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Insulin-induced genes INSIG1 and INSIG2 mediate oxysterol-dependent activation of the PERK-eIF2α-ATF4 axis.

ABSTRACT: Insulin-induced genes (INSIGs) encode endoplasmic reticulum-resident proteins that regulate intracellular cholesterol metabolism. Oxysterols are oxygenated derivatives of cholesterol, some of which orchestrate lipid metabolism via interaction with INSIGs. Recently, it was reported that expression of activating transcription factor-4 (ATF4) was induced by certain oxysterols; the precise of mechanism is unclear. Herein, we show that INSIGs mediate ATF4 upregulation upon interaction with oxysterol. Oxysterols that possess a high affinity for INSIG, such as 27- and 25-hydroxycholesterol (25HC), markedly induced the increase of ATF4 protein when compared with other oxysterols. In addition, ATF4 upregulation by these oxysterols was attenuated in INSIG1/2-deficient Chinese hamster ovary cells and recovered by either INSIG1 or INSIG2 rescue. Mechanistic studies revealed that the binding of 25HC to INSIG is critical for increased ATF4 protein via activation of protein kinase RNA-activated-like ER kinase and eukaryotic translation initiation factor 2α. Knockout of INSIG1 or INSIG2 in human hepatoma Huh7 cells attenuated ATF4 protein upregulation, indicating that only one of the endogenous INSIGs, unlike overexpression of intrinsic INSIG1 or INSIG2, was insufficient for ATF4 induction. Furthermore, ATF4 proactively upregulated the cell death-inducible gene expression, such as Chop, Chac1, and Trb3, thereby markedly reducing cell viability with 25HC. These findings support a model whereby that INSIGs sense an increase in oxysterol in the endoplasmic reticulum and induce an increase of ATF4 protein via the protein kinase RNA-activated-like ER kinase-eukaryotic translation initiation factor 2α pathway, thereby promoting cell death.

SUBMITTER: Watanabe Y

PROVIDER: S-EPMC8363831 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Selenite has emerged as an optional chemotherapeutic agent for hematological malignancies. Autophagy and apoptosis are both engaged in selenite-induced cell death. In a previous report, we have identified heat shock protein 90 (Hsp90) as a critical modulator of the balance between autophagy and apoptosis in selenite-treated leukemia cells. However, the mechanisms by which selenite mediates the crosstalk between autophagy and apoptosis remain largely unknown. Herein, we demonstrate that the endoplasmic reticulum (ER) stress-related PERK/eIF2α/ATF4 pathway and p38 are core modules for the selenite-induced switch to apoptosis from autophagy. We found that selenite activated PERK and eIF2α/ATF4 downstream to promote apoptosis. During this progression, p38 was dissociated from PERK-inhibiting Hsp90 and became autophosphorylated. Then, activated p38 further enhanced the docking of activating transcription factor 4 (ATF4) onto the CHOP (CCAAT/enhancer-binding protein homologous protein) promoter via eIF2α to enhance apoptosis. We also found that activated p38 suppressed the phosphorylation of eIF4E that directed ATF4 to bind to the MAP1LC3B (microtubule-associated protein 1 light chain 3B) promoter. Because of the deactivation of eIF4E, the association of ATF4 with the MAP1LC3B promoter was inhibited, and autophagy was compromised. Intriguingly, p53 played important roles in mediating the p38-mediated regulation of eIF2α and eIF4E. When activated by p38, p53 induced the phosphorylation of eIF2α and the dephosphorylation of eIF4E, particularly in the nucleus where the ATF4 transcription factor was modulated, ultimately resulting in differential expression of CHOP and LC3. Moreover, selenite exhibited potent antitumor effects in vivo. In an NB4 cell xenograft model, selenite induced apoptosis and hampered autophagy. In addition, related signaling proteins demonstrated similar changes to those observed in vitro. These data suggest that selenite may be a candidate drug for leukemia therapy.

Project description:BackgroundDue to the abnormal angiogenesis, cancer stem cells (CSCs) in esophageal cancer (EC) have the characteristics of a hypoxic microenvironment. However, they can resist hypoxia-induced apoptosis. the molecular mechanism underlying the resistance of esophageal CSCs to hypoxia-induced apoptosis is currently unclear. Therefore, this study will investigate the molecular mechanism based on CHOP-mediated endoplasmic reticulum stress.MethodsCD44+CD24- cells in EC9706 cells were screened by fluorescence-activated cell sorting (FACS). To clarify which apoptosis pathway esophageal CSCs resist hypoxia-induced cell apoptosis through, the effects of hypoxia on apoptosis were detected by nuclear staining, flow cytometry, and JC-1 reagent, the effects of hypoxia on the expression of apoptosis-related proteins were detected by western blotting (WB) assay and quantitative polymerase chain reaction (qPCR) assay. To clarify the mechanisms of CD44+CD24- cells resistance to hypoxia-induced apoptosis is achieved by inhibiting the activation of endoplasmic reticulum stress (ERS) pathway, silenced CHOP and PERK cell lines of EC9706 cells and overexpressed CHOP and PERK cell lines of CD44+CD24- cells were constructed, the effects of hypoxia on apoptosis, cell cycle, and mitochondrial membrane potential were detected by flow cytometry and JC-1 reagent. WB assay and qPCR assay were used to detect the expressions of apoptosis-related proteins and ERS-related proteins.ResultsHypoxia significantly induce apoptosis and cycle arrest of EC9706 cells (P<0.05), but did not affect apoptosis and cycle of CD44+CD24- cells (P>0.05). Hypoxia considerably induced the activation of mitochondrial and ERS apoptosis pathways in EC9706 cells (P<0.05), but did not affect Fas receptor apoptosis pathways (P>0.05). The three apoptosis pathways were not affected by hypoxia in CD44+CD24- cells (P>0.05). Silencing the CHOP and PERK gene inhibited hypoxia-induced apoptosis of EC9706 cells (P<0.05). CHOP and PERK overexpression promoted hypoxia-induced apoptosis of CD44+CD24- cells (P<0.05), whereas mitochondrial membrane permeability inhibitors inhibited hypoxia-induced apoptosis of CD44+CD24- cells overexpressed CHOP gene.ConclusionsCD44+CD24- tumor stem cells in EC resist to hypoxia-induced apoptosis by the inhibition of ERS-mediated mitochondrial apoptosis pathway, which suggested that ERS pathway can serve as a potential target for reducing EC treatment resistance in clinical treatment.

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Insulin-induced genes INSIG1 and INSIG2 mediate oxysterol-dependent activation of the PERK-eIF2α-ATF4 axis.

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