Regulation of glucocorticoids of arginase and argininosuccinate synthetase in cultured rat hepatoma cells.

We have examined and characterized the regulation by glucocorticoids of the levels of arginase and argininosuccinate synthetase in two rat hepatoma cell lines (H4-II-E-C3 and MH1C1). Hydrocortisone elevates the activity of both enzymes in a time- and dose-dependent fashion. This effect was blunted markedly by small amounts of ethanol (0.1 to 0.9% [v/v]) and blocked substantially by a high molar excess of the "anti-inducer" steroid fluoxymesterone. The other "optimal" inducers dexamethasone and corticosterone were as effective as hydrocortisone in elevating the levels of these enzymes at saturating concentrations. Inhibition of these stimulations by cycloheximide indicated that ongoing cellular protein synthesis was required for both effects, and the admixture of extracts from fully stimulated and basal cells gave no evidence for the existence of direct inhibitors or activators of either enzyme. The results corroborate findings from earlier whole-animal studies and provide evidence for the following conclusions. (i) This stimulation by hydrocortisone of urea-cycle enzymes in the cultured hepatoma cells is mediated by a classical glucocorticoid mechanism involving initial binding to specific cytoplasmic steroid receptors and the eventual accumulation of new enzyme molecules. (ii) These cell lines thus constitute valid experimental models for use in further detailed studies on the molecular mechanism(s) through which glucocorticoids and intermediary metabolites effect a selective modulation of arginase and argininosuccinate-synthetase gene expression in the differentiated mammalian liver.