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