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Fundamental and evolutionarily conserved role of autophagy is adaptation to starvation,however, the exact role of autophagy as nutrient recycle system remains unclear in mammals.We have investigated the role of autophagy in nutrient metabolism in mouse liver.The contribution of autophagy to protein synthesis was tested using the puromycin-labeled assay (SUNSET) and polysome profiling.In general, protein synthesis decreases upon starvation,however, it was maintained in the liver during 48-h fasting.The rate of protein synthesis was reduced in Atg5 KO liver compared to control mice, suggesting that amino acids supply through autophagic degradation is important for protein synthesis.In Atg5 KO liver, starvation-induced steatosis was also suppressed, suggesting that lipid metabolism is affected in the absence of autophagy.Nuclear transcription factors involved in lipid metabolism were found to be suppressed in knockout liver, suggesting that autophagy have an important impact on lipid metaboslim at transcription level.These data suggest that autophagy is involved in protein and lipid metabolism.