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

Abstract

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 À/À ) mouse fetuses. Both pretreatment and post-treatment with EPO counteracted glutamate-induced cell death in Stat5 þ / þ and Stat5 À/À neurons. Acute pharmacological inhibition of Janus kinase 2 (JAK2)/Stat signalling had no effect on EPO neuroprotection, whereas inhibition of phosphatidylinositol-3 0 kinase (PI3K)/Akt pathway abolished the protective effect of EPO in both Stat5 þ / þ and Stat5 À/À neurons. GH effectively protected Stat5 þ / þ cells against glutamate toxicity but had no effect in Stat5 À/À neurons or in Stat5 þ / þ neurons treated with JAK2/Stat or PI3K inhibitor. EPO and GH stimulated neurite outgrowth and branching of Stat5 þ / þ neurons by activating PI3K/Akt signalling but had no trophic effect in Stat5 À/À 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. The transcription factors signal transducer and activator of transcription 5a and 5b (Stat5) control cell fate decisions such as differentiation, proliferation and apoptosis. 1 Stat5 mediates cellular response to cytokines, growth factors and hormones. 1 Upon binding to their membrane receptors, growth factors such as erythropoietin (EPO) and growth hormone (GH) activate Janus kinase 2 (JAK2), causing activation of Stat5 by its phosphorylation, dimerization and translocation to the nucleus, where Stat5 induces expression of various antiapoptotic genes. 1 Indicative of an impaired functioning of EPO and GH receptor-associated signalling, Stat5a/b-knockout mice (Stat5 À/À ) are severely anaemic and growth retarded and the vast majority die perinatally. 2 Stat5 is expressed in the developing nervous system. 3,4 Recent studies suggest a role for Stat5 in neuronal migration and axon guidance in the developing brain 3 and a novel therapeutic potential for constitutively active Stat5 in reducing axonal outgrowth defects in spinal muscular atrophy-like motor neurons. 5 Since activation of Stat5 in neurons accompanies the antiapoptotic and neuroprotective effects of EPO in vitro and in vivo, 6-12 Stat5 has been suggested to mediate the protective effects of EPO in neuronal cells. Nevertheless, the molecular mechanisms of EPO receptor (EPOR) activation in neurons are complex, as multiple neuroprotective signalling pathways are activated downstream of EPOR/JAK2. In addition to Stat5, best characterized from these are the phosphatidylinositol-3 0 kinase (PI3K)/ Akt, nuclear factor-kB and Ras/mitogen-activated p...

Therapeutic Potential of Erythropoietin and its Structural or Functional Variants in the Nervous System

“…8,29 In neurons these include signal transducers and activators of transcription (Stat), phosphatidylinositol 3-kinase (PI3K)/Akt, Ras/extracellular signal regulated kinase (ERK1/2), nuclear factor-kappa-B (NF-B), and calcium. 6,7,30,31 Brines and Cerami 7 have proposed that the cytoprotective effects of EPO and its nonhematopoietic derivative, the carbamoylated EPO (CEPO) are mediated by a heteromeric receptor complex comprised of one EPOR subunit and a dimer of the common beta-chain shared by the members of the interleukin-3 receptor family. 32,33 The molecular assembly of this proposed heteroreceptor has been projected from that of the known stoichiometry of the granulocyte-macrophage colony-stimulating factor receptor complex.…”

Signal Transducer and Activator of Transcription-5 Mediates Neuronal Apoptosis Induced by Inhibition of Rac GTPase Activity

“…This finding is perhaps not surprising given that JSI-124 has been reported to exert effects outside of STAT3 inhibition (52). In fact, this compound was recently shown to inhibit the neuroprotective effects of growth hormone against glutamate-induced toxicity in hippocampal neurons, an effect that is dependent on STAT5 activity (53). In addition to the neuroprotective effects of JSI-124, we report that roscovitine similarly inhibits STAT5 phosphorylation induced by ToxB and protects CGNs against ToxB targeted Rac inhibition.…”

“…for 6 independent experiments. effects of both growth hormone in hippocampal neurons (53) and erythropoietin in differentiated neuroblastoma cells (55). Furthermore, STAT5 was shown to elicit a prosurvival effect via induction of Bcl-2 and Bcl-xL in neural progenitor cells subjected to apoptotic stimulation (56).…”

Endogenous Agents That Contribute to Generate or Prevent Ischemic Damage

“…Therefore, the authors concluded that STAT5 plays a role in Epo-mediated neuroprotection. However, in a very recent study, in an in vitro model of glutamate toxicity using hippocampal neuronal culture from STAT5 knockout mouse fetuses, STAT5 was not required for Epo-mediated neuroprotection (Byts et al, 2008). However, STAT5 was indispensable for the neurotrophic function of Epo.…”