Regulation and function of insulin and insulin-like growth factor receptor signalling

Abstract

Receptors of insulin and insulin-like growth factors (IGFs) are receptor tyrosine kinases whose signalling controls multiple aspects of animal physiology throughout life. In addition to regulating metabolism and growth, insulin-IGF receptor signalling has recently been linked to a variety of new, cell type-specific functions. In the last century, key questions have focused on how structural differences of insulin and IGFs affect receptor activation, and how insulin-IGF receptor signalling translates into pleiotropic biological functions. Technological advances such as cryo-electron microscopy have provided a detailed understanding of how native and engineered ligands activate insulin-IGF receptors. In this Review, we highlight recent structural and functional insights into the activation of insulin-IGF receptors, and summarize new agonists and antagonists developed for intervening in the activation of insulin-IGF receptor signalling. Furthermore, we discuss recently identified regulatory mechanisms beyond ligand-receptor interactions and functions of insulin-IGF receptor signalling in diseases.

Figure 1.. Structure and domain organization of insulin, IGFs, and IR family receptor.

a. IR family receptors are initially translated as a single polypeptide. During processing, each protomer is cleaved into an α-subunit and a β-subunit. Two α-subunits and two β-subunits are linked by multiple disulfide bonds and form a stable dimer. The extracellular domains (ECDs) of each protomer contain two leucine-rich repeats (L1 and L2), a cysteine-rich region (CR), three fibronectin type III domains (FnIII-1, −2, and −3), insert domain alpha and beta (IDα and IDβ), and a C-terminal tail of α-subunit (α-CT). The β-subunit traverses the plasma membrane (PM) through a single transmembrane (TM) domain that connects to a juxtamembrane (JM), a tyrosine kinase (TK), and a C-terminal (CT) domains. The exon 11 of IR encodes 12 amino acids (a.a.) at the α-CT. Long IR isoform (IR-B, +12 a.a.) contains exon 11, while short IR isoform (IR-A, −12 a.a.) does not. Disulfide bonds are indicated by red lines. b. The sequence alignment of mature insulin, IGF1, and IGF2 with domain information (top) and their structures (bottom) are shown. Disulfide bonds are indicated by red lines. A-chain (or domain), blue; B-chain (or domain), green; C-domain, yellow; D-domain, red. c. Ligand-activated IR and IGF1R trigger two major signaling cascades and control metabolism, growth, and proliferation. Insulin receptor (IR); IR substrate (IRS); phosphoinositide 3-kinase (PI3K); Ser/Thr-kinase phosphoinositide-dependent kinase (PDK); Protein kinase B (AKT); Mammalian target of rapamycin complex (mTORC); Forkhead family box (FOXO); Tuberous sclerosis 2 (TSC2); Glycogen synthase kinase 3b (GSK3b); RabGAP TBC1 domain family member 4 (TBC1D4); Ras-associated binding (RAB); Glucose transporter type 4 (GLUT4); Ras homolog enriched in brain (RHEB); Src homology and Collagen (SHC); Growth factor receptor-bound protein (GRB); GRB2-associated binding protein (GAP1); Son of Sevenless (SOS), Mitogen-activated protein kinase kinase (MEK); Extracellular signal-regulated kinase (ERK); Src homology-2 protein tyrosine phosphatase (SHP2); Protein tyrosine phosphatase-1B (PTP1B); Receptor protein tyrosine phosphatases (RPTPs); P, phosphorylation. d. When the plasma pH increases, activated IR-related receptor (IRR) boosts the expression of Cl⁻/HCO3⁻ exchanger, pendrin, which promotes bicarbonate excretion into the lumen to maintain acid-base equilibrium.

Figure 2.. Activation mechanisms of IR

a. The structures of IR in both inactive and active states. PDB ID, inactive IR: 4ZXB; active IR: 6PXV. Intracellular domains are shown as cartoon illustrations. Protomer 1 (blue), protomer 2 (green), site-1 insulin (yellow), and site-2 insulin (purple). b. The detailed binding mode of site-1 insulin (yellow) and site-2 insulin (purple) in the T-shaped active IR (6PXV). Site-1a (L1/α-CT’), site-1b (the top region of FnIII-1), and site-2 (the side surface of FnIII-1) are shown. c. Distinct active conformations of IR depending on the number of bound insulins. Protomer 1 (blue), protomer 2 (green), site-1 insulin (yellow), and site-2 insulin (purple). d. The proposed sequential model of IR activation under sub-saturated insulin concentrations. Insulin firstly binds to the exposed site-2 of apo-IR. The translocation of insulin from site-2 to site-1 completely breaks the apo-IR, allowing for the conformation changes of each IR protomer to form the active conformation. Protomer 1 (blue), protomer 2 (green), and site-1 insulin (yellow). e. The proposed conformation changes of IR at saturated insulin concentrations. Maximum four insulin molecules simultaneously bind to two site-1s and two site-2s, breaking the apo-IR and promoting the conformational changes of IR from a Λ-shape to a symmetric T-shape. Protomer 1 (blue), protomer 2 (green), site-1 insulin (yellow), and site-2 insulin (purple).

Figure 3.. Activation mechanisms of IGF1R and IRR

a. The structures of IGF1R in both inactive and active states. PDB ID, inactive IGF1R: 5U8R; active IGF1R: 6PYH. Protomer 1 (blue), protomer 2 (green), and IGF1 (yellow). b. The detailed binding mode of IGF1 (yellow) in the Γ-shaped active IGF1R (6PYH). Site-1a (L1/α-CT’) and site-1b (FnIII-1) are shown. The C-loop of IGF1 binds to CR domain of IGF1R and further stabilizes the active IGF1R conformation. c. The rigid connection between the two ID/α-CT motifs of IGF1R results in negative cooperativity between the two site-1s, preventing its structural transition to the symmetric T-shape. The distance between two membrane proximal domains is shown. d. The proposed conformation changes of IGF1R during IGF1-induced IGF1R activation. The binding of one IGF1 to IGF1R disrupts the apo-IGF1R and triggers a conformational rearrangement that leads to the formation of the Γ-shaped active IGF1R. Two protomers (blue and green) and IGF1 (yellow) are shown. e. The structures of IRR in both inactive and active states. PDB ID, inactive IRR: 7TYK, and active IRR: 7TYM. Two promoters are shown as green and blue. f. The proposed conformation changes of IRR during alkaline pH-induced IRR activation. The increase of pH disrupts the apo-IRR. Subsequently, the two relaxed IRR protomers undergo a rigid-body rotation to form a T-shaped active conformation. Two protomers are shown as blue and green.

Figure 4.. Regulation of insulin-IGF receptor signaling

a. IGFs form binary complexes with each of the six IGF-binding proteins (IGFBPs). IGFBP3 or IGFBP5 bound IGFs can further assemble into a ternary complex with acid-labile subunit (ALS). Protease, including pregnancy-associated plasma protein A and A2 (PAPP-A/A2), cleaves IGFBPs and increases IGFs bioavailability. Stanniocalcin 1 (STC1), STC2, and proform of the eosinophil major basic protein (proMBP) inhibit PAPP-A/A2 and control insulin-IGF signaling. The estimated half-lives of free IGFs, binary complexes, and ternary complexes are shown. b. Cryo-EM structure of IGF1/IGFBP/ALS ternary complex (PDB: 7WRQ). IGF1 (yellow), IGFBP3 (blue), and ALS (green). c. Cryo-EM structure of PAPP-A/IGFBP5 complex (PDB: 7UFG). PAPP-A dimers (blue and green), and IGFBP5 (red). d. Activated IR and IGF1R undergo clathrin- or caveolin-dependent endocytosis, which control spatiotemporal receptor signaling. Upon internalization, the receptors are either degraded or reinserted at the cell surface by recycling. Dynamic insulin-IGF receptors clusters present at the cell surface, in the cytosol, and in the nucleus. Budding uninhibited by benzimidazole-related 1 (BUBR1); Mitotic arrest deficient 2 (MAD2); p31^comet also known as MAD2L1 binding protein (MAD2L1BP); Carcinoembryonic antigen-related cell adhesion molecule 1 (CECAM1); Ephrin type-B receptor 4 (EPHBP4); Insulin inhibitory receptor (Inceptor); P, phosphorylation. e. Post-translational modifications of insulin-IGF receptors. Ubiquitination of insulin-IGF receptors promotes receptor endocytosis and degradation. S-nitroso-CoA (SNO-CoA) modification of IR by SNO-CoA-assisted nitrosylase (SCAN) inhibits IR signaling. Sumoylation of IGF1R controls IGF1R nuclear import. Casitas B-lineage lymphoma (c-CBL); neural precursorcell expressed developmentally down-regulated protein 4 (NEDD4); murine double minute 2 (MDM2); C-terminus of Hsp70-interacting protein (CHIP); Membrane-associated RING-CH-type finger 1 (MARCH1); Ub, ubiquitination; SNO, S-nitroso-CoA. SUMO, Sumoylation. f. Insulin-IGF receptors are cleaved by proteinases such as membrane type 1-matrix metallopeptidase (MT1-MMP), beta-site amyloid precursor protein cleaving enzyme (BACE1), and gamma-secretase, which controls the basal receptor levels and signaling. g. Integrin facilitates IGF1R signaling, while IGFBP7/MAC25 inhibits it. The IGF1R-Progerin interaction promotes IGF1R downregulation.

Figure 5.. Interventions through non-native ligands

a. Cryo-EM structure of IR/Vh-Ins-HALQ complex (PDB: 7MQO). Two protomers of IR are shown as blue and green. Vh-ins-HALQ is shown as yellow and the HALQ motif is highlighted in red. b. Cryo-EM structure of IGF1R/scLCDV1-VILP complex (PDB: 7U23). Two protomers of IGF1R are shown as blue and green. The unresolved regions in the IGF1R-ECD are indicated by ovals. scLCDV1-VILP (yellow). c. Cryo-EM structure of IR/IRPA-3 complex (PDB: 7MD4). Two protomers of IR are shown as blue and green. IRPA (yellow). d. Cryo-EM structure of insulin amyloid fibril (PDB: 8SBD). At the conditions of low pH, high concentrations, high temperature, and agitation, insulin tends to form an amyloid fibril and thus becomes inactive. However, specific insulin mutations, such as Thr-A8Arg, Ile-A10Arg, and Asn-A18Gln, significantly reduce its ability to form fibrils, offering a new generation of thermostable fibril resistance insulin. e. Cryo-EM structure of IR/S597 complex (PDB: 8DTL). Two protomers of IR are shown as blue and green. Site-1 and site-2 components of S597 are shown as yellow and pink. f. Cryo-EM structure of IR/IR-A62 complex (PDB: 7YQ6). Two protomers of IR are shown as blue and green. IR-A62 aptamer (yellow).