Tumor Necrosis Factor α (TNFα) Induces the Unfolded Protein Response (UPR) in a Reactive Oxygen Species (ROS)-dependent Fashion, and the UPR Counteracts ROS Accumulation by TNFα

Abstract

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes ER overload, resulting in ER stress. To cope with ER stress, mammalian cells trigger a specific response known as the unfolded protein response (UPR). Although recent studies have indicated cross-talk between ER stress and oxidative stress, the mechanistic link is not fully understood. By using murine fibrosarcoma L929 cells, in which tumor necrosis factor (TNF) ␣ induces accumulation of reactive oxygen species (ROS) and cell death, we show that TNF␣ induces the UPR in a ROS-dependent fashion. In contrast to TNF␣, oxidative stresses by H 2 O 2 or arsenite only induce eukaroytic initiation factor 2␣ phosphorylation, but not activation of PERK-or IRE1-dependent pathways, indicating the specificity of downstream signaling induced by various oxidative stresses. Conversely, the UPR induced by tunicamycin substantially suppresses TNF␣-induced ROS accumulation and cell death by inhibiting reduction of cellular glutathione levels. Collectively, some, but not all, oxidative stresses induce the UPR, and pre-emptive UPR counteracts TNF␣-induced ROS accumulation.Newly synthesized secretory and membrane-associated proteins are correctly folded and assembled in the endoplasmic reticulum (ER). 2Once ER function is perturbed by various pathological conditions, newly synthesized unfolded proteins accumulate in the ER, resulting in ER stress. To cope with accumulated unfolded ER proteins, mammalian cells trigger a specific response termed the unfolded protein response (UPR) (1-3). There are three distinct signaling pathways that are triggered in response to ER stress, mediated by PERK, ATF6, and IRE1. Under non-pathological conditions, all three components associate with the abundant lumenal chaperon Bip (also known as glucose-regulated protein 78) and this interaction keeps these signaling molecules in an inactive state (4, 5). Once unfolded proteins accumulate in the ER, Bip preferentially associates with the unfolded proteins instead of PERK, ATF6, and IRE1, resulting in activation of their downstream signaling molecules. PERK is an ER-resident serine/threonine protein kinase that phosphorylates the ␣ subunit of eukaryotic translation initiation factor 2 (eIF2␣) (6). Phosphorylation of eIF2␣ subsequently inhibits protein synthesis to prevent further influx of nascent proteins into an already saturated ER lumen. Paradoxically, eIF2␣ phosphorylation induces translation of a transcription factor ATF4 and subsequent expression of the ATF4 target genes, GADD34 and CHOP (7). The second signaling pathway is mediated by the basic leucine zipper-type transcription factor, ATF6. ATF6 is synthesized as a type II transmembrane precursor protein with a molecular mass with 90 kDa (p90 ATF6), and anchored to the ER membrane where it is retained by Bip. In response to ER stress, ATF6 is released from Bip and transported to the Golgi complex (8), where ATF6 undergoes sequential cleavages by two proteases, S1P and S2P (9). The processed form of ATF6 (p50ATF6) translocate...

The unfolded protein response controls induction and activation of ADAM17/TACE by severe hypoxia and ER stress

“…There is increasing evidence that ER stress and inflammation are interconnected by the increased production of ROS (Harding et al, 2003;Xue et al, 2005;Gorlach et al, 2006). Our results indicate that Thapsigargin-induced ER stress increased ROS levels that correlated with the kinetics of ADAM17 sheddase activity.…”

“…The involvement of the UPR may be of specific significance in this process. It has been reported that inflammatory cytokines, including TNFa (Xue et al, 2005), cause ER stress and therefore activate the UPR in various cancer cell lines. Thus, increased ADAM17 sheddase activity under ER stress may be a result of the negative feedback loop and an adaptive response desensitizing cells from stress-inducing stimulation.…”

Lipid signalling in disease

“…Remarkably, orally active chemical chaperones that stabilize protein conformation and improve ER folding capacity can alleviate ER stress and can function as potent antidiabetic agents, restoring glucose homeostasis in mouse models of type 2 diabetes 60 . The ER is also a major source of ROS and, consequently, oxidative stress 61,62 . Oxidative stress is emerging as a feature of obesity and an important factor in the development of insulin resistance, and is frequently associated with mitochondrial dysfunction [63][64][65] .…”

The relation between ER stress and HLA-B27 misfolding

“…Activated CREBH has enhanced production of proinflammatory mediators and also other transcription factors of the ER stress pathway can be participated in proinflammatory cytokine production such as XBP-1 and CHOP [48][49][50]. Proinflammatory cytokines, exclusively TNFα, can elevate ER stress and this event can cause a positive feedback mechanism that HLA-B27 misfolding lead to induce ER stress and proinflammatory cytokine production [51,52]. That feedback mechanism can contribute to understand the link between ER stress and a proinflammatory cytokine environment, also characteristic feature of AS patients.…”