Does exercise increase insulin sensitivity through angiotensin 1‐7?

Glucose homeostasis in humans and other mammals is regulated by the concerted contributions of several organs and systems. One of the quantitatively most important events for maintaining this equilibrium is glucose transport in skeletal muscle. The main physiological mediators that stimulate this process are insulin and exercise. Each triggers the translocation of glucose transporter type 4 (GLUT4), the predominant GLUT in muscle, from the interior of the cell to the plasma membrane by means of distinct mechanisms. In addition, exercise can increase insulin-mediated glucose transport in skeletal muscle and improve whole-body insulin sensitivity (Maarbjerg et al. 2011, Stanford & Goodyear 2014). Enhanced insulin sensitivity after an exercise session has been partially explained by the amplification of the insulin signalling pathway (distal) and GLUT4 translocation to the plasma membrane (Maarbjerg et al. 2011). Conversely, chronic exercise-induced insulin sensitization has been related to adaptations in skeletal muscle, such as improvements in mitochondrial function and increments in capillary density, oxidative fibres and GLUT4 protein expression (Stanford & Goodyear 2014). Current evidence shows that microvasculature plays a critical role in regulating insulin delivery to and action on post-exercised skeletal muscle; however, the mechanisms involved have not yet been elucidated. Here, we propose a novel hypothesis to explain how exercise through angiotensin (Ang) 1-7 signalling improves insulin sensitivity in skeletal muscle. The renin–angiotensin system (RAS), considered the master regulator of cardiovascular homeostasis, promotes the formation of two physiologically relevant peptides: Ang II and Ang 1-7. The synthesis process begins when the renin enzyme, released by juxtaglomerular cells in the kidney, cleaves angiotensinogen, which is synthesized and secreted by the liver, to form the inactive decapeptide Ang I. Subsequently, the angiotensin-converting enzyme (ACE), which is released by the lungs, cleaves Ang I to the active octapeptide Ang II. This peptide mediates its effects through two types of G-protein-coupled receptors. Some biological actions of Ang II via the Ang II type 1 receptor (AT1R) include vasoconstriction, inflammation, hypertrophy, oxidative stress and insulin resistance. Contrariwise, Ang II-mediated effects through the Ang II type 2 receptor (AT2R) involve vasodilatory, anti-inflammatory, antihypertrophic, antioxidant and insulin-sensitizing effects (Dominici et al. 2014). Thus, this ACE/Ang II/AT1R pathway represents the classical axis of the RAS. Ang 1-7, the other major active peptide of RAS, is formed in three distinct ways. It can be directly synthesized from Ang I by thimet oligopeptidase (TOP), neutral endopeptidase (NEP) or prolyl endopeptidase (PEP), or from Ang II by the ACE homolog, ACE2, which constitutes the most important enzyme for Ang 1-7 generation. Indirectly, this heptapeptide may also be formed from Ang I by ACE2, producing the intermediary peptide Ang 1-9, which in turn is cleaved by ACE or NEP. Ang 1-7, once formed, can interact with the G-protein-coupled Mas receptor (MasR) and, at a lesser proportion, AT2R. Last, Ang 1-7 is metabolized to Ang 1-5 by ACE. The majority of biological responses of Ang 1-7 through Mas and AT2 receptors are similar to each other, but dissimilar to those induced by Ang II via AT1R. Thus, the ACE2/Ang 1-7/MasR pathway, considered the new RAS axis, exerts counter-regulatory actions on the classical pathway, the ACE/Ang II/AT1R axis (Dominici et al. 2014). In addition to its systemic expression, local RAS is localized in several tissues, such as brain, heart, blood vessels, pancreas, liver, adipose tissue and skeletal muscle, where it regulates diverse functions. Recent studies show that RAS participates in some exercise-induced beneficial effects. After one or several exercise sessions, insulin-induced vasorelaxant responses are increased in isolated arteries from healthy and insulin-resistant rats (Fontes et al. 2014, Wang et al. 2015). In a similar fashion, swimming training improved the vasodilator effect of Ang 1-7 in thoracic aortae from spontaneous hypertensive rats (SHR) through a nitric oxide (NO)- and prostacyclin (PGI2)-dependent mechanism (Silva et al. 2011). In addition, Ang II-mediated vasoconstrictor responses were attenuated in several vascular beds from post-exercised rats and humans (Adams et al. 2005, Chies et al. 2013). These vascular adaptations after exercise have been associated with downregulation of the ACE/Ang II/AT1R axis and/or upregulation of the ACE2/Ang 1-7/MasR axis of RAS (Adams et al. 2005, Silva et al. 2011, Gu et al. 2014). Moreover, as in the vasculature and blood (Silva et al. 2011, Gomes-Santos et al. 2014, Gu et al. 2014), exercise training shifts the RAS towards the ACE2/Ang 1-7/MasR axis in skeletal muscle from rats with chronic heart failure (Gomes-Santos et al. 2014). In addition, the mechanical stretch in C2C12 myocytes downregulates messenger RNA (mRNA) expression of angiotensinogen and AT1R, while upregulating AT2R expression (Johnston et al. 2011). Although the functional implications of these findings are unknown, it was described that increased blood flow in rat skeletal muscle after a bout of exercise could be related to decreased ACE/Ang II/AT1R axis expression (Maeda et al. 2005). In summary, these results indicate that acute or chronic exercise upregulates the ACE2/Ang 1-7/MasR axis in both vasculature and skeletal muscle. Microvasculature plays a critical role in regulating insulin action in skeletal muscle. It represents the first barrier and rate-limiting step that insulin must overcome to promote glucose uptake in muscle. Insulin binds to its receptors in the capillary endothelium, leading to the activation of the insulin signalling pathway, IRS-1/PI3K/Akt/eNOS, which increases the production of NO, microvascular recruitment and transendothelial insulin transport to the muscle interstitium. Once insulin reaches muscle cells, it binds to its receptors, causing the activation of its signalling pathway, to promote GLUT4 translocation and glucose uptake in skeletal muscle (Wagenmakers et al. 2015). Our work group and others recently reported that acute infusion of Ang 1-7 in insulin-sensitive rats increases through MasR insulin-induced microvascular recruitment and glucose uptake in skeletal muscle in vivo (Echeverría-Rodríguez et al. 2014, Fu et al. 2014). In addition, acute treatment with captopril, an ACE inhibitor, enhanced insulin-stimulated glucose disposal in rat skeletal muscle through a MasR-dependent mechanism, suggesting that captopril-induced insulin sensitization involves the synthesis of Ang 1-7 (Echeverría-Rodríguez et al. 2014). In agreement with these findings, the AT1R antagonist losartan augmented insulin-mediated microvascular recruitment and glucose transport into skeletal muscle from insulin-resistant rats (Wang et al. 2013). In addition to its vascular actions, Ang 1-7 also regulates insulin sensitivity through direct actions in skeletal muscle. Ang 1-7 incubation, for example, amplifies insulin-stimulated glucose disposal in C2C12 myotubes (Takeda et al. 2013). Although the molecular mechanisms of this effect are unknown, it was described that Ang 1-7 at proximal level stimulates the phosphorylation of IRS-1 and Akt in human endothelial cells, skeletal muscle, adipose tissue and heart from rat (Muñoz et al. 2012, Verano-Braga et al. 2012, Dominici et al. 2014), whereas at distal level, it induces the phosphorylation of TBC1D4 (also known as Akt substrate of 160 kDa, AS160) in both human endothelial cells and rat skeletal muscle (Muñoz et al. 2012, Verano-Braga et al. 2012). Functional studies support that Ang 1-7 treatment reverses Ang II or fructose-induced insulin resistance by restoring the decreased activation of the insulin signalling pathway (Muñoz et al. 2012, Prasannarong et al. 2012). In addition, Ang 1-7 treatment upregulated GLUT4 protein expression in both mouse skeletal muscle and C2C12 cells (Takeda et al. 2013). Taken together, these findings show that Ang 1-7 improves insulin sensitivity by facilitating insulin delivery and action on skeletal muscle, possibly due to the amplification of the insulin signalling pathway in vasculature and muscle. Exercise also enhances insulin sensitivity through microvascular and metabolic actions, but the mechanisms involved have yet to be elucidated (Rattigan et al. 2001, Inyard et al. 2007, Wagenmakers et al. 2015). Considering the aforementioned findings, we propose that after acute or chronic exercise, some components of the ACE2/Ang 1-7/MasR axis of RAS are overexpressed in blood, microvasculature and/or skeletal muscle. Ang 1-7 signalling favoured by exercise will increase insulin-mediated vasodilatation, microvascular recruitment and transendothelial insulin transport to muscle interstitium. In addition, Ang 1-7 could also amplify insulin signalling in myocytes. As a result, exercise-induced Ang 1-7 signalling will augment insulin-stimulated GLUT4 translocation and glucose uptake in skeletal muscle (Fig. 1). To test this hypothesis, we propose to perform a hyperinsulinaemic euglycaemic clamp in rats subjected to an acute or chronic exercise scheme, to demonstrate whether, on antagonizing MasR with A-779, Ang 1-7 participates in exercise-induced insulin sensitization through microvascular actions. To assess whether Ang 1-7 synthesized after exercise improves insulin sensitivity through metabolic actions, glucose uptake assays should be performed in isolated muscles from sedentary and exercised rats, incubated or not with A-779. In summary, we propose that the enhanced Ang 1-7 signalling observed after one or several exercise sessions could contribute to the improvement of whole-body insulin sensitivity by facilitating insulin delivery to and action on skeletal muscle. In individuals with insulin resistance, as observed in obesity, hypertension, metabolic syndrome and type 2 diabetes, insulin-stimulated glucose uptake in skeletal muscle is markedly impaired (Hussain et al. 2013, Kongsted et al. 2014, Nellemann et al. 2014, Yonamine et al. 2014). Due to this action, exercise is able to delay or prevent the onset of type 2 diabetes (Stanford & Goodyear 2014). Thus, understanding the mechanisms underlying enhanced insulin sensitivity after exercise is of crucial importance for developing drugs with specific molecular targets and more effective treatments for ameliorating insulin resistance-associated disorders and decreasing the incidence of type 2 diabetes. None. OE-R is a postdoctoral fellow from DGAPA, UNAM, México. This work was supported by PAPIIT-DGAPA-UNAM, México (Grant No. IN219915).