αNAC depletion as an initiator of ER stress-induced apoptosis in hypoxia

Accumulation of unfolded proteins triggers endoplasmic reticulum (ER) stress and is considered a part of the cellular responses to hypoxia. The nascent polypeptide-associated complex (NAC) participates in the proper maturation of newly synthesized proteins. However, thus far, there have been no comprehensive studies on NAC involvement in hypoxic stress. Here, we show that hypoxia activates glycogen synthase kinase-3 beta (GSK-3 beta) and that the activated GSK-3 beta destabilizes alpha NAC with the subsequent apoptosis of the cell. Hypoxia of various cell types and the mouse ischemic brain was associated with rapid downregulation of alpha NAC and ER stress responses involving PERK, ATF4, gamma-taxilin, elF2 alpha, Bip, and CHOP. Depletion of alpha NAC by RNA interference specifically activated ER stress responses and caused mitochondrial dysfunction, which resulted in apoptosis through caspase activation. Interestingly, we found that the hypoxic conditions activated GSK-3 beta, and that GSK-3 beta inhibition prevented alpha NAC protein downregulation in hypoxic cells and rescued the cells from apoptosis. In addition, alpha NAC overexpression increased the viability of hypoxic cells. Taken together, these results suggest that alpha NAC degradation triggers ER stress responses and initiates apoptotic processes in hypoxic cells, and that GSK-3 beta may participate upstream in this mechanism. Cell Death and Differentiation (2009) 16, 1505-1514; doi:10.1038/cdd.2009.90; published online 17 July 2009