The Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor promotes endothelial cell survival through the activation of Akt/protein kinase B

The Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor (KSHV-GPCR) is a key molecule in the pathogenesis of Kaposi's sarcoma, playing a central role in the promotion of vascular endothelial growth factor (VEGF)-driven angiogenesis and spindle cell proliferation. We previously have shown that KSHV-GPCR has oncogenic potential when overexpressed in fibroblasts and is responsible for the expression and secretion of VEGF through the regulation of different intracellular signaling pathways (A. Sodhi et al., Cancer Res., 60: 4873-4880, 2000; C. Bais et al., Nature, 391: 86-89, 1998). Here, we describe that this constitutively active G protein-coupled receptor is able to promote cell survival in primary human umbilical vein endothelial cells and that this effect is independent of its ability to secrete VEGF because it is not prevented by the expression of antisense constructs for VEGF or the addition of VEGF-blocking antibodies. Instead we found that ectopic expression of KSHV-GPCR potently induces the kinase activity of Akt/protein kinase B in a dose-dependent manner and triggers its translocation to the plasma membrane. This signaling pathway requires the function of phosphatidylinositol 3'-kinase and is dependent on betagamma subunits released from both pertussis toxin-sensitive and -insensitive G proteins. Furthermore, we found that KSHV-GPCR is able to protect human umbilical vein endothelial cells from the apoptosis induced by serum deprivation and that both wortmannin and the expression of a kinase-deficient Akt K179M mutant are able to block this effect. Finally, we observed that the Akt K179M protein also inhibits the activation of nuclear factor-KB induced by KSHV-GPCR, suggesting that this transcription factor may represent one of the putative downstream targets for Akt in the survival-signaling pathway. These results provide further knowledge in the elucidation of the signal transduction pathways activated by KSHV-GPCR and support its key role in promoting the survival of viral-infected cells. Moreover, the present findings also emphasize the importance of this G protein-coupled receptor in the development of KSHV-related neoplasias.