The endoplasmic reticulum (ER) is the site of synthesis and folding of membrane and secretory proteins, which, collectively, represent a large fraction of the total protein output of a mammalian cell. Therefore, the protein flux through the ER must be carefully monitored for abnormalities, including the buildup of misfolded proteins. Mammalian cells have evolved an intricate set of signaling pathways from the ER to the cytosol and nucleus, to allow the cell to respond to the presence of misfolded proteins within the ER. These pathways, known collectively as the unfolded protein response, are important for normal cellular homeostasis and organismal development and may also play key roles in the pathogenesis of many diseases. This review provides background information on the unfolded protein response and discusses a selection of diseases whose pathogenesis involves ER stress.

Knockout studies have shown that the transcription factor Nrf1 is essential for embryonic development. Nrf1 has been implicated to play a role in mediating activation of oxidative stress response genes through the antioxidant response element (ARE). Because of embryonic lethality in knockout mice, analysis of this function in the adult knockout mouse was not possible. We report here that mice with somatic inactivation of nrf1 in the liver developed hepatic cancer. Before cancer development, mutant livers exhibited steatosis, apoptosis, necrosis, inflammation, and fibrosis. In addition, hepatocytes lacking Nrf1 showed oxidative stress, and gene expression analysis showed decreased expression of various ARE-containing genes, and up-regulation of CYP4A genes. These results suggest that reactive oxygen species generated from CYP4A-mediated fatty acid oxidation work synergistically with diminished expression of ARE-responsive genes to cause oxidative stress in mutant hepatocytes. Thus, Nrf1 has a protective function against oxidative stress and, potentially, a function in lipid homeostasis in the liver. Because the phenotype is similar to nonalcoholic steatohepatitis, these animals may prove useful as a model for investigating molecular mechanisms of nonalcoholic steatohepatitis and liver cancer. hepatocellular carcinoma ͉ oxidative stress ͉ knockout mouse T ranscription of many cytoprotective genes and phase-2 xenobiotic metabolizing genes is regulated through cis-active sequences known as antioxidant response elements (ARE) (1, 2). Regulation of ARE function is mediated by various basic leucine zipper (bZIP) transcription factors including members of the ''cap n collar'' (CNC)-bZIP and small-Maf family of proteins. Nrf1 and Nrf2 are CNC-bZIP proteins, and they function as obligate heterodimers by complexing with small-Maf and other bZIP proteins (3). An important role for Nrf2 in xenobiotic metabolism and oxidative stress response had been identified through knockout studies in mice (4-10). These and other studies indicate that Nrf2 is an important activator of AREs.In contrast, the function of Nrf1 is not fully understood. Mice deficient in Nrf1 function die during development (11). Analysis of nrf1 and nrf1::nrf2 mutant cells suggests that Nrf1 is also involved in the oxidative stress response (12, 13). However, the importance of Nrf1 in this function in an intact animal is not certain because early lethality precludes analysis of Nrf1-deficient animals beyond embryonic development. Chimeric mice generated with Nrf1-deficient embryonic stem cells showed widespread apoptosis in fetal livers at late gestation, demonstrating a cell autonomous role of Nrf1 in the survival of hepatocytes (14). This finding suggests that Nrf1 is required for normal function of hepatocytes. Based on these findings, we hypothesize that Nrf1 is critical to the oxidative stress response in the adult liver, and that it plays an important role in oxidative stress-induced liver disease. To bypass embryonic lethality, we used a Cre-lox system ...

The CNC-basic leucine zipper (CNC-bZIP) family is a subfamily of bZIP proteins identified from independent searches for factors that bind the AP-1-like cis-elements in the β-globin locus control region. Three members, p45-Nf-e2, Nrf-1 and Nrf-2 have been identified in mammals. Expression of p45-Nf-e2 is largely restricted to hematopoietic cells while Nrf-1 and Nrf-2 are expressed in a wide range of tissues. To determine the function of Nrf-1, targeted disruption of the Nrf-1 gene was carried out. Homozygous Nrf-1 mutant mice are anemic due to a non-cell autonomous defect in definitive erythropoiesis and die in utero.

The basal core promoter (BCP) of hepatitis B virus (HBV) controls the transcription of both the precore RNA and the core RNA. The precore RNA codes for the secreted e antigen, while the core RNA codes for the major core protein and the DNA polymerase and also is the pregenomic RNA. The double mutation of nucleotides 1762 and 1764 in the BCP from A and G to T and A, respectively, is frequently observed in HBV sequences isolated from chronic patients. Several papers have reported conflicting results regarding whether this double mutation is important for e antigen expression. In order to address this issue, we have introduced this double mutation into the HBV genome and studied its effects on HBV gene expression and replication. Our results indicate that the mutated BCP can no longer bind a liver-enriched transcription factor(s) and that the transcription of only precore RNA and, consequently, the expression of e antigen were reduced. The reduction of precore gene expression was accompanied by an increase in progeny virus production. This increase was found to occur at or immediately prior to the encapsidation of the pregenomic RNA. Thus, the results of our in vitro study resolve the discrepancy of previous clinical observations and indicate that this double mutation suppresses but does not abolish the e antigen phenotype. The implications of these findings in the pathogenesis of HBV are discussed.

Proteins that directly regulate TNFR signaling play critical roles in regulating cellular activation and survival. A20 Binding and Inhibitor of NFkB (ABIN-1) is a novel protein that is thought to inhibit NFkB signaling (1, 2). Here we show that mice deficient for ABIN-1 die during embryogenesis with fetal liver apoptosis, anemia and hypoplasia. ABIN-1 deficient cells are hypersensitive to TNF-induced PCD, and TNF deficiency rescues ABIN-1 deficient embryos. ABIN-1 inhibits caspase 8 recruitment to FADD in TNF-induced signaling complexes, preventing caspase 8 cleavage and PCD. Moreover, ABIN-1 directly binds polyubiquitin chains and this ubiquitin sensing activity is required for ABIN-1’s anti-apoptotic activity. These studies provide new insights into how ubiquitination and ubiquitin sensing proteins regulate cellular and organismal survival.