Am J Physiol Endocrinol Metab 290: E103–E113, 2006.
First published September 6, 2005; doi:10.1152/ajpendo.00605.2004.
Plasminogen activator inhibitor-1 modulates adipocyte differentiation
Xiubin Liang,1 Talerngsak Kanjanabuch,1 Su-Li Mao,1 Chuan-Ming Hao,2 Yi-Wei Tang,1,2
Paul J. Declerck,3 Alyssa H. Hasty,4 David H. Wasserman,4 Agnes B. Fogo,1,2 and Li-Jun Ma1
Departments of 1Pathology, 2Medicine, and 4Molecular Physiology and Biophysics, Vanderbilt
University School of Medicine, Nashville, Tennessee; and 3Laboratory for Pharmaceutical
Biology and Phytopharmacology, Katholieke Universiteit Leuven, Leuven, Belgium
Submitted 29 December 2004; accepted in final form 29 July 2005
Liang, Xiubin, Talerngsak Kanjanabuch, Su-Li Mao, ChuanMing Hao, Yi-Wei Tang, Paul J. Declerck, Alyssa H. Hasty, David
H. Wasserman, Agnes B. Fogo, and Li-Jun Ma. Plasminogen
activator inhibitor-1 modulates adipocyte differentiation. Am J
Physiol Endocrinol Metab 290: E103–E113, 2006. First published
September 6, 2005; doi:10.1152/ajpendo.00605.2004.—Increased
plasminogen activator inhibitor-1 (PAI-1) is linked to obesity and
insulin resistance. However, the functional role of PAI-1 in adipocytes
is unknown. This study was designed to investigate effects and
underlying mechanisms of PAI-1 on glucose uptake in adipocytes and
on adipocyte differentiation. Using primary cultured adipocytes from
PAI-1⫹/⫹ and PAI-1⫺/⫺ mice, we found that PAI-1 deficiency promoted adipocyte differentiation, enhanced basal and insulin-stimulated glucose uptake, and protected against tumor necrosis factor-␣induced adipocyte dedifferentiation and insulin resistance. These
beneficial effects were associated with upregulated glucose transporter 4 at basal and insulin-stimulated states and upregulated peroxisome proliferator-activated receptor-␥ (PPAR␥) and adiponectin
along with downregulated resistin mRNA in differentiated PAI-1⫺/⫺
vs. PAI-1⫹/⫹ adipocytes. Similarly, inhibition of PAI-1 with a neutralizing anti-PAI-1 antibody in differentiated 3T3-L1 adipocytes
further promoted adipocyte differentiation and glucose uptake, which
was associated with increased expression of transcription factors
PPAR␥, CCAAT enhancer-binding protein-␣ (C/EBP␣), and the
adipocyte-selective fatty acid-binding protein aP2, thus mimicking the
phenotype in PAI-1⫺/⫺ primary adipocytes. Conversely, overexpression of PAI-1 by adenovirus-mediated gene transfer in 3T3-L1 adipocytes inhibited differentiation and reduced PPAR␥, C/EBP␣, and
aP2 expression. This was also associated with a decrease in urokinasetype plasminogen activator mRNA expression, decreased plasmin
activity, and increased collagen I mRNA expression. Collectively,
these results indicate that absence or inhibition of PAI-1 in adipocytes
protects against insulin resistance by promoting glucose uptake and
adipocyte differentiation via increased PPAR␥ expression. We postulate that these PAI-1 effects on adipocytes may, at least in part, be
mediated via modulation of plasmin activity and extracellular matrix
components.
plasminogen activator inhibitor-1; peroxisome proliferator-activated
receptor-␥; adipocyte differentiation; glucose uptake; insulin resistance
PLASMINOGEN ACTIVATOR INHIBITOR-1
(PAI-1), a member of the
serine proteinase inhibitor family, is the major physiological
inhibitor of tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). PAI-1 not only inhibits
fibrinolysis but also has complex interactions with cellular
matrices, and further inhibits proteolysis (21, 37). Increased
Address for reprint requests and other correspondence: L.-J. Ma, MCN
C4209, Dept. of Pathology, Vanderbilt Univ. Medical Center, 21st and Garland
Ave., Nashville, TN 37232-2561 (e-mail: lijun.ma@vanderbilt.edu).
X. Liang and T. Kanjanabuch contributed equally to this study.
http://www.ajpendo.org
PAI-1 has been associated with fibrosis and thrombosis in
experimental models and humans (20, 28, 30, 39, 62).
Emerging evidences implicate PAI-1 as a significant risk
factor for macrovascular and microvascular complications of
diabetes. PAI-1 is also linked to insulin resistance. Indeed,
circulating PAI-1 levels are elevated at an early stage of
impaired glucose tolerance and continue to be elevated as
diabetes and the metabolic syndrome develop (29, 38). Plasma
PAI-1 correlates strongly with the degree of insulinemia, and
PAI-1 levels are further increased in type 2 diabetes even after
correction for insulin levels (2, 42, 61). Increased circulating
PAI-1 levels are also found in offspring and relatives of type 2
diabetics (22). Obesity also contributes significantly to elevated plasma PAI-1 levels. PAI-1 is overexpressed in adipose
tissue of obese mice and humans (1, 53). In contrast, surgical
fat removal or weight loss attributed to diet-mediated fat
reduction is associated with a decrease in plasma PAI-1 in
obese subjects (31, 45). In previous studies, increased PAI-1
levels were presumed to be consequent to obesity and insulin
resistance. Our recent data indicate that PAI-1 may not merely
increase in response to development of obesity and insulin
resistance but may have a direct causal role (40). Using a
high-fat diet-induced obesity and insulin resistance mouse
model, we found that PAI-1 deficiency completely prevented
development of obesity and insulin resistance (40).
Although adipose tissue only accounts for a relatively small
proportion (⬍10%) of the peripheral glucose utilization in
response to insulin in nonobese humans and animals, adipocytes may still play an important role in insulin resistance (57).
PAI-1, like tumor necrosis factor-␣ (TNF-␣), adiponectin, and
resistin, is highly expressed in adipose tissues of obese animals
and human subjects (3, 14, 27, 50). However, the functions of
PAI-1 in differentiated adipocytes remain unknown.
Adipocyte differentiation is a complex process. The differentiation of fibroblast-like preadipocytes to mature adipocytes
involves striking morphological and biochemical changes. Besides the key regulatory roles of two well-characterized adipogenic transcription factors, CCAAT enhancer-binding protein-␣ (C/EBP␣) and peroxisome proliferator-activated receptor-␥ (PPAR␥; see Ref. 49), multiple events occur during
adipocyte differentiation, including dynamic changes of cellmatrix interactions and extensive extracellular matrix (ECM)
remodeling (11). On the basis of the established role of PAI-1
in inhibition of ECM degradation, we hypothesized that alteration of PAI-1 will affect remodeling of ECM, adipose differentiation, and insulin resistance in adipocytes. The present
The costs of publication of this article were defrayed in part by the payment
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PAI-1 MODULATES ADIPOCYTE DIFFERENTIATION
study was therefore designed to investigate underlying mechanisms by which PAI-1 influences differentiation and glucose
homeostasis in adipocytes.
RESEARCH DESIGN AND METHODS
Cell culture. Two in vitro cell culture models were used in this
study. To determine the effects of PAI-1 deficiency on adipocyte
differentiation, glucose uptake, and TNF-␣-induced insulin resistance,
primary cultures of adipocytes were obtained from 4-wk-old male
PAI-1⫹/⫹ or PAI-1⫺/⫺ mice (both on C57BL/6 background) as
previously described (8, 24, 40). Differentiation of preadipocytes to
adipocytes was induced by addition of an adipogenic hormonal
cocktail (1 g/ml insulin, 0.25 M dexamethasone, and 0.5 mM
isobutylmethylxanthine) and confirmed morphologically by multiple
oil red O-stained fat droplets in the cytoplasm (40). Primary adipocytes at day 10 after induction of differentiation were used for this
study. Insulin-resistant primary adipocytes were obtained by incubating these differentiated 10-day adipocytes for an additional 3 days in
the presence of 3 ng/ml TNF-␣ (Sigma, St. Louis, MO) with or
without insulin stimulation for 10 min (52).
For studies of altered PAI-1 expression, we used murine 3T3-L1
preadipocytes (American Type Culture Collection, Manassas, VA)
grown in DMEM supplemented with 10% fetal bovine serum, 2 mM
L-glutamine, 50 U/ml penicillin, and 50 g/ml streptomycin at 37°C
in 5% CO2. After confluence was reached (2 days), differentiation was
initiated with adipogenic hormonal cocktail as described above for 2
days and then with DMEM containing insulin (1 g/ml) alone for 2
days, followed by an additional 2 days in medium without insulin.
These differentiated 3T3-L1 adipocytes at day 6 exhibited intracellular lipid droplets and were used for this study. To inhibit PAI-1,
3T3-L1 preadipocytes were treated for 6 days with a neutralizing
monoclonal antibody against PAI-1 (MA-33H1F7, 10 g/ml) along
with induction of differentiation (6, 7, 15). MA-33H1F7 inhibits
mouse and rat PAI-1 by converting PAI-1 to a noninhibitory substrate
for tPA (16). The inhibiting efficacy of this antibody on PAI-1 has
been demonstrated in vivo (6). The dose of anti-PAI-1 antibody was
chosen on the basis of its biological activity in inhibiting PAI-1 effects
on angiogenesis in vitro (46). A class-matched, noninhibitory monoclonal antibody (MA-32K3) was used as a control antibody.
Adenoviral infection of 3T3-L1 preadipocytes. Recombinant adenovirus bearing human PAI-1 (Ad-PAI-1) and control adenovirus
expressing Escherichia coli -galactosidase (Ad-lacz) were gifts from
Dr. Robert Gerard (University of Texas Southwestern Medical Center;
see Refs. 10 and 33). The recombinant viruses were propagated in
HEK 293 cells and purified by CsCl density gradient centrifugation.
3T3-L1 preadipocyte cultures (2 days postconfluence) in six-well
plates were infected with the Ad-PAI-1 or Ad-lacz by addition of 1 ⫻
109 plaque-forming units/well for 3 h before induction of differentiation. The medium containing free virus was then removed, fresh
DMEM with 10% fetal bovine serum was added, and cells were
induced to differentiate as above.
Oil Red O staining. Differentiation of preadipocytes to adipocytes
was monitored by measurement of intracellular lipid accumulation
using Oil Red O staining. After fixation with 10% formalin in PBS for
1 h, the cells were washed and stained with filtered 0.3% Oil Red O
in 55% isopropanol for 1 h (40), followed by counterstaining with
0.5% methyl green (Polysciences, Warrington, PA) in 0.1 M sodium
acetate, pH 7.4. Differentiation was calculated as percent cells with
Oil Red O positivity of total cells, assessed under ⫻100 magnification.
Glucose uptake. [2-3H]deoxyglucose uptake was measured as described previously (40, 43). Briefly, primary adipocytes (10 days
postdifferentiation) and 3T3-L1 adipocytes (6 days postdifferentiation) in six-well plates were cultured overnight in serum free-DMEM
with low glucose (1 g/l). After KRP buffer wash (containing 136 mM
NaCl, 4.7 mM KCl, 1 mM CaCl2, 1 mM MgSO4, 5 mM sodium
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pyrophosphate, 20 mM HEPES, and 1% BSA), cells were incubated
with 1 ml KRP buffer at 37°C for 20 min in the presence or absence
of insulin as indicated. [2-3H]deoxyglucose was added for a final
concentration of 0.1 mM (11.0 Ci/mmol; PerkinElmer Life Sciences,
Boston, MA) and incubated for 10 min at 37°C. The cells were
washed with cold KRP buffer and solubilized in 0.1% SDS. The
radioactivity of a 200-l aliquot was determined in a scintillation
counter. Glucose uptake was expressed as the degree of increase
compared with basal PAI-1⫹/⫹ or 3T3-L1 cells, normalized to protein
concentration in each sample.
RNA extraction and assessment. Total RNA was extracted from
cells as described previously (40, 41). Relative quantitation of expression of several murine genes in primary adipocytes and 3T3-L1
adipocytes was determined by a real-time, one-step RT-PCR assay
(TaqMan) using an ABI Prism 7700 sequence detection system
(Applied Biosystems, Foster City, CA). A 25-l reaction mixture
containing 2 g of total RNA, 0.5 M of each primer, and 0.2 M
TaqMan probe was mixed with 25 l of the TaqMan One-Step
RT-PCR 2⫻ Master Mix (Applied Biosystems), as described previously (17). Primers and probes designed to target mouse PPAR␥,
adiponectin, resistin, PAI-1, uPA, and collagen I genes are listed in
Table 1. The reaction conditions were designed as follows: RT at
48°C for 30 min and initial denaturation at 95°C for 10 min followed
by 40 cycles with 15 s at 95°C for denaturing and 1 min at 60°C for
annealing and extension. The threshold cycle (CT), i.e., the cycle
number at which the amount of amplified gene of interest reached a
fixed threshold, was subsequently determined. Relative quantification
of each target mRNA level was normalized to 18S rRNA or -actin
and calculated by the comparative CT method described elsewhere (36).
Immunofluorescence. 3T3-L1 cells cultured on cover slips were
infected with Ad-PAI-1 or Ad-lacz or not treated as described above.
After fixation in methanol-acetone (1:1) for 10 min at room temperature, the cells were permeabilized and blocked with 0.1% Triton
X-100 and 5% BSA in PBS for 10 min. After being washed, the cells
were then incubated with sheep anti-PAI-1 antibody (1:25; American
Diagnostica, Stamford, CT) or goat anti--Gal antibody (1:25; Bio-
Table 1. Nucleotide sequences of oligonucleotide primers
and probes (5⬘ to 3⬘) for real-time PCR
Genes
5⬘ to 3⬘ Oligonucleotide Sequences
Mouse PPAR␥
Sense
Antisense
Probe
Mouse adiponectin
Sense
Antisense
Probe
Mouse resistin
Sense
Antisense
Probe
Mouse PAI-1
Sense
Antisense
Probe*
Mouse uPA
Sense
Antisense
Probe*
Mouse collagen I
Sense
Antisense
Probe*
CTGTTATGGGTGAAACTCTGGGAG
ATAGGCAGTGCATCAGCGAA
TCCTGTTGACCCAGAGCATGGTGC
TGTTGGAATGACAGGAGCTGAA
CACACTGAAGCCTGAGCGATAC
CATAAGCGGCTTCTCCAGGCTCTCCT
TCGTGGGACATTCGTGAAGA
GCGGGCTGCTGTCCAG
AAAGTGTGTCACTGCCAGTGTGCAAGGAT
AACCCGGCGGCAGATC
CTTGAGATAGGACAGTGCTT
TGGCCCATGGCACCCTCC
TATTCTTCCGGGCA
CTTCGACTGACCCAGGTAGACAA
TCCCAAAGAAGGAAAAC
GGGCGRGTGCTGTGCTTT
CTCCTACATCTTCTGAGTTTGGTGATAC
TATTCTTCCGGGCA
*These probes were dually labeled with 6-carboxyfluorescein at the 5⬘ end
and minor groove binder at the 3⬘ end.
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PAI-1 MODULATES ADIPOCYTE DIFFERENTIATION
genesis) for 1 h at room temperature. FITC-conjugated rabbit antisheep IgG (DakoCytomation, Carprinteria, CA) or FITC-conjugated
rabbit anti-goat IgG antibodies (Dako) were then applied and incubated for 1 h. Internalization of inhibitory monoclonal PAI-1 antibody
or control antibody in 3T3-L1 cells was assessed by direct staining of
permeabilized cells with fluorochrome tetramethylrhodamine isothiocyanate-conjugated rabbit anti-mouse IgG (1:25; Dako). Images of
immunofluorescent cells were captured with a Zeiss AxioCam camera
attached to a Nikon Eclipse E400 microscope.
Western blotting. Adipocytes (primary and 3T3-L1) grown in
six-well plates were induced to differentiate along with treatments
indicated above. Cells were lysed in lysis buffer [containing 150 mM
NaCl, 50 mM Tris 䡠 HCl, pH 7.5, 5 mM EDTA, 1% Nonidet P-40,
0.5% sodium deoxycholate, 0.1% SDS, 100 g/ml phenylmethylsulfonyl fluoride, and 1:100 proteinase inhibitor cocktail tablet (Roche
Diagnostics, Mannheim, Germany)]. Total protein (30 g) was separated on SDS-PAGE and transferred to a nitrocellulose membrane.
Western blottings were performed with polyclonal rabbit antibodies
against PPAR␥ (catalog no. 2492; Cell Signaling Technology, Beverly, MA), C/EBP␣ (14AA; Santa Cruz Biotechnology, Santa Cruz,
CA), or fatty acid-binding protein (aP2, C-15; Santa Cruz Biotechnology). The blots were subsequently incubated with horseradish
peroxidase (HRP)-conjugated donkey anti-rabbit IgG (Amersham
Biosciences, Little Chalfont, UK) or HRP-conjugated bovine antigoat IgG (Santa Cruz Biotechnology). Immunoreactive proteins were
detected and visualized by using enhanced chemiluminescence detection reagents (Amersham Biosciences). The membranes were restripped for -actin by using monoclonal anti--actin antibody
(Sigma), as a control for normalization.
Plasmin activity. Total plasmin activity in 3T3-L1 cells lysis was
measured by a modified protocol as described previously (28) using a
plasmin-specific chromogenic substrate (Chromozym PL; Roche Molecular Biochemicals, Indianapolis, IN). This substance is specifically
cleaved by plasmin into a residual peptide and 4-nitroaniline, which
can be detected spectrophotometrically. 3T3-L1 adipocyte lysis (80
l) and 20 l of 3 mM Chromozym PL were added per reaction.
Absorbance was measured at 405 nm. A standard linear curve was
generated with serial dilutions of human plasmin (Roche). Results are
expressed as units per milligram protein.
Statistical analysis. Data are presented as means ⫾ SE, unless
otherwise noted. P values were calculated by ANOVA followed by
unpaired t-test as appropriate. A P value of ⬍0.05 was considered to
be significant.
RESULTS
PAI-1 deficiency or inhibition stimulates adipocyte differentiation. To address whether the absence of PAI-1 affects
adipocyte differentiation, primary cultured adipocytes from
PAI-1⫹/⫹ and PAI-1⫺/⫺ mice were used. Preadipocytes from
PAI-1⫺/⫺ showed more avid differentiation (75 ⫾ 1.2 vs. 60 ⫾
2.4%, P ⬍ 0.01) and smaller mature adipocytes vs. PAI-1⫹/⫹
at day 10 after induction (Fig. 1, A–D). Consistent with these
morphological observations, differentiated PAI-1⫺/⫺ adipocytes expressed higher levels of the adipocyte-related transcription factor C/EBP␣ (Fig. 2A) and adipogenic marker aP2
(Fig. 2B) compared with PAI-1⫹/⫹ at 10 days after induction
(Fig. 2, A and B). Exposure of the differentiated primary
PAI-1⫹/⫹ adipocytes to TNF-␣ (3 ng/ml) for 3 days resulted in
decreased C/EBP␣ and aP2, consistent with dedifferentiation
effects of TNF-␣ (47, 58). However, C/EBP␣ and aP2 protein
levels in response to TNF-␣ in PAI-1⫺/⫺ adipocytes were
relatively more preserved with less decrease vs. PAI-1⫹/⫹
adipocytes (Fig. 2, A and B).
In the present study, we confirmed again that PAI-1 deficiency in primary adipocytes significantly enhanced basal glucose uptake vs. PAI-1⫹/⫹ (5.8-fold increase in PAI-1⫺/⫺ vs.
PAI-1⫹/⫹, P ⬍ 0.01, Fig. 3A; see Ref. 40). PAI-1 deficiency
further enhanced insulin-stimulated glucose uptake over a
range of insulin doses in differentiated adipocytes compared
with differentiated PAI-1⫹/⫹ adipocytes (Fig. 3A). As ex-
Fig. 1. Effects of plasminogen activator inhibitor (PAI)-1 deficiency on adipocyte differentiation in primary cultured adipocytes.
Cells were stained with Oil Red O at day 10
after induction of differentiation and examined microscopically (left, ⫻400) and
macroscopically (right). Compared with
PAI-1⫹/⫹ adipocytes (A and B), PAI-1⫺/⫺
primary cultured adipocytes were more
differentiated and smaller (C and D).
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Fig. 2. PAI-1 deficiency protects against tumor necrosis factor (TNF)-␣induced adipocyte dedifferentiation. Representative Western blot analysis of
CCAAT enhancer-binding protein-␣ (C/EBP␣) and fatty acid-binding protein
(aP2) in day 10 differentiated PAI-1⫹/⫹ and PAI-1⫺/⫺ primary adipocytes in
the presence or absence of TNF-␣ (3 ng/ml) for 3 additional days.
pected, the response of PAI-1⫹/⫹ adipocytes to insulin (10 or
100 nM) stimulation in the presence of TNF-␣ was impaired,
and glucose uptake was almost completely abolished. In contrast, basal and insulin (10 or 100 nM)-stimulated glucose
uptake in PAI-1⫺/⫺ adipocytes was maintained at high levels
(Fig. 3A), indicating the absence of TNF-␣-mediated insulin
resistance in PAI-1⫺/⫺ cells.
To understand the effect of PAI-1 deficiency on enhanced
glucose uptake, we first measured total cellular GLUT4 levels
in primary adipocytes by Western blot analysis. In the insulinfree basal state, total cellular GLUT4 protein levels were
increased 1.6-fold in differentiated day 10 PAI-1⫺/⫺ vs. PAI1⫹/⫹ primary adipocytes (Fig. 3B). After insulin (100 nM)
stimulation, GLUT4 protein levels were further increased in
both PAI-1⫺/⫺ vs. PAI-1⫹/⫹ primary adipocytes vs. that seen
in the basal state (2.8- and 2.1-fold, respectively; Fig. 3B).
To further test whether inhibition of PAI-1 in 3T3-L1 cells
could reproduce the PAI-1 deficiency-associated phenotype
noted in primary adipocytes, 3T3-L1 cells were treated with a
neutralizing anti-PAI-1 antibody (MA-33H1F7) for 6 days
along with induction of differentiation. Oil Red O staining
showed an average of 60 ⫾ 4% of control 3T3-L1 preadipocytes were differentiated into adipocytes at day 6 after induction (Fig. 4A). The addition of anti-PAI-1 antibody to 3T3-L1
cells coupled with standard adipogenic-inducing cocktail led to
a further increase in adipocyte differentiation (87 ⫾ 7%; Fig.
4B) compared with control-induced 3T3-L1 cells (Fig. 4A) or
differentiated 3T3-L1 cells treated with a class-matched noninhibitory control antibody (Fig. 4C). None of the control
groups without induction showed differentiation, as evidenced
by lack of Oil Red O staining (Fig. 4, F–H).
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Glucose uptake was similar in day 6 differentiated 3T3-L1
adipocytes in all groups at baseline without insulin stimulation
and was increased significantly in response to insulin (100
nM). This increased glucose uptake in response to insulin
stimulation was further upregulated in anti-PAI-1 antibodytreated, differentiated 3T3-L1 adipocytes compared with those
differentiated 3T3-L1 adipocytes without antibody treatment or
treated with nonspecific control antibodies (Fig. 5).
Effects of PAI-1 deficiency or inhibition on expression of
genes related to insulin sensitivity and ECM modulation. We
next examined the expression of key genes that regulate insulin
sensitivity by real-time RT-PCR. The absence or presence of
PAI-1 in primary preadipocytes before induction had no significant effect on mRNA expressions of PPAR␥ or adiponectin
(Fig. 6, A and B). In contrast, when PAI-1⫹/⫹ adipocytes were
differentiated at day 10 after induction, PPAR␥ and adiponectin were both markedly increased compared with undifferentiated preadipocytes. Interestingly, differentiated adipocytes deficient in PAI-1 showed even further upregulation of PPAR␥
and adiponectin (1.8- and 1.3-fold of PAI-1⫹/⫹ differentiated
cells, respectively; Fig. 6, A and B). In contrast, resistin mRNA
was significantly downregulated in PAI-1⫺/⫺ (⬃7-fold) vs.
PAI-1⫹/⫹ adipocytes after differentiation (Fig. 6C).
We next examined the effects of inhibition of PAI-1 on
expression of the transcription factors PPAR␥ and C/EBP␣ and
the adipocyte differentiation marker aP2 in differentiated
3T3-L1 adipocytes after PAI-1 antibody treatment. Endogenous PAI-1 mRNA expression in 3T3-L1 cells was not different before or after induction of differentiation. Unexpectedly,
PAI-1 antibody treatment led to a decrease of PAI-1 mRNA
Fig. 3. PAI-1 deficiency protects against TNF-␣-induced insulin resistance in
adipocytes. A: representative glucose uptake in day 10 differentiated PAI-1⫹/⫹
(light gray bars) and PAI-1⫺/⫺ (black bars) primary adipocytes in response to
insulin stimulation (0, 10, and 100 nM) in the presence or absence of TNF-␣
(3 ng/ml) for 3 days (n ⫽ 3–5 in each group, *P ⬍ 0.01). NS, not significant.
B: effects of PAI-1 deficiency on total cellular GLUT4 protein in day 10
differentiated primary adipocytes in the presence or absence of insulin stimulation (100 nM) for 10 min.
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Fig. 4. Effect of modulation of PAI-1 on adipocyte differentiation of 3T3-L1 cells. Differentiated 3T3-L1 adipocytes at day 6 after induction (A) or
undifferentiated 3T3-L1 preadipocytes (F) served as controls. 3T3-L1 preadipocytes were treated with PAI-1 antibody (B and G) or control antibody (C and H)
for 6 days with (B and C) or without (G and H) induction of differentiation or were infected with adenovirus bearing human PAI-1 (Ad-PAI-1; D and I) or
adenovirus expressing Escherichia coli -galactosidase (Ad-lacz; E and J) with (D and E) or without (I and J) induction of differentiation for 6 days. Oil red
O staining was performed to assess the status of differentiation (A-J, ⫻100).
levels in differentiated 3T3-L1 cells (data not shown). PPAR␥
(Fig. 7, lane 1), C/EBP␣ (both 42- and 30-kDa isoforms), and
aP2 (Fig. 8, A and B, lane 1) were undetectable or low in
undifferentiated 3T3-L1 preadipocytes at baseline as assessed
by Western blot analyses. PPAR␥ (Fig. 7, lane 2), C/EBP␣,
and aP2 (Fig. 8, lane 2) were strongly induced in day 6
differentiated 3T3-L1 adipocytes compared with undifferentiated preadipocytes and were further upregulated when PAI-1
was inhibited via neutralizing anti-PAI-1 antibody (MA33H1F7; Figs. 7 and 8, lane 6). Noninhibitory control antibody
(MA-32K3) had no effects on PAI-1 (data not shown), PPAR␥,
C/EBP␣, and aP2 expressions (Figs. 7 and 8). Our data suggest
that this increased PPAR␥ and C/EBP␣ might be potential
contributors to the enhanced adipogenesis in PAI-1 antibodytreated 3T3-L1 adipocytes.
As described above, differentiated 3T3-L1 cells treated with
neutralized PAI-1 antibody had decreased PAI-1 mRNA levels
(data not shown). We postulated that PAI-1 antibodies may
Fig. 5. Effect of PAI-1 inhibition on insulin-stimulated glucose uptake in
differentiated 3T3-L1 adipocytes. [2-3H]deoxyglucose uptake was assessed in
day 6 differentiated 3T3-L1 adipocytes alone or in day 6 differentiated 3T3-L1
adipocytes treated for 6 days with neutralizing anti-PAI-1 antibody (MA33H1F7, 10 g/ml) or control antibody (MA-32K3, 10 g/ml) in the absence
or presence of insulin (100 nM) stimulation (n ⫽ 3 experiments for each
group). Ab, antibody; Cont, control.
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Fig. 6. Effects of PAI-1 deficiency on gene expression in primary adipocytes.
Peroxisome proliferator-activated receptor-␥ (PPAR␥; A), adiponectin (B), and
resistin (C) mRNA expression in primary preadipocytes (day 0 after induction)
and differentiated adipocytes (10 days after induction) was assessed by
real-time quantitative RT-PCR and normalized to 18S ribosomal RNA (n ⫽
3– 4 in each group).
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Fig. 7. Representative Western blot analysis
of PPAR␥ in 3T3-L1 adipocyte lysates.
PPAR␥ protein expression was undetectable
in undifferentiated cells alone (lane 1) or in
undifferentiated cells infected with AdPAI-1 (lane 3) or treated with neutralizing
anti-PAI-1 antibody (lane 5). PPAR␥ was
induced in differentiated cells (lane 2). AdPAI-1 infection in differentiated cells inhibited
PPAR␥ protein levels (lane 4). Conversely,
PPAR␥ protein expression was further increased in differentiated cells after PAI-1
inhibition with neutralizing anti-PAI-1 antibody treatment (lane 6). Control Ad-lacz
infection (lane 7) or control antibody treatment (lane 8) in differentiated cells had comparable PPAR␥ protein levels vs. normal
differentiated cells (lane 2).
have been internalized and thus interacted with PAI-1 protein,
with postulated feedback effects on PAI-1 mRNA expression.
Therefore, direct immunoflurescence staining was applied to
localize the neutralizing monoclonal anti-PAI-1 antibody or
control antibody in permeabilized 3T3-L1 cells with fluorochrome
TRITC-conjugated rabbit anti-mouse IgG (1:25; Dako). 3T3-L1
cells treated with either PAI-1 antibody or control antibody
displayed intracellular staining, whereas those cells without
antibody treatment showed no staining (data not shown). Of
note, these data do not clarify the molecular mechanism
whereby intracellular PAI-1 antibody affected PAI-1 mRNA
expression.
The effects of the neutralizing anti-PAI-1 antibody on intracellular plasmin activity in 3T3-L1 cells were also assessed. In
contrast to the decreased plasmin activity induced by PAI-1
overexpression, total plasmin activity was significantly increased (1.6-fold) in differentiated 3T3-L1 adipocytes cells
after treatment with anti-PAI-1 antibody compared with control, differentiated 3T3-L1 adipocytes (66 ⫾ 2 vs. 42 ⫾ 1 U/g
protein, P ⬍ 0.01).
Overexpression of PAI-1 inhibits adipocyte differentiation.
To investigate whether PAI-1 directly influences adipocyte
differentiation, PAI-1 was overexpressed in 3T3-L1 adipocytes
with the use of an adenovirus expression system. Low levels of
PAI-1 were seen in noninfected, differentiated 3T3-L1 cells
(Fig. 9A). Infectivity was assessed using an adenovirus expressing -gal (Ad-lacz), confirming 70 –90% infectivity at
24 h posttransfection in day 1 differentiated 3T3-L1 adipocytes
by indirect immunofluorescence (Fig. 9B). Comparable infection efficiencies of 70 –90% were also observed for adenovirus
expressing PAI-1 (Ad-PAI-1; Fig. 9, C and E). Immunofluorescence staining results further confirmed that PAI-1 protein
was markedly increased in Ad-PAI-1-infected 3T3-L1 cells at
both 1 day and 6 days after differentiation (Fig. 9, C and E,
respectively) vs. trace amounts in noninfected, differentiated
3T3-L1 cells (Fig. 9A). Control Ad-lacz had no effect on PAI-1
protein expressions in 3T3-L1 cells at either 1 day or 6 days
after differentiation (Fig. 9, D and F, respectively).
We next assessed the effect of overexpression of PAI-1 on
adipocyte differentiation. In contrast to enhanced adipocyte
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differentiation resulting from PAI-1 inhibition via anti-PAI-1
antibody treatment (Fig. 4B), PAI-1 overexpression inhibited
adipocyte differentiation, leading to only 15 ⫾ 5% of Ad-PAI1-infected 3T3-L1 preadipocytes undergoing differentiation at
day 6 after induction (P ⬍ 0.01; Fig. 4D). 3T3-L1 cells
infected with control adenovirus carrying lacz (Fig. 4E) had
comparable levels of differentiation as those cells treated
with adipogenic cocktail only (Fig. 4A). Adenovirus-infected
3T3-L1 adipocytes without induction of adipogenesis showed
no differentiation, as evidenced by lack of Oil Red O staining
(Fig. 4, I and J).
Effects of PAI-1 overexpression on adipogenic-related genes
and ECM modulators. PAI-1 mRNA was overexpressed (4.8and 6.1-fold, respectively) in both undifferentiated and differentiated 3T3-L1 adipocytes (6 days after differentiation) after
PAI-1 adenovirus infection compared with both undifferentiated preadipocytes and differentiated 3T3-L1 adipocytes (Fig.
10A). Control Ad-lacz had no effect on PAI-1 mRNA expression. The expressions of PPAR␥, C/EBP␣ (42- and 30-kDa
isoforms), and aP2 were significantly downregulated in AdPAI-1-infected differentiated cells compared with noninfected
differentiated cells (P ⬍ 0.01; Figs. 7 and 8).
We next determined whether genes of ECM and related
proteins were modulated after PAI-1 overexpression in differentiated 3T3-L1 cells. Quantitative real-time RT-PCR analysis
revealed that uPA mRNA was highly expressed in differentiated 3T3-L1 adipocytes vs. 3T3-L1 preadipocytes (Fig. 10B).
PAI-1-overexpressing undifferentiated 3T3-L1 cells showed
no change in uPA mRNA expression. However, overexpression of PAI-1 in differentiated 3T3-L1 adipocytes inhibited the
increase of uPA mRNA expression by 35% (P ⬍ 0.05 vs.
noninfected cells; Fig. 10B). In contrast, collagen I mRNA
expression was significantly decreased in differentiated 3T3-L1
cells vs. 3T3-L1 preadipocytes. Overexpression of PAI-1 in
differentiated 3T3-L1 adipocytes resulted in increased collagen
I mRNA expression compared with noninfected cells (2.6-fold
increase, P ⬍ 0.05; Fig. 10C). Of note, infection with adenovirus carrying lacZ had no effect on the expression of genes
examined (Fig. 10).
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PAI-1 MODULATES ADIPOCYTE DIFFERENTIATION
Fig. 8. Effects of modulation of PAI-1 on
expression of C/EBP␣ and aP2 in 3T3-L1
adipocyte lysates by Western blot. C/EBP␣
(A) and aP2 (B) were expressed at low levels
or undetectable in undifferentiated cells
alone (lane 1) or in undifferentiated cells
with indicated treatments (lanes 3, 5, 7, and
9). C/EBP␣ (A) and aP2 (B) were strongly
induced in day 6 differentiated adipocytes
(lane 2) and were downregulated in AdPAI-1 infected cells (lane 4), but were upregulated in differentiated cells after PAI-1
inhibition with neutralizing anti-PAI-1 antibody treatment (lane 6). Control Ad-lacz
infection (lane 8) or control antibody treatment (lane 10) in differentiated cells had
comparable C/EBP␣ (A) and aP2 (B) protein levels vs. normal differentiated cells
(lane 2).
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PAI-1 MODULATES ADIPOCYTE DIFFERENTIATION
Fig. 9. Overexpression of PAI-1 in 3T3-L1
cells detected by indirect immunofluorescence. A: low levels of PAI-1 were observed
in noninfected, differentiated 3T3-L1 adipocytes. B: 70 –90% infectivity of adenovirus
expressing -gal (Ad-lacz) in 3T3-L1 adipocytes at 24 h after induction of differentiation
was detected by indirect immunofluorescence
staining. PAI-1 was markedly increased in
3T3-L1 adipocytes infected with Ad-PAI-1 at
both 1 day (C) and 6 days (E) after differentiation vs. only trace amounts of PAI-1 expression in noninfected, differentiated
3T3-L1 cells (A). Control Ad-lacz had no
effect on PAI-1 protein expressions in cells at
either 1 day or 6 days after differentiation (D
and F, respectively).
To test our hypothesis that PAI-1 overexpression inhibits the
plasminogen system during adipocyte differentiation, plasmin
activity was measured. To this end, we compared differentiated
3T3-L1 cells with or without PAI-1 adenovirus infection.
PAI-1 overexpression in differentiated 3T3-L1 adipocytes inhibited total plasmin activity ⬃26% vs. noninfected cells
(31 ⫾ 2 vs. 42 ⫾ 6 U/g protein, P ⬍ 0.05). Plasmin levels
were comparable between noninfected and Ad-lacZ-infected
differentiated 3T3-L1 adipocytes (44 ⫾ 1, P ⫽ not significant).
DISCUSSION
Elevated plasma PAI-1 activity has been associated with
insulin resistance and closely correlated with visceral fat accumulation in human subjects (1, 12, 23, 34). These data
suggest that visceral adipose tissue can be an important contributor to the elevated plasma PAI-1 levels observed in obesity. Indeed, a large body of evidence indicates that adipose
tissue produces substantial amounts of PAI-1. Despite the
association between increased PAI-1 and obesity, the functional role of PAI-1 in differentiated adipocytes remains to be
elucidated. To address this question, we used both PAI-1deficient primary adipocytes and the preadipocyte 3T3-L1 cell
line. In this study, we provide evidence that PAI-1 deficiency
promotes adipocyte differentiation and protects against TNF␣-induced dedifferentiation and insulin resistance in primary
adipocytes. Inhibition of PAI-1 by an inhibitory anti-PAI-1
antibody in differentiated 3T3-L1 cells recaptured the phenoAJP-Endocrinol Metab • VOL
type of PAI-1 deficiency and enhanced adipocyte differentiation of, and glucose uptake in, 3T3-L1 cells. Conversely,
overexpression of PAI-1 in 3T3-L1 cells inhibited adipocyte
differentiation. Our results clearly indicate that PAI-1 plays an
important role in modulation of adipocyte differentiation.
In our previous studies, we found decreased fat pad weight
in PAI-1⫺/⫺ vs. PAI-1⫹/⫹ mice in response to a high-fat diet
(40). Furthermore, adipocytes were smaller and more differentiated, with more Oil Red O staining in PAI-1⫺/⫺ mice vs. the
wild type. Thus PAI-1 deficiency promoted adipocyte differentiation, but there was no overall increase in the mass of
adipose tissue. There are several possible explanations for the
discrepancy between positive correlation of PAI-1 with obesity
observed in vivo vs. PAI-1-induced inhibition of adipocyte
differentiation found in the present in vitro study. First, it could
reflect differences in the role of systemic PAI-1 vs. possible
local effects of PAI-1 in adipocytes. As evidenced in our
previous study in PAI-1⫺/⫺ mice on a high-fat diet (40),
systemic PAI-1 deficiency also increased resting metabolic
rates and total energy expenditure, which was associated with
a marked increase in uncoupling protein-3 expression in skeletal muscle, likely mechanisms contributing to prevention of
obesity in vivo (40). Second, it is likely that PAI-1 is not the
only factor influencing adipogenesis. Human obesity is controlled by many other factors.
Adipocyte differentiation is regulated by the coordinated
expression of various transcription factors, including PPAR␥
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PAI-1 MODULATES ADIPOCYTE DIFFERENTIATION
Fig. 10. Effects of overexpression of PAI-1 in 3T3-L1 cells on gene expression involved in extracellular matrix turnover. 3T3-L1 preadipocytes were
infected with adenovirus encoding -gal (Ad-lacz) or adenovirus expressing
PAI-1 (Ad-PAI-1). mRNA expressions of PAI-1 (A), urokinase-type plasminogen activator (uPA; B), and collagen I (C) were analyzed by quantitative
real-time RT-PCR at 6 days after induction of differentiation and compared
with cells without induction. Data are means ⫾ SE of values from 3 independent experiments.
and the C/EBP family. PPAR␥ is believed to be the master
regulator of adipocyte differentiation (35, 48, 49, 59, 65). Both
PPAR␥ and the C/EBP family also play important roles in the
regulation of insulin sensitivity (19, 32). Activation of PPAR␥
by synthetic and naturally occurring PPAR␥ activators regulates multiple genes involved in glucose and lipid metabolism,
thereby ameliorating insulin resistance and type 2 diabetes
(44). Conversely, dominant negative mutations in human
PPAR␥ and adipose-specific PPAR␥ gene deletion in mice are
associated with severe insulin resistance (4, 25). To investigate
whether the modulating effects of PAI-1 on adipocyte differentiation and glucose uptake in adipocytes are contributed to
by adipocyte-related key transcription factors PPAR␥ and
C/EBP␣, we examined expression of PPAR␥, C/EBP␣, and
PPAR␥ target genes, including aP2. In the present study, PAI-1
deficiency in primary adipocytes increased basal glucose upAJP-Endocrinol Metab • VOL
E111
take and adipocyte differentiation, which was associated with
increased PPAR␥ and C/EBP␣. Furthermore, PAI-1 inhibition
(achieved by an inhibitory anti-PAI-1 antibody) resulted in enhanced adipocyte differentiation and was also associated with
significantly upregulated PPAR␥, C/EBP␣, and aP2 expression
in differentiated 3T3-L1 adipocytes. Conversely, PAI-1 overexpression in differentiated 3T3-L1 adipocytes inhibited adipocyte differentiation and was accompanied by decreases in
PPAR␥, C/EBP␣, and aP2 levels. These observations imply
that modulating effects of PAI-1 on glucose uptake and adipocyte differentiation were mediated, at least in part, by
PPAR␥ and C/EBP␣. Glucose transport across the plasma
membrane is mediated by a family of glucose transporter
proteins (GLUTs). Upregulated GLUT4 in PAI-1-deficient
primary adipocytes may also contribute to the increased basal
glucose uptake.
TNF-␣ is produced in and secreted by adipocytes, and has
been implicated as an important mediator of insulin resistance
and adipocyte differentiation (63). Blocking of TNF-␣ functions by either genetic deletion of the TNF-␣ receptor or
neutralization of TNF-␣ in rodents protects from obesityinduced insulin resistance and increases insulin sensitivity (26,
27, 60). Interestingly, TNF-␣ induces PAI-1 expression in
adipose tissue in rodents and humans (13, 51). In vitro, longterm exposure to TNF-␣ induces adipocyte dedifferentiation
and suppresses insulin-stimulated glucose uptake in adipocytes
(18). Negative regulation of PPAR␥ and C/EBP gene expression by TNF-␣ contributes to its inhibition of adipocyte differentiation, as well as its induction of insulin resistance (32,
47, 67). In this study, we have demonstrated that PAI-1
deficiency protects against TNF-␣-induced adipocyte dedifferentiation, as shown by relatively maintained expression of
adipocyte transcription factor C/EBP␣ and the adipogenic
marker aP2. We have further demonstrated that PAI-1 deficiency protects against TNF-␣-induced insulin resistance, as
shown by the maintained high level of glucose uptake in
adipocytes. Our results suggest increased PAI-1 levels promote
insulin resistance in adipocytes, which may contribute to a role
of PAI-1 in systemic insulin resistance (5, 40, 54). Our data
also raise the interesting question that TNF-␣-induced insulin
resistance might, at least in part, be mediated through PAI-1.
Further study will be needed to address this hypothesis.
Adipocyte differentiation is a complex process. It has been
suggested recently that various factors in cell-cell and cellmatrix communications govern expression of adipocyte transcription factors and therefore regulate conversion of preadipocytes to adipocytes. During differentiation, expression of
ECM components, including collagens and proteases, is
changed (64, 66). Indeed, adipocytes produce and release a
variety of proteolytic proteins (9, 11). Plasminogen activators
tPA and uPA are serine proteinases that also play an important
role in regulating adipose tissue remodeling. tPA expression
and activity decreased, whereas uPA expression and activity
increased during adipocyte differentiation (55). Conversely,
inhibition of serine proteinase reduced adipocyte differentiation (56). Similarly, plasminogen deficiency suppressed differentiation of 3T3-L1 cells (56). The plasminogen cascade has
been postulated to foster adipocyte differentiation by degradation of the fibronectin-rich preadipocyte stromal matrix (56). In
the present study, our observations of decreased expression of
collagen I and increased expression of uPA after 3T3-L1
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PAI-1 MODULATES ADIPOCYTE DIFFERENTIATION
adipocyte differentiation are consistent with previous data (55,
66). Interestingly, the inhibited adipocyte differentiation in
PAI-1-overexpressing 3T3-L1 cells was associated with upregulated collagen I and downregulated uPA and plasmin.
Conversely, PAI-1 inhibition via neutralizing PAI-1 antibody
was associated with increased plasmin activity and increased
adipocyte differentiation.
Although there is no evidence that PAI-1 can directly affect
PPAR␥, indirect interactions of PAI-1 with PPAR␥ through
remodeling of ECM components are possible. We speculate
that PAI-1 might modulate the microenvironment and network
of ECM surrounding adipocytes via uPA and/or plasmin,
affecting cell-ECM interactions and transduction of extracellular signals to intracellular components, such as PPAR␥. We
further conclude that PAI-1 directly modulates adipocyte differentiation and glucose uptake. We postulate that PPAR␥ may
contribute to PAI-1-modulated adipocyte differentiation and
insulin sensitivity. Our data suggest that inhibition of PAI-1
might prove to be a novel anti-insulin resistance treatment.
ACKNOWLEDGMENTS
11.
12.
13.
14.
15.
16.
We thank HaiJing Li (Vanderbilt University School of Medicine) for
technical support in real-time PCR.
17.
GRANTS
This work was supported by a Research Award from the American Diabetes
Association (L.-J. Ma) and National Institutes of Health Grants DK-56942
(A. B. Fogo) and DK-50277 (D. H. Wasserman).
18.
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