Protein kinase C regulates macrophage tumor necrosis factor secretion: Direct protein kinase C activation restores tumor necrosis factor production in endotoxin tolerance

Abstract

Background. Macrophages pretreated in vitro with endotoxin (LPS p) secrete less tumor necrosis factor (TNF) in response to a second LPS activating (LPS a) stimulus. Protein kinase C (PKC) is required for TNF secretion in a macrophage stimulated with LPS a. In these experiments we examined the role of PKC in TNF signal transduction in naive and tolerant macrophages. Methods. Murine macrophages were cultured ± LPS p for 24 hours. Cultures were washed and treated for 1 hour with PKC inhibitors or phorbol myristate acetate (PMA), a direct PKC activator. Cells were then stimulated with a range of LPS a for 6 hours, and TNF was determined by bioassay. Results. LPS a-stimulated TNF secretion by nontolerant macrophages was inhibited by LPS p in the absence of PMA. PKC inhibitors decreased TNF by naive macrophages and exaggerated inhibition in tolerant cells. Depletion of PKC by 24 hours of PMA decreased TNF production by both naive and tolerant macrophages. PKC activation with PMA 1 hour before LPS a augmented TNF secretion in naive cells and reversed TNF inhibition of tolerant cells. Conclusions. Direct PKC activation with PMA restored TNF secretion in LPS-tolerant macrophages. Endotoxin tolerance may alter the LPS a signal transduction pathway between the LPS receptor and PKC activation.