Abstract
The activated amino acid response (AAR) and unfolded protein response (UPR) stress signaling pathways converge at the phosphorylation of translation initiation factor eIF2alpha. This eIF2alpha modification suppresses global protein synthesis but enhances translation of selected mRNAs such as that for activating transcription factor 4 (ATF4). An ATF4 target gene, SNAT2 (system A sodium-dependent neutral amino acid transporter 2), contains a C/EBP-ATF site that binds ATF4 and triggers increased transcription during the AAR. However, the present studies show that despite increased ATF4 binding to the SNAT2 gene during UPR activation in HepG2 human hepatoma cells, transcription activity was not enhanced. Hyperacetylation of histone H3 and recruitment of the general transcription factors at the HepG2 SNAT2 promoter occurred in response to the AAR but not the UPR. In contrast, the UPR did enhance transcription from a plasmid-based reporter gene driven by a SNAT2 genomic fragment containing the C/EBP-ATF site. Simultaneous activation of the AAR and the UPR pathways revealed that the UPR actually suppressed the increased SNAT2 transcription by the AAR pathway, demonstrating that the UPR pathway generates a repressive signal that acts downstream of ATF4 binding.
Highlights
Activation of either protein kinase-like endoplasmic reticulum kinase (PERK) or GCN2 leads to the induction of specific target genes, as a result of increased activating transcription factor 4 (ATF4) synthesis [1]
2) In HepG2 human hepatoma cells and mouse BNL-CL2 fetal hepatocytes SNAT2 transcriptional activity remains near basal levels despite a readily detected increase in ATF4 recruitment and binding to the C/EBPATF site within the SNAT2 gene that equals that observed during the amino acid response (AAR). 3) The lack of increased SNAT2 transcription by the unfolded protein response (UPR) results from an active repression of the gene rather than merely an absence of an activating signal
4) The mechanism of this repression is unknown, but it appears to occur at a step downstream of ATF4 binding; it is not triggered by the ATF6 or XBP1 arms of the UPR, and it can override the activation by the AAR pathway
Summary
Activation of either PERK or GCN2 leads to the induction of specific target genes, as a result of increased ATF4 synthesis [1]. Its adaptive regulation by substrate supply and hormones, as well as its increased expression in transformed cells and its role in diabetes, makes SNAT2 a potentially attractive therapeutic target Another ATF4-regulated gene is ASNS (asparagine synthetase). The experiments revealed that SNAT2 transcriptional activity remains at the basal level in the presence of ER stress despite increased synthesis of ATF4 and its subsequent enhanced binding to the SNAT2 C/EBP-ATF composite site. Chromatin immunoprecipitation (ChIP) analysis revealed no increase in histone H3 acetylation or general transcription factor (GTF) recruitment to the SNAT2 promoter following activation of the UPR pathway Simultaneous activation of both pathways indicated that the UPR generates a suppressive signal that blocks the AAR-induced SNAT2 transcription activity downstream of ATF4 binding
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
