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 th at the eta isoform of PKC plays a crucial role in mediating differenti ation of keratinocytes, Activation of PKC is associated with its intra cellular translocation from the cytoplasm to the plasma membrane, foll owed by down-regulation through proteolytic cleavage of the PKC molecu les. In the present study, we demonstrated that the eta isoform of PKC is unique in that it is not translocated nor down-regulated upon stim ulation. The level of the eta isoform, assayed by immunoblotting, rema ined unchanged during the first 12 h and then increased slightly up to 24 h when treated with tumor promoters or activators of PKC in consti tutively expressing normal human keratinocytes. The activity of the et a isoform also remained unchanged after the 12-O-tetradecanoyl-phorbo1 -13-acetate treatment, as judged by binding to an ATP analog, autophos phorylation, and phosphorylation of an exogenous substrate. The alpha isoform of PKC, however, was rapidly down-regulated and was undetectab le by 6 h after the treatment, These observations were further confirm ed by immunohistochemical staining of normal and transiently expressin g the alpha isoform human keratinocytes COS1 cells. In addition, altho ugh rapidly translocated to the plasma membrane, the eta isoform remai ned in the cytoplasm.