Manipulation of platelet aggregation by prostaglandins and their fatty acid precursors: pharmacological basis for a therapeutic approach

Addition of the one-, two- or three- series endoperoxide to human platelet-rich plasma tend to suppress aggregation, through the action of their respective non-enzymatic breakdown products PGE1, PGD2, or PGD3 all of which elevate cyclic AMP levels. On the other hand, these stable primary products do not arise in appreciable amounts from intrinsic endoperoxides generated from either endogenous or exogenous free fatty acids. 5,8,11,14,17-Eicosapentaenoic acid (EPA) suppresses arachidonic acid (5,8,11,14-eicosatetraenoic acid) conversion by cyclooxygenase (as well as lipoxygenase) to aggregatory metabolites in platelets. Exogenously added EPA was capable of inhibiting PRP aggregation induced either by exogenous or endogenous (released by ADP or collagen) arachidonate. The hypothetical combination of an EPA-rich diet and a thromboxane synthetase inhibitor might abolish production of the pro-aggregatory species, thromboxane A2, and enhance formation of the anti-aggregatory metabolite, prostacyclin. Whereas EPA is not detectably metabolized by platelets, dihomo-gamma-linolenic acid (8,11,14-eicosatrienoic acid) is primarily converted by cyclooxygenase and thromboxane synthetase into the inactive metabolite, 12-hydroxyheptadecadienoic (HHD) acid. Pretreatment of human platelet suspensions with the thromboxane synthetase inhibitor imidazole unmasks the aggregatory property of PGH1 and DLL which was partially compromised by the PGE1 formed. The combination of the thromboxane synthetase inhibitor and an adenylate cyclase inhibitor unmasks a complete irreversible aggregation by DLL or PGH1. The basis of a dietary strategy that replaces AA with DLL must rely on the production by the platelet of an inactive metabolite (HHD) rather than thromboxane A2.

Indomethacin inhibits bone resorption and lysosomal enzyme release from bone in organ culture

The effect of indomethacin on bone resorption was studied in an organ culture system, using calvarial bones from 6--7-day-old mice. It was found that indomethacin inhibited spontaneous bone resorption, as estimated by decreased release of 45Ca, Ca2+ and Pi. Indomethacin reduced the release of beta-glucuronidase, beta-galactosidase and beta-N-acetylglucosaminidase, diminished glucose consumption and lactate production, but showed no effect on the release of lactate dehydrogenase. No inhibitory effect of indomethacin on the release of 45Ca stimulated by parathyroid hormone, prostaglandin E2 or 1 alpha(OH)D3 could be registered. 5,8,11,14-eicosatetraynoic acid, an inhibitor of both cyclo- and lipoxygenase pathway of arachidonate metabolism, reduced the spontaneous release of 45Ca, whereas the selective lipoxygenase inhibitor 5,8,11-eicosatriynoic acid was without effect. The results presented indicate that indomethacin may have an inhibitory effect upon the osteoclasts, probably by decreased metabolism of arachidonic acid via the cyclo-oxygenase pathway. A possible relationship between this finding and the pathogenesis of rapid destruction of articular bone in osteoarthritic patients treated with indomethacin is discussed.

Inhibition of prostaglandin biosynthesis by eicosapentaenoic acid

Eicosapentaenoic acid [20 : 5 (n-3)] is not oxidized by the purified cyclooxygenase from sheep vesicular glands in the conditions of low peroxide tone in which arachidonate [20 : 4 (n-6)] is rapidly oxygenated. When the level of peroxide in incubation mixtures is allowed to rise, there is a dramatic change in reactivity of the cyclooxygenase to react with 20 : 5 (n-3) at one-halt the rate and one-third the extent observed with 20 : 4 (n-6). Overall, the low peroxide levels expected in vivo would most probably cause the (n-3) type of fatty acid to be a general inhibitor of prostaglandin formation, through both reversible and irreversible actions at the enzyme site.

Triene prostaglandins: prostaglandin D3 and icosapentaenoic acid as potential antithrombotic substances

Addition of the 3-series fatty acid precursor (icosapentaenoic acid, IPA), its endoperoxide [prostaglandin (PG)H(3)], or thromboxane A(3) to human platelet-rich plasma (PRP) does not result in aggregation of the platelets. In fact, preincubation of human PRP with exogenous PGH(3) actually inhibited aggregation by increasing platelet cyclic AMP concentrations. PGH(3) undergoes rapid spontaneous degradation to PGD(3) in human PRP. The PGD(3) so formed is adequate to account for the increase of platelet cAMP and inhibition of aggregation. Furthermore, addition of PGD-specific antisera to human PRP blocked the platelet inhibitory activity of exogenous PGH(3). PGD(3) has considerable potential as a circulating antithrombotic agent. Pretreatment of human PRP with the adenylate cyclase inhibitor 2',5'-dideoxyadenosine blocked the increase of platelet cyclic AMP and the inhibition of aggregation normally produced by PGI(2), PGE(1), PGD(2), PGH(3), and PGD(3). Furthermore, the dideoxyadenosine unmasked a direct but moderate reversible aggregatory effect in response to the subsequent addition of PGH(3). Similarly, the dideoxyadenosine markedly enhanced the aggregation produced by exogenous PGH(2). IPA is readily incorporated into tissue lipids but proved to be a poor substrate for kidney, blood vessel, or heart cyclooxygenase. IPA was previously shown to be a poor substrate for platelet cyclooxygenase. IPA is readily deacylated from the renal phospholipid pool in response to bradykinin, a substance that also stimulates the release of arachidonic acid. A diet that relies primarily on cold-water fish, as in the case of the Greenland Eskimos, lowers endogenous arachidonic acid and markedly increases the IPA content of tissue lipids. Thus, because IPA has the potential to act as an antagonist with arachidonic acid for platelet cyclooxygenase and lipoxygenase, the simultaneous release of IPA could suppress any residual arachidonic acid conversion to its aggregatory metabolites.

Neutrophil aggregation and degranulation. Effect of arachidonic acid

In response to aggregating and degranulating stimuli, platelets metabolize endogenous arachidonic acid to bioactive derivatives. These derivatives can stimulate platelets to degranulate and aggregate and, therefore, may be mediators of the platelet response. Because exogenous arachidonic acid also stimulates platelets to degranulate, aggregate, and form these mediators, we examined the effect of adding arachidonic acid to purified human neutrophil suspensions. Micromolar concentrations of arachidonic acid stimulated neutrophils to aggregate but not to degranulate. Cytochalasin B, a potentiator of neutrophil responses to chemotactic factors, also potentiated the arachidonic-acid-induced aggregation response; 5,8,11,14-eicosatetraynoic acid, an inhibitor of arachidonic acid metabolism, blocked this response. Aggregation of neutrophils, was not stimulated by several fatty acids with structural similarity to arachidonic acid. These results suggest that metabolic derivatives of arachidonic acid may be active in stimulating certain neutrophil responses. The role of these derivatives in mediating neutrophil responses to various stimuli needs to be examined.

Stimulation of hepatic lipogenesis by eicosa-5,8,11,14-tetraynoic acid in mice fed a high linoleate diet

Liver slices, from mice fasted for one day and then refed for three days either a 15% corn oil diet or a 15% corn oil diet containing eicosa-5,8,11,14-tetraynoic acid (TYA), were incubated with [1-14C] acetate or [3H] H2O to determine lipogenic capacity. Dietary TYA produced a twofold stimulation in fatty acid and cholesterol synthesis. TYA also caused an increase in the relative proportion of linoleate (C18:2) and a decrease in that of arachidonate (C20:4) in liver. Thus, (a) despite high levels of C18:2, hepatic lipogenesis can be increased, and (b) even short term feeding of TYA can alter the hepatic fatty acid composition presumably by inhibition of arachidonate synthesis from linoleate.

Hydrocortisone and the release of prostaglandins from spleen

An infusion of noradrenaline (1 mug/ml/min) released a PGE-like substance (PGEs) from superfused splenic strips of rabbits and from perfused cat spleen. The release of PGEs from rabbit splenic strips was not inhibited by the treatment of strips with hydrocortisone (40-150 mug/ml), but it was completely abolished in strips obtained from animals pretreated with hydrocortisone (1 mg/kg). The release of PGEs from the perfused cat spleen was reduced by hydrocortisone and abolished by indomethacin. It is concluded that the route of administration of hydrocortisone is essential for an appearance of its inhibitory effect on the PG release.

Inhibition of prostaglandin biosynthesis by triynoic acids

Prostaglandin biosynthesis from eicosa-8,11,14-trienoic acid in microsomes from bovine seminal vesicles is inhibited by acetylenic acids. Octadeca-6,9,12-triynoic acid and eicosa-8,11,14-triynoic acid are the most potent inhibitors. These acids both contain an omega-8 methylene group. Within the 20-carbon acetylenic acid series, inhibition decreases in the sequence eicosa-8,11,14-triynoic acid greater than eicosa-7,10,13-triynoic acid greater than eicosa-5,8,11-triynoic acid. Furthermore, eicosa-8,11,14-triynoic acid is a more potent inhibitor of arachidonic acid induced platelet aggregation than either eicosa-7,10,13-triynoic acid or eicosa-5,8,11-triynoic acid. The omega-8 methylene group is not the only determinent of inhibitory potency since docosa-10,13,16-triynoic acid is less potent than its 18 and 20 carbon analogs and all of these acids have an omega-8 methylene group.

Requirements for unsaturated fatty acids for the induction on respiration in Saccharomyces cerevisiae

Unsaturated fatty acids provided during the release from glucose repression were shown to be essential for derepression of respiration in an unsaturated fatty acid auxotroph of Saccharomyces cerevisiae (KD115). Cells derepressed in the presence of oleic acid contained three to six times as much cytochrome per cell as those derepressed in the absence of unsaturated fatty acid or those derepressed with eicosaenoic acid. The delta9 isomer was the most efficient of the cis-octadecenoic acid isomers in supporting that increase, and eicosaenoic acid supported an increase at only 15% the rate observed with oleic acid. Derepression, even in the presence of oleic acid, proceeded only after a lag of 3 hours. When glucose was removed prior to the addition of oleate, the lag was reduced by the time of the preincubation with glycerol. This result suggests that some processes necessary for increased respiration can proceed in the absence of an added unsaturated fatty acid, but these processes apparently require certain levels of unsaturated acids in the pre-existing lipids, since they occurred in cells whose membranes contained 50 mol % oleate, but not in cells containing only 20 mol %. These processes leading to eventual increased respiration were inhibited by cycloheximide but not chloramphenicol, suggesting that protein synthesis on cytoplasmic ribosomes but not mitochondrial ribosomes were required. Derepression in the absence of oleate for 3 hours lessened the inhibition or respiration induction by ethidium bromide. This result indicates that the transcription of mitochondrial DNA necessary for the induction of respiration may have occurred in the absence of added unsaturated fatty acid, but that some subsequent event required added esterified unsaturated fatty acid.