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Title: The role of hypoxia-inducible factor-1α in xenon preconditioning versus hypoxic-ischaemic organ injury
Author: Lim, Ta
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2008
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Background: The anaesthetic gas xenon provides long-lasting preservation of neuronal function when given several hours prior to neuronal injury. This phenomenon is described as preconditioning. However, little is known of the mechanisms by which xenon preconditioning mediates its protective effect. Interest has focused on the involvement of hypoxia-inducible factor-1α (HIF-1α) in preconditioning because: (i) Hypoxia activates HIF-1α and is an effective preconditioning stimulus; (ii) HIF-1α is a regulator of adaptive responses promoting cellular survival; (iii) Several genes that have hypoxia-responsive elements (HRE) for HIF-1 in the promoter region are capable of mediating preconditioning (e.g., erythropoietin). Therefore, this study speculates on the role of HIF-1α in xenon preconditioning. Method: Separate cohorts of male adult C57/BL6 mice were preconditioned by exposure to 75% xenon/25 % oxygen for 2 hours and thereafter immediately sacrificed for organ harvesting at 0-24 hours after xenon preconditioning. Semi-quantitative study of HIF-1α and EPO protein expression was performed by Western blot analysis, and RT-PCR was used as a measure of gene transcription. Results: Xenon preconditioning provokes a time-dependent increase in HIF-1α protein in brain as well as kidney. Xenon preconditioning also caused a time-dependent increase in EPO (a HIF-1 target gene) transcription and protein expression in a corresponding time course to xenon-induced HIF-1α. To elucidate the mechanisms of xenon-induced HIF-1α accumulation, the effect of xenon preconditioning on HIF-1α transcription, translation and degradation was studied. Xenon preconditioning does not induce a change in HIF-1α mRNA expression, nor does it significantly attenuate the expression of the PHD2 enzyme, required for HIF-1α degradation. To explore translation-dependent pathways, mice were treated with rapamycin before xenon preconditioning, to inhibit translation of HIF-1α through the mTOR pathway. Inhibition of this pathway prevented the xenon-induced increase of HIF-1α protein. To ascertain whether HIF-lα is required for xenon preconditioning, siRNA was used to knockdown HIF-lα expression in the kidney and xenon's renoprotective properties were shown to be abolished. Conclusions: Over the same time course as xenon's protection against subsequent injury in both brain and kidney, xenon preconditioning induces expression of HIF-1α. Increased HIF-1α expression is also associated with increased activity as evidenced by enhanced transcription and translation of the downstream effector, EPO. Xenon preconditioning does not regulate HIF-1α at the transcriptional level, nor does it inhibit HIF-lα degradation. However, these results suggest that xenon preconditioning upregulates expression of HIF-1α through translation-dependent mechanisms. Furthermore, xenon's action on HIF-1α is shown to be causally related to its organ protective effect. If these data can be extrapolated to the clinical setting, exposure to xenon would be beneficial prior to procedures in which organ perfusion is interrupted, preventing hypoxic-ischaemic as well as ischaemic-reperfusion injury.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available