SPHINGOMYELINASE ACTIVATES PROTEOLYTIC I-KAPPA-B-ALPHA DEGRADATION INA CELL-FREE SYSTEM

Tumor necrosis factor (TNF) is one of the most potent physiological in ducers of the nuclear transcription factor kappa B (NF-kappa B). A key event in the activation of NF-kappa B is the rapid release of the inh ibitory subunit I kappa B-alpha. Various inhibitors of serine-like pro teases are shown to block TNF-mediated NF-kappa B activation as well a s the disappearance of I kappa B-alpha immunoreactivity in primary mur ine T lymphocytes and in various human leukemic cell lines. The protea se inhibitors did not block TNF induced activation of either phosphati dylcholine specific phospholipase C or acidic sphingomyelinase (SMase) , indicating that the putative protease operates rather downstream of TNF signal transduction processes. I kappa B-alpha degradation could b e directly induced by addition of sphingomyelinase or synthetic cerami de to a cell free system, indicating a stringent coupling of SMase to the NF-kappa B activation pathway. SMase-induced I kappa B-alpha degra dation was suppressed by the protease inhibitor dichloroisocoumarin. T ogether, the data suggest that a TNF-responsive sphingomyelinase trigg ers the rapid degradation of I kappa B-alpha through a serine-like pro tease, which appears to be crucial to the control of NF-kappa B activa tion.