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

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.The zinc finger transcription factor early growth response (Egr 5 )-1 is an immediate early response gene that couples extracellular signals to the induction of cellular programs of differentiation, growth, and cell death through changes in the expression of Egr-1 target genes. Although Egr-1 expression is low or undetectable in resting cells, it is rapidly and transiently induced by a wide variety of environmental signals including growth factors, cytokines, and toxic substances (1). Egr-1 in turn activates the transcription of numerous target genes including growth factors that themselves induce Egr-1, as e.g. platelet-derived growth factor (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. Nab proteins were first identified as Egr-1-binding proteins in a two-hybrid assay (3, 4). Nab2, which is also known as mader (melanoma-associated delayed early response gene), was independently identified as a nuclear protein overexpressed in human malignant melanoma (5). Binding of Nab1 and Nab2 to Egr-1 through interaction between the NCD1 (Nab conserved domain 1) and the R1 domain of Egr-1 has been shown to inhi...

Opposing regulation of T cell function by Egr‐1/NAB2 and Egr‐2/Egr‐3

“…Consistent with other cell types, NAB2 expression was delayed when compared with Egr-1, and like Egr-1 at the RNA level its expression was not affected by the presence of costimulation (Fig. 1D) [22]. This is consistent with our previous findings that costimulation leads to post-transcriptional increases in NAB2 protein expression [21].…”

“…For example, Egr-1 promotes the induction of NAB2, which in turn represses Egr-1 function, including the up-regulation of NAB2 [22]. Given the fact that Egr-1 and NAB2 promote T cell function and Egr-2 and Egr-3 inhibit T cell function we hypothesized that such negative feedback loops might also be present in T cells.…”

Placenta Growth Factor-induced Early Growth Response 1 (Egr-1) Regulates Hypoxia-inducible Factor-1α (HIF-1α) in Endothelial Cells

“…Moreover, Egr-1 plays important roles in development, growth control, and differentiation (45). Egr-1-mediated gene transcription is tightly regulated by the repressor proteins NAB1 and Nab2 (35,47). The function of Nab2 as Egr-1 regulator is more important because its expression is shown to be induced by the same signals that lead to EGR-1 expression, whereas NAB1 is constitutively expressed in most cells (36).…”

Early Growth Response-1 Induces and Enhances Vascular Endothelial Growth Factor-A Expression in Lung Cancer Cells

“…29 Lung adenocarcinomas often carry oncogenic mutant K-RAS or EGFR, 30,31 and the occurrence of K-RAS and EGFR mutations are strictly mutually exclusive. 32 Both activating K-RAS and EGFR activate the ERK pathway.…”

“…NAB-2 expression increased in conjunction with EGR-1 induction, whereas NAB-1 expression remained unchanged, as reported previously. 28,29 To examine whether EGR-1 and HIF-1␣ cooperatively induce VEGF-A expression, we next investigated the relationships among EGR-1, HIF-1␣, and VEGF-A expression in EGR-1-induced or EGR-1-uninduced TKB5pRevEGR-1 cells cultured under hypoxic conditions using Western blot analysis. As shown in Figure 1C To confirm close association of EGR-1 with VEGF-A expression, we next conducted an RNA interference experiment.…”