Effect of adipose tissue site, animal size, and fasting on lipolysis in bovine adipose tissue in vitro.

1. Lipolytic rates expressed as mumol glycerol released per mg protein increased with body weight in Holstein steers. 2. Lipolytic rates were greatest in both inner and outer back fat and lowest in omental, perirenal, and intermuscular fat depots. 3. Epinephrine stimulated overall glycerol release 3-5-fold. 4. Fasting resulted in greater basal lipolytic rates but epinephrine-stimulated rates tended to be greater for nonfasted steer adipose tissue. 5. Lipolytic activity in adipose tissue seems to increase with growth and fattening, and differences in lipolytic rates between various depots diminish with growth.     

Intracellular accumulation of free fatty acids in isolated white adipose cells

A simple, rapid, and accurate method was developed for measuring intracellular FFA levels in isolated white adipose cells using sucrose-(14)C or inulin carboxyl-(14)C as nontransportable, nonutilizable markers of the extracellular space. Following incubation, medium and cells were separated by centrifugation and the infranatant medium was removed by aspiration. The volume of medium trapped between cells was determined by measuring the amount of sucrose-(14)C or inulin carboxyl-(14)C retained in the floating packed adipose cells. In this way the FFA content of the adipose cells could be corrected for contamination by FFA bound to extracellular albumin. With this technique the initial events in hormone-activated lipolysis were studied under conditions of maximal and constant rates of triglyceride hydrolysis. The FFA content of isolated adipocytes of fed rats was 0.5 micro mole/g cell lipid. On addition of norepinephrine in the presence of medium albumin, the concentration of intracellular FFA rapidly increased and reached a plateau at a concentration of 2-2.5 micro moles/g cell lipid. In the presence of medium albumin an initial lag in glycerol release occurred and this was attributed to partial hydrolysis of triglyceride with retention of lower glycerides. After 5 min of incubation FFA and glycerol output was constant. In the absence of medium albumin norepinephrine-stimulated lipolysis was reduced more than 90% and extracellular FFA release was not detected. Nevertheless, intracellular FFA accumulation was identical to that seen in the presence of albumin. The data suggest that most of this intracellular pool of FFA is bound to cytoplasmic constituents.     

Different lipolytic effects of theophylline and dibutyryl cyclic AMP in vivo and in vitro.

Both dcAMP and theophylline are known to promote lipolysis in vitro by increasing intracellular cAMP. Although theophylline stimulates FFA mobilization in vivo as well, a report of low circulating FFA levels in the rat given dcAMP suggested that dcAMP may inhibit lipolysis in the intact animal. To explore this possibility, a comparison of the in vitro and in vivo lipolytic effects of theophylline and dcAMP was made in the young dog. Circulating glycerol and FFA levels rose following the administration of theophylline. While glycerol and FFA fell slightly in puppies given dcAMP, only the FFA change was significant. Epinephrine infusions given alone produced sustained elevations of glycerol and FFA. When theophylline was given in conjunction with ongoing epinephrine infusions, plasma glycerol and FFA levels remained high. On the other hand, epinephrine-stimulated lipolysis was markedly inhibited by dcAMP, as shown by pronounced falls of glycerol and FFA from the elevated levels found with epinephrine alone. In vitro studies involving fragments of puppy adipose tissue reveal that epinephrine, theophylline, and dcAMP promoted glycerol release. In contrast to the in vivo observations, lipolysis was also stimulated by combinations of both epinephrine and theophylline as well as by epinephrine and dcAMP. Thus, theophylline stimulates lipolysis in vitro and in vivo in the puppy. In contrast, dcAMP stimulates lipolysis in vitro but inhibits this action in the intact animal. This important difference in the two pharmacologic agents suggests the need for caution when using them in in vivo studies involving the action of cAMP.     

Antilipogenic but not lipolytic effects of recombinant DNA-derived bovine somatotropin treatment on ovine adipose tissue; variation with genetic type

1. Lambs from three breeds (East Friesland, Oxford and Texel) were treated with recombinant DNA-derived bovine somatotropin (BST) at 0.05, 0.10, 0.20 mg/kg per day and fat metabolism assessed in subcutaneous adipose tissue biopsy samples. 2. BST treatment decreased adipose cell volume, fatty acid synthesis and acylglycerol glycerol synthesis but did not alter lipolytic rates (basal or noradrenaline-stimulated). 3. Genetic type influenced metabolism in a number of ways, most notably East Friesland lambs had lower fatty acid esterification rates and responded poorly to BST in terms of reduced lipogenesis as compared to the Oxford and Texel lambs. 4. Blood urea concentration was decreased by BST treatment suggesting increased nitrogen retention. 5. These results emphasise the role of somatotropin as an inhibitor of adipose tissue lipogenesis but cast further doubt on a physiological role in regulating lipolysis.     

Lipolysis in brown adipose tissue of cold- and heat-acclimated hamsters

Rates of release of free fatty acids (FFA) and glycerol to the incubation medium by brown adipose tissue (BAT) slices isolated from heat-acclimated (H), cold-acclimated (C), and control (N) hamsters in the absence or presence of epinephrine (E) were studied. Rates of FFA and glycerol release by tissue slices isolated from H and N animals were similar. In tissue slices isolated from C animals rate of release of FFA and glycerol was three times as high. Addition of E to the incubation medium (200 microgram/ml) had no effect on the rate of FFA and glycerol release of slices from C animals, but tripled the rates of slices from N, resulting in similar values for the two groups. In slices from H animals the rate of release was lower than in the other two groups, increasing only 1.5-fold. Pretreatment of N animals with triiodothyronine (T3; 0.8 microgram/100 g daily for 7 days) doubled the rates of FFA and glycerol release. Addition of E to the medium affected both pretreated and nontreated slices similarly. Two possible mechanisms by which temperature acclimation controls the lipolytic rate of BAT are suggested by 1) the concentration of specific enzymes and 2) cellular metabolites and hormones which activate existing systems. It seems that both operate in temperature-acclimated hamsters.     

Sex difference in triglyceride/fatty acid substrate cycling of rat adipose tissue: indirect regulation by androgens.

Male Sprague-Dawley rats displayed significantly higher rates of triglyceride/fatty acid (TG/FFA) substrate cycling in subcutaneous, perigenital, and mesenteric white adipose tissue, compared to females. To investigate possible regulation via androgens and estrogens, male rats were treated with the androgen antagonist, cyproterone acetate (10 mg daily in subcutaneous injections), or estradiol polyphosphate (0.3 mg intramuscularly, given as a single dose). Estradiol treatment did not affect TG/FFA cycling. Treatment with cyproterone acetate significantly decreased TG/FFA cycling in perigenital (epididymal) tissue. This effect could however largely be ascribed to concomitant inhibition of food intake by cyproterone acetate. The effects of cyproterone acetate on the two axes of TG/FFA cycling (lipolysis and re-esterification) were further studied in vitro. Norepinephrine-stimulated glycerol release from perigenital adipocytes was inhibited, whereas activities of esterification enzymes (GPAT and PPH) was essentially unaffected. We conclude that androgens seem to affect TG/FFA cycling indirectly via the lipolytic axis.     

Plasma FFA responses to prolonged walking in untrained men and women

Gender differences in plasma FFA responses to 90 min of treadmill walking at 35% VO2max were investigated in six men and six women following an overnight fast. The subjects represented average values for maximal oxygen uptake and body fat percentage for age and gender. Mean plasma FFA concentration at 45 and 90 min of exercise were significantly (P less than 0.05) higher for women (0.82 mmol X 1(-1), 0.88 mmol X 1(-1)) than men (0.42 mmol X 1(-1), 0.59 mmol X 1(-1)). Lower R values for women throughout the exercise period indicated a greater percentage fat in total metabolism than for men while the FFA/glycerol results supported greater lipolytic activity for women. The uniformity of percent fat in metabolism for women from rest to exercise showed that FFA release from adipose tissue increased rapidly with the onset of exercise which was not the case for men. Comparison of metabolic data as well as a statistical analysis (ANCOVA) controlling for the influence of VO2max and percentage body fat on FFA plasma concentration suggested that gender differences in FFA responses to prolonged submaximal exercise can be expected to occur in untrained subjects.     

The Dietary Protein/Carbohydrate Ratio Differentially Modifies Lipogenesis and Protein Synthesis in the Mammary Gland,Liver and Adipose Tissue during Gestation and Lactation

During gestation and lactation, a series of metabolic changes that are affected by the diet occurs in various organs of the mother. However, little is known about how the dietary protein (DP)/carbohydrate (DCH) ratio regulates the expression of metabolic genes in the mother. Therefore, the purpose of this work was to study the effect of consuming different percentages of DP/DCH, specifically 10/73, 20/63 and 30/53%, on the expression of genes involved in lipogenesis and protein synthesis in the mammary gland, liver and adipose tissue during gestation and lactation in dams. While the amount of weight gained during gestation was similar for all groups, only dams fed with 30/53% DP/DCH maintained their weight during lactation. In the mammary gland, the expression of the genes involved in lipogenesis, specifically SREBP1 and FAS, was dramatically increased, and the expression of the genes involved in protein synthesis, such as mTOR1, and the phosphorylation of its target protein, S6K, were also increased throughout pregnancy and lactation, regardless of the concentration of DP/DCH. In the liver and adipose tissue, the expression of the genes and proteins involved in lipid metabolism was dependent on the proportion of DP/DCH. The consumption of a low-protein/high-carbohydrate diet increased the expression of lipogenic genes in the liver and adipose tissue and the amount of lipid deposition in the liver. Conversely, the consumption of a high-protein/low-carbohydrate diet increased the expression of genes involved in amino acid oxidation in the liver during gestation. The metabolic adaptations reflected by the changes in the expression of metabolic genes indicate that the mammary gland has a priority for milk synthesis, whereas the adaptations in the liver and adipose tissue are responsible for providing nutrients to the mammary gland to sustain milk synthesis.     

Relative importance of liver,kidney, and substrates in epinephrine-induced increased gluconeogenesis in humans

Splanchnic and renal net balance measurements indicate that lactate and glycerol may be important precursors for epinephrine-stimulated gluconeogenesis (GNG) in liver and kidney, but the effects of epinephrine on their renal and hepatic conversion to glucose in humans have not yet been reported. We therefore used a combination of renal balance and isotopic techniques in nine postabsorptive volunteers to measure systemic and renal GNG from these precursors before and during a 3-h infusion of epinephrine (270 pmol. kg-1. min-1) and calculated hepatic GNG as the difference between systemic and renal rates. During infusion of epinephrine, renal and hepatic GNG from lactate increased 4- to 6-fold and accounted for approximately 85 and 70% of renal and hepatic glucose release, respectively, at the end of study; renal and hepatic GNG from glycerol increased approximately 1.5- to 2-fold and accounted for approximately 7-9% of renal and hepatic glucose release at the end of study. The increased renal GNG from lactate and glycerol was due not only to their increased renal uptake (approximately 3.3- and 1.4-fold, respectively) but also increased renal gluconeogenic efficiency (approximately 1.8- and 1.5-fold). The increased renal uptake of lactate and glycerol was wholly due to their increased arterial concentrations, since their renal fractional extraction remained unchanged and renal blood flow decreased. We conclude that 1) lactate is the predominant precursor for epinephrine-stimulated GNG in both liver and kidney, 2) hepatic and renal GNG from lactate and glycerol are similarly sensitive to stimulation by epinephrine, and 3) epinephrine increases renal GNG from lactate and glycerol by increasing substrate availability and the gluconeogenic efficiency of the kidney.     

Whole body and abdominal lipolytic sensitivity to epinephrine is suppressed in upper body obese women

We measured whole body and regional lipolytic and adipose tissue blood flow (ATBF) sensitivity to epinephrine in 8 lean [body mass index (BMI): 21 +/- 1 kg/m(2)] and 10 upper body obese (UBO) women (BMI: 38 +/- 1 kg/m(2); waist circumference >100 cm). All subjects underwent a four-stage epinephrine infusion (0.00125, 0.005, 0.0125, and 0.025 microgram. kg fat-free mass(-1). min(-1)) plus pancreatic hormonal clamp. Whole body free fatty acid (FFA) and glycerol rates of appearance (R(a)) in plasma were determined by stable isotope tracer methodology. Abdominal and femoral subcutaneous adipose tissue lipolytic activity was determined by microdialysis and (133)Xe clearance methods. Basal whole body FFA R(a) and glycerol R(a) were both greater (P < 0.05) in obese (449 +/- 31 and 220 +/- 12 micromol/min, respectively) compared with lean subjects (323 +/- 44 and 167 +/- 21 micromol/min, respectively). Epinephrine infusion significantly increased FFA R(a) and glycerol R(a) in lean (71 +/- 21 and 122 +/- 52%, respectively; P < 0.05) but not obese subjects (7 +/- 6 and 39 +/- 10%, respectively; P = not significant). In addition, lipolytic and ATBF sensitivity to epinephrine was blunted in abdominal but not femoral subcutaneous adipose tissue of obese compared with lean subjects. We conclude that whole body lipolytic sensitivity to epinephrine is blunted in women with UBO because of decreased sensitivity in upper body but not lower body subcutaneous adipose tissue.     

Adipose tissue sensitization to insulin induced by troglitazone and MEDICA 16 in obese Zucker rats in vivo

The putative role played by insulin sensitizers in modulating adipose tissue lipolysis in the fasting state was evaluated in obese conscious Zucker rats treated with troglitazone or beta,beta'-tetramethylhexadecanedioic acid (MEDICA 16) and compared with nontreated lean and obese animals. The rates of appearance (R(a)) of glycerol and free fatty acid (FFA), primary intra-adipose reesterification, and secondary reuptake of plasma FFA in adipose fat were measured using constant infusion of stable isotope-labeled [(2)H(5)]glycerol, [2,2-(2)H(2)]palmitate, and radioactive [(3)H]palmitate. The overall lipolytic flux (R(a) glycerol) was increased 1.7- and 1.4-fold in obese animals treated with troglitazone or MEDICA 16, respectively, resulting in increased FFA export (R(a) FFA) in the troglitazone-treated rats. Primary intra-adipose reesterification of lipolysis-derived fatty acids was enhanced twofold by insulin sensitizers, whereas reesterification of plasma fatty acids was unaffected by either treatment. Despite the unchanged R(a) FFA in MEDICA 16 or the increased R(a) FFA induced by troglitazone, very low density lipoprotein production rates were robustly curtailed. Total adipose tissue reesterification, used as an estimate of glucose conversion to glyceride-glycerol, was increased 1.9-fold by treatment with the insulin sensitizers. Our results indicate that, in the fasting state, insulin sensitizers induce, in vivo, a significant activation rather than suppression of adipose tissue lipolysis together with stimulation of glucose conversion to glyceride-glycerol.     

No apparent suppression by insulin of in vivo skeletal muscle lipolysis in nonobese women

To investigate the antilipolytic effect of insulin in skeletal muscle and adipose tissue in vivo, the rates of glycerol release from the two tissues were compared in 10 nonobese women during a two-step euglycemic hyperinsulinemic clamp. Tissue interstitial glycerol levels were determined by microdialysis, and tissue blood flow was assessed with the (133)Xe clearance technique. Absolute rates of glycerol release were estimated according to Fick's principle. In both adipose tissue and muscle, glycerol levels decreased significantly already during the low insulin infusion rate. The fractional release of glycerol (difference between interstitial glycerol and arterialized venous plasma glycerol) was reduced by more than one-half in adipose tissue (P < 0.0001) in response to insulin, whereas it remained unaltered in skeletal muscle. Muscle blood flow rates increased by 60% (P < 0.02) during insulin infusion; in adipose tissue, blood flow rates did not change significantly in response to insulin. The basal rate of glycerol release from skeletal muscle amounted to approximately 15% of that from adipose tissue. After insulin infusion, the rate of adipose tissue glycerol release was markedly suppressed, whereas in skeletal muscle the rate of glycerol mobilization did not change significantly in response to insulin. It is concluded that insulin does not inhibit the rate of lipolysis in skeletal muscle of nonobese women.     

Interrelation of aerobic glycolysis and lipogenesis in isolated perfused liver of well-fed rats

The rates of glycolysis and lipogenesis in isolated perfused liver of well-fed rats were studied. When liver was allowed to synthesize [14C]glycogen prior to perfusion, no more than 9% of the degraded [14C]glycogen was recovered in lactate and 6% in lipid. Addition of glucose, fructose and sorbitol enhanced concomitantly the formation of lactate and pyruvate and the rate of release of triglyceride and free fatty acid. Glucose was less efficient than fructose or sorbitol. The incorporation of 14C from these 14C-labelled substrates into lactate, pyruvate and lipids confirmed their role as carbon sources. Incorporation of 14C into the glycerol moiety of neutral lipid exceeded that found in the fatty acids, suggesting that these substrates contributed largely to the esterification of fatty acids. The total rate of de novo fatty acid synthesis was correlated with the formation of lactate and pyruvate. It is concluded that increased rates of aerobic glycolysis are related to increased rates of lipogenesis.     

Effect of exercise training on in vivo lipolysis in intra-abdominal adipose tissue in rats

Intra-abdominal obesity is associated with cardiovascular disease and non-insulin-dependent diabetes mellitus, and physical training has been suggested to alleviate these conditions. We compared epinephrine-stimulated lipolysis in vivo in three intra-abdominal adipose tissues (ATs: retroperitoneal, parametrial, and mesenteric) and in subcutaneous AT, and we also studied the effect of physical training. Moreover, we studied the effect of physical training on epinephrine-stimulated lipolysis in muscle in vivo. Female rats were either swim trained (15 wk, n = 8) or sedentary (n = 7). Under anesthesia, a two-stage intravenous epinephrine infusion (60 min of 80 and 200 ng. kg(-1). min(-1), respectively) was carried out, and local interstitial glycerol concentration was measured by the microdialysis technique. Blood flow was measured by microspheres. Training increased blood flow in all ATs [on average: 73 +/- 12 (trained) vs. 14 +/- 4 (sedentary) ml. 100 g(-1). min(-1), P < 0. 05]; nevertheless, epinephrine-stimulated interstitial glycerol concentrations were increased or unchanged. Interstitial glycerol concentration was higher in intra-abdominal than in subcutaneous AT in both trained and sedentary rats. In skeletal muscle, interstitial glycerol concentration and blood flow did not differ between trained and sedentary rats. In conclusion, in vivo lipolysis is higher both in the basal state and during epinephrine-stimulation in intra-abdominal than in subcutaneous AT, and training may be beneficial in alleviating intra-abdominal obesity by enhancing lipolysis in intra-abdominal fat depots.     

Glycerolipid biosynthesis in rat adipose tissue. Influence of adipose-cell size and site of adipose tissue on triacylglycerol formation in lean and obese rats.

The rates of lipid formation were compared in different fat-depots from lean and obese rats by using [14C]glycerol 3-phosphate, [14C]glucose or [14C]acetate as substrates. In lean animals, subcutaneous adipose tissue showed significantly lower rates of lipid synthesis than did perirenal and gonadal fat-tissue. In obese animals, the rates of lipid synthesis were significantly higher and did not vary from one fat-depot to another. Differences in the rates of lipid formation between lean and obese rats disappeared during dietary restriction of obese animals. The isolated adipocyte preparation did not reflect the true metabolic activity of the adipose organ, since this preparation was mainly derived from smaller adipocytes that were metabolically less active than larger adipocytes. The present study suggests that it is better to use whole tissue preparations to measure lipogenesis and esterification reactions, because these measurements represent the contribution of both larger and smaller adipocytes towards lipid formation.     

Temperature and benzyl alcohol as probes of the antilipolytic mechanism of insulin action in adipocytes.

The temperature dependence of cAMP accumulation and glycerol release in response to epinephrine and insulin in adipocytes is examined. (1) Glycerol release in the presence of epinephrine demonstrated linear Arrhenius kinetics to 41 degrees C, and above 45 degrees C glycerol release was progressively inhibited. (2) In contrast, incubation of the cells with both epinephrine and insulin resulted in glycerol release rates that were relatively temperature insensitive. (3) Calculation of the efficacy of insulin to inhibit epinephrine-stimulated glycerol release as a function of temperature yielded a biphasic response, with a distinct optimum around 41 degrees C, in a similar manner to the effects of insulin on hexose transport activation determined previously. (4) A saturating dose of insulin (40 ng/ml) was found to have no significant effect on epinephrine-stimulated intracellular cAMP over the temperature range studied. (5) Addition of benzyl alcohol (to 40 mM) resulted in substantial inhibition of basal, epinephrine stimulated, and insulin inhibited glycerol release, without affecting the magnitude of insulin inhibition. We conclude from these studies that (a) insulin inhibition of glycerol release can not be mediated directly by intracellular cAMP modulation, (b) as in the case of hexose transport activation, the signalling mechanism by the occupied insulin receptor appears to be relatively independent of the membrane lipid environment.     

Adipocyte responses to adrenaline and insulin in active and former sportsmen

The rates of lipolysis and lipogenesis in adipocytes, isolated from biopsy samples of subcutaneous fat, was assessed by estimation of glycerol release during a 30-min incubation, and of the incorporation of 14C-glucose into lipids during a 1-h incubation at 37 degrees C, respectively. The subjects were six highly-qualified, active endurance sportsmen, eight former endurance sportsmen of international class, and six untrained young men. In the active sportsmen the basal rate of lipolysis was about half of that in the previously-active sportsmen and the untrained subjects, but after the addition of adrenaline (10(-4) or 5 x 10(-4) mol.l-1) the lipolysis rate was the highest. No differences were observed in the lipolytic rates in the former sportsmen compared to the untrained subjects. Gases of a comparatively high level of lipogenesis were found in the trained subjects. The addition of insulin (9 microU.ml-1) to isolated adipocytes caused a significant augmentation of individual rates of lipogenesis in the active sportsmen and the untrained persons but not in the previously-active sportsmen. In comparison with the active sportsmen, the previously active sportsmen revealed an increased basal rate of lipolysis and a reduced sensitivity to the lipogenic action of insulin. These findings suggest that these changes may have had significance in avoiding an increase of adipose tissue after a decrease in energy expenditure due to a change in physical activity.     

Reduced plasma FFA availability increases net triacylglycerol degradation, but not GPAT or HSL activity, in human skeletal muscle

Intramuscular triacylglycerols (IMTG) are proposed to be an important metabolic substrate for contracting muscle, although this remains controversial. To test the hypothesis that reduced plasma free fatty acid (FFA) availability would increase IMTG degradation during exercise, seven active men cycled for 180 min at 60% peak pulmonary O(2) uptake either without (CON) or with (NA) prior ingestion of nicotinic acid to suppress adipose tissue lipolysis. Skeletal muscle and adipose tissue biopsy samples were obtained before and at 90 and 180 min of exercise. NA ingestion decreased (P < 0.05) plasma FFA at rest and completely suppressed the exercise-induced increase in plasma FFA (180 min: CON, 1.42 +/- 0.07; NA, 0.10 +/- 0.01 mM). The decreased plasma FFA during NA was associated with decreased (P < 0.05) adipose tissue hormone-sensitive lipase (HSL) activity (CON: 13.9 +/- 2.5, NA: 9.1 +/- 3.0 nmol.min(-1).mg protein(-1)). NA ingestion resulted in decreased whole body fat oxidation and increased carbohydrate oxidation. Despite the decreased whole body fat oxidation, net IMTG degradation was greater in NA compared with CON (net change: CON, 2.3 +/- 0.8; NA, 6.3 +/- 1.2 mmol/kg dry mass). The increased IMTG degradation did not appear to be due to reduced fatty acid esterification, because glycerol 3-phosphate activity was not different between trials and was unaffected by exercise (rest: 0.21 +/- 0.07; 180 min: 0.17 +/- 0.04 nmol.min(-1).mg protein(-1)). HSL activity was not increased from resting rates during exercise in either trial despite elevated plasma epinephrine, decreased plasma insulin, and increased ERK1/2 phosphorylation. AMP-activated protein kinase (AMPK)alpha1 activity was not affected by exercise or NA, whereas AMPKalpha2 activity was increased (P < 0.05) from rest during exercise in NA and was greater (P < 0.05) than in CON at 180 min. These data suggest that plasma FFA availability is an important mediator of net IMTG degradation, and in the absence of plasma FFA, IMTG degradation cannot maintain total fat oxidation. These changes in IMTG degradation appear to disassociate, however, from the activity of the key enzymes responsible for synthesis and degradation of this substrate.     

The glycerol phosphate, dihydroxyacetone phosphate and monoacylglycerol pathways of glycerolipid synthesis in rat adipose-tissue homogenates.

1. Fat-free homogenates from the epididymal fat-pads of rats were used to measure the rate of palmitate esterification with different substrates. The effectiveness of the acyl acceptors decreased in the order glycerol phosphate, dihydroxyacetone phosphate, 2-octadecenyl-glycerol and 2-hexadecylglycerol. 2. Glycerol phosphate and dihydroxyacetone phosphate inhibited their rates of esterification in a mutually competitive manner. 3. The esterification of glycerol phosphate was also inhibited in a partially competitive manner by 2-octadecenylglycerol and to a lesser extent by 2-hexadecylglycerol. However, glycerol phosphate did not inhibit the esterification of 2-octadecenylglycerol. 4. The esterification of dihydroxyacetone phosphate and 2-hexadecylglycerol was more sensitive to inhibition by clofenapate than was that of glycerol phosphate. Norfenfluramine was more effective in inhibiting the esterification of 2-hexadecylglycerol than that of glycerol phosphate or dihydroxyacetone phosphate. 5 It is concluded that rat adipose tissue can synthesize glycerolipids by three independent routes.     

Homocysteine suppresses lipolysis in adipocytes by activating the AMPK pathway

Hyperhomocysteinemia (HHcy) is an independent risk factor for coronary artery disease. Emerging evidence suggests that HHcy is also associated with adipocyte tissue dysfunction. One of the principal functions of adipose tissue is to provide energy substrate via lipolysis. In the present study, we investigated the effects of homocysteine (Hcy) on lipolysis in adipocytes. We found that Hcy inhibited release of glycerol and fatty acids, two typical indicators of the lipolytic response, in primary adipocytes and fully differentiated 3T3-L1 adipocytes in a dose-dependent manner under both basal and isoproterenol-stimulated conditions. In differentiated 3T3-L1 adipocytes, decreased glycerol and free fatty acid (FFA) release was associated with elevation of intracellular TG content. Further studies showed that Hcy-mediated antilipolytic responses were independent of the cyclic AMP-PKA and MEK-ERK1/2 pathways. However, Hcy increased phosphorylation levels of AMP-activated protein kinase (AMPK) and its downstream enzyme acetyl-CoA carboxylase. Compound C, an AMPK inhibitor, abolished Hcy-induced reduction of glycerol and FFA release under both basal and isoproterenol-stimulated conditions. Furthermore, AMPKα1 siRNA reversed Hcy-inhibited glycerol release. Supplementation of exogenous Hcy in the diet for 2 wk lowered circulating glycerol and FFA levels. Moreover, Hcy supplementation was associated with elevated leptin levels and reduced adiponectin levels in plasma. These results show that Hcy inhibits lipolysis through a pathway that involves AMPK activation.