AUTOCRINE ENHANCEMENT OF LEUKOTRIENE SYNTHESIS BY ENDOGENOUS LEUKOTRIENE B-4 AND PLATELET-ACTIVATING-FACTOR IN HUMAN NEUTROPHILS

1 Platelet-activating factor (PAF) and leukotriene B-4 (LTB(4)), two p otent lipid mediators synthesized by activated neutrophils, are known to stimulate several neutrophil functional responses. In this study, w e have determined that endogenous LTB(4) and PAF exert autocrine effec ts on LT synthesis, as well as the underlying mechanism involved. 2 Pr etreatment of neutrophils with either pertussis toxin (PT), or with re ceptor antagonists for LTB(4) and PAF, resulted in an inhibition of LT synthesis induced by calcium ionophore, A23187. This inhibition was m ost marked at submaximal (100- 300 nM) A23187 concentrations, whilst i t was least at ionophore concentrations which induce maximal LT synthe sis (1-3 mu M). Thus newly-synthesized PAF and LTB(4) can enhance LT s ynthesis induced by A23187 under conditions where the LT-generating sy stem is not fully activated. 3 In recombinant human (rh) granulocyte-m acrophage colony-stimulating factor (GM-CSF)-primed neutrophils, LT sy nthesis in response to chemoattractants (fMet-Leu-Phe or rhC5a) was al so significantly inhibited by the LTB(4) receptor antagonist, and to a lesser extent by PAF receptor antagonists. 4 Further investigation re vealed that LTB(4) and/or PAF exert their effects on LT synthesis via an effect on arachidonic acid (AA) availability, as opposed to 5-lipox ygenase (5-LO) activation. Indeed, the receptor antagonists, as well a s PT, inhibited LT synthesis and AA release to a similar extent, where as 5-LO activation (assessed with an exogenous 5-LO substrate) was vir tually unaffected under the same conditions. Accordingly, we showed th at addition of exogenous LTB(4) could enhance AA availability in respo nse to chemoattractant challenge in rhGM-CSF-primed cells, without sig nificantly affecting the 5-LO activation status. 5 Our data show that newly-generated PAF and LTB(4) have the ability to positively feedback on LT synthesis by acting at the level of the phospholipase A(2)/re-e sterification component of the LT biosynthetic pathway in neutrophils. Such autocrine affects are likely to represent an important amplifica tion step of LT synthesis, and may as such contribute to the rapid ons et, as well as to the evolution, of inflammatory responses.