Signal transduction of constitutively active protein kinase C epsilon

The protein kinase C (PKC) family is the most prominent target of tumor-promoting phorbol esters. For the PKCε isozyme, different intracellular localizations and oncogenic potential in several but not all experimental systems have been reported. To obtain information about PKCε-signaling, we investigated the effects of constitutively active rat PKCε (PKCεA/E, alanine 159 is replaced by glutamic acid) in HeLa cells in a doxycycline-inducible vector. Upon induction of PKCεA/E expression by doxycycline, the major part of PKCεA/E was localized to the Golgi. This led (i) to phosphorylations of PKCεS729, Elk-1S383, PDK1S241 and RbS807/S811, (ii) to elevated expression of receptor of activated C kinase 2 (RACK2) after 12 h, and (iii) increased colony formation in soft agar, increased cell migration and invasion, but not to decreased doubling time. Following induction of PKCεA/E-expression by doxycycline for 24 h and additional short-term treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA), PKCεA/E translocated to the plasma membrane and increased phosphorylation of MARCKSS152/156. Treatment with doxycycline/TPA or TPA alone increased phosphorylations of Elk-1S383, PDK1S241, RbS807/S811, PKCδT505, p38MAPKT180/Y182, MEK1/2S217/S221 and ERK2T185/T187. MARCKS was not phosphorylated after treatment with TPA alone, demonstrating that in this system it is phosphorylated only by PKCε localized to the plasma membrane but not by PKCα or δ, the other TPA-responsive PKC isozymes in HeLa cells. These results demonstrate that PKCε can induce distinctly different signaling from the Golgi and from the plasma membrane.