Nutrient and energy sensing in skeletal muscle

Abstract

Nutrient overload and physical inactivity often leads to the development of obesity and type 2 diabetes. Acute over-nutrition can induce insulin resistance, while physical exercise enhances skeletal muscle insulin sensitivity. Like every living cell, skeletal muscle senses nutrient and energy signals and to adjust metabolic flux. This thesis focuses on some of the key nutrient and energy sensing (exercise/contraction-induced) pathways in skeletal muscle that regulate metabolism.

AMPK is a key energy sensing enzyme, composed of three different subunits, with several isoforms existing for each subunit. The role of the different AMPK subunits in the regulation of mTOR signaling was investigated. In EDL muscle from wild-type mice, AICAR (a chemical AMPK activator) completely inhibited insulin-mediated phosphorylation of S6K1 (Thr389), rpS6 (Ser235/236) and 4E-BP1 (Thr37/46). Thus, AMPK is negative regulator of mTOR signaling. The inhibitory effects of AICAR were partially blocked in skeletal muscle from alpha2 AMPK depleted (KO) and gamma3 AMPK KO mice, functional alpha2 AMPK and gamma3 AMPK subunits are required for the AICAR-mediated inhibition of mTOR signaling.

Excessive amino acid availability impairs insulin action in skeletal muscle. In primary human myotubes, supra-physiological leucine concentrations reduced insulin-stimulated Akt phosphorylation, glucose uptake and glucose incorporation into glycogen. These results indicate nutrient overload induced insulin resistance. Depletion of S6K1 using siRNA enhanced basal glucose uptake and protected against the development insulin resistance in response to leucine. Study II highlights a direct role for S6K1 plays in insulin action and glucose metabolism.

Several proteins are phosphorylated in skeletal muscle in response to acute exercise. The effect of cycling or resistance exercise on the phosphorylation of Akt substrates was determined using an antibody that recognizes a consensus Akt phosporylation motif (PAS). Proteins of 160 and 300 kDa were indentified as AS160 (TBC1D4) and filamin A, respectively. Acute endurance exercise increased phosphorylation of TBC1D4 and filamin A, with concomitant increase in phosphorylation of Akt Ser473, whereas acute resistance exercise was without effect. TBC1D4 and filamin A may provide link between acute exercise and metabolism in muscle.

Hypoxia is useful model to study effects of exercise/muscle contraction. In paper III, hypoxia-induced glucose transport was partially impaired in EDL muscle from gamma3 AMPK KO mice, indicating a role for the gamma3 AMPK subunit in glucose metabolism. These effects were uncoupled from AMPK and TBC1D1/D4 signaling, suggesting that an AMPK-and TBC1D1/D4-independent mechanism contributes to glucose transport in skeletal muscle. An interaction between AMPK and CaMK is implicated, since the CaMK inhibitor KN-93 had a more potent effect to reduce hypoxia-induced glucose transport in gamma3 AMPK KO mice.

Nitric oxide (NO) is implicated in exercise-induced signaling networks. Exposure of human skeletal muscle to an NO donor increased glucose uptake, with a concomitant increase in cGMP levels and alpha1-associated AMPK activity. Thus, NO/cGMP signaling may be part of a novel pathway that regulates skeletal muscle glucose uptake.

In conclusion, AMPK and mTOR signaling play important roles in regulation of skeletal muscle metabolism. AMPK appears to have a heterotrimer-specific action on skeletal muscle metabolism. Furthermore, contraction/exercise responsive signaling pathways including CaMK, NO-cGMP and Akt are important in the regulation of skeletal muscle glucose uptake.