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1 Division of Molecular Genetics, Department of Pediatrics, and the Naomi Berrie Diabetes Center, Columbia University College of Physicians and Surgeons, New York 10032; and 2 New York Obesity Research Center, St. Luke's-Roosevelt Hospital, Columbia University College of Physicians and Surgeons, New York, New York 10025
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The relationship of leptin
gene expression to adipocyte volume was investigated in lean
10-wk-old male C57BL/6J mice. mRNA levels for leptin, insulin receptor,
glucocorticoid receptor, and tumor necrosis factor (TNF)-
in
inguinal, epididymal, and retroperitoneal adipose tissues were
quantified and related to adipocyte volume. Leptin mRNA levels were
highly correlated with adipocyte volume within each fat depot. Multiple
regression analysis of pooled data from the three depots showed that
leptin mRNA levels were strongly correlated with adipocyte volumes
(
= 0.84, P < 0.001) and, to a smaller degree,
with glucocorticoid receptor mRNA levels ( = 0.36, P < 0.001). Depot of origin had no effect (P > 0.9). Rates of leptin secretion in vitro were
strongly correlated with leptin mRNA levels (r = 0.89, P < 0.001). mRNA levels for TNF-, insulin receptor,
and glucocorticoid receptor showed no significant correlation with
adipocyte volume. These results demonstrate that depot-specific
differences in leptin gene expression are mainly related to the volumes
of the constituent adipocytes. The strong correlation between leptin
gene expression and adipocyte volume supports leptin's physiological
role as a humoral signal of fat mass.
glucocorticoid receptor; insulin receptor; tumor necrosis
factor-
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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A CRITICAL QUESTION in the physiology of body weight regulation is how the size of somatic energy stores (fat mass) is signaled to the brain and other organs. Leptin is a compelling candidate for such a signal, in part, because its plasma concentration is highly correlated with adiposity in both humans and rodents (7, 15, 33, 44, 49). However, because of evident depot-specific differences in leptin gene expression (23, 30, 34, 37, 41, 45, 59), it is not clear how leptin production from each fat depot is coordinated to provide an accurate signal with regard to the total fat mass.
Depot-specific differences in leptin gene expression have been observed in both human and rodent adipocytes (23, 30, 34, 37, 41, 45, 59). Leptin mRNA levels are higher in rat gonadal and retroperitoneal (intra-abdominal depots) than in inguinal (subcutaneous) adipose tissues (30, 41, 59). In humans, leptin mRNA levels and secretion rates are three- to fivefold higher in subcutaneous than omental adipose tissue (23, 34, 37, 45, 52). Because omental adipocytes are smaller than subcutaneous adipocytes in humans, such depot-specific differences in leptin gene expression may be related to differences in adipocyte size (23, 52). Consistent with this idea, leptin mRNA levels and rates of leptin secretion in both subcutaneous and omental adipose tissues are positively correlated with body mass index, which is usually correlated with adipocyte volume (25, 52). Contrary to these findings, leptin mRNA levels and secretion rates are higher in subcutaneous adipocytes than in omental adipocytes from obese human subjects, even though average adipocyte volumes are similar between the two depots (45), suggesting that adipocyte size may not be the main determinant of leptin gene expression level in large adipocytes from severely obese patients. In addition, short-term fasting dramatically decreases plasma leptin and adipose tissue leptin mRNA before having significant effects on total adiposity or adipocyte size in both humans and rodents (43, 47, 59), suggesting that acute energy balance also plays an important role in determining leptin gene expression. Such effects may be mediated, in part, by circulating concentrations of insulin, glucocorticoids, and catecholamines.
In rodents, insulin administration increases leptin gene expression during fasting and prevents the decrease of leptin mRNA and protein levels induced by caloric restriction (47, 59). Insulin also increases leptin mRNA levels in 3T3-L1 and 3T3-F442A adipocytes in vitro (29, 32). However, insulin appears to have no acute effect on leptin mRNA levels in adipocytes isolated from fed rats in vitro (4, 47). Prolonged infusion of exogenous insulin in the context of a euglycemic clamp increases plasma leptin concentrations in humans (2, 25). Acute effects of insulin infusion at physiological concentrations in the presence of a euglycemic clamp on leptin production in human subjects appear to be more variable, ranging from no effect to increased plasma leptin concentration (2, 25, 46). It has been suggested that the effects of insulin on leptin gene expression are secondary to its effects on glucose and lipid metabolism (38, 40, 46, 48). Insulin also increases leptin secretion in rat and human adipose tissues in vitro in the absence of significant effects on leptin mRNA levels, suggesting that insulin may have effects on the release of a stored pool of leptin (4, 45).
Glucocorticoids increase leptin mRNA expression and secretion in human and rodent adipocytes both in vivo and in vitro (4, 17, 45, 50, 54). Glucocorticoid receptor mRNA levels are higher in omental (in humans) or epididymal (in rats) than subcutaneous adipocytes (36, 42). Consistent with these results, human omental, relative to subcutaneous, adipose tissues appear to be more responsive to dexamethasone's effects on leptin gene expression in vitro (45, 54). Together, these results suggest that adipocyte sensitivity to glucocorticoids may play a role in depot-specific differences in leptin gene expression.
Increased tumor necrosis factor (TNF)- gene expression in adipocytes
of obese and aging humans and rodents has been linked to insulin
resistance (21, 22). Correlations of adipocyte TNF-
mRNA and activity levels with adiposity and adipocyte volumes have been
observed in rats (39), suggesting that the level of gene
expression for both TNF- and leptin may be related to adipocyte volume in a similar way.
In the experiments reported here, we quantified mRNA levels of leptin,
insulin receptor, glucocorticoid receptor, and TNF- and related
these to cell volume and rates of leptin secretion in adipocytes from
three major fat depots of 10-wk-old C57BL/6J male mice. The
relationship of mRNA levels of insulin receptor, glucocorticoid
receptor, and TNF- to leptin mRNA levels was also analyzed. Lean
young adult male mice were used to minimize possible confounding
effects of obesity, aging, or cyclical changes in gonadal steroids. We
conclude that leptin mRNA levels and rates of leptin secretion are
highly correlated with adipocyte volume and that adipocyte volume is
the major factor accounting for differences in the levels of leptin
gene expression among fat depots in lean young mice.
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MATERIALS AND METHODS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Animals and adipose tissue sample collection. Male C57BL/6J mice were purchased from Jackson laboratory at 8 wk of age and were maintained in a barrier facility (12:12-h light-dark cycle) for another 2 wk with ad libitum access to regular rodent chow (Purina rodent chow 5035). Animals were deprived of food for 5 h before death and were killed by CO2 asphyxia at the 8th hour into the light cycle. Adipose tissues from inguinal, epididymal, and retroperitoneal pads were dissected and immediately frozen in liquid nitrogen for RNA extraction. Adipose tissue from the contralateral pad was dissected and fixed in Bouin's fixative for measurement of adipocyte size in paraffin sections. In another set of animals, these contralateral inguinal or epididymal fat pads were separately pooled and used to measure the rate of leptin secretion and leptin mRNA levels.
Determination of mRNA levels for leptin, insulin receptor,
glucocorticoid receptor, and TNF-.
Total RNA was extracted from adipose tissue using TriZol reagent
(GIBCO, Bethesda, MD). About 1-5 µg of total RNA was
reverse-transcribed into cDNA using a random hexamer and M-MLV reverse
transcriptase. cDNA was quantitatively amplified by PCR using specific
primers for leptin (21 cycles), insulin receptor (22 cycles),
glucocorticoid receptor (22 cycles), or TNF- (28 cycles) in
combination with primers for -actin (17 cycles) (as an internal
control) in the first PCR amplification as previously described
(58). Amplification of each cDNA was performed on the
linear portion of the amplification curve. Primer sequences for all
genes tested are listed in Table 1.
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were
expressed as a ratio to -actin mRNA to normalize for initial RNA input.
Determination of adipocyte volume.
The average diameter of adipocytes in each tissue sample was determined
using a photomicrographic method described by Di Girolamo et al.
(10) and validated by Ashwell et al.
(1). Briefly, paraffin-embedded adipose tissue was
cut into 7-µm sections and processed and stained with hematoxylin and
eosin. Fields of representatively sized cells were photographed at
×100 magnification. Care was taken to choose an area with minimal
stromal-vascular cells. The total number of cells in a 0.5 × 0.5-mm area (calibrated with micrometer) was tallied in each of three
discontinuous sections for every single tissue sample. Cells were
counted only if more than one-half of their area fell within the
defined perimeter. The mean adipocyte diameter for each sample was
calculated on the basis of the average number of cells in the defined
area using the following formula: diameter = 1.1 × 2 × [area/(cell number × 
)]1/2. The
correction factor of 1.1 was included in the calculation of the average
diameter of adipocytes as suggested by Ashwell et al. (1),
who compared the result obtained by the above photomicrographic method
to the result obtained by measuring the diameters of adipocytes released from a tissue matrix by collagenase digestion. The average adipocyte volume, expressed as micrograms lipid per cell, was then
calculated using the calculated average diameter and lipid density of
0.915 g/ml.
Measurement of the rate of leptin secretion. The rate of leptin secretion by adipose tissue was measured as described by Van Harmelen et al. (52). Briefly, fresh adipose tissue from respective depots of 10 mice was cut into 10- to 15-mg fragments and incubated in Krebs-Ringer bicarbonate buffer (gassed with 5% C02-95% O2 for 45 min before the incubation) containing 1 g/l glucose and 40 g/l fatty acid-free BSA. Inguinal adipose tissue (150-200 mg) or epididymal adipose tissue (200-300 mg) was incubated in 3 ml of buffer for 2 h at 37°C with constant shaking. After the incubation, 2.5 ml of the incubation medium was lyophilized and resuspended in 0.25 ml of water. Two 0.1-ml aliquots of each sample were assayed in duplicate for leptin using a radioimmunoassay for murine leptin (Linco, St. Charles, MO). Aliquots of 25-50 mg of adipose tissues obtained before and after the incubation were frozen in liquid nitrogen for leptin mRNA assay.
Statistical analyses.
Statistica version 6.0 was used for all analyses. One-way ANOVA was
used to assess differences among depots. Post hoc comparison (Newman-Keuls test) was used to compare the difference between any two
depots. With pooled data from the three depots, multiple regression
analysis was used to assess the effects of adipocyte volume and of mRNA
levels of insulin receptor, glucocorticoid receptor, and TNF-
(independent variables) on leptin mRNA levels in adipose tissue
(dependent variable). To further investigate depot-of-origin
effects, we included two dummy variables representing the three depots.
Simple correlation analysis was used to examine the relationships
between adipocyte volume and mRNA levels for leptin, insulin receptor,
glucocorticoid receptor, and TNF- and between the rates of leptin
secretion and leptin mRNA levels in adipose tissues within each depot
and in the pooled data. The differences in intercepts and slopes of the
regression lines relating leptin mRNA and adipocyte volume among depots
were tested using a multiple regression of leptin mRNA levels on
adipocyte volume, depot of origin (through dummy variables), and
interactions between depot of origin and adipocyte volume. The
coefficients for these interaction terms represent the differences in
slope among these depots. Similarly, the coefficients for the depots
represent the differences in intercepts. A two-tailed P
value <0.05 was considered to indicate statistical significance.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Depot-specific differences in adipocyte volume and in mRNA levels
for leptin, insulin receptor, glucocorticoid receptor, and TNF-.
Adipocyte volume and mRNA levels of leptin, insulin receptor,
glucocorticoid receptor, and TNF- in inguinal, epididymal, and
retroperitoneal adipose tissues of 10-wk-old male C57BL/6J mice
(n = 15) are shown in Fig.
1. Adipocyte volumes and leptin mRNA
levels were significantly lower in inguinal adipose tissue than in
epididymal and retroperitoneal adipose tissues (Fig. 1). Adipocyte
volumes were similar between epididymal and retroperitoneal adipose
tissues, but leptin mRNA levels were higher in epididymal adipose
tissue than in retroperitoneal adipose tissue, although that difference
was not statistically significant. Insulin receptor mRNA levels were
also lower in inguinal adipose tissue than in epididymal and
retroperitoneal adipose tissues, and the difference between inguinal
and retroperitoneal adipose tissues was statistically significant
(P < 0.05). Glucocorticoid receptor mRNA levels were significantly higher in epididymal adipose tissue than in inguinal and
retroperitoneal adipose tissues. TNF- mRNA levels were significantly higher in inguinal adipose tissue than in epididymal and
retroperitoneal adipose tissues. Average adipocyte volume was smaller
in inguinal adipose tissue than in epididymal and retroperitoneal
adipose tissues. Thus, unlike in rats (39), TNF-
expression levels were not positively correlated with adipocyte volumes
in these mice.
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Relationships of leptin mRNA levels to adipocyte volume and mRNA
levels of insulin receptor and glucocorticoid receptor.
Figure 2 shows correlations of adipocyte
volume with mRNA levels for leptin, insulin receptor, glucocorticoid
receptor, and TNF- in inguinal, epididymal, and retroperitoneal
adipose tissues. There were significant correlations between leptin
mRNA levels and adipocyte volumes in each of the three depots
(r = 0.63, P < 0.05 in inguinal
adipose tissue; r = 0.85, P < 0.001 in
epididymal adipose tissue; r = 0.83, P < 0.001 in retroperitoneal adipose tissue) (Fig. 2A). The
slopes of the regression lines differed significantly between
epididymal and retroperitoneal adipose tissues (P < 0.05) but not between epididymal and inguinal adipose tissues (P = 0.14) or between retroperitoneal and inguinal
adipose tissues (P = 0.92). Intercepts of these
regression lines were not significantly different among the three
depots. These data suggest that although the major proportion of
variance in leptin mRNA expression is accounted for by adipocyte
volume, there are detectable depot-specific effects as well. Leptin
mRNA levels and adipocyte volumes were also strongly correlated in
pooled data from the three depots (r = 0.80, P < 0.001) (solid bold line in Fig. 2A). In
contrast, there was no significant correlation between adipocyte
volumes and mRNA levels for insulin receptor, glucocorticoid receptor, or TNF- within each depot (Fig. 2, B-D)
or in the pooled data (solid bold lines in Fig. 2,
B-D). These results suggest that, unlike
leptin, depot-specific differences in the expression levels of these
genes were not related to adipocyte size. A significant negative
correlation was found between adipocyte volumes and mRNA level for
glucocorticoid receptor in retroperitoneal adipose tissue (r
= 
0.75, P < 0.01). The underlying mechanism for
this correlation is not clear but may be related to the anatomic
proximity of retroperitoneal adipose tissue to the adrenal grand, which
is the major source of circulating glucocorticoids.
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inhibits leptin gene expression
(27, 35, 51) and was also included as an independent
variable in the multiple regression. Depot of origin was also included
through dummy variables (Table 2). This
analysis confirmed the strong, positive correlation between adipocyte
volume and leptin mRNA levels ( = 0.84, P < 0.001) and also showed a significant positive correlation between leptin mRNA levels and glucocorticoid receptor mRNA levels ( = 0.36, P < 0.01). Adipocyte volume and glucocorticoid
receptor mRNA levels accounted for 64 and 9% of the variation in
leptin mRNA levels in these tissues, respectively (total
R2 = 0.80). Insulin receptor mRNA levels
( = 0.12, P = 0.23) and TNF- mRNA levels
( = 0.18, P = 0.10) showed no significant
correlation with leptin mRNA levels in these mice. Including depot of
origin as an independent variable in the regression analysis had no
significant effect on either the standardized correlation coefficients
( values) for adipocyte volume and glucocorticoid receptor mRNA levels or the total R2 value (Table 2),
suggesting that depot-specific factors other than adipocyte volume and
glucocorticoid receptor expression level play a minimal role in
depot-specific differences in leptin gene expression. Omitting
glucocorticoid receptor mRNA level in the multiple regression analysis
resulted in lower total R2 value (0.72) and
significant effects of depot of origin (epididymal vs. retroperitoneal
adipose tissues, P < 0.05), suggesting that the
depot-specific differences in leptin mRNA levels between epididymal and
retroperitoneal adipose tissues are accounted for, in part, by the
depot-specific differences in glucocorticoid receptor mRNA expression.
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Correlation between rates of leptin secretion and leptin mRNA
levels.
To further assess the relationship between leptin secretion rates and
leptin mRNA levels in adipose tissues from different depots, we
measured rates of leptin secretion and leptin mRNA levels in inguinal
and epididymal adipose tissues in a separate group of 10-wk-old male
C57BL/6J mice (n = 10). Due to the small amounts of
retroperitoneal adipose tissue in each mouse, these measures were not
done in retroperitoneal adipose tissue. Rates of leptin secretion were
highly correlated with leptin mRNA levels in either inguinal or
epididymal adipose tissues (r = 0.75, P < 0.05 for inguinal adipose tissue; r = 0.67, P < 0.05 for epididymal adipose tissue) (Fig.
3A). When the data from both
depots were pooled, there was also a significant correlation between
the rates of leptin secretion and leptin mRNA levels (r = 0.89, P < 0.001), suggesting that adipocyte volume
is the major determinant of both leptin mRNA levels and secretion rates
in these mice. Leptin mRNA levels measured before and after the 2-h
incubation (for leptin secretion assay) were highly correlated with
each other (r = 0.88, P < 0.001; Fig.
3B), although leptin mRNA levels were decreased by 35%
after the incubation. The regression parameters (slopes and intercepts)
in the two depots in Fig. 3, A and B, were not significantly different. No depot effect was detected in multiple regression analysis when the rate of leptin secretion was the dependent
variable and leptin mRNA level (measure taken either before or after
the 2-h incubation, or the average of the 2 measures) and depot of
origin were treated as independent variables. Leptin mRNA levels (pre-
or postincubation or the average of the 2 measures) accounted for
70-80% of the variation in rates of leptin secretion.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The central result of the present study is that rates of leptin
mRNA expression and secretion in three anatomically distinct adipose
tissue depots of lean young adult mice are primarily accounted for by
adipocyte volume. The strong correlation of mRNA levels with adipocyte
volume in the three depots is unique to leptin among the genes
examined. Expression levels of insulin receptor, glucocorticoid
receptor, and TNF- showed no significant correlation with
adipocyte volume in the same mice. The strong correlation between
leptin secretion rates and adipocyte leptin mRNA levels suggests that
the synthesis and release of leptin are closely coupled and that leptin
secretion is mainly regulated by transcription. The acute approximately
twofold increase in leptin secretion induced by insulin points to the
existence of an intracellular pool of leptin (4, 45). Our
results indicate that, if leptin is stored intracellularly, the size of
the intracellular pool for leptin storage is likely to be small.
Although it has been suggested that leptin gene expression may be
correlated with adipocyte size (23, 31, 52), this report
represents the first documentation of a strong correlation between
these two parameters in different fat depots without the potential
confounding effects of obesity or aging. The strong correlation between
leptin gene expression and adipocyte volume is consistent with the idea
that leptin production by adipocytes can report their volumes. Adipose
tissue mass is the product of adipocyte size and number, and hence the
strong correlation of circulating leptin with total fat mass.
In addition to the predominant effect of adipocyte volume on leptin mRNA levels, which accounts for 64% of the variation in leptin mRNA levels among adipose tissue depots, glucocorticoid receptor mRNA level also has a smaller but significant positive effect on leptin mRNA levels in adipocytes, accounting for 9% of the variation in leptin mRNA levels in these mice. The significantly higher glucocorticoid receptor mRNA levels in epididymal adipose tissue than in retroperitoneal and inguinal adipose tissues apparently account for the higher leptin mRNA levels in adipocytes of similar volumes in epididymal adipose tissue than in retroperitoneal adipose tissue (as indicated by the steeper slope of the regression line of leptin mRNA levels on adipocyte volumes in epididymal adipose tissue than in retroperitoneal and inguinal adipose tissues), constituting a depot-specific difference in leptin gene expression that is not related to adipocyte volume. These results are consistent with previous reports that glucocorticoids increase leptin gene expression in both human and rodent adipocytes and that glucocorticoid receptor mRNA levels are higher in epididymal adipose tissue than in inguinal adipose tissue in rats (4, 17, 26, 42, 45, 50).
The lack of correlation of insulin receptor mRNA levels with leptin mRNA levels suggests that the difference in the level of insulin receptor gene expression among anatomically distinct fat depots is not a major determinant of depot-specific differences in leptin gene expression in these mice. Because the mice in this study were in the postprandial state, this result is consistent with previous observations that insulin lacks direct, acute effects on leptin gene expression in the fed state (25, 47). However, differences in insulin receptor expression levels (as an indirect measure of insulin sensitivity) may have differential effects on leptin gene expression among anatomically distinct fat tissues in the fasted state. In lean rats and mice, a 24-h fast decreases leptin mRNA levels dramatically in epididymal and retroperitoneal adipose tissues but has little effect on leptin mRNA levels in inguinal adipose tissue (Ref. 59 and Zhang, unpublished results), consistent with our finding that insulin receptor mRNA levels are higher in epididymal and retroperitoneal adipose tissues than in inguinal adipose tissue in lean mice. These results suggest that leptin gene expression levels in adipocytes are proportional to adipocyte volume only in the fed state and that fasting or negative energy balance states may interrupt the relationship of adipocyte volume to leptin gene expression. In rats, fasting-induced decreases of leptin mRNA levels in epididymal and retroperitoneal adipose tissues can be largely prevented by insulin administration (47, 59), suggesting that in the fed state, insulin plays an important role in maintaining the high levels of leptin expression in epididymal and retroperitoneal adipose tissues.
There was no significant effect of depot of origin or TNF- mRNA
levels on leptin mRNA levels after adipocyte volumes and glucocorticoid
receptor mRNA levels were taken into account, suggesting TNF- and
other depot-specific factors play a minimal role in depot-specific
differences in leptin gene expression in these lean mice. TNF- mRNA
and activity levels are correlated with adipocyte volume and age in
rats (39). Our results in young mice suggest that the
increased TNF- gene expression in these rats may be related to age
or other physiological factors rather than to adipocyte volumes per se.
The mechanism underlying the strong correlation between leptin gene
expression and adipocyte volume in these mice is not clear. Changes in
membrane cholesterol concentration related to adipocyte size have been
proposed as regulators of gene expression in adipocytes (28). Decreased membrane cholesterol concentration may
result in increased sterol regulatory element binding protein 2 (SREBP-2) gene expression; both of these findings are characteristics
of hypertrophic adipocytes in rodents (3, 16). SREBP-2
may, in turn, function as a coordinate regulator of gene expression in
adipocytes. Manipulation of cholesterol concentrations in 3T3-L1 adipocytes does not alter leptin gene expression but does change expression patterns of several other genes, including TNF-
(28). Alternatively, leptin expression levels may not be
determined by adipocyte size per se, but by rates of nutrient uptake
(e.g., free fatty acid and glucose) and anabolic metabolism, which, in turn, may be correlated with adipocyte size under certain physiological conditions. As noted earlier, leptin mRNA expression is extremely sensitive to acute caloric restriction and fasting (43, 47, 59). We have also shown that leptin mRNA levels in white adipose tissues are responsive to caloric balance in rat pups
(57). UDP-N-acetylglucosamine, an end product
of the hexosamine biosynthetic pathway whose intracellular
concentration is increased when nutrient levels are high (in anabolic
state), increases leptin gene expression in muscle and adipocytes
(53). In contrast to prevailing notions of declining
responsiveness of adipocyte glucose uptake to insulin stimulation with
increasing fat cell size in older, more obese animals (5, 8, 9,
56), Hood et al. (20) reported that basal and
insulin-stimulated uptake rates for glucose and palmitate were
positively correlated with adipocyte size in 14-wk-old lean Zucker
rats. This result is of particular interest because the effects of
adipocyte size were examined in adipocytes isolated from a single
epididymal fat pad, whereas many studies of the effects of adipocyte
size on glucose uptake and insulin sensitivity have been done in
adipocytes from individual animals that vary in age, degree of
adiposity, or diet regimen. Positive correlations of adipocyte size and
rates of de novo fatty acid/acylglyceride synthesis have been reported
in 3-mo-old pigs (11).
In summary, we have shown that leptin gene expression is highly correlated with adipocyte volume in epididymal, inguinal, and retroperitoneal adipose tissues of fed, lean 10-wk-old male mice, and the depot-specific differences in rates of leptin gene expression are primarily accounted for by differences in adipocyte volume. Adipocyte size is a critical aspect of adipose tissue mass with regard to systemic metabolic changes because adipocyte number is relatively stable in adult humans and rodents (12, 13, 19). The strong correlation between levels of leptin mRNA expression/secretion and adipocyte size supports leptin's importance as a signal of changes in fat mass.
Perspectives
The so-called "lipostatic" hypothesis postulated primarily by Kennedy (24) and Hervey (18) nearly 50 yr ago suggested that somatic fat stores somehow signal the brain regarding the status of systemic energy balance. Hirsch (19) subsequently emphasized that total fat mass was the product of cell number and cell size and that cell size reflects short- and intermediate-term changes in total fat mass. Therefore, whatever signal is being provided by fat stores, it was predicted that it would be proportionate to fat cell volume. Insulin, in fact, has this characteristic and can provide signals to the central nervous system (hypothalamus) proportionate to fat cell volume (55). The parabiosis experiments of Coleman (6) suggested that the ob gene product (leptin) might also be a humoral signal proportionate to fat mass. The cloning and characterization of the leptin gene confirmed that inference (14). In the studies reported here, leptin gene expression and protein production are shown to closely obey the predicted relationship to adipocyte volume across anatomically distinct fat depots in ad libitum-fed nonobese mice. The responsiveness of leptin gene expression to caloric balance in adipocytes and to adipocyte volume enables this secreted molecule to provide a precise measure of the size of adipose tissue energy stores as well as early warning of potential deficits in these stores. This demonstration provides a critical link between the molecular genetics of energy homeostasis and its realization in systemic physiology. It is important to note that the molecular bases for the proportionality of circulating insulin and leptin to fat cell volume are unknown. The mechanisms are likely to be quite different, and answers to these questions would provide deep insights into the cell biology of energy homeostasis.| |
ACKNOWLEDGEMENTS |
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We thank Q. Li and M. Su for assistance in preparing and staining of adipose tissue sections, and Dr. Y. Yu for helpful discussion.
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FOOTNOTES |
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This work was supported in part by the Career Development Award from the American Diabetes Association (to Y. Zhang) and by National Institute of Diabetes and Digestive and Kidney Diseases Grants R01-DK-52431 and 2-P-30-DK-26687 (to R. L. Leibel).
Address for reprint requests and other correspondence: Y. Zhang, Div. of Molecular Genetics, Columbia Univ., Russ Berrie Pavilion, 1150 St. Nicholas Ave., New York, NY 10032 (E-mail: yz84{at}columbia.edu).
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.
10.1152/ajpregu.00392.2001
Received 9 July 2001; accepted in final form 24 September 2001.
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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), epididymal
(
), and retroperitoneal (
) adipose
tissues of 10-wk-old male C57BL/6J mice (for separate depots, fine
dashed or dotted lines). Regression lines (solid bold lines) and
corresponding r and P values of the pooled data
from the 3 depots are shown.
