Despite increased ATF4 binding at the C/EBP-ATF composite site following activation of the unfolded protein response, system A transporter 2 (SNAT2) transcription activity is repressed in HepG2 cells.
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.
Gjymishka
Altin
A
Department of Biochemistry and Molecular Biology, Genetics Institute, Shands Cancer Center and Center for Nutritional Sciences, University of Florida College of Medicine, Gainesville, Florida 32610.
Palii
Stela S
SS
Department of Biochemistry and Molecular Biology, Genetics Institute, Shands Cancer Center and Center for Nutritional Sciences, University of Florida College of Medicine, Gainesville, Florida 32610.
Shan
Jixiu
J
Department of Biochemistry and Molecular Biology, Genetics Institute, Shands Cancer Center and Center for Nutritional Sciences, University of Florida College of Medicine, Gainesville, Florida 32610.
Kilberg
Michael S
MS
Department of Biochemistry and Molecular Biology, Genetics Institute, Shands Cancer Center and Center for Nutritional Sciences, University of Florida College of Medicine, Gainesville, Florida 32610. Electronic address: mkilberg@ufl.edu.
D000107:Acetylation; D051701:Activating Transcription Factor 4 / Q000378:metabolism*; D026924:Amino Acid Transport System A / Q000096:biosynthesis*; D000818:Animals; D022762:CCAAT-Enhancer-Binding Proteins / Q000378:metabolism; D045744:Cell Line, Tumor; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism; D006657:Histones / Q000378:metabolism; D006801:Humans; D051379:Mice; D014176:Protein Biosynthesis / Q000502:physiology; D017510:Protein Folding; D012333:RNA, Messenger / Q000378:metabolism; D020218:Response Elements / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*; D014158:Transcription, Genetic / Q000502:physiology*
C497109:ATF4 protein, human; D026924:Amino Acid Transport System A; C081452:Atf4 protein, mouse; D022762:CCAAT-Enhancer-Binding Proteins; D015852:Eukaryotic Initiation Factor-2; D006657:Histones; D012333:RNA, Messenger; C498797:SLC38A2 protein, human; D051701:Activating Transcription Factor 4