Altered lipolytic response to glucagon and adenosine deaminase in adipocytes from starved rats.
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ABSTRACT:
SUBMITTER: Honnor RC
PROVIDER: S-EPMC1161958 | biostudies-other | 1980 Jun
REPOSITORIES: biostudies-other
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Project description:1. Adipocytes isolated from rats 6--9 days after adrenalectomy had significantly increased sensitivity to insulin action against noradrenaline-stimulated lipolysis. In the presence of adenosine deaminase there was no significant difference in insulin sensitivity between cells from adrenalectomized and sham-operated rats. 2. Adipocytes from adrenalectomized rats had decreased lipolytic responses to all concentrations of noradrenaline and glucagon tested and a decreased lipolytic response to low but not high concentrations of corticotropin. There was no difference in lipolytic response to theophylline after adrenalectomy. Adenosine deaminase corrected the differences in response to noradrenaline and glucagon resulting from adrenalectomy. 3. In the presence of adenosine deaminase rates of lipolysis, after stimulation by high concentrations of noradrenaline, glucagon, corticotropin or theophylline, were the same in cells from adrenalectomized or sham-operated rats. 4. These findings and previously reported effects of adenosine and adrenalectomy on adipocyte function are discussed. It is proposed that changes in adipocyte hormone responsiveness after adrenalectomy may result from changes in adenosine metabolism or release.
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Project description:Effects of adenosine deaminase and glucagon on insulin-stimulated 2-deoxyglucose uptake by rat adipocytes are reported. (1) Adenosine deaminase (10 micrograms/ml) caused a rightward shift in the dose-response curve for the stimulation by insulin of 2-deoxyglucose uptake, but the enzyme did not alter either the basal or the maximally insulin-stimulated uptake rate. (2) In adipocytes obtained from 24 h-starved rats, glucagon inhibited the effect of insulin on 2-deoxyglucose uptake in the presence (but not in the absence) of adenosine deaminase. Basal uptake rates were unaffected. (3) Glucagon inhibited insulin-stimulated 2-deoxyglucose uptake to a greater extent in cells isolated from starved rats than in cells from fed rats. (4) Adipocytes isolated from fed and from starved rats did not differ in their capacity for degradation of 125I-labelled glucagon. The results suggest that adenosine and glucagon are regulators of insulin action in adipose tissue.
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Project description:Control of fatty acid storage and release in adipose tissue is fundamental in energy homeostasis and the development of obesity and type 2 diabetes. We here take the whole signalling network into account to identify how insulin and β-adrenergic stimulation in concert controls lipolysis in mature subcutaneous adipocytes obtained from non-diabetic and, in parallel, type 2 diabetic women. We report that, and show how, the anti-lipolytic effect of insulin can be fully explained by protein kinase B (PKB/Akt)-dependent activation of the phosphodiesterase PDE3B. Through the same PKB-dependent pathway β-adrenergic receptor signalling, via cAMP and PI3Kα, is anti-lipolytic and inhibits its own stimulation of lipolysis by 50%. Through this pathway both insulin and β-adrenergic signalling control phosphorylation of FOXO1. The dose-response of lipolysis is bell-shaped, such that insulin is anti-lipolytic at low concentrations, but at higher concentrations of insulin lipolysis was increasingly restored due to inhibition of PDE3B. The control of lipolysis was not altered in adipocytes from diabetic individuals. However, the release of fatty acids was increased by 50% in diabetes due to reduced reesterification of lipolytically liberated fatty acids. In conclusion, our results reveal mechanisms of control by insulin and β-adrenergic stimulation - in human adipocytes - that define a network of checks and balances ensuring robust control to secure uninterrupted supply of fatty acids without reaching concentrations that put cellular integrity at risk. Moreover, our results define how selective insulin resistance leave lipolytic control by insulin unaltered in diabetes, while the fatty acid release is substantially increased.
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Project description:The anti-lipolytic effect of the adenosine analogue N6-L-phenylisopropyladenosine was studied with rat adipocytes incubated with a high concentration of adenosine deaminase (0.5 unit/ml, approx. 2.5 micrograms/ml) and concentrations of noradrenaline that were equieffective in different physiological states. These studies were performed to compare the fed and starved (24h) states and to compare a hypothyroid state (induced by feeding propylthiouracil + a low-iodine diet) with the euthyroid state. Starvation increased sensitivity of the cells to the lipolytic action of noradrenaline, while decreasing sensitivity to the antilipolytic action of phenylisopropyladenosine. Hypothyroidism resulted in decreased sensitivity to noradrenaline and increased sensitivity to phenylisopropyladenosine. Studies of the binding of [3H]phenylisopropyladenosine to adipocyte plasma membranes indicated heterogeneity of binding sites or negative co-operativity in the binding. Starvation did not change [3H]phenylisopropyladenosine binding to membranes, whereas hypothyroidism caused an unexpected decrease in both the number and affinity of the binding sites. These observations are discussed in terms of the dual regulation of adipose-tissue lipolysis by lipolytic and anti-lipolytic agents.
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Project description:Secretin (Sct), a classical gut hormone, is now known to play pleiotropic functions in the body including osmoregulation, digestion, and feeding control. As Sct has long been implicated to regulate metabolism, in this report, we have investigated a potential lipolytic action of Sct. In our preliminary studies, both Sct levels in circulation and Sct receptor (SctR) transcripts in adipose tissue were upregulated during fasting, suggesting a potential physiological relevance of Sct in regulating lipolysis. Using SctR knockout and Sct knockout mice as controls, we show that Sct is able to stimulate lipolysis in vitro in isolated adipocytes dose- and time-dependently, as well as acute lipolysis in vivo. H-89, a protein kinase A (PKA) inhibitor, was found to attenuate lipolytic effects of 1 ?M Sct in vitro, while a significant increase in PKA activity upon Sct injection was observed in the adipose tissue in vivo. Sct was also found to stimulate phosphorylation at 660(ser) of hormone sensitive lipase (HSL) and to bring about the translocation of HSL from cytosol to the lipid droplet. In summary, our data demonstrate for the first time the in vivo and in vitro lipolytic effects of Sct, and that this function is mediated by PKA and HSL.
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Project description:1. Adipocytes isolated from epididymal fat-pads of fed rats were incubated with different concentrations of glucagon, insulin, adrenaline and adenosine deaminase, and the effects of these agents on the ;initial' activity of acetyl-CoA carboxylase in the cells were studied. 2. Glucagon (at concentrations between 0.1 and 10nm) inhibited acetyl-CoA carboxylase activity. Maximal inhibition was approx. 70% of the ;control' activity in the absence of added hormone, and the concentration of hormone required for half-maximal inhibition was 0.3-0.5nm-glucagon. 3. Incubation of cells with adenosine deaminase resulted in a similar inhibition of acetyl-CoA carboxylase activity. Preincubation of adipocytes with adenosine deaminase did not alter either the sensitivity of carboxylase activity to increasing concentrations of glucagon or the maximal extent of inhibition. 4. Adrenaline inhibited acetyl-CoA carboxylase to the same extent as glucagon. Preincubation of the cells with glucagon did not alter the sensitivity of enzyme activity to adrenaline or the degree of maximal inhibition. 5. Insulin activated the enzyme by 70-80% of ;control' activity. Preincubation of the cells with glucagon did not alter the concentration of insulin required to produce half the maximal stimulatory effect (about 12muunits of insulin/ml). The effects of insulin and glucagon appeared to be mediated completely independently, and were approximately quantitatively similar but opposite. These characteristics resulted in the mutual cancellation of the effects of the two hormones when they were both present at equally effective concentrations. 6. The implications of these findings with regard to current concepts about the mechanism of regulation of acetyl-CoA carboxylase and to the regulation of the enzyme in vivo are discussed.
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