Cooperative and redundant signaling of leukotriene B4 and leukotriene D4 in human monocytes.

Background: Leukotriene B-4 (LTB4) and cysteinyl leukotrienes (cysLTs) are important immune mediators, often found concomitantly at sites of inflammation. Although some of the leukotriene-mediated actions are distinctive (e. g., bronchial constriction for cysLTs), many activities such as leukocyte recruitment to tissues and amplification of inflammatory responses are shared by both classes of leukotrienes. Objective: We used human monocytes to characterize leukotriene-specific signaling, gene expression signatures, and functions and to identify interactions between LTB4- and cysLTs-induced pathways. Methods: Responsiveness to leukotrienes was assessed using oligonucleotide microarrays, real-time PCR, calcium mobilization, kinase activation, and chemotaxis assays. Results: Human monocytes were found to express mRNA for high-and low-affinity LTB4 receptors, BLT1 and BLT2, but signal predominantly through BLT1 in response to LTB4 stimulation as shown using selective agonists, inhibitors, and gene knock down experiments. LTB4 acting through BLT1 coupled to G-protein a inhibitory subunit activated calcium signaling, p44/42 mitogen-activated protein kinase, gene expression, and chemotaxis. Twenty-seven genes, including immediate early genes (IEG), transcription factors, cytokines, and membrane receptors were significantly up-regulated by LTB4. LTB4 and LTD4 had similar effects on signaling, gene expression, and chemotaxis indicating redundant cell activation pathways but costimulation with both lipid mediators was additive for many monocyte functions. Conclusion: Leukotriene B-4 and LTD4 display both redundant and cooperative effects on intracellular signaling, gene expression, and chemotaxis in human monocytes. These findings suggest that therapies targeting either leukotriene alone may be less effective than approaches directed at both.