Adenosine Triphosphate-dependent Inhibition of Insulin-stimulated Glucose Transport in Fat Cells

Effect of experimental hyperinsulinaemia on intracellular glucose metabolism of isolated adipocytes

Glucose oxidation and lipogenesis were studied in isolated adipocytes from control and non-obese, experimentally hyperinsulinaemic rats. In cells from the hyperinsulinaemic animals oxidation of either [1(-14) C]- or [6(-14) C] glucose was increased in the presence or absence of insulin, at substrate concentrations from 0.1 to 20 mmol/l. Glucose incorporation into total triglycerides and fatty acids was also increased. These enhanced rates of glucose metabolism were due to increased activity of the glucose transport system in addition to increased activity of intracellular glucose metabolism. Therefore, these data indicate that insulin can influence long term glucose homoeostasis by augmenting the overall cellular capacity for glucose metabolism at several loci.

Regulation of lipogenesis in adipocytes. Independent effects of thyroid hormones, cyclic AMP and insulin on the uptake of deoxy-D-glucose

Thyroidectomy is known to enhance fat cell phosphodiesterase activity; as a result, the response to lipolytic hormones is markedly reduced. Thyroidectomy also stimulates overall lipogenesis and the uptake of glucose: the present experiments investigated whether there was a correlation between cyclic AMP and glucose uptake. The parameter measured was the transport and phosphorylation (uptake) of deoxy-D-glucose in the presence of two modifiers of the cyclic AMP pool: phosphodiesterase inhibitors and the analogue, dibutyryl cyclic AMP. The inhibition by methylxanthines and dibutyryl cyclic AMP of deoxy-D-glucose uptake observed, was the same in fat cells from normal and thyroidectomized rats: the latter nonetheless still maintained their enhanced glucose uptake. It was therefore concluded that thyroid hormones and cyclic AMP control this step by different, separate pathways. Insulin, well known for its lipogenic effect, enhanced deoxy-D-glucose uptake in fat cells from both normal and thyroidectomized rats to the same extent (about 40%). An additive effect of thyroidectomy and insulin on glucose uptake was thus demonstrated. These results imply that glucose uptake in the adipocyte is controlled by at least three factors: thyroid hormones, cyclic AMP and insulin, each of which can act independently. Maximum glucose uptake is achieved in the presence of a combination of low concentrations of cyclic AMP, of insulin, and in the absence of thyroid hormones.

Phosphoprotein phosphatase activity at the outer surface of intact normal and transformed 3T3 fibroblasts

Using 32P-labeled phosphocasein or phosphohistones as exogenous substrates it was possible to detect a phosphoprotein phosphatase activity on the outer surface of intact normal and transformed 3T3 fibroblasts. Incubation of monolayers of intact cells in buffered salt solution with the radioactively labeled substrate resulted in the release of alkali-labile 32P counts into the surrounding medium. The reaction was: (a) linear with time (at least up to 20 min); (b) proportional to the cell density; (c) dependent on the temperature and pH of the incubation medium; (d) stimulated by K+; and (e) inhibited by sodium fluoride, inorganic pyrophosphate, zinc chloride and relatively impermeant sulfhydryl reagents. Less than 2% of the externally located phosphoprotein phosphatase activity was detectable in pooled cell-free washings of the intact cell monolayer. Phosphocasein did not cause any detectable leakage of intracellular lactate dehydrogenase or soluble phosphoprotein phosphatase activity into the external medium; incubation of the cells with phosphohistones, on the other hand, resulted in appreciable leakage of both these cytoplasmic activities. Neoplastic transformation was associated with a nearly two-fold decrease in the activity of the surface phosphoprotein phosphatase. Addition of serum to either non-transformed 3T3 or spontaneously transformed 3T6 cells resulted in a rapid and remarkeable drop in the cell surface dephosphorylating activity. Acrylamide gel electrophoresis of the dephosphorylated casein or histone substrate revealed no proteolytic degradation or change in electrophoretic mobility. The intact cells showed no damage upon microscopic examination as a result of exposure to phosphocasein or phosphohistones.

Cyclic AMP and lipogenesis in fat cells from thyroidectomized rats

Thyroid hormones regulate lipid metabolism by affecting lipogenesis as well as lipolysis. The present paper discusses the way thyroidectomy induced an enhancement in lipogenesis in rat fat cells. The doubling in the conversion of glucose to CO2 and fatty acids seen after thyroidectomy was found to be due to a modification in the actual pathway of glucose metabolism: there was a preferential stimulation of the conversion of glucose to CO2 by the pentose cycle (utilisation of [1-14C]glucose) while the production of fatty acids and glyceride-glycerol proceeded, respectively, much more, or only slightly more, via the pathway of [6-14C]glucose metabolism. Studies employing the phosphodiesterase inhibitor MIX, or the cyclic AMP analogue, DBcAMP showed that the lipogenic process depends on cyclic AMP. As the stimulatory effect of thyroidectomy was not abolished, however, lipogenesis must be under the independent control of both cyclic AMP and absence of thyroid hormones. Insulin, a further mediator of lipogenesis was found to further enhance the already preexisting high conversion of glucose to CO2 in fat cells from thyroidectomized rats. It is concluded that at least three factors modify lipogenesis: thyroidectomy, cyclic AMP and insulin; each achieving its effect in an independent manner.

Effect of prolonged anaerobiosis on 125I-insulin binding to rat soleus muscle: permissive effect of ATP

The specific binding of 125I-insulin by rat soleus muscle was depressed when muscle ATP was depleted, either by prolonged anoxia or more rapidly with 2,4-dinitrophenol. Insulin binding was not eliminated in ATP-depleted muscle, but was reduced by 70--80%. Insulin binding by aerobic muscle could be resolved into two components; a high-affinity, low-capacity site (KD = 7.8 nM) and a low-affinity, high-capacity site (KD = 390 nM). The stimulatory effect of insulin on xylose uptake could be correlated with binding to the high-affinity site. These results indicate that there is some ATP-dependent process involved in the regulation of insulin binding by soleus muscle. It is suggested that this could be a phosphorylation-dephosphorylation system, acting either on the receptor itself or on some closely related membrane protein.

Calcium as modulator of the hormonal-receptors-biological-response coupling system. Effects of Ca2+ ions on the insulin activated 2-deoxyglucose transport in rat fat cells

Activation of 2-deoxyglucose transport in isolated rat fat cells by insulin is dependent upon the presence of Ca2+ in the external medium. When calcium concentration is kept below 100 micron, insulin acts like a partial agonist, giving only half of the maximal activation obtained normally with a millimolar concentration of this ion. Oxytocin, whose insulin-like action on adipocytes activates glucose oxidation by these cells, was found to be unable to affect the rate of 2-deoxyglucose transport. This, together with previous observations, suggests that calcium ions play a role in the mechanism of insulin action possibly by binding selectively to membrane sites involved in the transmission of the hormonal message to the glucose carrier. Oxytocin seems to trigger only intracellular glucose metabolism and it appears that there is an absolute requirement for calcium ions in the activation of a still unknown membrane signal.

Permissive effect of ATP on insulin-stimulated sugar transport by rat soleus muscle

The stimulatory effect of insulin (0.1 U/ml) on D-xylose uptake was progressively lost when rat soleus muscles were preincubated at 37 degrees C under anaerobic conditions for longer than 30 min; after 90 min these muscles were completely insensitive to insulin. This effect was associated with the loss of muscle ATP. When the breakdown of ATP was retarded either by lowering the preincubation temperature or by preincubation with 5 mM glucose, the effect of insulin in anaerobic muscle was correspondingly prolonged. Under certain conditions, externally added ATP promoted an effect of insulin in otherwise insulin-unresponsive muscles. This effect was small in magnitude and was complicated by the degradation of the added ATP in the incubation medium and by the fact that ATP also tended to inhibit insulin-stimulated xylose uptake. These results indicate that there is a relationship between insulin-stimulated sugar transport and muscle ATP levels. This supports the proposal that there may be some ATP-dependent reaction(s) involved in the mechanism whereby insulin promotes the process of muscle sugar transport.

Retention of insulin-stimulated D-glucose transport activity by adipocyte plasma membranes following extraction of extrinsic proteins

Plasma membrane vesicles prepared from adipocytes incubated with insulin exhibited accelerated D-glucose transport activity characteristic to insulin action on intact fat cells. Both control and insulin-stimulated D-glucose transport activities were inhibited by cytochalasin B and thiol reagents. Extraction of plasma membranes with dimethylmaleic anhydride eluted 80% of the protein from plasma membrane vesicles. The two major glycoprotein bands (94,000 and 78,000 daltons) and small amounts of a 56,000-dalton band were retained in dodecyl sulfate gels of the extracted membranes. Both control and insulin-activated D-glucose transport activities were retained by plasma membrane vesicles extracted with dimethylmaleic anhydride. Cytochalasin B binding activity was also retained by extracted membrane vesicles and D-glucose uptake into extracted vesicles derived from untreated or insulin-treated fat cells was inhibited by cytochalasin B. These results suggest that the modification of the adipocyte hexose transport system elicited by insulin action is not altered by a major purification step which involves quantitative extraction of extrinsic membrane proteins.

The combined action of insulin and phlorizin on transport and metabolism of sugars and nucleotide turnover in the isolated rat diaphragm

The combined effects of insulin and phlorizin have been analyzed on two parameters of insulin stimulation in the surviving rat diaphragm : transport and metabolism of sugars, turnover of the phosphate groups of mononucleotides. Phlorizin (c mM) inhibits glucose transport both in the presence and absence of insulin and displays a small additional inhibitory effect on glycogen biosynthesis; with the non metabolizable glucose anlogue 3-O-methyl-D-glucose transport inhibition is demonstrated solely in the presence of insulin. No correlation is demonstrated between the rates of sugar transport, which are strongly phlorizin-sensitive, and the rates of nucleotide turnover, which show no such sensitivity.

Effect of anoxia, 2,4-dinitrophenol and salicylate on xylose transport by isolated rat soleus muscle

1. These studies examined the theory that ATP served to regulate muscle sugar transport by a feedback mechanism. Xylose uptake by isolated rat soleus muscle was determined over a 5-min period following preincubation at 37 degrees C for various times in the presence of insulin (0.1 unit/ml), 2,4-dinitrophenol (0.5 or 0.05 mM) or salicylate (5 mM) or under anaerobic conditions. 2. Xylose uptake, measured in freshly isolated soleus muscles, was approximately 3.5--4.0 mumol/g per h. When the muscles were preincubated at 37 degrees C, this rate fell by 50% during the first 30 min and then slowly increased. 3. The stimulatory effect of insulin was evident within 2 min in freshly isolated soleus muscle and increased on preincubation, reaching a maximum value (approx. 14 mumol/g per h) after 20 min. 4. There was a 10-min lag period before xylose uptake was stimulated by anoxia. This lag period was approximately doubled when the incubation temperature was lowered from 37 degrees C. The stimulatory effect of anoxia was promptly reversed when muscles were transferred from anaerobic to aerobic conditions. 5. There was a 5-min lag period before xylose uptake was stimulated by 2,4-dinitrophenol (0.05 mM) or by sodium salicylate (mM). At a concentration of 0.5 mM, 2,4-dinitrophenol stimulated xylose uptake in freshly isolated muscle. Whereas the stimulatory effects of insulin, anoxia and salicylate all tended to plateau with time, the effect of 2,4-dinitrophenol tended to peak and then decline. 6. There was no obvious relationship between total muscle ATP levels and xylose uptake. The stimulatory effect of anoxia, 2,4-dinitrophenol or salicylate on xylose uptake was not preceded by the fall in muscle ATP. Similarly, ATP levels did not change when xylose uptake was stimulated by anoxia at 27 degrees C, or when xylose uptake was restored to basal values by transferring muscles from anaerobic to aerobic conditions. 7. It was argued that the presence of the myofibrils could act as a permeability barrier, which would limit the access of ATP produced within the interior of the cell to a regulatory site on, or close to, the sarcolemma. On the other hand, it is conceivable that the ATP produced on the periphery of the fibre by the subsarcolemmal mitochondria could play a more specific role in the feedback regulation of sugar transport. 8. Insulin stimulated xylose uptake in the presence of 2,4-dinitrophenol (0.5 mM) when this was measured in freshly isolated muscle, but not after a period of preincubation. This suggested that there may be some ATP-dependent process involved in the stimulatory effect of insulin.

Methods for rapidly altering the permeability of mammalian cells

Various agents alter mammalian cells so that they rapidly become nonspecifically permeable to substances that ordinarily do not penetrate intact cells. Thus, toluene renders liver cells permeable to nucleotides and macromolecules. Tween 80 and Tween 60 act on similar fashion, and the effect is reversible. Dextran sulfate reversibly alters the permeability of Ehrlich ascites tumor cells, which offers a tool for studying the control of macromolecular syntheses and other processes. Brief exposure to external ATP alters the permeability of certain transformed mouse cells but not of untransformed cells. The effect of ATP is rapidly reversible.

Regulation of plasma membrane protein phosphorylation in two mammalian cell types

The appreciation of protein phosphorylation as a ubiquitous mechanism for the post-translational control of protein function has drawn our attention to the phosphorylation of plasma membrane proteins. We have studied this phenomenon in the human erythrocyte and rat adipocyte, and have observed several features, common to the two systems, which may be of general significance. In examining protein phosphorylation in intact cells incubated with 32Pi, it is evident that the 32P-polypeptides of the plasma membrane are among the most highly labelled species in the cell, despite their minor contribution to overall protein content. The addition of epinephrine (to adipocytes) or cAMP (to erythrocytes) increases the phosphorylation of certain peptides, whereas others are unaffected. The protein kinases mediating these phosphorylations are present in the plasma membrane as isolated, and can be divided into two groups--cAMP dependent and cAMP independent. These two classes of kinase differ markedly in their substrate specificity toward endogenous and exogenous polypeptide substrates. Two classes of protein kinases with similar properties can be detected in the cytoplasm. The relationship between the membrane-bound and cytoplasmic enzymes is uncertain. The potential roles of the plasma membrane cAMP dependent protein kinases are evident from the diverse effects of cAMP on surface properties; however, the prevalence of plasma membrane proteins phosphorylated via cAMP independent pathways is striking. Thus, elucidation of the regulatory properties of the plasma membrane cAMP independent protein kinases may give new insight into the control of a variety of surface phenomena not mediated by cAMP.

Transport of D-allose by isolated fat-cells: an effect of adenosine triphosphate on insulin stimulated transport

D-allose, a glucose analogue, is not metabolized by isolated fat-cells and its distribution space at equilibrium in the cells is the same as that of triated water. Uptake of allose is inhibited by glucose and 3-O-methylglucose, stimulated by insulin and virtually eliminated by cytochalasan B. Counter transport of allose out of fat-cells against a concentration gradient can be induced by exogenous glucose but not by pyruvate. It is concluded that allose is transported into fat-cells by the same carrier mediated transport system as glucose and that it is a suitable analogue with which to study the glucose transport system. Insulin stimulated allose transport, into or out of the cell, but not basal transport, is inhibited by a brief exposure of isolated fat-cells to exogenous ATP or ADP (but not AMP or AMP-PNP). The antilipolytic effect of insulin is not affected. The ATP inhibition is slowly reversible. It is suggested that ATP phosphorylates a membrane component and thereby blocks transmission of signal from the insulin receptor to the carrier system. Indirect evidence suggests that ATP does not alter the affinity of the insulin or glucose binding sites. Insulin decreases the Km of glucose metabolism of CO2 and lipid in isolated fat-cells and increases the Vmax. However,the hormone has no effect on the Ki of glucose as an inhibitor of allose transport. The glucose analogue, 3-O-methyl-glucose, also inhibits both glucose metabolism and allose transport. The Ki for both these processes is similar and is not affected by insulin. These results support the view that the effect of insulin on glucose transport is to raise the Vmax without a change in the Km. It appears further that sugar transport is not the major rate limiting step in metabolism at high glucose concentrations in the absence of insulin, or at most glucose concentrations in the presence of the hormone.

Regulation of the D-glucose transport system in isolated fat cells

Recent technical advances have yielded considerable new biochemical insights into the hexose transport systems of both brown and white fat cells. In the present studies a novel filtration method was used to monitor initial rates of 3-O-(3H)methylglucose uptake in isolated white fat cells. Transport of 3-O-methylglucose, a non-metabolizable analogue of glucose, occurred by facilitated diffusion, was inhibited by glucose, phloridzin, cytochalasin B and dipyridamole, and was rapidly stimulated by insulin as well as lectins. Total 3-O-methylglucose uptake in white fat cells could be attributed to two kinetically distinct processes in addition to a certain degree of diffusion. Two important new features of glucose transport in fat cells have been discovered. First, in both brown and white fat cells transport per se does not appear to be necessarily rate-limiting for further glucose metabolism. Thus vitamin K5, which markedly increases glucose oxidation by brown fat cells, did not affect the glucose transport system activity. Glucose utilization can apparently be significantly enhanced in fat cells by agents which either increase transport system activity or intracellular enzyme activity. Second, the transport system itself, whether in the basal state or after activation by insulin, lectins, or oxidants, is resistant to sulfhydryl reagents such as N-ethylmaleimide, while the increase in transport activity due to these agents is exquisitely sensitive to sulfhydryl blockage. N-ethylmaleimide blocks the stimulatory effect of insulin on transport whereas addition of insulin to fat cells prior to the reagent completely protects against this inhibitory effect. Further, N-ethylmaleimide prevents the elevated rates of transport system activity due to insulin (or other agents) from returning to basal levels once the cells are washed free of hormone. These data are consistent with the concept that activation of the transport system involves oxidation of key membrane sulfhydryls to the disulfide form, but alternative models are also possible. In any case, these findings provide a possible biochemical clue for future studies designed to identify the specific component(s) involved in the regulatory mechanism which modulates transport of glucose in isolated fat cells.

The content of water and potassium in fat cells

The distribution spaces at equilibrium for 3H2O, [14C]urea and s-O-[14C]-methylglucose were measured in white fat cells using centrifugation through silicone oil at 2500 X g; no significant differences were observed. L-[14C]Glucose added immediately before the centrifugation was used as a marker for the extracellular water space. The calculated intracellular water content of the cells after the centrifugation through oil (e.g. 3H2O space minus L-[14C]glucose space) is an unbiased measure of the water content of the fat cells in suspension as judged by the following criteria: (1) The intracellular distribution space for 3-O-[14C]'methylglucose at equilibrium (methylglucose space minus L-glucose space) was not different from that calculated from a methylglucose wash-out curve. (2) The intracellular content of L-[14C]glucose (half time of efflux about 60 min) in cells preloaded during incubation of the tissue with collagenase was not different in cells recovered by (a) centrifugation through oil at 2500 X g, (b) centrifugation through oil at 600 X g, (c) centrifugation at 600 X g in the absence of oil and (d) filtration on Millipore filters. The intracellular content of water determined on cells from single rats weighing 120-150 g was 2.75 +/- 0.55 mul/100 mul fat cells (+/- S.D., n = 30). The intracellular content of potassium, determined on cells from the same rats, was 252 +/- 62 nmols/100 mul fat cells (+/- S.D., n = 30). The concentration of potassium in the intracellular water was calculated as 104 +/- 15 mM (+/- S.D., n = 30).

Regulation of uracil uptake in Escherichia coli by adenosine 3',5'-monophosphate

Culture of a wild-type strain of Escherichia coli in the presence of cyclic AMP leads to an impairment of uracil uptake. Half maximum inhibition of uracil uptake was observed at 1.5 mM cyclic AMP. The effect seems to be specific since no inhibition was found in cultures supplemented with ATP, ADP or 5'-AMP. Similarly the inhibition was not observed in cultures of a mutant deficient in the cyclic AMP receptor protein. The inhibition in uracil uptake, found in bacteria cultured in the presence of cyclic AMP, is not a consequence of a reduction in the growth rate. On the other hand, this inhibition was observed only in cultures containing glucose or pyruvate as carbon source.

Effect of exogenous ATP on sodium transport in mammalian red cells

The effect of exogenous adenosine triphosphate (ATP) and other nucleotides on the transport of Na in various mammalian red cells has been studied. While they have no effect on the transport of Na in human and cat red cells, in dog red cells adenosine and its mono-, di- and triphosphorylated forms were found to increase Na-influx. Of these, ATP has the most striking effect, causing a more than 8-fold increase at a concentration of 0.6 mM and exerting this effect at a dose range of 10(-5) to 10(-3) M. The effect of ATP is rapid (less than 5 minutes) and can be reversed by washing or the addition of calcium or magnesium. In contrast to the adenosine series other phosphorylated nucleotides (GTP, CTP, UDP, GDP and cAMP) have no effect. The well known volume dependent Na-transport in these cells is reversed in the presence of 0.6 mM ATP. It is suggested that ATP acts on passive cation movements either by chelation of membrane charge or by a direct interaction with membrane proteins and may be involved in the volume regulation of cation transport in the dog erythrocyte.

Irreversible stimulation of adenylate cyclase activity of fat cell membranes of phosphoramidate and phosphonate analogs of GTP

The ability of 5'-guanylylimidodiphosphate (Gpp(NH)p) to stimulate irreversibly the adenylate cyclease activity of fat cell membranes has been studied by preincubating the membranes with this or related analogs followed by assaying after thoroughly washing the membranes. Activation can occur in a simple Tris-HCl buffer, in the absence of added divalent cations and in the presence of EDTA. Dithiothreitol enhances the apparent degree of activation, perhaps by stabilization. The importance of utilizing optimal conditions for stabilizing enzyme activity, and of measuring the simultaneous changes in the control enzyme, is illustrated. The organomercurial, p-aminophenylmercuric acetate, inhibits profoundly the activity of the native as well as the Gpp(NH)p-stimulated adenylate cyclase, but in both cases subsequent exposure to dithiothreitol restores fully the original enzyme activity. However, the mercurial-inactivated enzyme does not react with Gpp(NP)p, as evidenced by the subsequent restoration of only the control enzyme activity upon exposure to dithiothreitol. Thus, reaction with Gpp(NH)p requires intact sulfhydryl groups, but the activated state is not irreversibly destroyed by the inactivation caused by sulfhydryl blockade. GTP and, less effectively, GDP and ATP inhibit activation by Gpp(NH)p, but interpretations are complicated by the facts that this inhibition is overcome with time and that GTP and ATP can protect potently from spontaneous inactivation. These two nucleotides can be used in the Gpp(NH)p preincubation to stabilize the enzyme. The Gpp(NH)p-activated enzyme cannot be reversed spontaneously during prolonged incubation at 30 degrees C in the absence or presence of GTP, ATP, MgCl2, glycine, dithiothreitol, NaF or EDTA. The strong nucleophile, neutral hydroxylamine, decreases the Gpp(NH)p-activated enzyme activity and no subsequent activation is detected upon re-exposure to the nucleotide.

Studies on plasma membranes. XXIII. Hormone-like action of concanavalin A on liver plasma membranes: inhibition of (Na+-K+)ATPase

Concanavalin A inhibits the (Na+-K+)-ATPase activity of isolated rat-liver plasma membranes, while leaving the Mg2+-ATPase unaffected. Glucagon and cyclic AMP act supplementary to the lectin in the inhibition. The lectin effect is counteracted by insulin and L-epinephrine, and is completely abolished by the beta-adrenergic blocking agent propranolol. Results are discussed on the basis of the known interactions of concanavalin A with plasma membrane components, including its hormone-like action.