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The Journal of Clinical Endocrinology & Metabolism Vol. 85, No. 7 2455-2462
Copyright © 2000 by The Endocrine Society

Original Studies

Adipose Tissue Metabolism in Young and Middle-Aged Men after Control for Total Body Fatness1

P. Imbeault, D. Prud’homme, A. Tremblay, J.-P. Després and P. Mauriège

Physical Activity Sciences Laboratory (P.I., D.P., A.T., P.M.), Department of Social and Preventive Medicine, Laval University, Québec G1K 7P4; Lipid Research Center (D.P., J.-P.D., P.M.), Laval University Medical Research Center, Québec G1V 4G2; and Québec Heart Institute (J.-P.D.), Laval Hospital, Québec G1V 4G5, Canada

Address correspondence and requests for reprints to: P. Mauriège, Ph.D., Lipid Research Center, Laval University Medical Research Center, 2705, boul. Laurier, Room TR-93, Ste-Foy, Québec, Canada, G1V 4G2. E-mail: diabolo{at}internetclub.fr


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
The aim of this study was to compare the sc adipose tissue metabolism of young (29 ± 4 yr) vs. middle-aged men (57 ± 5 yr), once the concomitant variation in total adiposity was taken into account. For this purpose, sc abdominal and femoral adipose tissue lipoprotein lipase activities, as well as fat cell lipolytic responses, were investigated in 2 groups of 16 men, differing in age but displaying similar adipose tissue mass (within 2 kg) and sc abdominal adipose tissue area, measured by computed tomography (within 15 cm2). No difference was observed in adipose tissue lipoprotein lipase activity of young vs. middle-aged subjects, regardless of the adipose region considered. Epinephrine induced antilipolysis at low concentrations (10-9 to 10-7 mol/L) and a net lipolytic response at higher doses (10-6 to 10-5 mol/L), regardless of the subjects’ age and the anatomic location of fat. In addition, the selective {alpha}2-adrenergic agonist, UK-14304, promoted a similar antilipolytic response in sc abdominal and femoral adipose cells from both groups. However, maximal lipolysis induced by isoproterenol (ß-adrenergic agonist) or by postadrenoceptor agents such as dibutyryl-cAMP, forskolin, and theophylline were lower in both adipose regions of middle-aged (as compared with young) men. No difference in the ß- or the {alpha}2-adrenoceptor sensitivity of sc adipose cells was observed between groups. These results indicate that there is, with age, a selective decrease in the lipolytic capacity to ß-adrenergic agonist, which seems to be caused by postadrenoceptor impairments. Because subjects in the 2 age-groups displayed similar body fatness, these alterations are independent from the age-expected increase in total adiposity.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
ADVANCING AGE is associated with a number of changes in body composition, such as a reduction in lean body mass and an increase in adiposity (1). We have recently reported that a 12-yr follow-up in adult subjects was accompanied by a body fat gain, even if both a decrease in the relative fat intake and an increase in the participation in physical activity occurred (2). These results suggest that age-related effects play an important role in the regulation of fat balance, because they predominate the beneficial lifestyle changes that should have promoted fat loss.

Human adipose tissue is heterogeneous in its metabolic activity, and regional variation in storage and/or mobilizing potencies of adipose cells has been suggested as contributing to local differences in adiposity (3, 4, 5). Storage of fatty acids in the adipocyte is almost entirely dependent on the uptake of fatty acids released from the hydrolysis of circulating triglyceride-rich lipoproteins by the lipoprotein lipase (LPL) (6). On the other hand, lipid mobilization in humans is stimulated by hormones, such as catecholamines, which act on cell-surface receptors and control cAMP production and, thus, lipolytic activity, through hormone-sensitive lipase activation (HSL) (7, 8). It is therefore possible that alterations in these regulatory aspects of adipose tissue metabolism could contribute to the age-related effects on body composition. In this regard, previous studies have already shown that catecholamine-induced lipolysis was reduced in adipocytes of elderly subjects (9, 10). More recently, Lönnqvist et al. (11) have proposed that this impaired lipolytic response of fat cells to catecholamines, with advancing age, might be attributable to a defective activation of the HSL complex. This hypothesis was supported by the fact that all the lipolytic agents used induced about 50% lower responses in elderly (as compared with young) subjects, even if both the {alpha}2- and ß-adrenoceptor number and affinity remained unchanged with age.

However, to the best of our knowledge, no study has attempted to identify primary alterations in sc adipose tissue metabolism with advancing age, once the concomitant variation in total adiposity is taken into account. Indeed, previous experiments that have been conducted on young and elderly subjects (9, 10, 11) have compared the lipolytic activity of adipocytes from individuals whose body fat distribution was different. Therefore, the aim of the present study was to examine whether differences in sc abdominal and femoral adipose tissue lipoprotein lipase (AT-LPL) activities and adipose cell lipolysis exist in 16 pairs of men who displayed similar body fatness and sc fat accumulation but differed in age.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

Thirty-two healthy Caucasian men were recruited through the media and gave their written informed consent to participate in this study, which was approved by the Laval University Medical Ethics Committee. Sixteen pairs of subjects who displayed similar levels of sc AT area (within 15 cm2), measured by computed tomography (CT), and adipose tissue mass (within 2 kg), but differed in age, were compared for potential differences in sc AT-LPL activity and adipose cell lipolysis. All individuals underwent a medical evaluation by a physician, which included a medical history. Subjects with cardiovascular disease, diabetes mellitus, or endocrine disorders, or those on medication that could have influenced triglyceride metabolism (ß-blockers, antihypertensive drugs, and others) were excluded from the study. All participants were sedentary (fewer than two exercise sessions of 30 min/week), nonsmokers, and moderate alcohol consumers. None had recently been on a diet or been involved in a weight-reducing program, and their body weight had been stable during the last 6 months before the study.

Total body fatness and regional fat distribution

Body density was determined by the underwater weighing technique, and percent body fat was derived from body density (12). Pulmonary residual volume was measured using the helium dilution method (13). Fat mass was calculated as total body weight minus fat free mass. Waist girth was measured according to the procedures recommended at the Airlie Conference (14). CT was performed on a Somatom DRH scanner (Siemens, Erlangen, Germany), according to the methodology previously described by Sjöström et al. (15). Briefly, subjects were examined in the supine position with both arms stretched above the head. CT scans were performed at the abdominal (between L4 and L5 vertebrae) level, using an abdominal scout radiograph to establish the position of the scan to the nearest millimeter. Total adipose tissue (AT) areas were calculated by delineating the abdomen with a graph pen and then computing the AT surfaces with an attenuation range of -190 to -30 Hounsfield units (16). Abdominal visceral AT (VAT) area was determined by drawing a line within the muscle wall surrounding the abdominal cavity. The abdominal sc AT area was calculated by subtracting the VAT area from the total abdominal AT area.

Adipocyte isolation and lipolysis

After an overnight fast, participants underwent biopsies of sc fat, one performed in the periumbilical region (abdominal site) and the other at the anterior midthigh level (femoral site). A small cutaneous incision (1 cm) was performed in both sites after local anesthesia (1% lidocaine, without epinephrine), and approximately 350 mg of sc adipose tissue were surgically removed from the two fat depots.

Samples of 250 mg of adipose tissue from each site were used for the measurement of fat cell lipolysis. Adipocytes were isolated according to the method of Rodbell (17) in a Krebs-Ringer bicarbonate buffer (pH 7.4) (KRB) containing 4% BSA and 5 mmol/L glucose (KRBA), plus 1 mg/mL collagenase, as previously described (18). Digestion took place in a shaking water bath under an air gas phase of 95% O2 and 5% CO2, for 40 min at 37 C. The suspension was then filtered, and the cellular filtrate obtained was rinsed three times with 5 mL KRBA. Isolated adipocytes were finally resuspended in KRBA, to obtain a final concentration of approximately 500 cells per 50 µL.

Extracellular glycerol release was used as the indicator of adipocyte lipolysis. Fifty-microliter aliquots of the continuously stirred cell suspension were placed in 1.5-mL conical tubes. Two of these tubes were used for cell counting and sizing; two others containing 10 µL KRB were immediately placed on ice, and they provided evaluation of the initial concentration of glycerol in the medium. Agents for lipolysis stimulation or inhibition were added just before starting the incubation in 10-µL portions, to obtain the desired final concentration. After a 2-h incubation at 37 C in a shaking water bath, under 95% O2-5% CO2 gas phase, 50 µL HCl (1N) were added to all tubes to stop the reaction, then 50 µL NaOH (1N) were added to neutralize the medium. All tubes were stored at -20 C until glycerol determination, and NADH concentration was measured by bioluminescence with luciferase solution, using an automated 2250 luminometer (Dynatech Laboratories Inc., Chantilly, VA) (18, 19). For each concentration of stimulator or inhibitor agents, the amount of glycerol was taken as the average of the quantities obtained from the two incubated tubes. Glycerol measurement by bioluminescence is very sensitive and especially well adapted when only small amounts of adipose tissue are available (18, 19). Adipose cell diameters were determined using a microscope equipped with a graduated ocular (Leitz, Rockleigh, NJ). Mean fat cell diameter was assessed from the measurement of at least 500 cells, and the density of triolein was used to transform adipose cell volume into fat cell weight, as previously described (20).

The lipolytic activity of isolated fat cells was tested with epinephrine, which is a mixed agonist ({alpha}2/ß) with a higher affinity for {alpha}2- than for ß-adrenoceptor (AR) sites (21), UK-14304 (selective {alpha}2-AR agonist), and isoproterenol (nonselective ß-AR agonist) (19). Ascorbic acid (0.1 mmol/L ) was included in the medium, to prevent catecholamine degradation. Some experiments were conducted with forskolin (direct activator of adenylate cyclase), dibutyryl adenosine 3', 5' cyclic monophosphate [dibutyryl-cAMP (DcAMP), stimulator of the protein kinase A HSL complex and phophodiesterase-resistant cAMP analogue], and theophylline (mainly inhibitor of cyclic GMP-inhibited phosphodiesterase, cGI-PDE) (19). When antilipolytic effects were investigated, the incubation buffer was supplemented with 5 µg/mL adenosine deaminase (ADA) to remove adenosine released into the incubation medium by the isolated fat cells, this procedure allowing better investigations of {alpha}2-AR-mediated antilipolytic effects (18, 19). Lipolysis was expressed either per cell number (i.e. in µmol of glycerol/106 cells·2 h) or per unit of cell surface area (i.e. in nmol of glycerol/µm2·108·2 h), the latter mode of expression being used to correct for variation in fat cell size, which is a well-known modulator of lipolysis (19). In cases where complete dose-response curves were obtained, they were compared for both responsiveness and sensitivity. The responsiveness was expressed as the difference between basal glycerol release and the lipolytic rate at maximum effective concentration of the agents tested (10-5 mol/L isoproterenol or forskolin, 10-3 mol/L DcAMP or theophylline). The ß-adrenergic sensitivity was considered as the concentration of isoproterenol giving half-maximal stimulation of lipolysis (EC50), whereas the {alpha}2-adrenergic sensitivity was calculated as the dose of UK-14304 that produced half-maximal inhibition lipolysis (IC50). Both were evaluated by logarithmic conversion of each dose-response curve. The higher was the EC50 (isoproterenol) or the IC50 (UK-14304) value, the lower was the ß- or the {alpha}2-adrenergic sensitivity, respectively.

AT-LPL activity

Samples of approximately 100 mg of adipose tissue from each region were immediately frozen in liquid nitrogen and stored at -80 C for later measurement of heparin-releasable LPL activity, according to Savard et al. (22). AT-LPL activity was expressed as micromoles of free fatty acids released per hour per 106 cells. Because AT-LPL activity is associated with fat cell size (6, 22), AT-LPL activity was also expressed per cell surface area (i.e. nmol of free fatty acids/h·µm2 x 108).

Drugs and chemicals

Collagenase, BSA, ADA, and enzymes for glycerol assays were obtained from Roche Molecular Biochemicals (Laval, Canada). Ascorbic acid, theophylline, forskolin, DcAMP, (-)-isoproterenol bitartrate, (-)-epinephrine bitartrate, and cold triolein were purchased from Sigma (St. Louis, MO). 14C-triolein was obtained from Dupont NEN Life Science Products (Missisauga, Canada). 5-Bromo-6-(2-imidazolin-2-ylamino)quinoxaline (UK-14304) was generously provided by Dr. D. A. Faulkner (Pfizer, Inc., Sandwich, UK). All other chemicals and organic solvents were of the highest purity grade commercially available. The same batches of hormones, pharmacological agents, collagenase, and albumin were used in all experiments.

Statistical analyses

Two subgroups of 16 men, displaying similar levels of sc abdominal AT (measured by CT) and adipose tissue mass, but differing in age, were compared. The Student’s t test was used for comparisons of anthropometric variables, basal and ADA-stimulated lipolysis, and AT-LPL activity between young and middle-aged subjects. The effects of age (young vs. middle-aged) and site (abdominal vs. femoral) on adipose tissue lipolytic curves were tested by a two-way ANOVA for repeated measures. Post hoc comparisons were handled with a Student’s t test. Lipolysis measurements, expressed per unit of cell surface area (i.e. nmol glycerol/µm2 x 108 x 2 h) were obtained by the following formula, by assuming that an adipocyte has a spherical shape:

where D represents the mean diameter (µm) of approximately 500 adipose cells, and 108 is a factor used to simplify presentation of data. All analyses were performed using the Jump version 3.2.2 program (SAS Institute, Inc., Cary, NC), adapted for Macintosh computers.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Subjects’ characteristics

Subjects’ physical characteristics are presented in Table 1Go. As expected, a significant difference was observed in the subjects’ age (P < 0.0001). Regarding the various body fatness and fat distribution variables, comparison between groups revealed that both young and middle-aged subjects displayed similar body weight, fat mass, waist girth, and sc abdominal adipose tissue accumulation, measured by CT. However, middle-aged subjects had a lower fat free mass and a higher VAT accumulation (P values ranging from 0.01–0.05), as compared with young individuals. As shown in Fig. 1Go, mean fat cell weights from both depots did not differ between groups, although sc abdominal adipocytes tended to be larger in middle-aged, than in young, subjects (P = 0.06). No regional variation in adipocyte size was found within both groups. To control for the trend observed in the variation of fat cell weight, all lipolysis measurements have been further corrected for variation in cell surface area. However, it should be noted that similar results were obtained when expressed on a per-cell basis (not shown).


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  		Table 1. Physical characteristics of subjects

 


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  		Figure 1. Distribution of sc abdominal and femoral fat cell weights of young and middle-aged men. Horizontal lines represent mean values.

 
Adipocyte function

As shown in Fig. 2Go, the basal lipolytic rate of both adipose regions was not affected by age. Moreover, no regional variation in basal lipolysis was observed within both groups. As expected, the addition of ADA (5 µg/mL ), in the incubation medium, increased the basal lipolytic rate by approximately 1.5- to 2.5-fold (Fig. 2Go), with no further increment at higher doses of this enzyme, in all cell types. ADA-stimulated lipolysis was similar in both adipose regions between young and middle-aged subjects. No regional variation was observed in the lipolytic response to this enzyme within groups.



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  		Figure 2. Basal lipolytic rate and adenosine deaminase (ADA)-stimulated lipolysis in isolated adipocytes from the sc abdominal and femoral regions of young and middle-aged subjects. Values are means ± SE of 16 experiments performed in duplicate.

 
The effect of epinephrine, which is known for its mixed agonist ({alpha}2/ß) properties, on lipolysis was examined in the presence of ADA. As shown in Fig. 3Go, the catecholamine initiated a similar biphasic responsiveness in sc abdominal and femoral adipocytes from both groups: antilipolysis was observed at low concentrations (10-9 to 10-7 mol/L), this effect being completely reversed at higher doses at which the hormone exerted a lipolytic response (10-6 to 10-5 mol/L).



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  		Figure 3. Effect of epinephrine (EPI) on ADA-stimulated lipolysis in isolated adipocytes from the sc abdominal and femoral regions of young and middle-aged subjects. Values are means ± SE of 16 experiments performed in duplicate. Glycerol release was expressed as the difference between stimulated (with EPI) and basal (i.e. in the presence of 5 µg/mL ADA) values. Negative values reflect inhibition of lipolysis.

 
The selective {alpha}2-agonist, UK-14304, was also tested on ADA-stimulated lipolysis, to characterize the {alpha}2-adrenoceptor component (Fig. 4Go). A similar antilipolytic effect was observed in sc abdominal and femoral adipocytes from both groups. The {alpha}2-adrenergic sensitivity (IC50) estimated as the half-maximal antilipolysis induced by UK-14304 was also similar in both adipose depots between groups (Table 2Go). Moreover, middle-aged subjects were characterized by a greater {alpha}2-adrenergic sensitivity in femoral, than in sc, abdominal adipose cells (P < 0.05) (Table 2Go), whereas such a regional variation was not observed in young men.



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  		Figure 4. UK14304-induced inhibition of ADA-stimulated lipolysis in isolated adipocytes from the sc abdominal and femoral regions of young and middle-aged subjects. Values are means ± SE of 16 experiments performed in duplicate. Fat cells were incubated in the presence of ADA (5 µg/mL ). Antilipolysis is given as the difference between values in the presence of UK and basal values (with ADA). Agonist concentrations required for IC50 were determined from these dose-response curves.

 

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  		Table 2. Sensitivity for {alpha}2- and ß-adrenoceptor agonists estimated from in vitro lipolysis studies on sc abdominal and femoral adipocytes of young and middle-aged subjects

 
To study the influence of the ß-adrenoceptor component, the lipolytic effect of the ß-agonist isoproterenol was examined on basal lipolysis (Fig. 5Go). The lipolytic responses observed at both 10-6 and 10-5 mol/L were significantly lower in middle-aged than in young subjects (P < 0.01). No variation between sites was observed for lipolysis initiated by isoproterenol in middle-aged subjects, whereas young subjects displayed a greater isoproterenol-induced lipolysis in the sc abdominal region (P < 0.05). Moreover, no difference between groups was observed in the ß-adrenergic adipose cell lipolytic sensitivity of both regions (Table 2Go). The ß-adrenergic sensitivity was also greater in sc abdominal than in femoral adipose cells of middle-aged subjects (P < 0.05), whereas such regional variation was not observed in young individuals.



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  		Figure 5. Isoproterenol (ISO)-induced lipolysis in isolated adipocytes from the sc abdominal and femoral regions of young and middle-aged subjects. Fat cells were incubated without ADA (i.e. standard conditions), and values are means ± SE of 16 experiments performed in duplicate. Agonist concentrations required for EC50 were determined from these dose-response curves. {dagger}, Significant difference between groups at P < 0.01.

 
Furthermore, because any step of the lipolytic cascade may be responsible for the differences observed in catecholamine responsiveness, the effects of agents acting at different postreceptor levels were also investigated (Fig. 6Go). The maximal lipolytic responses to either DcAMP (10-3 mol/L), forskolin (10-5 mol/L), or theophylline (10-3 mol/L) were markedly decreased in sc adipocytes of middle-aged, as compared with young subjects (P values ranging from 0.01–0.05). Moreover, no regional variation was observed when lipolysis was stimulated by these postreceptor agents within both groups.



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  		Figure 6. Lipolytic responsiveness to postadrenoceptor agents in isolated adipocytes from the sc abdominal and femoral regions of young and middle-aged subjects. Fat cells were incubated without ADA, in the presence of either DcAMP (10-3 mol/L), forskolin (FK; 10-5 mol/L), or theophylline (THEO; 10-3 mol/L). Previous experiments revealed that the concentrations of different drugs used were maximally effective doses. Values are means ± SE of 16 experiments performed in duplicate, and basal glycerol release has already been subtracted. *, Significant difference between groups at P < 0.05; {dagger}, significant difference between groups at P < 0.01.

 
Finally, no difference was observed in both abdominal and femoral AT-LPL activities between young and middle-aged subjects. Also, no regional variation in this enzyme activity was found in both groups of men (Fig. 7Go).



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  		Figure 7. sc abdominal and femoral AT-LPL activities, expressed per adipocyte surface area, of young (n = 15) and middle-aged men (n = 14). Values are means ± SE.

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
This study was performed to compare adipose tissue metabolism of young and middle-aged men who displayed similar sc fat accumulation and fat cell size. This matching procedure was important because of the well-known influence of regional fat distribution and fat cell size on adipose tissue metabolism (3, 4, 5). Our results clearly showed that middle-aged subjects were characterized by a lower lipolytic capacity in both sc abdominal and femoral adipose regions, as compared with young individuals.

The {alpha}2-antilipolytic adrenergic responses to UK-14304 and epinephrine were not altered by aging in the present study. The {alpha}2-adrenergic sensitivity (IC50) induced by UK-14304 was also similar in both adipose depots and groups, suggesting that intrinsic properties of {alpha}2-adrenoceptors in human fat cells do not seem to be influenced by age, a notion concordant with previous observations (11). Taken together, these results are consistent with the fact that the increase in fat cell size, rather than aging itself, is one critical factor modulating the {alpha}2-adrenergic responsiveness and the {alpha}2-adrenoceptor number (4, 23).

The fact that maximal lipolytic effect of epinephrine did not differ between groups may seem discordant with the lower isoproterenol-stimulated maximal adipose cell lipolysis observed in middle-aged individuals, as compared with young subjects. In this regard, it could be hypothesized that the strong {alpha}2-adrenergic antilipolytic component of catecholamine could not entirely be compensated by the ß-adrenergic activity of the hormone (18, 19, 21), even after matching subjects for fat cell size and sc fat accumulation. Such a finding remains difficult to explain, because the {alpha}2-adrenoceptor responsiveness assessed either by epinephrine (mixed {alpha}2/ß-agonist) or by UK-14304 (highly selective {alpha}2-agonist) did not seem to be more pronounced in sc adipose cells from middle-aged, as compared with young men.

Our results also demonstrate that maximal adipose cell lipolytic response to the ß-agonist, isoproterenol, was lower in middle-aged (than in young) individuals, whereas the ß-adrenergic receptor sensitivity was similar in both groups. Because only a few available ß-adrenoceptors need to be occupied to obtain a maximal effect, the alteration in responsiveness is usually linked to changes in hormone action at further intracellular steps in the pathway of the signal, whereas changes in sensitivity may reflect alterations in hormone action that are located at, or near, the receptor level (24). Therefore, the impaired ß-adrenergic response to isoproterenol observed in adipocytes from middle-aged individuals suggests a defect located at the postreceptor levels. In this regard, Lönnqvist et al. (11) have already proposed that HSL, the enzyme that hydrolyzes triacylglycerol from the lipid droplet of adipocytes in glycerol and nonesterified fatty acids (7, 8), could play a key role in the age-related difference in catecholamine-stimulated lipolysis. Indeed, because HSL activity is activated by agents that promote an increase of intracellular level of cAMP, such as isoproterenol, via its activation of ß-adrenoceptors, DcAMP, forskolin, or theophylline, a lower lipolytic effect of these agents could reflect a decreased enzyme activity. A recent study has also shown that the maximal lipolytic capacity, determined by the adipose cell lipolytic response to isoproterenol, was significantly correlated with HSL activity (25). Therefore, the decreased isoproterenol-induced lipolysis that we observed in adipose cells from middle-aged subjects is possibly the consequence of an altered HSL activity.

However, alterations located at postreceptor levels other than HSL cannot be excluded at the present time. Indeed, that basal lipolytic rate was similar between young and middle-aged individuals suggests that the suspected HSL defect in old subjects is not major, because basal lipolytic rate has been reported to be a strong correlate of basal HSL activity (26, 27). In this regard, the impaired maximal lipolytic effects of all postreceptor agents tested in adipocytes from middle-aged subjects do not exclude the possibility that these individuals are characterized by a high adipose cell phosphodiesterase activity. This finding has already been reported in old rats by some investigators (28), whereas others did not observe any change in this enzyme activity (29, 30). Further studies are therefore needed to clarify this issue.

The reduced maximal adipose cell lipolytic capacity of middle-aged subjects could be the consequence of an altered neuroendocrine profile related to an increased VAT accumulation. Indeed, VAT accumulation and age are two correlates of low testosterone levels in men (31, 32, 33), and it is possible that the important VAT accumulation observed in middle-aged subjects may have a direct effect on their impaired lipolytic capacity. One could suggest that high visceral levels observed in middle-aged men could lead to an activation of the corticotropin-releasing factor/adrenocorticotropin-cortisol axis, increasing thereby their glucocorticoids levels (34, 35). Activation of the axis could produce inhibition of gonadotropin secretion, which in turn, could decrease androgen levels in middle-aged men and possibly reduce their adipose tissue lipolytic capacity, because adipose tissue lipolysis is activated by testosterone (36). On the other hand, it is likely that the high VAT accumulation found in middle-aged men acts as a steroid reservoir and as a major site of peripheral steroid interconversion, because steroidogenic enzyme activities and messenger RNAs have been found in adipose tissue (37, 38, 39). Thus, the enlarged VAT accumulation reported in middle-aged men may contribute to their decreased testosterone levels, in response to the increased aromatization of androgens in adipose tissue, which may explain their altered adipose tissue lipolytic capacity. However, further studies will be needed to examine the contribution of altered steroid hormone levels in the impaired adipose tissue lipolytic capacity of middle-aged men.

Finally, there is extensive evidence showing that AT-LPL activity is an enzyme involved in the regulation of fat cell storage (4, 5, 6). Rebuffé-Scrive et al. (40) have already reported an absence of regional variation in AT-LPL activity of middle-aged men, a finding consistent with our results. However, no study, to our knowledge, has attempted to verify the impact of aging on this enzyme activity. Based on the hypothesis that AT-LPL activity may contribute to regional fat distribution (41), it was therefore expected to observe a similar sc AT-LPL activity in both groups of subjects, because of their similar sc fat accumulation. Therefore, AT-LPL activity did not seem to be influenced by advancing age, once the concomitant variation in body fatness is taken into account.

Conclusion

This study demonstrated that middle-aged men display a reduced lipolytic capacity in both sc abdominal and femoral adipocytes, when compared to young individuals of similar body weight and sc fat distribution. This age-related difference in lipolytic activity was mostly explained by alterations located at different postreceptor levels and is probably attributed to a decreased activation of the HSL complex. The important VAT accumulation observed in middle-aged subjects could also be a potential factor explaining their reduced sc adipose tissue lipolytic capacity. Taken together, these results indicate that advancing age is not associated with any major change in AP-LPL activity but to an altered lipid mobilizing capacity.


   Acknowledgments
 
We express our gratitude to Sylvie St-Pierre, Éric Doucet, Martine Marcotte, France Levasseur, and Henri Bessette for their collaboration at various stages of the study and to Drs. Gilles Lortie and Germain Thériault for their medical supervision. Thanks are also expressed to Suzanne Brulotte, of the Department of Radiology (Laval University Hospital, Québec, Canada), for her help with the use of the computed tomograph. The subjects are also gratefully acknowledged.


   Footnotes
 
1 Supported by the Medical Research Council of Canada and the Fonds FCAR-Québec. Back

Received August 13, 1999.

Revised February 2, 2000.

Accepted March 16, 2000.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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