Cytoprotective and nonprotective autophagy in cancer therapy.

Two primary forms of autophagy have been identified in the field of cancer therapy based on their apparent functions in the tumor cell; these are the cytoprotective form that could, in theory, be inhibited for the purpose of sensitization to radiation and chemotherapeutic drugs and the "cytotoxic" form that either mediates or contributes to the actions of these treatment modalities. Surprisingly, to date, no clear-cut biochemical or molecular characteristics have been identified that might serve to distinguish between these two forms. In this commentary, we develop the concept of an additional form of autophagy that is nonprotective in that its inhibition neither sensitizes the tumor cell to exogenous stress (again, chemotherapy or radiation) nor protects the cell from the impact of these treatments. This form of autophagy also fails to exhibit any characteristics that might distinguish it from the cytoprotective and/or cytotoxic forms of autophagy. However, the existence of nonprotective autophagy is of potential significance in that it contributes to the challenge of predicting when the strategy of autophagy suppression might prove to have therapeutic benefit in the clinical treatment of cancer.