The Role of the Pituitary-Adrenal Axis in G-CSF Therapy after Neonatal Hypoxia-Ischemia
Several reports indicate that the activity of the hypothalamic-pituitary-adrenal axis (HPA) as measured by the increased level of adrenocorticotropic hormone (ACTH), and corticosterone is increased after a brain insult. These hormones are the effectors secreted respectively by the pituitary and adrenal glands. It has been shown that the down-regulation of corticosterone levels can improve detrimental outcomes associated with ischemic brain injuries. Neonatal hypoxia-ischemia (HI) is a devastating perinatal event with a grim prognosis and limited therapeutic strategies. In recent studies, granulocyte-colony stimulating factor (G-CSF) has shown promise in neonatal HI investigations by improving neuromotor function and reducing apoptosis in the brain. Furthermore, G-CSF is shown in the naïve rat to regulate hormones of the HPA axis. Therefore we hypothesized that G-CSF may in part confer its neuroprotective properties by influencing the pituitary-adrenal response after neonatal HI. To test our hypothesis, metyrapone was administered to inhibit the release of rodent specific glucocorticoid, corticosterone, at the adrenal level. Dexamethasone, a synthetic glucocorticoid, was administered to agonize the effects of corticosterone. Following the Rice-Vannucci model, seven-day old rats (P7) were subjected to unilateral carotid ligation followed by 2.5 hours of hypoxia. Our results show that both G-CSF and metyrapone significantly reduced infarct volume via anti-apoptotic properties but lost its protective effect when combined with dexamethasone. Additionally, G-CSF suppressed the increase of corticosterone in the blood after HI. To investigate the mechanism by which G-CSF modulated corticosterone synthesis, we evaluated its effect in adrenal cortical cells in vitro. Our results indicate that G-CSF activated the JAK/PI3K/Akt/PDE3B pathway, which in turn inhibited corticosterone synthesis. The inhibitors of JAK/PI3K/PDE3B respectively Tyrphostin AG490, LY-294002, and 3-isobutyl-lmethylxanthine reversed the inhibitory effect of G-CSF on corticosterone synthesis. We report that G-CSF was neuroprotective in neonatal HI by reducing infarct volume, by suppressing the HI-induced increase of the Bax/Bcl-2 ratio, and by decreasing corticosterone at the adrenal level via the JAK/PI3K/PDE3B pathway. Metyrapone was able to confer similar neuroprotection as G-CSF while dexamethasone reversed the effects of G-CSF. In conclusion, we show that reducing corticosterone was neuroprotective after neonatal HI, which can be achieved by administering G-CSF.
Microbiology, Molecular Genetics, Hypoxia-Ischemia; Brain -- physiopatholog; Central Nervous System -- drug effects; Brain Infarction -- drug therapy; Blood-Brain Barrier; Pituitary-Adrenal System; Adrenocorticotropic Hormone; Hypothalamic Hormones; Granulocyte-Colony Stimulating Factor; Receptors, Cytokine; Immunity; Cellular -- drug effects; Enzyme-Linked Immunosorbent Assay; Animals; Newborn Rats; Sprague-Dawley; Analysis of Variance
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