The mechanism of action of soluble lymphocyte mediators. IV. Effect of migratio inhibitory factor (MIF) on macrophage cyclic AMP and on responsiveness to adenylate cyclase stimulators

Stimulation of beta-adrenoceptors inhibits calcium-dependent potassium-channels in mouse macrophages

K+ efflux in mouse macrophages exhibited a rate constant (kK) of 0.67 +/- 0.04 (h)-1 (mean +/- SEM of 16 experiments). This was strongly stimulated by increasing concentrations of the Ca2+ ionophore A23187 up to a maximal value of 4.01 +/- 0.25 (h)-1 with an IC50 of 7.6 +/- 1.9 microM (mean +/- SEM of 6 experiments). Similar results were obtained with the Ca2+ ionophore ionomycin. Binding experiments with 3H-dihydroalprenolol revealed a high density of beta-adrenergic receptors (97.5 +/- 5.2 fmol/mg protein) with apparent dissociation constant of 2.03 +/- 0.06 nM. Isoproterenol at a concentration of 10(-6)-10(-5) M induced a two- to threefold stimulation of endogenous levels of cyclic AMP (cAMP). A23187-stimulated K+ efflux was partially inhibited by stimulation of adenylate cyclase with isoproterenol, forskolin or, PGE1; exogenous cAMP; and inhibition of phosphodiesterase with MIX (1-methyl-3-isobutylxanthine). Maximal inhibition of K+ efflux was obtained by simultaneous addition of isoproterenol and MIX. In dose-response curves, the isoproterenol-sensitive K+ efflux was half-maximally inhibited (IC50) with 2-5 X 10(-10) M of isoproterenol concentration. Propranolol was able to completely block the effect of isoproterenol, with an IC50 of about 1-2 X 10(-7) M. Isoproterenol and MIX were also able to partially inhibit ionomycin-stimulated K+ efflux. Isoproterenol and MIX did not inhibit A23187-stimulated K+ efflux in an incubation medium where NaCl was replaced by sucrose (or choline), suggesting the involvement of an Na+:Ca2+ exchange mechanism. Our results show that stimulation of beta-adrenoceptors in mouse macrophages counterbalances the opening of K+ channels induced by the calcium ionophore A23187. This likely reflects a decrease in cytosolic free calcium content via a cAMP-mediated stimulation of Na+:Ca2+ exchange.

Activation of macrophages

The role of macrophages is essential in the development of a normal immune response. Not only are these cells involved in the initiation of this response by presenting antigens to lymphocytes and by producing Interleukin I, but they also participate in the various phenomena of cellular co-operation and regulation. It is also evident that macrophages can act as cytotoxic effector cells, especially against micro-organisms and tumor cells. This last function is restricted to activated macrophages. The aim of this review is to summarize our present knowledge concerning this "macrophage activation".

Effect of forskolin on prostaglandin synthesis by mouse resident peritoneal macrophages

The relationship between cyclic 3',5'-adenosine monophosphate (cAMP) and prostaglandin (PG) synthesis was investigated in the mouse resident peritoneal macrophage. Macrophages were established as monolayer cultures in petri dishes. After addition of various agents, culture media and cells were removed for PG and cAMP analysis by standard radiochromatographic techniques and radioimmunoassay respectively. Forskolin, a potent receptor-independent adenylate cyclase activator, rapidly increased cAMP synthesis within 5 min in a dose related fashion in both non-treated macrophages and macrophages treated with 12-O-tetradecanoylphorbol-13-acetate (TPA). Furthermore, forskolin markedly reduced both 6-keto-PGF1 alpha and PGE2 synthesis induced by TPA and this inhibition was inversely correlated with increases with cAMP generation. In contrast, cholera toxin failed to mimic the inhibitory action of forskolin on PG synthesis even though it induced similar increases in cAMP; however this increase was only evident after a lag period of at least 1 h. Additionally, forskolin, but no cholera toxin inhibited PG synthesis and zymosan phagocytosis when these cells were activated with zymosan particles. These observations, therefore, suggest that a rise in cAMP is not always correlated with a reduction in PG synthesis.

The mechanism of action of lymphokines. VII. Modulation of the action of macrophage migration inhibitory factor by antioxidants and drugs affecting thromboxane synthesis

Guinea pig peritoneal exudate macrophages are active producers of oxygen radicals in response to membrane stimulation. We examined the involvement of oxygen radicals in the inhibition of macrophage migration caused by the lymphokine, macrophage migration inhibitory factor (MIF). It was found that the presence of scavengers of superoxide and hydrogen peroxide did not influence MIF action. Exogenous horseradish peroxidase, however, significantly enhanced the migration inhibitory effect of MIF. Among six scavengers of hydroxyl radicals and singlet oxygen that were tested, L-methionine (20-80 mM) and L-histidine (40-80 mM) were capable of preventing MIF action. Macrophage responsiveness to MIF was also blocked by the lipid antioxidant propyl gallate (0.125-0.25 mM). Lymphokine-induced inhibition of migration could not be prevented by inhibitors of phospholipase A2, cyclooxygenase, or lipoxygenase. The inhibiting of thromboxane synthetase, however, by imidazole, 1-benzylimidazole and the prostaglandin endoperoxide analogs U-51605 and U-44069 effectively prevented MIF action. These findings are compatible with the hypothesis that the inhibiton of macrophage migration by MIF is the result of reversible autotoxic damage by a yet unidentified product of the oxidative burst, possibly hypochlorous acid. Thromboxane also appears to be involved in MIF action probably by its adhesiveness increasing and proaggregatory actions. The intracellular target of the autotoxic attack and the relative importance of oxygen-derived products and thromboxane in the mediation of MIF-induced migration inhibition are unknown.

Unsaturated fatty acids as second messengers of superoxide generation by macrophages

Chemically elicited guinea pig peritoneal exudate macrophages respond by superoxide (O2-) production to a large number of unrelated stimulants. It has been found that 8 out of 10 stimulants also induce arachidonic acid (20:4) liberation and thromboxane synthesis. The elicitation of O2- production by most stimulants was reduced or totally suppressed by three procedures that inhibit the activity of endogenous phospholipases: the use of drug p-bromophenacyl bromide, elevation of the cellular cyclic AMP level, and the removal of extracellular Ca2+. O2- production in response to concanavalin A, wheat germ agglutinin, and fMet-Leu-Phe were exquisitely sensitive to inhibition of phospholipase activity. Exogenously applied 20:4 as well as other unsaturated fatty acids (linolenic, linoleic, and oleic) induced massive and instantaneous O2- production in a dose-dependent manner. Saturated fatty acids (stearic) and methyl esters of unsaturated acids were inactive. Lysophosphoglycerides were also inactive. Incubation of macrophages with inhibitors of cyclooxygenase or lipoxygenase did not prevent the elicitation of O2- production by stimulants or fatty acids. On the contrary, O2- formation was enhanced by indomethacin and indomethacin by itself was capable of evoking O2- generation. Treatment of 20:4 with soybean lipoxygenase did not abolish its capacity to induce O2- production; native and lipoxygenase-treated 20:4 exhibited similar dose-response ratios. Purified 15-hydroxyeicosatetraenoic acid also elicited O2- production by macrophages with a potency comparable to but not exceeding that of 20:4. Equimolar amounts of prostaglandin E2 were inactive. These findings suggest that liberation of unsaturated fatty acid (principally, 20:4) from membrane phospholipids, as a consequence of phospholipase activation, is a necessary step in the elicitation of an oxidative burst in macrophages. O2- generation is stimulated by unesterified 20:4 and, possibly, by certain metabolites of 20:4. It appears that the lipoxygenase pathway may generate metabolites with stimulating capacity while the cyclooxygenase pathway is abortive.

Lymphokine-induced Macrophage aggregation: the possible role of cyclic nucleotides

The possible role of cyclic nucleotides in guinea pig macrophage aggregation, induced by human lymphokine (LK) has been investigated. Small increases were found in guanosine 3'5'-cyclic monophosphate (cGMP), but not adenosine 3'5'-cyclic monophosphate (cAMP), levels of lymphokine aggregated macrophages. Addition of exogenous dibutyryl (DB) cAMP, L-isoproterenol, or theophylline did not induce macrophage aggregation. By contrast, both exogenous DBcGMP and carbamyl-choline induced a macrophage aggregation; although DBcGMP was more effective. In addition, both L-isoproterenol and DBcAMP in the presence of theophylline decreased LK-induced macrophage aggregation, whereas D-isoproterenol and DBcGMP had no effect. The results obtained here are discussed in the context of the previously reported effects of cyclic nucleotides on migration inhibitory factor (MIF) activity and the possible role of these agents in the mechanism of action of MIF.