Fig. (1) Nutrient deprivation triggers pro-survival signals or cell death. Amino acid deprivation inhibits TOR activity, thus promoting autophagy and inhibition of protein synthesis. Activation of AMPK by a high AMP/ATP ratio promotes catabolism and inhibits anabolic processes such as protein and DNA synthesis; some of these effects are mediated by p53 activation or TOR inhibition. The unfolded protein response (UPR) is activated by ER stress caused by misfolded proteins when cells lack amino acids or glucose required for protein synthesis. Under some circumstances, lack of nutrients promotes cell death by classical apoptosis (left), apoptosis with atypical DNA condensation (middle) or necrosis (right).