RhoA-GTPase Facilitates Entry of Kaposi's Sarcoma-Associated Herpesvirus into Adherent Target Cells in a Src-Dependent Manner

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Abstract
Kaposi's sarcoma-associated herpesvirus (KSHV) (human herpesvirus 8) binds to adherent target cell surface heparan sulfate molecules via its envelope glycoproteins gB and gpK8.1A, to integrins via gB, to the transporter CD98/xCT complex, and possibly to another molecule(s). This is followed by virus entry overlapping with the induction of preexisting host cell signal pathways, such as focal adhesion kinase, Src, phosphatidylinositol 3-kinase (PI3-K), Rho-GTPases, protein kinase C-zeta, and extracellular signal-regulated kinase 1/2. Here, using hemagglutinin-tagged plasmids expressing wild-type, dominant-positive, and dominant-negative forms of RhoA in HEK (human embryonic kidney) 293 cells, we investigated the role of RhoA-GTPase in virus entry. The dominant-negative form of RhoA GTPase and treatment of target cells with Clostridium difficile toxin B (CdTxB), a specific inactivator of Rho-GTPases, significantly blocked KSHV entry. KSHV infection induced closely similar levels of FAK and PI3-K in all three cell types. In contrast, very strong Src activation was observed in KSHV-infected dominant-positive RhoA cells compared to wild-type cells, and only moderate Src activation was seen in dominant-negative cells. Inhibition of Src activation by CdTxB and reduction of RhoA activation by Src inhibitors suggest that KSHV-induced Src is involved in RhoA activation, which in turn is involved in a feedback-sustained activation of Src. Since the decreased entry in RhoA dominant-negative cells may be due to inefficient signaling downstream of RhoA, we examined the induction of RhoA-activated Dia-2, which is also known to induce Src. Dia-2 coimmunoprecipitated with activated Src, which was inhibited by Src inhibitors, in the infected cells. Together with the reduced virus entry in RhoA dominant-negative cells, these results suggest that activated RhoA-dependent Dia-2 probably functions as a link between RhoA and Src in KSHV-infected cells, mediating the sustained Src activation, and that KSHV-induced Src and RhoA play roles in facilitating entry into adherent target cells.