However, whereas nSMase1 exhibits in vitro SMase activity, MCF-7 cells overexpressing the protein did not show changes in SM metabolism. Instead, nSMase1 had effects on metabolism of lysoPAF, an inert metabolite of biologically active platelet-activating factor and a natural 1-alkly 2-acetyl analog of phosphatidylcholine. Importantly, nSMase1 acts preferentially as a lysoPAF phosphodiesterase in vitro (Sawai et al., 1999), suggesting nSMase1 may have an alternative biochemical function. Consistent with this, stimulation of nSMase1-overexpressing cells with TNF or H2O2 did not elevate ceramide levels or induce apoptosis (Tomiuk et al., 1998). Moreover, in Jurkat cells, nSMase1 was not responsible for ceramide generation during the execution phase of death receptor-induced apoptosis (Tepper et al., 2001). Importantly, nSMase1 knockout (KO) mice showed no abnormality in sphingolipid metabolism or lipid storage disease (Zumbansen and Stoffel, 2002).
