Insulin action at a molecular level - 100 years of progress

Abstract

The discovery of insulin 100 years ago and its application to the treatment of human disease in the years since have marked a major turning point in the history of medicine. The availability of purified insulin allowed for the establishment of its physiological role in the regulation of blood glucose and ketones, the determination of its amino acid sequence, and the solving of its structure. Over the last 50 years, the function of insulin has been applied into the discovery of the insulin receptor and its signaling cascade to reveal the role of impaired insulin signaling-or resistance-in the progression of type 2 diabetes. It has also become clear that insulin signaling can impact not only classical insulin-sensitive tissues, but all tissues of the body, and that in many of these tissues the insulin signaling cascade regulates unexpected physiological functions. Despite these remarkable advances, much remains to be learned about both insulin signaling and how to use this molecular knowledge to advance the treatment of type 2 diabetes and other insulin-resistant states.

Keywords: Insulin; Insulin receptor; Insulin resistance; Insulin signal transduction.

Figure 1

Schematic diagram of the mature insulin receptor, composed of two extracellular α-subunits (red and blue) and two covalently linked transmembrane β-subunits. Contiguous modules of the α-subunits are labeled with the relative location of disulfide bonds (S–S) between the α- and β-subunits. The high-affinity insulin binding site is created from the L1-CR and CTa’ or L1′-CR′ and CTa domains of the disulfide-linked α- and α′-subunits. The β-subunit is formed upon furin-mediated cleavage of the ID region into IDα and IDβ. The COOH terminus of the Fn_iii2 and Fn_iii3 domains forms after the furin cleavage site is separated from the intracellular juxtamembrane region by the hydrophobic transmembrane domain. The tyrosine kinase catalytic domain, including the canonical ATP binding site (Lys₁₀₃₀) and the activation loop with three tyrosine phosphorylation sites, follows immediately after the juxtamembrane region. The β-subunit ends with two tyrosine phosphorylation sites in the COOH terminus.

Figure 2

Comparison of (a) IRS1 and (b) IRS2. Alignments of IRS1 and IRS2 tyrosine phosphorylation sites relative to the amino-terminal pleckstrin homology (PH) and phosphotyrosine binding (PTB) domains. Conserved tyrosine phosphorylation motifs-including their number in the human protein and the surrounding amino acid sequences-are color-coded to highlight alignments between the isoforms: white boxes indicate unique sites in IRS1 or IRS2, including the KRLB (kinase regulatory loop binding) domain in IRS2 located around Y₆₂₄ in IRS2. The relative position of Ser/Thr phosphorylation sites in IRS1 or IRS2 revealed by MS/MS are indicated with red diamonds.

Figure 3

A canonical insulin/IGF signaling cascade. The InsR subunits are illustrated at the top in red and blue. InsR signals begin with tyrosine phosphorylation of the IRS or SHC. The IRS protein binds and activates the PI3K, which generates PI3,4P₂ and PI3,4,5P₃ that recruit PDK1, SIN1, and AKT to the plasma membrane. AKT is activated upon phosphorylation at T308 by PDK1 and at S473 by the SIN1•mTORC2 complex. mTORC1 is activated by Rheb^GTP, which accumulates upon inhibition of the GAP activity of the TSC1•TSC2 complex following AKT-mediated phosphorylation of TSC2. mTORC1-mediates phosphorylation of S6K and SREBP1, which promote protein and lipid synthesis, respectively. AKT phosphorylates many cellular proteins, inactivating PGC1α, p21^kip, GSK3β, BAD, and AS160 and activating PDE3b, PCK1, and eNOS. AKT-mediated phosphorylation of FOXO1 and FOXK causes their sequestration in the cytoplasm, which inhibits their influence upon transcriptional activity. GRB2•SOS can bind to IRS or SHC. The Grb2/SOS complex promotes GDP/GTP exchange on p21^ras, which activates the ras→raf→MEK→ERK1/2 cascade. Activated ERK stimulates transcriptional activity by direct phosphorylation of ELK1 (ETS domain-containing protein) and by indirect phosphorylation of cFOS through MAPKAPK1 (MAPK-activated protein kinase-1). MAPKAPK1 also phosphorylates other proteins, including S6 (ribosomal protein S6), NFκB, PP1, and MYT1. Insulin stimulates protein synthesis by altering the intrinsic activity or binding properties of key translation initiation and elongation factors (eIFs and eEFs, respectively) as well as critical ribosomal proteins. mTORC1-mediated phosphorylation of 4E-BP1 and S6K plays an important role in stimulating translation initiation and elongation [270]. Stimulatory phosphorylation sites are highlighted in green, and inhibitory sites are highlighted in red.

Figure 4

Tissue-specific insulin signaling. The insulin receptor is autophosphorylated on multiple tyrosine residues, allowing the docking and activation of multiple signaling molecules, most notably insulin receptor substrate (IRS) proteins. This in turn activates phosphatidylinositol-3-kinase (PI3K) and Akt to mediate the increases in glucose uptake and metabolism as well as changes in protein and lipid metabolism. While the general pathway is similar in all tissues, the final biological effects are specialized to the roles of insulin in muscle (a), liver (b), and adipose tissue (c).

Figure 5

The integrative role of IR/IGF1R→IRS2 signaling in pancreatic β-cell function. The diagram shows the relation between the IRS2-branch of the insulin signaling pathway and upstream and downstream mechanisms regulating β-cell growth and function. Since IR and IGF1R are constitutively active in β-cells, activation of GLP1→cAMP→PKA→CREB, glucose→Ca²⁺→CRTC2, and calcineurin→NFAT induce IRS2 expression to regulate PI3K→AKT cascade, which places β-cell growth, function, and survival under the control of glucose and incretins.

Figure 6

Schematic diagram of feedback and heterologous regulation of the insulin signaling cascade. Various kinases in the insulin signaling cascade mediate feedback of Ser/Thr phosphorylation of IRS1/2—including AKT, mTOR, S6K, ERK, and AKT [137]. Other kinases activated by heterologous signals, including IL6, INFγ, and TNFα are also illustrated. Serine phosphorylation of IRS1 can recruit CRL7, which can promote ubiquitination and degradation of IRS1 through the 26S proteasome. Many proinflammatory cytokines cause insulin resistance through SOCS1 or SOCS3, targeting phosphotyrosine-containing IRS1 or IRS2 for ubiquitination by a BC-containing ubiquitin ligase (E3) and degradation [271,272].

Figure 7

Schematic diagram showing some of the changes in phosphorylation observed in iPS cell–derived myoblasts from control and T2DM patients. The sites highlighted in orange are increased in either basal or stimulated phosphorylation in cells of the T2DM patients, whereas those highlighted in green are decreased in their phosphorylation. Note that many of the altered phosphorylations occur in pathways outside the pathways considered canonical insulin signaling. Figure was adapted from the data of Batista et al. [269].