Human CCAAT/enhancer-binding protein β gene expression is activated by endoplasmic reticulum stress through an unfolded protein response element downstream of the protein coding sequence

Abstract

CCAAT/enhancer-binding protein β (C/EBPβ) is a member of the bZIP family of transcription factors that contribute to the regulation of a wide range of important cellular processes. The data in the present study document that transcription from the human C/EBPβ gene is induced in response to endoplasmic reticulum stress, such as glucose deprivation, or treatment of cells with tunicamycin or thapsigargin. Transient transfection of C/EBPβ genomic fragments linked to a luciferase reporter gene demonstrated that the C/EBPβ BP13 promoter plays no major regulatory role. Instead, by deletion analysis it was discovered that a 46-bp region, located at a genomic site that corresponds to the 3′-untranslated region of tine C/EBPβ mRNA, harbored an element that was required for the stress response. Mutagenesis demonstrated that a cis-regulatory element located at nt +1614-1621 (5′-TGACGCAA-3′) is responsible for activation of the C/EBPβ gene. Electrophoresis mobility shift analysis revealed that proteins are bound to this element and that the amount of binding is increased following glucose deprivation. This element is homologous to a previously reported mammalian unfolded protein response element that binds XBP-1. Consistent with those data, overexpression of XBP-1 caused an increase in transcription that was mediated by the C/EBPβ mammalian unfolded protein response element.