Regulation of endothelin-1 in human non-pigmented ciliary epithelial cells by tumor necrosis factor-α

Endothelins (ET) are potent vasoactive peptides present in many ocular structures and are formed from precursor Big endothelins (Big ET-1) by the action of an endothelin-converting enzyme (ECE). ET-1 is thought to decrease intraocular pressure by contracting the ciliary muscle thus enhancing the outflow of aqueous humor through the Canal of Schlemm and trabecular meshwork. However, the mechanisms involved in the regulation of endothelin-1 (ET-1) synthesis and release in ocular tissues have not been fully characterized. In this study we examined the effect of tumor necrosis factor- α (TNF-α; 10 nM), a proinflammatory cytokine, on the cellular mechanisms leading to ET-1 synthesis and release in SV-40 transformed human ciliary non- pigmented epithelial cells (HNPE). ET-1 and Big endothelin-1 (Big ET-1) immunoreactivity was time-dependently increased following TNF-α treatment. Phorbol esters (PMA), activators of PKC, also raised the immunoreactive levels of ET-1 and Big ET-1 while, staurosporine, a PKC inhibitor (20 nM), decreased ET-1 levels in TNF-α-stimulated cells. Pre-treatment with phosphoramidon (1 μM) an ECE-inhibitor, followed by TNF-α stimulation, decreased ir-ET-1 levels. Cycloheximide (9 μM), a protein synthesis inhibitor, decreased TNF-α-stimulated levels for ir-ET-1 and ir-Big ET-1, suggesting that TNF-α may be directly regulating ET-1 expression at the ET- 1 gene. Our data indicates that TNF-α regulates ET-1 levels in HNPE cells possibly by activating PKC either to stimulate protein synthesis and/or to enhance ET-1 secretion. These results suggest that ET-1 released from the ciliary body may play an important role in aqueous humor dynamics following cytokine activation.