Human umbilical vein endothelial cells: specific binding of platelet-activating factor and cytosolic calcium flux

Platelet-activating factor synthesis and receptor-mediated signaling are downregulated in ovine newborn lungs: relevance in postnatal pulmonary adaptation and persistent pulmonary hypertension of the newborn

Platelet-activating factor (PAF) is a phospholipid with a wide range of biological activities. We studied PAF metabolism and PAF receptor (PAFR) signaling in perinatal ovine lungs to understand PAF's role in transition of the perinatal pulmonary hemodynamics and pathophysiology of persistent pulmonary hypertension of the newborn. We hypothesized that downregulation of PAF synthesis with upregulation of PAF catabolism by acetylhydrolase (PAF-Ah) in the newborn lung is needed for fetus-to-newborn pulmonary adaptation. Studies were conducted on fetal and newborn lamb pulmonary arteries (PA), veins (PV) and smooth muscle cells (SMC). PAF metabolism, PAFR binding and cell proliferation were studied by cell culture; gene expression was studied by qPCR. Fetal lungs synthesized 60% more PAF than newborn lungs. Compared with the fetal PVs and SMCs, PAF-Ah activity in newborn was 40-60% greater. PAF-Ah mRNA expression in newborn vessels was different from the expression by fetal PA. PAF-Ah gene clone activity confirmed deletion of hypoxia-sensitive site. PAFR mRNA expression by the PVs and SMC-PV of the fetus and newborn was greater than by corresponding PAs and SMC-PA. Q-PCR study of PAFR expression by the SMC-PV of both groups was greater than SMC-PA. Fetal SMCs bound more PAF than the newborn SMCs. PAFR antagonist, CV-3988, inhibited PAFR binding and DNA synthesis by the fetal SMCs, but augmented binding and DNA synthesis by newborn cells. We show different PAF-PAFR mediated effects in perinatal lungs, suggesting both transcriptional and translational regulation of PAF-Ah and PAFR expression in the perinatal lamb lungs. These indicate that the downregulation of PAF-mediated effects postnatally protects against persistent pulmonary hypertension of the newborn.

Platelet-activating factor (PAF) increases NO production in human endothelial cells-real-time monitoring by DAR-4M AM

BACKGROUND

Platelet-activating factor (PAF) is a potent inflammatory lipid mediator that increases vascular permeability and vasodilation. Several studies have addressed the effect of PAF on nitric oxide (NO) production from microvessels in vivo.

OBJECTIVE

The aim of present study was to evaluate the effect of PAF on NO production in primary cultured human vascular endothelial cells.

METHODS

Human umbilical vein endothelial cells (HUVECs) were loaded with diaminorhodamine-4M acetoxymethyl ester (DAR-4MAM), and the cells were stimulated with PAF. Intracellular NO production was monitored as increase in fluorescence intensity. Also, NO production was visualized at cellular levels using DAR-4M AM and fluorescence imaging.

RESULTS

Significant increases in NO production in HUVECs were soon after the PAF stimulation, reaching a plateau after 10 min of the stimulation. The increase of NO production at 10 min after the stimulation was statistically significant (p<0.05) for 0.01-10 microM PAF. PAF-induced NO production was abolished by pretreatment of HUVECs with a NOS inhibitor N(G)-monomethyl-L-arginine (L-NMMA) or PAF receptor antagonist BN 52021. LysoPAF, the inactive metabolite of PAF, did not exert a significant effect on intracellular NO levels.

CONCLUSIONS

These results provide direct evidence that PAF cause intracellular NO production via activation of PAF receptors in human vascular endothelial cells.

Cytokines and neutrophils as important mediators of platelet-activating factor-induced kinin B1 receptor expression

PAF injection into the rat paw is accompanied by the concomitant activation of NF-kappaB and neutrophil influx, which appears to be relevant to the up-regulation of kinin B1 receptors. Herein, we analyse the role of TNF-alpha and IL-1beta production for PAF-induced B1 receptor upregulation in the rat paw. Additionally, we evaluate how cytokine production and neutrophil migration fit into the temporal sequence of events leading to PAF-induced B1 receptor upregulation. In our experiments, treatment with PAF resulted in a marked increase of B1 receptor-mediated paw oedema and in situ production of TNF-alpha at 1 h and IL-1beta at 3 and 6 h later. B1 receptor-mediated paw oedema was significantly inhibited by anti-TNF-alpha antibody and by interleukin-1 receptor antagonist (IRA). TNF-alpha was necessary for the local PAF-induced IL-1beta production. NF-kappaB blocker PDTC prevented the production of both TNF-alpha and IL-1beta, indicating that cytokine production is NF-kappaB dependent. Depletion of neutrophils with an anti-PMN antibody prevented IL-1beta, but not TNF-alpha, production. Although both TNF-alpha and IL-1beta are relevant to functional B1 receptor upregulation, PAF-induced increase in B1 receptor mRNA was markedly suppressed by anti-TNF-alpha and, to a lesser extent, by IRA. B1 receptor mRNA expression was also prevented by the anti-PMN antibody. In conclusion, the activation of the TNF-alpha/neutrophil axis by PAF seems to be sufficient for B1 receptor mRNA production. However, the TNF-alpha/neutrophil axis is also necessary for IL-1beta production. These two processes might lead to the appearance of functional kinin B1 upregulation receptors in vivo after PAF treatment.

Expression and localization of platelet-activating factor receptor in human nasal mucosa

BACKGROUND

Platelet-activating factor (PAF) has been thought to be a potent mediator of allergic rhinitis because PAF was recovered from the nasal lavage fluid of patients with allergic rhinitis after allergen provocation. Furthermore, PAF receptor antagonist attenuates the antigen-induced increase in nasal airway resistance and nasal vascular permeability in sensitized guinea pigs.

OBJECTIVE

To clarify the expression of PAF receptor in human nasal mucosa by investigating PAF receptor messenger RNA (mRNA) expression and its protein localization using polymerase chain reaction (PCR) and immunohistochemical analyses, respectively.

METHODS

Human turbinates were obtained after turbinectomy from 6 patients with nasal obstruction refractory to medical therapy. Total RNA was isolated from human nasal mucosa, and PAF receptor mRNA was detected in these tissues by using reverse transcriptase-PCR analysis. To identify the cells expressing PAF receptor protein, double immunostaining was performed using anti-PAF receptor antibody and monoclonal antileukocyte antibodies.

RESULTS

Reverse transcriptase-PCR analysis of total nasal RNA demonstrated the expression of PAF receptor mRNA. The immunohistochemical studies revealed that anti-PAF receptor antibody-labeled eosinophils, macrophages, neutrophils, mast cells, lymphocytes, vascular endothelial cells, epithelial cells, and submucosal glands in nasal mucosa.

CONCLUSIONS

These results may have important clinical implications for understanding the role of PAF receptor on upper airway diseases such as allergic and nonallergic rhinitis.

Oxygen-dependent PAF receptor binding and intracellular signaling in ovine fetal pulmonary vascular smooth muscle

Circulating levels of platelet-activating factor (PAF) are high in the fetus, and PAF is active in maintaining high PVR in fetal hypoxia (Ibe BO, Hibler S, Raj J. J Appl Physiol 85: 1079-1085, 1998). PAF synthesis by fetal pulmonary vascular smooth muscle cells (PVSMC) is high in hypoxia, but how oxygen tension affects PAF receptor (PAF-r) binding in PVSMC is not known. We studied the effect of oxygen tension on PAF-r binding and signaling in fetal PVSMC. PAF binding was saturable. PAF-r density (B(max): fmol/10(6) cells; means +/- SE, n = 6), 25.2 +/- 0.77 during hypoxia (Po(2) <40 Torr), was higher than 13.9 +/- 0.44 during normoxia (Po(2) approximately 100 Torr). K(d) was twofold lower in hypoxia than normoxia. PAF-r protein expression, 35-40% greater in hypoxia, was inhibited by cycloheximide, a protein synthesis inhibitor, suggesting translational regulation. IP(3) release, an index of PAF-r-mediated cell signaling, was greater in hypoxia (EC(50): hypoxia, 2.94 +/- 0.61; normoxia, 5.85 +/- 0.51 nM). Exogenous PAF induced 50-90% greater intracellular calcium flux in cells during hypoxia, indicating hypoxia augments PAF-r-mediated cell signaling. PAF-r phosphorylation, with or without 5 nM PAF, was 40% greater in hypoxia. These data show 1) hypoxia upregulates PAF-r binding, PAF-r phosphorylation, and PAF-r-mediated intracellular signaling, evidenced by augmented IP(3) production and intracellular Ca(2+) flux; and 2) hypoxia-induced PAF-r phosphorylation results in activation of PAF-r-mediated signal transduction. The data suggest the fetal hypoxic environment facilitates PAF-r binding and signaling, thereby promoting PAF-mediated pulmonary vasoconstriction and maintenance of high PVR in utero.

Mechanisms underlying the modulatory action of platelet activating factor (PAF) on the upregulation of kinin B1 receptors in the rat paw

1. The present study evaluated the ability of the administration of platelet activating factor (PAF) to induce the upregulation of B(1) receptors in the rat paw. 2. Local treatment with PAF resulted in a time-dependent increase of oedema formation induced by the B(1) receptor agonist des-Arg(9)-BK (des-Arg(9)-bradykinin), but not by the B(2) receptor agonist tyrosine(8)-bradykinin. Functional upregulation of B(1) receptors was accompanied by a prominent increase of B(1) receptor mRNA expression in the rat paw. 3. In PAF-treated paws, des-Arg(9)-BK-induced oedema formation was significantly inhibited by the B(1) receptor antagonists des-Arg(9)-[Leu(8)]-BK and R-715. The effects of PAF pretreatment were receptor operated, as assessed by the effects of the PAF receptor antagonist WEB2086 or by desensitisation of PAF receptors. 4. The protein synthesis inhibitor cycloheximide, the anti-inflammatory steroid dexamethasone or the nuclear factor (NF-kappaB) blockers pyrrolidine-dithiocarbamate (PDTC) and Nalpha-tosyl-L-chloromethylketone significantly blocked the functional upregulation of B(1) receptors. 5. The selectin inhibitor fucoidin, an anti-CD18 antibody or an anti-rat neutrophil antiserum, also significantly prevented des-Arg(9)-BK-induced paw oedema in rats pretreated with PAF. 6. Intradermal injection of PAF induced a 25-fold increase of myeloperoxidase activity in the rat paw, a response that was significantly inhibited by fucoidin, anti-CD-18, anti-rat neutrophil antiserum or PDTC. 7. Local treatment with PAF also resulted in a marked increase of NF-kappaB activation, an effect largely prevented by PDTC or by the anti-rat neutrophil antiserum. 8. Collectively, the present results indicate that the induction of B(1) receptors following treatment with the chemotatic mediator PAF is dependent on the recruitment of neutrophils, an event that is under the control of adhesion molecules, protein synthesis and NF-kappaB activation. These findings provide new insights into the role played by cell migration and chemotatic factors on B(1) receptor upregulation in vivo.

Lysophosphatidylcholines prime the NADPH oxidase and stimulate multiple neutrophil functions through changes in cytosolic calcium

A mixture of lysophosphatidylcholines (lyso-PCs) are generated during blood storage and are etiologic in models of acute lung injury. We hypothesize that lyso-PCs stimulate polymorphonuclear neutrophils (PMNs) through Ca(2)(+)-dependent signaling. The lyso-PC mix (0.45-14.5 micro M) and the individual lyso-PCs primed formyl-Met-Leu-Phe (fMLP) activation of the oxidase (1.8- to 15.7-fold and 1.7- to 14.8-fold; P<0.05). Labeled lyso-PCs demonstrated a membrane association with PMNs and caused rapid increases in cytosolic Ca(2)(+). Receptor desensitization studies implicated a common receptor or a family of receptors for the observed lyso-PC-mediated changes in PMN priming, and cytosolic Ca(2)(+) functions were pertussis toxin-sensitive. Lyso-PCs caused rapid serine phosphorylation of a 68-kD protein but did not activate mitogen-activated protein kinases or cause changes in tyrosine phosphorylation. With respect to alterations in PMN function, lyso-PCs caused PMN adherence, increased expression of CD11b and the fMLP receptor, reduced chemotaxis, provoked changes in morphology, elicited degranulation, and augmented fMLP-induced azurophilic degranulation (P<0.05). Cytosolic Ca(2)(+) chelation inhibited lyso-PC-mediated priming of the oxidase, CD11b surface expression, changes in PMN morphology, and serine phosphorylation of the 68-kD protein. In conclusion, lyso-PCs affect multiple PMN functions in a Ca(2)(+)-dependent manner that involves the activation of a pertussis toxin-sensitive G-protein.

A two-insult in vitro model of PMN-mediated pulmonary endothelial damage: requirements for adherence and chemokine release

Lysophosphatidylcholines (lyso-PCs), generated during blood storage, are etiologic in a two-insult, sepsis-based model of transfusion-related acute lung injury (TRALI). Individually, endotoxin (LPS) and lyso-PCs prime but do not activate neutrophils (PMNs). We hypothesized that priming of PMNs alters their reactivity such that a second priming agent causes PMN activation and endothelial cell damage. PMNs were primed or not with LPS and then treated with lyso-PCs, and oxidase activation and elastase release were measured. For coculture experiments, activation of human pulmonary microvascular endothelial cells (HMVECs) was assessed by ICAM-1 expression and chemokine release. HMVECs were stimulated or not with LPS, PMNs were added, cells were incubated with lyso-PCs, and the number of viable HMVECs was counted. Lyso-PCs activated LPS-primed PMNs. HMVEC activation resulted in increased ICAM-1 and release of ENA-78, GRO alpha, and IL-8. PMN-mediated HMVEC damage was dependent on LPS activation of HMVECs, chemokine release, PMN adhesion, and lyso-PC activation of the oxidase. In conclusion, sequential exposure of PMNs to priming agents activates the microbicidal arsenal, and PMN-mediated HMVEC damage was the result of two insults: HMVEC activation and PMN oxidase assembly.

Inhibition of prostaglandin G/H synthase unveils a potent effect of platelet activating factor on the permeability of bovine aortic endothelial cells to albumin

Platelet Activating Factor (PAF) is a very potent stimulant of various cell functions but little is known about the mechanisms responsible for its marked effect on endothelial permeability. An in vitro assay system was used to assess the direct effect of PAF on the permeability of a bovine aortic endothelial cell (BAEC) monolayer to albumin. PAF produced a small but not significant increase of the permeability of BAEC monolayer to albumin. However, pre-treatment of the monolayer with indomethacin (10 microM) resulted in a significant increase of BAEC permeability following PAF administration. This increase was concentration-dependent up to a maximal effect of 105% above basal value (for 0.1 microM PAF). Addition of the PAF antagonist SRI 63 441ZI (5 microM) abolished this effect. Exogenous administration of PGE2 (10(-7) M) inhibited the effect of PAF on the BAEC permeability suggesting that prostaglandins synthesized by the endothelium behave as a negative autoregulatory factor. Compound SRI 63 441ZI also partially inhibited bradykinin-induced permeability to albumin but did not significantly modify the activity of thrombin. These findings show that PAF can increase endothelial permeability to albumin when the synthesis of prostaglandins is inhibited. Our results also show that PAF might have an autocrine activity by mediating part of BK-induced permeability.

Metabolism of platelet activating factor by intrapulmonary vascular smooth muscle cells. Effect of oxygen on phospholipase A2 protein expression and activities of acetyl-CoA acetyltransferase and cholinephosphotransferase

We have demonstrated that platelet activating factor (PAF) plays an important physiological role in the maintenance of high pulmonary vascular tone in fetal lambs, a role attributable to increased PAF receptor binding (J. Appl. Physiol. 85 (1998) 1079; Am J. Physiol. 278 (2000) H1168). In this study, we examined the possibility that increased PAF synthesis via de novo and remodeling pathways as well as decreased PAF catabolism in hypoxic state of fetal lungs may account for the PAF action in vivo. We investigated effect of oxygen tension on PAF synthesis by ovine fetal intrapulmonary venous (PV) and arterial (PA) smooth muscle cells pulsed with [3H]choline (de novo), or [3H]acetate (remodeling), while PAF catabolism was studied by assay of acetylhydrolase (PAF-Ah) activity. Hypoxia stimulated PAF synthesis by choline incorporation (pmol/10(6)cells) in both PVSMC (1.14+/-0.13 vs 0.53+/-0.05) and PASMC (0.39+/-0.12 vs 0.22+/-0.04). Hypoxia stimulated PAF synthesis via remodeling pathway only in PVSMC (408+/-32 vs 225+/-17) which was 5-fold greater than in PASMC (77+/-15 vs 105+/-24), however, with A23187 in remodeling pathway, PAF synthesis increased 5-fold compared to baseline conditions and then synthesis in hypoxia was greater than in normoxia in both cell types. Phospholipase A2 protein expression was significantly higher in hypoxia in both cells and was approximately 2-fold higher in PVSMC. PAF-Ah activity (nmol lyso-PAF/min/mg protein) was greater in hypoxia vs normoxia in PVSMC (0.81+/-0.24 vs 0.44+/-0.088), but in PASMC activity was less in hypoxia vs normoxia (1.68+/-0.24 vs 3.93+/-0.44). Compared to PVSMC PAF-Ah activity in PASMC was 4-fold higher in hypoxia. Our data demonstrate that (1) PAF synthesis in intrapulmonary SMC of fetal lambs occurs by both de novo studied by choline incorporation and remodeling pathways, the latter being predominant. (2) There is heterogeneity in PAF synthetic and catabolic activities in lung vasculature of fetal lambs. We conclude that increased PAF synthesis in veins by the two synthetic pathways coupled with decreased catabolism will result in a higher venous PAF levels in the hypoxic environment of fetal lungs. We speculate that in vivo, a high PAF level in veins will make more PAF available for binding to its receptors so as to sustain the desired high venous tone in the fetal pulmonary circulation.

Maturational changes in ovine pulmonary metabolism of platelet-activating factor: implications for postnatal adaptation

We recently reported that PAF acetylhydrolase (PAF-Ah) mRNA level and PAF-Ah activity in lamb lungs are up-regulated in the immediate newborn period, thereby facilitating the fall in postnatal PAF levels as well as a fall in pulmonary vascular resistance (B. O. Ibe, F. C. Sardar, and J. U. Raj, Mol Genet Metab 69:46-55, 2000). We have studied hypoxia effects on PAF synthesis and PAF-Ah activity in fetal lamb pulmonary arterial smooth muscle cells (FPASMC) and endothelial cells (FPAEC). We also studied PAF synthesis by platelets, and PAF-Ah activity in plasma of perinatal lambs at different ages. PAF synthesis (means +/- SEM, pmol/10(6) cells) by SMC in baseline was 168 +/- 27 and increased 3-fold on stimulation with A23187. Hypoxia augmented A23187-stimulated PAF synthesis by 30%. In FPAEC, baseline synthesis was 0.54 +/- 0.062 and increased 3-fold to 1.72 +/-.34. Hypoxia had no effect on PAF synthesis by EC. FPASMC produced over 300-fold more PAF than FPAEC. PAF synthesis by platelets was 47.02 +/- 7.1, 63.4 +/- 6.6, 71.5 +/- 9.9, and 62.2 +/- 5.2 for fetal, and newborn lambs <2 h, <1 day, and 6-12 days, old, respectively. PAF synthesis by platelets of <1 day-old lambs was different from that of fetal lambs. PAF-Ah activity (nmol lyso-PAF/min/mg protein) by FPASMC in normoxia was 3.41 +/- 0.38 which was 50% higher than the rate in hypoxia. Activity in FPAEC was 1.75 +/- 0.37 which was not different from hypoxia. PAF-Ah activity in fetal lamb plasma was 47.83 +/- 6.87 which was different from 155.32 +/- 12.10, the activity in plasma of newborn <1 day old. Activity in the other perinatal lambs did not differ from fetal or newborn <1 d. Our data suggest that lower pulmonary vascular PAF synthesis in normoxia together with higher PAF-Ah activity during immediate postnatal period is necessary to ensure rapid catabolism of PAF in vivo so as to facilitate postnatal adaptation of the pulmonary and systemic circulations.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Fluorometric detection of platelet activating factor receptor in cultured oviductal epithelial and stromal cells and endometrial stromal cells from bovine at different stages of the oestrous cycle and early pregnancy

During the oestrous cycle and early pregnancy, the oviduct and uterus undergo a variety of morphological and physiological modifications in which the platelet activating factor receptor (PAF-R) plays an important role. PAF-R levels were quantified in bovine oviductal epithelial and stromal cells and endometrial stromal cells at days 2 to 4, 12, and 20 of the estrous cycle and during early pregnancy. Cells were grown in vitro and their intracellular PAF-R concentration was measured by flow cytometry using a polyclonal anti-PAF-R antibody system. A significant increase (P < 0.05) in the portion of PAF-R-positive oviductal epithelial and stromal cells was detected in both non-pregnant and pregnant cattle on days 2 to 4 in comparison to day 12 and 20. In endometrial stromal cells derived from day 20 pregnant bovine, a significant increase (P < 0.05) in PAF-R staining was observed in comparison to the day 20 non-pregnant and days 2 to 4 or 12 pregnant and non-pregnant animals. The PAF-R was detected in oviductal cells by using immunoblotting and immuno-gold postembedding method. Positive binding of the anti-PAF-R antibody was found on the cell membrane and in the cytoplasm. We concluded that the increased PAF-R concentration measured in cultured oviductal epithelial and stromal cells of cyclic and pregnant heifers on days 2 to 4 was hormonally regulated. The increased PAF-R in endometrial stromal cells on day 20 of pregnant heifers was a pregnancy-specific effect and may mediate a local increase in endometrial vascular permeability known to precede the implantation.

Platelet-activating factor (PAF) receptor and genetically engineered PAF receptor mutant mice

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was initially recognized for its potential to induce platelet aggregation and secretion, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues, many of which also produce the molecule. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor. PAF receptor is linked to intracellular signal transduction pathways, including turnover of phosphatidylinositol, elevation in intracellular calcium concentration, and activation of kinases, resulting in versatile bioactions. On the basis of numerous pharmacological reports, PAF is thought to have many pathophysiological and physiological functions. Recently advanced molecular technics enable us not only to clone PAF receptor cDNAs and genes, but also generate PAF receptor mutant animals, i.e., PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice gave us a novel and specific approach for identifying the pathophysiological and physiological functions of PAF. This review also describes the phenotypes of these mutant mice and discusses them by referring to previously reported pharmacological and genetical data.

Platelet activating factor receptor (PAF-R) is found in a large endosomal compartment in human umbilical vein endothelial cells

In previous studies, we have localized the platelet activating factor receptor (PAF-R) in situ on the surface of the endothelium in a number of microvascular beds without providing information on its intracellular location. In the present study, we used human umbilical vein cells (HUVECs) as a model to immunolocalize PAF-R by light and electron microscopic procedures. We raised two different polyclonal antibodies against synthetic peptides of the C- and N-terminal of PAF-R and used them for immunolocalization studies. By immunofluorescence, we found that the anti-C-terminal antibody (CPAF-R) stains an extensive intracellular tubular network. By electron microscopy, using a preembedding staining procedure, we detected PAF-R on the surface of the plasmalemma in a staining pattern similar to that described on microvascular endothelia in situ, but at a considerably lower density. Immunogold labeling of thin frozen sections revealed the presence of PAF-R on the plasmalemma, and especially in an extensive network of tubular-vesicular elements and vesicles associated with it. No detectable amounts of PAF-R were found in the endoplasmic reticulum (ER) or in Golgi cisternae. Double immunofluorescence labeling with antibodies for compartment marker proteins and PAF-R revealed that PAF-R localizes in an endosomal compartment. Confocal microscopy showed that PAF-R colocalizes in this compartment together with the transferrin receptor (Tf-R) and the thrombin receptor (TH-R), but it also showed that the colocalization was partial rather than complete. These findings suggest that the endosomal network is either discontinuous or, conversely, that the proteins in its membrane do not have a fully randomized distribution.

Non-specific PAF binding to embryonal F9 cells and prostacyclin synthesis in human umbilical vein endothelial cells

Our data suggest non-specific Paf binding to intact embryonal F9 cells. Non-specific Paf binding might have some physiological relevance when Paf metabolites interfere with prostacyclin synthesis, for example, in umbilical vein endothelial cells.