Shiga toxin has the potential to induce expression of inflammation-associated genes, although the underlying mechanisms are not well understood. We examined the effects of subtilase cytotoxin (SubAB), an AB₅ toxin produced by some Shiga toxigenic Escherichia coli, on the activation of NF-κB. SubAB is known to be a protease which selectively degrades GRP78/Bip. Treatment of NRK-52E cells with SubAB caused rapid cleavage of GRP78. Following the degradation of GRP78, transient activation of NF-κB was observed with a peak at 6–12 h; the activation subsided within 24 h despite the continuous absence of intact GRP78. The activation of NF-κB was preceded by transient phosphorylation of Akt. Treatment of the cells with a selective inhibitor of Akt1/2 or an inhibitor of PI3K attenuated SubAB-induced NF-κB activation, suggesting that activation of Akt is an event upstream of NF-κB. Degradation of GRP78 caused the unfolded protein response (UPR), and inducers of the UPR mimicked the stimulatory effects of SubAB on Akt and NF-κB. SubAB triggered the three major branches of the UPR including the IRE1-XBP1, PERK, and ATF6 pathways. Dominant-negative inhibition of IRE1α, XBP1, or PERK did not attenuate activation of NF-κB by SubAB. In contrast, genetic and pharmacological inhibition of ATF6 significantly suppressed SubAB-triggered Akt phosphorylation and NF-κB activation. These results suggested that loss of GRP78 by SubAB leads to transient phosphorylation of Akt and consequent activation of NF-κB through the ATF6 branch of the UPR.

Cadmium triggers apoptosis of LLC-PK1 cells through induction of endoplasmic reticulum (ER) stress. We found that cadmium caused generation of reactive oxygen species (ROS) and that cadmium-induced ER stress was inhibited by antioxidants. In contrast, suppression of ER stress did not attenuate cadmium-triggered oxidative stress, suggesting that ER stress occurs downstream of oxidative stress. Exposure of the cells to either , H₂O₂, or ONOO^- caused apoptosis, whereas ER stress was induced only by or ONOO^-. Transfection with manganese superoxide dismutase significantly attenuated cadmium-induced ER stress and apoptosis, whereas pharmacological inhibition of ONOO^- was ineffective. Interestingly, transfection with catalase attenuated cadmium-induced apoptosis without affecting the level of ER stress. caused activation of the activating transcription factor 6-CCAAT/enhancer-binding protein-homologous protein (CHOP) and the inositol-requiring ER-to-nucleus signal kinase 1-X-box-binding protein 1 (XBP1) proapoptotic cascades, and overexpression of manganese superoxide dismutase attenuated cadmium-triggered induction of both pathways. Furthermore, phosphorylation of proapoptotic c-Jun N-terminal kinase by or cadmium was suppressed by dominant-negative inhibition of XBP1. These data elucidated 1) cadmium caused ER stress via generation of ROS, 2) was selectively involved in cadmium-triggered, ER stress-mediated apoptosis through activation of the activating transcription factor 6-CHOP and inositol-requiring ER-to-nucleus signal kinase 1-XBP1 pathways, and 3) phosphorylation of JNK was caused by -triggered activation of XBP1.

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.

Background and Purpose— Ischemic injury in neurons can be strongly reduced by a preceding sublethal ischemic episode, of which the mechanism is poorly understood. Although changes in the expression of apoptosis-related proteins (Bcl-2, Bcl-xl, and Bax) have been considered to be crucially important in ischemic injury, the roles these proteins play in ischemic preconditioning induced by sublethal forebrain ischemia have not been elucidated. Therefore, we investigated the transcription and expression of Bcl-2, Bcl-xl, and Bax in striatum of mice subjected to sublethal forebrain ischemia and lethal ischemia, with or without ischemic preconditioning. Methods— Sublethal forebrain ischemia was induced in C57Black/Crj6 (C57BL/6) mice by 6 minutes of bilateral common carotid artery occlusion. The transcription and expression of Bcl-2 family genes were detected by reverse transcription–polymerase chain reaction, Western blot, and immunofluorescent staining. Results— No detectable neuronal loss was induced in striatum by 6 minutes of bilateral common carotid artery occlusion. Transcription and expression of Bcl-2 and Bcl-xl were increased after sublethal forebrain ischemia, which attenuated the DNA fragmentation induced by lethal ischemia. The transcription and expression of Bax remained unchanged. Conclusions— Upregulation of Bcl-2 and Bcl-xl but not Bax may have a role in protective ischemic preconditioning. This result indicates a potential strategy for further ischemic neuronal injury therapies.