Regulation of insulin signalling, glucose uptake and metabolism in rat skeletal muscle cells upon prolonged exposure to resistin

Abstract

Aims/hypothesis: Debate exists regarding the role of resistin in the pathophysiology of insulin resistance. The aim of this study was to directly assess the effects of resistin (0-24 h) on basal and insulin-stimulated glucose uptake and metabolism in skeletal muscle cells and to investigate the mechanisms responsible for the effects of resistin. Methods: We used L6 rat skeletal muscle cells and examined [ 3 H]2-deoxyglucose uptake, GLUT4 translocation and GLUT protein content. We assessed glucose metabolism by measuring the incorporation of D-[U-14 C] glucose into glycogen, 14 CO 2 and lactate production, as well as the phosphorylation level and total protein content of insulin signalling proteins, including insulin receptor β-subunit (IRβ), insulin receptor substrate (IRS), Akt and glycogen synthase kinase-3β (GSK-3β). Results: Treatment of L6 rat skeletal muscle cells with recombinant resistin (50 nmol/l, 0-24 h) reduced levels of basal and insulin-stimulated 2-deoxyglucose uptake and decreased insulin-stimulated GLUT4myc content at the cell surface, with no alteration in the production of GLUT4 or GLUT1. Resistin also decreased glycogen synthesis and GSK-3β phosphorylation. Insulin-stimulated oxidation of glucose via the Krebs cycle was reduced by resistin, whereas lactate production was unaltered. Although insulin receptor protein level and phosphorylation were unaltered by resistin, production of IRS-1, but not IRS-2, was downregulated and a decreased tyrosine phosphorylation of IRS-1 was detected. Reduced phosphorylation of Akt on T308 and S473 was observed, while total Akt and Akt1, but not Akt2 or Akt3, production was decreased. Conclusions/ interpretation: Our data show that resistin regulates the function of IRS-1 and Akt1 and decreases GLUT4 translocation and glucose uptake in response to insulin. Selective decreases in insulin-stimulated glucose metabolism via oxidation and conversion to glycogen were also induced by resistin. These observations highlight the potential role of resistin in the pathophysiology of type 2 diabetes in obesity.

Resistin disrupts glycogen synthesis under high insulin and high glucose levels by down-regulating the hepatic levels of GSK3β

“…The levels of neither insulin receptor β nor its phosphorylated form were altered by treatment of rat hepatocytes with resistin ( Fig. 2), results which are similar to those previously observed in muscle L6 cells (Palanivel et al 2006). IRS1 and IRS2 are two major insulin receptor effectors that contribute to the signaling that regulates glucose homeostasis (Thirone et al 2006), while tumor necrosis factor-alpha (TNFα) is an important mediator of insulin resistance in obesity and diabetes, as it induces insulin resistance at the level of the IRS proteins (Ishizuka et al 2007;Lorenzo et al 2008;Nieto-Vazquez et al 2008).…”

“…Other studies, however, suggested that basal and insulin-stimulated glycogen synthesis was decreased in L6 rat skeletal muscle cells treated with resistin (Palanivel et al 2006;Niederwanger et al 2007). In our research, we first explored the effect of recombinant murine resistin on liver glycogen content.…”

“…The activation of PI3K by insulin is mediated by the p85 regulatory subunit binding to tyrosine-phosphorylated insulin receptor substrate (IRS) proteins Myers et al 1992). Akt, a downstream target of PI3-kinase, has been reported to mediate insulin-induced glycogen synthase activation and glucose uptake, and is activated mainly at two regulatory sites, Thr 308 and Ser 473 (Palanivel et al 2006). We examined the effect of resistin on the protein levels of PI3Kp85 and Akt and no change in the protein level of PI3Kp85 was detected between experimental and control cells, however, resistin treatment of hepatocytes lead to a decrease in the level of Akt phosphorylated at S473 under high insulin (100 nM) and glucose (30 mM) stimulation (Fig.…”

“…For example, recombinant resistin attenuates insulin-stimulated glucose uptake by inducing the expression of SOCS3 in 3T3-L1 adipocytes (Steppan et al 2001;Steppan et al 2005). Basal and insulin-stimulated glucose uptake, oxidation and glycogen synthesis in L6 rat skeletal muscle cells were also decreased in the presence of resistin (Palanivel et al 2006). In addition, resistin has been shown to impair glucose-induced insulin secretion and induce insulin resistance in the islet β-cells of mice, by increasing SOCS3 expression and decreasing the phosphorylation level of Akt (Nakata et al 2007).…”

Naphthalenemethyl ester derivative of dihydroxyhydrocinnamic acid, a component of cinnamon, increases glucose disposal by enhancing translocation of glucose transporter 4

“…After centrifugation, pellets were dissolved in 500 μl water, 1.2 ml ethanol was added, and samples were incubated at −20°C for 20 min. Samples were centrifuged, pellets were dissolved in 500 μl water, and the radioactivity in 400 μl of solution was measured by scintillation counting [22,23].…”