Possible mechanisms by which adipocyte lipolysis is enhanced in exercise-trained rats

A possible mechanism(s) behind exercise training-enhanced lipolysis was investigated in rat adipocytes. Exercise training (9 weeks; running) enhanced the activity of cAMP-dependent protein kinase (PKA) and the protein expressions of PKA subunits (catalytic, RII alpha, and RII beta) in P(40) fraction (sedimenting at 40,000g), but not in I(40) fraction (infranatant of 40,000g) of adipocyte homogenate. The expression of PKA-anchoring protein 150 (AKAP150) in P(40) fraction was greater in exercise-trained (TR) than in control (C) rats. Hormone-sensitive lipase (HSL) activities in both fractions were also greater in TR. On the other hand, stimulated lipolysis was accompanied by increased activities of HSL in P(40) but not in I(40) fraction. The decreases in stimulated lipolysis due to St-Ht31 were greater in TR rats. Thus, the mechanisms behind exercise training-enhanced adipocyte lipolysis could involve the increased activities of PKA and HSL with enhanced expressions of AKAP150 and some subunits of PKA, all of which may be compartmentalized within adipocytes.     

Enhanced coupling of adenylate cyclase to lipolysis in permeabilized adipocytes from trained rats.

Digitonin-permeabilized adipocytes were used to study the coupling of adenylate cyclase (AC) to lipolysis in exercise-trained rats. Isoproterenol-(IPR) stimulated lipolysis in permeabilized cells was significantly greater in trained than in control rats. Under essentially identical conditions, the dose-response curve for IPR stimulation of AC activity in the absence of 3-isobutyl-1-methylxanthine was similar in trained and control rats. However, the potency of stimulation by IPR as a percentage of the basal level was greater in trained rats. AC activity and lipolysis in the presence of 3-isobutyl-1-methylxanthine were also significantly greater in trained than in control rats. Least-squares analysis by plotting the log AC vs. lipolysis values showed that the regression coefficient was about three-fold greater in trained than in control rats. The concentration of endogenous adenosine 3',5'-cyclic monophosphate (cAMP) needed to produce a half-maximal lipolytic response was 18.58 and 10.81 pmol.min-1.10(6) cells-1 in control and trained rats, respectively. Thus a positive relationship existed between lipolysis and AC activity, with a tighter coupling in trained rats. Lipolysis in response to exogenous cAMP tended to be greater in trained than in control rats, and the difference was statistically significant for 50 microM and 10 mM cAMP. Our finding support the concept that the major mechanism of enhanced lipolysis in trained rats was an increase in the activity of enzymatic step(s) distal to cAMP.     

beta-Adrenergic stimulated lipolysis in pony adipocytes is exclusively via a beta2-subtype and is not affected by lactation

Catecholamines are important lipolytic agents in horses and ponies but the nature of the adrenergic receptor subtype distribution in their adipocytes is uncertain. A first objective was to identify the beta-adrenergic receptor subtype(s) present in adipocytes from horses and ponies. A second objective was to evaluate if the lipolytic responsiveness of isolated adipocytes to beta-adrenergic agonists is altered during lactation, a condition known to affect markedly maternal fat metabolism. Isoproterenol and salbutamol elicited strong lipolytic responses in adipocytes isolated from horse and pony subcutaneous adipose tissue. There were weak lipolytic responses to norepinephrine, dobutamine and BRL37344. The weak lipolytic response to NE compared to isoproterenol or salbutamol suggests an antilipolytic action from alpha2-adrenergic receptors. The relative order of potency for the beta-adrenergic agonists was isoproterenol>/=salbutamol>dobutamine=BRL37344. There was expression of beta2-adrenergic receptor mRNA in pony and horse adipose tissues, as estimated by relative RT-PCR, but no expression of mRNAs for beta1- or beta3-adrenergic receptors. Early lactation did not alter the lipolytic responses to beta-adrenergic agonists, nor the expression of beta2-adrenergic receptor mRNA. Thus, these results indicate a dominant if not exclusive presence of beta2-adrenergic receptors in pony and horse adipocytes that is not affected by lactation.     

Nitric oxide production from rat adipocytes is modulated by beta3-adrenergic receptor agonists and is involved in a cyclic AMP-dependent lipolysis in adipocytes.

It is established that the modulation of beta(3)-adrenoceptor function could be associated with impairment of lipolysis in white fat and be responsible for disturbed lipid metabolism. Though two isoforms of nitric oxide synthase (NOS) were reported in adipocytes, the role of nitric oxide (NO) in adipose tissue is still ambiguous. The present work was directed to study the interplay between NO production and beta-adrenoceptor/cyclic AMP (cAMP) pathway on lipid mobilization (glycerol and nonesterified fatty acids, NEFA) in cultures of rat adipocytes isolated from epididymal white adipose tissue. beta-Nonselective (isoprenaline) and beta(3)-selective (BRL-37344) agonists and the postadrenoceptor agents such as dibutyryl-cAMP, forskolin, and 3-isobutyl-1-methylxanthine significantly increased nitrite, glycerol, and NEFA levels with BRL-37344 being the most potent. Conversely, addition of beta-nonselective (propranolol) or beta(3)-selective (bupranolol) antagonist or the adenylyl cyclase inhibitor (SQ 22,536) significantly reduced beta-agonist-induced NO production and lipolysis. For beta-adrenoceptor agonists, antagonists, and their pairs, there was a positive correlation between medium nitrite and glycerol or NEFA with r(2) being 0.90 and 0.84, respectively. The possible relationship between NO and lipolysis was revealed after adipocyte treatment with nonspecific (N(omega)-nitro-l-arginine methyl ester, l-NAME) and specific (aminoguanidine) NOS inhibitors. Both l-NAME and aminoguanidine significantly inhibited the lipolytic effect of BRL-37344. Moreover, NO-donor (S-nitroso-N-acetylpenicillamine) at higher concentration increased basal glycerol and NEFA levels. 8-bromo-cyclic GMP had no effect on adipocyte lipolysis. These data suggest that beta-adrenergic lipolysis, specifically beta(3)-adrenoceptor effect, which is realized via the adenylyl cyclase/cAMP/protein kinase A signaling cascade, involves NO production downstream of beta(3)-adrenoceptor/cAMP pathway.     

Peptide hormones and lipolysis in rabbit adipocytes

Fifty peptides and hormones from the hypophysis, hypothalamus, gastrointestinal tract and from other origins were tested for lipolytic activity in the isolated rabbit fat cell. Eight peptides derived from the precursor hormone proopiocortin stimulated glycerol release while all the other peptides and hormones showed no lipolytic activity. The most potent lipolytic peptide was alpha-MSH which also had the lowest minimal effective dose, followed by beta-lipotropin, ACTH and beta-MSH. The lipolytic activity was not influenced by the use of different collagenases or the cells from different breeds of rabbits.     

Targets for TNF-alpha-induced lipolysis in human adipocytes

BACKGROUND: Tumor necrosis factor-alpha (TNF-alpha)-induced lipolysis may be important for insulin resistance in both obesity and cachexia. In rodent cells TNF-alpha enhances lipolysis through down-regulation of the expression of the membrane proteins Galpha(i) and the lipid droplet-associated protein perilipin (PLIN). In human (but not murine) adipocytes TNF-alpha stimulates lipolysis through the mitogen activated protein kinases (MAPKs) p44/42 and JNK although it is unclear whether this is mediated via PLIN and/or Galpha(i). METHODS: Galpha(i) and PLIN as down-stream effectors of MAPKs were assessed in human adipocytes stimulated with TNF-alpha in the absence or presence of specific MAPK inhibitors. RESULTS: A 48-h incubation with TNF-alpha resulted in a pronounced increase in lipolysis, which was paralleled by a decrease in the mRNA and protein expression of PLIN. Both these effects were inhibited in a concentration-dependent manner in the presence of MAPK inhibitors specific for p44/42 (PD98059) and JNK (SP600125). However, TNF-alpha did not affect Galpha(i) mRNA or protein expression. Furthermore, experiments with pertussis toxin demonstrated that inhibition of Galpha(i) signaling did not affect TNF-alpha-mediated lipolysis. CONCLUSIONS: Our results suggest that TNF-alpha-mediated lipolysis is dependent on down-regulation of PLIN expression via p44/42 and JNK. This could be an important mechanism for the development of insulin resistance in both obesity and cachexia. However, in contrast to findings in rodent cells, Galpha(i) does not appear to be essential for TNF-alpha-induced lipolysis in human adipocytes.     

Streptozotocin induces lipolysis in rat adipocytes in vitro

Streptozotocin (STZ) is used to induce experimental diabetes in animals and is also applied for the treatment of patients with insulinoma. The aim of the present work was to investigate the direct effect of STZ on lipolysis in isolated rat adipocytes. After the isolation, the cells were incubated in a Krebs-Ringer buffer of pH 7.4, at the temperature 37 degrees C for 90 min with different concentrations of STZ: 0.5, 1 or 2 mmol/l. STZ caused a significant rise in basal values (99%, 199%, and 377%, respectively) and epinephrine-stimulated (1 micromol/l) lipolysis (15%, 24% and 46%, respectively). Augmentation of basal lipolysis by STZ was neither restricted by insulin (1 nmol/l) nor by H-89 (an inhibitor of protein kinase A, 50 micromol/l). These results indicate the stimulatory influence of STZ on the action of hormone-sensitive lipase in isolated cells of white adipose tissue. The obtained outcomes suggest that in studies employing STZ, it is necessary to consider its direct effect upon lipolysis in adipocytes.     

Augmentation of lipolysis in adipocytes from fed rats, but not from starved rats, by inhibition of rolipram-sensitive phosphodiesterase 4

The sensitivity of adipocytes to lipolytic agents is increased after starvation. In this study, we found that LY294002, an inhibitor of phosphatidylinositol-3 kinase (PI3K), in the concentration of more than 50 microM potentiates lipolysis induced by adenosine deaminase in adipocytes from fed rats (f-adipocytes), but not from starved rats (s-adipocytes). It also enhanced the sensitivity to lipolytic action of isoproterenol in f-adipocytes much more than s-adipocytes. The target of LY294002 may be an anti-lipolytic regulator expressed in response to food intake. Since another PI3K inhibitor, wortmannin, or a phosphodiesterase 3 (PDE3) inhibitor, cilostamide, failed to cause any specific effect to f-adipocytes, the PI3K-PDE3B pathway cannot be a target of LY294002. We found that LY294002 inhibits efficiently the cytoplasmic PDE activity of adipocytes. Rolipram, a specific inhibitor of PDE4, also inhibited the cytoplasmic PDE and caused a preferential increase of lipolysis in f-adipocytes. LY294002 blunted the actions of rolipram on lipolysis and the PDE activity. LY294002 accelerated protein kinase A activation. These data suggest that the rolipram-sensitive PDE4 is an anti-lipolytic enzyme expressed according to food intake. LY294002 may potentiate lipolysis through inhibition of the PDE4.     

Hormone-sensitive lipase is critical mediators of acute exercise-induced regulation of lipolysis in rat adipocytes

The purpose of the present study was to investigate the effect of acute exercise on lipolysis via coordination of hormone-sensitive lipase (HSL) and scaffold proteins, i.e., perilipin A and comparative gene identification-58 (CGI-58), in rat primary adipocytes. Glycerol release was significantly elevated immediately (0 h) and three hours (3 h) after exercise. Both activity and localization to the pellet of HSL were significantly greater in the pellet fraction, which is included in lipid droplet associated-proteins, than in the supernatant fraction. In the pellet fraction, although neither perilipin A nor CGI-58 protein level changed, level of perilipin A/CGI-58 complex was significantly reduced, accompanied by up-regulated association of perilipin A/HSL at 0 h and 3 h after exercise. On the other hand, there were no changes in these molecules at 24 h after exercise, despite a significant decrease in lipolysis that was observed in response to isoproterenol. These findings suggest that acute exercise enhances lipolysis up to at least 3 h after exercise in a manner dependent on modification of HSL and its association with and alteration in scaffold protein.     

Several agents and pathways regulate lipolysis in adipocytes

Adipose tissue is the only tissue capable of hydrolyzing its stores of triacylglycerol (TAG) and of mobilizing fatty acids and glycerol in the bloodstream so that they can be used by other tissues. The full hydrolysis of TAG depends on the activity of three enzymes, adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL) and monoacylglycerol lipase, each of which possesses a distinct regulatory mechanism. Although more is known about HSL than about the other two enzymes, it has recently been shown that HLS and ATGL can be activated simultaneously, such that the mechanism that enables HSL to access the surface of lipid droplets also permits the stimulation of ATGL. The classical pathway of lipolysis activation in adipocytes is cAMP-dependent. The production of cAMP is modulated by G-protein-coupled receptors of the Gs/Gi family and cAMP degradation is regulated by phosphodiesterase. However, other pathways that activate TAG hydrolysis are currently under investigation. Lipolysis can also be started by G-protein-coupled receptors of the Gq family, through molecular mechanisms that involve phospholipase C, calmodulin and protein kinase C. There is also evidence that increased lipolytic activity in adipocytes occurs after stimulation of the mitogen-activated protein kinase pathway or after cGMP accumulation and activation of protein kinase G. Several agents contribute to the control of lipolysis in adipocytes by modulating the activity of HSL and ATGL. In this review, we have summarized the signalling pathways activated by several agents involved in the regulation of TAG hydrolysis in adipocytes.     

Ontogenetic studies on adrenergic lipolysis in rat adipocytes from subcutaneous adipose tissue

The in vitro release of free fatty acids from small adipocytes (mean diameter 24.4 +/- 0.4 micrometer; 11-42 micrometer) isolated from subcutaneous adipose tissue of rats aged 5, 14, 21 and 35 days was induced by l-isoprenaline (ISO), l-noradrenaline (NA) or l-adrenaline (ADR). The results were related to a cell count of 10(6) and compared to results in adult rats [both sexes, fed ad libitum, mean weight 180-210 g). The cell yield decreased with development, i.e. it was highest in the youngest rats. In adult rats, increasing catecholamine (CA) concentrations produced pronounced concentration-dependent lipolysis in the potency order ISO greater than NA greater than ADR which is typical of beta 1-adrenergic functions. Qualitatively the same results in 5- and 35-day-old rats were observed, the latter showing greater sensitivity towards CA. Practically no lipolytic response was recorded in 14 and 21-day-old animals, however. Not one of the CA, nor any of the concentrations used, could evoke lipolysis in these animals. The experiments were carried out simultaneously on different age groups, technical errors can therefore be excluded.     

Inducible nitric oxide synthase modulates lipolysis in adipocytes

The role of inducible nitric oxide synthase (iNOS) in the modulation of adipocyte lipolysis was investigated. Treatment of white and brown adipose cell lines and mouse adipose explants with a mixture of tumor necrosis factor-alpha, interferon-gamma, and lipopolysaccharide (LPS) doubled the lipolytic rate, and this was associated with marked induction of iNOS expression and nitric oxide (NO) production. iNOS inhibition by 1400W, aminoguanidine, or L-NIL pretreatment further increased the cytokine/LPS-mediated lipolysis by 30% (P < 0.05) in cultured adipocytes and in adipose explants. However, this potentiating effect of iNOS inhibition was abolished in adipose explants isolated from iNOS knockout mice. Pharmacological inhibitors of adenylyl cyclase or protein kinase A reduced cytokine/LPS-induced lipolysis and also blunted the potentiating effect of iNOS inhibition on the lipolytic rate. Furthermore, addition of the antioxidants l-cystine and l-glutathione to cytokine/LPS-stimulated adipocytes mimicked the lipolytic effect of iNOS inhibition. In conclusion, inhibition of iNOS activity in adipocytes potentiates cytokine/LPS-induced lipolysis. This effect was fully reversed by adenylyl cyclase and protein kinase A inhibitors but was mimicked by cellular antioxidants. These data suggest that iNOS-mediated NO production counteracts cytokine/LPS-mediated lipolysis in adipocytes and that this feedback mechanism involves an oxidative process upstream of cAMP production in the signaling pathway.     

Catecholamine-stimulated lipolysis in adipocytes of spontaneously hypertensive rats

We investigated the catecholamine-stimulated lipolytic response of perirenal adipocytes isolated from spontaneously hypertensive (SHR) and normotensive (C) rats of the Sprague-Dawley (SD) and Wistar-Kyoto (WKY) strain. Younger rats (10-17 weeks) were matched with respect to age and body weight. Age-matched SHR rats were smaller than their C counterparts, had equal-size adipocytes, and demonstrated lipolytic responses equal to C cells. Weight-matched SHR rats were older than normotensive controls, had larger adipocytes, and showed depressed norepinephrine (NE)-stimulated responses with a rightward shift in the dose-response curve. Rates of lipolysis of SHR and C cells were not different in the simultaneous presence of norepinephrine and theophylline. Nine- to ten-month-old rats were of comparable body weight and adipocyte size regardless of blood pressure status; however, SHR cells still showed a significantly blunted response to catecholamine stimulation. We conclude that (1) the NE-stimulated lipolytic response of adipocytes of SHR rats is significantly less than that elicited from C cells; (2) this function difference seems unrelated to a size difference between cells of younger, SHR and C rats, thus implicating the adrenergic system; and (3) whole body growth (as reflected by body weight) and perirenal adipocyte growth do not proceed in parallel in actively growing SHR rats.     

MicroRNA-425 controls lipogenesis and lipolysis in adipocytes

An increasing number of studies have demonstrated that some microRNAs participate in the regulation of growth and development of adipocytes. The present study shows that microRNA-425-5p (miR-425) is a novel strong regulator of adipogenesis and adipolysis in adipocytes. Forced expression of miR-425 in mice promoted body fat accumulation and the development of obesity due to high-fat intake, whereas silencing miR-425 prevented mice from being obese. Mechanically, the expression of miR-425 is controlled by PPARγ during the adipogenesis process in adipocytes. MiR-425 overexpression resulted in a reduction in the proliferation of 3t3-L1 pre-adipocytes but significantly accelerated cellular adipogenic differentiation. Mapk14, a negative regulator of adipogenesis, was predicted and confirmed as a real target gene of miR-425. Moreover, knocking down miR-425 remarkably intensified intracellular lipolysis and promoted lipid oxidation, which is related to the activation of AMPK, a monitor for intracellular energy balance. MiR-425 activated AMPK not only by decreasing cellular ATP concentrations but also by targeting the gene of Cab39, which is an upstream co-activator of AMPK. The findings of the present study suggest that miR-425 could control adipogenesis and adipolysis in adipocytes by simultaneously triggering multidirectional targets.     

Enhanced metabolic response to caffeine in exercise-trained human subjects

The effect of caffeine on resting metabolic rate (RMR) was investigated in eight trained and eight nontrained young male subjects. The ingestion of 4 mg/kg caffeine produced a greater increase of RMR in trained subjects. This effect was associated with a greater increase in plasma free fatty acids and a larger fall in respiratory quotient, indicating an enhanced lipid oxidation following caffeine in exercise-trained subjects. An initial fall in plasma glucose was observed but only in trained subjects, and caffeine did not change plasma insulin in either group studied. Caffeine caused a significant fall in plasma norepinephrine and an increase in plasma epinephrine in both groups of subjects, but this action was significantly greater in trained subjects. It is suggested that the greater increase in RMR observed in trained subjects following caffeine ingestion is related to an enhanced lipid mobilization, possibly produced by a greater epinephrine secretion and by subsequent increased lipid oxidation.     

Rosiglitazone regulates IL-6-stimulated lipolysis in porcine adipocytes

Interleukin (IL)-6, a proinflammatory cytokine, stimulates adipocyte lipolysis and induces insulin resistance in obese and diabetic subjects. However, the effects of the anti-diabetic drug rosiglitazone on IL-6-stimulated lipolysis and the underlying molecular mechanism are largely unknown. In this study, we demonstrated that rosiglitazone suppressed IL-6-stimulated lipolysis in differentiated porcine adipocytes by inactivation of extracellular signal-related kinase (ERK). Meanwhile, rosiglitazone enhanced the lipolysis response of adipocytes to isoprenaline. In addition, rosiglitazone significantly reversed IL-6-induced down-regulation of several genes such as perilipin A, peroxisome proliferators activated receptor gamma (PPARγ), and fatty acid synthetase, as well as the up-regulation of IL-6 mRNA. However, mRNA expression of PPARγ coactivator-1 alpha (PCG-1α) was enhanced by rosiglitazone in IL-6-stimulated adipocytes. These results indicate that rosiglitazone suppresses IL-6-stimulated lipolysis in porcine adipocytes through multiple molecular mechanisms.     

Effect of endothelin-1 on lipolysis in rat adipocytes

Objective : To explore the role of endothelin‐1 (ET‐1) on lipid metabolism, we examined the effect of ET‐1 on lipolysis in rat adipocytes. Research Methods and Procedure : Adipocytes isolated from male Sprague‐Dawley rats, weighing 400 to 450 grams, were incubated in Krebs‐Ringer buffer with or without 10^?7 M ET‐1 for various times or with various concentrations of ET‐1 for 4 hours; then glycerol release into the incubation medium was measured. In addition, selective ET_AR and ET_BR blockers were used to identify the ET receptor subtype involved. We also explored the involvement of cyclic adenosine monophosphate (cAMP) in ET‐1‐stimulated lipolysis using an adenylyl cyclase inhibitor and by measuring changes in intracellular cAMP levels in response to ET‐1 treatment. To further explore the underlying mechanism of ET‐1 action, we examined the involvement of the extracellular signal‐regulated kinase (ERK)‐mediated pathways. Results : Our results showed that ET‐1 caused lipolysis in rat adipocytes in a time‐ and dose‐dependent manner. BQ610, a selective ET_AR blocker, blocked this effect. The adenylyl cyclase inhibitor, 2′, 5′‐dideoxyadenosine, had no effect on ET‐1‐stimulated lipolysis. ET‐1 did not induce an increase in intracellular cAMP levels. In addition, ET‐1‐induced lipolysis was blocked by inhibition of ERK activation using PD98059. Coincubation of cells with ET‐1 and insulin suppressed ET‐1‐stimulated lipolysis. Discussion : These findings show that ET‐1 stimulates lipolysis in rat adipocytes through the ET_AR and activation of the ERK pathway. The underlying mechanism is cAMP‐independent. However, this non‐conventional lipolytic effect of ET‐1 is inhibited by the anti‐lipolytic effect of insulin.