From GlaxoSmithKline Research and Development,
Research Triangle Park, North Carolina 27709 and ¶ Division of
Biological Sciences and Department of Nutrition, Harvard School of
Public Health, Boston, Massachusetts 02115
Received for publication, April 13, 2001, and in revised form, May 21, 2001
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ABSTRACT |
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Elevated levels of the hormone resistin, which is
secreted by fat cells, are proposed to cause insulin resistance and to
serve as a link between obesity and type 2 diabetes. In this report we
show that resistin expression is significantly decreased in the white
adipose tissue of several different models of obesity including the
ob/ob, db/db, tub/tub, and KKAy mice compared
with their lean counterparts. Furthermore, in response to several
different classes of antidiabetic peroxisome proliferator-activated receptor Adipocytes secrete a number of molecules such as tumor necrosis
factor- Resistin expression was also shown to be regulated by glitazones, a
class of insulin-sensitizing drugs approved for the treatment of type 2 diabetes (5). Rosiglitazone and other glitazones lower glucose and
lipid levels in patients with type 2 diabetes by activating the nuclear
receptor peroxisome proliferator-activated receptor In this report, we have examined resistin expression in several
different rodent models of obesity and its regulation in response to
different classes of PPAR Experimental Animals and Protocols--
All procedures performed
were in compliance with the Animal Welfare Act, United States
Department of Agriculture regulations and approved by the
GlaxoSmithKline and Harvard University Institutional Animal Care and
Use Committee. Animals were housed at 72 °F and 50% relative
humidity with a 12-h light and dark cycle and fed chow diet (Formulab
Diet 5008; PMI Feeds Inc., Richmond, IN). Age (9 weeks)- and
glucose-matched male Zucker diabetic fatty rats (Genetic Models, Inc.,
Indianapolis, IN) were gavaged twice daily for 7 days with vehicle
(0.05 M N-methylglucamine), GW1929 (5.0 mg/kg),
or rosiglitazone (3.0 mg/kg). Glucose, triglycerides, and
non-esterified fatty acids were measured as described previously (8). Insulin-treated animals received a mixture of Humulin®N and
Humulin®R (Lilly) by subcutaneous injection and were sacrificed 6 h later. Genetically obese ob/ob mice were from a colony
maintained at Harvard; the db/db, tub/tub, and KKAy mice
were from Jackson Laboratories (Bar Harbor, ME). All mice were
maintained on standard rodent chow. MCC-555 (20 mg/kg) was administered
by daily gavage for 10 days. Rosiglitazone (5 mg/kg) and GW1929 (5 mg/kg) were administered by daily intraperitoneal injections. Ambient
blood samples were obtained in the beginning and at the end of the
treatment period, and tissues were collected for further analyses 4 h
after food withdrawal.
RNA Preparation and Northern Blot Analysis--
Total RNA from
epididymal white adipose tissue was prepared from ZDF rats, resolved on
agarose gels, and blotted as described previously (9). Filters were
prehybridized at 68 °C in Express-Hyb (CLONTECH
Laboratories, Inc., Palo Alto, CA) for 60 min, followed by
hybridization to specific 32P-labeled cDNA probes at a
concentration of 1 × 106 cpm/ml for 2 h at
68 °C. Filters were washed twice in 2× SSC/0.1% SDS for 20 min,
followed by a single wash for 20 min in 0.1× SSC/0.1% SDS at
60 °C. A rat resistin cDNA clone was isolated from rat adipose
tissue RNA by reverse transcriptase PCR using nucleotide sequence
reported by Kim et al. (6). PCR oligonucleotide sequences used were as follows: coding strand, CTGAGCTCTCTGCCACGTACT;
non-coding strand, GCTCAGTTCTCAATCAACCGTCC. The cDNA was
subcloned into pUC18 (Amersham Pharmacia Biotech), sequenced to
confirm its identity, and used in Northern blot analysis. Image
analyses and quantitation from the phosphor screen were performed with
a Storm optical scanner using the ImageQuant software package
(Molecular Dynamics Inc., Sunnyvale, CA). Mouse resistin cDNA was
cloned by reverse transcriptase PCR based on the published sequence,
cloned and sequenced to confirm its identity, and used in Northern blot
analysis as described (10).
Resistin Expression in Obese Mice--
Because resistin is
identified as a gene negatively regulated by the insulin-sensitizing
drug rosiglitazone, and its protein level is increased in the
circulation of ob/ob and db/db mice relative to wild type controls (5),
it is reasonable to postulate that its expression in adipose tissue
would also be increased in obesity. To address this, we examined
resistin mRNA expression in several different genetic models of
obesity/diabetes including the ob/ob, db/db, tub/tub, and
KKAy mice compared with their age-matched lean littermates.
Northern blot analysis under high stringency conditions revealed a
single, 0.8-kilobase pair resistin mRNA as reported
previously (Fig. 1) (5). The resistin
mRNA was readily detectable in the adipose tissue of all lean mice
(Fig. 1). Unexpectedly, resistin levels were severely decreased in the
epididymal WAT of all models of obese mice relative to lean controls
(Fig. 1). This suppression was most dramatic in the tub/tub (35-fold)
and KKAy (50-fold) mice and was also very substantial in
the ob/ob (20-fold) and db/db (15-fold) animals. A similar suppression
in adipose tissue resistin mRNA expression was also observed in
mice with diet-induced obesity (data not shown). Thus, obesity
correlated with severely decreased WAT expression of resistin in these
mouse models.
Resistin Expression Is Stimulated by PPAR
We next examined the regulation of resistin in the WAT of ZDF rats
treated with either rosiglitazone or GW1929. Treatment with
rosiglitazone or GW1929 resulted in 46 and 74% decreases in glucose
levels, respectively, relative to vehicle-treated animals (data not
shown). Northern blot analysis with a resistin-specific probe revealed
two transcripts ~0.8 and 1.4 kilobase pairs in length (Fig.
3A). These transcripts are the
same size as those reported previously for rat resistin (6). Similar
to the ob/ob mice, Northern blot analysis revealed that
rosiglitazone or GW1929 treatment resulted in an increase in resistin
expression in WAT of ZDF rats (Fig. 3, A and B).
In agreement with a previous report (6), resistin expression was also
induced by insulin treatment (Fig. 3A). Thus both insulin
itself and insulin sensitizers stimulate the expression of resistin in
WAT.
Resistin has been proposed to serve as a link between obesity and
diabetes, with elevated levels of resistin promoting insulin resistance
(5). Moreover, PPAR agonists, adipose tissue resistin expression is
increased in both ob/ob mice and Zucker diabetic fatty rats. These data demonstrate that experimental obesity in rodents is associated with
severely defective resistin expression, and decreases in resistin
expression are not required for the antidiabetic actions of
peroxisome proliferator-activated receptor
agonists.
INTRODUCTION
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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS AND DISCUSSION
REFERENCES
, leptin, and free fatty acids that can influence the ability
of the body to respond to insulin and metabolize glucose (1-4).
Recently, a novel 12.5-kDa cysteine-rich protein, termed resistin, was
shown to be secreted by adipocytes (5). Resistin expression was
markedly induced during the conversion of 3T3-L1 cells to mature
adipocytes (5, 6). Administration of resistin to wild type mice
impaired glucose tolerance and insulin action, and resistin levels were
reported to be increased in genetic and diet-induced forms of obesity
(5). When an antibody against resistin was administered to obese mice,
an increase in systemic insulin sensitivity was noted (5). Based on
these data, it was suggested that resistin serves as a hormonal link
between obesity and peripheral insulin resistance in diabetes
(5).
(PPAR
)1 (7). Rosiglitazone
treatment was shown to reduce resistin expression in 3T3-L1 adipocytes
in vitro and in the white adipose tissue (WAT) of mice fed a
high fat diet (5). These data raised the interesting possibility that
decreases in resistin levels might be integral to the antidiabetic
actions of PPAR
agonists.
agonists. Surprisingly, we find that
resistin expression is decreased in obese mice and increased in ob/ob
mice and Zucker diabetic fatty (ZDF) rats in response to PPAR
agonists.
MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS AND DISCUSSION
REFERENCES
RESULTS AND DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS AND DISCUSSION
REFERENCES
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Fig. 1.
Expression and regulation of resistin
mRNA in murine genetic models of obesity. Northern blot
analysis of adipose tissue resistin expression in male obese
(O) ob/ob, db/db, tub/tub, and KKAy mice or their
lean (L) counterparts. Adipsin and aP2 mRNA expression
are shown as controls. Ethidium bromide (EtBr) staining is
shown as a control for loading and integrity of RNA.
Agonists--
We next
examined the regulation of resistin expression in the WAT of male ob/ob
mice treated with different PPAR
agonists including the
thiazolidinediones rosiglitazone and MCC-555 and the tyrosine
derivative GW1929. Rosiglitazone and GW1929 are full PPAR
agonists
(8, 11). MCC-555 profiles as a low affinity full PPAR
agonist in
cell-based assays but acts as a potent antidiabetic agent in
vivo (12). Treatment with rosiglitazone, GW1929, or MCC-555
resulted in 50, 50, and 30% reductions in serum glucose levels,
respectively, relative to treatment with vehicle alone and significant
increases in insulin sensitivity (data not shown). As expected,
Northern blot analysis demonstrated that each of these compounds
stimulated the expression of the PPAR
target genes fatty acid
transporter protein (FATP) and phosphoenolpyruvate carboxykinase
(PEPCK) (Fig. 2A) in WAT.
Surprisingly, treatment with each compound also resulted in an increase
in resistin expression in WAT (Fig. 2, A and B).
MCC-555 resulted in a greater increase (8.4-fold) in resistin
expression compared with either rosiglitazone (3.4-fold increase) or
GW1929 (2.2-fold increase). These data indicate that decreases in
resistin expression are not required for the antidiabetic actions of
three different PPAR
agonists in a standard genetic model of insulin
resistance.
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Fig. 2.
Northern blot analysis of resistin, FATP, and PEPCK
expression in WAT of male 14-week-old ob/ob mice treated with the
PPAR agonist MCC-555 (A). Effects of three
PPAR
agonists, MCC-555, rosiglitazone, and GW1929, on WAT resistin
expression in ob/ob mice (B). Each column shows
the mean ± S.E. obtained from four animals in each group.
Ethidium bromide (EtBr) staining is shown as a control for
loading and integrity of RNA. *, p < 0.01 relative to
vehicle treatment.
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Fig. 3.
Northern blot analysis of resistin and FATP
expression in WAT of male ZDF rats treated with the PPAR agonists
rosiglitazone or GW1929 for 7 days or insulin for 6 h
(A). Quantitation of the rosiglitazone and GW1929 Northern
data shown in panel A (B). Data represent the
mean ± S.E. *, p < 0.01 relative to vehicle
treatment. Ethidium bromide (EtBr) staining is shown as a
control for loading and integrity of RNA.
agonists have been proposed to enhance insulin
sensitivity by decreasing resistin expression (5). Our data do not
support either of these proposals. We show that insulin resistance in
several common rodent genetic models is associated with decreases in
resistin expression. In addition, we demonstrate that different PPAR
agonists all stimulate resistin expression in two standard rodent
models of type 2 diabetes. Although we were unable to determine
resistin protein levels, it is unlikely that post-transcriptional
regulation could account for the magnitude of differences observed in
our study. Further studies are needed to determine the mode of
regulation and biological functions of resistin and whether it is an
effector of insulin resistance in obesity.
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FOOTNOTES |
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* This work was supported in part by Grant DK52539 from the National Institutes of Health (to G. S. H.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
§ Contributed equally.
To whom correspondence should be addressed: Harvard School of
Public Health, 665 Huntington Ave., Boston, MA 02115. Tel.: 617-432-1950; Fax: 617-432-1941; E-mail:
ghotamis@hsph.harvard.edu.
Published, JBC Papers in Press, May 23, 2001, DOI 10.1074/jbc.C100189200
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ABBREVIATIONS |
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The abbreviations used are:
PPAR, peroxisome
proliferator-activated receptor
;
WAT, white adipose tissue;
ZDF, Zucker diabetic fatty;
PCR, polymerase chain reaction;
FATP, fatty acid
transporter protein;
PEPCK, phosphoenolpyruvate carboxykinase.
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REFERENCES |
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