Stimulation of protein kinase C during Ca(2+)-induced keratinocyte differentiation. Selective blockade of MARCKS phosphorylation by calmodulin.

Raising the external Ca2+ concentration from 0.05 to 1.8 mM stimulated membrane-associated protein kinase Cs (PKCs) activity as strongly as the specific PKCs activator, 12-O-tetradecanoyl phorbol-13-acetate (TPA) in BALB/MK mouse keratinocytes. This was indicated by the increased phosphorylation of a PKC-selective peptide substrate, Ac-FKKSFKL-NH2, by membranes isolated from the Ca(2+)- or TPA-stimulated keratinocytes. Raising the external Ca2+ concentration to 1.8 mM also triggered a 4-fold rise in the intracellular free Ca2+ concentration. As reported elsewhere (Moscat, J. Fleming, T. P., Molloy, C. J. Lopez-Barahona, M., and Aaronson, S. A. (1989) J. Biol. Chem. 264, 11228-11235), TPA stimulated the phosphorylation of the PKCs substrate, the 85-kDa myristoylated alanine-rich kinase C substrate (MARCKS) protein, in intact keratinocytes, but Ca2+ did not. Furthermore, Ca(2+)-pretreatment reduced the TPA-induced phosphorylation of the 85-kDa protein in intact cells. There was no significant increase in MARCKS phosphorylation when keratinocytes were treated with a Ca2+.CaM-dependent phosphatase inhibitor, cyclosporin A, before stimulation with 1.8 mM Ca2+.Ca2+.calmodulin suppressed the ability of isolated membranes to phosphorylate the 85-kDa MARCKS holoprotein in vitro in the presence of phosphatase inhibitors such as fluoride, pyrophosphate, and vanadate, and this inhibition was overcome by a calmodulin antagonist, the calmodulin-binding domain peptide. Thus, the ability of 1.8 mM Ca2+ to strongly stimulate the membrane PKCs activity without stimulating the phosphorylation of the MARCKS protein in keratinocytes is consistent with the possibility of Ca2+.calmodulin complexes, formed by the internal Ca2+ surge, binding to, and blocking the phosphorylation of, this PKC protein substrate.