Interactions of Endothelin-1 with Dexamethasone in Primary Cultured Human Trabecular Meshwork Cells

PURPOSE. Concentrations of aqueous humor endothelin (ET)-1 are increased in patients with primary open-angle glaucoma (POAG) as well as in animal models of glaucoma. Glucocorticoids have also been associated with glaucoma, in that topical administration of glucocorticoids can increase intraocular pressure by increasing outflow resistance in the trabecular mesh-work (TM) in some individuals. Recent research has shown that dexamethasone (Dex), a synthetic glucocorticoid, can increase the release of ET-1 from human nonpigmented ciliary epithelial (HNPE) cells, a source of aqueous ET-1. In the present study, the downstream interaction of ET-1 with Dex in target TM cells, an action that may alter outflow resistance, was investigated. METHODS. A normal primary human TM (NTM) cell line and a TM cell line derived from a glaucomatous eye (GTM) were used. The cells were treated with vehicle or Dex. The mRNA levels of prepro-ET-1, endothelin receptor A (ET_A), and endothelin receptor B (ET_B) were measured by quantitative RT-PCR (QPCR). The protein expression of ET_A and ET_B receptors were investigated by Western blot analysis using polyclonal anti-ET_A and anti-ET _B antibodies, respectively, on plasma membrane fractions. Intracellular Ca²⁺ ([Ca²⁺]_i) mobilization mediated by ET-1 was measured using the Fura-2 AM fluorescent probe technique as an index of ET receptor function. ET-1-stimulated nitric oxide (NO) release was measured using a Griess colorimetric NO synthase assay kit. RESULTS. Both NTM and GTM cultured cells expressed prepro-ET-1 mRNA less abundantly than did HNPE cells, and Dex treatment had no effect on the mRNA expression of the ET-1 gene. TM cells expressed mRNA of ET_A receptors as detected by QPCR, whereas the ET_B message was not clearly delineated. Western blot analysis showed that both ET_A and ET_B receptor proteins were present. The ET_A receptor was linked to calcium mobilization as ET-1 produced an increase in intracellular calcium release, and this increase was blocked with a selective ET_A receptor antagonist. Dex failed to induce any change in the expression of the ET_A receptor in both NTM and GTM cells, and this was supported by the absence of a Dex effect on the ET-1-induced calcium response. However, Dex treatment diminished ET_B receptor protein expression and produced a decrease in ET-1-stimulated release of NO, a response mediated by ET_B receptors in TM cells. CONCLUSIONS. The Dex-induced increase in ET-1 released by HNPE cells coupled to the downstream Dex-induced specific suppression of ET_B receptor protein expression and declines in ET-1-mediated increase in NO released by TM cells could increase contraction and decrease relaxation of the TM and contribute to the declines in conventional aqueous humor outflow and increases in intraocular pressure that occur with glucocorticoids.