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The constitutive signaling of KSHV-GPCR causes heterologus desensitization of responses mediated by other GPCRS in Xenopus oocytes1Department of Physiology & Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv 69978, Israel, 2Weill College of Medicine, Cornell University, New York, NY 10021AIMS: To study interactions between Kaposi sarcoma-associated herpesvirus 8 G-protein coupled receptor (KSHV-GPCR) and other GPCRs. METHODS: Various GPCRs (e.g. thyrotropin-releasing hormone receptor-TRH-R) were expressed in Xenopus oocytes or in cultured mammalian cells, either alone or together with KSHV-GPCR. Responses to stimulation of GPCRs by cognate agonists were measured assessed by measuring calcium-activated chloride currents and by labeled calcium efflux (in oocytes) and by fura 2 calcium imaging (in mammalian cells). RESULTS: Co-expression of KSHV-GPCR with other GPCRs in oocytes and in mammalian cells resulted in 50-80% inhibition of responses (heterologous desensitization) elicited by their cognate agonists (e.g. TRH). KSHV-GPCR, expressed alone in oocytes, caused 4.4-fold increase in labeled calcium basal efflux, which was almost fully inhibited by IP10, demonstrating the receptor's constitutive signaling and the ability of IP10 to serve as an inverse agonist. IP10 also caused significant recovery of responses mediated by co-expressed GPCR's. Expression of KSHV-GPCR impaired the response to the calcium-mobilizing messenger inositol 1,4,5-trisphosphate. Assay of calcium stores with thapsigargin indicated pronounced depletion in oocytes expressing the viral receptor. CONCLUSIONS: KSHV-GPCR is a constitutively signaling receptor, which responds well to Groa and is inhibited by the interferon-inducible protein 10 (IP10). Its constitutive signaling causes heterologous desensitization of responses mediated by other GPCRs, via chronic depletion of calcium stores. We suggest that this aspect of the viral receptor's activity usurps cellular control from other signals mediated by GPCRs coupled to the calcium mobilization pathway. KEY WORDS: Inverse agonism, Calcium stores depletion. For more information, contact mmeiri@post.tau.ac.il Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Paris, France; February 9 - 12, 2002; in the section on Viral Oncogenesis. |
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