Essential role for Stat5 in the neurotrophic but not in the neuroprotective effect of erythropoietin.

The transcription factors signal transducer and activator of transcription 5a and 5b (Stat5) are activated by the neuroprotective and neurotrophic cytokines, erythropoietin (EPO) and growth hormone (GH). Here, we show a dissociation of the intracellular pathway mediating the protective effect of EPO against glutamate toxicity from that needed for its neurotrophic activity using hippocampal neuronal cultures from Stat5a/b-knockout (Stat5(-/-)) mouse fetuses. Both pretreatment and post-treatment with EPO counteracted glutamate-induced cell death in Stat5(+/+) and Stat5(-/-) neurons. Acute pharmacological inhibition of Janus kinase 2 (JAK2)/Stat signalling had no effect on EPO neuroprotection, whereas inhibition of phosphatidylinositol-3' kinase (PI3K)/Akt pathway abolished the protective effect of EPO in both Stat5(+/+) and Stat5(-/-) neurons. GH effectively protected Stat5(+/+) cells against glutamate toxicity but had no effect in Stat5(-/-) neurons or in Stat5(+/+) neurons treated with JAK2/Stat or PI3K inhibitor. EPO and GH stimulated neurite outgrowth and branching of Stat5(+/+) neurons by activating PI3K/Akt signalling but had no trophic effect in Stat5(-/-) cells. We conclude that in hippocampal neurons, Stat5 is not required for neuroprotection by EPO but is together with Akt essential for its neurotrophic activity. Both Stat5 and Akt are needed for neuroprotective and neurotrophic signalling of GH in neurons.