Absence of Down-Regulation and Translocation of The η Isoform of Protein Kinase C in Normal Human Keratinocytes

Among 11 isoforms of protein kinase C (PKC), we previously reported that the η isoform of PKC plays a crucial role in mediating differentiation of keratinocytes. Activation of PKC is associated with its intracellular translocation from the cytoplasm to the plasma membrane, followed by down-regulation through proteolytic cleavage of the PKC molecules. In the present study, we demonstrated that the η isoform of PKC is unique in that it is not translocated nor down-regulated upon stimulation. The level of the η; isoform, assayed by immunoblotting, remained unchanged during the first 12h and then increased slightly up to 24h when treated with tumor promoters or activators of PKC in constitutively expressing normal human keratinocytes. The activity of the η isoform also remained unchanged after the 12-O-tetradecanoyl-phorbol-13-acetate treatment, as judged by binding to an ATP analog, autophosphorylation, and phosphorylation of an exogenous substrate. The a isoform of PKC, however, was rapidly down-regulated and was undetectable by 6h after the treatment. These observations were further confirmed by immunohistochemical staining of normal human keratinocytes and transiently expressing COS1 cells. In addition, although the α isoform rapidly translocated to the plasma membrane, the η isoform remained in the cytoplasm