Role of cytoplasmic tail phosphorylation sites of platelet-activating factor receptor in agonist-induced desensitization

Hydroxysafflor Yellow A Alleviates Ovalbumin-Induced Asthma in a Guinea Pig Model by Attenuateing the Expression of Inflammatory Cytokines and Signal Transduction

Hydroxysafflor yellow A (HSYA) is an effective ingredient of the Chinese herb Carthamus tinctorius L. In this study, we aimed to evaluate the effects of HSYA on ovalbumin (OVA)-induced asthma in guinea pigs, and to elucidate the underlying mechanisms. We established a guinea pig asthma model by intraperitoneal injection and atomized administration OVA. Guinea pigs were injected intraperitoneally with HSYA (50, 75, 112.5 mg/kg) once daily from days 2 to 22 before OVA administration. We examined biomarkers including lung function, pulmonary histopathology, immunoglobulin E (IgE), Th1/Th2 relative inflammatory mediators, and related pathways. Pathological changes in lung tissues were detected by hematoxylin and eosin and periodic acid-Schiff staining. Phosphorylation levels of JNK mitogen-activated protein kinase (MAPK), p38 MAPK, ERK MAPK, and inhibitor of nuclear factor κBα (IκBα) were detected by western blot. plasma levels of total IgE, platelet-activating factor (PAF), and interleukin (IL)-3 were detected by enzyme-linked immunosorbent assay (ELISA). Expression levels of tumor necrosis factor (TNF)-α, IL-1β, IL-2, IL-4, IL-5, IL-6, IL-13, and interferon (IFN)-γ were detected by ELISA and real-time quantitative polymerase chain reaction. HSYA significantly reduced airway resistance, improved dynamic lung compliance, and attenuated the pathologic changes. HSYA also inhibited the phosphorylation of JNK MAPK, p38 MAPK, ERK MAPK, and IκBα, and inhibited the OVA-induced elevations of IgE, PAF, IL-1β, IL-6, IL-4, IL-5, and IL-13 and the decreases in TNF-α, IFN-γ, IL-2, and IL-3. These findings suggest that HSYA has a protective effect on OVA-induced asthma through inhibiting the Th1/Th2 cell imbalance and inhibiting activation of the MAPK signaling pathway.

Hydroxysafflor Yellow A Suppresses Platelet Activating Factor-Induced Activation of Human Small Airway Epithelial Cells

Hydroxysafflor yellow A (HSYA) is a chemical component isolated from the Chinese medicine Carthamus tinctorius L. HSYA has numerous pharmacological effects, including protecting against and mitigating some respiratory diseases such as acute lung injury and chronic obstructive pulmonary disease; however, its effect on asthma remains unclear. We previously found that HSYA attenuated ovalbumin-induced allergic asthma in guinea pigs. Platelet activating factor (PAF) is a phospholipid mediator of inflammation and an important factor in the pathological process of asthma. In this study, we investigated the anti-inflammatory effects of HSYA and its underlying mechanisms in PAF-induced human small airway epithelial cells (HSAECs). PAF-activated cells were pretreated with HSYA and/or the PAF receptor inhibitor, ginkgolide B, and we observed changes in the expression of interleukin (IL)-1β, IL-6, and tumor necrosis factor alpha, monolayer permeability of HSAECs, and inflammatory signaling pathways. HSYA attenuated the PAF-induced increase in expression of inflammatory factors and destruction of cell-barrier function, and inhibited the expression of protein kinase C, mitogen-activated protein kinases, activator protein-1, and nuclear factor-κB activation induced by PAF. These findings suggest that HSYA may represent a potential new drug for the treatment of asthma.

Negative regulation of parathyroid hormone (PTH)-activated phospholipase C by PTH/PTH-related peptide receptor phosphorylation and protein kinase A

PTH binding to the PTH/PTHrP receptor activates adenylate cyclase/protein kinase A (PKA) and phospholipase C (PLC) pathways and increases receptor phosphorylation. The mechanisms regulating PTH activation of PLC signaling are poorly understood. In the current study, we explored the role of PTH/PTHrP receptor phosphorylation and PKA in PTH activation of PLC. When treated with PTH, LLCPK-1 cells stably expressing a green fluorescent protein (GFP)-tagged wild-type (WT) PTH/PTHrP receptor show a small dose-dependent increase in PLC signaling as measured by inositol trisphosphate accumulation assay. In contrast, PTH treatment of LLCPK-1 cells stably expressing a GFP-tagged receptor mutated in its carboxyl-terminal tail so that it cannot be phosphorylated (PD-GFP) results in significantly higher PLC activation (P<0.001). The effects of PTH on PLC activation are dose dependent and reach maximum at the 100 nm PTH dose. When WT receptor-expressing cells are pretreated with H89, a specific inhibitor of PKA, PTH activation of PLC signaling is enhanced in a dose-dependent manner. H89 pretreatment in PD-GFP cells causes a further increase in PLC activation in response to PTH treatment. Interestingly, PTH and forskolin (adenylate cyclase/PKA pathway activator) treatment causes an increase in PLCbeta3 phosphorylation at the Ser1105 inhibitory site and that increase is blocked by the PKA inhibitor, H89. Expression of a mutant PLCbeta3 in which Ser1105 was mutated to alanine (PLCbeta3-SA), in WT or PD cells increases PTH stimulation of inositol 1,4,5-trisphosphate formation. Altogether, these data suggest that PTH signaling to PLC is negatively regulated by PTH/PTHrP receptor phosphorylation and PKA. Furthermore, phosphorylation at Ser1105 is demonstrated as a regulatory mechanism of PLCbeta3 by PKA.

Identification and characterization of a new interleukin-8 receptor in bovine species

Mastitis is an inflammation of the mammary gland, most of the time caused by invading pathogens. Phagocytosis by neutrophils is a crucial defense of the mammary gland and the prompt recruitment of these phagocytes from blood to milk compartments is essential for the outcome of the infection. ELR+ CXC chemokines, ligands of the two interleukin-8 receptors (IL-8R), CXCR1 and CXCR2, are likely to be involved in the initiation of the inflammatory response and also in the migration of neutrophils. Recently, the polymorphism of bovine CXCR2 has been associated with resistance to mastitis. However, as the bovine IL-8R are not functionally defined, their contribution to the recruitment of neutrophils remains undetermined. In this study, the RNA ligase-mediated (RLM)-RACE method was used to clone a novel bovine interleukin-8 receptor (nIL-8R) of the bovine species. We showed that both bovine IL-8R (nIL-8R and the published CXCR2) are functional since bovine IL-8 induced migration of HEK-293 cells expressing either IL-8R. In addition, comparisons of full-length sequences suggested that the published CXCR2 sequence was improperly annotated and that the sequences of the nIL-8R and the published CXCR2 are homologous to human CXCR2 and CXCR1, respectively. This was confirmed by binding assays with labeled IL-8 and GRO-beta and calcium (Ca) flux responses of transfected cells. Moreover, the C-terminal of both bovine IL-8R showed 100% identity, whereas they differ in most other species, suggesting that the two bovine IL-8R initiate similar signal transduction. These results constitute a basis to improve our understanding of the molecular mechanisms implicated in the recruitment of bovine neutrophils.

Identification of novel cytosolic phospholipase A(2)s, murine cPLA(2){delta}, {epsilon}, and {zeta}, which form a gene cluster with cPLA(2){beta}

Phospholipase A(2) hydrolyzes the sn-2 ester bond of glycerophospholipids that produce free fatty acids and lysophospholipids. Cytosolic phospholipase A(2)s (cPLA(2), group IV) are a subgroup of enzymes that act on the intracellular phospholipid membrane. The best investigated cPLA(2)alpha (group IVA) is a key enzyme for lipid mediator production in vivo. Here we report cloning and characterization of novel murine cPLA(2)s: cPLA(2)delta (group IVD), cPLA(2)epsilon (group IVE), and cPLA(2)zeta (group IVF), that form a gene cluster with cPLA(2)beta (group IVB). The deduced amino acid sequences of cPLA(2)delta, epsilon, and zeta demonstrated a conserved domain structure of cPLA(2), i.e. one C2 domain and one lipase domain. The potential catalytic dyad, Ser and Asp, was conserved for these newly cloned cPLA(2)s along with relatively high conservation for the surrounding residues. Transcripts of murine cPLA(2)delta, epsilon, and zeta appeared to be enriched in certain organs rather than ubiquitous distribution. Major Northern signals for cPLA(2)delta were detected in placenta, cPLA(2)epsilon in thyroid, heart, and skeletal muscle, and cPLA(2)zeta in thyroid. Recombinant proteins expressed in human embryonic kidney 293 cells demonstrated molecular sizes of about 100 kDa by Western blotting and exhibited Ca(2+)-dependent PLA(2) activities on 1-palmitoyl-2-[(14)C]arachidonoyl-phosphatidylcholine substrate. In contrast to cPLA(2)alpha, cPLA(2)zeta preferred phosphatidylethanolamine to phosphatidylcholine. Intracellular localization was visualized by green fluorescent-tagged proteins. Each molecule showed specific localization, and cPLA(2)delta translocated from the cytosol to the perinuclear region by calcium-ionophore stimulation. We thus discovered these functional novel cPLA(2) genes, which cluster on murine chromosome 2E5.

Components of a platelet-activating factor-signaling loop are assembled in the ovine endometrium late in the estrous cycle

Pulsatile release of uterine prostaglandin F(2alpha) (PGF(2alpha)) induces luteolysis in ruminants. Exogenous PAF is well known to cause PGF(2alpha) release from the ovine uterus. This study examines whether the components of a PAF-signaling loop exist in sheep at the time luteolysis is normally initiated. Day 14 of the cycle was the first day the uterus responded to an infusion of PAF, inducing a significant short-term increase in circulating levels of the PGF(2alpha) metabolite. There was a significant increase of PAF concentration (P < 0.001) in the endometrium and PAF release by tissue explants (P < 0.001) from day 10 to day 16 of the cycle. Endometrial tissue PAF receptor mRNA expression was induced (P < 0.01) by estradiol and progesterone treatment of animals, and transcripts were present between days 10 and 16 of the estrous cycle. Western analysis of endometrial tissue showed marked upregulation of PAF receptor protein expression from day 14 of the cycle, and immunolocalization studies showed that the receptor expression was predominantly around the endometrial glands. PAF:acetylhydrolase was primarily located within the lumen of the endometrial glands. The study shows that a PAF-signaling loop was assembled within the ovine endometrium at the time that PGF(2alpha) pulsatility was first observed.

Platelet-activating factor, a pleiotrophic mediator of physiological and pathological processes

Platelet-activating factor (PAF) is a potent proinflammatory phospholipid with diverse pathological and physiological effects. This bioactive phospholipid mediates processes as diverse as wound healing, physiological inflammation, apoptosis, angiogenesis, reproduction and long-term potentiation. Recent progress has demonstrated the participation of MAP kinase signaling pathways as modulators of the two critical enzymes, phospholipase A2 and acetyltransferase, involved in the remodeling pathway of PAF biosynthesis. The unregulated production of structural analogs of PAF by non-specific oxidative reactions has expanded this superfamily of signaling molecules to include "PAF-like" lipids whose mode of action is identical to that of authentic PAF. The action of members of this family is mediated by the PAF receptor, a G protein-coupled membrane-spanning molecule that can engage multiple signaling pathways in various cell types. Inappropriate activation of this signaling pathway is associated with many diseases in which inflammation is thought to be one of the underlying features. Inactivation of all members of the PAF superfamily occurs by a unique class of enzymes, the PAF acetylhydrolases, that have been characterized at the molecular level and that terminate signals initiated by both regulated and unregulated PAF production.

Activation of ERK1/2 by platelet-activating factor receptor is independent of receptor internalisation and G-protein activation

Platelet-activating factor (PAF) is a potent pro-inflammatory phospholipid mediator involved in a broad range of physiological and pathophysiological processes. The receptor of PAF (PAFR) is a heptahelical G-protein-coupled receptor. We have shown previously that upon agonist stimulation, PAFR internalised through clathrin-coated vesicles in an arrestin-dependent, but G-protein-coupling-independent manner. In the current report, we demonstrate that PAF stimulates Erk1/2 phosphorylation and: (1). dominant negative mutants of arrestins and dynamin do not influence Erk1/2 activation, (2). hypertonic conditions do not decrease the extent of Erk1/2 phosphorylation, (3). internalisation-deficient and/or G-protein-coupling-deficient mutants of PAFR activate Erk1/2 as efficiently as the wild-type PAFR, and (4). inhibition of epidermal growth factor receptor (EGFR) does not block Erk1/2 activation. Taken together, our results suggest that PAFR-mediated activation of mitogen-activated protein kinases Erk1/2 does not require receptor endocytosis, receptor tyrosine kinase transactivation or G-protein activation. In addition, our studies reveal that PAFR-mediated signals of G-protein activation, receptor internalisation and MAPK activation are differentially regulated by receptor structure and/or conformation.

The platelet-activating factor signaling system and its regulators in syndromes of inflammation and thrombosis

OBJECTIVES

To review the platelet-activating factor (PAF) signaling system, its regulation, and its dysregulation in acute inflammation and thrombosis and in syndromes that involve these cascades, including sepsis.

DATA SOURCES

A summary of published literature from MEDLINE search files and published reviews. DATA EXTRACTION, SYNTHESIS, AND SUMMARY: PAF, a phospholipid signaling molecule, transmits outside-in signals to intracellular transduction systems and effector mechanisms in a variety of cell types, including key cells of the innate immune and hemostatic systems: neutrophils, monocytes, and platelets. Thus, the PAF signaling system is a point of convergence at which injurious stimuli can trigger and amplify both acute inflammatory and thrombotic cascades. The biological activities of PAF are regulated by several precise mechanisms that, together, constrain and control its action in physiologic inflammation. Unregulated synthesis of PAF or defects in the mechanisms that limit its biological activities have the potential to cause pathologic inflammation and thrombosis. In addition, nonenzymatic generation of oxidized phospholipids that are recognized by the PAF receptor can trigger inflammatory and thrombotic events. There is evidence that the PAF signaling system is dysregulated in sepsis, shock, and traumatic injury and that interruption or termination of its effector responses leads to beneficial outcomes. Plasma PAF acetylhydrolase, an enzyme that hydrolyzes PAF and structurally related oxidized phospholipids, yielding products that are no longer recognized by the PAF receptor, may be a particularly important signal terminator.

CONCLUSION

The PAF signaling system can trigger inflammatory and thrombotic cascades, amplify these cascades when acting with other mediators, and mediate molecular and cellular interactions (cross talk) between inflammation and thrombosis. Evidence from in vitro experiments, studies of experimental animals, and clinical observations in humans indicates that the PAF signaling system is important in sepsis and other syndromes of inflammatory injury and that therapeutic strategies to interrupt or terminate signaling via the PAF signaling system may be useful in these conditions.

Agonist-induced internalization of the platelet-activating factor receptor is dependent on arrestins but independent of G-protein activation. Role of the C terminus and the (D/N)PXXY motif

As with most G-protein-coupled receptors, repeated agonist stimulation of the platelet-activating factor receptor (PAFR) results in its desensitization, sequestration, and internalization. In this report, we show that agonist-induced PAFR internalization is independent of G-protein activation but is dependent on arrestins and involves the interaction of arrestins with a limited region of the PAFR C terminus. In cotransfected COS-7 cells, both arrestin-2 and arrestin-3 could be coimmunoprecipitated with PAFR, and agonist stimulation of PAFR induced the translocation of both arrestin-2 and arrestin-3. Furthermore, coexpression of arrestin-2 with PAFR potentiated receptor internalization, whereas agonist-induced PAFR internalization was inhibited by a dominant negative mutant of arrestin-2. The coexpression of a minigene encoding the C-terminal segment of the receptor abolished PAF-induced arrestin translocation and inhibited PAFR internalization. Using C terminus deletion mutants, we determined that the association of arrestin-2 with the receptor was dependent on the region between threonine 305 and valine 330 because arrestin-2 could be immunoprecipitated with the mutant PAFRstop330 but not PAFRstop305. Consistently, stop330 could mediate agonist-induced arrestin-2 translocation, whereas stop305 could not. Two other deletion mutants with slightly longer regions of the C terminus, PAFRstop311 and PAFRstop317, also failed to induce arrestin-2 translocation. Finally, the PAFR mutant Y293A, containing a single substitution in the putative internalization motif DPXXY in the seventh transmembrane domain (which we had shown to be able to internalize but not to couple to G-proteins) could efficiently induce arrestin translocation. Taken together, our results indicate that ligand-induced PAFR internalization is dependent on arrestins, that PAFR can associate with both arrestin-2 and -3, and that their translocation involves interaction with the region of residues 318-330 in the PAFR C terminus but is independent of G-protein activation.

The CXC chemokine NAP-2 mediates differential heterologous desensitization of neutrophil effector functions elicited by platelet-activating factor

During early inflammation, the chemoattractants neutrophil-activating peptide-2 (NAP-2), platelet-activating factor (PAF), and complement component C5a are rapidly generated within the vasculature and potently induce effector functions in neutrophils, such as chemotaxis and degranulation. We investigated whether these mediators would cross-desensitize each other's activities on isolated neutrophils. We demonstrate that NAP-2 and C5a desensitize degranulation of neutrophils in response to PAF. However, whereas C5a-mediated desensitization correlated with the downregulation of PAF binding sites on neutrophils, NAP-2 did not regulate PAF receptors, nor did it modulate their calcium signaling. In further contrast to C5a, which desensitized PAF-induced neutrophil chemotaxis, NAP-2 did not affect the chemotatic response to PAF. These observations indicate that NAP-2 mediates selective desensitization of PAF-induced neutrophil degranulation by a mechanism downstream from PAF receptors, still allowing migration toward PAF. Thus, a role for NAP-2 may be to limit PAF-dependent vascular tissue damage by preventing degranulation of neutrophils without affecting their migration into the inflamed tissue.

Comparison of the coding sequence of the platelet-activating factor receptor gene in three species

The actions of platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) are mediated through the PAF receptor (PAFr), which is a member of G-protein coupled superfamily of receptors. Our laboratory has data showing PAF has a role(s) in reproduction in domestic animals. Porcine, bovine and caprine PAFr genes cloned in BAC vectors were sequenced. Each PAFr coding sequence (cds) in these three species is 1029 nucleotides long and contains no intervening sequences. The deduced amino acid sequences (AAS) appear to contain seven putative transmembrane domains with an extracellular N-terminus in each species. There is a common glycosylation site at the fourth asparagine residue of N-terminus. In the tail of each deduced amino acid sequence, five to six serines and five threonine residues could act as phosphorylation sites, which play an important role in rapid receptor desensitization. The degree of homology of the three species is from 89 to 96% in nucleotide sequences (NtS), and 87-96% in identities (I) and 94-97% in positives (P) in amino acid sequences (AAS). The degree of homology with human, guinea pig, mouse and rat is 84-87, 82-88 and 83-88% in NtS, 77-84 (I) or 85-90 (P), 77-84 (I) or 85-90 (P) and 75-83 (I) or 87-90% (P) in AAS for caprine, bovine and pig, respectively. Southern blotting results suggested that the PAFr gene exists as a single copy in the genome of pig.

Platelet-activating factor and related lipid mediators

Platelet-activating factor (PAF) is a phospholipid with potent, diverse physiological actions, particularly as a mediator of inflammation. The synthesis, transport, and degradation of PAF are tightly regulated, and the biochemical basis for many of these processes has been elucidated in recent years. Many of the actions of PAF can be mimicked by structurally related phospholipids that are derived from nonenzymatic oxidation, because such compounds can bind to the PAF receptor. This process circumvents much of the biochemical control and presumably is regulated primarily by the rate of degradation, which is catalyzed by PAF acetylhydrolase. The isolation of cDNA clones encoding most of the key proteins involved in regulating PAF has allowed substantial recent progress and will facilitate studies to determine the structural basis for substrate specificity and the precise role of PAF in physiological events.

Activation via multiple signaling pathways induces down-regulation of platelet-activating factor receptors on human B lymphocytes

Platelet-activating factor receptor (PAFR) has been identified in B cell lines and primary human B cells, but the regulation of PAFR during B cell activation has not been completely elucidated. In the present study, we have investigated the effects of B cell activation on PAFR binding parameters, PAFR mRNA and PAF-triggered intracellular calcium mobilization. The human B lymphoid cell line LA350 was shown to exhibit high levels of PAFR (48,550 +/- 4,310 sites/cell) as determined by radio-ligand binding assay with PAFR antagonist [3H]WEB2086. Treatment with phorbol 12,13-dibutyrate caused a biphasic reduction of PAFR binding. The early phase was inhibited by the protein kinase C inhibitor bisindolylmaleimide I (BIM), whereas the late phase was not blocked by BIM, protein tyrosine kinase inhibitor genistein, or the mitogen-activated protein kinase/extracellular signal-related kinase inhibitor PD98059. However, staurosporine, a broad-spectrum protein kinase inhibitor, completely inhibited the late phase down-regulation. Ionomycin also decreased [3H]WEB2086 binding sites, whereas the combination of PDB and ionomycin induced a greater reduction than either agent alone. Cross-linking of B cell receptor by anti-IgM Ab also induced down-regulation of PAFR, which was abolished by genistein or PD98059, but not by BIM or staurosporine. The decrease in surface PAFR number was closely paralleled by the reduction in PAFR mRNA both in LA350 cells and human tonsillar B cells, and was associated with decreased response to PAF indicated by decreased intracellular calcium mobilization. These data show that multiple signaling pathways are involved in down-regulating PAFR expression during B cell activation and development.

Deletion of the serotonin 5-HT2C receptor PDZ recognition motif prevents receptor phosphorylation and delays resensitization of receptor responses

Phosphorylation-deficient serotonin 5-HT(2C) receptors were generated to determine whether phosphorylation promotes desensitization of receptor responses. Phosphorylation of mutant 5-HT(2C) receptors that lack the carboxyl-terminal PDZ recognition motif (Ser(458)-Ser-Val-COOH; DeltaPDZ) was not detectable based on a band-shift phosphorylation assay and incorporation of (32)P. Treatment of cells stably expressing DeltaPDZ or wild-type 5-HT(2C) receptors with serotonin produced identical maximal responses and EC(50) values for eliciting [(3)H]inositol phosphate formation. In calcium imaging studies, treatment of cells expressing DeltaPDZ or wild-type 5-HT(2C) receptors with 100 nm serotonin elicited initial maximal responses and decay rates that were indistinguishable. However, a second application of serotonin 2.5 min after washout caused maximal responses that were approximately 5-fold lower with DeltaPDZ receptors relative to wild-type 5-HT(2C) receptors. After 10 min, responses of DeltaPDZ receptors recovered to wild-type 5-HT(2C) receptor levels. Receptors with single mutations at Ser(458) (S458A) or Ser(459) (S459A) decreased serotonin-mediated phosphorylation to 50% of wild-type receptor levels. Furthermore, subsequent calcium responses of S459A receptors were diminished relative to S458A and wild-type receptors. These results establish that desensitization occurs in the absence of 5-HT(2C) receptor phosphorylation and suggest that receptor phosphorylation at Ser(459) enhances resensitization of 5-HT(2C) receptor responses.

Phosphorylation and regulation of a Gq/11-coupled receptor by casein kinase 1alpha

Agonist-mediated receptor phosphorylation by one or more of the members of the G-protein receptor kinase (GRK) family is an established model for G-protein-coupled receptor (GPCR) phosphorylation resulting in receptor desensitization. Our recent studies have, however, suggested that an alternative route to GPCR phosphorylation may be an operation involving casein kinase 1alpha (CK1alpha). In the current study we investigate the involvement of CK1alpha in the phosphorylation of the human m3-muscarinic receptor in intact cells. We show that expression of a catalytically inactive mutant of CK1alpha, designed to act in a dominant negative manner, inhibits agonist-mediated receptor phosphorylation by approximately 40% in COS-7 and HEK-293 cells. Furthermore, we present evidence that a peptide corresponding to the third intracellular loop of the m3-muscarinic receptor (Ser(345)-Leu(463)) is an inhibitor of CK1alpha due to its ability to both act as a pseudo-substrate for CK1alpha and form a high affinity complex with CK1alpha. Expression of this peptide was able to reduce both basal and agonist-mediated m3-muscarinic receptor phosphorylation in intact cells. These results support the notion that CK1alpha is able to mediate GPCR phosphorylation in an agonist-dependent manner and that this may provide a novel mechanism for GPCR phosphorylation. The functional role of phosphorylation was investigated using a mutant of the m3-muscarinic receptor that showed an approximately 80% reduction in agonist-mediated phosphorylation. Surprisingly, this mutant underwent agonist-mediated desensitization suggesting that, unlike many GPCRs, desensitization of the m3-muscarinic receptor is not mediated by receptor phosphorylation. The inositol (1,4, 5)-trisphosphate response did, however, appear to be dramatically potentiated in the phosphorylation-deficient mutant indicating that phosphorylation may instead control the magnitude of the initial inositol phosphate response.

Role of third intracellular loop of galanin receptor type 1 in signal transduction

To determine the domains essential for G-protein coupling of the human galanin receptor type 1 (GalR1), we have used both GalR1 mutants and synthetic receptor-derived peptides in(125)I-galanin and [(35)S]-GTPgammaS binding studies. Replacement of potential phosphorylation sites by Leu in the third intracellular loop (IC3) of GalR1 did not affect K(D)values for the receptor. Peptides derived form the IC3 loop, and especially the N-terminal part of it were able to increase the rate of [(35)S]-GTPgammaS binding to the trimeric Gialpha1beta1gamma2, but not to Gsalphabeta1gamma2, whereas the peptides corresponding to the IC1 and IC2 loops had no such effect. IC3 loop peptides also inhibited the binding of(125)I-galanin to GalR1 in membranes from Rin m5F cells. Our results suggest that the IC3 loop of GalR1, especially its N-terminal part, defines the coupling of the receptor to the Gialpha1beta1gamma2 protein and consequently, to the signal transduction cascade.

Lack of a C-terminal tail in the mammalian gonadotropin-releasing hormone receptor confers resistance to agonist-dependent phosphorylation and rapid desensitization

The mammalian gonadotropin-releasing hormone receptor (GnRH-R) is, at present, the only G-protein-coupled receptor that activates phospholipase C and lacks a C-terminal tail. We have previously demonstrated that this unique structural feature is associated with resistance to rapid desensitization of phosphoinositide signaling in COS-7 and HEK-293 cells (Heding, A., Vrecl, M., Bogerd, J., McGregor, A., Sellar, R., Taylor, P. L., and Eidne, K. A. (1998) J. Biol. Chem. 273, 11472-11477). Using receptors tagged with a nonapeptide of the influenza hemagglutinin protein to enable immunoprecipitation, we now demonstrate that the mammalian GnRH-R is not phosphorylated in an agonist-dependent manner. In contrast, the mammalian thyrotropin-releasing hormone receptor and the African catfish GnRH-R, both of which have a C-terminal tail, are phosphorylated in response to agonist challenge. Furthermore, chimeras of the mammalian GnRH-R with the C-terminal tail of either the mammalian thyrotropin-releasing hormone receptor or the catfish GnRH-R are also phosphorylated in an agonist-dependent manner. Only those receptors having C-terminal tails showed desensitization of phosphoinositide responses within 5-10 min of agonist challenge. We also show that the internalization of all these receptors when expressed transiently in COS-7 cells is similar. This dissociates receptor internalization from rapid desensitization and demonstrates that the lack of a C-terminal tail in the mammalian GnRH-R results in an inability of the receptor to undergo agonist-dependent phosphorylation and that this results directly in a resistance to rapid desensitization.

The proximal portion of the COOH terminus of the oxytocin receptor is required for coupling to g(q), but not g(i). Independent mechanisms for elevating intracellular calcium concentrations from intracellular stores

As the oxytocin receptor plays a key role in parturition and lactation, there is considerable interest in defining its structure/functional relationships. We previously showed that the rat oxytocin receptor transfected into Chinese hamster ovary cells was coupled to both G(q/11) and G(i/o), and that oxytocin stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis via protein kinase C activity. In this study, we show that deletion of 51 amino acid residues from the carboxyl terminus resulted in reduced affinity for oxytocin and a corresponding rightward shift in the dose-response curve for oxytocin-stimulated [Ca(2+)](i). However, oxytocin-stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis did not occur in cells expressing the truncated receptor. Oxytocin also failed to increase phospholipase A activity or activate protein kinase C, indicating that the mutant receptor is uncoupled from G(q)-mediated pathways. The Delta51 receptor is coupled to G(i), as oxytocin-stimulated Ca(2+) transients were inhibited by pertussis toxin, and a Gbetagamma sequestrant. Preincubation of Delta51 cells with the tyrosine kinase inhibitor, genistein, also blocked the oxytocin effect. A Delta39 mutant had all the activities of the wild type oxytocin receptor. These results show that the portion between 39 and 51 residues from the COOH terminus of the rat oxytocin receptor is required for interaction with G(q/11), but not G(i/o). Furthermore, an increase in intracellular calcium was generated via a G(i)betagamma-tyrosine kinase pathway from intracellular stores that are distinct from G(q)-mediated inositol trisphosphate-regulated stores.

Analysis of domain responsible for desensitization of beta1-adrenergic receptor

When the wild type beta1-adrenergic receptor (WT-beta1AR) was expressed in Sf9 cells, the beta1AR-stimulated adenylyl cyclase activities were desensitized by prior treatment with isoproterenol. The extent of beta1AR desensitization was not modified, and the onset was not promoted by the overexpression of G protein-coupled receptor kinase 2 (GRK2), GRK5 or GRK6. However, overexpression of the dominant negative mutant of GRK2 appeared to inhibit desensitization of the beta1AR. The change of the potential protein kinase A phosphorylation site located at the intracellular third loop did not affect beta1AR desensitization. Desensitization of the truncated mutant, in which nearly all of the serine and threonine residues from the carboxyl terminus were eliminated, was the same as that of the WT-beta1AR. A deletion mutant that lacked serine and threonine residues of the intracellular third loop was also desensitized by isoproterenol stimulation. Furthermore, the deletion of serine and threonine residues from both the intracellular third loop and carboxyl terminus did not affect desensitization of the beta1AR. These results suggested that phosphorylation by endogenous GRKs in Sf9 cells contributed to desensitization of the beta1AR and that the regions other than third intracellular loop and carboxyl terminus may be responsible for beta1AR desensitization.

Calcium-dependent PAF-stimulated generation of reactive oxygen species in a human keratinocyte cell line

During inflammation and other pathological states, the lipid mediator platelet-activating factor (PAF) and reactive oxygen species (ROS) are both generated. We have been investigating the effect of exogenous PAF on ROS formation in the human keratinocyte cell line (HaCaT). ROS production, measured using luminol-enhanced chemiluminescence (CL), proved to be rapid, transient, PAF receptor-mediated, and totally dependent on an increase in intracellular Ca2+ ([Ca2+]i) and on the presence of extracellular Ca2+. Repeated administration of PAF resulted in refractoriness to the agonist in terms of both capacities to increase [Ca2+]i and generate ROS. The cells, however, continued to respond fully to other stimulants (bradykinin, epidermal growth factor, thapsigargin). The PAF-induced increases in [Ca2+]i (monitored using the fluorescent probe Fluo-3) were also rapid and transient and paralleled those of ROS generation. Relatively specific inhibitors of potential ROS-producing systems were administered in an attempt to characterize the ROS producing system(s). Inhibitors of xanthine oxidase, phospholipase A2, lipoxygenase, cyclooxygenase and NO synthase did not interfere with PAF evoked ROS. The flavoprotein inhibitor diphenyleneiodonium and the mitochondrial cytochrome oxidase inhibitor KCN, prevented generation of ROS, making NAD(P)H a candidate for the electron source of the ROS and the mitochondria a potential major site of formation.

G-protein coupled receptor kinases as modulators of G-protein signalling

G-protein coupled receptors (GPCRs) comprise one of the largest classes of signalling molecules. A wide diversity of activating ligands induce the active conformation of GPCRs and lead to signalling via heterotrimeric G-proteins and downstream effectors. In addition, a complex series of reactions participate in the 'turn-off' of GPCRs in both physiological and pharmacological settings. Some key players in the inactivation or 'desensitization' of GPCRs have been identified, whereas others remain the target of ongoing studies. G-protein coupled receptor kinases (GRKs) specifically phosphorylate activated GPCRs and initiate homologous desensitization. Uncoupling proteins, such as members of the arrestin family, bind to the phosphorylated and activated GPCRs and cause desensitization by precluding further interactions of the GPCRs and G-proteins. Adaptor proteins, including arrestins, and endocytic machinery participate in the internalization of GPCRs away from their normal signalling milieu. In this review we discuss the roles of these regulatory molecules as modulators of GPCR signalling.

Critical duration of intracellular Ca2+ response required for continuous translocation and activation of cytosolic phospholipase A2

When cells are exposed to certain external stimuli, arachidonic acid (AA) is released from the membrane and serves as a precursor of various types of eicosanoids. A Ca2+-regulated cytosolic phospholipase A2 (cPLA2) plays a dominant role in the release of AA. To closely examine the relation between Ca2+ response and AA release by stimulation of G protein-coupled receptors, we established several lines of Chinese hamster ovary cells expressing platelet-activating factor receptor or leukotriene B4 receptor. Measurement of intracellular Ca2+ concentration ([Ca2+]i) demonstrated that cell lines capable of releasing AA elicited a sustained [Ca2+]i increase when stimulated by agonists. The prolonged [Ca2+]i elevation is the result of Ca2+ entry, because this elevation was blocked by EGTA treatment or in the presence of Ca2+ channel blockers (SKF 96365 and methoxyverapamil). cPLA2 fused with a green fluorescent protein (cPLA2-GFP) translocated from the cytosol to the perinuclear region in response to increases in [Ca2+]i. When EGTA was added shortly after [Ca2+]i increase, the cPLA2-GFP returned to the cytosol, without liberating AA. After a prolonged [Ca2+]i increase, even by EGTA treatment, the enzyme was not readily redistributed to the cytosol. Thus, we propose that a critical time length of [Ca2+]i elevation is required for continuous membrane localization and full activation of cPLA2.

Replacement of threonine 394 by alanine facilitates internalization and resensitization of the rat mu opioid receptor

Signaling of G protein-coupled receptors is terminated by phosphorylation of intracellular serine and threonine residues. Resensitization of these receptors requires internalization and subsequent dephosphorylation. We have recently shown that the resensitization rate of the rat micro opioid receptor (MOR) isoforms MOR1 and MOR1B is mainly determined by the amino acid composition of their alternatively spliced C-terminal tails. Upon agonist stimulation, MOR1B passes through an accelerated cycle of receptor endocytosis and reactivation, which in turn promotes a greater resistance to agonist-induced desensitization, as compared with MOR1. Given the fact that MOR1B lacks only one putative phosphorylation site (T394 of MOR1), we replaced this threonine by an alanine and stably expressed the wild-type MOR1 and its T394A mutant in mouse neuroblastoma Neuro2a cells. We show that during prolonged [D-Ala2, MePhe4, Gly5-ol]enkephalin exposure (5 h), the T394A receptor mutant desensitized at a slower rate than MOR1. In contrast, T394A is more rapidly removed from the cell surface than MOR1, as determined by flow cytometry using epitope-tagged receptors. This fast internalization was followed by immediate resensitization of T394A during 20 min of agonist removal while the wild-type MOR1 remained inactive. Similar to MOR1B, rapid internalization and reactivation of T394A may explain its delayed desensitization. These findings suggest that T394 represents a negative regulatory signal for MOR1 internalization. Furthermore, phosphorylation of this threonine residue may influence the time course of micro opioid receptor resensitization.

Conditional expression of the dual-specificity phosphatase PYST1/MKP-3 inhibits phosphorylation of cytosolic phospholipase A2 in Chinese hamster ovary cells

Stimulation of platelet-activating factor (PAF) receptor induces activation of extracellular signal-regulated kinase (ERK) and cytosolic phospholipase A2 (cPLA2) and release of arachidonic acid in Chinese hamster ovary cells. To determine whether the dual-specificity protein phosphatase PYST1/MKP-3 inhibits phosphorylation of cPLA2, we have generated a cell line that conditionally expresses PYST1 under the control of a tetracycline-regulated inducible system. We found that induction of PYST1 suppressed phosphorylation and activation of cPLA2 as well as ERK. Arachidonic acid release was also reduced by about 30%. Pretreatment of cells with an MEK inhibitor, PD98059, had similar effects on PAF-induced cPLA2 phosphorylation and arachidonic acid release. These experiments demonstrate that expression of PYST1 prevents phosphorylation of a cytoplasmic substrate for ERK. Thus, this inducible system may offer a valuable means of investigating physiological roles of ERK in vivo.

Inhibition of Salmonella typhimurium invasion by host cell expression of secreted bacterial invasion proteins

Pathogenic Salmonella species initiate infection of a host by inducing their own uptake into intestinal epithelial cells. An invasive phenotype is conferred to this pathogen by a number of proteins that are components of a type III secretion system. During the invasion process, the bacteria utilize this secretion system to release proteins that enter the host cell and apparently interact with unknown host cell components that induce alterations in the actin cytoskeleton. To investigate the role of secreted proteins as direct modulators of invasion, we have evaluated the ability of Salmonella typhimurium to enter mammalian cells that express portions of the Salmonella invasion proteins SipB and SipC. Plasma membrane localization of SipB and SipC was achieved by fusing carboxyl- and amino-terminal portions of each invasion protein to the intracellular carboxyl-terminal tail of a membrane-bound eukaryotic receptor. Expression of receptor chimeras possessing the carboxyl terminus of SipB or the amino terminus of SipC blocked Salmonella invasion, whereas expression of their chimeric counterparts had no effect on invasion. The effect on invasion was specific for Salmonella since the perturbation of uptake was not extended to other invasive bacterial species. These results suggest that Salmonella invasion can be competitively inhibited by preventing the intracellular effects of SipB or SipC. In addition, these experiments provide a model for examining interactions between bacterial invasion proteins and their host cell targets.

Role of C-terminal serines in desensitization and phosphorylation of the mouse thromboxane receptor

To investigate the role of C-terminal hydroxyamino acids in desensitization of the receptor for thromboxane A2 (TxA2), we created a mutant TxA2 receptor (TP receptor) in which serines at positions 321, 322, and 328 were replaced with either alanine or glycine. Mutant and wild type receptors were expressed in a mesangial cell line, and clones expressing similar numbers of receptors were studied. Affinity and specificity of TxA2 binding to the mutant receptor were identical to wild type receptors. In contrast, TxA2-induced inositol trisphosphate generation by the mutant receptor was enhanced compared with the wild type. Prior treatment with the TxA2 agonist U46619 reduced subsequent U46619-induced increases in inositol trisphosphate generation by both receptors; however, the extent of desensitization was significantly reduced in the receptor mutant. Protein kinase C (PKC) inhibitors attenuated TxA2-induced desensitization of wild type receptors, but had little effect on TxA2-induced desensitization of mutant receptors. Pretreatment with the phorbol ester phorbol 12, 13-dybutyrate (PDBu) (100 nM) decreased subsequent responsiveness of wild type but not mutant TP receptors. -induced desensitization of wild type receptors was associated with enhanced phosphorylation of receptor proteins. This agonist-specific phosphorylation of the TP receptor was largely prevented by inhibitors of PKC. Treatment with 100 nM PDBu increased phosphorylation of both wild type and mutant TP receptors, but the extent of phosphorylation of the receptor mutant was reduced compared with the wild type. Increasing the concentration of PDBu from 100 nM to 1 microM PDBu reduced responsiveness of both mutant and wild type receptors without enhancing phosphorylation of either of the receptor proteins. These data suggest that 1) phosphorylation of C-terminal serines contributes to agonist-specific desensitization of the TP receptor, 2) PKC-induced desensitization of TP receptors is caused, in part, by phosphorylation of C-terminal serines, and 3) desensitization of TP receptors by PKC is complex and involves mechanisms that may not require direct phosphorylation of receptor proteins.

G protein-coupled receptor kinases

G protein-coupled receptor kinases (GRKs) constitute a family of six mammalian serine/threonine protein kinases that phosphorylate agonist-bound, or activated, G protein-coupled receptors (GPCRs) as their primary substrates. GRK-mediated receptor phosphorylation rapidly initiates profound impairment of receptor signaling, or desensitization. This review focuses on the regulation of GRK activity by a variety of allosteric and other factors: agonist-stimulated GPCRs, beta gamma subunits of heterotrimeric GTP-binding proteins, phospholipid cofactors, the calcium-binding proteins calmodulin and recoverin, posttranslational isoprenylation and palmitoylation, autophosphorylation, and protein kinase C-mediated GRK phosphorylation. Studies employing recombinant, purified proteins, cell culture, and transgenic animal models attest to the general importance of GRKs in regulating a vast array of GPCRs both in vitro and in vivo.

Activation of the Epidermal Platelet-Activating Factor Receptor Results in Cytokine and Cyclooxygenase-2 Biosynthesis

Recent studies suggest that the lipid mediator platelet-activating factor (PAF) is involved in keratinocyte function and skin inflammation. Indeed, PAF is found in association with inflammatory skin diseases, intradermal injections of PAF induce inflammation, and keratinocytes express functional PAF receptors (PAF-R). One mechanism by which the keratinocyte PAF-R could contribute to epidermal functions and inflammatory states would be through the synthesis of inflammatory regulators, such as PAF, PGs, and cytokines. The ability of the epidermal PAF-R to induce the synthesis of these immunomodulators was tested using a model system created by transduction of the PAF-R-negative human epidermal cell line KB with the PAF-R. Activation of this epidermal PAF-R resulted in arachidonic acid release, and the biosynthesis of PAF and PGE2. In addition, the KB PAF-R triggered increased levels of mRNA and protein for the inducible isozyme of cyclooxygenase (COX-2) as well as IL-6 and IL-8, both of which have been implicated in skin inflammatory processes. Studies with the human keratinocyte-derived epidermal cell line HaCaT revealed that activation of the endogenous PAF-R led to the increased accumulation of COX-2, IL-6, and IL-8 mRNA similar to that seen with the KB PAF-R model system. Finally, treatment of HaCaT keratinocytes with IL-8 resulted in PAF biosynthesis, indicating the existence of a positive feedback loop between IL-8 and PAF in epidermal cells. These studies suggest involvement of PAF and the PAF-R in the epidermal cytokine network.

Identification and pharmacological characterization of platelet-activating factor and related 1-palmitoyl species in human inflammatory blistering diseases

Through its pro-inflammatory effects on leukocytes, endothelial cells, and keratinocytes, the lipid mediator platelet-activating factor (PAF) has been implicated in cutaneous inflammation. Although the 1-alkyl PAF species has been considered historically the most abundant and important ligand for the PAF receptor (PAF-R), other putative ligands for this receptor have been described including 1-acyl analogs of sn-2 acetyl glycerophosphocholines. Previous bioassays have demonstrated a PAF-like activity in lesions of the autoimmune blistering disease bullous pemphigoid. To assess the actual sn-2 acetyl glycerophosphocholine species that result in this PAF agonistic activity, we measured PAF and related sn-2 acetyl GPCs in fresh blister fluid samples from bullous pemphigoid and noninflammatory (suction-induced) bullae by mass spectrometry. We report the presence of 1-hexadecyl as well as the 1-acyl PAF analog 1-palmitoyl-2-acetyl glycerophosphocholine (PAPC) in inflammatory blister fluid samples. Because PAPC is the most abundant sn-2 acetyl glycerophosphocholine species found in all samples examined, the pharmacological effects of this species with respect to the PAF-R were determined using a model system created by transduction of a PAF-R-negative epidermoid cell line with the PAF-R. Radioligand binding and intracellular calcium mobilization studies indicated that PAPC is approximately 100x less potent than PAF. Though a weak agonist, PAPC could induce PAF biosynthesis and PAF-R desensitization. Finally, intradermal injections of PAF and PAPC into the ventral ears of rats demonstrated that PAPC was 100x less potent in vivo. These studies suggest possible involvement of PAF and related species in inflammatory bullous diseases.

Dose-dependent agonist and antagonist effects of the platelet-activating factor analogue 1-palmitoyl-2-acetoyl-sn-glycero-3-phosphocholine on B lymphocytes

BACKGROUND

Platelet activating-factor (PAF), an ether-linked phospholipid, is a potent activator of B lymphocyte cell lines. The related ester-linked phospholipid, 1-palmitoyl-2-acetoyl-sn-glycero-3-phosphocholine (PAGPC), is synthesized by tissues important in B-cell development.

OBJECTIVES

We examined whether PAGPC was capable of influencing immunoglobulin synthesis in B lymphocytes and compared its action with that of PAF. We also examined the interaction of the two mediators as agonists or competitive antagonists.

METHODS

Ramos, an IgM-secreting immature B-cell line that expresses PAF receptor, was used in these experiments. The effect of PAF, PAGPC, or both mediators together on IgM secretion and anti-IgM-mediated apoptosis was measured.

RESULTS

Both PAF and PAGPC stimulated IgM production in Ramos cells in a dose-dependent fashion, with PAGPC being approximately three logs less potent than PAF. The effect of both mediators was inhibited by specific PAF receptor antagonists. Preincubation with suboptimal concentrations of PAGPC inhibited the ability of PAF to increase IgM secretion by Ramos cells. Additionally, preincubation with low concentrations of PAGPC prevented PAF from rescuing Ramos cells from apoptosis induced by cross-linking the B-cell receptor with anti-IgM antibodies. PAGPC caused PAF receptor desensitization because displacement of bound PAGPC with high concentrations of bovine serum albumin did not reverse its PAF antagonist effect.

CONCLUSIONS

PAF and PAGPC are biologically active phospholipids, but PAF is approximately 1000 times more potent. At high concentrations, PAGPC acts similarly to PAF, whereas at lower concentrations, PAGPC acts as a functional PAF antagonist. Because it is secreted at sites of inflammation and allergic reactions, PAGPC may be an endogenous regulator of the effects of PAF.

The role of receptor kinases and arrestins in G protein-coupled receptor regulation

G protein-coupled receptors (GPRs) play a key role in controlling hormonal regulation of numerous second-messenger pathways. However, following agonist activation, most GPRs rapidly lose their ability to respond to hormone. For many GPRs, this process, commonly referred to as desensitization, appears to be primarily mediated by two protein families: G protein-coupled receptor kinases (GRKs) and arrestins. GRKs specifically bind to the agonist-occupied receptor, thereby promoting receptor phosphorylation, which in turn leads to arrestin binding. Arrestin binding precludes receptor/G protein interaction leading to functional desensitization. Many GPRs are then removed from the plasma membrane via clathrin-mediated endocytosis. Recent studies have implicated endocytosis in the resensitization of GPRs and have linked both GRKs and arrestins to this process. In this review, we discuss the role of GRKs and arrestins in regulating agonist-specific signaling and trafficking of GPRs.

Signal transduction, desensitization, and recovery of responses to thyrotropin-releasing hormone after inhibition of receptor internalization

Three independent methods were used to block internalization of the TRH receptor: cells were infected with vaccinia virus encoding a dominant negative dynamin, incubated in hypertonic sucrose, or stably transfected with a receptor lacking the C-terminal tail. Internalization was blocked in all three paradigms as judged by microscopy using a fluorescently labeled TRH agonist and biochemically. The initial inositol trisphosphate (IP3) and Ca2+ responses to TRH were normal when internalization was inhibited. The IP3 increase was sustained rather than transient, however, in cells expressing the truncated TRH receptor, implying that the C-terminal tail of the receptor may be important for uncoupling from phospholipase C. After withdrawal of TRH, cells were refractory to TRH until both ligand dissociation and resensitization of the receptor had occurred. When surface-bound TRH was removed by a mild acid wash, which did not impair receptor function, neither wild-type nor truncated receptors were able to generate full IP3 responses for about 10 min. The rate of recovery was not altered by blocking internalization. Recovery of intracellular Ca2+ responses also depended on the rate of Ca2+ pool refilling. In summary, in the continued presence of TRH, phospholipase C activity declines quickly due to receptor uncoupling; this desensitization does not take place for the truncated receptor. After TRH is withdrawn, cells are refractory to TRH. Before cells can respond, TRH must dissociate and a resensitization step, which takes place on the plasma membrane and does not require the C-terminal tail of the receptor, must occur.

Agonist-induced sequestration, recycling, and resensitization of platelet-activating factor receptor. Role of cytoplasmic tail phosphorylation in each process

Agonist-induced sequestration, recycling, and resensitization of platelet-activating factor (PAF) receptor were characterized in transfected Chinese hamster ovary cells. Exposure of the cells to PAF led to rapid sequestration of the receptors into the intracellular compartment and desensitization of the response to PAF. The sequestration was inhibited by pretreatments that perturbed the clathrin-mediated pathway. Subsequent removal of PAF by washing with receptor antagonists led to rapid recycling of the sequestered receptors to the cell surface accompanied by resensitization to PAF. To evaluate the potential role of phosphorylation in the receptor cytoplasmic tail during these processes, mutant receptors in which the tails were truncated or substituted, so as to lack serine/threonine residues, were created. PAF phosphorylated the wild-type receptor rapidly and strongly, but the mutants did not. The maximal extent of sequestration of each mutant was lower than that of the wild-type, and one of the substituted mutants showed no sequestration. Furthermore, the sequestration-defective mutant showed evidence of desensitization after agonist stimulation but not resensitization after agonist removal. Thus, agonist-induced phosphorylation of the cytoplasmic tail facilitates but is not essential for receptor sequestration, and sequestration/recycling appears important in receptor resensitization.

Mutation of a putative amphipathic alpha-helix in the third intracellular domain of the platelet-activating factor receptor disrupts receptor/G protein coupling and signaling

Platelet-activating factor (PAF) is a potent phospholipid mediator that interacts with G protein-coupled PAF receptors to elicit diverse physiological and pathophysiological actions. We recently demonstrated that the third intracellular domain of the rat PAF receptor (rPAFR) is a critical determinant in its coupling to phosphoinositide phospholipase C-activating G proteins. Here, we report identification of a putative amphipathic helix in the third intracellular domain of the rPAFR and the effects of mutational disruption of its amphipathic character on G protein coupling of and signaling by the rPAFR. Modeling of the third intracellular domain and adjacent transmembrane regions of the rPAFR identified a single amphipathic helix located in the amino-terminal region of the third intracellular domain of the receptor. Baby hamster kidney cells were transiently transfected with cDNAs encoding the rPAFR or rPAFR mutants in which nonconserved substitutions were made separately in the hydrophobic or polar face of this amphipathic helix. The number and affinity of binding sites for specific PAF receptor antagonist WEB2086 were identical in membranes prepared from rPAFR and amphipathic helix mutant PAFR transfectants. However, only membranes derived from rPAFR transfectants possessed high affinity PAF binding sites that were sensitive to the G protein-uncoupling effects of guanosine-5'-O-(3-thio)triphosphate. These results show that substitutions into either face of the amphipathic helical domain abolished the ability of the rPAFR to undergo coupling to G proteins to form a high affinity agonist/receptor/G protein ternary complex. To examine the effects of these mutations on rPAFR signaling, PAF-stimulated inositol phosphate accumulation was determined in cells transfected with cDNAs encoding the wild-type or amphipathic helix mutant PAFRs. Although PAF stimulated 10-fold increases in inositol phosphate accumulation in rPAFR transfectants, it had no effects on inositol phosphate accumulation in amphipathic helix mutant PAFR transfectants. These results suggest that an amphipathic helix located in the amino-terminal region of the third intracellular domain of the rPAFR is required for its coupling to and activation of G proteins. This study provides the first insight into the structure of the receptor interface for G protein coupling of a PAFR and suggests a conserved role of amphipathic helices in G protein coupling of receptors ranging from those for biogenic amines to the phospholipid mediator PAF.

Phagocyte chemoattractant receptors

Myeloid cells are attracted and activated by a variety of chemoattractants that bind to G protein-coupled receptors. In the past few years, the receptors for the classical chemoattractants (fMLF, C5a, PAF) and the chemotactic cytokines, known as C-X-C and C-C chemokines, have been cloned from myeloid cells. This review briefly describes recent advances in structure-function relationships of chemotactic receptors in human leukocytes as well as activation of signaling pathways and regulation of receptor function. In neutrophils, the binding of chemoattractants mainly activates the Gi2 protein inducing PIP2 hydrolysis and activation of the MAP kinase pathway. The C-C chemokine receptor, CC CKR5, and a chemokine receptor homologue, named fusin, have been shown to be the major cofactors for HIV-1 entry in macrophages and T cells. Recent studies suggest that the phosphorylation of chemoattractant receptors is a key event that regulates their biological function.

Regulation of human chemokine receptors CXCR4. Role of phosphorylation in desensitization and internalization

Members of the chemokine receptor family CCR5 and CXCR4 have recently been shown to be involved in the entry of human immunodeficiency virus (HIV) into target cells. Here, we investigated the regulation of CXCR4 in rat basophilic leukemia cells (RBL-2H3) stably transfected with wild type (Wt CXCR4) or a cytoplasmic tail deletion mutant (DeltaCyto CXCR4) of CXCR4. The ligand, stromal cell derived factor-1 (SDF-1) stimulated higher G-protein activation, inositol phosphate generation, and a more sustained calcium elevation in cells expressing DeltaCyto CXCR4 relative to Wt CXCR4. SDF-1 and phorbol 12-myristate 13-acetate (PMA), but not a membrane permeable cAMP analog induced rapid phosphorylation as well as desensitization of Wt CXCR4. Phosphorylation of DeltaCyto CXCR4 was not detected under any of these conditions. Despite lack of receptor phosphorylation, calcium mobilization by SDF-1 in DeltaCyto CXCR4 cells was partially desensitized by prior treatment with SDF-1. Of interest, the rapid release of calcium was inhibited without affecting the sustained calcium elevation, indicating independent regulatory pathways for these processes. PMA completely inhibited phosphoinositide hydrolysis and calcium mobilization in Wt CXCR4 but only partially inhibited these responses in DeltaCyto CXCR4. cAMP also partially inhibited these responses in both Wt CXCR4 and DeltaCyto CXCR4. SDF-1, PMA, and cAMP caused phosphorylation of phospholipase Cbeta3 in Wt and DeltaCyto CXCR4 cells. Both SDF-1 as well as PMA induced rapid internalization of Wt CXCR4. SDF-1 but not PMA induced internalization of DeltaCyto CXCR4 albeit at reduced levels relative to Wt CXCR4. These results indicate that signaling and internalization of CXCR4 are regulated by receptor phosphorylation dependent and independent mechanisms. Desensitization of CXCR4 signaling, independent of receptor phosphorylation, appears to be a consequence of the phosphorylation of phospholipase Cbeta3.

Agonist-induced desensitization of the mu opioid receptor is determined by threonine 394 preceded by acidic amino acids in the COOH-terminal tail

To identify the structural determinants necessary for mu opioid receptor desensitization, we serially ablated potential phosphorylation sites in the carboxyl tail of the receptor and examined their effects on [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin (DAMGO)-induced desensitization. First, we replaced Thr394 with alanine (T394A) and stably expressed this mutant receptor in Chinese hamster ovary cells. The T394A receptor did not desensitize after 1 h of treatment with DAMGO, indicating that Thr394 is required for agonist-induced early desensitization. To test whether Thr394 was the only residue necessary, we investigated the importance of 7 potential phosphorylation sites between residues 363 and 383, which were all replaced by alanines with the Thr394 maintained. This mutant (AT) showed partial loss of desensitization (30%), which was attributable to the Ala mutation at Thr383, since complete desensitization was achieved by restoring Thr383 (ATT). These results suggest that Thr394 is the primary recognition site for G protein-coupled receptor kinases, but Thr383 is also required for complete agonist-induced desensitization. The specificity of Thr394 as the primary initiation site appears to be dependent on the preceding acidic amino acid stretch, because in a mutant in which glutamic acid residues at 388, 391, and 393 were replaced by glutamines (EQ), agonist-induced desensitization was completely abolished, identical to the T394A mutant.

Structural and functional requirements for agonist-induced internalization of the human platelet-activating factor receptor

The receptor for platelet-activating factor (PAF) is a member of the G-protein-coupled receptor family. To study the structural elements and mechanisms involved in the internalization of human PAF receptor (hPAFR), we used the following mutants: a truncated mutant in the C-terminal tail of the receptor (Cys317 --> Stop) and mutations in the (D/N)P(X)2,3Y motif (Asp289 --> Asn,Ala and Tyr293 --> Phe,Ala). Chinese hamster ovary cells expressing the Cys317 --> Stop mutant exhibited a marked reduction in their capacity to internalize PAF, suggesting the existence of determinants important for endocytosis in the last 26 amino acids of the cytoplasmic tail. Substitution of Asp289 to alanine abolished both internalization and G-protein coupling, whereas substitution of Tyr293 to alanine abolished coupling but not internalization. Inhibition or activation of protein kinase C did not significantly affect the internalization process. Receptor sequestration and ligand uptake was, at least in part, blocked by concanavalin A and blockers of endocytosis mediated by clathrin-coated pits. Our data suggest that the internalization of a G-protein-coupled receptor and coupling to a G-protein can be two independent events. Moreover, the C terminus tail of hPAFR, but not the putative internalization motifs, may be involved in the internalization of hPAFR.

Role of the C terminus in histamine H2 receptor signaling, desensitization, and agonist-induced internalization

To evaluate the role of the histamine H2 receptor C terminus in signaling, desensitization, and agonist-induced internalization, canine H2 receptors with truncated C termini were generated. Wild-type (WT) and truncated receptors were tagged at their N termini with a hemagglutinin (HA) epitope and expressed in COS7 cells. Most of the C-terminal intracellular tail could be truncated (51 of 70 residues, termed T308 mutant) without loss of functions: cAMP production, tiotidine binding, and plasma membrane targeting. In fact, the T308 mutant produced more cAMP than the WT when cell-surface expression per cell was equivalent. Pretreatment of cells with 10(-5) M histamine desensitized cAMP productions via WT and T308 receptors to similar extents. Incubation of cells expressing WT receptors with 10(-5) M histamine reduced cell-surface anti-HA antibody binding by approximately 30% (by 30 min, t1/2 approximately 15 min), but did not affect the Bmax of tiotidine in membrane fractions, which represents total receptor amounts, suggesting that WT receptors were internalized from the cell surface. In contrast, no internalization was observed with T308 receptors following histamine treatment. A mutant with a deletion of the 30 C-terminal amino acids, termed T329, was functional but was as potent as the WT in terms of cAMP production. Apart from being desensitized by histamine, the internalization of the receptor was indistinguishable from that of the WT. Internalization was observed in the T320 but not in T313 mutant, narrowing the region involved in internalization to that between Glu314 and Asn320 (ETSLRSN). Of these seven residues, either Thr315, Ser316, or both, were replaced with Ala. Thr315 and Ser316 are conserved among species. The mutation at Thr315 (but not that at Ser316) abolished internalization. Taken together, these results demonstrate that Thr315 is involved in agonist-induced internalization. Furthermore, the finding that T308 receptors were desensitized in the absence of internalization suggests that internalization and desensitization are meditated by independent mechanisms.

Role of phospholipase Cbeta3 phosphorylation in the desensitization of cellular responses to platelet-activating factor

Platelet-activating factor (PAF) stimulates a diverse array of cellular responses through receptors coupled to G proteins that activate phospholipase C (PLC). Truncation of the cytoplasmic tail of the receptor to remove phosphorylation sites (mutant PAF receptor, mPAFR) results in enhancement of PAF-stimulated responses. Here we demonstrate that PAF or phorbol 12-myristate 13-acetate (PMA) pretreatment inhibited wild type PAFR-induced PLC-mediated responses by approximately 90%, whereas these responses to the phosphorylation-deficient mPAFR were inhibited by approximately 50%, despite normal G protein coupling, suggesting a distal inhibitory locus. PAF and PMA, as well as a membrane permeable cyclic AMP analog, stimulated phosphorylation of PLCbeta3. A protein kinase C (PKC) inhibitor blocked phosphorylation of PLCbeta3 stimulated by PAF and PMA but not by cAMP. Activation of protein kinase A (PKA) by cAMP did not result in inhibition of Ca2+ mobilization stimulated by PAF. In contrast, cAMP did inhibit the response to formylpeptide chemoattractant receptor. These data suggest that homologous desensitization of PAF-mediated responses is regulated via phosphorylation at two levels in the signaling pathway, one at the receptor and the other at PLCbeta3 mediated by PKC but not by PKA. Phosphorylation of PLCbeta3 by PKA could explain the inhibition of formylpeptide chemoattractant receptor signaling by cAMP. As PAF and formylpeptide chemoattractant receptors activate PLC via different G proteins, phosphorylation of PLCbeta3 by PKC and PKA could provide distinct regulatory control for classes of G protein-coupled receptors.

Expression of the PAF receptor in human monocyte-derived macrophages is downregulated by oxidized LDL: relevance to the inflammatory phase of atherogenesis

Human monocyte-derived macrophages play a major role in the initiation and progression of atherosclerotic lesions as a result of the production of a wide spectrum of proinflammatory and prothrombotic factors. Among such factors is a potent inflammatory phospholipid, platelet-activating factor (PAF), which is produced after macrophage activation. Because the cells involved in PAF biosynthesis are typically targets for the bioactions of PAF via specific cell surface receptors, we evaluated the expression of the PAF receptor in human monocyte-derived macrophages. Oxidized LDL (oxLDL) exerts multiple cellular effects that enhance lesion progression; we therefore investigated the potential modulation of expression of the macrophage PAF receptor by oxLDL. [3H]PAF bound to adherent human macrophages with a K(d) of 2.1 nmol/L and a B(max) of 19 fmol/10(6) cells; approximately 5300 binding sites per cell were detected. OxLDL (100 microg protein per milliliter) induced a twofold decrease in cellular PAF binding after 3 hours at 37 degrees C. Analysis of macrophage mRNA by reverse transcription-polymerase chain reaction (RT-PCR) revealed two forms corresponding to the PAF receptor, of which the leukocyte type (type 1 promoter) predominated. Expression of PAF receptor mRNA, evaluated by quantitative RT-PCR using an actin or a GAPDH mimic, was progressively reduced (up to 70%) by oxLDL up to 6 hours and remained low for at least 24 hours. Such downregulation was reversible after incubation of the cells for 24 hours in oxLDL-free medium. Addition of forskolin (3 micromol/L) or dibutyryl cAMP (1 mmol/L) to macrophage cultures reproduced the oxLDL-mediated inhibition of PAF receptor expression; carbamyl PAF reduced PAF binding and PAF mRNA to a similar degree (approximately 50%). These data demonstrate that atherogenic oxLDL downregulates the expression of both cellular PAF receptors and PAF receptor mRNA in macrophages, consistent with both a diminished bioresponse to PAF and decreased cell motility. Such diminished bioresponse to a powerful antacoid reflects the suppression of an acute inflammatory reaction, thereby leading to chronic, low-level inflammation, such as that characteristic of fatty streaks and more advanced atherosclerotic plaques.

The long cytoplasmic carboxyl terminus of the prostaglandin E2 receptor EP4 subtype is essential for agonist-induced desensitization

The 488-amino acid human prostaglandin E2 receptor EP4 subtype, which couples to stimulation of adenylyl cyclase, shares the major structural features of G protein-coupled receptors, having seven putative transmembrane domains, an extracellular amino terminus, and a cytoplasmic carboxyl terminus. The latter is composed of 156 amino acids and contains 38 serine and threonine residues, which are potential phosphorylation sites. The carboxyl terminus may be important in receptor function; in some receptors, truncation of the cytoplasmic tail abolishes desensitization. In others, truncation leads to constitutive activity, and in other receptors, truncation has no effect on receptor function. To investigate the role of the long cytoplasmic tail of the EP4 receptor, we constructed a mutant EP4 that lacks the last 138 amino acids at the carboxyl terminus, including 36 serine and threonine residues. The truncated EP4 receptor was stably expressed in Chinese hamster ovary cells at levels comparable to that of the wild-type receptor and exhibited a Kd value for [3H]PGE2 binding similar to that of the wild-type receptor. PGE2-mediated adenylyl cyclase activity as a function of PGE2 concentration was similar in cells expressing the wild-type and truncated EP4 receptors. Neither the wild-type receptor nor the truncated form showed any constitutive activity. However, the wild-type EP4 receptor underwent PGE2-induced desensitization fully within 15-20 min, whereas the truncated EP4 receptor, lacking 36 of the 38 carboxyl-terminal serines and threonines, displayed a sustained activation. Despite the continuous presence of PGE2, the rate of cAMP synthesis via stimulation of the truncated receptor remained constant over > or = 20 min. These findings suggest that the cytoplasmic tail of EP4 plays an important role in agonist-induced desensitization.

Functional consequences of the phosphorylation of the gonadotropin receptors

When target cells are exposed to a hormone, their responsiveness wanes with time, in spite of the continuous presence of the hormone. This phenomenon, referred to as desensitization, is due to regulatory steps that occur at the level of the hormone receptor as well as at post-receptor steps. While post-receptor events may be specific for the metabolic pathways activated in a given target cell, receptor events are more general in nature and conserved within a given family of receptors. There are at least two categories of regulatory events that occur at the level of the receptor and contribute to the process of desensitization. One of them, referred to as receptor uncoupling, will be used here to denote a change in the functional properties of a constant number of receptors resulting in a reduction in the ability of the receptor to activate its most proximal effector system. The other, referred to as receptor down-regulation, will be used here to denote a reduction in the density of cell surface receptors. Uncoupling is generally faster than down-regulation, and is believed to be due to post-translational modifications of the receptor. Down-regulation, on the other hand, is slower, and could be due to an increase in the rate of receptor internalization, sequestration or degradation, and/or to a decrease in the rate of receptor externalization, processing or synthesis. In this paper, recent studies from my laboratory designed to directly address the potential involvement of gonadotropin receptor phosphorylation in the process of uncoupling have been reviewed.

Effect of administered ethanol on protein kinase C in human platelets

There are numerous reports of the effect of ethanol on protein kinase C (PKC) in animals or with in vitro systems. However, the effect of ethanol on PKC in humans has not been extensively investigated despite the large number of studies involving PKC and human platelets. In this study, we administered ethanol to human volunteers and determined the level of PKC before and after a 0.4 g/kg dose of ethanol. We studied Native Americans and Caucasians of both sexes. There was an increases in PKC activity 60 min after ethanol administration. There were no ethnic, age, nor gender differences detected, nor was there any correlation between family history of alcoholism and the basal or stimulated platelet PKC levels. Neither was there any correlation of basal or stimulated PKC activity with the genotypes for ADH2, ADH3, ALDH2, CYP2E1, and CYP1A2.

Cross-desensitization among receptors for platelet activating factor and peptide chemoattractants. Evidence for independent regulatory pathways

Cross-desensitization among receptors for peptide chemoattractants have been shown to involve two independent processes, receptor phosphorylation and inhibition of phospholipase C (PLC) activation. Receptors for lipid chemoattractants, i.e. platelet activating factor (PAF) and leukotriene B4, did not inhibit the responses of peptide chemoattractant receptors, suggesting distinct signaling pathways. To examine cross-desensitization between receptors for lipid and peptide chemoattractants, cDNA encoding the PAF receptor (PAFR) was co-expressed into RBL-2H3 cells with cDNAs encoding receptors for either formylated peptides (FR), a product of the fifth component of complement (C5aR) or interleukin-8 A (IL-8RA). PAFR was homologously phosphorylated and desensitized by PAF, and cross-phosphorylated and cross-desensitized by fMet-Leu-Phe, C5a, and IL-8. In contrast, the receptors for peptide chemoattractants were neither cross-phosphorylated nor cross-desensitized by PAF. Staurosporine blocked cross-phosphorylation and cross-desensitization of the PAFR by peptide chemoattractants. Truncation of the cytoplasmic tail of PAFR (mPAFR) abolished its homologous and cross-phosphorylation. mPAFR was also resistant to cross-desensitization by peptide chemoattractants at the level of PLC activation. Interestingly, mPAFR mediated a sustained Ca2+ mobilization in response to PAF and was more active in inducing GTPase activity, phosphoinositide hydrolysis, secretion, and phospholipase D activation than the wild type PAFR. In contrast to PAFR, stimulation of the mPAFR cross-phosphorylated and cross-desensitized responses to IL-8RA. As expected, FR, which is resistant to cross-phosphorylation by C5aR and IL-8RA, was not phosphorylated by mPAFR. However, unlike C5aR and IL-8RA, mPAFR did not inhibit the ability of FR to activate PLC. Blocking Ca2+ influx inhibited mPAFR-mediated sustained Ca2+ response, phospholipase D activation and secretion, but not phosphoinositide hydrolysis and cross-phosphorylation and cross-desensitization of IL-8RA. The data herein suggest that cross-desensitization of PAFR by peptide chemoattractants is solely due to receptor phosphorylation. The PAFR and the peptide chemoattractant receptors do not cross-regulate each other at the level of PLC, suggesting distinct regulatory pathways.