A neutral sphingomyelinase resides in sphingolipid-enriched microdomains and is inhibited by the caveolin-scaffolding domain: potential implications in tumour necrosis factor signalling

Sphingomyelinases hydrolyse sphingomyelin to ceramide, a process involved i n signal-transduction routes leading to apoptosis and various other cellula r responses. In the present study, we investigated the sphingomyelinase con tent of caveolae, invaginated plasma-membrane microdomains that contain a v ariety of signalling molecules. These structures are highly enriched in sph ingomyelin as well as in ceramide, which suggests that metabolism of these lipids might, to some extent, occur locally. By cell fractionation, we demo nstrate that, in addition to a previously reported minute amount of acidic sphingomyelinase activity, a substantial amount of neutral sphingomyelinase activity resides in caveolae of human skin fibroblasts. This caveolar neut ral sphingomyelinase activity was also detected in Niemann-Pick disease typ e A fibroblasts, which are completely devoid of functional acidic sphingomy elinase. Neutral (but not acidic) sphingomyelinase activity was specificall y inhibited by a peptide that corresponds to the scaffolding domain of cave olin, which suggests a direct molecular interaction between the two protein s. In addition, this finding implies a cytosolic orientation of the caveola r neutral sphingomyelinase. Interestingly, stimulation of fibroblasts with tumour necrosis factor alpha (TNF alpha) resulted in a partial shift of its p55 receptor to caveolin-enriched membrane fractions and the appearance of caveolin-sensitive neutral sphingomyelinase activity in the non-caveolar f ractions. These results suggest that (part of) the presently identified cav eolar neutral sphingomyelinase activity is involved in TNF alpha signalling .