Endothelial cell inflammatory responses to tumor necrosis factor alpha. Ceramide-dependent and -independent mitogen-activated protein kinase cascades.

Ceramide generation by stimulated sphingomyelinase activity has been implicated in tumor necrosis factor alpha (TNF) signaling of apoptosis and differentiation. We examined the role of ceramide in a major action of TNF: the initiation of inflammatory events. Sphingomyelinase C at high levels induced inflammatory protein expression in endothelial cells resulting in leukocyte adhesion, but the pattern of induction of adhesion molecules (E-selectin, ICAM-1, VCAM-1) and cytokines (interleukins 6 and 8) differed from that induced by TNF. TNF induced only a small increase in ceramide: using lower doses of sphingomyelinase to mimic this we found that small amounts of ceramide did not induce protein expression, but still rapidly activated Raf-1, mitogen-activated protein/extracellular regulated kinase (ERK) kinase (MEK) and ERKs. TNF additionally caused rapid p38 and JNK-1 mitogen-activated protein kinase activation and efficient NF-kappaB 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 could be ceramide-dependent; and ceramide-independent efficient NF-kappaB translocation and activation of p38 and JNK-1 mitogen-activated kinases.