Cytostatic autophagy: an underappreciated form of autophagy and its ramifications in cancer.
Macroautophagy/autophagy is a well-established homeostatic mechanism that contributes to the integrity of multiple regulatory biological activities including but not limited to the gastro-intestinal tract and cognitive integrity. Autophagy also plays a central role in tissue regeneration, metamorphosis and development whereas defects in autophagy are associated with a wide range of disorders including metabolic diseases such as diabetes, organ pathophysiologies including liver, lung and heart disease, cancer, and microbial infection. In the field of cancer therapy, most research efforts have focused on cytoprotective autophagy, with substantial preclinical and clinical studies designed to interrogate the outcomes of pharmacologically (or genetically in preclinical work) inhibiting autophagy to enhance the efficacy of chemotherapeutic agents. There is lesser but nevertheless robust evidence for the cytotoxic function of autophagy while our laboratory and a few others have identified the nonprotective form of this cellular response. However, cytostatic autophagy, a distinct functional outcome of autophagy characterized by sustained proliferative arrest, has remained relatively underexplored. Cytostatic autophagy can be defined as a cellular condition in which autophagy activation coincides with durable proliferative arrest, and in which genetic or pharmacological inhibition of autophagy relieves the growth-arrest phenotype without inducing overt cytotoxicity. In this review, we provide the first comprehensive synthesis of the scientific literature addressing cytostatic autophagy, tracing its historical development and consolidating the experimental evidence that led to its current conceptual definition. We further discuss the molecular mechanisms underlying cytostatic autophagy, including the selective degradation of key cell-cycle regulators and the interplay between autophagy and senescence-associated signaling pathways.