Shutdown of translation: lethal or protective? Unfolded protein response versus apoptosis.

Shutdown of translation is a highly conserved response of cells to a severe form of metabolic, thermal, or physical stress. After the metabolic stress induced by transient cerebral ischemia, translational recovery is observed only in cells that withstand the transient interruption of blood supply, implying that restoration of translation critically determines the final outcome. On the other hand, apoptosis is believed to play a role in ischemia-induced cell death. Apoptosis is an active process that is blocked by agents known to suppress protein synthesis. Thus, the question arises whether stress-induced suppression of protein synthesis is protective or toxic for the affected cells. Accepting the notion that endoplasmic reticulum (ER) dysfunction is the mechanism underlying shutdown of translation after transient cerebral ischemia, an attempt may be made to try to solve the protein synthesis paradox by understanding the role of protein synthesis suppression in conditions associated with ER dysfunction. Endoplasmic reticulum dysfunction-induced accumulation of unfolded proteins in the ER lumen is the trigger of two signal transduction pathways: PKR-like ER kinase-induced shutdown of translation to suppress new synthesis of proteins that cannot be correctly folded, and IRE1-induced expression of ER stress genes, a protein synthesis-dependent pathway needed to restore ER functions. Together these comprise the unfolded protein response. They are also induced after transient ischemia, implying a dual effect of protein synthesis suppression, a protective and a pathologic effect during early and prolonged reperfusion.