Activation of a calcium-calmodulin-dependent protein kinase I cascade in PC12 cells

It has been observed that the activity of Ca²⁺-calmodulin (CaM)- dependent protein kinase I is enhanced up to 50-fold by its phosphorylation in vitro by a distinct CaM kinase I kinase (Lee, J. C., and Edelman, A.M. (1994) J. Biol. Chem. 269, 2158-2164). It has, however, been unclear whether this event represents an acute form of cellular regulation. We demonstrate here the phosphorylation and activation of CaM kinase I in PC12 pheochromocytoma cells in response to elevation of intracellular Ca²⁺. Treatment of PC12 cells with the Ca²⁺-ionophore, ionomycin, or with a depolarizing concentration of KCl, led to rapid, biphasic phosphorylation of CaM kinase I and to increases in CaM kinase I activity of 5.1- and 7.3-fold, respectively. Depolarization-induced activation of CaM kinase I was reduced by ~80% by blockade of Ca²⁺ influx through L-type voltage-dependent Ca²⁺ channels and completely abolished by removal of extracellular Ca²⁺. The ability of PC12 cell CaM kinase I to be phosphorylated and activated by purified CaM kinase I kinase in vitro was markedly reduced by prior depolarization of the cells, consistent with intracellular phosphorylation and activation of CaM kinase I by CaM kinase I kinase. These results demonstrate the existence in PC12 cells of a CaM kinase I cascade, the function of which may be to sensitize cells to signal-induced elevations of intracellular Ca²⁺.