Ammonia: a diffusible factor released by proliferating cells that induces autophagy.

During autophagy, portions of the cytoplasm are sequestered into autophagosomes and digested by lysosomal hydrolases. Massive autophagy can be induced in mammalian tissues in a coordinated fashion through nutrient deprivation, which has prompted the search of soluble metabolites that can stimulate autophagy. Ammonia, which is generated as a by-product of glutaminolysis, has been identified as a diffusible factor that stimulates autophagy. Intriguingly, cancer cells increase the rate glutaminolysis and the interstitial fluid of cancers contains higher-than-normal physiological concentrations of ammonia, suggesting a previously unknown pathway through which tumor cells can condition their microenvironment.