Glycogen synthase kinase-3β is required for the induction of skeletal muscle atrophy

Abstract

Skeletal muscle atrophy commonly occurs in acute and chronic disease. The expression of the muscle-specific E3 ligases atrogin-1 (MAFbx) and muscle RING finger 1 (MuRF1) is induced by atrophy stimuli such as glucocorticoids or absence of IGF-I/insulin and subsequent Akt signaling. We investigated whether glycogen synthase kinase-3β (GSK-3β), a downstream molecule in IGF-I/Akt signaling, is required for basal and atrophy stimulus-induced expression of atrogin-1 and MuRF1, and myofibrillar protein loss in C(2)C(12) skeletal myotubes. Abrogation of basal IGF-I signaling, using LY294002, resulted in a prominent induction of atrogin-1 and MuRF1 mRNA and was accompanied by a loss of myosin heavy chain fast (MyHC-f) and myosin light chains 1 (MyLC-1) and -3 (MyLC-3). The synthetic glucocorticoid dexamethasone (Dex) also induced the expression of both atrogenes and likewise resulted in the loss of myosin protein abundance. Genetic ablation of GSK-3β using small interfering RNA resulted in specific sparing of MyHC-f, MyLC-1, and MyLC-3 protein levels after Dex treatment or impaired IGF-I/Akt signaling. Interestingly, loss of endogenous GSK-3β suppressed both basal and atrophy stimulus-induced atrogin-1 and MuRF1 expression, whereas pharmacological GSK-3β inhibition, using CHIR99021 or LiCl, only reduced atrogin-1 mRNA levels in response to LY294002 or Dex. In conclusion, our data reveal that myotube atrophy and myofibrillar protein loss are GSK-3β dependent, and demonstrate for the first time that basal and atrophy stimulus-induced atrogin-1 mRNA expression requires GSK-3β enzymatic activity, whereas MuRF1 expression depends solely on the physical presence of GSK-3β.

A cell-autonomous role for the glucocorticoid receptor in skeletal muscle atrophy induced by systemic glucocorticoid exposure

“…We also examined FoxO1 and GSK-3␤, two important downstream targets of Akt. GSK-3␤, an inhibitor of protein synthesis and glycogen synthesis, was recently reported to play an important role in muscle atrophy resulting from impaired insulin/IGF-1 signaling or GC treatment (58). However, no changes in GSK-3␤ total or phosphorylated forms were observed with DEX treatment, consistent with a lack of effect on upstream Akt activity.…”

Stress-induced Skeletal Muscle Gadd45a Expression Reprograms Myonuclei and Causes Muscle Atrophy

“…Previous studies demonstrated important roles for reduced PGC-1␣ expression (18), reduced Akt signaling (40), increased ATF4 expression (21), increased GSK-3␤ signaling (48), increased caspase-3 activity (17), and increased autophagy (15,16). In addition, microarray studies showed that atrophying muscles contain very high levels of Gadd45a mRNA (4 -8, 21).…”

“…S4A). We therefore examined the effects of Gadd45a on Akt kinase activity, which inhibits muscle atrophy and increases protein synthesis by multiple mechanisms, including phosphorylation (inhibition) of its target GSK-3␤ (24,35,40,41,47,48). We found that Gadd45a reduced Akt and GSK-3␤ phosphorylation in cultured myotubes (Fig.…”

Synergistic stimulation of myogenesis by glucocorticoid and IGF-I signaling

“…IGF-I was used at 5 nM, which is sufficient to stimulate myogenic differentiation (51). Dex was used at a relatively low dose of 10 M compared with other studies (8,45,52). Furthermore, Pred, the prescribed oral GC in the post hoc stratification analyzed RCT (10), was used in equimolar and equipotent [fivefold compared with Dex (5, 6, 15)] doses.…”