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

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-alpha)-induced activation of NF-kappa B and NF-kappa B-dependent chemokine expression. To elucidate underlying mechanisms, the involvement of C/EBP and Akt, putative regulators of NF-kappa B, was investigated. Among members of the C/EBP family, SubAB preferentially induced C/EBP beta. Overexpression of C/EBP beta suppressed TNF-alpha-induced NF-kappa B activation, and knockdown of C/EBP beta attenuated the suppressive effect of SubAB on NF-kappa B. We identified that the ATF6 branch of the UPR plays a crucial role in the induction of C/EBP beta. In addition to this effect, SubAB depressed basal and TNF-alpha-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-kappa B by TNF-alpha, suggesting that the mTOR-Akt pathway is another target for SubAB-initiated, UPR-mediated NF-kappa B suppression. These results elucidated that SubAB blunts activation of NF-kappa B through ATF6-dependent mechanisms, i.e., preferential induction of C/EBP beta and mTOR-dependent dephosphorylation of Akt.