De novo lipogenesis and lipolysis activities in normal (Dw) and dwarf (dw) White Leghorn laying hens

1. In vitro activities of glucose oxidation, de novo lipogenesis and lipolysis were compared in normal (Dw) and dwarf (dw) laying hens. 2. Dwarfism reduced the hepatic glucose oxidation while de novo lipogenesis was not altered. As liver weight was depressed, total liver lipogenesis capacity was probably reduced by dwarfism. 3. As compared to normal hens, de novo lipogenesis and basal or stimulated lipolysis were lower in dwarf adipose tissue while its lipid content was enhanced in dwarfs. 4. Results suggest that in laying hens dwarfism reduces the adipose tissue lipid mobilization but probably also the liver de novo lipogenesis.     

Resveratrol directly affects in vitro lipolysis and glucose transport in human fat cells

Resveratrol is a naturally occurring polyphenol found in many dietary sources and red wine. Recognized as a cancer chemoprevention agent, an anti-inflammatory factor and an antioxidant molecule, resveratrol has been proposed as a potential anti-obesity compound and to be beneficial in diabetes. Most of the studies demonstrating the anti-adipogenic action of resveratrol were performed as long-term treatments on cultured preadipocytes. The aim of this study was to analyse the acute effects of resveratrol on glucose uptake and lipolysis in human mature adipocytes. Samples of subcutaneous abdominal adipose tissue were obtained from overweight humans and immediately digested by liberase. Fat cells were incubated (from 45 min to 4 h) with resveratrol 1 μM–1 mM. Then, glycerol release or hexose uptake was determined. Regarding lipolysis, the significant effects of resveratrol were found at 100 μM, consisting in a facilitation of isoprenaline stimulation and an impairment of insulin antilipolytic action. At 1 and 10 μM, resveratrol only tended to limit glucose uptake. Resveratrol 100 μM did not change basal glucose uptake but impaired its activation by insulin or by benzylamine. This inhibition was not found with other antioxidants. Such impairment of glucose uptake activation in fat cells may led to a reduced availability of glycerol phosphate and then to a decreased triacylglycerol assembly. Therefore, resveratrol increased triacylglycerol breakdown triggered by β-adrenergic activation and impaired lipogenesis. Consequently, our data indicate that resveratrol can be considered as limiting fat accumulation in human fat cells and further support its use for the mitigation of obesity.     

Hexosamine Biosynthesis Is a Possible Mechanism Underlying Hypoxia’s Effects on Lipid Metabolism in Human Adipocytes

Introduction

Hypoxia regulates adipocyte metabolism. Hexosamine biosynthesis is implicated in murine 3T3L1 adipocyte differentiation and is a possible underlying mechanism for hypoxia’s effects on adipocyte metabolism.

Methods

Lipid metabolism was studied in human visceral and subcutaneous adipocytes in in vitro hypoxic culture with adipophilic staining, glycerol release, and palmitate oxidation assays. Gene expression and hexosamine biosynthesis activation was studied with QRTPCR, immunofluorescence microscopy, and Western blotting.

Results

Hypoxia inhibits lipogenesis and induces basal lipolysis in visceral and subcutaneous human adipocytes. Hypoxia induces fatty acid oxidation in visceral adipocytes but had no effect on fatty acid oxidation in subcutaneous adipocytes. Hypoxia inhibits hexosamine biosynthesis in adipocytes. Inhibition of hexosamine biosynthesis with azaserine attenuates lipogenesis and induces lipolysis in adipocytes in normoxic conditions, while promotion of hexosamine biosynthesis with glucosamine in hypoxic conditions slightly increases lipogenesis.

Conclusions

Hypoxia’s net effect on human adipocyte lipid metabolism would be expected to impair adipocyte buffering capacity and contribute to systemic lipotoxicity. Our data suggest that hypoxia may mediate its effects on lipogenesis and lipolysis through inhibition of hexosamine biosynthesis. Hexosamine biosynthesis represents a target for manipulation of adipocyte metabolism.     

Adipogenic and lipolytic effects of chronic glucocorticoid exposure

Glucocorticoids have been proposed to be both adipogenic and lipolytic in action within adipose tissue, although it is unknown whether these actions can occur simultaneously. Here we investigate both the in vitro and in vivo effects of corticosterone (Cort) on adipose tissue metabolism. Cort increased 3T3-L1 preadipocyte differentiation in a concentration-dependent manner, but did not increase lipogenesis in adipocytes. Cort increased lipolysis within adipocytes in a concentration-dependent manner (maximum effect at 1-10 μM). Surprisingly, removal of Cort further increased lipolytic rates (～320% above control, P < 0.05), indicating a residual effect on basal lipolysis. mRNA and protein expression of adipose triglyceride lipase and phosphorylated status of hormone sensitive lipase (Ser563/Ser660) were increased with 48 h of Cort treatment. To test these responses in vivo, Sprague-Dawley rats were subcutaneously implanted with wax pellets with/without Cort (300 mg). After 10 days, adipose depots were removed and cultured ex vivo. Both free fatty acids and glycerol concentrations were elevated in fed and fasting conditions in Cort-treated rats. Despite increased lipolysis, Cort rats had more visceral adiposity than sham rats (10.2 vs. 6.9 g/kg body wt, P < 0.05). Visceral adipocytes from Cort rats were smaller and more numerous than those in sham rats, suggesting that adipogenesis occurred through preadipocyte differentiation rather than adipocyte hypertrophy. Visceral, but not subcutaneous, adipocyte cultures from Cort-treated rats displayed a 1.5-fold increase in basal lipolytic rates compared with sham rats (P < 0.05). Taken together, our findings demonstrate that chronic glucocorticoid exposure stimulates both lipolysis and adipogenesis in visceral adipose tissue but favors adipogenesis primarily through preadipocyte differentiation.     

Resveratrol,a naturally occurring diphenolic compound,affects lipogenesis,lipolysis and the antilipolytic action of insulin in isolated rat adipocytes

Resveratrol is a naturally occurring diphenolic compound exerting numerous beneficial effects in the organism. The present study demonstrated its short-term, direct influence on lipogenesis, lipolysis and the antilipolytic action of insulin in freshly isolated rat adipocytes. In fat cells incubated for 90 min with 125 and 250 μM resveratrol (but not with 62.5 μM resveratrol), basal and insulin-induced lipogenesis from glucose was significantly reduced. The antilipogenic effect was accompanied by a significant diminution of CO₂ release and enhanced production of lactate. The inhibition of glucose conversion to lipids found in the presence of resveratrol was not attenuated by activator of protein kinase C. However, acetate conversion to lipids appeared to be insensitive to resveratrol.In adipocytes incubated for 90 min with epinephrine, 10 and 100 μM resveratrol significantly enhanced lipolysis, especially at lower concentrations of the hormone. However, the lipolytic response to dibutyryl-cAMP, a direct activator of protein kinase A, was unchanged. Further studies demonstrated that, in cells stimulated with epinephrine, 1, 10 and 100 μM resveratrol significantly enhanced glycerol release despite the presence of insulin or H-89, an inhibitor of protein kinase A. The influence of resveratrol on epinephrine-induced lipolysis and on the antilipolytic action of insulin was not abated by the blocking of estrogen receptor and was accompanied by a significant (with the exception of 1 μM resveratrol in experiment with insulin) increase in cAMP in adipocytes. It was also revealed that resveratrol did not change the proportion between glycerol and fatty acids released from adipocytes exposed to epinephrine.Results of the present study revealed that resveratrol reduced glucose conversion to lipids in adipocytes, probably due to disturbed mitochondrial metabolism of the sugar. Moreover, resveratrol increased epinephrine-induced lipolysis. This effect was found also in the presence of insulin and resulted from the synergistic action of resveratrol and epinephrine. The obtained results provided evidence that resveratrol affects lipogenesis and lipolysis in adipocytes contributing to reduced lipid accumulation in these cells.     

Regulation of lipolysis in isolated adipocytes of rainbow trout (Oncorhynchus mykiss): the role of insulin and glucagon

In the present study, we have examined the effects of insulin and glucagon on the lipolysis of rainbow trout (Oncorhynchus mykiss). To this end, adipocytes were isolated from mesenteric fat and incubated in the absence (basal lipolysis) or presence of different concentrations of insulin and glucagon. In addition, to further elucidate the effects of these hormones in vivo on adipocyte lipolysis, both fasting and intraperitoneal glucagon injection experiments were performed. Basal lipolysis, measured as the glycerol released in the adipocyte medium, increased proportionally with cell concentration and incubation time. Cell viability was verified by measuring the release of lactate dehydrogenase (LDH) activity in the medium. Insulin (at doses of 35 and 350 nM) decreased lipolysis in isolated adipocytes of rainbow trout in vitro, while glucagon was clearly lipolytic at concentrations of 10 and 100 nM. Furthermore, hypoinsulinemia induced by fasting, as well as glucagon injection, significantly increased lipolysis in isolated adipocytes approximately 1.5- and 1.4-fold, respectively, when compared with adipocytes from control fish. Our data demonstrate that lipolysis, as measured in isolated adipocytes of rainbow trout, can be regulated by both insulin and glucagon. These results not only indicate that insulin is an important hormone in lipid deposition via its anti-lipolytic effects on rainbow trout adipocytes, but also reveal glucagon as a lipolytic hormone, as shown by both in vitro and in vivo experiments.     

Adipose triglyceride lipase regulates basal lipolysis and lipid droplet size in adipocytes

In adipocytes, lipid droplet (LD) size reflects a balance of triglyceride synthesis (lipogenesis) and hydrolysis (lipolysis). Perilipin A (Peri A) is the most abundant phosphoprotein on the surface of adipocyte LDs and has a crucial role in lipid storage and lipolysis. Adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) are the major rate-determining enzymes for lipolysis in adipocytes. Each of these proteins (Peri A, ATGL, and HSL) has been demonstrated to regulate lipid storage and release in the adipocyte. However, in the absence of protein kinase A (PKA) stimulation (basal state), the lipases (ATGL and HSL) are located mainly in the cytoplasm, and their contribution to basal rates of lipolysis and influence on LD size are poorly understood. In this study, we utilize an adenoviral system to knockdown or overexpress ATGL and HSL in an engineered model system of adipocytes in the presence or absence of Peri A. We are able to demonstrate in our experimental model system that in the basal state, LD size, triglyceride storage, and fatty acid release are mainly influenced by the expression of ATGL. These results demonstrate for the first time the relative contributions of ATGL, HSL, and Peri A on determination of LD size in the absence of PKA stimulation.     

Rab18 dynamics in adipocytes in relation to lipogenesis, lipolysis and obesity

Lipid droplets (LDs) are organelles that coordinate lipid storage and mobilization, both processes being especially important in cells specialized in managing fat, the adipocytes. Proteomic analyses of LDs have consistently identified the small GTPase Rab18 as a component of the LD coat. However, the specific contribution of Rab18 to adipocyte function remains to be elucidated. Herein, we have analyzed Rab18 expression, intracellular localization and function in relation to the metabolic status of adipocytes. We show that Rab18 production increases during adipogenic differentiation of 3T3-L1 cells. In addition, our data show that insulin induces, via phosphatidylinositol 3-kinase (PI3K), the recruitment of Rab18 to the surface of LDs. Furthermore, Rab18 overexpression increased basal lipogenesis and Rab18 silencing impaired the lipogenic response to insulin, thereby suggesting that this GTPase promotes fat accumulation in adipocytes. On the other hand, studies of the β-adrenergic receptor agonist isoproterenol confirmed and extended previous evidence for the participation of Rab18 in lipolysis. Together, our data support the view that Rab18 is a common mediator of lipolysis and lipogenesis and suggests that the endoplasmic reticulum (ER) is the link that enables Rab18 action on these two processes. Finally, we describe, for the first time, the presence of Rab18 in human adipose tissue, wherein the expression of this GTPase exhibits sex- and depot-specific differences and is correlated to obesity. Taken together, these findings indicate that Rab18 is involved in insulin-mediated lipogenesis, as well as in β-adrenergic-induced lipolysis, likely facilitating interaction of LDs with ER membranes and the exchange of lipids between these compartments. A role for Rab18 in the regulation of adipocyte biology under both normal and pathological conditions is proposed.     

Pertussis toxin effects on adenylate cyclase activity, cyclic AMP accumulation and lipolysis in adipocytes from hypothyroid, euthyroid and hyperthyroid rats

Adipocytes from hypothyroid rats have a decreased responsiveness to agents that activate adenylate cyclase, whereas cells from hyperthyroid rats have an increased responsiveness as compared to the controls. This is reflected in cyclic AMP accumulation as well as lipolysis. Administration of pertussis toxin to rats or its in vitro addition to adipocytes increased basal lipolysis and cyclic AMP accumulation as well as the response to norepinephrine or forskolin. The effects of thyroid status was not abolished by toxin treatment. Pertussis toxin-catalyzed ADP ribosylation of Ni was increased in adipocyte membranes from hypothyroid rats as compared to those from euthyroid rats. However, no change in sensitivity to N6-(phenylisopropyl)adenosine was observed. The data suggest that the amount of Ni might not be rate-limiting for the inhibitory action of adenosine. A consistent decrease in maximal lipolysis was observed in freshly isolated adipocytes from hypothyroid animals as compared to those from the controls. Such defective maximal lipolysis was not corrected by adenosine deaminase or in vivo administration of pertussis toxin. The relationship between cyclic AMP levels and lipolysis suggests that in fat cells from hypothyroid rats either the cyclic AMP-dependent protein kinase or the lipase activity itself may limit maximal lipolysis. There appears to be multiple effects of thyroid status on lipolysis involving factors other than those affecting adenylate cyclase activation.     

Is there an optimal dose for dietary linoleic acid? Lessons from essential fatty acid deficiency supplementation and adipocyte functions in rats

Differential effects of n-3 and n-6 polyunsaturated fatty acids (PUFAs) have been demonstrated on adipose tissue physiology. Facing to the widely recognized beneficial effects of n-3 PUFAs, the n-6 PUFA effects remain controversial. Thus, we further analyzed the linoleic acid (LA) influence on adipocyte functions. To this aim, we treated by LA supplementation at three distinct doses (1, 2.5, or 5 % of energy intake) rats with essential fatty acids deficiency (EFAD). PUFA composition was determined in blood and white adipose tissue (WAT), while lipolytic and lipogenic activities were measured in isolated adipocytes. EFAD rats exhibited reduced WAT mass and increased EFAD biomarkers. WAT mass was completely recovered after supplementation, irrespective of LA dose. However, neither body mass nor EFAD biomarkers returned to control with 1 % LA, while LA abundance doubled in adipocytes from rats supplemented with 5 % LA. Regarding lipolysis, 2.5 % LA normalized the EFAD-induced alterations. A trend to decrease the maximal stimulation of lipolysis was observed with 1 and 5 % LA. Regarding lipogenesis, the lower and higher LA doses increased basal activity and hampered insulin to further stimulate glucose incorporation into lipids whereas 2.5 % LA normalized the basal or insulin-stimulated levels. Our results show that dietary linoleate at 2.5 % restored anatomical, biochemical, and functional disturbances induced by EFAD. At higher dose, LA tended to reduce triacylglycerol breakdown, to increase triacylglycerol assembly, and to provoke insulin resistance. Therefore, LA influence on adipocyte functions does not appear to follow a typical dose–response relationship, adding further complexity to the definition of its dietary requirement.     

Regulation of lipogenesis by glucocorticoids and insulin in human adipose tissue

Patients with glucocorticoid (GC) excess, Cushing's syndrome, develop a classic phenotype characterized by central obesity and insulin resistance. GCs are known to increase the release of fatty acids from adipose, by stimulating lipolysis, however, the impact of GCs on the processes that regulate lipid accumulation has not been explored. Intracellular levels of active GC are dependent upon the activity of 11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1) and we have hypothesized that 11β-HSD1 activity can regulate lipid homeostasis in human adipose tissue (Chub-S7 cell line and primary cultures of human subcutaneous (sc) and omental (om) adipocytes. Across adipocyte differentiation, lipogenesis increased whilst β-oxidation decreased. GC treatment decreased lipogenesis but did not alter rates of β-oxidation in Chub-S7 cells, whilst insulin increased lipogenesis in all adipocyte cell models. Low dose Dexamethasone pre-treatment (5 nM) of Chub-S7 cells augmented the ability of insulin to stimulate lipogenesis and there was no evidence of adipose tissue insulin resistance in primary sc cells. Both cortisol and cortisone decreased lipogenesis; selective 11β-HSD1 inhibition completely abolished cortisone-mediated repression of lipogenesis. GCs have potent actions upon lipid homeostasis and these effects are dependent upon interactions with insulin. These in vitro data suggest that manipulation of GC availability through selective 11β-HSD1 inhibition modifies lipid homeostasis in human adipocytes.     

Effects of mitochondrial uncoupling on adipocyte intracellular Ca(2+) and lipid metabolism

Previous data from this laboratory demonstrate that increased intracellular Ca(2+) ([Ca(2+)]i) coordinately regulates human and murine adipocyte lipid metabolism by stimulating lipogenesis and inhibiting lipolysis. However, recent data demonstrate metabolic uncoupling increases [Ca(2+)]i but inhibits lipogenesis by suppressing fatty acid synthase (FAS) activity. Accordingly, we have evaluated the interaction between mitochondrial uncoupling, adipocyte [Ca(2+)]i, and adipocyte lipid metabolism. Pretreatment of 3T3-L1 cells with mitochondrial uncouplers (DNP or FCCP) amplified the [Ca(2+)]i response to depolarization with KCl by 2-4 fold (p <0.001), while this increase was prevented by [Ca(2+)]i channel antagonism with lanthanum. Mitochondrial uncouplers caused rapid (within 4hr) dose-dependent inhibition of FAS activity (p <0.001), while lanthanum caused a further additive inhibition. The suppression of FAS activity induced by uncoupling was reversed by addition of ATP. Mitochondrial uncouplers increased FAS expression significantly while [Ca(2+)]i antagonism with lanthanum decreased FAS expression (P <0.001). In contrast, mitochondrial uncouplers independently inhibited basal and isoproterenol-stimulated lipolysis (20-40%, p <0.001), while this inhibition was fully reversed by lanthanum. Thus, mitochondrial uncoupling exerted short-term regulatory effects on adipocyte [Ca(2+)]i and lipogenic and lipolytic systems, serving to suppress lipolysis via a Ca(2+) -dependent mechanism and FAS activity via a Ca(2+)-independent mechanism.     

C-Phycocyanin Modulates Selenite-Induced Cataractogenesis in Rats

The present investigation is aimed to evaluate the anticataractogenic potential of C-phycocyanin (C-PC), extracted and purified from Spirulina platensis. Enucleated rat lenses were maintained in vitro in Dulbecco’s modified Eagle medium (DMEM). Group I contained DMEM, Group II and Group III contained 100 μM of sodium selenite, Group III was subdivided into three viz IIIa, IIIb, IIIc supplemented with 100, 150, 200 μg of C-PC respectively. In the in vivo study, on tenth day post partum: Group I rat pups received an intraperitoneal injection of saline, Group II, IIIa, IIIb, and IIIc rat pups received a subcutaneous injection of sodium selenite (19 μmol/kg bodyweight) Group IIIa, IIIb, IIIc also received an intraperitoneal injection of 100, 150, 200 mg/kg body weight of C-PC, respectively, from postpartum days?9–14. On termination of the experiment, the lenses from both in vitro and in vivo studies were subjected to morphological examination and subsequently processed to estimate the activities of antioxidant enzymes namely superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, levels of reduced glutathione and lipid peroxidation products. Sodium selenite-exposed, C-PC-treated rat lenses (Group IIIc), showed significant restoration of antioxidant enzyme activity (p?<?0.05) when compared to their counterpart Group II. Group IIIc conserved the levels of GSH and lipid peroxidation products at near to normal levels as compared with Group II. Results conclude the possible role of C-PC in modulating the antioxidant enzyme status, thereby retarding sodium selenite-induced cataract incidence both in vitro and in vivo.     

A low-protein, high-carbohydrate diet increases the adipose lipid content without increasing the glycerol-3-phosphate or fatty acid content in growing rats

The amount of triacylglycerol (TAG) that accumulates in adipose tissue depends on 2 opposing processes: lipogenesis and lipolysis. We have previously shown that the weight and lipid content of epididymal (EPI) adipose tissue increases in growing rats fed a low-protein, high-carbohydrate (LPHC) diet for 15 days. The aim of this work was to study the pathways involved in lipogenesis and lipolysis, which ultimately regulate lipid accumulation in the tissue. De novo fatty acid synthesis was evaluated in vivo and was similar for rats fed an LPHC diet or a control diet; however, the LPHC-fed rats had decreased lipoprotein lipase activity in the EPI adipose tissue, which suggests that there was a decreased uptake of fatty acids from the circulating lipoproteins. The LPHC diet did not affect synthesis of glycerol-3-phosphate (G3P) via glycolysis or glyceroneogenesis. Glycerokinase activity - i.e., the phosphorylation of glycerol from the hydrolysis of endogenous TAG to form G3P - was also not affected in LPHC-fed rats. In contrast, adipocytes from LPHC animals had a reduced lipolytic response when stimulated by norepinephrine, even though the basal adipocyte lipolytic rate was similar for both of the groups. Thus, the results suggest that the reduction of lipolytic activity stimulated by norepinephrine seems essential for the TAG increase observed in the EPI adipose tissue of LPHC animals, probably by impairment of the process of activation of lipolysis by norepinephrine.     

Inhibition of lipolysis may contribute to the acute regulation of plasma FFA and glucose by FGF21 in ob/ob mice

FGF21 is a unique member of the fibroblast growth factors (FGFs) and a novel hormone that regulates glucose, lipid, and energy homeostasis. The beneficial effects of FGF21 reported thus far have mostly been from chronic treatments. In order to better understand the mechanism for FGF21 action, we evaluated the acute effects of FGF21 in vivo and in vitro. Here we report that a single injection of FGF21 acutely reduced plasma free fatty acid levels similar to its acute effects on plasma glucose in ob/ob mice. In vitro, FGF21 inhibited lipolysis in adipocytes during a short treatment and reduced total lipase activity. These results demonstrate the potential importance of adipocyte lipolysis to the observed acute improvements in plasma parameters.     

Loss of P2X7 nucleotide receptor function leads to abnormal fat distribution in mice

The P2X7 receptor is an ATP-gated cation channel expressed by a number of cell types. We have shown previously that disruption of P2X7 receptor function results in downregulation of osteogenic markers and upregulation of adipogenic markers in calvarial cell cultures. In the present study, we assessed whether loss of P2X7 receptor function results in changes to adipocyte distribution and lipid accumulation in vivo. Male P2X7 loss-of-function (KO) mice exhibited significantly greater body weight and epididymal fat pad mass than wild-type (WT) mice at 9 months of age. Fat pad adipocytes did not differ in size, consistent with adipocyte hyperplasia rather than hypertrophy. Histological examination revealed ectopic lipid accumulation in the form of adipocytes and/or lipid droplets in several non-adipose tissues of older male KO mice (9–12 months of age). Ectopic lipid was observed in kidney, extraorbital lacrimal gland and pancreas, but not in liver, heart or skeletal muscle. Specifically, lacrimal gland and pancreas from 12-month-old male KO mice had greater numbers of adipocytes in perivascular, periductal and acinar regions. As well, lipid droplets accumulated in the renal tubular epithelium and lacrimal acinar cells. Blood plasma analyses revealed diminished total cholesterol levels in 9- and 12-month-old male KO mice compared with WT controls. Interestingly, no differences were observed in female mice. Moreover, there were no significant differences in food consumption between male KO and WT mice. Taken together, these data establish novel in vivo roles for the P2X7 receptor in regulating adipogenesis and lipid metabolism in an age- and sex-dependent manner.     

Mechanisms of the antilipolytic response of human adipocytes to tyramine,a trace amine present in food

Tyramine is found in foodstuffs, the richest being cheeses, sausages, and wines. Tyramine has been recognized to release catecholamines from nerve endings and to trigger hypertensive reaction. Thereby, tyramine-free diet is recommended for depressed patients treated with irreversible inhibitors of monoamine oxidases (MAO) to limit the risk of hypertension. Tyramine is a substrate of amine oxidases and also an agonist at trace amine-associated receptors. Our aim was to characterize the dose-dependent effects of tyramine on human adipocyte metabolic functions. Lipolytic activity was determined in adipocytes from human subcutaneous abdominal adipose tissue. Glycerol release was increased by a fourfold factor with classical lipolytic agents (1 μM isoprenaline, 1 mM isobutylmethylxanthine) while the amine was ineffective from 0.01 to 100 μM and hardly stimulatory at 1 mM. Tyramine exhibited a partial antilipolytic effect at 100 μM and 1 mM, which was similar to that of insulin but weaker than that obtained with agonists at purinergic A1 receptors, α₂-adrenoceptors, or nicotinic acid receptors. Gi-protein blockade by Pertussis toxin abolished all these antilipolytic responses save that of tyramine. Indeed, tyramine antilipolytic effect was impaired by MAO-A inhibition. Tyramine inhibited protein tyrosine phosphatase activities in a manner sensitive to ascorbic acid and amine oxidase inhibitors. Thus, millimolar tyramine restrained lipolysis via the hydrogen peroxide it generates when oxidized by MAO. Since tyramine plasma levels have been reported to reach 0.2 μM after ingestion of 200 mg tyramine in healthy individuals, the direct effects we observed in vitro on adipocytes could be nutritionally relevant only when the MAO-dependent hepato-intestinal detoxifying system is overpassed.     

Daidzein, coumestrol and zearalenone affect lipogenesis and lipolysis in rat adipocytes

Daidzein, coumestrol and zearalenone - compounds called phytoestrogens, considered as active biological factors affecting many important physiological and biochemical processes appeared to be also significant regulators of adipocyte metabolism. In our experiments the influence of daidzein (0.01, 0.1 and 1 mM), coumestrol (0.001, 0.01 and 0.1 mM), zearalenone (0.01, 0.1 and 1 mM) and estradiol (0.01, 0.1 and 1 mM) on basal and insulin-stimulated (1 nM) lipogenesis from glucose and acetate was tested in adipocytes isolated from growing (160 +/- 5 g b.w) male Wistar rats. All tested compounds significantly attenuated glucose conversion to lipids. In the case of daidzein and coumestrol, this effect was probably due to inhibition of glycolysis. Daidzein (0.01, 0.1 and 1 mM), coumestrol (0.01 and 0.1 mM) and zearalenone (0.01, 0.1 and 1 mM) affected also basal and epinephrine-stimulated (1 microM) lipolysis. Daidzein (0.01 and 1 mM) augmented basal glycerides breakdown in adipocytes. The epinephrine-induced lipolysis was dependent on daidzein concentration and its stimulatory (0.1 mM) or inhibitory (1 mM) influence was observed. Zearalenone changed lipolysis only at the concentration of 1 mM and its effect was contradictory in the absence or presence of epinephrine (the stimulatory or inhibitory effect, respectively). Results obtained in experiments with inhibitors (insulin, 1 nM and H-89, 50 microM) and activators (dibutyryl-cAMP, 1 mM and forskolin, 1 microM) of lipolysis allowed us to assume that daidzein augmented basal lipolysis acting on PKA activity. The inhibitory effect of daidzein and zearalenone on epinephrine-induced lipolysis is probably due to restriction of HSL action. The influence of coumestrol on glycerides breakdown was less marked. Estradiol augmented only epinephrine-stimulated lipolysis.