Structural differences between liver- and muscle-derived insulin receptors in rats

2'-O-Methylperlatolic Acid Enhances Insulin-Regulated Blood Glucose-Lowering Effect through Insulin Receptor Signaling Pathway

PURPOSE

Insulin receptor (InsR) sensitizers represent a new type of therapeutic agent for the treatment of diabetes, with 2'-O-methylperlatolic acid (2-O-M) being a potential InsR targeting drug. The purpose of this study was to determine whether 2-O-M functions as an activator of the insulin signaling pathway, regulating glucose hemostasis through the InsR and exerting a glucose-lowering effect in an animal model of diabetes.

METHODS

SPR-based analyses were used to detect the binding of different concentrations of 2-O-M to the InsR. The protein levels of IR-β, p-IR, AKT, and p-AKT in Hepa and C2C12 cell lines and liver and muscle tissues were determined by western blotting. Glucose uptake capacity was determined in C2C12 cells. Streptozotocin-induced diabetic mice were randomly divided into four groups: the control, insulin treated, 2-O-M treated, and combined insulin and 2-O-M treated. Mice were injected with 2-O-M or normal saline and the average blood glucose concentration after 120 min, and the serum levels of insulin, glucagon, and C-peptide were measured. Next, qRT-PCR was performed to detect the mRNA expression of genes involved in lipid and glucose metabolism in the liver and muscle tissues.

RESULTS

2-O-M binds to the extracellular domain of the InsR. Moreover, combination treatment with 2-O-M and insulin resulted in significant activation of the insulin signaling pathway in vitro and significant stimulation of the glucose uptake capacity of C2C12 myotubes. In mice with streptozotocin-induced diabetes, 2-O-M significantly prolonged the blood glucose-lowering effect of insulin, significantly reduced the secretion of exogenous insulin, and reduced the blood glucose concentration in vivo. In addition, treatment with 2-O-M alone significantly enhanced the phosphorylation of AKT in muscle tissue, which enhanced glucose uptake in C2C12 myotubes. Further, 2-O-M significantly increased glucagon secretion and enhanced liver gluconeogenesis to prevent hypoglycemia.

CONCLUSION

2-O-M enhances the hypoglycemic effect of insulin through the insulin signaling pathway and can be used as a complement to insulin. This synergetic effect may lower the required dose of insulin and protect β cells.

Glucose metabolism during the early "flow phase" after burn injury

BACKGROUND

Burn injury (BI) is associated with insulin resistance (IR) and hyperglycemia which complicate clinical management. We investigated the impact of BI on glucose metabolism in a rabbit model of BI using a combination of positron emission tomography (PET) and stable isotope studies under euglycemic insulin clamp (EIC) conditions.

MATERIALS AND METHODS

Twelve male rabbits were subjected to either full-thickness BI (B) or sham burn. An EIC condition was established by constant infusion of insulin, concomitantly with a variable rate of dextrose infusion 3 d after treatment. PET imaging of the hind limbs was conducted to determine the rates of peripheral O(2) and glucose utilization. Each animal also received a primed constant infusion of [6,6-(2)H(2)] glucose to determine endogenous glucose production.

RESULTS

The fasting blood glucose in the burned rabbits was higher than that in the sham group. Under EIC conditions, the sham burn group required more exogenous dextrose than the B group to maintain blood glucose at physiological levels (22.2 ± 2.6 versus 13.3 ± 2.9 mg/min, P < 0.05), indicating a state of IR. PET imaging demonstrated that the rates of O(2) consumption and (18)F 2-fluoro-2-deoxy-D-glucose utilization by skeletal muscle remained at similar levels in both groups. Hepatic gluconeogenesis determined by the stable isotope tracer study was found significantly increased in the B group.

CONCLUSIONS

These findings demonstrated that hyperglycemia and IR develop during the early "flow phase" after BI. Unsuppressed hepatic gluconeogenesis, but not peripheral skeletal muscular utilization of glucose, contributes to hyperglycemia at this stage.

Demonstration of a relatively hepatoselective effect of covalent insulin dimers on glucose metabolism in dogs

Insulin analogues with relatively greater effect on hepatic glucose production than peripheral glucose disposal could offer a more physiological approach to the treatment of diabetes mellitus. The fact that proinsulin exhibits this property to a minor degree may suggest that analogues with increased molecular size may be less able than insulin to obtain access to peripheral receptor sites. Covalent insulin dimers have previously been shown to possess lower hypoglycaemic potencies than predicted by their in vivo receptor binding affinities. Reduced rates of diffusion to peripheral target tissues might be an explanation for the lower in vivo potency compared to insulin. To test the relative hepatic and peripheral effects of covalent insulin dimers, glucose clamp procedures with D-[3-3H]glucose tracer infusions were used in anaesthetised greyhounds to establish dose-response curves for rates of hepatic glucose production and glucose disposal with insulin, N alpha B1, N alpha B'1,-suberoyl-insulin dimer, and N epsilon B29, N epsilon B'29,-suberoyl-insulin dimer. With N alpha B1, N alpha B'1,-suberoyl-insulin dimer molar potencies relative to insulin were 68%, (34-133) (mean and 95% fiducial limits), for inhibition of hepatic glucose production and 14.7%, (10.3-20.9) for glucose disposal. With N epsilon B29,N epsilon B'29,-suberoyl-insulin dimer potencies were 75%, (31-184) and 2.5%. (1.5-4.3), for inhibition of hepatic glucose production and for glucose disposal, respectively. The demonstration that both dimers exhibit a significantly greater effect on glucose production than on glucose disposal supports the suggestion that analogues with increased molecular size may exhibit reduced ability to gain access to peripheral target cells.

Binding of human, porcine and bovine insulin to insulin receptors from human brain, muscle and adipocytes and to expressed recombinant alternatively spliced insulin receptor isoforms

Previous studies have suggested that human and porcine insulin exert identical effects on blood glucose and counter-regulatory hormones but elicit different neurophysiological reactions. A major goal of the present study was to investigate whether this could be caused by different relative affinities of the insulins from different species to insulin receptors from the brain compared to other tissues. Insulin receptors isolated from human brain, muscle or adipocytes as well as from cultured cells over-expressing either of the human insulin receptor isoforms (exon 11- or exon 11 +) were immobilized to microwells coated with monoclonal anti-insulin receptor antibody. Subsequently the binding of human, porcine and bovine insulin was measured. While the receptors derived from the different tissues had different affinities for insulin, there were no tissue-specific differences in the relative binding of the insulins of the three species. The insulins of the three species were also not different with regard to their binding to the receptor isoforms. Finally, in human brain homogenates no differences in the degradation rates for human, porcine and bovine insulin were detected. Thus, our data do not support the hypothesis that different neurophysiological reactions during hypoglycaemia due to human or porcine insulin are caused by differences of the binding of the insulins to human brain insulin receptors or their degradation in the human brain.

Structural and functional heterogeneity of insulin receptors

It was long believed that the effects of insulin are mediated by a unique insulin receptor. However, there is considerable evidence suggesting that insulin receptors in brain, liver, adipocytes, and lymphocytes are heterogeneous in structure and function. This evidence is based on comparisons of concentration response curves in cells and tissues, and on comparisons of binding and effects of insulin-derivatives and receptor antibodies. Two receptor isoforms (IR-A and IR-B) generated by alternative mRNA splicing have been identified, but cannot fully account for the observed differences in ligand binding and receptor function. It is suggested that the differences in ligand binding reflect yet to be defined post-translational modifications, and that post-receptor events are responsible for the observed heterogeneity of insulin action.

Tissue-specific expression of two alternatively spliced isoforms of the human insulin receptor protein

Two insulin receptor mRNA species are expressed in human tissues as a result of alternative splicing of exon 11. This event is regulated in a tissue-specific manner. To date, there is little information about the relative abundance of the two receptor protein isoforms on the cell surface. The aim of the present investigation was to assess whether the tissue-specific expression of the two insulin receptor mRNA species is paralleled by a similar pattern of expression of the two receptor protein isoforms. To this end, we assessed the relative distribution of the two receptor variants in various human tissues at the mRNA and protein levels. A PCR-based technique was used to measure the relative abundance of the two mRNA species, and two immunological assays were used to measure the relative steady-state expression of the two receptor protein isoforms. The expression of the two insulin receptor protein isoforms followed the tissue-specific pattern of expression of the two mRNA species.

Differential metabolic actions of biosynthetic insulin analogues in normal man assessed by stable isotopic tracers

Insulin analogues have been produced with high affinity for the insulin receptor and with affinity lower than that of native insulin, but differences in activity when administered in vivo to man are unconvincing. We have used very low dose insulin (0.005 units kg-1 h-1) to investigate possible differences in effect of these insulin analogues on lipolysis in seven healthy subjects. Only minor effects on blood glucose concentration were observed and glucose turnover measured isotopically with 6,6 2H glucose and leucine turnover measured with 1-13C leucine did not change significantly. Fatty acid levels decreased with insulin (area under curve, median (range) -23 (-41-10) mmol l-1) and with the low affinity analogue (-28 (-42-19) mmol l-1 h,), but the high affinity analogue had no significant effect compared with controls (high affinity analogue -8 (-28-35) mmol l-1 h; control +15 (11-53) mmol l-1). Glycerol production measured isotopically decreased with insulin (-0.54 (-1.50-0.63) mumol kg-1 min-1) and with the low affinity analogue (-0.74 (-1.76-0.72) mumol kg-1 min-1), but the high affinity analogue at these doses had no significant effect on glycerol turnover (-0.19 (-0.74-1.13) mumol kg-1 min-1). Thus at these low infusion rates insulin itself and the low affinity analogue suppressed lipolysis, as assessed by glycerol turnover and by circulating fatty acid concentrations. The high affinity analogue was cleared rapidly from the circulation producing no measurable increase in immunoreactive insulin concentrations, and no effect was observed on lipolysis.

Metabolic effects of monomeric insulin analogues of different receptor affinity

The effects of two monomeric insulin analogues of differing receptor affinities (human insulin = 100%) B9Asp-B27Glu-insulin (18%) and B10Asp-insulin (327%) were each compared with human insulin in two groups of 10 normal men when infused at equimolar low doses (1.0 and 2.0 pmol kg-1 min-1). The metabolic clearance rate under steady state conditions was highest for the analogue with the highest receptor affinity, 26.8 +/- 0.8 (+/- SE) vs 19.8 +/- 0.7 ml kg-1 min-1 for insulin (p less than 0.001), and lowest for the analogue with the lowest receptor affinity, 13.3 +/- 0.8 vs 25.1 +/- 2.0 ml kg-1 min-1 for insulin (p less than 0.001). The apparent plasma half-life was prolonged for the low affinity analogue compared with human insulin (12.6 +/- 0.6 vs 1.9 +/- 0.2 min, p less than 0.001), and significantly shorter for the higher affinity analogue (1.6 +/- 0.1 vs 3.1 +/- 0.4 min, p less than 0.05). The three insulins gave similar falls in blood glucose, non-esterified fatty acids, glycerol, and total ketone bodies over the infusion period. Thirty minutes after the end of the infusion, the rise in blood glucose for the low affinity analogue was significantly less than for human insulin (0.5 +/- 0.2 vs 0.9 +/- 0.1 mmol l-1, p less than 0.05). Despite different receptor affinities, these analogues have similar in vivo effects in normal men, but the time-course of their actions may differ when they are infused intravenously.

Influence of low- and high-protein diets on insulin and insulin-like growth factor-1 binding to skeletal muscle and liver in the growing rat

The influence of protein content of the diet on the plasma concentrations and binding to skeletal muscle and liver of insulin and insulin-like growth factor-1 (IGF-1), was studied in growing rats. Animals with a starting body-weight of 80 g received for an 11 d period isoenergetic diets containing (g/kg dry matter) 155 protein as controls (MP), or 55 (LP) or 300 (HP) protein. Food was offered as six equal meals/d. Daily food intakes provided adequate amounts of energy. Total plasma IGF-1 increased linearly as a function of dietary protein intake. Plasma insulin was lower in the LP than in the MP and HP groups. Hormone binding was studied in wheat-germ agglutinin (WGA) partially purified skeletal muscle receptor preparations. Each 125I-labelled hormone binding was competed for by increasing amounts of homologous and heterologous unlabelled hormone; this displacement needed lower concentrations of homologous than heterologous hormone. When compared with MP-diet feeding, the LP diet resulted in an increased ligand concentration for half-maximal binding. In addition the specific 125I-labelled insulin and 125I-labelled IGF-1 binding increased at all hormone concentrations and, as revealed by Scatchard analysis, the hormone binding capacity also rose (only significant for low-affinity insulin receptors and high-affinity IGF-1 receptors). The HP diet had little effect on hormone binding, except to increase insulin binding at very low insulin concentrations. Hormone binding was further studied in WGA partially purified liver receptor preparations. Those preparations did not exhibit any detectable specific 125I-labelled IGF-1 binding. The specific 125I-labelled insulin binding was not altered by dietary protein level. It is concluded that the increase in skeletal muscle insulin and IGF-1 binding along with a decrease in insulin and IGF-1 in the blood from rats fed on the LP diet, is consistent with the concept of an inverse relationship between plasma hormone and hormone binding. The physiological significance with respect to metabolic adaptation of muscle remains to be established.

Insulin receptor activity and insulin sensitivity in mammary gland of lactating rats

The mammary gland is a tissue that is extremely sensitive to insulin during lactation; during weaning, the effect of insulin is rapidly abolished. The purpose of this study was to characterize the mammary gland insulin receptors and their kinase activity in lactating and weaned mammary gland. The apparent molecular weight of the alpha-subunit was slightly lower in the mammary gland than in liver and white adipose tissue (127,000 vs. 134,000), but the apparent molecular weight of the beta-subunit was similar in the three tissues (95,000). Insulin induced a 10-fold increase in beta-subunit autophosphorylation, and the half-maximal effect was achieved at 2 nM insulin. After 24 h of weaning, the number of insulin receptors was decreased by 30%, but the kinase activity of the beta-subunit was unchanged. During the euglycemic hyperinsulinemic clamp, insulin entirely activated pyruvate dehydrogenase in lactating rat mammary gland, whereas after 24 h of weaning it was unable to increase the proportion of the enzyme in the active form. These results suggest that the site of alteration in the action of insulin on the mammary gland during weaning is distal to the receptor.

Kinetics of insulin binding and kinase activity of the partially purified insulin receptor from human skeletal muscle

The kinetics of insulin binding and kinase activity of soluble, partially purified insulin receptors from human skeletal muscle are considered. An equilibrium for insulin binding was obtained within 2 h at 37 degrees C. At lower temperatures the equilibrium for insulin binding was less clearly defined. Dissociation of 125I-labelled insulin was incomplete unless an excess amount of unlabelled insulin was added. Insulin-stimulatable autophosphorylation of the 95 kDa subunit was verified by gel electrophoresis. The kinase activity was measured with the synthetic polypeptide poly(Glu-Tyr(4:1] as a phosphoacceptor. The insulin receptor kinase activity correlated significantly (r = 0.92, P less than 0.0001) to the concentration of high-affinity insulin binding sites in the eluate. Autophosphorylation of the insulin receptor was necessary for the activation of the receptor kinase. When activated the receptor kinase activity was stable for at least 60 min at 21 degrees C with a pH optimum of approx. 7.8, similar to the pH optimum for insulin binding. The non-ionic detergent Triton X-100 inhibited the sensitivity of the receptor kinase to insulin. Insulin stimulated the Vmax of the kinase reaction about 3-fold, decreased the Km for ATP from 35 +/- 5 microM (mean +/- S.E.) to 8 +/- 1 microM (P less than 0.02) and induced a positive cooperativity to ATP with an increase in the Hill coefficient from 1.00 +/- 0.02 to 1.37 +/- 0.07 (P less than 0.05). According to the Hill plots, insulin itself showed no cooperativity with respect to receptor binding or kinase activation.

A 180,000 molecular weight glycoprotein substrate of the insulin receptor tyrosine kinase is present in human placenta and in rat liver, muscle, heart and brain plasma membrane preparations

Cell signalling for insulin may include insulin receptor tyrosine kinase catalysing the phosphorylation of one or more cell proteins. Since temporally the insulin receptor will encounter plasma membrane proteins first, we have studied the in vitro phosphorylation of purified plasma membrane preparations. Two proteins were immunoprecipitated with anti-phosphotyrosine antibody from rat liver, muscle, heart and brain membranes and from human placenta membranes: the insulin receptor (detected as a phosphorylated-beta-subunit) and a 180,000 molecular weight protein (pp180). pp180 is a monomeric glycoprotein that in the absence of dithiothreitol migrated in denaturing gels like a 150,000 molecular weight protein. pp180 was a substrate for the insulin receptor: (i) receptor and pp180 phosphorylation followed a similar insulin dose-response, although fold-stimulation of autophosphorylation was greater; and (ii) removal of insulin receptors with monoclonal antibodies prevented subsequent pp180 phosphorylation. Insulin-activated receptors increased the extent, but not the rate, of pp180 phosphorylation; the increased phosphate was incorporated into tyrosine and appeared to do so in three or four of pp180's 12 tryptic phosphopeptides. Some data suggest that pp180 is the same protein in each of the tested tissues. The occurrence of pp180, an insulin receptor substrate, in plasma membranes of several insulin responsive tissues suggests that it has a role in insulin signalling.

Differential expression of the receptors for epidermal growth factor and insulin in the developing human placenta

The detailed cellular distribution of epidermal growth factor (EGF) receptors and insulin receptors during the development of the human placenta was examined. We show that EGF receptors are expressed by villous cytotrophoblast cells in first trimester human placentae. However, where these cells proliferate to form extravillous cytotrophoblast cell columns, there is a dramatic decrease in EGF receptor expression. There is no such differential expression of insulin receptors on this cell population. In contrast, both EGF-and insulin-receptors are present throughout gestation on the microvillous membrane of the terminally differentiated and non-proliferative syncytiotrophoblast although, at term, EGF-but not insulin-receptors are also found on the basolateral membrane of this epithelium. We further show that EGF receptors isolated from first trimester and term human placentae have functional tyrosine kinase activities but differ in their extent of glycosylation. These results suggest that EGF receptors probably play several distinct functional roles in these epithelial cells depending on their proliferative capacity and differentiation status.

Antipeptide antibodies toward the extracellular domain of insulin receptor beta-subunit

In order to investigate structure and function of beta-subunit extracellular portion, four polyclonal antibodies (AP1, AP2, AP3 and AP4) toward peptides comprised in this region were generated. None of them recognizes native human and rat insulin receptor both in vitro and in whole cells. Two antibodies, AP1 and AP2, immunoprecipitate isolated (DTT-reduced) human beta-subunits and bind to human IM-9 cell after alpha-subunit tryptic cleavage. Only AP1 recognizes rat beta-subunit both in vitro and in trypsin treated rat FAD cells. These findings suggest that: (i) the extracellular portion of the insulin receptor beta-subunit is partially covered by the alpha-subunit in human and rat native insulin receptors; (ii) human and rat beta-subunit extracellular domains are different, at least in the amino acid sequence corresponding to residues 785-796 of the human insulin receptor.

Biological activity of des-(B26-B30)-insulinamide and related analogues in rat hepatocyte cultures

Short-term and long-term biological activities were studied in adult rat hepatocytes cultured in the presence of the insulin analogues des-(B26-B30)-insulinamide, [TyrB25]des-(B26-B30)-insulinamide and [HisB25]des-(B26-B30)-insulinamide. When compared to insulin, full potency of des-(B26-B30)-insulinamide has been reported in rat adipocytes and an enhanced potency has been reported for the other analogues. Steady state binding characteristics of the analogues to hepatocytes were indistinguishable from those of native insulin with half-maximal binding occurring at concentrations of about 0.8 nmol/l. Half-maximal effects for the stimulation of glycolysis and inhibition of basal and glucagon-activated glycogenolysis required identical concentrations for insulin and all 3 analogues. Induction of the key glycolytic enzymes glucokinase and pyruvate kinase as well as the inhibition of glucagon-dependent induction of phosphenolpyruvate carboxy-kinase also required identical concentrations of insulin and the 3 analogues. These data confirm that in cultured hepatocytes the C-terminal amidation of des-(B26-B30)-insulin results in a molecule with full in vitro potency. In contrast to data obtained in adipocytes, the des-(B26-B30)-insulin-amidated analogues with tyrosine or histidine substitutions at position B25 are equally as potent as native insulin in eliciting biological responses in rat hepatocyte culture.

Alternative splicing of human insulin receptor messenger RNA

The polymerase chain reaction has been used to examine alternative splicing of human insulin receptor (hINSR) mRNA. Alternative splicing of a 36 base pair exon, exon 11, generates hINSR transcripts encoding receptor isoforms which differ in sequence at the C-terminal end of the insulin-binding alpha-subunit. This process appears to be tissue-specific and, in addition, may be developmentally regulated.

The platelet insulin receptor: detection, partial characterization, and search for a function

A direct demonstration and partial characterization of the insulin receptor on human platelets was obtained by immunoprecipitation with a monoclonal antibody against the human insulin receptor. Two subunits were detected, one with a MW of 95 KD, which dose-dependently was phosphorylated upon challenge of the platelets with insulin and another with a MW of 69 KD that does not become phosphorylated. Retention on wheat germ agglutinin indicates that at least part of the receptor protein is glycosylated. In a search for cellular effects provoked by insulin on platelets, no changes could be detected in cAMP formation or degradation, inositol phosphate formation or phosphorylation of proteins other than the receptor itself.

Distinct receptors for insulin-like growth factor I in rat renal glomeruli and tubules

Purified preparations of renal glomeruli and tubules were obtained by a procedure involving perfusion of rat kidneys with magnetic iron oxide particles to selectively separate the iron-containing glomeruli from the nonmagnetic tubules. Detergent-soluble extracts of both renal glomerular and tubular membranes showed high-affinity, specific binding of 125I-labeled insulin-like growth factor I (125I-IGF-I), whereas degradation of this peptide hormone was minimal during a 90-min incubation at 22 degrees C in the presence of 2.5 mM EDTA and 5 mM N-ethylmaleimide. The affinity of these receptors for IGF-I appeared identical in the two types of renal tissue, since 50% inhibition of 125I-IGF-I binding to both glomerular and tubular tissue occurred in the presence of approximately 3 x 10(-9) M unlabeled IGF-I. In contrast, insulin was much less effective at blocking 125I-IGF-I binding to either tissue, with 1 x 10(-6) M insulin required to produce 50% inhibition of binding. Relative to 125I-IGF-I binding, 125I-insulin binding to glomerular and tubular tissue was significantly lower per milligram protein. 125I-IGF-I was specifically cross-linked to a glomerular receptor subunit that migrated as two discrete bands with relative molecular weight (Mr) of 140,000-150,000 on sodium dodecyl sulfate polyacrylamide gels in the presence of 40 mM dithiothreitol. In contrast, 125I-IGF-I was cross-linked to a tubular receptor subunit that migrated as two discrete bands but at a slightly different position, with Mr of 120,000-140,000.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential binding of monoiodinated insulins to muscle and liver derived receptors and activation of the receptor kinase

The binding affinity of monoiodoinsulin analogues to receptors purified from rat skeletal muscle and liver were compared. Insulin iodinated at tyrosine B26 bound to both muscle and liver derived insulin receptors with higher affinity than the A14-iodoisomer or native insulin. The affinity of the B26-iodoanalogue was greater for muscle than for liver derived receptors; by Scatchard analysis the affinity ratio B26/A14 was 2.8 for muscle and 1.3 for liver. The affinity of muscle and liver derived receptors for A14-iodoinsulin was not different. Dose response curves of autophosphorylation and exogenous tyrosine kinase activation showed significantly increased sensitivity to the B26-iodoanalogue (compared to the A14-iodoisomer or native insulin) in muscle derived receptors, but not in liver. The difference in affinity between muscle and liver derived insulin receptors towards B26-monoiodotyrosyl-insulin likely reflects the observed structural difference between the insulin receptor alpha-subunits from muscle and liver.

Peptide mapping on Northern blot analyses of insulin receptors in brain and adipocytes

Previous studies have demonstrated differences in the size of insulin receptor subunits in brain and adipocytes that appear to involve variations in glycosylation of the proteins. In this report, we examined the degree of homology in the protein backbones of insulin receptors in both tissues by peptide mapping and compared the mRNAs encoding the receptors by Northern blot analysis. Photoaffinity-labeled insulin receptors from rat brain and adipocytes were deglycosylated and then subjected to partial proteolysis by five different enzymes with differing substrate specificities. The intact receptors and their proteolytic fragments were analyzed by electrophoresis and autoradiography. Each enzyme yielded a unique pattern of fragments ranging from 70 to 11 kDa. In all cases, there was a striking similarity in the peptide maps generated from insulin receptors in brain and adipocytes. Northern hybridization experiments were carried out using poly(A)+ RNA from rat brain, rat adipocytes, and human hepatocarcinoma (HEP G2) cells. In rat brain, two bands of 9.5 and 7.4 kb were detected and, in rat adipocytes, the same two bands were observed. The two mRNA bands observed in rat tissues represented only two of the five mRNA species seen in human HEP G2 cells. The results indicate that the protein domains and the mRNAs encoding of insulin receptors in brain and adipocytes are very similar, if not identical.

Heterogeneity of human liver, muscle, and adipose tissue insulin receptor

We have studied the structure and function of the human insulin receptor in liver, skeletal muscle and adipose tissue. The alpha-subunit of the insulin receptor for liver, muscle and adipose tissue migrated on SDS-PAGE with Mrs 137632 +/- 216, 134034 +/- 1080, and 133575 +/- 165, respectively (p less than 0.05). Treatment of these receptors with neuraminidase decreased their molecule sizes and eliminated the relative size differences between the receptors. Three monoclonal antibodies (5A1, 10D9, and 20H3), directed towards different epitopes of the human insulin receptor alpha-subunit were used to probe immunological differences among the receptors. Antibodies 5A1 and 20H3 recognized all the receptors, whereas 10D9 recognized muscle and adipose tissue receptors but not liver receptors. The mobility of insulin receptor beta-subunit in the absence of insulin was the same in all tissues with a similar phosphorylation-induced decrease in mobility in SDS-PAGE in the presence of insulin. However, insulin stimulated autophosphorylation per receptor was different being greatest (p less than 0.05) in muscle (334 +/- 104 32P cpm) and similar in adipose tissue (114 +/- 10) and liver (183 +/- 68). These studies indicate, therefore, that the human insulin receptor is heterogeneous among the major target tissues for insulin, and raise the possibility that this heterogeneity may account for tissues' specific differences in insulin's biological messages.

Characterization of the chicken muscle insulin receptor

Insulin receptors are present in chicken skeletal muscle. Crude membrane preparations demonstrated specific 125I-insulin binding. The nonspecific binding was high (36-55% of total binding) and slightly lower affinity receptors were found than are typically observed for crude membrane insulin binding in other chicken tissues. Affinity crosslinking of 125I-insulin to crude membranes revealed insulin receptor alpha-subunits of Mr 128K, intermediate between those of liver (134K) and brain (124K). When solubilized and partially purified on wheat germ agglutinin (WGA) affinity columns, chicken muscle insulin receptors exhibited typical high affinity binding, with approximately 10(-10) M unlabeled insulin producing 50% inhibition of the specific 125I-insulin binding. WGA purified chicken muscle insulin receptors also exhibited insulin-stimulated autophosphorylation of the beta-subunit, which appeared as phosphorylated bands of 92- and 81K. Both bands were immunoprecipitated by anti-receptor antiserum (B10). WGA purified membranes also demonstrated dose-dependent insulin-stimulated phosphorylation of the exogenous substrate poly(Glu,Tyr)4:1. However, unlike chicken liver, chicken muscle insulin receptor number and tyrosine kinase activity were unaltered by 48 hr of fasting or 48 hr of fasting and 24 hr of refeeding. Thus, despite the presence of insulin receptors in chicken muscle showing normal coupling to receptor tyrosine kinase activity, nutritional alterations modulate these parameters in a tissue-specific manner in chickens.

Comparison of insulin and insulin-like growth factor I receptors from rat skeletal muscle and L-6 myocytes

Insulin and IGF-I receptors were solubilized from fused L-6 myocytes, a rat skeletal muscle derived cell line, and compared to rat skeletal muscle receptors. In skeletal muscle, 125I-insulin binding was competed by insulin greater than IGF-I greater than MSA, whereas in L-6 cells IGF-I greater than insulin greater than MSA. 125I-IGF-I binding was competed by IGF-I greater than insulin = MSA in both tissues. On electrophoresis, differences in Mr were observed between skeletal muscle and L-6 derived receptors both in the alpha- and beta-subunits. Six antibodies directed against the human insulin receptor beta-subunit recognized the rat skeletal muscle insulin receptor, while only two reacted strongly with L-6 derived receptors. Skeletal muscle has receptors with relative specificity for insulin and IGF-I respectively; L-6 cells also have two classes of receptors, one is kinetically similar to the IGF-I receptor from skeletal muscle; the other, which binds insulin with relatively high affinity has even greater affinity for IGF-I. This unusual receptor may represent a developmental stage in muscle or the transformed nature of L-6 cells.