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Clinical Diabetology (B.R.-L., I.C., N.K., A.M., J.P.) and the Department of Morphology (P.M., A.C.), Centre Médical Universitaire, CH-1211 Geneva 4, Switzerland; the Laboratory of Physiopathology of the Nutritition, Centre National de la Recherche Scientifique, URA 307, Université Paris VII (C.M., A.K.), F-75291 Paris Cedex 05, France
Address all correspondence and requests for reprints to: Beate Ritz-Laser, Ph.D., Clinical Diabetology, Centre Médical Universitaire, 1 rue Michel Servet, CH-1211 Geneva 4, Switzerland. E-mail: laser{at}cmu.unige.ch
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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The mechanisms by which palmitate affects preproinsulin gene expression implicate both preproinsulin mRNA stability and transcription, as suggested by an actinomycin D decay assay, quantification of primary preproinsulin transcripts, and transient transfection experiments in Min6 cells. Metabolism of palmitate is not required to obtain these effects, inasmuch as they can be reproduced by 2-bromopalmitate. However, oleate and linoleate did not significantly influence preproinsulin mRNA levels. We conclude that insulin release and preproinsulin gene expression are not coordinately regulated by palmitate and that chronically elevated FFA levels may interfere with ß-cell function and be implicated in the development of noninsulin-dependent diabetes.
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Chronic exposure of islets of Langerhans to FFAs leads to an increase in basal insulin secretion that compensates for peripheral insulin resistance and to an inhibition of glucose-stimulated insulin secretion (2, 10). The mechanisms by which FFAs alter the ß-cell response to glucose are not yet understood (11). In addition, FFAs induce long term changes in ß-cell metabolism by altering the expression of key enzymes. The levels of the messenger RNA (mRNA) coding for acetyl-coenzyme A (CoA) carboxylase, which catalyzes the formation of malonyl-CoA, decrease in response to the changes in ß-oxidation. Simultaneously, the gene encoding carnitine palmitoyltransferase (CPT-1), the rate-limiting enzyme for ß-oxidation of FFAs, is transcriptionally up-regulated (12, 13). The increased fat oxidation is then thought to interfere with glucose metabolism via the Randle cycle.
To investigate whether the effects of FFAs on basal and glucose-stimulated insulin secretion are accompanied by changes in preproinsulin gene expression, we analyzed the effect of palmitate on both pancreatic rat islets and the insulin-producing cell line Min6. We found that this saturated FFA induces a rise in insulin release and a decrease in cellular insulin content and preproinsulin mRNA levels at high glucose levels. The latter effect is fully reversible after 48 h. As suggested by an actinomycin D decay assay, the palmitate effect on preproinsulin mRNA levels results in part from an increase in mRNA half-life. In addition, studies on relative transcription rates and transient transfections with DNA constructs containing the rat preproinsulin I gene promoter linked to the chlormaphenicol acetyltransferase (CAT) reporter gene indicate that palmitate impairs the glucose- mediated increase in transcriptional activity and that the DNA sequence elements that mediate the transcriptional response to palmitate are located within the first 410 bp of the rat preproinsulin I gene promoter. Palmitate is likely to act on preproinsulin gene expression either as the free acid or via its acyl-CoA ester, inasmuch as the nonoxidizable analog 2-bromopalmitate can reproduce the effects observed with palmitate.
Thus, exposure of insulin-producing cells to high FFA concentrations leads to dysregulation of insulin secretion and gene expression in response to glucose and may be a critical factor in the development of NIDDM.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Cell culture, incubation, and transfection
Insulin-producing mouse MIN6 cells (15), provided by Dr. J.-I.
Miyazaki (Kumamoto University Medical School, Kumamoto, Japan), were
grown in DMEM (Sigma Chemical Co., Basel, Switzerland)
supplemented with 15% heat-inactivated FBS, 100 U/ml penicillin, 100
µg/ml streptomycin, and 50 µM 2-mercaptoethanol. Before
all experiments, Min6 cells were preincubated in RPMI 1640 (Sigma Chemical Co.) containing 5 mM glucose, 10%
heat-inactivated FBS, 100 U/ml penicillin, and 100 µg/ml streptomycin
for 24 h. For mRNA analysis, cells were incubated 6–24 h at 11
mM glucose in the presence or absence of fatty acids and
either harvested directly or incubated for another 24–48 h in the
presence of 1% BSA for cell recovery. Transfection was performed by
the diethylaminoethyl-dextran technique (16) with 3 µg plasmid
DNA/6-cm petri dish using 8.25 µg Transfectam (Promega Corp., Zurich, Switzerland) and 12 µg DNA/well of a 24-well
tissue culture plate. Reporter plasmid -410 InsCAT consisted of 410 bp
of the rat preproinsulin I gene 5'-flanking sequence linked to the CAT
gene (17). Rous sarcoma virus-CAT (18) served as a positive
control.
CAT and protein assays
After transfection, cells were incubated for 48 h in RPMI
medium containing 11 mM glucose and, for the last 12
h, BSA or BSA-bound palmitate. Cell extracts were prepared 48 h
after transfection and analyzed for CAT activity as described
previously (19), except that quantification of acetylated and
nonacetylated forms was performed using a PhosphorImager
(Molecular Dynamics, Inc.). All assays were carried out a
minimum of five times. Protein concentrations were determined with a
Bio-Rad protein assay kit (Bio-Rad, Hercules, CA).
Preproinsulin mRNA stability
Palmitate effects on preproinsulin mRNA half-life were
determined by an actinomycin D decay assay. After preincubation for
24 h in RPMI 1640 with 5 mM glucose, cells were
switched to 11 mM glucose and 5 µg/ml actinomycin D.
After 30 min, BSA or palmitate was added to the incubation medium (time
zero). Cells were harvested after 3, 6, 12, and 24 h and analyzed
by Northern blot.
Northern blot analysis
Total RNA from rat islets or Min6 cells was isolated by the
guanidine thiocyanate method followed by a cesium chloride gradient
(20). Northern blotting and hybridization were performed according to
standard protocols (21) using random labeled rat CPT-1 (22), mouse
ß-actin, and rat preproinsulin complementary DNA (cDNA) probes or the
oligonucleotide 5'-GCCGTCCCTCTTAATCATGGCCTCAGTTCC-3', which is
complementary to 18S ribosomal RNA (rRNA). Steady state RNA levels were
quantified using a PhosphorImager (Molecular Dynamics, Inc., Sunnyvale, CA).
Ribonuclease (RNase) protection analysis of primary preproinsulin
transcripts
Total RNA (2 µg) from Min6 cells incubated 24 h in the
presence or absence of 2 mM palmitate was deoxyribonuclease
I treated and analyzed by RNase protection (21) to determine the
relative proportions of primary and mature preproinsulin mRNAs. The
in vitro synthesized antisense riboprobes corresponded to
codon 220–303 of the mouse ß- actin cDNA (Ambion, Inc., Lugano, Switzerland) and to a 402-bp
(BamHI/KpnI fragment including 177 and 225 bp of
exon 2 and intron 2, respectively) of the rat preproinsulin II gene
(23). Protected fragments were quantified using a PhosphorImager.
Measurement of insulin release and content
Min6 cells were plated (105 cells/well) in 96-well
tissue culture plates. After 2 days of culture, cells were preincubated
24 h in 5 mM glucose and then for 24 h in 11
mM glucose in the presence of BSA or palmitate. Insulin
released into the incubation medium was determined by RIA, using a
charcoal separation step and rat insulin as a standard. For measurement
of total insulin content, Min6 cells were washed with PBS buffer, and
immunoreactive insulin was determined after acid-ethanol (1.5% HCl and
75% ethanol) extraction (24). A similar protocol was used for analysis
of insulin release and content of isolated islets.
Statistical analysis
All results are expressed as the mean ± SEM.
Analysis of statistical difference between groups was carried out using
ANOVA and the paired, two-tailed Student’s t test.
P < 0.05 was considered significant.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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, palmitate induced a 2- to 15-fold
rise in insulin release; the maximal increase was observed at 5
mM glucose, whereas at high glucose only a 2-fold
stimulation was noted. Our observation of enhanced insulin release at
high glucose, although of smaller magnitude compared with that at 2-
and 5 mM glucose, is in contrast to the reported blunting
of glucose-stimulated insulin secretion (2, 25), but reflects the 24-h
effects of combined high glucose and palmitate rather than an acute
glucose stimulation after FFA exposure. In the presence of 11
mM glucose, insulin release was already stimulated by 0.25
mM palmitate, and higher FFA concentrations did not
increase this stimulation further (Fig. 1A).
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). In contrast, islets incubated in 5 and 11
mM glucose showed significant 40% and 65% decreases in
insulin content, respectively. As observed for insulin release, the
palmitate effect on insulin content was near maximal at 0.25
mM palmitate (Fig. 1B).
To investigate the effects of palmitate on preproinsulin gene
expression, total RNA was isolated from rat islets, cultured as
described above, and analyzed by Northern blot. Increasing glucose
concentrations led to a rise in the steady state level of preproinsulin
mRNA (Fig. 2A). Addition of 1
mM palmitate had no effect in the presence of 2 and 5
mM glucose, but inhibited by about 50% the rise in
preproinsulin mRNA induced by 11 mM glucose. This
inhibition coupled with the palmitate-induced enhanced insulin release
may explain the depletion of insulin stores that was observed under
such conditions (Fig. 1B). The level of ß-actin mRNA was not affected
by the addition of palmitate (normalized to 18S rRNA; data not shown).
A dose-response analysis of the effect of palmitate on preproinsulin
mRNA in the presence of 11 mM glucose showed that the
decrease in steady state preproinsulin mRNA levels was already near
maximal at 0.25 mM FFA (Fig. 2B). Thus, the range of
palmitate concentrations used in our study had similar effects on both
insulin release and preproinsulin gene expression in isolated rat
islets.
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, A and B). Increasing concentrations of
palmitate reduced the steady state level of preproinsulin transcripts
in a dose-dependent manner between 0.25–2 mM (Fig. 3C).
Maximal effects on the expression of the preproinsulin gene in Min6
cells required a higher palmitate concentration compared with that
needed for insulin release. In addition, Min6 cells behaved differently
from rat islets, where maximal inhibition of both insulin release and
gene expression occurred at 0.25–0.5 mM palmitate.
Qualitatively, palmitate had the same effects in both Min6 cells and
rat islets.
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), a
decrease more pronounced than that observed in the dose curve (Fig. 3C)
that we attribute to a different preparation of palmitate; we have
indeed observed quantitative variations in the effect of palmitate
depending on batch preparations.
In control incubations with 2% BSA, the steady state level of
preproinsulin transcripts was not affected. Moreover, as shown in Fig. 4A, the decreased preproinsulin mRNA
levels observed after a 24-h incubation with 1 mM palmitate
were partially and fully restored after an additional incubation with
1% BSA for 24 and 48 h, respectively indicating that the
palmitate effects are reversible. The palmitate-mediated inhibition
appeared specific for preproinsulin mRNA, inasmuch as in two different
insulin-producing cell lines, MIN6 and INS-1 cells, as well as in
pancreatic islets, the mRNA levels of ß-actin remained constant, and
those of CPT-1 were increased 3- to 5-fold as previously reported (12)
(Fig. 4B). From the results obtained with both rat islets and
insulin-producing cell lines, we conclude that palmitate impairs
preproinsulin gene expression by decreasing steady state preproinsulin
mRNA levels and that the effect is fully reversible.
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). The initial increase in preproinsulin
mRNA levels at early time points (3 and 6 h) may be due to the
change in glucose concentration (5–11 mM) at the beginning
of the experiment; glucose indeed increases both preproinsulin gene
expression and transcript stability (27, 28) and might therefore induce
an increase in preproinsulin mRNA levels at early time points when
actinomycin D is not yet fully active. The 30% decrease observed after
24 h was less pronounced, however, than the effect of palmitate on
the steady state levels of preproinsulin mRNA, as measured in the
dose-response and time-course experiments (Fig. 3), suggesting that
palmitate affects preproinsulin gene expression also at the
transcriptional level. We therefore measured preproinsulin transcripts
from Min6 cells that were incubated 24 h in the presence of either
2 mM palmitate or 2% BSA by RNase protection analysis. The
preproinsulin antisense probe generated corresponded to 177 nucleotides
(nt) of exon 2 and 225 nt of intron 2 of the rat preproinsulin II gene,
thus allowing for distinction between the primary preproinsulin II
transcripts and the mature preproinsulin mRNA. Protected fragments were
quantified with a PhosphorImager and expressed as the insulin/ß-actin
ratio. As shown in Fig. 6, palmitate
drastically decreased primary preproinsulin II transcripts by 73%. As
the approximate half-life of preproinsulin transcripts containing
intron 2 is about 8 min (29), their relative abundance is likely to
reflect present transcription rates. A similar reduction was observed
in the insulin/ß-actin ratio of mature preproinsulin transcripts
(65% of control levels); this value corresponds to the decrease in
steady state preproinsulin mRNA levels we observed in the time course
of palmitate effects (Fig. 3B).
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Palmitate metabolism is not required to affect preproinsulin
mRNA
To analyze whether the decrease in preproinsulin mRNA levels
observed in incubations with palmitate was also inducible by other long
chain FFAs, we incubated rat islets and Min6 cells for 24 h with
BSA, palmitate, the monounsaturated FFA oleate (C18:1), or the
polyunsaturated FFA linoleate (C18:2). As shown in Fig. 7, oleate and linoleate did not
significantly affect preproinsulin mRNA levels. The observed decrease
in the preproinsulin/ß actin ratio in incubations of rat islets with
oleate was actually due to a relative increase in ß-actin mRNA, which
was only noted with oleate; the preproinsulin mRNA/18S rRNA ratio for
rat islets incubated with oleate was, in fact, 95% of the control
levels, whereas this ratio was 65% and 101% for palmitate and
linoleate, respectively. These results are surprising, inasmuch as both
oleate and linoleate stimulated insulin secretion as much as palmitate
in rat islets cells; we indeed observed a 2-fold stimulation of insulin
release with palmitate and oleate and a 1.5-fold stimulation by
linoleate (data not shown); the lower insulinotropic effect of
linoleate compared with palmitate and oleate has been described
previously (30). Similarly, all FFAs stimulated insulin release in Min6
cells, with the most potent effect obtained with palmitate (3.8-fold
increase in release/content vs. 1.8-fold by oleate or
linoleate) (2, 13). The data indicate that FFA effects on insulin
release and preproinsulin gene expression at high glucose are not
necessarily coordinated and uniform.
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).
Thus, palmitate and bromopalmitate reduce preproinsulin gene
transcription by a mechanism that does not involve mitochondrial
ß-oxidation. It has recently been reported that palmitate, but not its nonoxidizable fatty acid analog 2-bromopalmitate (Br-C16:0), decreases the mRNA of Pdx1, one of the major transcription factors binding to and trans-activating the preproinsulin gene promoter, and impairs binding of Pdx1 to its recognition element (31). We therefore performed gel-shift analyses using nuclear extracts from Min6 cells incubated in the presence of 1% BSA, 1 mM palmitate, or 1 mM 2-bromopalmitate and the FLAT element of the rat preproinsulin I gene promoter. Surprisingly, neither FFA impaired Pdx1 binding to its recognition element (data not shown). Our results are in contrast to those reported by Gremlich and co-workers (31), who observed a decrease in Pdx1 binding from rat islets in the presence of 0.6 mM palmitate. This discrepancy might be explained by the fact that we used Min6 cells incubated at 11 mM glucose and not rat islets incubated at 30 mM glucose. However, as only palmitate, but not bromopalmitate, affects Pdx1 mRNA levels (31), whereas both FFAs reduce preproinsulin mRNA levels, preproinsulin gene transcription may be impaired by a mechanism that probably does not involve Pdx1.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Short term exposures of HIT cells to 0.2–1 mM palmitate induce a dose-dependent increase in basal and glucose-stimulated insulin secretion (32). In contrast, chronic incubations of rat islets with palmitate have been reported to stimulate basal release, but to inhibit its glucose-induced increase (2, 10). We measured total insulin release after a 24-h incubation period and show that palmitate leads to enhanced basal insulin release from isolated rat islets regardless of the prevalent glucose concentration. However, the lower relative increase in insulin release induced by palmitate at high glucose, compared with that observed in the presence of 2 and 5 mM, also suggests that palmitate may also interfere with glucose-stimulated insulin release. FFAs have been described to interfere with glucose metabolism in pancreatic islets via a Randle effect with increased ß-oxidation leading to impairment of glucose-induced insulin secretion (2). In long term incubations of insulin-producing cells with high glucose concentrations, oxidative phosphorylation of glucose is repressed, but the increased concentration of long chain acyl-CoA ester may directly stimulate insulin secretion via an interaction with certain PKC isoforms and ion channels (33), thus leading to insulin hypersecretion.
Contrasting with its effects on insulin release, palmitate inhibits preproinsulin gene expression only at high glucose by impairing the glucose-induced rise in preproinsulin mRNA levels. Both effects were observed in the presence of physiologically relevant FFA levels, but persistently elevated FFA concentrations, such as those found in obesity and diabetes, may inhibit insulin biosynthesis in response to glucose and lead to further enhancement of the diabetic state (5). Our results are in apparent contrast with previous reports showing that a 24-h exposure of rat islets to 0.125 mM oleate and 5.6 mM glucose increases preproinsulin mRNA levels 2- to 3-fold, and that incubation of rat islets in the presence of 8 mM glucose and of 2 mM of an oleate-palmitate mixture (2:1) leads to a 2.4-fold rise in preproinsulin levels (2, 34). However, two points should be considered with respect to our observations. 1) The glucose concentration in the incubation medium appears to be critical for the FFA-mediated effect; preproinsulin gene expression was only affected by palmitate at high glucose concentration, whereas oleate might increase preproinsulin gene expression selectively at low and normal, but not high, glucose. 2) Saturated (palmitate), monounsaturated (oleate), and polyunsaturated (linoleate) fatty acids exert different effects on preproinsulin gene expression. In both rat islets and Min6 cells, only palmitate and its analog 2-bromopalmitate decreased preproinsulin mRNA levels, whereas oleate and linoleate had no significant effect (this study and Ref. 31). Although the length and the degree of saturation of FFAs are critical for the stimulation of insulin secretion (30), glucose oxidation has been shown to be repressed to the same extent by oleate and palmitate in long term incubations of pancreatic islets (2). These data indicate that the differential effects of palmitate and bromopalmitate vs. oleate and linoleate on preproinsulin gene expression may not be due to a greater inhibition of glucose metabolism. Alternatively, an intracellular mediator could interact differentially with saturated, monounsaturated, and polyunsaturated FFAs. The latter class of FFAs has been shown to induce specific effects on the transcription and mRNA stability of genes involved in lipid metabolism (35), and similarly, saturated FFAs may interfere with preproinsulin gene expression via a mechanism that is not influenced by unsaturated FFAs.
To investigate the mechanism by which palmitate impairs ß-cell function, we analyzed its effect on preproinsulin mRNA levels in the mouse cell line Min6. Exposure of Min6 cells to palmitate led to a decrease in the steady state levels of preproinsulin mRNA that was both dose and time dependent. Moreover, palmitate specifically decreased the preproinsulin mRNA half-life in long term incubations. An influence of glucose on preproinsulin mRNA stability has been previously reported (28). Thus, palmitate may accelerate degradation of preproinsulin transcripts, as described for the yeast OLE1 mRNA that encodes an enzyme of lipid metabolism (36). RNase protection analysis of primary preproinsulin II RNAs and transient transfections using 410 bp of the preproinsulin I gene promoter indicate that palmitate also affects the transcriptional regulation of the preproinsulin gene. The quantitative difference observed in the decrease in preproinsulin I and II gene transcription by palmitate may be due to a differential regulation of the two promoters. Indeed, Ling et al. (37) recently showed that in rat islet ß-cells, preproinsulin I and II gene are differentially transcribed in the presence of elevated glucose levels. As palmitate specifically inhibits the glucose-induced rise in preproinsulin mRNA levels, it may interfere with the transcriptional activation of the preproinsulin gene promoter by glucose. In previous studies, a glucose-responsive element was identified in the rat preproinsulin I gene flanking sequence (38), and the binding affinity of the homeobox transcription factor Pdx1 to the two A-boxes of the element was shown to be modified by glucose via phosphorylation/dephosphorylation reactions (39). However, we did not observe variations induced by palmitate or bromopalmitate in the binding of Pdx1 to its recognition element (data not shown); the effect of palmitate on the preproinsulin gene promoter is thus more likely to be mediated by another transcription factor.
To analyze whether palmitate metabolism is required for the reduction of preproinsulin mRNA levels, we used 2-bromopalmitate. 2-Bromopalmitate is readily activated to 2-bromopalmitoyl-CoA by the long chain acyl-CoA synthetase, binds in this form irreversibly to the mitochondrial CPT-1, and is not further metabolized. CPT-1 is considered the rate-limiting step for ß-oxidation, and its interaction with 2-bromopalmitate specifically inhibits the mitochondrial uptake and ß-oxidation of long chain FFAs without affecting the metabolism of medium or short chain FFAs (40, 41). Bromopalmitate was more efficient in repressing preproinsulin mRNA levels than palmitate; this difference might be explained by several mechanisms.
Dose dependence
In Ins1 cells, the palmitate concentration decreases by 30% after
10 h, whereas the concentration of 2-bromopalmitate remains
constant (12). The difference in the effective concentrations of both
FFAs should be even greater after 24 h, the point at which we
measured preproinsulin mRNA levels.
Effector molecule
Palmitate is activated to its CoA ester and then imported into the
mitochondria after transformation into palmitoylcarnitine. This
transition results in a relatively low concentration of palmitoyl-CoA.
In contrast, bromopalmitoyl-CoA is not further metabolized, it
accumulates in the cytoplasm and may therefore specifically interact
with secondary effector molecules. In addition, oxidation of all
endogenous long chain FFAs is blocked by bromopalmitate, but not by
palmitate, which could interfere with cellular metabolism.
Cytotoxic effect
We cannot exclude a specific effect of the bromo residue in
bromopalmitate with other enzymes or a limited toxic effect of
bromopalmitate. A toxic effect is, however, not likely in the
concentrations and the cell system used in our study, inasmuch as we
did not observe changes in either cell morphology or cell number after
incubation of Min6 cells with the different FFAs (including
bromopalmitate) or BSA. In addition, we obtained the same amount of
total RNA from rat islets or Min6 cells after different incubation
conditions (palmitate, bromopalmitate, or BSA), and the levels of
ß-actin mRNA were unaffected in all experiences involving
bromopalmitate.
Potential mediators of the palmitate effect on the expression of
the preproinsulin gene may be the CCAAT/enhancer- inding protein-ß,
the peroxisomal proliferator-activated nuclear receptors (PPARs),
and hepatic nuclear factor 4
(HNF-4). 1) CCAAT/enhancer-binding
protein-ß has recently been implicated in the glucose toxicity of
pancreatic ß-cells and in the development of diabetes in the Zucker
diabetic fatty rat (42). 2) FFAs have been shown to be natural ligands
and activators of PPARs (43, 44), which regulate genes controlling
lipid and glucose metabolism, as well as adipogenesis (45). Three types
of PPARs are expressed in the endocrine pancreas (46), and the three
types of long chain FFAs are PPAR ligands; mono- and polyunsaturated
FFAs are more effective PPAR trans-activators than saturated
FFAs (43, 44). This is in contrast to their impact on insulin gene
expression, arguing against PPARs as regulators of insulin
biosynthesis. 3) Fatty acyl CoA esters are ligands for the orphan
transcription factor HNF-4, and agonistic/antagonistic effects on
transcriptional activity are induced depending on the chain length and
the degree of saturation (47). Interestingly, an excess of exogenous
oleoyl-CoA or linoleoyl-CoA has no effect on glucose-induced
preproinsulin mRNA levels in our study, whereas palmitoyl-CoA impairs
this response. However, direct binding site of HNF-4 on the
preproinsulin gene promoters (rat 1+2, human) has not been identified,
and preliminary data from our laboratory do not indicate the
implication of HNF-4 in the palmitate effects on preproinsulin gene
transcription. Thus, an as yet unidentified secondary messenger may
interact specifically with palmitate or palmitoyl-CoA, mediating the
repression of glucose-induced preproinsulin gene transcription.
In conclusion, whereas low levels of FFAs may be crucial for both insulin secretion (30, 48, 49) and preproinsulin gene expression, long term exposure to high FFA concentrations leads to peripheral insulin resistance and hypersecretion of insulin. By exceeding insulin gene expression, this excessive secretion results in the depletion of insulin stores (4, 13, 34). In addition, our data indicate that at least some FFAs exert detrimental effects on pancreatic ß-cell function by impairing preproinsulin gene expression under conditions of high insulin requirement. Therefore, long term exposure to elevated FFA levels may be a critical factor in the development of NIDDM due to their effects on peripheral insulin resistance, insulin secretion, and preproinsulin gene expression.
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Footnotes |
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2 Present address: Laboratoire des Régulations
Energétiques, Cellulaires et Moléculaires, Centre National
de la Recherche Scientifique, UMR 5578, Université Claude Bernard
I, F-69622 Villeurbanne, Cedex, France.
Received January 11, 1999.
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References |
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