Activation of NF-kappa B by the human herpesvirus 8 chemokine receptor ORF74: Evidence for a paracrine model of Kaposi's sarcoma pathogenesis

Infection with human herpesvirus 8 (HHV-8), also known as Kaposi's sarcoma (KS)-associated herpesvirus, is necessary for the development of KS. The HH V-8 lytic-phase gene ORF74 is related to G protein-coupled receptors, parti cularly interleukin-8 (IL-8) receptors. ORF74 activates the inositol phosph ate/phospholipase C pathway and the downstream mitogen-activated protein ki nases, JNK/SAPK and p38. We show here that ORF74 also activates NF-KB indep endent of ligand when expressed in KS-derived HHV-8-negative endothelial ce lls or primary vascular endothelial cells. NF-KB activation was enhanced by the chemokine GRO alpha, but not by IL-8. Mutation of Val to Asp in the OR F74 second cytoplasmic loop did not affect ligand-independent signaling act ivity, but it greatly increased the response to GRO alpha. ORF74 upregulate d the expression of NF-kappaB-dependent inflammatory cytokines (RANTES, IL- 6, IL-8, and granulocyte-macrophage colony-stimulating factor) and adhesion molecules (VCAM-1, ICAM-1, and E-selectin). Supernatants from transfected KS cells activated NF-KB signaling in untransfected cells and elicited the chemotaxis of monocytoid and T-lymphoid cells. Expression of ORF74 conferre d on primary endothelial cells a morphology that was strikingly similar to that of spindle cells present in KS lesions. Taken together, these data, de monstrating that ORF74 activates NF-KB and induces the expression of proang iogenic and proinflammatory factors, suggest that expression of ORF74 in a minority of cells in KS lesions could influence uninfected cells or latentl y infected cells via autocrine and paracrine mechanisms, thereby contributi ng to KS pathogenesis.