Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

INSR

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101671

Synonyms

Historical Background

Discovery and isolation of insulin from pancreatic extract in 1922 (Banting et al. 1922) offered life-saving opportunities to diabetes mellitus patients and sparked active research on its mechanism of action. It was not until 1949 that Levine et al. (1949) identified significant reduction of blood sugar following intravenous administration of insulin to dogs, suggesting that insulin facilitates transfer of hexoses across cell membranes. This raised “the question of mechanism by which a molecule of the insulin type may affect the transfer of substances” (Levine et al. 1949). The idea of insulin receptor (IR) was hovering in the air for over two decades. In 1970, binding of radiolabeled insulin to rat liver plasma membranes was demonstrated (House and Weidemann 1970). Soon after, saturable, reversible, and high-affinity (KD = 7 × 10−11 M to 3 × 10−9 M) binding of insulin to membranes of hepatocytes and adipocytes was documented (Freychet et al. 1971; Cuatrecasas et al. 1971; Gammeltoft and Gliemann 1973), pointing to the existence of IR and triggering an avalanche of studies aimed at characterization of this intriguing molecule. IR was shown to be a receptor tyrosine kinase (RTK) that undergoes insulin-induced autophosphorylation at multiple sites (Kasuga et al. 1982). Human INSR gene (22 exons and 21 introns) was shown to be located on chromosome 19, and its cDNA was cloned and sequenced in 1985 (Ebina et al. 1985; Ullrich et al. 1985). IR preproreceptor was predicted to comprise 1370 amino acids (Ullrich et al. 1985) or 1382 amino acids (Ebina et al. 1985). This difference reflected the existence of two IR isoforms resulting from alternative splicing of exon 11: Isoform Long, or IR-B, and Isoform Short, or IR-A (UniProt accession numbers P06213-1 and P06213-2, respectively). Cleavage of the proreceptor by furin and glycosylation during maturation were documented subsequently. Further studies characterized the IR molecule in great detail, identifying the 27-residue signal sequence, the N-terminal extracellular α-subunit, and the C-terminal membrane-spanning β-subunit with the kinase domain and phosphorylation sites, the RKRR furin cleavage site, the α-β and the intermolecular α-α disulfide bonds, as well as similarities with other RTKs such as the epidermal growth factor receptor. The first IR substrate protein (IRS1) to be tyrosine phosphorylated by activated IR was discovered (White et al. 1985) and was later cloned and shown to mediate phosphatidylinositol 3-kinase pathway of cell signaling, thus providing critical insight into the molecular machinery by which insulin regulates cellular metabolism and growth.

IR Structure

IR belongs to the protein kinase superfamily, RTK family, and IR subfamily. The other two members of the latter are the insulin-like growth factor 1 receptor (IGF1R) and the insulin receptor-related receptor (IRR). All three are expressed in vertebrates, except that IRR has not been found in teleost fish, whereas invertebrates have just one IR homolog, Daf-2. IR is produced as a single-chain preproreceptor that sheds the signal sequence co-translationally and undergoes calnexin- and calreticulin-assisted folding in the endoplasmic reticulum (ER) and posttranslational modifications as the protein travels from ER to the Golgi complex and to the plasma membrane as a mature (αβ)2 homodimer. What makes the IR family members special is that they form covalent (disulfide-bridged) homodimers that stay in the basal, inactive state until hormone binding and activation, as opposed to other RTK’s that undergo noncovalent dimerization and activation upon hormone binding. (The author apologizes for skipping important references (available upon request); here the number of references is strictly limited.)

The amino acid sequence of the IR-B proreceptor is presented in Fig. 1. The proreceptor chain is composed of distinct structural domains, i.e., two leucine-rich repeat domains (L1 and L2), an intervening cysteine-rich domain (CR), three fibronectin type III domains (FnIII-1, FnIII-2, and FnIII-3), a transmembrane domain (TM), a juxtamembrane domain (JM), a tyrosine kinase domain (TK), and a C-terminal tail (CT). The FnIII-2 domain is divided into two parts, FnIII-2a and FnIII-2b, by an insert domain (ID) that contains the 12-residue product of exon 11 (KTSSGTGAEDPR), which is present only in IR-B, the furin cleavage site RKRR↓, cysteines involved in receptor dimerization, and the C-terminal part of the α-subunit involved in hormone binding. Proreceptor dimerization occurs in the ER through Cys524 (FnIII-1 domain) and at least one of Cys682, Cys683, and Cys685 (ID domain) (Sparrow et al. 1997). It has been proposed that Cys682 and Cys685 form an intrachain disulfide, while Cys683 is involved in interchain disulfide (Croll et al. 2016). Proteolytic cleavage by furin occurs in the trans-Golgi network, resulting in the α- and β-subunits of IR (residues 1–731 and 736–1355, respectively, human IR-B numbering) connected via a single disulfide bond between Cys647 (α-chain) and Cys872 (β-chain) (Sparrow et al. 1997). The tetrabasic stretch RKRR that appears at the C-terminus of the α-chain following furin cleavage is apparently removed by carboxypeptidases, but no published data have been found documenting such trimming. The proreceptor dimer, without cleavage into α- and β-chains, binds insulin but undergoes less efficient autophosphorylation. IR-B proreceptor binds insulin tighter than IR-A proreceptor. In certain species, such as some cartilaginous fishes, IR is unprocessed into α- and β-chains yet binds insulin and insulin-like growth factor 1 (IGF1) with high affinity and functions normally.
INSR, Fig. 1

Amino acid sequence of IR proreceptor long isoform (human IR-B). Loops between domains are included into domains in order to maintain sequence continuity, which may result in ±5 amino acid residue uncertainty in domain boundaries. The insert domain (underlined) invades FnIII-2, dividing it into FnIII-2a and FnIII-2b segments. The furin cleavage site RKRR in the ID domain is shown with white letters, highlighted magenta. The 12-residue sequence resulting from exon 11 is highlighted yellow (this stretch is absent in the short isoform, IR-A). Cys524 (FnIII-1 domain) and at least one of Cys682,683,685 (ID), highlighted cyan, are involved in inter-α-chain disulfide bonding. Cys647 (FnIII-2a domain) and Cys872 (Cys860 in IR-A) (FnIII-3 domain), highlighted green, form the α-β disulfide bond. The residues in the L1 domain and the αCT segment at the C-terminus of the α-chain that are involved in insulin binding are colored red (Menting et al. 2013; De Meyts 2015; UniProt entry P06213-1). Asn residues that are N-glycosylated are shown in white letters, highlighted dark green. Thr and Ser residues involved in O-glycosylation are shown in yellow letters, highlighted dark green. Tyr972 in the JM domain and Tyr1158,1162,1163 in the TK domain (colored red, highlighted gray) become autophosphorylated upon hormone binding (Tyr972 plays a major role in binding downstream signaling molecules, including IRS1 and SHC1). Other tyrosines in JM and CT domains that undergo autophosphorylation are colored white, highlighted blue (UniProt). Residues of the TK domain involved in ATP binding are colored green. The catalytic Asp1132 in the TK domain is highlighted red. Amino acid numbering of IR-A is the same up to residue Arg717 and is reduced by 12 units thereafter (residues 718–729 of IR-B are absent in IR-A)

IR is N-glycosylated at 18 or 19 asparagines and O-glycosylated at six threonines and serines of the ectodomain, i.e., the α-subunit and the extracellular part of the β-subunit. Most N-glycans are mannose rich, and some are complex oligosaccharides containing galactose, fucose, N-acetylglucosamine, and sialic acid. Glycosylation facilitates protein processing, folding, and trafficking.

X-ray crystal structures of a set of IR fragments, including the ectodomain or the intracellular TK domain, have been solved, but determination of the structure of the full-length receptor has not been achieved thus far (Cabail et al. 2015; Croll et al. 2016; reviewed by Tatulian 2015). The structure of IR ectodomain revealed a ∧-shaped dimeric conformation with the two protomers associated in an antiparallel manner through a twofold rotational axis, where L1, CR, and L2 domains make one foot and the three FnIII domains the other foot of the ∧ shape (PDB entries 2dtg and 3loh, superseded by 4zxb; see Croll et al. 2016). The quality of the electron density map prevented construction of the ID domain, including the αCT stretch at the C-terminus of the α-chain that plays an important role in hormone binding. Based on higher resolution diffraction data and novel analysis techniques, the structures of FnIII-1 and FnIII-3 domains have been improved recently, the ID structure was modeled, intermolecular (α-α) disulfides were revised, and all N-glycans have been added to the structure (Croll et al. 2016). The results of this revision have led to two structures, one in which atom coordinates are based on reliable electron densities (PDB entry 4zxb) and another (Model S1) that has all residues but involves tentative modeling (Fig. 2). A prominent new feature of 4zxb vs. 2dtg is the presence of the α-helical segment αCT (residues 698–709), and a major component of Model S1 is the ID domain, which makes two large loops, outside and inside the ∧ shape.
INSR, Fig. 2

Ectodomain structure of IR isoform A. (a) Ectodomain dimer shown in ribbon format based on Model S1 from Croll et al. (2016). One protomer is colored gray. The color code for the other protomer is as follows: L1, yellow; CR, green; L2, red; FnIII-1, indigo; FnIII-2, cyan; ID, magenta; FnIII-3, rose. Amino acid residues of L1 domain of one protomer and ID of the other involved in hormone primary binding site (shown in red letters in Fig. 1) are shown in CPK format, gray in one protomer and according to atom type in the other (carbons, gray; oxygens, red; nitrogens, blue; sulfurs, orange; hydrogens not shown). Cys524 (upper part of central axis), Cys682,683,685 (central part), and Cys647–Cys860 (Cys647–Cys872 in IR-B) disulfides (lower left and right) are shown in ball and stick format, colored according to atom type. According to this model, Cys524 and Cys683 form α-α intermolecular disulfide bonds, Cys682 and Cys685 form an intramolecular disulfide bond, and Cys647–Cys860 is a disulfide bond between the α- and β-chains (b) The structure shown in panel a is turned about the central vertical axis by 90 degrees. (c) Model S1 ectodomain dimer structure with the glycans present. Protein chains are shown in van der Waals surface format, and the glycans in CPK format. One protomer is shown in light green, glycans cyan, and the other protomer is shown highlighting electrostatic surfaces (negative, red; positive, blue; neutral, white), glycans according to atom type. (d) The structure shown in panel c is turned about the central vertical axis by 90°

The structures of various fragments of the cytoplasmic TK domain of IR have been determined by X-ray crystallography and suggested important conformational changes in the β-chain upon receptor phosphorylation (Hubbard et al. 1994; Hubbard 1997). Studies on a construct that involved the juxtamembrane region in addition to the tris-phosphorylated TK domain uncovered noncovalent dimerization between the JM domain of one protomer and the N-lobe of the TK domain of the other (Cabail et al. 2015). The dimeric structure was speculated to play a key role in receptor activation. According to the proposed mechanism, in the basal, unphosphorylated state, the JM-N-lobe interaction occurs within same hemireceptor protomers, thus keeping the receptor in an inactive conformation. Receptor phosphorylation and accompanying conformational changes lead to cross dimerization and structural rearrangements, facilitating ATP binding and phosphorylation of substrate proteins (Cabail et al. 2015).

Many key questions regarding the structural basis for IR activation still remain unanswered. It is not clear what prevents the kinase activity of the receptor in the basal state, spatial dissociation of the TK domains, or formation of a nonproductive complex (Cabail et al. 2015). The fundamental questions of what conformational changes occur upon insulin binding and how these changes trigger receptor autophosphorylation are awaiting clarification (De Meyts 2015).

Insulin Binding and Receptor Activation

Both A and B isoforms of IR bind insulin as well as insulin-like growth factors 1 and 2, IGF1 and IGF2. The difference between the two isoforms in terms of ligand-binding properties is that the IR-A (exon 11–) isoform binds IGF2 with much higher affinity than IR-B (exon 11+) isoform and can be involved in cancer-related mitogenic activity (Belfiore et al. 2009; Vigneri et al. 2016). Both isoforms bind IGF1 with relatively low affinity (Hale and Coward 2013). In addition to IR and IGF1R, hybrid receptors, composed of one αβ hemireceptor of IR and one of IGF1R, are produced in all tissues that express both receptors. They can be activated by all three hormones, i.e., insulin, IGF1, and IGF2, but their physiological importance remains to be characterized.

Mechanisms of binding of insulin, IGF1, and IGF2 to IR, IGF1R, or the hybrid receptor share common features. The receptor presents two hormone binding sites, 1 and 2 on one protomer and 1′ and 2′ on the other. The “primary” binding site 1 (or 1′) involves the L1 domain of one protomer and the αCT region of the second, and site 2 (or 2′) involves the boundary between FnIII-1 and FnIII-2 domains of the second protomer (De Meyts 2015). The antiparallel arrangement of the receptor dimer positions binding site 1 against 2′ and 2 against 1′ (Fig. 3). Insulin can bind to the primary site 1 (or 1′) with 6 nM affinity and to site 2′ (or 2) with a lower affinity of around 400 nM, resulting in a divalent binding with 0.2 nM overall affinity. This disrupts the symmetric dimeric arrangement of IR, resulting in a closer contact between sites 1 and 2′ (or 1′ and 2) on one side of the ectodomain and dislodging of respective sites on the other side. Low-affinity binding then can take place on each of the displaced binding sites. This mechanism has been put forward by De Meyts and coworkers (see De Meyts 2015 and references therein) and has been confirmed by a plethora of biochemical and structural data. It explains the negative cooperativity of hormone binding to IR and IGF1R, as well as the bell-shaped hormone concentration dependence of hormone dissociation rate for IR (for more details see De Meyts 2015; Tatulian 2015; Croll et al. 2016).
INSR, Fig. 3

Cartoon for the domain structure, dimerization, and hormone-induced conformational rearrangement of IR. In panel a, the domain structure of IR αβ hemireceptor is shown. Each domain is presented as a distinct shape and labeled according to domain names. Domains FnIII-1, FnIII-2, and FnIII-3 are labeled F1, F2, and F3, respectively. Domains that constitute the α-subunit are colored with shades of blue/purple, and those of the β-subunit are colored with shades of green. The F2 domain is split into F2a and F2b by an insert region (shown in gray) that contains the furin cleavage site and the sites of intermolecular disulfide bridging. Panel b shows an IR (αβ)2 dimer with a central twofold rotational axis, stabilized by at least two disulfide bonds between the two α-subunits. The disulfide bond between the α-subunit’s F2a domain and the β -subunit’s F3 domain and those between the two α-subunits are shown as black bars. Panel c shows the proposed conformational change in IR upon hormone binding, albeit very schematically. Both IR αβ protomers are colored gray. The primary hormone binding site (part of L1 domain and αCT region) is colored cyan in one protomer and purple in the other, and the secondary hormone binding site (junction between FnIII-1 and FnIII-2 domains) is colored green in one protomer and yellow in the other. The hormone molecule is shown as a blue hexagon, which binds to a binding site and causes conformational rearrangement in the receptor, resulting in a productive interaction between the TK domains and trans-autophosphorylation. Phosphates added to tyrosines are shown by red letters P. The lipid bilayer membrane is shown schematically

Structural data indicate that IR in the basal state does not provide an open hormone binding pocket (Fig. 2), suggesting that the receptor needs to undergo conformational rearrangements to accommodate high-affinity hormone binding. Crystallographic studies identified that insulin interacts with the αCT segment of IR, which is repositioned against the L1 domain of same protomer, and opens up in a way to allow the C-terminal segment of its B chain to interact with L1 domain of the other IR protomer (Menting et al. 2013, 2014). Thus, both the hormone and the receptor undergo significant conformational changes upon interaction, leading to an “induced fit” and “detachment” model for hormone binding to IR (De Meyts 2015).

Based on the crystal structures of IR and IGF1R ectodomains with and without the bound hormone, Kavran et al. (2014) proposed that the ligand-free, inactive state of these receptors is stabilized by tight interaction between the L1 domain of one protomer and the FnIII-2 and FnIII-3 domains of the other, holding the ∧-shaped ectodomain in a wide open conformation. Engagement of the L1 domain by the hormone frees the FnIII-2/FnIII-3 module, allowing it to swing toward the central axis of the receptor, thereby moving the downstream TM, JM, and TK domains closer to each other and facilitating trans-autophosphorylation (Kavran et al. 2014), as schematically shown in Fig. 3c. This receptor activation mechanism has not been unequivocally supported by structural data, however, because the hormone-bound structure was determined for a construct lacking the FnIII-2/FnIII-3 segments (Menting et al. 2013). Therefore it should be considered hypothetical rather than an established mechanism.

Physiological Function of IR

In mammals, the IR-A isoform is expressed ubiquitously, including nearly all organs, and is upregulated during prenatal development (Belfiore et al. 2009), and IR-B is mainly expressed in the skeletal muscle, liver, adipose tissue, placenta, and kidneys (Hale and Coward 2013). In humans, IR-B expression (at mRNA level) is 85–90% of total IR in the liver, 78% in skeletal muscle, up to 70% in adipose tissue, and 45–50% in placenta and is undetectable in lymphocytes (Benecke et al. 1992). The level of expression of IR in different tissues and organs varies in a wide range. For example, erythrocytes contain only around 40 IR molecules per cell, whereas more than 200,000 IR molecules are present on adipocytes and hepatocytes (White and Kahn 1993).

The anabolic peptide hormone insulin is produced by pancreatic β-islet cells in response to food intake and is spread throughout the organism by the circulation system. Binding of insulin to IR triggers conformational changes in IR and transmembrane signal transduction to the cytosolic β-chains, culminating in trans-autophosphorylation at multiple tyrosine residues in the JM, TK, and CT domains of IR (Figs. 1, 3, and 4). Tyrosine phosphorylation sites recruit various SH2 domain-containing signaling molecules leading to multiple functions of IR. Given the limited space, only a brief summary of IR signaling and downstream cellular processes is presented below.
INSR, Fig. 4

Simplified presentation of the insulin signaling cascade. Binding of insulin to IR causes conformational changes in the receptor, resulting in trans-autophosphorylation of its cytoplasmic β-chains at multiple sites. The insulin-induced asymmetric conformational change in IR is thought to involve inward motion of the membrane-proximal segments of the ectodomain and subsequent intermolecular contact between the β-chains. IRS1, phosphorylated by IR, mobilizes the Akt and/or the MAPK signaling branches. In the Akt pathway, phosphorylated IRS1 activates PI-3K, which is followed by phosphorylation of PIP2 to PIP3, membrane docking of PDK1 and Akt2 through their PH domains, and phosphorylation of Akt2 by PDK1. Activated Akt2 acts as activator of several targets, leading to membrane translocation of the glucose transporter GLUT4 and glucose import, and regulation of glycogen synthesis. Akt2 is also involved in adipogenesis, lipolysis, triglyceride synthesis, and protein synthesis in various tissues. In the MAPK pathway, phosphorylated. IRS1 or SHC1 activates the GRB2-SOS proteins, which mobilize the Ras-Raf-MAPK cascade resulting in transcriptional regulation of gene expression and cellular survival/mitogenic effects

The proteins that immediately interact with IR following its autophosphorylation are insulin receptor substrate (IRS) molecules. They act as adapter proteins that provide communications between cell signaling components and mediate pathways of normal physiology as well as pathology. IRS1 binds to phosphorylated Tyr972 (IR-B numbering) in the JM domain in a sequence (NPXY)-specific manner (X is any residue). Tyrosine phosphorylation at multiple sites of IRS1makes it an interaction partner for more than a dozen signaling molecules and thereby triggers a downstream polykinase cascade that includes processes ranging from glucose intake and glycogen synthesis to transcription regulation and mitogenesis. Phosphorylated Tyr972 recruits other proteins as well, such as the mitogenic protein SHC1, the transcription factor STAT5B, and the suppressor of cytokine-3 (SOCS3) that is involved in leptin and insulin resistance (http://www.genecards.org/cgi-bin/carddisp.pl?gene=INSR). Phosphorylated tyrosines 1158, 1162, and 1163 in the TK domain bind cytokine suppressor SOCS1 protein and MAPK3 inhibitor adapter protein GRB14. Tyrosine phosphorylation in the CT domain recruits IRS2. Thus, the events downstream to IR autophosphorylation can be divided into two major branches, metabolic, or Akt pathway, and mitogenic, or MAPK pathway.

IR primarily performs a metabolic function and thereby controls nutrient homeostasis, cellular metabolism and growth. High affinity of IR-A for both insulin and IGF2 is important for prenatal growth through the IGF2 pathway and postnatal metabolism through the insulin pathway. These patterns can vary among mammals, however.

The metabolic, or Akt pathway itself, has several components, e.g., glucose import, regulation of glycogen synthesis, gluconeogenesis, and protein synthesis. In this pathway, phosphorylated IRS1 binds to the regulatory subunit of the phosphatidylinositol-3 kinase (PI-3K), which then phosphorylates phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 3,4,5-trisphosphate (PIP3) in the cytoplasmic face of the cell plasma membrane. PIP3 recruits the PH domain-containing proteins, pyruvate dehydrogenase lipoamide kinase isoform 1 (PDK1) and Akt2. PDK1 phosphorylates and activates PIP3-anchored Akt2, which mediates plasma membrane translocation of the glucose transporter GLUT4 by phosphorylation of C2 domain-containing protein CDP138, followed by glucose influx (Xie et al. 2011). Another function of Akt2 is phosphorylation of glycogen synthase kinase 3 (GSK3), leading to regulation of glycogen synthesis in the liver (Fig. 4). Akt2 also mediates adipogenesis, lipolysis, and triglyceride synthesis in adipocytes (Shearin et al. 2016). Phosphorylation of the forkhead box O (FoxO) family transcription factors by Akt regulates glucose intake, glucose synthesis in hepatocytes, and protein synthesis in skeletal muscle (Martins et al. 2016). Thus, Akt plays a central role in the metabolic branch of IR signaling.

The mitogenic, MAPK pathway, can be triggered through both IRS1 and SHC1 mechanisms of IR signaling. In either case, phosphorylated IRS1 or SHC1 binds to and phosphorylates growth factor receptor-bound protein 2 (GRB2). GRB2 engages the son of sevenless (SOS) protein, which in turn activates the Ras protein and thereby sets in motion the Ras-Raf-MAPK cascade. This results in translocation into the nucleus of MAPK and regulation of various transcription factors, leading to gene expression, mitogenesis, cell survival, and proliferation (Fig. 4) (Vigneri et al. 2016). The mitogenic pathway is usually mobilized by growth factors and their receptors but can be hijacked by IR, depending on many factors, including hormone availability and concentration, receptor expression levels and functional state, and duration of hormone exposure.

Role of IR in Disease

Defects in insulin signaling pathways, coupled with impaired insulin secretion by the β cells, lead to hyperglycemia and insulin resistance, i.e., type 2 diabetes mellitus (T2DM). T2DM constitutes approximately 90% of all diabetes cases and affects around 400 million people worldwide. Diabetes is associated with many pathologies including, but not limited to, metabolic syndrome, obesity, chronic inflammation, dyslipidemia, atherosclerosis, hypertension, nephropathy, retinopathy and blindness, coronary heart disease, stroke, Alzheimer’s disease, and cancer. A fraction of T2DM is caused by mutations or dysfunction of proteins in the insulin signaling cascade, including missense mutations in the hormone binding or the TK domains of IR (Hubbard et al. 1994; Sesti et al. 2001; LeRoith and Accili 2008; Menting et al. 2013). UniProt reports 56 naturally occurring mutations in human IR associated with insulin resistance through defects in receptor processing and trafficking, hormone binding, or the catalytic kinase function of IR.

Adipocyte-specific IR knockout mice showed adipose and hepatic abnormalities and had significantly shortened lifespan (Friesen et al. 2016), whereas induction of hepatic expression of IR, especially the A isoform, in diabetic mice ameliorated glucose intolerance (Diaz-Castroverde et al. 2016). Total prevention of IR expression in humans resulted in severe growth retardation and diabetes, while homozygous IR-null mice or those expressing kinase-deficient IR rapidly developed hyperglycemia and did not survive beyond a few days of postnatal life (Accili 1995; Accili et al. 1996; Lauro et al. 1999). These data underscore the importance of IR in preventing diabetes and improving survival and life expectancy (LeRoith and Accili 2008).

Apart from genetic factors, T2DM can arise from sedentary lifestyle coupled with high-calorie diet. These factors cause obesity, which may be associated with chronic inflammation and T2DM. The latter in turn can cause endothelial dysfunction and defective vascular homeostasis through altered gene expression and cell signaling in vascular endothelium, leading to atherosclerosis, the major life-shortening factor for diabetic patients (Rask-Madsen and King 2013). High affinity of IR-A for IGF2 contributes to atherogenic processes and vascular damage. IGF2 and TNF-α induce overexpression of IR-A and IGF1R and increase the level of IR-A/IGF1R hybrid receptors in vascular smooth muscle cells, which augment atherosclerosis (Gómez-Hernández et al. 2013).

Risk of cardiovascular diseases such as myocardial infarction, heart failure, and ischemic stroke is significantly higher among patients with metabolic syndrome and/or T2DM. Diabetes-related cardiomyopathy has been documented in numerous experimental models using diabetic rodents (Fuentes-Antrás et al. 2015). These adverse effects are related to both proximal defects in the insulin signaling pathways and distal events such as transcription regulation by FoxO1 in the liver resulting in reduced levels of plasma adiponectin and high-density lipoprotein as well as overproduction of very low-density lipoprotein.

Insulin and IR are present in the brain where they support glucose metabolism and downregulate neuronal apoptosis and thereby contribute to neuronal functions such as cognition, memory, and learning. Insulin resistance and hyperinsulinemia are risk factors for Alzheimer’s disease (AD) and dementia possibly through promoting amyloid β (Aβ) peptide deposition (Willette et al. 2015), as well as reduced Aβ degradation and elevated tau protein phosphorylation (Morales-Corraliza et al. 2016). Insulin therapy could improve the cognitive function, but systemic (e.g., intravenous) insulin administration may cause adverse effects such as hypoglycemia. Intranasal administration of insulin to AD patients, which ends up mostly in the brain, indeed significantly ameliorates the memory and cognition of the patients and is associated with increased cerebrospinal Aβ levels and decreased tau-to-Aβ ratios (Craft et al. 2012). (Higher cerebrospinal Aβ means lower brain deposits.) Aβ oligomers were more neurotoxic in the absence than in the presence of insulin, suggesting that direct effect of insulin on the Aβ peptide may play a role in the correlation between T2DM and AD (Luo et al. 2016).

Strong link has been established between aberrant IR signaling and cancer. T2DM and hyperinsulinemia lead to activation of IR-A in tumor cells and promote various cancers (Vigneri et al. 2016). Owing to both its abundance in neoplasm and its high affinity for IGF2, IR-A favors mitogenesis and thus contributes to tumor growth and malignancy. In addition, overexpression of IR-A increases the level of IR-A/IGF1R hybrid receptors, which exert mitogenic effect and support cancer growth through both insulin and IGF pathways (Belfiore et al. 2009; Lundby et al. 2015). Anticancer drugs targeting growth hormone receptors, including IGF1R, have failed or proved ineffective because of a bypass route for the mitogenic activity of IGF2 through the IR-A pathway (Singh et al. 2014; Chan et al. 2016). Anti-IGF1R immunotherapy in transgenic mice with pancreatic carcinogenesis did not significantly affect tumor growth, but the same therapy in IR knockout mice resulted in strong reduction in tumor burden, increased apoptosis, and recovered sensitivity to anti-IGF1R therapy (Ulanet et al. 2010). Likewise, antibodies or small-molecule drugs aimed at IGF1R alone or IR-A alone showed little benefit in breast cancer in humans and mice, whereas combined inhibition of both IGF1R and IR-A was more effective (Rostoker et al. 2013; Chan et al. 2016). Dual IGF1R/IR therapy has also been more successful in treatment of prostate, lung, and colorectal cancers compared to targeted inhibition of IGF1R (Dayyani et al. 2012; Vincent et al. 2013; Vidal et al. 2015; Puzanov et al. 2015; Bendell et al. 2015).

Summary

IR is a special RTK that forms a disulfide-linked (αβ)2 type homodimer of two αβ hemireceptors. Insulin is the primary ligand of IR, and insulin binding to IR triggers a signaling cascade necessary for glucose uptake and metabolism, regulation of glycogen synthesis, gluconeogenesis, protein synthesis, lipolysis, and other metabolic functions. IR-A also binds IGF2 with high affinity, which contributes to mobilization of mitogenic pathways of IR signaling, often leading to various pathologies.

Unlike other RTKs, IR and the other two members of the IR family receptors (IGF1R and IRR) are covalent dimers. Receptor activation takes place through hormone-induced conformational changes in the ectodomain that are transmitted to the cytoplasmic TK domains, resulting in receptor trans-autophosphorylation and activation (IRR is less extensively studied in this respect). Much has been elucidated about the structural basis of insulin-induced transmembrane signaling of IR, primarily by X-ray crystal structures of the ectodomain and the TK domain of the receptor. The structure of the full-length IR has not been achieved yet, leaving important questions about the IR signaling mechanism unanswered.

There is strong evidence that IR, especially its A isoform, is involved in progression of a variety of diseases, ranging from T2DM to cardiovascular disease, atherosclerosis, heart failure, stroke, Alzheimer’s disease, and cancer. Similarities between the growth hormone receptor IGF1R and IR pose serious problems in cancer therapy targeting the former because IR-A hijacks the IGF2 pathway of cell proliferation. Drugs designed for dual inhibition of both receptors open promising prospects for treatment of certain cases of cancer.

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Authors and Affiliations

  1. 1.Department of Physics, College of SciencesUniversity of Central FloridaOrlandoUSA