Muscle stem cell (satellite cell) activation post?muscle injury is a transient and critical step in muscle regeneration. a p21Cip1/Waf1-independent manner in satellite cells [79], helps the cells to overcome cell cycle 1M7 blockage. HGF can activate satellite cells rapidly after trauma by promoting EMT to release them from their quiescent prone niche and removing cell cycle blockages. NO NO is a freely diffusible small messenger capable of pleiotropic cellular functions, such as survival, stress resistance, and neurotransmission [81]. NO is produced in skeletal muscle through reactions catalyzed 1M7 by nitric oxide synthase (NOS). Within 6?h post-injury, NOS mRNA levels are significantly increased in both damaged muscle fibers and the infiltrating macrophages, therefore elevating the NO levels at the injury site [82C84]. In iNOS(?/?) mice, satellite cells fail to proliferate and differentiate after injury [85], suggesting that NO is required for normal muscle reparation after injury. NO plays multiple roles during the muscle regeneration process. At the early stage of muscle damage, it promotes macrophages 1M7 to lyse damaged muscle cells in a reactive oxygen species (ROS)-independent manner to protect cells from further ROS damage [86], and stimulates the release of HGF, together with other growth factors and cytokines to activate satellite cells [87]. At the second stage of muscle regeneration, NO inhibits neutrophil-mediated lysis of muscle cells and reduces ROS generated from prolonged inflammation, protecting the activated satellite cells Rabbit Polyclonal to EPN2 from ROS stress and apoptosis [82]. NO activates satellite cells not only by facilitating the release of HGF, but also by antagonizing the inhibitory effects of TGF- on satellite cells. The administration of L-NAME, an NOS inhibitor, at the injury site in rat muscle leads to abnormally elevated TGF- level that induces fibrosis [88]. IGF and FGFs Insulin-like growth factor (IGF) is a circulating hormone critical for development and regeneration of almost every organ [89]. IGF signaling is initiated by binding of IGF to the IGF receptor (IGFR) to activate its tyrosine kinase activity and autophosphorylation, which in turn phosphorylates insulin receptor substrate 1 (IRS-1). Phosphorylated IRS-1 recruits the regulatory subunit of PI3K and activates it. Activated PI3K phosphorylates Akt, which then activates mTOR and p70S6 kinase to turn on the IGF-PI3K/Akt-mTOR-S6K axis of signaling pathway. This signaling process has been shown to be important for muscle mass maintenance [90]. Six IGF binding proteins, named IGFBP1-6, bind IGF in the extracellular fluid and the circulation to further regulate IGF activities [91]. The expression of IGF and all six IGFBPs has been detected in regenerating skeletal muscle [92], suggesting their roles in muscle wound healing. Muscle damage induces the expression of alternative splicing isoforms of IGF named mechano-growth factor (MGF) and IGF-IEa [93]. MGF is only expressed in 1M7 the damaged muscle and its expression is correlated with 1M7 the activation of quiescent satellite cells [94]. IGF-IEa is expressed later than MGF during muscle regeneration, correlating with myoblast proliferation and differentiation [95, 96]. MGF elevates the activity of superoxide dismutase, the enzyme required for decreasing the level of ROS [97], thus protecting the satellite cells from ROS-induced damage. IGFBP6 is an IGF sequester, which increases the expression levels of IGF isoforms. However, its expression level is dramatically decreased at the early stage of muscle regeneration to allow more IGF available to activate satellite cells and promote their proliferation [98]. IGF-IR heterozygous mice display decreased the levels of MyoD expression and satellite cell activation [99], further confirming the importance of IGF in the satellite cell activation process. The mechanism of IGF-mediated satellite cell activation has not been fully elucidated but may involve the upregulation of Myf5 expression upon injury. After muscle injury, an influx of calcium triggers calcineurin and calmodulin kinase through calcium binding to calmodulin, to activate Myf5 expression. IGF can activate Myf5 through the calcium-mediated activation pathway [100]. In addition, it could also activate Myf5 expression through PI3K/Akt and ERK signaling pathways [100, 101]. It can also activate expression of cyclin D2 to promote entry to cell cycle and cell.