Anti-inflammatory strategies are often used to reduce muscle pain and soreness that can result from high-intensity muscular activity. However, studies indicate that components of the acute inflammatory response may be required for muscle repair and growth. The hypothesis of this study was that cyclooxygenase (COX)-2 activity is required for compensatory hypertrophy of skeletal muscle. We used the synergist ablation model of skeletal muscle hypertrophy, along with the specific COX-2 inhibitor NS-398, to investigate the role of COX-2 in overload-induced muscle growth in mice. COX-2 was expressed in plantaris muscles during compensatory hypertrophy and was localized mainly in or near muscle cell nuclei. Treatment with NS-398 blunted the increases in mass and protein content in overloaded muscles compared with vehicle-treated controls. Additionally, the COX-2 inhibitor decreased activity of the urokinase type plasminogen activator, macrophage accumulation, and cell proliferation, all of which are required for hypertrophy after synergist ablation. Expression of insulin-like growth factor-1 and phosphorylation of Akt, mammalian target of rapamycin, and p70S6K were increased following synergist ablation, but were not affected by NS-398. Additionally, expression of atrogin-1 was reduced during hypertrophy, but was also not affected by NS-398. These results demonstrate that COX-2 activity is required for skeletal muscle hypertrophy, possibly through facilitation of extracellular protease activity, macrophage accumulation, and cell proliferation.

Adult skeletal muscle possesses remarkable potential for growth in response to mechanical loading; however, many of the cellular and molecular mechanisms involved remain undefined. The hypothesis of this study was that the extracellular serine protease, urokinase-type plasminogen activator (uPA), is required for muscle hypertrophy, in part by promoting macrophage accumulation in muscle subjected to increased mechanical loading. Compensatory muscle hypertrophy was induced in mouse plantaris (PLT) muscles by surgical ablation of synergist muscles. Following synergist ablation, PLT muscles in wild-type mice demonstrated edema and infiltration of neutrophils and macrophages but an absence of overt muscle fiber damage. Sham procedures resulted in no edema or accumulation of inflammatory cells. In addition, synergist ablation was associated with a large increase in activity of uPA in the PLT muscle. uPA-null mice demonstrated complete abrogation of compensatory hypertrophy associated with reduced macrophage accumulation, indicating that uPA is required for hypertrophy. Macrophages isolated from wild-type PLT muscle during compensatory hypertrophy expressed uPA and IGF-I, both of which may contribute to hypertrophy. To determine whether macrophages are required for muscle hypertrophy, clodronate liposomes were administered to deplete macrophages in wild-type mice; this resulted in reduced muscle hypertrophy. Decreased macrophage accumulation was associated with reduced cell proliferation but did not alter signaling through the mammalian target of rapamycin pathway. These data indicate that uPA and macrophages are required for muscle hypertrophy following synergist ablation.

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