Activation of MAP2-kinase in B lymphocytes by calcium ionophores.

The role of increases in intracellular calcium levels on tyrosine phosphorylation in human B lymphocytes was studied. Stimulation of normal, resting B lymphocytes or B lymphoblastoid cells with the calcium ionophores ionomycin or A23187 induced the tyrosine phosphorylation and the enzymatic activation of microtubule-associated protein-2 kinase (MAP2-K). Treatment of these cells with PMA induced tyrosine phosphorylation of a protein with the identical mobility, as well as the enzymatic activation of MAP2-K. Stimulation of these cells with ionomycin also resulted in increased ribosomal S6 kinase activity. Activation of MAP2-K in B lymphocytes by calcium ionophore was rapid (detectable within 1 min), transient (returning to background levels by 45 min), and dependent on extracellular calcium. These results demonstrate that transmembrane calcium flux induced by calcium ionophore results in the tyrosine phosphorylation and enzymatic activation of MAP2-K in human B cells.