Glucose-Regulated Gene Expression Maintaining the
Glucose-Responsive State of ␤-Cells
Frans Schuit, Daisy Flamez, Anick De Vos, and Daniel Pipeleers
The mammalian ␤-cell has particular properties that
synthesize, store, and secrete insulin in quantities that
are matched to the physiological demands of the organism. To achieve this task, ␤-cells are regulated both
acutely and chronically by the extracellular glucose
concentration. Several in vivo and in vitro studies indicate that preservation of the glucose-responsive state
of ␤-cells is lost when the extracellular glucose concentration chronically deviates from the normal physiological condition. Experiments with the protein synthesis
inhibitor cycloheximide suggest that the maintenance of
the functional state of ␤-cells depends on protein(s)
with rapid turnover. Analysis of newly synthesized proteins via two-dimensional gel electrophoresis and highdensity gene expression microarrays demonstrates that
the glucose-dependent preservation of ␤-cell function is
correlated with glucose regulation of a large number of
␤-cell genes. Two different microarray analyses of glucose regulation of the mRNA profile in ␤-cells show that
the sugar influences expression of multiple genes involved in energy metabolism, the regulated insulin
biosynthetic/secretory pathway, membrane transport,
intracellular signaling, gene transcription, and protein
synthesis/degradation. Functional analysis of some of
these regulated gene clusters has provided new evidence for the concept that cataplerosis, the conversion
of mitochondrial metabolites into lipid intermediates, is
a major metabolic pathway that allows ␤-cell activation
independently of closure of ATP-sensitive potassium
channels. Diabetes 51 (Suppl. 3):S326 –S332, 2002
ifferentiated pancreatic ␤-cells exhibit a phenotype enabling a complex cellular response to
changes in the extracellular glucose concentration within the physiological range (i.e., between 3 and 10 mmol/l). This response can be dissected
into several pathways and orders of magnitude of the
following time kinetics. Within minutes of exposure to
elevated extracellular glucose, profound effects are observed at the level of metabolic flux, adenine nucleotide
potential ([ATP]/[ADP] as well as [NAD(P)H/[NAD(P)⫹]),
cellular electrical activity, and cytosolic [Ca2⫹] (1). As a
D
From the Diabetes Research Center, Faculty of Medicine, Vrije Universiteit
Brussel, Brussels, Belgium.
Address correspondence and reprint requests to Frans C. Schuit, MD, PhD,
Diabetes Research Center, Vrije Universiteit Brussel, Laarbeeklaan 103,
B-1090 Brussels, Belgium. E-mail: [email protected].
Received for publication 2 April 2002 and accepted in revised form 8 May
2002.
FACS, fluorescence-activated cell sorter; K⫹ATP channel, ATP-sensitive
potassium channel.
The symposium and the publication of this article have been made possible
by an unrestricted educational grant from Servier, Paris.
S326
consequence, exocytosis of a readily releasable pool of
insulin secretory granules is triggered (1). After this first
phase of insulin release (⬃10 min), a second wave of
release builds up by recruiting new granules into the pool
that is exocytosis competent (1,2). In the meantime,
ribosomes accelerate translation (3) by speeding up both
initiation (4) and elongation (4,5) steps. Translational rates
of certain proteins such as preproinsulin accelerate preferentially over that of other (housekeeping) proteins (3).
This result could be explained (at least in part) because
there is some indication that within 1 hour of incubation in
high glucose, transcriptional activity of the insulin gene (6)
is increased, enlarging the pool of transcripts to be translated. After hours of glucose stimulation, the activity of
many other genes is altered, resulting in an altered ␤-cell
phenotype, as evidenced by the cellular responsiveness to
glucose. Glucose itself seems responsible for the maintenance of this phenotype, at least in vitro (7). Exposing rat
␤-cells for 9 days to low glucose (6 mmol/l) reduces the
cellular responsiveness to glucose compared with cells
cultured at 10 mmol/l glucose (7). Preferential translation
of preproinsulin is diverted into synthesis of non-insulin
proteins (7). When cultured at even lower glucose concentrations for several days, the cells are committed to
programmed cell death (8). Conversely, ␤-cells exposed
for days to weeks to a supra-physiological glucose concentration lose part of their differentiated phenotype as
well (9 –11) and assume a “basal” state of hyperactivity (7).
Depending on the genetic background, this adaptation can
result in activation of signaling pathways that promote
growth and proliferation (12). Glucose-stimulated growth
of the islet ␤-cell mass as well as glucose protection
against apoptosis (8) imply that glucose not only regulates
the phenotype of existing ␤-cells, but also regulates cell
number.
A major challenge in diabetes research is understanding
the pleiotropic effects of glucose on ␤-cells in molecular
terms, understanding their sites of dysregulation in diabetic patients, and finding drugs that can cure disease by
restoring abnormal function. From this point of view,
understanding the function of various proteins that are
directly responsible for the glucose-responsive ␤-cell phenotype is of great interest. Also, it is relevant to investigate
which of this set is a primary culprit in the pathogenesis of
diabetes, either by loss or by abnormal gain of function.
Consequentially, it should be explored how abnormal
function can be restored to normal by selective drugs.
With the exception of the diverse subtypes of maturityonset diabetes of the young (13), this level of molecular
understanding of the ␤-cell and diabetes is still largely
lacking. The present review will focus on glucose regulaDIABETES, VOL. 51, SUPPLEMENT 3, DECEMBER 2002
F. SCHUIT AND ASSOCIATES
FIG. 1. Functional clusters of glucose-responsive genes in FACS-purified rat ␤-cells.
To assess which genes are responsible for
the glucose-responsive phenotype of rat
␤-cells, mRNA profiles of ␤-cells cultured
for 24 h in 3 or 10 mmol/l glucose, were
compared via Affymetrix rat Genome U34A
oligonucleotide arrays, as described by Flamez et al. (16).
tion of ␤-cell gene expression related to the maintenance
of the differentiated phenotype.
MULTIPLE GENES ARE REGULATED IN ␤-CELLS BY
GLUCOSE
In general, the acute minute-to-minute effects of glucose
regulation of ␤-cells primarily depend on the allosteric or
covalent modification of existing effector or regulator
proteins. Examples of this type of regulation are the
closure of ATP-sensitive potassium channels (K⫹ATP channels) by a rise in the cellular [ATP]/[ADP] ratio (14) and
GTP-induced activation of the small G-protein Rab-3a,
which controls vesicle budding and exocytosis (15). On
the other hand, it is widely assumed that the subacute
(hours) and chronic (days) effects of glucose require, in
addition to the acute regulatory effects, changes in the
expression level of the very genes that encode proteins
involved in the acute regulation of ␤-cell function. In line
with the above examples, we have recently shown that
24-h exposure of rat ␤-cells to 10 mmol/l glucose increases
mRNA encoding both subunits of the K⫹ATP channels
(SUR1 and Kir6.2) (16), whereas at the same time, mRNA
encoding the Rab-geranylgeranyl transferase is reduced
compared with cells cultured in 3 mmol/l glucose (D.F., V.
Berger, M. Kruhoeffer, T. Orntoft, D.P., F.S., unpublished
data). The number and identity of genes undergoing glucose regulation in ␤-cells is not yet known, but knowledge
in this field is expanding rapidly (16 –18; D.F., V. Berger, M.
Kruhoeffer, T. Orntoft, D.P., F.S., unpublished data).
In pioneering studies, Collins et al. (19,20) analyzed via
two-dimensional gel electrophoresis 35S-methionine–
labeled proteins extracted from whole islets that were
exposed in vivo or in vitro to either low or high glucose.
Using this technique, ⬃2,000 different islet protein spots
were identified. From this pool of islet proteins, ⬃1.5%
was reported to be regulated by glucose in the physiological concentration range (20). Assuming that the total
number of proteins that are expressed in ␤-cells may be as
high as 10,000, the number of glucose-regulated proteins
DIABETES, VOL. 51, SUPPLEMENT 3, DECEMBER 2002
may exceed a few hundred. The need to identify such large
numbers of genes involved has shifted research efforts
from the “classical” candidate gene approach to representational difference analysis (differential display [18]) or
microarrays allowing gene expression analysis (16,17;
D.F., V. Berger, M. Kruhoeffer, T. Orntoft, D.P., F.S., unpublished data). Although the candidate gene approach has
given useful insights with respect to regulation of insulin
(21,22), IAPP (23,24), PC1/3 (25), GLUT2 (26), GAD65 (27),
and IA-2 (28) genes, it will not allow full comprehension of
the scale and details of a glucose-regulated gene network.
The prediction that glucose probably regulates hundreds
of ␤-cell genes is borne out of a recent large-scale mRNA
expression analysis. The possibility of undertaking such
screening depended on high-density microarrays, allowing
the measurement of thousands of different transcripts in
one particular sample. Using the glucose-responsive murine ␤-cell line MIN6, Webb et al. (17) identified 75
transcripts from which abundance was regulated by altering the glucose concentration in the culture medium from
5.5 to 25 mmol/l. Following a similar approach in primary
cultures of fluorescence-activated cell sorter (FACS)purified rat ␤-cells, our laboratory recently identified more
than 150 glucose-regulated transcripts for which abundance depended on the glucose concentration (3 vs. 10
mmol/l) during the 24-h tissue culture (16; D.F., V. Berger,
M. Kruhoeffer, T. Orntoft, D.P., F.S., unpublished data).
Messenger RNAs that were more abundant in G10 cells
than in G3 cells (n ⫽ 76) and transcripts with a higher
abundance in cells cultured at 3 mmol/l glucose (n ⫽ 103)
could be classified into several functional groups (Fig. 1).
The glucose-upregulated transcripts encode a number of
proteins from which expression was previously reported
to be glucose dependent in the candidate gene approach.
Examples are both preproinsulins (21), the prohormone
convertase PC1/3 (25), the glucose transporter GLUT2
(26), and the major ␤-cell autoantigens GAD65 (27) and
IA-2 (28). The analysis of MIN6 (17) and the FACS-purified
␤-cell genes (16) was performed with Affymetrix mouse/
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GLUCOSE-REGULATED GENES IN PANCREATIC ␤-CELLS
FIG. 2. Time-dependent loss of the glucose-responsive insulin secretory state by culture in low glucose or high glucose plus cycloheximide.
The protein synthesis inhibitor cycloheximide (10 ␮mol/l) caused a
time-dependent decline of the insulin secretory response to a 30-min
stimulation with 10 mmol/l glucose (G10) plus 10 nmol/l glucagon-like
peptide 1 when added to the culture medium in the presence of 10
mmol/l glucose (f). Cells cultured at 3 mmol/l glucose (G3) (E) are
shown in comparison. Control cells (F) were cultured in medium
containing 10 mmol/l glucose without cycloheximide. Means ⴞ SE of
five experiments are shown. *P < 0.05 vs. control cells.
rat A arrays that cover only part of all genes in these
murine genomes. Consequently, it can be predicted that
the number of glucose-regulated transcripts in ␤-cells may
be a lot higher than currently reported.
AN OPTIMAL GLUCOSE CONCENTRATION IS REQUIRED
FOR MAINTENANCE OF THE GLUCOSE-REGULATED
INSULIN SECRETORY STATE
The phenotype of differentiated ␤-cells is remarkable not
only for its production, storage, and secretion of insulin,
but also for its unique metabolic organization that may be
key to the exquisite glucose responsiveness of the cells.
Several aspects of glucose metabolism in the ␤-cells have
been previously reviewed (29 –32). Studies on FACS-purified rat ␤-cells have contributed to four particular aspects
of metabolic organization in these cells: a low hexokinaseover-glucokinase ratio (33), little anaerobic glycolysis (34),
a low [ATP]/[ADP] ratio at basal (5 mmol/l) glucose (14),
and high rates of anaplerosis/cataplerosis (16,34). As mentioned above, the insulin secretory phenotype of rat ␤-cells
to glucose can be maintained chronically in tissue culture
if a glucose concentration of 10 mmol/l is present (7). Both
lowering (6 mmol/l) and augmenting (20 mmol/l) the
glucose concentration during the culture period resulted
in a marked loss of the capacity of the cells to synthesize,
store, and secrete insulin upon further acute glucose
stimulation (7). ␤-Cells can also be dedifferentiated by
chronic hyperglycemia in vivo (10), strongly indicating
that this phenomenon is not an artifact of the in vitro
S328
serum-free culture system. Similar effects can be obtained
by exposing ␤-cells for at least 2 h to either low (3 mmol/l)
glucose (16) (Fig. 2) or high glucose in the presence of the
protein synthesis inhibitor cycloheximide (35) (Fig. 2).
From these results, it seems likely that the observed
changes in the ␤ phenotype (shift from a glucose-responsive to a glucose-unresponsive state) are linked to an
altered presence of at least one critical (regulatory or
effector) protein for which function is required for insulin
secretion. To explore this possibility, we performed twodimensional gel electrophoresis of 35S-methionine–labeled
proteins synthesized over 4 h at 10 mmol/l glucose in
FACS-purified rat ␤-cells after 10 days of culture in 6, 10,
or 20 mmol/l glucose (Fig. 3). In these conditions, the
amount of total protein synthesized per ␤-cell increased
from 7.6 ⫾ 2.5 cpm/␤-cell at 6 mmol/l glucose preculture to
13.9 ⫾ 4.0 cpm/␤-cell at 10 mmol/l glucose preculture and
remained the same (12.7 ⫾ 6.0 cpm/␤-cell) after culture in
20 mmol/l glucose (mean ⫾ SE; n ⫽ 3). Laser densitometric scanning of spot intensities could distinguish between
several different patterns of glucose regulation. A first
pattern, concentration-dependent upregulation by glucose
(6 –20 mmol/l), was obtained for the proinsulin 2 protein
(marked “a” in Fig. 3). Intensity of the two combined
proinsulin 2 spots (localized in two-dimensional gels of
immunoprecipitated extracts; data not shown) was increased sixfold between 6 and 10 mmol/l glucose and
another 2.7-fold between 10 and 20 mmol/l glucose culture.
The second pattern exhibited an optimum glucose concentration (10 mmol/l) for translation and lower 35S-incorporation in cells cultured at 6 or 20 mmol/l glucose. This
pattern is exemplified by four 82-kDa proteins (labeled “b”
in Fig. 3) from which synthesis was between 2.5- and 4-fold
higher at 10 mmol/l glucose culture than in the other two
culture conditions. This expression pattern is of potential
interest because it coincides with the apparent optimal
glucose level to maintain the differentiated state of primary ␤-cells both in tissue culture (7–9) and in vivo
(10,11). A third pattern (illustrated by “c” in Fig. 3) shows
suppression of translation when the culture glucose concentration increases. The experimental setup did not allow
further mass spectrometric analysis. Nevertheless, these
data underline the point (7) that the protein biosynthetic
activity of ␤-cells depends on the glucose concentration to
which the cells are chronically exposed. The shift in
phenotype (7) seems to be the result of a change in the
type of protein that is synthesized, which in turn depends
on the mRNA profile of the cells.
In our analysis of 24-h glucose-regulated transcripts in
FACS-purified rat ␤-cells (16), we observed that most of
the 76 transcripts for which abundance was higher after
culture in 10 mmol/l glucose encode proteins that are
related to specialized ␤-cell function (Fig. 1). Examples
are enzymes of glucose, lipid, and amino acid metabolism
(23 transcripts) as well as proteins involved in regulated
insulin synthesis and secretion (12 transcripts upregulated), signaling proteins (13 upregulated in 10 mmol/l
glucose), and channels/transporters (8 upregulated). On
the contrary, most of the 103 transcripts for which abundance was higher after culture in 3 mmol/l glucose encode
proteins involved in general cellular maintenance, repair,
protein synthesis/degradation, transcription, and RNA
DIABETES, VOL. 51, SUPPLEMENT 3, DECEMBER 2002
F. SCHUIT AND ASSOCIATES
FIG. 3. Effect of chronic glucose concentration on protein synthesis in
rat ␤-cells. Rat FACS-purified ␤-cells were cultured for 10 days in
HAM’s F-10 medium containing 6, 10, and 20 mmol/l glucose, after
which the cells were labeled for 4 h in the presence of 10 mmol/l glucose
and 35S-methionine. Samples of newly synthesized proteins from the
cell lysates (106 cpm per gel; corresponding to 4.5, 2.5, and 2.0 ⴛ 104
cells, respectively) were separated via two-dimensional gel electrophoresis using precast Immobiline DryStrip gels (pH range 4 –7) for the
first dimension and precast ExcelGel SDS gels (gradient 8 –18%) for
the second dimension. Gels were exposed to autoradiographic films
(␤-max Hyperfilm; Amersham, Bucks, U.K.) for 60 h at room temperature. Spot intensities were quantified using a laser densitometer
(Ultroscan XL; Pharmacia, Uppsala, Sweden) and Gelscan XL Software.
Intensity of the two spots in area “a” [coinciding with (pro)insulin 2]
was upregulated more than 10-fold between 6 and 20 mmol/l glucose.
The four upper spots in area “b” (molecular weight ⬃82 kDa) were
maximally expressed at 10 mmol/l glucose. Several spots in “c” represent proteins for which synthesis was suppressed by culture in 10 or 20
mmol/l glucose.
DIABETES, VOL. 51, SUPPLEMENT 3, DECEMBER 2002
splicing. To further study the functional role of these
glucose-regulated transcripts in rat ␤-cells, we recently
concentrated on two gene clusters that appear involved in
the triggering and amplifying pathway of glucose-induced
insulin release (16). For the triggering pathway, we observed higher expression of GLUT2, suppression of inactivating kinase of the pyruvate dehydrogenase complex,
and upregulation of several subunits of the respiratory
chain (16). These changes correlated to higher rates of
glucose oxidation. Moreover, transcripts encoding both
subunits (SUR1 and Kir6.2) of the K⫹ATP channels and the
calcium sensor calmodulin were more abundant in cells
cultured at 10 mmol/l glucose. In the same study, we
reported that expression of multiple genes involved in
cataplerosis, the export of mitochondrial metabolites into
the cytosol for the production of lipid intermediates, was
higher in cells cultured at 10 mmol/l glucose than at 3
mmol/l glucose (16). The identified transcripts encode the
first common step, ATP-citrate lyase, several enzymes of
fatty acid synthesis and desaturation, and cholesterol or
isoprene biosynthesis. These changes correlated well with
enhanced abundance of the transcription factor ADD1
(SREBP1c) (36) as well as with enhanced rates of incorporation of 14C-labeled carbon from glucose into hydrophobic molecules that could be extracted from the cells.
The functional importance of the ATP-citrate lyase reaction for maintaining stimulated insulin release was documented using two pharmacological inhibitors, radicicol
and (⫺)-hydroxycitrate, which abrogated part of the acute
secretory capacity of the cells (16). These data support the
idea (37) that cataplerosis is important to maintain the
functional state of the ␤-cells.
In contrast to the well-defined triggering pathway of
insulin release (1,38), the molecular targets and precise
signaling molecules involved in cataplerosis remain poorly
understood. They can vary from increased metabolic flux
through various NADPH-generating cycles such as the
citrate/pyruvate and citrate/malate cycles. It was previously estimated (34,39 – 41) that pyruvate carboxylation
accounts for a high percentage of total pyruvate metabolism in ␤-cells. What fraction of carboxylated pyruvate
serves to feed citrate/pyruvate and citrate/malate cycles
may become clear by following a novel nuclear magnetic
resonance (NMR) spectroscopic approach (41). Interestingly, ␤-cells cultured in 10 mmol/l glucose simultaneously
upregulate expression of ATP-citrate lyase and downregulate expression of isocitrate dehydrogenase (16), an enzyme that would draw citrate substrate away from the two
above-mentioned cycles. Alternatively, the cataplerotic
flux through the ATP-citrate lyase reaction can serve
primarily to generate building blocks for membrane biogenesis, which most likely will be enhanced under conditions of increased insulin biosynthesis, processing, and
exocytosis. Balancing the relative contribution of fatty
acid unsaturation and cholesterol production would affect
significant functional membrane variables such as lipid
raft density, membrane fluidity, or protein lipidation (42–
44). The produced long-chain acyl-CoAs may also play a
role in signaling as direct metabolic precursors of diacylglycerol, an activator of protein kinase C, or eicosanoids.
Finally, it is conceivable that amplification effects are
generated at the level of an integrated transcriptional
S329
GLUCOSE-REGULATED GENES IN PANCREATIC ␤-CELLS
FIG. 4. Molecular pathways of glucoseregulated gene expression. Multiple
regulatory pathways can alter the concentration and/or the activity of transcription factors (TFs) when glucose
stimulates ␤-cells. For an explanation,
see MOLECULAR MECHANISM OF GLUCOSEREGULATED GENE EXPRESSION IN ␤-CELLS.
response because glucose metabolism not only induces
the synthesis of transcription factors that promote lipogenic gene expression (16), it generates (still unknown)
ligands for various nuclear receptors (for example, the
HNF family).
MOLECULAR MECHANISM OF GLUCOSE-REGULATED
GENE EXPRESSION IN ␤-CELLS
Relatively little is known about the molecular mechanisms
that are involved in glucose regulation of ␤-cell gene
expression. Over the past decades, insight into control of
mammalian insulin gene expression has markedly progressed, both at the level of gene transcription (45) and at
post-transcriptional steps (46); therefore, this information
can be taken as a paradigm of glucose-regulated genes in
␤-cells. Unraveling the daunting complexity of the regulatory network of metabolites, signaling molecules, kinases/
phosphatases, and transcription factors (Fig. 4) seems to
be a formidable task for one gene and almost impossible
for a whole network of interacting genes. At the transcriptional level, glucose metabolism can alter gene expression
directly by (in)activation of proteins that are already
bound to the preinitiation complex on the gene promoter.
This may be achieved via kinases/phosphatases that sense
the abundance of metabolic flux in the ␤-cells or the
overall energy status of the cell. Some of the (in)activation
pathways may depend on glucose-induced influx of calcium in the cells (which triggers activation of calciumdependent kinases and phosphatases) or, even more
S330
indirectly, on glucose activation of insulin release and
insulin interactions with insulin receptors on ␤-cells (Fig.
4). Changes in the concentration of the transcription
factor protein in the nucleus can be obtained via translocation of existing molecules or glucose-induced changes in
the balance between synthesis and degradation of transcription factors. A leverage effect can be obtained for
transcription factors encoded by glucose-responsive genes
because of the overall stimulatory effect of glucose on the
translation rate of produced mRNA (3,4).
An interesting example of interacting kinases in glucosestimulated gene expression is found in the L-type pyruvate
kinase gene, which is nutrient responsive in ␤-cells (47)
and in liver (48). The molecular mechanism that is involved has been studied in most detail in liver tissue
(48 –50). Transcription is stimulated by carbohydrate response element binding protein (48) via mechanisms involving both cAMP-mediated phosphorylation (49) and
inactivation of AMP-activated protein kinase (50). It was
suggested that glucose-dependent transcription of the
L-type pyruvate kinase gene in MIN6 cells also depends on
inactivation of AMP-activated protein kinase because the
henomenon can be suppressed by the AMP analog 5aminoimidazole-4-carboxamide (AICA)-riboside and mimicked by intracellular injection of antibodies directed against
the AMP-activated protein kinase ␣-2 or ␤-2 chains (51).
An example of glucose regulation of gene expression via
signaling pathways involving elevated intracellular calcium has recently been documented for the insulin gene
DIABETES, VOL. 51, SUPPLEMENT 3, DECEMBER 2002
F. SCHUIT AND ASSOCIATES
(52). Synergism between glucose (nutrient stimulation)
and cAMP (glucagon-like peptide 1 stimulation) at the
level of insulin gene expression was reported to integrate
at the level of nuclear factor of activated T-cells (NFAT).
Studies with the calcineurin inhibitor FK506 and the
calcium chelator BAPTA further suggest that the transcriptional response requires activation of calcineurin, a
Ca2⫹-calmodulin– dependent protein phosphatase. A long
signaling loop can be proposed in which high extracellular
glucose elevates intracellular calcium, stimulates exocytosis, raises extracellular insulin, and causes insulin activation of receptors on ␤-cells (Fig. 4). It has been suggested
(6) that such a signaling loop contributes in a rapid manner
to the glucose-induced pool of preproinsulin mRNA in the
␤-cell. The fast-time kinetics of this process seem to some
extent contradictory with earlier observations (53). This
potentially interesting novel observation suggests that
insulin action on ␤-cells contributes to the glucose-induced transcriptional response to glucose. This finding
could explain why mice with ␤-cell–specific disruption of
insulin receptor gene expression have defects in glucoseinduced insulin release and diabetes (54).
Glucose not only alters transcription of many genes but
also accelerates the translation of existing transcripts.
This was first demonstrated for proinsulin mRNA (4,5,53).
It is possible that glucose effects on overall rates of
translation require direct intervention of metabolite(s),
nutrient-derived signals or second messengers, or kinases/
phosphatases at the level of the ribosome. The nature of
these signals is yet unknown. The preferential translation
of preproinsulin mRNA over that of other ␤-cell mRNAs
(3) was proposed to depend on glucose-dependent relief of
the signal peptide recognition particle (SRP)-mediated
arrest of elongation (5). Alternatively, it may be linked to
interactions with (still poorly characterized) proteins on
the 3⬘ and 5⬘ untranslated regions of the transcript (55).
Translational effects of glucose on ␤-cells could also
interact with the long signaling loop that is outlined in Fig.
4, which requires the activation of insulin receptors,
phosphatidylinositol-3 kinases, and protein kinase B. This
pathway has been shown in other mammalian cell types to
require the mammalian target of rapamycin (mTOR),
which activates the initiation factor eIF-4E by phosphorylation of its binding protein 4E-BP1 (56). In line with this
idea, rat islets exposed to glucose exhibit rapid phosphorylation of 4E-BP1, which could be mimicked by adding
exogenous insulin and could be suppressed by rapamycin
(57).
In summary, a new area of ␤-cell research is rapidly
unfolding. The behavior of large groups of genes under
different conditions of ␤-cell stimulation can be studied in
vitro using relatively low amounts of RNA. So far, this
approach has been used for the study of glucose effects on
the mRNA profile of rodent ␤-cells. When applied to
human ␤-cells from nondiabetic and diabetic pancreases, a
novel strategy for drug discovery can be planned by
identifying innovating targets for pharmacological intervention in ␤-cells.
ACKNOWLEDGMENTS
This study was supported by grant 9.0130.99 from the
Flemish Fund for Scientific Research (FWO Vlaanderen),
DIABETES, VOL. 51, SUPPLEMENT 3, DECEMBER 2002
the Ministerie van de Vlaamse Gemeenschap, Departement Onderwijs (Geconcerteerde Onderzoeksactie 1807),
and the Belgian Program on Interuniversity Poles of
Attraction, initiated by the Belgian State (IUAP-PAI).
The authors thank E. Quartier and V. Berger for valuable
technical support.
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DIABETES, VOL. 51, SUPPLEMENT 3, DECEMBER 2002