Fig. 10. Highly simplified diagram of the signaling pathways activated by ERS in mammalian cells.

There are three major pathways mediated by the ER-resident transmembrane proteins ATF6, IRE1 and PERK. ERS increases GRP78 expression in the ER, and induces activation of one or more of the signaling pathways. After activation, ATF6 translocates to the Golgi, where it is cleaved by the Sit-1 and Sit-2 proteins releasing a 50-kDa transcription factor that translocates to the nucleus and bind ER stress response elements (ERSE). Activation of IRE1 causes IRE1-mediated splicing of XBP1 mRNA. Translation of spliced XBP1 mRNA produces a transcription factor that up regulates target genes via the ERSE promoter. Once stimulated, PERK phosphorylates eIF2α inhibiting protein translation. p-eIF2α enhances translation of ATF4, which induces expression of CHOP. CHOP, in turn, down regulates the expression of the anti-apoptotic protein Bcl-2, initiating mitochondria mediated apoptosis (not shown). As described in Results, APAP and NAPQI stimulates PERK-mediated pathway, but while APAP activates oxidative stress responses and increases levels of p-eIF2α, NAPQI decreases ROS production and p-eIF2α levels. Neither APAP nor NAPQI stimulate ATF4-mediated signaling, suggesting that they induce apoptosis via a different pathway.