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Keywords:

  • MEK1/2;
  • PI3K;
  • PKB/Akt;
  • HT22;
  • hippocampus;
  • FGF-1;
  • Egr-1;
  • U0126;
  • LY294002

Background and purpose:  The transcription factor early growth response-1 (Egr-1) and the acidic fibroblast growth factor (FGF-1) are involved in many regulatory processes, including hippocampus-associated learning and memory. However, the intracellular signalling mechanisms regulating Egr-1 in hippocampal cells are not entirely understood.

Experimental approach:  We used primary mouse hippocampal neurons and the mouse hippocampal neuronal cell line HT22 to investigate how FGF-1 transiently induces Egr-1 protein. This was accomplished by a range of techniques including Western blotting, immunofluorescence, specific protein kinase inhibitors and transfectable constitutively active protein kinase constructs.

Key results:  Protein kinase B (PKB) and mitogen-activated protein kinase (MAPK) were both initially phosphorylated and activated by FGF-1 treatment, but when phosphorylated MAPK reached maximal activation, phosphorylated PKB was at its lowest levels, suggesting an interaction between MAPK kinase (MEK-1/2) and phosphatidyl inositol-3-kinase (PI3K) during Egr-1 induction. Interestingly, pharmacological inhibition of MEK-1/2 resulted in a robust increase in the phosphorylation of PKB, which was repressed in the presence of increasing doses of a PI3K inhibitor. FGF-1-mediated Egr-1 induction was impaired by inhibition of MEK-1/2, but not of PI3K. However, elevated levels of PKB, induced by transfection of constitutively active PKB (myrAkt) into hippocampal neuronal HT22 cells, led to reduced levels of Egr-1 after FGF-1 application.

Conclusions and implications:  Our data indicate a contribution of inactive (dephosphorylated) PKB to FGF-1-mediated induction of Egr-1, and strongly suggest a functionally and pharmacologically interesting cross-talk between MEK-1/2 and PI3K signalling in hippocampal neurons after FGF-1 stimulation that may play a role in hippocampal synaptic plasticity.