Nuclear localization of neutral sphingomyelinase 1: biochemical and immunocytochemical analyses.

To examine the intracellular localization of neutral sphingomyelinase 1 (nSMase 1), a rabbit polyclonal antibody was raised against a recombinant form of the enzyme expressed in E. coli. It has been reported that, in rat liver or in ascites hepatoma AH7974, high activity of neutral sphingomyelinase (SMase) is found at the plasma membrane, with a lesser but significant amount in nucleus and cytoplasm. The biochemical properties, dithiothreitol requirement and high salt concentration dependency, of cloned and expressed nSMase 1 resemble those of previously described nuclear neutral SMase of AH7974. The present study was therefore focused on the nuclear localization of this enzyme. Western blotting of subcellular fractions using anti-rat nSMase 1 antibody revealed most nSMase 1 to be associated with the nuclei and some with microsomes, but not with plasma membranes. Consistently, neutral SMase activity in nuclear extract was immunoprecipitated by the antibody, while that of plasma membranes was not. The results indicate that nSMase 1 mainly resides in the nucleus and may thus differ from neutral SMase in plasma membrane. On gel-filtration column chromatography of nuclear extract, the profile of neutral SMase activity corresponded well with immunoreactive protein bands on western blotting, suggesting that a large part of nuclear neutral SMase may be nSMase 1. Removal of the nuclear envelope by treatment with Triton X-100 did not significantly decrease the amount of nuclear nSMase 1, and western blotting of subnuclear fractions (i.e. nuclear envelope, chromatin, and nuclear matrix) revealed nSMase 1 signal exclusively in the nuclear matrix. Immunocytochemistry with AH7974, as well as rat fibroblast cell line 3Y1, demonstrated nSMase 1 to be localized mainly in the nucleus, with some in the cytoplasm. Moreover, immuno-electron microscopy clearly showed the signal of nSMase 1 to be more dense in the nucleus than in the cytoplasm of AH7974.