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

Yanjun Ma,Linda M Hendershot

doi:10.1074/jbc.m313724200

Abstract

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.

Highlights

(Ire1/XBP-1 and ATF6-dependent), and one(s) shared by other cellular stresses (PERK/eIF-2␣ phosphorylationdependent)
Our results enable us to separate unfolded protein response (UPR) targets into three sets (Fig. 7), those that are specific to the UPR and downstream of ATF6 and Ire1/XBP-1, those that are downstream of PERK and are shared by other cellular stresses, and those that are dually regulated by both pathways
Identification of a Conserved C/EBP-ATF Composite Site in the Herp Promoter—To determine if the regulation of CHOP was unique among UPR targets, we searched the promoters of other genes up-regulated during endoplasmic reticulum (ER) stress for elements that might suggest they were dually regulated

Summary

Divergence and Convergence of the UPR

(13), and ATF6 overexpression leads to CHOP induction in cells [10]. The ATF4 transcription factor binds to the C/EBP-ATF composite site in vitro in an ER stress-dependent manner [22]. In PERK-null and eIF-2␣S51A knock-in cells, where eIF-2␣ phosphorylation is blocked during ER stress, the induction of CHOP and XBP-1 is completely lost and the up-regulation of BiP is reduced [11, 16, 26] These data support the existence of a cross-talk between the two pathways, where a PERK-dependent target is facilitating the activation of the ATF6 and Ire1/XBP-1-dependent branches of the UPR. Our results enable us to separate UPR targets into three sets (Fig. 7), those that are specific to the UPR and downstream of ATF6 and Ire1/XBP-1, those that are downstream of PERK and are shared by other cellular stresses, and those that are dually regulated by both pathways. The latter group is likely to be induced by all cellular stresses involving activation of an eIF-2␣ kinase with maximal levels of induction occuring during UPR activation

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION