Effects of monensin on insulin processing in adipocytes. Evidence that the internalized insulin-receptor complex has some physiological activities

Effect of inhibiting vacuolar acidification on insulin signaling in hepatocytes

Previous studies have shown that the endosomal apparatus plays an important role in insulin signaling. Inhibition of endosomal acidification leads to a decrease in insulin-insulin receptor kinase (IRK) dissociation and insulin degradation. Thus, vacuolar pH could function as a modulator of insulin signaling in endosomes. In the present study we show that in primary hepatocytes pretreated with bafilomycin, there is an inhibition of vacuolar acidification. Incubation of these cells with insulin was followed by an augmentation of IRK activity but an inhibition of phosphatidylinositol 3-kinase/Akt activity and a decrease in insulin-induced DNA and glycogen synthesis. Bafilomycin treatment inhibited IRK recycling to the plasma membrane without affecting IRK internalization. Impaired IRK recycling correlated with a decrease in insulin signaling. We suggest that inhibiting vacuolar acidification sequesters activated IRKs in an intracellular compartment(s) where signaling is inhibited. This implies that endosomal receptor trafficking plays a role in regulating signal transduction.

The sulfonylurea glimepiride regulates intracellular routing of the insulin-receptor complexes through their interaction with specific protein kinase C isoforms

Sulfonylureas may stimulate glucose metabolism by protein kinase C (PKC) activation. Because interaction of insulin receptors with PKC plays an important role in controlling the intracellular sorting of the insulin-receptor complex, we investigated the possibility that the sulfonylurea glimepiride may influence intracellular routing of insulin and its receptor through a mechanism involving PKC, and that changes in these processes may be associated with improved insulin action. Using human hepatoma Hep-G2 cells, we found that glimepiride did not affect insulin binding, insulin receptor isoform expression, and insulin-induced receptor internalization. By contrast, glimepiride significantly increased intracellular dissociation of the insulin-receptor complex, degradation of insulin, recycling of internalized insulin receptors, release of internalized radioactivity, and prevented insulin-induced receptor down-regulation. Association of PKC-betaII and -epsilon with insulin receptors was increased in glimepiride-treated cells. Selective depletion of cellular PKC-betaII and -epsilon by exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA) or treatment of cells with PKC-betaII inhibitor G06976 reversed the effect of glimepiride on intracellular insulin-receptor processing. Glimepiride increased the effects of insulin on glucose incorporation into glycogen by enhancing both sensitivity and maximal efficacy of insulin. Exposing cells to TPA or G06976 inhibitor reversed these effects. Results indicate that glimepiride increases intracellular sorting of the insulin-receptor complex toward the degradative route, which is associated with both an increased association of the insulin receptor with PKCs and improved insulin action. These data suggest a novel mechanism of action of sulfonylurea, which may have a therapeutic impact on the treatment of type 2 diabetes.

Intracellular hyperinsulinism: a metabolic characteristic of obesity with and without Type 2 diabetes: intracellular insulin in obesity and Type 2 diabetes

There is evidence that intracellular insulin may carry out some insulin mediated actions, including glucose transport. As intracellular insulin has never been quantitatively assessed in human cells, we evaluated its concentrations in monocytes from normal subjects (n = 7) and obese patients without (n = 9) and with Type 2 diabetes mellitus (n = 10). After the incubation of cells with labeled insulin for 60 min at 37 degrees C, intracellular intact insulin concentrations were measured by HPLC and expressed as pmol x 10(-6). Insulin concentrations were higher (ANOVA P < 0.01) within cells from obese (115.4 +/- 26.4 pmol x 10(-6)/2 x 10(5) cells) and obese diabetic patients (93.2 +/- 36.3 pmol x 10(-6)/2 x 10(5) cells) compared with normal cells (28.5 +/- 13.1 pmol x 10(-6)/2 x 10(5) cells). Moreover, after insulin was removed from the incubation medium the decrease of intracellular insulin was significantly lower (P < 0.01) in cells from both obese and obese diabetic patients than in normal subjects. Intracellular undissociated insulin-insulin receptor complexes on average, increased 2-fold (P < 0.01) in cells from insulin resistant patients compared with normal cells. Finally, in downregulated cells from obese and obese diabetic patients, the recycling of the internalized insulin receptor was completely disrupted. In conclusion, monocytes from obese patients with and without Type 2 diabetes mellitus, present increased intracellular insulin concentrations and these conditions are associated with a significant impairment of insulin receptor processing. Increased intracellular insulin concentration in cells from these patients may be necessary in order to overcome insulin resistance.

Inhibition of endosomal acidification in normal cells mimics the derangements of cellular insulin and insulin-receptor metabolism observed in non-insulin-dependent diabetes mellitus

Dissociation of the insulin-insulin receptor complex plays a crucial role in the processing of both insulin and the insulin receptor, and the acidification of endocytic vesicles may be the mechanism by which internalized insulin is dissociated from its receptor and properly sorted and processed. Internalized insulin-insulin receptor complexes are abnormally processed in cells from patients with non-insulin-dependent diabetes mellitus (NIDDM). Accordingly, to further investigate the mechanisms of the derangements observed in NIDDM cells, we examined the effects of the ionophore monensin, which inhibits endosomal acidification, on the cellular processing of insulin and insulin receptor in monocytes from control subjects (n = 12) and NIDDM patients (n = 14). This study confirms that monocytes from NIDDM patients, compared with cells from normal controls, had reduced binding (P < .01), internalization (P < .01), and degradation (P < .01) of insulin. In addition, the release of intracellular radioactivity was slower (P < .01), and recycling of the insulin receptor was inhibited (P < .01). Moreover, these defects were associated with a significant (P < .01) decrease of dissociation of the internalized insulin-insulin receptor complex. In cells from normal controls, incubation with monensin decreased insulin binding (P < .01), but not insulin internalization. High-performance liquid chromatography (HPLC) analysis of intracellular radioactivity showed that after monensin intracellular intact insulin significantly increased (P < .01), thus suggesting a decrease of intracellular insulin degradation. Moreover, insulin receptor recycling was completely disrupted. All of these derangements were associated with a significant decrease (P < .01) of dissociation of insulin-insulin receptor complexes. On the contrary, in diabetic monocytes, monensin had no significant additional effect on NIDDM-linked alterations. Comparison of the results obtained in cells from NIDDM patients to those found in monensin-treated normal cells demonstrates that NIDDM and monensin gave rise to a superimposable impairment of dissociation of the intracellular insulin-insulin receptor complex, associated with similar abnormal sorting and processing of insulin and its receptor. The only defect present in NIDDM cells but not in monensin-treated cells is the decrease of insulin internalization, which thus seems independent of the action of monensin on the processing of internalized insulin-insulin receptor complex. These results suggest that the impairment of dissociation of the insulin-insulin receptor complex may play a crucial role in the subsequent altered processing of insulin and insulin receptor. Moreover, they raise the question as to a possible similar alteration of the same intracellular mechanism by NIDDM and monensin, and point out that the derangements found in cells from NIDDM patients could be localized within the endosomal apparatus and consist mainly of a defective acidification of its interior.

Delayed intracellular dissociation of the insulin-receptor complex impairs receptor recycling and insulin processing in cultured Epstein-Barr virus-transformed lymphocytes from insulin-resistant subjects

Insulin-receptor internalization and processing are defective in insulin-resistant subjects. To assess the reversibility of these defects, we cultured Epstein-Barr virus-transformed-lymphoblasts from six normal, six obese, and six non-insulin-dependent diabetic (NIDDM) subjects in media containing low (5 mmol/l) or high (25 mmol/l) glucose concentrations, and studied the insulin-receptor internalization and processing in vitro. In cells from normal, obese, and NIDDM subjects cultured in low glucose concentrations, exposure to 100 nmol/l insulin for 30 min at 37 degrees C reduced cell-surface 125I-insulin binding to a similar extent (82 +/- 2, 77 +/- 5, and 82 +/- 5% of initial values, respectively). The same results were obtained with cells cultured in high glucose concentrations. In cells cultured under both glucose conditions, and exposed to 100 nmol/l insulin for 30 min at 37 degrees C, a complete recovery of the initial 125I-insulin binding was observed in normal but not in obese and NIDDM subjects. Release of intracellular insulin and its degradation in vitro was determined by incubating cells with 600 pmol/l of 125I-insulin for 60 min at 37 degrees C, acid washing cells, and re-incubating in insulin-free buffer at 37 degrees C. The radioactivity released by cells was characterized by trichloroacetic acid precipitability, Sephadex G-50 column chromatography, and re-binding to fresh cells. Rates of release of internalized radioactivity were reduced in obese and NIDDM subjects (t1/2 = 61 +/- 9 min, p < 0.02; 58 +/- 10 min, p < 0.05; and 38 +/- 4 min in obese, NIDDM, and normal subjects, respectively). The percentage of intact insulin released from cells was significantly higher in obese and NIDDM subjects than in the normal subjects. The t1/2 of intracellular dissociation of insulin-receptor complexes measured by a polyethylene glycol assay was lower in normal (6 +/- 1 min) than in obese (12 +/- 2 min, p < 0.03) and NIDDM subjects (14 +/- 3 min, p < 0.02). The results suggest that in insulin-resistant subjects a primary defect in intracellular dissociation of insulin is responsible for alterations of receptor recycling and insulin processing.

Effects of lasalocid and cationomycin on the evolution of certain parameters in the blood plasma of sheep

Six adult sheep were fed at maintenance level, successively over three experimental periods, 1100 g of a roughage-rich diet without supplement or containing 33 mg kg-1 of lasalocid or cationomycin. The feed was administered in eight equal meals daily, every three hours. Blood samples were taken in each animal from the jugular vein at 10.00 hours, 16.00 and 22.00 hours, one hour after the animals were fed. The ionophores did not affect the plasma concentrations of glucose, free fatty acids, total amino acids, insulin, acetate, Ca or Mg. They decreased beta-hydroxy butyrate content (P < 0.05) and increased that of albumin (P < 0.05). Lasalocid alone significantly decreased uremia, but the significant threshold was only reached at 16.00 hours (P < 0.01). With this exception, the two ionophores had similar effects. Samples taken in peripheral blood appear to be too far from nutrient absorption sites to give a clear indication of the effects of these molecules on the products absorbed or metabolised in the digestive tract.

Mechanism and role of insulin receptor endocytosis

Like many other cell surface receptors for nutrients and polypeptide hormones, the insulin receptor undergoes a complex endocytotic itinerary. Upon insulin binding, the receptor is activated as a tyrosine-specific protein kinase and autophosphorylates. This autophosphorylation is necessary for the receptor to internalize. After endocytosis, the ligand (insulin) and its receptor are dissociated. Most of the insulin is degraded, whereas the receptors are largely recycled to the cell surface. The signals in the receptor that control and specify its endocytotic pathway are beginning to be understood. Through the techniques of in vitro mutagenesis, noninternalizing receptors have been engineered and their structural and functional properties have been analyzed. For example, the immediate submembranous domain of the insulin receptor has been found to contain sequences (Gly-Pro-Leu-Tyr and, to a lesser extent, Asn-Pro-Gln-Tyr) that are necessary for normal endocytosis. Receptors deleted or mutated in these sequences retain tyrosine kinase activity but fail to undergo endocytosis. Unlike the better understood low density lipoprotein and transferrin receptors, however, these sequences are not sufficient for endocytosis. An insulin receptor with only these sequences exposed in the cytoplasm does not internalize. Tyrosine kinase activity is thought to be needed to lead to autophosphorylation and a conformational change that exposes the otherwise buried endocytosis sequences in the normally dimerized insulin receptor. Non-internalizing mutants of the insulin receptor have been used to examine the role of endocytosis in insulin action. It was found that an endocytosis-defective receptor could induce a short-term metabolic action of insulin (glycogen synthetase stimulation) as well as longer-term mitogenic effects of insulin. Furthermore, insulin action deactivated after the hormone was removed from the noninternalizing receptors. Apparently, endocytosis is not necessary for insulin action, but probably is important for removing the insulin from the cell so the target cell for insulin responds in a time-limited fashion to the hormone.

The effect of CP 68,722, a thiozolidinedione derivative, on insulin sensitivity in lean and obese Zucker rats

The effect of a new drug (CP 68,722, Pfizer) on parameters of insulin sensitivity in an established insulin-resistant animal model was examined. Rates of hepatic glucose production (HGP) and peripheral glucose uptake in obese Zucker (fa/fa) rats treated with a 10-day course of the medication using a two-step (2 and 10 mU/kg/min) euglycemic hyperinsulinemic clamp technique were measured. In addition, changes in substrate concentrations after drug treatment were examined. Basal HGP rates were similar in the lean versus the obese animals (37 +/- 3 v 39 +/- 3 mumol/kg/min); however, the obese animals had impaired insulin-induced suppression of HGP at both 2 mU/kg/min (36 +/- 3 v 23 +/- 4 mumol/kg/min) and 10 mU/kg/min (18 +/- 5 v 2 +/- 1 mumol/kg/min). Insulin stimulation of glucose disposal was also defective in the obese animals (37 +/- 2 v 88 +/- 7 mumol/kg/min at 2 mU/kg/min and 98 +/- 9 v 219 +/- 18 mumol/kg/min at 10 mU/kg/min). In addition, obese animals had elevated free fatty acid (FFA) and ketone levels, both of which were resistant to insulin-induced suppression. After drug treatment, few alterations in glucose or lipid metabolism were found in the lean animals. In the obese animals, insulin suppression of HGP was normalized during the higher insulin infusion rate (0 v 18 +/- 5 mumol/kg/min at 10 mU/kg/min), and peripheral glucose disposal was enhanced at both steps of the insulin clamp (54 +/- 4 v 37 +/- 2 mumol/kg/min at 2 mU/kg/min and 134 +/- 12 v 98 +/- 9 mumol/kg/min at 10 mU/kg/min).(ABSTRACT TRUNCATED AT 250 WORDS)

Mode of action of chloroquine in patients with non-insulin-dependent diabetes mellitus

Clinical studies have demonstrated that chloroquine and hydroxychloroquine improve glucose metabolism in patients with insulin-resistant diabetes mellitus. The mechanism of action has not been determined. We undertook a randomized double-blind placebo-controlled trial of 3 days of oral chloroquine phosphate, 250 mg four times daily, in 20 patients with non-insulin-dependent diabetes mellitus controlled by diet. Rates of glucose appearance (Ra) and disappearance (Rd) were evaluated by infusion of stable isotopically labeled D-glucose ([6,6-2H2]glucose) during hyperinsulinemic euglycemic clamps before and after treatment with chloroquine or placebo. Chloroquine significantly improved fasting plasma glucose from 199.8 +/- 8.6 to 165.6 +/- 7.6 mg/dl (P less than 0.01). Total exogenous glucose infusion required to maintain euglycemia significantly increased (1,792.6-2,040.1 mg.kg-1.330 min-1, P less than 0.05) due to an increase in Rd (2,348.0-2,618.9 mg.kg-1.330 min-1, P less than 0.01) without change in Ra. Metabolic clearance rate of insulin decreased by 39% from 14.4 +/- 1.3 to 11.0 +/- 0.6 ml.kg-1.min-1 (P less than 0.01) at plasma insulin levels of 150-200 mU/l but not at levels of 2,000-3,000 mU/l. In addition, chloroquine increased fasting C-peptide secretion by 17% and reduced feedback inhibition of C-peptide by 9.1 and 10.6% during low- and high-dose insulin infusions, respectively.

Processing of the lipid-mobilizing peptide beta-lipotropin in rabbit adipose tissue

beta-Lipotropin, a pituitary peptide, is a strong stimulator of lipolysis in rabbit adipose tissue. This polypeptide is shown to be degraded by intact fat pads, homogenized adipose tissue and adipocytes of the rabbit dependent on the amount of adipose tissue, time and the pH of the incubation medium. In subcellular fractions of rabbit adipocytes the proteolytic activity could be localized into the cytosol and the microsomal fraction. To obtain information about the processing of beta-lipotropin in its target cell lipolysis and degradation of this polypeptide were investigated in the presence of inhibitors of distinct cellular mechanisms and in different physiological states such as obesity and starvation. Thus, the stronger lipolytic response in adipocytes from obese rabbits respectively animals fed ad libitum was accompanied by a significantly increased degradation in comparison to lean respectively starved rabbits. The six lysosomotropic agents (chloroquine, NH4Cl, propranolol, quinacrine, acridine orange and tetracaine), the proteinase inhibitors alpha 2-macroglobulin and monodansylcadaverine, cellular ATP depletion by 2-deoxy-D-glucose and 2,4-dinitrophenol and the omission of Ca2+ ions from the incubation medium inhibited dose-dependently the lipolytic activity as well as the degradation of beta-lipotropin in intact and homogenized adipose tissue. Inhibitors of the cytoskeleton such as colchicine, cytochalasin B, vinblastine and concanavalin A also reduced lipolysis but only the degradation in intact adipose tissue. It can be concluded that after receptor-mediated uptake the cytoskeleton and lysosomal proteases are involved in the processing of beta-lipotropin.

Insulin processing and signal transduction in rat adipocytes

A glycine-HCl buffer (glycine, 50 mM/NaCl, 0.15 M/HCl, pH 3.5) was used to strip insulin bound to adipocyte cell surfaces. Adipocytes retained their integrity in the glycine buffer and their binding capacity for [125I]iodoinsulin could be completely recovered on transfer of the cells to physiological media. At 37 degrees C, [125I]iodoinsulin binds rapidly to plasma membrane receptors; maximal binding occurs within 10 min. At this temperature, the initial binding is followed by rapid internalization, degradation of the hormone and subsequent loss of label. Insulin treatment, at 37 degrees C, induced internalization of 37% of the plasma membrane insulin receptors. Phenylarsine oxide (PAO), a confirmed inhibitor of protein internalization, allowed insulin binding but completely inhibited degradation of the hormone. Monensin, a carboxylic ionophore which impairs uncoupling hormone-receptor complexes, effectively restricted insulin degradation over short time periods (less than 30 min). Addition of monensin to insulin-stimulated cells did not impair D-glucose uptake. It has previously been reported that PAO inhibits hexose transport through the direct interaction with the glucose transporters and low concentrations of PAO (1 microM) transiently inhibit insulin-stimulated glucose uptake. This recovery phenomenon was again observed when PAO was added to insulin-stimulated, monensin-treated adipocytes. The data suggests that lysosomal degradation of insulin is not requisite for signal transduction.

Endocytosis of follicle-stimulating hormone by ovarian granulosa cells: analysis of hormone processing and receptor dynamics

Suspensions of freshly isolated rat granulosa cells were used to study endocytosis and processing of radioiodinated ovine follicle-stimulating hormone (I-oFSH) and to analyze the dynamics of its receptor. Ovine FSH was iodinated to a specific activity of 26 microCi/micrograms as determined by radioreceptor self-displacement assays with maximum specific binding to excess membrane receptors of 46%. Radiolabeled oFSH was judged biologically equivalent to the unlabeled hormone since I-oFSH shows saturation-binding kinetics and stimulates steroidogenesis in a similar dose-related manner to unlabeled oFSH. Experiments designed to study the extent and time course of degradation involved continuous exposure of isolated granulosa cells to I-oFSH. Saturation of membrane receptors was achieved within 1.5 h of incubation, and internalization of FSH occurred in a linear manner for up to 6 h. The rate of internalization was equivalent to 2,780 FSH molecules/cell/h. Degradation of FSH became apparent after 6 h of incubation and increased to 86% of total cellular-associated radioactivity at 22 h. FSH degradation was inhibited by 100 microM chloroquine or 0.45 mM leupeptin. The measurement of cell surface I-oFSH binding in the combined presence of 100 microM chloroquine and 0.5 mM cycloheximide was unchanged for up to 22 h of incubation. This and other receptor binding data suggest that there is no reutilization of FSH receptors. Scatchard analyses of 4 degrees C binding assays on intact cells indicated that a two-site model best fit the data with association constants of K11 = 1.44 (+/- .42) X 10(10) and K12 = 4.35 (+/- .91) X 10(8). Receptor binding and activation studies for progesterone production yielded ED50s of 270 pM and 7.7 pM, respectively, and also indicated that 20% receptor occupancy is sufficient to stimulate maximal progesterone production. We conclude that after the initial binding event, FSH is endocytosed very slowly and is subsequently shuttled to the lysosomal compartment for degradation. The retarded rate of endocytosis may relate to novel pathways of hormone processing.

Intracellular pathways of insulin transport across vascular endothelial cells

Processing and transport of hormones across vascular endothelial cells may modulate hormone action at subendothelial tissue sites. Insulin was transported across cultured rat capillary and bovine aortic endothelial cells, after a delay of 5-10 min, at a constant rate for 60 min at 37 degrees C. 125I-labeled insulin transport was inhibited by 88 +/- 11% (SE, n = 4) and 75 +/- 18% (SE, n = 4) in the presence of anti-insulin receptor antibody and unlabeled insulin (at 10(-7) M), respectively. Reverse phase high-performance liquid chromatography showed 88% of the 125I-insulin transported over 60 min was indistinguishable from the 125I-insulin added to the cells at 4 degrees C. In aortic endothelial cells preincubated with 2.3 x 10(-9) M of insulin for 24 h, insulin receptor binding was downregulated by 67%, and 125I-insulin transport was decreased by 52 +/- 11%. The proton ionophore monensin (0.05 mM) increased the internalized insulin in bovine aortic endothelial cells by 78%, with a corresponding decrease in 125I-insulin released by 76 +/- 2% (SE, n = 4). 125I-insulin transport across the aortic endothelial cell monolayer was similarly decreased (54 +/- 12%, SE, n = 4) by monensin. In contrast, the lysosomal protease inhibitor leupeptin had no effect. Degradation and transport were similarly dissociated by low temperature. At 15 degrees C, no significant insulin degradation was detected, whereas 125I-insulin release from the cells continued at 30 +/- 3% of the rate at 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence of a defect in insulin-receptor recycling in adipocytes from older rats

Although insulin-stimulated glucose uptake is known to be decreased in adipocytes isolated from old obese rats, the cause of this defect is not totally understood. In the present study, we examined the possibility that insulin resistance is associated with defects in the intracellular processing of the insulin-receptor complex. Adipocytes were isolated from control (2-mo-old rats) and obese, insulin-resistant rats (12-mo-old rats), and the following measurements were made: 1) insulin-stimulated glucose uptake; 2) insulin binding; 3) insulin-receptor internalization and recycling; 4) accumulation of insulin within the cell; and 5) rate of loss of insulin from the cell. The results indicated that maximal insulin-stimulated glucose uptake was significantly reduced in adipocytes from obese, insulin-resistant rats (increase over basal value was 500 +/- 53% in obese rats and 1,200 +/- 96 in control rats, P less than 0.01). 125I-insulin (A14) binding (cell-associated radioactivity) and the internalization of the hormone-receptor complex were not different in the two groups of animals studied. In contrast, insulin-receptor recycling was significantly decreased in adipocytes from obese rats (72.0 +/- 6.1 vs. 93.6 +/- 2.6%, P less than 0.01). In addition, loss of intracellular radioactivity was significantly prolonged in insulin-resistant rats (t1/2 = 12.05 +/- 0.9 vs. 9.4 +/- 0.3 min, P less than 0.05). Thus adipocytes isolated from the older rats were resistant to the insulin effect on glucose uptake, and this defect was not associated with a reduction in insulin binding. However, there was a decrease in insulin receptor recycling, and this phenomenon may be related to the insulin resistance present in these cells.

Internalization of transforming growth factor-beta and its receptor in BALB/c 3T3 fibroblasts

The fate of 125I-labeled transforming growth factor-beta (125I-TGF beta) after binding to its cells surface receptor has been investigated in BALB/c 3T3 mouse fibroblasts. Binding of 125I-TGF beta to cellular receptors at 4 degrees C is pH-sensitive, being markedly decreased at pH less than 6. Most (approximately 90%) of the 125I-TGF beta bound to cells at 4 degrees C can be removed by a brief treatment with acidic medium but is converted into an acid-resistant state rapidly after shifting the cells to 37 degrees C. Cell-bound 125I-TGF beta is degraded at 37 degrees C and the degradation products are released into the medium. The lysosomotropic bases chloroquine, methylamine, and ammonium and the carboxylic ionophore monensin inhibit the degradation and release of 125I-TGF beta from the cells. Cells allowed to accumulate 125I-TGF beta intracellularly by the action of chloroquine or monensin were treated with the bifunctional agent disuccinimidyl suberate in the presence of detergent Triton X-100; this treatment caused the cross-linking of internalized 125I-TGF beta with the 280-kilodalton TGF beta receptor component. Under conditions in which sustained binding and degradation of saturating 125I-TGF beta concentrations occurs, there is no marked decrease in the binding capacity of the cells even when protein synthesis is blocked with cycloheximide. These results indicate that after TGF beta binding the TGF beta:receptor complex becomes rapidly internalized and that TGF beta is directed towards lysosomes where it is degraded and released. However, the cell surface is replenished with TGF beta receptors recycled after internalization or supplied by a large intracellular pool.

The reversible receptor binding of insulin in isolated rat adipocytes measured at 37 degrees C. The binding is not rate limiting for cellular uptake

The bimolecular binding reaction between mono[TyrA14-125I]iodoinsulin and the insulin receptor was investigated at 37 degrees C in intact isolated rat adipocytes in which membrane traffic was inhibited by 1 mM KCN. This treatment decreased the fraction of cell-associated radioactivity resistant to treatment at pH 3 (usually regarded as internalized ligand) from 70% to 17%. The total amount of tracer being cell-associated at steady state was reduced to about half of the control value partly because of a decreased apparent binding affinity. The t1/2 for the forward reaction was reduced from 414 s in the control cell to 26 s in the KCN treated cell. Likewise, the t1/2 for the dissociation was reduced from 461 s to 67 s. Both rate constants were pH sensitive, the association rate constant being 7-8-fold more than the dissociation rate constant. Since both rate constants for the bimolecular reaction were one order of magnitude greater than those for the uptake and the release of label in the untreated cell, other processes than binding constitute the rate-limiting step(s) in the cellular reaction with insulin.

A thermodynamic approach to hormone-receptor interaction; application to insulin binding to adipocytes, adipocyte plasma membranes and liposomes incorporating adipocyte insulin receptors

The binding of insulin to its receptor in rat adipocyte and isolated plasma membranes has been measured. The adipocyte insulin receptor has been reconstituted in lecithin liposomes and the binding of insulin investigated. A method of interpreting binding data presented as binding vs. the logarithm of free insulin concentration (binding isotherms) in terms of the binding potential concept of Wyman (1965) is described, and the results are compared with the commonly used Scatchard analysis of binding. The binding potential approach enables binding constants and Gibbs energies of formation of the insulin-receptor complex to be determined as a function of insulin bound. The limiting Gibbs energies of binding at 15 degrees C to intact cells, membranes and liposomes were found to be -55, -52 and -49 kJ mol-1 respectively. The affinity of the receptor for insulin decreases smoothly with increase in binding in all three systems. For intact adipocytes the number of insulin receptors per cell is found to be approximately 43,000.