Prior AICAR Stimulation Increases Insulin Sensitivity in Mouse Skeletal Muscle in an AMPK-Dependent Manner

Abstract

An acute bout of exercise increases glucose uptake in skeletal muscle by an insulin-independent mechanism. In the period after exercise, insulin sensitivity to increased glucose uptake is enhanced. The molecular mechanisms underpinning this phenomenon are poorly understood but appear to involve an increased cell surface abundance of GLUT4. While increased proximal insulin signaling does not seem to mediate this effect, elevated phosphorylation of TBC1D4, a downstream target of both insulin (Akt) and exercise (AMPK) signaling, appears to play a role. The main purpose of this study was to determine whether AMPK activation increases skeletal muscle insulin sensitivity. We found that prior AICAR stimulation of wild-type mouse muscle increases insulin sensitivity to stimulate glucose uptake. However, this was not observed in mice with reduced or ablated AMPK activity in skeletal muscle. Furthermore, prior AICAR stimulation enhanced insulin-stimulated phosphorylation of TBC1D4 at Thr 649 and Ser 711 in wild-type muscle only. These phosphorylation events were positively correlated with glucose uptake. Our results provide evidence to support that AMPK activation is sufficient to increase skeletal muscle insulin sensitivity. Moreover, TBC1D4 phosphorylation may facilitate the effect of prior AMPK activation to enhance glucose uptake in response to insulin.The effect of insulin on skeletal muscle glucose uptake is increased in the period after a single bout of exercise. This phenomenon is observed in muscle from both humans and rodents (1-6) and may persist for up to 48 h after exercise, depending on carbohydrate availability (7-9). Improved muscle insulin sensitivity postexercise is mediated by one or several local contraction-induced mechanisms (10) involving both enhanced transport and intracellular processing of glucose. This period is characterized by increased GLUT4 protein abundance at the plasma membrane and enhanced glycogen synthase activity (11,12). These changes occur independent of global protein synthesis (13), including both total GLUT4 and glycogen synthase protein content (4,11), and are independent of changes in proximal insulin signaling, including Akt activation (3,4,(13)(14)(15)(16)(17).AMPK is a heterotrimeric complex consisting of catalytic (a1/a2) and regulatory subunits (b1/b2 and g1/g2/g3). Of the 12 heterotrimeric combinations, only 3 and 5 combinations have been found in the skeletal muscle of human and mouse, respectively (18,19). AMPK is activated in response to various stimuli that increase cellular energy stress (e.g., metformin, hypoxia, hyperosmolarity, muscle contraction, and exercise) (20). With energy stress, intracellular concentrations of AMP and ADP accumulate. This activates AMPK allosterically and decreases the ability of upstream phosphatases to dephosphorylate Thr 172 , which further increases AMPK phosphorylation and activity (21). Like exercise, AICAR increases AMPK activity in skeletal muscle (22), which partly mimics the metabolic changes observed during muscle contraction (...

Mechanisms for greater insulin-stimulated glucose uptake in normal and insulin-resistant skeletal muscle after acute exercise

“…AICAR did not alter muscle glycogen concentration, indicating that glycogenolysis was not essential for increased insulin sensitivity. Similar results were reported for isolated mouse extensor digitorum longus (EDL) muscles incubated with AICAR plus serum (64). To test whether AMPK activation is essential for improved insulin sensitivity, experiments were performed using EDL muscles from mice genetically deficient in AMPK.…”

Partial Disruption of Lipolysis Increases Postexercise Insulin Sensitivity in Skeletal Muscle Despite Accumulation of DAG

“…In addition, recent findings by Kjøbsted et al (38), demonstrating that insulin-stimulated glucose uptake in skeletal muscle was increased 4 h after AICAR stimulation in WT mice, but not in mice where AMPK activity was blunted, further supporting the notion that AMPK activation is sufficient to increase skeletal muscle insulin sensitivity. Since we observed that AMPK and ACC phosphorylation were similar between genotypes, both at basal and POST EX and with insulin stimulation in both SOL and EDL muscles, AMPK activation cannot account for the higher insulin-stimulated glucose transport in the skeletal muscle of HSL KO Figure 6-GLUT4 protein expression in SOL muscle (m. soleus) (A) and EDL muscle (m. extensor digitorum longus) (D), hexokinase II protein expression in SOL muscle (B) and EDL muscle (E), and ATGL protein expression in SOL muscle (C) and EDL muscle (F) in the presence of no insulin and during 100 mU/mL insulin stimulation at rest (white bars) and 90-min POST EX (PEX) (black bars) in WT and HSL KO mice.…”

“…Entire SOL and EDL muscles were homogenized and subjected to SDS-PAGE and Western blotting analysis, as described previously (37,38 Figure 1-Glucose uptake in the presence of no insulin and during 100 mU/mL and 10,000 mU/mL insulin stimulation in SOL muscle (m. soleus) (A) and in EDL muscle (m. extensor digitorum longus) (B) in WT and HSL KO mice. Measured between the ages of 16 and 25 weeks (n = 3-4 in each group).…”

AICAR Prevents Acute Olanzapine-Induced Disturbances in Glucose Homeostasis

“…Numerous studies using cell culture and rodent models have shown that AICAR treatment increases the phosphorylation of AMPK and its downstream target, acetyl-CoA carboxylase (ACC) (Guigas et al, 2007;Gaidhu et al, 2009;Hasenour et al, 2014;O'Neill et al, 2014;Kjøbsted et al, 2015;Monaco et al, 2015). In order to confirm whether AICAR treatment leads to activation of AMPK in our hands, mice were injected with AICAR or an equivalent volume of saline, and the phosphorylation of AMPK and ACC were measured.…”

“…Published on March 26, 2018as DOI: 10.1124 at ASPET Journals on April 5, 2019 jpet.aspetjournals.org Downloaded from JPET #248393 6 energy-sensing enzyme AMP-activated protein kinase (AMPK). The activation of AMPK in skeletal muscle improves insulin sensitivity (O'Neill et al, 2014;Kjøbsted et al, 2015), stimulates glucose uptake (Jørgensen et al, 2004) and enhances fat oxidation (O'Neill et al, 2014), while AMPK activation in the liver leads to reductions in gluconeogenesis (Foretz et al, 2005). The effects of AICAR in the liver are likely not solely attributable to AMPK as the AICAR-mediated inhibition of glucose production was intact in liver-specific AMPK knockout mice (Hasenour et al, 2014).…”