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日  時: 平成13年10月26日(金) 16:00 - 18:00

場  所: 東京大学医科学研究所 アムジェンホール大会議室

講  師: Dr. Uh-Hyun Kim
       Department of Biochemistry&s_comma;
       Chonbuk National University Medical School
演  題: CD38/Cyclic ADP-ribose/Ca2+Signaling
概  要:
CD38&s_comma; a type II transmembrane glycoprotein widely expressed in vertebrate
cells&s_comma; 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&s_comma; cell death by apoptosis occurs.
During the exposure of activated T cells to NAD&s_comma; 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&s_comma; 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&s_comma; resulting in
apoptosis of the activated T cells. We&s_comma; next&s_comma; examined the effect of
ADP-ribosylation of CD38 in mouse pancreatic islet cells. ADP-ribosylation
of CD38 inactivated its ecto-enzyme activities&s_comma; and abolished
glucose-induced increase of cADPR production&s_comma; intracellular concentration of
Ca2+&s_comma; and insulin secretion&s_comma; 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&s_comma; 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&s_comma; C119K/C201E&s_comma; 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&s_comma; 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&s_comma; L-2-oxothiazolidine-4-carboxylic acid (OTC)&s_comma; a prodrug of cysteine&s_comma;
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&s_comma; insulin
secretion and morphology of islets&s_comma; compared to control mice. These data
indicate that OTC improves glucose tolerance by enhancing insulin secretion
via CD38/cADPR/Ca2+signaling machinery.