Overexpression of Fyn inhibits macroautophagy while activating the mTORC1-IRE1a pathway leading to cell death
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Skeletal muscle accounts for about 40% of total body weight in the human body. In addition to its function in whole body movements, skeletal muscle also plays an important role in the regulation of energy metabolism. Decreased muscle mass or atrophy occurs under a variety of pathophysiologic states. The loss of muscle mass under these conditions typically results from disturbance of the homeostasis between protein synthesis and degradation.;Fyn is a member of the Src family non-receptor tyrosine kinases. We previously reported that Fyn stimulates mTORC1 through inhibition of the LKB1-AMPK axis. Although mTORC1 activation associated with protein synthesis was detected, skeletal muscle specific Fyn transgenic mice (SKM-FynTg) displayed severe atrophy. In parallel, we detected inhibition of macroautophagy via a STAT3-dependent reduction in VPS34 protein level and disassembly of VPS34NPS15/Beclinl/ATG14 complex ' in SKM-FynTg mice. Moreover, physiological conditions of starvation/refeeding regulated Fyn kinase activity that positively correlates with STAT3 phosphorylation and decrease of VPS34 protein levels. These data demonstrated a Fyn/STAT3NPS34 pathway that is responsible for macroautophagy inhibiton leading to skeletal muscle atrophy. Additionally, the increase of Fyn protein level with age implies that Fyn may be involved in the process of sarcopenia.;Further, to better characterize the atrophy phenotype of this transgenic model, we examined the relationship between Fyn and endoplasmic reticulum stress. Overexpression of Fyn in skeletal muscle and in HEK293T cells resulted in the activation of IRE1alpha and JNK leading to cell death. Fyn synergized with the general endoplasmic reticulum stress inducer thapsigargin resulted in the activation of IRE1alpha and further accelerated cell death. Moreover, inhibition of mTORC1 with rapamycin suppressed IRE1alpha activation and resulted in protection against Fyn and thapsigargin-induced cell death. In vivo, rapamycin treatment reduced the skeletal muscle IRE1alpha activation in the SKM-FynTg mice. These data demonstrated the presence of a Fyn-induced endoplasmic reticulum stress that occurred at least in part through the activation of mTORC1, and subsequent activation of the IRE1alpha-JNK pathway driving cell death.;Thus, the' findings in this thesis demonstrated that Fyn plays as a central node that connects autophagy inhibition and ER stress both in vitro and in vivo.