Secretory and membrane proteins are correctly folded and glycosylated in the endoplasmic reticulum (ER) by ER-resident molecular chaperones. The ER stores intracellular calcium and is a major signal-transducing organelle releasing calcium in the cytoplasm. Exposure of cells to various pharmacological agents that disturb ER functions, such as calcium depletion from the ER lumen, inhibition of glycosylation or reduction of disulfide bonds, and in physiological conditions, expression of mutant proteins or overexpression of some wild-type proteins by viral infection, leads to an accumulation of proteins in the ER lumen. Under these conditions called ER stress, two distinct signal transduction pathways, termed the unfolded protein response (UPR) and the ER-overload response (EOR), are activated to transduce the signal from the ER to the nucleus. 1,2)The UPR leads to an increase in the expression of ER stress-response genes, including ER chaperones and CHOP/GADD153. 3,4) This response is mediated by an ERtransmembrane molecule, IRE1 (a and b), whose cytosolic domain contains a Ser/Thr protein kinase and an endoribonuclease domain with its C-terminus.5,6) Accumulation of unfolded proteins in the ER lumen initiates oligomerization and trans-autophosphorylation of IRE1, resulting in activation of the kinase/RNase activities. Activated IRE1 then cleaves off the intron of XBP-1, a transcription factor that binds to the ER stress-response element (ERSE), to increase the transcription of target genes. 7-10)The EOR triggers the activation of nuclear factor-kB (NFkB) through accumulation of proteins in the ER.11) NF-kB is activated by a wide variety of stimuli such as cytokines, oxidant-free radicals, ultraviolet irradiation, and viral products. The activation of NF-kB induced by ER stress has been suggested to require the release of calcium from the ER through overloading of the ER with accumulated proteins and subsequent production of reactive oxygen species (ROS).12) On the other hand, production of GRP78/Bip, an ER chaperone, as the UPR was independent of the activation of NF-kB, since overexpression of NF-kB subunits fails to induce GRP78/Bip. 11) Therefore, the EOR appears to be distinguishable from the UPR.c-Jun N-terminal kinase (JNK) is also activated by ER stress. 13,14) This signaling pathway is mediated by the association of IRE1s with tumor necrosis factor-a (TNF-a) receptor-associated factor 2 (TRAF2).15) However, it has been unclear whether ER stress-induced NF-kB activation depends on the IRE1-TRAF2-mediated response. A signal initiated by TNF-a diverges from TRAF2 to two pathways leading to NF-kB and JNK signaling. 16,17) Thus, IRE1 seems to activate NF-kB through TRAF2, as well as JNK. We here show that ER stress-induced NF-kB activation is mediated by IRE1 and TRAF2, similar to the JNK activation. MATERIALS AND METHODS Expression Vector ConstructionHuman full-length IRE1a (the region corresponding to amino acids; 1-977) and DRN-IRE1a (a truncated form of the C-terminal RNase domain; 1-786) with a hemagglutin...

Stresses that impair the function of the endoplasmic reticulum (ER) lead to an accumulation of unfolded protein in the ER. Under these conditions, the expression of a variety of genes involved in preventing the accumulation of the unfolded proteins is induced. Yeast Hrd1p is an ER stress-inducible ER membrane protein that acts as a ubiquitin ligase (E3) with a RING ¢nger motif and plays a role in the ubiquitination of proteins in the ER. We report here the identi¢cation and characterization of a human homolog to yeast Hrd1p. The predicted structures are highly conserved from yeast to humans. Indeed, human HRD1 was localized to the ER and ubiquitinated its substrates. Furthermore, it was found that human HRD1 was up-regulated by ER stress via IRE1 and ATF6, which are ER stress transducers. Interestingly, 293 cells stably expressing wild-type HRD1, but not the C329S mutant, a¡orded resistance to ER stress-induced apoptosis. These results suggest that the production of HRD1 is up-regulated to protect against ER stress-induced apoptosis by degrading unfolded proteins accumulated in the ER.

Endoplasmic reticulum (ER) stress is defined as an accumulation of unfolded proteins in the endoplasmic reticulum. 4-phenylbutyrate (4-PBA) has been demonstrated to promote the normal trafficking of the DF508 cystic fibrosis transmembrane conductance regulator (CFTR) mutant from the ER to the plasma membrane and to restore activity. We have reported that 4-PBA protected against cerebral ischemic injury and ER stress-induced neuronal cell death. In this study, we revealed that 4-PBA possesses chemical chaperone activity in vitro, which prevents the aggregation of denatured a-lactalbumin and bovine serum albumin (BSA). Furthermore, we investigated the effects of 4-PBA on the accumulation of Parkin-associated endothelin receptor-like receptor (Pael-R) pathologically relevant to the loss of dopaminergic neurons in autosomal recessive juvenile parkinsonism (AR-JP). Interestingly, 4-PBA restored the normal expression of Pael-R protein and suppressed ER stress induced by the overexpression of Pael-R. In addition, we showed that 4-PBA attenuated the activation of ER stress-induced signal transduction pathways and subsequent neuronal cell death. Moreover, 4-PBA restored the viability of yeasts that fail to induce an ER stress response under ER stress conditions. These results suggest that 4-PBA suppresses ER stress by directly reducing the amount of misfolded protein, including Pael-R accumulated in the ER.

Human HRD1 and SEL1 are components of endoplasmic reticulum-associated degradation (ERAD), which is a retrograde transport mechanism from the ER to the cytosol for removing unfolded proteins. The expression of HRD1 and SEL1 was induced by ER stress-inducing agents and overexpression of both ER stress-responsive transcription factors, ATF6 and XBP1. Inhibition of IRE1 and ATF6 revealed that ER stress-induced HRD1 and SEL1 expressions are mediated by IRE1-XBP1-and ATF6-dependent pathways, respectively. These results suggest that the ER stress-induced ERAD gene expressions are mediated by different pathways, which are attributed to the differences in the promoter regions.