LKB1-AMPK signaling in muscle from obese insulin-resistant Zucker rats and effects of training

Abstract

is a key regulator of fat and carbohydrate metabolism. It has been postulated that defects in AMPK signaling could be responsible for some of the metabolic abnormalities of type 2 diabetes. In this study, we examined whether insulin-resistant obese Zucker rats have abnormalities in the AMPK pathway. We compared AMPK and ACC phosphorylation and the protein content of the upstream AMPK kinase LKB1 and the AMPKregulated transcriptional coactivator PPAR␥ coactivator-1 (PGC-1) in gastrocnemius of sedentary obese Zucker rats and sedentary lean Zucker rats. We also examined whether 7 wk of exercise training on a treadmill reversed abnormalities in the AMPK pathway in obese Zucker rats. In the obese rats, AMPK phosphorylation was reduced by 45% compared with lean rats. Protein expression of the AMPK kinase LKB1 was also reduced in the muscle from obese rats by 43%. In obese rats, phosphorylation of ACC and protein expression of PGC-1␣, two AMPK-regulated proteins, tended to be reduced by 50 (P ϭ 0.07) and 35% (P ϭ 0.1), respectively. There were no differences in AMPK␣1, -␣2, -␤1, -␤2, and -␥3 protein content between lean and obese rats. Training caused a 1.5-fold increase in AMPK␣1 protein content in the obese rats, although there was no effect of training on AMPK phosphorylation and the other AMPK isoforms. Furthermore, training also significantly increased LKB1 and PGC-1␣ protein content 2.8-and 2.5-fold, respectively, in the obese rats. LKB1 protein strongly correlated with hexokinase II activity (r ϭ 0.75, P ϭ 0.001), citrate synthase activity (r ϭ 0.54, P ϭ 0.02), and PGC-1␣ protein content (r ϭ 0.81, P Ͻ 0.001). In summary, obese insulin-resistant rodents have abnormalities in the LKB1-AMPK-PGC-1 pathway in muscle, and these abnormalities can be restored by training.acetyl-CoA carboxylase; peroxisome proliferator-activated receptor-␥ coactivator-1␣; exercise AMP-ACTIVATED PROTEIN KINASE (AMPK) is an energy-sensing enzyme that is activated by increases in AMP-to-ATP ratios through mechanisms involving allosteric modification and phosphorylation by one or more upstream AMPK kinases (AMPKKs) (11,25). AMPK is a heterotrimer that is formed by a catalytic ␣-subunit and the regulatory ␤-and ␥-subunits. There are two isoforms of the ␣-and ␤-subunits and three isoforms of the ␥-subunit (11, 25). Upon activation, AMPK switches on catabolic pathways that generate ATP, such as glucose transport and fatty acid oxidation, coupled with the inhibition of ATP-consuming (anabolic) pathways that include triglyceride and cholesterol synthesis. It has become apparent in recent years that AMPK plays a key role in mediating the acute and long-term effects of exercise on carbohydrate and fat metabolism in skeletal muscle. AMPK mediates, at least in part, exercise-stimulated increases in muscle glucose transport (3,15,34,35) and fatty acid oxidation (53, 57). The stimulatory effect of AMPK on fatty acid oxidation results from the phosphorylation and inhibition of acetyl-CoA carboxylase (ACC) by AMPK (10, 53, 59). There is evidence that AM...

Modest PGC-1α Overexpression in Muscle in Vivo Is Sufficient to Increase Insulin Sensitivity and Palmitate Oxidation in Subsarcolemmal, Not Intermyofibrillar, Mitochondria

“…the motor unit size recruitment principle). These exercise studies (45)(46)(47)(51)(52)(53), and our present transfection studies, in which PGC-1␣ is up-regulated in only some muscle fibers, do not invalidate the observations that metabolic changes are induced on a per g or per whole muscle basis. In exercise studies (53), as well as in the present studies, there will be a considerable heterogeneity of responses in PGC-1␣, and in other parameters, among muscle fibers, ranging from no change to very large changes in some muscle fibers.…”

“…Others had already shown that there was not a strong relationship between insulin-signaling proteins and the metabolic characteristics of rodent skeletal muscle (72), that PGC-1␣ did not induce insulin-signaling proteins in L6 myotubes (3), and that exercise training, which increases PGC-1␣ (46), also failed to up-regulate insulin-signaling proteins (73). However, unlike studies in PGC-1␣-transfected L6 myotubes (3), we did observe a greater insulin-stimulated phosphorylation of Akt2 in PGC-1␣-transfected muscles.…”

Ablation of AMP-Activated Protein Kinase α2 Activity Exacerbates Insulin Resistance Induced by High-Fat Feeding of Mice

“…Other studies have shown that obese insulin-resistant Zucker rats have reduced AMPK activity and related signaling abnormalities in skeletal muscle (37)(38)(39). These reports indicate that skeletal muscle AMPK activity can be altered if the obesity and/or insulin resistance is severe, and, in this situation, lower AMPK activity may contribute to the obese and insulin-resistant phenotype (39).…”

“…These reports indicate that skeletal muscle AMPK activity can be altered if the obesity and/or insulin resistance is severe, and, in this situation, lower AMPK activity may contribute to the obese and insulin-resistant phenotype (39). Importantly, exercise training can reverse abnormalities of impaired AMPK signaling in obese Zucker rats, which may contribute to the beneficial effect of exercise on improvement of insulin resistance (39). Interestingly, it has also been reported that suppression of AMPK signaling is involved in tumor necrosis factor-induced skeletal muscle insulin resistance (40).…”

Effect of Acute Exercise on AMPK Signaling in Skeletal Muscle of Subjects With Type 2 Diabetes

“…In addition to its role on contraction-induced muscle glucose transport and fatty acid oxidation with a single bout of exercise, it has also been hypothesized that the repetitive increases in muscle AMPK activity that occur during physical training with each exercise bout lead to increased mitochondrial biogenesis and function (13,14). Increases in mitochondrial number/function with physical training are thought to occur, in part, through a mechanism that involves AMPK-mediated peroxisome proliferator-activated receptor coactivator-1 (PGC-1) gene expression (15).…”