Modulation of cell signaling pathways by kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8)

Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8, has been associated with the development of Kaposi's sarcoma, pleural effusion lymphoma, and multicentric Castleman's disease. KSHV is a double-stranded DNA virus that has been classified as a gammaherpesvirus. The viral genome is approx, 160 kb long and encodes for several genes that are involved in cell signaling pathways. These include genes that are unique to the virus as well as viral homologues of cellular genes. The latter are likely to have been usurped from the host genome and include both virokines and viral receptor proteins. This article reviews how these KSHV proteins modulate cellular signal transduction pathways.     

Cell signaling pathways engaged by KSHV

Kaposi's sarcoma herpesvirus (KSHV) is the eighth human herpesvirus discovered in 1994 from Kaposi's sarcoma lesion of an AIDS patient. The strong molecular and epidemiological links associating KSHV with Kaposi's sarcoma and certain lymphoproliferative disorders indicate that KSHV is required for the development of these malignancies. Although KSHV is equipped to manipulate and deregulate several cellular signaling pathways, it is not yet understood how this leads to cell transformation. Profound understanding of the interplay of viral and cellular factors in KSHV-infected cells will provide valuable information on the mechanisms of viral tumorigenesis and enable development of efficient targeted therapies for virus-induced cancers. This review focuses on the cellular signaling pathways that KSHV gene products impinge on and discusses their putative contribution to tumorigenesis.     

Identification of the DNA sequence interacting with Kaposi's sarcoma-associated herpesvirus viral interferon regulatory factor 1

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of Kaposi's sarcoma. The open reading frame (K9) of KSHV encodes viral interferon regulatory factor 1 (vIRF1), which functions as a repressor of interferon-mediated signal transduction. The amino-terminal region of vIRF1 displays significant homology to the DNA-binding domain of cellular interferon regulatory factors, supporting the theory that the protein interacts with specific DNA sequences. Here, we identify the consensus sequence of vIRF1-binding sites from a pool of random oligonucleotides. Moreover, our data show that vIRF1 interacts with the K3:viral dihydrofolate reductase:viral interleukin 6 promoter region in the KSHV genome.     

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.     

Kaposi's sarcoma biology

Cancer remains a major burden for HIV-infected individuals. The majority of cancers associated with HIV infection are driven by oncogenic viruses like Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus and human papillomavirus. Kaposi's sarcoma is a tumour of endothelium and is the most common malignancy in HIV infected individuals. AIDS-related cancers are providing critical insight into cancer immunity and viral oncogenesis.     

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.     

Interleukin-8 and growth-regulated oncogene alpha mediate angiogenesis in Kaposi's sarcoma

The development of the complex neoplasm Kaposi's sarcoma is dependent on infection with the Kaposi's sarcoma-associated herpesvirus (KSHV) and appears to be greatly enhanced by cytokines and human immunodeficiency virus type 1 (HIV-1) Tat. Interleukin-8 (IL-8) and growth-regulated oncogene alpha (GRO-alpha) are chemokines involved in chemoattraction, neovascularization, and stimulation of HIV-1 replication. We have previously demonstrated that production of GRO-alpha is stimulated by exposure of monocyte-derived macrophages (MDM) to HIV-1. Here we show that exposure of MDM to HIV-1, viral Tat, or viral gp120 leads to a substantial increase in IL-8 production. We also demonstrate that IL-8 and GRO-alpha are induced by KSHV infection of endothelial cells and are crucial to the angiogenic phenotype developed by KSHV-infected endothelial cells in cell culture and upon implantation into SCID mice. Thus, the three known etiological factors in Kaposi's sarcoma pathogenesis-KSHV, HIV-1 Tat, and cellular growth factors-might be linked, in part, through induction of IL-8 and GRO-alpha.     

Differential expression of viral Bcl-2 encoded by Kaposi's sarcoma-associated herpesvirus and human Bcl-2 in primary effusion lymphoma cells and Kaposi's sarcoma lesions

Expression of human herpesvirus 8 viral Bcl-2 protein was demonstrated in spindle cells of late-stage Kaposi's sarcoma lesions but not in primary effusion lymphoma cell lines. In contrast, strong expression of human Bcl-2 was found in stimulated primary effusion lymphoma cells, whereas in Kaposi's sarcoma lesions preferential mononuclear cells, and to a lesser extent spindle cells, stained positive.     

Molecular piracy of Kaposi's sarcoma associated herpesvirus

Kaposi's Sarcoma associated Herpesvirus (KSHV) is the most recently discovered human tumor virus and is associated with the pathogenesis of Kaposi's sarcoma, primary effusion lymphoma, and Multicentric Casttleman's disease. KSHV contains numerous open reading frames with striking homology to cellular genes. These viral gene products play a variety of roles in KSHV-associated pathogenesis by disrupting cellular signal transduction pathways, which include interferon-mediated anti-viral responses, cytokine-regulated cell growth, apoptosis, and cell cycle control. In this review, we will attempt to cover our understanding of how viral proteins deregulate cellular signaling pathways, which ultimately contribute to the conversion of normal cells to cancerous cells.     

Analysis of 4.3 kilobases of divergent locus B of macaque retroperitoneal fibromatosis-associated herpesvirus reveals a close similarity in gene sequence and genome organization to Kaposi's sarcoma-associated herpesvirus

We previously identified retroperitoneal fibromatosis-associated herpesvirus (RFHV) as a simian homolog of Kaposi's sarcoma-associated herpesvirus (KSHV) in a fibroproliferative malignancy of macaques that has similarities to Kaposi's sarcoma. In this report, we cloned 4.3 kb of divergent locus B (DL-B) flanking the DNA polymerase gene from two variants of RFHV from different species of macaque with a consensus degenerate hybrid oligonucleotide primer approach. Within the DL-B region of RFHV, viral homologs of the cellular interleukin-6, dihydrofolate reductase, and thymidylate synthase genes were identified, along with a homolog of the gammaherpesvirus open reading frame (ORF) 10. In addition, a homolog of the KSHV ORF K3, the modulator of immune recognition-1, was identified. Our data show a close similarity in sequence conservation, gene content, and genomic structure between RFHV and KSHV which strongly supports the grouping of these viral species within the same RV-1 rhadinovirus lineage and the hypothesis that RFHV is the macaque homolog of KSHV.     

Spindle cells and their role in Kaposi's sarcoma

Spindle cells represent the main cell type of the advanced final nodular stage of Kaposi's sarcoma lesions. Despite some clinical and epidemiological differences, the four Kaposi's sarcoma forms (classic, endemic, post-transplant and epidemic) display very similar histopathological features, with the proliferation of spindle cells (considered as the Kaposi's sarcoma tumor cells) associated with inflammation and neo-angiogenesis. Electron-microscopy and immuno-histochemistry studies have led to the consensus that the spindle cells originated from the endothelial lineage. However, only recently, studies that used specific lymphatic immunological markers (such as podoplanin) and molecular features (gene expression microarrays) strongly linked Kaposi's sarcoma spindle cells to the endothelium lymphatic cell lineage. Both hybridization and immuno-histochemistry techniques have demonstrated that human herpesvirus 8 also known as Kaposi's sarcoma associated herpesvirus was present in spindle cells at all stages of the disease (patch, plaque, nodule). Interestingly, while the human herpesvirus 8 latent genes are expressed in nearly all tumor spindle cells, only a small fraction of them expresses markers of viral lytic replication. Recent findings showing that nodular Kaposi's sarcoma lesions display all patterns of human herpesvirus 8 clonality support the model according to which this tumor begins as a polyclonal disease with a subsequent evolution to a mono/oligoclonal process involving infected spindle cells. Spindle cells appear to be the central masterpiece in KS tumorigenesis, however the exact respective role of each human herpesvirus 8 gene, in the initiation and the disease progression is still under investigation and the question of whether or not this tumor is a reactive process or a true malignant proliferation of spindle cells remains yet unclear.     

Mitotic chromosome-binding activity of latency-associated nuclear antigen 1 is required for DNA replication from terminal repeat sequence of Kaposi's sarcoma-associated herpesvirus

Latency-associated nuclear antigen 1 (LANA1) of Kaposi's sarcoma-associated herpesvirus (KSHV) is implicated in the persistence of the viral genome during latent infection. It has been suggested that LANA1 tethers the viral genome to the host chromosome and also participates actively in DNA replication from the terminal repeat of KSHV. Here we show by mutational analysis that the mitotic chromosome-binding activity of LANA1 is tightly coupled to its replication activity. Thus, KSHV appears to have evolved a unique tactic for its stable maintenance.     

Degradation of p27(Kip) cdk inhibitor triggered by Kaposi's sarcoma virus cyclin-cdk6 complex.

The Kaposi's sarcoma-associated human herpesvirus 8 (KSHV/HHV8) encodes a protein similar to cellular cyclins. This cyclin is most closely related to cellular D-type cyclins, but biochemically it behaves atypically in various respects. Complexes formed between the viral cyclin and the cyclin-dependent kinase subunit, cdk6, can phosphorylate a wider range of substrates and are resistant to cdk inhibitory proteins. We show here that the KSHV-cyclin-cdk6 complex phosphorylates p27(Kip) on a C-terminal threonine that is implicated in destabilization of this cdk inhibitor. Expression of the viral cyclin in tissue culture cells overcomes a cell cycle block by p27(Kip). However, full cell-cycle transit of these cells appears to depend on C-terminal phosphorylation of p27(Kip) and seems to involve transactivation of other cellular cyclin-dependent kinases. A p27(Kip)-phosphorylating cdk6 complex exists in cell lines derived from primary effusion lymphoma and in Kaposi's sarcoma, this indicating that virally induced p27(Kip) degradation may occur in KSHV-associated tumours.     

Kaposi's sarcoma-associated herpesvirus: a new human tumor virus, but how?

Kaposi's sarcoma-associated herpesvirus, or human herpesvirus 8, the most recently discovered human tumor virus, is involved in the pathogenesis of Kaposi's sarcoma, primary effusion lymphoma and some cases of multicentric Castleman's disease. It is non-pathogenic in the majority of otherwise healthy individuals but highly oncogenic in the context of HIV-1 infection and iatrogenic immune suppression, and other cofactors might exist. Several viral genes can interfere with normal cell growth and differentiation, but their precise role in oncogenesis is still under investigation.