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Lipopolysaccharide (LPS) stimulates macrophages to release inflammatory cytokines, interleukin-1 beta (IL-1), and tumor necrosis factor (TNF). LPS-induced TNF suppresses scavenger receptor functions in macrophages (van Lenten, B. J., and Fogelman, A. M. (1992) J. Immunol. 148, 112-116), which is regulated by TNF-mediated protein kinases (Hsu, H. Y., and Twu, Y. C. (2000) J. Biol. Chem. 275, 41035-41048). To examine the molecular mechanism for LPS induction of IL-1 in macrophages, we demonstrated that LPS quickly stimulated reactive oxygen species (ROS), and 3 h later induced prointerleukin-1 beta (pro-IL-1, precursor of IL-1) production and IL-1 secretion. LPS stimulated pro-IL-1 message/protein between 3 and 10 h; however, there was a 40% reduction of pro-IL-1 in preincubation of the antioxidant, N-acetylcysteine (NAC). Moreover, NAC moderated LPS-induced IL-1 secretion partially via interleukin 1-converting enzyme. The maximal activity of LPS-induced ERK, JNK, and p38 was 12- (30 min), 5- (30 min), and 16-fold (15 min), respectively. In contrast, NAC reduced ERK activity to 60% and decreased p38 activity to the basal level, but JNK activity was induced 2-fold. Furthermore, the pharmacological antagonists LY294002, SB203580, curcumin, calphostin C, and PD98059 revealed the diverse roles of LPS-mediated protein kinases in pro-IL-1. On the other hand, NAC and diphenyleneiodonium chloride partially inhibited LPS-induced Rac activity and protein-tyrosine kinase (PTK), indicating that LPS-mediated ROS and NADPH oxidase correspond to Rac activation and IL-1 expression. Our findings establish for the first time that LPS-mediated PTK/phosphatidylinositol 3-kinase/Rac/p38 pathways play a more important role than pathways of PTK/PKC/MEK/ERK and of PTK/phosphatidylinositol 3-kinase/Rac/JNK in the regulation of pro-IL-1/IL-1. The findings also further elucidate the critical role of LPS-mediated ROS in signal transduction pathways. Our results suggest that understanding LPS-transduced signals in IL-1 induction upon the antibacterial action of macrophages should provide a therapeutic strategy for aberrant inflammatory responses leading to severe cellular injury or concurrent multiorgan septic damage.
Hsu et al. (Sat,) studied this question.