Correction of diet-induced
hyperglycemia, hyperinsulinemia, and skeletal muscle
insulin resistance by moderate hyperleptinemia
Roland
Buettner1,2,
Christopher B.
Newgard1,2,3,
Christopher J.
Rhodes1,2,4, and
Robert M.
O'Doherty1,2
1 Gifford Laboratories for Diabetes Research,
and Departments of 2 Internal
Medicine, 3 Biochemistry, and
4 Pharmacology, University of Texas Southwestern
Medical Center, Dallas, Texas 75235
 |
ABSTRACT |
Human obesity and high fat feeding in rats are associated
with the development of insulin resistance and perturbed carbohydrate and lipid metabolism. It has been proposed that these metabolic abnormalities may be reversible by interventions that increase plasma
leptin. Up to now, studies in nongenetic animal models of obesity and
in human obesity have concentrated on multiple injection therapy with
mixed results. Our study sought to determine whether a sustained,
moderate increase in plasma leptin, achieved by administration of a
recombinant adenovirus containing the leptin cDNA (AdCMV-leptin) would
be effective in reversing the metabolic abnormalities of the obese
phenotype. Wistar rats fed a high-fat diet (HF) were heavier (P < 0.05), had increased fat mass and intramuscular triglycerides
(mTG), and had elevated plasma glucose, insulin, triglyceride, and free
fatty acids compared with standard chow-fed (SC) control animals (all
P < 0.01). HF rats also had impaired glucose tolerance and
were markedly insulin resistant, as demonstrated by a 40% reduction in
insulin-stimulated muscle glucose uptake (P < 0.001).
Increasing plasma leptin levels to 29.0 ± 1.5 ng/ml (from 7.0 ± 1.4 ng/ml, P < 0.001) for a period of 6 days decreased adipose
mass by 40% and normalized plasma glucose and insulin levels. In
addition, insulin-stimulated skeletal muscle glucose uptake was
normalized in hyperleptinemic rats, an effect that correlated closely
with a 60% (P < 0.001) decrease in mTG. Importantly, HF rats
that received a control adenovirus containing the
-galactosidase
cDNA and were calorically matched to AdCMV-leptin-treated animals
remained hyperglycemic, hyperinsulinemic, insulin resistant, and
maintained elevated mTG. We conclude that a gene-therapeutic
intervention that elevates plasma leptin moderately for a sustained
period reverses diet-induced hyperglycemia, hyperinsulinemia, and
skeletal muscle insulin resistance, and that these improvements are
tightly linked to leptin-induced reductions in mTG.
obesity; tissue triglycerides; leptin action; insulin
action
 |
INTRODUCTION |
OBESITY IS A METABOLIC DISORDER characterized by weight
gain, increased adiposity, insulin resistance, perturbed carbohydrate and lipid metabolism, and in many cases the development of
non-insulin-dependent diabetes mellitus (2, 3, 7). Administration of
leptin corrects the metabolic abnormalities of the ob/ob mouse,
which lacks the adipocyte hormone (17, 21, 24), whereas hyperleptinemia in the normal rat reduces plasma and tissue lipids and increases insulin sensitivity (1, 5, 26). On the basis of these observations it
has been proposed that leptin therapy may be effective in the treatment
of human obesity.
To date, studies on the efficacy of leptin therapy in nongenetic animal
models of obesity and in human obesity have concentrated on multiple
injection therapy, with mixed results. Thus in high fat fed (HF)
animals, a widely used model of obesity, intraperitoneal delivery of
recombinant leptin reduces food intake and weight gain (4, 8). However,
these studies did not address the ability of leptin therapy to reverse
the metabolic abnormalities associated with high fat feeding. Human
trials have reported weight loss in response to delivery of leptin by
injection (7), but the losses have generally been minor compared with
the weight of those individuals before the initiation of leptin
therapy. Leptin injection therapy may be complicated by the short
half-life of the peptide in the circulation (16, 31). Thus a more
effective therapy may require a sustained, steady-state increase in
leptin levels. Indeed, a recent study has demonstrated the greater
effectiveness of sustained leptin increases over intraperitoneal leptin
delivery in decreasing food intake and weight gain in the
ob/ob mouse (16).
We have demonstrated previously that sustained, moderate
hyperleptinemia can be achieved in normal rats by use of a recombinant adenovirus gene delivery system (5, 19). In the present study we have
used this system to induce hyperleptinemia in the HF rat and tested the
effects of this gene-therapeutic intervention on the metabolic
abnormalities associated with obesity. The results demonstrate that
hyperglycemia, hyperinsulinemia, and skeletal muscle insulin resistance
are corrected by a sustained, moderate increase in plasma leptin. The
reversal of skeletal muscle insulin resistance is strongly correlated
to a decrease in muscle triglyceride levels.
 |
MATERIALS AND METHODS |
Animal care and maintenance.
Male Wistar rats were purchased from Charles River, at a weight of
150-175 g. After arrival, rats were caged individually with free
access to water and either a high-fat diet (HF diet, 45% of calories
from fat, Harlan Teklad, Madison WI., TD 96001) or standard rat chow
diet (SC diet, 11% of calories from fat, Harlan Teklad), except where
indicated below, and food intake and body weight were recorded on a
daily basis. Animals were held on a 12:12-h light-dark cycle.
Experimental design.
After 6 wk of a HF or SC diet, animals were anesthetized with
pentobarbital sodium (50-100 mg/100 g body wt ip) between 9 and 10 AM. Samples of the medial part of the right gastrocnemius muscle were
rapidly excised and frozen in liquid nitrogen and used to gain a
representative measure of hindlimb muscle triglyceride levels. Blood
samples were collected into EDTA-rinsed vials for analysis of plasma
variables. Solei muscles were excised, and basal and insulin-stimulated
2-deoxyglucose uptake was measured as described below. Mesenteric,
perirenal, and epididymal fat pads were excised, patted dry of excess
fluid, and weighed. Additional groups of 6-wk HF- or SC-fed animals
received recombinant adenovirus containing either the rat leptin cDNA
(AdCMV-leptin) (5) or the Escherichia coli
-galactosidase
gene (AdCMV-
Gal) (10). Where indicated, food intake in control
groups was matched to animals receiving AdCMV-leptin. HF-fed or SC-fed
animals were infused with 1 × 1012 viral particles in
200-250 µl PBS into a tail vein via a 22-gauge catheter as
previously described (28). To reduce immunologic responses to the
infused viruses, animals were treated with 15 mg · kg
1 · day
1
cyclosporin A (Calbiochem, La Jolla, CA) intraperitoneally one day
before the virus infusion, on the infusion day, and on the following
day. Rats were also treated with 1.5 mg · kg
1 · day
1
methylprednisone (Pharmacia Upjohn) intramuscularly on the same days.
Four to five hours before the virus infusion, an extra injection of 3 mg/kg prednisone was given intramuscularly. No immunosuppression occurred for the five remaining days before an experiment. Six days
after administration of the recombinant adenoviruses, blood samples
were taken, solei and right medial gastrocnemius muscles were excised
to measure 2-deoxyglucose uptake and triglyceride levels, respectively,
and fat pads were isolated.
Oral glucose tolerance tests and measurement of insulin-stimulated
skeletal muscle glucose uptake.
Oral glucose tolerance tests were begun at 9:00 AM after an overnight
fast. The rats were weighed, and a whole blood sample was taken from
the capillary bed of the tail tip. Two grams of glucose/kg body wt were
administered orally by gavage in a 40% glucose solution. Whole blood
samples were taken 30, 60, 90, and 120 min after the gavage. Glucose
values were measured in the samples with a HemoCue glucometer (HemoCue
AB, Angelholm, Sweden). 2-Deoxyglucose uptake was measured in whole rat
soleus muscles by use of a modification of previously described methods
(9, 29). Briefly, muscles were quickly excised with animals under pentobarbital anesthesia, weighed, mounted onto stainless steel clips,
and subjected to a series of incubations in modified
Krebs-Ringer-Henseleit (KRH) buffer (118.4 mM NaCl, 1.19 mM
KH2PO4, 4.76 mM KCl, 1.19 mM MgSO4,
24.9 mM NaHCO3, 1.2 mM CaCl2, 0.1% fat-free
BSA) with additions, as described below, at 29°C in a shaking
metabolic waterbath and under 95% O2-5% CO2.
The muscles were first incubated in KRH buffer containing 8 mM glucose
and 32 mM mannitol for 30 min and then in KRH buffer containing 40 mM
mannitol for 10 min. They were transferred to KRH buffer containing 39 mM [14C]mannitol (NEN, 51.5 Ci/mmol) and 1 mM
2-deoxy[3H]glucose (NEN, 25.5 Ci/mmol) for 30 min in the presence or absence of insulin at a final concentration of 1 mU/ml. The muscles were then hydrolyzed at 70°C for 1 h in 1 N
NaOH. A 200-µl aliquot was assayed for radioactivity in scintillation
cocktail, and 2-deoxyglucose uptake was calculated as nanomoles of
2-deoxyglucose per microliters intracellular water per hour.
Skeletal muscle triglycerides.
Muscle triglycerides were determined as described previously (28) with
slight modifications. Briefly, frozen muscle samples were first
powdered under liquid nitrogen. Twenty to fifty milligrams of frozen
muscle powder were then weighed into 1 ml of a chloroform-methanol mix
(2:1) and incubated for 1 h at room temperature with occasional shaking
to extract the lipid. After addition of 200 µl H2O,
vortexing, and centrifugation for 5 min at 3,000 g, the lower
lipid phase was collected and dried at room temperature. The lipid
pellet was redissolved in 60 µl tert-butanol and 40 µl of a
Triton X-114-methanol (2:1) mix, and triglycerides were measured by
means of the GPO-triglyceride kit (Sigma, St. Louis, MO) with Lintrol
lipids as standards (Sigma).
Plasma measurements.
Plasma triglycerides and free fatty acids were measured with kits from
Sigma and Boehringer Mannheim, respectively. Plasma glucose was
measured with a HemoCue glucose analyzer (HemoCue AB). Plasma leptin
and insulin were measured using rat-specific RIA kits (Linco Research,
St. Charles, MO).
Statistical methods.
All results are expressed as means of 4-10 independent experiments ± SE. Statistical significance was determined by an unpaired Student's t-test by use of the statistics module of Microsoft Excel, Version 5.0 (Microsoft, Seattle, WA). Statistical significance was assumed at P < 0.05.
 |
RESULTS |
Fasting plasma variables, fat depots, and skeletal muscle triglyceride
levels in the high fat fed rat.
After 6 wk, marked alterations in carbohydrate and lipid metabolism
were observed in HF rats compared with SC control animals (Table
1). Fasting plasma insulin, glucose, and
leptin levels were increased in HF animals by 6.5-, 1.4-, and 7.0-fold,
respectively. In addition, skeletal muscle triglycerides were increased
3.6-fold, and there were marked increases in plasma triglycerides
(2.0-fold), free fatty acids (1.9-fold), and visceral fat
mass (2.7-fold). Finally, HF rats were significantly heavier then
SC controls. Taken together, these data demonstrate that the
HF rats display a metabolic profile consistent with obesity, insulin
resistance, and a perturbed state of lipid and carbohydrate
homeostasis.
Confirmation of insulin resistance induced by high fat feeding.
To confirm diet-induced insulin resistance, SC and HF animals were
fasted overnight and then subjected to an oral glucose tolerance test
and analysis of insulin-stimulated glucose uptake in skeletal muscle
(Figs. 1 and 2). Administration of a glucose bolus to HF animals
resulted in whole blood glucose levels that were significantly greater
than those of SC animals at all time points tested (Fig.
1). In addition, at 120 min postglucose
bolus, when blood glucose had returned to baseline in the SC group,
blood glucose in the HF animals had decreased only marginally from the maximum value at 60 min postglucose bolus (Fig. 1), demonstrating severe whole body glucose intolerance and implying the presence of
insulin resistance. Because a major site of glucose disposal is
skeletal muscle, basal and insulin-stimulated glucose uptakes were
measured in the isolated soleus muscle of HF and SC animals. Basal
glucose uptake was unchanged by high fat feeding; however, insulin-stimulated glucose uptake was reduced by 40% compared with
that of SC animals (Fig. 2). These data
demonstrate insulin resistance in skeletal muscle of HF rats.

View larger version (21K):
[in this window]
[in a new window]
|
Fig. 1.
Male Wistar rats were fed a high fat (HF) or a standard chow (SC) diet
for 6 wk; n = 4 animals for each condition. At end of period
all animals were fasted overnight (18 h) and then received 2.0 g/kg
glucose by gavage. Tail vein blood samples were taken 0, 30, 60, 90, and 120 min after gavage, and blood glucose levels were measured. *,
** Significant difference between HF and SC animals at P < 0.05 and P < 0.01, respectively.
|
|

View larger version (19K):
[in this window]
[in a new window]
|
Fig. 2.
Male Wistar rats were fed HF or SC diet for 6 wk. At end of period all
animals were fasted overnight (18 h), soleus muscles were excised, and
2-deoxyglucose (2-DG) uptake was measured in absence (basal) or
presence (+ insulin) of insulin. *** Significant difference between
HF and SC groups (P < 0.001). There were 9 independent
measurements for each condition.
|
|
Leptin action in the high fat fed rat.
We next determined the effects of a sustained, moderate increase in
leptin on the obese phenotype. Recombinant adenovirus containing the
leptin cDNA was administered to both SC and HF animals, and a number of
variables of leptin action were monitored (Fig.
3 and Table 2).
A control HF group received an adenovirus expressing
-galactosidase,
and food intake was subsequently matched to food intake in
hyperleptinemic HF animals. Hyperleptinemia decreased caloric intake in
both HF and SC animals (Fig. 3A), but the decrease was
significantly greater in SC animals (Table 2). Cumulative weight loss
induced by hyperleptinemia was significant in SC, but not in HF animals
(Table 2); however, when expressed as grams lost per day, HF rats lost
significant body wt from day 5 to day 6 of
hyperleptinemia (Fig. 3B). Visceral fat mass was markedly
decreased by hyperleptinemia in HF rats (Table 2), and the loss was
substantially greater in absolute terms compared with hyperleptinemic
SC and calorically matched, AdCMV-
Gal-treated HF rats.

View larger version (16K):
[in this window]
[in a new window]
|
Fig. 3.
Male Wistar rats were fed HF or SC diet for 6 wk. At end of period
animals received a recombinant adenovirus encoding leptin cDNA (HF-Lep,
n = 9 and SC-Lep, n = 7). Food intake (A) and
weight (B) were monitored for the next 6 days. Control HF group
received a recombinant adenovirus expressing -galactosidase and were
maintained on same caloric intake as HF-Lep (HF- Gal-CM, n = 5). * Significant difference between HF-Lep and HF- Gal-CM
(P < 0.05).
|
|
View this table:
[in this window]
[in a new window]
|
Table 2.
Effects of 6 days of hyperleptinemia or caloric matching on weight,
food intake, and visceral fat mass in HF and SC animals
|
|
Effects of hyperleptinemia on plasma metabolic variables and
skeletal muscle insulin sensitivity.
We next tested the hypothesis that sustained hyperleptinemia would
correct the metabolic abnormalities and skeletal muscle insulin
resistance associated with diet-induced obesity. First, hyperleptinemia
resulted in a reduction in plasma glucose and insulin in HF animals to
levels comparable with those in SC animals (Table
3). Importantly, these effects were
specific, because HF animals that received AdCMV-
Gal and were
calorically matched to the hyperleptinemic HF rats remained
hyperglycemic and hyperinsulinemic compared with SC and hyperleptinemic
HF rats (Table 3). As previously reported (5), hyperleptinemia in SC
rats also reduced plasma glucose and insulin (Table 3), an effect that
was not observed in SC calorically matched animals (data not shown).
These data suggest that insulin sensitivity is improved by
hyperleptinemia. To evaluate this directly, we measured
insulin-stimulated glucose uptake in skeletal muscle from
hyperleptinemic HF and control animals. Skeletal muscle insulin
sensitivity was restored in hyperleptinemic HF rats to levels that were
similar to those in SC animals (Fig. 4). It
is noteworthy that treatment of HF rats with AdCMV-
Gal and
subsequent caloric matching to hyperleptinemic HF rats did not correct
skeletal muscle insulin resistance. Finally, plasma triglycerides and
intramuscular triglycerides, but not free fatty acids, were decreased
in hyperleptinemic HF rats compared with HF controls (Table 3 and Fig.
5). Caloric matching of
AdCMV-
Gal-treated HF rats to hyperleptinemic rats also reduced
plasma triglycerides and free fatty acids; however, skeletal muscle
triglycerides in calorically matched animals remained at levels similar
to those in HF animals fed ad libitum. Because insulin resistance was
not corrected in calorically matched animals, these data suggest that the leptin-mediated reduction in skeletal muscle triglyceride stores
underlies improvements in insulin action. Indeed, a comparison of
tissue triglyceride levels to skeletal muscle insulin sensitivity in
individual animals demonstrated a negative correlation of r =
0.7 (Fig. 6), lending further
support to this possibility.

View larger version (18K):
[in this window]
[in a new window]
|
Fig. 4.
Male Wistar rats were treated as detailed in legends of Figs. 2 and 3.
After overnight fast, 2-DG uptake was measured in absence (basal) or
presence (+ insulin) of insulin in soleus muscle preparations from SC
(n = 9), HF (n = 9), SC-Lep (n = 5),
hyperleptinemic HF (HF-Lep, n = 9), and HF- Gal-CM
(n = 5). *** Significant difference between indicated and
corresponding HF groups (P < 0.001).
|
|

View larger version (14K):
[in this window]
[in a new window]
|
Fig. 5.
Male Wistar rats were treated as detailed in legends of Figs. 2 and 3.
After overnight fast, skeletal muscle triglyceride levels were measured
in SC (n = 9), HF (n = 9), SC-Lep (n = 5),
HF-Lep (n = 9), and HF- Gal-CM (n = 5).
*** Significant difference between indicated group and HF group
(P < 0.001).
|
|

View larger version (21K):
[in this window]
[in a new window]
|
Fig. 6.
Relationship between skeletal muscle triglyceride levels and
insulin-stimulated 2-DG uptake in HF, SC, hyperleptinemic HF (HF-Lep),
hyperleptinemic SC (SC-Lep), and HF rats that received
adenovirus containing -galactosidase cDNA (AdCMV- Gal) and were
subsequently calorically matched to HF-HL (HF- Gal-CM).
|
|
 |
DISCUSSION |
The ability of leptin administration to reverse metabolic abnormalities
in the ob/ob mouse (17, 21, 24) and improve insulin
action in normal animals (1, 5, 26) has led to the proposal that leptin
may serve as an effective therapy for human obesity. However, a number
of questions remain unanswered in this regard. First, it is not clear
that increasing plasma leptin levels will be sufficient to correct the
metabolic abnormalities associated with obesity, principally insulin
resistance and perturbed lipid and carbohydrate metabolism. Second,
leptin therapy involving multiple injections has had mixed results both
in animal models of obesity and in human trials, and this suggests that
alternative strategies such as sustained increases in plasma leptin
should be considered. The current study addressed these questions in the HF rat, a model of obesity that displays a metabolic phenotype similar to that in human obesity.
Previous studies in the HF obese model demonstrated that leptin
administration can alter food intake and weight gain (4, 8) but did not
address the capacity of leptin to correct the obese phenotype. This is
an important question, because correction of the metabolic
abnormalities of obesity with leptin has been demonstrated only in the
ob/ob mouse, a model that lacks endogenous leptin. Thus
the current study extends previous observations of leptin action in
obesity by demonstrating in a nongenetic model that hyperglycemia,
hyperinsulinemia, and skeletal muscle insulin resistance are corrected
by a sustained, moderate increase in plasma leptin mediated by
recombinant adenovirus administration. It is important that we do not
observe a similar correction of insulin action or plasma variables in
HF animals that received a control adenovirus expressing
-galactosidase and were calorically matched to the HF
hyperleptinemic animals.
The increased skeletal muscle insulin sensitivity induced by
hyperleptinemia in HF rats is associated with decreases in muscle triglyceride levels. It is noteworthy that muscle triglycerides remained elevated and insulin resistance was maintained in HF animals
calorically matched to HF hyperleptinemic animals, whereas plasma
variables of lipid metabolism (free fatty acids and triglycerides) were
similar in the two groups. A comparison of muscle triglycieride levels
with muscle insulin-stimulated glucose uptake demonstrated a strong
negative correlation (r =
0.7). Of interest in this regard is the clustering of insulin-"sensitive" groups (SC,
hyperleptinemic SC and hyperleptinemic HF) and
insulin-"resistant" groups (HF and AdCMV-
Gal-treated,
calorically matched HF). Although these observations do not demonstrate
a mechanistic link between tissue triglycerides and insulin
sensitivity, a number of other studies have demonstrated a close
relationship between perturbed lipid metabolism and insulin resistance
(see Ref. 15 for review). Moreover, recent studies in rat and humans
(12, 18, 20, 22) have demonstrated a similar relationship between
insulin sensitivity and skeletal muscle triglyceride depots. In the
Zucker diabetic fatty rat, increases in tissue triglycerides occur in parallel with the development of the insulin resistance characteristic of this model (13), whereas insulin resistance does not develop when
pharmaceutical interventions are used that reduce or prevent muscle
triglyceride accumulation during a high fat diet or in genetic models
of obesity (14, 27). Although the mechanistic link between triglyceride
accumulation and insulin resistance remains to be defined, lipids have
been implicated in altering elements of the insulin signaling pathway
(11, 33) and are known to regulate fuel disposal (23, 25, 32).
Additional studies are required to determine the mechanisms underlying
leptin-mediated increases in muscle insulin sensitivity.
Given that a subset of obese patients will likely be amenable to leptin
therapy, much as some insulin-resistant type II diabetics respond to
insulin therapy, an important consideration will be the method of
leptin delivery. Injection therapy and more sustained increases in
plasma leptin have been proposed as treatment methods. In humans,
weight loss after injection therapy has been minor compared with the
starting weight of the subjects (7). In animal models of obesity, a
diminished (8) or absent (30) response to peripherally injected leptin
has been reported. These effects may be explained by increases in
plasma leptin levels that are transitory, thus compromising the
efficiency of leptin action. Indeed, a recent study (16) in the
ob/ob mouse demonstrated that intraperitoneally
injected leptin was undetectable in plasma only 3 h after injection,
having reached a peak value of 335 ng · ml
1 · 1 h
1 after injection. This study also
demonstrated that intraperitoneal leptin injection was less effective
at promoting decreases in food intake and weight gain compared with the
effects of a sustained increase in plasma leptin. Concern about
temporary increases in leptin levels was circumvented in the current
study by use of a gene-therapeutic intervention that resulted in a
sustained moderate increase in leptin. This strategy is clearly
effective, because a quite modest increase in leptin from 7 to 29 ng/ml
had a substantial impact on fuel homeostasis. However, additional
studies are required to determine whether therapeutic strategies that
maintain constant elevated leptin levels will be an effective therapy
in the treatment of human obesity.
This study and others (4, 6, 8, 30) have demonstrated that diet-induced
obesity is associated with elevated plasma leptin levels, but with
accelerated weight gain and normal or increased caloric intake, leading
to speculation that animals on high fat diets are leptin resistant. The
data in the present study demonstrate that, if present, leptin
resistance can be overcome by an intervention that sustains a moderate
increase in plasma leptin levels above the levels present in the HF
rat. Thus food intake and visceral adiposity were decreased
substantially by hyperleptinemia in both SC and HF rats. Indeed,
visceral fat loss in HF hyperleptinemic rats was greater in absolute
terms than in hyperleptinemic SC and HF calorically matched animals.
This suggests that leptin was more effective at mobilizing lipid stores in the HF animals. Caloric intake in HF hyperleptinemic animals was
reduced but remained ~15% above that of SC hyperleptinemic rats.
Because caloric intake before hyperleptinemia and the level of
hyperleptinemia achieved were similar in the two groups, these data may
indicate that HF rats are partially resistant to leptin effects on food
intake. Although weight loss occurred in SC hyperleptinemic rats, it
did not reach significance in the HF hyperleptinemic animals. This may
be due to the higher caloric intake, the greater energy reserves in the
form of adipose tissue, or a partial resistance to weight loss in HF
hyperleptinemic animals. Given that leptin resistance has been proposed
to play a role in the pathogenesis of obesity, it is important that the
nature and extent of leptin resistance in the HF rat be defined.
 |
ACKNOWLEDGEMENTS |
These studies were supported by National Institutes of
Health Grant P50H2598801 (to C. B. Newgard) and Novo Nordisk.
 |
FOOTNOTES |
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. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: R. M. O'Doherty, Univ. of Pittsburgh Medical Center, E1112 Biomedical
Science Tower, Pittsburgh, PA 15261 (E-mail: odohertyr{at}msx.dept-med.pitt.edu).
Received 11 March 1999; accepted in final form 13 October 1999.
 |
REFERENCES |
1.
Barzilai, N.,
J. Wang,
D. Massilon,
P. Vuguin,
M. Hawkins,
and
L. Rossetti.
Leptin selectively decreases visceral adiposity and enhances insulin action.
J. Clin. Invest.
100:
3105-3110,
1997[Abstract/Free Full Text].
2.
Bjorntorp, P.,
and
B. N. Brodoff
(Editors).
Obesity. Philadelphia: Lippincott, 1992.
3.
Bray, G. (Editor). Obesity. Endocrinol. Metab. Clin.
North Am. 25, 1996.
4.
Campfield, L. A.,
F. J. Smith,
Y. Guisez,
R. Devos,
and
P. Burn.
Recombinant mouse OB protein: evidence for a peripheral signal linking adiposity and central neural networks.
Science
269:
546-549,
1995[ISI][Medline].
5.
Chen, G.,
K. Koyama,
X. Yuan,
Y. Lee,
Y. T. Zhou,
R. M. O'Doherty,
C. B. Newgard,
and
R. H. Unger.
Disappearance of body fat in normal rats induced by adenovirus-mediated leptin gene therapy.
Proc. Natl. Acad. Sci. USA
93:
14795-14799,
1996[Abstract/Free Full Text].
6.
Frederich, R. C.,
A. Hamann,
S. Anderson,
B. Lollman,
B. B. Lowell,
and
J. S. Flier.
Leptin levels reflect body lipid content in mice: evidence for diet-induced resistance to leptin action.
Nature Med.
1:
1311-1314,
1995[ISI][Medline].
7.
Friedman, J. M.,
and
J. L. Halaas.
Leptin and the regulation of body weight in mammals.
Nature
395:
763-770,
1998[ISI][Medline].
8.
Halaas, J. L.,
C. Boozer,
J. Blair-West,
N. Fidahusein,
D. A. Denton,
and
J. M. Friedman.
Physiological response to long-term peripheral and central leptin infusion in lean and obese mice.
Proc. Natl. Acad. Sci. USA
94:
8878-8883,
1997[Abstract/Free Full Text].
9.
Henriksen, E. J.,
and
M. E. Tischler.
Glucose uptake in rat soleus: effect of acute unloading and subsequent reloading.
J. Appl. Physiol.
64:
1428-1432,
1988[Abstract/Free Full Text].
10.
Herz, J.,
and
R. D. Gerard.
Adenovirus-mediated transfer of low density lipoprotein lipase receptor gene acutely accelerates cholesterol clearance in normal mice.
Proc. Natl. Acad. Sci. USA
90:
2812-2816,
1993[Abstract].
11.
Heydrick, S. J.,
D. Jullien,
N. Gautier,
J. F. Tanti,
S. Giorgetti,
E. van Obberghen,
and
Y. Le Marchand-Brustel.
Defect in skeletal muscle phosphatidylinositol-3-kinase in obese insulin resistant mice.
J. Clin. Invest.
91:
1358-1366,
1993[ISI][Medline].
12.
Koyama, K.,
G. Chen,
Y. Lee,
and
R. H. Unger.
Tissue triglycerides, insulin resistance, and insulin production: implications for hyperinsulinaemia of obesity.
Am. J. Physiol. Endocrinol. Metab.
273:
E708-E713,
1997[ISI][Medline].
13.
Lee, Y.,
M. Hirose,
J. H. Ohneda,
J. H. Johnson,
J. D. McGarry,
and
R. H. Unger.
-Cell lipotoxicity in the pathogenesis of non-insulin-dependent diabetes mellitus of obese rats: impairment of adipocyte-
-cell relationships.
Proc. Natl. Acad. Sci. USA
91:
10878-10882,
1994[Abstract/Free Full Text].
14.
Matsui, H.,
K. Okumura,
K. Kawakami,
M. Hibnio,
Y. Toki,
and
T. Ito.
Improved insulin sensitivity by bezafibrate in rats. Relationship to fatty acid composition of skeletal muscle triglycerides.
Diabetes
46:
348-353,
1997[Abstract].
15.
McGarry, J. D.
Disordered metabolism in diabetes: have we underemphasized the fat cell component?
J. Cell. Biochem.
555:
29-38,
1994.
16.
Morsy, M. A.,
M. C. Gu,
J. Z. Zhao,
D. J. Holder,
I. T. Rogers,
W. J. Pouch,
S. L. Motzel,
H. J. Klein,
S. K. Gupta,
X. Liang,
M. R. Tota,
C. I. Rosenblum,
and
C. T. Caskey.
Leptin gene therapy and daily protein administration: a comparative study in the ob/ob mouse.
Gene Ther.
5:
8-18,
1997[ISI].
17.
Murphy, J. E.,
Z. Shangzen,
K. Giese,
L. T. Williams,
J. A. Escobedo,
and
J. Dwarki.
Long-term correction of obesity and diabetes in genetically obese mice by a single intramuscular injection of recombinant adeno-associated virus encoding mouse leptin.
Proc. Natl. Acad. Sci. USA
94:
13921-13926,
1997[Abstract/Free Full Text].
18.
Oakes, N. D.,
G. J. Cooney,
S. Camilleri,
D. J. Chisholm,
and
E. W. Kraegen.
Mechanisms of liver and muscle insulin resistance induced by chronic high-fat feeding.
Diabetes
46:
1768-1774,
1997[Abstract].
19.
O'Doherty, R. M.,
P. R. Anderson,
A. Z. Zhao,
K. E. Bornfeldt,
and
C. B. Newgard.
Sparing effect of leptin on liver glycogen stores during the fed-to-fasted transition.
Am. J. Physiol. Endocrinol. Metab.
277:
E544-E550,
1999[Abstract/Free Full Text].
20.
Pan, D. A.,
A. D. Lillioja,
M. R. Kriketos,
L. A. Baur,
A. B. Bogardus,
A. B. Jenkins,
and
L. H. Storlein.
Skeletal muscle triglyceride levels are inversely related to insulin action.
J. Clin. Invest.
46:
983-987,
1997.
21.
Pelleymounter, M. A.,
M. J. Cullen,
M. B. Baker,
R. Hecht,
D. Winters,
T. Boone,
and
F. Collins.
Effects of the obese gene product on body weight regulation in ob/ob mice.
Science
269:
540-543,
1995[ISI][Medline].
22.
Phillips, D. I. W.,
S. Caddy,
V. Ilic,
K. N. Fielding,
A. C. Borthwick,
and
R. Taylor.
Intramuscular triglyceride and muscle insulin sensitivity: evidence for a relationship in nondiabetic subjects.
Metabolism
45:
947-950,
1996[ISI][Medline].
23.
Randle, P. J., P. B. Garland, C. N. Hales, and E. A. Newsholme. The glucose
fatty acid cycle: its role in insulin sensitivity and the metabolic
disturbances of diabetes mellitus. Lancet i: 785-794,
1963.
24.
Schwartz, M. W.,
D. G. Baskin,
T. R. Bukoski,
J. L. Kuijber,
D. Foster,
G. Lasser,
D. E. Prunkard,
D. Porte, Jr.,
S. C. Woods,
R. J. Seely,
and
D. S. Weigle.
Specificity of leptin action on elevated blood glucose levels and hypothalamic neuropeptide Y gene expression in ob/ob mice.
Diabetes
45:
531-535,
1996[Abstract].
25.
Shrago, E.,
G. Woldegiorgis,
A. E. Ruoho,
and
C. C. DiRusso.
Fatty acyl-CoA esters as regulators of cell metabolism.
Prostaglandins Leukotrienes Essent. Fatty Acids
52:
163-166,
1995[ISI][Medline].
26.
Sivitz, W. I.,
S. A. Walsh,
D. A. Morgan,
M. J. Thomas,
and
W. G. Haynes.
Effects of leptin on insulin sensitivity in normal rats.
Endocrinology
138:
3395-3401,
1997[Abstract/Free Full Text].
27.
Storlein, L. H.,
N. D. Oakes,
D. A. Pan,
M. Kusunoki,
and
A. B. Jenkins.
Syndromes of insulin resistance in the rat. Inducement by diet and amelioration with benfluorex.
Diabetes
42:
457-462,
1993[Abstract].
28.
Trinh, K.,
R. M. O'Doherty,
P. R. Anderson,
A. J. Lange,
and
C. B. Newgard.
Perturbation of fuel homeostasis caused by overexpression of the glucose-6-phosphatase catalytic subunit in liver of normal rats.
J. Biol. Chem.
273:
31615-31620,
1998[Abstract/Free Full Text].
29.
Turinsky, J.,
W. Nagel,
J. S. Elmendorf,
A. Damrau-Abney,
and
T. Smith.
Sphingomyelinase stimulates 2-deoxyglucose uptake by skeletal muscle.
Biochem. J.
313:
215-222,
1996[ISI][Medline].
30.
Van Heek, M.,
D. S. Compton,
C. F. France,
R. P. Tedesco,
A. B. Fawzi,
M. P. Graziano,
E. J. Sybertz,
C. D. Strader,
and
H. R. Davis, Jr.
Diet-induced obese mice develop peripheral, but not central, resistance to leptin.
J. Clin. Invest.
99:
385-390,
1997[Abstract/Free Full Text].
31.
Van Heek, M.,
D. E. Mullins,
M. A. Wirth,
M. P. Graziano,
A. B. Fawzi,
D. S. Compton,
C. F. France,
L. M. Hoos,
R. L. Casale,
E. J. Sybertz,
C. D. Strader,
and
H. R. Davis, Jr.
The relationship of tissue localization, distribution and turnover to feeding after intraperitoneal 125I-leptin administration to ob/ob and db/db mice.
Horm. Metab. Res.
28:
653-658,
1996[ISI][Medline].
32.
Wititsuwannakul, D.,
and
K. Kim.
Mechanism of palmitoyl coenzyme A inhibition of liver glycogen synthase.
J. Biol. Chem.
252:
7812-7817,
1977[Abstract].
33.
Zierath, J. R.,
K. L. Houseknecht,
L. Gnudi,
and
B. B. Kahn.
High fat feeding impairs insulin-stimulated GLUT4 recruitment via an early insulin-signaling defect.
Diabetes
46:
215-223,
1997[Abstract].
Am J Physiol Endocrinol Metab 278(3):E563-E569
0193-1849/00 $5.00
Copyright © 2000 the American Physiological Society