Microbial stimuli and atmospheric particulate matter (PM) interact to amplify the release of inflammatory and immune-modulating cytokines. The basis of this interaction, however, is not known. Cultured human lung fibroblasts (HLF) were used to determine whether various protein kinase pathways were involved in the release of IL-6 following combined exposure to the PM-derived metal, Ni, and M. fermentans-derived macrophage-activating lipopeptide 2 (MALP-2), a toll-like receptor 2 agonist. Synergistic release of IL-6 by MALP-2 and NiSO4 was obvious after 8h of co-stimulation and correlated with a late phase accumulation of IL-6 mRNA. Ni and MALP-2, alone or together, all led to rapid and transient phosphorylations of ERK(1/2) and JNK/SAPK of similar magnitude. p38 phosphorylation, however, was observed only after prolonged treatment of cells with both stimuli together. A constitutive level of PI3K-dependent Akt phosphorylation remained unchanged by Ni and/or MALP-2 exposure. IL-6 induced by Ni/MALP-2 co-exposure was partially dependent on activity of HIF-1alpha and COX-2 as shown by targeted knockdown using siRNA. IL-6 release in response to Ni/MALP-2 was partially sensitive to pharmacological inhibition of ERK(1/2), p38, and PI3K signaling. The protein kinase inhibitors had minimal or no effects on Ni/MALP-2-induced accumulation of HIF-1alpha protein, however, COX-2 expression and, more markedly PGE(2) production, were suppressed by LY294002, SB203580, and U0126. Thus, Ni/MALP-2 interactions involve multiple protein kinase pathways (ERK(1/2), p38, and PI3K) that modulate events downstream from the early accumulation of HIF-1alpha to promote IL-6 gene expression directly or secondarily, through COX-2-derived autocrine products like PGE(2).
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