CHARACTERIZATION OF THE PROSTANOID RECEPTOR(S) ON HUMAN BLOOD MONOCYTES AT WHICH PROSTAGLANDIN E-2 INHIBITS LIPOPOLYSACCHARIDE-INDUCED TUMOR-NECROSIS-FACTOR-ALPHA GENERATION

1 The prostanoid receptor(s) that mediates inhibition of bacterial lip opolsaccharide (LPS)-induced tumour necrosis factor-alpha (TNF alpha) generation from human peripheral blood monocytes was classified by use of naturally occurring and synthetic prostanoid agonists and antagoni sts. 2 In human monocytes that were adherent to plastic, neither prost aglandin D-2 (PGD(2)), prostaglandin E-2 (PGE(2)), prostaglandin F-2 a lpha (PGF(2 alpha)) nor the stable prostacyclin and thromboxane mimeti cs, cicaprost and U-46619, respectively, promoted the elaboration of T NF alpha-like immunoreactivity, as assessed with a specific ELISA, ind icating the absence of excitatory prostanoid receptors on these cells. 3 Exposure of human monocytes to LPS (3 ng ml(-1),similar to EC84) re sulted in a time-dependent elaboration of TNF alpha which was suppress ed in cells pretreated with prostaglandin E-1 (PGE(1)), PGE(2) and cic aprost. This effect was concentration-dependent with mean pIC(50) valu es of 7.14, 7.34 and 8.00 for PGE(1), PGE(2) and cicaprost, respective ly. PGD(2), PGF(2 alpha) and U-46619 failed to inhibit the generation of TNF alpha at concentrations up to 10 mu M. 4 With respect to PGE(2) , the EP-receptor agonists, 16,16-dimethyl PGE(2) (non-selective), mis oprostol (EP2/EP3-selective), 11-deoxy PGE(1) (EP2-selective) and buta prost (EP2-selective) were essentially full agonists as inhibitors of LPS-induced TNF alpha generation with mean pIC(50) values of 6.21, 6.0 2, 5.67 and 5.59, respectively. In contrast to the results obtained wi th butaprost and 11-deoxy PGE(1), another EP2-selective agonist, AH 13 205, inhibited TNF alpha generation by only 21% at the highest concent ration (10 mu M) examined. EP-receptor agonists which have selectivity for the EP1-(17-phenyl-omega-trinor PGE(2)) and EP3-receptor (MB 28,7 67, sulprostone) were inactive or only weakly active as inhibitors of TNF alpha generation. 5 Pretreatment of human monocytes with the TP/EP 4-receptor antagonist, AH 23848B, at 10, 30 and 100 mu M suppressed LP S-induced TNF alpha generation by 10%, 28% and 77%, respectively, but failed to shift significantly the location of the PGE(2) concentration -response curves. 6 Given that AH 13205 was a poor inhibitor of TNF al pha generation, studies were performed to determine if it was a partia l agonist and whether it could antagonize the inhibitory effect of PGE (2). Pretreatment of human monocytes with 10 and 30 mu M AH 13205 inhi bited the generation of TNF alpha by 31% and 53%, respectively, but fa iled to shift significantly the location of the PGE(2) concentration-r esponse curves at either concentration examined. 7 Since PGD(2) and 17 -phenyl-omega-trinor PGE(2) (EP1-agonist) did not suppress TNF alpha g eneration, the EP1/EP2/DP-receptor antagonist, AH 6809, was employed t o assess if EP2-receptors mediated the inhibitory effect of PGE(2). Pr etreatment of human monocytes with 10 mu M AH 6809 did not affect LPS- induced TNF alpha generation but produced a parallel 3.5 fold rightwar ds shift of the PGE(2) concentration-response curve. 8 Collectively, t hese data suggest that human peripheral blood monocytes express at lea st two distinct populations of inhibitory prostanoid receptors that me diate inhibition of LPS-induced TNF alpha generation. One of these pro bably represents IP receptors based upon the selectivity of cicaprost for this subtype. The other population has the pharmacology of EP-rece ptors, but the rank order of potency for a range of synthetic EP-recep tor agonists was inconsistent with an interaction with any of the curr ently defined subtypes. Given the pharmacological behaviour of butapro st, AH 6809 and AH 23848B in these cells, we propose that multiple (EP 2-and/or EP4-and/or IP) or novel EP-receptors mediate the inhibitory e ffect of PGE(2) on TNF alpha generation.