Investigation of the mechanisms of G protein: effector coupling by the human and mouse prostacyclin receptors. Identification of critical species-dependent differences

Inhibition of TRPM8 function by prostacyclin receptor agonists requires coupling to Gq/11 proteins

BACKGROUND AND PURPOSE

The TRPM8 ion channel is involved in innocuous cold sensing and has a potent anti-inflammatory action. Its activation by lower temperature or chemical agonists such as menthol and icilin induces analgesic effects, reversing hypersensitivity and reducing chronic pain. On the other hand, prostacyclin (PGI2) enhances pain and inflammation by activating the IP receptors. Due to the critical roles of TRPM8 and IP receptors in the regulation of inflammatory pain, and considering their overlapping expression pattern, we analysed the functional interaction between human TRPM8 and IP receptors.

EXPERIMENTAL APPROACH

We transiently expressed human TRPM8 channels and IP receptors in HEK293T cells and carried out intracellular calcium and cAMP measurements. Additionally, we cultured neurons from the dorsal root ganglia (DRGs) of mice and determined the increase in intracellular calcium triggered by the TRPM8 agonist, icilin, in the presence of the IP receptor agonist cicaprost, the IP receptor antagonist Cay10441, and the Gq/11 inhibitor YM254890.

KEY RESULTS

Activation of IP receptors by selective agonists (cicaprost, beraprost, and iloprost) inhibited TRPM8 channel function, independently of the Gs-cAMP pathway. The potent inhibition of TRPM8 channels by IP receptor agonists involved Gq/11 coupling. These effects were also observed in neurons isolated from murine DRGs.

CONCLUSIONS AND IMPLICATIONS

Our results demonstrate an unusual signalling pathway of IP receptors by coupling to Gq/11 proteins to inhibit TRPM8 channel function. This pathway may contribute to a better understanding of the role of TRPM8 channels and IP receptors in regulating pain and inflammation.

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

Apolipoprotein E favours the blunting by high-fat diet of prostacyclin receptor activation in the mouse aorta

BACKGROUND AND PURPOSE

NO-mediated, endothelium-dependent relaxations of isolated arteries are blunted by ageing and high-fat diets, as well as by apolipoprotein E deletion. The present study was designed to test the hypothesis that apolipoprotein E deletion impairs endothelium-dependent responses to prostacyclin (IP) receptor activation.

EXPERIMENTAL APPROACH

Five-week-old ApoE^+/+ and ApoE^-/- mice were fed normal chow or high-fat diet for 29 weeks. The aortae were isolated for the measurements of isometric tension in Halpern-Mulvany myographs. Levels of proteins were assessed by Western blotting and immunofluorescence, and cyclic nucleotide levels by elisa.

KEY RESULTS

The IP receptor agonist, iloprost, induced endothelium-, NO-synthase- and IP-dependent relaxations in aortae of young ApoE^+/+ mice. High-fat diet favoured activation of thromboxane receptors by iloprost, causing contraction. Apolipoprotein E was present in aortae of ApoE^+/+ mice, especially in endothelium. Its presence was augmented by high-fat diet. Its deletion potentiated iloprost-induced relaxations in aortae of young mice and prevented the blunting of this response by high-fat diet. Levels of cAMP were higher, but those of cGMP were lower in the aorta of ApoE^-/- than in ApoE^+/+ mice of the same age. The levels of IP receptor protein were not different between ApoE^+/+ and ApoE^-/- mice.

CONCLUSIONS AND IMPLICATIONS

Iloprost induced an endothelium-dependent relaxation in the aorta of young healthy mice which involved both the cGMP and cAMP pathways. This response was blunted by prolonged exposure to a high-fat diet. Apolipoprotein E deletion potentiated relaxations to IP receptor activation, independently of age and diet.

GPCRs in context: sexual dimorphism in the cardiovascular system

Cardiovascular disease (CVD) remains the largest cause of mortality worldwide, and there is a clear gender gap in disease occurrence, with men being predisposed to earlier onset of CVD, including atherosclerosis and hypertension, relative to women. Oestrogen may be a driving factor for female-specific cardioprotection, though androgens and sex chromosomes are also likely to contribute to sexual dimorphism in the cardiovascular system (CVS). Many GPCR-mediated processes are involved in cardiovascular homeostasis, and some exhibit clear sex divergence. Here, we focus on the G protein-coupled oestrogen receptor, endothelin receptors ET_A and ET_B and the eicosanoid G protein-coupled receptors (GPCRs), discussing the evidence and potential mechanisms leading to gender dimorphic responses in the vasculature. The use of animal models and pharmacological tools has been essential to understanding the role of these receptors in the CVS and will be key to further delineating their sex-specific effects. Ultimately, this may illuminate wider sex differences in cardiovascular pathology and physiology.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Lipid Mediators of Allergic Disease: Pathways, Treatments, and Emerging Therapeutic Targets

Bioactive lipids are critical regulators of inflammation. Over the last 75 years, these diverse compounds have emerged as clinically-relevant mediators of allergic disease pathophysiology. Animal and human studies have demonstrated the importance of lipid mediators in the development of asthma, allergic rhinitis, urticaria, anaphylaxis, atopic dermatitis, and food allergy. Lipids are critical participants in cell signaling events which influence key physiologic (bronchoconstriction) and immune phenomena (degranulation, chemotaxis, sensitization). Lipid-mediated cellular mechanisms including: (1) formation of structural support platforms (lipid rafts) for receptor signaling complexes, (2) activation of a diverse family of G-protein coupled receptors, and (3) mediating intracellular signaling cascades by acting as second messengers. Here, we review four classes of bioactive lipids (platelet activating factor, the leukotrienes, the prostanoids, and the sphingolipids) with special emphasis on lipid synthesis pathways and signaling, atopic disease pathology, and the ongoing development of atopy treatments targeting lipid mediator pathways.

Galanin activated Gi/o-proteins in human and rat central nervous systems

The neuropeptide galanin (GAL) is involved in the control of hormone secretion, nociception, feeding behavior, attention, learning and memory. The anatomical localization of galanin receptors in the brain has been described using autoradiography and immunohistochemistry, but both techniques are limited by the availability of specific radioligands or antibodies. Functional autoradiography provides an alternative method by combining anatomical resolution and information of the activity mediated by G-protein coupled receptors. The present study analyzes the functional GAL receptors coupled to Gi/o-proteins in human and rat brain nuclei using [(35)S]GTPγS autoradiography. The results show the anatomical distribution of Gi/o-proteins activated by GAL receptors that trigger intracellular signaling mechanisms. The activity mediated by GAL receptors in human and rat brain showed a good correlation of the net stimulation in areas such as spinal cord, periaqueductal gray, putamen, CA3 layers of hippocampus, substantia nigra and diverse thalamic nuclei. The functional GAL receptors coupled to Gi/o-proteins showed a similar pattern for both species in most of the areas analyzed, but some discrete nuclei showed differences in the activity mediated by GAL, such as the ventroposteromedial thalamic nucleus, or areas that regulate learning and memory processes in the hippocampus. Taken into consideration the present results, the rat could be used as an experimental model for the study of the physiological role of GAL-mediated neurotransmission and the modulation of GAL receptors activity in the human CNS.

New insights into structural determinants for prostanoid thromboxane A2 receptor- and prostacyclin receptor-G protein coupling

G protein-coupled receptors (GPCRs) interact with heterotrimeric G proteins and initiate a wide variety of signaling pathways. The molecular nature of GPCR-G protein interactions in the clinically important thromboxane A2 (TxA(2)) receptor (TP) and prostacyclin (PGI(2)) receptor (IP) is poorly understood. The TP activates its cognate G protein (Gαq) in response to the binding of thromboxane, while the IP signals through Gαs in response to the binding of prostacyclin. Here, we utilized a combination of approaches consisting of chimeric receptors, molecular modeling, and site-directed mutagenesis to precisely study the specificity of G protein coupling. Multiple chimeric receptors were constructed by replacing the TP intracellular loops (ICLs) with the ICL regions of the IP. Our results demonstrate that both the sequences and lengths of ICL2 and ICL3 influenced G protein specificity. Importantly, we identified a precise ICL region on the prostanoid receptors TP and IP that can switch G protein specificities. The validities of the chimeric technique and the derived molecular model were confirmed by introducing clinically relevant naturally occurring mutations (R60L in the TP and R212C in the IP). Our findings provide new molecular insights into prostanoid receptor-G protein interactions, which are of general significance for understanding the structural basis of G protein activation by GPCRs in basic health and cardiovascular disease.

Prostacyclin receptor-dependent inhibition of human erythroleukemia cell differentiation is STAT3-dependent

We have previously demonstrated that activation of prostacyclin (IP) receptors in human erythroleukemia (HEL) cells phosphorylates the signal transducer and activator of transcription 3 (STAT3) via Gα(s) and Gα(16) hybrid signalling. This current study was designed to determine if functional responses to cicaprost in HEL cells were dependent on STAT3 phosphorylation. Cicaprost significantly enhanced the rapid change in HEL cell morphology induced by phorbol-12-myristate-13-acetate (PMA), and this effect was inhibited by the IP receptor antagonist RO1138452 and a STAT3 inhibitory peptide. Other indicators of PMA-induced HEL cell differentiation, such as increased expression of CD41/CD61 and an increase in cell complexity/granularity, were inhibited by cicaprost in an IP receptor-dependent and STAT3-dependent manner. Although thrombopoietic cytokines promote megakaryocytic differentiation and platelet production via activation of STAT3, the predominant STAT3-dependent effects of cicaprost in HEL cells were inhibitory towards the process of PMA-induced megakaryocytopoeisis.

Identification of a novel endoplasmic reticulum export motif within the eighth α-helical domain (α-H8) of the human prostacyclin receptor

The human prostacyclin receptor (hIP) undergoes agonist-dependent trafficking involving a direct interaction with Rab11a GTPase. The region of interaction was localised to a 14 residue Rab11a binding domain (RBD) within the proximal carboxyl-terminal (C)-tail domain of the hIP, consisting of Val(299)-Val(307) within the eighth helical domain (α-H8) adjacent to the palmitoylated residues at Cys(308)-Cys(311). However, the factors determining the anterograde transport of the newly synthesised hIP from the endoplasmic reticulum (ER) to the plasma membrane (PM) have not been identified. The aim of the current study was to identify the major ER export motif(s) within the hIP initially by investigating the role of Lys residues in its maturation and processing. Through site-directed and Ala-scanning mutational studies in combination with analyses of protein expression and maturation, functional analyses of ligand binding, agonist-induced intracellular signalling and confocal image analyses, it was determined that Lys(297), Arg(302) and Lys(304) located within α-H8 represent the critical determinants of a novel ER export motif of the hIP. Furthermore, while substitution of those critical residues significantly impaired maturation and processing of the hIP, replacement of the positively charged Lys with Arg residues, and vice versa, was functionally permissible. Hence, this study has identified a novel 8 residue ER export motif within the functionally important α-H8 of the hIP. This ER export motif, defined by "K/R(X)(4)K/R(X)K/R," has a strict requirement for positively charged, basic Lys/Arg residues at the 1st, 6th and 8th positions and appears to be evolutionarily conserved within IP sequences from mouse to man.

The role of prostacyclin in lung cancer

Prostanoids are bioactive lipids that interact with 7-membrane-spanning G-protein-coupled receptors on target cells to impart their biologic effects. They include prostaglandins, prostacyclin, and thromboxane. Prostanoids are widely distributed; mediate several diverse biologic effects like platelet aggregation and smooth-muscle contraction; and are known to be involved in allergies, acquired immunity, and cancer metastasis. Prostanoids have also been associated with breast and endometrial cancer promotion, and with the inhibition of melanoma. The role of prostanoids in the development of lung disease has been poorly understood. In particular, prostacyclin possesses significant anti-inflammatory and antimetastatic properties and is the main product of cyclooxygenase-2 activity in the lung. In fact, the balance of the various members of the prostanoids family, specifically the prostaglandins PGE(2) and prostacyclin (PGI(2)), seems to play an increasingly important role in the development of lung cancer. Gaining a better understanding of prostanoids and their associated pathways is critical to the future development of molecular-based and pharmaceutical treatments of lung disease.

The human prostacyclin receptor from structure function to disease

Thirty years have passed since Vane and colleagues first described a substance, prostanoid X, from microsomal fractions (later called prostacyclin) that relaxed rather than contracted mesenteric arteries. The critical role of prostacyclin in many pathophysiological conditions, such as atherothrombosis, has only recently become appreciated (through receptor knockout mice studies, selective cyclooxygenase-2 inhibition clinical trials, and the discovery of dysfunctional prostacyclin receptor genetic variants). Additionally, important roles in such diverse areas as pain and inflammation, and parturition are being uncovered. Prostacyclin-based therapies, currently used for pulmonary hypertension, are accordingly emerging as possible treatments for such diseases, fueling interests in structure function studies for the receptor and signal transduction pathways in native cells. The coming decade is likely to yield many further exciting advances.

Immature and mature species of the human Prostacyclin Receptor are ubiquitinated and targeted to the 26S proteasomal or lysosomal degradation pathways, respectively

Background

The human prostacyclin receptor (hIP) undergoes agonist-induced phosphorylation, desensitisation and internalisation and may be recycled to the plasma membrane or targeted for degradation by, as yet, unknown mechanism(s).

Results

Herein it was sought to investigate the turnover of the hIP under basal conditions and in response to cicaprost stimulation. It was established that the hIP is subject to low-level basal degradation but, following agonist stimulation, degradation is substantially enhanced. Inhibition of the lysosomal pathway prevented basal and agonist-induced degradation of the mature species of the hIP (46-66 kDa). Conversely, inhibition of the proteasomal pathway had no effect on levels of the mature hIP but led to time-dependent accumulation of four newly synthesised immature species (38-44 kDa). It was established that both the mature and immature species of the hIP may be polyubiquitinated and this modification may be required for lysosomal sorting of the mature, internalised receptors and for degradation of the immature receptors by the 26S proteasomes through the ER-associated degradation (ERAD) process, respectively. Moreover, these data substantially advance knowledge of the factors regulating processing and maturation of the hIP, a complex receptor subject to multiple post-translational modifications including N-glycosylation, phosphorylation, isoprenylation, palmitoylation, in addition to polyubiquitination, as determined herein.

Conclusion

These findings indicate that the hIP is post-translationally modified by ubiquitination, which targets the immature species to the 26S proteasomal degradation pathway and the mature species to the lysosomal degradation pathway.

Molecular modeling and docking studies of human 5-hydroxytryptamine 2A (5-HT2A) receptor for the identification of hotspots for ligand binding

The serotonergic system has been implicated in emotional and cognitive function. In particular, 5-HT(2A) (5-hydroxytrytamine receptor 2A) is attributed to a number of disorders like schizophrenia, depression, eating disorders and anxiety. 5-HT(2A), being a GPCR (G-protein coupled receptor), is important in the pharmaceutical industry as a proven target for these disorders. Despite their extensive clinical importance, the structural studies of this protein is lacking due to difficulties in determining its crystal structure. We have performed sequence analysis and molecular modeling of 5-HT(2A) that has revealed a set of conserved residues and motifs considered to play an important role in maintaining structural integrity and function of the receptor. The analysis also revealed a set of residues specific to the receptor which distinguishes them from other members of the subclass and their orthologs. Further, starting from the model structure of human 5-HT(2A) receptor, docking studies were attempted to envisage how it might interact with eight of its ligands (such as serotonin, dopamine, DOI, LSD, haloperidol, ketanserin, risperidone and clozapine). The binding studies of dopamine to 5-HT(2A) receptor can bring up better understanding in the etiology of a number of neurological disorders involving both these two receptors. Our sequence analysis and study of interactions of this receptor with other ligands reveal additional residue hotspots such as Asn 363 and Tyr 370. The function of these residues can be further analyzed by rational design of site-directed mutagenesis. Two distinct binding sites are identified which could play important roles in ligand binding and signaling.

Intermolecular cross-talk between the prostaglandin E2 receptor (EP)3 of subtype and thromboxane A(2) receptor signalling in human erythroleukaemic cells

BACKGROUND AND PURPOSE

In previous studies investigating cross-talk of signalling between prostaglandin (PG)E(2) receptor (EP) and the TPalpha and TPbeta isoforms of the human thromboxane (TX)A(2) receptor (TP), 17-phenyl trinor PGE(2)-induced desensitization of TP receptor signalling through activation of the AH6809 and SC19220-sensitive EP(1) subtype of the EP receptor family, in a cell-specific manner. Here, we sought to further investigate that cross-talk in human erythroleukaemic (HEL) 92.1.7 cells.

EXPERIMENTAL APPROACH

Specificity of 17-phenyl trinor PGE(2) signalling and its possible cross-talk with signalling by TPalpha/TPbeta receptors endogenously expressed in HEL cells was examined through assessment of agonist-induced inositol 1,4,5-trisphosphate (IP)(3) generation and intracellular calcium ([Ca(2+)](i)) mobilization.

KEY RESULTS

While 17-Phenyl trinor PGE(2) led to activation of phospholipase (PL)Cbeta to yield increases in IP(3) generation and [Ca(2+)](i), it did not desensitize but rather augmented that signalling in response to subsequent stimulation with the TXA(2) mimetic U46619. Furthermore, the augmentation was reciprocal. Signalling by 17-phenyl trinor PGE(2) was found to occur through AH6809- and SC19920-insensitive, Pertussis toxin-sensitive, G(i)/G(betagamma)-dependent activation of PLCbeta. Further pharmacological investigation using selective EP receptor subtype agonists and antagonists confirmed that 17-phenyl trinor PGE(2)-mediated signalling and reciprocal cross-talk with the TP receptors occurred through the EP(3), rather than the EP(1), EP(2) or EP(4) receptor subtype in HEL cells.

CONCLUSIONS AND IMPLICATIONS

The EP(1) and EP(3) subtypes of the EP receptor family mediated intermolecular cross-talk to differentially regulate TP receptor-mediated signalling whereby activation of EP(1) receptors impaired or desensitized, while that of EP(3) receptors augmented signalling through TPalpha/TPbeta receptors, in a cell type-specific manner.

Novel signaling pathways promote a paracrine wave of prostacyclin-induced vascular smooth muscle differentiation

The important athero-protective role of prostacyclin is becoming increasingly evident as recent studies have revealed adverse cardiovascular effects in mice lacking the prostacyclin receptor, in patients taking selective COX-2 inhibitors, and in patients in the presence of a dysfunctional prostacyclin receptor genetic variant. We have recently reported that this protective mechanism includes the promotion of a quiescent differentiated phenotype in human vascular smooth muscle cells (VSMC). Herein, we address the intriguing question of how localized endothelial release of the very unstable eicosanoid, prostacyclin, exerts a profound effect on the vascular media, often 30 cell layers thick. We report a novel PKA-, Akt-1- and ERK1/2-dependent prostacyclin-induced prostacyclin release that appears to play an important role in propagation of the quiescent, differentiated phenotype through adjacent arterial smooth muscle cells in the vascular media. Treating VSMC with the prostacyclin analog iloprost induced differentiation (contractile protein expression and contractile morphology), and also up-regulated COX-2 expression, leading to prostacyclin release by VSMC. This paracrine prostacyclin release, in turn, promoted differentiation and COX-2 induction in neighboring VSMC that were not exposed to iloprost. Using siRNA and pharmacologic inhibitors, we report that this positive feedback mechanism, prostacyclin-induced prostacyclin release, is mediated by cAMP/PKA signaling, ERK1/2 activation, and a novel prostacyclin receptor signaling pathway, inhibition of Akt-1. Furthermore, these pathways appear to be regulated by the prostacyclin receptor independently of one another. We conclude that prevention of de-differentiation and proliferation through a paracrine positive feedback mechanism is a major cardioprotective function of prostacyclin.

Recycling of the human prostacyclin receptor is regulated through a direct interaction with Rab11a GTPase

The human prostacyclin receptor (hIP) undergoes agonist-induced internalization but the mechanisms regulating its intracellular trafficking and/or recycling to the plasma membrane are poorly understood. Herein, we conducted a yeast-two-hybrid screen to identify proteins interacting with the carboxyl-terminal (C)-tail domain of the hIP and discovered a novel interaction with Rab11a. This interaction was confirmed by co-immunoprecipitations in mammalian HEK293 and was augmented by cicaprost stimulation. The hIP co-localized to Rab11-containing recycling endosomes in both HEK293 and endothelial EA.hy 926 cells in a time-dependent manner following cicaprost stimulation. Moreover, over-expression of Rab11a significantly increased recycling of the hIP, while the dominant negative Rab11(S25N) impaired that recycling. Conversely, while the hIP co-localized to Rab4-positive endosomes in response to cicaprost, ectopic expression of Rab4a did not substantially affect overall recycling nor did Rab4a directly interact with the hIP. The specific interaction between the hIP and Rab11a was dependent on a 22 amino acid (Val(299)-Gln(320)) sequence within its C-tail domain and was independent of isoprenylation of the hIP. This study elucidates a critical role for Rab11a in regulating trafficking of the hIP and has identified a novel Rab11 binding domain (RBD) within its C-tail domain that is both necessary and sufficient to mediate interaction with Rab11a.

Prostacyclin receptor-induced STAT3 phosphorylation in human erythroleukemia cells is mediated via Galpha(s) and Galpha(16) hybrid signaling

Human prostacyclin receptor (hIP) stimulates STAT3 via pertussis toxin-insensitive G proteins in human erythroleukemia (HEL) cells. Since hIP can utilize G(s) and G(q) proteins for signal transduction and that both G proteins can induce STAT3 phosphorylation and activation via complex signaling networks, we sought to determine if one of them is predominant in mediating the hIP signal. Stimulation of STAT3 Tyr(705) and Ser(727) phosphorylations by the IP-specific agonist, cicaprost, was sensitive to inhibition of protein kinase A, phospholipase Cbeta, protein kinase C, calmodulin-dependent protein kinase II and Janus kinase 2/3. Unlike Galpha(16)-mediated regulation of STAT3 in the same cells, cicaprost-induced STAT3 Tyr(705) phosphorylation was resistant to inhibition of Src and MEK while STAT3 Ser(727) phosphorylation distinctly required phosphatidylinositol-3 kinase. This unique inhibitor-sensitivity pattern of STAT3 phosphorylation was reproduced in HEL cells by stimulating the G(16)-coupled C5a receptor in the presence of dibutyryl-cAMP, suggesting that the change in inhibitor-sensitivity was due to activation of the G(s) pathway. This postulation was confirmed by expressing constitutively active Galpha(16)QL and Galpha(s)QL in human embryonic kidney 293 cells and the inhibitor-sensitivity of Galpha(16)QL-induced STAT3 phosphorylations could be converted by the mere presence of Galpha(s)QL to resemble that obtained with cicaprost in HEL cells. In addition, the restoration of the Galpha(16)-mediated inhibitor-sensitivity upon cicaprost induction in Galpha(s)-knocked down HEL cells again verified the pivotal role of G(s) signal. Taken together, our observations illustrate that co-stimulation of G(s) and G(q) can result in the fine-tuning of STAT3 activation status, and this may provide the basis for cell type-specific responses following activation of hIP.

Agonist-dependent internalization and trafficking of the human prostacyclin receptor: a direct role for Rab5a GTPase

The human prostacyclin receptor (hIP) undergoes rapid agonist-induced internalization by largely unknown mechanism(s). Herein the involvement of Rab5 in regulating cicaprost-induced internalization of the hIP expressed in human embryonic kidney 293 cells was investigated. Over-expression of Rab5a significantly increased agonist-induced hIP internalization. Additionally, the hIP co-localized to Rab5a-containing endocytic vesicles in response to cicaprost stimulation and there was a coincident net translocation of Rab5 from the cytosol/soluble fraction of the cell. Co-immunoprecipitation studies confirmed a direct physical interaction between the hIP and Rab5a that was augmented by cicaprost. Whilst the dominant negative Rab5a(S34N) did not show decreased interaction with the hIP or fully impair internalization, it prevented hIP sorting to endocytic vesicles. Moreover, the GTPase deficient Rab5a(Q79L) significantly increased internalization and co-localized with the hIP in enlarged endocytic vesicles. While deletion of the carboxyl terminal (C)-tail domain of the hIP did not inhibit agonist-induced internalization, co-localization or co-immunoprecipitation with Rab5a per se, receptor trafficking was altered suggesting that it contains structural determinant(s) for hIP sorting post Rab5-mediated endocytosis. Taken together, data herein and in endothelial EA.hy 926 cells demonstrate a direct role for Rab5a in agonist-internalization and trafficking of the hIP and increases knowledge of the factors regulating prostacyclin signaling.

IL-1beta, BK, and TGF-beta1 attenuate PGI2-mediated cAMP formation in human pulmonary artery smooth muscle cells by multiple mechanisms involving p38 MAP kinase and PKA

We have previously shown that interleukin (IL)-1beta, transforming growth factor (TGF)-beta1, or bradykinin (BK) impair cAMP generation in response to prostacyclin analogs in human pulmonary artery smooth muscle (PASM), suggesting that inflammation can impair the effects of prostacyclin analogs on PASM in pulmonary hypertension. Here we explored the biochemical mechanisms involved. We found that IL-1beta, BK, and TGF-beta1 reduced adenylyl cyclase isoform 1, 2, and 4 mRNA, increased Galphai protein levels, and reduced prostacyclin receptor (IP receptor) mRNA expression. In contrast, Galphas protein levels were unchanged. Protein kinase A (PKA) (H-89, KT-2750, PKIm) and p38 mitogen-activated protein (MAP) kinase (SB-202190) inhibitors attenuated these effects, but protein kinase C (bisindolylmaleide) or phosphoinositol 3-kinase (LY-294002) inhibitors did not. Fluorescent kemptide assay and Western blotting confirmed that PKA and p38 MAP kinase were activated by IL-1beta, BK, and TGF-beta1. These studies suggest that IL-1beta, BK, and TGF-beta1 impair IP receptor-mediated cAMP accumulation by multiple effects on different components of the signaling pathway and that these effects are PKA and p38 MAP kinase dependent.

Cardioprotective prostacyclin signaling in vascular smooth muscle

Prostacyclin plays an important cardioprotective role, which has been increasingly appreciated in recent years in light of adverse effects of COX-2 inhibitors in clinical trials. This cardioprotection is thought to be mediated, in part, by prostacyclin inhibition of platelet aggregation. Multiple lines of evidence suggest that prostacyclin additionally protects from cardiovascular disease by pleiotropic effects on vascular smooth muscle. Genetic deletion of the prostacyclin receptor in mice revealed an important role for prostacyclin in preventing the development of atherosclerosis, intimal hyperplasia, and restenosis. In vitro studies have shown these effects may be due to prostacyclin inhibition of vascular smooth muscle cell proliferation and migration. Prostacyclin has also been shown to promote vascular smooth muscle cell differentiation at the level of gene expression through the Gs/cAMP/PKA pathway. Recently identified single nucleotide polymorphisms in the prostacyclin receptor that compromise receptor function suggest that some genetic variations may predispose individuals to increased cardiovascular disease. Herein, we review the literature on the cardioprotective effects of prostacyclin on vascular smooth muscle, and the underlying molecular signaling mechanisms. Understanding the role of prostacyclin and other eicosanoid mediators in the vasculature may lead to improved therapeutic and preventative options for cardiovascular disease.

Prostacyclin inhibits endothelial cell XIAP ubiquitination and degradation

To understand the role of prostacyclin (PGI(2)) in protecting endothelial cells from apoptosis, we evaluated the effects of carbaprostacyclin (cPGI(2)) on H(2)O(2)-induced human umbilical vein endothelial cell (HUVEC) apoptosis. cPGI(2) suppressed H(2)O(2)-induced annexin V-positive cells in a concentration- and time-dependent manner. Pre-treatment of HUVEC with 50 microM cPGI(2) for 4 h produced the maximal anti-apoptotic effect. Authentic PGI(2) generated by adenoviral transfer of PGI(2) synthetic genes exerted a similar protective effect. cPGI(2) inhibited Smac/DIABLO release from mitochondria, caspase 3 activation, focal adhesion protein degradation, and cell detachment. cPGI(2) selectively protected X-linked inhibitor of apoptosis protein (X-linked IAP, XIAP) from H(2)O(2)-induced ubiquitination, and preserved XIAP protein levels. PD-98059 but not H-89 abrogated the protective action of cPGI(2). cPGI(2) increased ERK phosphorylation which was blocked by PD-98059. HUVEC stably transfected with dominant negative Ras abrogated XIAP preservation by cPGI(2) while constitutive active Ras increased ERK phosphorylation and protected XIAP from degradation. Our results demonstrate for the first time that PGI(2) inhibits XIAP ubiquitination and degradation via the Ras/MEK-1/ERK signaling pathway. Preservation of XIAP proteins represents a key mechanism by which PGI(2) protects endothelial cells from oxidant-induced apoptosis.

Human prostacyclin receptor structure and function from naturally-occurring and synthetic mutations

Prostacyclin (PGI2) is released by vascular endothelial cells and serves as a potent vasodilator, inhibitor of platelet aggregation (anti-thrombotic), and moderator of vascular smooth muscle cell proliferation-migration-differentiation (anti-atherosclerotic). These actions are mediated via a seven transmembrane-spanning G-protein coupled receptor (GPCR), known as the human prostacyclin receptor or hIP. Animal studies using prostacyclin receptor knock-out (IP-/-) mice have revealed increased propensities towards thrombosis, intimal hyperplasia, atherosclerosis, restenosis, as well as reperfusion injury. Of further importance has been the world-wide withdrawal of selective COX-2 inhibitors, due to their discriminating suppression of COX-2-derived PGI2 and its cardioprotective effects, leading to increased cardiovascular events, including myocardial infarction and thrombotic stroke. Over the last decade, mutagenesis studies of the IP receptor, in conjunction with in vitro functional assays and molecular modeling, have provided critical insights into the molecular mechanisms of both agonist binding and receptor activation. Most recently, the discovery of naturally-occurring and dysfunctional mutations within the hIP has provided additional insights into the proposed cardioprotective role of prostacyclin. The aim of this review is to summarize the most recent findings regarding hIP receptor structure-function that have developed through the study of both synthetic and naturally-occurring mutations.

A Profile of the Residues in the First Intracellular Loop Critical for Gs-Mediated Signaling of Human Prostacyclin Receptor Characterized by an Integrative Approach of NMR-Experiment and Mutagenesis

The first intracellular loop (iLP1, residues 39-51) of human prostacyclin receptor (IP) was proposed to be involved in signaling via its interaction with the Galphas protein. First, evidence of the IP iLP1 interaction with the C-terminus of the Galphas protein was observed by the fluorescence and NMR spectroscopy using the synthetic peptide (Galphas-Ct) mimicking the C-terminal 11 residues of the Galphas protein in the presence of a constrained synthetic peptide mimicking the IP iLP1. Then, the residues (Arg42, Ala44, and Arg45) in the IP iLP1 peptide possibly involved in contacting the Galphas-Ct peptide were initially assigned by observation of the significant proton resonance shifts of the side chains of the constrained IP iLP1 peptide using 2D (1)H NMR spectroscopy. The results of the NMR studies were used as a guide for further identification of the residues in the IP important to the receptor signaling using a recombinant protein approach. A profile of the residues in the IP iLP1, including the residues observed from the NMR studies involved in the Galphas mediated signaling, was mapped out by mutagenesis. According to our results, it can be predicted that the seven residues (Arg42-Ala48) with the conserved Arg45 at the center will form an epitope with a specific conformation involved in the Galphas mediated signaling. The conservation of the basic residues (Arg45 in the IP) in all of the prostanoid receptors suggests that the iLP1 regions of the other prostanoid receptors may also contain the epitopes important to their signaling.

Prostacyclin signaling in the kidney: implications for health and disease

The balance between vasodilator and vasoconstrictor pathways is key to the maintenance of homeostasis and the outcome of disease. In the kidney, prostaglandins (PGs) uphold this balance and regulate renal function: hemodynamics, renin secretion, growth responses, tubular transport processes, and cell fate. With the advent of cyclooxygenase (COX)-2-selective inhibitors, targeted deletions in mice (COX knockouts, PG receptor knockouts), and the discovery of intracrine signaling options for PGs (peroxisome proliferator-activated receptors and perinuclear PGE2receptors: EP1,3,4), many advances have been made in the study of arachidonic acid metabolites. Although prostacyclin (PGI2) is a major product of the COX pathway, there is very little emphasis on its importance to the kidney. This review will discuss PGI2biology and its relevance to different aspects of renal disease (growth, fibrosis, apoptosis), highlighting the most significant research from the past decade of PGI2literature, what we have learned from other organ systems, while stressing the significance of cross talk between various PGI2signaling pathways and its implications for renal health and disease.

Prostacyclin receptor induces STAT1 and STAT3 phosphorylations in human erythroleukemia cells: a mechanism requiring PTX-insensitive G proteins, ERK and JNK

The ability of the human prostacyclin receptor (hIP) to regulate the activities of signal transducers and activators of transcription (STATs) has not yet been documented. In the present study, we have delineated the mechanism by which hIP induces STAT3 phosphorylations in human erythroleukemia (HEL) cells. Stimulation of endogenous hIP by its specific agonist, cicaprost, resulted in STAT3 Tyr705 and Ser727 phosphorylations in a time- and concentration-dependent manner. Cicaprost-induced STAT3 Tyr705 and Ser727 phosphorylations were resistant to pertussis toxin (PTX) treatment, suggesting that these responses were mediated through PTX-insensitive G proteins. In addition, extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK), but not p38 MAPK, were shown to be phosphorylated by cicaprost in a time- and concentration-dependent manner via PTX-insensitive G proteins. The levels of the interaction between STAT3, ERK and JNK were enhanced by cicaprost treatment. The involvement of Raf-1, MEK1/2 and JNK in cicaprost-induced phosphorylations of STAT3 was illustrated by the use of their selective inhibitors. In contrast, p38 MAPK did not appear to be required. Similar observations were obtained with STAT1 upon stimulation by cicaprost. Taken together, these results demonstrate for the first time that hIP activation by cicaprost can lead to STAT1 and STAT3 phosphorylations via signaling pathways involving PTX-insensitive G proteins, ERK and JNK.

Pharmacology and signaling of prostaglandin receptors: multiple roles in inflammation and immune modulation

Prostaglandins are lipid-derived autacoids that modulate many physiological systems including the CNS, cardiovascular, gastrointestinal, genitourinary, endocrine, respiratory, and immune systems. In addition, prostaglandins have been implicated in a broad array of diseases including cancer, inflammation, cardiovascular disease, and hypertension. Prostaglandins exert their effects by activating rhodopsin-like seven transmembrane spanning G protein-coupled receptors (GPCRs). The prostanoid receptor subfamily is comprised of eight members (DP, EP1-4, FP, IP, and TP), and recently, a ninth prostaglandin receptor was identified-the chemoattractant receptor homologous molecule expressed on Th2 cells (CRTH2). The precise roles prostaglandin receptors play in physiologic and pathologic settings are determined by multiple factors including cellular context, receptor expression profile, ligand affinity, and differential coupling to signal transduction pathways. This complexity is highlighted by the diverse and often opposing effects of prostaglandins within the immune system. In certain settings, prostaglandins function as pro-inflammatory mediators, but in others, they appear to have anti-inflammatory properties. In this review, we will discuss the pharmacology and signaling of the nine known prostaglandin GPCRs and highlight the specific roles that these receptors play in inflammation and immune modulation.

Clusters of transmembrane residues are critical for human prostacyclin receptor activation

Relaxation of vascular smooth muscle and prevention of blood coagulation are mediated by ligand-induced activation of the human prostacyclin (hIP) receptor, a seven-transmembrane-domain G-protein-coupled receptor (GPCR). In this study, we elucidate the molecular requirements for receptor activation within the region of the ligand-binding pocket, identifying transmembrane residues affecting potency. Eleven of 30 mutated residues in the region of the ligand-binding domain exhibited defective activation (decreased potency). These critical residues localized to four distinct clusters (analysis via a rhodopsin-based human prostacyclin receptor homology model). Residues Y75(2.65) (TMII), F95(3.28) (TMIII), and R279(7.40) (TMVII) comprised the immediate binding-pocket cluster and were shown to be essential for proper receptor activation, compared to equivalent expression levels of the wild-type hIP (WT EC(50) = 1.2 +/- 0.1 nM; Y75(2.65)A EC(50) = 347.3 +/- 62.8 nM, p < 0.001; F95(3.28)A EC(50) = 8.0 +/- 0.6 nM, p < 0.001; R279(7.40)A EC(50) = 130 +/- 63.0 nM, p < 0.001). Residues S20(1.39) (TMI), F24(1.43) (TMI), and F72(2.62) (TMII) were localized to a cluster involving P17(1.36), a critical residue thought to facilitate transmembrane movement during changes in activation conformation. A third cluster formed around amino acid D60(2.50) (TMII), containing the highly conserved (100% of prostanoid receptors) D288(7.49)/P289(7.50) motif located in TMVII. Last, a large hydrophobic cluster composed of aromatic residues F146(4.52) (TMIV), F150(4.56) (TMIV), F184(5.40) (TMV), and Y188(5.44) (TMV) was observed away from the ligand-binding pocket, but still necessary for hIP activation. These results assist in delineating the potential molecular requirements for agonist-induced signaling through the transmembrane domain. Such observations may be generally applicable, as many of these clusters are highly conserved among the prostanoid receptors as well as other class A GPCRs.

Prostacyclin receptor-mediated activation of extracellular signal-regulated kinases 1 and 2

The prostacyclin mimetic cicaprost increased phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in Chinese hamster ovary cells transiently expressing human (hIP-CHO) or mouse prostacyclin (mIP-CHO) receptors, but not in human neuroblastoma SK-N-SH cells or rat/mouse neuroblastoma-glioma NG108-15 cells which endogenously express IP receptors. Cicaprost stimulated ERK1/2 activity in hIP-CHO and mIP-CHO cells with EC50 values of 60 and 83 nM, respectively, and this response was significantly inhibited by protein kinase C inhibitors and agents which elevate cyclic AMP. A poor correlation was discovered between the level of ERK1/2 activity and the ability of agents to increase or decrease cyclic AMP production. The potent inhibitory effect of 3-isobutyl-1-methyl xanthine on cicaprost-stimulated phospho-ERK1/2 may be due to inhibition of phosphoinositide 3-kinase. Therefore, IP receptor-mediated activation of ERK1/2 in CHO cells occurs through a Gq/11/protein kinase C-dependent and a phosphoinoside 3-kinase-dependent process which is insensitive to IP receptor-generated cyclic AMP.

Protein kinase A-dependent coupling of mouse prostacyclin receptors to Gi is cell-type dependent

The ability of the prostacyclin (IP) receptor agonist cicaprost to activate Gs-, Gq/11- and Gi-mediated cell signalling pathways has been examined in Chinese hamster ovary (CHO) cells and human embryonic kidney 293 (HEK 293) cells expressing the cloned human (hIP) or mouse (mIP) prostacyclin receptor, and compared with data from NG108-15 and SK-N-SH cells that endogenously express rat/mouse and human IP receptors, respectively. Cicaprost stimulated [3H]cyclic AMP production with EC50 values of 1.5-22 nM, and stimulated [3H]inositol phosphate production (EC50 values 49-457 nM) in all but the SK-N-SH cells. Cicaprost failed to inhibit forskolin-stimulated [3H]cyclic AMP production in any of these cell lines. Therefore, although both human and mouse IP receptors couple to Gs and Gq/11-mediated signalling pathways in a cell type-dependent manner, we could find no evidence for IP receptor coupling to Gi.

Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors

For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.

Palmitoylation of the human prostacyclin receptor. Functional implications of palmitoylation and isoprenylation

We have previously established that isoprenylation of the prostacyclin receptor (IP) is required for its efficient G protein coupling and effector signaling (Hayes, J. S., Lawler, O. A., Walsh, M. T., and Kinsella, B. T. (1999) J. Biol. Chem. 274, 23707-23718). In the present study, we sought to investigate whether the IP may actually be subject to palmitoylation in addition to isoprenylation and to establish the functional significance thereof. The human (h) IP was efficiently palmitoylated at Cys(308) and Cys(311), proximal to transmembrane domain 7 within its carboxyl-terminal (C)-tail domain, whereas Cys(309) was not palmitoylated. The isoprenylation-defective hIP(SSLC) underwent palmitoylation but did not efficiently couple to G(s) or G(q), confirming that isoprenylation is required for G protein coupling. Deletion of C-tail sequences distal to Val(307) generated hIP(Delta307) that was neither palmitoylated nor isoprenylated and did not efficiently couple to G(s) or to G(q), whereas hIP(Delta312) was palmitoylated and ably coupled to both effector systems. Conversion of Cys(308), Cys(309), Cys(311), Cys(308,309), or Cys(309,311) to corresponding Ser residues, while leaving the isoprenylation CAAX motif intact, did not affect hIP coupling to G(s) signaling, whereas mutation of Cys(308,311) and Cys(308,309,311) abolished signaling, indicating that palmitoylation of either Cys(308) or Cys(311) is sufficient to maintain functional G(s) coupling. Although mutation of Cys(309) and Cys(311) did not affect hIP-mediated G(q) coupling, mutation of Cys(308) abolished signaling, indicating a specific requirement for palmitoylation of Cys(308) for G(q) coupling. Consistent with this, neither hIP(C308S,C309S), hIP(C308S,C311S), nor hIP(C308S,C309S,C311S) coupled to G(q). Taken together, these data confirm that the hIP is isoprenylated and palmitoylated, and collectively these modifications modulate its G protein coupling and effector signaling. We propose that through lipid modification followed by membrane insertion, the C-tail domain of the IP may contain a double loop structure anchored by the dynamically regulated palmitoyl groups proximal to transmembrane domain 7 and by a distal farnesyl isoprenoid permanently attached to its carboxyl terminus.

beta-Arrestin-mediated PDE4 cAMP phosphodiesterase recruitment regulates beta-adrenoceptor switching from Gs to Gi

Phosphorylation of the beta(2) adrenoreceptor (beta(2)AR) by cAMP-activated protein kinase A (PKA) switches its predominant coupling from stimulatory guanine nucleotide regulatory protein (G(s)) to inhibitory guanine nucleotide regulatory protein (G(i)). beta-Arrestins recruit the cAMP-degrading PDE4 phosphodiesterases to the beta(2)AR, thus controlling PKA activity at the membrane. Here we investigate a role for PDE4 recruitment in regulating G protein switching by the beta(2)AR. In human embryonic kidney 293 cells overexpressing a recombinant beta(2)AR, stimulation with isoprenaline recruits beta-arrestins 1 and 2 as well as both PDE4D3 and PDE4D5 to the receptor and stimulates receptor phosphorylation by PKA. The PKA phosphorylation status of the beta(2)AR is enhanced markedly when cells are treated with the selective PDE4-inhibitor rolipram or when they are transfected with a catalytically inactive PDE4D mutant (PDE4D5-D556A) that competitively inhibits isoprenaline-stimulated recruitment of native PDE4 to the beta(2)AR. Rolipram and PDE4D5-D556A also enhance beta(2)AR-mediated activation of extracellular signal-regulated kinases ERK12. This is consistent with a switch in coupling of the receptor from G(s) to G(i), because the ERK12 activation is sensitive to both inhibitors of PKA (H89) and G(i) (pertussis toxin). In cardiac myocytes, the beta(2)AR also switches from G(s) to G(i) coupling. Treating primary cardiac myocytes with isoprenaline induces recruitment of PDE4D3 and PDE4D5 to membranes and activates ERK12. Rolipram robustly enhances this activation in a manner sensitive to both pertussis toxin and H89. Adenovirus-mediated expression of PDE4D5-D556A also potentiates ERK12 activation. Thus, receptor-stimulated beta-arrestin-mediated recruitment of PDE4 plays a central role in the regulation of G protein switching by the beta(2)AR in a physiological system, the cardiac myocyte.