CD36 is important for adipocyte recruitment and affects lipolysis

Objective: The scavenger receptor CD36 facilitates the cellular uptake of long‐chain fatty acids. As CD36‐deficiency attenuates the development of high fat diet (HFD)‐induced obesity, the role of CD36‐deficiency in preadipocyte recruitment and adipocyte function was set out to characterize. Design and Methods: Fat cell size and number were determined in gonadal, visceral, and subcutaneous adipose tissue of CD36^?/? and WT mice after 6 weeks on HFD. Basal lipolysis and insulin‐inhibited lipolysis were investigated in gonadal adipose tissue. Results: CD36^?/? mice showed a reduction in adipocyte size in all fat pads. Gonadal adipose tissue also showed a lower total number of adipocytes because of a lower number of very small adipocytes (diameter <50 μm). This was accompanied by an increased pool of preadipocytes, which suggests that CD36‐deficiency reduces the capacity of preadipocytes to become adipocytes. Regarding lipolysis, in adipose tissue from CD36^?/? mice, cAMP levels were increased and both basal and 8‐bromo‐cAMP stimulated lipolysis were higher. However, insulin‐mediated inhibition of lipolysis was more potent in CD36^?/? mice. Conclusions: These results indicate that during fat depot expansion, CD36‐deficiency negatively affects preadipocyte recruitment and that in mature adipocytes, CD36‐deficiency is associated with increased basal lipolysis and insulin responsiveness.     

The effect of chronic fatty acid treatment on lipolysis in 3T3-L1 adipocytes.

Various saturated and unsaturated fatty acids were included in the culture medium to test their effects on lipolysis in 3T3-L1 adipocytes. Following prolonged incubation, only oleate was found to exert enhancing effect on basal and isoproterenol-stimulated lipolysis. The effect of oleate was concentration-dependent and was accompanied with increased intracellular cAMP content. Furthermore, the lipolytic response induced by isobutyl-methylxanthine, forskolin or dibutyryl cAMP was also increased in adipocytes treated with oleate. Thus, it appears that in addition to an increased cAMP accumulation, a step distal to cAMP production in the cells may be involved in inducing enhanced lipolysis in 3T3-L1 adipocytes by prolonged exposure to oleate.     

Effects of central or peripheral leptin administration on norepinephrine turnover in defined fat depots

Leptin preserves lean tissue but decreases adipose tissue by increasing lipolysis and/or inhibiting lipogenesis. The sympathetic nervous system (SNS) is a primary regulator of lipolysis, but it is not known if leptin increases norepinephrine turnover (NETO) in white adipose tissue. In this study, we examined the effect of leptin administered either as a chronic physiological dose (40 microg/day for 4 days from ip miniosmotic pumps) or as an acute injection in the third ventricle (1.5 microg injected two times daily for 2 days) on NETO and the size of brown and white fat depots in male Sprague Dawley rats. NETO was determined from the decline in tissue norepinephrine (NE) during 4 h following administration of the NE synthesis inhibitor alpha-methyl-para-tryrosine. The centrally injected leptin-treated animals demonstrated more dramatic reductions in food intake, body weight, and fat pad size and an increase in NETO compared with the peripherally infused animals. Neither route of leptin administration caused a uniform increase in NETO across all fat pads tested, and in both treatment conditions leptin decreased the size of certain fat pads independent of an increase in NETO. Similar discrepancies in white fat NETO were found for rats pair fed to leptin-treated animals. These results demonstrate that leptin acting either centrally or peripherally selectively increases sympathetic outflow to white fat depots and that a leptin-induced change in fat pad weight does not require an increase in NETO.     

Catestatin (chromogranin A(352-372)) and novel effects on mobilization of fat from adipose tissue through regulation of adrenergic and leptin signaling

Chromogranin A knock-out (Chga-KO) mice display increased adiposity despite high levels of circulating catecholamines and leptin. Consistent with diet-induced obese mice, desensitization of leptin receptors caused by hyperleptinemia is believed to contribute to the obese phenotype of these KO mice. In contrast, obesity in ob/ob mice is caused by leptin deficiency. To characterize the metabolic phenotype, Chga-KO mice were treated with the CHGA-derived peptide catestatin (CST) that is deficient in these mice. CST treatment reduced fat depot size and increased lipolysis and fatty acid oxidation. In liver, CST enhanced oxidation of fatty acids as well as their assimilation into lipids, effects that are attributable to the up-regulation of genes promoting fatty acid oxidation (Cpt1α, Pparα, Acox, and Ucp2) and incorporation into lipids (Gpat and CD36). CST did not affect basal or isoproterenol-stimulated cAMP production in adipocytes but inhibited phospholipase C activation by the α-adrenergic receptor (AR) agonist phenylephrine, suggesting inhibition of α-AR signaling by CST. Indeed, CST mimicked the lipolytic effect of the α-AR blocker phentolamine on adipocytes. Moreover, CST reversed the hyperleptinemia of Chga-KO mice and improved leptin signaling as determined by phosphorylation of AMPK and Stat3. CST also improved peripheral leptin sensitivity in diet-induced obese mice. In ob/ob mice, CST enhanced leptin-induced signaling in adipose tissue. In conclusion, our results implicate CST in a novel pathway that promotes lipolysis and fatty acid oxidation by blocking α-AR signaling as well as by enhancing leptin receptor signaling.     

Melatonin inhibits fatty acid transport in inguinal fat pads of hepatoma 7288CTC-bearing and normal Buffalo rats via receptor-mediated signal transduction.

Melatonin inhibits fatty acid uptake and linoleic acid-dependent growth in hepatoma 7288CTC in vivo in Buffalo rats. In this study we measured the effects of melatonin on arteriovenous differences for fatty acids across inguinal fat pads in fed and fasted rats to determine if fatty acid transport in white adipose tissue was also affected by melatonin. Intravenous infusion of melatonin in fasted tumor-bearing rats in vivo simultaneously and rapidly inhibited both fatty acid release from fat pads and fatty acid uptake by the tumors. Perfusion of fat pads in situ in normal rats with melatonin (0.1 nM) inhibited fatty acid release (fasted rats) and uptake (fed rats). Fatty acid transport was restored by addition of any of the following: a melatonin receptor antagonist (S 20928, 1.0 nM), pertussis toxin (0.5 microg/ml), forskolin (1 microM) or 8-Br-cAMP (10 microM). We conclude that fatty acid transport in inguinal fat pads requires cAMP and that melatonin inhibits this transport via a melatonin receptor-mediated, Gi protein-coupled signal transduction pathway. Melatonin has both anticachectic and lipid homeostatic actions in the white adipose tissue of inguinal fat pads.     

Continuous flow microcalorimetric measurement of heat production in white adipose tissue from obese (ob/ob) mice and their lean littermates

Heat production, free fatty acid and glycerol release from white adipose tissue fat pads from obese (ob/ob) mice and their lean littermates are determined. Heat production was significantly lower in obese mice compared to lean mice when expressed on wet weight basis but not when expressed on DNA basis. Noradrenaline significantly increased the heat production in fat pads from both groups of animals. However, the increase in heat production due to noradrenaline addition in fat pads from lean mice was significantly higher than in fat pads from obese mice. The release of free fatty acids and glycerol before incubation with noradrenaline was similar from fat pads from both groups of animals. Addition of noradrenaline to the fat pads increased the release of free fatty acids and glycerol in both groups of animals, but the increase was significantly larger from fat pads from lean mice. In the absence of noradrenaline the free fatty acid/glycerol ratio (mol/mol) in the effluent was 7.9:1 and 4.8:1 for lean mice and obese mice, respectively. In the presence of noradrenaline the ratio decreased to 3:1 for both groups of animals.     

ApoB100-LDL Acts as a Metabolic Signal from Liver to Peripheral Fat Causing Inhibition of Lipolysis in Adipocytes

Background

Free fatty acids released from adipose tissue affect the synthesis of apolipoprotein B-containing lipoproteins and glucose metabolism in the liver. Whether there also exists a reciprocal metabolic arm affecting energy metabolism in white adipose tissue is unknown.

Methods and Findings

We investigated the effects of apoB-containing lipoproteins on catecholamine-induced lipolysis in adipocytes from subcutaneous fat cells of obese but otherwise healthy men, fat pads from mice with plasma lipoproteins containing high or intermediate levels of apoB100 or no apoB100, primary cultured adipocytes, and 3T3-L1 cells. In subcutaneous fat cells, the rate of lipolysis was inversely related to plasma apoB levels. In human primary adipocytes, LDL inhibited lipolysis in a concentration-dependent fashion. In contrast, VLDL had no effect. Lipolysis was increased in fat pads from mice lacking plasma apoB100, reduced in apoB100-only mice, and intermediate in wild-type mice. Mice lacking apoB100 also had higher oxygen consumption and lipid oxidation. In 3T3-L1 cells, apoB100-containing lipoproteins inhibited lipolysis in a dose-dependent fashion, but lipoproteins containing apoB48 had no effect. ApoB100-LDL mediated inhibition of lipolysis was abolished in fat pads of mice deficient in the LDL receptor (Ldlr^−/−Apob ^100/100).

Conclusions

Our results show that the binding of apoB100-LDL to adipocytes via the LDL receptor inhibits intracellular noradrenaline-induced lipolysis in adipocytes. Thus, apoB100-LDL is a novel signaling molecule from the liver to peripheral fat deposits that may be an important link between atherogenic dyslipidemias and facets of the metabolic syndrome.     

Novel form of lipolysis induced by leptin.

Hyperleptinemia causes disappearance of body fat without a rise in free fatty acids (FFA) or ketones, suggesting that leptin can deplete adipocytes of fat without releasing FFA. To test this, we measured FFA and glycerol released from adipocytes obtained from normal lean Zucker diabetic fatty rats (+/+) and incubated for 0, 3, 6, or 24 h in either 20 ng/ml recombinant leptin or 100 nM norepinephrine (NE). Whereas NE increased both FFA and glycerol release from adipocytes of +/+ rats, leptin increased glycerol release in +/+ adipocytes without a parallel increase in FFA release. In adipocytes of obese Zucker diabetic fatty rats (fa/fa) with defective leptin receptors, NE increased both FFA and glycerol release, but leptin had no effect on either. Leptin significantly lowered the mRNA of leptin and fatty acid synthase of adipocytes (FAS) (p < 0.05), and up-regulated the mRNA of peroxisome proliferator-activated receptor (PPAR)-alpha, carnitine palmitoyl transferase-1, (CPT-1), and acyl CoA oxidase (ACO) (p < 0.05). NE (100 nM) also lowered leptin mRNA (p < 0.05) but did not affect FAS, PPARalpha, ACO, or CPT-1 expression. We conclude that in normal adipocytes leptin directly decreases FAS expression, increases PPARalpha and the enzymes of FFA oxidation, and stimulates a novel form of lipolysis in which glycerol is released without a proportional release of FFA.     

Lipoprotn lipase content and triglyceride fatty acid uptake in adipose tissue of rats of differing body weights

Increasing body weight appears to alter lipid metabolism in adipose tissue. We have measured the content of lipoprotein lipase and the uptake of chylomicron triglyceride fatty acids in epididymal fat pads of rats of different weights. In order that the results might be expressed in terms of cell numbers, the relationship between the weights of fat pads and the numbers and volumes of fat cells isolated from them was determined. Highly significant correlations were found between fat pad weight and both the number and the volume of the individual adipocytes. In rats weighing from 140 to 350 g, the increase in the size of fat pads was attributable almost equally to increases in cell size and in cell number. Lipoprotein lipase activity was measured in acetone powders of whole fat pads and of isolated fat cell preparations. With both, lipoprotein lipase activity per cell diminished significantly as the weight of fat tissue increased, i.e., larger fat cells contained less enzyme per cell than smaller cells. The uptake of triglyceride fatty acid radioactivity was measured after incubation of fat pads with radiolabeled rat lymph chylomicrons in flasks containing either buffer alone or with added glucose or glucose plus insulin. The addition of glucose and insulin led to a mean increase of 70% in the uptake of radioactivity, but larger adipocytes were stimulated less than smaller cells. This resulted in a significant negative correlation between the weights of fat pads and the uptake of radioactivity. Enlargement of fat cells also led to a diminution in their capacity to esterify fatty acids.     

Suppression of adipose lipolysis by long-chain fatty acid analogs

Agonist-induced lipolysis of adipose fat is robustly inhibited by insulin or by feedback inhibition by the long-chain fatty acids (LCFA) produced during lipolysis. However, the mode of action of LCFA in suppressing adipose lipolysis is not clear. β,β'-Tetramethyl hexadecanedioic acid (Mββ/ EDICA16) is a synthetic LCFA that is neither esterified into lipids nor β-oxidized, and therefore, it was exploited for suppressing agonist-induced lipolysis in analogy to natural LCFA. Mββ is shown here to suppress isoproterenol-induced lipolysis in the rat in vivo as well as in 3T3-L1 adipocytes. Inhibition of isoproterenol-induced lipolysis is due to decrease in isoproterenol-induced cAMP with concomitant inhibition of the phosphorylation of hormone-sensitive lipase and perilipin by protein kinase A. Suppression of cellular cAMP levels is accounted for by inhibition of the adenylate cyclase due to suppression of Raf1 expression by Mββ-activated AMPK. Suppression of Raf1 is further complemented by induction of components of the unfolded-protein-response by Mββ. Our findings imply genuine inhibition of agonist-induced adipose lipolysis by LCFA, independent of their β-oxidation or reesterification. Mββ suppression of agonist-induced lipolysis and cellular cAMP levels independent of the insulin transduction pathway may indicate that synthetic LCFA could serve as insulin mimetics in the lipolysis context under conditions of insulin resistance.     

Leptin: an essential regulator of lipid metabolism

This paper reviews the general mechanisms by which leptin acts as a regulator of lipid reserves through changes in food intake, energy expenditure and fuel selection, with an emphasis on its direct effects on cellular lipid metabolism. Briefly, when leptin levels increase, food consumption decreases via modulation of hypothalamic neuropeptides. As well, normal decreases in energy expenditures (e.g. with diurnal cycles or reduced caloric intake) do not occur. This is probably caused by an increase in mitochondrial proton leak mediated by leptin via increases in sympathetic nervous system stimulation and thyroid hormone release. The decrease in caloric input coupled with relatively higher energy expenditure, therefore, leads to negative energy balance. Leptin also changes the fuel source from which ATP is generated. Fuel preference switches from carbohydrate (glucose) to lipid (fatty acids). This effect arises through stimulation of triacylglycerol catabolism by leptin. In vitro studies show that leptin is a potent stimulator of lipolysis and fatty acid oxidation in adipocytes and other cell types. Consequently, leptin is also a regulator of cellular triacylglycerol content. Hormonal regulation of leptin, as well as its role in fasting and seasonal weight gain and energy expenditure are also briefly discussed.     

Cyclic AMP-mediated control of lipogenic enzyme synthesis during adipose differentiation of 3T3 cells

During the adipose differentiation of 3T3-F442A cells, there is an increase in the synthesis of numerous proteins, including the lipogenic enzymes glycerophosphate dehydrogenase, fatty acid synthetase and malic enzyme. Agents that increase cAMP content (Dibutyryl cAMP, theophylline, and isoproterenol) are known to induce lipolysis in fat cells; but the same agents are shown here to reduce the synthesis of the lipogenic enzymes during adipose differentiation. The extent of reduction depends on the agent used and differs for the three enzymes; fatty acid synthetase is most sensitive and its synthesis can be suppressed completely. In contrast to their effects on lipogenic enzyme synthesis, these agents do not affect morphological changes or the synthesis of several other proteins, of which some increase and others (such as actin) decrease during the differentiation. The effects of the agents on the synthesis of lipogenic enzymes are not dependent on lipolysis, since they take place to the same degree in cells not permitted to accumulate triglyceride. Translation in vitro of mRNA isolated from cells treated with the agents promoting cAMP accumulation indicates that the levels of functional mRNA for lipogenic enzymes are reduced. We conclude that, in addition to its activation of lipolysis, cAMP reduces specifically mRNA accumulation for lipogenic enzymes. These results also demonstrate the independent control of morphological change and enzyme synthesis during adipose differentiation.     

Effect of pH on lipolysis, cAMP and cAMP-dependent protein kinase activity in isolated rat fat cells

The effect of acidosis and alkalosis on lipolysis, cAMP production and cAMP-dependent protein kinase activity in isolated rat fat cells incubated in the presence of norepinephrine and norepinephrine plus theophylline has been investigated. The pH of the incubation medium was adjusted to 6.8, 7.4 and 7.8 respectively. Acidosis inhibited both norepinephrine- and norepinephrine plus theophylline-induced release of glycerol whereas alkalosis led to slight stimulation. Norepinephrine produced an increase in cAMP and cAMP-dependent protein kinase activity. However, comparison of both parameters in acidosis and alkalosis with those at pH 7.4 indicates that they were higher at pH 7.8 and lower at pH 6.8. Addition of theophylline in combination with norepinephrine increases cAMP production within 5 min, under acidosis to values similar to those obtained at pH 7.4 with norepinephrine. The same effect on protein kinase activity was obtained. In spite of this increment in cAMP and protein kinase activity produced by addition of norepinephrine plus theophylline, lipolysis remains inhibited by acidosis. Addition of theophylline at pH 7.4 and 7.8 induced a much higher cAMP production and cAMP-dependent protein kinase activity although at pH 7.8 there was a statistically significant increase in protein kinase activity at 10 min it did not induce a significant increase in lipolysis. This is discussed and possible mechanisms are suggested to explain the effect of acidosis and alkalosis on the lipolysis induced by norepinephrine in rat fat cells.     

FGF21 mediates alcohol-induced adipose tissue lipolysis by activation of systemic release of catecholamine in mice

Alcohol consumption leads to adipose tissue lipoatrophy and mobilization of FFAs, which contributes to hepatic fat accumulation in alcoholic liver disease. This study aimed to investigate the role of fibroblast growth factor (FGF)21, a metabolic regulator, in the regulation of chronic-binge alcohol-induced adipose tissue lipolysis. FGF21 KO mice were subjected to chronic-binge alcohol exposure, and epididymal white adipose tissue lipolysis and liver steatosis were investigated. Alcohol exposure caused adipose intracellular cAMP elevation and activation of lipolytic enzymes, leading to FFA mobilization in both WT and FGF21 KO mice. However, alcohol-induced systemic elevation of catecholamine, which is known to be a major player in adipose lipolysis by binding to the β-adrenergic receptor, was markedly inhibited in KO mice. Supplementation with recombinant human FGF21 to alcohol-exposed FGF21 KO mice resulted in an increase in fat loss in parallel with an increase of circulating norepinephrine concentration. Furthermore, alcohol consumption-induced fatty liver was blunted in the KO mice, indicating an inhibition of fatty acid reverse transport from adipose to the liver in the KO mice. Taken together, our studies demonstrate that FGF21 KO mice are protected from alcohol-induced adipose tissue excess-lipolysis through a mechanism involving systemic catecholamine release.     

Natriuretic peptides: a new lipolytic pathway in human fat cells

Human fat cell lipolysis was considered until recently to be an exclusive cAMP/protein-kinase A (PKA)-regulated metabolic pathway under the control of catecholamines and insulin. Moreover, exercise-induced lipid mobilization in humans was considered to mainly depend on catecholamine action and interplay between fat cell beta- and alpha2-adrenergic receptors controlling adenylyl cyclase activity and cAMP production. We have recently demonstrated that natriuretic peptides stimulate lipolysis and contribute to the regulation of lipid mobilization in humans. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) stimulate lipolysis in human isolated fat cells. Activation of the adipocyte plasma membrane type A guanylyl cyclase receptor (NPR-A), increase in intracellular guanosine 3',5'-cyclic monophosphate (cyclic GMP) levels and activation of hormone-sensitive lipase mediate the action of ANP. ANP does not modulate cAMP production and PKA activity. Increment of cGMP induces the phosphorylation of hormone-sensitive lipase and perilipin A via the activation of a cGMP dependent protein kinase-I (cGK-I). Plasma concentrations of glycerol and non-esterified fatty acids are increased by i.v. infusion of ANP in humans. Physiological relevance of the ANP-dependent pathway was demonstrated in young subjects performing physical exercise. ANP plays a role in conjunction with catecholamines in the control of exercise-induced lipid mobilization. This pathway becomes of major importance when subjects are submitted to chronic treatment with a beta-blocker. Oral beta-adrenoceptor blockade suppresses the beta-adrenergic component of catecholamine action in fat cells and potentiates exercise-induced ANP release by the heart. These findings may have several implications whenever natriuretic peptide secretion is altered such as in subjects with left ventricular dysfunction, congestive heart failure and obesity.     

Combined leptin actions on adipose tissue and hypothalamus are required to deplete adipocyte fat in lean rats: implications for obesity treatment

Intense hyperleptinemia completely depletes adipocyte fat of normal rats within 14 days. To determine the mechanism, epididymal fat pads from normal wild-type (+/+) and obese (fa/fa) Zucker Diabetic Fatty (ZDF) donor rats were transplanted into normal +/+ and fa/fa ZDF recipients. Hyperleptinemia induced by adenovirus-leptin administration depleted all fat from native fat pads and from fat transplants from +/+ donors but not from transplants from ZDF(fa/fa) donors with defective leptin receptors. In both native and transplanted +/+ fat pads, large numbers of mitochondria were apparent, and genes involved in fatty acid oxidation were up-regulated. However, +/+ fat pads transplanted into fa/fa recipients did not respond to hyperleptinemia, suggesting lack of an essential leptin-stimulated cohormone(s). In +/+ but not in fa/fa rats, plasma catecholamine levels rose, and both P-STAT3 and P-CREB increased in adipose tissue, suggesting that both direct and indirect (hypothalamic) leptin receptor-mediated actions of hyperleptinemia are involved in depletion of adipocyte fat.     

Dual roles of P2 purinergic receptors in insulin-stimulated leptin production and lipolysis in differentiated rat white adipocytes

ATP is co-localized with norepinephrine at the sympathetic nerve terminals and may be released simultaneously upon neuronal stimulation, which results in activation of purinergic receptors. To examine whether leptin synthesis and lipolysis are influenced by P2 purinergic receptor activation, the effects of ATP and other nucleotides on leptin secretion and glycerol release have been investigated in differentiated rat white adipocytes. Firstly, insulin-induced leptin secretion was inhibited by nucleotide treatment with the following efficacy order: 3'-O-(4-benzoyl)benzoyl ATP (BzATP) > ATP > UTP. Secondly, treatment of adipocytes with ATP increased both intracellular Ca(2+) concentration and cAMP content. Intracellular calcium concentration was increased by ATP and UTP, but not BzATP, an effect attributed to phospholipase C-coupled P2Y(2). On the other hand, cAMP was generated by treatment with BzATP and ATPgammaS, but not UTP, indicating functional expression of adenylyl cyclase-coupled P2Y(11) receptors in white adipocytes. Thirdly, lipolysis was significantly activated by BzATP and ATP, which correlated with the characteristics of the P2Y(11) subtype. Taken together, the data presented here suggest that white adipocytes express at least two different types of P2Y receptors and that activation of P2Y(11) receptor might be involved in inhibition of leptin production and stimulation of lipolysis, suggesting that purinergic transmission can play an important role in white adipocyte physiology.     

Response of white adipocyte of mouse and rabbit to catecholamines and ACTH

Summary Hormone stimulated lipolysis of mouse and rabbit adipocytes as measured by both free fatty acid and glycerol release, is proportionally elevated with increase in the adipocyte cAMP level up to 1 nmole/g. The correlation coefficients are 0.94 and 0.97 for FFA/cAMP and glycerol/cAMP respectively. Increments in cAMP greater than 1 nmole/g show no correlation with increase in lipolysis. The release of lipolytic products, glycerol and free fatty acids, from white adipocytes in response to ACTH, epinephrine or morepinephrine was measured using radiochemical assays in short term incubation systems, with cAMP levels measured at the same time and from the same cell sample. Under the conditions studied, epinephrine is a more effective lipolytic hormone than ACTH in mouse adipocyte, and ACTH is more effective than epinephrine in rabbit adipocyte. The effect of catecholamines on the rabbit adipocyte is not modified by phentolamine (10 μM), but it is potentiated by 1-methyl-3-isobutyl xanthine (0.1 mM). The results suggest that cAMP mediates the action of these lipolytic hormones in white adipocytes of mouse and rabbit.     

Stimulatory release of lipoprotein lipase activity from rat fat pads by vanadate

Vanadate stimulated the release of lipoprotein lipase (LPL) activity from rat fat pads into the medium in a time- and dose-dependent manner. It exerted the synergetic effect with heparin. The stimulatory effects of vanadate and heparin were decreased by incubation in Na+- or Ca2+-free media but were well preserved in K+-free medium. Amiloride inhibited the vanadate-stimulated release of LPL activity in a dose-dependent manner, but did not inhibit the heparin-stimulated release of LPL activity. Colchicine, antimycin A, and carbonyl cyanide m-chlorophenylhydrazone suppressed the stimulatory effect of vanadate, but cycloheximide did not. Preincubation of the fat pads with the tetrakis (acetoxymethyl) ester of quin 2 (quin 2-AM) inhibited the vanadate-stimulatory release of LPL activity without affecting basal activity. The concentration required for half-maximal inhibition of the action of vanadate by quin 2-AM was calculated to be 39 microM, suggesting that the action of vandate was dependent on intracellular Ca2+ concentration. The heparin-stimulated release, on the other hand, was not inhibited even at higher concentrations of quin 2-AM (up to 200 microM). These findings suggest that vanadate stimulates the release of LPL activity through mechanisms of action involving amiloride-sensitive and calcium-dependent pathways with a requirement of metabolic energy.     

Short photoperiod exposure increases adipocyte sensitivity to noradrenergic stimulation in Siberian hamsters

Siberian hamsters (Phodopus sungorus) exhibit a naturally occurring, reversible seasonal obesity with body fat peaking in long "summerlike" days (LDs) and reaching a nadir in short "winterlike" days (SDs). These SD-induced decreases in adiposity are mediated largely via sympathetic nervous system (SNS) innervation of white adipose tissue (WAT), as indicated by increased WAT norepinephrine (NE) turnover. We examined whether SDs also increase sensitivity to NE-stimulated lipolysis. This was accomplished by measuring NE- and beta3-adrenoceptor (beta3-AR) agonist (BRL-37344)-induced lipolysis (glycerol release) as well as NE-induced cAMP accumulation by inguinal, epididymal, and retroperitoneal WAT (IWAT, EWAT, and RWAT) in isolated adipocytes of LD- and SD-housed hamsters. SDs increased potency/efficacy of NE-triggered lipolysis in a temporally and fat pad-specific manner. Thus when WAT pad mass decreased most rapidly (5 wk of SDs), potency (sensitivity/EC50) and efficacy (maximal response asymptote) of NE-stimulated lipolysis were increased for all WAT pads and also at 10 wk for IWAT compared with their LD counterparts. SD enhancement of lipolysis was similar for NE and BRL-37344 in IWAT adipocytes. These results, coupled with our previous demonstration that SDs upregulate WAT beta3-AR mRNA expression, suggest that increased beta3-ARs mediated the SD-induced increased NE sensitivity. NE-stimulated adipocyte accumulation of cAMP was greater after 5 wk of SDs for IWAT and EWAT and after 10 wk of SDs for IWAT compared with LDs, with no photoperiod effect for RWAT. Therefore, the SD-induced increase in SNS drive to WAT and increased sensitivity to this drive may work together to increase lipolysis in SDs.