Differences in lipogenesis and lipolysis in obese and non-obese adult human adipocytes

It has been proposed that differences in adipocyte function and/or metabolism between obese and lean individuals may manifest themselves in functional adipose tissue abnormalities that lead to metabolic disorders in obesity. We studied lipogenesis and lipolysis of omental adipocytes from obese (OB) and non-obese (NOB) humans. The specific activity of the lipogenic marker enzyme G3PDH was 50% lower in total adipocytes of OB compared to that of NOB subjects. Omental adipocytes from OB subjects also had lower basal lipolytic levels, and a lower lipolytic response to beta-adrenergic stimulus. Cholesterol depletion of adipocyte plasma membrane using methyl b-cyclodextrin caused a lipolytic effect on adipocytes of both groups together, but when obese and lean subjects were analyzed separately, the response was significant only in the obese. We present evidence of a different lipogenic and lipolytic profile in obese individuals' omental adipocytes, and propose a relevant role of plasma membrane cholesterol, where the impact of its removal in OB and NOB adipocyte lipolysis differs.     

Effects of Dexamethasone on Primarily Cultured Newly Differentiated Rat Adipocytes from Different Adipose Tissue Regions

The effects of dexamethasone (dex) on newly differentiated adipocytes in primary culture derived from mesenteric, retroperitoneal, epididymal, and inguinal subcutaneous adipose tissues of male rats were studied. The degree of differentiation was similar in these adipose precursor cells derived from different regions as assessed by lipoprotein lipase (LPL) activity, an early marker of adipocyte differentiation. LPL activity was increased by addition of dex, and no differences in degree of activation were observed in cells from different adipose tissue regions. Development of both basal and isoproterenol-stimulated lipolysis was also similar in adipose precursor cells from different adipose tissue regions. Dex addition enhanced the isoproterenol-stimulated lipolysis with no regional differences. Studies of binding of [³H]-dex showed no regional differences in either binding affinity or maximal binding capacity. It is concluded that dex stimulates both LPL activity and lipolytic activity in newly differentiated rat adipocytes in primary culture. This seems, however, not to vary in magnitude in cells obtained from different adipose tissue regions. This might be due to the apparent similarity of number and affinity of glucocorticoid binding sites. Regional variations in glucocorticoid regulated LPL and lipolytic activity in adipose tissue might therefore not be due to inherent differences between adipocytes.     

Fatty acid binding protein expression in different human adipose tissue depots in relation to rates of lipolysis and insulin concentration in obese individuals

Two fatty acid binding proteins (FABPs) are expressed in adipose tissue, adipocyte lipid binding protein (ALBP) and keratinocyte lipid binding protein (KLBP). This study investigated FABP expression in visceral and subcutaneous human adipose tissue depots and associations with lipolytic differences between the depots and circulating insulin concentrations. ALBP and KLBP (protein and RNA) were quantified in subcutaneous and omental adipose tissue from obese individuals and expressed relative to actin. ALBP RNA and protein expression was significantly higher in subcutaneous compared to omental adipose tissue (both p < 0.05), whereas KLBP RNA and protein expression was no different between the two sites. There were significant inverse correlations between serum insulin concentrations and the ALBP/KLBP RNA ratio in both subcutaneous and omental adipose tissue (both p < 0.02). Basal rates of glycerol and fatty acid release measured in adipocytes isolated from subcutaneous and omental adipose tissue were significantly higher in the former (p 0.02). Therefore the relative ALBP/KLBP content of human adipose tissue is different in different adipose tissue depots and at the RNA level is related to the circulating insulin concentration, at least in obese subjects. The higher rates of basal lipolysis in adipocytes isolated from subcutaneous compared to omental adipose tissue might be related to the increased ALBP content of the former. Therefore adipose tissue FABPs are interesting candidates for investigation to further our understanding of the insulin resistance syndrome and regulation of lipolysis.     

Adrenergic lipolysis in human fat cells: properties and physiological role of alpha-adrenergic receptors (author's transl)

Lipolytic activity of human isolated fat cells from different fat deposits was studied. The purpose of the present investigations was to determine the epinephrine responsiveness, with regard to alpha- and beta-adrenergic receptor site activity, of omental and subcutaneous adipocytes (abdominal or from the lateral part of the thigh). Adipocytes were obtained from normal subjects or from obese subjects on iso- or hypocaloric diets. The lipolytic effect of epinephrine varied according to the fat deposits, while the beta-lipolytic effect of isoproterenol was more stable (Fig. 1). We explored the possible involvement of adrenergic alpha-receptors, in order to explain these results. The potentiating action of phentolamine on epinephrine-induced lipolysis, and the antilipolytic effect of alpha-agonists on basal or theophylline--induced lipolysis, were found to be a good indication of alpha-adrenergic activity. The alpha-adrenergic antilipolytic effect was most prominent in adipose tissue from the lateral part of the thigh, and less noticeable in omental adipocytes. In conclusion, the inability of epinephrine to induce lipolysis, and the epinephrine-induced inhibition of lipolysis observed when the basal rate of FFA release was spontaneously increased in subcutaneous fat-cells of the thigh, could be explained by an increased alpha adrenergic responsiveness (Fig. 2). Moreover, various alpha-adrenergic agonists (phenylephrine, noradrenaline and adrenaline) showed a clear inhibiting effect on theophylline-stimulated adipocytes from the thigh. The pharmacological study of the antilipolytic effect of epinephrine on theophylline-induced lipolysis showed that the inhibition was linked to a specific stimulation of the alpha-receptors of the subcutaneous adipocytes (Fig. 4). From the different sets of experiments, it is shown that the modifications in the lipolytic effect of epinephrine on adipocytes of different areas could be explained by the occurrence of a variable alpha-adrenergic effect initiated by catecholamine. Furthermore, theophylline stimulation of lipolysis provides an accurate system to investigate the alpha-inhibiting effect of catecholamines. Our study was completed by the investigation of the lipolytic activity of subcutaneous fat cells from obese subjects submitted to a hypocaloric diet (800-1 000 Cal/day). An increased alpha-inhibitory effect of epinephrine was shown on the increased basal lipolytic activity observed in the fat cells of obese subjects on a hypocaloric diet (Fig. 5); a similar effect was observed when these adipocytes were stimulated by theophylline. To conclude, these investigations allow the alpha-adrenergic effect to be considered as a regulator mechanism of the in vitro lipolytic activity in human adipose tissue, since the antilipolytic effect is operative whenever the basal rate of lipolysis is increased (spontaneously, after caloric restriction, or with a lipolytic agent such as theophylline).     

Metabolism of Different Adipose Tissues in Vivo in the Rat

To explore regional differences in triglyceride retention in white adipose tissues of growing male rats, the mass of adipocytes from epididymal, retroperitoneal, inguinal, and mesenteric tissues were followed with time. In order to attempt to explain regional differences, adipose tissue metabolism was studied in vivo and in vitro. (U-¹⁴ C) oleic acid in sesame oil was given by gastric gavage to conscious male and female rats, and accumulation and half-life of radioactivity measured. Lipoprotein lipase activity and lipolysis were studied in vitro. Adipocyte triglyceride mass increased linearly in all the depots during 4 months of observation. The increase in mass was more pronounced in retroperitoneal (0.31 μg) and epididymal (0.30 μg) than in mesenteric (0.11 μg) or inguinal (0.05 μg) adipocytes. In the fed state label from (U-¹⁴C) oleic acid first increased with time in liver, muscle, and adipose tissues. In the liver radioactivity peaked at 4 hours, and was not measurable in either liver or muscle after a time point between 24 hours to 1 week. In contrast label continued to increase in adipose tissues up to about 16 hours to 24 hours, suggesting transfer of label by recirculation from liver and muscle to adipose tissues. Thereafter the radioactivity decreased. When expressed per adipocyte uptake of label was not significantly different between white adipose tissues. The rate of decrease between 7 days and 4 months was, however, more rapid in mesenteric and inguinal than, particularly, epididymal, and, probably, retroperitoneal adipocytes. These results were partly parallel to in vitro data on lipoprotein lipase activity, which was not different between depots, and the rate of lipolysis, which was higher in mesenteric than other adipocytes. These results suggest that differences in weight increase of adipose tissue regions are due mainly to differences in the rate of mobilization of adipocyte triglycerides. When expressed per gram triglyceride, uptake and mobilization of label were clearly more rapid in mesenteric than other white adipose tissues. This is probably explained by a combination of a higher adipocyte density plus the metabolic characteristics of adipocytes in this depot. Since mesenteric adipose tissue is smaller than the other depots studied, the absolute contribution of this tissue to the energy supply of the body is probably not different from that of other adipose tissues, however. A large uptake and short half life was observed in interscapular adipose tissue. This region contains brown adipocytes, and the results therefore suggest that lipid uptake for thermogenic purposes is of a considerable magnitude. It was concluded that among white adipose tissues, the mesenteric tissue has a rapid turnover of triglyceride. This is probably due to a combination of a high density and specific metabolic characteristics of these adipocytes. Factors in the microenvironment of adipocytes probably contribute to the high turnover either directly, or by modification of cellular characteristics.     

Lipolysis in Intraabdorninal Adipose Tissues of Obese Women and Men

Intraabdominal fat in humans is located in two major depots, the omental and mesenteric. We compared basal and stimulated lipolysis in adipose tissue from these two depots and the subcutaneous abdominal depot of obese women and men. Omental fat cells of women are smaller and have lower rates of basal lipolysis than in men. Basal Iipolysis rates are significantly higher in subcutaneous than intraabdominal adipose tissues of both genders. In men, the incremental lipolytic response to norepinephrine is significantly greater in both intraabdominal fat depots than in the subcutaneous fat, while in women tlie response of tlie mesenteric is lower than tlie omental. In women, but not men, responsiveness to tlie beta agonist isoproterenol is also increased in omental tissue. Thus, in women, omental and mesenteric adipose tissues show distinctly different metabolic properties which may moderate the impact of intraabdominal obesity.     

Interactions of high density lipoprotein subclasses (HDL2 and HDLc) with dog adipocytes: selective effects of cholesterol and saturated fat feeding

Adipose tissue is a cholesterol storage organ and derives its cholesterol primarily from circulating lipoproteins. The present study shows that adipocytes isolated from canine omental fat tissue interact specifically with high density lipoprotein subfractions lacking or enriched in apolipoprotein E, namely canine high density lipoprotein-2 (HDL2) and HDLc, respectively. While 125I-labeled HDL2 binding was inhibited similarly by both excess unlabeled HDLc and HDL2, 125I-labeled HDLc interaction was inhibited by its homologous ligand only. Paired studies showed that the amount of HDLc associated with adipocytes was significantly higher compared to HDL2. The effect of a short-term cholesterol and saturated fat feeding on adipocyte-HDL interaction was examined using fat cells obtained from dogs before and again 3 weeks after a diet supplemented with cholesterol (1% w/w) and saturated fat (30% lard, w/w). Significant increases in body weight and omental fat cell weight occurred after fat feeding. The amount of 125I-labeled HDL2 that could be bound to adipocytes increased after the diet, whether expressed on a per cell basis (P less than 0.005) or per unit cell surface (P less than 0.025). The amount of cell-associated 125I-labeled HDLc, however, was not significantly affected by the cholesterol-rich diet. The characteristics of HDLc and HDL2 dissociation were assessed by examining the release of labeled lipoproteins from adipocytes preincubated with 125I-labeled HDLc and 125I-labeled HDL2. HDL2 dissociation from adipocytes was significantly decreased (P less than 0.05) following the diet and may explain in part the apparent increase in cell-associated 125I-labeled HDL2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prolonged fasting on the lipolytic activity of isolated adipocytes of the rat epididymis

Male Wistar rats, 6-8 week old, were fasted for 72 hours. The in vitro lipolytic activity of epididymal adipocytes was measured in the presence of adrenalin (a alpha and beta adrenergic agonist), isoprenaline (a pure beta agonist), theophylline (a phosphodiesterase inhibitor) or UK 14304 (a alpha 2 adrenoceptor agonist) associated with adenosine deaminase. The basal lipolytic activity, expressed per 100 mg lipids, was higher in fasted adipocytes than in fed ones. Its stimulation by adrenalin or isoproterenol was decreased by fast. The effects of these drugs were more potentiated by theophylline in fasted adipocytes than in fed ones. The UK 14304 inhibition of adenosine deaminase-stimulated lipolysis was about 20% in fasted adipocytes and 50% in fed adipocytes. The in vitro resistance of fasted adipocytes to the lipolytic effect of adrenalin or isoproterenol may be related to the hypothyroid status of fasted rats.     

Studies on hormonal regulation of lipolysis and lipogenesis in fat cells of various mammalian species

1. Corticotropin-stimulated lipolysis in adipocytes of rats, mice, hamsters, guinea pigs and rabbits. Melanotropins elicited high lipolytic activity only in guinea pig and rabbit adipocytes. Opiate peptides were active only in rabbit adipocytes. Pituitary and chorionic gonadotropins and somatotropin were lipolytic in guinea pig adipocytes. Other hormones tested including prolactin, somatostatin, substance P, neurotensin, angiotensin II, thyrotropin releasing hormone and pancreatic polypeptide were devoid of lipolytic activity in all of the adipocytes studied. 2. In the rabbit adipocytes gamma-melanotropin was lipolytic only at high doses. At these doses the peptide inhibited the lipolytic response to a high dose of corticotropin. 3. Lipolysis stimulated by vasoactive intestinal peptide and epinephrine in rat adipocytes was antagonized by insulin. The lipolytic hormones corticotropin, epinephrine, vasoactive intestinal peptide and secretin suppressed basal and insulin-stimulated lipogenesis.     

Catecholamine stimulated lipolysis in differentiated human preadipocytes in a serum-free,defined medium

Adipocyte precursors from the stromal vascular fraction of human adipose tissue were allowed to differentiate in serum-free defined medium, whereafter their catecholamine stimulated lipolytic response was compared to that of mature isolated human adipocytes. Seventy-five to ninety percent of the fibroblast-like cells accumulated lipid droplets and glycerol-3-phosphate dehydrogenase activities of 1,000–2,800 mU/mg protein were measured in cell homogenates of differentiated cells. Lipolysis could be stimulated by both isoproterenol and norepinephrine in both differentiated preadipocytes as well as mature adipocytes. The results obtained with β-adrenergic agents suggested the presence of a higher affinity receptor in differentiated preadipocytes as compared to mature adipocytes. Mature adipocytes responded well to β-adrenergic agents, but no antilipolytic α₂-adrenergic response was observed in the differentiated preadipocytes. The presence of Gi proteins in the differentiated preadipocytes was suggested by the antilipolytic effect of adenosine as well as the lipolytic activity generated by pertussis toxin. In conclusion, our medium supported the differentiation of a very high percentage of human preadipocytes which developed a sensitive β-adrenergic lipolytic response but which lacked an α₂-adrenergic antilipolytic response.     

Perilipin Expression in Human Adipose Tissues: Effects of Severe Obesity,Gender, and Depot

Objective: Perilipins are phosphoproteins that are localized to the surface of triacylglycerol droplets within adipocytes where they regulate the rate of lipolysis. We sought to determine the effects of severe obesity and depot [omental (Om) vs. subcutaneous (Sc)] on perilipin expression in the adipose tissue of individuals. Research Methods and Procedures: Samples of Om and Sc adipose tissues obtained at surgery from severely obese subjects and fat aspirations from nonobese subjects were analyzed for perilipin protein and mRNA levels by Northern and Western analysis. Results: Perilipin A (periA) was the major perilipin expressed in adipose tissues. periA mRNA relative abundance was significantly lower in Sc adipose tissue from severely obese compared to that from nonobese subjects. Western blotting of adipose tissue extracts showed that periA protein levels expressed relative to tissue protein or fat cell surface area were significantly lower (～ ?40%) in abdominal Sc adipose tissue from severely obese compared to that from nonobese subjects. However, the calculated mass of perilipin per fat cell did not differ between the two groups. Perilipin mRNA levels were higher in Sc compared to Om adipose tissue from obese individuals (p < 0.025; n = 26; 17 women, 9 men); however, periA protein levels did not differ. In addition, perilipin protein, but not mRNA, levels were higher in Sc adipose tissue from obese men than from women (p < 0.025). Discussion: Variations in perilipin expression may contribute to the higher basal lipolytic rates observed in obese compared to nonobese individuals and in obese women compared to obese men.     

Sex- and depot-specific lipolysis regulation in human adipocytes: interplay between adrenergic stimulation and glucocorticoids

The purpose of this investigation was to explore interactions between adrenergic stimulation, glucocorticoids, and insulin on the lipolytic rate in isolated human adipocytes from subcutaneous and omental fat depots, and to address possible sex differences. Fat biopsies were obtained from 48 nondiabetic subjects undergoing elective abdominal surgery. Lipolysis rate was measured as glycerol release from isolated cells and proteins involved in lipolysis regulation were assessed by immunoblots. Fasting blood samples were obtained and metabolic and inflammatory variables were analyzed. In women, the rate of 8-bromo-cAMP- and isoprenaline-stimulated lipolysis was approximately 2- and 1.5-fold higher, respectively, in subcutaneous compared to omental adipocytes, whereas there was no difference between the two depots in men. Dexamethasone treatment increased the ability of 8-bromo-cAMP to stimulate lipolysis in the subcutaneous depot in women, but had no consistent effects in fat cells from men. Protein kinase A, Perilipin A, and hormone sensitive lipase content in adipocytes was not affected by adipose depot, sex, or glucocorticoid treatment. In conclusion, catecholamine and glucocorticoid regulation of lipolysis in isolated human adipocytes differs between adipose tissue depots and also between sexes. These findings may be of relevance for the interaction between endogenous stress hormones and adipose tissue function in visceral adiposity and the metabolic syndrome.     

Characteristics of the alpha 2/beta-adrenoceptor-coupled adenylate cyclase system and their relationship with adrenergic responsiveness in hamster fat cells from different anatomical sites.

Various studies have shown that the lipolytic response of white adipocytes to catecholamines was dependent on the anatomical origin of these cells. To provide a biological explanation for this phenomenon, we compared hamster white adipocytes, from femoral subcutaneous and epididymal fat, for their lipolytic activities, cAMP responses and adrenoceptor-coupled adenylate cyclase system. Basal and maximal lipolytic responses to the beta-adrenergic (isoproterenol) and the mixed alpha 2/beta-adrenergic (epinephrine) agonists were lower in femoral subcutaneous cells than in epididymal cells, but the alpha 2-adrenergic antilipolytic response to 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline bi-tartate (UK14304) was slightly greater in femoral subcutaneous fat cells than in epididymal fat cells. Identical results were observed for cAMP responses, except for the alpha 2-adrenergic inhibitory response which was identical in both fat deposits. Adrenoceptors studies revealed higher density of inhibitory alpha 2-adrenoceptors 2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline ([3H]RX821002-binding sites) in femoral subcutaneous fat cells than in epididymal fat cells, but identical density of stimulatory beta-adrenoceptors (125I-cyanopindolol-binding sites) and similar subdivision into beta-adrenoceptor subtypes in both adipose deposits. Finally, the level of the alpha-subunits of the stimulatory and inhibitors guanine-nucleotide-binding regulatory proteins, as well as the adenylate cyclase catalytic activity were 40-50% lower in femoral subcutaneous fat cell membranes than in epididymal fat cell membranes. These results suggest that the differences in cAMP and lipolytic responses to catecholamines between epididymal and femoral subcutaneous adipocytes result at least in part from site-related differences in the adenylate cyclase system rather than in the adrenoceptor status.     

Cyclic AMP-dependent protein kinase activity and lipolysis in adipose tissue. Effect of fasting, oligomycin and iodoacetamide

The release of glycerol into the medium, the concentration of cAMP, and the cAMP-dependent protein-kinase activity were studied in adipocytes and in fat-pads obtained from epididymal adipose tissue of rats under different conditions of feeding. An increase in the tissue concentration of cAMP and in the protein-kinase activity was observed in vivo at 48 and 96 h of fasting. A diminished release of glycerol was found in adipocytes from rats fasted for 48 h, in the absence of glucose, and the maximum concentration of cAMP was inferior to that of fed rats. Oligomycin and iodoacetamide, in the presence of epinephrine and glucose, produce a diminution in the values of the parameters studied. No significant differences were observed, however, in the responses of tissue obtained from fed and fasting rats to these compounds. The present results confirm previous observations and show the dependence of the lipolytic process on carbohydrate metabolism.     

Characterization of somatostatin binding sites in isolated rat adipocytes

Somatostatin binding sites were characterized in isolated rat adipocytes. The binding was found to be saturable, reversible, and time- and temperature-dependent. The somatostatin binding sites are principally located on the cell surface. 125I-[Tyr11]somatostatin binding was not inhibited by glucagon and angiotensin II. By contrast, native somatostatin and somatostatin-28 displaced labeled peptide with a similar ED50: 50 nM. Scatchard analysis pointed to the existence of two classes of binding sites, with a Kd of 7.64 nM for the high-affinity sites and a Kd of 295 nM for the low-affinity ones. Comparison of somatostatin receptor binding and its lipolytic action in isolated rat adipocytes suggested that the spare receptor phenomenon cannot be applied to the lipolytic action of somatostatin in rat adipose tissue.     

The primary amine metabolite of sibutramine stimulates lipolysis in adipocytes isolated from lean and obese mice and in isolated human adipocytes.

Sibutramine is a satiety-inducing serotonin-noradrenaline reuptake inhibitor that acts predominantly via its primary and secondary metabolites. This study investigates the possibility that sibutramine and/or its metabolites could act directly on white adipose tissue to increase lipolysis. Adipocytes were isolated by a collagenase digestion procedure from homozygous lean (+/+) and obese-diabetic OB/OB mice, and from lean nondiabetic human subjects. The lipolytic activity of adipocyte preparations was measured by the determination of glycerol release over a 2-hour incubation period. The primary amine metabolite of sibutramine M2, caused a concentration-dependent stimulation of glycerol release by murine lean and obese adipocytes (maximum increase by 157+/-22 and 245+/-16%, respectively, p<0.05). Neither sibutramine nor its secondary amine metabolite M1 had any effect on lipolytic activity. Preliminary studies indicated that M2-induced lipolysis was mediated via a beta-adrenergic action. The non-selective beta-adrenoceptor antagonist propranolol (10 (-6) M) strongly inhibited M2-stimulated lipolysis in lean and obese murine adipocytes. M2 similarly increased lipolysis by isolated human omental and subcutaneous adipocytes (maximum increase by 194+/-33 and 136+/-4%, respectively, p<0.05) with EC50 values of 12 nM and 3 nM, respectively. These results indicate that the sibutramine metabolite M2 can act directly on murine and human adipose tissue to increase lipolysis via a pathway involving beta-adrenoceptors.     

Cyclic AMP metabolism and lipolysis in cultured human adipocyte precursors

Triacylglycerol breakdown (lipolysis) results from a series of reactions culminated by activation of "hormone-stimulated" triacylglycerol lipase, an enzyme unique to adipose tissue. We have studied various components of the lipolytic process in human omental adipocyte precursors differentiating in culture. The levels of cyclic AMP, the "second messenger" of lipolytic hormones, were about sixfold higher in fat cell precursors than those in abdominal skin fibroblasts. L-Isoproterenol resulted in significant elevation of cyclic AMP levels in both cell types. Preincubation of intact adipocyte precursors with insulin resulted in significant enhancement of "low Km" cyclic AMP phosphodiesterase activity; in contrast, this hormone had no effect on fibroblast phosphodiesterase activity, a distinctive biochemical difference despite the morphological similarities between the two cell types during the early stages of adipocyte precursor maturation. Incubation of adipocyte precursors with isoproterenol resulted in the release of fatty acids into the medium, findings indicative of "hormone-stimulated" lipase activity and, hence, the operation of the entire "lipolytic cascade"; isoproterenol-stimulated lipolysis was inhibited by insulin. Release of fatty acids from fibroblasts was not observed. Thus, "hormone-stimulated" lipolysis and insulin stimulation of cyclic AMP phosphodiesterase activity are expressed during early stages of human adipocyte precursor differentiation.     

Comparison of the lipolytic activity of adipocytes isolated from white adipose tissue in Zucker and Wistar rats. In vitro effects of 3,5,3'-triiodo-L-thyronine

The in vitro effects of 3,5,3'-triiodo-L-thyronine (L-T3) on (-) epinephrine stimulated lipolytic activity were studied in adipocytes isolated from white adipose tissue of Zucker and Wistar male rats. Basic lipolytic activity was small and nearly insensitive to epinephrine in Zucker genetically obese rats. On the other hand, lipolytic activity was stimulated by epinephrine in Zucker lean rats and Wistar rats in the same way. There was no synergistic interaction between epinephrine and L-T3 on lipolytic activity, since the released glycerol levels were nearly the same. These data corroborate the hypothyroid status of Zucker genetically obese rats.     

Human adipose tissue under in vitro inhibition of 11beta-hydroxysteroid dehydrogenase type 1: differentiation and metabolism changes.

In humans, oxoreducing 11beta-HSD-1 activity appears to be related to body fat distribution in male-type central obesity, but not in female-type peripheral obesity. We postulated that inhibition of 11beta-HSD-1 might have clinical therapeutic significance in oxoreducing mostly visceral fat and its metabolic activity. Our current study investigated the consequence at the cellular level of such inhibition. As an inhibitor of 11beta-HSD-1 activity, we used the licorice derivative carbenoxolone. Carbenoxolone has an inhibitory effect on the activity of both oxidizing 11beta-HSD-2, which converts cortisol to cortisone, and oxoreducing 11beta-HSD-1; yet, preadipocytes and adipocytes only express the latter. Preadipocytes were retrieved from omental and subcutaneous fat from healthy non-obese individuals and differentiated in vitro to mature adipocytes. Activity of 11beta-HSD-1 was assayed by measuring conversion of added 500 nM cortisone to cortisol. Expression of 11beta-HSD-1 mRNA was determined by real-time PCR, while lipolytic effects were determined by measuring glycerol and triglyceride concentration in the culture medium. Carbenoxolone decreased 11beta-HSD-1 activity in a dose-dependent manner with an IC-50 of 5X10 -6 M, but did not affect the expression of 11beta-HSD-1 mRNA. Cortisone stimulated subcutaneous, but not omental preadipocytes proliferation, an effect that was not abolished by carbenoxolone. Dexamethasone had a stimulatory effect on the maturation of both omental and subcutaneous preadipocytes. Carbenoxolone per se, either with or without cortisone, had a negative effect on preadipocyte maturation. Inhibiting 11beta-HSD-1 activity by carbenoxolone had no impact on leptin secretion. Thus, carbenoxolone has no effect on preadipocyte proliferation, but a dramatic inhibitory effect on preadipocyte differentiation into mature adipocytes. The mechanism is only partly related to its inhibitory effect on 11beta-HSD-1 activity. The present observations lend support to the presence of an intracrine loop of a hormone that is both produced from a precursor and active within the preadipocyte and adipocyte.     

Body fat-lowering effect of conjugated linoleic acid is not due to increased lipolysis

The ability of conjugated linoleic acid (CLA) to reduce adiposity may be due to changes in energy expenditure and/or direct effects on adipocyte lipid metabolism. The aim of the present work was to analyse if CLA supplementation modifies lipolytic activity in adipose tissue from hamsters fed on high-fat diet. Hamsters were divided into two groups and fed on diets supplemented with either 0.5% linoleic acid (control) or 0.5% trans-10,cis-12 CLA. After 6 weeks, animals were fasted overnight and adipose tissues were dissected and weighed. Adipocytes were isolated by collagenase digestion and incubated in Krebs-Ringer bicarbonate buffer with or without several agents acting at different levels of the lipolytic cascade. Adipocyte diameters were measured by microscopy. Adipose tissue DNA content was assessed by spectrophotometry. Animals fed on CLA diet showed significantly reduced adipose tissue mass. No differences between both groups was found for basal lipolysis, lipolytic effects of isoproterenol, forskolin, dibutyryl-cAMP and isobutylmethylxanthine, and pD2 for isoproterenol. A similar total DNA amount was found in adipose tissue of both groups, showing that CLA diet had no effect on total cell number per fat pad. Although DNA content per gram tissue, an indirect reverse index of cell size, was significantly increased in CLA fed hamsters, microscopy did not reveal differences in medium mature adipocyte diameter, nor in cell size distribution between both groups. These results suggest that adipose tissue size reduction induced by trans-10,cis-12 CLA intake is not due to changes in lipolysis. Reduced preadipocyte differentiation into mature adipocytes may account for this fat-lowering effect.