Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 envelope glycoprotein gB induces the integrin-dependent focal adhesion kinase-Src-phosphatidylinositol 3-kinase-Rho GTPase signal pathways and cytoskeletal rearrangements

Human herpesvirus 8 (HHV-8; Kaposi's sarcoma-associated herpesvirus) envelope glycoprotein gB possesses an RGD motif, interacts with 001 integrin, and uses it as one of the entry receptors. HHV-8 induces the integrin-dependent focal adhesion kinase (FAK), a critical step in the outside-in signaling pathways necessary for the subsequent phosphorylation of other cellular kinases, cytoskeletal rearrangements, and other functions. As an initial step toward deciphering the role of HHV-8 gB-integrin interaction in infection, signal pathways induced by gB were examined. A truncated form of gB without the transmembrane and carboxyl domains (gBDeltaTM), a gBDeltaTM mutant form (gBDeltaTM-RGA) with an RGD-to-RGA mutation, and inhibitors of cellular kinases were used. HHV-8 gBDeltaTM, but not gBDeltaTM-RGA, induced FAK phosphorylation in target cells, which was in part dependent on the presence of alpha3beta1 integrin. FAK was critical for the subsequent phosphorylation of Src by gBDeltaTM, and Src induction was essential for the phosphorylation of phosphatidyllinositol 3-kinase (PI-3K). HHV-8 gBDeltaTM-induced PI-3K was essential for the induction of RhoA and Cdc42 Rho GTPases that was accompanied by the cytoskeletal rearrangements. These gB-induced morphological changes were inhibited by the PI-3K inhibitors. Ezrin, one of the essential elements required to cross-link the actin cytoskeleton with the plasma membrane and to induce the morphological changes, was induced by the Rho GTPases. Inhibition of cellular tyrosine kinases by the brief treatment of cells with 4',5,7-trihydroxyisoflavone (genistein) blocked the entry of HHV-8 into target cells. These findings suggest that, independently of other viral glycoproteins and via its RGD motif, HHV-8 gB induces integrin-dependent pre-existing FAK-Src-PI-3K-Rho GTPase kinases. Since these signal pathways play vital roles in host cell endocytosis and movement of particulate materials in the cytoplasm, the early stages of HHV-8 gB interaction with host cells may provide a very conducive environment for the successful infection of target cells.