The prevalence of obesity and associated chronic diseases has increased significantly in recent years worldwide. A high-fat diet and physical inactivity have been imputed as precursors of insulin resistance, mainly in adipose, hepatic, and muscle tissue[1–3]. On the other hand, physical exercise has been described as a way of controlling glucose homeostasis and increasing insulin sensitivity in several tissues. Over the past several years, considerable progress has been made in understanding the molecular basis for these clinically important effects of physical exercise. In fact, there is now extensive epidemiological evidence demonstrating that long-term regular physical exercise can significantly reduce the body fat and insulin resistance. The molecular mechanism involved in sensitivity to insulin mediated by physical exercise is associated to the increase in levels of phosphorylation in pivotal protein related to insulin signal transduction, such as IR, IRSs, and Akt[19–24]. However, most studies have evaluated the therapeutic effects of physical exercise in individuals who already have obesity; whereas a few studies have demonstrated the impact of this intervention when applied simultaneously (protective manner) to those fed a high-fat (that wisely induces both obesity and insulin resistance). In the present study, we showed that mice which were fe with a high-fat diet and not submitted simultaneously to exercise training protocol have reduced levels of phosphorylation of IR, IRS, and Akt in hepatic, muscle, and adipose tissue, when compared with the chow standard control group. As widely expected, a high-fat diet will induce insulin resistance. On the other hand, when mice were fed with a high-fat diet and were simultaneously submitted to exercise training, the degree of insulin resistance was significantly reduced; that is, the levels of phosphorylation of IR, IRS, and Akt increased in the DIO-T group when compared with the DIO group.
Several mechanisms have been shown to link obesity and insulin resistance. Obesity has been strongly associated with a pro-inflammatory molecule (low-grade inflammation), including the IKK/NF-κB pathway. Increased activation of the IKK/NF-κB pathway results in increased serine 307 phosphorylation of IRS-1 that, ultimately, leads to impaired insulin signaling in several peripheral tissue. A previous study, including our group, showed that diet-induced obesity rats submitted to exercise training reduce activation of both the NF-κB and serine 307 phosphorylation of IRS-1, leading to a decreased resistance to insulin[24, 27]. In addition, evidence demonstrates that exogenous nitric oxide (NO) and the NO produced by inducible nitric oxide synthase (iNOS) can induce insulin resistance by S-nitrosation. So, Pauli and colleagues investigated whether this insulin resistance, mediated by S-nitrosation of proteins involved in early steps of the insulin signal transduction pathway, could be reversed by physical exercise. The authors observed that physical activity can revert insulin resistance through the reduction of S-nitrosation of the IR and IRS proteins. Other well-characterized molecules that induce insulin resistance and are induced by the high-fat diet are the phosphatase protein, such as PTP, PTEN, and SOCS3. These proteins have been reported to bind to the insulin receptor and prevent the coupling of IRS-1 with the insulin receptor, thereby inhibiting IRS-1 phosphorylation and downstream insulin signaling[31, 32]. Our group also showed that 12 weeks of exercise training reduced the expression of both phosphatase and SOCS3.
As described earlier, a gamma of results has shown that exercise training decreases molecules activity or protein levels that can lead to insulin resistance (PTEN, PTP1B, SOCS3, pro-inflammatory molecule, S-nitrosation, and others); it can also improve insulin resistance. In addition, insulin sensitivity may be improved by increased adiponectin levels. Adiponectin is adipokine that is predominantly secreted by differentiated adipocytes that are involved in energy homeostasis, insulin sensitivity, and the anti-inflammatory response. The physical exercise leads to higher levels of adiponectin, and it may also reduce insulin resistance for review see. Adiponectin is reduced in obesity and increased levels of this peptide by exercise training may improve insulin signal transduction. Co-treatment of C2C12 myotubes with adiponectin and insulin showed a synergistic increase in Akt phosphorylation, and this synergism disappeared in APPL1 knockdown cells. Hence, the involvement of adiponectin in this process is demonstrated. Here, we showed that mice which were fed with a high-fat diet and not submitted simultaneously to exercise training protocol (DIO group) have reduced adiponectin receptor R1 and 2 and APPL1 protein levels in hepatic, muscle, and adipose tissue, when compared with the chow standard control group. On the other hand, when mice were fed with a high-fat diet and simultaneously submitted to exercise training, the degree of reduction of ADIPOR1-2 and APPL1 was significantly smaller; that is, ADIPOR1-2 and APPL1 protein levels increased in the DIO-T group when compared with the DIO group.
Adiponectin exerts its action through its receptors ADIPOR1 and ADIPOR2. ADIPOR1 and ADIPOR2 interact with the adaptor protein that contains a pleckstrin homology domain, a phosphotyrosine domain, and a leucine zipper motif (APPL1), which bind the N-terminal intracellular domains of the receptors[10–13]. It has been demonstrated that the endosomal adaptor protein, APPL1, regulates the activity of Akt. The improvement found that insulin signaling in different tissues studied in the present study, least in part, can be attributed to recovery protein levels in adiponectin receptors and APPL1. Previous studies have demonstrated that exercise training induces increased adiponectin levels[34, 35], and ADIPOR1 protein levels in skeletal muscle. In parallel, Marinho et al. have found that exercise increases insulin action, at least in part, through the enhancement of APPL1 expression in the liver of obese mice. Our results confirm that an improvement in insulin action can be, least in part, via adipoR1-2/APPL1. Taken together, our data demonstrated that exercise training performed concomitantly to feeding with a high-fat diet reduces the degree of insulin resistance and improves adiponectin receptors 1 and 2 and APPL1 protein levels in the hepatic, adipose, and skeletal muscle tissue.