The role of platelet-activating factor in inflammation

PAF signaling plays a role in obesity-induced adipose tissue remodeling

BACKGROUND/OBJECTIVES

Platelet-activating factor receptor (PAFR) activation controls adipose tissue (AT) expansion in animal models. Our objective was twofold: (i) to check whether PAFR signaling is involved in human obesity and (ii) investigate the PAF pathway role in hematopoietic or non-hematopoietic cells to control adipocyte size.

MATERIALS/SUBJECTS AND METHODS

Clinical parameters and adipose tissue gene expression were evaluated in subjects with obesity. Bone marrow (BM) transplantation from wild-type (WT) or PAFR^-/- mice was performed to obtain chimeric PAFR-deficient mice predominantly in hematopoietic or non-hematopoietic-derived cells. A high carbohydrate diet (HC) was used to induce AT remodeling and evaluate in which cell compartment PAFR signaling modulates it. Also, 3T3-L1 cells were treated with PAF to evaluate fat accumulation and the expression of genes related to it.

RESULTS

PAFR expression in omental AT from humans with obesity was negatively correlated to different corpulence parameters and more expressed in the stromal vascular fraction than adipocytes. Total PAFR^-/- increased adiposity compared with WT independent of diet-induced obesity. Differently, WT mice receiving PAFR^-/--BM exhibited similar adiposity gain as WT chimeras. PAFR^-/- mice receiving WT-BM showed comparable augmentation in adiposity as total PAFR^-/- mice, demonstrating that PAFR signaling modulates adipose tissue expansion through non-hematopoietic cells. Indeed, the PAF treatment in 3T3-L1 adipocytes reduced fat accumulation and expression of adipogenic genes.

CONCLUSIONS

Therefore, decreased PAFR signaling may favor an AT accumulation in humans and animal models. Importantly, PAFR signaling, mainly in non-hematopoietic cells, especially in adipocytes, appears to play a significant role in regulating diet-induced AT expansion.

Thrombosis in Psoriasis: Cutaneous Cytokine Production as a Potential Driving Force of Haemostatic Dysregulation and Subsequent Cardiovascular Risk

Psoriasis (PsO) is a common T cell-mediated inflammatory disorder of the skin with an estimated prevalence of 2%. The condition manifests most commonly as erythematous plaques covered with scales. The aetiology of PsO is multifactorial and disease initiation involves interactions between environmental factors, susceptibility genes, and innate and adaptive immune responses. The underlying pathology is mainly driven by interleukin-17. In addition, various inflammatory mediators from specific T helper (T_H) cell subsets, namely T_H1, T_H17, and T_H22, are overexpressed in cutaneous lesions and may also be detected in the peripheral blood of psoriatic patients. Moreover, these individuals are also at greater risk, compared to the general population, of developing multiple comorbid conditions. Cardiovascular disease (CVD) has been recognised as a prominent comorbidity of PsO. A potential mechanism contributing to this association may be the presence of a hypercoagulable state in these individuals. Inflammation and coagulation are closely related. The presence of chronic, low-grade systemic inflammation may promote thrombosis – one of the major determinants of CVD. A pro-inflammatory milieu may induce the expression of tissue factor, augment platelet activity, and perturb the vascular endothelium. Altogether, these changes will result in a prothrombotic state. In this review, we describe the aetiology of PsO, as well as the pathophysiology of the condition. We also consider its relationship to CVD. Given the systemic inflammatory nature of PsO, we evaluate the potential contribution of prominent inflammatory mediators (implicated in PsO pathogenesis) to establishing a prothrombotic state in psoriatic patients.

Secondary Metabolites of Plants as Modulators of Endothelium Functions

According to the World Health Organization, cardiovascular diseases are the main cause of death worldwide. They may be caused by various factors or combinations of factors. Frequently, endothelial dysfunction is involved in either development of the disorder or results from it. On the other hand, the endothelium may be disordered for other reasons, e.g., due to infection, such as COVID-19. The understanding of the role and significance of the endothelium in the body has changed significantly over time—from a simple physical barrier to a complex system encompassing local and systemic regulation of numerous processes in the body. Endothelium disorders may arise from impairment of one or more signaling pathways affecting dilator or constrictor activity, including nitric oxide–cyclic guanosine monophosphate activation, prostacyclin–cyclic adenosine monophosphate activation, phosphodiesterase inhibition, and potassium channel activation or intracellular calcium level inhibition. In this review, plants are summarized as sources of biologically active substances affecting the endothelium. This paper compares individual substances and mechanisms that are known to affect the endothelium, and which subsequently may cause the development of cardiovascular disorders.

COVID-19, microthromboses, inflammation, and platelet activating factor

Recent articles report elevated markers of coagulation, endothelial injury, and microthromboses in lungs from deceased COVID-19 patients. However, there has been no discussion of what may induce intravascular coagulation. Platelets are critical in the formation of thrombi and their most potent trigger is platelet activating factor (PAF), first characterized by Demopoulos and colleagues in 1979. PAF is produced by cells involved in host defense and its biological actions bear similarities with COVID-19 disease manifestations. PAF can also stimulate perivascular mast cell activation, leading to inflammation implicated in severe acute respiratory syndrome (SARS). Mast cells are plentiful in the lungs and are a rich source of PAF and of inflammatory cytokines, such as IL-1β and IL-6, which may contribute to COVID-19 and especially SARS. The histamine-1 receptor antagonist rupatadine was developed to have anti-PAF activity, and also inhibits activation of human mast cells in response to PAF. Rupatadine could be repurposed for COVID-19 prophylaxis alone or together with other PAF-inhibitors of natural origin such as the flavonoids quercetin and luteolin, which have antiviral, anti-inflammatory, and anti-PAF actions.

Coronavirus 2019, Microthromboses, and Platelet Activating Factor

Recent articles have reported elevated markers of coagulation, endothelial injury, and microthromboses in lungs from deceased patients with coronavirus 2019 (COVID-19). Platelets are critical in the formation of thrombi, and their most potent trigger is platelet activating factor (PAF). PAF is produced by cells involved in host defense, and its biological actions bear similarities with COVID-19 disease manifestations, including pulmonary microthromboses and inflammation, possibly via activation of mast cells. The histamine1 receptor antagonist rupatadine was developed to have anti-PAF activity and inhibits activation of human mast cells in response to PAF. Rupatadine could be repurposed for COVID-19 prophylaxis.

Serum cytokines and clinical features in patients with fever and thrombocytopenia syndrome

PURPOSE

To explore the clinical, microbiological and immunological features of patients with fever and thrombocytopenia.

METHODS

Patients with unexplained fever and thrombocytopenia were enrolled. Viruses were detected using real-time PCR, and bacteria were measured by culturing methods. Serum cytokines, platelet antibody IgG (PA-IgG) and Helicobacter pylori (HP) were detected using ELISA.

RESULTS

Pathogens were detected in 74.68% of patients, which included single fungal/viral/bacterial infection and multiple infection. The pathogens could not be unidentified in 25.32% of cases. Cytokines including Interleukin (IL)-6, IL-10, interferon-γ(IFN-γ), platelet activating factor (PAF) and PA-IgG were significantly higher in patients as compared to healthy controls (P < .01 or P < .05). Principal component analyses extracted four groups of parameters that have a strong positive predicting value, revealing that disease status evaluation would be more accurate if we combined the platelet parameters and inflammatory biomarkers. While event-free survival (EFS) that indicates the time of platelet elevated after therapy was the highest in patients with single bacterial or fungal infection, EFS was affected by the levels of cytokines and PA-IgG.

CONCLUSIONS

Differences in immune function may be the main factors affecting the prognosis of patients with fever and thrombocytopenia, while treatment based on precise etiological diagnosis is important for therapeutic efficacy.

Markers of anaphylaxis - a systematic review

Anaphylaxis is defined as severe, life-threatening, systemic or general, immediate reaction of hypersensitivity, with repeatable symptoms caused by the dose of stimulus which is well tolerated by healthy persons. The proper diagnosis, immediate treatment and differential diagnosis are crucial for saving patient's life. However, anaphylaxis is relatively frequently misdiagnosed or confused with other clinical entities. Thus, there is a continuous need for identifying detectable markers improving the proper diagnosis of anaphylaxis. Here we presented currently known markers of anaphylaxis and discussed in more detail the most clinically valuable ones: tryptase, platelet activacting factor (PAF), PAF-acethylhydrolase, histamine and its metabolites.

Biomarkers of necrotizing enterocolitis: a review of literature

Necrotizing enterocolitis (NEC) is among the most serious gastrointestinal emergency in very low birth weight (VLBW), extremely low birth weight (ELBW) and extremely low gestational age neonates (ELGAN), affecting 7-14% of these neonates. Despite extensive research, the underlying aetiology of NEC still remains blurred. Due to high mortality, morbidity and its delayed presentation, early detection of NEC is considered to be lifesaving. A number of biomarkers have been studied for early detection and prediction of severity of NEC but till date, no ideal marker has been discovered. Molecular techniques like proteomic and metabolomic have recently emerged in the field for the development of biomarkers for early detection and understanding the pathophysiology of NEC. We did literature search for identifying all biomarkers that have been used for the detection of NEC and, in this review article, we discuss these biomarkers along with the available current evidence.

Metabolomics of reef benthic interactions reveals a bioactive lipid involved in coral defence

Holobionts are assemblages of microbial symbionts and their macrobial host. As extant representatives of some of the oldest macro-organisms, corals and algae are important for understanding how holobionts develop and interact with one another. Using untargeted metabolomics, we show that non-self interactions altered the coral metabolome more than self-interactions (i.e. different or same genus, respectively). Platelet activating factor (PAF) and Lyso-PAF, central inflammatory modulators in mammals, were major lipid components of the coral holobionts. When corals were damaged during competitive interactions with algae, PAF increased along with expression of the gene encoding Lyso-PAF acetyltransferase; the protein responsible for converting Lyso-PAF to PAF. This shows that self and non-self recognition among some of the oldest extant holobionts involve bioactive lipids identical to those in highly derived taxa like humans. This further strengthens the hypothesis that major players of the immune response evolved during the pre-Cambrian.

Metabolomics of pulmonary exacerbations reveals the personalized nature of cystic fibrosis disease

Background. Cystic fibrosis (CF) is a genetic disease that results in chronic infections of the lungs. CF patients experience intermittent pulmonary exacerbations (CFPE) that are associated with poor clinical outcomes. CFPE involves an increase in disease symptoms requiring more aggressive therapy. Methods. Longitudinal sputum samples were collected from 11 patients (n = 44 samples) to assess the effect of exacerbations on the sputum metabolome using liquid chromatography-tandem mass spectrometry (LC-MS/MS). The data was analyzed with MS/MS molecular networking and multivariate statistics. Results. The individual patient source had a larger influence on the metabolome of sputum than the clinical state (exacerbation, treatment, post-treatment, or stable). Of the 4,369 metabolites detected, 12% were unique to CFPE samples; however, the only known metabolites significantly elevated at exacerbation across the dataset were platelet activating factor (PAF) and a related monacylglycerophosphocholine lipid. Due to the personalized nature of the sputum metabolome, a single patient was followed for 4.2 years (capturing four separate exacerbation events) as a case study for the detection of personalized biomarkers with metabolomics. PAF and related lipids were significantly elevated during CFPEs of this patient and ceramide was elevated during CFPE treatment. Correlating the abundance of bacterial 16S rRNA gene amplicons to metabolomics data from the same samples during a CFPE demonstrated that antibiotics were positively correlated to Stenotrophomonas and Pseudomonas, while ceramides and other lipids were correlated with Streptococcus, Rothia, and anaerobes. Conclusions. This study identified PAF and other inflammatory lipids as potential biomarkers of CFPE, but overall, the metabolome of CF sputum was patient specific, supporting a personalized approach to molecular detection of CFPE onset.

Feed-Forward Reciprocal Activation of PAFR and STAT3 Regulates Epithelial-Mesenchymal Transition in Non-Small Cell Lung Cancer

Platelet-activating factor receptor (PAFR), a G-protein-coupled receptor, has been implicated in tumorigenesis, but its contributions to metastatic progression have not been investigated. Here, we show that PAFR is overexpressed in non-small cell lung cancer (NSCLC) as well as in breast, colorectal, and gastric carcinomas. Expression of PAFR correlates closely with clinical stages, survival time, and distant metastasis. In human NSCLC cells, activation of the PAF/PAFR signaling axis accentuated malignant character, including by stimulating epithelial-mesenchymal transition (EMT). In contrast, silencing PAFR in aggressive NSCLC cells inhibited these effects. Mechanistic investigations showed that PAFR stimulated EMT by activating STAT3 via upregulation of G-protein-dependent SRC or JAK2 kinase activity. Notably, STAT3 transcriptionally elevated PAFR expression. Thus, activation of PAFR in NSCLC cells initiated a forward feedback loop responsible for mediating the aggressive malignant character of NSCLC cells in vitro and in vivo. Reinforcing this reciprocal activation loop, PAF/PAFR signaling also upregulated IL6 expression and thereby STAT3 activation. Overall, our results elucidated an important role for PAFR dysregulation in the pathogenicity of NSCLC and unraveled a forward feedback loop between PAFR and STAT3 that acts to drive the malignant progression of NSCLC.

Macrophage derived platelet activating factor implicated in the resolution phase of gouty inflammation

Human blood derived in vitro differentiated monocytes or macrophages are a population of cells which have been investigated over the years to determine the role these cells play in the resolution phase of gout. Macrophages are able to phagocytose monosodium urate monohydrate (MSU) crystals without releasing inflammatory factors. This study analysed macrophage platelet activating factor secretion and its possible role in the pathway of gout resolution. Analysis of sunatants from in vitro differentiated macrophages stimulated with MSU crystals revealed the secretion of platelet activating factor (PAF)  1.54 ± 0.10 mean ± SEM; ng/mL per 10⁶ cells. This secretion was absent in immature monocytes treated similarly. When these monocytes were pretreated with recombinant human PAF-acetylhydrolase (rhuPAF-AH) and MSU crystals resulted in TNFα suppression. Addition of WEB2086, a platelet activating factor (PAF) antagonist, to differentiated macrophages with MSU crystals unmasked TNFα secretion 0.7 ± 0.06 mean ± SEM; ng/mL per 10⁶ cells. This study identifies a role for PAF and the PAF receptor antagonist in the pathway by which macrophages ingest MSU crystals and resolve the concomitant inflammation.

The 15-deoxy-δ12,14-prostaglandin J2 inhibits LPS‑stimulated inflammation via enhancement of the platelet‑activating factor acetylhydrolase activity in human retinal pigment epithelial cells

A well-recognized natural ligand of PPARγ, 15-deoxy-δ(12,14)-prostaglandin J(2) (15d-PGJ(2)) possesses immunomodulatory properties. The aim of this study was to elucidate whether 15d-PGJ(2) was able to attenuate lipopolysaccharide (LPS)-induced inflammatory responses in human retinal pigment epithelial (RPE) cells, which are involved in ocular immune responses. In addition, we examined whether the platelet activating factor (PAF) is associated with the anti-inflammatory activity of 15d-PGJ(2). ARPE19 cells treated with varying concentrations of 15d-PGJ(2) and a PAF antagonist (CV3988) were used in this study. The activity of PAF-acetylhydrolase (PAF-AH) was assayed by treatment with 15d-PGJ(2) and CV3988 in the presence of LPS. 15d-PGJ(2) and CV3988 inhibited the LPS-induced mRNA expression and protein production of interleukin-6 (IL-6), monocyte chemoattractant protein-1 (MCP-1), and intercellular adhesion molecule-1 (ICAM-1) in ARPE19 cells. These effects resulting from 15d-PGJ(2) were not abrogated by the PPARγ antagonist, indicating that the actions were PPARγ-independent. Furthermore, 15d-PGJ(2) and CV3988 enhanced the PAF-AH activity. Additionally, 15d-PGJ(2) inhibited the phosphorylation of the extracellular signal-regulated kinase (ERK) and the activation of nuclear transcription factor-κB (NF-κB). These results demonstrated that 15d-PGJ(2) reduced LPS-stimulated inflammatory responses in ARPE19 cells by enhancing the PAH-AH activity. These results suggest that 15d-PGJ(2) may have potent anti-inflammatory activity against ocular inflammation.

Lung endothelial cell platelet-activating factor production and inflammatory cell adherence are increased in response to cigarette smoke component exposure

An early event in the pathogenesis of emphysema is the development of inflammation associated with accumulation of polymorphonuclear leukocytes (PMN) in small airways, and inflammatory cell recruitment from the circulation involves migration across endothelial and epithelial cell barriers. Platelet-activating factor (PAF) promotes transendothelial migration in several vascular beds, and we postulated that increased PAF production in the airways of smokers might enhance inflammatory cell recruitment and exacerbate inflammation. To examine this possibility, we incubated human lung microvascular endothelial cells (HMVEC-L) with cigarette smoke extract (CSE) and found that CSE inhibits PAF-acetylhydrolase (PAF-AH) activity. This enhances HMVEC-L PAF production and PMN adherence, and adherence is blocked by PAF receptor antagonists (CV3988 or ginkgolide B). CSE also inhibited PAF-AH activity of lung endothelial cells isolated from wild-type (WT) and iPLA(2)β knockout mice, and with WT cells, CSE enhanced PAF production and RAW 264.7 cell adherence. In contrast, CSE did not affect PAF production or RAW 264.7 cell adherence to iPLA(2)β-null cells, suggesting that iPLA(2)β plays an important role in PAF production by lung endothelial cells. These findings suggest that inhibition of PAF-AH by components of cigarette smoke may initiate or exacerbate inflammatory lung disease by enhancing PAF production and promoting accumulation of inflammatory cells in small airways. In addition, iPLA(2)β is identified as a potential target for therapeutic interventions to reduce airway inflammation and the progression of chronic lung disease.

Tumor necrosis factor alpha and interleukin 6 productions in response to platelet-activating factor in chronic hepatitis B virus infection

OBJECTIVE

The aim of this study was to determine tumor necrosis factor alpha (TNF-alpha) and interleukin-6 (IL-6) release in response to platelet-activating factor (PAF) induction in peripheral blood mononuclear cells (PBMCs) from chronic hepatitis B virus (HBV) carriers.

METHODS

Subjects were grouped into three subgroups. The mean age was 37 +/- 10 years. Group A (n = 15), group B (n = 10) and group C (n = 9) subjects were HBV serology-negative, had natural immunity after recovery from an acute HBV infection, and were chronic HBV carriers, respectively.

RESULTS

Compared with group A, PBMCs from naturally immune subjects and chronic HBV carriers produced significantly higher amounts of TNF-alpha and IL-6 in response to PAF. In chronic HBV carriers, TNF-alpha (1,633.3 +/- 793.7) and IL-6 (2,533.3 +/- 466.3) production was statistically lower than TNF-alpha (2,630.0 +/- 727.3) and IL-6 (3,870.0 +/- 728.4) obtained from naturally immune subjects to HBV.

CONCLUSION

Differences of TNF-alpha levels between chronic HBV carriers and naturally immune subjects suggest that TNF-alpha may be a critical mediator of HBV clearance.

Is platelet-activating factor produced during hemodialysis with AN-69 polyacrylonitrile membrane?

BACKGROUND

Platelet-activating factor (PAF) production during hemodialysis (HD) with cuprophane (CU) membrane has previously been demonstrated, while the results regarding PAF production during HD with AN-69 polyacrylonitrile membrane are dubious. In this study an attempt is made to show that PAF is produced during HD using AN-69 membrane while comparing this production with the corresponding one from HD with CU. Since previous studies have indicated that PAF, like the complement system, could also be implicated in HD-related leukopenia and thrombocytopenia (especially when CU membrane is used), the circulating leukocyte and platelet counts as well as the C3a-desArg and SC5b-9 (soluble, nonlytic form of the terminal complement complex) levels were measured.

METHODS

Ten hemodialyzed patients were subjected to HD with CU and AN-69 membranes for 2 consecutive weeks (first week with CU and second with AN-69). During the third HD session of each week and at different times (0, 2, 5, 15, 30, 60, 180 and 240 min), the PAF levels in the blood as well as the leukocyte and platelet counts were measured, while the circulating levels of the C3a-desArg and SC5b-9 were measured at 0, 5, 15, 60 and 240 min. PAF was detected by ethanol extraction, followed by purification by column chromatography and high-pressure liquid chromatography and finally quantified by bioassay. The C3a-desArg and SC5b-9 fractions of the complement were measured by immunoassay while an autoanalyzer gave the leukocyte and platelet counts.

RESULTS

Circulating PAF levels were detected at all time intervals during HD with AN-69 (PAF(AN-69)) and CU (PAF(CU)) membranes. At all time intervals PAF(AN-69) < PAF(CU), however, statistically significant differences (s) between the two membranes existed only at 15, 30, 60, 180 and 240 min. The highest PAF(AN-69) and PAF(CU) occurred at 5 and 15 min into dialysis, respectively. The same observations were made for circulating C3a-desArg levels (s existed additionally at 5 min as well). The reduction of the circulating leukocytes had almost a mirror image with the C3a-desArg as well as PAF levels while the maximal reduction of platelets was observed after 2 min into dialysis with both membranes (i.e., simultaneously with the first increase in PAF secretion).

CONCLUSIONS

PAF is indeed produced during HD with AN-69 membrane, as it is during HD with CU. At all time intervals during the HD procedure, PAF(AN-69) < PAF(CU). PAF seems to contribute to HD-related leukopenia and thrombocytopenia with both membranes.

Detection of platelet-activating factor in gingival tissue surrounding failed dental implants

BACKGROUND

Dental implant therapy has entered routine clinical practice. However, the failure rate of implants at 5 years, due to biological factors, is still around 7%. The pathogenesis of implant loss involves a complex network of cells and inflammatory mediators. This study evaluated platelet-activating factor (PAF), a potent phospholipid mediator of inflammation, in soft tissue surrounding failed dental implants versus healthy implants.

METHODS

PAF was estimated on extracted lipids by bioassay on washed rabbit platelets; inflammatory cell populations were assessed semiquantitatively after staining, and microvessel density was evaluated after immunohistochemical staining.

RESULTS

Biologically active PAF was detected in the lipid extracts of samples excised from gingival tissue of patients with failed implants, but not in samples from patients with osseointegrated implants or from healthy edentulous subjects. The amount of PAF detected in failed implants was significantly higher than in healthy implants, suggesting a local production of this mediator.

CONCLUSIONS

The presence of PAF was associated with histopathological findings of local inflammation and increased blood vessel density.

Role of platelet-activating factor in cardiovascular pathophysiology

Platelet-activating factor (PAF) is a phospholipid mediator that belongs to a family of biologically active, structurally related alkyl phosphoglycerides. PAF acts via a specific receptor that is coupled with a G protein, which activates a phosphatidylinositol-specific phospholipase C. In this review we focus on the aspects that are more relevant for the cell biology of the cardiovascular system. The in vitro studies provided evidence for a role of PAF both as intercellular and intracellular messenger involved in cell-to-cell communication. In the cardiovascular system, PAF may have a role in embryogenesis because it stimulates endothelial cell migration and angiogenesis and may affect cardiac function because it exhibits mechanical and electrophysiological actions on cardiomyocytes. Moreover, PAF may contribute to modulation of blood pressure mainly by affecting the renal vascular circulation. In pathological conditions, PAF has been involved in the hypotension and cardiac dysfunctions occurring in various cardiovascular stress situations such as cardiac anaphylaxis and hemorrhagic, traumatic, and septic shock syndromes. In addition, experimental studies indicate that PAF has a critical role in the development of myocardial ischemia-reperfusion injury. Indeed, PAF cooperates in the recruitment of leukocytes in inflamed tissue by promoting adhesion to the endothelium and extravascular transmigration of leukocytes. The finding that human heart can produce PAF, expresses PAF receptor, and is sensitive to the negative inotropic action of PAF suggests that this mediator may have a role also in human cardiovascular pathophysiology.

Effect of platelet-activating factor receptor expression on CHO cell motility

Tumor cell migration may favor local mass expansion and metastasis dissemination. Several tumors were found to express the receptor for platelet-activating factor (PAF), a potent mediator of leukocyte chemotaxis and endothelial cell migration. However, its functional role on tumor cells is largely unexplored. In the present study, we evaluated the motogenic effect of PAF on Chinese hamster ovarian (CHO) cancer cells transfected with the human PAF-receptor cDNA (CHO PAF-R). By using time-lapse recording, we detected a rapid motogenic response to PAF stimulation on CHO PAF-R, whereas no effect was evident on vector-only transfected cells. Such an effect was observed on scattered cell motility, on cells seeded on a fibronectin- or collagen-coated surface, and on migration of confluent monolayer cells. Cell speed increased at 1 h and was maximal 6-8 h after PAF stimulation on CHO PAF-R. Concomitantly, PAF induced marked changes in cytoskeleton actin distribution with cell contraction, assembling of stress fibers, and polar foci of adhesion. In conclusion, the present study demonstrates that PAF is a potent inducer of tumor cell motility, thus suggesting a role for this mediator in tumor growth and dissemination.

Intestinal secretory factor released by macrophages stimulated with Clostridium difficile toxin A: role of interleukin 1beta

Clostridium difficile toxin A is associated with enterocolitis in animals and humans. However, the mechanisms of its secretory and damaging effects are not totally understood. In this work, we examined the intestinal secretion of electrolytes and water caused by supernatants from macrophages stimulated with toxin A in rabbit ileal mucosa mounted in Ussing chambers. We also investigated the mechanism by which the intestinal secretory factor (ISF) is released from stimulated macrophages. Supernatants from macrophages stimulated with toxin A caused potent intestinal secretion (change in short-circuit current [DeltaIsc], 76 microA x cm-2; P < 0.01). The release of the ISF was pertussis toxin sensitive (reduction, 61%; P < 0.01) and was also reduced (P < 0.05) by a protein synthesis inhibitor (67%), protease inhibitors (57%), a phospholipase A2 inhibitor (54%), a cyclo-oxygenase inhibitor (62%), a dual cyclo- and lipoxygenase inhibitor (48%), a platelet-activating factor (PAF) receptor antagonist (55%), and tumor necrosis factor alpha (TNF-alpha) synthesis inhibitors (48%). However, this release was not inhibited by a lipo-oxygenase inhibitor. Monoclonal anti-interleukin 1beta (IL-1beta) but not anti-IL-1alpha antibody blocked (72%; P < 0.01) the secretory action of the ISF, as did recombinant human IL-1 receptor antagonist (80%; P < 0.01). High levels of IL-1beta (3,476 pg/ml) were detected by an enzyme-linked immunosorbent assay in the above supernatants. Furthermore, the addition of IL-1beta to the serosal side caused a potent secretory effect (DeltaIsc, 80 microA x cm-2; P < 0.01). These results show that macrophages stimulated with toxin A release an ISF capable of provoking intestinal secretion. The regulation of this factor is dependent upon the activation of the G protein. In addition, prostaglandins, PAF, and TNF-alpha are involved in the release of the ISF. We conclude that IL-1beta is probably the ISF released by macrophages in response to toxin A.

Plasma and urinary platelet activating factor concentrations and leukotriene releasing activity of leukocytes in steroid sensitive nephrotic syndrome of childhood

Platelet activating factor (PAF) is synthesized and secreted by glomerular mesangial and endothelial cells. It increases glomerular basement membrane permeability and induces proteinuria. Leukotrienes (LT) are mediators released by either leukocytes or glomerular cells under the PAF effect. The possible role of PAF in steroid sensitive nephrotic syndrome (SSNS) of childhood was studied in 8 children with SSNS in the acute stage, 5 children in remission and 8 healthy controls. The PAF concentrations in urine and plasma were determined. Leukocytes were stimulated in vitro and the LT release in response to stimulation was determined. The urinary and plasma concentrations of PAF were significantly higher in the acute phase than in remission and in control patients. Children with SSNS were found to have peripheral leukocytes with increased LT releasing activity in vitro. These results are in accordance with clinical and experimental observations indicating that PAF originates in the kidney and plays a role in normal kidney physiology. Urinary PAF concentrations may be related to proteinuria because they were strongly correlated in the present study. Elevated plasma PAF concentrations in the acute stage of SSNS could result from either its secretion from the circulating leukocytes or decreased acetyl hidrolase activity needed for its hydrolysis in plasma. The increased LT release in vitro suggests that these cells might have been activated by PAF secreted from glomeruli. It is proposed that PAF and different LT in systemic and glomerular circulation are important mediators in childhood SSNS.

Bone Marrow Stroma-Derived Prolactin Is Involved in Basal and Platelet-Activating Factor–Stimulated In Vitro Erythropoiesis

Cooperation between in vitro exogenous prolactin (PRL), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-3 (IL-3) at an early step of in vitro erythroid differentiation has been shown in a previous study. To gain more insight into the role of PRL in in vivo hematopoiesis, we have now addressed the involvement of endogenous PRL in the growth of hematopoietic progenitors in a bone marrow (BM) stroma environment. The possible modulation of local PRL production by the inflammatory mediator platelet-activating factor (PAF), which is known to be produced by BM cells and to regulate pituitary PRL release, has also been evaluated. Development of burst-forming unit-erythroid (BFU-E) colonies from CD34+ hematopoietic progenitors cultured on a BM stroma cells (BMSC) layer was slightly, but significantly, reduced in the presence of an antihuman PRL antibody. Pretreatment of BMSC with PAF increased the BFU-E colony efficiency of cocultured CD34+ cells, and this effect was completely abrogated by the antiserum. PAF-modulated release of PRL by BMSC was confirmed by an enzyme-linked-immunospot (Elispot) technique. In addition, immunoprecipitation and Western blotting experiments showed two immunoreactive products in the BMSC culture medium. These corresponded to the nonglycosylated (23 kD) and glycosylated (25.5 kD) forms of pituitary PRL that are also expressed by the B-lymphoblastoid cell line IM9-P3. Specific increase of the nonglycosylated form and decrease of the glycosylated form was observed after PAF treatment. Polymerase chain reaction (PCR) amplification of reverse transcribed RNA using PRL-specific primers showed the presence of PRL message in BMSC and IM9-P3 cells. In situ hybridization experiments with a rat PRL cDNA probe cross-reacting with human PRL mRNA confirmed its presence in a small fraction of unstimulated BMSC and in the majority of PAF-stimulated BMSC. The enhancing effect of PAF on PRL-mediated colony formation, PRL release, and mRNA activation was counteracted by pretreating BMSC with the PAF-receptor (R) antagonist WEB 2170. Lastly, responsiveness of BMSC to PAF was substantiated by the presence of the PAF-R mRNA on these cells.

Bone Marrow Stroma-Derived Prolactin Is Involved in Basal and Platelet-Activating Factor–Stimulated In Vitro Erythropoiesis

Cooperation between in vitro exogenous prolactin (PRL), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-3 (IL-3) at an early step of in vitro erythroid differentiation has been shown in a previous study. To gain more insight into the role of PRL in in vivo hematopoiesis, we have now addressed the involvement of endogenous PRL in the growth of hematopoietic progenitors in a bone marrow (BM) stroma environment. The possible modulation of local PRL production by the inflammatory mediator platelet-activating factor (PAF), which is known to be produced by BM cells and to regulate pituitary PRL release, has also been evaluated. Development of burst-forming unit-erythroid (BFU-E) colonies from CD34+ hematopoietic progenitors cultured on a BM stroma cells (BMSC) layer was slightly, but significantly, reduced in the presence of an antihuman PRL antibody. Pretreatment of BMSC with PAF increased the BFU-E colony efficiency of cocultured CD34+ cells, and this effect was completely abrogated by the antiserum. PAF-modulated release of PRL by BMSC was confirmed by an enzyme-linked-immunospot (Elispot) technique. In addition, immunoprecipitation and Western blotting experiments showed two immunoreactive products in the BMSC culture medium. These corresponded to the nonglycosylated (23 kD) and glycosylated (25.5 kD) forms of pituitary PRL that are also expressed by the B-lymphoblastoid cell line IM9-P3. Specific increase of the nonglycosylated form and decrease of the glycosylated form was observed after PAF treatment. Polymerase chain reaction (PCR) amplification of reverse transcribed RNA using PRL-specific primers showed the presence of PRL message in BMSC and IM9-P3 cells. In situ hybridization experiments with a rat PRL cDNA probe cross-reacting with human PRL mRNA confirmed its presence in a small fraction of unstimulated BMSC and in the majority of PAF-stimulated BMSC. The enhancing effect of PAF on PRL-mediated colony formation, PRL release, and mRNA activation was counteracted by pretreating BMSC with the PAF-receptor (R) antagonist WEB 2170. Lastly, responsiveness of BMSC to PAF was substantiated by the presence of the PAF-R mRNA on these cells.

Transcriptional activation of the interleukin-8 gene by platelet-activating factor in human peripheral blood monocytes

Interleukin-8 (IL-8) is a member of the chemokine family and a potent neutrophil chemoattractant and activator. It is produced by a variety of cell types during inflammation. In the present work, we examined the regulation of IL-8 gene expression in monocytes by the pro-inflammatory lipid mediator, platelet-activating factor (PAF). Stimulation of human peripheral blood monocytes with PAF augmented their release of IL-8. The enhancement of IL-8 secretion was associated with an increase in IL-8 mRNA expression. PAF induced a concentration- and time-dependent augmentation of IL-8 mRNA accumulation. The response was maximal at PAF concentrations of 10-100 nM. The increased mRNA expression was evident after 1.5 hr of stimulation and persisted for 6 hr. Stimulation of monocytes with PAF, followed by arrest of de novo transcription with actinomycin D, indicated that PAF only marginally increased the stability of IL-8 mRNA. However, in vitro nuclear transcription demonstrated that the enhancement of IL-8 mRNA expression occurred mainly at the transcriptional level. The PAF-induced increase in IL-8 mRNA levels could be blocked with a PAF receptor antagonist. These results show, for the first time, that IL-8 gene expression and protein production can be upregulated by PAF. This interaction could be important in the development and amplification of the inflammatory response.

A pharmacophore for high affinity PAF antagonists. II. Hydrophobicity study using the molecular lipophilicity potential

Platelet-activating factor (PAF) is a powerful phospholipid-derived autacoid involved in many physiopathological mechanisms. Many PAF antagonists have been synthesized and evaluated as therapeutic candidates. In a previous report, we have described an electronic pharmacophore of PAF antagonists using the molecular electrostatic potential. In the present study, a molecular lipophilicity potential is used to compare the hydrophobic properties of 49 "heterocyclic sp2 nitrogen' highly potent PAF antagonists, belonging to six structurally different series (nine hetrazepines, five pyrrolo[1,2-c]thiazoles, 14 carboxamides, nine dihydropyridines, nine pyridinyl-thiazolidines and three imidazo[4,5-c]pyridines). Their common features consist of three hydrophilic (HYD2, HY14(3)B and HYD3) and two lipophilic zones (LIP3 and LIP4), defining the lipophilic pharmacophore of the antagonists. This pharmacophore is also characterized by several zone-to-zone distances: HYD3-HYD2 = 1.3 +/- 1.0 A, HY3B-HYD2 = 7.8 +/- 1.1, HYD3-HY3B = 5.1 +/- 1.1 A, LIP4-LIP3 = 5.4 +/- 1.1 A, LIP3-HYD2 = 11.3 +/- 1.6 A, LIP3-HY3B = 5.9 +/- 1.0 A, LIP3-HYD3 = 4.3 +/- 0.9 A, LIP4-HYD2 = 14.7 +/- 1.6 A, LIP4-HY3B = 8.1 +/- 1.2 A and LIP4-HYD3 = 3.9 +/- 1.1 A. These results represent a new step in the determination of a global pharmacophore for PAF antagonists.

PAF modulates eicosanoids and TNF release in immune-complex arthritis in rats

The participation of lipid mediators and tumor necrosis factor (TNF) on an experimental model of immune-complex arthritis was investigated. Male Wistar rats received intraarticular injection of rabbit antibodies to bovine serum albumin into the knee joint followed by i.v. injection of the antigen. The levels of eicosanoids and TNF released into the synovial exudates were then assessed using ELISA and the L929 lytic cell assay, respectively. Increase in the levels of LTB4, TXB2 and PGE2 were detected 5 min, 5 min, and 6 h after arthritis induction, respectively. Pretreatment with the PAF receptor antagonist WEB 2170 decreased the levels of PGE2 and increased those of LTB4, without altering TXB2 levels. Increase in the levels of TNF was detected at 3 h of arthritis. Pretreatment with either the cycloxygenase inhibitor indomethacin or the 5-lipoxygenase inhibitor L-663,536 had no effect on TNF levels. Pretreatment with WEB 2170 significantly decreased TNF levels. These results are the first demonstration of eicosanoids and TNF release in immune-complex arthritis. The data also suggest that PAF had both a positive and negative modulatory role on the release of PGE2 and LTB4, respectively. Moreover, TNF release into the synovial exudate did not depend on eicosanoids whereas platelet activating factor (PAF) appeared to mediate the release of this cytokine in the model.

Simultaneous determination of lysophospholipids by high-performance liquid chromatography with fluorescence detection

A high-performance liquid chromatography (HPLC) procedure for the separation of choline lysophospholipids including 1-acyl-lysophosphatidylcholines and 1-O-alkyl-lysophosphatidyl- cholines, like the lysoform of the platelet activating factor (2-lysoPAF), is described. The lysophospholipids are derivatized at the sn-2 position of the hydroxyl group by 7-diethylaminocoumarin-3-carbonylazide, which converts them into the corresponding carbamoyl derivatives. The derivatized compounds were well separated by reversed-phase HPLC and quantified by fluorimetric detection. This method shows a high sensitivity and allows the separation and quantification of mixtures of lysophospholipids at picomolar level. The method was applied to assay enzyme activities, like phospholipase A2 and PAF-acetylhydrolase, on single phospholipids or their mixtures.

A pharmacophore for high affinity PAF antagonists. I. Electronic model using molecular electrostatic potential

PAF is a powerful phospholipid-derived autacoid involved in many physio-pathological mechanisms. Many PAF antagonists have been synthesized and assayed for therapeutic purposes. In this study, molecular electrostatic potential is used to compare the electronic properties of 48 'heterocyclic sp2 nitrogen' highly potent PAF antagonists, belonging to six series (nine hetrazepines, five pyrrolo[1,2-c]thiazoles, 14 carboxamides, nine dihydropyridines, nine pyridinylthiazolidines and two imidazo[4,5-c]pyridines). Their common features consist of three main electronegative zones (A, B1 and B2) describing the electronic pharmacophore of these ligands. The high affinity of these PAF antagonists seems to be related to this electronegative system A-B(x), which is characterized by three distances A-B1 (9.3 +/- 1.0 A), A-B2 (13.4 +/- 0.7 A) and B1-B2 (4.9 +/- 0.9 A). Moreover, B1 and B2 may surround a common anchorage point in the binding site of the receptor.

Monitoring of immunotherapy with cytokines or monoclonal antibodies

Recombinant cytokines and monoclonal antibodies (mAbs) are increasingly used in the treatment of a number of human diseases. Monitoring of the clinical efficacy of these agents requires specific clinical and laboratory measurements. A number of these novel therapies share common side effects, ranging from fever, headache and general malaise to hypotension, the development of edema leading to the vascular leak syndrome, the occurrence of thromboembolic processes and, in severe cases, organ dysfunction. As an example of the pathogenesis of these side effects, recent data are presented which were obtained in patients receiving immunotherapy with high doses of the cytokine interleukin-2 as an anti-cancer treatment.

Platelet activating factor (PAF) and lyso-PAF in psoriasis

Previous studies have shown that scale from lesional psoriatic skin contains substantial amounts of platelet activating factor (PAF). In this study, PAF and its immediate precursor, lyso-PAF, were measured in exudates from abrasions on lesional and uninvolved psoriatic skin, and from skin of healthy subjects. The mean amounts of PAF recovered from lesional and uninvolved psoriatic skin (n = 13) and from healthy skin (n = 14) were not significantly different (range 0.05-2.14 pmol/sample). Mean recoveries of lyso-PAF from lesional psoriatic skin (n = 9) and skin of healthy subjects (n = 13) were also similar (9.5 +/- 1.9 and 11.0 +/- 1.9 pmol/sample, respectively), but significantly less lyso-PAF was found in exudates from the uninvolved psoriatic skin (n = 9; 3.1 +/- 0.4 pmol/sample; P < 0.01 relative to both lesional psoriasis and healthy skin). The finding of reduced lyso-PAF in uninvolved psoriatic skin was unexpected because increased phospholipase-A2 activity is associated with psoriasis. These results do not support the hypothesis that extracellular PAF contributes significantly to the inflammation associated with psoriasis.

Tumor necrosis factor alpha-induced angiogenesis depends on in situ platelet-activating factor biosynthesis

Tumor necrosis factor (TNF) alpha, a potent inhibitor of endothelial cell growth in vitro, is angiogenic in vivo. Therefore, it was suggested that the angiogenic properties of this agent might be consequent to the production of secondary mediators. Since TNF-alpha stimulates the synthesis of platelet-activating factor (PAF) by monocytes and endothelial cells, we investigated the possible involvement of PAF in the angiogenic effect of TNF-alpha. Angiogenesis was studied in a murine model in which Matrigel was used as a vehicle for the delivery of mediators. In this model the angiogenesis induced by TNF-alpha was shown to be inhibited by WEB 2170, a specific PAF receptor antagonist. Moreover, in mice injected with TNF-alpha, PAF was detected within the Matrigel, 6 and 24 h after TNF-alpha injection. The synthesis of PAF within the Matrigel was concomitant with the early migration of endothelial cells and infiltration of monocytes. No infiltration of lymphocytes or polymorphonuclear leukocytes was observed. Synthetic PAF as well as PAF extracted and purified from mice challenged with TNF-alpha induced a rapid angiogenic response, inhibited by WEB 2170. These results suggest that the angiogenic effect of TNF-alpha is, at least in part, mediated by PAF synthesized from monocytes and/or endothelial cells infiltrating the Matrigel plug.

Ethnic differences in the occurrence of the M1(ala213) haplotype of alpha-1-antitrypsin in asthmatic and non-asthmatic black and white South Africans

An ethnic study of 175 individuals, comprising 65 black and 110 white South Africans, has shown a conclusive difference in the frequency of the M1(ala213) haplotype of alpha 1-antitrypsin (P < 0.00001). The M1(ala213) haplotype occurred more frequently in the black group. In the latter group, the frequency of the M1(ala213) haplotype was the same in both controls (0.55) and asthmatics (0.53). However, there was a significant difference in the frequencies (0.19 and 0.36) for the respective white groups (P < 0.01), the frequency of the M1(ala213) haplotype being much higher in the asthmatics. Apart from the above differences, there was also a difference in the elastase-inhibitory capacities of the homozygote phenotypes M1(val213) vs M1(ala213) (P < 0.0001), this capacity being lower in the latter phenotype. We conclude that the occurrence of the M1(ala213) allele of alpha 1-antitrypsin differs in various ethnic groups and may play a role in asthma.

Streptokinase induces intravascular release of platelet-activating factor in patients with acute myocardial infarction and stimulates its synthesis by cultured human endothelial cells

BACKGROUND

Reocclusion of a successfully recanalized infarct-related artery may account for failure of thrombolytic therapy. Evidence suggests that the intravascular activation of platelets may limit the response to this treatment. The aim of the present study was to investigate whether platelet-activating factor (PAF), an ether lipid mediator with multiple potent biological activities, is synthesized during therapy with thrombolytic agents. Two sets of experiments were performed: (1) we extracted and quantified PAF in blood of patients with acute myocardial infarction treated or untreated with streptokinase (SK), and (2) since the endothelium/platelet interaction is thought to be at the basis of vascular reocclusion, we studied whether cultured human endothelial cells synthesize PAF after stimulation with SK or plasmin.

METHODS AND RESULTS

PAF was extracted from blood samples immediately after acidification to destroy the acid-labile PAF-acetylhydrolase in 25 patients with acute myocardial infarction treated (group A, n = 14) and untreated (group B, n = 11) with intravenous infusion of SK. PAF was detected in 10 of 14 patients of group A and none of group B. PAF began to be detectable 60 to 90 minutes after SK infusion and disappeared from the circulation within 120 to 180 minutes. Percent variation of platelet count over basal values correlated negatively with the amount of PAF present in the circulation at 90 minutes (r = -.719; P < .001) and at 120 minutes (r = -.652; P < .001). Cultured human umbilical cord vein-derived endothelial cells (ECs) synthesized PAF in a dose-dependent manner in response to SK and plasmin, with a synthesis that peaked at 15 minutes and persisted up to 30 minutes for SK and 2 hours for plasmin. PAF extracted from blood samples or from ECs was quantified by bioassay performed after purification by thin-layer chromatography and high-performance liquid chromatography (HPLC). PAF-bioactive material was characterized as PAF with physicochemical and enzymatic treatments, HPLC-tandem mass spectrometry, and specific PAF-receptor antagonists.

CONCLUSIONS

The observation that PAF was detectable in the blood of patients of group A only after treatment with SK and was not detectable in patients with a comparable infarct not treated with SK (group B) suggested that SK stimulates the synthesis of this mediator either directly or via plasmin generation. Indeed, cultured human ECs synthesize PAF after stimulation with both SK and plasmin. PAF production by ECs may promote platelet activation and interaction of these cells as well as of circulating leukocytes with endothelium. These events may limit the beneficial effects of thrombolytic therapy.

Development of a high-performance liquid chromatographic-mass spectrometric technique, with an ionspray interface, for the determination of platelet-activating factor (PAF) and lyso-PAF in biological samples

An HPLC-mass spectrometric technique with an ionspray interface was developed for the determination of platelet-activating factor (PAF) and PAF-related compounds in biological samples. HPLC separations were performed using a reversed-phase column. The mass spectra showed intense [M + H]+ ions. Collision-induced dissociation of protonated molecular ions gave characteristic daughter ions corresponding to the phosphorylcholine group. By selective-ion monitoring, a detection limit of 0.3 ng was obtained for all molecules; by multiple reaction monitoring, the same sensitivity was achieved for PAF whereas for lyso-PAF the limit was 3 ng. Finally, PAF was comparatively determined by bioassay and HPLC-MS after extraction from the cell pellets and the supernatants of human polymorphonuclear neutrophils unstimulated or stimulated with opsonized zymosan. The good correlation observed between these techniques indicated the reliability of HPLC-MS for biochemical studies on PAF and PAF-related molecules.

Platelet activating factor, lyso-platelet activating factor and arachidonic acid release in normal human skin and the influence of topical steroid treatment

1. Previous, in vitro, studies have established the synthesis of platelet activating factor (PAF) by the 're-modelling' pathways in which the activation of a phospholipase A2 (PLA2) enzyme catalyses the hydrolysis of an ether-acyl-phosphocholine to give concomitant release of lyso-PAF, the immediate precursor of PAF, and arachidonic acid, the precursor of the icosanoids. The aim of this study was to investigate the relationship between PAF and eicosanoid release in human skin, and to study the effect of treatment of skin with a topical steroid, on the release of PAF, lyso-PAF and arachidonic acid. 2. A novel assay procedure was developed for the simultaneous assay of PAF and lyso-PAF in skin exudates from abrasions and suction blisters in normal human skin. In addition we assayed arachidonic acid and prostaglandin E2 (PGE2), a representative eicosanoid. 3. The mean amounts of mediator recovered in the first 30 min period following abrasion were PAF 0.43, lyso-PAF 11.9, PGE2 25.7 and arachidonic acid 760 pmol/sample. The molar ratio of PAF:lyso-PAF:arachidonic acid in skin exudates from abrasions was 1:30:1800 and in suction blister exudates was 1:90:3660. 4. Time course studies showed a decline in the recoveries of arachidonic acid and lyso-PAF, of about 50% in 2 h. In contrast, PAF was recovered in exudates at a constant rate over 2 h but PGE2 release decreased by more than 90% after the initial 30 min period. 5. Topical application under occlusion, of 0.05% clobetasol propionate, a potent corticosteroid, significantly reduced lyso-PAF by 30% in suction blister exudates but did not significantly alter the concentrations of PAF or arachidonic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of platelet-activating factor in hypotension and platelet activation induced by infusion of thrombolytic agents in rabbits

Infusion of the thrombolytic agents streptokinase (SK, 666 units/kg per minute for 60 minutes) and tissue-type plasminogen activator (t-PA, 10 micrograms/kg per minute for 15 minutes) in rabbits induced a significant hypotension and decrease in platelet count that were completely prevented by treatment with platelet-activating factor (PAF) receptor antagonists SDZ 63-675 and WEB 2170. PAF synthesis by vascular tissue was suggested by its extraction from blood-free heart and aorta of rabbits treated in vivo with SK or t-PA but not of control rabbits. In contrast, PAF was not detected in peripheral blood. Ex vivo studies on platelet aggregation response to ADP and PAF performed on platelet-rich plasma obtained before and after SK and t-PA infusion demonstrated an early hyperaggregable phase, abrogated by PAF receptor antagonists and followed by reduced sensitivity of platelets to PAF. The ED50 values for the aggregation of washed rabbit platelets induced by PAF but not thrombin were significantly increased at 60 and 120 minutes after SK and t-PA infusion, suggesting a specific desensitization of platelets to PAF. In contrast to PAF receptor antagonists, aspirin did not significantly modify the hypotension and the platelet hyperaggregability induced by SK or t-PA or the platelet hypoaggregability induced by t-PA. Thrombocytopenia induced by t-PA, but not by SK, was partially prevented by aspirin. The effect of SK, t-PA, and plasmin on the aggregation of washed platelets from untreated rabbits and from humans was also studied. Whereas SK and t-PA were inactive, plasmin induced dose-dependent platelet aggregation that was inhibited by platelet pretreatment with PAF receptor antagonists. In conclusion, the effect of PAF receptor antagonists observed in the present experimental model suggests that the hypotension and activation of platelets induced by SK and t-PA infusion are mediated by PAF.