Independent actions of asparagine and high levels of free Ca²⁺ in the induction of ornithine decarboxylase

During growth stimulation of cells, Ca²⁺ and amino acids of the A, ASC and N transport systems are important for the induction of ornithine decarboxylase (ODC, l-ornithine carboxylyase, EC 4.1.1.17). In order to clarify the relationship between Ca²⁺ and amino acids, we studied the induction of ODC by asparagine under three different Ca²⁺ states in H-35 rat hepatoma cells. First, in normal cells, extracellular Ca²⁺ above 0.1 mM and 10 mM asparagine separately stimulated ODC activity and their effects were approximately additive. In these normal cells, asparagine could act in the absence of medium Ca²⁺. TMB-8, a sequestered-Ca²⁺ release antagonist, had no effect on ODC induction whilst the asparagine action is sensitive to treatment with W₇, a Ca-calmodulin antagonist, or lanthanum, a Ca²⁺ antagonist. Secondly, in cells treated with 0.5 mM EGTA in Ca²⁺-free medium, the asparagine action on ODC induction was blocked but the inhibition could be reversed by the addition of Ca²⁺ to the medium. Thirdly, ionomycin treatment in the absence of medium Ca²⁺ did not block the asparagine effect. Furthermore, in ionomycin-treated cells, the presence of high levels of medium Ca²⁺ increased ODC activity, but this increase was additive to, and could not replace, the action of asparagine. Our results indicate that the asparagine action does not depend on an increase of intracellular free-Ca²⁺.