Effects of dexamethasone and dibutyryl cyclic AMP on polyamine synthesizing enzymes in mouse lymphoma cells

S49.1 Lymphoma cells were arrested in G₁ phase of the cell cycle when treated with either 1 μM dexamethasone (Dex) or 0.5 mM N⁶, O²-dibutyryl cyclic adenosine 3′ :5′ -monophosphate (Bt₂cAMP) plus 0.2 mM theophylline. However, the two agents had markedly different effects on aspects of polyamine and cyclic nucleotide metabolism within the arrested cells. Bt₂cAMP had an early and pronounced inhibitory effect on ornithine decarboxylase (ODC) activity causing a decrease to 40% of control within 1 h. However, there was no significant inhibition of ODC activity in the Dex-treated cells until 4 h of exposure, at which time ODC activity was reduced to approximately 60% of the control value. Sadenosyl-L-methionine decarboxylase (SAMD) activity was reduced by both agents, Bt₂cAMP having the more pronounced inhibitory effect. The activity of SAMD was reduced to 40% of control after 10 h of Dex, whereas Bt₂cAMP reduced the activity to approximately 25% of control within 4 h. Intracellular polyamine pools were decreased rapidly in Dex-treated cells but not in those exposed to Bt₂cAMP. Bt₂cAMP decreased the amount of type I (PK_I) and type II (PK_II) cyclic AMP-dependent protein kinase (cAMP-PK) activity to 30% of control or less within 2 h. In contrast, Dex had very little effect on either PK_I or PK_II until 24 h, when cell viability was affected. The specific activity of both PK_I and PK_II remained significantly decreased in cells exposed to Bt₂cAMP for 6 h and then resuspended in fresh medium. The rapid decrease in ODC activity in response to Bt₂cAMP and the slow recovery after washout may be due to the marked decreases in total PK_I and PK_II activities. Dex, which had no effect on PK_I and PK_II specific activities, only slowly inhibited ODC activity and recovery of enzyme activity was rapid upon resuspension in fresh medium. These data further stress the importance of the maintenance of the cellular protein kinase pools in the regulation of the recovery time to growth inhibition in response to naturally occurring steroids and second messengers.