Suppressive Effects of Thyroxine on Glucocorticoid (GC)-induced Metabolic Changes and Cataract Formation on Developing Chick Embryos

Abstract

To prevent cataracts induced by glucocorticoids (GC) as a systemic disease, the suppression of oxidative stress caused by GC in the hepatic metabolism is of significant interest. In this study, to elucidate the formative mechanism of GC-induced cataracts, we examined the preventive effect and then analysed the mechanisms of thyroxine on GC-induced cataract formation. Fifteen day old chick embryos were administered with 0.25μ mol hydrocortisone succinate sodium (HC), and then 12–30nmol of thyroxine 4hr after HC administration. At the indicated time after HC treatment, we examined the incidence of cataract formation, the levels of serum glucose and lipids, lenticular and hepatic glutathione (GSH), and lipid peroxide (LPO) in the lens, blood and liver. Almost all lenses (96%) removed 48hr after HC administration were opaque. Thyroxine prevented HC-induced cataract formation effectively, and suppressed the elevations of serum glucose and LPO in the lens, blood and liver. The treatment prevented the decreased lenticular GSH level at 48hr, but the hepatic GSH level at 24hr remained lowered in contrast to the results of previous studies using insulin. Moreover, thyroxine did not decrease the elevated serum lipid level (triglyceride and non-esterified fatty acid) caused by HC. Under thyroxine treatment, in constant to insulin, acceleration of GSH-GSSG cycle rather than GSH de novo synthesis keeps a certain level of hepatic GSH necessary for diminishing the elevation of LPO as a risk factor of GC-induced cataract formation. The regulation of metabolic changes ensured the maintenance of hepatic GSH level, which is necessary to reduce oxidative stress produced by GC and to protect the lens from oxidative stress leading to opacification.