Egr-1 Induces the Expression of Its Corepressor Nab2 by Activation of the Nab2 Promoter Thereby Establishing a Negative Feedback Loop

Joerg Kumbrink,Marco Gerlinger,Judith P Johnson

doi:10.1074/jbc.m511079200

Joerg Kumbrink, Marco Gerlinger + Show 1 more

Open Access

https://doi.org/10.1074/jbc.m511079200

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Abstract

The transcription factor Egr-1 regulates the expression of numerous genes involved in differentiation, growth, and in response to environmental signals. Egr-1 activity is modulated in part through the binding of corepressors Nab1 and Nab2. Nab2 appears crucial for controlling Egr-1-mediated transactivation because it is a delayed early response gene, induced by the same stimuli that induce the immediate early gene Egr-1. To identify important elements regulating Nab2 expression, we cloned the human Nab2 gene and investigated the 5'-region. The TATA- and initiator-less Nab2 promoter, located from -679 to -74 bp, contains a total of 11 Egr binding sites, including a cluster of multiple overlapping Egr/Sp1 sites between -329 and -260 bp. This region is critical for basal promoter activity as well as for maximum induction by phorbol esters. Electromobility shifts show that Sp1 binds to this region in normal and stimulated cells, whereas stimulation induces binding of Egr-1. In addition Egr-1 activates the Nab2 promoter in a pattern similar to phorbol esters, suggesting that Egr-1 is a major inducer of protein kinase C-mediated Nab2 induction. Depletion of Egr-1 by each of two distinct Egr-1 short-interfering RNAs reduces Nab2 expression and inducibility, confirming that Egr-1 is an important regulator of Nab2 expression. Transfection experiments show that Egr-1-induced Nab2 promoter activity is itself repressed by Nab2. These results indicate that Egr-1 mediates the induction of its own repressor, thereby preventing a permanent transactivation of Egr-1 target genes and a damaging overreaction in response to environmental signals.

Highlights

(PDGF), thereby establishing a positive autocrine feedback loop
This positive feedback loop is controlled in part through the action of two proteins, Nab1 and Nab2, which bind to Egr-1 and inhibit its activity
As a delayed early response gene, Nab2 expression follows that of Egr-1 by several hours

Summary

Introduction

(PDGF), thereby establishing a positive autocrine feedback loop. Egr-1 activity is tightly regulated, in part through the function of two proteins, NGFI-A-binding protein (Nab)1 and Nab2, which prevents the permanent activation of Egr-1 target genes and signaling pathways such as those for PDGF [2], which could lead to cellular transformation. To identify elements in the Nab2 promoter mediating induction by phorbol esters the 5Ј-truncated fragments were transfected into Colo 320DM and Mel 888 cells and cultured in the presence or absence of PMA for 24 h (Fig. 5A).

Results

Conclusion