Glutamate Secretion and Metabotropic Glutamate Receptor 1 Expression during Kaposi's Sarcoma-Associated Herpesvirus Infection Promotes Cell Proliferation

Abstract

Kaposi's sarcoma associated herpesvirus (KSHV) is etiologically associated with endothelial Kaposi's sarcoma (KS) and B-cell proliferative primary effusion lymphoma (PEL), common malignancies seen in immunocompromised HIV-1 infected patients. The progression of these cancers occurs by the proliferation of cells latently infected with KSHV, which is highly dependent on autocrine and paracrine factors secreted from the infected cells. Glutamate and glutamate receptors have emerged as key regulators of intracellular signaling pathways and cell proliferation. However, whether they play any role in the pathological changes associated with virus induced oncogenesis is not known. Here, we report the first systematic study of the role of glutamate and its metabotropic glutamate receptor 1 (mGluR1) in KSHV infected cell proliferation. Our studies show increased glutamate secretion and glutaminase expression during de novo KSHV infection of endothelial cells as well as in KSHV latently infected endothelial and B-cells. Increased mGluR1 expression was detected in KSHV infected KS and PEL tissue sections. Increased c-Myc and glutaminase expression in the infected cells was mediated by KSHV latency associated nuclear antigen 1 (LANA-1). In addition, mGluR1 expression regulating host RE-1 silencing transcription factor/neuron restrictive silencer factor (REST/NRSF) was retained in the cytoplasm of infected cells. KSHV latent protein Kaposin A was also involved in the over expression of mGluR1 by interacting with REST in the cytoplasm of infected cells and by regulating the phosphorylation of REST and interaction with β-TRCP for ubiquitination. Colocalization of Kaposin A with REST was also observed in KS and PEL tissue samples. KSHV infected cell proliferation was significantly inhibited by glutamate release inhibitor and mGluR1 antagonists. These studies demonstrated that elevated glutamate secretion and mGluR1 expression play a role in KSHV induced cell proliferation and suggest that targeting glutamate and mGluR1 is an attractive therapeutic strategy to effectively control the KSHV associated malignancies.

Author Summary

Kaposi's sarcoma associated herpesvirus (KSHV), prevalent in immunosuppressed HIV infected individuals and transplant recipients, is etiologically associated with cancers such as endothelial Kaposi's sarcoma (KS) and B-cell primary effusion lymphoma (PEL). Both KS and PEL develop from the unlimited proliferation of KSHV infected cells. Increased secretion of various host cytokines and growth factors, and the activation of their corresponding receptors, are shown to be contributing to the proliferation of KSHV latently infected cells. Glutamate, a neurotransmitter, is also involved in several cellular events including cell proliferation. In the present study, we report that KSHV-infected latent cells induce the secretion of glutamate and activation of metabotropic glutamate receptor 1 (mGluR1), and KSHV latency associated LANA-1 and Kaposin A proteins are involved in glutaminase and mGluR1 expression. Our functional analysis showed that elevated secretion of glutamate and mGluR1 activation is linked to increased proliferation of KSHV infected cells and glutamate release inhibitor and glutamate receptor antagonists blocked the proliferation of KSHV infected cells. These studies show that proliferation of cancer cells latently infected with KSHV in part depends upon glutamate and glutamate receptor and therefore could potentially be used as therapeutic targets for the control and elimination of KSHV associated cancers.