The Journal of Clinical Endocrinology & Metabolism Vol. 82, No. 8 2660-2663
Copyright © 1997 by The Endocrine Society
Mechanisms of Defective Glucose-Induced Insulin Release in Human Pancreatic Islets Transplanted to Diabetic Nude Mice1
Décio L. Eizirik,
Leif Jansson,
Malin Flodström,
Claes Hellerström and
Arne Andersson
Department of Medical Cell Biology, Uppsala University, Biomedicum,
Uppsala, Sweden
Address all correspondence and requests for reprints to: Dr. Décio L. Eizirik, Department of Metabolism and Endocrinology, Vrije Universiteit Brussel, Laarbeeklaan 103, B-1090 Brussels, Belgium. E-mail: deizirik{at}mebo.vub.ac.be
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Abstract
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We have previously observed that human islets, transplanted under the
kidney capsule of hyperglycemic nude mice, show a long-lasting
impairment in glucose-induced insulin release. To investigate the
cause(s) of this phenomenon, we transplanted human islets into
normoglycemic or alloxan-diabetic nude mice for a 4- to 6-week period.
In a third experimental group, aimed at evaluating reversibility of
hyperglycemia effects, diabetic nude mice bearing a human islet graft
were cured by a second intrasplenic transplant of mouse islets, and the
human islets were exposed to a further 2 weeks of normoglycemia. Four
to 6 weeks of hyperglycemia induced a severe impairment of glucose- and
arginine-induced insulin release, as demonstrated by perfusion of the
graft-bearing kidney. This defective release was not restored by a
subsequent 2-week period of normoglycemia, and it was accompanied by
normal (pro)insulin biosynthesis, glucose oxidation, and expression of
insulin messenger RNA. Taken together with our previous study, these
observations indicate that impaired glucose metabolism, depletion of
insulin messenger RNA, decreased (pro)insulin biosynthesis, increased
glycogen accumulation, and depletion of insulin reserves cannot explain
the deleterious effects of the diabetic state on human islet insulin
release. This, and the similar inhibition of glucose- and
arginine-induced insulin release, suggest that prolonged hyperglycemia
may exert its deleterious effect on insulin release at a step distal to
closure of ATP-sensitive K-channels.
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Introduction
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EXPERIMENTAL DATA suggest that chronic
hyperglycemia induces ß-cell dysfunction (reviewed in Refs. 13).
This deleterious effect of glucose may be of relevance for the
defective insulin release observed in noninsulin-dependent diabetes
mellitus (non-IDDM) and in the early stages of IDDM. However, the
limited access to human islet tissue means that most of the studies
reviewed above have been performed in animal models. There are
important differences between human and rodent islets regarding glucose
transport (4) and mechanisms of ß-cell damage and repair (5, 6).
Thus, it is crucial to further characterize hyperglycemia-induced
ß-cell dysfunction in human islets. We have previously observed that
human islets exposed in vitro to high glucose concentration
for 7 days show a defective insulin release in response to glucose or
glucose plus theophylline (7). In a subsequent study, we transplanted
such islets under the kidney capsule of normoglycemic or diabetic nude
mice (8). After 4 weeks of exposure to a diabetic environment, human
islet cells showed a marked glycogen accumulation, depletion of insulin
reserves, and a lack of insulin release in response to 16.7 mmol/L
glucose. When these diabetic mice were cured by a second mouse islet
graft, and the human islets consequently exposed to 2-weeks of
normoglycemia, a restoration of insulin reserves and disappearance of
intracellular glycogen deposits was observed, but the defective
glucose-induced insulin release persisted (8).
The aim of the present study was to characterize some of the mechanisms
potentially involved in the loss of insulin response to glucose by
human islets exposed to a diabetic environment. For this purpose, human
islets were grafted into normoglycemic or hyperglycemic nude mice
(similar to the groups referred to above) and, after 46 weeks
in vivo, examined regarding (pro)insulin biosynthesis,
insulin messenger RNA (mRNA) content, glucose oxidation, and insulin
release in response to both glucose or glucose plus arginine.
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Materials and Methods
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Islet isolation, culture, and transplantation
Human islets were isolated from 24 adult heart-beating
nondiabetic organ donors at the Central Unit of the ß-Cell Transplant
(Medical Campus, Vrije Universiteit Brussel) (9). The mean age of the
donors was 43 ± 3 yr (mean ± SEM, range 764
yr). Examination by electron microscopy of the islet-enriched fraction
after islet isolation showed 7 ± 1% dead cells and 0.6 ±
0.3% acinar cells. Light microscopical characterization of
immunohistochemically stained islets (10) indicated the presence of
52 ± 3% insulin-positive cells and 15 ± 2%
glucagon-positive cells. The islet insulin content was 1.14 ±
0.13 ng insulin/ng DNA. After isolation, the islets were transported to
Uppsala and cultured as previously described (7, 8). The islets were
then implanted into athymic male nude mice (C57BL/6
[nu/nu] (Bomholtgaard, Ry, Denmark) weighing 2025 grams.
In some of the mice, diabetes was induced by iv injection of alloxan
(80 mg/kg BW; Sigma Chemical Co., St. Louis, MO) 35 days before
transplantation. For the transplantation procedure, the animals were
anesthetized with avertin (11) and the left kidney visualized via a
flank incision. Two small incisions were made under the kidney capsule,
and around 1.5 µL (circa 150 islets) of human islets were
implanted into each incision by means of a braking pipette. The 2
grafts were located, respectively, in the upper and lower kidney poles.
The transplantation of 300 human islets is usually not sufficient to
restore normoglycemia in alloxan-diabetic nude mice (8). Some diabetic
mice also received a second intrasplenic implant of approximately 200
islets from C57BL/6 (ob/ob) mice (8), which, combined with
the previously transplanted human islets, is sufficient to restore
normoglycemia (8). The experimental groups were as follow: 1)
normoglycemic recipients of human islets 46 weeks before killing
(4N); 2) alloxan-diabetic recipients of human islets 46 weeks before
killing (4H); 3) alloxan-diabetic recipients transplanted initially
with 300 human islets and, after 4 weeks of hyperglycemia, with a
second intrasplenic ob/ob mouse islet transplant, which
normalized blood glucose during the subsequent 2 weeks (4H + 2N). The
function of human islets transplanted into normoglycemic or diabetic
mice was similar after 4 or 6 weeks in vivo (data not
shown). Thus, data obtained at these 2 time points were pooled and are
presented together.
Perfusion of graft-bearing kidneys and graft insulin mRNA
content
Graft perfusion was performed as previously described (12). The
perfusions started with a 30-min period using Krebs Ringer bicarbonate
buffer (KRBH) containing 2.8 mmol/L glucose, followed by 20 min at 16.7
mmol/L glucose, 15 min at 2.8 mmol/L glucose, 10 min at 5.6 mmol/L
glucose + 10 mmol/L arginine, and finally, 15 min at 2.8 mmol/L
glucose. A 1.0-mL sample of the effluent medium was obtained 14 and 15
min after starting the equilibration period and then every 5 min, with
the exception of the first 10 min of perfusion with 16.7 mmol/L glucose
or 5.6 mmol/L glucose + 10 mmol/L arginine, when samples were collected
after 15, 7, and 10 min. The insulin concentration was determined by
RIA (13). After perfusion, the human islet grafts were carefully
dissected from the surrounding renal tissues (12) and analyzed for
insulin mRNA content by Northern blot (14). Total RNA was isolated from
45 pooled grafts by using an RNA Isolation Kit (RNAeasy for total RNA
extraction, Qiagen, Santa Clarita, CA). After extraction, the RNA
samples (5 µg) were electrophoresed on a 1% agarose gel containing
formaldehyde. After acridine orange staining of the gel to assure
similar sample loading, the RNA was transferred to a nylon membrane and
hybridized to a 32P-labeled complementary DNA probe coding
for the rat insulin gene (PRI-7; 15 . Hybridization to 18 S
ribosomal RNA, using a labeled rat genomic DNA clone (16), was used as
internal control.
(Pro)insulin biosynthesis and glucose oxidation
Graft (pro)insulin biosynthesis was determined as previously
described (12). Briefly, two grafts, retrieved from the same mouse,
were incubated for 2 h at 37 C in 100 µL KRBH containing either
1.7 or 16.7 mmol/L glucose and 100 µCi/mL
L-[4,5-3H]leucine (Amersham, Amersham, UK).
The tissue was then homogenized and part of the homogenate used for
determination of graft DNA content (17) while the newly synthesized
(pro)insulin was measured by determination of [3H]leucine
incorporation into immunoprecipitated (pro)insulin (18). Total protein
biosynthesis was measured after precipitation with trichloroacetic
acid. For determination of graft glucose oxidation rates (11, 19), the
two dissected grafts were incubated for 90 min in KRBH containing
D-[U-14C]glucose and nonradioactive glucose,
with a final concentration of 1.7 or 16.7 mmol/L glucose. After the
oxidation, the dry weight of the grafts was estimated and the results
expressed per µg dry weight (11).
Statistical analysis
Data are presented as means ± SEM, and groups
of data were compared by using paired or unpaired Students
t test or Wilcoxon signed-rank test. When multiple
comparisons were performed, the data were evaluated by ANOVA.
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Results
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The blood glucose concentration of the nonalloxan-treated mice
(4N) was 4.1 ± 0.3 mmol/L (n = 14). All alloxan-treated mice
in groups 4H and 4H + 2N had blood glucose levels above 15 mmol/L, 4
weeks after the human islet implantation (group 4H, 22.3 ± 0.2
mmol/L; n = 32). Implantation of the additional 200
ob/ob islets in the spleen of 4H + 2N mice normalized blood
glucose levels (3.7 ± 0.2 mmol/L; n = 28).
The contribution of (pro)insulin to the total pool of labeled proteins
in the human islets was in the range of 59%, independent of the
glucose concentration used, and in none of the groups studied was there
a significant glucose stimulation of (pro)insulin synthesis between 1.7
and 16.7 mmol/L glucose (Table 1
). Moreover, there were
differences neither in (pro)insulin biosynthesis nor in percentage of
(pro)insulin over total protein synthesis between human islets grafted
into 4N, 4H, or 4H + 2N mice. The apparent trend for higher
(pro)insulin synthesis in group 4N (n = 20), as compared with
group 4 H (n = 13) (P > 0.1), was not maintained
when the corresponding 13 mice (i.e. receiving islet grafts
from the same donor) from groups 4N and 4H were compared (data not
shown). In line with this lack of difference in (pro)insulin
biosynthesis, the expression of insulin mRNA was similar between human
islets grafted into 4N or 4H mice (Fig. 1
).
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Table 1. (Pro)insulin biosynthesis of human islets
transplanted under the renal capsule of normoglycemic or hyperglycemic
nude mice
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Figure 1. Northern blot analysis of insulin mRNA
expression in human islets grafted under the kidney capsule of
normoglycemic (4N) or hyperglycemic (4H) nude mice (46 weeks
in vivo). Total RNA (5 µg), pooled from human grafts
retrieved from five normoglycemic or hyperglycemic mice, were
sequentially hybridized with complementary DNAs for insulin (A, the
figure is representative of two separate Northern blots) and 18S (not
shown). Densitometric analysis of the blots are shown in B
(insulin/18S; data as arbitrary units of optical density).
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Graft glucose oxidation was increased 34 x (P
< 0.01) in all groups when comparing glucose metabolism at 1.7 and
16.7 mmol/L glucose (Table 2
). However, the graft
glucose oxidation rates did not differ between 4N, 4H, and 4H + 2N
mice.
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Table 2. Glucose oxidation of human islets transplanted under
the renal capsule of normoglycemic or hyperglycemic nude mice
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Grafts in normoglycemic recipients (4N) showed a well-preserved insulin
response to glucose or arginine stimulation in the perfusion
experiments (Fig. 2
). Exposure to hyperglycemia for 46
weeks (4H) led to total abolition of glucose-induced insulin release
and severely decreased the insulin response to arginine. In mice
hyperglycemic for 4 weeks, followed by 2 weeks of normoglycemia (4H +
2N), there was still a diminished insulin release in response to both
glucose and arginine. Using planimetry, we calculated the total insulin
output in the presence of 16.7 mmol/L glucose or 10 mmol/L arginine
plus 5.6 mmol/L glucose. Islets grafted into hyperglycemic animals (4H)
have a markedly decreased insulin response to both secretagogues, and
this was only partially restored by a subsequent 2 weeks of
normoglycemia (4H + 2N) (data not shown).

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Figure 2. Insulin concentration in the effluent medium
collected from human islet graft-bearing kidneys of nude mice 4 or 6
weeks after transplantation. Kidneys were perfused with a medium
containing 16.7 mmol/L glucose or 5.6 mmol/L glucose + 10 mmol/L
arginine, as indicated in the top of the figure. During
the periods before or after stimulation with 16.7 mmol/L glucose or 5.6
mmol/L glucose + 10 mmol/L arginine, the medium contained 2.8 mmol/L
glucose. The curves represent data from recipients
normoglycemic for 46 weeks (4N, open squares; n =
7), hyperglycemic for 4 weeks (4H, filled diamonds;
n = 8) or hyperglycemic for 4 weeks followed by normoglycemia for
2 weeks (4H + 2N, filled squares; n = 5).
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Discussion
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There are several hypotheses to explain why ß-cells, chronically
exposed to elevated glucose levels, lose their ability to release
insulin in response to glucose. Among them, a possible role has been
suggested for glycogen accumulation, depletion of insulin reserves,
decreased insulin mRNA expression with an associated impaired
(pro)insulin biosynthesis, and/or defective glucose transport and
oxidation (1, 2, 3, 20, 21). Moreover, there is evidence suggesting that
elevated levels of free fatty acids, which often go in parallel with
hyperglycemia in decompensated diabetes, contribute to defective islet
glucose metabolism and accompanying impaired insulin release (22, 23).
Most of the hypotheses described above are based on data obtained in
rodent islets under in vitro conditions. Thus, it is
imperative to test these hypotheses in human pancreatic islets, if
possible, exposed to long periods of hyperglycemia in vivo.
The observations that human islets grafted to normoglycemic nude mice
for 4 weeks maintain an elevated insulin release in response to glucose
(Ref. 8; present data), well preserved ß-cell ultrastructure (8) and
insulin mRNA expression (present data), and a 4-fold increase in
glucose oxidation in response to 16.7 mmol/L glucose (present data)
indicate that the present model offers an adequate alternative to study
these issues. It should be mentioned that reinnervation of islet grafts
into renal subcapsular space occurs slowly (24), which may affect both
insulin release and synthesis.
Using the above described preparations, we observed that exposure for
46 weeks of human islets to hyperglycemia induced a severe impairment
of insulin release in response to both glucose and arginine. This
defective release was not significantly improved by a subsequent 2-week
period of normoglycemia, achieved by transplantation of 200 islets
isolated from ob/ob mice. However, it is conceivable that
this second transplant, although sufficient to normalize basal
glycemia, is not enough to achieve normal glucose tolerance. If that is
the case, exaggerated glucose fluctuations over 24 h may
contribute to the defective recovery in ß-cell function. The decrease
in insulin release in the 4H and 4H + 2N was accompanied neither by a
decreased (pro)insulin biosynthesis nor by a decreased expression of
the insulin mRNA or by an impaired glucose oxidation. Previous
observations in the same model (8) suggested that glucose-induced
impairment in insulin release is independent of a depletion of insulin
content or glycogen accumulation. These combined observations suggest
that impaired glucose metabolism, decreased insulin mRNA expression and
(pro)insulin biosynthesis, glycogen accumulation, or depletion of
insulin stores (although of possible relevance for
hyperglycemia-induced rodent ß-cell dysfunction) cannot explain the
prolonged defect in glucose-induced insulin release observed in human
islets exposed to a diabetic environment for 46 weeks. Of course,
these observations do not exclude the possibility that more prolonged
exposure of human islets to hyperglycemia may lead to irreversible
impairment in insulin gene transcription, as described for a hamster
insulinoma cell line (2).
The mechanism by which arginine is supposed to induce insulin release
is by electrogenic transport into the ß-cell and direct membrane
depolarization, followed by Ca2+ entry and insulin
secretion (25, 26). Thus, the present observations that exposure of
human ß-cells to a diabetic environment impairs both glucose- and
arginine-induced insulin release, without affecting glucose metabolism,
suggest a dysfunction located distally to ATP production and closure of
ATP-sensitive K-channels, perhaps at the level of the exocytotic
process of insulin secretion (for a recent review on these processes,
see 27 . The fact that 2 weeks of normoglycemia did not suffice to
restore insulin response to glucose suggests that 46 weeks of
hyperglycemia induces a long-lasting depletion of a crucial
component(s) of the insulin exocytotic process. Identification of this
component(s) may provide new insights into the mechanisms of human
ß-cell dysfunction in non-IDDM and early IDDM.
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Acknowledgments
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The skilled technical assistance of B. Bodin, M. Engkvist, A.
Nordin and I.-B. Hallgren is gratefully acknowledged. We are grateful
to Professor D. Pipeleers, Coordinator of the ß-Cell Transplant, for
providing the human islet preparations and information on human islet
cell composition.
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Footnotes
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1 This study made use of human islets prepared by the Central Unit of
the B-Cell Transplant, supported by a Shared Costs Action of the
European Community. It was also supported by grants from the Juvenile
Diabetes Foundation International, Swedish Medical Research Council
(12X-9237; 12X-109; 12X-9886; 12X-6538; 12P-9287; and 12P-8982), the
Swedish Diabetes Association, the Novo-Nordisk Insulin Fund, the Family
Ernfors Fund, and the Göran Gustafsson Foundation. 
Received January 24, 1997.
Accepted April 17, 1997.
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