Targeting PKC-Mediated Signal Transduction Pathways Using Enzastaurin to Promote Apoptosis in Acute Myeloid Leukemia-Derived Cell Lines and Blast Cells

Recent studies in acute myeloid leukemia (AML) suggest activation of pro-proliferative signaling cascades including those mediated by protein kinase C (PKC) represent a poor prognostic factor for patients. The classical PKC isoforms a and beta generally support survival signaling and have emerged as important targets for anti-cancer therapy. Enzastaurin is a PKC beta inhibitor and is in clinical trials for lymphomas, gliomas, and lung cancer. Presently, it is not known if enzastaurin could be effective against AML. In the current study, we found that high dose enzastaurin was found to promote apoptosis in the AML-derived cell lines and in blast cells from AML patients. The mechanism of cell death, however, likely does not involve PKC beta as another PKC beta inhibitor was not toxic to AML cell lines and did not promote enzastaurin-induced cell killing. While enzastaurin is fairly specific for PKC beta, the agent can inhibit other PKC isoforms at higher concentrations. Enzastaurin was effective at inhibiting PKC alpha phosphorylation and membrane localization in the AML cell lines and suppressed phosphorylation of BCL2. Furthermore, enzastaurin suppressed activation of ERK (which can be activated by PKC alpha). Analysis of the serine/threonine phosphorylation profile in HL60 cells after enzastaurin treatment revealed that the drug inhibits the phosphorylation of a distinct set of proteins while promoting phosphorylation of another set of proteins. This suggests the drug may regulate multiple signaling pathways. Taken together, these findings suggest that enzastaurin could be effective in the therapy of AML. J. Cell. Biochem. 112: 1696-1707, 2011. (C) 2011 Wiley-Liss, Inc.