Departments of Pharmacology and Pathology, Amgen Center, Thousand Oaks, California 91320-1789
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ABSTRACT |
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We examined a possible
mechanistic interaction between leptin and thyroid hormones in rats
with hypothyroidism induced by thyroidectomy (TX) and propylthiouracil
administration. In study 1, the TX rats were treated by
vehicle (V, n = 9) or by recombinant murine leptin (L,
0.3 mg · kg1 · day
1,
n = 9) or were pair-fed (PF, n = 9)
against L. In study 2, the TX rats were all given
3,3'5'-triiodo-L-thyronine (T3) replacement (T,
5 µg · kg
1 · day
1) to
correct hypothyroidism. They were then subdivided into three groups,
namely, vehicle (T+V, n = 9), leptin (T+L,
n = 10), and pair-feeding (T+PF, n = 9), similar to study 1 except for T3 (T). Reduced food consumption and weight gain in the TX rats were reversed by T3 replacement. Leptin suppressed food intake in the TX
rats regardless of T3 replacement. O2
consumption (
O2) and CO2
production (
CO2) were reduced in TX rats
(P < 0.05 vs. normal) but were normalized by either
T3 or leptin treatment. T+L additively increased
O2 and
CO2 (P < 0.05 vs. TX,
T3, and L). The respiratory exchange ratio was unaltered in
TX rats, with and without T3, but was significantly reduced
by L or T+L treatments. These results indicate that the metabolic
actions of leptin are not dependent on a normal thyroid status and that
the effects of leptin and T3 on oxidative metabolism are additive.
hypothyroidism; indirect calorimetry; respiratory exchange ratio; oxygen consumption
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INTRODUCTION |
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THE OB GENE PRODUCT leptin (19, 53) acts as a negative feedback signal in controlling metabolism and adiposity (4, 5, 30). Leptin deficiency in ob/ob mice is associated with a reduction in resting energy expenditure (20, 34). Systemic administration of leptin induces a significant reduction of body weight and food consumption and causes an increase in energy expenditure (9, 21, 34). The biological mechanisms responsible for the effects of leptin in stimulating energy expenditure are still far from being resolved. Because thyroid hormone plays a fundamental role in regulation of energy metabolism, it is of great interest to explore the interrelationship between leptin and thyroid hormones. Recently, leptin was shown to prevent fasting-induced suppression of prothyrotropin-releasing hormone mRNA expression in the hypothalamic paraventricular nuclei (27). In our own studies, we have repeatedly documented elevated plasma thyroid hormone levels in rodents after prolonged leptin treatment (7, 49). Recent evidence shows that both leptin and thyroid hormone can stimulate uncoupling protein 3 mRNA expression in rodent muscle and brown adipose tissues (18). It is also well known that thyroid disorders in general are associated with changes in basal metabolic rate, oxygen consumption, appetite, and body weight (43). Thus the interactions between leptin and thyroid hormone were thought to be logical and may function as part of the leptin action mechanisms (25). However, the establishment of such interactions and mechanisms awaits more research evidence.
To examine our hypothesis that the effects of leptin on energy metabolism may be regulated independently of the thyrotropin-releasing hormone (TRH)/thyroid-stimulating hormone (TSH)/thyroid hormone axis, we have studied the effect of systemic administration of leptin on body weight, food consumption, and parameters of indirect calorimetry in thyroidectomized (TX) rats. The results obtained from the TX rats were compared with those obtained from normal control rats and also from TX rats with thyroid hormone replacement.
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MATERIALS AND METHODS |
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Animal preparation. Male Sprague-Dawley rats (250-300 g) were obtained from Harlan Sprague Dawley (San Diego, CA). Animals were maintained at ambient temperature (22°C) and with 12:12-h light-dark cycles. Animals were fed a balanced rodent diet with a metabolizable energy of 2.98 kcal/g (Rodent diet no. 8640, Harlan Teklad, Madison, WS). Drinking water was allowed ad libitum. Hypothyroidism was induced by thyroidectomy performed by the vendor (Harlan Sprague Dawley). The TX rats were given 2% calcium acetate in drinking water for 10 days to prevent the hypocalcemia that might occur as a result of potential surgical impairment of the parathyroid glands. Propylthiouracil was given in drinking water (0.02%) for 4 wk to obviate residual, if any, thyroid hormone production (22). The hypothyroid status of these TX rats was verified by blood hormone levels [3,3'5'-triiodo-L-thyronine (T3), thyroxine (T4), TSH] and the Oxymax indirect calorimetry system (Columbus Instruments, Columbus, OH).
Experimental protocols.
A total number of 55 TX rats were randomized into the following two
studies to determine the effects of leptin in hypothyroid rats with
(TX+T3) or without (TX) thyroid hormone replacement, respectively. The TX study was subdivided into three groups:
1) leptin treated (L, n = 9); 2)
PBS vehicle treated (V, n = 9); and 3)
vehicle treated and pair-fed, with a daily food ration matching the
average daily consumption by the T+L group (T+PF, n = 9). The TX+T3 study was performed in TX rats with
T3 replacement that were randomly divided into three
subgroups similar to those in the TX study. The three subgroups in the
TX+T3 study were named, accordingly, as T+L
(n = 10), T+V (n = 9), and T+PF
(n = 8). Recombinant murine leptin (r-metMuleptin;
Amgen, Thousand Oaks, CA) was administered (0.3 mg · kg1 · day
1)
subcutaneously via implanted Alzet osmotic pumps (model 2ML4, ALZA,
Palo Alto, CA). T3 (Sigma, St. Louis, MO) was dissolved in
PBS solution with 0.5% rat albumin (Sigma) and was administered subcutaneously via Alzet osmotic pumps (model 2004) at the rate of 5 µg · kg
1 · day
1. This
relatively low dose reportedly restored normal plasma T3 levels and suppressed TSH in the hypothyroid rat (13). All
animals were treated (leptin, T3, vehicle, and PF) for 3 wk. An additional group of eight healthy intact rats, matched for sex,
age, and body weight, were studied in parallel with all of the other
groups and served as the normal control group. All animals were
monitored daily for changes in food consumption and body weight. The
animal protocols were preapproved by the Laboratory Animal Research
Committee of Amgen.
Determination of metabolic rate.
The resting oxygen consumption (O2),
carbon dioxide production (
CO2), and
respiratory exchange ratio (RER) were determined by use of an Oxymax
indirect calorimetry system (Columbus Instruments). The effects of
leptin on these parameters were compared with those of the normal
controls and with the TX rats with and without T3 replacement. After a 4- to 5-h fast and under nonstressful conditions, the rats were individually placed into indirect calorimetric chambers with
1 h of adaptation time before the measurements. The measurements of flow, differential gas fractions (O2 and
CO2) across the chambers, were used to compute metabolic
parameters. At the end of the experiments, all animals were killed
under isoflurane anesthesia. Blood samples were collected via cardiac
puncture for determination of plasma hormones and metabolites.
Determination of plasma hormones and metabolites.
Baseline and terminal blood samples were obtained through tail artery
of the rats under light isoflurane anesthesia. Blood was collected into
heparinized (10 U/ml) polypropylene tubes and centrifuged immediately,
and the plasma fractions were stored at 80°C until assayed. The
biochemical parameters triglyceride, cholesterol, corticosterone,
-hydroxybutyrate (
-OHB), free fatty acids (FFA), lactate, and
glycerol were measured spectrophotometrically on a Hitachi 717 Clinical
Chemistry Autoanalyzer (Boehringer Mannheim, Indianapolis, IN). Plasma
insulin concentration was measured with a rat insulin radioimmunoassay
kit from Linco Research (St. Charles, MO) (31). Plasma
leptin concentration was determined by an enzyme-linked immunoassay
(51). The thyroid hormones (T3 and
T4) and TSH were determined in frozen plasma samples by
Anilytics (Gaithersburg, MD) with radioimmunoassay methods
(44).
Data analysis.
All data are reported as means ± SE. Because of the large number
of groups and various background conditions, two statistical approaches
were used to analyze the data. First, unpaired t-tests were
used to compare the values between the two pairs: 1) N vs. V
and 2) N vs. T+V. These comparisons identify the baseline
differences of the hypothyroid rat model, with and without thyroid
hormone replacement, compared with the euthyroid state. Second, two
sets of ANOVA and Bonferroni-Dunn multicomparison tests were used for the TX cohort (including V, L, and PF groups) and for the
TX+T3 cohort (including T+V, T+L, and T+PF groups),
respectively. An additional unpaired t-test was used to
compare the T+L combined treatment with L alone to identify the impact
of thyroid hormone replacement above leptin treatment. Statistical
significance is defined as < 5%. Calculations were done
using the StatView program (SAS Institute, Cary, NC).
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RESULTS |
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Effects of leptin on body weight and food consumption in
hypothyroid rats.
In contrast with the normal control rats that gained an average
of 2 g/day, untreated TX rats (V) had no change in their mean body
weight at the termination of the 3-wk experiment compared with the
initial body weight (designated as 0, Fig.
1). Leptin treatment in TX rats resulted
in a significant reduction in body weight compared with that of V
( =
1.81 g/day; P < 0.05, L vs. V; Fig. 1).
This reduction was 50% greater than that of the PF rats
(P < 0.01, PF vs. L). T3 replacement in TX
rats (T+V) caused a significant increase in body weight (
= 3.43 ± 0.20 g/day). However, such T3-induced
increment in body weight is attenuated by two-thirds by either leptin
treatment or restricted feeding (T+L, T+PF, both P < 0.05, vs. T+V). The mean daily energy intake was reduced in untreated
TX rats (V) by 37% compared with normal control animals (146.1 ± 2.4 kcal · kg
0.75 · day
1,
P < 0.05, vs. V). Leptin treatment (L) and its
corresponding pair-feeding (PF) further attenuated energy consumption
by 20-25% from that of V (both P < 0.05). The
reduced energy intake in TX rats (V) was significantly reversed by
T3 replacement with a 25% increment (T+V vs. V,
P < 0.05). The leptin (T+L) and the corresponding pair-feeding control (T+PF) groups consumed only 80% of the daily energy consumption by the T+V group (both P < 0.05 vs.
T+V).
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Effects of leptin on O2 and
CO2 in the hypothyroid rats.
The resting
O2 and
CO2 were reduced by 11.4 and 8%,
respectively, in untreated TX rats compared with the normal rats (both P < 0.05 vs. N) after 3 wk (Table
1). T3 replacement (T+V)
alone for 1 and 3 wk normalized
O2 (both P < 0.05 vs. V). Interestingly, leptin treatment alone (L) for 1 and 3 wk
was able to closely reproduce the effects of T3 in
normalizing
O2 [both P = nonsignificant (NS) vs. V+T; and P < 0.05 vs. V],
whereas the corresponding pair-feeding had no effect (P = NS, PF vs. V, for both 1 and 3 wk). More intriguingly, combined
treatment of leptin and T3 (T+L) resulted in an apparently additive effect in stimulating
O2
(15.34 ± 0.38 ml · kg
1 · min
1 at 3 wk,
P < 0.05 vs. L and T+V). The changes in
CO2 induced by 3 wk of T3
treatment (T+V) are qualitatively similar to the change from leptin
(L). Again, the combined treatment (T+L) for 3 wk resulted in the
greatest increment in
CO2
(P < 0.05 vs. L and T+V). Of note, reduced feeding in
hypothyroidism (PF) is associated with markedly reduced
CO2 (9.37-9.49
ml · kg
1 · min
1) from V,
whereas the
O2 was not different from
that of V, apparently in line with the hypothyroidism and reduced
feeding conditions. When T3-treated animals were
food-restricted (T+PF), the
O2 and
CO2 were initially normalized (at 1 wk).
However, the prolonged undernourishment (3 wk) results in a further
increase in
O2 but a decline in
CO2.
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Effect of leptin on RER in TX rats.
With both reduced O2 and
CO2, the RER in untreated TX rats was
not significantly different from that of normal control rats at both 1 and 3 wk of the study (Fig. 2). RER with
leptin treatment alone (L) was significantly reduced to 0.803 ± 0.01 at 1 wk (P < 0.05 vs. V) and regained slightly at
3 wk (0.844 ± 0.010). Reduced feeding in hypothyroid rats (PF)
resulted in 9.0% lower RER values than those in L (P < 0.001). T3 replacement (T+V) normalized RER (at 1 and 3 wk, P = NS vs. respective N values). Leptin treatment
combined with T3 replacement (T+L) displayed a biphasic
change in RER that was significantly reduced at 1 wk but became
elevated to the same level as the normal control (N) and
T3-treated (T+V) groups at 3 wk. The RER values were
initially elevated (0.913 ± 0.006, 1 wk) but later declined
(0.764 ± 0.025, 3 wk) in the T3-replaced
food-restricted (T+PF) rats, in contrast to those in the T+L group.
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Effects of leptin on hormones and metabolites in TX rats.
As shown in Table 2, markedly diminished
total and free T3 levels confirm the hypothyroid status in
all of the TX rats (V, L, and PF), which was totally normalized or
substantially improved in the TX+T3 rats (T+V, T+L, and
T+PF). The extremely diminished total and free T4 levels in
the TX animals were not affected by T3 replacement. All
hypothyroid rats had significantly elevated TSH levels (5- to 6-fold
increments in V, L, and PF) from the level of N (2.2 ± 0.35 ng/ml). The increment of TSH was attenuated by T3
replacement for 3 wk (1.6- to 4-fold increments in T+V, T+L, and T+PF),
with T+PF showing the least increment. These data support the validity
of our TX model and the replacement therapy. Plasma leptin
concentrations were not affected by hypothyroidism (V vs. N,
P = NS) but were significantly decreased
(P < 0.05) by reduced feeding with PF
(P < 0.05, vs. N and V) and T+PF (P < 0.05 vs. N and T+V). Leptin treatment significantly raised plasma leptin concentrations in both L and T+L groups, but the magnitude of
increment was far greater in L than in T+L. Corticosterone levels were
30% lower in the hypothyroid rats (9.92 ± 0.51) than in the
normal rats (14.55 ± 1.41). The low levels of corticosterone appeared to be elevated by reduced food intake with PF
(P < 0.05, vs. V) and T+PF (P < 0.05, vs. T+V), which resembled a mild state of food deprivation.
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DISCUSSION |
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The present study demonstrates for the first time that leptin is
capable of stimulating energy metabolism in the hypothyroid state in
the rat, evidenced by correction of reduced
O2 and
CO2. The effects of leptin are
comparable to those of thyroid hormone replacement therapy. Notably,
after just 1 wk of leptin treatment,
O2
was significantly elevated, whereas
CO2
remained as low as that of untreated hypothyroid rats. This suggests an overall state of increased energy metabolism, with a greater
contribution to calorigenesis by fat oxidation in the early stage of
leptin treatment (18, 20, 21). After 3 wk of leptin
treatment,
CO2 was also significantly
increased and brought back to the normal levels. This, together with a
normalized
O2 value, led to a normal
RER. These data indicate that the metabolic rate stimulated by leptin
was initially (at 1 wk) overweighted with fat oxidation but that later
(after 3 wk) was transitioned toward the use of more balanced energy
substrates (fat, carbohydrate, and protein). Our data strongly suggest
that the effects of leptin in modulating energy metabolism are not
mediated by secondary thyroid messages and are not dependent on a
normal thyroid status. We also demonstrate that a prolonged combined
treatment using leptin and T3 in hypothyroid rats results
in an additive enhancement in energy consumption, reflected by
substantially increased total
O2. A
relatively low but effective dose of T3 replacement
(13) was chosen in this study to avoid artificial
hyperthyroidism, which could obscure the biological actions of leptin.
Such slight underreplacement may affect a number of parameters,
including an insufficient increase in energy intake (seen in the T+V group).
Hypothyroidism is an insufficiency in thyroid hormone production and/or function, with elevated TSH secretion resulting from a lack of negative-feedback stimulation to the hypothalamus and pituitary. Thyroid insufficiency leads to a generalized slowing of calorigenic metabolism, including overall decreases in daily energy intake, oxygen consumption, and thermogenesis (8, 39). The untreated hypothyroid rats (V) appeared to be in a reduced yet balanced state of anabolism and catabolism, evidenced by virtually unchanged body weights, whereas energy consumption rates were substantially decreased. The lack of thyroid hormone also results in stunted growth in these young and otherwise growing rats, as shown in Fig. 1. The body weight increase in the T3-treated rats, especially with T+V, presumably reflects an enhanced anabolism and thus a regain of the retarded growth. This regain of body weight is feasible because T3 was not overreplaced while energy intake was increased. In the present study, leptin has retained its capability in stimulating catabolism and reducing food intake and body weight in the hypothyroid rats with or without T3 replacement. These effects of leptin are comparable to those seen in intact animals (42, 49). Body weight reduction by leptin has been mechanistically associated with increased expression of uncoupling proteins, increased sympathetic neural output, and escalated rates of lipid oxidation and thermogenesis (9, 10, 17, 20). These effects appear to coincide with the physiological role of the thyroid hormones (18, 26). Thus, it has been postulated that the TRH-TSH-T4 axis may be involved in the leptin action mechanism (18, 25, 27, 33).
Leptin has been found to influence thyroid hormone secretion or circulating levels (1, 27, 33). Systemic or central administration of leptin reportedly induces a myriad of neuroendocrine responses. Among all the neuroendocrine responses, neuropeptide Y and glucocorticoid hormones are thought to be implicated in modulation of thyroid activities (23, 40, 46, 50). Leptin can also impact on thyroid hormone secretion by preventing fasting-induced suppression of pro-TRH mRNA expression in the hypothalamic paraventricular nuclei (1, 27). In our previous studies, we have demonstrated significant increases in plasma T4 levels after leptin treatment in both normal (49) and diabetic rats (7). These observations seem to imply a significant involvement of the thyroid gland in the leptin action mechanism.
If thyroid hormones indeed constitute a major component of the action
mechanism of leptin, then removal of thyroid hormones would logically
obliterate or weaken leptin's metabolic actions. However, leptin
treatment alone, in the current study in the hypothyroid rats, was
nearly as potent as in the intact animals in reducing food intake and
body weight and in stimulating oxidative and calorigenic metabolism. On
the other hand, T3 replacement alone in TX rats was as
effective as exogenous leptin treatment in enhancing oxygen consumption
and CO2 production. Thus leptin and thyroid hormones appear
to run in parallel and act independently of each other to achieve
comparable calorigenic effects. These two hormones appear to invoke
distinct mechanisms and yet act in concert and additively to modulate
energy metabolism. The mechanistic distinction between leptin and
thyroid hormones can also be appreciated in the changes of energy
intake and body weight. The lower-than-normal daily energy intake in
hypothyroid (V) rats was further reduced by leptin (L, 25%), whereas
it was increased by 33% with T3 replacement (T+V).
Furthermore, the TX rats suffered an initial growth stunt due to
hypothyroidism. After treatment with T3, they appeared to
have recaptured part of the delayed growth. In contrast, leptin either
caused a net weight loss in TX rats or attenuated
T3-induced weight gain in T+L rats. These differences,
taking place in the face of comparable calorimetric enhancement by
leptin or T3, argue against a mechanistic dependence of the
leptin action on thyroid hormones. Although leptin may induce a
measurable increment in thyroid hormone release, the absence of thyroid
hormones does not hamper the metabolic actions of leptin. Thus the
thyroid gland does not constitute an essential link of the leptin
action pathway.
The values of the RER reflect the sources of oxidative energy
substrates. An RER value close to 1, or to 0.7, indicates a predominant
oxidation of carbohydrates, or fat, respectively (24, 28).
Among the hypothyroid groups with T3 replacement (TX+T3) for 1 wk, the RER values of the leptin-treated group (T+L) were significantly lower than the level of the vehicle control (T+V) and the
pair-fed (T+PF) groups. This indicates that the animals treated with
both leptin and T3 utilized more fat as the oxidative fuel.
This effect was nevertheless contributed primarily by leptin, which by
itself was able to reduce RER while increasing
O2. On the other hand, T3
replacement alone had no effect on RER values, because both
O2 and
CO2 increased in proportion. It may be deduced that leptin plays a more important role in metabolizing fat
than do the thyroid hormones. The discrepancies in the RER,
O2, and
CO2 values and in their
interrelationship invoked by leptin vs. thyroid hormone further support
our thesis that the two hormones are mechanistically distinct.
In light of the similar and yet mutually independent metabolic effects
of leptin and thyroid hormones, we hypothesize that a common downstream
effector site may exist as a converging point for both hormones. One
such possible downstream effector may be the sympathetic nervous system
(9). Leptin has been shown to stimulate thermogenic
metabolism (39), probably involving activated uncoupling
proteins (18, 29). Thyroid hormone can also potentiate the
sympathetic neural activities (36, 37) and enhance
uncoupling protein expression (15). An alternative
downstream site could be the growth hormone (GH), which can be
stimulated by either leptin (6) or thyroid hormones
(16). GH is affected by nutritional states and has well
established effects of stimulating oxidative metabolism
(O2) (32, 48). Although
these two potential factors are discussed, the actual mechanisms are
still poorly understood and await further and more direct evidence.
Leptin has been shown to raise circulating thyroid hormone levels in both normal (49) and diabetic rats (7), but the effect of altered thyroid status on leptin levels is rather controversial. Circulating leptin levels were reportedly elevated (35), decreased (12, 47), or unchanged (10, 45) in hypothyroidism. Body mass index and gender appear to remain the major correlates of circulating leptin concentrations in each of the altered thyroid states, as in the general population (10, 45). In the present study, leptin levels were not significantly affected by the hypothyroid state (P = NS, N vs. V). Thyroid hormones at pharmacological levels stimulate lipogenesis and ob gene expression in vitro (14, 52), whereas the slightly underreplaced T3 in these hypothyroid rats (TX+T3) did not significantly increase leptin levels. The lower leptin levels in the food-restricted groups (PF and T+PF) are likely the consequence of a net catabolic state and a negative weight gain. It is interesting that the leptin concentrations in leptin-treated TX rats (L) were 2.6-fold higher than those in rats with combined leptin and T3 treatment (T+L) despite the same rates of leptin delivery. Leptin is known to be almost totally cleared by the kidneys (11), and hypothyroidism is associated with a significantly impaired renal clearance for creatinine (38). Thus the correction of hypothyroidism by T3 replacement may presumably enhance renal clearance of leptin and attenuate the rise in leptin levels induced by exogenous leptin in the T+L group.
The reduction in plasma insulin levels by leptin is consistent with our previous observations in both intact (49) and diabetic rats (7) and relates to decreases in energy intake, body weight, and blood glucose levels. Reduced levels of corticosterone in the TX rats are likely due to an attenuated tone of the hypothalamic/pituitary/adrenal axis in hypothyroidism and appeared to be corrected by T3 replacement. Leptin treatment (L and T+L) attenuated the decrease of corticosterone in both TX and T+T3 rats and also minimized the fasting-induced rise (as seen with PF and T+PF) in circulating corticosterone, in agreement with the other reports (1, 3).
The significant body weight loss in leptin-treated rats has been
attributed mainly to the loss of visceral fat (2). This may also be the case in the hypothyroid rats, although body composition was not determined. The falling values of triglycerides in
leptin-treated rats (L and T+L) may be related to reduced production
(41), increased oxidative metabolism in the adipose tissue
(49), and a decrease in visceral fat mass
(2). Elevated cholesterol levels in hypothyroid rats were
not corrected by T3 treatment for 3 wk in our experimental
setting. The significant decrease in plasma triglyceride is in part due
to a decrease in triglyceride synthesis (7, 41) and in
part due presumably to an accelerated fat decomposition and oxidation
(49). The decrease in circulating FFA levels in TX rats
presumably resulted from a diminished rate of lipolysis, which appears
to be normalized by leptin but not by T3 replacement.
Although the L and T+L rats consumed the same amount of food as
their corresponding pair-fed groups (PF and T+PF groups), respectively,
leptin treatment appeared to be able to normalize the -OHB levels
regardless of whether hypothyroidism was corrected with T3.
This may be related to a counterbalance of both increased ketone body
production and utilization, because leptin is known to stimulate
ketosis in normal rats (49). The PF and T+PF groups
displayed a unique pattern of hormones (such as cortisol, leptin,
insulin, and T3) and metabolites (such as cholesterol,
triglyceride, glycerol,
-OHB, and FFA) that are distinct from those
of leptin treatment, although their food intake was equivalent. These
differences are in agreement with the current consensus that the
metabolic effects of leptin are largely independent of reduced food intake.
In summary, our study demonstrates for the first time that the effects of leptin in reducing food intake and body weight and in stimulating energy metabolism are not dependent on the presence of the thyroid hormones. The thyroid gland does not constitute an integral component of the leptin action mechanism. However, leptin and thyroid hormones may share some common downstream action sites and can thus act additively, although independently, to enhance calorigenic metabolism.
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ACKNOWLEDGEMENTS |
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We are grateful to Larry Ross and Sylvia Copon for their assistance in conducting the biochemical and hormonal assays, and Tina Langas for secretarial assistance.
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FOOTNOTES |
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This study was supported by Amgen, Inc.
Address for reprint requests and other correspondence: Z.-Q. Shi, Dept. of Pharmacology, Amgen Inc., Mail Stop 15-2-A, One Amgen Center Drive, Thousand Oaks, CA 91320-1789 (E-mail: jshi{at}amgen.com).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 20 January 2000; accepted in final form 7 August 2000.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Ahima, RS,
Prabakaran D,
Mantzoros C,
Qu D,
Lowell B,
Maratos-Flier E,
and
Flier JS.
Role of leptin in the neuroendocrine response to fasting.
Nature
382:
250-252,
1996[ISI][Medline].
2.
Barzilai, N,
Wang J,
Massilon D,
Vuguin P,
Hawkins M,
and
Rossetti L.
Leptin selectively decreases visceral adiposity and enhances insulin action.
J Clin Invest
100:
3105-3110,
1998
3.
Bornstein, SR,
Uhlmann K,
Haidan A,
Ehrhart-Bornstein M,
and
Scherbaum WA.
Evidence for a novel peripheral action of leptin as a metabolic signal to the adrenal gland: leptin inhibits cortisol release directly.
Diabetes
46:
1235-1238,
1997[Abstract].
4.
Campfield, LA,
Smith FJ,
and
Burn P.
Strategies and potential molecular targets for obesity treatment.
Science
280:
1383-1387,
1998
5.
Campfield, LA,
Smith FJ,
Guisez Y,
Devos R,
and
Burn P.
Recombinant mouse OB protein: evidence for a peripheral signal linking adiposity and central neural networks.
Science
269:
546-549,
1995[ISI][Medline].
6.
Carro, E,
Senaris R,
Considine RV,
Casanueva FF,
and
Dieguez C.
Regulation of in vivo growth hormone secretion by leptin.
Endocrinology
138:
2203-2206,
1997
7.
Chinookoswong, N,
Wang JL,
and
Shi ZQ.
Leptin restores euglycemia and normalizes glucose turnover in insulin-deficient diabetes in the rat.
Diabetes
48:
1487-1492,
1999[Abstract].
8.
Cimmino, M,
Mion F,
Goglia F,
Minaire Y,
and
Geloen A.
Demonstration of in vivo metabolic effects of 3,5-di-iodothyronine.
J Endocrinol
149:
319-325,
1996[Abstract].
9.
Collins, S,
Kuhn CM,
Petro AE,
Swick AG,
Chrunyk BA,
and
Surwit RS.
Role of leptin in fat regulation.
Nature
380:
676-677,
1996[ISI].
10.
Corbetta, S,
Englaro P,
Giambona S,
Persani L,
Blum WF,
and
Beccoz P.
Lack of effects of circulating thyroid hormone levels on serum leptin concentrations.
Eur J Endocrinol
137:
659-663,
1997[ISI][Medline].
11.
Cumin, F,
Baum HP,
and
Levens N.
Leptin is cleared from the circulation primarily by the kidney.
Int J Obes
20:
1120-1126,
1996[ISI][Medline].
12.
Diekman, MJM,
Romijn JA,
Endert E,
Sauerwein H,
and
Wiersinga WM.
Thyroid hormones modulate serum leptin levels: observations in thyrotoxic and hypothyroid women.
Thyroid
8:
1081-1086,
1998[ISI][Medline].
13.
Escobar-Morreale, HF,
Del Rey FE,
and
De Escobar MG.
Thyroid hormones influence serum leptin concentrations in the rat.
Endocrinology
138:
4485-4488,
1997
14.
Fain, JN,
and
Bahouth SW.
Effect of tri-iodothyronine on leptin release and leptin mRNA accumulation in rat adipose tissue.
Biochem J
332:
361-366,
1998[ISI][Medline].
15.
Fain, JN,
Coronel EC,
Beauchamp MJ,
and
Bahouth SW.
Expression of leptin and 3-adrenergic receptors in rat adipose tissue in altered thyroid states.
Biochem J
322:
145-150,
1997[ISI][Medline].
16.
Fisher, DA,
Hoath S,
and
Lakshmanan J.
The thyroid hormone effects on growth and development may be mediated by growth factors.
Endocrinol Exp
16:
259-271,
1982[ISI][Medline].
17.
Fruhbeck, G,
Aguado M,
and
Martinez JA.
In vitro lipolytic effect of leptin on mouse adipocytes: evidence for a possible autocrine/paracrine role of leptin.
Biochem Biophys Res Comm
240:
590-594,
1997[ISI][Medline].
18.
Gong, DW,
He Y,
Karas M,
and
Reitman M.
Uncoupling protein-3 is a mediator of thermogenesis regulated by thyroid hormone, beta3-adrenergic agonists, and leptin.
J Biol Chem
272:
24129-24132,
1997
19.
Halaas, JL,
Gajiwala KS,
Maffei M,
Cohen SL,
Chait BT,
Rabinowitz D,
Lallone RL,
Burley SK,
and
Friedman JM.
Weight-reducing effects of the plasma protein encoded by the obese gene.
Science
269:
542-546,
1995.
20.
Hwa, JJ,
Fawzi AB,
Graziano MP,
Ghibaudi L,
Williams P,
Van Heek M,
Davis H,
Rudinski M,
Sybertz E,
and
Strader CD.
Leptin increases energy expenditure and selectively promotes fat metabolism in ob/ob mice.
Am J Physiol Regulatory Integrative Comp Physiol
272:
R1204-R1209,
1997
21.
Hwa, JJ,
Ghibaudi L,
Compton D,
Fawzi AB,
and
Strader CD.
Intracerebroventricular injection of leptin increases thermogenesis and mobilizes fat metabolism in ob/ob mice.
Horm Metab Res
28:
659-663,
1996[ISI][Medline].
22.
Iossa, S,
Lionetti L,
Mollica MP,
Barletta A,
and
Liverini G.
Thermic effect of food in hypothyroid rats.
J Endocrinol
148:
167-174,
1996[Abstract].
23.
Kakucska, I,
Qi Y,
and
Lechan RM.
Changes in adrenal status affect hypothalamic thyrotropin-releasing hormone gene expression in parallel with corticotropin-releasing hormone.
Endocrinology
136:
2795-2802,
1995[Abstract].
24.
King, RF,
Almond DJ,
Oxby CB,
Holmfield JH,
and
McMahon MJ.
Calculation of short-term changes in body fat from measurement of respiratory gas exchange.
Metabolism
33:
826-832,
1984[ISI][Medline].
25.
Korbonits, M.
Lepin and the thyroida puzzle with missing pieces.
Clin Endocrinol
49:
569-572,
1998[ISI][Medline].
26.
Lanni, A,
Moreno M,
Lombardi A,
and
Goglia F.
3,5-Diiodo-L-thyronine and 3,5,3'-triiodo-L-thyronine both improve the cold tolerance of hypothyroid rats, but possibly via different mechanisms.
Eur J Physiol
436:
407-414,
1998[ISI][Medline].
27.
Legradi, G,
Emerson CH,
Ahima RS,
Flier JS,
and
Lechan RM.
Leptin prevents fasting-induced suppression of prothyrotropin-releasing hormone messenger ribonucleic acid in neurons of the hypothalamic paraventricular nucleus.
Endocrinology
138:
2569-2576,
1997
28.
Lindmark, L,
Rudholm L,
and
Lundholm K.
A method for measuring energy metabolism and whole body oxidation in small animals with special reference to experimental metabolism and nutrition.
Scand J Clin Lab Invest
46:
273-281,
1986[ISI][Medline].
29.
Liu, Q,
Bai C,
Chen F,
Wang R,
MacDonald T,
Gu M,
Zhang Q,
Morsy MA,
and
Caskey CT.
Uncoupling protein-3: a muscle-specific gene upregulated by leptin in ob/ob mice.
Gene
207:
1-7,
1998[ISI][Medline].
30.
Montague, CT,
Farooqi IS,
Whitehead JP,
Soos MA,
Rau H,
Wareham NJ,
Sewter CP,
Digby JE,
Mohammed SN,
Hurst JA,
Cheetham CH,
Earley AR,
Barnett AH,
Prins JB,
and
O'Rahilly S.
Congenital leptin deficiency is associated with severe early-onset obesity in humans.
Nature
387:
903-908,
1997[ISI][Medline].
31.
Morgan, CR,
and
Lazarow A.
Immunoassay of insulin: two antibody system. Plasma insulin levels in normal, subdiabetic and diabetic rats.
Diabetes
12:
115-126,
1963[ISI].
32.
Nass, R,
Huber RM,
Klauss V,
Muller OA,
Schopohl J,
and
Strasburger CJ.
Effect of growth hormone (hGH) replacement therapy on physical work capacity and cardiac and pulmonary function in patients with hGH deficiency acquired in adulthood.
J Clin Endocrinol Metab
80:
552-557,
1995[Abstract].
33.
Orban, Z,
Bornstein SR,
and
Chrousos GP.
The interaction between leptin and the hypothalamic-pituitary-thyroid axis.
Horm Metab Res
30:
231-235,
1998[ISI][Medline].
34.
Pelleymounter, MA,
Cullen MJ,
Baker MB,
Hecht R,
Winters D,
Boone T,
and
Collins F.
Effects of the obese gene product on body weight regulation in ob/ob mice.
Science
269:
540-543,
1995[ISI][Medline].
35.
Pinkney, JH,
Goodrick SJ,
Katz J,
Johnson AB,
Lightman SL,
Coppack SW,
and
Mohamed-Ali V.
Leptin and the pituitary-thyroid axis: a comparative study in lean, obese, hypothyroid and hyperthyroid subjects.
Clin Endocrinol
49:
583-588,
1998[ISI][Medline].
36.
Rubio, A,
Raasmaja A,
Maia AL,
Kim KR,
and
Silva JE.
Effects of thyroid hormone on norepinephrine signaling in brown adipose tissue. I. Beta 1- and beta 2-adrenergic receptors and cyclic adenosine 3',5'-monophosphate generation.
Endocrinology
136:
3267-3276,
1995[Abstract].
37.
Rubio, A,
Raasmaja A,
and
Silva JE.
Thyroid hormone and norepinephrine signaling in brown adipose tissue. II: Differential effects of thyroid hormone on beta 3-adrenergic receptors in brown and white adipose tissue.
Endocrinology
136:
3277-3284,
1995[Abstract].
38.
Sagawa, K,
Murer H,
and
Morris ME.
Effect of experimentally induced hypothyroidism on sulfate renal transport in rats.
Am J Physiol Renal Physiol
276:
F164-F171,
1999
39.
Scarpace, PJ,
Matheny M,
Pollock BH,
and
Tuemer N.
Leptin increases uncoupling protein expression and energy expenditure.
Am J Physiol Endocrinol Metab
273:
E226-E230,
1997
40.
Schwartz, MW,
Baskin DG,
Bukowski TR,
Kuijper JL,
Foster D,
Lasser G,
Prunkard DE,
Porte D, Jr,
Woods SC,
Seeley RJ,
and
Weigle DS.
Specificity of leptin action on elevated blood glucose levels and hypothalamic neuropeptide Y gene expression in ob/ob mice.
Diabetes
45:
531-535,
1996[Abstract].
41.
Shi, ZQ,
and
Nelson A.
Chronic leptin treatment decreases triglyceride production in the normal rat (Abstract).
Diabetes
46, Suppl1:
190A,
1997.
42.
Shi, ZQ,
Nelson A,
Whitcomb L,
Wang J,
and
Cohen A.
Intracerebroventricular administration of leptin markedly enhances insulin sensitivity and systemic glucose utilization in conscious rats.
Metabolism
47:
1274-1280,
1998[ISI][Medline].
43.
Silva, JE.
Thyroid hormone control of thermogenesis and energy balance.
Thyroid
5:
481-492,
1995[ISI][Medline].
44.
Spira, O,
Birkenfeld A,
Gross J,
and
Gordon A.
TSH synthesis and release in the thyroidectomized rats: a) effect of short and long-term hypothyroidism.
Acta Endocrinol
92:
489-501,
1979[ISI][Medline].
45.
Sreenan, S,
Carro JF,
and
Refetoff S.
Thyroid dysfunction is not associated with alterations in serum leptin levels.
Thyroid
7:
407-409,
1997[ISI][Medline].
46.
Stephens, TW,
Basinski M,
Bristow PK,
Bue-Valleskey JM,
Burgett SG,
Craft L,
Hale J,
Hoffmann J,
Hsiung HM,
Kriauciunas A,
Mackellar W,
Rosteck PR, Jr,
Schoner B,
Smith D,
Tinsley FC,
Zhang XY,
and
Heiman M.
The role of neuropeptide Y in the antiobesity action of the obese gene product.
Nature
377:
530-532,
1995[ISI][Medline].
47.
Vacavi, R,
Zini X,
Peino R,
Casanueva FF,
and
Dieguez C.
Influence of thyroid status on serum immunoreactive leptin levels.
J Clin Endocrinol Metab
82:
1632-1634,
1997
48.
Vahl, N,
Jorgensen JOL,
Jurik AG,
and
Christiansen JS.
Abdominal adiposity and physical fitness are major determinants of the age associated decline in stimulated GH secretion in healthy adults.
J Clin Endocrinol Metab
81:
2209-2215,
1996[Abstract].
49.
Wang, JL,
Chinookoswong N,
Scully S,
Qi M,
and
Shi ZQ.
Differential effects of leptin in regulation of tissue glucose utilization in vivo.
Endocrinology
140:
2117-2124,
1999
50.
Wang, Q,
Bing C,
Al-Barazanji K,
Mossakowaska DE,
Wang XM,
McBay DL,
Neville WA,
Taddayon M,
Pickavance L,
Dryden S,
Thomas ME,
McHale MT,
Gloyer IS,
Wilson S,
Buckingham R,
Arch JR,
Trayhurn P,
and
Williams G.
Interactions between leptin and hypothalamic neuropeptide Y neurons in the control of food intake and energy homeostasis in the rat.
Diabetes
46:
335-341,
1997[Abstract].
51.
Wu-Peng, XS,
Chua SC, Jr,
Okada N,
Liu SM,
Nicolson M,
and
Leibel RL.
Phenotype of the obese Koletsky (f) rat due to Tyr763Stop mutation in the extracellular domain of the leptin receptor (Lepr): evidence for deficient plasma-to-CSF transport of leptin in both the Zucker and Koletsky obese rat.
Diabetes
46:
513-518,
1997[Abstract].
52.
Yoshida, T,
Monkawa T,
Hayashi M,
and
Saruta T.
Regulation of expression of leptin mRNA and secretion of leptin by thyroid hormone in 3T3-L1 adipocytes.
Biochem Biophys Res Comm
232:
822-826,
1997[ISI][Medline].
53.
Zhang, Y,
Proenca R,
Maffei M,
Barone M,
Leopold L,
and
Friedman JM.
Positional cloning of the mouse obese gene and its human homologue.
Nature
372:
425-431,
1994[ISI][Medline].
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