Metabolites of Arachidonic Acid and Their Role in Inflammatory Disease

Arachidonic acid (AA), after it is released from cell membrane phospholipids, may be metabolized by the cyclo-oxygenase (CO) enzyme leading to formation of the prostaglandins (PG's) and thromboxanes (TX's) or by the 5-lipoxygenase (5-LO) leading to formation of the leukotrienes (LT's). These metabolites are collectively known as eicosanoids. The PG's,especially PGE2 and PGI2 (prostacyclin) have important roles in the development of the cardinal signs of inflammation including erythema, oedema and pain. However, PGE2 and PGI2 can also have anti-inflammatory actions. Thromboxane A2 (TXA2) is a vasoconstrictor and a potent inducer of platelet aggregation while PGI2 is a vasodilator and a potent inhibitor of platelet aggregation. The balance between TXA2 synthesis by platelets and PGI2 synthesis by vascular endothelium is thought to be an important mechanism in haemostasis while an imbalance is thought to play a role in the development of some disease states. Leukotriene B4 (LTB4) is thought to mediate leukocyte behaviour during the inflammatory response since it is a potent chemokinetic and chemotactic agent for polymorphonuclear leukocytes (PMN's) and also induces adhesion, aggregation and degranulation of PMN's. Although the individual properties of PGI2, TXA2 and LTB4 are well recognized, their precise role in specific inflammatory diseases is often not known, although such information may facilitate the development of effective treatments. The remit of this thesis was to use available methodology (although not uncritically), to measure levels of these mediators in several inflammatory disease states and to examine some other aspects of their participation, with a view to assessing their role in these conditions. The effect of various therapies on production of these mediators, both in vitro and ex vivo was also examined. The studies reported in Chapter 5 looked at the role of eicosanoids in some inflammatory vascular diseases. Theories regarding the pathogenesis of Raynaud's phenomenon (RP) and approaches to it's treatment have been numerous. This work has shown that in RP secondary to progressive systemic sclerosis (PSS), plasma levels of PGl2-metabolites were unexpectedly elevated. Furthermore, the platelet sensitivity to iloprost (a stable analogue of PGI2) in these patients was shown to be decreased, but was normalized after PG therapy. The beneficial effect of PGl2/iloprost infusion in PSS may therefore be considered as overcoming platelet resistance to PGI2 rather than supplementing an inadequate production of PGI2. The production of LTB4 from stimulated PMN's of patients with RP secondary to PSS and vibration-induced white finger (VWF) was also shown to be increased. The relevance of this observation to the development of vasospasm and vascular occlusion is discussed. The observation of increased levels in VWF supports the growing opinion that this condition is a true secondary Raynaud's syndrome. The finding of increased LTB4 production in PSS suggests that cellular resistance to the inhibitory effects of PGI2 may be a generalized phenomenon in this condition. A recent advent in the treatment of RP has been the use of iloprost. The effect of transdermal iloprost on LTB4 production from PMN's of normal volunteers and Raynaud's patients was investigated. It was concluded that iloprost had some inhibitory effect on LTB4 production, but that it's anti-platelet and vasodilatory effects may be more important in RP. The possibility that some of the beneficial effects of ketanserin (a serotonin antagonist) and stanozalol (an enhancer of fibrinolysis) in RP could be due to an effect on PMN LTB4 production was also examined. Henoch-Schonlein purpura (HSP) is a type of childhood vasculitis. Results are presented which show that plasma from these patients has a diminished or absent ability to support vascular PGI2 generation. Further experiments were undertaken to determine whether this was due to reduced stimulation of PGI2 synthesis or active inhibition. Chapter 6 examined LTB4 production in rheumatoid arthritis (RA) and psoriatic arthritis (PA). It was found that isolated PMN's from RA patients had an increased capacity to produce LTB4 and that this was not secondary to non-steroidal antiinflammatory drug (NSAID) therapy. Weak but significant correlations were found between LTB4 and markers for joint disease activity. Likewise, an increased capacity for LTB4 production was observed in PMN's from PA patients. The relevance of this to both the skin and joint manifestations of the disease is discussed