A dominant negative Egr inhibitor blocks nerve growth factor-induced neurite outgrowth by suppressing c-Jun activation: role of an Egr/c-Jun complex.

Members of the Egr family of transcription factors are rapidly and robustly induced by neurotransmitters and neurotrophins and have been implicated in mediating enduring changes in neuronal function elicited by these stimuli. Because we have found in previous studies that a dominant negative inhibitor of Egr action, the Egr zinc finger domain (ZnEgr), blocks NGF-induced neurite outgrowth in PC12 cells, we have used this preparation to help identify the downstream targets of Egr proteins involved in plasticity. Our investigation into the mechanism of action of ZnEgr indicates that it blocks NGF-induced neurite outgrowth by suppressing activation of c-Jun, a critical step in the signaling pathway mediating this response. Although we had assumed that ZnEgr exerts its effects by binding to the Egr response element (ERE) and thereby blocking target gene regulation by Egr proteins, this classical mode of action appears to be too slow to mediate the effects of Egr proteins on c-Jun activation. In evaluating alternative ERE-independent mechanisms of Egr (and ZnEgr) action, we found that Egr1 and c-Jun coprecipitate and that ZnEgr disrupts formation of the Egr1/c-Jun complex. Furthermore, mutations of ZnEgr that greatly impair or abolish its ability to bind to the ERE do not block its ability to suppress c-Jun activation or neurite outgrowth induced by NGF. Accordingly, our studies indicate that Egr and ZnEgr proteins regulate c-Jun activation via a novel mechanism, protein-protein interaction with c-Jun, rather than via their classical mode of action, binding to the ERE.