RESUMEN
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 1ß (IL-1ß), 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-1ß response domain. Furthermore, IL-1ß 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-1ß-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-1ß-mediated immune response and that ATF5 is a negative regulator for IL-1ß-induced expression of SAA1 and SAA2 in HepG2 cells.
Asunto(s)
Factores de Transcripción Activadores/metabolismo , Interleucina-1beta/metabolismo , Biosíntesis de Proteínas/fisiología , Factores de Transcripción Activadores/genética , Arsenitos/farmacología , Cloruro de Cadmio/farmacología , Inhibidores Enzimáticos/farmacología , Células Hep G2 , Humanos , Interacciones Hidrofóbicas e Hidrofílicas/efectos de los fármacos , Interleucina-1beta/genética , Biosíntesis de Proteínas/efectos de los fármacos , Estabilidad Proteica/efectos de los fármacos , Estructura Terciaria de Proteína , Proteína Amiloide A Sérica/biosíntesis , Proteína Amiloide A Sérica/genética , Compuestos de Sodio/farmacologíaRESUMEN
AIMS: Activating transcription factor (ATF) 5 is a member of the cAMP response element-binding protein (CREB)/ATF family of transcription factors. We have shown that ATF5 is a stress response transcription factor that responds to amino acid limitation, arsenite exposure, or cadmium exposure. In this study we investigated whether ATF5 is involved in the regulation of CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP) gene expression. MAIN METHODS: We used a transient transfection system to express ATF5 and analyzed the regulation of CHOP gene promoter in human hepatoma, HepG2 cells. We also studied the effect of ATF5 knockdown on arsenite-induced CHOP protein expression and arsenite-induced cell death of HepG2 cells. KEY FINDINGS: We showed that ATF5 activates the CHOP gene promoter in HepG2 cells. Both deletion analysis and point mutations of the promoter revealed that amino acid response element (AARE) 1 is responsible for ATF5-dependent promoter activation. Furthermore, the existence of either AARE1 or activating protein-1 (AP-1) site is sufficient for transcriptional activation of the CHOP gene promoter by arsenite exposure, although complete induction requires the existence of both elements. We also demonstrated that knockdown of ATF5 reduced arsenite-induced CHOP protein expression and arsenite-induced cell death of HepG2 cells. SIGNIFICANCE: These results suggested that the CHOP gene is a potential target for ATF5, and that ATF5 raises the arsenite-induced CHOP gene expression level via the AARE1 site in HepG2 cells.
