Hormone-receptor interactions are noncooperative: application to the beta-adrenergic receptor

Structural basis of allosteric ligand-receptor interactions in the insulin/relaxin peptide family: implications for other receptor tyrosine kinases and G-protein-coupled receptors

The insulin/relaxin superfamily of peptide hormones comprises 10 members in humans. The three members of the insulin-related subgroup bind to receptor tyrosine kinases (RTKs), while four of the seven members of the relaxin-like subgroup are now known to bind to G-protein-coupled receptors (GPCRs), the so-called relaxin family peptide receptors (RXFPs). Both systems have a long evolutionary history and play a critical role in fundamental biological processes, such as metabolism, growth, survival and longevity, and reproduction. The structural biology and ligand-binding kinetics of the insulin and insulin-like growth factor I receptors have been studied in great detail, culminating in the recent crystal structure of the insulin receptor extracellular domain. Some of the fundamental properties of these receptors, including constitutive dimerization and negative cooperativity, have recently been shown to extend to other RTKs and GPCRs, including RXFPs, confirming kinetic observations made over 30 years ago.

Cooperative binding of insulin-like Peptide 3 to a dimeric relaxin family peptide receptor 2

Insulin-like peptide 3 (INSL3) binds to a G protein-coupled receptor (GPCR) called relaxin family peptide receptor 2 (RXFP2). RXFP2 belongs to the leucine-rich repeat-containing subgroup (LGR) of class A GPCRs. Negative cooperativity has recently been demonstrated in other members of the LGR subgroup. In this work, the kinetics of INSL3 binding to HEK293 cells stably transfected with RXFP2 (HEK293-RXFP2) have been investigated in detail to study whether negative cooperativity occurs and whether this receptor functions as a dimer. Our results show that negative cooperativity is present and that INSL3-RXFP2 binding shows both similarities and differences with insulin binding to the insulin receptor. A dose-response curve for the negative cooperativity of INSL3 binding had a reverse bell shape reminiscent of that seen for the negative cooperativity of insulin binding to its receptor. This suggests that binding of INSL3 may happen in a trans rather than in a cis way in a receptor dimer. Bioluminescence resonance energy transfer (BRET(2)) experiments confirmed that RXFP2 forms constitutive homodimers. Heterodimerization between RXFP2 and RXFP1 was also observed.

Evidence for the lack of spare high-affinity insulin receptors in skeletal muscle

In this study, the relationship between the concentration of extracellular insulin, insulin binding and insulin action was evaluated in skeletal muscle. Initially we investigated the dose-response relationship of insulin action using three different experimental models that are responsive to insulin, i.e. the isolated perfused rat hindquarter, incubated strips of soleus muscle, and insulin receptors partially affinity-purified from skeletal muscle. We selected as insulin-sensitive parameters glucose uptake in the perfused hindquarter, lactate production in the incubated muscle preparation, and tyrosine receptor kinase activity in the purified receptor preparation. Our results showed that the dose-response curves obtained in the perfused hindquarter and in the incubated muscle were superimposable. In contrast, the dose-response curve for insulin-stimulated receptor tyrosine kinase activity in partially purified receptors was displaced to the left compared with the curves obtained in the perfused hindquarter and in the incubated muscle. The differences between the dose-response curve for receptor tyrosine kinase and those for glucose uptake and lactate production were not explained by a substantial insulin concentration gradient between medium and interstitial space. Thus the medium/interstitial insulin concentration ratio, when assayed in the incubated intact muscle at 5 degrees C, was close to 1. We also compared the dose-response curve of insulin-stimulated receptor tyrosine kinase with the pattern of insulin-binding-site occupancy. The curve of insulin-stimulated receptor kinase activity fitted closely with the occupancy of high-affinity binding sites. In summary, assuming that the estimation of the medium/interstitial insulin concentration ratio obtained at 5 degrees C reflects the actual ratio under more physiological conditions, our results suggest that maximal insulin action is obtained in skeletal muscle at insulin concentrations which do allow full occupancy of high-affinity binding sites. Therefore our data provide evidence for a lack of spare high-affinity insulin receptors in skeletal muscle.

Biochemical properties of the testicular follitropin-receptor system

The interaction of FSH with membrane receptors from rat and calf testis has been studied in some detail. The FSH receptor has been solubilized through use of the nonionic detergent Triton X-100 and highly purified by affinity chromatography on Affigel-10 coupled to ovine FSH. Hormone binding activity of the solubilized receptor has been preserved for extended periods through use of the structure-stabilizing agent glycerol. Other components of the FSH testes receptor system including the guanyl nucleotide binding protein and adenylate cyclase have been solubilized by nonionic detergents and also found to be stabilized by glycerol. FSH binding activity has been observed in testes cytosol and represents a putative class of receptors prepared from testes in the absence of detergent. The concentration of this buffer-soluble component decreased with age and increased concomitantly with loss of membrane receptors consequent to their down-regulation after administration of exogenous FSH. Phospholipids seem involved in the interaction of FSH with membrane-bound, detergent-solubilized, and buffer-soluble FSH binding activity. Phospholipids may maintain or stabilize a particular receptor conformation necessary for interaction with the hormone. A specific role for GTP seems indicated in regulation of FSH-stimulated adenylate cyclase activity in immature rat testis. Follitropin binding to testes receptor appears modulated by a variety of factors present in serum, testes extracts, follicular fluid, and seminal plasma, which are poorly understood at present. Inhibition of FSH binding by seminal plasma best-fit by a model proposing two hormone binding sites per receptor molecule, where binding to one site decreases the affinity of the other site for FSH. As a result of studies in this and other laboratories, the molecular endocrinology of FSH interaction with testis receptors is becoming increasingly understood.

Separation of two thyrotropin binding components from porcine thyroid tissue by affinity chromatography: characterization of high and low affinity sites

Two distinct thyrotropin (TSH) binding species have been separated from solubilized porcine thyroid membranes. Membranes was solubilized with 1% Triton X-100, and the supernatant was recovered by centrifugation at 105,000 X g. Scatchard analysis of thyrotropin binding to solubilized membranes (SM) yielded a nonlinear plot with Kd values for the high and low affinity components similar to those of intact membranes. Chromatography of the SM preparation on concanavalin A-Sepharose 4B resulted in the retention of 10-20% of the binding activity. Upon elution of the column, a peak of binding material (5-7% of total activity) was eluted at 0.3 M alpha-methyl-D-mannoside. This concanavalin A (Con A) bound fraction exhibited a linear Scatchard plot with a Kd value similar to that of the high affinity component of the SM. The protein fraction that did not bind to Con A (Con A unbound) also exhibited a linear Scatchard plot, but with affinity similar to that of the low affinity component of SM. Discontinuous sucrose density gradient ultracentrifugation revealed the presence of two major binding peaks in the solubilized membrane preparation. The slowly sedimenting peak corresponded to that seen in the Con A bound fraction, whereas the rapidly sedimenting peak corresponded to that of the Con A unbound fraction. Sepharose 6B chromatography indicated that in the case of the Con A unbound fraction, a single peak of specific binding activity was eluted in the void volume, and in the case of the Con A bound fraction, one major peak with an approximate Stokes radius of 67 A and several other minor peaks were eluted. These results demonstrate the physical separation of two distinct TSH binding species from thyroid membranes and provide further support for the model of multiple classes of binding sites.

Effects of octyl beta-glucoside on insulin binding to solubilized membrane receptors

Octyl beta-glucoside (1%), a dialyzable detergent, was used to solubilize the insulin receptor of the turkey erythrocyte membrane. Insulin binding capacity was stable for at least 1 week when the receptor was kept in 1% octyl beta-glucoside at 4 degrees C. The binding properties of the solubilized receptor were examined at detergent concentrations above (1%) and below (0.6%) the critical micelle concentration. A reduction in insulin binding occurred when the detergent concentration was raised above the critical micelle concentration, due to an apparent decrease in the number of binding sites. The specificity of the receptor for insulin analogues was preserved, and the relative affinity of the solubilized receptor, desoctapeptide insulin greater than proinsulin greater than porcine insulin, was similar in 0.6% and 1% detergent. Addition of divalent cations increased insulin binding to a similar extent at both detergent concentrations, but there was a slightly greater stimulation of binding in 0.6% detergent as compared to 1% detergent. The pH optimum for binding was not affected by changes in the detergent concentration. These results indicate that the insulin receptor can be successfully solubilized by octyl beta-glucoside and that the binding activity is quite stable. Therefore, octyl beta-glucoside may be a useful detergent for purification of this receptor. In addition, the data indicate that the binding properties of the insulin receptor can be affected by changes in the physical state of the octyl beta-glucoside.

Human insulin prepared by recombinant DNA techniques and native human insulin interact identically with insulin receptors

Human insulin synthesized from A and B chains separately produced in Escherichia coli from cloned synthetic genes (prepared by the Eli Lilly Research Laboratories, Indianapolis, IN) was characterized by examining its interaction with human cultured lymphocytes, human circulating erythrocytes in vitro, and isolated rat fat cells. The binding behavior of the biosynthetic insulin with human cells was indistinguishable from that of native human or porcine insulins, with respect to affinity, association and dissociation kinetics, negative cooperativity, and the down-regulation of lymphocyte receptors. Similarly, the biosynthetic insulin was as potent as the native insulins in stimulating lipogenesis in isolated rat fat cells. We also examined the receptor binding characteristics of 125I-labeled human and porcine insulins monoiodinated solely at Tyr-A14, which were obtained by means of high-performance liquid chromatography of the iodination reaction mixture (this material was prepared by B. Frank, Eli Lilly Research Laboratories). In all aspects studied, the pure [TyrA14-125I]iodoinsulins were superior as tracers to the monoiodoinsulin purified by the more conventional method of gel filtration.