Antagonism by ETYA of the effects of leukotrienes on ileum and lung parenchymal strips independent of effects on arachidonic acid metabolism

Free radical intermediates of phenytoin and related teratogens. Prostaglandin H synthase-catalyzed bioactivation, electron paramagnetic resonance spectrometry, and photochemical product analysis

Phenytoin and related xenobiotics can be bioactivated by embryonic prostaglandin H synthase (PHS) to a teratogenic free radical intermediate. The mechanism of free radical formation was evaluated using photolytic oxidation with sodium persulfate and by EPR spectrometry. Characterization of the products by mass spectrometry suggested that phenytoin photolyzes to a nitrogen-centered radical that rapidly undergoes ring opening to form a carbon-centered radical. PHS-1 was incubated with teratogen (phenytoin, mephenytoin, trimethadione, phenobarbital, and major metabolites) or its vehicle and the free radical spin trap alpha-phenyl-N-t-butylnitrone, and incubations were analyzed by EPR spectrometry. There was no alpha-phenyl-N-t-butylnitrone radical adduct in control incubations. For phenytoin, a putative unstable nitrogen-centered radical adduct and a stable carbon-centered radical adduct were detected. Free radical spin adducts also were detected for all other teratogens and metabolites except carbamazepine. The PHS inhibitor eicosatetraynoic acid abolished the free radical EPR signal. Incubation of 2'-deoxyguanosine with phenytoin and PHS-1 resulted in a 5-fold increase in its oxidation to 8-hydroxy-2'-deoxyguanosine. This is the first direct chemical evidence for PHS-catalyzed bioactivation of phenytoin and related teratogens to a free radical intermediate that initiates DNA oxidation, which may constitute a common molecular mechanism of teratologic initiation.

Ribozyme-mediated inhibition of expression of leukocyte-type 12-lipoxygenase in porcine aortic vascular smooth muscle cells

Activation of a leukocyte-type 12-lipoxygenase (12-LO) has been proposed to be an important mechanism for angiotensin II- and glucose-induced vascular smooth muscle cell growth. Currently, no specific pharmacological inhibitors for the leukocyte-type 12-LO are available to test this hypothesis. We have therefore designed a chimeric DNA-RNA hammerhead ribozyme to produce cleavage at the first GUC sequence at nucleotide 7 of porcine leukocyte 12-LO mRNA. The ribozyme was tested in vitro with a 206-base 12-LO mRNA as substrate. We observed that the ribozyme specifically and dose-dependently cleaved porcine leukocyte 12-LO mRNA at the predicted site under physiological temperature. Furthermore, we also efficiently delivered the ribozyme into porcine aortic vascular smooth muscle cells by transfection with cationic liposomes. The ribozyme caused a dose-dependent decrease in levels of porcine leukocyte-type 12-LO mRNA in these cells and was more potent than an antisense oligonucleotide directed against porcine leukocyte 12-LO. The 12-LO ribozyme also attenuated 12-LO protein levels in the cells. The action of the ribozyme was primarily a result of its catalytic activity, since a modified ribozyme that lacks catalytic activity showed reduced effects. This represents the first ribozyme directed against a mammalian LO pathway. These results demonstrate the potential utility of new ribozyme technology to generate novel agents for gene modulation experiments to modify the development or progression of vascular disease in humans.

Arachidonate lipoxygenase inhibitors in guinea-pig isolated trachea. Effects on contractions to antigen and various agonists

We compared the effects of the leukotriene (LT) D4 receptor antagonist FPL55712 and some lipoxygenase inhibitors on contractions of isolated guinea-pig trachea induced by antigen (ovalbumin, OA) and calcium ionophore A23187 in the presence of the cyclooxygenase inhibitor indomethacin (5 microM), and by arachidonic acid (AA), melittin and LTD4. FPL55712 (0.1 and 1 microM) inhibited contractions induced by AA (100 microM) and the phospholipase A2 activator melittin (3 micrograms/ml), while the lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA, 10 microM) was a more effective inhibitor of the melittin response than the AA response. FPL55712 inhibited contractions induced by OA (100 micrograms/ml) more than by A23187 (1 microgram/ml), and these inhibitory effects of FPL55712 were much less in the presence of l-serine-borate complex (45 mM), an inhibitor of LTC4 conversion to LTD4. NDGA (10 microM) had no significant effect on the OA response, whereas the lipoxygenase inhibitors 1-phenyl-3-pyrazolidone (phenidone, 10 microM) and 5,8,11,14-eicosatetraynoic acid (ETYA, 10 microM) clearly inhibited it. In contrast, NDGA and phenidone inhibited the A23187 response, but ETYA had no effect on it. FPL55712, phenidone and ETYA, but not NDGA, had a large inhibitory effect on LTD4-induced contractions, but these inhibitors had no effect on histamine-induced contractions. These results suggest that in the guinea-pig trachea inhibitors of LTD4-induced contractions decrease antigen-induced contractions, whereas lipoxygenase inhibitors reduce the contraction to A23187.

Lipoxygenase metabolites mediate increased airways responsiveness to histamine after acute platelet activating factor exposure in the guinea-pig

Platelet activating factor (Paf, 0.02 micrograms/kg, i.v. bolus) caused an acute increase in airways responsiveness to histamine in anaesthetized guinea-pigs prepared for recording airways resistance (RL) and dynamic compliance (Cdyn). Aspirin pretreatment (10 mg/kg, i.v.) attenuated the return of airways responsiveness to prechallenge levels. Pretreatment with the combined cyclooxygenase/lipoxygenase inhibitors BW 755C (20 mg/kg, i.v.) and ETYA (20 mg/kg, i.v.), or with the putative cysteinyl-containing leukotriene antagonist FPL 55712 (0.25 mg/kg/min, i.v.), or a Paf antagonist SRI 63441 (2.5 mg/kg, i.v.), prevented Paf-induced increased airways responsiveness. Inhibitors of leukotriene synthesis, BW 755C and ETYA, or action, FPL 55712, had variable effects on Paf-induced bronchoconstriction. These data suggest that lipoxygenase metabolites, possibly leukotrienes, may mediate an acute increase in airways responsiveness to histamine after Paf exposure.

Specific action of the lipoxygenase pathway in mediating angiotensin II-induced aldosterone synthesis in isolated adrenal glomerulosa cells

Angiotensin II (AII) in adrenal glomerulosa cells activates phospholipase C resulting in the formation of inositol phosphates and diacylglycerol rich in arachidonic acid (AA). Although glomerulosa cells can metabolize AA via cyclooxygenase (CO), this pathway plays little role in aldosterone synthesis. Recent evidence suggests that the lipoxygenase (LO) pathway may be important for hormonal secretion in endocrine tissues such as the islet of Langerhans. However, the capacity of the glomerulosa cell to synthesize LO products and their role in aldosterone secretion is not known. To study this, the effect of nonselective and selective LO inhibitors on AII, ACTH, and potassium-induced aldosterone secretion and LO product formation was evaluated in isolated rat glomerulosa cells. BW755c, a nonselective LO inhibitor dose dependently reduced the AII-stimulated level of aldosterone without altering AII binding (91 +/- 6 to 36 +/- 4 ng/10(6) cells/h 10(-4) M, P less than 0.001). The same effect was observed with another nonselective LO blocker, phenidone, and a more selective 12-LO inhibitor, Baicalein. In contrast U-60257, a selective 5-LO inhibitor did not change the AII-stimulated levels of aldosterone (208 +/- 11% control, AII 10(-9) M vs. 222 +/- 38%, AII + U-60257). The LO blockers action was specific for AII since neither BW755c nor phenidone altered ACTH or K+-induced aldosterone secretion. AII stimulated the formation of the 12-LO product 12-hydroxyeicosatetraenoic acid (12-HETE) as measured by ultraviolet detection and HPLC in AA loaded cells and by a specific RIA in unlabeled cells (501 +/- 50 to 990 +/- 10 pg/10(5) cells, P less than 0.02). BW755c prevented the AII-mediated rise in 12-HETE formation. In contrast, neither ACTH nor K+ increased 12-HETE levels. The addition of 12-HETE or its unstable precursor 12-HPETE (10(-9) or 10(-8) M) completely restored AII action during LO blockade. AII also produced an increase in 15-HETE formation, but the 15-LO products had no effect on aldosterone secretion. These studies suggest that the 12-LO pathway plays a key role as a new specific mediator of AII-induced aldosterone secretion.