TOR-dependent control of autophagy: biting the hand that feeds

Induction of autophagy in response to starvation is a highly conserved ability of eukaryotic cells, indicating a crucial and ancient role of this process in adapting to nutrient conditions. The target of rapamycin (TOR) pathway is major conduit for such signals, and in most cell types TOR activity is necessary and sufficient to suppress autophagy under favorable growth conditions. Recent studies have begun to reveal how TOR activity is regulated in response to nutritional cues, and are shedding new light on the mechanisms by which TOR controls the autophagic machinery. In addition, a variety of signals, stressors and pharmacological agents that induce autophagy independent of nutrient conditions have been identified. In some cases these signals appear to have been spliced into the core TOR pathway, whereas others are able to bypass the control mechanisms regulated by TOR. Increasing evidence is pointing to an important role for both positive and negative feedback loops in controlling this pathway, leading to an emerging view that TOR signaling not only regulates autophagy but is also highly sensitive to cellular rates of autophagy and other TOR-dependent processes.