Hexose transport in human adipocytes: factors influencing the response to insulin and kinetics of methylglucose and glucose transport

Historical perspectives in fat cell biology: the fat cell as a model for the investigation of hormonal and metabolic pathways

For many years, there was little interest in the biochemistry or physiology of adipose tissue. It is now well recognized that adipocytes play an important dynamic role in metabolic regulation. They are able to sense metabolic states via their ability to perceive a large number of nervous and hormonal signals. They are also able to produce hormones, called adipokines, that affect nutrient intake, metabolism and energy expenditure. The report by Rodbell in 1964 that intact fat cells can be obtained by collagenase digestion of adipose tissue revolutionized studies on the hormonal regulation and metabolism of the fat cell. In the context of the advent of systems biology in the field of cell biology, the present seems an appropriate time to look back at the global contribution of the fat cell to cell biology knowledge. This review focuses on the very early approaches that used the fat cell as a tool to discover and understand various cellular mechanisms. Attention essentially focuses on the early investigations revealing the major contribution of mature fat cells and also fat cells originating from adipose cell lines to the discovery of major events related to hormone action (hormone receptors and transduction pathways involved in hormonal signaling) and mechanisms involved in metabolite processing (hexose uptake and uptake, storage, and efflux of fatty acids). Dormant preadipocytes exist in the stroma-vascular fraction of the adipose tissue of rodents and humans; cell culture systems have proven to be valuable models for the study of the processes involved in the formation of new fat cells. Finally, more recent insights into adipocyte secretion, a completely new role with major metabolic impact, are also briefly summarized.

Adipose tissue IL-6 content correlates with resistance to insulin activation of glucose uptake both in vivo and in vitro

Obesity and type 2 diabetes are associated with insulin resistance, the mechanisms of which remain poorly understood. A significant correlation between circulating IL-6 level and insulin sensitivity has recently been found in humans. Because adipose tissue could be a significant source of IL-6, we analyzed the relationship between the levels of adipose tissue IL-6 and insulin action in vivo, during a hyperinsulinemic normoglycemic clamp, and in vitro by measuring glucose transport in adipocytes from 12 obese subjects with (n = 7) or without (n = 5) diabetes. We observed an inverse correlation between adipose tissue IL-6 content and maximal insulin-responsiveness measured in vivo (P < 0.02) and in vitro (P < 0.02). Conversely, there was no significant correlation between these two later parameters and adipose tissue leptin or tumor necrosis factor-alpha protein contents. Furthermore, we showed, for the first time, the presence of immunoreactive IL-6 receptors in the plasma membrane of human abdominal sc adipocytes. This suggests that locally secreted IL-6 could act on adipocytes by an autocrine/paracrine mechanism. In conclusion, increased IL-6 production by sc adipose cells might participate to the insulin-resistant state observed in human obesity.

Tissue insulin sensitivity and body weight in polycystic ovary syndrome

OBJECTIVE

Polycystic ovarian syndrome (PCOS) and obesity both affect insulin sensitivity. This study was designed to investigate the biochemical indices of PCOS and tissue insulin sensitivity in groups of lean and obese women with clinically equivalent degrees of the syndrome, relative to control subjects.

DESIGN

A prospective study of in vivo parameters and in vitro study of adipocytes to assess insulin sensitivity.

PATIENTS

Six lean and 14 overweight patients fulfilling formal diagnostic criteria for PCOS were studied. The degree of hirsutism and amenorrhoea was similar in each group. Eight control subjects were also studied.

MEASUREMENTS

Endocrine and metabolic parameters were measured in lean and overweight patients with PCOS and control subjects. In vitro studies of adipocyte insulin receptor binding and adipocyte insulin action were performed.

RESULTS

The mean plasma LH level was elevated in both groups of PCOS but was significantly higher in the lean group (LH levels were 25.1 +/- 3.1 and 14.5 +/- 1.6 iu/l in lean PCOS and obese PCOS, respectively (P = 0.01)). There was a strong inverse correlation between BMI and LH levels (R = - 0.70, P = 0.001). Fasting insulin levels were elevated in both lean and obese groups (11.5 +/- 2.8 and 26.8 +/- 8.1 mU/l, respectively; P = 0.068). Mean serum testosterone and serum androstenedione levels were also elevated in PCOS compared to control subjects but there was no difference between the two groups of PCOS subjects. Insulin receptor binding in amenorrhoeic subjects with PCOS was low in both lean and obese patients with PCOS but was not significantly different between the two groups (0.79 +/- 0.17% and 0.66 +/- 0.07% per 10 cm2 cell membrane, respectively). Maximally insulin-stimulated rates of 3-O-methylglucose transport were low in both groups compared to previously studied normal subjects (0.96 +/- 0.21 and 0.64 +/- 0.07 pmol per 10 cm2 membrane in lean and obese PCOS subjects, respectively; P = NS).

CONCLUSIONS

Lean subjects with a given phenotypic expression of PCOS have an equivalent degree of tissue insulin resistance compared to obese PCOS subjects. This implies that the insulin resistance may be a primary feature of PCOS. If this is so, a similar clinical degree of the syndrome may be brought about by genetically determined insulin resistance in lean subjects or by insulin resistance which is secondary to obesity.

Adipocyte insulin action following ovulation in polycystic ovarian syndrome

The role of anovulation and insulin resistance in the pathogenesis of polycystic ovarian syndrome (PCOS) remains to be determined. The aim of this study was to investigate whether the metabolic abnormality of insulin resistance in PCOS reflects, rather than causes, the ovarian dysfunction. Eight subjects with classical PCOS were studied on two occasions. Adipocyte insulin sensitivity together with hormonal and metabolic changes were investigated in patients with PCOS following prolonged amenorrhoea and then again in the early follicular phase after ovulation. Insulin receptor binding in amenorrhoeic subjects with PCOS was low at 0.78 +/- 0.08% and this increased to 1.18 +/- 0.19% after an ovulatory cycle (P < 0.05). Maximal insulin stimulated 3-O-methylglucose uptake was 0.70 +/- 0. 14 during amenorrhoea and increased to 1.08 +/- 0.25 pmol/10 cm(2) cell membrane (P < 0.05). Plasma testosterone fell (4.0 +/- 0.4 to 2. 3 +/- 0.2 nmol/l; P < 0.001), luteinizing hormone fell (17.6 +/- 2.3 to 6.7 +/- 0.8 IU/l; P < 0.001) but plasma insulin concentrations remained unchanged following ovulation (14.6 +/- 1.9 and 15.7 +/- 3. 8 pmol/l during amenorrhoea and after ovulation respectively). The results of this study suggest that chronic anovulation per se appears to modify the factors contributing to cellular insulin resistance seen in PCOS.

Severe diabetic ketoacidosis: the need for large doses of insulin

A 21-year-old female with Type 1 diabetes mellitus (DM) presented in ketoacidosis. She received intravenous normal saline and insulin at 6 U/h and 1.26% sodium bicarbonate solution. After the blood glucose had fallen to 9.5 mmol/l, the saline infusion was changed to 5% glucose solution and the insulin infusion rate to 2 to 3 U/h. The next day the patient became more drowsy (Glasgow coma scale 13/15, later falling to 4/15). Computed tomography (CT) scan suggested cerebral oedema and the patient was treated with dexamethasone and mannitol. She remained critically ill for 48 h, eventually making a full recovery. Insulin was given at rates of 8 to 14 U/h, with 10% or 20% glucose infusion to maintain the blood glucose above 5 mmol/l; despite this it was not until the fifth day that her serum bicarbonate became normal. Textbooks usually advise starting insulin at 6 U/h and reducing the infusion rate to 1-4 U/h when the blood glucose falls below a certain level. In this case, even with high rates of insulin infusion, it took 5 days before the patient's serum bicarbonate returned to normal. Thus, in severe diabetic ketoacidosis (DKA), protocols should advise that the insulin infusion be continued at high dose (4 to 6 U/h or more), with appropriate glucose infusion to prevent hypoglycaemia, until the serum bicarbonate is normal or nearly so.

The effects of insulin on the level and activity of the GLUT4 present in human adipose cells

Human adipose cells are much less responsive to insulin stimulation of glucose transport activity than are rat adipocytes. To assess and characterize this difference, we have determined the rates of 3-O-methyl-D-glucose transport in human adipose cells and have compared these with the levels of glucose transporter 4 (GLUT4) assessed by using the bis-mannose photolabel, 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(D-mannos- 4-yloxy)-2-propyl-amine, ATB-BMPA. The rates of 3-O-methyl-D-glucose transport and the cell-surface level of GLUT4 are very similar in the human and rat adipocyte in the basal state. The Vmax for 3-O-methyl-D-glucose transport in fully insulin-stimulated human adipose cells is 15-fold lower than in rat adipose cells. Photolabelling of GLUT4 suggests that this low transport activity is associated with a low GLUT4 abundance (39 x 10(4) sites/cell; 19.9 x 10(4) sites at the cell surface). The turnover number for human adipose cell GLUT4 (5.8 x 10(4) min-1) is similar to that observed for GLUT4 in rat adipose cells and the mouse cell line, 3T3L1. Since 50% of the GLUT4 is at the cell surface of both human and rat adipose cells in the fully insulin-stimulated state, an inefficient GLUT4 exocytosis process cannot account for the low transport activity. The intracellular retention process appears to have adapted to release, in the basal state, a greater proportion of the total-cellular pool of GLUT4 to the cell surface of the larger human adipocytes. These cell-surface transporters are presumably necessary to provide the basal metabolic needs of the adipocyte.(ABSTRACT TRUNCATED AT 250 WORDS)

Severe impairment of insulin action in adipocytes from amenorrheic subjects with polycystic ovary syndrome

Adipose tissue was used to characterize the metabolic abnormality of insulin resistance in polycystic ovary syndrome (PCOS). Nine patients with PCOS were studied during a period of amenorrhea and confirmed to be chronically anovulatory by vaginal ultrasound and plasma progesterone measurements. These were compared with six age- and body mass index (BMI)-matched controls (BMI, 27.2 +/- 2.2 in PCOS and 24.7 +/- 2.3 in control subjects). Insulin receptor binding was measured and insulin action was assessed by measuring initial rates of 3-O-methylglucose uptake and by inhibition of lipolysis. The maximum specific insulin receptor binding was 0.62% +/- 0.12% and 1.78% +/- 0.18% per 10-cm2 cell surface (mean +/- SEM) in PCOS and control subjects, respectively (P < .001). Maximum rates of glucose transport were also impaired as compared with controls, with 3-O-methylglucose transport being 0.90 +/- 0.15 versus 1.57 +/- 0.28 pmol/10 cm2/5 s, respectively (P < .05). The concentration of insulin required for half-maximal stimulation of glucose uptake was 165 +/- 36 versus 32 +/- 10 pmol in PCOS and control subjects, respectively (P < .05). The maximum percentage lipolysis inhibition (mean +/- SEM) was 9.5% +/- 1.6% in PCOS and 28.3% +/- 7.2% in control patients, respectively (P < .01). These data demonstrate that there are both insulin binding and postreceptor defects in adipocytes from amenorrheic PCOS subjects. The degree of defect in adipocyte insulin action is greater than would have been anticipated from in vivo data.

The glucose transporter family: structure, function and tissue-specific expression

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Divergent regulation of the Glut 1 and Glut 4 glucose transporters in isolated adipocytes from Zucker rats

We have studied the relationship between glucose uptake rate and Glut 1 and Glut 4 protein and mRNA levels per fat cell in lean (FA/FA) and obese (fa/fa) Zucker rats at 5, 10, and 20 wk of age, and after induction of acute diabetes with streptozotocin. 5 wk obese rats exhibit insulin hyperresponsive glucose uptake, whereas 20 wk obese rats show insulin resistant glucose uptake. The relative abundance of Glut 1 and Glut 4 mRNA and protein per equal amount of total RNA and total membrane protein, respectively, is lower in adipocytes from obese rats. However, at all ages the enlargement of fat cells from obese rats is accompanied by a severalfold increase in total RNA and total membrane protein per cell. Thus, on a cellular basis, mRNA and protein levels of Glut 4 increases in young obese rats and gradually declines as a function of age. Basal glucose uptake is increased severalfold in fat cells from obese rats, and in parallel Glut 1 expression per cell in obese rats is two- to threefold increased over lean rats at all ages. Acute diabetes in 20 wk obese rats causes a profound downregulation of glucose uptake and a concomitant reduction of both Glut 1 and Glut 4 protein levels. Thus, changes in Glut 4 expression are a major cause of alteration in insulin-stimulated glucose uptake of adipocytes during evolution of obesity and diabetes in Zucker rats.

Effect of prolonged hyperinsulinaemia on adipocyte insulin binding and action in normal man

Prolonged moderate insulinaemia over 20 h in normal humans produces a significant reduction of in vivo insulin action. To examine the effects in man of such moderate hyperinsulinaemia on in vitro glucose metabolism, insulin-receptor binding, glucose transport and incorporation of glucose into lipid were determined in adipocytes isolated from paired gluteal fat biopsies, taken from six normal human volunteers before and 1 h after 20 h of exogenous hyperinsulinaemia (plasma insulin 38 +/- 3 mU/l). Specific binding of insulin to isolated adipocytes was not altered (6.7 +/- 0.7 versus 7.6 +/- 1.5% per 10(5) cells). Basal glucose transport of 3-O-[14C]methylglucose in the absence of insulin was reduced after hyperinsulinaemia (5.2 +/- 1.1 versus post 4.3 +/- 1.3 pmol/7 s per 10(5) cells, 0.1 less than P greater than 0.05; or expressed as percent of maximal response: 60 +/- 5 versus post 49 +/- 1%, P less than 0.05), but maximal transport (9.2 +/- 1.5 versus post 8.7 +/- 1.5 pmol/7 s per 10(5) cells) and ED50 (87 +/- 17 versus 67 +/- 15 pmol/l) were unchanged. Conversion of glucose into lipid, using 3-D-[3H]glucose, was unchanged basally, at maximal response or ED50 of the dose response curve. In conclusion, moderate 20 h in vivo hyperinsulinaemia in normal humans induces only a small change in basal vitro adipocyte glucose transport, and no change in insulin-receptor binding or in vitro incorporation of glucose into lipid. These data suggest that the adipocyte does not contribute to the impaired insulin action produced by in vivo moderate hyperinsulinaemia in man.

The effects of metformin on adipocyte insulin action and metabolic control in obese subjects with type 2 diabetes

To investigate the mechanisms of action of metformin, insulin receptor binding and the activity of several insulin-controlled metabolic pathways were measured in adipocytes taken from 10 obese Type 2 diabetic patients treated for 4 weeks with either metformin (0.5 g x 3 daily) or matching placebo using a double-blind crossover design. Metformin therapy was associated with a significant fall in serum fructosamine levels (3.1 +/- 0.4 vs 2.8 +/- 0.4 mmol l-1, p less than 0.02) as well as fasting (10.8 +/- 2.4 vs 9.4 +/- 2.1 mmol l-1) and daytime (11.5 +/- 2.4 vs 10.0 +/- 2.2 mmol l-1) plasma glucose concentrations (p less than 0.05). Fasting and postprandial plasma levels of C-peptide and insulin were unchanged. While fasting plasma lactate concentrations remained unaltered after metformin, a rise was noted in response to meals (from 1.4 +/- 0.1 to 1.8 +/- 0.2 mmol l-1, p less than 0.05). Adipocyte insulin receptor binding was unaffected by drug treatment. Moreover, no insulin-like effects or post-binding potentiation of insulin action could be found on adipocyte glucose transport, glucose oxidation, lipogenesis, glycolysis or antilipolysis. A complementary in vitro study using adipocytes from non-obese healthy volunteers failed to show any direct effect of metformin on adipocyte insulin binding or glucose transport and metabolism, at media drug concentrations corresponding to therapeutic plasma levels.

A mixed meal potentiates the insulin sensitivity of glucose transport and metabolism in adipocytes from patients with type 2 diabetes mellitus

Post-glucose enhancement of insulin action may represent a physiological mechanism for the acute regulation of insulin sensitivity of target tissues. To clarify whether a similar mechanism is operative in the insulin-resistant diabetic state we have investigated the effects of a mixed meal on adipocyte insulin action in eight patients with Type 2 diabetes mellitus. Ninety minutes after ingestion of breakfast insulin binding to fat cells increased by 21% (p less than 0.05). In the fasting state 6 patients had a significant response of glucose transport and lipogenesis to insulin whereas two exhibited non-responsiveness. In the 6 responders insulin sensitivity, as estimated by the insulin concentration at which half-maximal effect was achieved, increased for glucose transport (before, 260 +/- 46 pmoll-1; after, 105 +/- 21 pmol l-1; p less than 0.05) and for lipogenesis (before, 36 +/- 9 pmol l-1; after, 9 +/- 2 pmol l-1; p less than 0.05). No significant changes occurred in basal or maximal glucose transport or lipogenesis. In the two primary non-responders intake of the meal was associated with average increase in maximal insulin responsiveness of 52% for glucose transport and 28% for lipogenesis. Intake of a mixed meal is associated with a slight increase of insulin binding to adipocytes from patients with Type 2 diabetes mellitus but a marked increase of adipocyte insulin sensitivity at the post-binding levels of glucose transport and metabolism.

Use of adipose tissue for metabolic studies

Open biopsy of adipose tissue from volunteer subjects has led to a greater understanding of the mechanisms of adipose-tissue insulin resistance in various clinical states. Studies of adipose tissue obtained during surgical operations have allowed development of techniques and exploration of adipocyte physiology. This has been particularly valuable in examining the relationship between cellular insulin binding and action. Examination of the lipid stores and of enzyme activities has been possible by using the more convenient technique of needle biopsy. Regional differences in adipose tissue metabolism have been identified and must be considered in experimental design. It is now clear that the insulin sensitivity of any one metabolic pathway does not necessarily reflect that of others, and care must be taken to avoid inappropriate extrapolation of data both between metabolic pathways in the adipocyte itself and from the adipocyte to the whole body.

Fasting-mediated alteration studies in insulin action on lipolysis and lipogenesis in obese women

The effects of fasting on insulin-induced antilipolysis and lipogenesis were investigated in vitro in isolated human fat cells of 11 obese females. Glycerol release and lipogenesis were determined simultaneously in the same test tube and related to methylglucose transport and specific insulin binding. Insulin binding and sensitivity and the responsiveness (maximum effect) of insulin-induced antilipolysis were enhanced by fasting. The latter was strongly correlated with an enhancement in the lipolysis rate. The effects of fasting on antilipolysis were not dependent on the glucose concentration, unlike insulin-stimulated lipogenesis. At 1 mumol/l of glucose, where hexose transport is rate limiting, sensitivity and responsiveness of insulin-induced lipogenesis were inhibited by fasting. Similar results were obtained with methylglucose transport. At 1-10 mmol/l of glucose, where hexose metabolism is rate limiting, insulin stimulated lipogenesis before fasting but was totally ineffective after fasting. In conclusion, fasting induces multiple alterations in insulin action on lipolysis and lipogenesis in adipocytes. Antilipolysis is enhanced because of stimulation at the receptor and postreceptor levels, which may be associated with an enhanced rate of lipolysis. Fasting inhibits the lipogenic effect of insulin due to postreceptor changes involving both transport and metabolism of glucose, making lipogenesis unresponsive to insulin at physiological glucose concentrations.

Monoclonal antibodies to the human insulin receptor that activate glucose transport but not insulin receptor kinase activity

Three mouse monoclonal antibodies were produced that reacted with the alpha subunit of the human insulin receptor. All three both immunoprecipitated 125I-labeled insulin receptors from IM-9 lymphocytes and competitively inhibited 125I-labeled insulin binding to its receptor. Unlike insulin, the antibodies failed to stimulate receptor autophosphorylation in both intact IM-9 lymphocytes and purified human placental insulin receptors. Moreover, unlike insulin, the antibodies failed to stimulate receptor-mediated phosphorylation of exogenous substrates. However, like insulin, two of the three antibodies stimulated glucose transport in isolated human adipocytes. One antibody, on a molar basis, was as potent as insulin. These studies indicate, therefore, that monoclonal antibodies to the insulin receptor can mimic a major function of insulin without activating receptor kinase activity. They also raise the possibility that certain actions of insulin such as stimulation of glucose transport may not require the activation of receptor kinase activity.

Distribution of glucose transporters in membrane fractions isolated from human adipose cells. Relation to cell size

We examined insulin's effects on glucose transport and on subcellular transporter distribution in isolated human omental adipocytes of various sizes. Insulin stimulated 3-O-methylglucose transport by twofold in small cells, while a smaller and insignificant effect was measured in large cells. In the small cells, basal concentrations of glucose transporters were 2.9 and 17.2 pmol/mg membrane protein in the plasma and the low density microsomal membranes, respectively. Increasing cell size was associated with a 50% decrease in the concentration of transporters in each fraction, with no change in their total number per cell. Insulin stimulated the translocation of transporters from the intracellular pool to the plasma membranes, irrespective of cell size. Thus, insulin resistance at the postreceptor level, observed in human obesity, may be associated with a relative depletion of total transporters per cell together with a reduction in their intrinsic activity at the plasma membrane level.

Postbinding defects of insulin action in human adipocytes from uremic patients

It is now well established that longstanding human uremia is associated with impaired in vivo insulin action on glucose utilization of peripheral target tissues. In an attempt to define the cellular basis of the uremic insulin resistance we studied insulin action in adipocytes from eight patients with undialyzed chronic uremia and from eight matched healthy controls. (125I)-Insulin binding to fat cells from uremic patients was normal. In contrast (14C)-D-glucose transport exhibited decreased sensitivity to insulin. The concentrations of insulin that elicited half-maximal response was 422 +/- 95 pmoles/liter in uremic patients and 179 +/- 38 pmoles/liter in normal subjects (P less than 0.01). The noninsulin- and the maximal insulin-stimulated glucose transport of adipocytes from uremic patients with normal. (14C)-D-glucose conversion to total lipids was also measured in these cells in the absence and presence of various insulin concentrations. Similar to the findings in transport studies the lipogenesis of fat cells from uremic patients had depressed sensitivity to insulin (half-maximal stimulation at 38 +/- 8 pmoles/liter in uremic patients and at 11 +/- 3 pmoles/liter in normal subjects, P less than 0.01) with unchanged noninsulin and maximal insulin-stimulated lipogenesis. Taken together these results suggest that the insulin resistance of adipocytes from patients with chronic uremia may be accounted for primarily by postbinding defects localized to glucose transport and metabolism.

Subcellular translocation of glucose transporters: role in insulin action and its perturbation in altered metabolic states

In this article we have described the hypothesis that insulin stimulates glucose transport through glucose transporter translocation from an intracellular pool to the plasma membrane. In addition, we have shown that changes in the numbers and subcellular distributions of glucose transporters correlate with alterations in insulin-stimulated glucose transport activity in several experimental models of insulin resistance and hyperresponsiveness. However, in experiments with counterregulatory hormones and with hyperresponsive states induced by nutritional repletion following deprivation, changes in insulin responsiveness cannot be fully explained by such alterations in the numbers and/or subcellular distribution of glucose transporters. Thus, evidence has been presented for changes in glucose transporter intrinsic activity that both inhibit and augment insulin-stimulated glucose transport rates. Finally, we have discussed data suggesting that the translocation process is applicable to human tissue and that significant changes in adipose cell glucose transport activity have been correlated with total glucose disposal in various metabolic states in humans. Determining the physiologic factors involved in modulating these events at the cellular level is an important area for further investigation.

Improvement in in vitro insulin action after one month of insulin therapy in obese noninsulin-dependent diabetics. Measurements of glucose transport and metabolism, insulin binding, and lipolysis in isolated adipocytes

It has been previously reported that maximum insulin-stimulated glucose transport and utilization were both decreased, while basal lipolysis was increased in adipocytes from obese subjects with noninsulin-dependent diabetes mellitus (NIDDM). To determine whether these values can be returned towards those obtained in equally obese subjects with normal glucose tolerance, these measures of adipocyte metabolism were quantified in 10 NIDDM subjects before and after control of hyperglycemia with insulin. The results demonstrate that maximum insulin-stimulated glucose transport (P less than 0.02) and glucose incorporation into triglyceride (P less than 0.01) and CO2 (P less than 0.05) (at 5.5 mM glucose) increased and basal lipolysis decreased (P less than 0.05) after 4 wk of insulin treatment. In contrast, glucose incorporation into lactate and other glycolytic metabolites (at 5.5 mM glucose), and sensitivity of glucose transport to insulin, did not improve with insulin therapy. The latter occurred despite an increase in insulin binding (P less than 0.01). Finally, the improvement in maximal insulin-stimulated glucose transport correlated with the fall in fasting hyperglycemia (r = 0.77, P less than 0.01). These findings demonstrate that several of the abnormalities of carbohydrate and lipid metabolism recently noted to be present in adipocytes from patients with NIDDM can be shown to significantly improve with insulin treatment.

Increased insulin binding to adipocytes and monocytes and increased insulin sensitivity of glucose transport and metabolism in adipocytes from non-insulin-dependent diabetics after a low-fat/high-starch/high-fiber diet

Nine non-insulin-dependent diabetics were studied before and after 3 weeks on an isoenergetic high-fiber/high-starch/low-fat diet (alternative diet), and nine non-insulin-dependent diabetics were studied on their usual diet. In the group that ate the alternative diet, the intake of fiber and starch increased 120% and 53%, whereas fat intake decreased 31%. Diabetes control improved as demonstrated by decreased fasting plasma glucose (P less than 0.05) and 24-hour urinary glucose excretion (P less than 0.05). The in vivo insulin action increased (KIVITT increased, P less than 0.05) with no change in fasting serum insulin levels. In fat cells obtained from patients in the alternative-diet group, insulin receptor binding increased (P less than 0.05) after the change of diet. Insulin binding to purified monocytes (more than 95% monocytes) also increased (P less than 0.05), whereas no change was found in insulin binding to erythrocytes. When lipogenesis was studied at a tracer glucose concentration at which glucose transport seems to be rate limiting, insulin sensitivity increased (P less than 0.02). This is the predicted consequence of increased receptor binding. Moreover, when CO2 production and lipogenesis were studied at a higher glucose concentration, where steps beyond transport seem to be rate limiting for glucose metabolism, increased insulin sensitivity was also observed. In contrast, no change was found in maximal insulin responsiveness. Fat and blood cells from the patients who continued on their usual diet showed no changes of the mentioned quantities.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro insulin resistance of human adipocytes isolated from subjects with noninsulin-dependent diabetes mellitus

To assess possible cellular mechanisms of in vitro resistance in noninsulin-dependent diabetes mellitus (NIDDM), maximum insulin-stimulated glucose transport and utilization and insulin binding were measured in adipocytes isolated from weight-matched normal glycemic subjects and patients with NIDDM. Glucose transport rate was determined by measuring the amount of [U-14C]-D-glucose taken up by incubating adipocytes at trace concentrations of glucose (300 nM), and glucose metabolism by estimating the amount of lactate, CO2, triglyceride, and total glucose carbons retained in the cells following incubating at 5.5 mM glucose. Insulin binding was measured at 50, 100, and 200 pM [mono125I-tyrosinyl A14]insulin. Both maximum insulin-stimulated glucose transport and utilization in adipocytes from diabetic subjects were 40% (P less than 0.01) and 32% (P less than 0.05) lower, respectively, than values obtained for subjects with normal glucose tolerance. In addition, the maximum capacity of glucose transport was correlated with the maximum capacity of glucose utilization (r = 0.81, P less than 0.001). Furthermore, fasting plasma glucose concentrations of diabetic subjects were negatively correlated with both maximum insulin-stimulated glucose transport (r = -0.56, P less than 0.05) and glucose utilization (r = -0.67, P less than 0.05). Since basal glucose transport in adipocytes from diabetic subjects was also 33% lower than in adipocytes from normal subjects, there was no change in the relative ability of insulin to stimulate glucose transport. However, there was a 64% decrease in the sensitivity of the glucose transport system to insulin (P less than 0.05), unrelated to concomitant changes in insulin binding. These results demonstrate that both maximal insulin-stimulated glucose transport and utilization, and the sensitivity of the glucose transport system to insulin, was decreased in adipocytes isolated from subjects with NIDDM. These in vitro defects were associated with impaired glucose metabolism in vivo, consistent with the view that the metabolic alterations observed at the cellular level may contribute to the in vivo insulin resistance of NIDDM.

Calcium-dependence of insulin receptor phosphorylation

Phosphorylation of the insulin receptor of isolated rat adipocytes in response to insulin has been studied. Immunoprecipitation of adipocyte membrane protein demonstrated increased incorporation of 32P after exposure to insulin for 15 min, but this was dependent on the presence of physiological concentrations of Ca2+ and Mg2+. Autoradiography of solubilized immunoprecipitated membrane protein after sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed that most of the 32P incorporation occurred in a band corresponding to Mr 95 000, which has been identified previously as the beta-subunit of the insulin receptor. 32P incorporation was inhibited by 2,4-dinitrophenol and trifluoperazine. It is suggested that insulin-receptor phosphorylation is an energy-requiring process that is Ca2+-dependent and may be modulated by calmodulin. Phosphorylation may proceed independently of glucose transport.

Marked increase in insulin sensitivity of human fat cells 1 hour after glucose ingestion

The effect of glucose ingestion on insulin action was investigated in isolated human fat cells. Subcutaneous adipose tissue was obtained from eight normal adult volunteers before and 1 h after oral intake of 100 g of glucose. Lipolysis (glycerol release) and specific insulin receptor binding were determined. Insulin binding increased significantly by 20-30% after glucose ingestion. This was due to an increase in insulin binding affinity, without any change in the receptor number. The concentration of insulin producing half-maximum inhibition (ED(50)) of basal lipolysis was 50 muU/ml before and 0.25 muU/ml after glucose ingestion (P < 0.01), which represented a 200-fold difference. As regards isoprenaline-induced lipolysis, the ED(50) for insulin inhibition was 30 muU/ml before and 2.5 muU/ml after oral glucose (P < 0.01), which was a 12-fold difference. The maximum insulin-induced inhibition of basal and isoprenaline-induced lipolysis were not altered after oral glucose. It is concluded that glucose ingestion is accompanied by a marked increase in insulin sensitivity of human fat cells and this may be an important modulating factor in the overall scheme of insulin action.

Insulin receptor binding and insulin action in human fat cells: effects of obesity and fasting

We have studied (125I)-insulin binding and insulin dose response relationships of (14C)-methylglucose transport conversion of (14C)-glucose to CO2 and total lipids, and lipolysis at 37 degrees C and pH 7.4 in adipocytes from obese patients before (n = 15) and after fasting for 10 days (n = 6). Studies of adipocytes from obese before fasting showed a significant reduction of insulin binding when expressed to cell surface area and rightward shifts of the insulin dose response curves (decreased insulin sensitivity) for glucose transport, glucose oxidation, lipogenesis and antilipolysis. The decreased insulin sensitivity of adipocytes from obese was most likely the functional consequence of the impaired insulin binding. Moreover, decreased maximal glucose transport capacities were present in rat cells from obese both in the basal and maximally insulin stimulated states. Similarly, the percentage response above basal level to maximal insulin stimulation of glucose oxidation and lipogenesis was impaired to these cells. The latter findings suggest post receptor defects localized both to the transport system per se and to intracellular mechanisms involved in the metabolism of glucose. Conversely, the post receptor pathways for the insulin induced antilipolysis was intact in fat cells from obese man. Studies after fasting showed an increase of adipocyte insulin binding accompanied by an increased sensitivity to the antilipolytic effect of insulin with unchanged maximal responsiveness. However, due to marked post receptor alterations, the insulin stimulated glucose utilization was severely blunted. Thus, the glucose transport system of adipocytes from all fasted subjects was totally unresponsive to insulin, while some of the fasted patients had a slight response of glucose oxidation and lipogenesis in the presence of insulin in maximally effective concentrations.

Insulin binding and action on fat cells from young healthy females and males

Insulin binding and action were studied in fat cells from the gluteal region of young healthy subjects. Fat cells from females were larger than those of males, had higher insulin receptor binding and higher rates of noninsulin-stimulated and maximally insulin-stimulated rates of methylglucose transport and glucose metabolism when these data were expressed per cell number. However, when insulin binding and insulin effects were expressed per cell surface, which may be physiologically more relevant, no sex differences were found in insulin binding and glucose transport, whereas noninsulin-stimulated and maximally insulin-stimulated glucose metabolism was still significantly increased in female fat cells. The latter indicates postreceptor differences in glucose metabolism between females and males. The insulin concentrations causing half-maximal responses (a measure of the sensitivity to insulin) of glucose transport, glucose metabolism and lipolysis were similar in fat cells from the two sexes, which is consistent with the comparable values of insulin receptor binding when adjusted to cell surface. Studies of rate-determining steps for the glucose utilization of human fat cells showed that glucose transport was not the rate-limiting step at physiological glucose concentrations. Moreover, at physiological glucose levels, glucose metabolism exhibited a decreased maximal insulin responsiveness and an increased insulin sensitivity when compared with glucose metabolism at low glucose concentrations at which glucose transport is rate limiting for the fat cell glucose utilization.