Protein kinase C zeta plays a central role in activation of the p42/44 mitogen-activated protein kinase by endotoxin in alveolar macrophages

Human alveolar macrophages respond to endotoxin (LPS) by activation of a nu mber of mitogen-activated protein kinase pathways, including the p42/44 (ex tracellular signal-related kinase (ERK)) kinase pathway. In this study, me evaluated the role of the atypical protein kinase C (PKC) isoform, PKC zeta , in LPS-induced activation of the ERK kinase pathway. Kinase activity assa ys showed that LPS activates PKC zeta, mitogen-activated protein/ERK kinase (MEK, the upstream activator of ERK), and ERK, LPS did not activate Raf-1, the classic activator of MEK. Pseudosubstrate-specific peptides with attac hed myristic acid are cell permeable and can be used to block the activity of specific PKC isoforms in vivo. We found that a peptide specific for PKC zeta partially blocked activation of both MEK and ERK by LPS, We also found that this peptide blocked in vivo phosphorylation of MER after LPS treatme nt. In addition, me found that LPS caused PKC zeta to bind to MEK in vivo. These observations suggest that MEK is an LPS directed target of PKC zeta P KC zeta has been shown in other systems to be phosphorylated by phosphatidy linositol (PI) 3-kinase-dependent kinase, We found that LPS activates PI 3- kinase and causes the formation of a PKC zeta/PI 3-kinase-dependent kinase complex. These data implicate the PI 3-kinase pathway as an integral part o f the LPS-induced PKC zeta activation, Taken as a whole, these studies sugg est that LPS activates ERK kinase, in part, through activation of an atypic al PKC isoform, PKC zeta.