STIMULATION OF THE CERAMIDE PATHWAY PARTIALLY MIMICS LIPOPOLYSACCHARIDE-INDUCED RESPONSES IN MURINE PERITONEAL-MACROPHAGES

Recent studies have suggested that lipopolysaccharide (LIPS) stimulate s cells by mimicking the second-messenger function of ceramide, a lipi d generated in the cell by the action of sphingomyelinase (SMase), To examine this possibility further, we compared the abilities of LPS, SM ase, and/or ceramide analogs to induce cytokine secretion, modulate ge ne expression, and induce endotoxin tolerance in macrophages, SMase an d LPS induced secretion of tumor necrosis factor alpha (TNF-alpha) to comparable degrees; however, unlike LPS, SMase failed to stimulate det ectable interferon activity, Cell-permeable analogs of ceramide induce d the expression of many LPS-inducible genes; however, the expression of interferon-inducible protein 10 (IP-10) and interferon consensus se quence-binding protein (ICSBP) mRNAs was significantly lower than that induced by LPS. Both SMase-induced TNF-alpha secretion and LPS-induce d TNF-alpha secretion were inhibited by pretreatment with a serine/thr eonine phosphatase inhibitor, calyculin A. Macrophages preexposed in v itro to LPS to induce a well-characterized state of endotoxin toleranc e secreted little or no TNF-alpha upon secondary challenge with either LPS or SMase, whereas macrophages preexposed to SMase secreted high l evels of TNF-alpha upon secondary stimulation with LPS or SMase, Colle ctively, these results suggest that ceramide activates a subset of LPS -induced signaling pathways in murine peritoneal exudate macrophages.