Complex Gangliosides as Cell Surface Inhibitors for the Ecto-NAD+Glycohydrolase of CD38

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Abstract
Leukocyte cell surface antigen CD38 is a single-transmembrane protein whose extracellular domain has catalytic activity for NAD+ glycohydrolase (NADase). We previously reported that b-series gangliosides inhibit the NADase activity of the extracellular domain of CD38 expressed as a fusion protein [Hara-Yokoyama, M., Kukimoto, I., Nishina, H., Kontani, K., Hirabayashi, Y., Irie, F., Sugiya, H., Furuyama, S., and Katada, T. (1996) J. Biol. Chem.271, 12951−12955]. In the present study, we examined the effect of exogenous gangliosides on the NADase activity of CD38 on the surface of retinoic acid-treated human leukemic HL60 cells and CD38-transfected THP-1 cells. After incubation of the cells with GT1b, inhibition of NADase activity was observed. The time course of inhibition was slower than that of the incorporation of GT1b into the cells, suggesting that incorporation into the cell membranes is a prerequisite for inhibition. Inhibition occurred efficiently when GT1b and CD38 were present on the same cells (cis interaction) rather than on different cells (trans interaction). Although gangliosides may affect localization of cell surface proteins, indirect immunofluorescence intensity due to CD38 was not affected after GT1b treatment. Comparison of the effect of GT1b and GD1a indicates that the tandem sialic acid residues linked to the internal galactose residue of the gangliotetraose core are crucial to the inhibition. These results suggest a novel role of complex gangliosides for the first time as cell surface inhibitors of CD38 through specific and cis interaction between the oligosaccharide moiety and the extracellular domain.