Egr-1 and Hipk2 are required for the TrkA to p75(NTR) switch that occurs downstream of IGF1-R.

The aging program mediated by IGF1-R is responsible for a naturally occurring TrkA to p75(NTR) switch that leads to activation of the second messenger ceramide and increased production of the Alzheimer's disease amyloid beta-peptide. Biochemical and genetic approaches that target IGF1-R signaling, p75(NTR), or ceramide are able to block the above events. Here, we show that the transcription factors Egr-1 and Hipk2 are required elements for the TrkA to p75(NTR) switch downstream of IGF1-R signaling. Specifically, Egr-1 is required for the upregulation of p75(NTR), whereas Hipk2 is required for the downregulation of TrkA. In fact, gene silencing of Egr-1 abolished the ability of IGF1 to upregulate p75(NTR), whereas similar approaches directed against Hipk2 blocked the downregulation of TrkA. In addition, IGF1 treatment favored binding of Egr-1 and Hipk2 to the promoter of p75(NTR) and TrkA, respectively. Finally, the expression levels of both Egr-1 and Hipk2 are upregulated in an age-dependent fashion. Such an event is opposed by caloric restriction, a model of delayed aging, and favored by the p44 transgene in p44(+/+) animals, a model of accelerated aging.