CD38/Cyclic ADP-ribose/Ca2+ Signaling

CD38, a type II transmembrane glycoprotein widely expressed in vertebrate cells, is a bifunctional ectoenzyme that catalyzes both the formation of cyclic ADP-ribose (cADPR) from NAD and the hydrolysis of cADPR to ADP-ribose. cADPR is a second messenger that releases Ca²⁺ from intracellular stores. T lymphocytes can be induced to express CD38 when activated with antibodies against specific antigen receptors. If the activated T cells are then exposed with NAD, cell death by apoptosis occurs. During the exposure of activated T cells to NAD, the CD38 is modified by ecto-mono-ADP-ribosyltransferases (ecto-mono-ADPRTs) specific for cysteine- and arginine residues. Arginine-ADP-ribosylation results in inactivation of both cyclase and hydrolase activities of CD38, whereas cysteine-ADP-ribosylation results only in the inhibition of the hydrolase activity. The arginine (Arg269)-ADP-ribosylation causes a decrease in intracellular cADPR and a subsequent decrease in Ca²⁺ influx, resulting in apoptosis of the activated T cells. We, next, examined the effect of ADP-ribosylation of CD38 in mouse pancreatic islet cells. ADP-ribosylation of CD38 inactivated its ecto-enzyme activities, and abolished glucose-induced increase of cADPR production, intracellular concentration of Ca²⁺, and insulin secretion, indicating that ecto-cyclase activity of CD38 to produce intracellular cADPR seems to be indispensable for insulin secretion. Internalization of CD38 has been proposed to address the topological problem of the ectocellular production of the intracellular Ca²⁺ mobilizing second messenger. CD38 was reported to undergo internalization upon incubating CD38⁺ cells with appropriate stimuli, including agonistic antibody against CD38. We have found that CD38 could be ADP-ribosylated on its two cysteine residues (Cys119 and Cys201). Hela cells transfected with CD38 mutants, C119K/C201E, showed a complete inhibition of the internalization induced by anti-CD38 antibody. The enhancing effect of anti-CD38 antibody on intracellular cADPR concentration and Ca²⁺ levels was completely inhibited in the C119K/C201E mutant. Furthermore, thiol compound induced not only to dimerize CD38 but also to enhance intracellular cADPR and Ca²⁺ concentrations. These results suggest that the dimerization of CD38 through its Cys119 and Cys201 is essential for CD38 internalization and that the disulfide bond between Cys119 and Cys201 in CD38 may be involved in the CD38 dimerization and internalization. We tested the effect of reducing agent, L-2-oxothiazolidine-4-carboxylic acid (OTC), a prodrug of cysteine, on CD38 internalization in pancreatic islets. OTC enhanced insulin release from isolated islets as well as CD38 internalization and cytoplasmic Ca²⁺ level. Intake of OTC in db/db mice ameliorated glucose tolerance, insulin secretion and morphology of islets, compared to control mice. These data indicate that OTC improves glucose tolerance by enhancing insulin secretion via CD38/cADPR/Ca²⁺ signaling machinery.