Mifepristone and Photoreceptor Survival

CUBILLA, MA; CASTAÑEDA M; OLIVERA, M; BACHOR, TP; SUBURO AM

Fort Lauderdale

Congreso; ARVO Annual Meeting; 2011

ARVO

Purpose:Glucocorticoids (GCs) secreted from the adrenal cortex easily pass through the blood brain barrier and bind to intracellular glucocorticoid (GRα) receptors in neurons and glia. They are critical for survival and viability of neurons and can prevent, or ameliorate, light-induced and inherited retinal degeneration. GRα antagonists, such as mifepristone (MFP or RU-486), have been advocated for treatment of serous chorioretinopathy. We have previously shown that MFP aggravates light-induced retinal degeneration, therefore, we have now studied effects of this GRα antagonist under standard illumination conditions. Methods:Balb-c male mice were bred and cared in accordance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. Standard illumination conditions are < 60 lux, cyclic 12 h/day. Animals were left in complete darkness during 24 hours and randomly separated into experimental groups receiving: MFP (4, 10 or 30 mg/kg), dexamethasone (DEX, 4 or 10 mg/kg), MFP + DEX, or vehicle alone (VHC). Mice were euthanatized after 6 hours. Retinas were processed for Fragment End Labeling (FragEL) of DNA, immunohistochemistry or Western blots using antibodies against GRα, glutamine synthase (GS), glial fibrillary acidic protein (GFAP), Bcl-XL, Bcl-2 and cleaved caspase-3 (CC-3). Results:Within 6 hours, MFP increased GRα in all retinal nuclear layers. A subpopulation of strongly immunoreactive nuclei showed in the INL. Müller cells displayed stronger GS immunoreactivity, but no GFAP. FragEL showed numerous apoptotic nuclei that were exclusively localized in the ONL. Control retinas contained high levels of Bcl-XL, whereas Bcl-2 was undetectable. MFP depleted Bcl-XL and induced Bcl-2 expression. CC-3 was not detected in control retinas but high levels appeared after MFP. Simultaneous treatment with DEX partially reduced CC-3 protein but had no effect on Bcl-XL and Bcl-2. Conclusions:MFP induced cell death in photoreceptors, most likely executed by activation of caspase-3. DEX blocked photoreceptor death, indicating that endogenous glucocorticoids would be essential for photoreceptor survival. However, DEX did not completely reverse MFP effects, suggesting that the progesterone receptor might also be involved in photoreceptor survival. Keywords: apoptosis/cell death ? corticosteroids ? photoreceptors