Hypoxia augments TNF-α-mediated endothelin-1 release and cell proliferation in human optic nerve head astrocytes

The effect of hypoxia (24h) on TNF-α-mediated release of endothelin-1 (ET-1) from human optic nerve head astrocytes (hONAs) and TNF-α- and ET-1-induced hONA proliferation was determined. ET-1 synthesis and release was quantitated using ELISA while TNF-α (10nM)- and ET-1 (100nM)-mediated hONA proliferation was assessed by CellTiter 96 aqueous one-solution cell proliferation assay, respectively. hONAs appeared to be more rounded with fewer processes following 24h hypoxia compared to thodr seen in normoxia. Hypoxia enhanced TNF-α-mediated ET-1 synthesis and release (by 5-fold) and also significantly increased TNF-α- and ET-1-mediated hONA proliferation. PD142893 (1μM), an ET_A/B receptor antagonist, blocked ET-1-mediated hONA proliferation both under normoxia and hypoxia, while doing so only under normoxia following TNF-α treatment. Also, U0126 (10μM; an upstream ERK1/2 inhibitor) completely blocked agonist-induced hONA proliferation in normoxia and partially blocked the same in hypoxia. These results demonstrate for the first time that hONAs secrete ET-1 and that TNF-α and hypoxia can regulate its levels. Moreover, hypoxia augments the proliferative responses of hONAs to TNF-α and ET-1. These agonist-mediated effects following hypoxia could contribute to astroglial activation as seen in glaucomatous optic nerve heads.