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

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.

卡波西肉瘤相关疱疹病毒（Kaposi's sarcoma associated herpesvirus, KSHV）在病因学上与内皮细胞型卡波西肉瘤（endothelial Kaposi's sarcoma, KS）以及B细胞增殖性原发性渗出性淋巴瘤（B-cell proliferative primary effusion lymphoma, PEL）密切相关，此类恶性肿瘤常见于免疫缺陷的HIV-1感染者体内。此类癌症的进展由KSHV潜伏感染细胞的增殖所驱动，而该过程高度依赖感染细胞分泌的自分泌与旁分泌因子。
谷氨酸及其受体现已被证实为细胞内信号通路与细胞增殖的关键调控因子，但它们是否参与病毒诱导的肿瘤发生相关的病理过程，目前尚不清楚。本研究首次系统探讨了谷氨酸及其代谢型谷氨酸受体1（metabotropic glutamate receptor 1, mGluR1）在KSHV感染细胞增殖中的作用。
研究发现，在内皮细胞新发KSHV感染阶段，以及KSHV潜伏感染的内皮细胞与B细胞中，均存在谷氨酸分泌与谷氨酰胺酶表达的上调。在KSHV感染的KS与PEL组织切片中，也检测到mGluR1表达升高。感染细胞中c-Myc与谷氨酰胺酶的表达上调，由KSHV潜伏期相关核抗原1（latency associated nuclear antigen 1, LANA-1）介导。
此外，调控mGluR1表达的宿主RE1沉默转录因子/神经元限制性沉默因子（RE-1 silencing transcription factor/neuron restrictive silencer factor, REST/NRSF）在感染细胞的细胞质中发生滞留。KSHV潜伏蛋白Kaposin A同样参与了mGluR1的过表达：其可在感染细胞的细胞质中与REST结合，并通过调控REST的磷酸化及其与β-TRCP的相互作用以介导泛素化过程。在KS与PEL组织样本中，也观察到Kaposin A与REST的共定位现象。
谷氨酸释放抑制剂与mGluR1拮抗剂可显著抑制KSHV感染细胞的增殖。上述研究证实，谷氨酸分泌上调与mGluR1表达升高在KSHV诱导的细胞增殖中发挥关键作用，提示靶向谷氨酸与mGluR1或是有效控制KSHV相关恶性肿瘤的极具前景的治疗策略。