Protein Kinase Cμ Regulation of the JNK Pathway Is Triggered via Phosphoinositide-dependent Kinase 1 and Protein Kinase Cε*

Abstract

The protein kinase C (PKC)-related enzyme PKCμ/PKD (protein kinase D) is activated by activation loop phosphorylation through PKCη. Here we demonstrate that PKCμ is activated by the direct phosphorylation of PKCε. PKCμ colocalizes with PKCε in HEK293 and MCF7 cells as shown by confocal immunofluorescence analyses. PDK1, known as the upstream kinase for several PKC isozymes, associates intracellularly with PKCε and PKCη. PKCη is phosphorylated by PDK1 in vitro, leading to kinase activation as similarly reported for PKCε activation by PDK1. Coexpression of PDK1, PKCε and PKCμ in HEK293 cells results in PKCμ activation. In contrast, the coexpression of PDK1 and PKCη with PKCμ does not activate PKCη or consequently PKCμ. PDK1/PKCε-triggered activation of PKCμ inhibits JNK, a downstream effector of PKCμ, whereas upon transient expression of PDK1, PKCη, and PKCμ, JNK is not affected. These data implicate PKCε as the biologically important upstream kinase for PKCμ in HEK293 cells, regulating downstream effectors. Our results further indicate a PDK1/PKCη/PKCμ controlled negative regulation of PKCη kinase activity. In this study, we show that differentially activated kinase cascades involving PDK1 and novel PKC isotypes are responsible for the regulation of PKCμ activity and consequently inhibit the JNK pathway.