The Synthesis and Degradation of Poly(A)‐containing mRNAs in Mouse Neuroblastoma Cells Treated with Dibutyryl cAMP or with Ro20‐1724 *

Abstract: The relative rates of synthesis and turnover of poly(A)‐containing mRNAs were determined in the S‐20 (cholinergic) and NIE‐115 (adrenergic) neuroblastoma clones after 3 days treatment with dibutyryl cAMP or the phos‐phodiesterase inhibitor N‐4‐(3‐butoxy‐4‐methoxybenzyl)‐2‐imidazolidinone (Ro20‐1724) in an attempt to correlate transcriptional and posttranscriptional changes with observed cAMP‐induced changes in the differentiated state. Treatment of S‐20 cells with dibutyryl cAMP for 3 days caused a fourfold increase in intracellular cAMP and a 30% decrease in growth rate compared with the levels in control cells. However, there was no difference in the synthesis of the poly(A)‐containing RNAs, relative to the ribosomal RNAs, after either 2‐ or 18‐h labeling with ³H‐adenosine. There was also no increase in the steady‐state mRNA levels. Treatment with Ro20‐1724 caused only a transient twofold increase in intracellular cAMP level when S‐20 cells were plated at a low density and no increase at higher densities although growth was inhibited by 60%. There was, again, no increase in the relative synthesis of poly(A)‐mRNAs with 2‐h label in cells treated with Ro20‐1724, but there was a twofold increase after labeling for 18 h. Optical density measurements, however, showed that this increased incorporation of ³H‐adenosine did not result in an increase in the absolute amount of unlabeled poly(A)‐containing mRNAs. NIE‐115 cells showed an 8‐fold increase in intracellular cAMP after 3 days of treatment with Ro20‐1724 and an 80% inhibition of growth rate. However, the relative rates of synthesis of the poly(A)‐containing mRNAs were identical to those obtained using S‐20 cells. These results show that neither increased intracellular cAMP levels nor growth inhibition necessarily result in a relative increase in the synthesis of the poly(A)‐containing mRNAs in these neuroblastoma clones. There was no change in the synthesis or processing of the poly(A) regions of the mRNAs in the S‐20 control and treated cells. In contrast to results with other cells, the poly(A) profiles were heterogeneous, even after 1 hour of labeling. The average size of the poly(A) region decreased similarly in control and drug‐treated cells after 1, 2 and 18 h labeling.