Activation and hypoxia inducible factor 1 by insulin-like growth factor in the rat brain

Hypoxia inducible factor 1 (HIF-1) is a heterodimeric transcription factor containing an oxygen-regulated HIF-1α subunit and a constitutively expressed HIF-1β subunit. During hypoxia the HIF-1α subunit accumulates due to inhibition of its proteolytic degradation. In the first part of this study, we analyzed HIF-1α expression in the rat cerebral cortex after transient global ischemia induced by cardiac arrest and resuscitation. HIF-1α accumulation was observed as early as 1h after cardiac arrest and resuscitation and persists for at least 7 days. The expression of Epo and Glut-1, both HIF-1 target genes, was also induced at 12h–7d of recovery. A possible explanation for the sustained HIF-1 activation might be that the brain remained hypoxic for prolonged periods after resuscitation. However, the use of the in-vivo hypoxic marker EF5 revealed that the brain was hypoxic only during the first hours of recovery, but it was no longer hypoxic at 2 days. Thus, the initial ischemic episode must have activated other non-hypoxic mechanisms that maintain prolonged HIF-1α accumulation.; We then explore the possibility that IGF-1 was responsible for the prolonged HIF-1 activation in the ischemic brain. Our data showed that IGF-1 expression was upregulated after cardiac arrest and resuscitation and inhibition of the IGF-1R prevented HIF-1α accumulation. In addition, IGF-1 was able to induce HIF-1α in PC12 and cultured neurons as well as in the brain of rats that received exogenous intra-cerebroventricular and systemic IGF-1 infusion.; In the second part we analyzed the potential mechanisms involved in the IGF-1-dependent activation of HIF-1 in cultured neuronal cells. Our results showed that the IGF-1 effect on HIF-1 activation was independent of transcriptional mechanisms and ongoing protein synthesis. Moreover, the data demonstrate that IGF-1 prevented the interaction between HIF-1α and VHL probably due to inhibition of neuronal prolyl hydroxylase activity. In contrast to hypoxia, the phosphatidyl inositol 3-kinase (PI3K)/Akt pathway was required for the IGF-1 dependent HIF-1 activation. However, the mammalian target of rapamycin (mTOR) and the MEK/ERK dependent signaling did not contribute significantly to the activation of HIF-1 by IGF-1. In conclusion, this dissertation work demonstrated that in the rat brain, IGF-1 is a non-hypoxic activator of HIF-1 and this effect, at least in cultured neurons, requires the PI3K/Akt pathway and inhibition of HIF-1α/VHL interaction.