SPECIFIC PROTEIN-KINASE-C ISOZYMES MEDIATE THE INDUCTION OF KERATINOCYTE DIFFERENTIATION MARKERS BY CALCIUM

The maturation of epidermal keratinocytes is a tightly regulated, step wise process which requires protein kinase C (PKC) activation. We inve stigated the effect of elevated extracellular Ca2+, a potent different iation signal which increases cellular sn-1,2-diacylglycerol levels, o n the PKC isozyme profile of cultured murine keratinocytes. Five PKC i sozymes (alpha, delta, epsilon, xi, and eta) were detected by immunobl otting. During Ca2+-induced differentiation, total cellular PKC alpha decreased, PKC epsilon and eta increased 3-5-fold, and the level of ot her PKC isozymes was relatively unchanged. PKC alpha underwent a progr essive translocation from the soluble to the particulate fraction foll owing elevation of extracellular Ca2+. The kinetics of PKC alpha trans location corresponded with the induction of keratinocyte differentiati on markers. Both PKC delta and epsilon were selectively lost from the soluble fraction of keratinocytes exposed to elevated extracellular Ca 2+, resulting in an increase in the proportion of these isoforms in th e particulate fraction. PKC eta increased in both the soluble and part iculate fractions, while PKC xi did not change in amount or distributi on during keratinocyte differentiation. Selective down-regulation of P KC isoforms by either 12-deoxyphorbol-13-phenylacetate or bryostatin 1 inhibited Ca2+-induced expression of differentiation markers at doses most specific for the down-regulation of PKC alpha. Taken together, t hese observations suggest that the induction of keratinocyte different iation by Ca2+ results in the activation of specific PKC isozymes.