Independent induction of tyrosine aminotransferase activity by dexamethasone and glucagon in isolated rat liver parenchymal cells in suspension and in monolayer culture in serum-free media

Abstract

The regulation of tyrosine aminotransferase (TAT) activity by dexamethasone and glucagon in isolated parenchymal cells from adult rat liver has been studied. The characteristics of TAT induction were compared in cells isolated from a single rat liver and used immediately in suspension culture and after 48 h in monolayer culture. TAT activity was induced by dexamethasone and by glucagon, alone and in combination, in cells under both incubation conditions in a serum-free medium. Each hormone induced a higher level of TAT activity in suspension cultures than in monolayers in absolute terms. However, since control TAT activity was considerably lower in monolayers, the inducibility of TAT activity appears to be greater in monolayer cultures. The K a for TAT induction by dexamethasone (2 × 10 −7 M) in monolayers was about 100-fold higher than that in suspension cultures, whereas that for glucagon (10 −8–10 −9 M) was essentially unchanged. Only in monolayers was there a good correlation between the level of TAT activity and of cAMP induced by glucagon. The induction of maximum TAT activity by glucagon in suspension cultures appeared to require considerably higher cAMP levels than in monolayers. Dexamethasone and glucagon interacted synergistically in TAT induction in both suspensions and monolayers. The data indicate that the characteristics of enzyme induction in isolated hepatocytes may vary according to whether the cells are maintained as fresh suspensions or as monolayer cultures.