Specific protein kinase C isozymes mediate the induction of keratinocyte differentiation markers by calcium

The maturation of epidermal keratinocytes is a tightly regulated, stepwise process which requires protein kinase C (PKC) activation. We investigated the effect of elevated extracellular Ca2+, a potent differentiation signal which increases cellular sn-1,2-diacylglycerol levels, on the PKC isozyme profile of cultured murine keratinocytes. Five PKC isozymes (alpha, delta, epsilon, zeta, and eta) were detected by immunoblotting. During Ca(2+)-induced differentiation, total cellular PKC alpha decreased, PKC epsilon and eta increased 3-5-fold, and the level of other PKC isozymes was relatively unchanged. PKC alpha underwent a progressive translocation from the soluble to the particulate fraction following elevation of extracellular Ca2+. The kinetics of PKC alpha translocation corresponded with the induction of keratinocyte differentiation markers. Both PKC delta and epsilon were selectively lost from the soluble fraction of keratinocytes exposed to elevated extracellular Ca2+, resulting in an increase in the proportion of these isoforms in the particulate fraction. PKC eta increased in both the soluble and particulate fractions, while PKC zeta did not change in amount or distribution during keratinocyte differentiation. Selective down-regulation of PKC isoforms by either 12-deoxyphorbol-13-phenylacetate or bryostatin 1 inhibited Ca(2+)-induced expression of differentiation markers at doses most specific for the down-regulation of PKC alpha. Taken together, these observations suggest that the induction of keratinocyte differentiation by Ca2+ results in the activation of specific PKC isozymes.