Constitutive signaling by Kaposi's sarcoma-associated herpesvirus G-protein-coupled receptor desensitizes calcium mobilization by other receptors

Constitutive activation of T cells by γ2-herpesviral GPCR through the interaction with cellular CXCR4

Members of the herpesviral family use multiple strategies to hijack infected host cells and exploit cellular signaling for their pathogenesis and latent infection. Among the most intriguing weapons in the arsenal of pathogenic herpesviruses are the constitutively active virally-encoded G protein-coupled receptors (vGPCRs). Even though vGPCRs contribute to viral pathogenesis such as immune evasion and proliferative disorders, the molecular details of how vGPCRs continuously activate cellular signaling are largely unknown. Here, we report that the vGPCR of Herpesvirus saimiri (HVS), an oncogenic γ2-herpesvirus, constitutively activates T cells via a heteromeric interaction with cellular CXCR4. Constitutive T cell activation also occurs with expression of the vGPCR of Kaposi's sarcoma-associated herpesvirus (KSHV), but not the vGPCR of Epstein-Barr virus. Expression of HVS vGPCR down-regulated the surface expression of CXCR4 but did not induce the degradation of the chemokine receptor, suggesting that vGPCR/CXCR4 signaling continues in cytosolic compartments. The physical association of vGPCR with CXCR4 was demonstrated by proximity ligation assay as well as immunoprecipitation. Interestingly, the constitutive activation of T cells by HVS vGPCR is independent of proximal T cell receptor (TCR) signaling molecules, such as TCRβ, Lck, and ZAP70, whereas CXCR4 silencing by shRNA abolished T cell activation by vGPCRs of HVS and KSHV. Furthermore, previously identified inactive vGPCR mutants failed to interact with CXCR4. These findings on the positive cooperativity of vGPCR with cellular CXCR4 in T cell activation extend our current understanding of the molecular mechanisms of vGPCR function and highlight the importance of heteromerization for GPCR activity.

Modulation of cellular signaling by herpesvirus-encoded G protein-coupled receptors

Human herpesviruses (HHVs) are widespread infectious pathogens that have been associated with proliferative and inflammatory diseases. During viral evolution, HHVs have pirated genes encoding viral G protein-coupled receptors (vGPCRs), which are expressed on infected host cells. These vGPCRs show highest homology to human chemokine receptors, which play a key role in the immune system. Importantly, vGPCRs have acquired unique properties such as constitutive activity and the ability to bind a broad range of human chemokines. This allows vGPCRs to hijack human proteins and modulate cellular signaling for the benefit of the virus, ultimately resulting in immune evasion and viral dissemination to establish a widespread and lifelong infection. Knowledge on the mechanisms by which herpesviruses reprogram cellular signaling might provide insight in the contribution of vGPCRs to viral survival and herpesvirus-associated pathologies.

Piracy of prostaglandin E2/EP receptor-mediated signaling by Kaposi's sarcoma-associated herpes virus (HHV-8) for latency gene expression: strategy of a successful pathogen

Kaposi's sarcoma-associated herpes virus (KSHV) is implicated in the pathogenesis of KS, a chronic inflammation-associated malignancy. Cyclooxygenase-2 (COX-2) and its metabolite prostaglandin E2 (PGE2), two pivotal proinflammatory/oncogeneic molecules, are proposed to play roles in the expression of major KSHV latency-associated nuclear antigen-1 (LANA-1). Microsomal PGE2 synthase, PGE2, and its receptors (EP1, EP2, EP3, and EP4) were detected in KS lesions with the distinct staining of EP2/EP4 in KS lesions. In latently infected endothelial TIVE-LTC cells, EP receptor antagonists downregulated LANA-1 expression as well as Ca(2+), p-Src, p-PI3K, p-PKCzeta/lambda, and p-NF-kappaB, which are also some of the signal molecules proposed to be important in KS pathogenesis. Exogenous PGE2 and EP receptor agonists induced the LANA-1 promoter in 293 cells, and YY1, Sp1, Oct-1, Oct-6, C/EBP, and c-Jun transcription factors seem to be involved in this induction. PGE2/EP receptor-induced LANA-1 promoter activity was downregulated significantly by the inhibition of Ca(2+), p-Src, p-PI3K, p-PKCzeta/lambda, and p-NF-kappaB. These findings implicate the inflammatory PGE2/EP receptors and the associated signal molecules in herpes virus latency and uncover a novel paradigm that shows the evolution of KSHV genome plasticity to use inflammatory response for its survival advantage of maintaining latent gene expression. These data also suggest that potential use of anti-COX-2 and anti-EP receptor therapy may not only ameliorate the chronic inflammation associated with KS but could also lead to elimination of the KSHV latent infection and the associated KS lesions.

Lysophosphatidic acid decreases the nuclear localization and cellular abundance of the p53 tumor suppressor in A549 lung carcinoma cells

Lysophosphatidic acid (LPA) is a bioactive lipid that promotes cancer cell proliferation and motility through activation of cell surface G protein-coupled receptors. Here, we provide the first evidence that LPA reduces the cellular abundance of the tumor suppressor p53 in A549 lung carcinoma cells, which express endogenous LPA receptors. The LPA effect depends on increased proteasomal degradation of p53 and it results in a corresponding decrease in p53-mediated transcription. Inhibition of phosphatidylinositol 3-kinase protected cells from the LPA-induced reduction of p53, which implicates this signaling pathway in the mechanism of LPA-induced loss of p53. LPA partially protected A549 cells from actinomycin D induction of both apoptosis and increased p53 abundance. Expression of LPA(1), LPA(2), and LPA(3) receptors in HepG2 hepatoma cells, which normally do not respond to LPA, also decreased p53 expression and p53-dependent transcription. In contrast, neither inactive LPA(1) (R124A) nor another G(i)-coupled receptor, the M(2) muscarinic acetylcholine receptor, reduced p53-dependent transcription in HepG2 cells. These results identify p53 as a target of LPA action and provide a new dimension for understanding how LPA stimulates cancer cell division, protects against apoptosis, and thereby promotes tumor progression.

Virus-encoded chemokines, chemokine receptors and chemokine-binding proteins: new paradigms for future therapy

Over millions of years of coevolution with their hosts, viruses have learned the finest artifices of the immune system defense mechanisms and developed a variety of strategies for evading them. The chemokine system has been a primary target of these viral efforts because of the critical role it plays in the development of effective immune responses. Not only do chemokines control cellular recruitment at the site of infection, they also regulate the magnitude and character of the immune responses. Several viruses, and large DNA viruses in particular, have exploited the chemokine system by hijacking and reprogramming chemokine or chemokine-receptor genes, and/or secreting chemokine-binding proteins. In the past few years there has been intense investigation in this area, driven not only by the prospect of gaining a better understanding of viral-immune evasion mechanisms, but also by the possibility of targeting these molecules as part of future antiviral therapeutic approaches, as well as exploiting viral strategies of chemokine interference as novel therapies for inflammatory or neoplastic diseases.

Carboxyl Tail Cysteine Mutants of the Thyrotropin-Releasing Hormone Receptor Type 1 Exhibit Constitutive Signaling: Role of Palmitoylation

We studied the role of carboxyl tail cysteine residues and their palmitoylation in constitutive signaling by the thyrotropin-releasing hormone (TRH) receptor type 1 (TRH-R1) in transfected mammalian cells and in Xenopus laevis oocytes. To study palmitoylation, we inserted a factor Xa cleavage site within the third extracellular loop of TRH-R1, added a carboxyl-terminal C9 immunotag and expressed the mutant receptor in Chinese hamster ovary cells. We identified TRH-R1-specific palmitoylation in the transmembrane helix-7/carboxyl-tail receptor fragment mainly at Cys-335 and Cys-337. In contrast to a mutant truncated at Cys-335 that was reported previously to be constitutively active, a receptor truncated at Lys-338 (K338Stop), which preserves Cys-335 and Cys-337, and C337Stop and N336Stop, which preserve Cys-335, did not exhibit increased constitutive signaling. TRH-R1 mutants substituted singly by Gly or Ser at Cys-335 or Cys-337 did not exhibit constitutive signaling. By contrast, substitution of both cysteines (C335G/C337G or C335S/C337S) yielded TRH-R1 mutants that exhibited marked constitutive signaling in mammalian cells. In the oocyte, constitutive signaling by C335G/C337G resulted in homologous (of C335G/C337G) and heterologous (of M1 muscarinic receptor) desensitization. Because both Cys-335 and Cys-337 have to be substituted or deleted for constitutive signaling, we propose that a single palmitoylation site in the proximal carboxyl tail is sufficient to constrain TRH-R1 in an inactive conformation.

HCMV-encoded chemokine receptor US28 employs multiple routes for internalization

The human cytomegalovirus-encoded G protein-coupled receptor homologue US28 binds inflammatory chemokines and sequesters them from the environment of infected cells. Low surface deposition and endocytosis are dependent on constitutive C-terminal phosphorylation, suggesting a requirement for beta-arrestin binding in receptor internalization. In this report, a US28-dependent redistribution of beta-arrestin into vesicular structures occurred, although internalization of US28 was independent of beta-arrestin. Internalization of US28 was dynamin-dependent, and US28 partially partitioned into the detergent-resistant membrane fraction. Endocytosis was diminished by cholesterol depletion, yet sucrose inhibition was even stronger. The relevance of the clathrin-coated pit pathway was supported by colocalization of beta(2)-adaptin and US28 in endocytic compartments. Exchange of the C-terminal dileucine endocytosis motif inhibited rapid endocytosis, indicating a direct interaction of US28 with the AP-2 adaptor complex. We suggest that the arrestin-independent, dynamin-dependent internalization of US28 reveals a differential sorting of beta-arrestins and the virally encoded chemokine receptor homologue.

Down-regulation of inositol 1,4,5-trisphosphate receptor in cells stably expressing the constitutively active angiotensin II N111G-AT(1) receptor

The diverse cellular changes brought about by the expression of a constitutively active receptor are poorly understood. QBI-human embryonic kidney 293A cells stably expressing the constitutively active N111G-AT(1) receptor (N111G cells) showed elevated levels of inositol phosphates and frequent spontaneous intracellular Ca(2+) oscillations. Interestingly, Ca(2+) transients triggered with maximal doses of angiotensin II were much weaker in N111G cells than in wild-type cells. These blunted responses were observed independently of the presence or absence of extracellular Ca(2+) and were also obtained when endogenous muscarinic and purinergic receptors were activated, revealing a heterologous desensitization process. The desensitized component of the Ca(2+) signaling cascade was neither the G protein G(q) nor phospholipase C. The intracellular Ca(2+) store of N111G cells and their mechanism of Ca(2+) entry also appeared to be intact. The most striking adaptive response of N111G cells was a down-regulation of their inositol 1,4,5-trisphosphate receptor (IP(3)R) as revealed by reduced IP(3)-induced Ca(2+) release, lowered [(3)H]IP(3) binding capacity, diminished IP(3)R immunoreactivity, and accelerated IP(3)R degradation involving the lysosomal pathway. Treatment with the inverse agonist EXP3174 reversed the desensitized phenotype of N111G cells. Down-regulation of IP(3)R represents a reversible adaptive response to protect cells against the adverse effects of constitutively active Ca(2+)-mobilizing receptors.