Differences in the mode of regulation of ornithine decarboxylase and tyrosine aminotransferase in H-35 rat hepatoma cells shown by varying the medium composition

Abstract

The effects of dibutyryl cyclic AMP, dexamethasone, and asparagine as inducers of ornithine decarboxylase (EC 4.1.1.17) and tyrosine aminotransferase (EC 2.6.1.5) in H-35 rat hepatoma cells were studied; a comparison was made between cells incubated in Eagle's minimal essential medium and cells incubated in Earle's balanced salt solution (salts/glucose solution). The addition of either 1 mM dibutyryl cyclic AMP or 1 μM dexamethasone increased both ornithine decarboxylase and tyrosine aminotransferase activities of H-35 hepatoma cells maintained in Eagle's minimal essential medium, whereas 10 mM asparagine increased the activity of ornithine decarboxylase and had no effect on tyrosine aminotransferase activity. The actions of dibutyryl cyclic AMP and dexamethasone but not asparagine, as inducers of ornithine decarboxylase activity, appeared to be dependent on the incubation medium used; dibutyryl cyclic AMP and dexamethasone were ineffective in increasing ornithine decarboxylase activity of H-35 hepatoma cells maintained in the salts/glucose solution, while asparagine was fully effective. Parallel studies on tyrosine aminotransferase demonstrated that both dibutyryl cyclic AMP and dexamethasone elicited maximal increases of tyrosine aminotransferase activity of H-35 hepatoma cells maintained in the salts/glucose solution. Experiments on the activation of cyclic AMP-dependent protein kinase and the uptake/binding of [ 3H]dexamethasone to whole cells revealed no significant difference between H-35 hepatoma cells maintained in Eagle's minimal essential medium and cells maintained in the salts/glucose solution. The half-lives of ornithine decarboxylase of cells maintained in minimal essential medium and salts/glucose solution were 45 and 38 min, respectively; the corresponding values for tyrosine aminotransferase were 2.5 and 1.5 h, respectively. It is proposed that the actions of dibutyryl cyclic AMP and dexamethasone as inducers of ornithine decarboxylase may involve the participation of component(s), possibly amino acids, present in the Eagle's minimal essential medium but not in the salts/glucose solution. These components may either mediate or modulate the actions of dibutyryl cyclic AMP and dexamethasone in the induction of ornithine decarboxylase activity.