Insulin signaling meets mitochondria in metabolism

Abstract

Insulin controls nutrient and metabolic homeostasis via the IRS-PI3K-AKT signaling cascade that targets FOXO1 and mTOR. Mitochondria, as the prime metabolic platform, malfunction during insulin resistance in metabolic diseases. However, the molecular link between insulin resistance and mitochondrial dysfunction remains undefined. Here we review recent studies on insulin action and the mechanistic association with mitochondrial metabolism. These studies suggest that insulin signaling underpins mitochondrial electron transport chain integrity and activity by suppressing FOXO1/HMOX1 and maintaining the NAD(+)/NADH ratio, the mediator of the SIRT1/PGC1α pathway for mitochondrial biogenesis and function. Mitochondria generate moderately reactive oxygen species (ROS) and enhance insulin sensitivity upon redox regulation of protein tyrosine phosphatase and insulin receptor. However, chronic exposure to high ROS levels could alter mitochondrial function and thereby cause insulin resistance.

Figure 1

Comparison of IRS1 and IRS2 protein sequences, including the relative location of the amino-terminal pleckstrin homology (PH) and phosphotyrosine binding (PTB) domains, and numerous known (*, phosphorylation sites revealed by MS/MS) or expected tyrosine phosphorylation sites. The amino acid sequences surrounding tyrosine sites are shown, and motifs conserved between IRS1 and IRS2 are coded with a similar background color. The kinase regulatory loop-binding (KRLB) domain in IRS2 is indicated by a yellow box, which includes the tyrosine residue that binds in the ATP binding pocket (Y₆₂₁).

Figure 2

Insulin and insulin-like signaling cascade. Two main branches propagate signals generated via the IRS-proteins: PI3K→PDK1→AKT and GRB2/SOS→RAS kinase cascades. Activation of the receptors for insulin and IGF-1 results in tyrosine phosphorylation of the IRS-proteins, which bind PI3K and GRB2/SOS. 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 and by indirect phosphorylation of fos through p90^rsk. The activation of PI3K by IRS-protein recruitment produces PI3,4P₂ and PI3,4,5P₃ (antagonized by the action of PTEN or SHIP2), which recruit PDK1 and AKT to the plasma membrane. AKT is activated via phosphorylation at T308 by PDK1 and at S473 by mTOR in complex with rictor. The mTOR kinase is activated by Rheb^GTP, which accumulates upon inhibition of the GAP activity of the TSC1–TSC2 complex following PKB-mediated phosphorylation of TSC2. mTOR is also activated by AKT-mediated PRAS40 phosphorylation. The S6K is primed through mTOR-mediated phosphorylation for activation by PDK1. AKT phosphorylates many cellular proteins, and this inactivates PGC1α, p21^kip, GSK3β, BAD and AS160, and activates PDE3b and eNOS. mTOR also promote cleavage and activation of SREBP1c (Clv’d SREBP1C), which stimulates the expression of genes needed for lipid synthesis. AKT-mediated phosphorylation of forkhead proteins, including FOXO1, results in their sequestration in the cytoplasm, which inhibits their influence upon transcriptional activity. 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 crucial ribosomal proteins. Components of the translation machinery that are targets of insulin regulation include eIF2B, eIF4E, eEF1, eEF2 and the S6 ribosomal protein [117]. TNFα activates JNK which can phosphorylate IRS1, inhibiting its interaction with the insulin receptor and its subsequent tyrosine phosphorylation. IRS2 expression is promoted by nuclear FOXO, which increases IRS2 concentration in the fasted liver.

Figure 3

Mitochondria integrate glucose and lipid metabolism for energy generation. Glycolysis (glucose metabolism) and fatty acid oxidation (from lipid) generate acetyl-CoA, the substrate for tricarboxylic acid (TCA) cycle that oxidize the substrates into carbon dioxide and reducing products NADH and FADH₂. NADH and FADH₂ donate electrons to complexes I and II, respectively, of the respiration chain that generate an electrochemical gradient (membrane potential) by pumping protons in matrix across the inner membrane (IM) into the intermembrane space (IMS). The electrochemical gradient drives ATP generation at complex V and pumps protons back into the matrix. Moderately reactive oxygen species (such as O₂^•−) can be generated as a second messenger under physiological conditions, but a significant ROS burst can occur during mitochondrial hyperpolarization.

Figure 4

Mechanistic association of insulin signaling with mitochondrial function. Insulin elicits the IR→IRS→PI3K→AKT signaling cascade and inhibits the transcriptional factor FOXO1 under normal conditions. During insulin resistance, including genetic deletion of IRS1 and IRS2, or physiological challenge of obesity, FOXO1 is hyperactivated and induces HMOX1. HMOX1 oxidizes heme to biliverdin (BV) and free Fe³⁺. Because heme is essential for the function and stability of electron transport proteins, insulin resistance impairs the ETC activity that is essential for NADH oxidation. Consequently, NAD⁺ levels decrease and the NAD⁺/NADH ratio increases, and this can inhibit the activity of the NAD⁺-dependent deacetylase SIRT1. Therefore, mitochondrial function and biogenesis are impaired under insulin-resistant conditions owing to the relative inactivity of SIRT1. Moreover, mitochondria can generate ROS (e.g., O₂^•− and H₂O₂) as a second messenger to regulate IR-mediated signaling cascade. ROS functions by oxidizing the β chain of the insulin receptor to facilitate its autophosphorylation (activation) or through oxidative modification of protein tyrosine phosphatases, especially PTP1B and PTEN, which leads to hyperphosphorylation of the insulin receptor and IRS1/2, and increased activity of the PI 3-kinase. Suppression of mitochondrial ROS causes insulin resistance, whereas knockout of the ROS-scavenger enzyme improves insulin responsiveness [–105,118].