IGF-I neuroprotection in the immature brain after hypoxia-ischemia, involvement of Akt and GSK3β?

Insulin-like growth factor I (IGF-I) is a neurotrophic factor that promotes neuronal growth, differentiation and survival. Neuroprotective effects of IGF-I have previously been shown in adult and juvenile rat models of brain injury. We wanted to investigate the neuroprotective effect of IGF-I after hypoxia-ischemia (HI) in 7-day-old neonatal rats and the mechanisms of IGF-I actions in vivo. We also wanted to study effects of HI and/or IGF-I on the serine/threonine kinases Akt and glycogen synthase kinase 3 beta (GSK3 beta) in the phophatidylinositol-3 kinase (PI3K) pathway. Immediately after HI, phosphorylated Akt (pAkt) and phosphorylated GSK3 beta (pGSK3 beta) immunoreactivity was lost in the ipsilateral and reduced in the contralateral hemisphere. After 45 min, pAkt levels were restored to control values, whereas pGSK3 beta remained low 4 h after HI. Administration of IGF-I (50 mu g i.c.v.) after HI resulted in a 40% reduction in brain damage (loss of microtubule-associated protein) compared with vehicle-treated animals. IGF-I treatment without HI was shown to increase pAkt whereas pGSK3 beta decreased in the cytosol, but increased in the nuclear fraction. IGF-I treatment after HI increased pAkt in the cytosol and pGSK3 beta in both the cytosol and the nuclear fraction in the ipsilateral hemisphere compared with vehicle-treated rats, concomitant with a reduced caspase-3- and caspase-9-like activity. In conclusion, IGF-I induces activation of Akt during recovery after HI which, in combination with inactivation of GSK3 beta, may explain the attenuated activation of caspases and reduction of injury in the immature brain.