Evolution of the insulin receptor family and receptor isoform expression in vertebrates.

The molecular phylogeny of the vertebrate insulin receptor (IR) family was reconstructed under maximum likelihood (ML) to establish homologous relationships among its members. A sister group relationship between the orphan insulin-related receptor (IRR) and the insulin-like growth factor 1 receptor (IGF1R) to the exclusion of the IR obtained maximal bootstrap support. Although both IR and IGF1R were identified in all vertebrates, IRR could not be found in any teleost fish. The ancestral character states at each position of the receptor molecule were inferred for IR, IRR + IGF1R, and all 3 paralogous groups based on the recovered phylogeny using ML in order to determine those residues that could be important for the specific function of IR. For 18 residues, ancestral character state of IR was significantly distinct (probability >0.95) with respect to the corresponding inferred ancestral character states both of IRR + IGF1R and of all 3 vertebrate paralogs. Most of these IR distinct (shared derived) residues were located on the extracellular portion of the receptor (because this portion is larger and the rate of generation of IR shared derived sites is uniform along the receptor), suggesting that functional diversification during the evolutionary history of the family was largely generated modifying ligand affinity rather than signal transduction at the tyrosine kinase domain. In addition, 2 residues at positions 436 and 1095 of the human IR sequence were identified as radical cluster-specific sites in IRR + IGF1R. Both Ir and Irr have an extra exon (namely exon 11) with respect to Igf1r. We used the molecular phylogeny to infer the evolution of this additional exon. The Irr exon 11 can be traced back to amphibians, whereas we show that presence and alternative splicing of Ir exon 11 seems to be restricted exclusively to mammals. The highly divergent sequence of both exons and the reconstructed phylogeny of the vertebrate IR family strongly indicate that both exons were acquired independently by each paralog.

Insulin receptors and glucose uptake in the human parasite Schistosoma mansoni.

Very little is known about insulin signalling in schistosomes despite its potential importance in host-parasite molecular dialogue and parasite growth and development. The recent characterization of two insulin receptors (SmIR-1 and SmIR-2) in Schistosoma mansoni has led us to reconsider the question of the potential importance of insulin in host-schistosome interactions. In this work, we demonstrated that insulin could regulate glucose uptake in schistosomes and we investigated the implication of SmIR-1 and SmIR-2 in this process. The possibility that specific inhibitors of SmIR-1 and SmIR-2 tyrosine kinase activities could be developed to target schistosomes is discussed.

Prospects for kinase activity modulators in the treatment of diabetes and diabetic complications.

The worldwide population afflicted with diabetes is growing at an epidemic rate. There are almost five times the number of people suffering from this disease today as compared to 10 years ago and the worldwide diabetic population is expected to exceed 300 million by the year 2028. This trend appears to be driven by the world's adoption of a "western lifestyle" comprising a combination of unhealthy dietary habits and a sedentary daily routine. Today, diabetes is the sixth leading cause of death in the United States and the death rates associated with diabetes have increased by 30% over the last decade. While medications are available to reduce blood glucose, approximately one third of the patients on oral medications will eventually fail to respond and require insulin injections. Consequently, there is a tremendous medical need for improved medications to manage this disease that demonstrate superior efficacy. Emerging knowledge regarding the underlying mechanisms that impair glucose-stimulated insulin secretion and the action of insulin on its target tissues has grown tremendously over the last two decades. During that same period of time, an understanding of the important role that phosphorylation state plays in signal transduction has drawn attention to several kinases as attractive approaches for the treatment of diabetes. Recent advances include the discovery of a"small molecule" allosteric binding site on the insulin receptor, inhibitors of glycogen synthase kinase-3(GSK-3) which improve insulin sensitivity in diabetic animal models and inhibitors of protein kinase C- beta that are presently being evaluated in clinical trials for diabetic retinopathy. This review will detail these recent discoveries and highlight emerging biological targets that hold potential to normalize blood glucose and prevent the progression of diabetes related complications.

A model to explore the interaction between muscle insulin resistance and beta-cell dysfunction in the development of type 2 diabetes.

Type 2 diabetes is a polygenic disease characterized by defects in both insulin secretion and insulin action. We have previously reported that isolated insulin resistance in muscle by a tissue-specific insulin receptor knockout (MIRKO mouse) is not sufficient to alter glucose homeostasis, whereas beta-cell-specific insulin receptor knockout (betaIRKO) mice manifest severe progressive glucose intolerance due to loss of glucose-stimulated acute-phase insulin release. To explore the interaction between insulin resistance in muscle and altered insulin secretion, we created a double tissue-specific insulin receptor knockout in these tissues. Surprisingly, betaIRKO-MIRKO mice show an improvement rather than a deterioration of glucose tolerance when compared to betaIRKO mice. This is due to improved glucose-stimulated acute insulin release and redistribution of substrates with increased glucose uptake in adipose tissue and liver in vivo, without a significant decrease in muscle glucose uptake. Thus, insulin resistance in muscle leads to improved glucose-stimulated first-phase insulin secretion from beta-cells and shunting of substrates to nonmuscle tissues, collectively leading to improved glucose tolerance. These data suggest that muscle, either via changes in substrate availability or by acting as an endocrine tissue, communicates with and regulates insulin sensitivity in other tissues.

Origin of insulin receptor-like tyrosine kinases in marine sponges.

One autapomorphic character restricted to all Metazoa including Porifera [sponges] is the existence of transmembrane receptor tyrosine kinases (RTKs). In this study we screened for molecules from one subfamily within the superfamily of the insulin receptors. The subfamily includes the insulin receptors (InsR), the insulin-like growth factor I receptors, and the InsR-related receptors--all found in vertebrates--as well as the InsR-homolog from Drosophila melanogaster. cDNAs encoding putative InsRs were isolated from the hexactinellid sponge Aphrocallistes vastus, the demosponge Suberites domuncula, and the calcareous sponge Sycon raphanus. Phylogenetic analyses of the catalytic domains of the putative RTKs showed that the sponge polypeptides must be grouped with the InsRs. The relationships revealed that all sponge sequences fall into one branch of this group, whereas related sequences from mammals (human, mouse, and rat), insects and molluscs, and polypeptides from one cephalochordate, fall together into a second branch. We have concluded that (i) the InsR-like molecules evolved in sponges prior to the "Cambrian Explosion" and contributed to the rapid appearance of the higher metazoan phyla; (ii) the sponges constitute a monophyletic taxon, and (iii) epidermal growth factor (EGF)-like domains are present in sponges, which allows the insertion of this domain into potential receptor and matrix molecules.

Insulin receptor sub-types in a human lymphoid-derived cell line (IM-9): differential regulation by insulin, dexamethasone and monensin.

The cells of the human IM-9 lymphocyte-derived line contain a subpopulation of insulin binding sites which differ from classical insulin binding sites in their higher binding affinity for insulin-like growth factor II (IGF-II) and insulin-like growth factor I (IGF-I). These atypical insulin binding sites are identified on IM-9 cells by [125I]IGF-II binding. To determine whether the atypical and classical insulin receptors of IM-9 cells were subject to different modes of in vivo regulation, we treated IM-9 cells with agents known to alter the surface expression of insulin receptors--insulin, dexamethasone and monensin. We then measured insulin and IGF-II binding to the surface of the washed cells. Pretreatment of IM-9 cells with 1 microM insulin for 20 h at 37 degrees C induced a 44-48% decrease in the number of high affinity insulin binding sites, but no change in the number of IGF-II binding sites. In contrast, the surface expression of both insulin and IGF-II binding sites (classical and atypical insulin receptors) increased 1.3 to 1.7-fold after treatment with dexamethasone (200 nM) and decreased 30 to 45% after monensin (1 microM). These results suggest that atypical and classical insulin receptors are differentially susceptible to down-regulation by insulin.

Classification of the insulin-like growth factor binding proteins into three distinct categories according to their binding specificities.

Competitive binding experiments with insulin-like growth factor (IGF)-1, IGF-2 and des-(1-3)-IGF-1 have confirmed the interpretation based on limited amino-terminal sequence analysis that at least three types of IGF binding protein occur. In addition to the acid stable subunit of the large serum binding protein which exhibits des-(1-3)-IGF-1 binding only slightly less than IGF-1, the small IGF binding proteins can be separated into two classes based on differences in des-(1-3)-IGF-1 and IGF-2 binding potencies.

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)

Identification of insulin receptors in epithelial cells from duodenum, jejunum, ileum, caecum, colon and rectum in the rat.

Specific insulin binding was demonstrated in isolated epithelial cells of rat intestine, from the duodenum to the rectum. In every segment tested, insulin binding exhibited similar properties relating to kinetics and specificity and could be interpreted in terms of two populations of receptors possessing different affinities and binding capacities. Insulin receptors were unequally distributed along the intestinal tract, suggesting that the hormone may have varying importance at different points along the intestinal axis. These observations together with previous data on insulin regulation of a number of intestinal epithelial activities suggest that the pancreatic hormone may play an important role in gut physiology from the duodenum to the rectum.

Characterization of solubilized insulin receptors from rat liver microsomes. Existence of two receptor species with different binding properties.

Insulin receptors were solubilized from rat liver microsomes by the nonionic detergent Triton X-100. After gel filtration of the extract on Sepharose CL-6B, two insulin-binding species (peak I and peak II) were obtained. The structure and binding properties of both peaks were characterized. Gel filtration yielded Stokes radii of 9.2 nm (peak I) and 8.0 nm (peak II). Both peaks were glycoproteins. At 4 degrees C peak I showed optimal insulin binding at pH 8.0 and high ionic strength. In contrast, peak II had its binding optimum at pH 7.0 and low ionic strength, where peak I binding was minimal. For peak I the change in insulin binding under different conditions of pH and ionic strength was due to a change in receptor affinity only. For peak II an additional change in receptor number was found. Both peaks yielded non-linear Scatchard plots under most of the buffer conditions examined. At their binding optima at 4 degrees C the high affinity dissociation constants were 0.50 nM (peak I) and 0.55 nM (peak II). Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of peak I revealed five receptor bands with Mr 400 000, 365 000, 320 000, 290 000, and 245 000 under non-reducing conditions. For peak II two major receptor bands with Mr 210 000 and 115 000 were found. The peak II receptor bands were also obtained after mild reduction of peak I. After complete reduction both peaks showed one major receptor band with Mr 130 000. The reductive generation of the peak II receptor together with molecular mass estimations suggest that the peak I receptor is the disulfide-linked dimer of the peak II receptor. Thus, Triton extracts from rat liver microsomes contain two receptor species, which are related, but differ considerably in their size and insulin-binding properties.