Interaction of platelet-activating factor with endothelial and vascular smooth muscle cells in coculture

Before platelets: the production of platelet-activating factor during growth and stress in a basal marine organism

Corals and humans represent two extremely disparate metazoan lineages and are therefore useful for comparative evolutionary studies. Two lipid-based molecules that are central to human immunity, platelet-activating factor (PAF) and Lyso-PAF were recently identified in scleractinian corals. To identify processes in corals that involve these molecules, PAF and Lyso-PAF biosynthesis was quantified in conditions known to stimulate PAF production in mammals (tissue growth and exposure to elevated levels of ultraviolet light) and in conditions unique to corals (competing with neighbouring colonies over benthic space). Similar to observations in mammals, PAF production was higher in regions of active tissue growth and increased when corals were exposed to elevated levels of ultraviolet light. PAF production also increased when corals were attacked by the stinging cells of a neighbouring colony, though only the attacked coral exhibited an increase in PAF. This reaction was observed in adjacent areas of the colony, indicating that this response is coordinated across multiple polyps including those not directly subject to the stress. PAF and Lyso-PAF are involved in coral stress responses that are both shared with mammals and unique to the ecology of cnidarians.

Intravascular adenovirus-mediated lipoprotein-associated phospholipase A2 gene transfer reduces neointima formation in balloon-denuded rabbit aorta

Postangioplasty restenosis is a multifactorial process and involves mechanisms such as inflammation and stimulation of the expression of growth factors. Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) can modify inflammatory responses by hydrolyzing phospholipids with shortened and/or oxidized sn-2 residues. In this study, we tested a hypothesis that adenovirus-mediated Lp-PLA(2) gene transfer can reduce restenosis in rabbits. Aortas of cholesterol-fed NZW rabbits were balloon-denuded and intra-arterial gene transfer was performed using Dispatch catheter with Lp-PLA(2) or LacZ adenoviruses (1.15 x 10(10)pfu). Intima/media ratio (I/M), histology and cell proliferation were analyzed. Two weeks after the gene transfer I/M in the LacZ-transduced control group was 0.45+/-0.05 but Lp-PLA(2) gene transfer reduced I/M to 0.25+/-0.03. At four weeks time point I/M in the Lp-PLA(2) group (0.34+/-0.05) was also lower than in the LacZ group (0.53+/-0.06). Plasma Lp-PLA(2) activity was increased in the Lp-PLA(2) group (48.2+/-4.2) as compared to the LacZ group (33.6+/-3.51) at two weeks time point. Transgene expression was detected in the arterial wall two and four weeks after the procedure. Apoptosis was higher in the control vessels than in the Lp-PLA(2) group at two weeks time point. In conclusion, local adenovirus-mediated Lp-PLA(2) gene transfer resulted in a significant reduction in neointima formation in balloon-denuded rabbit aorta and may be useful for the prevention of restenosis after arterial manipulations.

Platelet-associated and secreted PAF-acetylhydrolase activity in patients with stable angina: sequential changes of the enzyme activity after angioplasty

BACKGROUND

Platelet-activating factor (PAF), the lipid mediator of inflammation and potent platelet agonist, can be hydrolysed and inactivated by PAF-acetylhydrolase (PAF-AH). We investigated the PAF-AH activity in relation to PAF formation in platelets from patients with stable angina undergoing elective percutaneous transluminal coronary angioplasty (PTCA).

DESIGN

Twenty-seven patients with stable angina, undergoing PTCA, and 30 age- and sex-matched controls were studied. The platelet-associated and secreted PAF-AH activity was measured, before PTCA, as well as at 4 h, 48 h and 6 months afterwards. PAF formation by thrombin-stimulated platelets and the platelet aggregation responses to PAF and ADP were also determined.

RESULTS

The PAF-AH activity secreted by thrombin-stimulated platelets before PTCA (in pmol/10(9) cells/h) was significantly higher compared to controls (892 +/- 222 vs. 624 +/- 144, P < 0.001). The enzyme activity was not altered at 4 h after PTCA, but was significantly increased at 48 h (1284 +/- 312, P < 0.005) to return to the levels observed in the control group 6 months afterwards. Detectable levels of PAF in thrombin-stimulated platelets were found only at 6 months after PTCA. Furthermore, the cell-associated enzyme activity in resting platelets before PTCA was significantly lower compared with controls. Unlike in controls, the platelet-associated enzyme activity in the patient group was not increased after stimulation with thrombin and it was associated by a platelet hyperaggregability to PAF. Both the intact cell-associated activity and the platelet hyper-reactivity to PAF were restored at 6 months after PTCA.

CONCLUSIONS

Alterations in the platelet PAF-AH activity, which affect the PAF formation in thrombin-stimulated platelets and are associated by an increased aggregatory response to PAF, are observed in patients with stable angina and are completely restored after PTCA.

Relationship of molecular structure to the mechanism of lysophospholipid-induced smooth muscle cell proliferation

We previously reported that oxidized low-density lipoprotein and one of its constituents, lysophosphatidylcholine (lysoPC), caused smooth muscle cell proliferation that was inhibitable by vitamin E and by a neutralizing antibody against basic fibroblast growth factor-2 (FGF-2). We now show that the mitogenic activity of lysolipids is highly dependent on structure. Phospholipids with palmitoyl fatty acid and phosphocholine induced DNA synthesis optimally. Shorter and longer fatty acids were significantly less potent, as were phosphoserine and phosphoethanolamine head groups. Structurally related phospholipids [platelet-activating factor (PAF) and lysoPAF] were also mitogens and acted via an analogous FGF-2-dependent, vitamin E-inhibitable mechanism. The mechanism of lysoPC stimulation was distinct from that of another phospholipid mitogen, lysophosphatidic acid (lysoPA), in that lysoPC stimulation was not pertussis toxin inhibitable. Furthermore, lysoPA stimulation was not inhibitable by vitamin E. Despite its distinct cellular pathway for stimulation, lysoPA also ultimately led to FGF-2 release. Our data show that specific structural attributes of lysoPC, PAF, and lysoPAF enable these agents to mediate smooth muscle cell release of FGF-2, which in turn stimulates proliferation.

Platelets and restenosis

Restenosis is currently the major limitation of percutaneous transluminal coronary angioplasty (PTCA). Factors such as elastic recoil, migration of vascular smooth muscle cells from media to intima, neointimal proliferation and vascular remodeling underly the restenotic process. Presently there is no effective therapy available for restenosis. The role of platelets in the development of thrombosis and abrupt closure after PTCA is well recognized. However, the effects of platelets in PTCA extend well beyond the early phase. Although antiplatelet agents such as glycoprotein IIb/IIIa antagonists have been reported to reduce target vessel revascularization, major unresolved controversies still exist. This report reviews the potential role of platelets in restenosis. Various drugs, successfully tested in experimental studies and in a small number of human studies, that inhibit the effect of platelets on the restenotic process are also reviewed.

Overexpression of human superoxide dismutase inhibits oxidation of low-density lipoprotein by endothelial cells

Oxidation of LDL in the subendothelial space has been proposed to play a key role in atherosclerosis. Endothelial cells produce superoxide anions (O2.-) and oxidize LDL in vitro; however, the role of O2.- in endothelial cell-induced LDL oxidation is unclear. Incubation of human LDL (200 microg/mL) with bovine aortic endothelial cells (BAECs) for 18 hours resulted in a 4-fold increase in LDL oxidation compared with cell-free incubation (22.5+/-1.1 versus 6.3+/-0.2 [mean+/-SEM] nmol malondialdehyde/mg LDL protein, respectively; P<0.05). Under similar conditions, incubation of LDL with porcine aortic endothelial cells resulted in a 5-fold increase in LDL oxidation. Inclusion of exogenous copper/zinc superoxide dismutase (Cu/ZnSOD, 100 microg/mL) in the medium reduced BAEC-induced LDL oxidation by 79%. To determine whether the intracellular SOD content can have a similar protective effect, BAECs were infected with adenoviral vectors containing cDNA for human Cu/ZnSOD (AdCu/ZnSOD) or manganese SOD (AdMnSOD). Adenoviral infection increased the content and activity of either Cu/ZnSOD or MnSOD in the cells and reduced cellular O2.- release by two thirds. When cells infected with AdCu/ZnSOD or AdMnSOD were incubated with LDL, formation of malondialdehyde was decreased by 77% and 32%, respectively. Two other indices of LDL oxidation, formation of conjugated dienes and increased LDL electrophoretic mobility, were similarly reduced by SOD transduction. These data suggest that production of O2.- contributes to endothelial cell-induced oxidation of LDL in vitro. Furthermore, adenovirus-mediated transfer of cDNA for human SOD, particularly Cu/ZnSOD, effectively reduces oxidation of LDL by endothelial cells.

Differential signaling pathways in platelet-activating factor-induced proliferation and interleukin-6 production by rat vascular smooth muscle cells

Vascular smooth muscle cells (SMCs) can be induced to proliferate in response to several cytokines and growth factors, including interleukin (IL)-6. Platelet-activating factor (PAF) also has been shown to induce SMC proliferation. Because PAF can stimulate IL-6 production in monocytes, macrophages, and endothelial cells, our study was undertaken to determine whether PAF could induce IL-6 production by SMCs and to define the underlying signaling pathways. Exposure of rat aortic SMCs to picomolar concentrations of PAF resulted in enhanced production of IL-6. The effect was concentration dependent, selective for the active form of PAF, and mediated by specific PAF receptors. Pretreatment of the cells with Bordatella pertussis toxin (PTX) prevented the effect of PAF, suggesting the involvement of alpha i-type subunits of G proteins in the signal-transduction pathway. PAF-dependent IL-6 production was also prevented by inhibition of tyrosine kinases with genistein or erbstatin. Inhibition of eicosanoid production by blocking either phospholipase A2 or cyclooxygenase also abrogated the effect of PAF on IL-6 production. Moreover, inhibition of Ca2+-calmodulin activity with W7 or blocking of calcium channels with verapamil or nifedipine prevented PAF-mediated enhancement of IL-6 production. Whereas PAF-induced signal-transduction pathways leading to IL-6 production and SMC proliferation were partially common, they appeared to diverge downstream of PLA2 activation: inhibition of cyclooxygenase had no effect on proliferation, whereas augmentation of cyclic adenosine monophosphate (cAMP) levels or activation of protein kinase A inhibited proliferation, in contrast to IL-6 production. Our findings suggest a role for PAF in modulating vascular function by stimulating local production of IL-6 by SMCs and promoting their proliferation. The two effects are, however, associated with partially divergent signaling pathways and may not be causally related.

Cell response to hyperbaric oxygen treatment

Wound healing involves matrix deposition, angiogenesis, and new tissue growth. Cellular activity during healing is related to tissue oxygen levels. Since wound healing requires oxygen, the purpose of this study was to investigate the effects of hyperbaric oxygen (HBO) on cells involved in wound healing. Cultured endothelial cells and fibroblasts were exposed to HBO. The effect of varied partial pressure, oxygen saturation, and duration and frequency of exposure to HBO on cell proliferation was determined by 3H-labeled thymidine incorporation. HBO causes an increase in the partial pressure of oxygen in the medium of cultured cells, leading to increased endothelial cell and fibroblast proliferation. Increased endothelial cell proliferation occurred after 15 min of HBO. Fibroblasts required 120 min of HBO to produce a response. A second exposure to HBO on the same day produced no additional increase in cell proliferation. A 120-min HBO exposure stimulated fibroblast proliferation for 72 h after the exposure. An increase in pressure from 2.4 to 4.0 atmospheres absolute did not enhance the proliferative response. These studies begin to elucidate the effects of HBO on cells involved in wound healing and establish a scientific foundation upon which to develop more efficacious and cost-effective HBO therapeutic protocols.

Oxidatively modified LDL contains phospholipids with platelet-activating factor-like activity and stimulates the growth of smooth muscle cells

Oxidative modification of lipoproteins is believed to be important in the genesis of atherosclerosis. We established cultures of smooth muscle cells (SMC) and exposed them to native LDL or oxidized LDL. Oxidized LDL, but not native LDL, was mitogenic as measured by incorporation of [3H]-thymidine into DNA. This effect was concentration dependent, averaged 288% of control, and was blocked by a platelet-activating factor (PAF) receptor antagonist. We hypothesized that phospholipids with PAF-like activity were generated during the oxidation of LDL. To test this hypothesis we extracted phospholipids from copper-oxidized LDL and assayed for PAF-like activity. Phospholipids extracted from oxidized LDL and purified by HPLC induced neutrophil adhesion equivalent to PAF (10 nM) and were mitogenic for smooth muscle cells. These effects were not seen with phospholipids extracted from native LDL and were blocked by two structurally different, competitive antagonists of the PAF receptor. The effects of these lipids were also abolished by pretreating them with PAF acetylhydrolase. Finally, we used Chinese hamster ovary cells that had seen stably transfected with a cDNA for the PAF receptor to confirm that phospholipids from oxidized LDL act via this receptor. We found that PAF (control) and the oxidized phospholipids each induced release of arachidonic acid from the transfected cells, but had no effect on wildtype Chinese hamster ovary cells, which lack the PAF receptor. This effect was also blocked by a PAF receptor antagonist. Thus, phospholipids generated during oxidative modification of LDL may participate in atherosclerosis by stimulating SMC proliferation and leukocyte activation.

Characterization of platelet-activating factor synthesis in glomerular endothelial cell lines

Platelet-activating factor synthesis in two transformed lines of glomerular endothelial cells was characterized and contrasted with platelet-activating factor production in macrovascular-derived endothelial cells as well as with glomerular cells of mesenchymal origin. Platelet-activating factor synthesis was assessed in intact cells and in cell-free preparations. Glomerular endothelial cells constitutively synthesize bio-active alkyl-PAF, and this basal activity can be chronically augmented by various inflammatory and thrombotic agents. In contrast, thrombin-mediated platelet-activating factor formation in bovine pulmonary aortic endothelial cells as well as in glomerular mesangial cells is acute and transient. The potential role of anti-inflammatory prostanoids to function as negative feedback modulators of thrombin- or endothelin-mediated platelet-activating factor synthesis was also investigated, as the synthesis of platelet-activating factor is often associated with the formation of these prostanoids. Indomethacin augmented receptor-mediated platelet-activating factor synthesis while prostanoids of the E and I series reduced agonist-stimulated PAF synthesis. In summary, the unique capacity of glomerular endothelial cells to respond to inflammatory stimuli with sustained platelet-activating factor synthesis is a clear indication of this cell's pivotal role in augmenting the inflammatory response in the limited environment of the glomerulus.

Tumor necrosis factor release by murine macrophages stimulated by the cytotoxic ether lipid 1-O-hexadecyl-2-O-methyl-SN-glycero-3-phosphorylcholine (ET-18-O-OCH3)

The cytotoxic ether lipid 1-O-hexadecyl-2-O-methyl-SN-glycero-3-phosphorylcholine (ET-18-O-OCH3) is a structural analog of the mediator of inflammation platelet-activating factor (PAF). Recent studies demonstrated that ET-18-O-OCH3 activates human monocytes selectively at non-cytotoxic concentrations. The current studies determined the capacity of ET-18-O-OCH3 to stimulate release of TNF alpha by murine peritoneal macrophages. Macrophage receptors for ET-18-OCH3 and PAF were also assessed. ET-18-O-OCH3 and PAF stimulated TNF alpha release by resident BALB/c macrophages in the presence of LPS but not in the absence of this co-factor. In contrast, both ET-18-O_OCH3 and PAF stimulated TNF alpha release by thioglycollate-elicited macrophages in the absence of LPS although release was greater in the presence of this co-stimulus. Optimal stimulation of TNF alpha release occurred at 10(-14)-10(-11) M ET-18-O-OCH3 and PAF. Elicited macrophages and splenic macrophages from C57Bl/6 mice, unlike those from BALB/c mice, did not respond to 10(-15)-10(-8) M ET-18-O-OCH3 or PAF without or with LPS. Scatchard analysis of [3H]PAF binding to elicited BALB/c macrophages revealed the existence of high affinity receptors for PAF. In contrast, there was no evidence for receptors for ET-18-O-OCH3. ET-18-O-OCH3 did not compete with PAF for binding; macrophage activation by ET-18-O-OCH3 was not stereospecific; and, binding studies using [3H]ET-18-O-OCH3 did not reveal saturable binding characteristic of binding to specific receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

13-HODE increases intracellular calcium in vascular smooth muscle cells

13-Hydroxyoctadecadienoic acid (HODE) (2 microM) consistently increased porcine aortic and pulmonary artery smooth muscle cell calcium concentrations ([Ca2+]i), whereas 9-HODE and linoleic acid had no significant effect in the aortic cells and a much lesser effect in the pulmonary artery cells. A transient increase in [Ca2+]i occurred with as little as 50 nM 13-HODE. Structural specificity for elevation of [Ca2+]i also was seen with the monohydroxyeicosatetraenoic acids (HETEs), with 12-HETE but not 5- or 15-HETE increasing [Ca2+]i. 13-HODE, but not 9-HODE, increased smooth muscle cell guanosine 3',5'-cyclic monophosphate (cGMP) levels. The [Ca2+]i increase produced by 13-HODE was dependent on extracellular calcium and was inhibited by the calcium channel blockers verapamil and nifedipine and by KT-5823, a cGMP-dependent kinase inhibitor. A similar increase in [Ca2+]i was produced by 8-bromo-cGMP. These results suggest that 13-HODE, a 15-lipoxygenase product formed from linoleic acid, can act as a lipid mediator in vascular smooth muscle. It raises smooth muscle cGMP, causing a secondary increase in [Ca2+]i due to Ca2+ influx through a cGMP kinase-dependent L-type channel.

Effect of platelet-activating factor (PAF) receptor blockers on smooth muscle cell replication in vitro and allograft arteriosclerosis in vivo

Platelet-activating factor (PAF) stimulates smooth muscle cell (SMC) replication both in vivo and in vitro. In this study we have investigated whether PAF receptor-blocking molecules modulate SMC replication in vitro and the generation of allograft arteriosclerosis in vivo. SMC cultures were established from baby rat aorta media and fibroblast control cultures from the adventitia. Identification of the cultured cell types was determined both by immunohistochemistry and electron microscopy. Both cell types replicated in culture with 10% fetal calf serum (FCS). The addition of PAF-C18 enhanced, and the addition of three PAF receptor inhibitors--WEB 2086, WEB 2170, and BN 50739--reduced, SMC replication and protein synthesis in a dose-dependent fashion in vitro until toxic concentrations were reached. The most potent of these drugs, WEB 2170, was then delivered at the rate of 12 mg/kg per day to recipients of rat aortic allografts. The responses were quantitated by autoradiography after short-term labeling of the recipients with tritium-labeled thymidine (3H-TdR) and by quantitative morphology. Administration of the PAF receptor blocker had no impact on the replication of the inflammatory cells in the allograft adventitia nor on the replication of SMCs in the media and intima. Administration of the PAF receptor blocker delayed the generation of allograft arteriosclerosis slightly, but not significantly. These results suggest that PAF is not an essential component in the inflammatory cascade leading to allograft arteriosclerosis.

Platelet-activating factor stimulates phosphatidic acid formation in cultured rat mesangial cells: roles of phospholipase D, diglyceride kinase, and de novo phospholipid synthesis

Platelet-activating factor (PAF) stimulates phospholipase C (PLC)-induced hydrolysis of phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2). Yet, PAF-stimulated diglycerides (DG) are still elevated at time points where inositol polyphosphates have returned to basal levels. Thus, other signal transduction pathways that hydrolyze phosphatidylcholine (PtdCho) or phosphatidylethanolamine (PtdEth) and form DG and phosphatidic acids (PA) through either PLC or phospholipase D (PLD) may also mediate PAF-stimulated cellular responses. Initially the effects of PAF upon 32P-PA generation in mesangial cells (MC) were assessed. PA formation may be indicative of several metabolic pathways including PLD and DG kinase activities as well as de novo phospholipid synthesis. PAF (10(-7) M) increased 32P-PA formation as early as 5 seconds and this elevation persisted up to 15 minutes. When MC were pretreated with the DG kinase inhibitor-R59022, PAF-induced 32P-PA formation was diminished at early but not late time points, demonstrating that the initial component of PA formation may be due, in part, to PLC activation and subsequent phosphorylation of DG. The reciprocal reaction, PA phosphohydrolase, which dephosphorylates PA to from DG was not stimulated by PAF, suggesting that the sustained elevation of DG induced by PAF is primarily a reflection of PLC. 3H-glycerol pulse-labeling experiments suggest that PAF also stimulates de novo phospholipid synthesis which also contributes to PA formation. Conclusive proof for PLD in the generation of PA was obtained by assessing the formation of 3H-phosphatidyl-ethanol (PEt) from 3H-alkyl-lyso-glycero phosphocholine (GPC) and exogenous ethanol. PAF stimulated alkyl-PEt generation in the presence but not the absence of 0.5% ethanol. Also, PAF induced a concomitant elevation of alkyl-PA at 15 minutes and this elevation of alkyl-PA was reduced when the cells were exposed to exogenous ethanol, reflecting the formation of PEt. Corroborating evidence suggests that PAF stimulates 3H-choline and 3H-ethanolamine release, suggesting that PtdCho and PtdEth are substrates for PLD. Thus, these data demonstrate that MC respond to PAF with elevated PLD and DG kinase activities as well as with an increased rate of de novo lipid synthesis which increases PA, a potential intracellular signal.

Lysophosphatidylcholine causes cGMP-dependent verapamil-sensitive Ca2+ influx in vascular smooth muscle cells

Lysophosphatidylcholine (lyso-PC) is a vasoactive phospholipid present in oxidized low-density lipoprotein. We used a coculture model of the vascular wall to study its interaction with endothelial cells (EC) and vascular smooth muscle cells (SMC). Lyso-PC was taken up readily by SMC and gradually acylated to phosphatidylcholine. Low concentrations (< or = 1 microM) of lyso-PC present in the interstitial medium of an EC-SMC coculture system were taken up primarily by the SMC. Lyso-PC produced a rapid two- to three-fold increase in SMC guanosine 3',5'-cyclic monophosphate (cGMP) levels, reaching a maximum in 1 min. This increase was associated with decreased SMC proliferation and increased calcium influx. The increase in intracellular calcium was inhibited by verapamil and KT5823, a specific cGMP-dependent kinase inhibitor, while a similar increase was produced by the membrane-permeant cGMP analogue 8-bromoguanosine 3',5'-cyclic monophosphate. These studies suggest that SMC are the primary target for the biological effects of lyso-PC present in the vessel wall and that the responses are mediated by calcium influx, possibly due to opening of a verapamil-sensitive cGMP kinase-dependent channel.

Nitrovasodilators inhibit thrombin-induced platelet-activating factor synthesis in human endothelial cells

In response to inflammatory agents such as thrombin, cultured endothelial cells produce platelet-activating factor (PAF), which has been linked with most inflammatory and immune processes, and is a potent coronary constrictor. Sodium nitroprusside (SNP) and SIN-1 (3-morpholinosydnonimine), which spontaneously release the free radical nitric oxide (NO), cause direct relaxation of blood vessels and inhibition of platelet aggregation by activating soluble guanylate cyclase. In the present study we report that in human umbilical vein endothelial cells (HUVEC) these compounds stimulate the production of cGMP and inhibit thrombin-induced PAF synthesis in a concentration-dependent manner. 8-bromo-cGMP, a permeant non-hydrolysable analogue of cGMP, mimics the inhibitory effect of NO-generating vasodilators. PAF synthesis requires phospholipase A2-mediated hydrolysis of membrane precursors to lyso-PAF, which is in turn converted into PAF by an acetyltransferase. The thrombin-elicited activation of both enzymes is inhibited in a dose-dependent way in HUVEC pretreated with SNP and SIN-1. The inhibitory effect of SNP and SIN-1 on the thrombin-mediated PAF synthesis suggests a new mechanism of action whereby the endogenous NO can affect vascular tone and endothelium-dependent intercellular adhesion. Moreover, PAF production in endothelial cells appears to be an important target for the pharmacological action of nitrovasodilators.

Effect of antineoplastic agents on smooth muscle cell proliferation in vitro: implications for prevention of restenosis after transluminal angioplasty

The effect of several antineoplastic agents on vascular smooth muscle cell proliferation was studied in vitro. Both fluorouracil and cytarabine produced significant concentration-dependent inhibition of smooth muscle cell proliferation in cultured porcine pulmonary artery in vitro, while cyclophosphamide stimulated growth. For fluorouracil, inhibition was near maximal at a concentration of 13.0 microgram/mL and was seen with both coincubation and 2-hour preincubation of fluorouracil with quiescent cells. Fluorouracil is a promising agent for inhibition of intimal proliferation. Further work is warranted to determine its effect in vivo.

Platelet activating factor receptor blockade ameliorates murine systemic lupus erythematosus

Untreated 16-week-old MRL/MpJ-lpr/lpr (lpr) mice, when compared to congenic MRL/MpJ-+/+ (+/+) mice, are characterized by a systemic lupus erythematosus syndrome, including severe glomerulonephritis, proteinuria and reduction of renal function. We hypothesized that platelet activating factor (PAF), a potent chemotactic and proinflammatory phospholipid mediator synthesized and released by circulating cells, glomerular mesangial and renal medullary interstitial cells, may play a role in the development of renal injury in lupus mice. We assessed renal PAF synthesis in lpr as well as +/+ mice and the effect of treatment with a PAF receptor blocking agent. Treatment with the PAF receptor antagonist L659,989 for four weeks, starting at 12 weeks of age, significantly reduced acute glomerular infiltration and proliferation, and prevented chronic glomerular histological changes; proteinuria and serum creatinine levels were also significantly reduced in treated mice. Renal PAF production was increased in lpr when compared to +/+ mice, and treatment with L659,989 restored renal PAF synthesis to the control levels. Our results support the hypothesis that PAF can be one of the mediators of glomerular injury characteristic of murine lupus nephritis, and indicate the possible therapeutic utility of PAF receptor antagonists in immunologic renal diseases.

Interleukin-1 and tumour necrosis factor alpha induced polymorphonuclear leukocyte-endothelial cell adhesion and transendothelial migration in vitro: the effect of apical versus basal monolayer stimulation

The cytokines interleukin-1 (IL-1) and tumour necrosis factor alpha (TNF alpha) enhance polymorphonuclear leukocyte (PMNL) adhesion to vascular endothelium by an endothelial cell dependent mechanism in vitro and induce PMNL infiltration in vivo In this study, we employed human umbilical vein endothelium (HUVE) cultured on microporous membrane filters to form a monolayer, a system in which PMNL adherence and PMNL transendothelial migration could be measured using 51Cr-labelled human PMNL. In this system, it was found that PMNL adhesion and migration were dependent on prior treatment of the HUVE monolayer with IL-1 or TNF alpha for at least 2 h and that cytokine could be removed prior to the addition of PMNL without any effect on the response. PMNL adherence to the HUVE was maximal by 30 min and was followed by progressive migration of PMNL across the monolayer and the membrane filter into the lower chamber. The effect of apical surface versus basal surface exposure of the HUVE monolayer to IL-1 alpha and TNF alpha on subsequent PMNL interaction with the HUVE monolayer in the absence of cytokine was examined. Apical or basal stimulation induced comparable PMNL adherence at 30 min following addition of PMNL (35.5% and 43.1%). However, basal (i.e., abluminal) exposure to IL-1 or TNF alpha of the HUVE induced significantly greater PMNL transendothelial migration (e.g., 27.8% vs. 15.4%; P less than 0.01). The expression of endothelial-leukocyte adhesion molecules ELAM-1 and ICAM-1 following apical versus basal stimulation was determined by ELISA on viable cells. These adhesion molecules were upregulated to a similar extent under both conditions. These observations suggest that spacial localization or orientation of adhesion molecules may be influenced by basal versus apical cytokine stimulation or that other mechanisms are responsible for the preferential PMNL migration with basal stimulation. These findings may have implications for the in vivo interactions of PMNL with vascular endothelium, depending on whether the endothelium is exposed to IL-1 of TNF alpha via the blood on the luminal (apical) surface or via the extravascular space on the abluminal (basal) surface.

Intercellular transport of lysosomal acid lipase mediates lipoprotein cholesteryl ester metabolism in a human vascular endothelial cell-fibroblast coculture system

We present results from studies of human cell culture models to support the premise that the extracellular transport of lysosomal acid lipase has a function in lipoprotein cholesteryl ester metabolism in vascular tissue. Vascular endothelial cells secreted a higher fraction of cellular acid lipase than did smooth muscle cells and fibroblasts. Acid lipase and lysosomal beta-hexosaminidase were secreted at approximately the same rate from the apical and basolateral surface of an endothelial cell monolayer. Stimulation of secretion with NH4Cl did not affect the polarity. We tested for the ability of secreted endothelial lipase to interact with connective tissue cells and influence lipoprotein cholesterol metabolism in a coculture system in which endothelial cells on a micropore filter were suspended above a monolayer of acid lipase-deficient (Wolman disease) fibroblasts. After 5-7 d, acid lipase activity in the fibroblasts reached 10%-20% of the level in normal cells; cholesteryl esters that had accumulated from growth in serum were cleared. Addition of mannose 6-phosphate to the coculture medium blocked acid lipase uptake and cholesterol clearance, indicating that lipase released from endothelial cells was packaged into fibroblast lysosomes by a phosphomannosyl receptor-mediated pathway. Supplementation of the coculture medium with serum was not required for lipase uptake and cholesteryl ester hydrolysis by the fibroblasts, but was necessary for cholesterol clearance. Results from our coculture model suggest that acid lipase may be transported from intact endothelium to cells in the lumen or the wall of a blood vessel. We postulate that delivery of acid hydrolases and lipoproteins to a common endocytic compartment may occur and have an impact on cellular lipoprotein processing.

1-O-alkyl-2-acetyl-sn-glycerol: a platelet-activating factor metabolite with biological activity in vascular smooth muscle cells

Platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine; PAF) is a potent vasoactive ether lipid produced by activated blood cells and endothelial cells. Vascular smooth muscle cells partially convert exogenous PAF to 1-O-alkyl-2-acetyl-sn-glycerol (AAG), a biologically active diacylglycerol analogue. AAG is formed rapidly (less than 15 s) after exposure of the smooth muscle cells and does not appear to be a substrate for diacylglycerol kinase in these cells. Although most of the compound is metabolized to 1-O-alkyl-sn-glycerol, a small quantity remains as AAG for greater than or equal to 6 h. AAG inhibits phorbol ester binding, and it is as effective an activator of protein kinase C as diolein in an in vitro assay. Furthermore, AAG and PAF produce the same pattern of effects on smooth muscle cell proliferation. These observations suggest that at least some of the actions of PAF in vascular smooth muscle may be mediated through the formation of AAG, a stable, bioactive metabolite that appears to function as a diacylglycerol analogue.