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R. Lai, L. Wheeler; Protective Mechanism of Alpha-2 Adrenergic Receptor Agonist Brimonidine. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4931.
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Brimonidine has been reported to be protective in a number of retinal injury and disease models, although the mechanism of this protective effect is not fully understood. Brimonidine is a selective alpha-2 adrenergic receptor agonist. In the present study, we utilized a cell line (CHO) transfected with the adrenergic receptor and the Muller glial cells which constitutively express the adrenergic receptor to investigate the protective mechanism of brimonidine against oxidative stress.
CHO cells overexpressing the adrenergic receptor and primary cultured Muller glial cells were subjected to one of two forms of oxidative stress: UV irradiation exposure or hydrogen peroxide. Cell death and cell survival were visualized through ethidium homodimer/ calcein AM staining, and quantified by LDH and Cell-titer glo luminescence assays. Caspase activation was detected with Caspase glo luminescence assay. Phosphorylation of Akt, Erks, p70S6K, FOXO1 proteins were analyzed with Western blots using specific antibodies.
Oxidative stress by UV or hydrogen peroxide resulted apoptotic cell death of the CHO and Muller cells. Brimonidine ameliorated the cell death in a dose-dependent fashion. This protection could be inhibited by alpha-2 adrenergic antagonists rauwolscine or yohimbine. This effect also could be inhibited by pertussis toxin, Erk inhibitor U0126, PI3 kinase inhibitor Wortmannin, and Akt inhibitor. In cells that did not express the alpha-2 adrenergic receptor, brimonidine failed to elicit any cell protection. Western blot analysis showed that brimonidine activated the PI3/Akt and MAPK pathways, which further activated the FOXO and p70S6K pathways downstream.
Brimonidine dose-dependently protects cells from apoptotic cell death caused by oxidative damage. The effect is mediated through the activation of Gi-coupled alpha-2 adrenergic receptor which further activates downstream Akt survival signaling pathways.
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