N-terminal Hydrophobic Amino Acids of Activating Transcription Factor 5 (ATF5) Protein Confer Interleukin 1β (IL-1β)-induced Stabilization

Takanori Abe,Masaki Kojima,Satoshi Akanuma,Hiromi Iwashita,Takashi Yamazaki,Ryuichi Okuyama,Kenji Ichikawa,Mariko Umemura,Haruo Nakano,Shigeru Takahashi,Yuji Takahashi

doi:10.1074/jbc.m113.491217

Abstract

Activating transcription factor 5 (ATF5) is a stress-response transcription factor that responds to amino acid limitation and exposure to cadmium chloride (CdCl2) and sodium arsenite (NaAsO2). The N-terminal amino acids contribute to the destabilization of the ATF5 protein in steady-state conditions and serve as a stabilization domain in the stress response after CdCl2 or NaAsO2 exposure. In this study, we show that interleukin 1β (IL-1β), a proinflammatory cytokine, increases the expression of ATF5 protein in HepG2 hepatoma cells in part by stabilizing the ATF5 protein. The N-terminal domain rich in hydrophobic amino acids that is predicted to form a hydrophobic network was responsible for destabilization in steady-state conditions and served as an IL-1β response domain. Furthermore, IL-1β increased the translational efficiency of ATF5 mRNA via the 5' UTRα and phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α). ATF5 knockdown in HepG2 cells up-regulated the IL-1β-induced expression of the serum amyloid A 1 (SAA1) and SAA2 genes. Our results show that the N-terminal hydrophobic amino acids play an important role in the regulation of ATF5 protein expression in IL-1β-mediated immune response and that ATF5 is a negative regulator for IL-1β-induced expression of SAA1 and SAA2 in HepG2 cells.

Highlights

The N-terminal region of Activating transcription factor 5 (ATF5) is responsible for its CdCl2- and NaAsO2-induced expression
Our results show that the N-terminal hydrophobic amino acids play an important role in the regulation of ATF5 protein expression in IL-1␤-mediated immune response and that ATF5 is a negative regulator for IL-1␤-induced expression of serum amyloid A 1 (SAA1) and serum amyloid A2 (SAA2) in HepG2 cells
To determine whether IL-1␤ regulated the stability of ATF5 protein, we monitored the levels of basal and IL-1␤-induced ATF5 protein in HepG2 cells after treatment with the protein synthesis inhibitor cycloheximide

Summary

Background

The N-terminal region of ATF5 is responsible for its CdCl2- and NaAsO2-induced expression. The N-terminal amino acids contribute to the destabilization of the ATF5 protein in steady-state conditions and serve as a stabilization domain in the stress response after CdCl2 or NaAsO2 exposure. Our results show that the N-terminal hydrophobic amino acids play an important role in the regulation of ATF5 protein expression in IL-1␤-mediated immune response and that ATF5 is a negative regulator for IL-1␤-induced expression of SAA1 and SAA2 in HepG2 cells. Highly phosphorylated eIF2␣ decreases ribosome assembly and favors reinitiation of translation at the ATF5 ORF downstream of uORF2, resulting in elevated ATF5 protein production [15]. We demonstrated that the N-terminal 21 amino acids of ATF5 protein serve as a destabilization domain in steady-state conditions and function as a stress response stabilization domain after CdCl2 and NaAsO2 exposure. This study provides new insights about the roles of ATF5 in the immune response

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION