Enzyme regulation in neuroblastoma cells in a salts/glucose medium: Induction of ornithine decarboxylase by asparagine and glutamine

Abstract

L-Asparagine is necessary and sufficient for the maximal induction of ornithine decarboxylase (ODC) (L-ornithine carboxy-lyase, EC 4.1.1.17) activity in confluent N18 mouse neuroblastoma cells in a salts/glucose medium; L-asparagine also induces maximal ODC activity when added to a tissue culture medium. L-Glutamine is about one-half as effective as asparagine. Cholera toxin and agents that are known to raise intracellular cyclic AMP concentrations have no effect on the induction of ODC activity unless suboptimal concentrations of asparagine are present in the salts/glucose medium. Whereas actinomycin D does not inhibit induction of ODC activity by asparagine, it inhibits the induction of ODC activity in association with cyclic AMP. In the salts/glucose medium, the rate of loss of ODC activity following the inhibition of protein synthesis by cycloheximide or puromycin depends upon the presence or absence of asparagine; loss is rapid only in the absence of asparagine and does not appear to be related to the inhibition of protein synthesis. These results are discussed in the context that the overlay of the growth medium tends to mask the minimal requirements for enzyme induction, because the composition of the medium defines: (a) the requirements for the induction of ODC activity; (b) the effect, or lack of effect, of cyclic AMP (and of inducers of intracellular cyclic AMP) on the induction of ODC activity; (c) the effect, or lack of effect, of actinomycin D on the induction of ODC activity; and (d) the action of puromycin and of cycloheximide on the rate of loss of ODC activity. It will be interesting to determine whether these results are uniquely applicable to ODC, whether many of the reactions attributed to cyclic AMP in the literature may be mediated by asparagine and glutamine, and whether actinomycin D, cycloheximide, and puromycin can be relied upon to differentiate between transcriptional and post-transcriptional control.