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Original Articles |
While Stimulating That of PPAR
in Mononuclear Cells in Obese Subjects1
Division of Endocrinology, Diabetes, and Metabolism, State University of New York at Buffalo and Kaleida Health, Buffalo, New York 14209
Address all correspondence and requests for reprints to: Paresh Dandona, M.D., Ph.D., Diabetes-Endocrinology Center of WNY, 3 Gates Circle, Buffalo, New York 14209. E-mail: pdandona{at}kaleidahealth.org
Abstract
We have recently demonstrated that troglitazone exerts an
anti-inflammatory effect in the insulin resistant obese in
vivo in parallel with its insulin-sensitizing effect. Because
these effects are thought to be mediated through peroxisome
proliferator-activated receptors
and
(PPAR
and PPAR
), we
have now examined the possibility that troglitazone may
modulate the expression of PPAR
and PPAR
. Seven obese
hyperinsulinemic subjects were administered 400 mg
troglitazone daily for 4 weeks. Fasting blood samples were
obtained before and during troglitazone therapy at 1, 2,
and 4 weeks. Fasting insulin concentrations fell at week 1 and
persisted at lower levels till 4 weeks. PPAR
expression fell
significantly at week 1 and fell further at weeks 2 and 4. In contrast,
PPAR
expression increased significantly at week 2 and further at
week 4. 9- and 13-hydroxyoctadecanoic acid, products of linoleic acid
peroxidation and agonists of PPAR
, decreased during
troglitazone therapy. We conclude that
troglitazone, an agonist for both PPAR
and PPAR
, has
significant but dramatically opposite effects on PPAR
and PPAR
.
These effects may be relevant to its insulin sensitizing and
anti-inflammatory effects.
TROGLITAZONE IS A thiazolidinedione, a class of drugs known to increase sensitivity to the action of insulin. Thiazolidinediones are thus used in type 2 diabetes to reduce insulin resistance and to lower blood glucose concentrations (1, 2). We have recently demonstrated an antioxidant effect of troglitazone with a reduction in reactive oxygen species (ROS) generation by leukocytes and a reduction in lipid peroxidation in vivo (3).
More recently, we have also demonstrated that
troglitazone may have a potent anti-inflammatory action
with a suppressive action on nuclear factor
B (NF
B) in
mononuclear cells (MNC) and a stimulatory action on I
B, the
inhibitor of NF
B (4). These
changes in anti-inflammatory effects are observed within 7 days of
starting troglitazone and parallel similar falls in
fasting insulin concentrations. This suggests that both insulin
sensitization and anti-inflammatory effects of
troglitazone probably mediated by peroxisome
proliferator-activated receptors
and
(PPAR
and PPAR
) are
rapid and occur in parallel (5, 6). We have therefore now
examined the possibility that PPAR
and PPAR
may be modulated by
troglitazone in MNC. This communication describes the
sequential effect of troglitazone on PPAR
and
PPAR
in MNC. Plasma concentrations of 9- and
13-hydroxyoctadecanoic acid (9-HODE and 13-HODE) were also measured
because these oxidatively damaged products of linoleic acid are now
known to be agonists of PPAR
(7).
Subjects, Materials, and Methods
Subjects
Seven obese subjects (age range 3252 yr, mean 40.6 ± 8.0
yr), all with body mass index (BMI) greater than 37
kg/m2, (BMI range 37.060.9, mean: 46.1 ±
8.7 kg/m2) were included in this study (Table 1
). None of the obese subjects was on
vitamin E or C or any other antioxidant therapy. The subjects were not
advised any special diet, and none of them was actively trying to lose
weight during the period of the study. The Institutional Review Board
approved the study. Written informed consent was obtained from all
subjects.
|
MNC isolation
Three and a half milliliters of the anticoagulated blood sample were carefully layered over 3.5 mL of PMN medium (Robbins Scientific Corp., Sunnyvale, CA). The sample was centrifuged at 450 x g for 30 min at 22 C. At the end of the centrifugation, two bands separate out at the top of the red blood cell pellet. The top band consists of MNC, and the bottom consists of PMN. The bands were harvested, repeatedly washed with HBSS, and reconstituted to a concentration of 4 x 105 cells/mL in HBSS. This method provides yields greater than 95% pure PMN and MNC suspensions. This was tested repeatedly to validate this method. Thereafter, random checks were made to ensure the purity of the preparations.
13-HODE and 9-HODE measurements
Hydroxy polyunsaturated fatty acids were measured by a modification of the high-performance liquid chromatography based method as previously described (3).
PPAR
and PPAR
Western blotting
MNC cell lysates were prepared by adding 1 mL boiling lysis
buffer (1% SDS, 1 mM sodium ortho-vanadate, 10
mM Tris, pH 7.4) to the MNC pellets. Total protein
concentrations were determined using BCA protein assay (Pierce Chemical Co., Rockland, IL). Sixty micrograms of total cell
lysate were electrophoresed on 8% SDS polyacrylamide gels. The
proteins were transferred to polyvinylidene difluoride membrane,
blocked for 1 h in 5% nonfat dry milk, and then incubated for
1 h with polyclonal antibodies against PPAR
or PPAR
(Affinity BioReagents, Inc., Golden, CO). Finally, the
membrane was washed and developed using super signal, chemiluminescence
reagent (Pierce Chemical Co.). Densitometry was performed
using Bio-Rad Laboratories, Inc. molecular analyst
software (Hercules, CA). These measurements were carried out at 0, 1,
2, and 4 weeks.
Plasma immunoreactive insulin, glucose
Insulin was measured from fasting plasma samples using an enzyme-linked immunosorbent assay kit (Diagnostics Systems Laboratories, Inc., Webster, TX). Glucose was measured by Hexokinase method.
Statistical analysis
Statistical analysis was performed using SigmaStat software (Jandel Scientific, San Rafael, CA). Kruskal-Wallis one-way ANOVA on ranks was used to compare the indices measured in this study.
Results
Plasma glucose, immunoreactive insulin, and lipid concentrations
Plasma glucose concentrations did not alter significantly following troglitazone (week 0: 5.28 ± 0.83 mmol/L; week 1: 5.17 ± 0.49 mmol/L; week 2: 5.20 ± 0.59 mmol/L; and week 4: 5.13 ± 0.62 mmol/L). Plasma insulin concentrations fell significantly at week 1 and fell further by week 4. Plasma insulin concentrations fell from 31.2 ± 26.9 µU/mL at baseline to 14.2 ± 10.5 µU/mL at week 1, 6.9 ± 2.8 µU/mL at week 2 and 7.3 ± 4.9 µU/mL at week 4. Serum lipids, including total cholesterol and triglycerides, did not alter significantly.
Plasma 9-HODE and 13-HODE concentrations
Plasma 9-HODE concentrations fell from 787.4 ± 52.4 to 754.8 ± 48.5 at week 2 and 720.36 ± 66.7 nmol/L at 4 weeks (P < 0.05). Plasma 13-HODE concentrations fell from 713.1 ± 44.7 to 692.1 ± 67.0 at week 2 and 675.2 ± 65.0 nmol/L at week 4 (P < 0.05).
PPAR
and PPAR
expression
PPAR
protein content fell markedly from a basal level of 100%
to 70 ± 5.4% at week 1, 21 ± 4.4% at week 2 and 38
± 14.9% at week 4. This decrease was significant at 1, 2, and 4 weeks
following troglitazone intake (P < 0.05;
Fig. 1
). PPAR
protein content
increased significantly at week 1 and continued to increase until week
4. It increased from a basal level of 100% to 148 ± 28% at week
1, 306 ± 108% at week 2 and 437 ± 167% at week 4
(P < 0.05; Fig. 2
).
|
|
Our data demonstrate for the first time that PPAR
is expressed
abundantly in MNC in the obese and that its expression falls rapidly
within 1 week following treatment with a modest dose (400 mg) of
troglitazone, in parallel with a fall in insulin. The fall
persisted throughout 4 weeks of troglitazone
administration. This change was observed in each of the seven subjects.
Because glucose and lipid concentrations did not alter, this effect was
independent of plasma glucose, cholesterol, and triglycerides. Whether
PPAR
expression is a function of insulin resistance and whether it
falls in parallel with processes governing insulin resistance like
NF
B will require further investigation. Our data also show that
PPAR
expression increases within 1 week of
troglitazone, which is also known to be a PPAR
agonist.
Whether this effect is directly attributable to
troglitazone action or is mediated by a fall
in insulin resistance and insulin concentrations or by other events
like NF
B inhibition is not clear. It is known that PPAR
may have
an anti-inflammatory role (5), and, thus, NF
B- or
NF
B-mediated effects may modulate both the expression of PPAR
and
the effects of PPAR
. 9-HODE and 13-HODE are also known to be ligands
of PPAR
(7), and their decrease following
troglitazone administration may contribute to alterations
in PPAR
expression. Indeed, 9-HODE and 13-HODE are constituents of
oxidized low-density lipoprotein (ox-LDL), which modulates both the
expression of PPAR
in the monocyte and the formation of foam cells
(7, 8). Previous work shows that troglitazone
prevents the decrease in the expression of PPAR
induced by
proinflammatory cytokines in mature rat adipocytes. This effect was not
observed in the absence of the inhibitory effect of proinflammatory
cytokines like tumor necrosis factor
(TNF
) (9). It
is of interest that in human MNC in vivo, the fall in
PPAR
expression occurs in association with a fall in TNF
induced
by troglitazone (4). The diametrically
opposite effect of troglitazone on PPAR
expression in
the rat adipocytes in vitro, when compared with that on
human MNC in vivo, could either be due to the difference in
the behavior of the two cell types or a species difference. It could
also be due to the concentrations of troglitazone used.
Our data are naturally more relevant to human pathophysiology because
our experiments were carried out in humans in vivo using
modest therapeutic doses of troglitazone.
Although there are hitherto no data on the modulation of PPAR
and
PPAR
expression in the human in vivo, PPAR
1 and
PPAR
2 expression in adipocytes in some models of obesity has been
shown to be increased. On the other hand, a fast causes a rapid fall in
PPAR
expression in adipocytes by as much as 6080% over a period
of 48 h (10).
It has been claimed that human monocytes do not express PPAR
(11), but macrophages in atherosclerotic lesions express
PPAR
(12). In our studies, we have used mononuclear
cell populations, which contain a mix of monocytes and lymphocytes. We
tested this population because all of our previous work on ROS
generation and the anti-inflammatory effects of glucocorticoids
(13), ß-blockers carvedilol and nadolol (14, 15), and troglitazone (3, 4) has used
this cell population. Because it is known that oxidized LDL induces the
expression of PPAR
(7), it is possible that increased
lipid peroxidation and an increase in 9-HODE and 13-HODE concentrations
in the obese previously described by us (3, 16), may
contribute to the expression of PPAR
in the mononuclear cell
population from the obese. Activation of PPAR
has been shown to
inhibit the induced expression of endothelin-1 in endothelial cells,
cyclooxygenase-2 and interleukin-6 in vascular smooth muscle cells,
TNF
and interleukin-2 in monocytes, and class A scavenger receptor
and inducible nitric oxide synthase in activated macrophages
(17, 18, 19). These actions are achieved through interference
with the action of the proinflammatory transcription factors, NF
B,
and activator protein-1 (17, 19). We have recently
demonstrated that serum TNF
, soluble intercellular adhesion
molecule-1, monocyte chemoattractant protein-1, and C-reactive
protein are all diminished following treatment with
troglitazone in the obese (4). These
observations would be consistent with PPAR
and PPAR
mediated
anti-inflammatory effects of troglitazone. This is
probably also achieved through ox-LDL-induced increase in PPAR
expression in the monocyte/macrophage as well as simultaneous increase
in the scavenger receptor CD36, which binds and internalizes ox-LDL and
may lead to foam cell formation. Because class A scavenger
receptor-mediated uptake of ox-LDL and the additional actions of
activator protein-1 and NF
B (protection from apoptosis) also
participate in the process of foam cell formation, there may be other
roles for PPAR
and its agonists because thiazolidinediones have now
been shown to be inhibitors of NF
B (4, 17).
To test the role of PPAR
in the pathogenesis of insulin resistance,
Miles et al. (20) have investigated PPAR
knockout mice.
Although the homozygous PPAR
-/- mouse is unable to survive
(incompatible with life), the heterozygous PPAR
± mouse is
hypersensitive to insulin. Thus, PPAR
± mice with 50% PPAR
have lower insulin concentrations following a glucose challenge than
the wild-type +/+ mice. This suggests that PPAR
may exert an effect
that increases insulin resistance. This is paradoxical because PPAR
with its ligand mediates sensitization to insulin. It is possible that
although PPAR
with an appropriate ligand has a particular
transcriptional activity, the receptor without a ligand binds to DNA
without inducing transcription, thus blocking transcription. Our data
appear consistent with this concept because in the insulin-resistant
obese, PPAR
expression is high. Following treatment with
troglitazone, PPAR
content falls in parallel with a
fall in fasting insulin concentrations in plasma. It is thus possible
that in the absence of a ligand, PPAR
increases to further increase
insulin resistance.
In contrast to PPAR
suppression following troglitazone,
the role of PPAR
enhancement following this drug has obvious
clinical implications. PPAR
modulates the expression of fatty
acid-binding protein and peroxisomal acyl-CoA oxidase
(21). Acyl CoA-oxidase is an enzyme responsible for fatty
acid oxidation (22), and thus an increase in PPAR
may
enhance the metabolism of fatty acids and thus lower fatty acid
concentration, which is believed to be cardinal to the action of
troglitazone.
We conclude that troglitazone, a PPAR
and a PPAR
agonist, has profound and diametrically opposite effects on the
expression of PPAR
(inhibitory) and PPAR
(stimulatory). This may
have implications in the mediation of its anti-inflammatory effects and
its effects on the restoration of insulin sensitivity in the obese.
These effects may also have relevance to atherogenesis in the long
term.
The study was completed 6 months before the withdrawal of troglitazone from the market. The subjects were informed about the potential hepatotoxicity of the drug while signing the contract. The Institutional Review Board was informed about the association of potential hepatotoxicity of the drug.
Acknowledgments
We gratefully acknowledge the support of McGowan Charitable Fund.
Footnotes
1 Supported by a grant from Parke-Davis (now Pfeifer). ![]()
Received October 30, 2000.
Revised February 27, 2001.
Accepted March 6, 2001.
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