CHANGES IN LEVELS OF ARGININOSUCCINATE LYASE MESSENGER-RNA DURING INDUCTION BY GLUCAGON AND CYCLIC-AMP IN CULTURED FETAL-RAT HEPATOCYTES

During the perinatal period, the activity of the urea-cycle enzyme argininosuccinate lyase (ASL) is regulated by glucocorticoids, glucagon and insulin. In this study, the effects of glucagon and cyclic AMP (cAMP) analogues were examined on the synthesis of ASL and on the level of its corresponding mRNA in cultured foetal hepatocytes. Northern-blot analysis revealed that these agents only gave a transient induction of ASL mRNA amount, which reached a peak at 6 h and declined thereafter. This induction preceded the increase in enzyme activity and amount which could be observed for 2 or 3 days of culture. Stimulation of ASL mRNA accumulation by a combination of cAMP analogues and dexamethasone was additive, indicating that glucocorticoids and cAMP are both necessary to promote hepatocyte differentiation and that inductions could occur via independent pathways. Induction by cAMP analogues could be abolished by actinomycin D, suggesting a control mechanism at the transcriptional level. Puromycin was without effect on ASL mRNA induction by cAMP, indicating that no ongoing protein synthesis was required in the stimulation process.