Kaposi's sarcoma-associated herpesvirus (KSHV) causes Kaposi's sarcoma, primary effusion lymphoma and multicentric Castleman's disease. KSHV infection of cells produces both latent and lytic cycles of infection. In vivo, the virus is found predominantly in the latent state. In vitro, a lytic infection can be induced in KSHV-infected cells by treating with phorbol ester (TPA). However, the exact signalling events that lead to the reactivation of KSHV lytic infection are still elusive. Here, a role is demonstrated for B-Raf/MEK/ERK signalling in TPA-induced reactivation of KSHV latent infection. Inhibiting MEK/ERK signalling by using MEK-specific inhibitors decreased expression of the TPA-induced KSHV lytic-cycle gene ORF8. Transfection of BCBL-1 cells with B-Raf small interfering RNA inhibited TPA-induced KSHV lytic infection significantly. Additionally, overexpression of MEK1 induced a lytic cycle of KSHV infection in BCBL-1 cells. The significance of these findings in understanding the biology of KSHV-associated pathogenesis is discussed.

Human herpesvirus-8 (HHV-8/KSHV) is etiologically associated with Kaposi's sarcoma (KS) and other tumors. Constitutive activation of the mitogen-activated protein kinase (MAPK) signaling pathway has been associated with a variety of tumors, including AIDS-related KS. The oncoprotein Raf is situated at a pivotal position in regulating the MAPK pathway. Hence, we analysed the effect of oncoprotein Raf on HHV-8 infectious entry into target cells. Here we report Raf expression to significantly enhance HHV-8 infection of target cells. These findings implicate a role for Raf not only in the infectious entry of HHV-8 but also in modulating KS pathogenesis.

IntroductionKaposi sarcoma-associated herpesvirus/human herpesvirus 8 (KSHV/HHV-8) is the latest addition to the list of human herpesviruses. KSHV was first isolated from Kaposi sarcoma (KS) lesions in persons suffering from acquired immunodeficiency syndrome (AIDS) in 1994. 1 The cancerous conditions that are etiologically associated with KSHV are KS, primary effusion lymphoma (PEL), and multicentric Castleman disease (MCD). 2 Apart from the inflammatory cytokines (ICs)/growth factors (GFs), lytic KSHV infection plays an instrumental role in the progression of KS lesions. 3 The lytic cycle of infection is also critical for the spread of KSHV to different organs. Successful KSHV infection is characterized by both virus entry and the ability of the virus to establish latency.In our earlier study, we provided evidence to show that the overexpression of Raf specifically enhanced KSHV infection of target cells at the level of entry. 4 Regulation of Raf is crucial for the proper maintenance of cell growth, proliferation, apoptosis, and differentiation. 5 Of the 3 isoforms, B-Raf has gained a lot of focus over the last couple of years due to the detection of B-Raf somatic missense mutations in malignant melanomas (66%), colon cancers (15%), and at lower frequencies in a wide variety of human cancers. [6][7][8] The ability of Raf to regulate vascular endothelial growth factor-A (VEGF-A) has also been demonstrated. 4 In separate studies, we found VEGF-A to enhance KSHV infection of fibroblasts and epithelial cells. 9,10 Interestingly, constitutive activation of the components (Ras/Raf) of the mitogen-activated protein kinase (MAPK) pathway of signaling as well as VEGF expression has been a common feature with KSHV pathogenesis. 11,12 Hence, in this study we attempted to analyze the relationship between the expression of B-Raf and VEGF-A in KSHV-infected hematopoietic cells. We provide evidences for the existence of a Raf-dependent VEGF-A expression in KSHV-infected hematopoietic cells. Materials and methods Cell cultureHuman foreskin fibroblasts (HFFs; Clonetics, Walkersville, MD), BCBL-1, BC-1 (American Type Culture Collection [ATCC], Manassas, VA; CRL-2230), BC-2 (ATCC CRL-223), BCP-1 (ATCC CRL-2294), and BJAB cells were used in this study. Target cells were grown in either phenol red-free Dulbecco modified Eagle medium (DMEM) or RPMI medium (Invitrogen, Carlsbad, CA) containing 10% charcoal-stripped fetal bovine serum (FBS; Atlanta Biologicals, Lawrenceville, GA), L-glutamine, and antibiotics. 13,14 Dermal microvascular endothelial cells (HMVEC-Ds; CC-2543; Clonetics) were propagated in EGM MV-microvascular endothelial cell medium (Clonetics) as per standard protocols. 13 The passage numbers for HFFs and HMVEC-Ds used in this study ranged between 6 and 10, and 5 and 9, respectively. HFF/pBabePuro3, HFF/⌬B-Raf [DD] :ER, and HFF/⌬B-Raf [FF] : ER cells were cultured as per standard protocols. 4,10 ␤-Estradiol (1 M) stimulation of these cells results in the activation of ⌬B-Raf:ER fusion proteins. 4Inhibitors PD98059 w...

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Human herpesvirus-8 (HHV-8/KSHV) is etiologically associated with Kaposi's sarcoma (KS) and other tumors. The Raf oncoprotein enhances HHV-8 infection of target cells. In addition, we have previously demonstrated that Raf induces vascular endothelial growth factor (VEGF) expression. VEGF is a growth factor that has autocrine growth activity and has been implicated in the formation of the spindle shape cell morphology characteristic of Kaposi's sarcoma (KS). The aim of this study was to test the hypothesis that VEGF enhances infection of HHV-8. Herein, we demonstrate that the soluble VEGF enhanced green florescence protein encoding (GFP)-HHV-8 (rKSHV.152) infection of human foreskin fibroblasts (HFF) and not of 293 cells. We found this to be in part, due to the fact that HFF inherently produces significantly lower concentrations of VEGF when compared to 293 cells. Treating 293 cells (but not HFF) with a VEGF receptor (VEGFR) inhibitor significantly lowered infection. Furthermore, transfecting 293 cells with VEGF specific si-RNA did not alter the binding of HHV-8 to cells; but significantly lowered VEGF expression and thus GFP-HHV-8 infection. Interestingly, lowering VEGF expression in 293 cells wtih VEGF specific si-RNA did not completely inhibit GFP-HHV-8 infection. We conclude that VEGF is not a requirement for HHV-8 infection; but VEGF plays a major role in augmenting infection at a post binding stage of entry. These findings suggest that targeting VEGF/VEGFR may prove efficacious in controlling HHV-8 associated pathogenesis.

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SummaryKaposi's sarcoma-associated herpesvirus (KSHV) has two modes of replications: latent and lytic replications. Reactivation from latency is dictated, in part, by the cell cycle. Herein, we have attempted to delineate the importance of cell cycle in KSHV pathogenesis by exploring the expression pattern of cellsurface receptors during different phases of the cell cycle. aV integrin expression is augmented during S phase in fibroblasts, epithelial and KSHV-infected cells. Using a Matrigel system, we pioneer the concept that KSHV-infected primary effusion lymphoma cells can attach to extracellular matrix proteins. This attachment is mediated primarily via aV integrins or virally encoded gB, and occurs preferentially in cells from S phase or cells from S phase actively supporting a lytic infection respectively. Such an ability of infected B cells to attach to endothelial cells may also aid in the dissemination of infection. The keystone of this work is that for the first time, we describe the ability of KSHV-infected B cells to preferentially use cellular (aV) or viral (gB) receptors to specifically bind cells, depending upon the stage of the cell cycle and infection.