Cell-specific regulation of expression of plasma-type platelet-activating factor acetylhydrolase in the liver

Lipoprotein-associated phospholipase A2: The story continues

Inflammation is thought to play an important role in the pathogenesis of vascular diseases. Lipoprotein-associated phospholipase A2 (Lp-PLA2) mediates vascular inflammation through the regulation of lipid metabolism in blood, thus, it has been extensively investigated to identify its role in vascular inflammation-related diseases, mainly atherosclerosis. Although darapladib, the most advanced Lp-PLA2 inhibitor, failed to meet the primary endpoints of two large phase III trials in atherosclerosis patients cotreated with standard medical care, the research on Lp-PLA2 has not been terminated. Novel pathogenic, epidemiologic, genetic, and crystallographic studies regarding Lp-PLA2 have been reported recently, while novel inhibitors were identified through a fragment-based lead discovery strategy. More strikingly, recent clinical and preclinical studies revealed that Lp-PLA2 inhibition showed promising therapeutic effects in diabetic macular edema and Alzheimer's disease. In this review, we not only summarized the knowledge of Lp-PLA2 established in the past decades but also emphasized new findings in recent years. We hope this review could be valuable for helping researchers acquire a much deeper insight into the nature of Lp-PLA2, identify more potent and selective Lp-PLA2 inhibitors, and discover the potential indications of Lp-PLA2 inhibitors.

Clinical application of elevated platelet-activating factor acetylhydrolase in patients with hepatitis B

Background

The aim of this study was to investigate the variation of platelet-activating factor acetylhydrolase (PAF-AH) in patients with various stages of hepatitis B infection and evaluate the association between PAF-AH activity and chronic severe hepatitis B (CSHB) and mortality in patients with hepatitis B.

Methods

Serum PAF-AH activity was measured in 216 patients with hepatitis B and in 152 healthy controls using an automatic biochemical analysis system. Spearman correlation was used to investigate the correlation between PAF-AH activity and other biochemical indicators. The receiver operating characteristic (ROC) curve and multivariable logistic regression analysis were used to evaluate the ability of PAF-AH activity to predict CSHB and mortality in patients with hepatitis B.

Results

The PAF-AH activities in patients with CSHB (1320 ± 481 U/L) were significantly higher than those in healthy controls and in other hepatitis B groups (all P < 0.01). In patients with hepatitis B, PAF-AH activity correlated with total bilirubin (r = 0.633), total bile acid (r = 0.559), aspartate aminotransferase (r = 0.332), apolipoprotein B (r = 0.348), high-density lipoprotein cholesterol (r = −0.493), and apolipoprotein AI (r = −0.530). The areas under the ROC curves for the ability of PAF-AH activity to predict CSHB and mortality in patients with hepatitis B were 0.881 (95% confidence interval (CI): 0.824–0.937, P < 0.001) and 0.757 (95% CI: 0.677–0.837, P < 0.001), respectively. Multivariate logistic regression analysis showed PAF-AH activity to be an independent factor predicting CSHB with an odds ratio of 1.003 (95% CI: 1.002–1.005, P < 0.001).

Conclusion

Elevated PAF-AH in patients with hepatitis B was significantly associated with liver damage. Thus, serum PAF-AH could be used as a novel indicator for predicting CSHB and mortality in patients with hepatitis B. Further, PAF-AH activity was an independent factor predicting CSHB.

Pseudomonas aeruginosa toxin ExoU induces a PAF-dependent impairment of alveolar fibrin turnover secondary to enhanced activation of coagulation and increased expression of plasminogen activator inhibitor-1 in the course of mice pneumosepsis

Background

ExoU, a Pseudomonas aeruginosa cytotoxin with phospholipase A₂ activity, was shown to induce vascular hyperpermeability and thrombus formation in a murine model of pneumosepsis. In this study, we investigated the toxin ability to induce alterations in pulmonary fibrinolysis and the contribution of the platelet activating factor (PAF) in the ExoU-induced overexpression of plasminogen activator inhibitor-1 (PAI-1).

Methods

Mice were intratracheally instilled with the ExoU producing PA103 P. aeruginosa or its mutant with deletion of the exoU gene. After 24 h, animal bronchoalveolar lavage fluids (BALF) were analyzed and lung sections were submitted to fibrin and PAI-1 immunohistochemical localization. Supernatants from A549 airway epithelial cells and THP-1 macrophage cultures infected with both bacterial strains were also analyzed at 24 h post-infection.

Results

In PA103-infected mice, but not in control animals or in mice infected with the bacterial mutant, extensive fibrin deposition was detected in lung parenchyma and microvasculature whereas mice BALF exhibited elevated tissue factor-dependent procoagulant activity and PAI-1 concentration. ExoU-triggered PAI-1 overexpression was confirmed by immunohistochemistry. In in vitro assays, PA103-infected A549 cells exhibited overexpression of PAI-1 mRNA. Increased concentration of PAI-1 protein was detected in both A549 and THP-1 culture supernatants. Mice treatment with a PAF antagonist prior to PA103 infection reduced significantly PAI-1 concentrations in mice BALF. Similarly, A549 cell treatment with an antibody against PAF receptor significantly reduced PAI-1 mRNA expression and PAI-1 concentrations in cell supernatants, respectively.

Conclusion

ExoU was shown to induce disturbed fibrin turnover, secondary to enhanced procoagulant and antifibrinolytic activity during P. aeruginosa pneumosepsis, by a PAF-dependent mechanism. Besides its possible pathophysiological relevance, in vitro detection of exoU gene in bacterial clinical isolates warrants investigation as a predictor of outcome of patients with P. aeruginosa pneumonia/sepsis and as a marker to guide treatment strategies.

Lipopolysaccharide and platelet-activating factor stimulate expression of platelet-activating factor acetylhydrolase via distinct signaling pathways

OBJECTIVES

This study was designed to investigate and characterize the ability of platelet-activating factor (PAF) to induce the expression of platelet-activating factor acetylhydrolase (PAF-AH).

METHODS

Ribonuclease protection assays and quantitative real-time PCR were used to investigate the ability of lipopolysaccharide (LPS) and PAF to regulate PAF-AH mRNA expression in human monocyte-macrophage 6 (MM6) cells. Pharmacological inhibitors of mitogen activated protein kinases (MAPK) and PAF receptor antagonists were used to investigate the mechanism of regulation of PAF-AH.

RESULTS

PAF-AH mRNA levels were increased upon exposure to LPS or PAF in a dose-dependent manner. LPS elicited a more potent and rapid increase in PAF-AH expression than the PAF-stimulated response. However, when administered concomitantly, PAF augmented the LPS-stimulated response. LPS-stimulated PAF-AH expression was susceptible to partial inhibition by a p38 MAPK inhibitor and PAF receptor antagonists. PAF-induced up-regulation of PAF-AH levels was solely mediated via the PAF receptor and was p38 MAPK-independent.

CONCLUSION

The proinflammatory mediators, LPS and PAF, increased levels of PAF-AH mRNA via distinct signaling pathways.

Serum platelet-activating factor acetylhydrolase activity: relationship with metabolic syndrome in women with history of gestational diabetes mellitus

OBJECTIVE

The aim of this study was to evaluate plasma platelet-activating factor acetylhydrolase (PAF-AH) activity in euglycaemic women with history of gestational diabetes (GDM), and to explore whether this activity is associated with metabolic syndrome (MS) in this group of women.

METHODS

The cross-sectional study included 36 women with history of GDM and 40 women with history of normal glucose tolerance in pregnancy (control group).

RESULTS

Compared to the controls, the GDM group had significantly higher mean values for serum glucose, insulin, HOMA-IR, triglyceride, GGT and plasma PAF-AH activity, and a statistically higher prevalence of MS. Within the GDM group, women diagnosed with MS had significantly higher PAF-AH activity than those without MS (p=0.002).

CONCLUSION

This is the first study to have shown that plasma PAF-AH activity and GGT levels may be significant for evaluating atherosclerosis risk and metabolic hepatic damage in women with history of GDM.

Recent progress in phospholipase A₂ research: from cells to animals to humans

Mammalian genomes encode genes for more than 30 phospholipase A₂s (PLA₂s) or related enzymes, which are subdivided into several classes including low-molecular-weight secreted PLA₂s (sPLA₂s), Ca²+-dependent cytosolic PLA₂s (cPLA₂s), Ca²+-independent PLA₂s (iPLA₂s), platelet-activating factor acetylhydrolases (PAF-AHs), lysosomal PLA₂s, and a recently identified adipose-specific PLA. Of these, the intracellular cPLA₂ and iPLA₂ families and the extracellular sPLA₂ family are recognized as the "big three". From a general viewpoint, cPLA₂α (the prototypic cPLA₂ plays a major role in the initiation of arachidonic acid metabolism, the iPLA₂ family contributes to membrane homeostasis and energy metabolism, and the sPLA₂ family affects various biological events by modulating the extracellular phospholipid milieus. The cPLA₂ family evolved along with eicosanoid receptors when vertebrates first appeared, whereas the diverse branching of the iPLA₂ and sPLA₂ families during earlier eukaryote development suggests that they play fundamental roles in life-related processes. During the past decade, data concerning the unexplored roles of various PLA₂ enzymes in pathophysiology have emerged on the basis of studies using knockout and transgenic mice, the use of specific inhibitors, and information obtained from analysis of human diseases caused by mutations in PLA₂ genes. This review focuses on current understanding of the emerging biological functions of PLA₂s and related enzymes.

Decrease of plasma platelet-activating factor acetylhydrolase activity in lipopolysaccharide induced mongolian gerbil sepsis model

Platelet-activating factor (PAF) plays an important role in the pathogenesis of sepsis, and the level of plasma PAF acetylhydrolase (pPAF-AH), which inactivates PAF, decreases in sepsis patients except for the sepsis caused by severe leptospirosis. Usually, increase of pPAF-AH activity was observed in lipopolysaccharide (LPS)-induced Syrian hamster and rat sepsis models, while contradictory effects were reported for mouse model in different studies. Here, we demonstrated the in vivo effects of LPS upon the change of pPAF-AH activity in C57BL/6 mice and Mongolian gerbils. After LPS-treatment, the clinical manifestations of Mongolian gerbil model were apparently similar to that of C57BL/6 mouse sepsis model. The pPAF-AH activity increased in C57BL/6 mice after LPS induction, but decreased in Mongolian gerbils, which was similar to that of the human sepsis. It thus suggests that among the LPS-induced rodent sepsis models, only Mongolian gerbil could be used for the study of pPAF-AH related to the pathogenesis of human sepsis. Proper application of this model might enable people to clarify the underline mechanism accounted for the contradictory results between the phase II and phase III clinical trials for the administration of recombinant human pPAF-AH in the sepsis therapy.

Differential expression of platelet-activating factor acetylhydrolase in lung macrophages

Platelet-activating factor (PAF) acetylhydrolase plays a crucial role inactivating the potent inflammatory mediator, PAF. PAF is implicated in the initiation and propagation of acute lung injury. Although PAF acetylhydrolase is a constitutively active plasma protein, increased PAF production during inflammatory events may necessitate an increase in PAF acetylhydrolase activity in the local environment. A series of experiments were conducted to determine whether the systemic administration of LPS to Sprague-Dawley rats resulted in enhanced expression of PAF acetylhydrolase in lung tissue. Ribonuclease protection assays revealed a dramatic increase in PAF acetylhydrolase mRNA, which peaked at 24 h following in vivo LPS administration. The increase in PAF acetylhydrolase mRNA was dose dependent and was detected when as little as 10 microg/kg of LPS was administered. Western blot analyses of lung tissue homogenates confirmed an increased production of PAF acetylhydrolase protein in response to LPS. In addition, Western blot analyses revealed the rat PAF acetylhydrolase protein exhibited heterogeneous molecular weights with predominant species migrating at 63 and 67 kDa. Some of the molecular weight heterogeneity likely resulted from extensive glycosylation of the secreted protein. Immunohistochemical analyses of lung tissue sections and colocalization experiments revealed a heterogenous population of cells that express the plasma-type PAF acetylhydrolase. Lung interstitial macrophages were PAF acetylhydrolase positive, but surprisingly, alveolar macrophages did not increase expression of PAF acetylhydrolase in response to systemic LPS administration. In addition, rat granulocytes consisting primarily of neutrophils were strongly positive for PAF acetylhydrolase in the LPS-exposed lung tissue. The absence of immunoreactive PAF acetylhydrolase in alveolar macrophages obtained from bronchial alveolar lavage confirmed that systemic LPS administration resulted in enhanced PAF acetylhydrolase expression only in a subset of lung macrophages.

Serum activity of platelet-activating factor acetylhydrolase is a potential clinical marker for leptospirosis pulmonary hemorrhage

Pulmonary hemorrhage has been recognized as a major, often lethal, manifestation of severe leptospirosis albeit the pathogenesis remains unclear. The Leptospira interrogans virulent serogroup Icterohaemorrhagiae serovar Lai encodes a protein (LA2144), which exhibited the platelet-activating factor acetylhydrolase (PAF-AH) activity in vitro similar to that of human serum with respect to its substrate affinity and specificity and thus designated L-PAF-AH. On the other hand, the primary amino acid sequence of L-PAF-AH is homologous to the alpha1-subunit of the bovine brain PAF-AH isoform I. The L-PAF-AH was proven to be an intracellular protein, which was encoded unanimously and expressed similarly in either pathogenic or saprophytic leptospires. Mongolian gerbil is an appropriate experimental model to study the PAF-AH level in serum with its basal activity level comparable to that of human while elevated directly associated with the course of pulmonary hemorrhage during severe leptospirosis. Mortality occurred around the peak of pulmonary hemorrhage, along with the transition of the PAF-AH activity level in serum, from the increasing phase to the final decreasing phase. Limited clinical data indicated that the serum activity of PAF-AH was likely to be elevated in the patients infected by L. interrogans serogroup Icterohaemorrhagiae, but not in those infected by other less severe serogroups. Although L-PAF-AH might be released into the micro-environment via cell lysis, its PAF-AH activity apparently contributed little to this elevation. Therefore, the change of PAF-AH in serum not only may be influential for pulmonary hemorrhage, but also seems suitable for disease monitoring to ensure prompt clinical treatment, which is critical for reducing the mortality of severe leptospirosis.

Platelet activating factor (PAF) antagonism with ginkgolide B protects the liver against acute injury. importance of controlling the receptor of PAF

Platelet activating factor (PAF) is an ubiquitous phospholipid that acts as a mediator of numerous pathophysiological conditions, including hepatotoxicity. The present study has been conducted to evaluate the eventual role of the platelet activating factor in post-acetaminophen intoxication of liver, using ginkgolide B, BN52021, a selective PAF receptor antagonist. One group of rats was treated with a toxic dose of acetaminophen (APAP) (3.5 g/kg b.w.) (control group) and a second one with the same dose of APAP followed by a dose of ginkgolide B, BN52021 (10 mg/kg b.w.) (BN52021-treated group). The animals were killed at 8, 16, 24, 32 and 40 h after treatment. APAP was found to cause an acute hepatic injury, evident by alterations of biochemical (serum enzymes: ALT, AST and ALP) and liver histopathological (degree of inflammation and apoptosis) indices, which was followed by liver regeneration evident by three independent indices ([3H] thymidine incorporation into hepatic DNA, liver thymidine kinase activity and hepatocyte mitotic index). Hepatic levels of malondialdehyde and serum cholesterol/HDL cholesterol fraction were also measured as parameters of oxidant-antioxidant balance. The protected effects of ginkgolide B were qualified during post treatment time by: (1) reduction of oxidative stress, (2) high decrease of hepatic injury, and (3) decrease of regenerating activity. These results indicate that PAF may play an important role in APAP-induced liver injury and regeneration, and that the use of ginkgolide B attenuates liver damage providing important means of improving liver function following acetaminophen intoxication.

Platelet-activating factor-acetylhydrolase A379V (exon 11) gene polymorphism is an independent and functional risk factor for premature myocardial infarction

BACKGROUND

Oxidation of low density lipoproteins is an initial step of atherogenesis that generates pro-inflammatory phospholipids, including platelet-activating factor (PAF). PAF is degraded by PAF-acetylhydrolase (PAF-AH), which has been postulated to be a risk factor for myocardial infarction (MI). The role of PAF-AH for the onset of premature MI is unclear.

METHODS

Polymorphisms located in putatively functional regions were investigated in a cohort of patients having premature MI onset prior to 46 years of age (n = 200) and a sex-age-matched control group (n = 200). The activity of PAF-AH and coronary angiograms were evaluated for the severity of coronary atherosclerosis.

RESULTS

The V allele of A379V (exon 11) polymorphism on PAF-AH gene was more frequent in patients with premature MI (P = 0.001). This V allele polymorphism was also associated with a lower activity of plasma PAF-AH and a more complex coronary atherosclerosis (p Trends <0.05). Multiple logistic regression analysis showed that this polymorphism was an independent risk factor (Odds Ratio [OR] 1.66, 95% CI 1.14.1 to 5.80, P = 0.008) as well as smoking (OR 3.72, 95% CI 1.77 to 9.28, P = 0.001), diabetes mellitus (OR 2.25, 95% CI 1.40 to 5.32, P = 0.007) and hypertension (OR 1.88, 95% CI 1.25 to 5.36, P = 0.003) for the onset of premature MI.

CONCLUSION

We conclude that a functional and significant association between the A379V polymorphism on exon 11 of PAF-AH gene and premature MI exists in this Taiwanese population. This polymorphism is significantly associated with the PAF-AH activity and the severity of coronary atherosclerosis.

Effect of platelet-activating factor (PAF) receptor antagonist (BN52021) on acetaminophen-induced acute liver injury and regeneration in rats

BACKGROUND

Platelet-activating factor (PAF) is an endogenous lipid mediator that plays a key role in catalyzing various pro-inflammatory processes associated with acute liver injury. In the present study, the possible influence of PAF-R antagonist (BN52021) on the protection of liver injury after 4-hydroxyacetanilide, N-acetyl-p-aminophenol, paracetamol (APAP) intoxication was investigated.

METHODS

Thereby, one group of rats was treated with a toxic dose of APAP (3.5 g/kg body weight (b.w.). The animals were killed at 56, 66, 72, 84 and 96 h after treatment.

RESULTS

APAP was found to cause an acute hepatic injury, evident by alterations of biochemical (serum enzymes: aspartate aminotransferase (AST), alanine aminotransferase (ALT) and alkaline phosphatase) and liver histopathological (degree of necrosis and apoptosis) indices, which was followed by liver regeneration, evident by three independent indices ([3H] thymidine incorporation into hepatic DNA, liver thymidine kinase activity and hepatocyte mitotic index). The protective effects of BN52021 were qualified during post-treatment time by: (1) significant reduction of hepatic injury as showed by all biochemical and histological parameters, (2) high decrease of regenerating activity showed by three regenerative markers and (3) remarkable increase of PAF-acetylhydrolase (PAF-AH) activity.

CONCLUSION

These results suggest that PAF may play an important role in APAP-induced liver injury and regeneration, and PAF-R antagonist (BN52021) attenuates liver damage.

Modulating effects of cholesterol feeding and simvastatin treatment on platelet-activating factor acetylhydrolase activity and lysophosphatidylcholine concentration

BACKGROUND

Platelet-activating factor acetylhydrolyse (PAF-AH) is an enzyme that degrades PAF and bioactive oxidized lipids. However, it has been reported to be a risk factor for coronary heart disease. The present study examined the effects of cholesterol feeding and simvastatin treatment on plasma PAF-AH activity.

METHODS

Japanese White rabbits (n=22) were fed a diet containing 0.3% cholesterol and 3% corn oil for 1 month, and then divided into two groups that continued to receive this diet with (treated) or without (control) treatment with simvastatin (0.01%) for another 2 months.

RESULTS

Cholesterol feeding increased and simvastatin treatment decreased apolipoprotein (apo) B-containing lipoprotein subfractions as characterized by capillary isotachophoresis, serum levels of total cholesterol, phospholipids, LDL-C, apoE, plasma and LDL-associated PAF-AH (LDL-PAF-AH) activities, and plasma lyso-PC concentration. Cholesterol feeding also increased apoB levels but decreased the LDL-PAF-AH/LDL-C ratio and did not change the plasma PAF-AH/lyso-PC ratio. Simvastatin treatment did not affect apoB levels and only slightly increased the LDL-PAF-AH/LDL-C ratio. Secretion of PAF-AH activity from monocyte-derived macrophages was increased by cholesterol feeding but not affected by simvastatin treatment. These results indicate that PAF-AH activity is increased by cholesterol feeding due to increased secretion of PAF-AH activity from macrophages and that PAF-AH activity is decreased by simvastatin due to increased removal of lipid and enzyme contents of LDL particles.

CONCLUSION

Cholesterol elevation by cholesterol feeding and cholesterol-lowering by simvastatin modulate plasma PAF-AH activity by different mechanisms.

The effect of statin therapy on lipoprotein associated phospholipase A2 levels

Lipoprotein associated phospholipase A2 (Lp-PLA2), a biomarker of oxidation and inflammation, has been associated with increased coronary heart disease (CHD) risk. To date, data examining the effect of HMG CoA reductase inhibitors on Lp-PLA2 are few. We evaluated the effect of pravastatin 40 mg daily versus placebo on Lp-PLA2 levels among 481 subjects free of cardiovascular disease (pravastatin N=246 and placebo N=235) who participated in the Pravastatin Inflammation/CRP Study (PRINCE). After 12 weeks, Lp-PLA2 levels decreased by 22.1% among pravastatin treated participants and by 7.8% among those randomized to placebo (p<0.001). These results were similar in all subgroups evaluated according to age, blood pressure, lipid parameters, diabetic status, smoking status, aspirin use, body mass index and gender. There were correlations between change in Lp-PLA2 levels and baseline Lp-PLA2 levels (r=-0.63, p<0.001), total cholesterol change (r=-0.26, p<0.001), LDL-C change (r=-0.32, p<0.001) and C-reactive protein (CRP) change (r=-0.13, p=0.05). Multivariate linear regression models that assessed the relationship between the log difference in Lp-PLA2 at 12 weeks and treatment revealed a beta-coefficient of 0.15 for the treatment variable (p<0.01). However, adjustment for change in LDL-C substantially attenuated the beta-coefficient associated with treatment to 0.07 (P<0.005) and after additional control for other potential confounders, the effect of treatment was no longer significant. Thus, Lp-PLA2 levels were significantly reduced at 12 weeks by pravastatin, an effect that was significantly related to LDL cholesterol reduction accounting for about 6% of the variability in this response. Moreover, pravastatin induced reduction in Lp-PLA2 was no longer significant after taking the latter into account.

Platelet-activating factor (PAF) involvement in acetaminophen-induced liver toxicity and regeneration

Acetaminophen-induced toxicity has been attributed to cytochrome P-450-generated metabolites, which covalently modify target proteins. However, the mechanism of liver injury pathogenesis needs to be further elucidated. Platelet-activating factor (PAF) is one of the mediators involved in inflammatory tissue alterations associated with acute liver failure. In this study, alterations in blood PAF levels and the serum activity of PAF-acetylhydrolase (PAF-AH) were investigated over the time course of liver injury and regeneration induced by acetaminophen treatment in rats. The administration of a toxic dose of acetaminophen (3.5 g/kg) in rats caused acute hepatic injury, as evident by alterations of biochemical (serum enzymes: ALT, AST and ALP) and liver histopathological (degree of inflammation and apoptosis) indices between 20 and 40 h post-treatment. The hepatic damage was followed by liver regeneration, made evident by three independent indices ([3H]thymidine incorporation into hepatic DNA, liver thymidine kinase activity and hepatocyte mitotic index), presenting a peak at 72 h. The PAF levels were elevated at 24 and 28 h, presenting a remarkable peak at 32 h post-treatment. PAF-AH activity presented different kinetics to that of PAF. The enzyme activity was relatively low at all time points examined before the rise in PAF activity, peaking later, at 72, 84 and 96 h. Our data demonstrate that PAF is involved in the pathogenesis of acute liver failure and in augmented compensatory liver tissue repair post-acetaminophen treatment. However, the putative role of PAF during liver toxicity and regeneration remains to be established.

Regulating inflammation through the anti-inflammatory enzyme platelet-activating factor-acetylhydrolase

Platelet-activating factor (PAF) is one of the most potent lipid mediators involved in inflammatory events. The acetyl group at the sn-2 position of its glycerol backbone is essential for its biological activity. Deacetylation induces the formation of the inactive metabolite lyso-PAF. This deacetylation reaction is catalyzed by PAF-acetylhydrolase (PAF-AH), a calcium independent phospholipase A2 that also degrades a family of PAF-like oxidized phospholipids with short sn-2 residues. Biochemical and enzymological evaluations revealed that at least three types of PAF-AH exist in mammals, namely the intracellular types I and II and a plasma type. Many observations indicate that plasma PAF AH terminates signals by PAF and oxidized PAF-like lipids and thereby regulates inflammatory responses. In this review, we will focus on the potential of PAF-AH as a modulator of diseases of dysregulated inflammation.

The p38 MAPK pathway mediates transcriptional activation of the plasma platelet-activating factor acetylhydrolase gene in macrophages stimulated with lipopolysaccharide

Administration of lipopolysaccharide (LPS) to experimental animals results in the up-regulation of expression of the plasma form of platelet-activating factor acetylhydrolase (PAF AH) in tissue macrophages. To investigate the mechanism underlying induction of PAF AH by LPS we used murine RAW264.7 and human THP-1 macrophages as model systems. We found that the p38 mitogen-activated protein kinase (p38 MAPK) pathway mediates transcriptional activation of the PAF AH gene through the participation of nucleotides -68/-316 relative to the transcriptional initiation site. This promoter region spans two Sp1/Sp3 binding sites (SP-A and SP-B) and is necessary and sufficient for the observed effect. Disruption of these Sp binding sites significantly reduces promoter activity in LPS-stimulated cells. The ability of LPS to induce transcriptional activation of PAF AH is not due to enhanced Sp1/Sp3 binding to the promoter but involves enhanced transactivation function of Sp1 via p38 MAPK activation. These studies characterize the mechanism by which LPS modulates expression of PAF AH at the transcriptional level, and they have important implications for our understanding of responses that occur during the development of LPS-mediated inflammatory diseases.

Molecular characterization of the constitutive expression of the plasma platelet-activating factor acetylhydrolase gene in macrophages

Plasma platelet-activating factor acetylhydrolase (PAF-AH) is a phospholipase that inactivates platelet-activating factor (PAF) and PAF-like lipids to generate products with little or no biological activity. The levels of circulating PAF-AH correlate with several disease syndromes. We previously reported that mediators of inflammation regulate the expression of the human PAF-AH gene at the transcriptional level. In the present paper, we characterize the constitutive expression of plasma PAF-AH using the mouse gene as a model system, and we report comparative results obtained using human and mouse promoter constructs. We first cloned, sequenced and analysed the promoter region of the murine plasma PAF-AH (mPAF-AH) gene and found that this gene lacks a canonical TATA box. We demonstrated that the cis -elements required for basal transcription are localized within the -316 to -68 bp region. In vitro band-shift and supershift assays showed that Sp1 and Sp3 transcription factors from RAW264.7 and J774A.1 macrophage nuclear extracts bound strongly to a distal GC-rich site within -278/-243 [specificity protein (Sp-A)] and to a proximal TC-rich motif within -150/-114 (Sp-B). In addition, we observed weak binding to a GA-rich site within -110/-82 (Sp-C). The regions containing Sp-B and Sp-C are highly conserved between the human and mouse genes. Forced expression of Sp1 or Sp3 in Sp-lacking Drosophila SL2 cells induced markedly the activity of the exogenous mPAF-AH promoter in a dose-dependent manner, and this induction was dependent on the presence of intact Sp-A and Sp-B. Interestingly, we found that the Sp1- and Sp3-associated DNA-binding activities increased during the maturation of primary human monocytes into macrophages in cell culture. These results demonstrate that Sp1 and Sp3 are key factors that contribute to the basal, constitutive transcription of the plasma PAF-AH gene in macrophages.

Plasma platelet activating factor-acetylhydrolase (PAF-AH)

The platelet-activating factor-acetylhydrolase (PAF-AH) is an enzyme which catalyzes the hydrolysis of acetyl ester at the sn-2 position of PAF. The family of PAF-AHs consists of two intracellular isoforms (Ib and II), and one secreted isoform (plasma). These PAF-AHs show different biochemical characteristics and molecular structures. Plasma PAF-AH and intracellular isoform, II degrade not only PAF but also oxidatively fragmented phospholipids with potent biological activities. Among these PAF-AHs, plasma PAF-AH has been the target of many clinical studies in inflammatory diseases, such as asthma, sepsis, and vascular diseases, because the plasma PAF-AH activity in the patients with these diseases is altered when compared with normal individuals. Finding a genetic deficiency in the plasma PAF-AH opened the gate in elucidating the protecting role of this enzyme in inflammatory diseases. The most common loss-of-function mutation, V279F, is found in more than 30% of Japanese subjects (4% homozygous, 27% heterozygous). This single nucleotide polymorphism in plasma PAF-AH and the resulting enzymatic deficiency is thought to be a genetic risk factor in various inflammatory diseases in Japanese subjects. Administration of recombinant plasma PAF-AH or transfer of the plasma PAF-AH gene improves pathology in animal models. Therefore, substitution of plasma PAF-AH would be an effective in the treatment of the patients with the inflammatory diseases and a novel clinical approach. In addition, the detection of polymorphisms in the plasma PAF-AH gene and abnormalities in enzyme activity would be beneficial in the diagnosis of the inflammatory diseases.

Inflammation, bioactive lipids and atherosclerosis: potential roles of a lipoprotein-associated phospholipase A2, platelet activating factor-acetylhydrolase

It is well established that inflammation is an integral feature of atherosclerosis and of the cardiovascular diseases which it underlies. Oxidative stress is also recognized as a key actor in atherogenesis, in which it is closely associated with the inflammatory response and bioactive lipid formation. Several bioactive lipids have been identified in the atherosclerotic plaque, including the potent inflammatory mediator platelet activating factor (PAF), PAF-like lipids, oxidised phospholipids (oxPL) and lysophosphatidylcholine (lyso-PC). Recent evidence has established a central role of two phospholipases (PL) in atherogenesis, the non-pancreatic Type II secretory phospholipase A(2) (sPLA(2)) and the lipoprotein-associated PLA(2)-alternatively termed as PAF-acetylhydrolase (PAF-AH). sPLA(2) is calcium-dependent and hydrolyses the sn-2 acyl group of glycerophospholipids of lipoproteins and cell membranes to produce lyso-PC and free fatty acids. It is also implicated in isoprostane production from oxPL. sPLA(2) is an acute phase reactant, which is upregulated by inflammatory cytokines and may represent a new independent risk factor for coronary heart disease. In contrast to sPLA(2), PAF-AH is calcium-independent and is specific for short acyl groups at the sn-2 position of the phospholipid substrate and with the exception of PAF, can equally hydrolyze oxPL to generate lyso-PC and oxidized fatty acids. Thus PAF-AH plays a key role in the degradation of proinflammatory oxPL and in the generation of lyso-PC and oxidized fatty acids. PAF-AH equally can also hydrolyze short-chain diacylglycerols, triacylglycerols, and acetylated alkanols, and displays a PLA(1) activity. Whereas sPLA(2) may represent a new independent risk factor for coronary artery disease, the potential relevance of PAF-AH to atherosclerosis remains the subject of debate, and recent results suggest that the potential role of the LDL-associated PAF-AH in atherogenesis may be distinct to that of the HDL-associated enzyme. This review is focused on the main structural and catalytic features of plasma PAF-AH, on the association of the enzyme with distinct lipoprotein particle subspecies, on its cellular sources, and finally on the potential significance of this lipoprotein-associated PLA(2) in cardiovascular disease.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.

Platelet-activating factor acetylhydrolases in health and disease

The platelet-activating factor (PAF) acetylhydrolases catalyze hydrolysis of the sn-2 ester bond of PAF and related pro-inflammatory phospholipids and thus attenuate their bioactivity. One secreted (plasma) and four intracellular isozymes have been described. The intracellular isozymes are distinguished by differences in primary sequence, tissue localization, subunit composition, and substrate preferences. The most thoroughly characterized intracellular isoform, Ib, is a G-protein-like complex with two catalytic subunits (alpha1 and alpha2) and a regulatory beta subunit. The beta subunit is a product of the LIS1 gene, mutations of which cause Miller-Dieker lissencephaly. Isoform II is a single polypeptide that is homologous to the plasma PAF acetylhydrolase and has antioxidant activity in several systems. Plasma PAF acetylhydrolase is also a single polypeptide with a catalytic triad of amino acids that is characteristic of the alpha/beta hydrolases. Deficiency of this enzyme has been associated with a number of pathologies. The most common inactivating mutation, V279F, is found in >30% of randomly surveyed Japanese subjects (4% homozygous, 27% heterozygous). The prevalence of the mutant allele is significantly greater in patients with asthma, stroke, myocardial infarction, brain hemorrhage, and nonfamilial cardiomyopathy. Preclinical studies have demonstrated that recombinant plasma PAF acetylhydrolase can prevent or attenuate pathologic inflammation in a number of animal models. In addition, preliminary clinical results suggest that the recombinant enzyme may have pharmacologic potential in human inflammatory disease as well. These observations underscore the physiological importance of the PAF acetylhydrolases and point toward new approaches for controlling pathologic inflammation.

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.

The expression and localization of plasma platelet-activating factor acetylhydrolase in endotoxemic rats

The production of platelet-activating factor (PAF) and PAF-like phospholipids that also bind the PAF receptor are implicated in numerous pathological situations including bacterial endotoxemia and injury-induced oxidative damage. PAF and PAF-like phospholipids are hydrolyzed and inactivated by the enzyme PAF acetylhydrolase. In the intact rat, infusion of lipopolysaccharide (LPS) into a mesenteric vein served as an acute, liver-focused model of endotoxemia. We determined that the liver responds to LPS exposure with the production of plasma-type PAF acetylhydrolase mRNA and protein expression specifically in the resident macrophages of the liver. Liver macrophages, defined immunohistochemically using antibodies against ED1, present in livers from saline-treated animals contained no detectable PAF acetylhydrolase. Twenty-four hours following in vivo LPS administration, immunohistochemistry detected a slight increase in the number of ED1 staining cells and the ED1-positive cells now contained an abundance of PAF acetylhydrolase. The systemic administration of LPS resulted in increased expression of PAF acetylhydrolase in several tissues. Of the tissues examined, the greatest increase in PAF acetylhydrolase expression was observed in lung followed by increases in spleen, liver, kidney, and thymus. Additionally, the expression of PAF acetylhydrolase mRNA increased in circulating leukocytes and in peritoneal macrophages in response to systemic exposure to LPS. We examined the regulation of PAF acetylhydrolase expression and demonstrated the administration of the PAF receptor antagonists, BN 50739 and WEB 2170, inhibited by 50% the increase in PAF acetylhydrolase expression in response to LPS. The up-regulation of the plasma-type PAF acetylhydrolase expression constitutes an important mechanism for elevating the local and systemic ability to inactivate PAF and oxidized phospholipids in order to minimize PAF-mediated pathophysiology consequent from exposure to endotoxin. The abundance of PAF acetylhydrolase production in the liver lobule likely limits endotoxin-mediated tissue damage due to PAF synthesis.

Infection and inflammation-induced proatherogenic changes of lipoproteins

Epidemiologic studies suggest a link between infection/inflammation and atherosclerosis. During the acute-phase response to infection and inflammation, cytokines induce tissue and plasma events that lead to changes in lipoprotein. Many of these changes are similar to those proposed to promote atherogenesis. The changes of lipoproteins during infection and inflammation are reviewed with a focus on those that are potentially proatherogenic. Hypertriglyceridemia, elevated triglyceride-rich lipoproteins, the appearance of small dense low-density lipoproteins, increased platelet-activating factor acetylhydrolase activity, and secretory phospholipase A(2), sphingolipid-enriched lipoproteins, and decreased high-density lipoprotein (HDL) cholesterol are changes that could promote atherogenesis. Moreover, alterations of proteins associated with HDL metabolism (e.g., paraoxonase, apolipoprotein A-I, lecithin:cholesterol acyltransferase, cholesterol ester transfer protein, hepatic lipase, phospholipid transfer protein, and serum amyloid A) could decrease the ability of HDL to protect against atherogenesis through antioxidation and reverse cholesterol transport mechanisms. These proatherogenic changes of lipoproteins may contribute to the link between infection/inflammation and atherosclerosis.

Correlations between plasma platelet-activating factor acetylhydrolase (PAF-AH) activity and PAF-AH genotype, age, and atherosclerosis in a Japanese population

Platelet-activating factor acetylhydrolase (PAF-AH), a plasma enzyme that hydrolyzes PAF and oxidized phospholipids, is thought to be involved in protecting cells against oxidative stress. A G(994) (M allele)-->T (m allele) mutation in the plasma PAF-AH gene, which results in a Val(279)-->Phe substitution in the mature protein, leads to a loss of catalytic activity. To elucidate the relationships among PAF-AH enzyme activity, genotype, age, and atherosclerosis, we assayed these parameters in a large Japanese population (n=3932) that consisted of three groups; a control group (healthy individuals; n=1684), a risk-factor group (individuals having at least one conventional risk factor for atherosclerosis; n=1398), and a diseased group (patients who had suffered a myocardial infarction or stroke; n=850). We observed a significantly increased frequency of the m allele in the diseased group as compared with the control or risk-factor groups. Plasma PAF-AH activity increased significantly with age in women in the control group with the MM and Mm genotypes, and in men in the control group with the MM genotype, but not in men with the Mm genotype. In both the risk-factor and diseased groups, however, no correlation was observed between plasma PAF-AH activity and age in subjects with either genotype. These results suggest that in individuals with the MM genotype, plasma PAF-AH activity may be increased in response to stresses induced by PAF and/or oxidized phospholipids that might accumulate with age, but that this response is not evident or reduced in healthy individuals with the m allele, or in subjects with atherosclerotic disease, or having risk factors. Together with our previous findings, the G(994)-->T mutation in the PAF-AH gene may be one of the genetic determinants for atherosclerotic disease in the Japanese population.

Plasma platelet-activating factor acetylhydrolase activity in human immunodeficiency virus infection and the acquired immunodeficiency syndrome

Platelet-activating factor (PAF) acetylhydrolase (PAF-AH) catalyzes the hydrolysis of PAF, a mediator of inflammation, as well as other biologically active oxidized phospholipids. In humans, plasma PAF-AH activity is bound to low-density lipoprotein (LDL) and high-density lipoprotein (HDL). Higher levels of plasma PAF-AH activity have been found in a variety of diseases, and are thought to be a defense mechanism against the toxic effects of PAF and oxidized phospholipids. We studied plasma PAF-AH activity in patients with human immunodeficiency virus (HIV) infection and acquired immunodeficiency syndrome (AIDS), a disease characterized by chronic HIV infection and a systemic host response. Plasma PAF-AH activity was significantly greater in AIDS patients compared with control subjects (25.2 +/- 2.0 v 17.0 +/- 0.8 nmol/min/mL, P < .001). The higher levels of plasma PAF-AH activity were found in LDL (28.2 +/- 2.2 v 18.3 +/- 1.0 nmol/min/mL for AIDS v controls, respectively, P = .0005), but not in HDL. Plasma PAF-AH activity in AIDS correlated with circulating interferon alfa (r = .575, P = .005) and plasma triglycerides (r = .556, P < .0025). The presence of secondary infection in AIDS did not significantly change plasma PAF-AH activity. The initiation of a new antiretroviral regimen with either a protease inhibitor or the nucleoside analog lamivudine did not significantly decrease plasma PAF-AH activity, despite successful suppression of HIV RNA levels. Plasma PAF-AH activity may be a sensitive marker of the host response to infection, and the higher levels of plasma and LDL-associated PAF-AH activity in patients with HIV infection and AIDS may be a physiological response to protect the host against oxidative injury from PAF and oxidized phospholipids.

In vivo regulation of plasma platelet-activating factor acetylhydrolase during the acute phase response

Plasma platelet-activating factor acetylhydrolase (PAF-AH) hydrolyzes PAF and oxidized phospholipids and is associated with lipoproteins in the circulation. Endotoxin [lipopolysaccharide (LPS)], a potent inducer of the acute phase response (APR), produces marked changes in several proteins that play important roles in lipoprotein metabolism. We now demonstrate that LPS produces a 2.5- to 3-fold increase in plasma PAF-AH activity in Syrian hamsters. The plasma PAF-AH activity is found in the high-density lipoprotein (HDL) fraction and is increased threefold with LPS treatment despite a decrease in plasma HDL levels, indicating that plasma PAF-AH activity is increased per HDL particle. LPS markedly increased PAF-AH mRNA levels in liver, spleen, lung, and small intestine. The maximal increase in plasma PAF-AH activity and mRNA expression in liver and spleen is seen 24 h after LPS treatment. Both tumor necrosis factor and interleukin-1 modestly increased plasma PAF-AH activity and mRNA levels in liver and spleen, suggesting that they may partly mediate the effect of LPS on PAF-AH. Surgical removal of spleen had no effect on basal or LPS-induced plasma PAF-AH activity, suggesting that spleen per se may not contribute to plasma PAF-AH activity. Finally, LPS, turpentine and zymosan increased plasma PAF-AH activity in mice and/or rats, indicating that multiple APR inducers upregulate plasma PAF-AH and this effect is consistent across different rodent species. Taken together, our results indicate that plasma PAF-AH activity and mRNA expression is markedly upregulated during the host response to infection and inflammation. An increase in plasma PAF-AH may enhance the degradation of PAF as well as alter the structure and function of HDL during infection and inflammation.

Hepatic regulation of platelet-activating factor acetylhydrolase and lecithin:cholesterol acyltransferase biliary and plasma output in rats exposed to bacterial lipopolysaccharide

Normal rat bile contains secretory platelet-activating factor acetylhydrolase (PAF-AH), the enzyme capable of hydrolyzing the inflammatory mediator platelet-activating factor (PAF), and phospholipids containing oxidized truncated fatty acids. Because lecithin:cholesterol acyltransferase (LCAT) possesses intrinsic PAF-AH-like activity, it also may represent a potential anti-inflammatory enzyme. The behavior of PAF-AH and LCAT in hepatobiliary inflammatory responses in vivo has not been characterized. We therefore investigated the biliary and plasma secretion and pharmacological characteristics of these enzymes in rats subjected to intraportal bacterial endotoxin exposure (lipopolysaccharide [LPS], Escherichia coli, 055:B5). Portal vein LPS infusion (1 mg/kg, bolus) resulted in a maximal 4- to 5-fold increase in bile PAF-AH-specific activity with a gradual decline to baseline by 18 hours. Biliary PAF-AH hydrolyzed also the truncated sn-2-succinoyl and sn-2-glutaroyl analogs of PAF, indicating a broader activity of PAF-AH in bile toward byproducts of glycerophospholipid peroxidation. Plasma PAF-AH activity was not altered 5 hours after LPS injection compared with saline injection, but it was significantly elevated 18 hours after endotoxin exposure. The levels of LCAT in bile were low and declined to nearly undetectable values by 5 hours after cannulation in both control and LPS-exposed rats. Plasma LCAT activity was significantly increased after 5 hours and decreased 18 hours after LPS injection. In summary, hepatic exposure to endotoxin results in a rapid increase in biliary secretion of PAF-AH followed by elevation of LCAT and PAF-AH levels in plasma. We propose that biliary secretion of PAF-AH may be involved in the hepatic response to endotoxic insult by counteracting potential inflammatory damage in the biliary tree and gastrointestinal tract, whereas plasma increases in LCAT and PAF-AH may promote elimination of excess PAF and oxidized phospholipids in the circulation.

Secretory PAF-acetylhydrolase of the rat hepatobiliary system: characterization and partial purification

Hepatocytes and Kupffer cells in primary culture both secrete plasma-type platelet-activating factor-acetylhydrolase (pPAF-AH) into serum-free culture medium. The rate of secretion of pPAF-AH by Kupffer cells was 20 to 25 times higher than from hepatocytes, and Kupffer cells expressed a higher level of pPAF-AH mRNA than did hepatocytes. Purified liver cell-secreted pPAF-AH exhibited a major protein band of 65-67 kDa on SDS-PAGE; this was the band predominantly labeled when the enzyme catalytic center was reacted with [3H]diisopropylfluorophosphate ([3H]DFP). Rat bile collected from cannulated bile ducts contained significant PAF-AH activity, and bile samples possessed a prominent band at 30-32 kDa, which was the exclusive target for [3H]DFP. Experiments using tunicamycin, an inhibitor of N-linked glycosylation, and endoglycosidase H suggested that pPAF-AH secreted constitutively by cultured hepatocytes and Kupffer cells is glycosylated. The present study supports the notion that hepatic secretion of pPAF-AH into the blood contributes to the regulation of PAF and oxidized phospholipid levels in the circulation, whereas secretion of PAF-AH into the bile may allow hepatic control of these phospholipid signaling molecules in the gastrointestinal tract.

Expression of plasma platelet-activating factor acetylhydrolase is transcriptionally regulated by mediators of inflammation

Platelet-activating factor (PAF) is a potent phospholipid with diverse physiological and pathological actions, and it is inactivated by PAF acetylhydrolase. In this study, we analyzed the tissue distribution of the plasma PAF acetylhydrolase mRNA in humans. We isolated a 3.5-kilobase fragment containing the 5' genomic sequence of the plasma PAF acetylhydrolase gene and further characterized the promoter activity. We determined the transcriptional initiation site by primer extension. We then prepared constructs containing various lengths of 5' genomic fragments fused to a luciferase reporter gene and transfected these constructs into COS-7 cells. We found that there is more than one region in the 1.3-kilobase 5' genomic sequence conferring promoter activity and that a very short 5'-flanking region (72 base pairs) is sufficient for more than 65% of the basal activity. In parallel, we examined the regulation of expression of the PAF acetylhydrolase gene. We found that interferon-gamma (IFNgamma) and lipopolysaccharide (LPS) significantly inhibited synthesis of PAF acetylhydrolase, whereas other cytokines, including IFNalpha, interleukin (IL) 1alpha, IL4, IL6, tumor necrosis factor-alpha, granulocyte/macrophage colony-stimulating factor, and macrophage colony-stimulating factor, had a smaller or no effect in human monocyte-derived macrophages. Furthermore, transfection of the promoter/reporter construct into macrophage RAW264.7 cells revealed that IFNgamma and LPS decreased the promoter activity by 35% and 50%, respectively, whereas PAF stimulated it by 52% via its receptor. The promoter activity was much lower in monocytic U937 cells compared with the basal level in COS-7 cells, while the activities in P388D1 and RAW264.7 macrophagic cells were considerably higher than the basal level in COS-7 cells. There are multiple regions in the PAF acetylhydrolase promoter that contain responsive elements for signal transducer and activators of transcription-related proteins, and also for myeloid-specific transcription factors. Our data indicate that the opposite of mRNA expression in monocytes versus macrophages is due to inhibition of the promoter activity in the former and activation in the latter cells.