the nine AML patients were examined by Western analysis . Patients 1, 2, 4, and 7 exhibited robust phosphorylation of PKC a and/or PKC b. Interestingly, three of these patients showed the least sensitivity to enzastaurin as indicted by Annexin V staining . While the significance of expression of these PKC isoforms and sensitivity to the drug is not clear, the data does suggest that the Triciribine drug can be effective at killing blast cells in a subset of AML patients and sensitivity might be influenced by PKC activity. DISCUSSION The cPKC family members PKC a and PKC b play important roles in promoting cell survival and chemoresistance in leukemia cells . PKC b has been shown to be important in the development of colon cancer and plays key roles in B cell development.
The kinase is important in B cell receptor and Notch signaling and thus likely plays a role in lymphoid cancers . It was logical that enzastaurin as a PKC b inhibitor would be used as an anti neoplastic agent for B cell lymphomas and it is not surprising Rocuronium clinical trial that the drug has shown promise in treating this disease . While there is a well founded rationale for using enzastaurin in treating lymphoid malignancies, the question arises whether targeting PKC b in myeloid diseases would be similarly effective. Gene expression of PKC b is significantly elevated in a number of leukemia cell lines and in blast cells from newly diagnosed AML patients compared to counterpart cells from normal bone marrow donors . Enzastaurin promoted apoptosis in two AML derived cell lines and primary AML blast cells but only at concentrations well above the IC50 for the suppression of PKC b.
Consistent with the notion that the drug acted on a target other than PKC b, a specific PKC b inhibitor from Calbiochem showed no toxicity to OCI AML3 cells and the drug did not affect toxicity of enzastaurin . A recent article by Bcr-Abl inhibitor cancer Meng et al. suggest that inhibition of PKC b by means including the use of enzastaurin supports death ligand induced apoptosis in AML and other leukemia cells. Furthermore, that study found that phorbol ester mediated protective effect against death ligand induced apoptosis in leukemia cells appears to require PKC b . Perhaps PKC b plays a greater role in modulating the extrinsic Rolipram solubility apoptotic pathway in AML cells.
Considering that enzastaurin is effective at inhibiting other PKC isoforms at concentrations used in clinical trials , the drug may be effective at suppressing PKC kinases that are relevant for AML cell survival such as PKC a . PKC a suppresses apoptosis induced liberty by HDAC inhibitors in a variety of multidrug resistant leukemia cells . IL 3 supports PKC a activation and promotes cell survival by mechanisms involving a number of pro survival molecules such as AKT and BCL2 . PKC a is emerging as an important prognostic factor in AML, particularly in the context of BCL2 and other pro survival kinases . The cPKC inhibitor Go6976 has been found to be effective at killing AML cells though it cannot be ruled out that inhibition of non PKC kinases may be involved . Enzastaurin had little effect on PKC b phosphorylation or PKC b localization . The drug, however, was effective at suppressing PKC a phosphorylation and PKC a membrane localization , and the drug prevented phosphorylation of the PKC .