Kaposi's Sarcoma-Associated Herpesvirus-Encoded G Protein-Coupled Receptor ORF74 Constitutively Activates p44/p42 MAPK and Akt via G_iand Phospholipase C-Dependent Signaling Pathways

Abstract

The G protein-coupled receptor encoded by Kaposi's sarcoma-associated herpesvirus, also referred to as ORF74, has been shown to stimulate oncogenic and angiogenic signaling pathways in a constitutively active manner. The biochemical routes linking ORF74 to these signaling pathways are poorly defined. In this study, we show that ORF74 constitutively activates p44/p42 mitogen-activated protein kinase (MAPK) and Akt via G iand phospholipase C (PLC)-mediated signaling pathways. Activation of Akt by ORF74 appears to be phosphatidylinositol 3-kinase (PI3-K) dependent but, interestingly, is also mediated by activation of protein kinase C (PKC) and p44/p42 MAPK. ORF74 may signal to Akt via p44/p42 MAPK, which can be activated by G i , through activation of PI3-K or through PKC via the PLC pathway. Signaling of ORF74 to these proliferative and antiapoptotic signaling pathways can be further modulated positively by growth-related oncogene (GRO␣/ CXCL1) and negatively by human gamma interferon-inducible protein 10 (IP-10/CXCL10), thus acting as an agonist and an inverse agonist, respectively. Despite the ability of the cytomegalovirus-encoded chemokine receptor US28 to constitutively activate PLC, this receptor does not increase phosphorylation of p44/p42 MAPK or Akt in COS-7 cells. Hence, ORF74 appears to signal through a larger diversity of G proteins than US28, allowing it to couple to proliferative and antiapoptotic signaling pathways. ORF74 can therefore be envisioned as an attractive target for novel treatment of Kaposi's sarcoma.

The KSHV G protein-coupled receptor signals via multiple pathways to induce transcription factor activation in primary effusion lymphoma cells

“…Some crosstalk, however, between the Gq-and Gi-coupled limbs is evident since inhibition of ERK-1/2 with PD98059 led to an obvious inhibition of vGPCRinduced phosphorylation of Akt as seen in Figure 3e. Our data support the apparent ERK-1/2-dependent Akt activation by vGPCR that was first reported by Smit et al (2002) using the ERK-1/2 inhibitor U0126 in COS-7 cells .…”

“…Figure 3b shows, by Western blot, that vGPCRinduced phosphorylation of ERK-1/2 is not inhibited by either the PI3K inhibitor wortmannin or by pertussis. This is in contrast to data from Smit et al (2002) that show Gi-mediated ERK-1/2 activation in COS-7 cells. Although our data do not rule out minor Gi-mediated contribution to vGPCR-mediated ERK-1/2 activation in PEL cells, they do show that it is likely negligible compared to that mediated by Gq coupling.…”

A requirement for NF-κB induction in the production of replication-competent HHV-8 virions

“…Overexpression of the viral G-protein-coupled receptor (vGPCR, ORF 74) has also been shown to induce NF-kB (Montaner et al, 2001;Pati et al, 2001;Schwarz and Murphy, 2001;Chiou et al, 2002;Smit et al, 2002) through the activation of the PI3K/AKT and IKK pathways (Montaner et al, 2001;Pati et al, 2001). vGPCR has been detected in HHV-8-infected PEL cells undergoing lytic replication, consistent with its classification as an early lytic gene product (Sun et al, 1999;Chiou et al, 2002).…”

“…This interaction results in the induction of a survival pathway that effectively inhibits caspase-mediated apoptosis via engagement of the Fas receptor. Overexpression of the early lytic protein vGPCR (Bais et al, 1998;Yang et al, 2000;Guo et al, 2003) has also been shown to induce NF-kB (Montaner et al, 2001;Pati et al, 2001;Schwarz and Murphy, 2001;Chiou et al, 2002;Smit et al, 2002) through the activation of PI3-K/AKT and IKK pathways (Montaner et al, 2001;Pati et al, 2001).…”

Cell transformation mediated by the Epstein–Barr virus G protein-coupled receptor BILF1 is dependent on constitutive signaling

“…Furthermore, both US28 and ORF74 are constitutively active (Arvanitakis et al, 1997;Waldhoer et al, 2002). US28 signals through Ga q , Ga s and Ga 12/13 , further activating kinases and transcription factors, whereas ORF74 signals through Ga q , Ga s and Ga i , virtually activating the same downstream signal transduction pathways as US28 (Smit et al, 2002;Waldhoer et al, 2002;Streblow et al, 2003;McLean et al, 2004). Furthermore, ORF74 and US28 stimulate the secretion of cytokines and growth factors, including vascular endothelial growth factor (VEGF) (Bais et al, 1998;Sodhi et al, 2000;Pati et al, 2001;Maussang et al, 2006).…”