J M Olefsky

Receptor and Postreceptor Defects Contribute to the Insulin Resistance in Noninsulin-dependent Diabetes Mellitus

We have assessed the mechanisms involved in the pathogenesis of the insulin resistance associated with impaired glucose tolerance and Type II diabetes mellitus by exploring, by means of the euglycemic glucose-clamp technique, the in vivo dose-response relationship between serum insulin and the overall rate of glucose disposal in 14 control subjects; 8 subjects with impaired glucose tolerance, and 23 subjects with Type II diabetes. Each subject had at least three studies performed on separate days at insulin infusion rates of 40, 120, 240, 1,200, or 1,800 mU/M2 per min. In the subjects with impaired glucose tolerance, the dose-response curve was shifted to the right (half-maximally effective insulin level 240 vs. 135 microunits/ml for controls), but the maximal rate of glucose disposal remained normal. In patients with Type II diabetes mellitus, the dose-response curve was also shifted to the right, but in addition, there was a posal. This pattern was seen both in the 13 nonobese and the 10 obese diabetic subjects. Among these patients, an inverse linear relationship exists (r = -0.72) so that the higher the fasting glucose level, the lower the maximal glucose disposal rate. Basal rates of hepatic glucose output were 74 +/- 4, 82 +/- 7, 139 +/- 24, and 125 +/- 16 mg/M2 per min for the control subjects, subjects with impaired glucose tolerance, nonobese Type II diabetic subjects, and obese Type II diabetic subjects, respectively. Higher serum insulin levels were required to suppress hepatic glucose output in the subjects with impaired glucose tolerance and Type II diabetics, compared with controls, but hepatic glucose output could be totally suppressed in each study group. We conclude that the mechanisms of insulin resistance in patients with impaired glucose tolerance and in patients with Type II noninsulin-dependent diabetes are complex, and result from heterogeneous causes. (a) In the patients with the mildest disorders of carbohydrate homeostasis (patients with impaired glucose tolerance) the insulin resistance can be accounted for solely on the basis of decreased insulin receptors. (b) In patients with fasting hyperglycemia, insulin resistance is due to both decreased insulin receptors and postreceptor defect in the glucose mechanisms. (c) As the hyperglycemia worsens, the postreceptor defect in peripheral glucose disposal emerges and progressively increases. And (d) no postreceptor defect was detected in any of the patient groups when insulins ability to suppress hepatic glucose output was measured.

Mechanisms of insulin resistance in human obesity: evidence for receptor and postreceptor defects.

UNLABELLED To assess the mechanisms of the insulin resistance in human obesity, we have determined, using a modification of the euglycemic glucose clamp technique, the shape of the in vivo insulin-glucose disposal dose-response curves in 7 control and 13 obese human subjects. Each subject had at least three euglycemic studies performed at insulin infusion rates of 15, 40, 120, 240, or 1,200 mU/M2/min. The glucose disposal rate was decreased in all obese subjects compared with controls (101 +/- 16 vs. 186 +/- 16 mg/M2/min) during the 40 mU/M2/min insulin infusion. The mean dose-response curve for the obese subjects was displaced to the right, i.e., the half-maximally effective insulin concentration was 270 +/- 27 microU/ml for the obese compared with 130 +/- 10 microU/ml for controls. In nine of the obese subjects, the dose-response curves were shifted to the right, and maximal glucose disposal rates (at a maximally effective insulin concentration) were markedly decreased, indicating both a receptor and a postreceptor defect. On the other hand, four obese patients had right-shifted dose-response curves but reached normal maximal glucose disposal rates, consistent with decreased insulin receptors as the only abnormality. When the individual data were analyzed, it was found that the lease hyperinsulinemic, least insulin-resistant patients displayed only the receptor defect, whereas those with the greatest hyperinsulinemia exhibited the largest post-receptor defect, suggesting a continuous spectrum of defects as one advances from mild to severe insulin resistance. When insulins ability to suppress hepatic glucose output was assessed, hyperinsulinemia produced total suppresssion in all subjects. The dose-response curve for the obese subjects was shifted to the right, indicating a defect in insulin receptors. Insulin binding to isolated adipocytes obtained from the obese subjects was decreased, and a highly significant inverse linear relationship was demonstrated between insulin binding and the serum insulin concentration required for halfmaximal stimulation of glucose disposal. IN CONCLUSION (a) decreased cellular insulin receptors contribute to the insulin resistance associated with human obesity in all subjects; (b) in the least hyperinsulinemic, insulin-resistant patients, decreased insulin receptors are the sole defect, whereas in the more hyperinsulinemic, insulin-resistant patients, the insulin resistance is the result of a combination of receptor and postreceptor abnormalities; (c) all obese patients were insensitive to insulins suppressive effects on hepatic glucose output; this was entirely the result of decreased insulin receptors; no postreceptor defect in this insulin effect was demonstrated.

The Effect of Insulin Treatment on Insulin Secretion and Insulin Action in Type II Diabetes Mellitus

We have studied the effects of 3 wk of continuous subcutaneous insulin infusion (CSII) on endogenous insulin secretion and action in a group of 14 type II diabetic subjects with a mean (±SEM) fasting glucose level of 286 ± 1 7 mg/dl. Normal basal and postprandial glucose levels were achieved during insulin therapy at the expense of marked peripheral hyperinsulinemia. During the week of posttreatment evaluation, the subjects maintained a mean fasting glucose level of 155 ± 11 mg/dl off insulin therapy, indicating a persistent improvement in carbohydrate homeostasis. Adipocyte insulin binding and in vivo insulin doseresponse curves for glucose disposal using the euglycemic clamp technique were measured before and after therapy to assess the effect on receptor and postreceptor insulin action. Adipocyte insulin binding did not change. The insulin dose-response curve for overall glucose disposal remained right-shifted compared with age-matched controls, but the mean maximal glucose disposal rate increased by 74% from 160 ± 14 to 278 ± 18 mg/m2min (P < 0.0005). The effect of insulin treatment on basal hepatic glucose output was also assessed; the mean rate was initially elevated at 159 ± 8 mg/m2/min but fell to 90 ± 5 mg/m2/min in the posttreatment period (P < 0.001), a value similar to that in control subjects. Endogenous insulin secretion was assessed in detail and found to be improved after exogenous insulin therapy. Mean 24-h integrated serum insulin and C-peptide concentrations were increased from 21,377 ± 2766 to 35,584 ± 4549 μU/ml/ min (P < 0.01) and from 1653 ± 215 to 2112 ± 188 pmol/ml/min (P < 0.05), respectively, despite lower glycemia. Second-phase insulin response to an intravenous (i.v.) glucose challenge was enhanced from 170 ± 53 to 1022 ± 376 μU/ml/min (P < 0.025), although first-phase response remained minimal. Finally, the mean insulin and C-peptide responses to an i.v. glucagon pulse were unchanged in the posttreatment period, but when glucose levels were increased by exogenous glucose infusion to approximate the levels observed before therapy and the glucagon pulse repeated, responses were markedly enhanced. Simple and multivariate correlation analysis showed that only measures of basal hepatic glucose output and the magnitude of the postbinding defect in the untreated state could be related to the respective fasting glucose levels in individual subjects. We conclude that after 3 wk of intensive insulin therapy, diabetic subjects maintain lower glucose values concomitant with: (1) partial reversal of the postbinding defect in peripheral insulin action, (2) near-normalization of basal hepatic glucose output, and (3) enhanced insulin secretory responses. First-phase insulin response remained minimal and may be a marker for the diabetic state. Correlation analysis could only implicate basal hepatic glucose output and the postbinding defect in the untreated state as direct determinants of the fasting glucose level.

Effects of Weight Loss on Mechanisms of Hyperglycemia in Obese Non-Insulin-Dependent Diabetes Mellitus

To quantitate the effects of weight loss on the mechanisms responsible for hyperglycemia in non-insulin-dependent diabetes mellitus (NIDDM), eight obese subjects with NIDDM were studied before and after weight reduction with posttreatment assessment after 3 wks of isocaloric (weight maintenance) refeeding. After weight loss of 16.8 ± 2.7 kg (mean ± SE), the fasting plasma glucose level decreased 55% from 277 ± 21 to 123 ± 8 mg/dl. The individual fasting glucose levels were highly correlated with the elevated basal rates of hepatic glucose output (HGO) (r = 0.91, P < .001), which fell from 138 ± 11 to 87 ± 5 mg · m2 · min1 after weight loss. The change in fasting plasma glucose also correlated significantly with the change in the basal rates of HGO (r = 0.74, P < .05). This was associated with reduced fasting serum levels of glucagon (from 229 ± 15 to 141 ± 12 pg/ml), reduced free fatty acids (from 791 ± 87 to 379 ± 35 μeq/L), and unchanged basal insulin levels (17 ± 4 to 15 ± 2 fill/ml). Peripheral glucose disposal, assessed by the euglycemic glucose-clamp technique, at insulin infusion rates of 120 and 1200 mU · m2 · min1 increased between 135 and 165%, from 128 ± 17 to 288 ± 24 mg · m2 · min1 during the 120-mU · m2 · min1 studies and from 159 ± 19 to 318 ± 24 mg · m2 · min1 during the 1200-mU · m2 · min1 clamp studies, despite comparable steady-state serum insulin levels at each infusion rate before and after weight loss. HGO during the 120-mU · m2 · min1 clamp studies increased from 85% to complete (100%) suppression after treatment. Adipocyte size was reduced 44% (851 ± 91 to 475 ± 48 pi), whereas surface area decreased by 32% (4.30 × 104 to 2.92 × 104 μm2cell). Insulin binding to isolated adipocytes was unchanged, whereas basal in vitro rates of 3-O-methylglucose transport increased from 0.21 ± 0.13 to 0.53 ± 0.24 pmol/(2 × 109 μm2) × (10 s−1) and maximal glucose transport rates increased from 0.64 ± 0.29 to 1.18 ± 0.48 pmol/(2 × 105 cells) × (10 s−1) and 0.42 ± 0.20 to 1.04 ± 0.30 pmol/(2 × 109 μm2) × (10 s−1). Finally, absolute serum insulin levels during oral glucose-tolerance tests and meal-tolerance tests were unchanged by weight reduction, whereas plasma glucose levels were markedly reduced. We conclude that in obese NIDDM, weight loss results in improved glucose homeostasis with 1) reducedbasal HGO predominently responsible for the lowering of fasting glucose levels; 2) improved postprandial glucose excursions with marked amelioration of peripheral insulin resistance due to improved postreceptor insulin action, which is at least partly due to enhanced glucose transport system activity; and 3) unchanged absolute insulin levels in the face of markedly reduced glycemia, indicative of enhanced p-cell sensitivity to insulinogenic stimuli.

Lilly Lecture 1980: Insulin Resistance and Insulin Action: An In Vitro and In Vivo Perspective

Insulin is produced in the pancreatic B-cell as the primary biosynthetic product pre-proinsulin. This peptide is rapidly converted to proinsulin (MW ~ 9000) which is, in turn, converted to insulin (MW ~ 6000) plus C-peptide (MW ~ 3000) by specific proteolytic steps within the B-cell secretory granule. The normal secretory products are, therefore, insulin, an equimolar amount of C-peptide, and a small amount (~5%) of unconverted proinsulin. After a brief circulation time (t1/2 6-10 min) the hormone interacts with target tissues to exert its biologic effects. One of insulins major biologic effects is to promote overall glucose metabolism, and abnormalities of this aspect of insulin action can lead to a number of important clinical and pathophysiologic states. Insulin resistance exists when a given, known quantity of insulin produces less than the normal expected biologic effect. Since insulin travels from the B-cell, through the circulation, to the target tissue, events at any one of these loci can influence the ultimate action of the hormone. Therefore, it is useful to categorize insulin resistance according to known etiologic mechanisms, and such a classification is presented in Table 1. Insulin resistance can be due to three general categories of causes: (1) an abnormal B-cell secretory product, (2) circulating insulin antagonists, or (3) a target tissue defect in insulin action. Within each of these categories, subclassifications exist.

Decreased kinase activity of insulin receptors from adipocytes of non-insulin-dependent diabetic subjects.

The tyrosine kinase activity of the insulin receptor was examined with partially-purified insulin receptors from adipocytes obtained from 13 lean nondiabetics, 14 obese nondiabetics, and 13 obese subjects with non-insulin-dependent diabetes (NIDDM). Incubation of receptors at 4 degrees C with [gamma-32P]ATP and insulin resulted in a maximal 10-12-fold increase in autophosphorylation of the 92-kDa beta-subunit of the receptor with a half maximal effect at 1-3 ng/ml free insulin. Insulin receptor kinase activity in the three experimental groups was measured by means of both autophosphorylation and phosphorylation of the exogenous substrate Glu4:Tyr1. In the absence of insulin, autophosphorylation and Glu4:Tyr1 phosphorylation activities, measured with equal numbers of insulin receptors, were comparable among the three groups. In contrast, insulin-stimulated kinase activity was comparable in the control and obese subjects, but was reduced by approximately 50% in the NIDDM group. These findings indicate that the decrease in kinase activity in NIDDM resulted from a reduction in coupling efficiency between insulin binding and activation of the receptor kinase. The insulin receptor kinase defects observed in NIDDM could be etiologically related to insulin resistance in NIDDM and the pathogenesis of the diabetic state.

The Acute and Chronic Effects of Sulfonylurea Therapy in Type II Diabetic Subjects

Although sulfonylurea agents have been used in the clinical management of type II diabetes (non-insulin-dependent diabetes mellitus, NIDDM) for over two decades, the mechanisms responsible for their hypoglycemie action remain controversial. We have quantitated glycemie control, endogenous insulin secretion in response to mixed meals, adipocyte insulin binding, insulin-mediated peripheral glucose disposal, and basal hepatic glucose output in 17 type II diabetic subjects before and after 3 mo of therapy with the second-generation, sulfonylurea compound glyburide in an attempt to identify the factors responsible for the clinical response to the drug. In addition, 9 subjects were treated for an additional 15 mo to see if the response to the drug changed with time. The mean fasting serum glucose level fell from an initial value of 264 ± 17 mg/dl to 178 ± 16 mg/dl after 3 mo of drug therapy. Endogenous insulin secretion increased in all subjects, but the increase was most marked in those subjects who continued to exhibit fasting hyperglycemie (fasting serum glucose > 175 mg/dl) after 3 mo of therapy. Adipocyte insulin binding was unchanged after 3 mo of therapy, while the maximal rate of peripheral glucose disposal was increased by 23%, indicating enhancement of peripheral insulin action at a postreceptor site(s). Basal hepatic glucose output showed a significant correlation with the fasting serum glucose level both before and after therapy (r = 0.86, P < 0.001) and fell from 141 ±12 mg/m2/min before therapy to 107 ± 11 mg/m2/min after 3 mo of therapy. A significant correlation was also found between the decrease in the fasting glucose level and both the reduction in basal hepatic glucose output (r = 0.81, P < 0.001) and the enhancement of postreceptor function in peripheral tissues (r = 0.68, P < 0.005). After 18 mo of therapy, those subjects exhibiting an initial good response to the drug demonstrated a slight decrease in endogenous insulin secretion compared with the levels seen at 3 mo, adipocyte insulin binding had increased to the normal range, postreceptor function was further enhanced, and basal hepatic glucose output remained unchanged from the levels observed after 3 mo of therapy. We conclude that (1) glyburide therapy increases endogenous insulin secretion, increases adipocyte insulin binding after 18, but not 3, mo of therapy, enhances peripheral insulin action by acting primarily at a post-receptor site, and reduces basal hepatic glucose output; (2) the increase in postreceptor function and the reduction of basal hepatic glucose output appear to be the crucial determinants of the clinical response to the sulfonylurea agent; and (3) the response pattern to sulfonylurea compounds in terms of these various parameters can vary as a function of the duration of treatment.

Insulin Treatment Reverses the Insulin Resistance of Type II Diabetes Mellitus

Type II diabetic subjects are both insulin-deficient and insulin-resistant. Recent studies suggest that the insulin resistance is due to a combined receptor and postreceptor defect with the postreceptor defect being the predominant lesion. In the present study, we examined the effects of exogenous insulin therapy upon these defects in insulin action in six untreated type II diabetic subjects. Glycemic control and adipocyte insulin binding were measured and in vivo insulin dose-response curves for overall glucose disposal and suppression of hepatic glucose output were constructed before treatment. Following these initial studies, the diabetic subjects were treated with twice-daily injections of regular and NPH purified pork insulin for 14 days and the pretreatment studies repeated. Glycemic control was significantly improved by this treatment regimen. The mean fasting serum glucose level (±SE) fell from 287 ± 20 to 125 ± 13 mg/dl, the mean glycosylated hemoglobin level (± SE) decreased from 14.2 ± 1.1% to 8.3 ± 0.5%, and the mean 24-h urinary glucose excretion (±SE) declined from 65.6 ± 40.3 to 0.6 ± 0. 1 g/24 h. Adipocyte insulin binding did not change significantly during the treatment period. In contrast, the 14-day period of insulin treatment produced a 72% increase (P < 0.005) in the maximal rate of insulin-stimulated glucose disposal, 321 ± 32 mg/M2/min compared with 187 ± 32 mg/M2/min before treatment, indicating that the postreceptor defect in insulin action was significantly ameliorated by insulin treatment. The dose-response curve for insulin-mediated suppression of hepatic glucose output was rightshifted, consistent with the decrease in insulin binding, with no decrease in the maximal effect before treatment and not significantly changed following insulin treatment. In conclusion, the postreceptor defect in insulin-stimulated glucose disposal is largely ameliorated by exogenous insulin treatment, suggesting that this defect in insulin action is an acquired abnormality which is secondary to some aspect of the insulin-deficient state.

Reversibility of defective adipocyte insulin receptor kinase activity in non-insulin-dependent diabetes mellitus. Effect of weight loss.

Insulin-stimulated kinase activity of adipocyte-derived insulin receptors is reduced in subjects with non-insulin-dependent diabetes mellitus (NIDDM) but normal in obese nondiabetics. To assess the reversibility of the kinase defect in NIDDM, insulin receptor kinase activity was measured before and after weight loss in 10 NIDDM and 5 obese nondiabetic subjects. Peripheral insulin action was also assessed in vivo by glucose disposal rates (GDR) measured during a hyperinsulinemic (300 mU/M2 per min) euglycemic clamp. In the NIDDMs, insulin receptor kinase activity was reduced by 50-80% and rose to approximately 65-90% (P less than 0.01) of normal after 13.2 +/- 2.0 kg (P less than 0.01) weight loss; comparable weight loss (18.2 +/- 1.5 kg, P less than 0.01) in the nondiabetics resulted in no significant change in insulin receptor kinase activity. Relative to GDR measured in lean nondiabetics, GDR in the NIDDMs was 35% of normal initially and 67% (P less than 0.01) of normal after diet therapy; weight loss in the nondiabetics resulted in an increase in GDR from 53 to 76% of normal (P less than 0.05). These results indicate that the insulin receptor kinase defect that is present in NIDDM is largely reversible after weight reduction. In contrast, the improvement in GDR, in the absence of any change in insulin receptor kinase activity in the nondiabetics, suggests that the main cause of insulin resistance in obesity lies distal to the kinase.

Metabolic Consequences of Very-Low-Calorie Diet Therapy in Obese Non-insulin-dependent Diabetic and Nondiabetic Subjects

To determine the effects of very-low-calorie diets on the metabolic abnormalities of diabetes and obesity, we have studied 10 obese, non-insulin-dependent diabetic (NIDDM) and 5 obese, nondiabetic subjects for 36 days on a metabolic ward during consumption of a liquid diet of 300 kcal/day with 30 g of protein. Rapid improvement occurred in the glycemie indices of the diabetic subjects, with mean (± SEM) fasting plasma glucose falling from 291 ± 21 to 95 ± 6 mg/dl (P < 0.001) and total glycosylated hemoglobin from 13.1 ± 0.7% to 8.8 ± 0.3% (P < 0.001) (normal reference range 5.5–8.5%). Lipid elevations were normalized with plasma triglycerides reduced to <100 mg/dl and total plasma cholesterol to < 150 mg/dl in both groups. Hormonal and substrate responses were also comparable between groups with reductions in insulin and triiodothy-ronine and moderate elevations in blood and urinary ketoacid levels without a corresponding rise in free fatty acids. Electrolyte balance for sodium, potassium, calcium, and phosphorus was initially negative but approached equilibrium by completion of the study. Magnesium, in contrast, remained in positive balance in both groups throughout. Total nitrogen loss varied widely among all subjects, ranging from 70 to 367 g, and showed a strong positive correlation with initial lean body mass (N = 0.83, P < 0.001) and total weight loss (N = 0.87, P < 0.001). The nondiabetic group, which had a significantly greater initial body weight and lean body mass than the diabetic group, also had a significantly greater weight loss of 450 ± 31 g/day compared with 308 ± 19 g/day (P < 0.01) in the diabetic subjects. The composition of the weight lost at completion was similar in both groups and ranged from 21.6% to 31.3% water, 3.9% to 7.8% protein, and 60.9% to 74.5% fat. The contribution of both water and protein progressively decreased and fat increased, resulting in unchanged caloric requirements during the diet. This study demonstrates that short-term treatment with a very-low-calorie diet in both obese diabetic and nondiabetic subjects results in: (1) safe and effective weight loss associated with the normalization of elevated glucose and lipid levels, (2) a large individual variability in total nitrogen loss determined principally by the initial lean body mass, and (3) progressive increments in the contribution of fat to weight loss with stable caloric requirements and no evidence of a hypometabolic response.