Induction of Ornithine Decarboxylase in Guinea Pig Lymphocytes by the Divalent Cation Ionophore A23187 and Phytohemagglutinin

Abstract

Ornithine decarboxylase activity increased rapidly in guinea pig lymphocytes and reached a peak 5 h after the addition of A23187 to the cell culture. The optimum dose of A23187 for the induction of ornithine decarboxylase was similar to that for the stimulation of DNA synthesis. Induction of ornithine decarboxylase by A23187 or phytohemagglutinin was inhibited by the addition of ethylene glycol bis(β-aminoethylether)-N,N-tetraacetic acid to the cell cultures and the inhibition was overcome by the addition of CaCl2, but not MgCl2. The effects of phytohemagglutinin on Ca2+ influx and efflux were less than those of A23187. In contrast, phytohemagglutinin was more potent than A23187 in stimulating ornithine decarboxylase activity. These results suggest that Ca2+ is necessary to induce ornithine decarboxylase, but the extent of increase of cytosolic Ca2+ concentration does not necessarily coincide with that of ornithine decarboxylase activity. Treatment with low concentrations of cytochalasin B inhibited A23187-induced ornithine decarboxylase activity, but not phytohemagglutinin-induced activity. The addition of di-butyryl cAMP or cholera toxin to cell cultures inhibited phytohemagglutinin-induced ornithine decarboxylase activity, while it rather enhanced A23187-induced activity. Phytohemagglutinin and A23187 in combination failed to induce ornithine decarboxylase activity more effectively than phytohemagglutinin alone, but in combination with lipopolysaccharide, A23187 had an additive effect. These results suggest that there are substantial differences between A23187 and phytohemagglutinin as regards the mechanism of induction of ornithine decarboxylase, although both mitogens act on the same lymphocyte population.