Sphingomyelinase activates proteolytic I kappa B-alpha degradation in a cell-free system

Tumor necrosis factor (TNF) is one of the most potent physiological inducers 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 inhibitory subunit I kappa B-alpha. Various inhibitors of serine-like proteases are shown to block TNF-mediated NF-kappa B activation as well as the disappearance of I kappa B-alpha immunoreactivity in primary murine T lymphocytes and in various human leukemic cell lines. The protease inhibitors did not block TNF-induced activation of either phosphatidylcholine-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 be directly induced by addition of sphingomyelinase or synthetic ceramide to a cell-free system, indicating a stringent coupling of SMase to the NF-kappa B activation pathway. SMase-induced I kappa B-alpha degradation was suppressed by the protease inhibitor dichloroisocoumarin. Together, the data suggest that a TNF-responsive sphingomyelinase triggers the rapid degradation of I kappa B-alpha through a serine-like protease, which appears to be crucial to the control of NF-kappa B activation.