Stressed to death: regulation of apoptotic signaling pathways by the heat shock proteins.

Cellular damage can engage two fundamental cellular responses: apoptosis, a precisely regulated form of cell death; and the heat shock protein (Hsp), or stress response, which functions to protect cells and to mediate an accelerated recovery following damage. The coordinated balance between these two opposing pathways governs the ultimate fate of the cell--whether it lives or dies. The self-destruction of a cell is mediated by one of many signaling pathways culminating in the activation of the caspase proteases. The Hsps regulate the activity of multiple intracellular signaling intermediates, many of which are intimately involved in the execution of the apoptotic signaling pathways. This review addresses whether the antiapoptotic activities of several Hsps, including Hsp70, Hsp90, and Hsp27, can be attributed to their collective ability to regulate the activities, expression, or both of apoptotic signaling molecules. In summary, the functional interface between the ancient heat shock or stress protein response and the highly conserved biochemical pathways leading to the activation of apoptosis governs the susceptibility of a cell to damaging stimuli.