Herp is dually regulated by both the endoplasmic reticulum stress-specific branch of the unfolded protein response and a branch that is shared with other cellular stress pathways

The mammalian unfolded protein response (UPR) includes two major branches: one(s) specific to ER stress (Ire1/XBP-1 and ATF6-dependent), and one(s) shared by other cellular stresses (PERK/eIF-2alpha phosphorylation-dependent). Here, we demonstrate that the ER-localized protein Herp represents a second target, in addition to CHOP, that is dually regulated by both the shared and the ER stress-specific branches during UPR activation. For the first time, we are able to assess the contribution of each branch of the UPR in the induction of these targets. We demonstrate that activation of the shared branch of the UPR alone was sufficient to induce Herp and CHOP. ATF4 was not required during ER stress when both branches were used but did contribute significantly to their induction. Conversely, stresses that activated only the shared branch of the UPR were completely dependent on ATF4 for CHOP and Herp induction. Thus, the shared and the ER stress-specific branches of the UPR diverge to regulate two groups of targets, one that is ATF6 and Ire1/XBP-1-dependent, which includes BiP and XBP-1, and another that is eIF-2alpha kinase-dependent, which includes ATF4 and GADD34. The two branches also converge to maximally up-regulate targets like Herp and CHOP. Finally, our studies reveal that a PERK-dependent target other than ATF4 is contributing to the cross-talk between the two branches of the UPR that has previously been demonstrated.     

ATF6 as a transcription activator of the endoplasmic reticulum stress element: thapsigargin stress-induced changes and synergistic interactions with NF-Y and YY1

ATF6, a member of the leucine zipper protein family, can constitutively induce the promoter of glucose-regulated protein (grp) genes through activation of the endoplasmic reticulum (ER) stress element (ERSE). To understand the mechanism of grp78 induction by ATF6 in cells subjected to ER calcium depletion stress mediated by thapsigargin (Tg) treatment, we discovered that ATF6 itself undergoes Tg stress-induced changes. In nonstressed cells, ATF6, which contains a putative short transmembrane domain, is primarily associated with the perinuclear region. Upon Tg stress, the ATF6 protein level dropped initially but quickly recovered with the additional appearance of a faster-migrating form. This new form of ATF6 was recovered as soluble nuclear protein by biochemical fractionation, correlating with enhanced nuclear localization of ATF6 as revealed by immunofluorescence. Optimal ATF6 stimulation requires at least two copies of the ERSE and the integrity of the tripartite structure of the ERSE. Of primary importance is a functional NF-Y complex and a high-affinity NF-Y binding site that confers selectivity among different ERSEs for ATF6 inducibility. In addition, we showed that YY1 interacts with ATF6 and in Tg-treated cells can enhance ATF6 activity. The ERSE stimulatory activity of ATF6 exhibits properties distinct from those of human Ire1p, an upstream regulator of the mammalian unfolded protein response. The requirement for a high-affinity NF-Y site for ATF6 but not human Ire1p activity suggests that they stimulate the ERSE through diverse pathways.