LANA tethers the viral episome to the host chromosome, thus ensuring efficient persistence of the viral genome during successive rounds of cell division. Besides episome maintenance, LANA modulates the expression of genes of various cellular and viral pathways, including those of retinoblastoma protein and p53. Herpesvirus saimiri (HVS), another gamma-2-herpesvirus, primarily infects New World primates. Orf73, encoding the nuclear antigen of HVS, is the positional homolog of the LANA gene, and the ORF73 protein has some sequence homology to KSHV LANA. However, the function of ORF73 of HVS has not been thoroughly investigated. In this report, we show that HVS ORF73 may be important for episome persistence and colocalizes with the HVS genomic DNA on metaphase chromosomes. Furthermore, HVS terminal repeats (TRs) contain a cis-acting sequence similar to that in KSHV TRs, suggesting that the LANA binding sequence is conserved between these two viruses. This cis-acting element is sufficient to bind HVS ORF73 from strains C488 and A11, and plasmids containing the HVS C488 TR element are maintained and replicate in HVS C488 ORF73-expressing cells.

The herpesvirus saimiri ORF73 gene product interacts with host-cell mitotic chromosomes and self-associates via its C terminus

The herpesvirus saimiri (HVS) ORF73 gene product shares limited homology with the ORF73 protein of Kaposi's sarcoma-associated herpesvirus (KSHV). ORF73 is expressed in an in vitro model of HVS latency, where the genome persists as a non-integrated circular episome. This suggests it may have a similar role to KSHV ORF73 in episomal maintenance, by tethering viral genomes to host-cell chromosomes. Here, the association of ORF73 with host mitotic chromosomes is described. Deletion analysis demonstrates that the distal 123 aa of the ORF73 protein are required for mitotic chromosomal localization and for self-association. Moreover, deletion of the extreme C terminus disrupts both self-association and host mitotic chromosome colocalization. This suggests that HVS ORF73 has a similar role to KSHV ORF73 in episomal maintenance and that association of ORF73 to host mitotic chromosomes is dependent on its ability to form multimers.

KSHV vFLIP binds to IKK-gamma to activate IKK

When expressed in heterologous cells, the viral FLIP protein (vFLIP) of Kaposi's-sarcoma-associated herpesvirus (KSHV) has been reported both to block Fas-mediated apoptosis and to activate the NF-kappaB activation pathway by interaction with IkappaB kinase (IKK). In a yeast-two-hybrid screen, we identified IKKgamma as an interacting partner of vFLIP. We expressed fragments of IKKgamma in mammalian cells and bacteria, and identified the central CCR3/4 (amino acids 150-272) as the vFLIP binding region. To investigate the proteins interacting with vFLIP in a KSHV-infected primary effusion lymphoma (PEL) cell line, we immunoprecipitated vFLIP and identified four associated proteins by mass spectrometry: IKK components IKKalpha, beta and gamma, and the chaperone, Hsp90. Using gel filtration chromatography, we demonstrated that a single population of vFLIP in the cytoplasm of PEL cells co-eluted and co-precipitated with an activated IKK complex. An inhibitor of Hsp90, geldanamycin, inhibited IKK's kinase activity induced by vFLIP and killed PEL cells, suggesting that vFLIP activation of IKK contributes to PEL cell survival.

Human herpesvirus-8-encoded signalling ligands and receptors

Analysis of the genome of human herpesvirus 8 (HHV-8) led to the discovery of several novel genes, unique among the characterized gammaherpesviruses. These include cytokines (interleukin-6 and chemokine homologues), two putative signal-transducing transmembrane proteins encoded by genes K1 and K15 at the genome termini, and an OX-2 (CD200) receptor homologue that had not previously been identified in a gammaherpesvirus. HHV-8 also specifies a diverged version of the gammaherpesvirus-conserved G protein-coupled chemokine receptor (vGCR) and a latently expressed protein unique to HHV-8 specified by open reading frame (ORF) K12. These cytokine and receptor homologues mediate signal transduction or modulate the activities of other endogenous cytokines and receptors to enhance viral productive replication, regulate latent-lytic switching, evade host attack, or mediate cell survival. The viral signalling ligands and receptors are also potential contributors to virus-associated diseases, Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease, and so represent potentially important targets for therapeutic and antiviral drugs. Understanding these proteins' modes of action and functions in viral biology and disease is therefore of considerable importance, and the subject of this review.

Surface downregulation of major histocompatibility complex class I, PE-CAM, and ICAM-1 following de novo infection of endothelial cells with Kaposi's sarcoma-associated herpesvirus

Under selective pressure from host cytotoxic T lymphocytes, many viruses have evolved to downregulate major histocompatibility complex (MHC) class I and/or T-cell costimulatory molecules from the surface of infected cells. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes two proteins, MIR-1 and MIR-2, that serve this function during lytic replication. In vivo, however, KSHV exists in a predominantly latent state, with less than 5% of infected cells expressing discernible lytic gene products. Thus, mechanisms of immune evasion that depend on genes expressed only during lytic replication are unlikely to be active in most KSHV-infected cells. As a result, we searched for evidence of similar defensive strategies extant during latency, employing culture systems that strongly favor latent KSHV infection. We measured cell surface levels of immunomodulatory proteins on both primary dermal microvascular endothelial cells (pDMVEC) infected through coculture with induced primary effusion lymphoma cells and telomerase-immortalized DMVEC infected directly with cell-free virus. Employing a panel of antibodies against several endothelial cell surface proteins, we show that de novo infection with KSHV leads to the downregulation of MHC class I, CD31 (PE-CAM), and CD54 (ICAM-I) but not CD58 (LFA-3) or CD95 (Fas). Furthermore, flow cytometry with a fluorescently labeled monoclonal antibody to the latency-associated nuclear antigen (LANA) revealed that downregulation occurred predominantly on KSHV-infected (LANA-positive) cells. Although the vast majority of infected cells displayed this downregulation, less than 1% expressed either immediate-early or late lytic proteins detectable by immunofluorescence. Together, these results suggest that downregulation of immunomodulatory proteins on the surface of target cells may represent a constitutive mode of immune evasion employed by KSHV following de novo infection.

Molecular genetics of Kaposi's sarcoma-associated herpesvirus (human herpesvirus-8) epidemiology and pathogenesis

Kaposi's sarcoma had been recognized as unique human cancer for a century before it manifested as an AIDS-defining illness with a suspected infectious etiology. The discovery of Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus-8, in 1994 by using representational difference analysis, a subtractive method previously employed for cloning differences in human genomic DNA, was a fitting harbinger for the powerful bioinformatic approaches since employed to understand its pathogenesis in KS. Indeed, the discovery of KSHV was rapidly followed by publication of its complete sequence, which revealed that the virus had coopted a wide armamentarium of human genes; in the short time since then, the functions of many of these viral gene variants in cell growth control, signaling apoptosis, angiogenesis, and immunomodulation have been characterized. This critical literature review explores the pathogenic potential of these genes within the framework of current knowledge of the basic herpesvirology of KSHV, including the relationships between viral genotypic variation and the four clinicoepidemiologic forms of Kaposi's sarcoma, current viral detection methods and their utility, primary infection by KSHV, tissue culture and animal models of latent- and lytic-cycle gene expression and pathogenesis, and viral reactivation from latency. Recent advances in models of de novo endothelial infection, microarray analyses of the host response to infection, receptor identification, and cloning of full-length, infectious KSHV genomic DNA promise to reveal key molecular mechanisms of the candidate pathogeneic genes when expressed in the context of viral infection.

Transcription mapping of human herpesvirus 8 genes encoding viral interferon regulatory factors

The human herpesvirus 8 (HHV-8) genome contains four tandemly arranged genes encoding viral interferon regulatory factors (vIRF-1 to 4) located between genes 57 and 58. Transcript mapping techniques were employed to determine the sizes, ends and splicing patterns of mRNAs specified by these genes in HHV-8-infected cell lines untreated or chemically induced into the lytic growth cycle. Depending on the cell line used, vIRF-3 transcription was minimally or not induced (i.e. expressed with latent kinetics), whereas the other vIRFs were inducible (i.e. expressed with lytic kinetics). Each gene possessed its own promoter (or promoters) and polyadenylation sites, and all but vIRF-1 were spliced from two exons. vIRF-1 was transcribed in uninduced and induced cells from a single initiation site preceded by a TATA box, with the possible use of an additional TATA box and initiation site in uninduced cells. In induced cells, vIRF-2 was transcribed from a single major initiation site preceded by a TATA box, and vIRF-4 was expressed from two sites each preceded by a TATA box. Transcripts for these genes were insufficiently abundant in uninduced cells to map the 5'-ends. vIRF-3 lacks an obvious TATA box and exhibited heterogeneous 5'-ends in uninduced and induced cells. These data clarify and extend our understanding of the structure and transcription of the HHV-8 vIRF genes.

Kaposi's sarcoma-associated herpesvirus encoded vFLIP induces cellular IL-6 expression: the role of the NF-kappaB and JNK/AP1 pathways

The Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a FADD-like interferon converting enzyme or caspase 8 (FLICE) inhibitory protein (vFLIP) that prevents death receptor-mediated apoptosis by inhibiting the recruitment and activation of FLICE. Since vFLIP physically interacts with tumor necrosis factor receptor associated factor 2 (TRAF2) and TRAF2 mediates activation of the jun NH(2)-terminal kinase (JNK)/activation protein 1 (AP1) pathway, we hypothesized that vFLIP might also activate this pathway. To evaluate this hypothesis, we transiently and stably transfected a vFLIP expression construct and performed several complementary assays to document that vFLIP activates the JNK/AP1 pathway and does so in a TRAF-dependent fashion. As vFLIP also activates the nuclear factor kappaB (NF-kappaB) signaling pathway and the NF-kappaB and JNK/AP1 pathways both modulate cellular interleukin-6 (cIL-6) expression, we postulated that vFLIP induces expression of this cytokine. We show that vFLIP induces cIL-6 expression and activates the cIL-6 promoter, and maximal activation of the cIL-6 promoter by vFLIP requires NF-kappaB and AP1 activation. In addition, vFLIP and latency-associated nuclear antigen (LANA), another KSHV-encoded latent protein, potentiate each other's ability to activate the cIL-6 promoter. Gene silencing experiments by RNA interference demonstrate that vFLIP in BCBL-1 endogenously infected primary effusion lymphoma (PEL) cells mediates JNK/AP1 activation and cIL-6 expression. Thus, we conclude that vFLIP, in addition to its known effects on NF-kappaB activation, also modulates the JNK/AP1 pathway and induces gene expression from the cIL-6 promoter in a JNK/AP1-dependent fashion.

The latency-associated nuclear antigen homolog of herpesvirus saimiri inhibits lytic virus replication

Herpesvirus saimiri (HVS), a T-lymphotropic tumor virus of neotropical primates, and the Kaposi's sarcoma-associated human herpesvirus 8 (KSHV) belong to the gamma-(2)-herpesvirus (Rhadinovirus) subfamily and share numerous features of genome structure and organization. The KSHV latency-associated nuclear antigen (LANA) protein appears to be relevant for viral persistence, latency, and transformation. It binds to DNA, colocalizes with viral episomal DNA, and presumably mediates efficient persistence of viral genomes. LANA further represses the transcriptional and proapoptotic activities of the p53 tumor suppressor protein. Here we report on the ORF73 gene of HVS strain C488, which is the positional and structural homolog of KSHV LANA. The ORF73 gene in OMK cells can encode a 62-kDa protein that localizes to the nucleus in a pattern similar to that of LANA. We show that the ORF73 gene product can regulate viral gene expression by acting as a transcriptional modulator of latent and lytic viral promoters. To define the HVS ORF73 function in the background of a replication-competent virus, we constructed a viral mutant that expresses ORF73 under the transcriptional control of a mifepristone (RU-486)-inducible promoter. The HVS ORF73 gene product efficiently suppresses lytic viral replication in permissive cells, indicating that it defines a critical control point between viral persistence and lytic replication.

Split genes and their expression in Kaposi's sarcoma-associated herpesvirus

A split or interrupted gene is defined as a gene consisting of introns and exons. Removal (splicing) of the intron(s) from a primary transcript (pre-mRNA) is essential for creating a mRNA. Initial assignment of a potential protein coding region in the KSHV genome was based on the initiation codon context and predicted protein size larger than 100 amino acids, but the gene discontinuity was disregarded. Experimental investigation of the assigned ORFs has demonstrated that there are up to 25 split genes, more than one fourth of the total KSHV genes described in the KSHV genome. This includes the genes involved in all phases (latent, immediate early, early and late) of KSHV infection. The complexity of a split gene expression depends upon the availability of a proximal promoter and polyadenylation (pA) signal. Sharing a single promoter or a single pA signal by two or three genes is not uncommon in the expression of KSHV split genes and the resulting transcripts are usually polycistronic. Among those of KSHV split genes, 15 genes express a bicistronic or tricistronic RNA and 10 genes express a monocistronic RNA. Alternative RNA splicing could happen in a particular pre-mRNA due to intron or exon inclusion or skipping or the presence of an alternative 5' splice site or 3' splice site. This may, respectively, result in at least 8 species of K8 and 14 species of K15 transcripts. This appears to be related to cell differentiation and stages of the virus infection, presumably involving viral cis elements and trans splicing factors.

Ovine herpesvirus 2 lytic cycle replication and capsid production

Ovine herpesvirus 2 (OvHV-2) causes malignant catarrhal fever in cattle, pigs and deer. We have observed intact circular and linear OvHV-2 genomes in infected T cell lines derived from cows and rabbits. Bovine T cell lines were predominantly latently infected but rabbit T cell lines supported OvHV-2 productive cycle gene expression and virus capsids were demonstrated for the first time.

The Herpesvirus Saimiri open reading frame 73 gene product interacts with the cellular protein p32

The role of the gamma-2 herpesvirus open reading frame (ORF) 73 gene product has become the focus of considerable interest. It has recently been shown that the Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA) is expressed during a latent infection and can modulate both viral and cellular gene expression. The herpesvirus saimiri (HVS) ORF 73 gene product has some sequence homology to LANA; however, the role of HVS ORF 73 is unknown. We have previously demonstrated that HVS ORF73 is expressed in a stably transduced human carcinoma cell line, where HVS genomes persist as nonintegrated circular episomes. This implies that there may be some functional homology between these proteins. To further investigate the role of the HVS ORF 73 protein, the yeast two-hybrid system was employed to identify interacting cellular proteins. We demonstrate that ORF 73 interacts with the cellular protein p32 and triggers the accumulation of p32 in the nucleus. Using reporter gene-based transient-transfection assays, we demonstrate that ORF 73 can transactivate a number of heterologous promoter constructs and also upregulate its own promoter. Moreover, ORF 73 and p32 act synergistically to transactivate these promoters. The binding of ORF 73 to p32 is mediated by an amino-terminal arginine-rich domain, which contains two functionally distinct nuclear localization signals. The p32 binding domains are required for ORF 73 transactivating abilities and for ORF 73 to induce nuclear accumulation of p32. These results suggest that ORF 73 can function as a regulator of gene expression and that p32 is involved in ORF 73-dependent transcriptional activation.

Tissue specificity of the Kaposi's sarcoma-associated herpesvirus latent nuclear antigen (LANA/orf73) promoter in transgenic mice

Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8) is a human-oncogenic herpesvirus. Cells from KSHV-associated tumors, such as Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL), are of endothelial and B-cell origin, respectively. KSHV persists indefinitely in these cell lineages during latent infection. Indeed, cellular latency is a hallmark of all herpesviruses that is intimately linked to their pathogenesis. We previously characterized the promoter for the KSHV latency-associated nuclear antigen LANA/orf73. LANA is required for latent episome maintenance and has also been implicated in oncogenesis. Hence, regulation of LANA expression is critical to KSHV persistence. We find that a region extending to bp -1299 upstream of the LANA transcription start site is able to drive lacZ-reporter gene expression in several lines of transgenic mice. In agreement with KSHV's natural tropism, we detected reporter gene expression in CD19-positive B cells but not in CD3-positive T cells. We also detected expression in the kidney and, at a lower level, in the liver. In contrast to KS tumors, transgene expression was localized to kidney tubular epithelium rather than vascular endothelial cells. This suggests that our promoter fragment contains all cis-regulatory elements sufficient for B-cell specificity but not those required for endothelial specificity. Alternatively, while the trans-acting factors required for LANA expression in B cells are evolutionarily conserved, those that regulate endothelial cell-specific expression are unique to humans. Our in vivo studies address a conundrum in KSHV biology: in culture, KSHV is able to infect a variety of cell types indiscriminately, while in healthy latent carriers KSHV is found in B lymphocytes. The transgenic-mouse experiments reported here suggest that tissue-restricted LANA gene expression could explain B-cell-specific viral persistence.

Kinetics of expression of rhesus monkey rhadinovirus (RRV) and identification and characterization of a polycistronic transcript encoding the RRV Orf50/Rta, RRV R8, and R8.1 genes

Rhesus monkey rhadinovirus (RRV) is a close relative of Kaposi's sarcoma-associated herpesvirus (KSHV; human herpesvirus 8). RRV serves as an in vitro and an in vivo model for KSHV, and the mapping of its transcription program during lytic replication is significant since it represents de novo infection in the absence of stimulation with phorbol esters. Further, the RRV lytic system facilitates the making of recombinant viruses, and hence transcription profiling of the wild-type virus is important. Currently, the kinetics of lytic gene expression of RRV, the function of the RRV Orf50/Rta gene, and the presence of the RRV R8 and R8.1 genes are not known. This study details the transcription profile seen during RRV lytic replication and shows that RRV latency-associated nuclear antigen, viral FLIP (vFLIP), and vCyclin are transcribed during the RRV lytic phase. In addition, this study describes the identification of three new spliced products of the RRV Orf50, R8, and R8.1 genes, which are structural homologs of the KSHV Orf50, K8, and K8.1 genes, respectively. Characterization of the RRV Orf50 protein identifies it as a strong transcriptional transactivator capable of activating three early RRV promoters. Interestingly, the KSHV Orf50 transactivator can also activate these simian virus promoters, suggesting that there exists a conservation of gene function between the key transcription factors of KSHV and RRV.

Charting latency transcripts in Kaposi's sarcoma-associated herpesvirus by whole-genome real-time quantitative PCR

The division into a latent or lytic life cycle is fundamental to all herpesviridae. In the case of Kaposi's sarcoma-associated herpesvirus (KSHV) (human herpesvirus 8), latent genes have been implicated in cell autonomous transformation, while certain lytic genes procure a tumor friendly milieu through paracrine mechanism. To query KSHV transcription, we devised and validated a high-throughput, high-specificity, high-sensitivity, real-time quantitative reverse transcription-PCR array. This novel methodology is applicable to many human pathogens. Its first use demonstrated that the mRNA levels for KSHV LANA, v-cyclin, and v-FLIP do not increase at any time after viral reactivation. The mRNA for LANA-2/vIRF-3 is similarly resistant to viral reactivation. In contrast, every other latent or lytic message was induced. Hence, LANA, v-FLIP, v-cyclin, and LANA-2 constitute a group of uniquely regulated transcripts in the KSHV genome.

Kaposi's sarcoma-associated herpesvirus K8 exon 3 contains three 5'-splice sites and harbors a K8.1 transcription start site

Kaposi's sarcoma-associated herpesvirus (KSHV) K8 and K8.1 open reading frames are juxtaposed and span from nucleotide (nt) 74850 to 76695 of the virus genome. A K8 pre-mRNA overlaps the entire K8.1 coding region, and alternative splicing of KSHV K8 and K8.1 pre-mRNAs each produces three isoforms (alpha, beta, and gamma) of the mRNAs. We have mapped the 5' end of the K8.1 RNA in butyrate-induced KSHV-positive JSC-1 cells to nt 75901 in the KSHV genome and have shown that exon 3 of the K8 pre-mRNA in JSC-1 cells covers most part of the intron 3 defined previously and has three 5'-splice sites (ss), respectively, at nt 75838, 76155, and 76338. Selection of the nt 75838 5'-ss dictates the K8 mRNA production and overwhelms the RNA processing. Alternative selection of other two 5'-ss is feasible and leads to production of two additional bicistronic mRNAs, K8/K8.1alpha and -beta. However, the novel bicistronic K8/K8.1 mRNAs translated a little K8 and no detectable K8.1 proteins in 293 cells. Data suggest that production of the K8/K8.1 mRNAs may be an essential way to control K8 mRNAs, especially K8alpha, to a threshold at RNA processing level.

Patterns of gene expression and a transactivation function exhibited by the vGCR (ORF74) chemokine receptor protein of Kaposi's sarcoma-associated herpesvirus

The ORF74 or vGCR gene encoded by Kaposi's sarcoma-associated herpesvirus (KSHV; also called human herpesvirus 8) has properties of a ligand-independent membrane receptor signaling protein with angiogenic properties that is predicted to play a key role in the biology of the virus. We have examined the expression of vGCR mRNA and protein in primary effusion lymphoma (PEL) cell lines, PEL and multicentric Castleman's disease (MCD) tumors, Kaposi's sarcoma lesions and infected endothelial cell cultures. The vGCR gene proved to be expressed in PEL cell lines as a large spliced bicistronic mRNA of 3.2 kb that also encompasses the upstream vOX2 (K14) gene. This mRNA species was induced strongly by phorbol ester (TPA) and sodium butyrate treatment in the BCBL-1 cell line, but only weakly in the HBL6 cell line, and was classified as a relatively late and low-abundance delayed early class lytic cycle gene product. A complex bipartite upstream lytic cycle promoter for this mRNA was nestled within the intron of the 5'-overlapping but oppositely oriented latent-state transcription unit for LANA1/vCYC-D/vFLIP and responded strongly to both TPA induction and cotransfection with the KSHV RNA transactivator protein (RTA or ORF50) in transient reporter gene assays. A vGCR protein product of 45 kDa that readily dimerized was detected by Western blotting and in vitro translation and was localized in a cytoplasmic and membrane pattern in DNA-transfected Vero and 293T cells or adenovirus vGCR-transduced dermal microvascular endothelial cells (DMVEC) as detected by indirect immunofluorescence assay (IFA) and immunohistochemistry with a specific rabbit anti-vGCR antibody. Similarly, a subfraction of KSHV-positive cultured PEL cells and of KSHV (JSC-1) persistently infected DMVEC cells displayed cytoplasmic vGCR protein expression, but only after TPA or spontaneous lytic cycle induction, respectively. The vGCR protein was also detectable by immunohistochemical staining in a small fraction (0.5 to 3%) of the cells in PEL and MCD tumor and nodular Kaposi's sarcoma lesion specimens that were apparently undergoing lytic cycle expression. These properties are difficult to reconcile with the vGCR protein's playing a direct role in spindle cell proliferation, transformation, or latency, but could be compatible with proposed contributions to angiogenesis via downstream paracrine effects. The ability of vGCR to transactivate expression of both several KSHV promoter-driven luciferase (LUC) reporter genes and an NFkappaB motif containing the chloramphenicol acetyltransferase (CAT) reporter gene may also suggest an unexpected regulatory role in viral gene expression.

The molecular pathology of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is the eighth and most recently identified human herpesvirus (HHV-8). KSHV was discovered in 1994 by Chang et al. who used representational difference analysis to search for DNA sequences present in AIDS-associated KS but not in adjacent normal skin [1]. The virus has since been shown to be specifically associated with all forms of this disease and has fulfilled all of Hill's criteria for causation (reviewed in ). KSHV is also found in all cases of primary effusion lymphoma and in a plasmablastic variant of multicentric Castleman's disease. Over the last few years a wealth of data has been gained on the role of KSHV genes during infection. This review is an attempt to assemble this information into a more complete picture of how KSHV may cause disease.

Isolation and expression of three open reading frames from ovine herpesvirus-2

Ovine herpesvirus-2 (OvHV-2), a member of the gammaherpesviruses (genus Rhadinovirus), asymptomatically infects its natural host, the sheep, but causes malignant catarrhal fever (MCF) in susceptible hosts, such as cattle, deer and pigs. A permissive cell culture system for virus replication has not been identified but viral DNA is present within lymphoblastoid cell lines (LCLs) established from cases of MCF. During this study, a cDNA expression library generated from LCLs was screened with sheep sera and two cDNAs were isolated. One cDNA contained two open reading frames (ORFs) that show similarity to ORFs 58 and 59 of alcelaphine herpesvirus-1 (AlHV-1), a closely related gammaherpesvirus that also causes MCF. Both ORFs 58 and 59 are conserved throughout the gammaherpesviruses. ORF 58 is predicted to be a membrane protein, while ORF 59 has been shown to be an early lytic gene that functions as a DNA polymerase processivity factor. The second cDNA clone contained a partial ORF showing limited similarity to AlHV-1 ORF 73, a homologue of the latency-associated nuclear antigen of human herpesvirus-8, which is associated with latent infections. The full-length OvHV-2 ORF 73 was cloned subsequently by PCR. The ORFs isolated from the library were cloned into a bacterial expression vector and the recombinant proteins tested for their reactivity to sera from OvHV-2-infected animals. An ORF 59 fusion protein was recognized specifically by sera from OvHV-2-infected cattle and will be used to develop a sero-diagnostic test.