ABC | Volume 113, Nº6, December 2019

Review Article Ferrari et al. Exercise-mediated glucose uptake Arq Bras Cardiol. 2019; 113(6):1139-1148 binding of the hormone to a specific membrane receptor, named insulin receptor, composed by four subunits: two α subunits located in the external part of the membrane, and two transmembrane, β subunits. Insulin binds to the α subunits, and activate the kinase activity of beta subunits, which promotes the self-phosphorylation of tyrosine residues in the intracellular region of insulin receptor. 16 This generates the recruitment of adaptor proteins and phosphorylation of several protein substrate, including members of the insulin receptor substrate family – IRS-1, 2, 3 and 4. 17 Among these members, phosphorylation of IRS-1 and IRS-2 into tyrosine – by addition of a phosphate group – bind to and activate Src homology-2 (SH2) domains, such as the phosphoinositide 3-kinase (PI3K). The SH2 domain exhibits approximately 100 amino acids and is able to recognize and bind to phosphorylated tyrosine. 18 PI3K, in turn, catalyzes the formation of phosphatidylinositol (3,4,5)-trisphosphate (PI3P), 19 an allosteric regulator of phosphoinositide-dependent kinase (PDK). 20 PDK activates one of the isoforms of protein kinase B (PKB), also known as Akt, and the atypical protein kinase C (aPKC). 21 There is evidence that aPKC is essential for insulin‑stimulated glucose transport in skeletal muscle; its activation seems to be compromised in IR, 22 and potentialized by PE. 23 Among the aPKC isoforms, the aPKC lambda/iota has shown an important role in glucose transport. This enzyme phosphorylates the double C2-like domain-containing protein (DOC2b), which regulates the soluble N-ethylmaleimide- sensitive factor attached protein receptor (SNARE), facilitating the interaction with syntaxin-4 and promoting the fusion of GLUT4-containing vesicles with the plasma membrane. 24 In addition to aPKC, other PKC isoforms are also involved in GLUT4 translocation, including PKC α and PKC θ , which are activated by the increase in intracellular calcium. 25 Besides the PKC isoforms, the Akt enzyme promotes the phosphorilation of the RabGTPase-activating proteins (RabGAPs), that involve the TBC1 domain family member 4 (TBC1D4) and TBC1 domain family member 1 (TBC1D1). This enables the dissociationof theRabprotein, andconsequently, increaseduptake of glucose by increased GLUT4 translocation. 26 The TBC1D1 and the TBC1D4proteins act cooperatively regulating the translocation of GLUT4 in response to a stimulus, since both are co-expressed in skeletal muscle. 27 In summary, TBC1D4, previously known as Akt substrate of 160 kDa (AS160), is a protein that, when phosphorylated into treonin-642, helps in the translocation of GLUT4-containing vesicles to the membrane, in GLUT4 expression, leading to increased glucose uptake. 28 Akt also induces the phosphorylation of serine/threonine kinase with an atypical placement of the catalytic lysine, called with-no-lysine kinase (WNK1), with omnipresent expression, including the skeletal muscle. WNK1, in turn, phosphorylates the TBC1D4 enzyme, promoting the translocation of GLUT4 in the skeletal muscle. 29 Therefore, activation of the cascade that involves PI3K/Akt enzymes allows the entry of glucose into cells by facilitated diffusion, by stimulation of translocation of GLUT4 from intracellular vesicles to the plasma membrane. 30 In addition to GLUT4 translocation, PI3K simultaneously stimulates the synthesis of hepatic and muscle glycogen. 31 In this context, another important mechanism was proposed. Previous studies using cell cultures have shown that inhibition of the endogenous Rac1 (member of the Rho-family of GTPases) blocked the insulin-induced GLUT4 translocation. 32,33 Rac1, in turn, was described as essential in the stimulation of insulin-mediated glucose uptake in skeletal muscle and glucose homeostasis in the whole body, 34,35 exerting a preponderant role in the regulation of insulin-induced GLUT4 translocation, as observed in cultured muscle cells. 36 Also, when endogenous production of insulin is compromised (or in state of very high insulin resistance), the role of PE is even more important due to its insulin- independent hypoglycemic effect. 37 Physical exercise in the regulation of glucose uptake in skeletal muscle During PE, the utilization of energy substrates (mainly glucose and free fatty acids) considerably increases in relation to rest. These substrates originate from intramuscular stores, hepatic production and fat tissue mobilization by hormone-sensitive lipase. 38 Both acute aerobic exercise and chronic exercise training can potentiate the action of insulin, and evidence from animal models has helped us to understand the mechanisms involved. In rats fed a high-fat diet, acute PE seems to affect the activation of insulin receptor, since a unique session of exercise increases insulin-stimulated IR phosphorylation in skeletal muscles. 39 In obese rats, both high-volume exercise (six-hour duration) and low-volume exercise (45 minutes) were effective in increasing insulin sensitivity, by increased phosphorylation of IR, IRS-1 and Akt. 40 Another experiment with rats showed an improvement in insulin sensitivity in adipocytes after seven weeks of daily aerobic exercise (60-minute duration), mediated by increased tyrosine phosphorylation in IRS-1 and IRS-2 and greater association of IRS-1 with PI3K and, consequently, increased phosphorylation of Akt protein. 41 In addition, PE can increase glucose uptake in the muscle by other pathways that involve a key enzyme activated by muscle contraction, named AMP-activated protein kinase (AMPK). AMPK is a heterotrimeric molecule composed of a catalytic subunit (alpha) and two regulatory subunits (beta and gamma), with the following isoforms β 1, β 2, γ 1, γ 2 and γ 3. It is activated by phosphorylation of a threonine-172 residue within the activation loop of the α subunit. 42 The activation of AMPK can result from an energy imbalance caused by muscle contraction. 43 Among the proteins that regulate AMPK, liver kinase B1 (LKB1) is currently considered the main protein involved in AMPK phosphorylation. 44 The activation of AMPK and LKB1 during exercise has been widely demonstrated in animals and humans. 43,45 It is worth pointing out AMPK-stimulated glucose transport seems to be mediated by multiple factors – by increase of intracellular concentrations of Ca ++ and bradykinin (plasma polypeptide that causes vasodilation), increased activity of endothelial nitric oxide synthase (which increases vasodilation and the availability of nitric oxide), by activation of mitogen-activated protein kinase (MAPK), activation of Ca 2+ /calmodulin-dependent protein kinase (CaMK), activation of protein kinase C (PKC), and even hypoxia. 46,47 All these factors are necessary for an effective translocation of GLUT4 and consequent entry of glucose into the cells. 1140