Signaling pathways initiated in macrophages after engagement of type A scavenger receptors

Scavenger receptors are macrophage cell surface molecules associated with endocytic uptake of lipoproteins and binding of microbial ligands. Macrophage class A scavenger receptors (SR-As) interact with ligands to induce cellular signaling leading to gene transcription and cytokine release. We used inhibitors of early and late signaling to block SR-A-mediated polyinosinic-polycytidilic acid (poly I:C) and lipoteichoic acid (LTA) activation of RAW 264.7 macrophages. Effects of multiple inhibitors on tumor necrosis factor (TNF)-alpha release were monitored to determine requirements for inflammatory cytokine production. Cycloheximide, monodansylcadaverine, and cytochalasin B all blocked TNF-alpha release from macrophages stimulated with LTA or poly I:C, whereas monensin only nominally reduced TNF-alpha production. Selected inhibitors of downstream signaling events reduced SR-A-dependent TNF-alpha release by >95% after stimulation with either ligand, whereas others were ineffective. The PKC inhibitor H7 reduced LTA-dependent secretion of TNF-alpha by 94% but inhibited poly I:C-dependent TNF-alpha production only by 50%. Priming of RAW 264.7 cells with interferon-gamma potentiated the response to poly I:C but did not alter inhibitor effects. These results demonstrated that for both ligands tested here, early events of receptor internalization are requisite for cellular activation. The response pattern suggests that tyrosine phosphorylation and activation of the MAP kinase pathway are key components of SR-A-mediated signal transduction cascades.