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Journal of Steroid Biochemistry & Molecular Biology 111 (2008) 262–267
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Journal of Steroid Biochemistry and Molecular Biology
journal homepage: www.elsevier.com/locate/jsbmb
Anti-obesity, anti-diabetic, and lipid lowering effects of the thyroid
receptor ␤ subtype selective agonist KB-141
Galina Bryzgalova a , Suad Effendic a , Akhtar Khan a , Stefan Rehnmark b ,
Peter Barbounis c , Jamie Boulet c , Gao Dong c , Rajni Singh c , Sue Shapses d ,
Johan Malm b , Paul Webb e , John D. Baxter e , Gary J. Grover c,f,∗
a
Department of Molecular Medicine and Surgery, Karolinska Institute, Stockholm SE-171 76, Sweden
Karo Bio AB, Novum, Huddinge SE-141 57, Sweden
c
Eurofins-Product Safety Laboratories, 2394 Highway 130, Dayton, NJ 08810, USA
d
Department of Nutritional Sciences, Rutgers, The State University of New Jersey, 96 Lipman Drive, New Brunswick, NJ 08901-8525, USA
e
Department of Medicine, University of California, San Francisco, CA 94143, USA
f
Department of Physiology and Biophysics, Robert Wood Johnson Medical School, Piscataway, NJ 08559, USA
b
a r t i c l e
i n f o
Article history:
Received 14 February 2008
Received in revised form 16 May 2008
Accepted 16 June 2008
Keywords:
Thyroid hormones
Obesity
KB-141
Cholesterol
a b s t r a c t
Selective thyroid hormone receptor subtype-␤ (TR␤) agonists have received attention as potential treatments for hypercholesterolemia and obesity, but have received less attention as treatments for diabetes,
partly because this condition is not improved in thyroid hormone excess states. The TR␤ selective agonist KB-141 induces 5–10% increases in metabolic rate and lowering of plasma cholesterol levels without
tachycardia in lean rats, unlike the major active thyroid hormone, T3 . In the current study, we determined
whether KB-141 promotes weight loss in obese animals and whether it exhibits anti-diabetogenic effects.
Body weight, adiposity (DEXA), and lipid levels were examined following p.o. administration of KB-141 to
obese Zucker fa/fa rats at 0.00547–0.547 mg/kg/day for 21 days, and in ob/ob mice at 0.5 mg/kg/day KB-141
for 7 days. In rats, KB-141 reduced body weight by 6 and 8%, respectively, at 0.167 and 0.0547 mg/kg/day
without tachycardia and adiposity was reduced at 0.167 mg/kg/day (5–6%). In ob/ob mice, KB-141 lowered
serum cholesterol (35%), triacylglycerols (35%) and both serum and hepatic free fatty acids (18–20%) without tachycardia. Treatment of ob/ob mice with KB-141 (0.0547 or 0.328 mg/kg/day over 2 weeks) improved
glucose tolerance and insulin sensitivity in a dose-dependent manner with no effect on heart rate. Thus,
KB-141 elicits anti-obesity, lipid lowering and anti-diabetic effects without tachycardia suggesting that
selective TR␤ activation may be useful strategy to attenuate features of the metabolic syndrome.
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction
The anti-obesity and lipid-lowering activity of thyroid hormones has long tantalized pharmacologists and physicians [1–5].
The increase in adiposity and cholesterol seen in hypothyroid
patients can be reduced by well-managed replacement therapy
[6]. Thyroid hormone induced increases in metabolic rate and
fatty acid utilization [7–9] contribute to the anti-obesity actions of
the hormones. Thyroid hormone analogs reduce serum cholesterol
through effects on high-density lipoprotein (HDL) and low-density
lipoprotein pathways [10,11]. Unfortunately, treatment of obesity
∗ Corresponding author at: Eurofins-PSL, 2394 Highway 130, Dayton, NJ 08810,
USA. Tel.: +1 732 438 4100; fax: +1 732 355 3275.
E-mail address: [email protected] (G.J. Grover).
0960-0760/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsbmb.2008.06.010
and hyperlipidemia in euthyroid subjects with naturally occurring
thyroid hormones, T3 and T4 , is not viable because of deleterious
cardiac effects, including tachycardia, arrhythmias and heart failure (8,12), and excessive increases in metabolic rate (8,12). Indeed,
the lack of selectivity of these agents resulted in failure of an illfated series of studies using dinitrophenol with or without thyroid
hormones [1].
While selective thyroid hormone receptor (TR) agonists that
modestly increase metabolic rate over a large dose range, with
minimal effects on the heart, could potentially be used as antiobesity and/or lipid-lowering agents, selectivity has been hard to
achieve because restricted structure–activity relationships around
the diphenyl-ether scaffold allow limited chemical modifications
[13]. Recent discoveries of novel means of separating TR activities
has led to development of compounds, including KB-141, GC-1, and
MB07811 with pharmacologically selective profiles different from
G. Bryzgalova et al. / Journal of Steroid Biochemistry & Molecular Biology 111 (2008) 262–267
263
T3 [12–15]. These efforts have been facilitated by determinations
of the three dimensional structures of the ligand binding domains
of the ␣ and ␤ isoforms of the thyroid hormone receptor (TR)
[2,7,13].
One way to separate beneficial effects of thyroid hormone
analogs from harmful is to tailor ligands to particular TR subtypes
[2]. There are two primary ligand binding TR subtypes (␣, ␤) that
are products of distinct genes [7]. Both are ubiquitously expressed
[7,15]. However, a major action of the predominant ligand regulated
form of TR␣, TR␣1 , is on heart rate [7,14–17], whereas the predominant form of TR␤, TR␤1 , display preferential effects on cholesterol
lowering through the liver [8,15–17]. Accordingly, the TR␤ selective
agonist KB-141 and a similar TR␤ selective compound (GC-1) that
also displays tissue and liver selective uptake reduce cholesterol
at doses that do not affect heart [8,17,18]. Both compounds exhibit
10–30-fold dose windows in which 5–10% increases in metabolic
rate and 80% reductions in cholesterol are seen without tachycardia in lean rats [8,17–19]. Villicev et al. [20] showed larger increases
in metabolic rate with GC-1 in lean Wistar rats (>50–70%) after 6
weeks of treatment and found that this effect was accompanied
by loss of body weight with no increases in food consumption,
unlike T3 . Significant weight reduction was also seen for KB-141
and GC-1 in lean primates [8], and Erion et al. [14] reported that
KB-141 reduced body weight in a mouse model of diet induced
obesity (DIO mice). However, it has not been reported whether KB141 or GC-1 can promote weight loss in other models of obesity
and whether this occurs via loss of adiposity or lean body mass.
Thus, in current studies, we addressed whether we could show a
reduction of weight via loss of adiposity in Zucker fatty rats without promoting cardiac acceleration. We also addressed whether
KB-141 can promote weight loss in an animal model of obesity
that more closely mimics the metabolic syndrome, namely ob/ob
mice that are obese and have diabetes resembling type 2 diabetes
in man.
Effects of TR␤ selective ligands on glucose homeostasis have
not been investigated in detail. Since obesity and hyperlipidemia
are prevalent in the metabolic syndrome and are regulated by the
TR␤ selective ligands, the question arises as to whether TR␤ selective compounds could affect glucose homeostasis. Hyperthyroid
patients can experience a worsening of diabetes [21]; thus it is not
obvious that selective activation of TR signaling pathways would
have any beneficial effect. Thus, in the current study, we determined
effects of KB-141 (Fig. 1) on glucose homeostasis in the diabetic
ob/ob mouse.
used, as all of our previous work used females and we tend to
observe less variability in weight change with females. To assess
effects on cholesterol as an indication of TR␤ effects, these animals were put on a high cholesterol diet (1.5% cholesterol + 0.5%
cholic acid) ad lib for 2 weeks prior to drug administration and
the amount consumed daily at the end of this period was maintained throughout the remainder of the study. Diet restriction was
used because a previous study using GC-1 showed differences in
food consumption and we wanted to remove this as a variable [20].
In all groups, animals completely consumed their food throughout the drug treatment period. KB-141 or vehicle was given p.o.
once daily for 3 weeks by oral gavage. Vehicle was 10% m-pyrol, 5%
ethanol, 5% cremaphor and 80% water. Rats were divided into the
following groups with n = 10 per group: vehicle, 0.00547 mg/kg/day
KB-141, 0.0547 mg/kg/day KB-141, or 0.547 mg/kg/day KB-141.
Doses were chosen based on molar doses previously used
for metabolic rate determination (0.00547 mg/kg = 15.4 nmol/kg;
0.0547 = 154 nmol/kg; 0.547 = 1540 nmol/kg) [8]. Body weights and
food consumption were measured daily. Two hours after the last
dose, rats were anesthetized with 30 mg/kg pentobarbital i.p. and
HR was determined for 5 min using a lead II ECG. Blood was
withdrawn for measurement of serum cholesterol (Cholesterol kit,
Roche Diagnostics), serum chemistries, and liver function tests
(alanine aminotransferase; ALT, aspartate aminotransferase; AST,
Idexx, MA, USA).
To determine the effect of KB-141 on body composition (adiposity) and TSH, Zucker rats (425–450 g) were cholesterol-fed as
described above for 2 weeks before treatment for 21 days with
vehicle (n = 10) or 0.164 mg/kg/day KB-141 (470 nmol/kg; n = 10)
and diet restriction as described above. This dose was chosen to
be clinically relevant with 1–2% body weight loss per week. Body
weights and food consumption were monitored daily and at the
end of the study, the rats were anesthetized and HR determined
as described above. A blood sample was withdrawn for measurement of rat serum TSH (Idexx, rat TSH ELISA) and blood chemistries
(Idexx, MA, USA). The carcasses were frozen in a normal reclining
position and body composition was measured with a densitometer (DEXA), the Prodigy Advanced system (GE-LUNAR Corp.) with
small animal software [22]. From this, changes in percent adiposity
were determined. We were unable to find a dose of T3 that reduced
body weight without inducing florid thyrotoxicosis leading to mortality. The heart rates in these animals are always very high and the
animals are most likely more susceptible to heart failure than lean
animals [8].
2. Methods
2.2. Effect of KB-141 on hyperlipidemia in ob/ob mice
2.1. Effects of KB-141 on Zucker fa/fa rats: effect on body weight
and adiposity
The studies in this section were approved by the EurofinsProduct Safety Laboratories Institutional Animal Care and Use
Committee. Zucker fa/fa (fatty, nondiabetic) female rats had fasting glucose levels of 95 ± 8 mg/dL, were relatively healthy and
were 425–450 g (Harlan, Indianapolis, IN, USA). Female rats were
Fig. 1. Chemical structures of T3 (3,5,3 -triiodo-l-thyronine) and KB-141.
The studies in this section were approved by the KaroBio Institutional Animal Care and Use Committee. Female ob/ob mice (37–43 g,
Harlan) were obtained from the animal colony at the Karolinska
Institute and were fed standard rodent chow diet (Lactamin AB,
Kimstad, Sweden) ad lib and had free access to water. They were
cholesterol-fed for 2 weeks by incorporating 1.5% cholesterol and
0.5% cholic acid into their regular chow. They were then given
0.547 mg/kg KB-141 via oral gavage daily for 7 days and maintained
on the high cholesterol diet ad lib for the duration of the study. Body
weights and food consumption were measured daily. After 7 days,
blood was collected from the vena cava by a syringe and livers were
removed. Serum and livers were analyzed for cholesterol, triacyglycerols, and NEFA using Roche Diagnostics kits. An additional group
(n = 6) was run at the highest dose (0.547 mg/kg/day) and tested for
heart rate changes as described above. Seven days of treatment was
chosen so that significant lipid-lowering can be observed. We chose
to use ob/ob mice because this species is more sensitive to lipidlowering effects of thyromimetics than Zucker rats. The lower limit
264
G. Bryzgalova et al. / Journal of Steroid Biochemistry & Molecular Biology 111 (2008) 262–267
Table 1
Absolute body weight data for the beginning of the study (pre-drug, 2 weeks post-cholesterol feeding) and at day 21 of KB-141 treatment (last day of study) for body weight,
heart rate and serum cholesterol for KB-141 or vehicle treated Zucker rats
Vehicle
KB-141 (0.00547 mg/kg/day)
KB-141 (0.0547 mg/kg/day)
KB-141 (0.547 mg/kg/day)
Body weight (g)
Pre-drug
Post-drug
523 ± 6
572 ± 9
527 ± 7
540 ± 11
524 ± 8
543 ± 9
528 ± 7
527 ± 7*
Heart rate (BPM)
Serum cholesterol (mg/dL)
340 ± 5
256 ± 10
330 ± 5
267 ± 11
327 ± 20
240 ± 10
355 ± 8
218 ± 11*
Mean ± S.E.M.
*
Significantly different from vehicle group value (p < 0.05). N = 10 per group.
of quantitation (LLQ) for cholesterol was 0.2 ␮g/mL and the LLQ for
triglycerides was 0.1 nmol. The measurements were repeatable and
the standard errors low as will be seen in Section 3.
2.3. Effect of KB-141 on diabetes in ob/ob mice
The studies in this section were approved by the Karolinska Institute Animal Care and Use Committee. Female ob/ob
mice (42–46 g) were given KB-141 (0.0547 mg/kg or 0.328 mg/kg;
924 nmol/kg) dissolved in 99% carboxymethylcellulose (0.25%)/1%
ethanol via oral gavage, once daily for 14 days. The doses were based
on the molar doses reported previously for KB-141 [8]. Vehicletreated animals received an equal volume of solvent. Body weight
and food intake were measured every second day in mice treated
with KB-141 at 0.328 mg/kg/day dose, but only the pre-test and final
body weights were measured for the lower dose.
After 14 days of vehicle or KB-141 treatment (n = 8 per group),
an intraperitoneal glucose tolerance test (IPGTT) was performed in
overnight fasted mice. Blood glucose was measured at the basal
state (0 min) and at 10, 30, 60 and 120 min after a glucose load
(2 g/kg, i.p.). Blood glucose concentrations were measured using a
MediSence glucose analyzer (Abbott Scandinavia AB, Solna, Sweden) as blood was collected from the tail vein. Two days later
an intraperitoneal insulin tolerance test (IPITT) was performed
in the same animals. For this purpose, blood glucose concentrations were measured at the basal state, and mice were injected
with insulin (0.25 U/kg, i.p.). Blood glucose concentrations were
measured 10 min after insulin injection (0 min), followed by administration of glucose (1 g/kg, ip). Blood glucose concentrations were
measured at 15, 30, 60, 90 and 120 min after the glucose load as
described above. The area under the curve (AUC) for the glucose
excursion was calculated from the SigmaPlot 2001 for Windows
Version 7.101 (SPSS Inc.).
A group of ob/ob mice was tested at the 0.328 mg/kg dose (n = 6)
for the same duration of the study for determination of the effect of
KB-141 on heart rate. Heart rate was determined using lead II ECG
as described above.
2.4. Statistics
For comparisons of two groups, Student’s t-test was used with
p < 0.05 being considered significant. With more than two groups
or variables were compared, analysis of variance was used with a
Newman–Keuls post hoc test with p < 0.05 being considered significant.
KB-141 reduced body weight gain relative to the vehicle treated
group (Table 1). Weight loss, expressed as percent change from their
pre-treatment values, achieved statistical significance for the 0.547
(9.4%) and 0.0547 (5.7%) mg/kg/day doses of KB-141.
Serum cholesterol was reduced in a dose-dependent manner.
This effect was smaller than previously seen in lean cholesterol-fed
rats (8); cholesterol reduction (14.8%) was statistically significant
only at the highest dose (0.547 mg/kg/day) of KB-141. No changes
in BUN, liver enzymes, creatinine, or electrolytes were seen for any
group (data not shown). No changes in liver enzymes, electrolytes,
pH, or creatinine levels suggested normal liver and renal function
in these animals.
We addressed whether KB-141 induced weight loss seen is due
to loss of adiposity in another group of Zucker rats treated with
0.164 mg/kg/day KB-141 for 21 days. As shown above, this dose
gives significant weight reduction in the clinically relevant range
of 1–2% per week (Table 2). Pre-treatment weights were similar for
vehicle and drug groups (data not shown). Significant reduction
of weight (6.7%) was observed for KB-141 compared to vehicletreated animals. Percent adiposity was also significantly reduced
by 5.4% confirming that the decrease in weight gain arises primarily from loss of adipose tissue rather than lean body mass (no loss
of lean body mass was detected). No effect on HR was observed.
Serum cholesterol was not significantly reduced in this study, but
TR␤ receptors were activated as shown by TSH suppression. Blood
chemistries for this group of animals were normal suggesting a lack
of toxicity and confirming the lack of muscle wasting (BUN, creatine
phosphokinase) in these animals as seen with the DEXA. Plasma ALT
and AST levels were normal suggesting a lack of liver toxicity and
creatinine levels were normal suggesting normal kidney function
(data not shown).
3.2. Lipid-lowering studies in ob/ob mice
In ob/ob mice, KB-141 reduced serum levels of total cholesterol
(36%), triacylglyerols (28%), NEFA (18%) (Table 3), and liver triacylglycerols (30%), but not hepatic cholesterol levels. KB-141 also
caused a significant decrease in weight but did not affect heart
rate at this dose. The body weights of the two groups were similar before treatment (36.1 ± 0.5 and 35.9 ± 0.4 g for vehicle and
Table 2
Effect of 0.164 mg/kg KB-141 or vehicle on Zucker fa/fa (nondiabetic) rats treated for
21 days on heart rate, serum TSH (thyroid stimulating hormone), body weight and
percent adiposity as measured by DEXA
Vehicle
KB-141
(0.164 mg/kg/day)
3. Results
62.1
507
2.4
397
±
±
±
±
1.1
3
0.7
2
58.7
478
0.6
398
±
±
±
±
1.1*
7*
0.2*
5
Percent change
from vehicle
−5.4
−6.8
−75
0
3.1. Effects of KB-141 on body weights and adiposity in Zucker
obese rats
Percent adiposity
Body weight (g)
TSH (ng/mL)
Heart rate (beats/min)
Body weights were similar for all groups at the beginning of the
study. The vehicle treated animals gained weight over time and
Mean ± S.E.M.
*
Significantly different from vehicle group value (p < 0.05). n = 10 per group.
G. Bryzgalova et al. / Journal of Steroid Biochemistry & Molecular Biology 111 (2008) 262–267
265
Table 3
Effect of 7 days treatment with vehicle or 0.547 mg/kg KB-141 on serum and liver cholesterol, serum and liver triacylgycerols, and serum non-esterified fatty acids (NEFA) in
cholesterol-fed ob/ob mice
Serum cholesterol (mg/dL)
Vehicle
506 ± 12
0.547 mg/kg KB-141 336 ± 14*
Serum triacylglyerols (mg/dL)
Liver cholesterol (mg/g tissue)
Liver triacylglycerols (mg/g tissue)
Serum NEFA (mM)
447 ± 5
323 ± 14*
113 ± 3
119 ± 4
42 ± 9
25 ± 7*
5.1 ± 0.2
4.2 ± 0.2*
Mean ± S.E.M.
*
Significantly different from vehicle group value (p < 0.05). n = 8 per group.
drug, respectively), but KB-141 significantly reduced body weight
by approximately 9% (38.6 ± 0.4 and 35.3 ± 0.6 g for vehicle and KB141, respectively). Food consumption was the same in all animals.
No overt signs of toxicity were noted and blood chemistries were
normal so the animals did not appear to be losing weight due to
toxicity.
ning and the end of the study for the 0.054 mg/kg dose: 44 ± 0.5
and 45.1 ± 0.4 g pre-study weights for vehicle and B-141, respectively, and post-treatment and 47.7 ± 1.0 and 43.1 ± 0.4 g for vehicle
and KB-141, respectively, showing approximately 8% weight loss
for KB-141 versus vehicle. Food intake was similar in KB-141 and
vehicle-treated mice and KB-141 did not cause any change in heart
rate.
3.3. Anti-diabetic effects of KB-141
4. Discussion
We determined effects of a 2-week KB-141 treatment on glucose
tolerance and insulin sensitivity in ob/ob mice. Assessed by IPGTT,
the 0.0547 and 0.328 mg/kg/day doses of KB-141 led to significant
decreases in fasting blood glucose levels (Fig. 2). Similar effects
were also observed following glucose challenge with a significantly
(p < 0.05) reduced AUC (by 33 and 47%, respectively) compared to
vehicle-treated animals. Therefore, KB-141 improves glucose tolerance in ob/ob mice.
After insulin and glucose challenge (IPITT study), done 2 days
after the IPGTT test, KB-141 significantly lowered basal (0 time),
peak (15 min) and decline (30–120 min) blood glucose concentrations (Fig. 3). AUCs for the 0.328 and 0.0547 mg/kg/day doses of
KB-141 were significantly (p < 0.05) lower (67 and 54%, respectively)
compared to vehicle-treated animals. Hence, insulin sensitivity was
also improved in the KB-141-treated mice.
Weight reduction, not the primary end point of this study,
was significant in the KB-141 treated animals. With 0.054 and
0.328 mg/kg/day groups KB-141 reduced body weight by 8% (only
pre-and post-study weights taken for low dose) and 18% (Fig. 4)
compared to vehicle-treated mice at the end of study. For the
low dose group body weights were only measured at the begin-
Fig. 2. IPGTT in ob/ob mice treated with KB-141141 at dose of 0.054 mg/kg/day (open
circle), 0.328 mg/kg (black triangle) or vehicle-treated group (black circle) for 14
days. Blood glucose concentrations were measured before and after glucose load
(2 g/kg, i.p.) at indicated time points. Data are presented as mean ± S.E.M., n = 8,
***
p < 0.001.
In current studies, we addressed whether KB-141 could influence obesity, cholesterol and diabetes in animal models that better
reflect the metabolic syndrome in humans than those used in the
past. We used Zucker fa/fa rats which, unlike lean rats that we used
in our earlier studies [8], exhibit a predictable progression of obesity to examine effects on adiposity and cholesterol. We also used
ob/ob mice, which exhibit obesity and impaired glucose tolerance,
to determine effects on diabetes, cholesterol, NEFA and weight gain.
To examine effects on plasma cholesterol levels, both species were
cholesterol-fed to increase plasma LDL-cholesterol levels relative
to the predominance of HDL-cholesterol prior to the cholesterolfeeding [8].
For Zucker rats, we chose a 3-week treatment period because
this is sufficient to obtain metabolic rate changes that should affect
body composition [8], and we restricted diet to control for altered
food intake [19]. Relative to vehicle treated animals, 0.00547 and
0.0547 mg/kg/day doses of KB-141 modestly reduced body weight
and 0.164 and 0.547 mg/kg/day doses yielded significant weight
reductions. Using a 0.164 mg/kg/day dose of KB-141 to produce significant weight reductions in the clinically relevant range of 1–2%
per week, we found that the reduction in adiposity matched the
loss of body weight, and that there was no reduction in lean body
Fig. 3. IPITT in ob/ob mice treated with KB-141 at dose of 0.054 mg/kg/day (open
circle), 0.328 mg/kg/day (black triangle) or vehicle-treated group (black circle) for
14 days. Animals were injected first with insulin (25 U/kg, i.p.) and 10 min later with
glucose (1.0 g/kg, i.p.). Blood glucose levels were measured at basal conditions and
at different time points after glucose load. Data are presented as mean ± S.E.M., n = 8,
***
p < 0.001.
266
G. Bryzgalova et al. / Journal of Steroid Biochemistry & Molecular Biology 111 (2008) 262–267
Fig. 4. Body weight in ob/ob mice treated with KB-141 at dose of 0.328 mg/kg/day
(open circle) or vehicle-treated animals (black circle) for 14 days. Data are presented
as mean ± S.E.M., n = 8, * p < 0.05, ** p < 0.001 and *** p < 0.001.
mass. This was confirmed by the lack of significant findings in the
blood chemistry data where BUN, creatine phosphokinase, creatinine, electrolytes and liver function tests were normal, indicative of
little muscle wasting or other toxicity. This weight loss is similar to
findings previously noted in lean rats and primates [8] and in DIO
mice [14]. In lean primates in a stable growth phase, we previously
showed that KB-141 caused dose-dependent weight reduction that
was not associated with loss of lean body mass or overt toxicity [8].
Thus, KB-141 exhibits anti-obesity effects in relevant animal models. Important was the lack of tachycardia seen over a large dose
range for KB-141.
We only observed modest effects of KB-141 on serum cholesterol
in the Zucker rat model (14.8%); this effect was obtained at a high
dose that resulted in 7.9% decrease in body weight relative to control. The fact that these effects are weaker than obtained in other
studies, for example, with GC-1, may be related to the fact that KB141 exhibits less preferential accumulation in liver than GC-1 [18].
Accordingly, reduction of cholesterol was associated with lowering
of plasma TSH levels (75%; Table 2), presumably via actions on the
hypothalamic pituitary axis. However, the studies could also imply
that peripheral effects of the TR␤ selective agonists are optimal for
promotion of weight loss.
We could not obtain direct comparisons of KB-141 with T3 in
Zucker rats. While we did perform studies to assess effects of T3 on
body weight, we could not find a dose at which weight reduction
was observed in the absence of florid thyrotoxicosis leading to significant mortality. These observations could point to the utility of
the TR␤ selectivity of KB-141 in promoting weight loss.
Unfortunately, we did not have the ability to measure metabolic
rate in these studies and the precise contributions of this parameter to the loss of adiposity are not established. However, we suggest
that it is likely that weight loss is related to enhanced metabolic
rate; in previous studies, KB-141-induced weight loss was correlated with increases in metabolic rate [8] and the amount of food
consumed was similar in all groups.
KB-141 exhibited several potentially useful effects in ob/ob
mice. We observed reductions in serum levels of total cholesterol
(36%), triacylglycerides (28%) and NEFA (18%), and in hepatic levels of triglycerides (30%) but not hepatic cholesterol. These data
are consistent with previously published data showing that thyromimetics increase fatty acid utilization [7,9,23–25] and the fact
that we observe these effects in the absence of TR␣1 dependent
effects on heart rate suggests that this could be a TR␤ mediated
phenomenon. Interestingly, thyromimetics are known to increase
metabolic rate, in part, by increasing fatty acid cycling causing dissipation of energy [7,9,23–25] and our data raise the possibility
that this effect contributes to loss of adiposity seen with KB-141.
The observed decrease in hepatic triacylglycerols could also have
relevance for prevention/treatment of both diabetes and hepatic
steatosis, both of which are related to hepatic triacylglycerol content. Confirming reports of others [14] we also found that KB-141
promoted weight reduction that was independent of food intake
even though the animals were treated for 2 weeks and were ad lib
fed.
Whereas some effects of the TR ligands, such as those on obesity
and hepatic triaclyglycerol levels, might be expected to have a indirect beneficial effect on diabetes [13,26–28], previous experience
has not suggested that thyroid hormone administration or hyperthyroidism amelioriates this condition [21]. Thus, of great interest
was that KB-141 increased insulin-sensitivity and improved glucose tolerance as assessed by the IPGTT and insulin sensitivity as
assessed by the IPITT in the ob/ob mice. Of note is that Erion et
al. previously reported that KB-141 and the liver and TR␤ selective
agonist MB07811 reduced blood glucose levels in DIO mice and that
MB07811 also reduced blood glucose in lean Sprague–Dalwey rats
[14]. These investigators did not measure insulin sensitivity and
did not draw attention to the fact that TR ligands might be used as
anti-diabetic agents. Of further importance in the current study is
that the effects of KB-141 on diabetes were observed in a few days,
prior to fat loss, indicating that anti-diabetic actions are independent of obesity lowering activities. Thus, the data are supportive
of the concept that TR␤ selective ligands might be potent agents
to treat diabetes and should receive further investigation in this
respect.
In conclusion, the TR selective ligand KB-141 elicits several
effects that deserve further exploration. It elicits reductions in body
weight and adiposity in obese Zucker rats and reductions in body
weight in ob/ob mice that are similar to those observed previously
by others using another TR agonist in DIO mice [14] and in lean
rodents and primates [8]. KB-141 also elicits anti-diabetic actions
that may be related to weight loss, but effects on glucose uptake
may also be critical. These effects were seen despite a lack of effect
on heart rate. These actions deserve further study as potential uses
for treatment of metabolic syndrome in man.
Disclosure
GG, JB, GD, RS, PB, JD, SS, PW have nothing to declare; JM, SR are
employees of KaroBio AB; JB has equity ownership in KaroBio and
has taken consulting fees from KaroBio AB; GB, SE, AK received a
grant from KaroBio to perform the diabetes study.
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