ENDOTHELIAL-CELL INFLAMMATORY RESPONSES TO TUMOR-NECROSIS-FACTOR-ALPHA - CERAMIDE-DEPENDENT AND CERAMIDE-INDEPENDENT MITOGEN-ACTIVATED PROTEIN-KINASE CASCADES

Ceramide generation by stimulated sphingomyelinase activity has been i mplicated in tumor necrosis factor alpha (TNF) signaling of apoptosis and differentiation. We examined the role of ceramide in a major actio n of TNF: the initiation of inflammatory events. Sphingomyelinase C at high levels induced inflammatory protein expression in endothelial ce lls resulting in leukocyte adhesion, but the pattern of induction of a dhesion molecules (E-selectin, ICAM-1, VCAM-1) and cytokines (interleu kins 6 and 8) differed from that induced by TNF. TNF induced only a sm all increase in ceramide: using lower doses of sphingomyelinase to mim ic this we found that small amounts of ceramide did not induce protein expression, but still rapidly activated Raf-1, mitogen-activated prot ein/extracellular regulated kinase (ERK) kinase (MEK) and ERKs. TNF ad ditionally caused rapid p38 and JNK-1 mitogen-activated protein kinase activation and efficient NF-kappa B translocation, which could not be achieved even by high levels of ceramide. Thus activation of the ERK: cascade alone is an incomplete endothelial cell stimulus, and the TNF receptor generates at least two signals: Raf-1 activation, which coul d be ceramide-dependent; and ceramide-independent efficient NF-kappa B translocation and activation of p38 and JNK-1 mitogen-activated kinas es.