FUNCTIONAL EVIDENCE FOR AN EXTRACELLULAR CALCIUM RECEPTOR MECHANISM TRIGGERING TYROSINE KINASE ACTIVATION-ASSOCIATED WITH MOUSE KERATINOCYTE DIFFERENTIATION

Calcium-induced keratinocyte differentiation is associated with tyrosi ne phosphorylation of a p62 protein which associates with the ras-GTPa se activating protein (GAP). We have examined the nature of the cal du m signal triggering p62 phosphorylation. EGTA, a specific chelator of calcium, was able to completely block calcium induced p62 phosphorylat ion, even after using conditioned medium from calcium-treated keratino cytes. Preventing calcium-induced cell-cell contacts by anti-cadherin antibodies did not inhibit tyrosine phosphorylation. Slight increases in extracellular calcium concentrations (0.15 or 0.30 mM) were already sufficient to induce p62 phosphorylation. Other divalent cations, suc h as magnesium, zinc, nickel, and cobalt, but not the trivalent cation lanthanum, induced p62 phosphorylation to a similar extent as calcium . There was no close correlation between the ability of the various io ns to induce p62 phosphorylation and increase free intracellular calci um. Similarly, treatment of primary keratinocytes with the calcium ion ophores A23187 or X537A did not induce p62 phosphorylation, although i t increased free intracellular calcium levels. Finally, blockers of po tassium uptake, which is induced by calcium, did not inhibit p62 phosp horylation. Thus, in keratinocyte differentiation, calcium is likely t o provide the primary signal for p62 tyrosine phosphorylation and may act directly at the cell membrane through a ''cationic receptor mechan ism'' analogous to that described in other cell types.