Endoplasmic reticulum stress triggers an acute proteasome‐dependent degradation of ATF6

Abstract

ATF6, a 670 amino acid endoplasmic reticulum (ER) transmembrane glycoprotein with the electrophoretic mobility of a 90 kDa protein, is a key transcriptional activator of the unfolded protein response (UPR) that allows mammalian cells to maintain cellular homeostasis when the cells are subjected to a variety of environmental and physiological stress. Previous studies have established that ATF6 is a short-lived protein, the activation of which involves relocation from the ER to the Golgi where it is cleaved by the S1P/S2P protease system to generate a nuclear form that acts as a transcriptional activator for ER-stress inducible target genes such as Grp78/BiP. We report here that in addition to this process, ER-stress mediated by thapsigargin triggers an acute proteasomal degradation of the pre-existing pool of p90ATF6 independent of S1P/S2P cleavage. We showed that ATF6 is a direct target of proteasome-ubiquitin pathway, and this process can be suppressed by proteasome inhibitors, ALLN and MG115. We further observed that in non-stressed cells, p90ATF6 can be stabilized by MG115 but not ALLN and that treatment of cells with MG115 results in Grp78 induction in the absence of ER stress. These studies suggest that ER-stress induced acute, transit degradation of p90ATF6 could represent a novel cellular defense mechanism to prevent premature cell death resulting from p90ATF6 activation. Further, inhibition of proteasome activity can result in chaperone protein gene induction through stabilization of p90ATF6 as well as accumulation of malfolded proteins.

Celecoxib upregulates endoplasmic reticulum chaperones that inhibit celecoxib-induced apoptosis in human gastric cells

“…It was suggested that GRP78 binds to PERK (and also IRE1 and ATF6) and inactivates PERK's ER stress response inducing capabilities under nonstress conditions; when ER stressors accumulate unfolded proteins in the ER, PERK (and also IRE1 and ATF6) is activated to induce the ER stress response by releasing GRP78 from PERK, as a result of the binding of unfolded proteins to GRP78 (Bertolotti et al, 2000;Shen et al, 2002a). Induction of GRP78 by proteasome inhibitors has been reported (Hong et al, 2004). It was reported that overexpression of GRP78 inhibited the phosphorylation of PERK in the presence of an ER stressor (thapsigargin) (Bertolotti et al, 2000).…”

Biology of Endoplasmic Reticulum Stress in the Heart

“…ER stress triggered by Ca 2ϩ depletion induces the formation of a nascent, partially glycosylated form of ATF6␣ with reduced interaction with calreticulin and faster rate of traverse to the Golgi, resulting in higher transactivation of N-ATF6␣ gene targets. 42 Because an accumulation of underglycosylated proteins in the ER has the ability to induce the UPR, the glycosylation status of ATF6␣ may serve as a novel sensor of glycoprotein homeostasis, leading to activation of the UPR. There are 2 isoforms of ATF6 (ATF6␣ and ATF6␤) that are characterized by their divergent transcriptional activity domains.…”

West Nile Virus Infection Activates the Unfolded Protein Response, Leading to CHOP Induction and Apoptosis

“…Additionally, we analyzed endogenous atf6 mRNA levels in WNV-infected SK-N-MC cells and did not observe any changes, indicating that the difference occurs at a posttranscriptional step (data not shown). Previous studies have shown rapid degradation of ATF6 upon ER stress, in a proteasomedependent manner (16). To confirm if ATF6 degradation in WNV infection occurs in a similar fashion, WNV-infected cells were treated at 36 h p.i.…”