OBJECTIVE Skeletal muscle undergoes rapid and profound atrophy in response to decreased mechanical loading,e.g.,limb immobilization and bed rest.Phosphatidylinositol 3 kinase(PI3K)/Akt signaling pathway is critical for regulating the balance between protein synthesis and degradation during disuse/inactivity-induced muscle atrophy.The present study aimed to investigate whether natural product Icaritin(ICT)required PI3K/Akt signaling to exert counteractive effect on skeletal muscle atrophy following mechanical unloading.METHODS Two oral dosages of ICT(80and 120mg·kg-1·d-1)were administrated daily to adult male rats with or without daily injection of PI3K/Akt signaling inhibitor wortmannin(15μg·kg-1·d-1)during 28-d hindlimb suspension(HS).Ex vivo muscle functional testing,histological and immunohistochemical analysis were performed to determine the changes of soleus muscle function,mean muscle fiber cross-sectional area(CSA)and fiber type distribution.Western blot and real-time PCR analysis were also performed to evaluate the protein or mRNA expression of the markers involved in PI3K/Akt signaling pathway.RESULTS After 28-d HS,soleus muscle underwent profound muscle atrophy(-52.7% muscle mass vs.pre-HS baseline).The high dose ICT treatment significantly attenuated the decreases in soleus muscle mass(+22.6% vs.HS),muscle fiber CSA(+52.8% vs.HS),as well as the muscle functional testing parameters during the unloading.Molecularly,the high dose ICT treatment significantly attenuated the decreases in protein synthesis markers at protein levels(phosphorylation of Akt and its downstream proteins)during the unloading,whereas the increases in protein degradation markers at mRNA(atrogin-1and MuRF-1)and protein(nuclear FOXO1 and FOXO3a)levels during the unloading were significantly attenuated by the high dose ICT treatment.The low dose ICT treatment moderately attenuated the above changes induced by the unloading.Mechanistically,Wortmannin could abolish the above effects of ICT.CONCLUSION ICT requires participation of PI3K/Akt signaling to counteract skeletal muscle atrophy following mechanical unloading in a dose-dependent manner. OBJECTIVE Skeletal muscle undergoes rapid and profound atrophy in response to decreased mechanical loading, eg, limb immobilization and bed rest. Phosphatidylinositol 3 kinase (PI3K) / Akt signaling pathway is critical for regulating the balance between protein synthesis and degradation during disuse / inactivity-induced muscle atrophy. The present study aimed to investigate whether natural product Icaritin (ICT) required PI3K / Akt signaling to exert counteractive effect on skeletal muscle atrophy following mechanical unloading. METHODS Two oral dosages of ICT (80 and 120 mg · kg -1 · d -1 ) were administrated daily to adult male rats with or without daily injection of PI3K / Akt signaling inhibitor wortmannin (15 μg · kg -1 · d -1) during 28-d hindlimb suspension (HS) .Ex vivo muscle functional testing, histological and immunohistochemical analysis were performed to determine the changes of soleus muscle function, mean muscle fiber cross-sectional area (CSA) and fiber type distribution. Western blot and real-time PCR a nalysis were also performed to evaluate the protein or mRNA expression of the markers involved in PI3K / Akt signaling pathway .RESULTS After 28-d HS, soleus muscle underwent profound muscle atrophy (-52.7% muscle mass vs. pre-HS baseline) high dose ICT treatment significantly attenuated the decrease in soleus muscle mass (+ 22.6% vs. HS), muscle fiber CSA (+ 52.8% vs. HS), as well as the muscle functional testing parameters during the unloading. Molecularly, the high dose ICT treatment significantly attenuated the decrease in protein synthesis markers at protein levels (phosphorylation of Akt and its downstream proteins) during the unloading, while the increasing in protein degradation markers at mRNA (atrogin-1 and MuRF-1) and protein (nuclear FOXO1 and FOXO3a ) levels during the unloading were significantly attenuated by the high dose ICT treatment. The low dose ICT treatment moderately attenuated the above changes induced by the unloading. Mechanistically, Wortmannin could abolish the above effects of ICT.CCONCLUSION ICT requires participation of PI3K / Akt signaling to counteract skeletal muscle atrophy following mechanical unloading in a dose-dependent manner.