The inducible zinc finger transcription factors EGR1, EGR2, and EGR3 regulate the expression of numerous genes involved in differentiation, growth, and response to extracellular signals. Their activity is modulated in part through NAB2 which is induced by the same stimuli. In melanoma and carcinoma cells EGR1 activates NAB2 expression. In T lymphocytes EGR2 and EGR3 have been shown to inhibit NAB2 expression. Therefore, we investigated the influence of EGR2 and EGR3 on NAB2 expression in melanoma and carcinoma cells. Here, we show that like EGR1, EGR2 and EGR3 induced NAB2 expression in these cells. EGR1 and EGR3 act in concert on the NAB2 promoter and are more potent activators of NAB2 transcription than EGR2. EGR1-, EGR2-, and EGR3-induced NAB2 promoter activity is mediated through similar cis-regulatory elements and the activation by each EGR is repressed by NAB2. Kinetic studies suggest that induction of EGR1 leads to low NAB2 expression, while EGR2 and EGR3 are necessary for maximal and sustained expression. As already shown for EGR1, reduction of EGR2 or EGR3 expression by siRNAs reduced endogenous NAB2 levels. Depletion of EGR3 also resulted in a reduction of EGR2 levels confirming EGR2 as a target gene of EGR3. Our results suggest that in many cells of neuroectodermal and epithelial origin EGR1, EGR2, and EGR3 activate NAB2 transcription which is in turn repressed by NAB2, thus establishing a negative feedback loop. This points to a complex relationship between the EGR factors and NAB2 expression likely depending on the cellular context.
(c) 2010 Wiley-Liss, Inc.