| Research Abstract |
1.Skeletal muscle expresses two catalytic subunits, alpha1 and alpha2, of the 5'-AMP-activated protein kinase (AMPK), which is implicated in exercise-stimulated metabolic events, such as glucose transport, fatty acid oxidation and insulin sensitivity. AMPK is activated in response to metabolic stresses, such as contraction, hypoxia, and the inhibition of oxidative phosphorylation. In this research we hypothesized that acute oxidative stress stimulates AMPK in skeletal muscle. Our data suggest that oxidative stress activates alpha1, but not alpha2, in skeletal muscle via an AMP-independent mechanism and leads to increases in the rate of glucose transport and acetyl CoA carboxylase (ACC) phosphorylation.
2.Muscle contraction activates alpha2, but this activation may occur with or without activation of alpha1, suggesting that alpha2 is the major isoform responsible for contraction-induced metabolic events. In this research we found that 1)AMPK alpha1 is the predominant isoform activated by
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AMP-independent phosphorylation in low-intensity contracting muscle, 2)AMPK alpha2 is activated by an AMP-dependent mechanism in high-intensity contracting muscle, and 3)activation of each isoform enhances glucose transport and ACC phosphorylation in skeletal muscle.
3.We previously reported that transgenic mice overexpressing leptin in the liver (LepTg) exhibit hyperleptinemia and enhanced insulin sensitivity. In this research we found that phosphorylation of AMPK was chronically augmented in LepTg soleus, with a concomitant decrease in tissue triglyceride. Despite preexisting hyperleptinemia, high-fat diet (HFD)-fed LepTg developed obesity, insulin-resistance, and hyperlipidemia. In parallel, elevated soleus AMPK in regular diet-fed LepTg was attenuated, and tissue triglyceride was increased in those given HFD. Substitution of HFD with regular diet caused a robust recovery of soleus AMPK in LepTg, with a higher rate of body weight reduction and a regain of insulin sensitivity. We conclude that soleus AMPK in LepTg changes in parallel with its insulin sensitivity under dietary modification, suggesting a close association between skeletal muscle AMPK activity and sensitivity to leptin. Less
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