Granulocyte colony-stimulating factor (G-CSF) supports the proliferation, differentiation and survival of myeloid cells by stimulating the activation of several signaling cascades including the serine/threonine kinase Akt pathway. Akt activation has been shown to be important for G-CSF-induced survival and granulocytic differentiation. Although significant progresses have been made in our understanding of the molecular mechanisms by which Akt is activated, much less is known about the signaling events that negatively regulate Akt activation. Interestingly, G-CSF-induced activation of Akt was completely inhibited when myeloid 32D cells transfected with the wild type G-CSF receptor were incubated with phorbol-12-myristate 13-acetate (PMA), a PKC activator. PMA-mediated inhibition of Akt activation occurred with 5 min and lasted at least 1 hour. Previously, it has been shown that a carboxyl terminally truncated G-CSF receptor (D715), whose expression is associated with the development of acute myeloid leukemia in patients with severe congenital neutropenia (SCN), mediates significantly prolonged Akt activation. Notably, Akt activation by G-CSF in 32D cells expressing the D715 receptor mutant was rapidly downregulated by PMA treatment. The inhibitory effect of PMA on Akt activation was abolished by pretreatment of cells with the specific PKC inhibitor GF109203X, suggesting that PKC-dependent pathway negatively regulates Akt activation. Ro-31-7549, a specific inhibitor of PKCe, also abrogated PMA-mediated inhibition of Akt activation whereas rottlerin and Go6976, inhibitors of PKCd and PKC a/bI,, respectively, displayed no effect. Together, these results identified PKCe as being critically involved in PMA-mediated inhibition of Akt activation. Experiments are currently under way to determine the mechanism by which PKCe downregulates Akt activation and the role of PKCe in the regulation of cell proliferation, differentiation and survival in response to G-CSF.