Date Published: March 22, 2017
Publisher: Public Library of Science
Author(s): Barbara Lipert, Mateusz Wilamowski, Andrzej Gorecki, Jolanta Jura, Yoon Ki Kim.
CCAAT/enhancer-binding protein beta (C/EBPβ) is a transcription factor controlling a broad range of genes essential for homeostasis, including genes related to immune functions, inflammation, metabolism and growth. Monocyte chemoattractant protein-1-induced protein 1 (MCPIP1) also called as Regnase-1 is an RNase and has been shown to decrease the stability of short-lived transcripts coding for inflammation-related proteins, including IL-1β, IL-6, IL-2, IL-8, IL-12b, IER-3, c-Rel. We found previously that the half-life of the C/EBPβ transcript is regulated by MCPIP. To understand the mechanism driving down-regulation of C/EBPβ by MCPIP1, we applied an in vitro cleavage assay, followed by a luciferase-reporter assay and RNA immunoprecipitation (RIP). We demonstrated that MCPIP1 recognizes regions of the 3’UTR of C/EBPβ mRNA and promotes its decay by introducing direct endonucleolytic cleavage.
Modulation of mRNA stability has been found to be responsible for 40–50% of all changes in gene expression [1,2]. Research in this field has revealed the importance of cis-acting elements located predominantly in the 3’ untranslated region of an mRNA (3’UTR). These elements enable the interaction of mRNA transcripts with an mRNA-decay complex where unspecific nucleases promote degradation. Since the degradation involves 5’-to-3’ and/or 3’-to-5’ exonucleases, their attack must be preceded by a hydrolysis of the 7-metylo-guanine cap on the 5’ end and/or removal of a poly(A)-tail on the 3’ end of an mRNA . However, an efficient way for instantaneous exposition of both mRNA ends for degradation is an intramolecular cleavage by endonucleases. A recently discovered endonuclease capable of mRNA cleavage is known as MCPIP1. This protein is essential for the degradation of short-lived transcripts coding for inflammation-related proteins, including IL-1β, IL-6, IL-2, IL-8, IL-12b, IER-3, c-Rel [4–8]. It has been shown that MCPIP1 binding of an mRNA depends on a conserved stem-loop structure in the 3’UTR. The ribonucleolytic activity of MCPIP1 has been attributed to a PIN (PilT N terminus) like domain, where four Asp residues (D141, D225, D226, D244) in the catalytic center determine RNase activity . Interestingly, the PIN-like domains of two MCPIP1 molecules have been shown to interact with each other and this seems to be critical for MCPIP1 activity in vitro .
C/EBPβ belongs to a family of leucine-zipper transcriptions factors. According to the Human Protein Atlas, C/EBPβ is a ubiquitous protein with the highest expression levels in intestine, lungs, skin, adipose tissue, breast, skeletal muscle and liver . Since C/EBPβ activates a broad range of genes essential for homeostasis, including genes related to immune function, metabolism, and growth, the regulation of C/EBPβ activity must be accurate. It has been shown that C/EBPβ activity is controlled on transcriptional and post-translational levels. However, little is known about the post-transcriptional control of C/EBPβ expression. The HuR protein has been shown to bind the C/EBPβ transcript and enhance its stability [24,25], but according to our knowledge no destabilizing agent for the C/EBPβ transcript has been described so far.