| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
ARTICLES |
) and PPAR-
Messenger Ribonucleic Acid Expression in the Liver in Murine Obesity: Troglitazone Induces Expression of PPAR--Responsive Adipose Tissue-Specific Genes in the Liver of Obese Diabetic Mice1
Departments of Medicine and Psychiatry (L.H.T., K.N.), University of California, San Francisco, California 94143; and Metabolism Section, Medical Service, Department of Veterans Affairs Medical Center, San Francisco, California 94121
Address all correspondence and requests for reprints to: Riaz A. Memon, Ph.D., Department of Veterans Affairs Medical Center, Metabolism Section 111F, 4150 Clement Street, San Francisco, California 94121. E-mail: rmemon{at}itsa.ucsf.edu
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
, 
, and 
, have distinct tissue
distribution patterns. PPAR- is predominantly present in the liver,
and PPAR- in adipose tissue, whereas PPAR- is ubiquitously
expressed. A recent study reported increased PPAR- messenger RNA
(mRNA) expression in the liver in ob/ob mice; however,
it is not known whether increased PPAR- expression in the liver has
any functional consequences. The expression of PPAR- and - in the
liver in obesity has not been determined. We have now examined the mRNA
levels of PPAR-, -, and - in three murine models of obesity,
namely, ob/ob (leptin-deficient), db/db
(leptin-receptor deficient), and serotonin 5-HT2c receptor (5-HT2cR)
mutant mice. 5-HT2cR mutant mice develop a late-onset obesity that is
associated with higher plasma leptin levels. Our results show that
PPAR- mRNA levels in the liver are increased by 2- to 3-fold in all
three obese models, whereas hepatic PPAR- mRNA levels are increased
by 7- to 9-fold in ob/ob and db/db mice
and by 2-fold in obese 5-HT2cR mutant mice. PPAR- mRNA expression is
not altered in ob/ob or db/db mice. To
determine whether increased PPAR- expression in the liver has any
functional consequences, we examined the effect of
troglitazone treatment on the hepatic mRNA levels of
several PPAR--responsive adipose tissue-specific genes that have
either no detectable or very low basal expression in the liver. The
treatment of lean control mice with troglitazone
significantly increased the expression of adipocyte fatty acid-binding
protein (aP2) and fatty acid translocase (FAT/CD36) in the liver. This
troglitazone-induced increase in the expression of aP2 and
FAT/CD36 was markedly enhanced in the liver in ob/ob
mice. Troglitazone also induced a pronounced increase in
the expression of uncoupling protein-2 in the liver in
ob/ob mice. In contrast to the liver,
troglitazone did not increase the expression of aP2,
FAT/CD36, and uncoupling protein-2 in adipose tissue in lean or
ob/ob mice. Taken together, our results suggest that the
effects of PPAR- activators on lipid metabolism and energy
homeostasis in obesity and type 2 diabetes may be partly mediated
through their effects on PPAR- in the liver. |
Introduction |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
, , and , that have
distinct tissue distribution patterns. PPAR- is mainly present in
liver, heart, and kidney (3). PPAR- is ubiquitously
expressed, whereas PPAR- is predominantly expressed in adipose
tissue and to a lesser extent in spleen, cells of the hemopoietic
system, liver, and skeletal muscle (3, 4, 5). PPARs form
heterodimers with the retinoid X receptor and bind to specific PPAR
response elements in the promoter region of their target genes
(1, 2). Several endogenous ligands and drugs have been
identified that bind to PPARs and activate gene transcription. Fatty
acids such as arachidonic acid and its analog,
5,8,11,14-eicosatetraynoic acid; hypolipidemic drugs such as
fibrates and Wy-14,463; and leukotriene B4 preferentially bind to
PPAR- (1, 2). On the other hand, PPAR- is activated
by polyunsaturated fatty acids, prostaglandin J2, and the
thiazolidinedione group of drugs that includes
troglitazone, rosiglitazone, and pioglitazone
(1, 2). Thiazolidinediones are insulin-sensitizing
agents (6) that have recently been approved for the
treatment of type 2 diabetes.
PPARs play an important role in the regulation of genes involved in
lipid utilization and storage, lipoprotein metabolism, adipocyte
differentiation, and insulin action (7, 8, 9, 10). Specifically,
PPAR- regulates the expression of genes that encode for enzymes
involved in peroxisomal proliferation and fatty acid oxidation in
peroxisomes and mitochondria (7). Some of the these
PPAR--responsive genes include acyl-coenzyme A (acyl-CoA)
synthetase, acyl-CoA oxidase, acyl-CoA dehydrogenase, and carnitine
palmitoyltransferase I. Additionally, genes coding for apoliopoproteins
AI, AII, and CIII are also altered by PPAR- (8)
suggesting a role for PPAR- in the regulation of lipoprotein
metabolism.
In contrast to PPAR-, PPAR- plays an important role in the
regulation of genes involved in adipocyte differentiation, lipid
storage, and glucose metabolism. It has been shown that PPAR-
regulates the transcription of adipocyte fatty acid-binding protein
(aP2), lipoprotein lipase, and phosphoenolpyruvate carboxykinase in
adipose tissue (9, 10). Additionally, PPAR-
activators up-regulate the expression of acyl-CoA synthetase, fatty
acid transporters, and uncoupling protein-2 (UCP2), and down-regulate
the expression of leptin and tumor necrosis factor- in adipocytes
(9, 10).
Obesity and type 2 diabetes in both humans and animal models are
associated with several abnormalities of lipid metabolism and insulin
resistance (11). Despite the significant role of PPARs in
the regulation of lipid metabolism and insulin action, limited data are
available on the expression and regulation of PPAR- and/or PPAR-
in obesity and diabetes. PPAR- messenger RNA (mRNA) levels are
increased in streptozotocin-induced diabetic rat liver
(12), whereas PPAR- expression is reduced in
streptozotocin-induced diabetic mouse adipose tissue (13).
The mRNA expression of PPAR- is not altered in adipose tissue in
leptin-deficient ob/ob or gold thioglucose-treated obese
mice (13). Similarly, studies in humans have found no
difference in the mRNA levels of PPAR-, -, or - in adipose
tissue among lean controls or obese or type 2 diabetic subjects
(14).
The liver plays a pivotal role in the regulation of fatty acid and
lipoprotein metabolism. Although PPAR- is abundantly expressed in
liver, basal expression of PPAR- in the liver is very low
(3, 4, 5). A recent study reported increased expression of
PPAR- mRNA in the liver in ob/ob mice (15).
However, it is not known whether other rodent models of obesity display
similar changes or whether increased PPAR- expression in the liver
in ob/ob mice has any functional consequences. The
expression of PPAR- and PPAR- in the liver in rodent models of
obesity has not been reported. In the present study we examined the
mRNA expression of PPAR-, -, and - in the liver in three
murine models of obesity, namely, ob/ob (leptin-deficient),
db/db (leptin-receptor deficient), and serotonin 5-HT2c
receptor (5-HT2cR) mutant mice. 5-HT2cR mutant mice develop a
late-onset obesity that is associated with higher plasma leptin levels
(16). To determine whether increased hepatic PPAR- mRNA
expression has any functional consequences, we compared the effects of
troglitazone (a PPAR- activator) treatment on the
expression of several PPAR--responsive genes in liver and adipose
tissue of lean control and ob/ob mice.
|
Materials and Methods |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
-32P]Deoxy-CTP (3000 Ci/mmol, 10
mCi/ml) was purchased from NEN Life Science Products
(Boston, MA). The multiprime DNA labeling system was purchased from
Amersham International (Aylesbury, UK); minispin G-50
columns were obtained from Worthington Biochemical Corp.
(Freehold, NJ); Oligo(deoxythymidine) cellulose type 77F was purchased
from Pharmacia LKB Biotechnology AB (Uppsala, Sweden);
nitrocellulose and Nytran were obtained from Schleicher & Schuell, Inc. (Keene, OH). The complementary DNA (cDNA) clones
for PPAR-, -, and - were provided by Dr. Nathan M. Bass,
University of California (San Francisco, CA). Kodak XAR5
film (Eastman Kodak Co., Rochester, NY) was used for
autoradiography.
Animals
Male ob/ob (strain C57BL/6J ob/ob),
db/db (strain C57BL/KsJ db/db), and their
respective age-matched lean littermates were purchased from The Jackson Laboratory (Bar Harbor, ME). Mice were maintained in a
normal light cycle room (light from 0600–1800 h; dark from 1800–0600
h) at controlled temperature conditions and were provided with standard
rodent chow (Simonsen Laboratories, Gilroy, CA) and water ad
libitum. Mice were allowed to acclimatize for at least 1 week
before study. To examine the effect of PPAR- activation, control and
ob/ob mice were fed powdered chow or powdered chow
containing troglitazone (Parke-Davis, Ann
Arbor, MI) at a concentration of 200 mg/100 g chow for 10 days. The
food consumption of mice was monitored daily, and the chow was
replaced. Food consumption of both groups of mice remained constant
throughout the experiment. At the end of all experiments mice were
anesthetized with halothane, then tissues were obtained, frozen in
liquid nitrogen, and stored at -70 C until further analysis. All
animal experimentations were carried out in accordance with the NIH
Guidelines for the Care and Use of Laboratory Animals and were approved
by an institutional review board.
Generation of obese mice lacking serotonin 5-HT2c receptors
5-HT2cR mutant mice were generated by introducing a nonsense
mutation into exon 5 of the cognate gene (17), thereby
placing a stop codon within the fifth putative
trans-membrane segment of the receptor and eliminating the
carboxyl-terminal half of the protein. To reduce genetic heterogeneity,
the 5-HT2cR mutant mice were backcrossed for 13–14 generations to a
C57BL/6 background. Genotyping was performed by PCR analysis of DNA
from tail tissue. The growth curves of the resultant mutant mice
demonstrated the development of obesity of late onset, with body
weights of mutant mice diverging from wild-type mice after about 20
weeks of age despite the fact that both young and old mutant mice were
hyperphagic (16). Both 3-month-old (hyperphagic but not
obese) and 9-month-old (hyperphagic and obese) mutant mice were used
for measuring PPAR- and - mRNA levels in the liver.
Isolation of RNA and Northern blotting
Total RNA was isolated by a variation of the guanidinium
thiocyanate method (18) as described previously
(19). Total RNA from adipose tissue was used for Northern
blotting, whereas polyadenylated [poly(A)+] RNA
from liver was isolated using oligo(deoxythymidine) cellulose. Total or
poly(A)+ RNA was quantified by measuring the
absorption at 260 nm. Equal amounts of total (20 µg/lane) or
poly(A)+ RNA (10 µg/lane) were loaded on 1%
agarose-formaldehyde gels and electrophoresed. The uniformity of sample
application was checked by UV visualization of the acridine
orange-stained gels before transfer to Nytran membranes. The cDNA probe
hybridization was performed as described previously (19).
The blots were exposed to x-ray films at -70 C for various durations
(indicated in the figure legends) to ensure that measurements were made
on the linear portion of curve, and the bands were quantified by
densitometry. The blots were also probed for cyclophilin as a
housekeeping gene, and the densitometric values were normalized
relative to cyclophilin.
Serum chemistry and total hepatic lipids
Serum glucose, triglycerides, cholesterol, FFA, alanine
aminotransferase (ALT), and aspartate aminotransferase (AST) levels
were measured using standard enzyme assays kits. The total lipid
contents in the liver in control and ob/ob mice treated with
troglitazone were measured by the Bligh-Dyer technique
(20).
Statistics
The results are presented as the mean ± SEM.
Statistical significance between two groups was determined using
Student’s t test. Comparisons among several groups were
performed by ANOVA, and statistical significance was calculated by
using Bonferroni’s multiple comparison test.
|
Results |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
). PPAR- mRNA was abundantly
expressed in the liver in control animals and was increased by 2.3-fold
in ob/ob and by 2.2-fold in db/db mice (Fig. 1A).
On the other hand, the basal expression of PPAR- was relatively low
in control animals, but PPAR- mRNA levels were increased by 7.9-fold
in ob/ob and by 9.4-fold in db/db mice compared
with those in their lean littermates (Fig. 1B). PPAR- was also
abundantly present in the liver in control animals; however, PPAR-
mRNA levels in ob/ob or db/db mice were not
significantly different from those in controls (Fig. 1C). As reported
by others (13), PPAR- mRNA levels in adipose tissue
were not significantly different between control and ob/ob
or db/db mice (data not shown).
|
and PPAR- mRNA levels in young (3
months) and old (9 months) 5-HT2cR mutant mice to those in their
wild-type littermates. 5-HT2cR mutant mice develop late-onset obesity
(16). Although both young and old 5-HT2cR mutant mice are
hyperphagic, only old (9 months) mice are modestly obese (40%
overweight compared with genetically identical, wild-type littermates).
The data presented in Fig. 2A demonstrate
that PPAR- mRNA levels in wild-type and young 5-HT2cR mutant mice
are the same; however, there is a 2.1-fold increase in PPAR- mRNA
levels in the old obese 5-HT2cR mutant mice compared with those in the
age-matched wild-type littermates. Similarly, PPAR- mRNA levels were
comparable in young control and 5-HT2cR mutant mice, whereas there was
a 2.2-fold increase in PPAR- mRNA expression in the liver in old
obese 5-HT2cR mutant mice (Fig. 2B). These data suggest that the
increase in both PPAR- and PPAR- mRNA expression is related to
the development of obesity in this model.
|
and - mRNA expression in the liver, we next
examined PPAR- and - mRNA levels in the liver of ob/ob
mice at the age of 5 and 10 weeks. The ob/ob mice were only
modestly obese (body weight 40% over that in age-matched controls) at
the age of 5 weeks, whereas they weighed almost 150% over age-matched
controls at the age of 10 weeks. The data presented in Fig. 3A demonstrate that PPAR- mRNA levels
in the liver were comparable between control and ob/ob mice
at the age of 5 weeks and were increased by 2.8-fold in
ob/ob mice at the age of 10 weeks. On the other hand,
PPAR- mRNA levels in the liver were increased by 7.7- and 9.3-fold
in 5- and 10-week-old ob/ob mice, respectively (Fig. 3B).
These data suggest that the increase in PPAR- mRNA expression in the
liver occurs earlier than that in PPAR- during the course of
obesity.
|
-responsive genes in
liver and adipose tissue of ob/ob mice and are known to
activate the transcription of genes that contain PPAR response elements
(6). To determine whether increased expression of PPAR-
in the liver in ob/ob mice has any functional consequences,
we examined the effect of troglitazone treatment (200
mg/100 g chow for 10 days) on several PPAR--responsive genes,
including aP2, FAT/CD36, and UCP2 mRNA levels in the liver. Like
PPAR-, all of these proteins are predominantly expressed in adipose
tissue and have either no detectable or very low basal expression in
the liver of control animals. We and others have recently shown that
FAT/CD36 and UCP2 are overexpressed in the liver of ob/ob
mice (19, 21, 22). We hypothesized that if PPAR- in the
liver of ob/ob mice is functionally active, then
troglitazone treatment should induce mRNA expression of
aP2, FAT/CD36, and UCP2 in the liver of ob/ob mice. The data
presented in Table 1 demonstrate that, as
expected, 10 days of troglitazone treatment significantly
lowered serum glucose, triglyceride, and FFA levels in the
ob/ob mice. As clinical studies have shown that
troglitazone may produce liver damage in a small
percentage of patients undergoing therapy (23), we also
measured serum transaminase levels in these animals. Both serum ALT and
AST levels were significantly higher in ob/ob mice (Table 1); however, troglitazone had no significant effect on
serum ALT and AST levels (Table 1) in lean or obese mice.
|
presents Northern blots showing
the expression of aP2, FAT/CD36, and UCP2 in the liver in control and
ob/ob mice treated with troglitazone, and the
densitometric analysis of the each Northern blot is presented in fig. 4, B–D. Because the basal expression of these genes is very low in
liver, the fold increase observed here should only be considered a
rough estimate of induction. Northern blots showing the expression of
PPAR- (the target receptor for troglitazone) and
cyclophilin (a housekeeping gene used as an internal control) are also
presented in Fig. 4A and demonstrate that PPAR- and cyclophilin mRNA
levels are not altered by troglitazone treatment in
control and ob/ob mice. As shown previously (19, 21, 22), there was a marked increase in FAT/CD36 (32-fold) and UCP2
(3.5-fold) expression in the liver in ob/ob mice, whereas
there was a slight increase in aP2 expression in the liver in
ob/ob mice. In lean controls, troglitazone
significantly increased aP2 (6-fold) and FAT/CD36 (8-fold) mRNA
expression in the liver, but had no effect on hepatic UCP2 expression.
Compared with lean mice, troglitazone produced a marked
increase in aP2 (170-fold), FAT/CD36 (62-fold), and UCP2 (7-fold) mRNA
levels in the liver of ob/ob mice. Compared with untreated
ob/ob mice, the troglitazone-induced increase
was 2-fold for FAT/CD36 and UCP2 and 57-fold for aP2 expression in the
liver of ob/ob mice. These data suggest that activation of
hepatic PPAR- by troglitazone induces the transcription
of PPAR--responsive genes in the liver.
|
-responsive
genes in the liver is selective and is not due to adipocyte
contamination of the ob/ob liver.
We also examined the effect of troglitazone treatment on
PPAR--responsive genes, including acyl-CoA oxidase (ACO) and
carnitine palmitoyltransferase (CPT I). These genes
contain PPAR response elements, and it has been suggested that PPAR-
agonists may also activate PPAR-responsive genes
(15). The data presented in Fig. 5A show that the mRNA levels of ACO in
the liver are slightly higher in untreated ob/ob mice, and
are increased (5-fold) by troglitazone treatment in
ob/ob mice. On the other hand, CPT I mRNA levels were 2-fold
higher in the liver in untreated ob/ob mice and were not
further increased by troglitazone in ob/ob mice
(Fig. 5B). The mRNA levels of apolipoprotein CIII, another
PPAR--responsive gene, were neither altered in ob/ob mice
nor influenced by troglitazone treatment (data not shown),
suggesting that only some of the PPAR--responsive genes are altered
by treatment with troglitazone. The mRNA levels of
PPAR- itself were also not altered by troglitazone
treatment in lean or obese mice (data not shown).
|
). The livers from
ob/ob mice contained 59% more total lipids on a per g basis
(P < 0.001). Although troglitazone
treatment had no significant effect on total hepatic lipid content in
control mice, it produced a 19% decrease in total lipid content in the
liver of ob/ob mice (P < 0.05), suggesting
that troglitazone may enhance the utilization of lipids in
the liver of obese mice.
|
expression
is not altered in adipose tissue in obese mice. Figure 7A presents Northern blots showing the
expression of aP2, FAT/CD36, and UCP2 in adipose tissue in control and
ob/ob mice treated with troglitazone, and the
densitometric analysis of the each Northern blot is presented in Fig. 7, B–D. As shown previously (19, 22), the mRNA levels of
FAT/CD36 and UCP2 were increased in adipose tissue of untreated
ob/ob mice, whereas aP2 mRNA expression was slightly
decreased (Fig. 7A). In lean controls, troglitazone
increased aP2 mRNA levels by 40%, and a similar trend was seen in
ob/ob mice (Fig. 7B). Troglitazone
significantly decreased FAT/CD36 mRNA levels in adipose tissue of both
control and ob/ob mice (Fig. 7C). Troglitazone
slightly decreased UCP2 mRNA levels in adipose tissue of control mice,
and a similar trend was seen in ob/ob mice (Fig. 7D). Hence,
consistent with no significant change in PPAR- expression in adipose
tissue of obese mice, there was no significant difference in
troglitazone action on adipose tissue between lean and
obese mice.
|
|
Discussion |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
and obesity and reported that PPAR-
expression was not altered in the adipose tissue in either genetic
(ob/ob mice) or acquired (gold-thioglucose-induced) models
of obesity (13). Similarly, no differences were found in
PPAR- expression in adipose tissue between lean controls and Zucker
fatty rats (24). However, a recent study reported that
PPAR- expression is increased in the liver of ob/ob mice
(15).
In the present study we have extended these observations and report
that both PPAR- and PPAR- mRNA levels are markedly increased in
the liver of ob/ob (leptin-deficient) and db/db
(leptin receptor deficient) mice, whereas hepatic PPAR- mRNA
expression and adipose tissue PPAR- expression are not altered in
these models. The increase in PPAR- mRNA levels in the liver in
ob/ob and db/db mice is more pronounced compared
with the increase in PPAR- mRNA expression. Moreover, the
up-regulation of PPAR- in the liver of ob/ob mice appears
to be more sensitive to the development of obesity, as it is seen at an
earlier age (5 weeks) when these animals are only 40% overweight
compared with their lean littermates. The hepatic expression of
PPAR- is not elevated at this age in ob/ob mice.
Our results also demonstrate that both PPAR- and PPAR- mRNA
levels in the liver are increased in old (obese) 5-HT2cR mutant mice,
but are not altered in young (lean) 5-HT2cR mutant mice. The lack of
change in PPAR- and PPAR- mRNA levels in young 5-HT2cR mutant
mice and the increase in old 5-HT2cR mutant mice further support the
concept that the increased expression of PPAR- and PPAR- mRNA in
the liver is related to the development of obesity. The obesity in
5-HT2cR mutant mice is different from that in ob/ob and
db/db mice in several respects. First, 5-HT2cR mice develop
a modest obesity (30–40% increase in body weight), whereas
ob/ob and db/db mice are markedly obese
(100–150% increase in body weight). Second, obesity develops late in
5-HT2cR mice (by 8–9 months of age), whereas ob/ob and
db/db mice develop obesity as early as 4 weeks of age.
Third, young 5-HT2cR mutant mice have normal plasma leptin levels and
are sensitive to the anorectic effects of exogenous leptin, whereas old
5-HT2cR mutant mice have higher plasma leptin levels and are partially
resistant to the anorectic effect of exogenous leptin
(16). In contrast, ob/ob and db/db
mice are leptin deficient and leptin receptor deficient, respectively
(25, 26). Fourth, although obese 5-HT2cR mutant mice
develop glucose intolerance and insulin resistance, they do not develop
hyperlipidemia (16), which is commonly seen in both
ob/ob and db/db mice. Despite these differences,
a common denominator among these three murine models is that
development of obesity is accompanied by increased expression of
PPAR- and PPAR- in the liver.
In the present study we also demonstrate that up-regulation of hepatic
PPAR- allows the activation of several PPAR--responsive genes
(aP2, FAT/CD36, and UCP2) in the liver by troglitazone.
Like PPAR- itself, these genes are predominantly expressed in
adipose tissue and have either no detectable or very low basal
expression in the liver of lean control animals. We and others have
recently reported that FAT/CD36 and UCP2 mRNA expression is
significantly increased in the liver in ob/ob mice
(19, 21, 22), whereas aP2 expression in the liver is not
significantly altered (present study) in ob/ob mice. Our
present results demonstrate that troglitazone, a synthetic
ligand for PPAR-, significantly increases mRNA levels of aP2 and
FAT/CD36 in the liver of lean control mice. Moreover,
troglitazone markedly enhances the mRNA expression of aP2,
FAT/CD36, and UCP2 in the liver of ob/ob mice. It is
possible that the increase in FAT/CD36 and UCP2 expression in the liver
of untreated ob/ob mice (19, 21, 22) is due to
activation of elevated hepatic PPAR- by endogenous ligands.
Similarly, the marked induction of aP2, FAT/CD36, and UCP2 in the
ob/ob liver by troglitazone can be attributed
to its ability to bind to hepatic PPAR-, resulting in the enhanced
transcription of these genes in the liver. These results raise the
possibility that the effects of PPAR- activators on lipid metabolism
and energy balance may partly be mediated through their effects in the
liver. Our data demonstrating that total hepatic lipid content is
significantly decreased in the liver of obese mice after
troglitazone treatment suggest that PPAR- activators
may increase the utilization of lipids in the liver in obese diabetic
mice, and this may be an additional mechanism by which PPAR-
activators decrease serum lipid levels. Our hypothesis is further
supported by the studies by Burant et al. (27),
who demonstrated that troglitazone improved insulin
sensitivity and lowered serum lipid levels in a transgenic mouse model
in which white and brown adipose tissues were ablated through targeted
expression of diphtheria toxin A chain under the control of the
adipocyte-specific aP2 promoter.
Because PPAR- and PPAR- recognize similar DNA response elements,
Edvardsson et al. proposed that thiazolidinediones may
activate PPAR--responsive genes in the liver of obese mice
(15). This hypothesis was based on semiquantitiative data
obtained from analysis of two-dimensional gels of proteins from livers
of ob/ob mice treated with agonists of PPAR- (WY14,643)
and PPAR- (BRL 49653). Their results showed that spots
with the mobilities of ACO, peroxisomal bifunctional enzyme, and
3-ketoacyl thiolase were up-regulated by WY14,643 and BRL
49653 (15). Our results demonstrate that
troglitazone, a well characterized PPAR- agonist,
increases the mRNA expression of ACO, but has no effect on CPT I or
apolipoprotein CIII mRNA levels. It is important to note that ACO is
involved in peroxisomal oxidation of fatty acids, whereas CPT I is the
major regulatory protein in mitochondrial fatty acid oxidation
(7). On the other hand, apolipoprotein CIII is a potent
inhibitor of lipoprotein lipase (8). All of these genes
contain PPAR response elements and are activated by PPAR- agonists
(1, 2, 7, 8). The ability of troglitazone to
induce some of the PPAR--responsive genes (i.e. ACO)
raises the possibility that troglitazone may be an
activator of both PPAR- and PPAR- isoforms or that activation of
PPAR- is able to interact with response elements in certain
PPAR--responsive genes. The inability of troglitazone
to induce other PPAR--responsive genes such as CPT I and
apolipoprotein CIII suggests that these proteins may require the
presence of other coactivators or may be under the dominant regulatory
control of other transcription factors.
Although others have previously shown that various thiazolidinediones
induce aP2 mRNA expression in differentiating cultured white adipocytes
(28), the results obtained in vivo have been
different. For example, BRL 49653 increased aP2 mRNA
levels in adipose tissue in rats fed a high fat diet, but not in
chow-fed rats (29). Similarly, although PPAR-
activators increased UCP2 expression in cultured white and brown fat
cells (30, 31), they had no effect on UCP2 mRNA levels in
white adipose tissue in lean or Wistar fatty rats (32, 33). Our results demonstrate that despite producing a marked
increase in aP2, FAT/CD36 and UCP2 mRNA expression in the liver,
troglitazone had no significant stimulatory effect on the
expression of these same genes in white adipose tissue from chow fed
ob/ob mice. In fact, the expression of FAT/CD36 and UCP2 was
significantly decreased, whereas mRNA levels of aP2 were not
significantly different in adipose tissue of ob/ob mice
after troglitazone treatment. The inability of
troglitazone to induce the same set of PPAR--responsive
genes that are normally abundant in adipose tissue suggests that the
stimulatory effect of troglitazone in vivo may
depend upon several factors, such as the basal expression of specific
genes as well as the local expression of PPAR-, and the presence of
endogenous ligands. It is possible that because of higher basal
expression of PPAR- in adipose tissue and abundant presence of
endogenous ligands, i.e. fatty acids, these genes are
already maximally induced, and hence, troglitazone is
unable to produce any further increase in their transcription. A
decrease in the expression of some of these genes by
troglitazone in adipose tissue can be partly explained by
the recently proposed hypothesis of Miles et al.
(34) that PPAR- activators may act as partial
agonist-antagonists in vivo and thus compete with putative
endogenous agonists.
Because the exact physiological functions of aP2, UCP2, and FAT/CD36
are not known, we can only speculate on the consequences of their
up-regulation in the ob/ob liver by
troglitazone. aP2 is a member of family of intracellular
fatty acid-binding proteins that binds long chain fatty acids with high
affinity (35). It is possible that a
troglitazone-induced increase in aP2 in the liver in
ob/ob mice prevents the adverse effects of FFA on cells and
membranes. UCPs have been proposed to be involved in the regulation of
energy balance in part by increasing FA utilization (36).
A troglitazone-induced increase in UCP2 expression could
have both beneficial as well as harmful effects. On the one hand, UCP2
could dispose of excess energy by uncoupling mitochondrial respiration
from oxidative phosphorylation, thereby increasing energy expenditure
and using fatty acids. On the other hand, an increase in UCP2 could
deplete hepatic ATP stores and make the liver more susceptible to
injurious stimuli (21). Finally, FAT/CD36 is believed to
facilitate fatty acid transport, promote phagocytosis of apoptotic
cells, and function as a receptor for oxidized LDL (37).
Recent studies by Aitman et al. have shown that a syndrome
of spontaneous hypertension, hypertriglyceridemia, abdominal obesity,
and insulin resistance in rats is due to a defective CD36 gene
(38). Moreover, CD36 knockout mice have higher serum
triglyceride and FFA levels (39). Conversely,
overexpression of FAT/CD36 in transgenic mice reduces their serum
triglyceride and FFA levels (40). It is possible that the
troglitazone-induced increase in FAT/CD36 in the livers of
obese mice may be an additional mechanism by which
troglitazone lowers serum triglyceride levels and improves
insulin sensitivity. As both PPAR- and PPAR- are up-regulated in
the liver in rodent models of obesity, it is possible that combination
therapy with PPAR- and PPAR- agonists may have a synergistic
effect in lowering serum lipid levels and improving insulin
sensitivity.
In summary, in this study we have shown that PPAR- and PPAR- mRNA
expression is up-regulated in the liver in three distinct murine models
of obesity, whereas PPAR- expression is not altered. Moreover,
treatment with troglitazone induces the expression of
several PPAR--responsive genes, including aP2, FAT/CD36, and UCP2,
in the liver of ob/ob mice, raising the possibility that the
effects of PPAR- agonists on lipid metabolism and energy balance may
partly be mediated through their effects in the liver.
|
Footnotes |
|---|
Received May 15, 2000.
|
References |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
, -ß and - in the adult rat. Endocrinology 137:354–366[Abstract]
(PPAR ) and its
coactivators steroid receptor coactivator-1 and PPAR-binding protein
PBP in the brown fat, urinary bladder, colon and breast of the mouse.
Am J Pathol 153:349–354 (PPAR-). J Biol Chem 273:5678–5684 deficient
mice. J Biol Chem 272:27307–27312: adipogenic regulator
and thiazolidinedione receptor. Diabetes. 47:507–514
: bigger than fat. Curr Opin Genet Dev 8:576–581[CrossRef][Medline]
in diabetic rat liver. Mol Cell Biochem 194:227–234[CrossRef][Medline]
gene expression by nutrition and obesity in rodents. J Clin Invest 97:2553–2561[Medline]
in humans: no
alterations in adipose tissue of obese and NIDDM patients. Diabetes 46:1319–1327[Abstract]
selective agonist, causes peroxisome proliferator like liver effects in
obese mice. J Lipid Res 40:1177–1184.
Biochim Biophys Acta 1484:41–50[Medline]
gene is not altered in the fatty (fa/fa)
Zucker rat. Metabolism 47:1494–1498[CrossRef][Medline]
and
aP2 in adipose tissue of high-fat fed rats. Biochem Biophys Res Commun 229:752–757[CrossRef][Medline]
and
mediate in vivo regulation of uncoupling protein (UCP-1,
UCP-2, UCP-3) gene expression. Endocrinology 139:4920–4927
deficiency. J Clin Invest 105:287–292[Medline]
This article has been cited by other articles:
 |
|
 |
|
  S. M. Rahman, I. Qadri, R. C. Janssen, and J. E. Friedman Fenofibrate and PBA prevent fatty acid-induced loss of adiponectin receptor and pAMPK in human hepatoma cells and in hepatitis C virus-induced steatosis J. Lipid Res., November 1, 2009; 50(11): 2193 - 2202. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. K. Badman, A. R. Kennedy, A. C. Adams, P. Pissios, and E. Maratos-Flier A very low carbohydrate ketogenic diet improves glucose tolerance in ob/ob mice independently of weight loss Am J Physiol Endocrinol Metab, November 1, 2009; 297(5): E1197 - E1204. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 N. H. Rogers, J. W. Perfield II, K. J. Strissel, M. S. Obin, and A. S. Greenberg Reduced Energy Expenditure and Increased Inflammation Are Early Events in the Development of Ovariectomy-Induced Obesity Endocrinology, May 1, 2009; 150(5): 2161 - 2168. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. P. Thyfault, R. S. Rector, G. M. Uptergrove, S. J. Borengasser, E. M. Morris, Y. Wei, M. J. Laye, C. F. Burant, N. R. Qi, S. E. Ridenhour, et al. Rats selectively bred for low aerobic capacity have reduced hepatic mitochondrial oxidative capacity and susceptibility to hepatic steatosis and injury J. Physiol., April 15, 2009; 587(8): 1805 - 1816. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Beysen, E. J. Murphy, H. Nagaraja, M. Decaris, T. Riiff, A. Fong, M. K. Hellerstein, and P. J. Boyle A pilot study of the effects of pioglitazone and rosiglitazone on de novo lipogenesis in type 2 diabetes J. Lipid Res., December 1, 2008; 49(12): 2657 - 2663. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B. N. Reid, G. P. Ables, O. A. Otlivanchik, G. Schoiswohl, R. Zechner, W. S. Blaner, I. J. Goldberg, R. F. Schwabe, S. C. Chua Jr., and L.-S. Huang Hepatic Overexpression of Hormone-sensitive Lipase and Adipose Triglyceride Lipase Promotes Fatty Acid Oxidation, Stimulates Direct Release of Free Fatty Acids, and Ameliorates Steatosis J. Biol. Chem., May 9, 2008; 283(19): 13087 - 13099. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Nonogaki, K. Nozue, and Y. Oka Social Isolation Affects the Development of Obesity and Type 2 Diabetes in Mice Endocrinology, October 1, 2007; 148(10): 4658 - 4666. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H.-X. Hao, C. M. Cardon, W. Swiatek, R. C. Cooksey, T. L. Smith, J. Wilde, S. Boudina, E. D. Abel, D. A. McClain, and J. Rutter PAS kinase is required for normal cellular energy balance PNAS, September 25, 2007; 104(39): 15466 - 15471. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y.-L. Zhang, A. Hernandez-Ono, P. Siri, S. Weisberg, D. Conlon, M. J. Graham, R. M. Crooke, L.-S. Huang, and H. N. Ginsberg Aberrant Hepatic Expression of PPAR{gamma}2 Stimulates Hepatic Lipogenesis in a Mouse Model of Obesity, Insulin Resistance, Dyslipidemia, and Hepatic Steatosis J. Biol. Chem., December 8, 2006; 281(49): 37603 - 37615. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. Degrace, B. Moindrot, I. Mohamed, J. Gresti, Z.-Y. Du, J.-M. Chardigny, J.-L. Sebedio, and P. Clouet Upregulation of liver VLDL receptor and FAT/CD36 expression in LDLR-/- apoB100/100 mice fed trans-10,cis-12 conjugated linoleic acid J. Lipid Res., December 1, 2006; 47(12): 2647 - 2655. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 Y. P. Turmelle, O. Shikapwashya, S. Tu, P. W. Hruz, Q. Yan, and D. A. Rudnick Rosiglitazone inhibits mouse liver regeneration FASEB J, December 1, 2006; 20(14): 2609 - 2611. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 L. Wang, J. Huang, P. Saha, R. N. Kulkarni, M. Hu, Y. Kim, K. Park, L. Chan, A. S. Rajan, I. Lee, et al. Orphan Receptor Small Heterodimer Partner Is an Important Mediator of Glucose Homeostasis Mol. Endocrinol., November 1, 2006; 20(11): 2671 - 2681. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S.-C. Hsu and C.-j. Huang Reduced Fat Mass in Rats Fed a High Oleic Acid-Rich Safflower Oil Diet Is Associated with Changes in Expression of Hepatic PPAR{alpha} and Adipose SREBP-1c-Regulated Genes J. Nutr., July 1, 2006; 136(7): 1779 - 1785. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. Qiao, P. S. MacLean, H. You, J. Schaack, and J. Shao Knocking Down Liver CCAAT/Enhancer-Binding Protein {alpha} by Adenovirus-Transduced Silent Interfering Ribonucleic Acid Improves Hepatic Gluconeogenesis and Lipid Homeostasis in db/db Mice Endocrinology, June 1, 2006; 147(6): 3060 - 3069. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Lopez-Soriano, C. Chiellini, M. Maffei, P. A. Grimaldi, and J. M. Argiles Roles of Skeletal Muscle and Peroxisome Proliferator-Activated Receptors in the Development and Treatment of Obesity Endocr. Rev., May 1, 2006; 27(3): 318 - 329. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. Patsouris, J. K. Reddy, M. Muller, and S. Kersten Peroxisome Proliferator-Activated Receptor {alpha} Mediates the Effects of High-Fat Diet on Hepatic Gene Expression Endocrinology, March 1, 2006; 147(3): 1508 - 1516. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. L. Gruen, M. R. Plummer, W. Zhang, K. A. Posey, M. Linton, S. Fazio, and A. H. Hasty Persistence of high density lipoprotein particles in obese mice lacking apolipoprotein A-I J. Lipid Res., September 1, 2005; 46(9): 2007 - 2014. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. R. Jones, C. Barrick, K.-A. Kim, J. Lindner, B. Blondeau, Y. Fujimoto, M. Shiota, R. A. Kesterson, B. B. Kahn, and M. A. Magnuson Deletion of PPAR{gamma} in adipose tissues of mice protects against high fat diet-induced obesity and insulin resistance PNAS, April 26, 2005; 102(17): 6207 - 6212. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Z. Strowski, Z. Li, D. Szalkowski, X. Shen, X.-M. Guan, S. Juttner, D. E. Moller, and B. B. Zhang Small-Molecule Insulin Mimetic Reduces Hyperglycemia and Obesity in a Nongenetic Mouse Model of Type 2 Diabetes Endocrinology, November 1, 2004; 145(11): 5259 - 5268. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Sharma, A. Pradeep, L. Wong, A. Rana, and B. Rana Peroxisome Proliferator-activated Receptor {gamma} Activation Can Regulate {beta}-Catenin Levels via a Proteasome-mediated and Adenomatous Polyposis Coli-independent Pathway J. Biol. Chem., August 20, 2004; 279(34): 35583 - 35594. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. Yajima, E. Ikeshima, M. Shiraki, T. Kanaya, D. Fujiwara, H. Odai, N. Tsuboyama-Kasaoka, O. Ezaki, S. Oikawa, and K. Kondo Isohumulones, Bitter Acids Derived from Hops, Activate Both Peroxisome Proliferator-activated Receptor {alpha} and {gamma} and Reduce Insulin Resistance J. Biol. Chem., August 6, 2004; 279(32): 33456 - 33462. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Sharma, A. Pradeep, R. G. Pestell, and B. Rana Peroxisome Proliferator-activated Receptor {gamma} Activation Modulates Cyclin D1 Transcription via {beta}-Catenin-independent and cAMP-response Element-binding Protein-dependent Pathways in Mouse Hepatocytes J. Biol. Chem., April 23, 2004; 279(17): 16927 - 16938. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Zhang, D. I. W. Phillips, C. Wang, and C. D. Byrne Human skeletal muscle PPAR{alpha} expression correlates with fat metabolism gene expression but not BMI or insulin sensitivity Am J Physiol Endocrinol Metab, February 1, 2004; 286(2): E168 - E175. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Muurling, A. M. van den Hoek, R. P. Mensink, H. Pijl, J. A. Romijn, L. M. Havekes, and P. J. Voshol Overexpression of APOC1 in obob mice leads to hepatic steatosis and severe hepatic insulin resistance J. Lipid Res., January 1, 2004; 45(1): 9 - 16. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. C. Albarado, J. McClaine, J. M. Stephens, R. L. Mynatt, J. Ye, A. W. Bannon, W. G. Richards, and A. A. Butler Impaired Coordination of Nutrient Intake and Substrate Oxidation in Melanocortin-4 Receptor Knockout Mice Endocrinology, January 1, 2004; 145(1): 243 - 252. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
O. Gavrilova, M. Haluzik, K. Matsusue, J. J. Cutson, L. Johnson, K. R. Dietz, C. J. Nicol, C. Vinson, F. J. Gonzalez, and M. L. Reitman Liver Peroxisome Proliferator-activated Receptor {gamma} Contributes to Hepatic Steatosis, Triglyceride Clearance, and Regulation of Body Fat Mass J. Biol. Chem., September 5, 2003; 278(36): 34268 - 34276. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C.-H. Lee, P. Olson, and R. M. Evans Minireview: Lipid Metabolism, Metabolic Diseases, and Peroxisome Proliferator-Activated Receptors Endocrinology, June 1, 2003; 144(6): 2201 - 2207. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Yu, K. Matsusue, P. Kashireddy, W.-Q. Cao, V. Yeldandi, A. V. Yeldandi, M. S. Rao, F. J. Gonzalez, and J. K. Reddy Adipocyte-specific Gene Expression and Adipogenic Steatosis in the Mouse Liver Due to Peroxisome Proliferator-activated Receptor gamma 1 (PPARgamma 1) Overexpression J. Biol. Chem., January 3, 2003; 278(1): 498 - 505. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. M. Watkins, P. R. Reifsnyder, H.-j. Pan, J. B. German, and E. H. Leiter Lipid metabolome-wide effects of the PPAR{gamma} agonist rosiglitazone J. Lipid Res., November 1, 2002; 43(11): 1809 - 1817. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. H. Lane, N. K. MacLennan, J. L. Hsu, S. M. Janke, and T. D. Pham Increased Hepatic Peroxisome Proliferator-Activated Receptor-{gamma} Coactivator-1 Gene Expression in a Rat Model of Intrauterine Growth Retardation and Subsequent Insulin Resistance Endocrinology, July 1, 2002; 143(7): 2486 - 2490. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Endocrinology | Endocrine Reviews | J. Clin. End. & Metab. |
| Molecular Endocrinology | Recent Prog. Horm. Res. | All Endocrine Journals |
