Deferroxamine Preconditioning Promotes Long-Lasting Retinal Ischemic Tolerance

Objective: Ischemic tolerance can be induced in the retina by preconditioning with brief periods of non-injurious retinal ischemia or systemic hypoxia. The present study was undertaken to assess whether tolerance can be induced pharmacologically by deferroxamine (DFX), an iron chelator, which promotes the expression of the transcription factor, hypoxia-inducible factor 1-alpha (HIF-1 alpha), and to identify potential HIF-1 alpha-induced effectors of this endogenous protective response. Methods: ND4 Swiss-Webster mice were preconditioned with DFX (200 mg/kg, intraperitoneally) as a single dose (SDP) or as repetitive doses (RDP; 6 doses over 2 weeks) and then subjected to 30 min of retinal ischemia (by intraocular pressure elevation) 1 or 4 weeks later. Retinal layer thicknesses and cell counts were quantified 1 week after ischemia. Retinae of additional mice were obtained at various times after SDP or RDP to examine protein-level expression of HIF-1 alpha and adrenomedullin (ADM), a HIF-1 alpha gene target, by immunoblotting and immunohistochemistry. Results: Ischemia-induced injury was significantly attenuated by SDP 1 week earlier, but not when SDP occurred 4 weeks earlier. However, RDP performed 4 weeks earlier was potently neuroprotective. DFX robustly induced HIF-1 alpha protein expression throughout the inner retina, and levels of HIF-1a protein remained significantly elevated over the 1- and 4-week periods of time between the respective SDP and RDP stimulus and the induction of retinal ischemia. Increases in ADM protein expression were evident throughout the retina following both preconditioning treatments. Conclusions: DFX preconditions the retina against ischemic injury and multiple doses promote a long-lasting, ischemia-protective phenotype. The widespread and protracted elevations in HIF-1 alpha protein levels and the robust expression of one of its neuroprotective, prosurvival gene targets, ADM, strongly suggest that DFX-induced preconditioning is HIF-1 alpha-dependent. The ability to pharmacologically induce ischemic tolerance in the retina by a clinically well-tolerated drug underscores the potential therapeutic utility of preconditioning for retinal protection in various ischemic retinopathies.