Hypoxia upregulates hypoxia inducible factor (HIF) pathway and ER stress signal, unfolded protein response (UPR). The crosstalk of both signals affects the pathogenic alteration by hypoxia. Here, we showed that ER stress induced by tunicamycin or thapsigargin suppressed inducible (CoCl(2) or hypoxia) transcription of erythropoietin (EPO), a representative HIF target gene, in HepG2. This suppression was inversely correlated with UPR activation, as estimated by expression of UPR regulator GRP78, and restored by an ER stress inhibitor, salubrinal, in association with normalization of the UPR state. Importantly, the decreased EPO expression was also observed in HepG2 overexpressing UPR transcription factor ATF4. Overexpression of mutated ATF4 that lacks the transcriptional activity did not alter EPO transcriptional regulation. Transcriptional activity of EPO 3' enhancer, which is mainly regulated by HIF, was abolished by both ER stressors and ATF4 overexpression, while nuclear HIF accumulation or expression of other HIF target genes was not suppressed by ER stress. Chromatin immunoprecipitation analysis identified a novel ATF4 binding site (TGACCTCT) within the EPO 3' enhancer region, suggesting a distinct role for ATF4 in UPR-dependent suppression of the enhancer. Induction of ER stress in rat liver and kidney by tunicamycin decreased the hepatic and renal mRNA and plasma level of EPO. Collectively, ER stress selectively impairs the transcriptional activity of EPO but not of other HIF target genes. This effect is mediated by suppression of EPO 3' enhancer activity via ATF4 without any direct effect on HIF, indicating that UPR contributes to oxygen-sensing regulation of EPO.
"Aldolase (Ald) transcription start site resulted in clear Atf4-dependent up-regulation of GFP during ER stress (Figure 5A), but mutation of the 2 CREs within this reporter had no effect on its regulation (Figure 5B). These results open up the possibility that Atf4 also regulates glycolytic genes through alternative binding sites (Fawcett et al. 1999; Gombart et al. 2007; Gjymishka et al. 2008; Kode et al. 2012; Chiang et al. 2013; Han et al. 2013) or via both direct and indirect mechanisms. "
"sion of transcriptio n factors, pCAX-F-hXBP1 (sp) , pCAX-hATF6(1-373)-F , and pCAX-F-mATF 4  were used. In the reporter assays, pGL4.10 (Promega) was used as a negative control without promoter. "
[Show abstract][Hide abstract] ABSTRACT: IRE1 α is an ER-located transmembrane RNase whose activation leads to the production of the transcriptional factor, XBP1. Recently, many studies report that IRE1 α-XBP1 pathway has novel and significant roles in placenta. However, its molecular details have been still unknown. To address this point, we have focused on the molecular linkage between IRE1 α -XBP1 pathway and Cts7 and Cts8, which are essential cathepsins for placenta formation. In cellular model, this pathway positively contributed to their expression at transcriptional level. In addition, the disruption of IRE1 α or XBP1 in animal model significantly attenuated their transcripts in placenta. These results indicated that IRE1 α -XBP1 pathway function as a specific program supporting the placenta formation by ensuring the moderate expression of specific subset of placental cathepsins.
Biochemical and Biophysical Research Communications 03/2013; 433(4). DOI:10.1016/j.bbrc.2013.02.119 · 2.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Pathogenic pain is a common sign of many diseases. The mechanism is unclear. Activating transcription factor 4 (ATF4) plays a critical role in cell activation. Brain-derived neurotrophic factor (BDNF) is an important molecule in pathogenic pain. This study aims to investigate the role of ATF4 in inducing BDNF release from microglial cells. In this study, mouse microglial cells were cultured. The levels of BDNF in the culture medium were determined by enzyme-linked immunosorbent assay. Overexpression of ATF4 in microglial cells was performed by gene transfection. The apoptosis of microglial cells was assessed by flow cytometry. The results showed that microglial cells expressed ATF4 and protease-activated receptor-2 (PAR2). BDNF was detectable in the culture medium of microglial cells, which was significantly increased in the ATF4-overexpressing microglial cells. The ATF4-overexpressing microglial cells showed a high frequency of apoptotic cells, which could be inhibited by exposure to the PAR2 agonist tryptase in the culture. The tryptase-treated ATF4-overexpressing microglial cells kept higher secretion of BDNF. We conclude that the activation of ATF4 can increase BDNF release from microglial cells.
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