Defective lipopolysaccharide-dependent ERK 1/2 activation in endotoxin tolerant murine macrophages is reversed by direct protein kinase C stimulation.

Lipopolysaccharide (LPSp) pretreatment inhibits TNF secretion in endotoxin-tolerant macrophages via alterations in signal transduction pathways of LPS activation (LPSa). Protein kinase C inhibitors prevent TNF release in response to LPSa and direct protein kinase C activation with phorbol myristate acetate (PMA) restores TNF secretion after LPSp. In the current experiments the effect of protein kinase C modulation on LPSa-stimulated ERK 1/2 activation was investigated. Murine macrophage TNF production was determined after stimulation with 100 ng/mL of LPSa, +/- 24 h pretreatment with 10 ng/mL of LPSp. Direct protein kinase C activators (PMA or indolactam) or inhibitors (H7 or bisindolylmaleimide) were added 1 h before LPSa. Diphosphorylated ERK 1/2 was assayed after LPSa stimulation by Western blot. LPS tolerance after LPSp was characterized by inhibition of LPSa-stimulated TNF and accompanied by impaired ERK 1/2 activation by LPSa. Protein kinase C activation with PMA or indolactam restored ERK 1/2 activation and TNF secretion. Inhibition of protein kinase C with H7 or bisindolylmaleimide prevented TNF secretion and ERK 1/2 activation by LPSa. These findings suggest that both ERK 1/2 and protein kinase C are required for TNF production in nontolerant macrophages and that LPS tolerance may be associated with an inability to phosphorylate ERK 1/2.