Salubrinal is a selective inhibitor of cellular complexes that dephosphorylate eukaryotic translation initiation factor 2α (eIF2α). In previous reports, salubrinal was shown to have the potential to inhibit the activation of nuclear factor-κB (NF-κB) by several stimuli. However, the effects of salubrinal on NF-κB signaling are largely unknown. In this study, we investigated whether and how salubrinal affects NF-κB activation induced by tumor necrosis factor (TNF)-α and interleukin (IL)-1β. We found that salubrinal selectively blocked TNF-α-but not IL-1β-induced activation of NF-κB. This inhibitory effect occurred upstream of transforming growth factor (TGF)-β-activated kinase 1 (TAK1). Further experiments revealed that salubrinal blocked TNF-α-triggered NF-κB activation independent of its action on eIF2α because knockdown of eIF2α by small interfering RNA (siRNA) did not reverse the inhibitory effect of salubrinal on NF-κB. Moreover, guanabenz, a selective inhibitor of the regulatory subunit of protein phosphatase (PP) 1, also preferentially inhibited TNF-α-triggered activation of NF-κB. These findings raise the possibility that salubrinal may selectively block TNF-α-triggered activation of the NF-κB pathway through inhibition of the PP1 complex.Key words salubrinal; nuclear factor-κB; eukaryotic translation initiation factor 2α; protein phosphatase (PP) 1; guanabenz Salubrinal is a small molecule that inhibits the dephosphorylation of eukaryotic translation initiation factor 2α (eIF2α) through the suppression of protein phosphatase (PP) 1 catalytic subunits, resulting in sustained phosphorylation of eIF2α on Ser51. Through this effect, salubrinal protects cells from endoplasmic reticulum (ER) stress-mediated apoptosis.1) It has been shown that salubrinal influences several signaling pathways involved in apoptosis, autophagy and bone homeostasis.2-4) The nuclear factor-κB (NF-κB) pathway is also a target of salubrinal. Recently, Huang et al. reported that salubrinal protected against β-amyloid-triggered neuronal cell death and microglial activation through the suppression of NF-κB activation. 5) Moreover, Hamamura et al. suggested that salubrinal reduced the inflammatory cytokine-driven expression of matrix metalloproteinase 13 via blockade of the p38 mitogen-activated protein kinase pathway and the NF-κB pathway.6) Currently, however, the mechanism by which salubrinal regulates NF-κB remains largely unknown.The Rel/NF-κB family consists of five different members, namely, p50, p52, p65 (RelA), RelB and c-Rel, and functions as a transcription factor through the formation of homo-or heterodimers.7) NF-κB plays crucial roles in the regulation of inflammation, immune responses and apoptosis. 8) In unstimulated cells, NF-κB is retained in the cytoplasm through binding to inhibitors of NF-κB (IκBs). However, in response to several stimuli such as inflammatory cytokines, oxidative stress, ultraviolet light and bacterial components, IκB kinase (IKK) complex including IKKα, IKKβ and IKKγ (also called NF-κB essential modu...