Historical Background Of Kshv Biology Essay

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Kaposis sarcoma, the most common cancer in HIV-infected person, was primarily discovered by a Hungarian dermatologist Moritz Kaposi in 1872. He describing the skin lesion in five men in his publication entitle Idiopathisches multiples Pigmentsarkom der Haut (idiopathic multiple pigmented sarcoma of the skin). After two decades another premium dermatologist, Kobner, designated the disease as Kaposi's sarcoma which is now commonly known as classic KS (cited in Antman and Chang, 2000). Over decades of study on the etiology and pathogenesis of Kaposi's sarcoma (KS) has now enlightened us the tumorigenesis of KS, although it is still in the midst of journey today. The identification of causative agents was not intensively study until early 90s when KS was dramatically increased in AIDS patients. This sudden increase in prevalence strongly suggested an involvement of infectious agents in KS development. This agent was first described in 1994 by Yuan Chang and Patrick Moore at Columbia University, who used representational difference analysis or RDA (PCR based techniques) to characterize the DNA sequence of KS biopsies and found that novel Human γ herpes virus DNA sequences were constantly present in the KS lesions. This new virus become known as Kaposi's sarcoma-associated herpesvirus (KSHV) or Herpesvirus 8 that is associated with several human cancers including Kaposi's sarcoma (Chang et al, 1994), multicentric Castleman's diseases (Soulier et al, 1995) And primary effusion lymphoma (PEL) (Cesarman et al, 1995). Successful sequencing of KSHV genome (Russo et al, 1996) has further sophisticated the knowledge of functions of viral encoded proteins, mechanisms of oncogenesis in molecular, clinical research and potential targets for therapeutic intervention. KSHV, composed with 165Kb DNA sequences which encode up to 90 viral products (Russo et al, 1996), undergoes two distinguishable viral life cycle; the latent and lytic infection. During infection by KSHV, viral infection being latently well established before progression to full-blown disease (Martin et al, 1998) in endothelial lineage-derived spindle cells (Boshoff et al, 1995). Although it is predominantly in latent cycle, several evidences support that lytic viral infection is also important in Kaposi's sarcoma development. First, the incidence of KS is ten times higher in KSHV seropositive patients who have PBMC (peripheral blood mononuclear cell) viremia than those without virus in the peripheral blood (Engels et al, 2003). Second, treatment of KS risked AIDS patients with ganciclovir, a nucleoside analogue that competitively inhibits herpesviral DNA polymerase proteins and slows the elongation of DNA chains and blocks lytic replication, strikingly reduces the incidence of KS development (Martin et al, 1999). Last, the viral load in peripheral blood mononuclear cells is directly proportionate with disease progression to clinical KS (Ambroziak et al, 1995). These findings hypothesize that switch from latency to lytic activation and subsequence replications are important events in KS pathogenesis. This would likely be part of the underlying mechanism in association between HIV infection and KS.

Epidemiology of Kaposi's sarcoma

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Kaposi's sarcoma can be generally grouped into 4 epidemiological forms. Classic KS are characterized by unusual multifocal neoplasm with dark purple lesions and predominant cell being the spindle cell and this is predominantly affecting the elderly men of Mediterranean, Eastern European and Jewish origins. Prior to the HIV/AIDS era, Kaposi's sarcoma was mainly seen in Sub-Saharan Africans, southern Italy or Mediterranean regions and in immunosuppressed HIV (+) patients or transplant recipients (Ahmed et al, 2001), but rarely (less than 5%) seen in Western countries (Gao et al, 1996). This raises the hypothesis that development of Kaposi's sarcoma is highly associated with some geographical distribution. However, since the first identification of disseminated form of skin lesions in young homosexual with AIDS in 1981 it is classified as AIDS-related epidemic KS (AKS) (Friedman-Kein et al, 1981). Endemic forms (EKS) was first identified in 1950s and it is more aggressive than classic forms. This is highly prevalence in equatorial zones of Africa and mainly affected to young men and children. In fact, AIDS epidemic has played an important role in the prevalence of KS in Africa; i.e. 10% of all cancer in this area (Schwartz et al, 2008). The last type termed iatrogenic KS occurred mainly among immunosuppressive and organ transplant recipients although is not as high as risks seen in HIV infection (Regamey et al, 1998). This suggests that damage in immune system would be predisposing factors for KSHV infection and subsequent KS development. The observation of the geographic variation in incidence of KS suggests that there may be association with host genetic variation. Several studies have claimed the potential contribution of genetic polymorphisms of immune response genes in different types of KS even though it is slightly low significant in overall risks. Diplotypes of interleukin-8 receptor-β (IL8RB), IL-13 (Brown et al, 2006) and certain human leukocyte antigen (HLA) haplotypes (Dorak et al, 2005) is suggested to associate with classic KS. Iatrogenic KS is associated with IL6 promoter polymorphism (Gazouli et al, 2004).

Molecular biology of KSHV

3.1 KSHV genomic organization and structure

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Like others rhadinoviruses, KSHV genomic organization consists of two peripheral sequences rich in GC DNA (H DNA) flanking a central segment of low-GC DNA (L DNA) (Russo et al, 1996) which contains long unique region of open reading frames (ORFs) encoding more than 87 genes (Moore et al. 1996b). KSHV genome exists in a linear, double-stranded form in the viral capsid, but following infection, like other herpesviruses, rapidly circularize and become as a circular double-stranded episome within the host nucleus. KSHV genome consist short, scattered region of specific genes subfamily which separating the four major highly conserved gene blocks. Conserved ORFs such as (ORF25 for the major capsid protein, ORF9 for the DNA polymerase) usually encode either structural proteins or genes that needed for viral replication and gene expression regulation. The KSHV genome with any unique ORF non-homologous to Herpesvirus or other rhadinoviruses were prefix with a "K" in their names such as those genes encoding viral interferon regulatory factors (IRFs) (Gao et al, 1997). Study in PEL cell lines for KSHV viral transcription have classified genes transcripts into three classes based on responsiveness of 12-O-tetradecanoyl phorbol-13-acetate (TPA) treatment (Sarid et al, 1998); class I transcripts which are constitutively express without upregulated by chemical treatment (LANA, vCyc, vFLIP), class II transcripts are those with basal expression and also inducible by chemicals (vIRFs, vIL6, vMIP II, bZIP, ORF57 and Kaposin), and class III transcripts which are only expressed with chemical treatment. By using viral life cycle expression kinetics profiling in PELs, KSHV lytic genes can be further classified into three categories (Sun et al, 1999); immediate-early (IE) genes, early genes, and late genes (Fig.1). Five latency genes have been identified in latent state, includes kaposin, v-FLIP, v-cyclin, LANA (latency-associated nuclear antigens), and vIRF-2 or LANA-2 (Burysek and Pitha, 2001; Saveliev et al, 2002). The IE genes are transcribed earliest after viral infection or reactivation from latent to lytic replication which encode proteins for regulation of genes expression during infection and reactivation. Synthesis of IE genes do not required de nova protein synthesis and also not sensitive to the protein synthesis inhibitor cycloheximide. The most important IE genes in carcinogenesis are ORF50 which encode the expression of replication and transcription activator (RTA), the major mediator of lytic replication. Delay early or early lytic genes include those expressed after IE genes, encoding viral proteins required for DNA replication. Although their replication is independent on the viral DNA synthesis, they are controlled by protein from IE gene expression (i.e. RTA). Late lytic genes are those encoding structural proteins, such as capsid proteins, packaging proteins that are needed for assembly of viral particles (virions) and they can be inhibited by cycloheximide or DNA synthesis inhibitors (Saveliev, Zhu and Yuan 2002).

3.2 KSHV life cycle

3.2.1 Latent life cycle

KSHV can execute two distinct life cycles, known as long persistent latent infection and temporary lytic growth. Latent gene expression enables the herpesvirus to immune escape and has an important role in maintaining viral genomic structure as well as in establishing the infected cells (Boshoff et al, 1995). After viral infection, KSHV initiate abortive lytic replication i.e. it expresses some viral proteins to counteract the virus from host cell immune defence system and induce pre-existing host transcription mechanisms (apoptosis, inflammation, angiogenesis) (Greene et al., 2007). in order to establish long latency period in primary reservoir CD19+ B-cells in lymphoid system (Antman and Chang, 2000; Naranatt et al, 2004) (Figure 3). Endothelial, epithelial cells, foreskin fibroblast cells, keratinocytes and monocytes are also other favours for viral infection. During this period, viral gene expression is strongly restricted, and expression of these latent genes is quite distinct within gammaherpesviruses. In PEL-derived cell lines only handful amount of genes such as the viral FLICE inhibitory protein (vFLIP), viral cyclin (v-cyclin), LANA, kaposin and vIRF-3/LANA-2 are being expressed. These molecules are critical for persistence of virus in the host cells and KS tumorigenesis. Although there is different in the expression profiles between lytic genes and latent genes, some protein such as Kaposin are able to transcribe in both phases of life cycle (Jenner et al, 2001; Nakamura et al, 2003). One of the major predominant latent proteins, which are consistently shown to be highly expressed in all types of KSHV associated malignancies, is LANA (Dupin et al, 1999; Cotter et al, 1999). KSHV LANA, 222-234KDa nuclear phosphoprotein encoded by ORF73, is essential for maintaining of genome integrity and persistency. It maintains the viral episome by binding to the KSHV terminal-repeat (TR) regions and tethers the viral episome to the host chromosome (Grundhoff et al, 2003). LANA contains a central repeat region, which has been shown to inhibit the tumour-suppressor functions of the Rb and p53 Proteins (Radkov et al, 2000). Furthermore, LANAs can upregulate expression of β-catenin by sequestering on glycogen synthase kinase 3β (GSK-3β), an inhibitor of β-catenin (Fujimori et al, 2003). Lan et al, (2003) has also demonstrated that LANA has transcription regulatory proteins by its role in inhibition of ORF 50-mediated lytic replication which suggest that balance between latency and lytic are highly depend on the interaction between LANA and Rta.

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Similarly, KSHV vFLIP has been shown to produce antiapoptotic effects and lead to morphologic changes that give characteristic spindle-cell in KS and have differential role in RTA-induced lytic reactivation (Brown et al, 2003) by inducing the expression of the antiapoptotic transcription factor NF-κB (Matta et al, 2003) (Figure 2). Although latency has been established with important role in KS pathogenesis, it is however observed that latency in KSHV is not immortalizing as in EBV (Grundhoff and Ganem, 2004). Thus, it raises the question of the role of lytic replication in viral pathogenesis. Role of lytic replication can be practically achieved by the ability of lytic inducible of latently infected PEL cells by certain chemicals (such as: phorbol esters TPA or HDAC inhibitors) (Arvanitakis et al, 1997; Yu et al, 1999). These results, in some aspect, suggested that lytic replications are regulated by nucleosome organization.

3.2.2 Lytic life cycle

There are several evidences shown that dissemination of viral load by lytic replication is critical for progression of KS (Engels et al, 2003; Keller et al, 2001; Moore et al, 1996c). The program of gene expression during lytic replication was revealed by DNA array expression profiling of PEL cell lines (Murphy, 2001). After the induction of lytic replication, groups of regulators gene expression including the immediate early transactivators ORF50 (RTA), K8 (Zta or K-bZIP) and ORF57 (post-transcriptional regulator of gene expression) has expressed. This is followed by the expression of sets of genes, which include homologs of human cyclin D (v-CYC) (Boshoff et al, 1997), G protein-coupled receptor (v-GPCR) (Swanton et al, 1997), chemokine homologs (v-MIPs) (McGeoch et al, 1999), homolog of IL-6, protein with similarity to interferon (IFN) regulatory factor (v-IRF) (Moore et al, 1996a) and a bcl-2 homolog (Cheng et al, 1997). These genes provide supporting functions to the virus for survival and replication by modulating the normal cellular pathways. The structural genes and those involved in virus expression and maturation are expressed later, generally after 24 h post-infection (Jenner et al, 2001). Studies have demonstrated that an ORF50 of KSHV functions as an immediate early transactivator of KSHV lytic replication with its capability of stimulating KSHV lytic replication in vitro similar to the Zta and RTA proteins of EBV (Renne et al, 1996). The molecular events and genes that control latent to lytic switching have been studied best in EBV-infected B lymphoma cells. Although latent infection has essential role in sustained viral infection and tumorigenesis, lytic reactivation has been implicated to be important for KS development (Martin et al, 1999).

3.3 Reactivation of KSHV

The majority of KSHV infection allows the host cell to grow indefinitely by establishing in latent infection and it is remarkably stable in most circumstances, whereas only a tiny fraction of cells will switch to the lytic mode (reactivates). Reactivation of KSHV latency can be achieved in PEL cell cultures by induction with several chemical substances such as phorbol esters, and sodium butyrate. This was approved from northern blot analysis on TPA induced PEL cells which showed that the ORF50 transcript (the earliest lytic gene) can be detected within few hour after the experiment (Lukac et al, 1999). In addition to chemical induction, physiologically, it can be induced by inflammatory factors and environmental factors. In 2001, Davis et al, reported that PEL cell lines treated with hypoxia induced HIF which binds to promoter region of RTA. This is therefore not surprising that KS lesions have primarily dominant in lower extremities (Hengge et al, 2002). Extensive studies between inflammatory and mitogenic induced cell cycle progression, particularly KS development has also given a remarkable understanding about its role in lytic reactivation. There is a hypothesis that lytic reactivation have inter-relationship with latency in KSHV-KS paracrine oncogenesis which is believed to play an important role in KS development. These paracrine acting manner of cellular or viral cytokines, growth factors and chemokines are largely supplied by lytic replication. Several the cytokines (interferon γ, vIL6) and viral encoded chemokines such as vCCL-1/vMIP-I (ORF K6), vCCL-2/vMIP-II (ORF K4), vCCL-3/vMIP-III (ORF K4.1) and vIRFs (vIRF1-4) are replicated during transient lytic reactivation and drive the latently infected cell to mitogenesis, proliferation, angiogenesis, immune escape, and inflammation which further recruit neighbouring uninfected cells and form KS tumour ultimately. Although latent proteins possess oncogenic properties, latency alone does not appear to sufficient for immortalizing, suggesting that lytic reactivation also has a role in KS pathogenesis. There are several studies that the lytic cycle is likely to aid tumorigenesis. First, virions and lytic viral proteins (vGPCR, vMIPs) are increased in a minority of cells within KS lesions (Staskus et al, 1997; Neipel, Albrecht and Fleckenstein, 1997). Second, immunosuppression increases KSHV lytic re-activation. Third, clinical studies showed that inhibiting lytic replication by immune reconstitution or by anti-lytic herpes antivirals, such as ganciclovir successfully reduced the occurrence of new KS tumors in patients with advanced AIDS, signifying that lytic replication has a critical role maintaining of KS tumorigenesis (Martin et al, 1998). Fourth, lytic infection play a role to support viral episomal maintenance by the recruitment of new host cells to latency to replace those that have segregated from their viral episome (Grundhoff et al, 2004). Last, co-injection of vGPCR is necessary to induce tumour formation by cells with only latent genes expression (Montaner et al, 2006). Finally, lytic replication provides the auxiliary function for latently infected cells unless the non-persistence KSHV episome would reduce the viral population during cell division (Pauk et al, 2000). Moreover, lytic gene such as growth modulators and immune evasion genes, expression also play vital roles in KS disease pathogenesis (Parkin et al, 2006).

3.3.1 General structure of RTA protein

The lytic reactivation in HHV-8 can be initiated by the HHV-8 ORF50 gene, a homologue of EBV RTA (Sun et al, 1998). The HHV-8 RTA is one of the earliest IE genes expressed during viral reactivation (Sun et al, 1999). Study of the ORF50 mRNA has shown that ORF50 gene encodes several transcripts with one major 3.6kb sense transcripts in which K8 are encoded and three minor antisense transcripts. The first major ORF50 transcripts encode for a protein of 691aa, with molecular mass of 73.7 kDa. However, analysis by Lukac et al, 1999 on Western blotting appears to give 110KDa, suggesting that there could be post-translational modifications such as phosphorylation in RTA expression. The study further proposed that this modification could be due to its ST (serine/threonine) rich C-terminal domain. The remaining three transcripts from similar study observed that they do not encode any viral proteins and thus their functions are still unclear. Similar result applied in the work of Saveliev et al, 2002. The basic RTA protein consists of an N-terminal 272 amino acids sequenced DNA binding domain that bind to promoter of KSHV DNA (Lukac et al, 2001), two nuclear localization signals (NLS), a central dimerization domain which is rich in Leucine and Threonine, and a C-terminal acidic activation domain (aa 531-691) which is potent transactivator (Figure 3). Deletion of the activation domain sequences, which containing part the DNA binding domain, has been shown to maintain DNA-binding activity for RTA responsive promoters but no longer express lytic genes (transdominant-negative mutant) (Lukac et al, 1999). By generating of this mutant, Lukac et al, (1999) was able to prove that Rta is necessary for lytic reactivation, by suppression viral reactivation stimulated by TPA, n-butyrate.

3.3.2 Role of RTA in lytic reactivation

Studied in PEL cells for many viral proteins in their ability to reactivate KSHV has done , however, analysis with Northern and Western blotting, and immunofluorescence have shown that only Rta/Orf50 expression was necessary to complete productive viral lytic replication (Lukac et al, 1998; Sun et al, 1998; Gradoville et al, 2000; Xu et al, 2005). This has been further proved by Lukac et al, (1999), with expression of domain-negative mutant in PELs that RTA is necessary for lytic reactivation. Moreover, expression of RTA is independent on the de novo protein synthesis and it is sufficient for ectopic lytic reactivation in PEL cell line (Lukac et al, 1998, 1999). Taken together of these characteristics suggest that HHV-8 RTA is a major, sufficient and essential regulator for lytic reactivation and various reactivation signals all activated through the ORF50/Rta to successful viral replication. This essential role of RTA in the lytic reactivation is well conserved within gammaherpesvirus including MHV-68, and EBVs (Goodwind et al, 2001). RTA can activate cascades of expression of its target genes by two mechanisms. The simplest way is by directly bound to RRE (RTA-responsive element) of target genes, which is stated in the work of Chang et al, 2002 in study of interaction between Rta and viral PAN and K12 expression. The second way is by indirectly bound to cellular factors. (Chen et al, 2009). In fact, RTA has shown to activate several downstream lytic expression genes such as ORF57, polyadenylated nuclear (PAN) RNA, ORF71/72, vIRF1, K1, and itself by binding on the RTA binding sites of these genes. As initiation of lytic cycle in KSHV requires binding of Origin-binding protein (Rta) to ori-Lyt DR-L and DR-R (Fig: 1). Recently, with an in vivo ChIP-on-chip approach, Ziegelbauer et al, 2006; Chen et al, 2009) has identified more genes that directly activated by RTA: ORF4.1, ORFK5, ORF16, ORF29, ORF45, ORF50, ORF59, miRNA cluster, ORF74.

3.3.3 Role of RTA proteins in association with lytic gene expression

It has been demonstrated that the ORF50/Rta cooperates with KSHV ORF57/Mta protein to reactivate the virus from latency in PEL models of infection (Diana et al, 2007). Similar study also confirmed that ectopic expression of RTA alone was sufficient to reactivate the virus, but ORF57/Mta was less likely to induce KSHV reactivation in the absence of RTA expression (Malik et al, 2004; Diana et al, 2007). The HHV-8 ORF57 is an early gene which is expressed in early phase of lytic replication after reactivation from latency and essential for both early and late lytic gene expression (Duan et al., 2001). In fact, the major function of ORF57 is regulating mRNA processing and facilitating the transport of the mRNAs to the cytoplasm. Duan et al, (2001) have also shown that the expression of ORF57 is activated by the addition of TPA and is upregulated by RTA expression. Lukac et al, in 2001 proposed that expression of ORF57 required collaboration of active domain of RTA to specific 16 bps sequence in promoter regions of ORF57, and deletion of which will ablated RTA responsiveness to lytic replication in baculovirus Sf9 cells study. It is interestingly found that suppression of Rta-mediated lytic replication can be achieved by several repressors (IRF-7, K-RBP) by competitive binding to lytic genes promoter (ORF57). Yang and Wood, 2007 also proved that knocking down of K-RBP expression in latently infected cells will lead to increase KSHV-RTA-mediated lytic genes expression (figure 4B).

RTA also has interaction with another IE protein called KbZIP/RAP (replication association protein). KSHV KbZIP is a homologue of EBV-ZTA protein, but functionally different from it by which KSHV KbZIP are insufficient to activate lytic cycle (Lukac et al, 1999; Polson et al, 2001). In fact, Izumiya et al, (2003) has shown that K8/KZIP has inhibitory role (as trans-repressor) in TPA induced KSHV lytic replication in BCBL-1 cell lines. In contrast, one of the studies in 2003 by Wu et al has stated that KbZIP has actively promoted the cell cycle arrest by activating p21 CDKI. Similar study done by Izumiya et al, (2003b) has also shown that binding of KbZIP to cyclin A/E-cdk2 and subsequent phosphorylation of cdks can slow down the cell cycle (G1 progression) which favour for lytic replication. Although it has repressive role in Rta-mediated lytic reactivation in some genes such as ORF-57, this inhibitory function still require interaction with RTA. Moreover, according to Wang et al, (2006) study, K8/KbZIP also act as Origin-binding proteins (OBP) and associate with RTA in forming replication complex to unwind latent viral episome for lytic replication.

3.3.4 Role of cellular and viral factors in regulation of RTA-mediated lytic

reactivation

It has been found that Rta alone is not necessary for initiation the replication and several viral replication factors are suggested to be involved in formation of replication complex. Thus, it is suggested that RTA utilizes more than one mechanism to downregulate target proteins. RTA protein composed of several transcription factor binding sites; KRBP, RBP-Jκ, C/EBP-α, CBP, HDAC, Oct-1 (Fig: 3). Despite the fact that detailed mechanism is unclear, several evidences proposed that these cellular factors have been found to play important roles in RTA-mediated transactivation (table 2). For instant, study suggested that the expression of the KSHV ORF50 protein augment the expression of the potential regulatory proteins ORF57 and K-bZIP with the interaction of sequence-specific DNA binding protein RBP-Jκ, a major protein of the Notch signaling pathway (Liang and Ganem, 2003; Persson and Wilson, 2010) (Fig: 4A). To this end, reactivation of lytic replication is established by the RTA transactivation, however, it is not accomplished unless cooperatively interact with several cellular transcription factors.

It is the common feature of KSHV to establish latency in their infected host cells which suggests that RTA expression is regulated in one or another way. Several numbers of cellular (HDAC, IRF-7) and viral factors (CBP, K-RBP, LANA, KbZIP proteins, vIL6, kaposin and IRF7) are likely to involve in RTA downregulation (West and Wood, 2003). One of the major cellular repressor for RTA is K-RBP which has shown to negatively regulate the RTA-mediated KSHV reactivation (Zhilong and Charles, 2007). It is also shown that K-BRP mediated RTA-regulation is dose dependent in nature (Wang et al, 2001). Zhilong and Charles (2007) also denoted that DNA-binding zinc finger domain of K-RBP has important role in suppression of RTA-mediated ORF57 expression. Same study also present that reactivation of RTA require ability of degradation of repressor and balance between repression and reactivation by degradation of repression by RTA control the virulence of KS pathogenesis. It is said that RTA processes E3 ubiquitin ligase activity and has pivotal role in IRF-7 ubiquitination and degradation (Yu et al, 2005) and also in K-RBP ubiquitination and degradation (Zhilong et al, 2008) (Fig: 4C). Degradation of RTA to IRF-7 has supressed IRF-7-mediated IFN-β activation and similarly RTA can downregulate NF-κB activation; both are known to involve in regulation of type I interferon production, thus evade innate immune response in viral infection (Yu, Wang and Hayward, 2005; Seth, Sun and Chen, 2006). Interestingly, Yang, Yan and Wood (2008) have found that RTA has positive regulation on NF-κB in late stage of lytic replication and it may suggest that RTA is not solely mediator in suppressor degradation. In addition to K-RBP, studies also indicated that RTA can be downmodulated several cellular factors during lytic replication, such as KbZIP (Izumiya, et al, 2003a), LANA (Lan et al, 2004), by direct degradation (Yang, Yan and Wood, 2008).

Regulation of RTA expression

Through the compelling evidences on the study of RTA concluded that there are a number of factors can trigger the activation of RTA to exaggerate the lytic reactivation. During the latency, expression of RTA is highly supressed (Katano et al., 2001a). Evidence suggests that KSHV latency can achieve by methylation of RTA promoter at CEBP/α sites (Chen et al, 2001). Highly methylated RTA promoter region promote the activity for histone deacetylation by HDAC which further supress the RTA transcription (Lu et al., 2003; Shamay et al., 2006). Thus butyrate, (HDAC inhibitor) and TPA (CBP/p300 inducer) which have HDAC inhibitory action can reactivate the viral cells in PEL cell line experiments. Recently, Gould et al, 2009 has proposed that RTA is repressed by Hey1 during latency in association with other cellular factors and this repressive activity can be overcome by proteosomal degradation induced by RTA itself during late lytic reactivation. Thus, RTA fine-tunes the switch between latency and lytic reactivation by equilibrating between it and its regulators, cellular/viral factors and environmental factors and this could be, in the future, a target area for therapeutic intervention.

Therapeutic approaches and future perspective

Treatment of KS is currently categorised as sarcoma rather than viral disease. With strong evidence of the association of KS with AIDS and immunosuppression, HAART has been well documented for both reducing the incidence of AIDS-KS, and regression of existing tumours (Sgadari et al, 2002). Mechanisms based on the ability to decreasing circulating HIV associated pro-inflammatory cytokines and pro-angiogenic factors (Bourboulia et al, 2004). This gives the evidence that immune system play an important role in KS pathogenesis and it may potentially prove to be useful for therapeutic and preventive purposes. Thus it can be assumed that establishing design vaccines for preventing KSHV infection would also be possible therapeutic approach for KS treatment. However, the anti-retro therapy itself has some pitfalls such as it took several months for patients to restore immune systems and potential increase resistance of patients to drugs. In fact, KS still remains a problem with high recurrence risk after HAART in Western world. In addition to anti-retro therapy, radiation, cryotherapy, surgery for localized lesions, and systemic conventional chemotherapy either single or combinations, for disseminated diseases has been approved with good result outcomes. The findings of many inflammatory factors, angiogenesis factors and antiapoptotic factors that play critical role in contribution of KS have led to the use of drugs which have been shown to block these pathways such as thalidomide (Little et al, 2000), IM862 (Tulpule et al, 2000), and systemic retinoid (Aboulafia et al, 2003). Knowledge of KSHV molecular mechanism has now enlightened the possibility of using targeted therapy for treatment of KS. Several drugs such as ganciclovir, foscarnet has shown to be well established in clinical trial of viral therapy, and recently Casper et al, (2008) has also shown that valganciclovir has magnificent effect in inhibition of KSHV replication. However these anti-herpesviral medications, has not yet been shown to succeed in cancer therapy. The reason is still unclear but it is likely due to toxicity in prolonged treatment and variable of drug activity in each form of KS-associated disease. Interferon-α has now approved in treatment of KS by inhibits KSHV replication which specifically target to the antigens expressed by latently infected cells (Godfrey et al, 2005). Another clinical trial with rapamycin on renal-transplant patients done by Stallone et al, 2005, has also shown effectively suppress the KS lesions. The mechanism is thought to be mediated from mTOR pathway, although its role in KS biology is still not fully understood. Together with the clinical trials and animal model studies, it is pointing to a potentially fruitful line of new research on molecular and cellular biology of KS and designing a high efficient therapeutic management for KS.

Conclusion remarks

Although it is still controversial that KSHV alone is sufficient for KS lesions, the strong correlation between this virus with all forms of KS including PEL and MCD suggests that KSHV is hallmark of viral tumorigenesis. Over million years of successful establishment of herpesviruses in mammalian host cells acknowledge that latent infection is essential for viral survival. However, it is unlikely for the virus to be virulence unless it allowed contribution of lytic replication and transmission of viruses. Moreover, latency programs of KSHV are not immortalizing at all and thus more studies focus on the role of lytic replication which is to spread the virus from primary reservoir to sites of disease progression. The actual mechanisms that HHV-8 RTA stimulates viral expression are still not clear, however, compelling evidences of studies shown that RTA can stimulate transcription by either directly binding to target gene sequences or indirectly interacting with cellular or viral transcriptional factors or co-factors. Thus lytic infection may contribute viral pathogenesis by replacing dying cells and Cells with lytic infection may thus not only serve as sources of infectious virus but also for viral pathogenesis. In clinical point of view, these pathogenic mechanisms of viral initiation, episomal maintenance, inflammatory infiltration or tumour aggressiveness are all characteristic the KS lesions. For this reason, studies are now focusing on the elucidating of ORF50 and its molecular mechanism of action and expecting to provide optimal target-orientated therapeutic strategies with less toxicity and better efficacy in near future.