Requirement of Prolonged Presence of a High Intracellular Level of Cyclic AMP for Induction of Serine Dehydratase in Primary Cultured Rat Hepatocytes

Abstract

The effects of glucagon, dexamethasone (Dex), and isoproterenol (Ip) on induction of serine dehydratase [EC 4. 2. 1. 13] in primary cultured hepatocytes were investigated. Both glucagon and Dex were necessary for induction of this enzyme. Activity started to increase after a 6-h lag period and reached a maximum 24 h after addition of the hormones. This lag period could be diminished by pretreating the cells with Dex alone for 12h and then adding glucagon. The reverse order of additions of hormones did not cause enzyme induction and the loss of induction was due to desensitization of response to glucagon at a step of cyclic AMP (cAMP) formation.
In the presence of Dex, glucagon induced serine dehydratase, whereas Ip did not. This was because glucagon could maintain a high level of cAMP in the cells for 6h, whereas Ip increased the cAMP level for only a short time, which was not sufficient to induce serine dehydratase. Thus, when the cells were treated with Ip plus 1-methyl-3-isobutylxanthine (MIX), the intracellular cAMP concentration remained high for over 6h and serine dehydratase was induced to the maximum level. Similarly, the active state of cAMP-dependent protein kinase [EC 2. 7. 1. 37] was maintained for over 6 h when the cells were treated with either glucagon or Ip plus MIX, but not with Ip alone. When cells were treated with various concentrations of glucagon, the intracellular concentration of cAMP at 3 h correlated well with the induced level of enzyme activity at 24 h after hormone treatment.
These results indicate that for induction of serine dehydratase, the action of glucocorticoid is a prerequisite, and that it is followed by a cAMP-dependent action of glucagon. Maintenance of a high cAMP level for a prolonged period is necessary for induction of this enzyme, unlike for the inductions of other enzymes such as tryptophan oxygenase [EC 1. 13. 11. 11] and tyrosine aminotransferase [EC 2. 6. 1. 5].