PGC-1alpha protects skeletal muscle from atrophy by suppressing FoxO3 action and atrophy-specific gene transcription

Abstract

Maintaining muscle size and fiber composition requires contractile activity. Increased activity stimulates expression of the transcriptional coactivator PGC-1alpha (peroxisome proliferator-activated receptor gamma coactivator 1alpha), which promotes fiber-type switching from glycolytic toward more oxidative fibers. In response to disuse or denervation, but also in fasting and many systemic diseases, muscles undergo marked atrophy through a common set of transcriptional changes. FoxO family transcription factors play a critical role in this loss of cell protein, and when activated, FoxO3 causes expression of the atrophy-related ubiquitin ligases atrogin-1 and MuRF-1 and profound loss of muscle mass. To understand how exercise might retard muscle atrophy, we investigated the possible interplay between PGC-1alpha and the FoxO family in regulation of muscle size. Rodent muscles showed a large decrease in PGC-1alpha mRNA during atrophy induced by denervation as well as by cancer cachexia, diabetes, and renal failure. Furthermore, in transgenic mice overexpressing PGC-1alpha, denervation and fasting caused a much smaller decrease in muscle fiber diameter and a smaller induction of atrogin-1 and MuRF-1 than in control mice. Increased expression of PGC-1alpha also increased mRNA for several genes involved in energy metabolism whose expression decreases during atrophy. Transfection of PGC-1alpha into adult fibers reduced the capacity of FoxO3 to cause fiber atrophy and to bind to and transcribe from the atrogin-1 promoter. Thus, the high levels of PGC-1alpha in dark and exercising muscles can explain their resistance to atrophy, and the rapid fall in PGC-1alpha during atrophy should enhance the FoxO-dependent loss of muscle mass.

Fig. 1.

Inhibition of PGC-1α expression in various types of muscle atrophy. (a) Rat models of muscle atrophy: acute streptozotocin-induced diabetes mellitus, chronic renal failure induced by subtotal nephrectomy, and cancer cachexia induced by Yoshida ascities hepatoma. Animal models were described in depth elsewhere (22, 36, 37). Samples were taken at times when the muscles were undergoing rapid weight loss. PGC-1α was assayed by quantitative PCR. (b) Denervation by unilateral transection of the sciatic nerve in mouse. Results are expressed relative to mRNA levels in the contralateral enervated muscle.

Fig. 2.

PGC-1α-transgenic mice are protected from denervation- and fasting-induced muscle atrophy. (a) Fiber size in control and denervated wild-type mice. (Left) Succinate dehydrogenase staining of mock-transected (Upper) and denervated (Lower) tibialis anterior muscle. (Right) Fiber size distribution of tibialis anterior muscles. Red bars, denervated; black bars, mock-transected. (b) As in a, using MCK–PGC-1α-transgenic mice. (c) Mean cross-sectional area of denervated and control wild-type and MCK–PGC-1α transgenic tibialis anterior muscles. (d) Mean cross-sectional area of tibialis anterior muscles from food-deprived and fed wild-type and MCK–PGC-1α transgenic animals.

Fig. 3.

PGC-1α reduces transcription of key atrogenes involved in protein degradation. (a) Expression of ubiquitin-ligases atrogin-1, MuRF-1, and lysosomal hydrolase cathepsin L after denervation for 12 days, analyzed by real-time PCR. (b) Expression of ubiquitin-ligases atrogin-1, MuRF-1, and lysosomal hydrolase cathepsin L, after food deprivation for 2 days, analyzed by real-time PCR. (c) Expression of genes involved in energy metabolism after 12 days of denervation. ∗, P < 0.05 between control and transgenic mice, by Student's t test.

Fig. 4.

PGC-1α suppresses FoxO3 action. (a Left) Plasmids bearing a luciferase reporter driven by the atrogin-1 promoter, c.a.FoxO3A, and PGC-1α were electroporated into the intact tibialis anterior muscles of adult mice, and luciferase activity was measured in extracts from the muscles 1 week later. (a Right) As in a Left, but using a canonical FoxO sequence (DAF16) driving the luciferase gene. (b) c.a.FoxO3 binding to a FoxO response element in the atrogin-1 promoter is blocked by PGC-1α. Muscles were collected 8 days after c.a.FoxO3 transfection with or without FLAG-PGC-1α, and ChIP assays were performed as described. (c) Constitutively active HA-tagged FoxO3A with or without FLAG-PGC-1α was electroporated into the intact tibialis anterior muscles of adult mice. Muscles were harvested after 8 days, serially sectioned, and subjected to immunohistochemistry (Left) or fiber size quantification (Right).

Fig. 5.

Mechanisms for inhibition of atrophy and growth promotion by muscle activity. With repeated muscle contractions, IGF-1 production by the muscle increases, which stimulates protein synthesis and fiber hypertrophy through activation of PI3K and AKT kinases. AKT also causes phosphorylation and nuclear exclusion of FoxO 1, 3, and 4, which suppresses atrogene expression and proteolysis. In addition, PGC-1α is induced, leading to increased production of mitochondria and a shift to slow-twitch, oxidative fibers. PGC-1α also inhibits transcriptional activity of FoxO3, which suppresses atrogene expression and protein degradation.