| Kaposi's sarcoma (KS) is a unique neoplasm that is the most common AIDS-related tumor. However, its pathogenesis is not well understood. In 1994, Kaposi's sarcoma-associated herpesvirus (KSHV, or human herpesvirus 8, HHV8) was found to be consistently present in all forms of KS. While it is believed that KSHV is necessary for tumorigenesis, other co-factors, such as HIV and immunosuppression, are most likely involved. The genome of KSHV contains a number of cellular homologs that are putative mediators of viral pathogenesis. Open reading frame 74 (ORF 74) of the KSHV genome encodes a G protein coupled receptor (KSHV-GPCR) that is expressed in KS lesions. KSHV-GPCR has the highest homology to chemokine receptors, in particular CXCR1 and CXCR2. KSHV-GPCR, a pirated viral chemokine receptor, may promote disease by subverting signaling pathways activated by chemokine receptors.;To determine if KSHV-GPCR encodes a functional receptor, KSHV-GPCR was expressed in a transient cell system to study its binding and signaling properties. KSHV-GPCR was found to bind both CXC and CC chemokines, signal independently of ligand, and induce cellular proliferation. Several chemokines were tested to determine their effects on the constitutive signaling activity by KSHV-GPCR. Two CXC chemokines, IP-10 and SDF-1alpha, and a CC chemokine analog encoded by KSHV, vMIP-II, were found to inhibit KSHV-GPCR signaling. In contrast, IL-8 and Gro-alpha, two CXC chemokines containing an ELR motif in their NH2 terminus, were able to activate KSHV-GPCR signaling. The ELR motif of these chemokines was shown to be required for activation of KSHV-GPCR.;In order to study the biological effects of KSHV-GPCR in human cells, a double stable HEK 293 cell line (HEK Tet-KSHV-GPCR) was established based upon the tetracycline regulatable system. In HEK Tet-KSHV-GPCR cells, expression of functional KSHV-GPCRs was induced by the tetracycline analogue, doxycycline (dox). In HEK Tet-KSHV-GPCR cells, proliferation was stimulated by dox with a time course consistent with signaling by KSHV-GPCRs. Induction of KSHV-GPCR expression in these cells led to parallel but independent activation of endogenous phosphatidylinositol (PI) 3-kinase and its downstream effector Akt/PKB, and of p38 MAPK. SB 202190, an inhibitor of p38 MAPK, did not inhibit activation of PI 3-kinase, and LY 294002, an inhibitor of PI 3-kinase, did not inhibit activation of p38 MAPK. However, SB 202190 or LY 294002 virtually abolished HEK Tet-KSHV-GPCR-stimulated cell proliferation/survival. These results show that induction of KSHV-GPCR expression stimulates cell proliferation/survival via two parallel but independent pathways that include PI 3-kinase or p38 MAPK. Thus, one mechanism by which KSHV-GPCR may be involved in human tumorigenesis is through activation of PI 3-kinase and p38 MAP kinase that in turn stimulates proliferation/survival of KSHV-infected cells. |
