Regulation of Nerve Growth Factor Action on Nsp100 Phosphorylation in PC12h Cells by Calcium

Previous work from these laboratories has shown that in PC12 cells the phosphorylation of a specific soluble protein is decreased by treatment with nerve growth factor. This protein, designated Nsp100, and its kinase have been separated and partially purified from PC12 cells. The present studies have been designed to investigate the role of calcium in this action of nerve growth factor. It is shown here, using PC12h cells, that A23187, a calcium ionophore, and high levels of K+, a depolarizing stimulus, also decrease phosphorylation of Nsp100. Furthermore, the actions of nerve growth factor as well as those of A23187 and high levels of K+ are prevented by treatment of the cells with the calcium chelator EGTA. It is also shown that agents that raise levels of cyclic AMP in the cells, specifically dibutyryl cyclic AMP and cholera toxin, also decrease phosphorylation of Nsp100 but, in addition, increase phosphorylation of tyrosine hydroxylase. The action of these latter agents on Nsp100 is blocked by EGTA, but their action on tyrosine hydroxylase is not, indicating that even agents such as cholera toxin act on Nsp100 through a Ca2+-dependent mechanism.