Abstract
The protein serine/threonine kinase Akt/protein kinase B has been recognized as a critical signaling mediator for multiple cell systems. The function of Akt in skeletal muscle is not well understood, and whether contractile activity stimulates Akt activity has been controversial. In the current study, contraction in situ, induced via sciatic nerve stimulation, significantly increased Akt Ser(473) phosphorylation in multiple muscle types including the extensor digitorum longus (13-fold over basal), plantaris (5.8-fold), red gastrocnemius (4.7-fold), white gastrocnemius (3.3-fold), and soleus (1.6-fold). In addition to increasing phosphorylation, contraction in situ significantly increased the activity of all three Akt isoforms (Akt1 > Akt2 > Akt3) with maximal activation occurring at 2.5 min and returning to base line with 15 min of contraction. Akt phosphorylation and activity were also increased when isolated muscles were contracted in vitro in the absence of systemic factors, although to a much lesser extent. The phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002 fully inhibited contraction-stimulated Akt phosphorylation and activity but did not diminish contraction-stimulated glycogen synthase kinase-3 phosphorylation and glycogen synthase activity. These results demonstrate that contraction increases Akt phosphorylation and activity in skeletal muscle and that this stimulation is rapid, transient, muscle fiber type-specific, and wortmannin- and LY294002-inhibitable. Akt signaling is not necessary for the regulation of glycogen synthase activity in contracting skeletal muscle.
Highlights
The protein serine/threonine kinase Akt/protein kinase B has been recognized as a critical signaling mediator for multiple cell systems
The signal transduction mechanism by which insulin induces these metabolic responses has been the focus of intense research, and it is well established that the activation of phosphatidylinositol 3-kinase (PI3K)1 is critical for insulin actions, as most physiological responses of mammalian cells to insulin are prevented by pharmacologic inhibition or by overexpression of dominant negative mutants
What is known is that insulin and contraction utilize different signaling pathways leading to glucose uptake and glycogen synthesis in skeletal muscle because contraction-stimulated glucose uptake occurs through a PI3K-independent mechanism [2,3,4], and contraction-stimulated glycogen synthase activation can occur in the absence of PI3K activation [5]
Summary
Akt signaling is not necessary for the regulation of glycogen synthase activity in contracting skeletal muscle. Several studies have reported that Akt can be activated in some cell systems by a mechanism independent of PI3K activation, for example in response to growth hormone treatment [15], and increases in intracellular calcium or cAMP [16, 17]. Other studies have shown significant activation or phosphorylation of Akt in intact skeletal muscles in response to contraction [28, 29] These discrepant findings suggest that contraction may regulate Akt in an intensity- and time-dependent manner, or perhaps Akt stimulation in muscle may be fiber type-specific. We report that contraction clearly stimulates Akt activity and phosphorylation in skeletal muscle and that this occurs in a timeand fiber type-specific manner. Contraction-induced Akt activation occurs through a wortmannin-sensitive mechanism, and Akt does not appear to be necessary for the ability of muscle contraction to regulate glycogen synthase activity
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