Ceramide-binding and activation defines protein kinase c-Raf as a ceramide-activated protein kinase

Interleukin 1 is the prototype of an inflammatory cytokine, and evidence suggests that it uses the sphingomyelin pathway and ceramide production to trigger mitogen-activated protein kinase (MAPK) activation and subsequent gene expression required for acute inflammatory processes. To identify downstream signaling targets of ceramide, a radioiodinated photoaffinity labeling analog of ceramide ([I-125]3-trifluoromethyl-3-(m-iodophenyl)diazirine ceramide) was employed. It is observed that ceramide specifically binds to and activates protein kinase c-Raf, leading to a subsequent activation of the MARK cascade. Ceramide does not bind to any other member of the MAPK module nor does it bind to protein kinase C-zeta. These data identify protein kinase c-Raf as a specific molecular target for interleukin 1 beta-stimulated ceramide formation and demonstrate that ceramide is a lipid cofactor participating in regulation of c-Raf activity.