Photoaffinity labelling of the insulin receptor in intact rat hepatocytes, mouse soleus muscle, and cultured human lymphocytes

Effect of an alpha-glycosidase inhibitor on experimentally-induced obesity in mice

The effect of prolonged treatment with acarbose, an inhibitor of alpha-glycosidase, has been studied in mice made obese and hyperinsulinaemic by goldthioglucose. After the onset of obesity, one month after goldthioglucose administration, mice were then treated, with or without a 10% sucrose supplement, for four months with acarbose, added to the diet at 50 mg/100 g food. When mice received a standard diet, acarbose had no effect on body weight, blood glucose or insulin levels. In contrast, in the control obese mice receiving a 10% sucrose-enriched diet, it decreased the body weight gain, and prevented the rise in glycaemia and insulinaemia. Basal (non insulin-stimulated) glucose uptake, which is decreased in isolated soleus muscle from untreated obese mice, returned to normal values under acarbose treatment. However, muscle insulin resistance was not improved in acarbose-treated obese mice at maximal and submaximal effective concentrations, despite a higher insulin binding in muscles of acarbose-treated obese than in control obese animals. Furthermore, insulin receptor autophosphorylation and tyrosine kinase activity were altered similarly in treated and untreated obese mice compared to lean mice.

The structure of the hepatic insulin receptor and insulin binding

Hepatocytes or hepatic plasma membranes were photoaffinity-labelled with radioiodinated N epsilon B29-monoazidobenzoyl-insulin. Analysis of the samples by SDS/polyacrylamide-gel electrophoresis and autoradiography revealed the insulin receptor as a predominant band of 450 kDa. When hepatic plasma membranes were first treated with clostridial collagenase and then photolabelled, the insulin receptor appeared as a predominant band of 360 kDa. This effect of collagenase treatment on the insulin receptor was due to Ca2+-dependent heat-labile proteinases contaminating the preparation of collagenase, and it could be mimicked by elastase. The decrease in size of the insulin receptor to 360 kDa resulted from the loss of a receptor component that was inaccessible to photolabelling. In contrast, the size of the insulin receptor of intact cells was not affected by collagenase treatment. This suggests that the site sensitive to proteolysis was located on the cytoplasmic side of the plasma membrane. In hepatic plasma membranes that were treated with collagenase or elastase, and contained the 360 kDa form of the insulin receptor, the binding affinity for insulin was increased by up to 2-fold. These findings support the concept that a component which is either a part of, or closely associated with, the insulin receptor may regulate its affinity for insulin.

Intracellular pathway followed by the insulin receptor covalently coupled to 125I-photoreactive insulin during internalization and recycling

After it interacts with a specific receptor on the cell surface, insulin is internalized in its target cell by an adsorptive endocytotic process and eventually degraded in lysosomes. It was also recently shown that the initial surface interaction between the hormone and its receptor is followed by an internalization of the receptor, which later is recycled back to the cell surface. In the present study the insulin receptor was tagged with a 125I-photoreactive insulin analogue that can be covalently coupled to the insulin receptor by ultraviolet irradiation. Using this tool we could trace by quantitative electron microscope autoradiography the intracellular pathway followed by this labeled receptor. The quantitative analysis of the intracellular distribution of the labeled material as a function of incubation time at 37 degrees C supports the following sequence of events: association first with clear vesicles, second with multivesicular bodies, third with dense bodies, and fourth, a return to the cell surface via clear vesicles. This insulin receptor recycling process is inhibited by monensin but unaffected by cycloheximide.

The subunit structure of the insulin receptor and molecular interactions with major histocompatibility complex antigens

Insulin receptors were labeled with 125I-photoreactive insulin (specifically labeling alpha-subunits) and by insulin-stimulated autophosphorylation (specifically labeling beta-subunits). The results show that the insulin receptor exists under different free and disulfide-linked combinations of alpha and beta subunits. Moreover, the insulin receptor is closely associated to class I antigens of the major histocompatibility complex to form a high molecular weight multi-molecular membrane complex.

Photoaffinity labelling of hepatic plasma membranes suggests two classes of hepatic insulin receptor

Photoaffinity labelling of hepatic insulin receptors revealed specifically-labelled bands of 130, 90 and 40 kDa. Endogenous protease activity in hepatic plasma membranes, as well as contaminating proteases present in preparations of clostridial collagenase, degraded some of the 130-kDa insulin-binding subunit to a 115-kDa form. However, a large proportion of the 130-kDa subunits were resistant to degradation, suggesting the presence of two classes of insulin receptor in hepatic plasma membranes. In one class the 130-kDa subunit was sensitive to proteolysis, while in the other it was not. In contrast, the 130-kDa receptor subunits of adipose tissue were all resistant to such degradation. Scatchard analysis of control and collagenase-treated plasma membranes demonstrated that conversion of the 130-kDa subunit to a 115-kDa form did not affect the insulin-binding characteristics of the receptor. It was also apparent that insulin binds to a single class of high-affinity sites in hepatic plasma membranes.

The endosomal compartment of rat hepatocytes. Its characterization in the course of [125I]insulin internalization

When freshly isolated hepatocytes are incubated with [125I]insulin in the presence of the microtubule-disrupting agent colchicine, internalization of the labelled hormone is not significantly altered. However, the drug limits the endocytosis of the labelled material to a peripheral band of cytoplasm extending 1 micron beyond the plasma membrane. Both in the presence and absence of colchicine, internalized [125I]insulin preferentially associates with clear vesicles (endosomes) and lysosome-like structures, but the relative amount of labelled material associated with clear vesicles is higher in the presence of the drug than in its absence. An inverse pattern is observed for the lysosome-like structures. As demonstrated by cytochemical methods, clear vesicles do not contain the lysosomal enzyme aryl sulfatase. Moreover, colchicine induces an increase of the clear vesicle diameter without affecting their frequency, while it perturbs multivesicular bodies and dense bodies in an opposite way by increasing their frequency without affecting their size. By reducing and/or delaying the fusion between internalized endocytotic vesicles and lysosomes, colchicine allows better characterization of the endosomal compartment of isolated rat hepatocytes and allows it to be distinguished from other compartments, such as multivesicular bodies and the Golgi apparatus.

Appearance of a functional insulin receptor during rabbit embryogenesis

The domain structure of the insulin receptor was investigated in liver and brown adipose tissue of developing rabbits. The structure of the binding domain (alpha-subunit) was analysed after covalent labelling with a 125I photo-reactive insulin analogue. The structure of the tyrosine kinase domain (beta-subunit) and the transmission of the hormonal signal from the alpha-to the beta-subunit were analysed by stimulating with insulin the autophosphorylation of the beta-subunit. Finally, the immunoreactivity of the receptor in developing tissues was assessed with anti-receptor antibodies. The results show that a functional insulin receptor can be detected at the early stages of fetal development in both tissues and is conserved throughout ontogenesis to adulthood.

Identification of functional insulin receptors on membranes from an insulin-producing cell line (RINm5F)

Insulin receptors on RINm5F cell membranes (an insulin-producing rat pancreatic cell line) were studied. To study the insulin receptor alpha-subunit, 125I-labelled photoreactive insulin was covalently bound to the membranes in the absence or presence of unlabelled insulin. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis under reducing conditions showed specific labelling of an Mr 130 000 protein. The receptor beta-subunit was studied by using a cell-free phosphorylation assay. Analysis under reducing conditions showed a phosphoprotein of Mr 95 000 whose level of phosphorylation was selectively increased by insulin, and which was specifically immunoprecipitated by antibodies to the insulin receptor. Further, covalent hormone-receptor complexes purified with anti-insulin antibodies were able to undergo autophosphorylation, indicating the existence of operational receptor subunit arrangements. RINm5F cell insulin receptors (and, by analogy, possibly those of native B-cells) thus display structural and functional integrity comparable with those of conventional insulin target cells.

Characterization of skeletal muscle insulin receptor

The insulin receptor from rat skeletal muscle was characterized. Treatment of muscle membranes with the photoactive insulin analog, 125I[N-epsilonB29-monoazidobenzoyl]-insulin revealed a single protein band of 135,000 Da, the alpha subunit. Iodination of total membrane protein followed by Triton X-100 solubilization and immunoprecipitation demonstrated the presence of a protein band of 90,000 Da, the beta subunit, together with a protein band of 190,000 Da, which may be the receptor precursor. In partially purified receptor preparations, the beta subunit exhibited dose-dependent, insulin-stimulated phosphorylation with incorporation of phosphate solely into tyrosine residues, which was also observed in the 190,000-Da receptor precursor. Purified plasma membranes contained a large amount of insulin-degrading activity which had to be inactivated prior to performing insulin-binding studies. If degradation of insulin was not prevented, apparent enhanced binding in the presence of unlabeled insulin was observed.

Interaction between insulin receptors and major histocompatibility complex antigens in mouse liver membranes

Insulin receptors from H-2k mouse liver membranes were specifically labelled using a photoreactive insulin analogue coupled to the alpha-subunit (Mr 130,000) of the receptor by ultraviolet-irradiation. Up to 25% of the labelled insulin receptors recovered after membrane solubilization and precipitable by anti-insulin receptor antibodies could be immunoprecipitated by various monoclonal antibodies reacting with H-2Kk antigens. None of these monoclonal antibodies inhibited insulin binding to its receptor. These results show that insulin receptors and H-2Kk antigens interact in liver plasma membranes to form non-covalent complexes. This association between insulin receptors and major histocompatibility complex antigens could be involved in the pathogenesis of certain forms of insulin resistance.

Immunoprecipitation of insulin receptors by antibodies against Class 1 antigens of the murine H-2 major histocompatibility complex

Insulin receptors from C57BL/6J mouse (H-2b) liver membranes were specifically labeled with 125I-photo-reactive insulin by UV irradiation. Membranes were solubilized and the capacity of various antibodies reacting with the major histocompatibility complex to immunoprecipitate insulin receptors was tested. About 5% of the labeled receptors were immunoprecipitated by a conventional mouse antiserum against H-2b histocompatibility antigens and by a monoclonal antibody against Class 1 antigens of the H-2b haplotype (Kb and Db). No immunoprecipitation was obtained with a monoclonal antibody against Class 2 antigens of I-Ab or against Class 1 antigens of the H-2k haplotype. Insulin receptors can thus be specifically immunoprecipitated by antibodies against class I histocompatibility antigens.