Involvement of Protein Kinase Cε in the Negative Regulation of Akt Activation Stimulated by Granulocyte Colony-Stimulating Factor

Stimulation of cells with G-CSF activates multiple signaling cascades, including the serine/threonine kinase Akt pathway. We show in this study that G-CSF-induced activation of Akt in myeloid 32D was specifically inhibited by treatment with PMA, a protein kinase C (PKC) activator. PMA treatment also rapidly attenuated sustained Akt activation mediated by a carboxy truncated G-CSF receptor, expressed in patients with acute myeloid leukemia evolving from severe congenital neutropenia. The inhibitory effect of PMA was abolished by pretreatment of cells with specific PKC inhibitor GF109203X, suggesting that the PKC pathway negatively regulates Akt activation. Ro31-8820, a PKCε inhibitor, also abrogated PMA-mediated inhibition of Akt activation, whereas rottlerin and Go6976, inhibitors of PKCδ and PKCαβI, respectively, exhibited no significant effects. Furthermore, overexpression of the wild-type and a constitutively active, but not a kinase-dead, forms of PKCε markedly attenuated Akt activation, and inhibited the proliferation and survival of cells in response to G-CSF. The expression of PKCε was down-regulated with G-CSF-induced terminal granulocytic differentiation. Together, these results implicate PKCε as a negative regulator of Akt activation stimulated by G-CSF and indicate that PKCε plays a negative role in cell proliferation and survival in response to G-CSF.