Selective Abrogation of BiP/GRP78 Blunts Activation of NF-κB through the ATF6 Branch of the UPR: Involvement of C/EBPβ and mTOR-Dependent Dephosphorylation of Akt

Abstract

Subtilase cytotoxin (SubAB) that selectively cleaves BiP/GRP78 triggers the unfolded protein response (UPR) and protects mice from endotoxic lethality and collagen arthritis. We found that pretreatment of cells with SubAB suppressed tumor necrosis alpha (TNF-α)-induced activation of NF-κB and NF-κB-dependent chemokine expression. To elucidate underlying mechanisms, the involvement of C/EBP and Akt, putative regulators of NF-κB, was investigated. Among members of the C/EBP family, SubAB preferentially induced C/EBPβ. Overexpression of C/EBPβ suppressed TNF-α-induced NF-κB activation, and knockdown of C/EBPβ attenuated the suppressive effect of SubAB on NF-κB. We identified that the ATF6 branch of the UPR plays a crucial role in the induction of C/EBPβ. In addition to this effect, SubAB depressed basal and TNF-α-induced phosphorylation of Akt via the UPR. It was mediated by the induction of ATF6 and consequent activation of mTOR that dephosphorylated Akt. Inhibition of Akt attenuated activation of NF-κB by TNF-α, suggesting that the mTOR-Akt pathway is another target for SubAB-initiated, UPR-mediated NF-κB suppression. These results elucidated that SubAB blunts activation of NF-κB through ATF6-dependent mechanisms, i.e., preferential induction of C/EBPβ and mTOR-dependent dephosphorylation of Akt.

ACKNOWLEDGMENTS

We thank Masayuki Miura (University of Tokyo), Ronald C. Wek (Indiana University School of Medicine), Kazutoshi Mori (Kyoto University), Antonio Leonardi (University of Naples), Ez-Zoubir Amri (CNRS), Yoshihiko Nishio (Shiga University of Medical Science), Laurie H. Glimcher (Harvard Medical School), David Ron (New York University School of Medicine), Kenneth Walsh (Boston University School of Medicine), Satoshi Ogawa (Kanazawa University), Kazunori Imaizumi (University of Miyazaki), Pierre Fafournoux (INRA de Theix), Jacob Friedman (University of Colorado), and Takao Iwawaki (RIKEN) for providing us with expression plasmids.

This work was supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan (no. 20390235) to M. Kitamura.