1 Obesity Research Center, St. Luke's-Roosevelt Hospital Center, College of Physicians and Surgeons, Columbia University, New York, New York 10025; and 2 Department of Nutritional Sciences, Rutgers University, New Brunswick, New Jersey 08901
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Visceral obesity is associated with resistance to the antilipolytic effect of insulin in vivo. We investigated whether subcutaneous abdominal and gluteal adipocytes from viscerally obese women exhibit insulin resistance in vitro. Subjects were obese black and white premenopausal nondiabetic women matched for visceral adipose tissue (VAT), total adiposity, and age. Independently of race and adipocyte size, increased VAT was associated with decreased sensitivity to insulin's antilipolytic effect in subcutaneous abdominal and gluteal adipocytes. Absolute lipolytic rates at physiologically relevant concentrations of insulin or the adenosine receptor agonist N6-(phenylisopropyl)adenosine were higher in subjects with the highest vs. lowest VAT area. Independently of cell size, abdominal adipocytes were less sensitive to the antilipolytic effect of insulin than gluteal adipocytes, which may partly explain increased nonesterified fatty acid fluxes in upper vs. lower body obese women. Moreover, increased VAT was associated with decreased responsiveness, but not decreased sensitivity, to insulin's stimulatory effect on glucose transport in abdominal adipocytes. These data suggest that insulin resistance of subcutaneous abdominal and, to a lesser extent, gluteal adipocytes may contribute to increased systemic lipolysis in both black and white viscerally obese women.
abdominal and gluteal adipocytes; lipolysis; glucose transport; adipose tissue
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
INCREASED VISCERAL ADIPOSE TISSUE (VAT) has been associated with the insulin-resistance syndrome in upper body (abdominal) obesity (14). However, abdominal obesity is associated not only with excess VAT but also with increased upper body subcutaneous adipose tissue (SAT). A stronger relationship of in vivo insulin resistance to subcutaneous truncal fat than to intraperitoneal fat was shown in moderately obese men (3) and recently in a multiethnic group of lean and obese men and women (20). The relative contributions of visceral and subcutaneous abdominal adipose tissue metabolism to insulin resistance and its associated metabolic disorders are still uncertain (18, 47).
The mechanisms linking visceral obesity to decreased whole body insulin-stimulated glucose utilization are incompletely understood, but the increased availability of nonesterified fatty acids (NEFA) has been strongly implicated (9). Systemic NEFA flux has been shown to be increased in upper body obesity, as defined by an increased waist-to-hip ratio (WHR > 0.85; Ref. 24). Albu et al. (4) have also recently demonstrated that increased VAT, independently of total body fat, is associated with resistance to the systemic antilipolytic effect of insulin measured in vivo in obese women. Although visceral fat is known to exhibit a high lipolytic capacity in vitro (11, 36, 44), this depot has been shown to account for only 15% of systemic NEFA flux in women with upper body obesity (32). Therefore, the majority of NEFA released in these women was thought to derive from upper body SAT (21, 32).
A number of lines of evidence indicate that adipocytes from abdominal
SAT are more lipolytically active in subjects with upper body or
visceral obesity. Adipocytes from abdominal SAT of viscerally obese
women were found to exhibit elevated basal and -adrenergically stimulated lipolysis (34, 39). Also, a recent study
(33) found that adipocytes from abdominal SAT of
viscerally obese men were more sensitive to
-adrenergic agonists and
that this was linked to higher fasting insulin levels.
Additionally, insulin action in adipocytes from abdominal SAT has been shown to vary by race, as well as by WHR (17, 28). Adipocytes from abdominal SAT exhibited insulin resistance in premenopausal Caucasian (non-Hispanic white) women with a WHR > 0.85 but not in similarly obese African-American (black) women with a WHR > 0.85 (17). Obese black women, however, have less VAT for the same WHR compared with equally obese white women (5, 13, 30). Whether racial variation in VAT accumulation explains racial differences in insulin action in adipocytes from upper body SAT is not clear.
In the current study, we tested whether increased VAT is associated with impaired insulin action in adipocytes from SAT and whether this relationship differs by race. A related aim was to determine whether insulin action differs in adipocytes from upper vs. lower body SAT in these women. We studied insulin action in subcutaneous abdominal and gluteal adipocytes from nondiabetic black and white premenopausal women who had a wide range of VAT area measured at midwaist and a wide range of insulin sensitivity (SI) measured in vivo by the minimal model of Bergman et al. (8).
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Subjects. Potential subjects were 20- to 49-yr-old white (non-Hispanic Caucasian) or black (African-American), obese [body mass index (BMI) = 27-45 kg/m2], premenopausal women. The extent of racial admixture was assessed from self-report; subjects were included in the study if all four grandparents were of either Caucasian or African ancestry. Obese women of both races with a similarly wide range of visceral adiposity were recruited, as described in an earlier publication (5). All subjects were in good health, nonparous, taking no prescription medications, weight stable ( ±2 kg) for at least the 6 mo before the study, and regularly menstruating at the time of the study. Absence of diabetes according to the National Diabetes Data Group criteria (38) was based on a screening oral glucose tolerance test (OGTT). The study was approved by the Institutional Review Board of the Health Sciences Institute at St. Luke's-Roosevelt Hospital Center. All subjects provided written consent before participation in the study.
Anthropometric measurements. Fasting weight and height were measured with subjects wearing only their undergarments. Minimum waist circumference (minimum circumference between the lower rib margin and the iliac crest, usually the midpoint or midwaist) and maximum hip circumference (below the iliac crest, with subject viewed from the front) were measured while the subjects were standing with heels together.
Body composition. Total body fat mass, fat-free mass (FFM), and percent body fat were determined by hydrodensitometry in the fasted state. The coefficient of variation for percent body fat by hydrodensitometry at our center is 2%. Different densities of FFM were used for black (1.106 g/cm3) and white (1.100 g/cm3) women in calculating percent body fat with the Siri equation. Our center quantified the density of FFM using a four-compartment model of body composition and found FFM density to be slightly greater in black than in white women, primarily due to heavier bone mass (41).
Visceral fat measurements. Areas of VAT and SAT were measured by magnetic resonance imaging (MRI; G.E. System Signa Advantage 5.3 Scanner, G.E. Medical Systems, Milwaukee, WI) as described (5). Adipose tissue areas (VAT and SAT) were determined from images obtained at midwaist (the midpoint between the last rib and the iliac crest at the L2-L3 level). The measurement of VAT volume by a similar technique has been validated in human cadavers (1). VAT area measured on a transverse body scan has been shown to correlate highly with total VAT volume (45, 50), especially at the L2-L3 level (2).
Three subjects who declined MRI measurements because of the discomfort associated with confinement were measured by computed tomography scans of the same body regions. Images were read on the G.E. System Independent Physicians Console (G.E. Medical Systems) and on a Picker System/Voxel Q software system (Picker International, Highland Heights, OH).OGTT. OGTT was performed after a 12-h overnight fast. Plasma glucose (Beckman glucose analyzer, Fullerton, CA; coefficient of variation <4%) and insulin were measured in blood samples taken at 30-min intervals for 2 h after glucose ingestion (75 g dextrose, Baxter Healthcare, Valencia, CA). Plasma insulin was measured by radioimmunoassay using the charcoal extraction technique (coefficient of variation < 12%; Ref. 22). This method does not distinguish insulin from proinsulin levels. The integral glucose and insulin areas under the curve (AUC) of the 2-h OGTT were estimated by the trapezoid method.
Frequently sampled intravenous glucose tolerance test. Sensitivity to insulin's effect on systemic glucose utilization (SI) was measured according to the tolbutamide-modified frequently sampled intravenous glucose tolerance test of Bergman et al. (8). The frequently sampled intravenous glucose tolerance test was performed after a 12-h overnight fast. All subjects had this measurement within 10 days of the onset of their menstrual cycle. Plasma glucose and insulin were measured on frequently obtained samples, and SI was calculated from these values with the nonlinear mathematical model of glucose disappearance (MINMOD program, copyright R.N. Bergman, 1986).
SAT biopsy. Twenty-two white and 22 black women agreed to have needle aspirations of SAT; these were performed at 9 AM after a 12-h fast. The biopsies were performed without regard to phase of the menstrual cycle. Previous studies did not find significant differences between the follicular and luteal phases in insulin action on glucose transport and lipolysis in adipocytes from premenopausal women (31). The skin was anesthetized (2% lidocaine; Elkins-Sinn, Cherry Hill, NJ), and adipose tissue was obtained using a blunt-ended needle designed for liposuction (3-mm Spirotri cannulas; Unitech Instruments, Fountain Valley, CA) from subcutaneous abdominal and gluteal sites, with no longer than 10 min elapsing between the abdominal and gluteal aspirations (17).
Adipocyte isolation. Adipocytes were isolated as described previously (17). Aspirated adipose tissue was washed with PBS (37°C, pH 7.4) and digested with collagenase (1 mg/ml, lot no. 47K121, Worthington Biochemical, Freehold, NJ) in Krebs-Henseleit HEPES (25 mM) buffer, pH 7.4, containing bicarbonate (10 mM), 5% albumin (CRG-7, lot no. H54707, Intergen, Purchase, NY) (KHBH-A buffer), and 0.5 mM glucose. After 45-60 min of collagenase digestion in a shaking water bath (60 cycles/min, 37°C), isolated adipocytes were filtered through a 250-µm nylon mesh (Tetko, Briarcliff Manor, NY) and washed four times by flotation with collagenase-free KHBH-A buffer at 37°C.
Determination of adipocyte size. Diameters of at least 200 cells from each adipocyte suspension were measured directly using a microscope with ocular micrometer (15). A frequency distribution plot of cell diameters was used to determine the mean fat cell diameter and standard deviation about the mean; from these, the mean fat cell volume and surface area were then determined using standard equations (19, 52). Mean fat cell weight (µg lipid/cell) was calculated from cell volume, assuming that the density of lipid is equal to that of triolein (0.915 g/l). The lipid content of an aliquot of cell suspension was determined by extraction (16). Fat cell density in suspension was calculated by dividing the lipid content of the cell suspension by the mean fat cell weight.
In vitro lipolysis measurements. Rates of lipolysis in isolated adipocytes were estimated by glycerol appearance as described (17). Adipocytes (10,000-20,000 cells/ml) were incubated with gentle agitation (37°C, 60 cycles/min) for 2 h in KHBH-A buffer containing glucose (5 mM) and adenosine deaminase (ADA; 2 µg/ml). After incubation, cell-free medium was obtained by centrifugation of the cell layer through silicon oil. Glycerol was assayed in neutralized perchloric acid extracts of the incubation medium (12). Data were expressed as net glycerol release per 105 cells during 2-h incubation; glycerol present at the start of incubation was subtracted from all samples. Baseline incubations (ADA alone) were performed in quadruplicate. Incubations to determine the antilipolytic effects of insulin (25, 50, 100, and 400 pM) and of the nonhydrolyzable adenosine receptor agonist N6-(phenylisopropyl)adenosine (PIA: 10, 20, 50, and 100 nM) were performed in triplicate.
Determination of glucose transport. Rates of basal and insulin-stimulated glucose uptake were measured in isolated subcutaneous abdominal adipocytes according to the method of Kashiwagi et al. (26). This assay reflects rates of cellular glucose transport measured directly with 3-O-methylglucose (26). Isolated adipocytes suspended in KHBH-A (20,000-40,000 cells/ml) were preincubated in triplicate with insulin (0, 25, 50, 100, 400, and 16,000 pM) in a shaking water bath (15 min, 37°C). A trace amount (300 nM) of D-[U-14C]glucose (0.1 µCi/ml) was then added, and the incubation was continued for 1 h at 37°C. The incubation was terminated by centrifuging the fat cell suspension through silicon oil in polyethylene microcentrifuge tubes. The tubes were cut through the oil layer containing the cell pellet and added to scintillation vials. Radioactivity associated with the fat cell layer was determined in a Packard Tri-Carb 1600TR liquid scintillation analyzer (Packard Instrument, Meriden, CT). Data were expressed as glucose clearance rate in femtoliters per cell per second.
In vitro calculations. The antilipolytic responses of insulin and PIA to ADA-stimulated lipolysis were assessed by dose-response curves. Sensitivity [dose giving half-maximal response (ED50)] was defined as the concentration of insulin (or PIA) at which 50% suppression of lipolysis was observed: this was determined from log-logit plots of glycerol release rate vs. insulin (or PIA) concentration (6). In these calculations, the difference between lipolysis at baseline and at 400 pM insulin (or 100 nM PIA) was set at 100%. The maximum response was calculated as the absolute difference between the lowest rate of lipolysis observed under antilipolytic suppression and the ADA-stimulated lipolysis (when no insulin or PIA was present). This difference was also expressed as a percentage of the ADA-stimulated lipolysis without antilipolytic agent (% of baseline). A high ED50 denotes decreased antilipolytic sensitivity. The sensitivity and maximum response of glucose transport to insulin were assessed in a similar manner (26).
Statistical analyses.
Data are reported as means ± SD (see text and Tables 1-3)
or, for illustrative reasons, as means ± SE (see Figs.
1-6). One-way ANOVA was used to compare subject data
(anthropometric, body composition, and visceral fat measurements) and
in vitro data, both between races and between groups with high and low
VAT areas. Lipolysis and glucose transport rates were log transformed
if the data were not normally distributed.
|
|
|
|
|
|
|
|
|
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Subjects.
The subject characteristics are shown in Table
1. The 22 black and 22 white women did
not differ by age, BMI, body fat mass and percentage, VAT and SAT
areas, insulin AUC during OGTT, or SI. WHR was higher
(P < 0.05), and glucose AUC during OGTT was lower
(P < 0.01) in the black than in the white group, as
previously reported (5). Both groups displayed a wide
range of visceral obesity (VAT range = 33-239 and 25-258
cm2) and SI (range = 0.1-8.4 and
0.5-8.6
104 · min
1 · µU · ml
1,
for black and white women, respectively). The mean subcutaneous cell
size (wt) varied widely within black and white groups (abdominal ranges
0.36-0.97 and 0.32-1.24 µg, gluteal ranges 0.44-1.09
and 0.39-1.02 µg, respectively) but did not differ between the
two races (P = nonsignificant for abdominal or gluteal
adipocytes; Table 1). The mean subcutaneous abdominal cell size was
smaller than the mean gluteal cell size in black women
(P < 0.05) but did not differ by depot in white women
(P = 0.6). The race by depot interaction for mean cell
size was significant (P < 0.01).
Lipolysis: effects of race and visceral obesity. Insulin inhibition of lipolysis was measured in adipocytes from abdominal SAT of 22 black and 20 white women and in gluteal adipocytes from a subset of 17 black and 14 white women. Measurements in both adipose tissue sites of the same subject were done in a subset of 16 black and 12 white women. The absolute rates of lipolysis and sensitivity to the antilipolytic effect of insulin, in either subcutaneous abdominal or gluteal adipocytes, did not differ by race (Fig. 1, A and B, respectively). When lipolysis data were expressed per cell surface area to adjust for depot and racial differences in mean cell size, similar results were obtained (not shown).
In the entire group of women, subcutaneous abdominal adipocytes were significantly less sensitive (higher ED50) to the antilipolytic effect of insulin compared with gluteal adipocytes (Fig. 2; ED50 = 47.7 ± 21.3 vs. 33.1 ± 13.7 pM, respectively, P < 0.01). No significant race by depot interaction was found for ED50 (P = 0.9). To determine whether decreased sensitivity of adipocytes to insulin (higher ED50) was associated with variation in either mean fat cell size, total body fat, or visceral fat distribution, the correlations between lipolysis indexes (ADA-stimulated lipolysis, ED50, and maximum response) and indexes of total or visceral obesity (adipocyte size, VAT, VAT-to-SAT ratio, and total fat mass) were calculated (Table 2). Because the slopes and intercepts of these relationships did not differ by race, data for all women are shown. Sensitivity to the antilipolytic effect of insulin was decreased (higher ED50), in both adipose tissue sites, in women with increasing visceral obesity. The ED50 in subcutaneous abdominal adipocytes was positively correlated with VAT area (P < 0.01; Table 2 and Fig. 3A) and with the VAT-to-SAT ratio (P < 0.01). These correlations were present in gluteal adipocytes, albeit weaker (Table 2). The ED50 values did not vary as a function of mean cell sizes or total body fat mass. However, the rates of basal (ADA-stimulated) lipolysis and the maximum antilipolytic response to insulin were greater with an increase in the respective subcutaneous cell sizes (Table 2). Neither total nor visceral adiposity was significantly correlated with the maximum response to insulin, expressed either as the absolute decrement in lipolysis from baseline (Table 2) or as a percentage of baseline lipolysis (not shown). To assess the impact of visceral adiposity on subcutaneous abdominal adipocyte lipolysis at varying concentrations of insulin, absolute rates of lipolysis were compared for women in the highest (200 ± 34 cm2, n = 11) and lowest (75 ± 27 cm2, n = 11) quartiles of VAT area measurements (Fig. 3B). Both the absolute rates of lipolysis and the ED50 values were higher in the highest compared with the lowest VAT group (Fig. 3B: P = 0.054 for overall rates of lipolysis; P < 0.05 for lipolysis rates at 50 and 100 pM insulin and for ED50). There was no significant interaction between VAT group and insulin concentration (P = 0.11). The women in the highest VAT group were older and heavier than those in the lowest VAT group (age = 38 ± 5 vs. 32 ± 5 yr; total body fat mass = 49 ± 7 vs. 39 ± 9 kg; FFM = 56 ± 5 vs. 51 ± 5 kg, respectively; all P < 0.05). The women with the highest VAT also had larger subcutaneous abdominal cell size (P < 0.05) and a slightly but not significantly higher SAT area (P > 0.06) compared with the women with the lowest VAT. The increased lipolytic rates in subcutaneous abdominal adipocytes from women in the highest VAT quartile were largely due to increased mean cell size, because after an adjustment was made for mean cell size by analysis of covariance (or when data were expressed per cell surface area), the difference in lipolytic rates was no longer significant. However, the ED50 was still significantly higher in the highest VAT group (P < 0.05) after adjusting for subcutaneous abdominal cell size. Because the women with highest VAT were fatter and older than the women with lowest VAT, standard multiple regression analysis was performed in the entire group of subjects, to determine whether VAT area (and the VAT-to-SAT ratio) predicted ED50 independently of age and/or total body fat mass. ED50 was related to VAT area and to the VAT-to-SAT ratio independent of age and fat mass (P < 0.05 for subcutaneous abdominal; P < 0.07 for gluteal). Total fat mass alone did not independently predict either subcutaneous abdominal or gluteal ED50 (P > 0.3 both sites). When SAT was substituted for total body fat mass in parallel multiple regression analyses, VAT remained independently related to ED50 in both subcutaneous abdominal and gluteal adipocytes. Thus the decreased sensitivity to the antilipolytic effect of insulin in adipocytes of women with highest VAT was independent of the level of total fatness. Lipolytic flux across the abdominal SAT would thus be expected to be highest when visceral obesity is accompanied by subcutaneous fat cell hypertrophy due to higher rates of basal lipolysis (Table 2) and lower sensitivity to the antilipolytic effect of insulin (Fig. 3A). The antilipolytic effect of PIA in abdominal SAT was also attenuated in viscerally obese women. Dose-response curves for the antilipolytic effect of PIA in subcutaneous abdominal adipocytes from women in the highest and lowest quartiles of VAT area are shown in Fig. 4. Analyses were performed similarly to those for insulin inhibition of lipolysis, as described in the previous paragraphs. Rates of lipolysis at 10 nM PIA were higher for the women in the highest VAT group compared with those in the lowest VAT group (P < 0.03). This difference was not significant when lipolysis rates were expressed per cell surface area or after adjustment for mean cell size by analysis of covariance. Sensitivity to the antilipolytic effect of PIA (ED50) in abdominal SAT was positively correlated with VAT (r = 0.37, P < 0.05), but this relationship was no longer significant after controlling for total body fat mass (partial P = 0.14).Glucose transport: effects of race and visceral obesity. Insulin-stimulated rates of glucose transport were measured in subcutaneous abdominal adipocytes only, in 22 black and a subset of 20 white women. The anthropometric and regional fat distribution characteristics of these women were similar to those of the larger group. Black and white groups did not differ with regard to absolute rates of insulin-stimulated glucose transport nor to the maximum response and sensitivity of glucose transport to insulin (Fig. 5). Similar results were obtained when glucose transport data were expressed per cell surface area (not shown). There were no significant interactions by race in the relationship of VAT to insulin's effect on glucose transport in vitro; therefore, we combined racial groups for further analysis.
For both groups combined, the maximum response to insulin's stimulatory effect on glucose transport was inversely related to VAT area (n = 39, r =Correlations of in vivo and in vitro measurements. Because visceral obesity per se is associated with hyperinsulinemia and with resistance to insulin's ability to stimulate glucose disposal in vivo, we examined the relationships of insulin levels (fasting and during OGTT) and SI to measurements of insulin sensitivity in vitro (Table 3). In both subcutaneous abdominal and gluteal adipocytes, reduced sensitivity to the antilipolytic effect of insulin (elevated ED50) was strongly related to elevated fasting insulin levels and insulin AUC and to decreased SI (all P < 0.05). Additionally, the maximum response to insulin's stimulatory effect on glucose transport in abdominal adipocytes was highly correlated with SI (P = 0.004) and was inversely related to fasting insulin and insulin AUC (P < 0.03).
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
We demonstrate for the first time in this study that subcutaneous adipocytes from black and white women with visceral obesity are less sensitive to the antilipolytic effects of insulin and the adenosine agonist PIA. Resistance to the antilipolytic effect of insulin in vitro was correlated with increased VAT accumulation, independently of the size of subcutaneous adipocytes and/or the level of total body fat mass. Thus the alterations in insulin action on subcutaneous adipocytes appear to be a specific feature of visceral obesity in premenopausal women and are not wholly attributable to increased adiposity per se.
Additionally, when measured at physiologically relevant concentrations
of insulin or adenosine, absolute lipolytic rates were increased in
subcutaneous fat cells from women with visceral obesity. These elevated
rates of lipolysis were proportional to increased adipocyte size. In
two studies (34, 39), viscerally obese women exhibited
elevated rates of basal and -adrenergic receptor-mediated lipolysis
in subcutaneous adipocytes; however, these studies disagreed on whether
these associations were independent of adipocyte size. Our data
indicate that increased rates of lipolysis in the SAT of obese women
may be independently influenced by adipocyte size, as well as by the
presence of increased VAT.
These data further suggest that the increases in baseline systemic NEFA flux previously reported in upper body obese women (24) are at least partly attributable to 1) decreased sensitivity to the antilipolytic effect of insulin, independent of fat cell size; and to 2) increased lipolytic rates associated with subcutaneous fat cell hypertrophy. These in vitro data are also consistent with previous observations that NEFA flux after a meal is less well suppressed in upper body obesity (21, 46).
Albu et al. (4) have recently reported that women with greater degrees of visceral obesity are more resistant to the in vivo antilipolytic effect of insulin. Our in vitro data suggest that, at least partly, this systemic antilipolytic resistance may originate in SAT. The mechanisms responsible for this association are not known at the present time. Assessment of intracellular mechanisms for the insulin action abnormalities in SAT could have been helpful in better understanding the association with enlarged VAT. Future studies should examine whether insulin receptor binding capacity, autophosphorylation, or signal transduction (51) are impaired in subcutaneous adipocytes from viscerally obese individuals.
In animal experiments, it has been shown that the presence or absence
of an enlarged specific fat depot may influence adipose tissue
metabolism in another fat depot, as well as the systemic insulin
levels. Sprague-Dawley rats that had epididymal and perinephric fat
pads surgically removed were shown to have decreased plasma insulin
levels relative to sham-operated rats and decreased expression of the
cytokines tumor necrosis factor- and leptin in other SAT (7). However, circulating NEFA levels were unchanged by
this experimental manipulation (7). Expression of tumor
necrosis factor-
and leptin has been shown to be regulated by
circulating lipids and insulin (40, 42).
From these results we might hypothesize that, in viscerally obese
humans, chronically elevated insulin levels (5) may
directly lead to insulin insensitivity in subcutaneous adipocytes.
Indeed, statistically, the relationship between VAT and diminished
sensitivity to the antilipolytic effect of insulin in SAT in this study
can be entirely explained by elevated insulin levels (fasting or during OGTT), independent of total body fatness. Hyperinsulinemia has also
been statistically associated with increased sensitivity to
-adrenergic agonists in subcutaneous abdominal adipocytes from
viscerally obese men (33).
In a previous report, Dowling et al. (17) assessed the impact of race and upper body obesity on insulin's antilipolytic action in subcutaneous adipocytes. Upper body obese white women, characterized by an increased WHR, exhibited resistance to the antilipolytic effect of insulin in abdominal and gluteal adipocytes. However, black women with upper body obesity by the WHR criteria did not exhibit alterations in insulin action, when lower body obese black women of similar adiposity were used as controls. VAT areas were not determined in that study. In the current study, we found no racial differences in the relationship of VAT to absolute rates of lipolysis or glucose transport or to insulin responsiveness or sensitivity, in subcutaneous adipocytes. Thus the results of our previous study (17) could be explained if obese black women have less VAT for the same WHR compared with equally obese white women, as we and others have previously reported (5, 13).
Furthermore, we have demonstrated in this study that subcutaneous abdominal adipocytes are more resistant to the antilipolytic effect of insulin than gluteal adipocytes, independent of cell size. This in vitro finding may explain why, in vivo, the abdominal adipose tissue makes a greater contribution to postmeal systemic NEFA flux than does the gluteal adipose tissue, at least in lean and upper body obese women (21, 23). Viscerally obese subjects have increased abdominal SAT deposition relative to gluteal fat deposition. This could further account for an overall decreased systemic antilipolytic effect of insulin. It was not possible to detect this in the current study, because abdominal SAT was not measured in its entirety. However, using whole body MRI measurements, Kovera et al. (27) have recently reported that VAT is in fact associated with upper body rather than lower body SAT accumulation.
There has been considerable disagreement in previous in vitro studies
regarding regional differences in sensitivity to insulin's antilipolytic effect. Some investigators (43, 48) have
found greater sensitivity to the antilipolytic effect of insulin in subcutaneous abdominal adipocytes and were unable to demonstrate any
measurable insulin antilipolytic effect in lower body subcutaneous adipocytes. The lack of insulin effect in these studies may have been
due to the use of norepinephrine to stimulate lipolysis, which resulted
in significantly higher lipolytic rates in subcutaneous abdominal
adipocytes due to regional differences in the distribution of - and
-adrenergic receptors (35). Generally, when in vitro studies have been performed without catecholamine stimulation, subcutaneous upper and lower body (gluteal and femoral) fat cells appear to exhibit similar sensitivities to insulin's antilipolytic effect (10, 17, 37).
Previously, Dowling et al. (17) reported that subcutaneous abdominal adipocytes were more resistant to the antilipolytic effect of insulin than gluteal adipocytes but only in lower body obese black women. In the present study, the decreased antilipolytic effect of insulin in abdominal compared with gluteal SAT was present in both black and white women, when analyses were done separately by race. We may have selected subjects, in both races, who had a broader abdominal vs. gluteal SAT distribution than in our previous study, where upper and lower body obese groups were selected by extremes of WHR.
Subcutaneous abdominal adipocytes from the viscerally obese women in this study also exhibited a decreased suppression of lipolysis at 10 nM PIA, an adenosine A1-receptor agonist. In the previous study by Dowling et al. (17), upper body obese white women exhibited reduced adenosine sensitivity in vitro compared with similarly obese white women with lower body obesity. The interstitial concentration of adenosine in human adipose tissue normally ranges from 25 to 300 nM, with an average of ~130 nM (29). Because PIA exhibits a sixfold higher binding affinity relative to adenosine (49), these data suggest that altered adenosine sensitivity may further contribute to the elevated baseline lipolytic rates in viscerally obese subjects under physiological conditions. Decreased sensitivity to adenosine may be related to increased adenosine content in SAT and subsequent downregulation of adenosine receptor number (25). However, it is not known whether the SAT of women with visceral obesity has an increased adenosine content.
Additionally, subcutaneous abdominal adipocytes from viscerally obese women were less responsive to the stimulatory effect of insulin on glucose transport. However, sensitivity (ED50) was not affected. The maximum response to insulin's ability to stimulate glucose transport in subcutaneous abdominal adipocytes was decreased in women with increased VAT, particularly after adjusting for cell size. These results were similar to previous findings by Dowling et al. (17) in white women with various degrees of fat distribution assessed by WHR. It is unclear why the sensitivity to insulin's antilipolytic effect was diminished in subcutaneous adipocytes from viscerally obese women, while the sensitivity to insulin's ability to stimulate glucose transport was not. This pathway specificity of alterations in SI with visceral obesity suggests differences in the coupling of insulin receptor activation to signaling pathways regulating glucose transport and lipolysis or possibly divergence in these pathways downstream of insulin receptor autophosphorylation (51).
In summary, excess visceral adiposity is associated with insulin resistance of subcutaneous fat cells from both abdominal and gluteal depots. Subcutaneous adipocytes from viscerally obese women are less sensitive to the antilipolytic effect of insulin and less responsive to the stimulatory effect of insulin on glucose transport. In addition, adipocytes from upper body SAT are more resistant to the antilipolytic effect of insulin than are adipocytes from lower body SAT, providing a mechanism for the lesser effect of lower vs. upper body obesity on systemic NEFA flux. Moreover, visceral adiposity has similar effects in black and white women. These findings provide a cellular basis for explaining the increased systemic lipolysis seen in upper body and, to a greater extent, in visceral obesity. These data support the hypothesis that increased nonesterified fatty acids derived from subcutaneous adipocytes may contribute to the relationship of visceral obesity with metabolic complications, including insulin resistance and dyslipidemia.
![]() |
ACKNOWLEDGEMENTS |
---|
We thank the women who agreed to participate in this study, and we thank Yim Dam and Kangping Chen for technical assistance.
![]() |
FOOTNOTES |
---|
This study was supported in part by National Institute of Diabetes and Digestive and Kidney Diseases Grants RO1-DK-40414, DK-26687, and K08-DK-02155, and the National Center for Research Resources Grant RR-OO645-25.
Portions of this work were presented at the annual meeting of the Federation of American Societies for Experimental Biology (Experimental Biology '96), Washington, DC, 1996, and at the 16th International Diabetes Federation Congress, Helsinki, Finland, 1998.
Address for reprint requests and other correspondence: J. Albu, Obesity Research Center, St. Luke's-Roosevelt Hospital Center, 1111 Amsterdam Ave., New York, NY 10025 (E-mail: jba1{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.
Received 20 December 1999; accepted in final form 24 August 2000.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Abate, N,
Burns D,
Peshock RM,
Garg A,
and
Grundy SM.
Estimation of adipose tissue mass by magnetic resonance imaging: validation against dissection in human cadavers.
J Lipid Res
35:
1490-1496,
1994[Abstract].
2.
Abate, N,
Garg A,
Peshock RM,
and
Grundy SM.
Prediction of abdominal adipose tissue masses using a single axial magnetic resonance imaging (MRI) slice in men (Abstract).
J Investig Med
43:
244A,
1995.
3.
Abate, N,
Garg A,
Peshock RM,
Stray-Gundersen J,
and
Grundy SM.
Relationships of generalized and regional adiposity to insulin sensitivity in men.
J Clin Invest
96:
88-98,
1995[ISI][Medline].
4.
Albu, JB,
Curi M,
Shur M,
Murphy L,
Matthews DE,
and
Pi-Sunyer FX.
Systemic resistance to the antilipolytic effect of insulin in black and white women with visceral obesity.
Am J Physiol Endocrinol Metab
277:
E551-E560,
1999
5.
Albu, JB,
Murphy L,
Frager DH,
Johnson JA,
and
Pi-Sunyer FX.
Visceral fat and race-dependent health risks in obese nondiabetic premenopausal women.
Diabetes
46:
456-462,
1997[Abstract].
6.
Arner, P,
Bolinder J,
Hellmér J,
and
Engfeldt P.
Studies on human fat cell metabolism in small adipose tissue samples.
In: Methods in Diabetes Research VII. Clinical Methods, edited by Clarke WL,
Larner J,
and Pohl SL.. New York: John Wiley & Sons, 1986, p. 233-258.
7.
Barzilai, N,
She L,
Liu B-Q,
Vuguin P,
Cohen P,
Wang J,
and
Rosetti L.
Surgical removal of visceral fat reverses hepatic insulin resistance.
Diabetes
48:
94-98,
1999[Abstract].
8.
Bergman, RN,
Prager R,
Volund A,
and
Olefsky JM.
Equivalence of the insulin sensitivity index in man derived by the minimal model method and the euglycemic glucose clamp.
J Clin Invest
79:
790-800,
1987[ISI][Medline].
9.
Boden, G.
Role of fatty acids in the pathogenesis of insulin resistance and NIDDM.
Diabetes
46:
3-10,
1997[Abstract].
10.
Bolinder, J,
Engfeldt P,
Östman J,
and
Arner P.
Site differences in insulin receptor binding and insulin action in subcutaneous fat of obese females.
J Clin Endocrinol Metab
57:
455-461,
1983[Abstract].
11.
Bolinder, J,
Kager L,
Östman J,
and
Arner P.
Differences at the receptor and postreceptor levels between human omental and subcutaneous adipose tissue in the action of insulin on lipolysis.
Diabetes
32:
117-123,
1983[Abstract].
12.
Boobis, LH,
and
Maugham RJ.
A simple one-step enzymatic fluorometric method for the determination of glycerol in 20 µL of plasma.
Clin Chim Acta
132:
173-179,
1983[ISI][Medline].
13.
Conway, JM,
Yanovski SZ,
Avila NA,
and
Hubbard VS.
Visceral adipose tissue differences in black and white women.
Am J Clin Nutr
61:
765-771,
1995[Abstract].
14.
Després, JP.
Abdominal obesity as important component of insulin-resistance syndrome.
Nutrition
9:
452-459,
1993[ISI][Medline].
15.
DiGirolamo, M,
Mendlinger S,
and
Fertig JW.
A simple method to determine fat cell size and number in four mammalian species.
Am J Physiol
221:
850-858,
1971[ISI][Medline].
16.
Dole, VP.
A relation between non-esterified fatty acids in plasma and the metabolism of glucose.
J Clin Invest
35:
150-154,
1956[ISI].
17.
Dowling, HJ,
Fried SK,
and
Pi-Sunyer FX.
Insulin resistance in adipocytes of obese women: effects of body fat distribution and race.
Metabolism
44:
987-995,
1995[ISI][Medline].
18.
Frayn, KN,
Samra JS,
and
Summers LKM
Visceral fat in relation to health: is it a major culprit or simply an innocent bystander?
Int J Obes
21:
1191-1192,
1997[ISI].
19.
Goldrick, RB.
Morphological changes in the adipocyte during fat deposition and mobilization.
Am J Physiol
212:
777-782,
1967[ISI][Medline].
20.
Goodpaster, BH,
Thaete FL,
Simoneau J-A,
and
Kelley DE.
Subcutaneous abdominal fat and thigh muscle composition predict insulin sensitivity independently of visceral fat.
Diabetes
46:
1579-1585,
1997[Abstract].
21.
Guo, Z,
Hensrud DD,
Johnson CM,
and
Jensen MD.
Regional postprandial fatty acid metabolism in different obesity phenotypes.
Diabetes
48:
1586-1592,
1999[Abstract].
22.
Herbert, V,
Lau KS,
Crottlieb LW,
and
Bleicher SJ.
Coated charcoal immunoassay of insulin.
J Clin Endocrinol Metab
25:
1375-1384,
1965[ISI][Medline].
23.
Jensen, MD.
Gender differences in regional fatty acid metabolism before and after meal ingestion.
J Clin Invest
96:
2297-3303,
1995[ISI][Medline].
24.
Jensen, MD,
Haymond MW,
Rizza RA,
Cryer PE,
and
Miles JM.
Influence of body fat distribution on free fatty acid metabolism in obesity.
J Clin Invest
83:
1168-1173,
1989[ISI][Medline].
25.
Kaartinen, JM,
Hreniuk SP,
Martin LF,
Ranta S,
LaNoue KF,
and
Ohisalo JJ.
Attenuated adenosine-sensitivity and decreased adenosine-receptor number in adipocyte plasma membranes in human obesity.
Biochem J
279:
17-22,
1991[ISI][Medline].
26.
Kashiwagi, A,
Verso MA,
Andrews J,
Vasquez B,
Reaven G,
and
Foley JE.
In vitro insulin resistance of human adipocytes isolated from subjects with noninsulin-dependent diabetes mellitus.
J Clin Invest
72:
1246-1254,
1983[ISI][Medline].
27.
Kovera, A,
Tan Y,
Gallagher D,
Heymsfield S,
and
Albu J.
Age and BMI are important determinants of fat distribution in women (Abstract).
Obes Res
7, Suppl1:
107S,
1999.
28.
Landin, K,
Lönnroth P,
Krotkiewski M,
Holm G,
and
Smith U.
Increased insulin resistance and fat cell lipolysis in obese but not lean women with a high waist/hip ratio.
Eur J Clin Invest
20:
530-535,
1990[ISI][Medline].
29.
Lönnroth, P,
Jansson P-A,
Fredholm BB,
and
Smith U.
Microdialysis of intercellular adenosine concentration in subcutaneous tissue in humans.
Am J Physiol Endocrinol Metab
256:
E250-E255,
1989
30.
Lovejoy, JC,
de la Bretonne JA,
Klemperer M,
and
Tulley R.
Abdominal fat distribution and metabolic risk factors: effects of race.
Metabolism
45:
1119-1124,
1996[ISI][Medline].
31.
Marsden, PJ,
Murdoch A,
and
Taylor R.
Adipocyte insulin action during the normal menstrual cycle.
Hum Reprod
11:
968-974,
1996[Abstract].
32.
Martin, ML,
and
Jensen MD.
Effects of body fat distribution on regional lipolysis in obesity.
J Clin Invest
88:
609-613,
1991[ISI][Medline].
33.
Mauriège, P,
Brochu M,
Prud'homme D,
Tremblay A,
Nadeau A,
Lemieux S,
and
Després JP.
Is visceral adiposity a significant correlate of subcutaneous adipose cell lipolysis in men?
J Clin Endocrinol Metab
84:
736-742,
1999
34.
Mauriège, P,
Després JP,
Marcotte M,
Ferland M,
Tremblay A,
Nadeau A,
Moorjani S,
Lupien PJ,
Thériault G,
and
Bouchard C.
Abdominal fat cell lipolysis, body fat distribution, and metabolic variables in premenopausal women.
J Clin Endocrinol Metab
71:
1028-1035,
1990[Abstract].
35.
Mauriège, P,
Galitzky J,
Berlan M,
and
LaFontan M.
Heterogeneous distribution of beta and alpha-2 adrenoceptor binding sites in human fat cells from various fat deposits: functional consequences.
Eur J Clin Invest
17:
156-165,
1987[ISI][Medline].
36.
Mauriège, P,
Marette A,
Atgie C,
Bouchard C,
Thériault G,
Bukowiecki LJ,
Marceau P,
Biron S,
Nadeau A,
and
Després JP.
Regional variation in adipose tissue metabolism of severely obese premenopausal women.
J Lipid Res
36:
672-684,
1995[Abstract].
37.
Mauriège, P,
Prud'homme D,
Lemieux S,
Tremblay A,
and
Després JP.
Regional differences in adipose tissue lipolysis from lean and obese women: existence of postreceptor alterations.
Am J Physiol Endocrinol Metab
269:
E341-E350,
1995
38.
National Diabetes Data Group.
Classification and diagnosis of diabetes mellitus and other categories of glucose intolerance.
Diabetes
28:
1039-1057,
1979[ISI][Medline].
39.
Nicklas, BJ,
Rogus EM,
Colman EG,
and
Goldberg AP.
Visceral adiposity, increased adipocyte lipolysis, and metabolic dysfunction in obese postmenopausal women.
Am J Physiol Endocrinol Metab
270:
E72-E78,
1996
40.
Nisoli, E,
Carruba MO,
Tonello C,
Macor C,
Federspil G,
and
Vettor R.
Induction of fatty acid translocase/CD36, peroxisome proliferator-activated receptor 2, leptin, uncoupling proteins 2 and 3, and tumor necrosis factor-
gene expression in human subcutaneous fat by lipid infusion.
Diabetes
49:
319-324,
2000[Abstract].
41.
Ortiz, O,
Russell M,
Daley TL,
Baumgartner RN,
Waki M,
Lichtman S,
Wang J,
Pierson RN,
and
Heymsfield SB.
Differences in skeletal muscle and bone mineral mass between black and white females and their relevance to estimates of body composition.
Am J Clin Nutr
55:
8-13,
1992[Abstract].
42.
Pratley, RE,
Ren K,
Milner MR,
and
Sell SM.
Insulin increases leptin mRNA expression in abdominal subcutaneous adipose tissue in humans.
Mol Genet Metab
70:
19-26,
2000[ISI][Medline].
43.
Rebuffé-Scrive, M,
Lönnroth P,
Mårin P,
Wesslau C,
Björntorp P,
and
Smith U.
Regional adipose tissue metabolism in men and postmenopausal women.
Int J Obes
11:
347-355,
1987[ISI][Medline].
44.
Richelsen, B,
Pedersen SB,
Møller-Pedersen T,
and
Bak JF.
Regional differences in triglyceride breakdown in human adipose tissue: effects of catecholamines, insulin, and prostaglandin E2.
Metabolism
40:
990-996,
1991[ISI][Medline].
45.
Ross, R,
Leger L,
Morris D,
DeGuise J,
and
Guardo R.
Quantification of adipose tissue by MRI: relationship with anthropometric variables.
J Appl Physiol
72:
787-795,
1992
46.
Roust, LR,
and
Jensen MD.
Postprandial free fatty acid kinetics are abnormal in upper body obesity.
Diabetes
42:
1567-1573,
1993[Abstract].
47.
Seidell, JC,
and
Bouchard C.
Visceral fat in relation to health: is it a major culprit or simply an innocent bystander?
Int J Obes
21:
626-631,
1997[ISI].
48.
Smith, U,
Hammersten J,
Björntorp P,
and
Kral JG.
Regional differences and effect of weight reduction on human fat cell metabolism.
Eur J Clin Invest
9:
327-332,
1979[ISI][Medline].
49.
Trost, T,
and
Schwabe U.
Adenosine receptors in fat cells: identification by (-)-N6-[3H]phenylisopropyladenosine binding.
Mol Pharmacol
19:
228-235,
1981[Abstract].
50.
Van der Kooy, K,
and
Seidell JC.
Techniques for the measurement of visceral fat: a practical guide.
Int J Obes
17:
187-196,
1993[ISI].
51.
Zierath, JR,
Livingston JN,
Thörne A,
Bolinder J,
Reynisdottir S,
Lönnqvist F,
and
Arner P.
Regional difference in insulin inhibition of non-esterified fatty acid release from human adipocytes: relation to insulin receptor phosphorylation and intracellular signaling through the insulin receptor substrate-1 pathway.
Diabetologia
41:
1343-1354,
1998[ISI][Medline].
52.
Zinder, O,
and
Shapiro B.
Effect of cell size on epinephrine- and ACTH-induced fatty acid release from isolated fat cells.
J Lipid Res
12:
91-95,
1971