The Role Of Insulin-like Growth Factor 1 Axis In Autophagy And Muscle Atrophy In Chronic Kidney Disease And Influence Of Autophagy Intervention

BackgroundMuscle atrophy is a frequent complication of chronic kidney disease(CKD)and is associated with increased morbidity and mortality.Insulin-like growth factor 1(IGF-1)resistance has been considered to be one of mechanisms of skeletal muscel atrophy in CKD,but the underlying molecular mechanisms remain elusive.IGF-1 acts both in endocrine and autocrine/paracrine fashion.Herein we focused on understanding the biological roles of each mode of IGF-1 in skeletal muscle of CKD.The activation of autophagy has been shown in the skeletal muscle of 5/6 nephrectomized rats,and low protein diet supplemented with ?-ketoacids(LPD+KA)improves the muscle atrophy by abolishing the activation of autophagy.We hypothesized that activation of autophagy is assosiated with impaired tissue IGF-1 axis in skeletal muscle.The aim of this study was to investigate molecular mechanisms of CKD related aberrant activation of autophagy in skeletal muscle.We focused on the main signaling pathway known to control protein turnover on skeletal muscle,consisting of the IGF-1,the kinase Akt and its downstream effectors,and the effects of low-protein diet supplemented with ketoacids on the IGF1-Akt-FoxOs pathway in the skeletal muscle of rats with CKD.Furthermore,we focused on mitophagy disturbance in CKD and the influence of Mitsugumin 53(MG53).Methods 240 patients with CKD stage 1-5 were included from Augest 2016 to February 2017.Serum IGF-1 and IGFBP-3 were measured using chemiluminescence,and the influencing factors of serum IGF-1 and IGFBP-3 were analyzed.Besides,male Sprague-Dawley rats(200-250g)were randomly assigned to either the sham-operated control group(Control,n=15)and the 5/6 nephrectomy group(n=45).Surgery was performed according to a previous method.Rats with 5/6 nephrectomy were randomly divided into three groups,and fed with either 22%protein(normal-protein diet;NPD,n=15),6%protein(LPD,n=15)or 5%protein plus 1%KA(LPD+KA,n=15),for 24 weeks.Sham-operated rats were fed with NPD.Expression of autophagy markers and IGF1-Akt-FoxO pathway were evaluated at the level of transcript and protein by real-time PCR and western blot.Expressions of Ambral and MG53 in skeletal muscle of CKD rats were determined by Western blot.Furthermore,C2C12 myoblasts were randomly divided into five groups:un-stretched control,pcDNA3.1,MG53 overexpression,si-scramble and siMG53 group.Autophagic flux was analysed by mRFP-GFP-LC transfection.ResultsThe findings showed that the level of serum IGF-1 did not decrease or increase with the decrease of estimated glomerular filtration rate(eGFR).However,after multivariable adjustment,concentrations of serum IGFBP-3 were associated with eGFR in adult CKD patients.In animal experiments,the activation of autophagy and muscle atrophy were observed in the skeletal muscle of 5/6 nephrectomized rats.Decreased tissue IGF-1 levels caused by CKD caused defects in IGF-1-stimulated intracellular signaling that suppresses IGF-1R activity leading to decreased phosphorylation of Akt(p-Akt).A low p-Akt 1ed to decreased phosphorylation of forkhead transcription factors(FoxOs)which enter the nucleus to stimulate the expression of autophagy markers that can be associated with muscle atrophy in CKD.Expression of Ambral ? MG53 decreased in the skeletal muscle of 5/6 nephrectomized rats compared with the control group.In vitro experiments,overpression of MG53 promoted the expression Ambral and mitophagy,which may help to eliminate abnormal mitochondria and relieve muscle atrophy.ConclusionsIn contrast to the endocrine effects,the autocrine/parscrine action of IGF-1 in skeletal muscle may play a more important role in skeletal muscle atrophy of CKD.The results indicate that activation of autophagy and muscle atrophy were associated with impaired IGF-1/Akt signaling and activation of FoxOs.Low protein diet supplemented with ?-ketoacids lighten the activation of autophagy induced by CKD,possibly by ameliorating IGF-1/Akt pathway and suppression of FoxOs activity in the LPD and LPD+KA group,which may be responsible for its effect on muscle atrophy in CKD.Impaired autophagic flux may also play a important role in skeletal muscle atrophy of CKD.In vitro experiments,overpression of MG53 could activate mitophagy in C2C12 myoblasts.Whether MG53 can relief muscle atrophy via regulation of mitophagy in CKD remains elusive.