ATF6alpha optimizes long-term endoplasmic reticulum function to protect cells from chronic stress

In vertebrates, three proteins--PERK, IRE1alpha, and ATF6alpha--sense protein-misfolding stress in the ER and initiate ER-to-nucleus signaling cascades to improve cellular function. The mechanism by which this unfolded protein response (UPR) protects ER function during stress is not clear. To address this issue, we have deleted Atf6alpha in the mouse. ATF6alpha is neither essential for basal expression of ER protein chaperones nor for embryonic or postnatal development. However, ATF6alpha is required in both cells and tissues to optimize protein folding, secretion, and degradation during ER stress and thus to facilitate recovery from acute stress and tolerance to chronic stress. Challenge of Atf6alpha null animals in vivo compromises organ function and survival despite functional overlap between UPR sensors. These results suggest that the vertebrate ATF6alpha pathway evolved to maintain ER function when cells are challenged with chronic stress and provide a rationale for the overlap among the three UPR pathways.     

Gestational low protein diet selectively induces the amino acid response pathway target genes in the liver of offspring rats through transcription factor binding and histone modifications

The amino acid response (AAR) pathway detects a deficiency of dietary amino acid or protein. To investigate the impact of gestational protein restriction on the AAR pathway in offspring, pregnant Sprague-Dawley rats were fed a control (C) or low protein (LP) diet during gestation. Livers of female offspring were collected on postnatal d 38. The mRNA amount of Atf3 in LP offspring increased significantly compared with C offspring, while Asns did not differ between the two groups. ATF4 and p-eIF2α were both induced in LP offspring, whereas p-ERK was significantly decreased. Additionally, amino acid limitation (-AA) in HepG2 cells increased p-ERK and AAR pathway-related genes, while U0126 decreased p-ERK but did not completely reverse the activation of AAR pathway-related genes. Chromatin immunoprecipitation assay demonstrated an increased association of both RNA polymerase II (Pol II) and ATF4 at the Atf3 promoter in LP offspring, while acetylated histone H4, tri-methyl histone H3 at lysine 9, ATF4, ATF3, C/EBPβ, and CHOP, but not Pol II, were all increased at the Asns promoter in LP offspring. In -AA HepG2 cells, C/EBPβ siRNA treatment did not prevent the activation of either ATF3 or ASNS in response to -AA, while ATF4 siRNA prevented the activation of ASNS but not ATF3. Our data demonstrates that a maternal LP diet programs the AAR pathway in the liver of offspring rats. The differential priming of the downstream target genes of the AAR pathway in response to maternal LP diet presents a novel regulatory mechanism related to nutrient-gene interactions.     

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.