HCMV-Encoded Chemokine Receptor US28 Mediates Proliferative Signaling Through the IL-6–STAT3 Axis

A viral G protein–coupled receptor may initiate a positive feedback loop to promote tumor proliferation and vascularization. A Viral Pathway to Tumor Development Human cytomegalovirus (HCMV), a widespread human herpesvirus that persists in a latent form, is associated with pathological processes in immunocompromised hosts and has been implicated in the development of several forms of cancer, including glioblastoma. HCMV encodes a G protein–coupled receptor, US28, that resembles a chemokine receptor and constitutively activates signaling pathways associated with cell proliferation. Slinger et al. expressed US28 in cultured cells to explore the mechanisms through which it could promote tumor development. They found that US28 stimulated the production and secretion of both vascular endothelial growth factor (VEGF) and the cytokine interleukin-6 (IL-6) and defined a signaling pathway whereby US28 increased cell proliferation through IL-6–dependent activation of the JAK1-STAT3 axis. IL-6 is itself a target of STAT3, leading the authors to propose that US28-dependent production and secretion of IL-6 and consequent autocrine and paracrine STAT3 activation lead to establishment of a positive feedback loop that promotes proliferation of both infected and neighboring cells. Analyses of human glioblastoma tissue revealed US28 and activated STAT3 in cells lining blood vessels, suggesting that US28 may play a role in tumor vascularization. US28 is a viral G protein (heterotrimeric guanosine triphosphate–binding protein)–coupled receptor encoded by the human cytomegalovirus (HCMV). In addition to binding and internalizing chemokines, US28 constitutively activates signaling pathways linked to cell proliferation. Here, we show increased concentrations of vascular endothelial growth factor and interleukin-6 (IL-6) in supernatants of US28-expressing NIH 3T3 cells. Increased IL-6 was associated with increased activation of the signal transducer and activator of transcription 3 (STAT3) through upstream activation of the Janus-activated kinase JAK1. We used conditioned growth medium, IL-6–neutralizing antibodies, an inhibitor of the IL-6 receptor, and short hairpin RNA targeting IL-6 to show that US28 activates the IL-6–JAK1–STAT3 signaling axis through activation of the transcription factor nuclear factor κB and the consequent production of IL-6. Treatment of cells with a specific inhibitor of STAT3 inhibited US28-dependent [3H]thymidine incorporation and foci formation, suggesting a key role for STAT3 in the US28-mediated proliferative phenotype. US28 also elicited STAT3 activation and IL-6 secretion in HCMV-infected cells. Analyses of tumor specimens from glioblastoma patients demonstrated colocalization of US28 and phosphorylated STAT3 in the vascular niche of these tumors. Moreover, increased phospho-STAT3 abundance correlated with poor patient outcome. We propose that US28 induces proliferation in HCMV-infected tumors by establishing a positive feedback loop through activation of the IL-6–STAT3 signaling axis.