The inflammatory response to acute muscle injury

The overall goal of this dissertation was to examine inflammatory and regenerative responses to acute skeletal muscle damage and to define molecular mediators of repair. Study I examined the effects of an oral anti-inflammatory supplement on exercise-induced muscle damage (EIMD) and systemic inflammation in a human model. Quercetin has been shown in animal and in vitro models to downregulate nuclear factor-kappa beta (NF-κB) nuclear transactivation and monocyte chemoattractant protein 1 (MCP-1) secretion, which regulate muscle regeneration and inflammatory signaling between muscle and immune cells after injury. Subjects ingested quercetin (N=15) or placebo (N=15) before and after performing 24 eccentric contractions of the elbow flexors. Subjects experienced moderate strength losses and delayed onset muscle soreness, indicating damage, but no supplementation effect was observed. The null effect of quercetin in the human model (with its complex inflammatory response) encouraged us to explore basic injury-induced inflammation in a controlled in vitro model, to better understand the post-injury roles of NF-κB and MCP-1. Study II used an in vitro injury model (scratch of C 2C12 myotubes) to identify the roles and interplay of NF-κB and MCP-1 in muscle regeneration and inflammation following acute injury. Protein expression changes of NF-κB and MCP-1, and morphological changes in regenerating muscle cultures were monitored for 24-72 hours (h) post-injury (3-6 replicates per experiment). NF-κB activation was significantly downregulated (-30±1.4% to -44±1.1%) at 6-12h post-injury. Pharmacological blockade of NF-κB downregulated satellite cell proliferation by 19±9% after 19h and 72h, evidence for a role of NF-κB signaling in post-injury regeneration. Furthermore, NF-κB activation strongly correlated (R=0.69) with MCP-1 secretion from injured muscle cultures, and blockade of NF-κB reduced MCP-1 secretion at 1-24h (-33±0.1%) and strongly correlated (R=0.74) with NF-κB activation. These data support recent in vivo findings to demonstrate that NF-κB and MCP-1 signaling are critical regulators of inflammatory and regenerative responses following muscle injury. Moreover, this work provides the first kinetic profile of early (\u3c24 hours) molecular responses of NF-κB and MCP-1 to acute muscle injury, and introduces novel evidence that NF-κB regulates MCP-1 protein secretion, indicating an indispensible role of NF-κB signaling in muscle inflammation in vitro