PAF Receptors

PAF Receptors Peter M. Henson* Department of Pediatrics, National Jewish Medical Research Center, 1400 Jackson Street, Room D-508, Denver, CO 80206-2761, USA * corresponding author tel: 303-398-1380, fax: 303-398-1381, e-mail: [email protected] DOI: 10.1006/rwcy.2000.23003.

SUMMARY Platelet-activating factor (PAF) reacts with a specific seven transmembrane, G protein-linked receptor with two promoter splice forms showing tissue-specific regulation and for which a knockout mouse has recently been created. The receptor is widely expressed on hematopoietic cells, endothelial cells, keratinocytes, and cells of the central nervous system. A huge number of receptor antagonists have been synthesized and many are being explored in vitro, in animal models, and in a variety of clinical trials. In addition, the complexity of the PAF story is increased by the observation that in the majority of cells that synthesize it, most of the PAF remains within the cell, leading to speculation that it plays an intracellular role, perhaps even on an intracellular receptor, in addition to its role as extracellular communication molecule.

BACKGROUND

Discovery Following a large body of work on PAF receptors determined by a variety of binding assays, a candidate plasma membrane receptor from the guinea pig was cloned in 1991 by expression cloning in Xenopus eggs (Honda et al., 1991), followed soon after by its human counterpart (Nakamura et al., 1991; Ye et al., 1991; Kunz et al., 1992; Chase et al., 1993).

Structure The receptor is a seven transmembrane, heterotrimeric G protein-linked receptor (see Figure 2).

Main activities and pathophysiological roles As discussed in the PAF chapter, much of the PAF that is synthesized does not gain access to the extracellular environment. The presence of PAF within synthesizing cells as well as the recent interest in oxidized phospholipids and questions about the roles of intracellular acetylhydrolases, have rekindled research into the intracellular effects of this family of molecules. While there little direct evidence to date for such effects and even less information on specific receptors or signaling pathways, a number of intriguing (although speculative) possibilities and questions might be considered. Oxidized phospholipids would be expected to have potential toxic effects within cells leading to the propagation of redox alterations and alteration in normal membrane structure and function. Potent catabolic enzymes may have developed to counteract such effects. These acetylhydrolases (as well as the action of phospholipases A2) generate lysophosphatides, themselves potentially toxic because of their amphipathic properties. The transacylase and acetyltransferase involved in PAF biosynthesis could therefore further protect the cell, ultimately making use of an abundant ingredient, acetyl-CoA. The extremely wide distribution of such pathways (including plants, fungi, and bacteria) might be in keeping with such a concept. Additional activities of these molecules might originally have derived from this need for inactivation but now could include effects on intracellular receptors and signaling pathways. No such receptor has yet been definitively described although there were some early reports of such molecules at the 1998 PAF and Related Lipid Mediators Meeting in New Orleans. It has been suggested that the antagonist BN-50730

2184 Peter M. Henson may be effective against possible intracellular PAF receptors (Bazan et al., 1997) and its use has also raised the possibility of such effects in a number of neuronal systems. This is indeed an area that needs more investigation.

GENE

Accession numbers GenBank: Human: S52624, D10202, D31736, U11032 Mouse: D50872 Rat: U04740 Guinea pig: X56736

from leukocytes and brain; one is upregulated by glucocorticoids, the other by retinoids, etc. On the other hand, pharmacological studies have consistently suggested a more complex system with the likelihood of more than one receptor (see, for example, Hwang, 1988, 1991; Kato et al., 1994; LeVan et al., 1997; Voelkel et al., 1986). At this point, if another membrane receptor is found, it will probably be quite different from the cloned version since attempts to find genes related to the cloned PAFR have been uniformly unsuccessful. Glycosylation of the receptor is required for its expression (Garcia Rodriguez et al., 1995a). An intron in the 50 untranslated region of the human gene has also been described (Chase et al., 1993).

PROTEIN

Sequence

Description of protein

See Figure 1.

Chromosome location and linkages Only one gene has been identified (on chromosomes 1 and 4 in human or mouse respectively), although alternative splice forms are seen with an identical coding region but differing upstream. 50 sequences and different transcription initiation sites (Mutoh et al., 1993; Shimizu and Mutoh, 1997). One is found in most cells and tissues, the other is selectively absent

The receptor appears to be a relatively standard seven transmembrane, heterotrimeric G protein-linked receptor (Mutoh et al., 1993; Shimizu and Mutoh, 1997) comprising 342 amino acids (in human). The cysteines at positions 90 and 173 form a disulfide bond and Cys95 also seems important ± mutation of these alters membrane expression and function (Le Gouill et al., 1997). Mutation of Leu231 to Arg led to a constituitively active receptor with higher binding affinity for PAF, whereas conversion of the adjacent

Figure 1 Deduced sequences of rat, guinea pig, human, and mouse PAF receptor. From Nakamura et al. (1991), Bito et al. (1994), Honda et al. (1991), Ishii et al. (1996). RA: GP: HU: MU:

MEQNGSPRVDSEFRYTLFPIVYSVIFVLG VVANGYVLWVFATLYPSKKLNEIKIFMVNLTVADLLFLMTLPLWIVYYSNE NELNSSSRVDSEFRYTLFPIVYSIIFVLGIIANGYVLWVFARLYPSKKLNEIKIFMVNLTVADLLFLITLPLWIVYYSNQ MEPHDSSHMDSEFRYTLFPIVYSIIFVLGVIANGYVLWVFARLYPCKKFNEIKIFMVNLTMADMLFLITLPLWIVYYQNQ MEHNGSFRVDSEFRYTLFPIVYSVIFILGVVANGYVLWVFANLYPSKKLNEIKIFMVNLTMADLLFLITLPLWIVYYYNE GDWIVHKFLCNLAGCLFFINTYCSVAFLGVITYNRYQAVAYPIKTAQATTRKRGITLSLVIWISIAATASYFLATDSTNV GNWFLPKFLCNLAGCLFFINTYCSVAFLGVITYNRFQAVKYPIKTAQATTRKRGIALSLVIWVAIVAAASYFLVMDSTNV GNWILPKFLCNVAGCLFFINTYCSVAFLGVITYNRFQAVTRPIKYAQANTRKRGISLSLVIWVAIVGAASYFLILDSTNT GATILPNFLCNVAGCLFFINTYCSVAFLGVITYNRYQAVAYPIKTAQATTRKRGISLSLIIWVSIVATASYFLATDSTNL VPKKDGSGNITRCFEHYEPYSVPILVV HIFITSCFELVFFLIFYCNMVIIHTLLTRPVRQQRKPEVKRRA LWMVCTVLAV VSNKAGSGNITRCFEHYEKGSKPVLIIHICIVLGFFIVFLLILFCNLVIIHTLLRQPVKQQRNAEVRRRALWMVCTVLAV VPDSAGSGNVTRCFEHYEKGSVPVLIIHIFIVFSFFLVFLIILFCNLVIIRTLLMQPVQQQRNAEVKRRALWMVCTVLAV VPNKDGSGNITRCFEHYEPYSVPILVVHVFIAFCFFLVFFLIFYCNLVIIHTLLTQPMRQQRKAGVKRRALWMVCTVLAV FVICFVPHHVVQLPWTLAELGYQ-TNFHQAINDAHQITLCLLSTNCVLDPVIYCFLTKKFRKHLSEKFYSMRSSRKCSRA FVICFVPHHMVQLPWTLAELGMWPSSNHQAINDAHQVTLCLLSTNCVLDPVIYCFLTKKFRKHLSEKLNIMRSSQKCSRV FIICFVPHHVVQLPWTLAELGFQDSKFHQAINDAHQVTLCLLSTNCVLDPVIYCFLTKKFRKHLTEKFYSMRSSRKCSRA FIICFVPHHVVNLPWTLAGLGTQ-TNFHQAINDAHQITLCLLSTNCVLDPVIYCFLTKKFRKHLSEKFYSMRSSRKCSRA TSDTCTEVMMPANQTPVLPLKN TTDTGTEMAIPINHTPVNPIKN TTLTVTEVVVPFNQIPGNSLKN TSDTCTEVIVPANQTPIVSLKN

PAF Receptors 2185 Ala230 to Glu had the opposite effect (Parent et al., 1996), suggesting an important influence of this region of the third intracellular loop on configuration. An N-glycosylation site is present on the second extracellular loop and appears important for membrane expression but not for function (Garcia Rodriguez et al., 1995b).

ment as a phospholipid from the outer membrane leaflet and probably only partitioned into the aqueous environment on a carrier such as albumin. It has been estimated that plasma albumin has four binding sites for PAF (probably of different affinities) and suggested that PAF is usually presented to its receptor on this protein (Clay et al., 1990). An alternative source of PAF activity for interaction with target cells may be the shedding of PAF-containing membrane vesicles from the synthesizing cells (Patel et al., 1992), perhaps providing an explanation for early studies of PAF activity in the absence of an albumin carrier. As indicated, there are still a significant number of discrepancies in our understanding of PAF-receptor interactions. Addition of PAF to cells results in nonreceptormediated association/insertion into the outer membrane leaflet (e.g. Bratton et al., 1992). From there it can be flipped to the inner leaflet by the action of the phospholipid scramblase. What is not clear is whether, in either of these configurations, it can interact with the PAF receptor or whether such binding can only occur when it is presented in solution (i.e. on a carrier) and/or whether it can interact with the receptor on one cell when presented in the membrane of another. It should be noted that PAF can also be internalized through interaction with its receptor (Gerard and Gerard, 1994), making it

Relevant homologies and species differences The differences between the PAF receptor in the three species from which it has been cloned are subtle (see Figure 1). The mouse and rat protein has one less amino acid than human or guinea pig. Figure 2 illustrates these differences in the context of the distribution across the membrane.

Affinity for ligand(s) PAF is an amphiphilic phospholipid. Accordingly, its mode of interaction with a surface receptor might be expected to be complex. At concentrations above micromolar in aqueous solution (without a carrier) it is likely to form micelles. When `secreted' by cells it is thought to be presented to the extracellular environ-

Figure 2 PAF receptor showing amino acids common between human, mouse, rat, and guinea pig. Adapted from Shimizu and Mutoh (1997). Extracellular D

W

S S

E F R Y

M E

G

F L N

Y

Y I V I V P V L Y L P S T I F F L L L G D A A T N G L Y N V M L V W F V I F K A I L E Y N P K K F

N

C

N

T L

E H C F Y E R T N G S V T S

V P

I

K T

A

L

L

L

Q

E

Cytoplasmic 342 (341)

A

H

L

T

Q

P L T L C L Q S T H T C P H N V F F V L V P F I D C F V F I Y A V I F C L T C I V C N T M V V K C K W S I Q Q L K R R I S A F S R R T T P R R K L M L V H D L T W

H

A

D A

G

P

S A F I W I S L L I G R K R T

Q A

N S

D

Y

R

I

G

A

C G L F F N I Y T S C A V L F G I V T Y N

H Q A

I

N K

V

E

P

K

T E

N

P

2186 Peter M. Henson imperative to distinguish these pathways in any studies of uptake. An intriguing observation that may have little to do with PAF or its analogs directly is the suggestion that Streptococcus pneumoniae can bind to cells, including epithelial cells, via the PAF receptor, utilizing the phosphorylcholine on the bacterial teichoic acid (Cundell et al., 1995).

Cell types and tissues expressing the receptor The PAF receptor and/or PAF binding has been found throughout the body, including the central nervous system on many different cell types, including neutrophils, eosinophils, basophils, mononuclear phagocytes, some lymphocytes, mast cells, endothelial cells (Domingo et al., 1994; Predescu et al., 1996; Sasaki et al., 1996), and keratinocytes (Shimada et al., 1998a). It can be seen associated with an endosomal compartment within endothelial cells (Ihida et al., 1999) but this could represent endocytosed receptor. Transcripts for the receptor are present from very early in embryogenesis (Stojanov and O'Neill, 1999) and are also present on spermatozoa (Reinhardt et al., 1999).

Regulation of receptor expression The human PAF receptor appears to be transcribed by two distinct promoters, each of which has distinct transcriptional initiation sites (Mutoh et al., 1993). These exons are alternatively spliced to a third that contains the total open reading frame, thus generating two different transcripts. Transcript 1 has consensus sequences (three) for NFB and SP-1, and the initiator (Inr) sequence (see also Pang et al., 1995, and see GenBank accession number U11032). Transcript 2 also contains consensus sequences for AP-1, AP-2, and SP-1. Both transcripts were reported in heart, lung, spleen, and kidney, whereas only transcript 1 was found in granulocytes and brain. A variety of cytokines have been reported to upregulate PAF receptor expression. Thus increased PAFR mRNA expression was seen in eosinophils from patients with asthma and was upregulated by exposure to IL-3, IL-5, and GM-CSF (Kishimoto et al., 1996a, 1996b, 1997). In endothelial cells, mechanical stress also increases mRNA expression (Okahara et al., 1998; Chaqour et al., 1999). PAF itself has been reported to do the same (Wang et al.,

1997). All of these have been suggested to act through NFB, three binding sites for which are seen in the promoter between ÿ571 and ÿ459 bp (Mutoh et al., 1994). On the other hand, PAF itself has also been suggested to downregulate PAF receptor mRNA expression (Nakao et al., 1997) and oxidized LDL has been reported to do likewise (Stengel et al., 1997). How this occurs is not yet clear but the contrasting observations certainly raise the possibility of complex, possibly multifactorial regulation. By a presumably different mechanism, the receptor is upregulated by TGF 2 acting as a consequence of a protein synthetic step on a response element in the PAFR promoter between ÿ44 and ÿ17 bp (Yang et al., 1997).

SIGNAL TRANSDUCTION

Cytoplasmic signaling cascades Signaling While the PAF receptor may be considered a relatively conventional member of the large seven transmembrane, heterotrimeric G protein-coupled receptor family, it may show differences from other chemoattractant or inflammatory mediator receptors in its presumed G protein usage, e.g. partial pertussis insensitivity (Hwang, 1988; Barzaghi et al., 1989; Yue et al., 1992; Honda et al., 1994), implying use of multiple G proteins (Ali et al., 1994). Its signaling pathways have been investigated in a number of cells with native receptor (e.g. neutrophils, eosinophils, or platelets) and after transfection into cells not normally expressing this molecule (see Boulay et al., 1997; Honda et al., 1994; Shimizu and Mutoh, 1997). On the other hand, a number of differences have been noted between fMLP and PAF signaling in neutrophils (e.g. Kadiri et al., 1990; M'Rabet et al., 1999), including relative selectivity towards p38 among the MAP kinases for the PAF receptor (Nick et al., 1997). Eicosanoid production often accompanies the generation of PAF, both in vitro and in vivo. The first step in biosynthesis of eicosanoids is the action of cPLA2 on the arachidonyl moiety esterified in membrane phospholipids. A proportion of this available substrate is in phosphatidylcholine (PC) and some of this, in turn, alkyl arachidonyl PC. Synthetic coupling of the two mediator classes would, therefore, be expected since substrate (arachidonate or lyso-PAF) for both synthetic pathways is generated simultaneously. In addition, however, PAF can activate cPLA2 in target cells via upstream signal pathways, including activation of the MAP kinase

PAF Receptors 2187 ERK (Hirabayashi et al., 1998), which could in turn lead to both more eicosanoids and more PAF.

Priming In neutrophils, and particularly eosinophils, PAF is directly chemotactic in vitro and also induces the related alterations in cell shape driven by cortical assembly of F-actin. On the other hand, with respect to other cell responses the PAF receptor often generates an incomplete signal, requiring additional cofactor stimulation for effective responses. In neutrophils this process was described as priming (Guthrie et al., 1984) and was first shown with LPS as a `priming' agent. However, PAF stimulation is the prime example of this effect (e.g. Gay, 1993). For example, by itself, it does not initiate the oxidative burst but rather induces alterations in the cell responsiveness such that other stimuli, including other chemoattractants as well as FcR ligation or even phorbol ester activation are all enhanced as a consequence (e.g. Bass et al., 1988; Kitchen et al., 1996). Other PAF-primable responses include eicosanoid production (Bauldry et al., 1991; Shindo et al., 1996, 1997), actin assembly (Shalit et al., 1987), secretion (Partrick et al., 1997; Vercellotti et al., 1988), etc. The mechanisms for this priming phenomenon are still not known but it does not seem to involve upregulation or significant alteration of the receptors for the triggering stimuli. One intriguing observation in this regard is that priming stimuli, especially PAF, are particularly effective at activating the p38 subfamily of MAP kinases (Nick et al., 1997) although its relationship to priming has still to be determined. (Note that PAF can also stimulate cells primed with other agents (e.g. Brunner et al., 1991), i.e. the processes are often reciprocal.) The suggested importance of priming is that stimuli such as PAF are presumed to initiate inflammatory cell accumulation but, by themselves, not to stimulate the cells further, thus minimizing potential damage to the vessel wall during the migration. Once in the tissues however, the primed cells would be hyperresponsive to additional triggering stimuli such as other inflammatory mediators, bacteria, immune complexes, etc. This is not to imply that PAF is only, or even primarily, a proinflammatory mediator since, as discussed below, it acts on many other cells and systems and, in vivo, may have equally or more important roles in a number of physiologic processes. Rather, the point here is that PAF receptor signaling probably intersects significantly with other signaling pathways ± a concept that may be of critical importance in consideration of some of its other potential roles, such as in

the neurologic or vascular systems, cell growth and differentiation, etc. Receptor Desensitization From studies in inflammatory cells again, it has long been appreciated that PAF responses are often even more transient that those seen with other inflammatory mediators and chemokines, i.e. the receptor seems to be particularly susceptible to desensitization. In fact, this process was earlier used to show PAF action in vivo in a model of IgE-mediated anaphylactic shock wherein the rapid and transient thrombocytopenia was suggested to be PAF-dependent because the platelets that returned to the circulation after 60 minutes had become selectively unresponsive to PAF (Henson and Pinckard, 1977). The PAF receptor can be directly phosphorylated (Ali et al., 1994), probably by a PKC, PKA, and a receptor kinase, leading to homotypic desensitization (Takano et al., 1994). The serine/threonine phosphorylation seems to enhance receptor internalization and thereby plays an additional role in desensitization (Ishii et al., 1998a). A further mechanism for desensitization has been described in which PKA-driven phosphorylation of PLC 3 is suggested to lead to unresponsiveness of that enzyme, and because of its relatively selective usage by the PAF receptor, unresponsiveness of the cell to PAF (Ali et al., 1997, 1998). The potential importance of rapid and effective downregulation is the transience of PAF responses, perhaps related to the potency of the mediator and strength of its effects.

DOWNSTREAM GENE ACTIVATION Incomplete information.

BIOLOGICAL CONSEQUENCES OF ACTIVATING OR INHIBITING RECEPTOR AND PATHOPHYSIOLOGY The enormous heterogeneity of the possible effects of PAF in vitro and ultimately in vivo makes it difficult (a) to summarize these in simple terms in one chapter and (b) to determine which of these possibilities is actually operative, or even more critical, important, in the whole animal. To determine a specific role for a potential mediator, such as PAF (and its analogs), in

2188 Peter M. Henson any given process, it would need to be present at the site and shown able to induce the response if introduced; the effect should be abrogated if the mediator is removed, blocked or absent and return when reintroduced. As indicated above, the availability of a number of PAF receptor antagonists has allowed various combinations of these criteria to be applied and has suggested the many potential actions. The presence and wide distribution of potent inactivating enzymes has also led to speculative roles for PAF in vivo and, in some cases, genetic variants of these enzymes have been shown to have disease associations. The field was reviewed in 1994 by Stewart (Stewart, 1994) but most of the more recent reviews have addressed specific areas of potential action. In this section we will discuss some of the likely areas of PAF effect, not with any expectation of completeness but rather to emphasize some specific points, concepts, and areas for future development. As a general example, the potential importance of oxidized phospholipids in atherosclerosis and the recognition that some of these have PAF-like activity opens up a number of important questions that are crying out for more detailed investigation. A receptor knockout mouse has recently been created with a phenotype that calls into question some of these wide-ranging effects ± at least as far as those mediated through this receptor are concerned (see below).

Unique biological effects of activating the receptors When added to isolated cells with functioning PAF receptors (e.g. neutrophils or eosinophils) the types of cellular responses that are seen are, in general, typical of rapid mediator action via seven transmembrane, G protein-coupled receptors. These include calcium mobilization, shape change (morphologic polarization), actin polymerization, chemokinesis and chemotaxis, priming for NADPH oxidase activation and eicosanoid production, adhesion, and integrin regulation. Platelet responses are also relatively standard, for example, potent aggregation and some release reaction. Interestingly, phosphatidylserine expression is not observed (Henson and Landes, 1976) and, as a consequence, procoagulant effects are minimal. There has been some suggestion that PAF is relatively selective as a proadhesive and chemotactic agent for eosinophils (Stewart, 1994; Wardlaw et al., 1986), but it certainly has similar activity against other leukocytes in vitro and its administration to animals does

not result in an exclusively eosinophilic response. On the other hand, the addition of IL-5 increases the eosinophil response selectively (e.g. Warringa et al., 1992), thus emphasizing the importance of mediator cooperation in selecting specific cells for activation. Longer term cellular responses in inflammatory cells have been less often investigated except for those classified under the rubric of `priming' responses, i.e. an enhancing function in cooperation with other stimuli. Even here there is much room for further investigation, for example in the de novo synthesis of new mediators, regulatory enzymes of the eicosanoid pathways, etc. A similar confusing story is seen when questions of PAF action in cell proliferation and maturation are raised. Reported effects include growth inhibition (Huang et al., 1999; Shimada et al., 1998b) and downregulation of myelopoesis (Dupuis et al., 1998) or, on the other hand, evidence of growth stimulation (Bennett and Birnboim, 1997; Rougier et al., 1997). It is presumed that responses will vary between cell types and, for the most part, will be mediated via secondary production or inhibition of regulatory molecules. PAF as a Cofactor and the Concept of Mediator Networks A common statement is that `PAF is a mediator of inflammation'. What is specifically meant by this is not always clear. The presence of PAF receptors on inflammatory cells (including platelets) as well as on the endothelium provides for an action on these cells via the cell responses listed above. In artificial systems PAF can be shown to induce chemotaxis, transendothelial migration (e.g. Casale et al., 1993) and, in conjunction with other stimuli, production and secretion of oxidants, other lipid mediators and proinflammatory mediator proteins. In a significant number of circumstances in vivo, the observed PAF effect is actually mediated by one of these downstream. products, often an eicosanoid ± i.e. PAF is an intermediary, perhaps contributing an amplification step to the overall proinflammatory signal. A potential theme that might be considered for PAF action is that it is, for the most part, a cofactor. Its role as a priming agent for neutrophil oxidant production is well established. Taking this as a model then, the ability of signaling pathways initiated through the PAF receptor to integrate with signaling from other, often more cell-specific, receptors could serve to explain many of the proposed PAF actions. For example, its role as an eosinophil chemoattractant may involve interaction with the relatively eosinophil-selective agent IL-5. Similarly with monocytes

PAF Receptors 2189 in the presence of endothelial P-selectin (Zimmerman et al., 1996). The idea of selective recognition resulting from integrated networks rather than exclusively cell- and ligand-specific receptors is gaining general ground (see, for example, the latest concepts of selectivity in odorant recognition (Mombaerts, 1999)). Similar processes could apply in inflammation where the complex network of mediators may drive specific, directed, and sequential cell responses by the integrated action of numerous, intrinsically generally acting, mediators on a given cell type. A cofactor role for PAF would mean that its antagonism could dampen down many specific responses (as seen in the studies with antagonists) but also that specific essential actions would be very hard to pin down. It would also explain the relative absence of phenotype in the knockout mice, in which compensatory mechanisms within the network would be particularly prone to take over.

Phenotypes of receptor knockouts and receptor overexpression mice Such mice developed normally and, when adult, showed no obvious physical or physiologic abnormalities (Ishii et al., 1998b). The animals reproduced normally despite a suggested role for PAF in reproduction. Interestingly, the mean arterial blood pressure was unchanged in the PAFRÿ/ÿ mice which is not easily consistent with a role for PAF in normal vasomotor tone, although confirmation of this would require more detailed investigation. There is a longstanding suggestion that PAF is involved in anaphylactic shock and endotoxic shock. In keeping with the former, intravascular antigen administration to sensitized PAFRÿ/ÿ animals did indeed result in markedly diminished shock and also the concomitant bronchial constriction and airways resistance. By contrast, endotoxin effects in knockout mice were indistinguishable from the control PAFR‡/‡ animals. Since a wide variety of PAF receptor antagonists have been shown to be capable of reducing or abolishing endotoxic shock, this finding was indeed a surprise. Even more so since PAF receptor overexpressing mice showed increased susceptibility to LPS (Ishii et al., 1997). Admittedly, many of the pharmacological studies with PAF in endotoxin shock were performed in rats and there are marked species differences in LPS responses. Additionally, possible compensatory mechanisms are well known to have the ability to confound conclusions drawn from knockout mice. Despite the caveats, at first glance we may have to reassess our concepts of PAF action

and/or reconsider the issue of alternative receptors or modes of action for this group of molecules. Finally, there are many areas of pathophysiology in which PAF has been invoked that have not yet been tested rigorously in PAFRÿ/ÿ mice, in particular, acute and chronic inflammation, platelet effects, neurophysiological processes, and responses mediated through putative intracellular PAF effects. To date then, the receptor knockout animals show alterations only in IgE-mediated responses ± anaphylaxis and bronchoconstriction.

THERAPEUTIC UTILITY

Effects of inhibitors (antibodies) to receptors Over 50 compounds have been described with PAF receptor antagonist activity (see Hwang, 1994; Koltai et al., 1994; Negro Alvarez et al., 1997, for example). They range widely in chemical composition and have often been synthesized on a semi-rational basis (Lamotte-Brasseur et al., 1991; Lamouri et al., 1993; Heymans et al., 1997). Some are structurally related to PAF but many are not. Some are of natural origin, including a variety of compounds extracted from the evolutionarily ancient tree, Gingko biloba, but most are synthetic. Of importance for antagonism of a phospholipid mediator, some are lipophilic (CV62091) but others, including the widely used WEB2086, are hydrophilic and presumably do not enter the lipid bilayer, but rather act on the receptor from the aqueous phase. A representative, but by no means exhaustive, sample of these antagonists is listed in Table 1, each with a relatively recent reference as a potential access to its bibliography. In most cases the antagonists have been shown to act in one or more of the myriad in vivo models wherein PAF has been felt to play a part. In fact, it is from these studies of antagonist effect that, for the most part, the complexity and breadth of potential PAF actions have been derived. In many cases the specificity for PAF (and related phospholipids) is quite remarkable and with newer generation compounds, the activity has increased significantly. Because of the plethora of receptor antagonists, little emphasis has been placed on PAF synthesis inhibitors. Not surprisingly from the cPLA2 knockout data, inhibitors of this enzyme do reduce PAF production in stimulated cells and drugs have been described that block both PAF and leukotriene production at this step (e.g. Farina et al., 1994). However, in the light of new evidence for multiple

2190 Peter M. Henson Table 1 Some of the compounds described with PAF receptor antagonist activity Source

Name/number

Reference

Ginkolides

Heller et al., 1998; Lamouri et al., 1993; Simon et al., 1987

Kadsurenone

Ko et al., 1992; Shen et al., 1985

Gliotoxins

Okamoto et al., 1986a, 1986b; Yoshida et al., 1988; Tanaka et al., 1995

Natural compounds

FR-900452 FR-49175 Synthetic compounds Structurally related to PAF

Structurally unrelated to PAF

CV-3988

Adachi et al., 1997; Dupuis et al., 1998; Khoury and Langleben, 1996; Okano et al., 1995

SR-27417

Bernat et al., 1992; Herbert, 1992; Kravchenko et al., 1995

ONO-6240

Toyofuku et al., 1986; Terashita et al., 1987; Ando et al., 1990

RO-19-3704

Wallace et al., 1987; Lagente et al., 1988; Mounier et al., 1993

E-5880

Nakatani et al., 1996; Ono et al., 1996; Takada et al., 1998

RP-48740

Ferreira et al., 1991; Lefort et al., 1988; Weissman et al., 1993

RP-59227

Zhang and Decker, 1994; Auchampach et al., 1998

WEB-2086

Liu et al., 1998; Latorre et al., 1999; Mori et al., 1999; Okayama et al., 1999; Rainsford, 1999

WEB-2170 BN-50730

Singh et al., 1997; Bazan, 1998

TCV-309

Kawamura et al., 1994; Ide et al., 1995; Adachi et al., 1997; Izuoka et al., 1998;

YM-264

Arima et al., 1995; Nagaoka et al., 1997

A-85783, ABT-299

Albert et al., 1996; Kruse-Elliott et al., 1996; Travers et al., 1998

Adapted from Heller et al. (1998).

pathways (and even nonenzymatic mechanisms) for generation of PAF-like molecules, disinterest in this approach seems even more reasonable. Despite all the receptor inhibitors, there is to date no example of an effective therapeutic role for any of these drugs. A few clinical trials have been reported in, for example, asthma (Hozawa et al., 1995; Evans et al., 1997), arthritis (Hilliquin et al., 1995, 1998), sepsis (Dhainaut et al., 1994, 1998; Froon et al., 1996), and psoriasis (Elbers et al., 1994), with equivocal results. This has led to some disillusionment on the part of the pharmaceutical industry. Part of the problem may be the misconception that any one mediator, PAF in particular, plays such a key role that its blockade will, in and of itself, prevent a given disease process. For reasons of historical accident, the early history of the PAF field may have given a disproportionate emphasis to its role as mediator

of allergic reactions and asthma (resulting in many complex schemes with PAF and/or leukotrienes as major participants in these processes). Had the molecule been developed out of its possible role in modulation of the vascular system, or in the category of ether lipid antitumor agents a different emphasis might have ensued. More realistically, PAF is an accessory molecule, playing key but not necessarily sole, roles in a variety of complex mediator networks from inflammation to parturition, neuronal function to vascular motor tone. On the other hand, there is also a perception among some investigators that the reason for the problems with early clinical trials is not only the possible choice of disease processes to examine but, more importantly, the relatively low efficacy of the first-generation antagonists. Some of the later compounds are reputedly in early trials and may show promise in the future.

PAF Receptors 2191

References Adachi, T., Aoki, J., Manya, H., Asou, H., Arai, H., and Inoue, K. (1997). PAF analogues capable of inhibiting PAF acetylhydrolase activity suppress migration of isolated rat cerebellar granule cells. Neurosci Lett. 235, 133±136. Albert, D. H., Conway, R. G., Magoc, T. J., Tapang, P., Rhein, D. A., Luo, G., Holms, J. H., Davidsen, S. K., Summers, J. B., and Carter, G. W. (1996). Properties of ABT-299, a prodrug of A85783, a highly potent platelet activating factor receptor antagonist. J. Pharmacol. Exp. Ther. 277, 1595±1606. Ali, H., Richardson, R. M., Tomhave, E. D., DuBose, R. A., Haribabu, B., and Snyderman, R. (1994). Regulation of stably transfected platelet activating factor receptor in RBL-2H3 cells. Role of multiple G proteins and receptor phosphorylation. J. Biol. Chem. 269, 24557±24563. Ali, H., Fisher, I., Haribabu, B., Richardson, R. M., and Snyderman, R. (1997). Role of phospholipase Cbeta3 phosphorylation in the desensitization of cellular responses to platelet-activating factor. J. Biol. Chem. 272, 11706±11709. Ali, H., Sozzani, S., Fisher, I., Barr, A. J., Richardson, R. M., Haribabu, B., and Snyderman R. (1998). Differential regulation of formyl peptide and platelet-activating factor receptors. Role of phospholipase Cbeta3 phosphorylation by protein kinase A. J. Biol. Chem. 273, 11012±11016. Ando, M., Suginami, H., and Matsuura S. (1990). Pregnancy suppression by a platelet activating factor antagonist, ONO-6240, in mice. Asia Oceania J. Obstet. Gynaecol. 16, 169±174. Arima, M., Yukawa, T., and Makino S. (1995). Effect of YM264 on the airway hyperresponsiveness and the late asthmatic response in a guinea pig model of asthma. Chest 108, 529±534. Auchampach, J. A., Pieper, G. M., Cavero, I., and Gross G. J. (1998). Effect of the platelet-activating factor antagonist RP 59227 (Tulopafant) on myocardial ischemia/reperfusion injury and neutrophil function. Basic Res. Cardiol. 93, 361±371. Barzaghi, G., Sarau, H. M., and Mong, S. (1989). Platelet-activating factor-induced phosphoinositide metabolism in differentiated U-937 cells in culture. J. Pharmacol. Exp. Ther. 248, 559±566. Bass, D. A., McPhail, L. C., Schmitt, J. D., Morris-Natschke, S., McCall, C. E., and Wykle, R. L. (1988). Selective priming of rate and duration of the respiratory burst of neutrophils by 1, 2diacyl and 1-O-alkyl-2-acyl diglycerides. Possible relation to effects on protein kinase C. J. Biol. Chem. 263, 19610±19617. Bauldry, S. A., Wykle, R. L., and Bass, D. A. (1991). Differential actions of diacyl- and alkylacylglycerols in priming phospholipase A2, 5-lipoxygenase and acetyltransferase activation in human neutrophils. Biochim. Biophys. Acta 1084, 178±184. Bazan, N. G. (1998). The neuromessenger platelet-activating factor in plasticity and neurodegeneration. Progr. Brain Res. 118, 281±291. Bazan, N. G., Packard, M. G., Teather, L., and Allan, G. (1997). Bioactive lipids in excitatory neurotransmission and neuronal plasticity. Neurochem. Int. 30, 225±231. Bennett, S. A., and Birnboim, H. C. (1997). Receptor-mediated and protein kinase-dependent growth enhancement of primary human fibroblasts by platelet activating factor [see comments]. Mol. Carcinog. 20, 366±375. Bernat, A., Herbert, J. M., Salel, V., Lespy, L., and Maffrand, J. P. (1992). Protective effect of SR 27417, a novel PAF antagonist, on PAF- or endotoxin-induced hypotension in the rat and the guinea-pig. J. Lipid Mediat. 5, 41±48. Bito, H., Honda, Z., Nakamura, M., and Shimizu, T. (1994). Cloning, expression and tissue distribution of rat platelet-activating-factor-receptor cDNA. Eur. J. Biochem. 221, 211±218.

Boulay, F., Naik, N., Giannini, E., Tardif, M., Brouchon., and Brouchon, L. (1997). Phagocyte chemoattractant receptors. Ann. N.Y. Acad. Sci. 832, 69±84. Bratton, D., Dreyer, E., Kailey, J., Fadok, V., Clay, K., and Henson, P. (1992). The mechanism of internalization of PAF in activated human neutrophils: enhanced transbilayer movement across the plasma membrane. J. Immunol. 148, 514±523. Brunner, T., de Weck, A. L., and Dahinden C. A. (1991). Plateletactivating factor induces mediator release by human basophils primed with IL-3, granulocyte-macrophage colony-stimulating factor, or IL-5. J. Immunol. 147, 237±242. Casale, T. B., Erger, R. A., and Little, M. M. (1993). Plateletactivating factor-induced human eosinophil transendothelial migration: evidence for a dynamic role of the endothelium. Am. J. Respir. Cell Mol Biol. 8, 77±82. Chaqour, B., Howard, P. S., Richards, C. F., and Macarak, E. J. (1999). Mechanical stretch induces platelet-activating factor receptor gene expression through the NF-kappaB transcription factor. J. Mol. Cell. Cardiol. 31, 1345±1355. Chase, P. B., Halonen, M., and Regan, J. W. (1993). Cloning of a human platelet-activating factor receptor gene: evidence for an intron in the 50 -untranslated region. Am. J. Respir. Cell. Mol. Biol. 8, 240±244. Clay, K., Johnson, C., and Henson, P. (1990). Binding of platelet activating factor to albumin. Biochim. Biophys. Acta 1046, 309±314. Cundell, D. R., Gerard, N. P., Gerard, C., Cidanpaan-Heikkila, I., and Tuomanen, E. I. (1995). Streptococcus pneumoniae anchor to activated human cells by the receptor for platelet-activating factor. Nature 377, 435±438. Dhainaut, J. F., Tenaillon, A., Le Tulzo, Y., Schlemmer, B., Solet, J. P., Wolff, M., Holzapfel, L., Zeni, F., Dreyfuss, D., Mira, J. P., de Vathaire, F., and Guinot, P. (1994). Plateletactivating factor receptor antagonist BN 52021 in the treatment of severe sepsis: a randomized, double-blind, placebo- controlled, multicenter clinical trial. BN 52021 Sepsis Study Group. Crit. Care Med. 22, 1720±1728. Dhainaut, J. F., Tenaillon, A., Hemmer, M., Damas, P., Le Tulzo, Y., Radermacher, P., Schaller, M. D., Sollet, J. P., Wolff, M., Holzapfel, L., Zeni, F., Vedrinne, J. M., de Vathaire, F., Gourlay, M. L., Guinot, P., and Mira, J. P. (1998). Confirmatory platelet-activating factor receptor antagonist trial in patients with severe gram-negative bacterial sepsis: a phase III, randomized, double-blind, placebo-controlled, multicenter trial. BN 52021 Sepsis Investigator Group [see comments]. Crit. Care Med. 26, 1963±1971. Domingo, M. T., Spinnewyn, B., Chabrier, P. E., and Braquet, P. (1994). Changes in [3H]PAF binding and PAF concentrations in gerbil brain after bilateral common carotid artery occlusion: a quantitative autoradiographic study. Brain Res. 640, 268±276. Dupuis, F., Gachard, N., Allegraud, A., Dulery, C., Praloran, V., and Denizot Y. (1998). Effect of platelet-activating factor on the growth of human erythroid and myeloid CD34‡ progenitors. Mediators Inflamm. 7, 99±103. Elbers, M. E., Gerritsen, M. J., and van de Kerkhof, P. C. (1994). The effect of topical application of the platelet-activating factorantagonist, Ro 24±0238, in psoriasis vulgaris ± a clinical and immunohistochemical study. Clin. Exp. Dermatol. 19, 453±457. Evans, D. J., Barnes, P. J., Cluzel, M., and O'Connor, B. J. (1997). Effects of a potent platelet-activating factor antagonist, SR27417A, on allergen-induced asthmatic responses. Am. J. Respir. Crit. Care Med. 156, 11±16. Farina, P. R., Graham, A. G., Hoffman, A. F., Watrous, J. M., Borgeat, P., Nadeau, M., Hansen, G., Dinallo, R. M.,

2192 Peter M. Henson Adams, J., Miao, C. K., Lazer, E. S., Parks, T. P., and Homon, C. A. (1994). BIRM 270: a novel inhibitor of arachidonate release that blocks leukotriene B4 and platelet-activating factor biosynthesis in human neutrophils. J. Pharmacol. Exp. Ther. 271, 1418±1426. Ferreira, M. G., Braquet, P., and Fonteles, M. C. (1991). Effects of PAF antagonists on renal vascular escape and tachyphylaxis in perfused rabbit kidney. Lipids 26, 1329±1332. Froon, A. M., Greve, J. W., Buurman, W. A., van der Linden, C. J., Langemeijer, H. J., Ulrich, C., and Bourgeois. M. (1996). Treatment with the platelet-activating factor antagonist TCV309 in patients with severe systemic inflammatory response syndrome: a prospective, multi-center, double-blind, randomized phase II trial. Shock 5, 313±319. Garcia Rodriguez, C., Cundell, D. R., Tuomanen, E. I., Kolakowski Jr, L. F., Gerard, C., and Gerard, N. P. (1995a). The role of N-glycosylation for functional expression of the human platelet-activating factor receptor. Glycosylation is required for efficient membrane trafficking. J. Biol. Chem. 270, 25178±25184. Garcia Rodriguez, C., Cundell, D. R., Tuomanen, E. I., Kolakowski Jr, L. F., Gerard, C., and Gerard N. P. (1995b). The role of N-glycosylation for functional expression of the human platelet-activating factor receptor. Glycosylation is required for efficient membrane trafficking. J. Biol. Chem. 270, 25178±25184. Gay, J. C. (1993). Mechanism and regulation of neutrophil priming by platelet-activating factor. J. Cell Physiol. 156, 189±197. Gerard, N. P., and Gerard, C. (1994). Receptor-dependent internalization of platelet-activating factor. J. Immunol. 152, 793± 800. Guthrie, L., McPhail, L., Henson, P., and Johnston, Jr. R. (1984). Priming of neutrophils for enhanced release of oxygen metabolites by bacterial lipopolysaccharide. Evidence for increased activity of the superoxide-producing enzyme. J. Exp. Med. 160, 1656±1671. Heller, A., Koch, T., Schmeck, J., and van Ackern, K. (1998). Lipid mediators in inflammatory disorders. Drugs 55, 487±496. Henson, P., and Landes, R. (1976). Activation of platelets by platelet activating factor (PAF) derived from IgE-sensitized basophils. IV. PAF does not activate platelet factor 3 (PF3). Br. J. Haematol. 34, 269±282. Henson, P., and Pinckard R. (1977). Basophil-derived plateletactivating factor (PAF) as an in vivo mediator of acute allergic reactions: demonstration of specific desensitization of platelets to PAF during IgE-induced anaphylaxis in the rabbit. J. Immunol. 119, 2179±2184. Herbert, J. M. (1992). Characterization of specific binding sites of 3H-labelled platelet- activating factor ([3H]PAF) and a new antagonist, [3H]SR 27417, on guinea-pig tracheal epithelial cells. Biochem. J. 284, 201±206. Heymans, F., Dive, G., Lamouri, A., Bellahsene, T., Touboul, E., Huet, J., Tavet, F., Redeuilh, C., and Godfroid J. J. (1997). Design and modeling of new platelet-activating factor antagonists. 3. Relative importance of hydrophobicity and electronic distribution in piperazinic series. J. Lipid Mediat. Cell Signal. 15, 161±173. Hilliquin, P., Guinot, P., Chermat-Izard, V., Puechal, X., and Menkes, C. J. (1995). Treatment of rheumatoid arthritis with platelet activating factor antagonist BN 50730. J. Rheumatol. 22, 1651±1654. Hilliquin, P., Chermat-Izard, V., and Menkes, C. J. (1998). A double blind, placebo controlled study of a platelet activating factor antagonist in patients with rheumatoid arthritis. J. Rheumatol. 25, 1502±1507.

Hirabayashi, T., Kume, K., and Shimizu, T. (1998). Conditional expression of the dual-specificity phosphatase PYST1/MKP-3 inhibits phosphorylation of cytosolic phospholipase A2 in Chinese hamster ovary cells. Biochem. Biophys. Res. Commun. 253, 485±488. Honda, Z., Nakamura, M., Miki, I., Minami, M., Watanabe, T., Seyama, Y., Okado, H., Toh, H., Ito, K., Miyamoto, T., and Shimizu, T. (1991). Cloning by functional expression of plateletactivating factor receptor from guinea-pig lung. Nature 349, 342±346. Honda, Z., Takano, T., Gotoh, Y., Nishida, E., Ito, K., and Shimizu, T. (1994). Transfected platelet-activating factor receptor activates mitogen- activated protein (MAP) kinase and MAP kinase in Chinese hamster ovary cells. J. Biol. Chem. 269, 2307±2315. Hozawa, S., Haruta, Y., Ishioka, S., and Yamakido, M. (1995). Effects of a PAF antagonist, Y-24180, on bronchial hyperresponsiveness in patients with asthma. Am. J. Respir. Crit. Care Med. 152, 1198±1202. Huang, Y. H., Schafer-Elinder, L., Wu, R., Claesson, H. E., Frostegard., and J. (1999). Lysophosphatidylcholine (LPC) induces proinflammatory cytokines by a platelet-activating factor (PAF) receptor-dependent mechanism. Clin. Exp. Immunol. 116, 326±331. Hwang, S. B. (1988). Identification of a second putative receptor of platelet-activating factor from human polymorphonuclear leukocytes. J. Biol. Chem. 263, 3225±3233. Hwang, S. B. (1991). High affinity receptor binding of plateletactivating factor in rat peritoneal polymorphonuclear leukocytes. Eur. J. Pharmacol. 196, 169±175. Hwang, S.-B. (1994). In ``Lipid Mediators'' (ed F. M. Cunningham), Platelet activating factor: receptors and receptor antagonists, pp. 181±192. Academic Press, London. Ide, S., Kawahara, K., Takahashi, T., Sasaki, N., Shingu, H., Nagayasu, T., Yamamoto, S., Tagawa, T., and Tomita, M. (1995). Donor administration of PAF antagonist (TCV-309) enhances lung preservation. Transplant. Proc. 27, 570±573. Ihida, K., Predescu, D., Czekay, R. P., and Palade, G. E. (1999). Platelet activating factor receptor (PAF-R) is found in a large endosomal compartment in human umbilical vein endothelial cells. J. Cell Sci. 112, 285±295. Ishii, S., Matsuda, Y., Nakamura, M., Waga, I., Kume, K., Izumi, T., and Shimizu, T. (1996). A murine platelet-activating factor receptor gene: cloning, chromosomal localization and upregulation of expression by lipopolysaccharide in peritoneal resident macrophages. Biochem. J. 314, 671±678. Ishii, S., Nagase, T., Tashiro, F., Ikuta, K., Sato, S., Waga, I., Kume, K., Miyazaki, J., and Shimizu, T. (1997). Bronchial hyperreactivity, increased endotoxin lethality and melanocytic tumorigenesis in transgenic mice overexpressing platelet-activating factor receptor. EMBO J. 16, 133±142. Ishii, I., Saito, E., Izumi, T., Ui, M., and Shimizu, T. (1998a). Agonist-induced sequestration, recycling, and resensitization of platelet-activating factor receptor. Role of cytoplasmic tail phosphorylation in each process. J. Biol. Chem. 273, 9878±9885. Ishii, S., Kuwaki, T., Nagase, T., Maki, K., Tashiro, F., Sunaga, S., Cao, W. H., Kume, K., Fukuchi, Y., Ikuta, K., Miyazaki, J., Kumada, M., and Shimizu T. (1998b). Impaired anaphylactic responses with intact sensitivity to endotoxin in mice lacking a platelet-activating factor receptor. J. Exp. Med. 187, 1779±1788. Izuoka, T., Takayama, Y., Sugiura, T., Taniguchi, H., Tamura, T., Kitashiro, S., Jikuhara, T., and Iwasaka T. (1998). Role of platelet-activating factor on extravascular lung water after coronary reperfusion in dogs. Jpn J. Physiol. 48, 157±161.

PAF Receptors 2193 Kadiri, C., Cherqui, G., Masliah, J., Rybkine, T., Etienne, J., and Bereziat, G. (1990). Mechanism of N-formyl-methionyl-leucylphenylalanine- and platelet-activating factor-induced arachidonic acid release in guinea pig alveolar macrophages: involvement of a GTP-binding protein and role of protein kinase A and protein kinase C. Mol. Pharmacol. 38, 418±425. Kato, K., Clark, G. D., Bazan, N. G., and Zorumski, C. F. (1994). Platelet-activating factor as a potential retrograde messenger in CA1 hippocampal long-term potentiation. Nature 367, 175±179. Kawamura, M., Kitayoshi, T., Terashita, Z., Fujiwara, S., Takatani, M., and Nishikawa, K. (1994). Effects of TCV-309, a novel PAF antagonist, on circulatory shock and hematological abnormality induced by endotoxin in dogs. J. Lipid Mediat. Cell Signal. 9, 255±265. Khoury, J., and Langleben, D. (1996). Platelet-activating factor stimulates lung pericyte growth in vitro. Am. J. Physiol. 270, L298±304. Kishimoto, S., Shimadzu, W., Izumi, T., Shimizu, T., Fukuda, T., Makino, S., Sugiura, T., and Waku K. (1996a). Regulation by IL-5 of expression of functional platelet-activating factor receptors on human eosinophils. J. Immunol. 157, 4126±4132. Kishimoto, S., Shimazu, W., Izumi, T., Shimizu, T., Fukuda, T., Makino, S., Sugiura, T., and Waku K. (1996b). Enhanced expression of platelet-activating factor receptor on human eosinophils by interleukin-3, interleukin-5 and granulocyte-macrophage colony-stimulating factor. Int. Arch. of Allergy Immunol. 111, 63±65. Kishimoto, S., Shimadzu, W., Izumi, T., Shimizu, T., Sagara, H., Fukuda, T., Makino, S., Sugiura, T., and Waku, K. (1997). Comparison of platelet-activating factor receptor mRNA levels in peripheral blood eosinophils from normal subjects and atopic asthmatic patients. Int. Arch. Allergy Immunol. 114, 60±63. Kitchen, E., Rossi, A. G., Condliffe, A. M., Haslett, C., and Chilvers, E. R. (1996). Demonstration of reversible priming of human neutrophils using platelet- activating factor. Blood 88, 4330±4337. Ko, F. N., Wu, T. S., Liou, M. J., Huang, T. F., and Teng, C. M. (1992). PAF antagonism in vitro and in vivo by aglafoline from Aglaia elliptifolia Merr. Eur. J. Pharmacol. 218, 129±135. Koltai, M., Guinot, P., Hosford, D., and Braquet, P. G. (1994). Platelet-activating factor antagonists: scientific background and possible clinical applications. Adv. Pharmacol. 28, 81±167. Kravchenko, V. V., Pan, Z., Han, J., Herbert, J. M., Ulevitch, R. J., and Ye, R. D. (1995). Platelet-activating factor induces NFkappa B activation through a G protein-coupled pathway. J. Biol. Chem. 270, 14928±14934. Kruse-Elliott, K. T., Albert, D. H., Summers, J. B., Carter, G. W., Zimmerman, J. J., and Grossman, J. E. (1996). Attenuation of endotoxin-induced pathophysiology by a new potent PAF receptor antagonist. Shock 5, 265±273. Kunz, D., Gerard, N. P., and Gerard, C. (1992). The human leukocyte platelet-activating factor receptor. cDNA cloning, cell surface expression, and construction of a novel epitopebearing analog. J. Biol. Chem. 267, 9101±9106. Lagente, V., Desquand, S., Hadvary, P., Cirino, M., LellouchTubiana, A., Lefort, J., and Vargaftig, B. B. (1988). Interference of the Paf antagonist Ro (19-3704 with Paf and antigen- induced bronchoconstriction in the guinea-pig. Br. J. Pharmacol. 94, 27±36. Lamotte-Brasseur, J., Heymans, F., Dive, G., Lamouri, A., Batt, J. P., Redeuilh, C., Hosford, D., Braquet, P., and Godfroid, J. J. (1991). PAF receptor and `Cache-oreilles' effect. Simple PAF antagonists. Lipids 26, 1167±1171. Lamouri, A., Heymans, F., Tavet, F., Dive, G., Batt, J. P., Blavet, N., Braquet, P., and Godfroid, J. J. (1993). Design

and modeling of new platelet-activating factor antagonists. 1. Synthesis and biological activity of 1, 4-bis(30 , 40 , 50 - trimethoxybenzoyl)-2-[[(substituted carbonyl and carbamoyl)oxy]methyl]piperazines. J. Med. Chem. 36, 990±1000. Latorre, E., Aragones, M. D., Fernandez, I., and Catalan, R. E. (1999). Platelet-activating factor modulates brain sphingomyelin metabolism. Eur. J. Biochem. 262, 308±314. Le Gouill, C., Parent, J. L., Rola-Pleszczynski, M., and Stankova, J. (1997). Role of the Cys90, Cys95 and Cys173 residues in the structure and function of the human platelet-activating factor receptor. FEBS Lett. 402, 203±208. Lefort, J., Sedivy, P., Desquand, S., Randon, J., Coeffier, E., Maridonneau-Parini, I., Floch, A., Benveniste, J., and Vargaftig B. B. (1988). Pharmacological profile of 48740 R. P., a PAF-acether antagonist. Eur. J. Pharmacol. 150, 257±268. LeVan, T. D., Dow, S. B., Chase, P. B., Bloom, J. W., Regan, J. W., Cunningham, E., and Halonen, M. (1997). Evidence for platelet-activating factor receptor subtypes on human polymorphonuclear leukocyte membranes. Biochem. Pharmacol. 54, 1007±1012. Liu, L., Zuurbier, A. E., Mul, F. P., Verhoeven, A. J., Lutter, R., Knol, E. F., and Roos D. (1998). Triple role of platelet-activating factor in eosinophil migration across monolayers of lung epithelial cells: eosinophil chemoattractant and priming agent and epithelial cell activator. J. Immunol. 161, 3064±3070. M'Rabet, L., Coffer, P. J., Wolthuis, R. M., Zwartkruis, F., Koenderman, L., and Bos, J. L. (1999). Differential fMet-LeuPhe- and platelet-activating factor-induced signaling toward ral activation in primary human neutrophils. J. Biol. Chem. 274, 21847±21852. Mombaerts, P. (1999). Molecular biology of odorant receptors in vertebrates. Annu. Rev. Neurosci. 22, 487±509. Mori, N., Horie, Y., Gerritsen, M. E., and Granger, D. N. (1999). Ischemia-reperfusion induced microvascular responses in LDLreceptorÿ/ÿ mice. Am. J. Physiol. 276, H1647±1654. Mounier, C., Hatmi, M., Faili, A., Bon, C., and Vargaftig, B. B. (1993). Competitive inhibition of phospholipase A2 activity by the platelet- activating factor antagonist Ro 19-3704 and evidence for a novel suppressive effect on platelet activation. J. Pharmacol. Exp. Ther. 264, 1460±1467. Mutoh, H., Bito, H., Minami, M., Nakamura, M., Honda, Z., Izumi, T., Nakata, R., Kurachi, Y., Terano, A., and Shimizu, T. (1993). Two different promoters direct expression of two distinct forms of mRNAs of human platelet-activating factor receptor. FEBS Lett. 322, 129±134. Mutoh, H., Ishii, S., Izumi, T., Kato, S., and Shimizu, T. (1994). Platelet-activating factor (PAF) positively auto-regulates the expression of human PAF receptor transcript 1 (leukocytetype) through NF-kappa B. Biochem. Biophys. Res. Commun. 205, 1137±1142. Nagaoka, H., Hara, H., Suzuki, T., Takahashi, T., Takeuchi, M., Matsuhisa, A., Saito, M., Yamada, T., Tomioka, K., and Mase, T. (1997). 2-(3-Pyridyl)thiazolidine-4-carboxamides. 1. Novel orally active antagonists of platelet-activating factor (PAF). Chem. Pharm. Bull. (Tokyo) 45, 1659±1664. Nakamura, M., Honda, Z., Izumi, T., Sakanaka, C., Mutoh, H., Minami, M., Bito, H., Seyama, Y., Matsumoto, T., Noma, M., and Simizu, T. (1991). Molecular cloning and expression of platelet-activating factor receptor from human leukocytes. J. Biol. Chem. 266, 20400±20405. Nakao, A., Watanabe, T., Bitoh, H., Imaki, H., Suzuki, T., Asano, K., Taniguchi, S., Nosaka, K., Shimizu, T., and Kurokawa, K. (1997). cAMP mediates homologous downregulation of PAF receptor mRNA expression in mesangial cells. Am. J. Physiol. 273, F445±450.

2194 Peter M. Henson Nakatani, T., Sakamoto, Y., Ando, H., and Kobayashi, K. (1996). Effects of platelet-activating factor antagonist E5880 on intrahepatic and systemic metabolic responses to transient hepatic inflow occlusion and reperfusion in the rabbit. World J. Surg. 20, 1060±1067; discussion 1067±1068. Negro Alvarez, J. M., Miralles Lopez, J. C., Ortiz Martinez, J. L., Abellan Aleman, A., and Rubio del Barrio, R. (1997). Plateletactivating factor antagonists. Allergol. Immunopathol. 25, 249±258. Nick, J. A., Avdi, N. J., Young, S. K., Knall, C., Gerwins, P., Johnson, G. L., and Worthen, G. S. (1997). Common and distinct intracellular signaling pathways in human neutrophils utilized by platelet activating factor and FMLP. J. Clin. Invest. 99, 975±986. Okahara, K., Sun, B., Kawasaki, T., Monden, M., and Kambayashi, J. (1998). Expression of platelet-activating factor receptor transcript-2 is induced by shear stress in HUVEC. Prostaglandins Other Lipid Mediat. 55, 323±329. Okamoto, M., Yoshida, K., Nishikawa, M., Ando, T., Iwami, M., Kohsaka, M., and Aoki, H. (1986a). FR-900452, a specific antagonist of platelet activating factor (PAF) produced by Streptomyces phaeofaciens. I. Taxonomy, fermentation, isolation, and physico-chemical and biological characteristics. J. Antibiot. (Tokyo) 39, 198±204. Okamoto, M., Yoshida, K., Uchida, I., Kohsaka, M., and Aoki, H. (1986b). Studies of platelet activating factor (PAF) antagonists from microbial products. II. Pharmacological studies of FR49175 in animal models. Chem. Pharm. Bull. (Tokyo) 34, 345±348. Okano, S., Tagawa, M., and Urakawa, N. (1995). Effect of platelet activating factor antagonist (CV-3988) on 6-keto- PGF1 alpha and thromboxane B2 in dogs with experimental endotoxin- induced shock. J. Vet. Med. Sci. 57, 81±85. Okayama, N., Coe, L., Oshima, T., Itoh, M., and Alexander, J. S. (1999). Intracellular mechanisms of hydrogen peroxide-mediated neutrophil adherence to cultured human endothelial cells. Microvasc. Res. 57, 63±74. Ono, S., Mochizuki, H., and Tamakuma, S. (1996). A clinical study on the significance of platelet-activating factor in the pathophysiology of septic disseminated intravascular coagulation in surgery. Am. J. Surg. 171, 409±415. Pang, J. H., Hung, R. Y., Wu, C. J., Fang, Y. Y., and Chau, L. Y. (1995). Functional characterization of the promoter region of the platelet-activating factor receptor gene. Identification of an initiator element essential for gene expression in myeloid cells. J. Biol. Chem. 270, 14123±14129. Parent, J. L., Le Gouill, C., de Brum-Fernandes, A. J., RolaPleszczynski, M., and Stankova, J. (1996). Mutations of two adjacent amino acids generate inactive and constitutively active forms of the human platelet-activating factor receptor. J. Biol. Chem. 271, 7949±7955. Partrick, D. A., Moore, E. E., Moore, F. A., Barnett, C. C., and Silliman C. C. (1997). Lipid mediators up-regulate CD11b and prime for concordant superoxide and elastase release in human neutrophils. J. Trauma 43, 297±302; discussion 302±293. Patel, K. D., Zimmerman, G. A., Prescott, S. M., and McIntyre, T. M. (1992). Novel leukocyte agonists are released by endothelial cells exposed to peroxide. J. Biol. Chem. 267, 15168±15175. Predescu, D., Ihida, K., Predescu, S., and Palade, G. E. (1996). The vascular distribution of the platelet-activating factor receptor. Eur. J. Cell Biol. 69, 86±98. Rainsford, K. D. (1999). Inhibition by leukotriene inhibitors, and calcium and platelet-activating factor antagonists, of

acute gastric and intestinal damage in arthritic rats and in cholinomimetic-treated mice. J. Pharm. Pharmacol. 51, 331±339. Reinhardt, J. C., Cui, X., and Roudebush, W. E. (1999). Immunofluorescent evidence of the platelet-activating factor receptor on human spermatozoa. Fertil. Steril. 71, 941±942. Rougier, F., Dupuis, F., Cornu, E., Dulery, C., Praloran, V., and Denizot, Y. (1997). Platelet-activating factor and antagonists modulate DNA synthesis in human bone marrow stromal cell cultures. J. Lipid Mediat. Cell Signal. 16, 147±153. Sasaki, T., Tohyama, T., Oda, K., Toyama, H., Ishii, S., Senda, M., Karasawa, K., Satoh, N., Setaka, M., Nojima, S., Nozaki, T., and Braquet, P. (1996). A distribution study of 11C platelet-activating factor (PAF) analogs in normal and tumorbearing mice. Nucl. Med. Biol. 23, 309±314. Shalit, M., Dabiri, G. A., and Southwick, F. S. (1987). Plateletactivating factor both stimulates and `primes' human polymorphonuclear leukocyte actin filament assembly. Blood 70, 1921±1927. Shen, T. Y., Hwang, S. B., Chang, M. N., Doebber, T. W., Lam, M. H., Wu, M. S., Wang, X., Han, G. Q., and Li, R. Z. (1985). Characterization of a platelet-activating factor receptor antagonist isolated from haifenteng (Piper futokadsura): specific inhibition of in vitro and in vivo platelet-activating factorinduced effects. Proc. Natl Acad. Sci. USA 82, 672±676. Shimada, A., Ota, Y., Sugiyama, Y., Sato, S., Kume, K., Shimizu, T., and Inoue, S. (1998a). In situ expression of platelet-activating factor (PAF)-receptor gene in rat skin and effects of PAF on proliferation and differentiation of cultured human keratinocytes. J. Invest. Dermatol. 110, 889±893. Shimada, A., Ota, Y., Sugiyama, Y., Sato, S., Kume, K., Shimizu, T., and Inoue, S. (1998b). In situ expression of platelet-activating factor (PAF)-receptor gene in rat skin and effects of PAF on proliferation and differentiation of cultured human keratinocytes. J. Invest. Dermatol. 110, 889±893. Shimizu, T., and Mutoh, H. (1997). Structure and regulation of platelet activating factor receptor gene. Adv. Exp. Med. Biol. 407, 197±204. Shindo, K., Koide, K., Hirai, Y., Sumitomo, M., and Fukumura, M. (1996). Priming effect of platelet activating factor on leukotriene C4 from stimulated eosinophils of asthmatic patients. Thorax 51, 155±158. Shindo, K., Koide, K., and Fukumura, M. (1997). Enhancement of leukotriene B4 release in stimulated asthmatic neutrophils by platelet activating factor. Thorax 52, 1024±1029. Simon, M. F., Chap, H., Braquet, P., and Douste-Blazy, L. (1987). Effect of BN 52021, a specific antagonist of platelet activating factor (PAF-acether), on calcium movements and phosphatidic acid production induced by PAF-acether in human platelets. Thromb. Res. 45, 299±309. Singh, N., Sharma, A., and Singh, M. (1997). Effects of BN-50730 (PAF receptor antagonist) and physostigmine (AChE inhibitor) on learning and memory in mice. Meth. Find Exp. Clin. Pharmacol. 19, 585±588. Stengel, D., Antonucci, M., Arborati, M., Hourton, D., Griglio, S., Chapman, M. J., and Ninio, E. (1997). Expression of the PAF receptor in human monocyte-derived macrophages is downregulated by oxidized LDL: relevance to the inflammatory phase of atherogenesis. Arterioscler. Thromb. Vasc. Biol. 17, 954±962. Stewart, A. G. (1994). In ``Lipid Mediators'' (ed F. M. Cunningham), Biological properties of platelet activating factor, pp. 181±192. Academic Press, London. Stojanov, T., and O'Neill, C. (1999). Ontogeny of expression of a receptor for platelet-activating factor in mouse preimplantation embryos and the effects of fertilization and culture in vitro on its expression. Biol. Reprod. 60, 674±682.

PAF Receptors 2195 Takada, Y., Taniguchi, H., Fukunaga, K., Yuzawa, K., Otsuka, M., Todoroki, T., Iijima, T., and Fukao, K. (1998). Prolonged hepatic warm ischemia in non-heart-beating donors: protective effects of FK506 and a platelet activating factor antagonist in porcine liver transplantation. Surgery 123, 692± 698. Takano, T., Honda, Z., Sakanaka, C., Izumi, T., Kameyama, K., Haga, K., Haga, T., Kurokawa, K., and Shimizu, T. (1994). Role of cytoplasmic tail phosphorylation sites of platelet-activating factor receptor in agonist-induced desensitization. J. Biol. Chem. 269, 22453±22458. Tanaka, T., Tokumura, A., and Tsukatani, H. (1995). Plateletactivating factor (PAF)-like phospholipids formed during peroxidation of phosphatidylcholines from different foodstuffs. Biosci. Biotechnol. Biochem. 59, 1389±1393. Terashita, Z., Imura, Y., Takatani, M., Tsushima, S., and Nishikawa, K. (1987). CV-6209, a highly potent antagonist of platelet activating factor in vitro and in vivo. J. Pharmacol. Exp. Ther. 242, 263±268. Toyofuku, T., Kubo, K., Kobayashi, T., and Kusama, S. (1986). Effects of ONO-6240, a platelet-activating factor antagonist, on endotoxin shock in unanesthetized sheep. Prostaglandins 31, 271±281. Travers, J., Pei, Y., Morin, S. M., and Hood, A. F. (1998). Antiinflammatory activity of the platelet-activating factor receptor antagonist A-85783. Arch. Dermatol. Res. 290, 569± 573. Vercellotti, G. M., Yin, H. Q., Gustafson, K. S., Nelson, R. D., and Jacob, H. S. (1988). Platelet-activating factor primes neutrophil responses to agonists: role in promoting neutrophilmediated endothelial damage. Blood 71, 1100±1107. Voelkel, N., Chang, S., Pfeffer, K., Worthen, G., McMurtry, I., and Henson, P. (1986). PAF antagonists: different effects on platelets, neutrophils, guinea pig ileum and PAF-induced vasodilation in isolated rat lung. Prostaglandins 32, 359±372. Wallace, J. L., Steel, G., Whittle, B. J., Lagente, V., and Vargaftig, B. (1987). Evidence for platelet-activating factor as a mediator of endotoxin- induced gastrointestinal damage in the rat. Effects of three platelet-activating factor antagonists. Gastroenterology 93, 765±773. Wang, H., Tan, X., Chang, H., Gonzalez-Crussi, F., Remick, D. G., and Hsueh, W. (1997). Regulation of platelet-activating factor receptor gene expression in vivo by endotoxin, platelet-activating

factor and endogenous tumour necrosis factor. Biochem. J. 322, 603±608. Wardlaw, A. J., Moqbel, R., Cromwell, O., and Kay, A. B. (1986). Platelet-activating factor. A potent chemotactic and chemokinetic factor for human eosinophils. J. Clin. Invest. 78, 1701± 1706. Warringa, R. A., Mengelers, H. J., Kuijper, P. H., Raaijmakers, J. A., Bruijnzeel, P. L., and Koenderman, L. (1992). In vivo priming of platelet-activating factor-induced eosinophil chemotaxis in allergic asthmatic individuals. Blood 79, 1836±1841. Weissman, D., Poli, G., Bousseau, A., and Fauci, A. S. (1993). A platelet-activating factor antagonist, RP 55778, inhibits cytokine-dependent induction of human immunodeficiency virus expression in chronically infected promonocytic cells. Proc. Natl Acad. Sci. USA 90, 2537±2541. Yang, H. H., Pang, J. H., Hung, R. Y., and Chau, L. Y. (1997). Transcriptional regulation of platelet-activating factor receptor gene in B lymphoblastoid Ramos cells by TGF-beta. J. Immunol. 158, 2771±2778. Ye, R. D., Prossnitz, E. R., Zou, A. H., and Cochrane, C. G. (1991). Characterization of a human cDNA that encodes a functional receptor for platelet activating factor. Biochem. Biophys. Res. Commun. 180, 105±111. Yoshida, K., Okamoto, M., Shimazaki, N., and Hemmi, K. (1988). PAF inhibitors of microbial origin. Studies on diketopiperazine derivatives. Progr. Biochem. Pharmacol. 22, 66±80. Yue, T. L., Stadel, J. M., Sarau, H. M., Friedman, E., Gu, J. L., Powers, D. A., Gleason, M. M., Feuerstein, G., and Wang, H. Y. (1992). Platelet-activating factor stimulates phosphoinositide turnover in neurohybrid NCB-20 cells: involvement of pertussis toxin-sensitive guanine nucleotide-binding proteins and inhibition by protein kinase C. Mol. Pharmacol. 41, 281±289. Zhang, F., and Decker, K. (1994). Platelet-activating factor antagonists suppress the generation of tumor necrosis factoralpha and superoxide induced by lipopolysaccharide or phorbol ester in rat liver macrophages. Eur. Cytokine Netw. 5, 311±317. Zimmerman, G. A., Elstad, M. R., Lorant, D. E., McIntyre, T. M., Prescott, S. M., Topham, M. K., Weyrich, A. S., and Whatley, R. E. (1996). Platelet-activating factor (PAF): signalling and adhesion in cell±cell interactions. Adv. Exp. Med. Biol. 416, 297±304.