In vivo lipolysis in adipose tissue from two anatomical locations measured by microdialysis

In this study, the difference in lipolytic response in inguinal subcutaneous and epididymal adipose tissues of male Sprague-Dawley rats was assessed in vivo by microdialysis. Probes were perfused with Ringer solution in which increasing concentrations of isoproterenol (10(-7) - 10(-4) mol/L) were added. Glycerol release, expressed as extracellular glycerol concentration, was used as lipolytic index. The effect of isoproterenol on local blood flow was investigated using the ethanol technique. No differences were found in the interstitial glycerol concentration between both adipose tissues under basal conditions. When isoproterenol was perfused, a dose-response increase in glycerol production was induced in both tissues. Interstitial glycerol concentration from epididymal adipose tissue was higher than that of inguinal subcutaneous depot at all isoproterenol concentrations. No vasodilatory effect of isoproterenol was found. These results suggest that epididymal adipose tissue is more responsive in vivo to beta-adrenergic lipolysis stimulation than is subcutaneous fat pad from the inguinal region.     

In vivo nitric oxide suppression of lipolysis in subcutaneous abdominal adipose tissue is greater in obese than lean women

Mounting evidence suggests there is a reduced mobilization of stored fat in obese compared to lean women. It has been suggested that this decreased lipid mobilization may lead to, or perpetuate, the obese state; however, there may be a beneficial effect of reduced lipolysis, either by allowing for a sink of excess fatty acids, or by limiting a potentially harmful rise in interstitial and circulating fatty acid concentration. Nitric oxide (NO) may be responsible for a portion of the reduced in vivo rates of lipolysis in obese women because NO reduces adipose tissue lipolysis and adipose tissue nitric oxide synthase (NOS) mRNA is higher in obese than lean individuals. The purpose of this study was to determine if the inhibition of NOS by L-N(g)-monomethyl-L-arginine (L-NMMA) in the absence and presence of lipolytic stimulation would result in a larger increase in lipolytic rate in obese (OB) than lean (LN) women. Microdialysis probes were inserted into the subcutaneous abdominal adipose tissue of seven obese and six lean women to monitor lipolysis. Dialysate glycerol concentration increased in response to L-NMMA in OB (basal 125 ± 26 μmol/l; L-NMMA 225 ± 35 μmol/l) to a greater extent than in LN (basal 70 ± 18 μmol/l; L-NMMA 84 ± 20 μmol/l) women (P < 0.05). Dialysate glycerol increased to a similar extent in OB and LN in response to adrenergic stimulation by isoprenaline or norepinephrine in the presence of L-NMMA. The differential glycerol responses to L-NMMA between obese and lean could not be explained by differential blood flow responses. It can be concluded that NO suppresses basal lipolysis in obese women to a greater extent than in lean women.     

Inhibition of growth hormone-induced lipolysis by 3',5'-guanosine monophosphate in chicken adipose tissue in vitro

The influence of cyclic 3',5'-guanosine monophosphate (cGMP) on the lipolytic and antilipolytic (inhibition of glucagon-stimulated lipolysis) responses to GH (1 microgram/ml) was examined in chicken adipose tissue in vitro. Both 8-bromo-cGMP (0.1 mM) and sodium nitroprusside (1 mM) (a guanyl cyclase stimulator) completely inhibited the lipolytic effect of GH. A cGMP-lowering agent, LY83583 (10 microM), reversed the inhibitory effect of sodium nitroprusside on GH-stimulated lipolysis. Furthermore, the suppressive effects of insulin (100 ng/ml), insulin-like growth factor I (IGF-I) (100 ng/ml), or insulin-like growth factor II (IGF-II/MSA) (100 ng/ml), but not somatostatin (1 ng/ml), on GH-stimulated lipolysis were prevented by LY83583 addition. Neither 8-bromo-cGMP, sodium nitroprusside, nor LY83583 altered GH-induced inhibition of glucagon (1 ng/ml)-stimulated lipolysis. It is proposed that cGMP may mediate inhibitory control of GH-stimulated lipolysis by insulin, IGF-I, and IGF-II in chicken adipose tissue.     

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

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

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

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

Regional and gender variations in adipose tissue lipolysis in response to weight loss.

Catecholamine-induced lipolysis was investigated in 32 obese subjects (14 men and 18 premenopausal women), aged 36-50 years, whose body mass index ranged from 30 to 42 kg/m(2). Isolated subcutaneous (subc) abdominal and femoral adipocytes were studied before and after a 15-week weight reducing program, during which mean body weight loss averaged 9 vs. 10 kg in women and men, respectively (P < 0.0001). Participants were re-examined when they were weight-stable. Fat cell weight decreased by about 15;-20% in both depots (P values ranging from 0.01 to 0.05). Epinephrine (mixed alpha2-/beta-adrenoceptor (AR) agonist) induced antilipolysis at low concentrations and a net lipolytic response at higher doses, irrespective of subjects' fatness and anatomic location of fat. Basal lipolysis, maximal lipolytic responses to isoprenaline (beta-AR agonist), dobutamine and procaterol (beta1- and beta2-AR agonists, respectively) as well as maximal antilipolytic effects of epinephrine or UK-14304 (alpha2-AR agonist) were similar before and after weight reduction. However, both beta- and beta2-AR lipolytic sensitivities and the beta-AR density were increased in both genders after weight reduction, this effect being more marked in subc abdominal than in femoral adipocytes (P values ranging from 0.001 to 0.05). The alpha2-AR antilipolytic sensitivity was reduced in adipose cells from both regions in women, but only in subc abdominal adipocytes in men (P < 0.05), although the alpha2-AR density remained unchanged after weight reduction. In conclusion, a moderate weight loss leads to a higher adipose cell lipolytic efficiency which is associated with changes at receptor levels (mainly an increased beta2- and a decreased alpha2-AR sensitivities), in both genders.     

Acute exposure to long-chain fatty acids impairs {alpha}2-adrenergic receptor-mediated antilipolysis in human adipose tissue

The acute in vitro and in vivo effects of long-chain fatty acids (LCFAs) on the regulation of adrenergic lipolysis were investigated in human adipose tissue. The effect of a 2 h incubation, without or with LCFA (200 mumol/l), on basal and hormonally induced lipolysis was tested in vitro on isolated fat cells. The lipolytic response to epinephrine was enhanced by suppression of the antilipolytic alpha(2)-adrenergic effect. Then, healthy lean and obese male subjects performed a 45 min exercise bout at 50% of their heart rate reserve either after an overnight fast or 3 h after a high-fat meal (HFM: 95% fat, 5% carbohydrates). Subcutaneous adipose tissue lipolysis was measured by microdialysis in the presence or absence of an alpha-antagonist (phentolamine). In vivo, a HFM increased plasma levels of nonesterified fatty acids in lean and obese subjects. In both groups, the HFM did not alter hormonal responses to exercise. Under fasting conditions, the alpha(2)-adrenergic antilipolytic effect was more pronounced in obese than in lean subjects. The HFM totally suppressed the alpha(2)-adrenergic antilipolytic effect in lean and obese subjects during exercise. LCFAs per se, in vitro as well as in vivo, suppress alpha(2)-adrenergic-mediated antilipolysis in adipose tissue. LCFA-mediated suppression of antilipolytic pathways represents another mechanism whereby a high fat content in the diet might increase adipose tissue lipolysis.     

Lipolysis response to endoplasmic reticulum stress in adipose cells

In obesity and diabetes, adipocytes show significant endoplasmic reticulum (ER) stress, which triggers a series of responses. This study aimed to investigate the lipolysis response to ER stress in rat adipocytes. Thapsigargin, tunicamycin, and brefeldin A, which induce ER stress through different pathways, efficiently activated a time-dependent lipolytic reaction. The lipolytic effect of ER stress occurred with elevated cAMP production and protein kinase A (PKA) activity. Inhibition of PKA reduced PKA phosphosubstrates and attenuated the lipolysis. Although both ERK1/2 and JNK are activated during ER stress, lipolysis is partially suppressed by inhibiting ERK1/2 but not JNK and p38 MAPK and PKC. Thus, ER stress induces lipolysis by activating cAMP/PKA and ERK1/2. In the downstream lipolytic cascade, phosphorylation of lipid droplet-associated protein perilipin was significantly promoted during ER stress but attenuated on PKA inhibition. Furthermore, ER stress stimuli did not alter the levels of hormone-sensitive lipase and adipose triglyceride lipase but caused Ser-563 and Ser-660 phosphorylation of hormone-sensitive lipase and moderately elevated its translocation from the cytosol to lipid droplets. Accompanying these changes, total activity of cellular lipases was promoted to confer the lipolysis. These findings suggest a novel pathway of the lipolysis response to ER stress in adipocytes. This lipolytic activation may be an adaptive response that regulates energy homeostasis but with sustained ER stress challenge could contribute to lipotoxicity, dyslipidemia, and insulin resistance because of persistently accelerated free fatty acid efflux from adipocytes to the bloodstream and other tissues.     

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.     

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.     

Stress-induced alteration in the lipolytic response to beta-adrenoceptor agonists in rat white adipocytes.

We analysed the sensitivity to beta-adrenoceptor agonists in epididymal adipose cells from rats submitted to a stress protocol previously reported to induce alterations in sensitivity to catecholamines in cardiac tissue from rats. Food intake and body weight were lower, whereas adipocytes basal lipolysis was higher (control: 0.59 +/- 0.04; stress: 1.00 +/- 0.11, micromol glycerol/100 mg total lipids/100 min) in stressed compared to control rats. The responses to isoprenaline (pD(2) control: 7.46 +/- 0.11; stress: 8.11 +/- 0.17), adrenaline (pD(2) control: 5.78 +/- 0. 20; stress: 6.13 +/- 0.18), and salbutamol (pD(2) control: 5.64 +/- 0.28; stress: 5.92 +/- 0.34) were sensitized, and the lipolytic responses to norepinephrine (pD(2) control: 6.98 +/- 0.13; stress: 6. 41 +/- 0.12) and to BRL37344 (pD(2) control: 8.43 +/- 0.19; stress: 7.54 +/- 0.21) were desensitized. Responses to the higher concentration (100 microm) of isoprenaline (control: 1.80 +/- 0.18; stress: 2.24 +/- 0.10 micromol glycerol/100 mg total lipids/100 min), epinephrine (control: 1.64 +/- 0.17; stress: 2.24 +/- 0.14 micromol glycerol/100 mg total lipids/100 min), salbutamol (control: 0.65 +/- 0.11; stress: 1.21 +/- 0.41 micromol glycerol/100 mg total lipids/100 min), and d-butyryl-cAMP (control: 1.59 +/- 0.17; stress: 2.72 +/- 0.25) were significantly enhanced in adipocytes from stressed rats. pD(2) or maximum response to CGP12177 were not altered. Supersensitivity to isoprenaline was abolished by 50 nm ICI118,551 but was not modified by 100 nm metoprolol. However, subsensitivity to norepinephrine and to BRL37344 was abolished by 100 nM metoprolol. Our results suggest that in epididymal adipocytes from stressed rats there is a desensitization of the response to adrenoceptor agonists mediated by beta(1)-adrenoceptors together with a sensitization of the response mediated by beta(2)-adrenoceptors. beta(3)-adrenoceptors seem to be resistant to the stress effect.     

In vitro and in vivo impairment of alpha2-adrenergic receptor-dependent antilipolysis by fatty acids in human adipose tissue.

The aim of the present study was to study the influence of fatty acids on the adrenergic control of lipolysis both in vitro and in vivo. Human subcutaneous adipose tissue explants were cultured for 48 h in the presence of 100 microM bromopalmitate (BrPal), and lipolysis was measured in isolated adipocytes. In control conditions, beta-AR-dependent activation of lipolysis by epinephrine was almost undetectable, and could be fully restored by pharmacological blockade of alpha2-AR-dependent antilipolysis. After BrPal treatment, epinephrine became fully lipolytic and was no longer influenced by alpha2-AR-blockade. Radioligand binding analysis revealed that BrPal treatment led to a significant reduction in the coupling of alpha2-AR to G proteins. In parallel, a chronic and significant increase in plasma fatty acids resulting from a 4-day high-fat diet (HFD) was accompanied by an impairment of the amplifying effect of the alpha2-AR antagonist phentolamine on exercise-induced lipolysis (measured in the subcutaneous adipose tissue with the use of a microdialysis probe) normally observed after a low-fat diet. In conclusion, in vitro and in vivo studies showed that fatty acids impair alpha2-AR-dependent antilipolysis.     

Extracellular vesicles-released parathyroid hormone-related protein from Lewis lung carcinoma induces lipolysis and adipose tissue browning in cancer cachexia

Cancer cachexia is a metabolic disorder characterized by skeletal muscle wasting and white adipose tissue browning. Specific functions of several hormones, growth factors, and cytokines derived from tumors can trigger cachexia. Moreover, adipose tissue lipolysis might explain weight loss that occurs owing to cachexia. Extracellular vesicles (EVs) are involved in intercellular communication. However, whether EVs participate in lipolysis induced by cancer cachexia has not been thoroughly investigated. Using Lewis lung carcinoma (LLC) cell culture, we tested whether LLC cell-derived EVs can induce lipolysis in 3T3-L1 adipocytes. EVs derived from LLC cells were isolated and characterized biochemically and biophysically. Western blotting and glycerol assay were used to study lipolysis. LLC cell-derived EVs induced lipolysis in vivo and vitro. EVs fused directly with target 3T3-L1 adipocytes and transferred parathyroid hormone-related protein (PTHrP), activating the PKA signaling pathway in 3T3-L1 adipocytes. Blocking PTHrP activity in LLC-EVs using a neutralizing antibody and by knocking down PTHR expression prevented lipolysis in adipocytes. Inhibiting the PKA signaling pathway also prevents the lipolytic effects of EVs. In vivo, suppression of LLC-EVs release by knocking down Rab27A alleviated white adipose tissue browning and lipolysis. Our data showed that LLC cell-derived EVs induced adipocyte lipolysis via the extracellular PTHrP-mediated PKA pathway. Our data demonstrate that LLC-EVs induce lipolysis in vitro and vivo by delivering PTHrP, which interacts with PTHR. The lipolytic effect of LLC-EVs was abrogated by PTHR knockdown and treatment with a neutralizing anti-PTHrP antibody. Together, these data show that LLC-EV-induced lipolysis is mediated by extracellular PTHrP. These findings suggest a novel mechanism of lipid droplet loss and identify a potential therapeutic strategy for cancer cachexia.Subject terms: Oncogenes, Mechanisms of disease     

Cardiotrophin-1 stimulates lipolysis through the regulation of main adipose tissue lipases

Cardiotrophin-1 (CT-1) is a cytokine with antiobesity properties and with a role in lipid metabolism regulation and adipose tissue function. The aim of this study was to analyze the molecular mechanisms involved in the lipolytic actions of CT-1 in adipocytes. Recombinant CT-1 (rCT-1) effects on the main proteins and signaling pathways involved in the regulation of lipolysis were evaluated in 3T3-L1 adipocytes and in mice. rCT-1 treatment stimulated basal glycerol release in a concentration- and time-dependent manner in 3T3-L1 adipocytes. rCT-1 (20 ng/ml for 24 h) raised cAMP levels, and in parallel increased protein kinase (PK)A-mediated phosphorylation of perilipin and hormone sensitive lipase (HSL) at Ser660. siRNA knock-down of HSL or PKA, as well as pretreatment with the PKA inhibitor H89, blunted the CT-1-induced lipolysis, suggesting that the lipolytic action of CT-1 in adipocytes is mainly mediated by activation of HSL through the PKA pathway. In ob/ob mice, acute rCT-1 treatment also promoted PKA-mediated phosphorylation of perilipin and HSL at Ser660 and Ser563, and increased adipose triglyceride lipase (desnutrin) content in adipose tissue. These results showed that the ability of CT-1 to regulate the activity of the main lipases underlies the lipolytic action of this cytokine in vitro and in vivo, and could contribute to CT-1 antiobesity effects.     

Regional variation in adipose tissue lipolysis in lean and obese men.

Biopsies of adipose tissue were obtained from two subcutaneous regions (abdominal and femoral) in a sample of 54 men (32 obese and 22 lean subjects). Clonidine-induced antilipolysis in femoral adipose cells was similar in both groups, whereas subcutaneous abdominal adipocytes of obese individuals showed a higher alpha 2-adrenergic response than did subcutaneous abdominal adipose cells from lean subjects. In addition, epinephrine had a biphasic effect in subcutaneous abdominal adipocytes from obese individuals, as it induced antilipolysis at low concentrations, and a net lipolytic response at higher doses. In contrast, the physiological amine promoted lipolysis in subcutaneous abdominal adipose cells of lean subjects. Epinephrine- and clonidine-induced antilipolysis of subcutaneous abdominal adipocytes was positively associated with the level of subcutaneous abdominal fat measured by computed tomography (CT). Finally, men with a high alpha 2-adrenergic response of subcutaneous abdominal fat cells were fatter than those with a low alpha 2-adrenergic component. These results suggest that, in men with a wide range of body fatness, variations in the lipolytic response of subcutaneous abdominal adipose cells to epinephrine appear to involve changes in the functional balance between alpha 2- and beta-adrenoceptors.     

Effect of oestradiol on catecholamine-stimulated lipolysis

The authors studied the effect of a single in vivo dose of oestradiol (OE) on adrenergic lipolysis in the epididymal adipose tissue of adult and juvenile male rats, and the effect of OE on plasma free fatty acids (FFA), cholesterol and beta-lipoprotein levels at various intervals after its administration. It was found that OE injected 24 h beforehand in vivo (s.c.), in doses of 100 and 200 micrograms X kg-1 body weight, significantly potentiated the lipid-mobilizing action of the catecholamines noradrenaline (NOR) and isoprenaline (ISO) in adult rats (the action of ISO was potentiated more intensively); in addition, the adipose tissue became more sensitive to the action of NOR, but not of ISO. Raising the dose of OE to 400 micrograms X kg-1 did not enhance the potentiation of the lipolytic action of the catecholamines any further; on the contrary, the lipid mobilizing effect of the catecholamines was potentiated less than after half this dose. Following the s.c. injection of an oily OE solution, the lipolytic effect was potentiated after more than 7 h; the potentiation was strongest after 12 h, but only as far as the maximum attainable degree of lipolysis was concerned. Potentiation of adrenergic lipolysis was found only in adult male rats. In male rats weighing 130-150 g the lipolytic effect of catecholamines (in mumol/g adipose tissue) was significantly greater than in adult animals and the pre-administration of OE did not potentiate adrenergic lipolysis any further. Determination of plasma FFA, cholesterol and beta-lipoprotein levels 1, 2, 4 and 6 hours after the s.c. injection of OE showed only nonsignificant changes (an increase in FFA and a decrease in cholesterol). The authors consider it important to distinguish between the effect of OE on catecholamine-stimulated lipolysis in depot adipose tissue and its effect on lipid metabolism. In their opinion, the dose-dependent effect of OE on muscular and metabolic adrenergic reactions could be one of the factors co-reversible for certain side reactions to steroid contraceptives.     

Parathyroid hormone-induced lipolysis in human adipose tissue

Relative lipolytic activity of human parathyroid hormone-(1-34) (hPTH-(1-34], hPTH-(3-34), desamino-Ser1-hPTH-(1-34), and rat PTH-(1-34) was compared in human subcutaneous adipose tissues in vitro. Human PTH-(1-34), rat PTH-(1-34), and desamino-Ser1-hPTH-(1-34) stimulated in vitro lipolysis significantly above basal level at the concentration of 10(-6) M. Average increments of lipolytic rate were 2.39, 1.82, and 0.87 mumol/g per 2 hr, respectively, being significantly different among the three groups. On the other hand, hPTH-(3-34)-induced lipolytic rate was 0.83 +/- 0.18 mumol/g per 2 hr, not significantly different from the basal level (0.71 +/- 0.20 mumol/g per 2 hr). The effect of hPTH-(3-34) on glycerol release stimulated by hPTH-(1-34), isoproterenol, or forskolin was subsequently investigated. Human PTH-(3-34) produced a dose-dependent inhibition of hPTH-(1-34)-stimulated lipolysis. In contrast, isoproterenol- and forskolin-induced lipolytic rates were not influenced by hPTH-(3-34). The effect of propranolol on hPTH-(1-34)- or isoproterenol-induced lipolysis was also studied. Propranolol dose-dependently inhibited isoproterenol-induced lipolysis but had no effect on lipolysis stimulated by hPTH-(1-34). These results suggest that the amino acids at positions 1 (serine) and 2 (valine) of PTH are critical for the stimulation of lipolysis in human adipose tissue. Human PTH-(1-34) causes lipolysis after binding to receptors distinct from beta-adrenergic receptors of fat cells and possibly hPTH-(3-34) inhibits hPTH-(1-34)-stimulated lipolysis by competing at the level of PTH receptor.     

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.     

Insulin inhibits lipolytic activity and degradation of beta-lipotropin in rabbit adipose tissue

beta-Lipotropin, a pituitary peptide, is a potent stimulator of lipolysis in rabbit adipose tissue in vitro and in vivo. Insulin inhibited the beta-lipotropin (1-100 nM)-stimulated glycerol release from rabbit adipocytes and fat pads significantly at concentrations of 10 and 100 microM. Both these concentrations of insulin also decreased the degradation of beta-lipotropin in intact adipose tissue to the same extent as the lipolytic activity. Furthermore, insulin reduced the degradation of beta-lipotropin in rabbit adipose tissue homogenate. Like insulin, several lysosomotropic agents also decreased significantly the degradation and the lipolytic activity of beta-lipotropin. On the other hand, insulin-like growth factor I in lower concentrations (1-100 nM) did not effect degradation and lipolytic activity of beta-lipotropin in rabbit adipose tissue. Thus, a direct influence of insulin on lysosomal enzymes degrading beta-lipotropin in rabbit adipose tissue can be suggested.     

Effects of Growth Hormone on the Function of β‐Adrenoceptor Subtypes in Rat Adipocytes

Objective: The influence of growth hormone (GH) on the regulation of lipolytic response to specific agonists to β‐adrenoceptors and several post‐receptor steps in the lipolytic cascade were investigated. Research Methods and Procedures: Adipose tissues from rats were incubated with or without GH (1.38 nM). After a 24‐hour incubation, isolated adipocytes were prepared for different assays. Rats were hypophysectomized. One week after operation, l‐thyroxine and hydrocortisone acetate was given to hypophysectomized rats. One group of rats was treated with GH (1.33 mg/kg, daily). After 1 week of hormonal treatment, adipose tissues were removed for different studies. Results: GH treatment increased both basal lipolysis and lipolytic sensitivity to dobutamine and CGP 12177 in adipocytes. The lipolytic sensitivity to terbutaline was not influenced by GH treatment. GH treatment increased the maximal lipolytic response to dobutamine and CGP 12177, but not to terbutaline as determined with absolute values of lipolysis. Forskolin‐induced lipolysis was increased by addition of GH to tissues. Moreover, GH treatment resulted in enhanced expression of hormone‐sensitive lipase. GH treatment in hypophysectomized rats influenced neither the expressions of Gαs protein and cholera toxin‐catalyzed adenosine diphosphate‐ribosylation of Gαs protein, nor cholera toxin‐induced 3′, 5′‐cyclic adenosine monophosphate accumulation. However, the expression of Gαi protein was decreased after GH treatment. Discussion: These and previous results suggest that GH increases lipolysis in rat adipocytes partly through the β‐adrenergic system, including increases in both β₁‐ and β₃‐adrenergic receptor function, and partly through enhanced adenylate cyclase function, and expression of hormone‐sensitive lipase, perhaps via a decrease in Gαi protein expression.