Control of glutamine synthetase messenger RNA by hydrocortisone in the embryonic chick retina

Abstract

Hydrocortisone (HC) and related glucocorticoids elicit an age-dependent induction of glutamine synthetase (GS) in the embryonic chick retina. We show here that HC has no effect either on the rate of synthesis of GS or on the low intracellular level of GS mRNA in the younger preinducible (7-day) retina. In the inducible (12-day) retina, GS induction appears to be a primary effect of HC since an increase in the rate of synthesis of GS is detectable as early as 45-60 min after exposure to HC. When these retinas are cultured with HC, the rate of GS synthesis as well as the amount of GS mRNA in the polysomal and in the nonpolysomal cytoplasmic fractions increased progressively up to 10 hr in culture and declined subsequently. The increase in the level of polysomal GS mRNA is not due to loading of new GS mRNA into polysomes from the nonpolysomal fraction and is thus a result of an enhanced supply of GS mRNA from nuclei. These data are compatible with the hypothesis that, in the inducible retina, HC induces GS by initiating or accelerating the transcription of structural genes for GS and/or increasing the stability of GS mRNA in the nuclei. We conclude that the development of the age-related inducibility of GS in the embryonic retina is due to the ability of HC to evoke elevated levels of GS mRNA in the polysomes of the inducible retina, resulting from an increased supply of GS mRNA from the nucleus. The preinducible retina is insensitive to this particular effect.