Giancarlo Folco

Eicosanoid Transcellular Biosynthesis: From Cell-Cell Interactions to in Vivo Tissue Responses

The biosynthesis of the biologically active metabolites of arachidonic acid involves a number of enzymes that are differentially expressed in cells. Prostaglandins and thromboxanes are derived from the chemically unstable prostaglandin (PG) H2 intermediate synthesized by PGH synthases (cyclooxygenase-1/2) and leukotrienes from chemically unstable leukotriene A4 by 5-lipoxygenase. Additional enzymes transform these reactive intermediates to a variety of chemical structures known collectively as the lipid mediators. Although some cells have the complete cassette of enzymes required for the production of biolog-ically active prostaglandins and leukotrienes, the actual biosynthetic events often are a result of cell-cell interaction and a transfer of these chemically reactive intermediates, PGH2 and leukotriene A4, between cells. This process has come to be known as transcellular biosynthesis of eicosanoids and requires a donor cell to synthesize and release one component of the biosynthetic cascade and a second, accessory cell to take up that intermediate and process each into the final biologically active product. This review focuses on the evidence for transcellular biosynthetic events for prostaglandins, leukotrienes, and lipoxins occurring during cell-cell interactions. Evidence for arachidonic acid serving as a transcellular biosynthetic intermediate is presented. Experiments for transcellular events taking place in vivo that reveal the true complexity of eicosanoid biosynthesis within tissues are also reviewed.

Simultaneous investigation of the neuronal and vascular compartments in the guinea pig brain isolated in vitro

We describe a new method for studying the interactions between vascular tone changes and neuronal activity in the arterially perfused isolated brain of the adult guinea pig maintained in vitro. Electrophysiological recordings were performed in the piriform and entorhinal cortices with the entire arterial bed preserved or after vascular restriction to the territories of median and posterior cerebral arteries of one hemisphere. The changes in vascular tone were measured by means of a pressure transducer. The arterial pressure was 53.77+/-12.74 mmHg in control conditions at 30 degreesC. Intraluminal application of vasoactive drugs, such as the tromboxane A2 receptor agonist U46619 (0.1 microM) and 5-HT (3 microM), induced an increase in the resistance to perfusion pressure that was prevented by the selective antagonists. The preservation of the endothelial function was verified by inducing the release of endogenous endothelial relaxant factor after intraluminal application of 1 microM acetylcholine. The study of the reciprocal interactions between neuronal activity and vascular tone modifications demonstrated that evoked responses in the piriform and entorhinal cortices were not modulated by rapid changes of the vascular tone. A sustained and elevated plateau of vasoconstriction maintained for several minutes determined a cortical spreading depression. Epileptiform discharges induced in limbic cortices by GABAa receptor blockade were consistently associated with a vasodilation (8.26+/-2.8 mmHg). The results demonstrate that the in vitro isolated guinea pig brain preparation can be exploited for studying simultaneously neuronal activity and cerebrovascular motility.

RELEASE OF LEUKOTRIENE A4 VERSUS LEUKOTRIENE B4 FROM HUMAN POLYMORPHONUCLEAR LEUKOCYTES

The reactive intermediate formed by 5-lipoxygenase metabolism of arachidonic acid, leukotriene A4, is known to be released from cells and subsequently taken up by other cells for biochemical processing. The objective of this study was to determine the relative amount of leukotriene A4 synthesized by human polymorphonuclear leukocytes (PMNL) that is available for transcellular biosynthetic processes. This was accomplished by diluting cell suspensions and measuring the relative amounts of enzymatic versus nonenzymatic leukotriene A4-derived metabolites after challenge with the Ca2+ ionophore A23187. Nonenzymatic leukotriene A4-derived metabolites were used as a quantitative index of the amount of leukotriene A4 released into the extracellular milieu. The results obtained demonstrated that in human PMNL, the relative amounts of nonenzymatic versus enzymatic leukotriene A4-derived metabolites increased with decreasing cell concentrations. After a 20-fold dilution of PMNL in cell preparations, a doubling in the amount of nonenzymatic leukotriene A4-derived metabolites was observed following challenge (from 53.9 ± 1.3 to 110.4 ± 8.9 pmol/106 PMNL, p < 0.01). Reduction of possible cell-cell interactions by dilution suggested that over 50% of leukotriene A4 synthesized is released from the PMNL. These data provide evidence that, in human PMNL preparations, transfer of leukotriene A4 to neighboring PMNL is taking place, resulting in additional formation of leukotriene B4 and its ω-oxidized metabolites 20-hydroxy- and 20-carboxy-leukotriene B4. Neutrophil reuptake of extracellular leukotriene A4 leads to an underestimation of the fraction of leukotriene A4 that is in fact available for transcellular metabolism when tight cell-cell interactions occur, such as during PMNL adhesion to the microvascular endothelium and diapedesis.

Prostacyclin (PGI2) in pregnant human uterus

Prostacyclin lowers the tonus and reduces the spontaneous motility of isolated pregnant human myometrium. This effect seems to be related to coclic-AMP accumulation, since PGI2 increases the formation of this cyclic nucleotide in incubated minces of pregnant and non-pregnant uterus. The ability of this tissue to generate a labile substance which inhibits platelets aggregation, has been demonstrated and discussed.

The Cycloxygenase-2 inhibitor SC58236 is neuroprotective in an in vivo model of focal ischemia in the rat.

Focal ischemia was induced in the fronto-parietal region of rat brain, by injection of Rose Bengal, followed by light activation. Focal ischemia was accompanied by formation of PGD(2) peaking 60-90 min post irradiation and declining thereafter. Increased Cycloxygenase-2 (COX-2) expression was also observed. Control ischemic rats showed distinct morphological alterations with necrosis of neurons, glial cells and blood vessels, surrounded by a halo with pyknotic cells with cytoplasm swelling and vacuolization. Compound SC58236, a selective COX-2 inhibitor, dose-dependently prevented, ischemia-induced eicosanoid formation (area under the curve (AUC) of controls: 3.11 +/- 0.87; AUC of 20 mg/kg SC58236: 0.39 +/- 0.24), and caused significant reduction of damaged area (30.7 and 18.9% at SC58236 20 and 6.6 mg/kg), suggesting that selective inhibitors of COX-2 are neuroprotective.

Cysteinyl-leukotrienes receptor activation in brain inflammatory reactions and cerebral edema formation: a role for transcellular biosynthesis of cysteinyl-leukotrienes

We studied the effect of intravascular activation of human neutrophils on the synthesis of cysteinyl leukotrienes (cysLT) and the formation of cerebral edema in guinea‐pig brains. Challenge with the chemotactic formylated tripeptide fMLP (0.1 µM) of neutrophil‐perfused brain in vitro resulted in blood‐brain barrier disruption associated with a significant increase of cysLT. Both events were completely prevented by neutrophil pretreatment with a specific 5‐lipoxygenase (5‐LO) inhibitor. Perfusion with the 5‐LO metabolite leukotriene B4 (10 nM), together with neutrophils treated with the 5‐LO inhibitor, did not restore the alteration in permeability observed upon perfusion with untreated and activated neutrophils. The dual cysLT1‐cysLT2 receptor antagonist BAYu9773 was more potent and more effective than a selective cysLT1 antagonist in preventing the brain permeability alteration induced by neutrophil activation. RT‐PCR showed significant expression of cysLT2 receptor mRNA in human umbilical vein endothelial cells. Intravital microscopy in mice showed that inhibition of leukotriene synthesis significantly reduced firm adhesion of neutrophils to cerebral vessels without affecting rolling. These data support the hypothesis that neutrophil and endothelial cells cooperate toward the local synthesis of cysLT within the brain vasculature and, acting via the cysLT2 receptor on endothelial cells, may represent a contributing pathogenic mechanism in the development of cerebral inflammation and edema.

A new class of furoxan derivatives as NO donors: mechanism of action and biological activity

1 The mechanism of action and biological activity of a series of R‐substituted and di‐R‐substituted phenylfuroxans is reported. 2 Maximal potency as vasodilators on rabbit aortic rings, precontracted with noradrenaline (1 μm), was shown by phenyl‐cyano isomers and by the 3,4‐dicyanofuroxan, characterized by a potency ratio 3–10 fold higher than glyceryl trinitrate (GTN). This effect was reduced upon coincubation with methylene blue or oxyhaemoglobin (10 μm). 3 The furoxan derivatives showing maximal potency as vasodilators were also able to inhibit collagen‐induced platelet aggregation, with IC50 values in the sub‐micromolar range. 4 The furoxan derivatives were able to stimulate partially purified, rat lung soluble guanylate cyclase; among the most active compounds, the 3‐R‐substituted isomers displayed a higher level of stimulatory effect than the 4‐R analogues. 5 Solutions (0.1 mm) of all the tested furoxans, prepared using 50 mm phosphate buffer, pH 7.4, (diluting 10 mm DMSO stock solutions) did not release nitric oxide (NO) spontaneously; however in presence of 5 mm l‐cysteine, a significant NO‐releasing capacity was observed, which correlated significantly with their stimulation of the guanylate cyclase activity.

Mediator release after endobronchial antigen challenge in patients with respiratory allergy

The aim of the present study was to evaluate the release of some potential mediators of allergic reactions, such as histamine, peptide leukotrienes (LTs), LTB4 and prostaglandin D2 (PGD2), in bronchoalveolar lavage (BAL) fluids from 11 patients with respiratory allergy (eight with bronchial asthma and three with allergic rhinitis), who underwent specific endobronchial challenge. Histamine, peptide LT, and PGD2 levels in BAL fluids increased significantly after antigen stimulation both in patients with asthma and in patients with rhinitis. By contrast, LTB4 concentration was always below the limits of detection of the radioimmunoassay. In patients with asthma, histamine concentration increased from 5.3 +/- 0.6 ng/ml in lavages obtained before provocation to 20.2 +/- 5.8 ng/ml (mean +/- SEM; p less than 0.04) 5 minutes after bronchoprovocation. Peptide LTs increased from 0.32 +/- 0.08 to 0.82 +/- 0.21 ng/ml (p less than 0.02) and PGD2 from 0.06 +/- 0.01 ng/ml to 0.36 +/- 0.09 ng/ml (p less than 0.02). Elevated histamine, peptide LT, and PGD2 concentrations were also found in the 15-minute postchallenge BAL fluids. Similar results were obtained in patients with rhinitis. Histamine concentration was 3.4 +/- 0.6 ng/ml in prechallenge bronchial lavages and 11.3 +/- 1.7 ng/ml in postchallenge lavages; peptide LTs increased from 0.13 +/- 0.008 ng/ml to 0.73 +/- 0.21 ng/ml, and PGD2 from 0.05 +/- 0.01 ng/ml to 0.26 +/- 0.06 ng/ml.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by lipoxygenase products of TXA2-like responses of platelets and vascular smooth muscle: 14-Hydroxy from 22:6N-3 is more potent than 12-HETE

Lipoxygenase products, which are formed in great amounts in platelets during their activation, have been prepared from arachidonic acid (20:4n-6), the main polyunsaturated fatty acid (PUFA) esterified in platelet phospholipids, and from two major PUFAs of fish fat, eicosapentaenoic (20:5n-3) and docosahexaenoic (22:6n-3) acids. These compounds have been synthesized using platelet suspension as enzymic source, purified by high performance liquid chromatography, and their structure were checked by gas chromatography-mass spectrometry. Their effects were investigated in vitro upon human platelet aggregation induced by 11,9-epoxy-methano-analogue of PGH2 (U-46619) and upon thromboxane A2-induced vasoconstriction of rabbit aorta. All hydroxylated fatty acids inhibited U-46619-induced aggregation in a concentration-dependent fashion. Compounds issued from 22:6n-3 were the most potent inhibitors and their IC50 differed significantly from that of 12-hydroxy-eicosatetraenoic acid (12-HETE). Among them, 14-hydroxy-docosahexaenoic acid (14-OH-22:6) was the most effective anti-aggregating molecule (IC50:0.45 microM). 10 microM 12-HETE and 14-OH-22:6 inhibited 60% and 75% of smooth muscle contraction induced by TXA2-like material, respectively. At 1 microM, solely 14-OH-22:6 had an inhibitory effect on adrenaline-, angiotensine- or histamine-induced contraction. Since thromboxane receptors in platelets and vascular smooth muscle cells present strong similarities, it is concluded that hydroxylated fatty acids can antagonize prostanoid action probably by interfering with their receptor sites.

Bronchoconstriction by histamine and bradykinin in guinea pigs: Relationship to thromboxane A2 generation and the effect of aspirin

Histamine 2.5, 5, 10 or 20 microgram/kg i.v. induce a pronounced bronchospasm in guinea-pigs, accompanied by a dose-related increase of TXA2 in arterial blood, as revealed by contraction of rabbit isolated aorta and by radioimmunoassay. Aspirin 10 mg/kg prevented formation of TXA2-like material without significantly modifying the severity of the bronchospasm. Bradykinin 0.5, 1 or 2 microgram/kg i.v. acted similarly, except that pretreatment with aspirin blocked both the increased airway resistance and release of TXA2. Aspirin also blocked the increase in blood pressure and heart rate caused by histamine or bradykinin.