Inhibition of dexamethasone-induced cytoskeletal changes in cultured human trabecular meshwork cells by tetrahydrocortisol

Purpose. To determine the cellular mechanism of action of the intraocular pressure (IOP) lowering steroid tetrahydrocortisol (THF). Methods. Tetrahydrocortisol was evaluated for glucocorticoid antagonist activity using in vitro and in vivo assays. Systemically administered THF was evaluated for its ability to inhibit dexamethasone-induced body weight loss and systemic hypertension in rats. In vitro receptor antagonism was tested using the supernatant fraction of IM9 cells as the source of soluble glucocorticoid receptor in ³H-dexamethasone displacement binding assays. In addition, six different primary human trabecular meshwork (TM) cell lines were cultured for 0 to 14 days in the absence or presence of dexamethasone (10^-7 M) and/or THF (10^-6 to 10^-8 M). The effects these steroids on the TM cytoskeleton were determined by epifluorescent microscopy and by transmission electron microscopy. Results. Tetrahydrocortisol was unable to inhibit the dexamethasone (DEX)-induced systemic hypertension and decrease in both mass in rats and was unable to displace ³H-DEX from the soluble human glucocorticoid receptor. However, THF inhibited the DEX-induced formation of cross-linked actin networks in cultured human TM cells in a progressive and dose-dependent manner (PIC₅₀=5.7 x 10 ⁷ M). Dexamethasone caused changes in the TM cell microtubules that were reviewed partially by concomitant treatment with THF. Tetrahydrocortisol alone appeared to increase microfilament bundling to TM cells. Conclusions. Tetrahydrocortisol was not a glucocorticoid antagonist at the level of the classical glucocorticoid receptor and did not appear to antagonize systemically mediated glucocorticoid activity in the rat. Tetrahydrocortisol inhibited DEX-induced changes in the TM microfilaments and microtubules. These results may explain partially the IOP lowering activity of THF because glucocorticoid-mediated changes in the TM cytoskeleton have been proposed to be involved in the generation of ocular hypertension.