In Y1 adrenocortical
tumor cells,
corticotropin (
ACTH),
cyclic AMP, and
8-bromoadenosine 3',5'-monophosphate (8BrcAMP) stimulated
ornithine decarboxylase activity (
L-ornithine carboxy-lyase, EC 4.1.1.17) and steroidogenesis. The concentrations required for half-maximal activation of
ornithine decarboxylase were 60 pM for
ACTH and 1 mM for 8BrcAMP; the concentrations required for half-maximal activation of steroidogenesis were 50 pM for
ACTH and 0.2 mM for 8BrcAMP.
Ornithine decarboxylase activity increased 1.5 hr after the addition of these agents, reached a maximum between 4 and 6 hr, and then declined. Mutant clones with impaired
ACTH-responsive
adenylate cyclase systems [
ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1]did not respond to
ACTH with increased
ornithine decarboxylase activity, but they responded normally to added
cyclic AMP. These results indicate that
adenylate cyclase and
cyclic AMP are necessary for the stimulation of
ornithine decarboxylase activity by
ACTH. In a series of Y1(Kin) mutants with altered
cyclic AMP-dependent protein kinase activities (
ATP:protein phosphotransferase, EC 2.7.1.37), the effects of
ACTH on
ornithine decarboxylase also were attenuated. These findings suggest that
cyclic AMP-dependent protein kinase also plays a necessary role in the stimulation of
ornithine decarboxylase activity by
ACTH. The effects of
ACTH on
ornithine decarboxylase in the Kin mutants, however, were quantitatively different from the effects on steroidogenesis and did not closely reflect the degree of defect in
cyclic AMP-dependent protein kinase activity. These differences suggest that the pathways of
ACTH action leading to stimulation of steroidogenesis and
ornithine decarboxylase activity diverge subsequent to activation of the
protein kinase.