Possible mechanism of regulating adenylate cyclase activity in adipocyte membranes from exercise-trained male rats

Habitual exercise training acts as a physiological stimulator for constant activation of lipolytic enzymes in rat primary white adipocytes

It is widely accepted that lipolysis in adipocytes are regulated through the enzymatic activation of both hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) via their phosphorylation events. Accumulated evidence shows that habitual exercise training (HE) enhances the lipolytic response in primary white adipocytes with changes in the subcellular localization of lipolytic molecules. However, no study has focused on the effect that HE exerts on the phosphorylation of both HSL and ATGL in primary white adipocytes. It has been shown that the translocation of HSL from the cytosol to lipid droplet surfaces requires its phosphorylation at Ser-563. In primary white adipocytes obtained from HE rats, the level of HSL and ATGL proteins was higher than that in primary white adipocytes obtained from sedentary control (SC) rats. In HE rats, the level of phosphorylated ATGL and HSL was also significantly elevated compared with that in SC rats. These differences were confirmed by Phos-tag SDS-PAGE, a technique used to measure the amount of total phosphorylated proteins. Our results suggest that HE can consistently increase the activity of both lipases, thereby enhancing the lipolysis in white fat cells. Thus, HE helps in the prevention and treatment of obesity-related diseases by enhancing the lipolytic capacity.

The Molecular Mechanism Underlying Continuous Exercise Training-Induced Adaptive Changes of Lipolysis in White Adipose Cells

Physical exercise accelerates the mobilization of free fatty acids from white adipocytes to provide fuel for energy. This happens in several tissues and helps to regulate a whole-body state of metabolism. Under these conditions, the hydrolysis of triacylglycerol (TG) that is found in white adipocytes is known to be augmented via the activation of these lipolytic events, which is referred to as the "lipolytic cascade." Indeed, evidence has shown that the lipolytic responses in white adipocytes are upregulated by continuous exercise training (ET) through the adaptive changes in molecules that constitute the lipolytic cascade. During the past few decades, many lipolysis-related molecules have been identified. Of note, the discovery of a new lipase, known as adipose triglyceride lipase, has redefined the existing concepts of the hormone-sensitive lipase-dependent hydrolysis of TG in white adipocytes. This review outlines the alterations in the lipolytic molecules of white adipocytes that result from ET, which includes the molecular regulation of TG lipases through the lipolytic cascade.

Higher levels of ATGL are associated with exercise-induced enhancement of lipolysis in rat epididymal adipocytes

BACKGROUND

In adipose cells, adipose triglyceride lipase (ATGL) catalyzes the first step in adipocyte triacylglyceride hydrolysis, thereby regulating both basal and hormone-stimulated lipolysis. However, little is known about the molecular mechanism(s) underlying habitual exercise-induced adaptive modulation of ATGL in white adipocytes via alteration in transcription regulator and lipolytic cofactors.

METHODOLOGY/PRINCIPAL RESULTS

Male Wistar rats were randomly divided into 2 groups a sedentary control group (CG) and a habitual exercise group (EG). The EG was subjected to running on a treadmill set at 5 days per week for 9 weeks. The CG was not subjected to running on a treadmill. In the EG, levels of ATGL mRNA and protein were elevated with a significant increase in lipolysis compared with the CG, accompanied by a significant increase in associations of CGI-58 with ATGL protein. Under these conditions, an upregulation of peroxisome proliferation-activated receptorg-2 (PPARg-2) was observed. In the EG, the addition of rosiglitazone further significantly increased the levels of ATGL protein compared with the CG. However, attenuated levels of the ATGL protein in adipocytes were obtained by the addition of insulin, which is known to inhibit the expression of ATGL, in both types of groups. Actually, levels of plasma insulin were significantly reduced in the EG compared with the CG.

CONCLUSIONS

These data suggest that elevated levels of ATGL are involved in the exercise-induced enhancement of lipolysis in primary adipocytes. The exact mechanism(s) underlying this phenomenon is associated, at least in part, with upregulated transcriptional activation of PPARg-2. In addition, exercise-induced lower circulation levels of insulin also correlate with habitual exercise-induced higher levels of ATGL in primary adipocytes.

Preventive and improvement effects of exercise training and supplement intake in white adipose tissues on obesity and lifestyle-related diseases

Recent increases in the number of obese individuals and individuals suffering from lifestyle-related diseases, such as type 2 diabetes, that accompany obesity have become a serious social problem. White adipose tissue (WAT) is more than a mere organ for storage of energy; it is also a highly active metabolic and endocrine organ that secretes physiologically active substances collectively known as adipokines, including tumor necrosis factor-α and adiponectin. Dysregulated expression of adipokines in WAT that is hypertrophied by obesity has been closely associated with the phenomenon of insulin resistance. Therefore, WAT is currently considered to be one of the tissues that promote lifestyle-related diseases. Reduction of excess WAT that results from obesity is seen as an important strategy in preventing and improving lifestyle-related diseases. This review shows that exercise training as well as intake of supplements, such as polyphenols, is one strategy for this, because this regimen can result in reduction of WAT mass, which affects the expression and secretory response of adipokines.

Exercise training decreases expression of inflammation-related adipokines through reduction of oxidative stress in rat white adipose tissue

Increased oxidative stress in adipocytes causes dysregulated expression of inflammation-related adipokines. We have examined the effects of exercise training on oxidative stress in rat white adipose tissue (WAT), especially focusing on inflammation-related adipokines. The levels of lipid peroxidation in WAT of exercise-trained (TR) rats were lower than those in control (C) rats. The content of manganese-containing superoxide dismutase in WAT of TR rats was increased as compared with those in C rats. In contrast, the expression of the NADPH oxidase NOX2 protein in WAT was downregulated by exercise training. Moreover, the levels of inflammation-related adipokines, such as tumor necrosis factor-alpha and monocyte chemoattractant protein-1, in WAT of TR rats were lower than those in C rats. The effects of exercise training were more remarkable in visceral WAT than in subcutaneous. These results suggest that exercise training decreases the expression of inflammation-related adipokines by reducing oxidative stress in WAT.

Reduced lipolysis and increased lipogenesis in adipose tissue from pinealectomized rats adapted to training

The current study investigated the effects of chronic training and pinealectomy on the lipogenic and lipolytic activity of adipose tissue. Pinealectomized and sham-operated adult male Wistar rats were distributed in to four subgroups: pinealectomized untrained, pinealectomized trained, control untrained and control trained. At the end of the training period (8 wk) the rats were killed. Blood samples were collected for glucose, insulin and leptin determinations. Peri-epididymal adipocytes were isolated for measurement of in vitro rates of lipolysis and incorporation of substrates (D-[U-14C]-glucose, L-[U-14C]-lactate, [2-14C]-acetate and [1-14C]-palmitate) into lipids, and samples of epididymal adipose tissue were homogenized for evaluation of glucose-6-phosphate dehydrogenase maximal activity. Pinealectomy resulted in a significantly increased lipolytic capacity in response to isoproterenol and a decrease in circulating leptin levels without affecting the rates of incorporation of different substrates into lipids. However, only in the intact control group did training promote a higher basal and isoproterenol-stimulated lipolysis, increase the incorporation of palmitate (esterification), decrease the incorporation of acetate (lipogenesis) into lipids and diminish circulating leptin levels. These effects of exercise training were not seen in pinealectomized rats. However, pinealectomized trained animals showed a marked reduction in lipolysis and an increased rate of acetate incorporation. In conclusion, we demonstrated for the first time that the pineal gland plays an important role in the regulation of lipid metabolism in such a way that its absence caused a severe alteration in the balance between lipogenesis and lipolysis, which becomes evident with the adaptation to exercise training.

Exercise training enhances tumor necrosis factor-alpha-induced expressions of anti-apoptotic genes without alterations in caspase-3 activity in rat epididymal adipocytes

The effect of exercise training on tumor necrosis factor-alpha (TNF-alpha) signaling was investigated in rat epididymal adipocytes. Incubation of isolated adipocytes with TNF-alpha (20 ng/ml) for 5 h enhanced the expression of the inhibitor apoptosis protein 2 (IAP2) gene without any enhancement of caspase-3 activity in both the sedentary control (C) and exercise-trained (TR) groups. However, the ability of TNF-alpha to enhance IAP2 gene expression was significantly greater in TR than in C rats. The basal expression of the IkappaB kinase beta (IKK beta) gene and that of the BCL-x(L) gene were also higher in TR than in C rats. Mn-superoxidedismutase contents in adipocytes were higher in TR than in C rats. Moreover, no apoptotic nucleuses of adipocytes in response to acute exercise were observed in either group at least up to 5 h after exercise. Exercise training also enhanced the inhibitory effect of TNF-alpha on the gene expression of the fatty acid synthase (FAS), a lipogenic enzyme, suggesting that fatty acid synthesis may be reduced. Thus, exercise training enhanced TNF-alpha signaling directed toward the expressions of survival signals and the suppression of FAS gene expression.

Desensitization of the inhibitory effect of norepinephrine on insulin secretion from pancreatic islets of exercise-trained rats

The effect of exercise training (9 weeks of running) on norepinephrine-induced inhibition of insulin secretion was examined in rat islets. Insulin secretions from islets in the presence of glucose (> or =5.5 mmol/L) were significantly lower in trained (TR) than in control rats (CR). Norepinephrine inhibited 5.5 mmol/L glucose-stimulated insulin secretions and cyclic adenosine monophosphate (cAMP) contents in a dose-dependent manner in CR. Norepinephrine (10 micromol/L)-induced inhibition of insulin secretion was reversed by the blockade of the alpha(2)-adrenergic receptor in CR, but not in TR. Exercise training substantially shifted the dose-dependent curve for clonidine-induced inhibition of insulin secretions and that of cAMP contents to the right. Exercise training did not alter the density of the alpha(2)-adrenergic receptor either per islet or per protein of islet crude membrane. However, exercise training significantly reduced the protein expression of G alpha i-2 without change in G alpha i-2 mRNA. In CR but not in TR, norepinephrine significantly inhibited insulin secretions elicited by a combination of high glucose, a protein kinase C activator, and an adenylate cyclase activator under Ca(2+)-free conditions. Thus, exercise training appears to provoke a decreased expression of G alpha i-2 protein. This, at least in part, results in loss of the inhibitory effect of norepinephrine either on cAMP content or on insulin secretion at the post-calcium events in stimulus-secretion coupling, which, in turn, leads to the blunted inhibitory effects of norepinephrine on insulin secretion.

Possible role of nitric oxide on adipocyte lipolysis in exercise-trained rats

A possible role of nitric oxide (NO) on adipocyte lipolysis was studied in exercise-trained (9 weeks of running) rats. Lipolysis in adipose tissue tended to be greater in trained rats than in control rats. A treatment of adipose tissue with 5 mM N(G)-nitro-L-arginine methyl ester (L-NAME) showed that basal and isoproterenol-stimulated lipolysis were both significantly greater in trained rats than in control rats. In contrast, in isolated adipocytes L-NAME had no effect on lipolysis in either group of rats, though the lipolysis of isolated adipocytes was significantly greater in trained rats than in control rats. Training significantly reduced nitrite/nitrate production in adipocytes, but not in tissue. On the other hand, training increased the protein expression of endothelial nitric oxide synthase (eNOS), but not that of inducible NOS (iNOS) in the extracts of tissue homogenates. In tissue homogenates, eNOS activity but not iNOS activity was significantly greater in trained rats than in control rats. In cellular extracts, training significantly reduced the activities of both NOS's, but the mRNA expressions of both NOS's were not different between groups. The NO donors, S-nitroso-N-acetyl-penicillamine (SNAP) and 1-propamine, 3-(2-hydroxy-2-nitroso-1-propyl-hydrazine) (PAPA-NONOate), significantly inhibited adipocyte lipolysis in response to isoproterenol in both groups. This inhibitory effect of SNAP, but not that of PAPA-NONOate, was greater in the adipocytes of trained rats than in those of the control rats. Thus it is possible that NO is involved in the regulation of lipolysis and that exercise training enhances the responsiveness of adipocytes to extracellular NO with the reduced production of nitrite/nitrate in adipocytes because of decreased activities of NOS's. On the other hand, it is also possible that exercise increases either the activity or the protein expression of eNOS in adipose tissue.

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.

Exercise training increases membrane bound form of tumor necrosis factor-alpha receptors with decreases in the secretion of soluble forms of receptors in rat adipocytes

We examined the effects of exercise training (treadmill running over 9 weeks) on the ability of isolated adipocytes to secrete tumor necrosis factor-alpha (TNF-alpha) and type 1 soluble TNF receptor (sTNFR1) in vitro in Wistar rats. We also examined the effects of exercise training on the expression of membrane bound forms of type 1 TNF receptor (mTNFR1) in adipocyte crude membranes of the same rat subjects. Exercise training significantly increased the secretions of TNF-alpha from isolated adipocytes. Treatment with a cyclooxygenase inhibitor, either indomethacin (100 microM) or eicosatetraynoic acid (100 microM), significantly blocked the release of TNF-alpha from adipocytes in both exercise-trained rat group and sedentary control rat group, suggesting that some cyclooxygenase metabolite(s) acts as a ligand in TNF-alpha synthesis. Decreased amounts of TNF-alpha were found to be significantly greater in both exercise-trained rat group than in sedentary control rat group after incubation with inhibitors. Thus, the inhibitory effect of both indomethacin and eicosatetraynoic acid was significantly greater in adipocytes from exercise-trained rats. Both plasma sTNFR1 levels and adipocytes-derived sTNFR1 were found to be significantly less in the exercise-trained rat group. Western blot analysis revealed that exercise training remarkably increased the expressions of mTNFR1 in adipocyte crude membrane. Thus, exercise training enhanced the ability of isolated adipocytes to secrete TNF-alpha with reduced secretion of sTNFR1, and provoked the greater expressions of mTNFR1 in adipocyte crude membrane. These alterations may induce enhanced the autocrine effects of TNF-alpha within adipocytes in exercise-trained rats.

The effect of exercise training on hormone-sensitive lipase in rat intra-abdominal adipose tissue and muscle

1. Adrenaline-stimulated lipolysis in adipose tissue may increase with training. The rate-limiting step in adipose tissue lipolysis is catalysed by the enzyme hormone-sensitive lipase (HSL). We studied the effect of exercise training on the activity of the total and the activated form of HSL, referred to as HSL (DG) and HSL (TG), respectively, and on the concentration of HSL protein in retroperitoneal (RE) and mesenteric (ME) adipose tissue, and in the extensor digitorum longus (EDL) and soleus muscles in rats. 2. Rats (weighing 96 +/- 1 g, mean +/- S.E.M.) were either swim trained (T, 18 weeks, n = 12) or sedentary (S, n = 12). Then RE and ME adipose tissue and the EDL and soleus muscles were incubated for 20 min with 4.4 microM adrenaline. 3. HSL enzyme activities in adipose tissue were higher in T compared with S rats. Furthermore, in RE adipose tissue, training also doubled HSL protein concentration (P < 0.05). In ME adipose tissue, the HSL protein levels did not differ significantly between T and S rats. In muscle, HSL (TG) activity as well as HSL (TG)/HSL (DG) were lower in T rats, whereas HSL (DG) activity did not differ between groups. Furthermore, HSL protein concentration in muscle did not differ between T and S rats (P > 0.05). 4. In conclusion, training increased the amount of HSL and the sensitivity of HSL to stimulation by adrenaline in intra-abdominal adipose tissue, the extent of the change differing between anatomical locations. In contrast, in skeletal muscle the amount of HSL was unchanged and its sensitivity to stimulation by adrenaline reduced after training.

Cellular adaptations in fat tissue of exercise-trained miniature swine: role of excess energy intake

This study examined the influence of energy expenditure and energy intake on cellular mechanisms regulating adipose tissue metabolism. Twenty-four swine were assigned to restricted-fed sedentary, restricted-fed exercise-trained, full-fed sedentary, or full-fed exercise-trained groups. After 3 mo of treatment, adipocytes were isolated and adipocyte size, adenosine A(1) receptor characteristics, and lipolytic sensitivity were measured. Swine were infused with epinephrine during which adipose tissue extracellular adenosine, plasma fatty acids, and plasma glycerol were measured. Results revealed that adipocytes isolated from restricted-fed exercised swine had a smaller diameter, a lower number of A(1) receptors, and a greater sensitivity to lipolytic stimulation, compared with adipocytes from full-fed exercised swine. Extracellular adenosine levels were transiently increased on infusion of epinephrine in adipose tissue of restricted-fed exercised but not full-fed exercised swine. These results suggest a role for adenosine in explaining the discrepancy between in vitro and in vivo lipolysis findings and underscore the notion that excess energy intake dampens the lipolytic sensitivity of adipocytes to beta-agonists and adenosine, even if accompanied by exercise training.

Effects of endurance training on superoxide dismutase activity, content and mRNA expression in rat muscle

1. The purpose of the present study was to investigate the changes in superoxide dismutase (SOD) isoenzyme (Mn(2+)-SOD and Cu2+, Zn(2+)-SOD) activities, contents and mRNA expressions in rat skeletal muscle during endurance training and a single bout of exercise. 2. Thirty-eight male Wistar rats were divided into untrained (U) and trained (T) groups. The T group rats were treadmill-trained for 9 weeks. The activity, content and mRNA expression of Mn(2+)-SOD and Cu2+, Zn(2+)-SOD were determined in the soleus muscle of each rat. 3. Mn(2+)-SOD activity and content in the T group were significantly higher than in the U group, both at rest (22 and 21%, respectively) and after exercise (24 and 46%, respectively), while a single bout of exercise affected neither the activity nor content of Mn(2+)-SOD in either group. 4. The content of Cu2+,Zn(2+)-SOD in both groups was not different at rest and after exercise, although its activity at rest was significantly higher in the T group than in the U group (by 29%). 5. After exercise, the expression of Mn(2+)-SOD mRNA was markedly attenuated only in the U group (49%); the expression of Cu2+,Zn(2+)-SOD mRNA was not influenced by exercise. 6. Our results suggest that adequate endurance training increases both the activity and content of Mn(2+)-SOD and that untrained rats are rather susceptible to oxidative stress during physical exercise. It thus appears that Mn(2+)-SOD provides a reliable index of physical training. 7. The results obtained in the present study also suggest that muscle has the capacity of responding to training in such a manner as to reduce the potential harm arising from the accumulation of oxygen free radicals resulting from enhanced metabolic activity.

Adenylyl cyclase system is affected differently by endurance physical training in heart and adipose tissue

Adaptive changes in the beta-adrenergic adenylyl cyclase (AC) system in response to endurance training were studied in heart and adipose tissue. Training was performed by making male Wistar rats run on a motor-driven treadmill. The changes following exercise training were opposite in the two tissue studied. The density of beta-adrenergic receptors in left ventricular membranes of trained rats showed a marked decrease. Comparison of AC activities in cardiac membranes prepared from trained and sedentary rats revealed a depressing effect of endurance training on: 1. the beta-adrenergic stimulatory pathway and the inhibition of AC via receptor; 2. the Gs component and the Gs-adenylyl cyclase coupling, as shown by the response of adenylyl cyclase to GppNHp and NaF; and 3. the enzyme catalytic activity in the presence of Mn2+ or forskolin. The levels of Gsalpha subunits in the left ventricle, as measured in terms of ADP-ribosylated and immunologically reactive proteins, were decreased by endurance exercise, whereas immunodetectable levels of Gialpha2 increased in the membranes of trained myocardium. In contrast to the diminished sensitivity that characterizes the behavior of the cardiac beta-adrenergic-AC system, endurance physical training increased sensitivity of this signal transduction system in adipose tissue. Thus, the density of beta-ARs as well as AC activity and the beta-adrenergic stimulatory pathway were increased in adipose membranes of trained rats compared with the corresponding sedentary controls. In addition, the levels of Gsalpha subunits were higher in the adipose plasma membranes of trained rats. However, immunodetectable levels of Gi1alpha and Gi3alpha increased with training, whereas the amount of Gi2alpha decreased in membranes of trained rats. In conclusion, the present study shows that chronic exercise is associated with a tissue-specific adaptation of the beta-adrenergic AC system.

Effect of running training on uncoupling protein mRNA expression in rat brown adipose tissue

The effect was investigated of endurance training on the expression of uncoupling protein (UCP) mRNA in brown adipose tissue (BAT) of rats. The exercised rats were trained on a rodent treadmill for 5 days per week and a total of 9 weeks. After the training programme, a marked decrease in BAT mass was found in terms of weight or weight per unit body weight; there was a corresponding decrease in DNA content and a downward trend in RNA and glycogen levels. The UCP mRNA was present at a markedly decreased level in BAT of trained animals. In consideration of the reduced levels of mRNAs for hormone-sensitive lipase and acyl-CoA synthetase, the brown adipose tissue investigated appeared to be in a relatively atrophied and thermogenically quiescent state.

Effect of endurance training on angiogenic activity in skeletal muscles

The effect of endurance training on angiogenic activity in skeletal muscle was investigated using cell culture. The exercised rats were trained on a rodent treadmill, 5 days a week for 9 weeks. After the training program, a significant increase (7.8%) in mass was found in soleus muscle (MS), although we noted no apparent increase in gastrocnemius muscle and extensor digitorum longus muscle. The extracts from all the muscles (especially from the MS) of trained rats considerably increased the capillary growth in an in vitro angiogenesis model, in which microvascular fragments and myofibroblastic (Mf) cells isolated from lipid tissues were grown in co-culture. In addition, the extract from the MS of trained rats increased the DNA synthesis and growth of bovine capillary endothelial (BCE) cells, although the extract did not stimulate the growth of Mf cells. Moreover, the extract from the MS of trained rats notably enhanced the activity of plasminogen activator in the conditioned medium of BCE cells compared with that of sedentary rats. On the other hand, extracts from the MS of both sedentary and trained rats increased the synthesis of collagen by Mf cells and metalloproteinase activity in the conditioned medium of BCE cells to the same degree. These results suggest that endurance training increases the angiogenic activity in skeletal muscles.

Role of guanine nucleotide regulatory proteins in insulin stimulation of glucose transport in rat adipocytes. Influence of bacterial toxins

The potential role of guanine nucleotide regulatory proteins (G-proteins) in acute insulin regulation of glucose transport was investigated by using bacterial toxins which are known to modify these proteins. Cholera-toxin treatment of isolated rat adipocytes had no effect on either 2-deoxyglucose transport or insulin binding. Pertussis-toxin treatment resulted in an inhibition of both insulin binding and glucose transport. Insulin binding was decreased in pertussis-toxin-treated cells by up to 40%, owing to a lowering of the affinity of the receptor for hormone, with no change in hormone internalization. The dose-response curve for insulin stimulation of glucose transport was strongly shifted to the right by pertussis-toxin treatment [EC50 (half-maximally effective insulin concn.) = 0.31 +/- 0.04 ng/ml in control cells; 2.29 +/- 1.0 in treated cells), whereas cholera toxin had only a small effect (EC50 = 0.47 +/- 0.02 ng/ml). Correcting for the change in hormone binding, pertussis toxin was found to decrease the coupling efficiency of occupied receptors (50% of maximal insulin effect with 928 molecules bound/cell in control and 3418 in treated cells). Pertussis-toxin inhibition of insulin sensitivity was slow in onset, requiring 2-3 h for completion. Under conditions where pertussis-toxin inhibition of insulin sensitivity was maximal, a 41,000 Da protein similar to the alpha subunit of Gi (the inhibitory G-protein) was found to be fully ribosylated. These results are consistent with the concept that pertussis-toxin-sensitive G-protein(s) can modify the insulin-receptor/glucose-transport coupling system.