Regulation of Tumor Necrosis Factor-α and IL-1β Synthesis by Thromboxane A₂ in Nonadherent Human Monocytes

Synthesis of TNF-α and IL-1β by monocytes/macrophages can be partially regulated by the eicosanoid, PGE₂. We report here that inhibition of both PGE₂ and thromboxane A₂ (TXA₂) synthesis by a prostaglandin H synthase inhibitor, NS-398, had no effect on the synthesis of either TNF-α or IL-1β, even though the addition of PGE₂ to these treated cells dose-dependently inhibited TNF-α and IL-1β synthesis. Because TXA₂ is a major eicosanoid product of stimulated human monocytes, we examined its influence on cytokine production. Inhibition of thromboxane synthase by carboxyheptyl imidazole (CI) resulted in inhibition of TNF-β (61 ± 4.3%; n = 8; p < 0.001) and IL-1β (54 ± 4.2%; n = 8; p < 0.001) synthesis by serum-treated zymosan-stimulated nonadherent human monocytes. This effect was observed when cytokine production was measured by ELISA or bioactivity assays. Furthermore, the addition of a TXA₂ agonist, carbocyclic TXA₂, to CI-treated monocytes dose-dependently restored the levels of TNF-α and IL-1β synthesis to those found with serum-treated zymosan stimulation alone. Inhibition of TXA₂ activity by the thromboxane receptor antagonists, pinane TXA₂ or SQ 29,548, also inhibited the production of TNF-α (67 ± 2.4% and 65 ± 2.7%, respectively; n = 8; p < 0.001) and IL-1β (59 ± 3.3% and 70 ± 1.2%, respectively; n = 8; p < 0.001). Treatment with CI resulted in a partial decrease in TNF-α mRNA levels (60 ± 12.0%; n = 4), but had little or no effect on IL-1β mRNA levels. These novel observations implicate TXA₂ as an important paracrine or autocrine facilitator of TNF-α and IL-1β production in stimulated human monocytes and suggest that levels of TNF-α and IL-1β synthesis are determined in part by the balance between TXA₂ and PGE₂ production in human monocytes.