The general amino acid control pathway regulates mTOR and autophagy during serum/glutamine starvation

Organisms have evolved elaborate mechanisms to adjust intracellular nutrient levels in response to fluctuating availability of exogenous nutrients. During starvation, cells can enhance amino acid uptake and synthesis through the general amino acid control (GAAC) pathway, whereas nonessential cellular contents are recycled by autophagy. How these two pathways are coordinated in response to starvation is currently unknown. Here we show that the GAAC pathway couples exogenous amino acid availability with autophagy. Starvation caused deactivation of mTOR, which then activated autophagy. In parallel, serum/glutamine starvation activated the GAAC pathway, which up-regulated amino acid transporters, leading to increased amino acid uptake. This elevated the intracellular amino acid level, which in turn reactivated mTOR and suppressed autophagy. Knockdown of activating transcription factor 4, the major transcription factor in the GAAC pathway, or of SLC7A5, a leucine transporter, caused impaired mTOR reactivation and much higher levels of autophagy. Thus, the GAAC pathway modulates autophagy by regulating amino acid uptake and mTOR reactivation during serum/glutamine starvation.