The structure of the hepatic insulin receptor and insulin binding

Insulin receptors in Xenopus laevis liver and forelimb regenerates and the effects of local insulin deprivation on regeneration

As forelimb regeneration in Xenopus laevis is mainly a cell proliferative event which results in a spike-shaped appendage, we set out to examine the possibility that insulin is a growth-promoting factor in this process. The objectives were 1) to detect the presence of insulin receptors (IRs) in the liver (a specific target organ for insulin) and IRs in the forelimb regenerates of X. laevis, 2) to determine whether the receptor is similar to IRs identified in other organisms, and 3) to absorb insulin locally by implanting anti-insulin antibody-soaked hydrolyzed polyacrylamide beads into regenerating forelimb outgrowths in order to assess the effects of insulin deprivation on regeneration. The results show that IRs are present in Xenopus liver plasma membranes (XLPM) as well as in plasma membranes of 21 day forelimb regenerates. Insulin binding to this receptor is time-dependent and specific, as unlabeled bovine insulin competes with radioiodinated insulin for binding to XLPM more effectively than insulin-like growth factor-I, guinea pig insulin, or glucagon. Scatchard analysis of insulin binding to XLPM describes a two binding site receptor possessing a low affinity (0.16 nM-1), high capacity (3.2 +/- 0.9 pM/mg) binding site and a high affinity (2.7 nM-1), low capacity (0.5 +/- 0.3 pM/mg) binding site. The holoreceptor has a molecular mass of 380 kDa. The reduced receptor has subunits of 130 kDa and 95 kDa. The 95 kDa subunit undergoes autophosphorylation following insulin stimulation. Implantation of hydrolyzed polyacrylamide beads, saturated with anti-insulin antibody, into regenerating Xenopus forelimbs significantly impeded development of the regenerates and, therefore, demonstrates that insulin is required for growth of Xenopus forelimb regenerates.

Detection of insulin receptors in newt liver and forelimb regenerates and the effects of local insulin deprivation on epimorphic regeneration

Previous in vivo and in vitro studies indicate that insulin is required in adult newt forelimb regeneration. The objectives of the current study were 1) to detect insulin receptors in the liver (a classical target organ for insulin) and once verified, detection of insulin receptors in the adult newt forelimb regenerate; and 2) to determine whether locally implanting insulin antibody-soaked hydrolyzed polyacrylamide beads (hypa beads) into a regenerating forelimb blastema would affect its growth and/or differentiation. The results show that insulin receptors are detectable in the plasma membranes of newt liver and forelimb regenerates. Radioiodinated bovine insulin binding is time-dependent and specific; unlabeled bovine insulin competes with labeled insulin for binding to NLPM more effectively than does insulin-like growth factor-I, guinea pig insulin, and glucagon. The newt hepatic insulin receptor binds insulin with high affinity (1.1 nM-1) and low capacity (63 +/- 8 fmoles/mg). The size of the alpha subunit of the newt insulin receptor is 130 kDA and that of the beta subunit is 95 kDa. The beta subunits undergo insulin-stimulated phosphorylation in response to insulin. An autoantibody against the human insulin receptor recognizes the newt receptor protein. Insulin receptors are also detectable in 15 and 20 day newt forelimb regenerates. Specific immunogold labelling of the receptor-bound antibody appears to be restricted to the cellular processes of the regenerate. Implanting hypa beads soaked with purified insulin antibody into regenerating adult newt forelimbs results in abnormal growth and differentiation of the regenerates, confirming that insulin plays an essential role in adult newt forelimb regeneration.

Binding of mutant insulins to a mutated insulin receptor

We studied the binding of mutant insulins to both the normal human insulin receptor and an insulin receptor in which the sequence 240-250 of the receptor alpha subunit was mutated to provide an additional net positive charge. One mutant insulin (AspB10), which has an additional negative charge, bound to both types of receptors with a higher affinity than native insulin. Moreover, this mutant insulin was more effective in activating the tyrosine kinase activity of both types of receptors. This study suggests, therefore, that charge interactions between insulin and its receptor may play a role in insulin receptor binding and action.

The endogenous functional turkey erythrocyte and rat liver insulin receptor is an alpha 2 beta 2 heterotetrameric complex

Previous studies have indicated that turkey erythrocyte and rat liver membranes contain endogenous alpha beta heterodimeric insulin receptors in addition to the disulphide-linked alpha 2 beta 2 heterotetrameric complexes characteristic of most cell types. We utilized 125I-insulin affinity cross-linking to examine the structural properties of insulin receptors from rat liver and turkey erythrocyte membranes prepared in the absence and presence of sulphydryl alkylating agents. Rat liver membranes prepared in the absence of sulphydryl alkylating agents displayed specific labelling of Mr 400,000 and 200,000 bands, corresponding to the alpha 2 beta 2 heterotetrameric and alpha beta heterodimeric insulin receptor complexes respectively. In contrast, affinity cross-linking of membranes prepared with iodoacetamide (IAN) or N-ethylmaleimide identified predominantly the alpha 2 beta 2 heterotetrameric insulin receptor complex. Similarly, affinity cross-linking and solubilization of intact turkey erythrocytes in the presence of IAN resulted in exclusive labelling of the alpha 2 beta 2 heterotetrameric insulin receptor complex, whereas in the absence of IAN both alpha 2 beta 2 and alpha beta species were observed. Turkey erythrocyte alpha 2 beta 2 heterotetrameric insulin receptors from IAN-protected membranes displayed a 3-4-fold stimulation of beta subunit autophosphorylation and substrate phosphorylation by insulin, equivalent to that observed in intact human placenta insulin receptors. Turkey erythrocyte alpha beta heterodimeric insulin receptors, prepared by defined pH/dithiothreitol treatment of IAN-protected membranes, were also fully competent in insulin-stimulated protein kinase activity compared with alpha beta heterodimeric human placenta receptors. In contrast, endogenous turkey erythrocyte alpha beta heterodimeric insulin receptors displayed basal protein kinase activity which was insulin-insensitive. These data indicate that native turkey erythrocyte and rat liver insulin receptors are structurally and functionally similar to alpha 2 beta 2 heterotetrameric human placenta insulin receptors. The alpha beta heterodimeric insulin receptors previously identified in these tissues most likely resulted from disulphide bond reduction and denaturation of the alpha 2 beta 2 holoreceptor complexes during membrane preparation.

Insulin stimulates proteolysis of the alpha-subunit, but not the beta-subunit, of its receptor at the cell surface in rat liver

Insulin receptors in rat liver plasma membranes contain two alpha- and two beta-subunits held together by interchain disulphide bonds ([alpha beta]2 receptors). Affinity-labelled receptors were digested with chymotrypsin or elastase and then exposed to dithiothreitol before solubilization from membranes and SDS/polyacrylamide-gel electrophoresis. This resulted in partial reduction and isolation of Mr-225,000 alpha beta, Mr-200,000 alpha 1 beta, Mr-165,000 alpha beta 1 and Mr-145,000 alpha 1 beta 1 receptor halves containing intact (alpha, beta) or degraded (alpha 1, beta 1) subunits. The ability to identify half-receptor complexes containing intact or degraded subunits made it possible to assay each subunit simultaneously for insulin-induced proteolysis in isolated plasma membranes or during perfusion of rat liver in situ with insulin. In liver membranes, insulin binding increased the fraction of receptors containing degraded alpha-subunits to about one-third of the total population during 2 h of incubation at 23 degrees C. beta-Subunit proteolysis increased only minimally during this time. Plasma membranes isolated from livers perfused with insulin at 37 degrees C contained degraded alpha-subunits but only intact beta-subunits, showing that insulin induced cell-surface proteolysis of the binding, but not the kinase, domain of its receptor. Since previous observations [Lipson, Kolhatkar & Donner (1988) J. Biol. Chem 263, 10495-10501] have shown that receptors containing degraded alpha-subunits are internalized but do not recycle, it is possible that cell-surface degradation may play a role in the regulation of insulin-receptor number in hepatic tissue. Proteolysis of the beta-subunit is not a likely mechanism by which receptor-kinase activity may be attenuated under physiological conditions.

Insulin processing in primary endosomes is not responsible for insulin resistance observed in parametrial adipocytes from lactating rats

The fate of [125I insulin and the insulin receptor after internalization was characterized in parametrial adipocytes from virgin rats. Parallel experiments were carried out on parametrial adipocytes from 2-4-day lactating rats, which are insulin resistant. Similar results were obtained in adipocytes from either group of animals. Insulin caused 10% of the plasma membrane insulin receptor to be translocated to a compartment resistant to extracellular trypsin. The intracellularly located insulin receptor rapidly recycled to the plasma membrane at 37 degrees C. An endosomal compartment involved in both the endocytosis and subsequent recycling of [125I]insulin and the insulin receptor to the plasma membrane was identified on sucrose density floatation gradients. [125I]Insulin internalized at 37 degrees C accumulated in a fraction of modal density 1.12 g/ml. Crosslinking experiments revealed the presence of intact [125I]insulin-insulin receptor complexes in endosomes. After a pulse with [125I]insulin, 55-60% of the 125I radioactivity recovered in the endosome compartment was intact [125I]insulin. The remainder was composed of low molecular weight degradation products. Endosomal 125I radioactivity was rapidly retroendocytosed to the medium with a mean half-life of 6 min. These results suggest: (1) [125I]insulin and the insulin receptor are internalized by parametrial adipocytes into an early endosomal compartment (primary endosomes), from which the receptor, intact [125I]insulin, and [125I]tyrosine are returned to the cell surface; and (2) the damping of the insulin signal observed in parametrial adipocytes from lactating rats is not expressed at the level of altered endocytotic processing of [125I]insulin and the insulin receptor.

A short incubation time in insulin binding experiments leads to disappearance of negative cooperativity in H35 hepatoma cells

The effects of different periods of incubation (8 min vs 20 min) on insulin binding kinetics were examined in a H35 hepatoma cell line. Scatchard plots from cells incubated for 8 min were linear (r = 0.987 +/- 0.006), in contrast to curvilinear Scatchard plots from cells incubated for 20 min. Hill plots showed a slope of 1.006 +/- 0.024 for the 8 min incubation, whereas the slope was 0.827 +/- 0.0026 (p less than 0.0005) for the 20 min incubation. TCA precipitation of the medium showed minimal insulin degradation products at 8 min with a significant increase at 20 min (1.38 +/- 0.11% vs. 3.06 +/- 0.37%, p less than 0.0005). Internalized insulin was also significantly increased at 20 min as compared to 8 min incubation (48.9 +/- 5.6% vs. 32.4 +/- 3.0%, p less than 0.0005) These data indicate that after 8 min of incubation no appreciable cooperativity of insulin binding was present, while negative cooperativity was present after 20 min of incubation. As significantly more insulin degradation has taken place after prolonged incubation these data support the hypothesis that insulin degradation leads to negative cooperativity of insulin receptors.

Separation and characterization of three insulin receptor species that differ in subunit composition

Partially purified human placental insulin receptor preparations give rise to three distinct insulin-binding peaks when eluted from a Mono Q high-performance liquid chromatography anion-exchange column. We analyzed the basis for this phenomenon by affinity cross-linking of insulin to each peak, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We find that the three insulin-binding peaks represent different molecular weight complexes with the following subunit composition: (alpha beta)2, (alpha beta)(alpha beta'), and (alpha beta')2, where beta' represents a proteolytically derived fragment of the beta subunit. This analysis of subunit composition was confirmed by silver staining of affinity-purified insulin receptor following resolution of the forms on a Mono Q column as described previously. We have characterized the three isolated insulin receptor forms with regard to ligand binding by LIGAND and Scatchard analysis. We also measured insulin-stimulatable autophosphorylation and exogenous kinase activity directed toward poly(Glu/Tyr) (4:1). The three forms of the insulin receptor exhibit similar KD's for insulin binding to the high- and low-affinity sites. The (alpha beta)2 and (alpha beta)(alpha beta') forms of the insulin receptor display superimposable curvilinear Scatchard plots. In contrast, only the intact holoreceptor (alpha beta)2 form demonstrates insulin-stimulatable autophosphorylation and exogenous kinase activity. The (alpha beta)(alpha beta') form has reduced basal kinase activity which was not increased by prior incubation with insulin. The (alpha beta')2 form lacks a kinase domain and consequently demonstrated no kinase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of angiotensinogen production by H4 rat hepatoma cells in serum-free culture

A serum-free, hormone and attachment factor supplemented culture for rat H4 hepatoma cells was established. In the defined medium (Dulbecco's Modified Eagle's + Ham's F12 + insulin, transferrin, fibronectin liver cell growth factor, and sodium selenite), H4 cells grew equally well as in 10% fetal bovine serum supplemented medium. H4 cells in either defined or serum-containing culture conditions produce transferrin but not albumin or alpha-fetoprotein. In this paper we have studied the effect of various hormones and pressor peptides on the production of angiotensinogen by H4 cells cultured in defined conditions. Only glucocorticoid hormone had a significant effect on the production of angiotensinogen, whereas other hormones previously reported to exert their effect on angiotensinogen production had little or no effect.

The insulin-receptor interaction: is the kinetic approach for inferring negative-cooperative site-site interactions valid?

The dissociation of insulin from its receptor is reportedly enhanced when the dissociation is induced by dilution in the presence of insulin. This experiment is frequently conducted when curvilinear Scatchard plots of insulin binding are observed in order to infer negative cooperative site-site interactions amongst insulin receptors. However, when insulin binding to purified liver plasma membranes was measured at 15 degrees C in 50 mM Tris, pH 7.5 containing 0.1% bovine serum albumin and 100 U/ml bacitracin, the insulin binding data was characterised by a linear Scatchard plot and a Hill plot with a slope equal to unity. Thus, under the conditions of this binding assay, insulin apparently bound to a single non-interacting class of homogeneous binding sites. But, despite the apparent absence of cooperative interactions under these specific conditions, the dissociation of receptor-bound insulin was still enhanced when the dissociation of insulin from its receptor was induced by dilution in the presence of insulin. This result cast serious doubt on the validity of inferring negative-cooperative site-site interactions amongst insulin receptors based solely on the observation that the dissociation of receptor-bound insulin is enhanced by dilution in the presence of insulin.