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

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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 1/2 as effective as asparagine. Cholera toxin and agents that are known to raise intracellular cyclic [c]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 cAMP. In the salts/glucose medium, the rate of loss of ODC activity following inhibition of protein synthesis by cycloheximide or puromycin depends on the presence or absence of asparagine. Loss is rapid only in the absence of asparagine and does not appear to be related to the inhbition 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: requirements for the induction of ODC activity; the effect, or lack of effect, of cAMP (and inducers of intracellular cAMP) on the induction of ODC activity; the effect, or lack of effect, of actinomycin D on the induction of ODC activity and the action of puromycin and 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 cAMP in the literature may be mediated by asparagine and glutamine and whether actinomycin D, cycloheximide and puromycin can be relied on to differentiate between transcriptional and post transcriptional control.