NAD+-dependent internalization of the transmembrane glycoprotein CD38 in human Namalwa B cells

CD38 is a transmembrane glycoprotein involved as an orphan receptor in many physiological processes of lymphocytes. It is also a bifunctional enzyme that catalyzes at its ectocellular domain the synthesis from NAD+ (cyclase) and the hydrolysis (hydrolase) of the calcium-mobilizing metabolite cyclic ADP-ribose (cADPR). A still unexplained paradox concerns the relationship between ectocellular localization of CD38 and intracellular calcium-releasing activity of its intermediate product cADPR. Incubation of CD38+ human Namalwa B cells with external NAD+ elicited extensive membrane down-regulation of CD38 and its internalization in non-clathrin-coated vesicles. Since the internalized CD38 was demonstrated to be enzymatically active, this NAD+-dependent process is a hitherto unrecognized means for shifting cADPR metabolism from the cell surface to the intracellular environment.