Healthy skeletal muscle atrophy and/or metabolism via FoxOs signaling pathway protects against starvation-induced fatty liver
Authors: Oyabu Mamoru , Goto Tsuyoshi , Yoshioka Kiyoshi , Kawaguchi Runa , Kwon Jungin , Ohira Yuto , Ishihara Kengo , Suganami Takayoshi , Miura Shinji , Kamei Yasutomi 
Published: May 2023

Objective: Non-alcoholic fatty liver disease (NAFLD) with excessive triglyceride accumulation in the liver prevails in approximately 24% of adults worldwide. Prolonged fasting rapidly causes hepatic steatosis. However, the etiology of hepatic steatosis during starvation is unclear. During starvation, skeletal muscle, the central tissue for metabolism, physical exercise, and storage for protein/amino acids, adapts to energy deficit by promoting muscle proteolysis, which leads to muscle atrophy. The critical factors that cause muscle atrophy are the Forkhead Box O family transcription factors (FoxO1, 3 and 4). Although skeletal muscle FoxOs are necessary to adapt to starvation, the specific function of skeletal muscle FoxOs at the whole-body level is unclear. We hypothesized that skeletal muscle atrophy and/or metabolism would be associated with the etiology of starvation-induced fatty liver and aimed to elucidate the inter-organ crosstalk from skeletal muscle to liver via FoxOs signaling pathway. Methods: We previously generated skeletal muscle-specific FoxO1,3,4-triple knockout mice (mFoxO1,3,4-/-) using human skeletal α-actin promoter-driven Cre recombinase expression (Oyabu et al. 2022. FASEB J 36: e22152). The control FoxO1,3,4flox/flox mice (wild-type mice) and mFoxO1,3,4-/- mice were fasted for 48 h to mimic prolonged fasting. Results: Skeletal muscle mass was significantly reduced in 48 h-fasted wild-type mice. This reduction was prevented by the skeletal muscle FoxO-triple deletion. Fasted mFoxO1,3,4-/- mice showed reduced blood glucose levels, suggesting the lack of energy in 48 h-fasted mFoxO1,3,4-/- mice. Interestingly, starvation-induced lipid-droplet formation in the liver, accumulation of hepatic triglyceride, and the expression of hepatic lipid-droplet-marker protein (Plin2) were significantly increased in 48 h-fasted mFoxO1,3,4-/- mice compared to fasted wild-type mice, indicating that hepatic steatosis was accelerated by skeletal muscle FoxO-triple deletion, despite of the retention from skeletal muscle loss. This suggests that skeletal muscle FoxOs are essential for protection against starvation-induced hepatic steatosis for adaptation to starvation. Starvation induced the upregulation of genes involved in not only muscle proteolysis, but also triglyceride uptake and metabolism in skeletal muscle, which were abrogated by the skeletal muscle-specific FoxO-triple deletion. In addition, the muscle triglyceride level was lower in 48 h-fasted mFoxO1,3,4-/- mice than fasted wild-type mice. Conclusion: Here, we demonstrated a critical connection between liver and skeletal muscle via FoxOs transcription factors. Our data indicate that healthy skeletal muscle atrophy and/or metabolism via FoxOs signaling pathway is critical for prevention of starvation-induced hepatic steatosis. This study sheds light on the skeletal muscle-liver inter-organ crosstalk as the potential therapeutic targets of malnutrition-induced NAFLD. This study was supported by Grants-in-Aids for Scientific Research KAKENHI from the Japan Society for the Promotion of Science. This study was also supported by Mishima Kaiun Memorial Foundation and Grants-in-Aids for the Second Dream Challenge Planning Awards from JSBBA. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.