Defining the underlying defect in insulin action in type 2 diabetes

Abstract

Insulin resistance is one of the earliest defects in the pathogenesis of type 2 diabetes. Over the past 50 years, elucidation of the insulin signalling network has provided important mechanistic insights into the abnormalities of glucose, lipid and protein metabolism that underlie insulin resistance. In classical target tissues (liver, muscle and adipose tissue), insulin binding to its receptor initiates a broad signalling cascade mediated by changes in phosphorylation, gene expression and vesicular trafficking that result in increased nutrient utilisation and storage, and suppression of catabolic processes. Insulin receptors are also expressed in non-classical targets, such as the brain and endothelial cells, where it helps regulate appetite, energy expenditure, reproductive hormones, mood/behaviour and vascular function. Recent progress in cell biology and unbiased molecular profiling by mass spectrometry and DNA/RNA-sequencing has provided a unique opportunity to dissect the determinants of insulin resistance in type 2 diabetes and the metabolic syndrome; best studied are extrinsic factors, such as circulating lipids, amino acids and other metabolites and exosomal microRNAs. More challenging has been defining the cell-intrinsic factors programmed by genetics and epigenetics that underlie insulin resistance. In this regard, studies using human induced pluripotent stem cells and tissues point to cell-autonomous alterations in signalling super-networks, involving changes in phosphorylation and gene expression both inside and outside the canonical insulin signalling pathway. Understanding how these multi-layered molecular networks modulate insulin action and metabolism in different tissues will open new avenues for therapy and prevention of type 2 diabetes and its associated pathologies.

Keywords: Cell-autonomous; Insulin action; Insulin resistance; Phosphorylation; Review; The metabolic syndrome; Tissue crosstalk; Type 2 diabetes; iPS cells.

Fig. 1

Insulin signalling in classical tissues. Insulin binding to the insulin receptor leads to activation of intrinsic tyrosine kinase activity and multisite insulin receptor and IRS phosphorylation. Tyrosine-phosphorylated IRS serves as docking sites for PI3K leading to PIP₃ formation and PDK-dependent Akt activation, which in turn promotes nutrient utilisation, storage and other anabolic processes, and concomitantly suppresses catabolic pathways in (a) skeletal muscle, (b) adipose tissue and (c) liver. aPKC, atypical PKC; ATGL, adipose triglyceride lipase; CAP, Cbl-associated protein; CBL, Cbl proto-oncogene; ChREBP, carbohydrate-responsive element binding protein; CREB, cAMP responsive element binding protein; CRTC2, CREB-regulated transcription coactivator 2; FATP, long-chain fatty acid transport protein; G3P, glyceraldehyde 3-phosphate; G6Pase, glucose-6-phosphatase; GRB2, growth factor receptor bound protein 2; GS, glycogen synthase; HSL, hormone-sensitive lipase; MAPK, mitogen-activated protein kinases; MEK, MAPK kinase; OXPHOS, oxidative phosphorylation; PDE3B, phosphodiesterase 3B; PHK, phosphorylase kinase; PKA, protein kinase A; RAC1, RAC family small GTPase 1; RAF, RAF proto-oncogene serine/threonine kinase; Ras, Ras GTPase; SHC, SH2 domain containing transforming protein; S6K, p70 ribosomal S6 kinase; SOS, son of sevenless homolog; TBC1D1, TBC1 domain family member 1; TC10, Rho-related GTP binding protein RhoQ; TCA, tricarboxylic acid; TSC2, tuberous sclerosis 2. This figure is available as part of a downloadable slideset.

Fig. 1

Insulin signalling in classical tissues. Insulin binding to the insulin receptor leads to activation of intrinsic tyrosine kinase activity and multisite insulin receptor and IRS phosphorylation. Tyrosine-phosphorylated IRS serves as docking sites for PI3K leading to PIP₃ formation and PDK-dependent Akt activation, which in turn promotes nutrient utilisation, storage and other anabolic processes, and concomitantly suppresses catabolic pathways in (a) skeletal muscle, (b) adipose tissue and (c) liver. aPKC, atypical PKC; ATGL, adipose triglyceride lipase; CAP, Cbl-associated protein; CBL, Cbl proto-oncogene; ChREBP, carbohydrate-responsive element binding protein; CREB, cAMP responsive element binding protein; CRTC2, CREB-regulated transcription coactivator 2; FATP, long-chain fatty acid transport protein; G3P, glyceraldehyde 3-phosphate; G6Pase, glucose-6-phosphatase; GRB2, growth factor receptor bound protein 2; GS, glycogen synthase; HSL, hormone-sensitive lipase; MAPK, mitogen-activated protein kinases; MEK, MAPK kinase; OXPHOS, oxidative phosphorylation; PDE3B, phosphodiesterase 3B; PHK, phosphorylase kinase; PKA, protein kinase A; RAC1, RAC family small GTPase 1; RAF, RAF proto-oncogene serine/threonine kinase; Ras, Ras GTPase; SHC, SH2 domain containing transforming protein; S6K, p70 ribosomal S6 kinase; SOS, son of sevenless homolog; TBC1D1, TBC1 domain family member 1; TC10, Rho-related GTP binding protein RhoQ; TCA, tricarboxylic acid; TSC2, tuberous sclerosis 2. This figure is available as part of a downloadable slideset.

Fig. 1

Insulin signalling in classical tissues. Insulin binding to the insulin receptor leads to activation of intrinsic tyrosine kinase activity and multisite insulin receptor and IRS phosphorylation. Tyrosine-phosphorylated IRS serves as docking sites for PI3K leading to PIP₃ formation and PDK-dependent Akt activation, which in turn promotes nutrient utilisation, storage and other anabolic processes, and concomitantly suppresses catabolic pathways in (a) skeletal muscle, (b) adipose tissue and (c) liver. aPKC, atypical PKC; ATGL, adipose triglyceride lipase; CAP, Cbl-associated protein; CBL, Cbl proto-oncogene; ChREBP, carbohydrate-responsive element binding protein; CREB, cAMP responsive element binding protein; CRTC2, CREB-regulated transcription coactivator 2; FATP, long-chain fatty acid transport protein; G3P, glyceraldehyde 3-phosphate; G6Pase, glucose-6-phosphatase; GRB2, growth factor receptor bound protein 2; GS, glycogen synthase; HSL, hormone-sensitive lipase; MAPK, mitogen-activated protein kinases; MEK, MAPK kinase; OXPHOS, oxidative phosphorylation; PDE3B, phosphodiesterase 3B; PHK, phosphorylase kinase; PKA, protein kinase A; RAC1, RAC family small GTPase 1; RAF, RAF proto-oncogene serine/threonine kinase; Ras, Ras GTPase; SHC, SH2 domain containing transforming protein; S6K, p70 ribosomal S6 kinase; SOS, son of sevenless homolog; TBC1D1, TBC1 domain family member 1; TC10, Rho-related GTP binding protein RhoQ; TCA, tricarboxylic acid; TSC2, tuberous sclerosis 2. This figure is available as part of a downloadable slideset.

Fig. 2

Reciprocal regulation of FOX transcription factors by insulin. (a) Under feeding or other conditions where insulin action is high, FOXOs are phosphorylated by Akt on serine residues, creating interaction sites for 14-3-3 proteins, leading to cytoplasmic retention and inhibited transcriptional activity. Under these conditions, increased Akt and mTORC1 activity inhibits GSK3 signalling and relieves FOXKs from inhibitory GSK3-mediated phosphorylation, leading to increased nuclear translocation and FOXK transcriptional activity. Since FOXKs and the insulin receptor co-immunoprecipitate together [21], it is possible that some of FOXK’s transcriptional effects are due to insulin receptor–FOXK complexes in the nucleus (dotted line). (b) Under fasting or other conditions of insulin deficiency or insulin resistance, low activity of PI3K/Akt/mTORC1 pathways results in FOXO hypophosphorylation leading to increased nuclear localisation and transcriptional activity. Under these conditions, increased GSK3 activity leads to increased FOXK phosphorylation and interaction with phosphoserine-binding 14-3-3 proteins, resulting in cytoplasmic retention and decreased transcriptional activity. Line thickness indicates strength of signalling activity, with thicker lines indicating stronger signalling activity. Faded shading of text boxes indicates lower FOXO/FOXK abundance at the nucleus or cytoplasm within in each state, and arrows between the nucleus and cytoplasm indicate the direction of translocation. This figure is available as part of a downloadable slideset.

Fig. 3

Extrinsic factors contributing to insulin resistance. Several environmental factors may lead to systemic changes affecting multiple tissues and contributing to impaired insulin signalling. Obesity negatively correlates with circulating levels of adiponectin [118] and signalling lipids with beneficial properties, such as 12,13-dihydroxy-9Z-octadecenoic acid (12,13-diHOME) [119] and branched fatty acid esters of hydroxy fatty acids (FAHFAs) [120]. Overnutrition leads to adipose tissue expansion and increased release of cytokines and other inflammatory mediators (e.g., retinol binding protein 4 [RBP4]) by macrophages and adipocytes themselves; these mediators bind to cytokine receptors on metabolic tissues and downregulate proximal insulin signalling due to activation of Ser/Thr kinases (e.g. JNK, IKK and novel PKCs [nPKCs]) and increased IRS serine phosphorylation, and due to increased transcription of SOCS proteins, which interfere with IRS tyrosine phosphorylation. Adipose tissue insulin resistance is associated with ectopic lipid accumulation, mitochondrial dysfunction and reactive oxygen species (ROS) generation, and ER stress in insulin-sensitive tissues. All of these mechanisms contribute to activation of Ser/Thr kinases and IRS serine phosphorylation. Adipose tissue expansion in obesity may also have an impact on systemic metabolism through altered release of exosomal miRNAs. Insulin signalling proteins are shown in blue and intracellular mediators of cytokine receptors and other stress signals are shown in orange. Changes in adipocyte lipid/cytokine release during obesity are indicated font/arrow size, with bigger font/thicker arrows representing increased release and smaller font/thinner arrows representing decreased release. DAG, diacylglycerol; IRE1, inositol-requiring enzyme 1; JAK, Janus kinasePI3K, phosphoinositide 3-kinase; STAT, signal transducer and activator of transcription; TLR4, Toll-like receptor 4; TNFR, TNF-α receptor; UPR, unfolded protein response; XBP1, X-box binding protein 1. This figure is available as part of a downloadable slideset.

Fig. 3

Extrinsic factors contributing to insulin resistance. Several environmental factors may lead to systemic changes affecting multiple tissues and contributing to impaired insulin signalling. Obesity negatively correlates with circulating levels of adiponectin [118] and signalling lipids with beneficial properties, such as 12,13-dihydroxy-9Z-octadecenoic acid (12,13-diHOME) [119] and branched fatty acid esters of hydroxy fatty acids (FAHFAs) [120]. Overnutrition leads to adipose tissue expansion and increased release of cytokines and other inflammatory mediators (e.g., retinol binding protein 4 [RBP4]) by macrophages and adipocytes themselves; these mediators bind to cytokine receptors on metabolic tissues and downregulate proximal insulin signalling due to activation of Ser/Thr kinases (e.g. JNK, IKK and novel PKCs [nPKCs]) and increased IRS serine phosphorylation, and due to increased transcription of SOCS proteins, which interfere with IRS tyrosine phosphorylation. Adipose tissue insulin resistance is associated with ectopic lipid accumulation, mitochondrial dysfunction and reactive oxygen species (ROS) generation, and ER stress in insulin-sensitive tissues. All of these mechanisms contribute to activation of Ser/Thr kinases and IRS serine phosphorylation. Adipose tissue expansion in obesity may also have an impact on systemic metabolism through altered release of exosomal miRNAs. Insulin signalling proteins are shown in blue and intracellular mediators of cytokine receptors and other stress signals are shown in orange. Changes in adipocyte lipid/cytokine release during obesity are indicated font/arrow size, with bigger font/thicker arrows representing increased release and smaller font/thinner arrows representing decreased release. DAG, diacylglycerol; IRE1, inositol-requiring enzyme 1; JAK, Janus kinasePI3K, phosphoinositide 3-kinase; STAT, signal transducer and activator of transcription; TLR4, Toll-like receptor 4; TNFR, TNF-α receptor; UPR, unfolded protein response; XBP1, X-box binding protein 1. This figure is available as part of a downloadable slideset.

Fig. 4

Intrinsic factors contributing to insulin resistance. Cell-autonomous insulin resistance is associated with defects in glucose transport, mitochondrial metabolism and insulin signalling. Global phosphoproteomics of iMyos from individuals with type 2 diabetes reveal a network of signalling defects that underlie skeletal muscle insulin resistance [100]. Proteins linked to insulin action and metabolism are indicated in blue and site-specific effects of type 2 diabetes evidenced by increased and decreased basal phosphorylation are shown in orange and green, respectively. Groups of multiple proteins of the same category are shown in purple and non-labelled circles indicate groups of up- or downregulated phosphosites. All signalling events are derived from MS-based phosphoproteomics except for Akt^T308, GSK3α^S21 and FOXO1^T24/FOXO3^T32, which are from immunoblot analysis. Faded shading of text boxes indicates lower cytoplasm abundance. Ac, acetyl group; ARHGAP, Rho GTPase activating protein; ARHGEF, Rho guanine nucleotide exchange factor; HDAC, histone deacetylase; KAT, lysine acetyltransferase; KDM, lysine demethylase; Me, methyl group; MEF2C, myocyte enhancer factor 2C; PDHA1, pyruvate dehydrogenase E1 subunit alpha 1; SETD, SET domain containing histone lysine methyltransferase; SR, serine- and arginine-rich splicing factor; T2D, type 2 diabetes; TBC1D1, TBC1 domain family member 1; TSC2, tuberous sclerosis 2; U1, U1 small nuclear ribonucleoprotein complex; U2, U2 small nuclear ribonucleoprotein complex. This figure is available as part of a downloadable slideset.