Activation of signal transduction pathways by HHV-8 chemokine receptor homologue ORF74: Evidence for a paracrine mechanism of Kaposi's sarcoma pathogenesis

Abstract

Infection with Human Herpesvirus 8 (HHV-8), also known as Kaposi's sarcoma (KS)-associated herpesvirus (KSHV), is necessary for the development of KS. The HHV-8 lytic phase gene ORF74 is related to G protein-coupled receptors, particularly CXCR2. ORF74 has been shown to activate inositol phosphate/phospholipase C and the downstream MAP kinases. We show here that ORF74 activates the transcription factor NF-kappaB, independent of ligand, when expressed in KS-derived HHV-8 negative endothelial cells, primary vascular endothelial cells, or T-lymphoid cells. We found that activation of NF-kappaB by ORF74 occurs primarily through the PI-3 Kinase/Akt pathway. ORF74 also results in the activation of a number of factors downstream of Akt, other than NF-kappaB, which are relevant to KS pathogenesis. Activation of NF-kappaB and the Akt pathway by ORF74 was enhanced by the addition of HIV-1 Tat protein, suggesting a collaborative role between the two in the promotion of AIDS KS. Mutation of Val to Asp in the ORF74 second cytoplasmic loop did not affect ligand independent signaling activity, but greatly increased the response to GROalpha. In KS cells, endothelial cells, and T-lymphoid cells, ORF74 upregulated the expression of NF-kappaB-dependent inflammatory cytokines and adhesion molecules. KS cells and T cells expressing ORF74 showed increased levels of adhesion to one another. Supernatants from transfected KS cells activated NF-kappaB signaling in untransfected cells and elicited the chemotaxis of monocytoid and T lymphoid cells. Expression of ORF74 conferred a morphology on primary endothelial cells that was strikingly similar to that of spindle cells present in KS lesions. Taken together, these data, demonstrating that ORF74 activates signaling pathways and induces the expression of pro-angiogenic and pro-inflammatory factors, suggest that expression of ORF74 in a minority of cells in KS lesions could influence uninfected cells or latently infected cells via autocrine and paracrine mechanisms, thereby contributing to KS pathogenesis.