A potential alpha-helix motif in the amino terminus of LANA encoded by Kaposi's sarcoma-associated herpesvirus is critical for nuclear accumulation of HIF-1alpha in normoxia

Single molecule analysis of replicated DNA reveals the usage of multiple KSHV genome regions for latent replication

Kaposi's sarcoma associated herpesvirus (KSHV), an etiologic agent of Kaposi's sarcoma, Body Cavity Based Lymphoma and Multicentric Castleman's Disease, establishes lifelong latency in infected cells. The KSHV genome tethers to the host chromosome with the help of a latency associated nuclear antigen (LANA). Additionally, LANA supports replication of the latent origins within the terminal repeats by recruiting cellular factors. Our previous studies identified and characterized another latent origin, which supported the replication of plasmids ex-vivo without LANA expression in trans. Therefore identification of an additional origin site prompted us to analyze the entire KSHV genome for replication initiation sites using single molecule analysis of replicated DNA (SMARD). Our results showed that replication of DNA can initiate throughout the KSHV genome and the usage of these regions is not conserved in two different KSHV strains investigated. SMARD also showed that the utilization of multiple replication initiation sites occurs across large regions of the genome rather than a specified sequence. The replication origin of the terminal repeats showed only a slight preference for their usage indicating that LANA dependent origin at the terminal repeats (TR) plays only a limited role in genome duplication. Furthermore, we performed chromatin immunoprecipitation for ORC2 and MCM3, which are part of the pre-replication initiation complex to determine the genomic sites where these proteins accumulate, to provide further characterization of potential replication initiation sites on the KSHV genome. The ChIP data confirmed accumulation of these pre-RC proteins at multiple genomic sites in a cell cycle dependent manner. Our data also show that both the frequency and the sites of replication initiation vary within the two KSHV genomes studied here, suggesting that initiation of replication is likely to be affected by the genomic context rather than the DNA sequences.

Viruses as key regulators of angiogenesis

Angiogenesis is an important physiological process that is controlled by a precise balance of growth and inhibitory factors in healthy tissues. However, environmental and genetic factors may disturb this delicate balance, resulting in the development of angiogenic diseases, tumour growth and metastasis. During the past decades, extensive research has led to the identification and characterization of genes, proteins and signalling pathways that are involved in neovascularization. Moreover, increasing evidence indicates that viruses may also regulate angiogenesis either directly, by (i) producing viral chemokines, growth factors and/or receptors or (ii) activating blood vessels as a consequence of endothelial cell tropism, or indirectly, by (iii) modulating the activity of cellular proteins and/or (iv) inducing a local or systemic inflammatory response, thereby creating an angiogenic microenvironment. As such, viruses may modulate several signal transduction pathways involved in angiogenesis leading to changes in endothelial cell proliferation, migration, adhesion, vascular permeability and/or protease production. Here, we will review different mechanisms that may be applied by viruses to deregulate the angiogenic balance in healthy tissues and/or increase the angiogenic potential of tumours.

The single RBP-Jkappa site within the LANA promoter is crucial for establishing Kaposi's sarcoma-associated herpesvirus latency during primary infection

Kaposi's sarcoma-associated herpesvirus (KSHV; or human herpesvirus 8 [HHV8]) is implicated in the pathogenesis of many human malignancies including Kaposi's sarcoma (KS), multicentric Castleman's disease (MCD), and primary effusion lymphoma (PEL). KSHV infection displays two alternative life cycles, referred to as the latent and lytic or productive cycle. Previously, we have reported that the replication and transcription activator (RTA), a major lytic cycle transactivator, contributes to the development of KSHV latency by inducing latency-associated nuclear antigen (LANA) expression during early stages of infection by targeting RBP-Jκ, the master regulator of the Notch signaling pathway. Here, we generated a bacterial artificial chromosome (BAC) KSHV recombinant virus with a deletion of the RBP-Jκ site within the LANA promoter to evaluate the function of the RBP-Jκ cognate site in establishing primary latent infection. The results showed that genetic disruption of the RBP-Jκ binding site within the KSHV LANA promoter led to enhanced expression of the KSHV-encoded immediate early RTA, resulting in an increase in lytic replication during primary infection of human peripheral blood mononuclear cells (PBMCs). This system provides a powerful tool for use in indentifying additional cellular and viral molecules involved in LANA-mediated latency maintenance during the early stages of KSHV infection.

Amplification of the angiogenic signal through the activation of the TSC/mTOR/HIF axis by the KSHV vGPCR in Kaposi's sarcoma

BACKGROUND

Kaposi's sarcoma (KS) is a vascular neoplasm characterized by the dysregulated expression of angiogenic and inflammatory cytokines. The driving force of the KS lesion, the KSHV-infected spindle cell, secretes elevated levels of vascular endothelial growth factor (VEGF), essential for KS development. However, the origin of VEGF in this tumor remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report that the KSHV G protein-coupled receptor (vGPCR) upregulates VEGF in KS through an intricate paracrine mechanism. The cytokines secreted by the few vGPCR-expressing tumor cells activate in neighboring cells multiple pathways (including AKT, ERK, p38 and IKKβ) that, in turn, converge on TSC1/2, promoting mTOR activation, HIF upregulation, and VEGF secretion. Conditioned media from vGPCR-expressing cells lead to an mTOR-dependent increase in HIF-1α and HIF-2α protein levels and VEGF upregulation. In a mouse allograft model for KS, specific inhibition of the paracrine activation of mTOR in non-vGPCR-expressing cells was sufficient to inhibit HIF upregulation in these cells, and abolished the ability of the vGPCR-expressing cells to promote tumor formation in vivo. Similarly, pharmacologic inhibition of HIF in this model blocked VEGF secretion and also lead to tumor regression.

CONCLUSIONS/SIGNIFICANCE

Our findings provide a compelling explanation for how the few tumor cells expressing vGPCR can contribute to the dramatic amplification of VEGF secretion in KS, and further provide a molecular mechanism for how cytokine dysregulation in KS fuels angiogenesis and tumor development. These data further suggest that activation of HIF by vGPCR may be a vulnerable target for the treatment of patients with KS.

Update on KSHV epidemiology, Kaposi Sarcoma pathogenesis, and treatment of Kaposi Sarcoma

Much has been learned since the discovery of KSHV in 1994 about its epidemiology and pathology but much of what has been learned has yet to be translated into clinical practice. In this review, we survey the current state of knowledge on KSHV epidemiology and KS pathogenesis and highlight therapeutic opportunities in both the developed and developing world.

Oxygen homeostasis

Metazoan life is dependent upon the utilization of O(2) for essential metabolic processes and oxygen homeostasis is an organizing principle for understanding metazoan evolution, ontology, physiology, and pathology. Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that is expressed by all metazoan species and functions as a master regulator of oxygen homeostasis. Recent studies have elucidated complex mechanisms by which HIF-1 activity is regulated and by which HIF-1 regulates gene expression, with profound consequences for prenatal development, postnatal physiology, and disease pathogenesis.

Development of an oncolytic herpes simplex virus using a tumor-specific HIF-responsive promoter

We exploited the differential activation of hypoxia-inducible factor (HIF)-dependent gene expression in tumors versus normal tissue for the design of a targeted oncolytic Herpes simplex virus type-1 (HSV-1). A gene that is essential for viral replication, ICP4, was placed under the regulation of a HIF-responsive promoter and then introduced into the thymidine kinase locus (U_L23) of HSV d120 which contains partial deletions in the two endogenous ICP4 genes. Recombinant HIF-HSV were isolated and their derivation from d120 was verified by expression of a truncated, nonfunctional form of ICP4 protein. Disruption of the U_L23 locus was confirmed by loss of thymidine kinase expression and resistance to acyclovir. Unexpectedly, HIF-HSV expressed ICP4 and induced tumor cell lysis at similar levels under normoxia and hypoxia. The lack of HIF-dependent ICP4 transgene expression by HIF-HSV was due to two factors that have not previously been reported- reversion of the ICP4 gene region to its wild-type configuration and increased HIF-transcriptional activity under normoxia when cells were infected with any strain of HSV-1. The findings that an oncolytic HSV-1 is genetically unstable and can activate a tumor-related promoter in a non-specific manner have important implications for any proposed use of this virus in cancer therapy.

Ubiquitin/SUMO modification regulates VHL protein stability and nucleocytoplasmic localization

Functional inactivation of the von Hippel-Lindau (VHL) tumor suppressor protein is linked to the development of several forms of cancer as well as oncogenic progression like sporadic renal clear-cell carcinomas (RCC). Despite the critical role played by VHL in destruction of hypoxia-inducible factor α (HIFα) via ubiquitin-mediated proteolysis, very little is known about the post-translational modification which regulates VHL activity. Our previous study showed that the SUMO E3 ligase PIASy interacts with VHL and induces VHL SUMOylation on lysine residue 171 (Cai et al, PLoS ONE, 2010, 5(3):e9720). Here we further report that VHL also undergoes ubiquitylation on both lysine residues 171 and 196, which is blocked by PIASy. Moreover, using a VHL-SUMO1 or ubiquitin fusion protein, we found that ubiquitylated VHL is localized predominantly in the cytoplasm, while SUMOylated VHL results in increased VHL protein stability and nuclear redistribution. Interestingly, substitution of lysine 171 and 196 to arginine of VHL abrogates its inhibitory function on the transcriptional activity of HIFα, and tube formation in vitro. This demonstrates that post-translational modifications like ubiquitylation and SUMOylation contributes to VHL protein stability and nucleocytoplasmic shuttling, and that the overall function of VHL in tumor suppression may require a precise and dynamically regulated process which involves protein modification.

Kaposi's sarcoma-associated herpesvirus inhibits interleukin-4-mediated STAT6 phosphorylation to regulate apoptosis and maintain latency

Cytokine-mediated JAK/STAT signaling controls numerous important biologic responses like immune function, cellular growth, and differentiation. Inappropriate activation of this signaling pathway is associated with a range of malignancies. Kaposi's sarcoma-associated herpesvirus (KSHV) is the infectious viral agent associated with Kaposi's sarcoma and may also contribute to B-cell disorders, which include primary effusion lymphoma (PEL) and multicentric Castleman's disease. However, regulation of cytokine-mediated lymphocytic immune response by KSHV is not fully understood. In this report, we demonstrate that KSHV suppresses the interleukin-4 (IL-4)-stimulated immune response of B-lymphocyte activation and cell proliferation. Moreover, we show that the latency-associated nuclear antigen (LANA) encoded by KSHV is essential for viral blocking of IL-4-induced signaling. LANA reduces phosphorylation of the signal transducers and activators of transcription 6 (STAT6) on Y-641 and concomitantly its DNA binding ability. Importantly, knockdown of endogenous STAT6 dramatically increases the sensitivity of PEL cells to low-serum stress or chemical-mediated cellular apoptosis and reactivation of KSHV from latent replication. Thus, these findings suggest that the IL-4/STAT6 signaling network is precisely controlled by KSHV for survival, maintenance of latency, and suppression of the host cytokine immune response of the virus-infected cells.

Hypoxia inactivates the VHL tumor suppressor through PIASy-mediated SUMO modification

The hypoxic microenvironment contributes to embryonic development and tumor progression through stabilization of the potent transcriptional factor HIFα. In normoxia, the tumor suppressor protein VHL acts as an E3 ubiquitin ligase to target HIFα for proteolytic destruction. Increasing evidence shows that VHL is a multifunctional adaptor involved in inhibition of HIFα-dependent and independent cellular processes. However, the molecular effect of hypoxic stress on VHL functions remains elusive. Here we report that PIASy, a SUMO E3 ligase upregulated in hypoxia, interacts with VHL and induces VHL SUMOylation on lysine residue 171. Moreover, PIASy-mediated SUMO1 modification induces VHL oligomerization and abrogates its inhibitory function on tumor cell growth, migration and clonogenicity. Knockdown of PIASy by small interfering RNA leads to reduction of VHL oligomerization and increases HIF1α degradation. These findings reveal a unique molecular strategy for inactivation of VHL under hypoxic stress.

Defining the role of hypoxia-inducible factor 1 in cancer biology and therapeutics

Adaptation of cancer cells to their microenvironment is an important driving force in the clonal selection that leads to invasive and metastatic disease. O2 concentrations are markedly reduced in many human cancers compared with normal tissue, and a major mechanism mediating adaptive responses to reduced O2 availability (hypoxia) is the regulation of transcription by hypoxia-inducible factor 1 (HIF-1). This review summarizes the current state of knowledge regarding the molecular mechanisms by which HIF-1 contributes to cancer progression, focusing on (1) clinical data associating increased HIF-1 levels with patient mortality; (2) preclinical data linking HIF-1 activity with tumor growth; (3) molecular data linking specific HIF-1 target gene products to critical aspects of cancer biology and (4) pharmacological data showing anticancer effects of HIF-1 inhibitors in mouse models of human cancer.

Molecular biology of Kaposi's sarcoma-associated herpesvirus and related oncogenesis

Kaposi's Sarcoma-associated Herpesvirus (KSHV), also known as human herpesvirus 8 (HHV-8), is the most recently identified human tumor virus,and is associated with the pathogenesis of Kaposi's sarcoma and two lymphoproliferative disorders known to occur frequently in AIDS patients-primary effusion lymphoma and multicentric Castleman disease. In the 15 years since its discovery, intense studies have demonstrated an etiologic role for KSHV in the development of these malignancies. Here, we review the recent advances linked to understanding KSHV latent and lytic life cycle and the molecular mechanisms of KSHV-mediated oncogenesis in terms of transformation, cell signaling, cell growth and survival, angiogenesis, immune invasion and response to microenvironmental stress, and highlight the potential therapeutic targets for blocking KSHV tumorigenesis.

LANA-1, Bcl-2, Mcl-1 and HIF-1alpha protein expression in HIV-associated Kaposi sarcoma

Human herpesvirus 8 (HHV8) is necessary for Kaposi sarcoma (KS) to develop, but whether the tissue viral load is a marker of KS progression is still unclear. Little is known about the level of expression of apoptosis-regulating proteins and of hypoxia-inducible factors (HIFs) in KS tumour cells relative to HHV8 expression. We therefore investigated the expression of the latency-associated nuclear antigen (LANA-1) of HHV8, Bcl-2, Mcl-1, Bax, Bcl-xL, caspase 3 and HIF-1alphain KS tissue specimens at different stages of the disease. The expression of these proteins was evaluated immunohistochemically using tissue microarrays (TMAs) in tissue specimens from 245 HIV-positive patients at different stages of the disease. Both LANA-1 and HIF-1alpha were expressed in KS biopsies taken at different stages, but their level increased throughout tumour progression. Additionally, the levels of Bcl-2 and Mcl-1 were higher in visceral KS lesions compared to levels observed in cutaneous and mucosal KS. This study demonstrates that late tumour stages of KS in tissues from HIV-positive patients are associated with high levels of LANA-1, HIF-1alpha and of the anti-apoptotic proteins, Bcl-2 and Mcl-1. Finally, the expression of these proteins can be potentially used as a tissue biomarker in defining patients with a higher risk of disease progression.

X-box binding protein 1 contributes to induction of the Kaposi's sarcoma-associated herpesvirus lytic cycle under hypoxic conditions

Kaposi's sarcoma-associated herpesvirus (KSHV), like other herpesviruses, has two stages to its life cycle: latency and lytic replication. KSHV is required for development of Kaposi's sarcoma, a tumor of endothelial origin, and is associated with the B-cell tumor primary effusion lymphoma (PEL) and the plasmablastic variant of multicentric Castleman's disease, all of which are characterized by predominantly latent KSHV infection. Recently, we and others have shown that the activated form of transcription factor X-box binding protein 1 (XBP-1) is a physiological trigger of KSHV lytic reactivation in PEL. Here, we show that XBP-1s transactivates the ORF50/RTA promoter though an ACGT core containing the XBP-1 response element, an element previously identified as a weakly active hypoxia response element (HRE). Hypoxia induces the KSHV lytic cycle, and active HREs that respond to hypoxia-inducible factor 1alpha are present in the ORF50/RTA promoter. Hypoxia also induces active XBP-1s, and here, we show that both transcription factors contribute to the induction of RTA expression, leading to the production of infectious KSHV under hypoxic conditions.

Characterization of the interaction between latency-associated nuclear antigen and glycogen synthase kinase 3beta

The latency-associated nuclear antigen (LANA) of Karposi's sarcoma-associated herpesvirus has been reported to interact with glycogen synthase kinase 3beta (GSK-3beta) and regulate its activity, leading to inhibition of GSK-3-dependent beta-catenin degradation. In this study, the interaction between LANA and GSK-3beta was characterized further. LANA was found to interact with GSK-3beta in vitro as well as in intact cells. However, LANA did not regulate GSK-3beta kinase activity and LANA-induced upregulation of beta-catenin was GSK-3beta independent. LANA did not regulate the stability of beta-catenin or of its reported interaction partners p53 and von Hippel-Lindau protein. Additional targets of LANA are likely to mediate its malignancy-promoting function.

Activation of the oxidative stress pathway by HIV-1 Vpr leads to induction of hypoxia-inducible factor 1alpha expression

The detection of biomarkers of oxidative stress in brain tissue and cerebrospinal fluid of patients with human immunodeficiency virus, type 1 (HIV)-associated dementia indicates the involvement of stress pathways in the neuropathogenesis of AIDS. Although the biological importance of oxidative stress on events involved in AIDS neuropathogenesis and the HIV-1 proteins responsible for oxidative stress remain to be elucidated, our results point to the activation of hypoxia-inducible factor 1 (HIF-1) upon HIV-1 infection and its elevation in brain cells of AIDS patients with dementia. HIF-1 is a transcription factor that is responsive to oxygen. Under hypoxic conditions, HIF-1alpha becomes stable and translocates to the nucleus where it dimerizes with aryl hydrocarbon receptor nuclear translocator and modulates gene transcription. Activation of HIF-1 can also be mediated by the HIV-1 accessory protein Vpr. In addition, cellular components, including reactive oxygen species, contribute to the induction of HIF-1alpha. Our results show that Vpr induces reactive oxygen species by increasing H(2)O(2) production, which can contribute to HIF-1alpha accumulation. Interestingly, increased levels of HIF-1alpha stimulated HIV-1 gene transcription through HIF-1 association with HIV-1 long terminal repeat. These observations point to the existence of a positive feedback interplay between HIF-1alpha and Vpr and that, by inducing oxidative stress via activation of HIF-1, Vpr can induce HIV-1 gene expression and dysregulate multiple host cellular pathways.

Epstein-Barr virus-encoded EBNA1 modulates the AP-1 transcription factor pathway in nasopharyngeal carcinoma cells and enhances angiogenesis in vitro

The Epstein-Barr virus (EBV)-encoded EBNA1 protein is expressed in all virus-associated tumours, including nasopharyngeal carcinoma (NPC), where it plays an essential role in EBV genome maintenance, replication and transcription. Previous studies suggest that EBNA1 may have additional effects relevant to oncogenesis, including enhancement of cell survival, raising the possibility that EBNA1 may influence cellular gene expression. We have recently demonstrated by gene expression microarray profiling in an NPC cell model that EBNA1 influences the expression of a range of cellular genes, including those involved in transcription, translation and cell signalling. Here, we report for the first time that EBNA1 enhances activity of the AP-1 transcription factor in NPC cells and demonstrate that this is achieved by EBNA1 binding to the promoters of c-Jun and ATF2, enhancing their expression. In addition, we demonstrate elevated expression of the AP-1 targets interleukin 8, vascular endothelial growth factor (VEGF) and hypoxia-inducible factor-1alpha in response to EBNA1 expression, which enhances microtubule formation in an in vitro angiogenesis assay. Furthermore, we confirm elevation of VEGF and the phosphorylated isoforms of c-Jun and ATF2 in NPC biopsies. These findings implicate EBNA1 in the angiogenic process and suggest that this viral protein might directly contribute to the development and aggressively metastatic nature of NPC.

Angiogenesis, Kaposi's Sarcoma and Kaposi's Sarcoma-Associated Herpesvirus

Tumor angiogenesis is the uncontrolled growth of blood vessels in tumors, serving to supply nutrients and oxygen, and remove metabolic wastes. Kaposi's sarcoma (KS), a multifocal angioproliferative disorder characterized by spindle cell proliferation, neo-angiogenesis, inflammation, and edema, is associated with infection by Kaposi's sarcoma-associated herpesvirus (KSHV). Recent studies indicate that KSHV infection directly promotes angiogenesis and inflammation through an autocrine and paracrine mechanism by inducing pro-angiogenic and pro-inflammatory cytokines. Many of these cytokines are also expressed in KS lesions, implicating a direct role of KSHV in the pathogenesis of this malignancy. Several KSHV genes are involved in KSHV-induced angiogenesis. These studies have provided insights into the pathogenesis of KS, and identified potential therapeutic targets for this malignancy.