ABSTRACT
The serine/threonine kinase mammalian target of rapamycin (mTOR) is crucial for cell growth and proliferation, and is constitutively activated in primary acute myeloid leukemia (AML) cells, therefore representing a major target for drug development in this disease. We show here that the specific mTOR kinase inhibitor AZD8055 blocked mTORC1 and mTORC2 signaling in AML. Particularly, AZD8055 fully inhibited multisite eIF4E-binding protein 1 phosphorylation, subsequently blocking protein translation, which was in contrast to the effects of rapamycin. In addition, the mTORC1-dependent PI3K/Akt feedback activation was fully abrogated in AZD8055-treated AML cells. Significantly, AZD8055 decreased AML blast cell proliferation and cell cycle progression, reduced the clonogenic growth of leukemic progenitors and induced caspase-dependent apoptosis in leukemic cells but not in normal immature CD34+ cells. Interestingly, AZD8055 strongly induced autophagy, which may be either protective or cell death inducing, depending on concentration. Finally, AZD8055 markedly increased the survival of AML transplanted mice through a significant reduction of tumor growth, without apparent toxicity. Our current results strongly suggest that AZD8055 should be tested in AML patients in clinical trials.
Subject(s)
Leukemia, Myeloid, Acute/metabolism , Leukemia, Myeloid, Acute/prevention & control , Morpholines/pharmacology , Proteins/antagonists & inhibitors , Transcription Factors/antagonists & inhibitors , Adaptor Proteins, Signal Transducing/metabolism , Animals , Apoptosis/drug effects , Autophagy/drug effects , Blotting, Western , Cell Cycle/drug effects , Cell Cycle Proteins , Cell Proliferation/drug effects , Cells, Cultured , Humans , Immunoenzyme Techniques , Immunoprecipitation , Leukemia, Myeloid, Acute/mortality , Mechanistic Target of Rapamycin Complex 1 , Mice , Mice, Nude , Multiprotein Complexes , Phosphatidylinositol 3-Kinases/metabolism , Phosphoproteins/metabolism , Phosphorylation/drug effects , Proteins/metabolism , Survival Rate , TOR Serine-Threonine Kinases , Transcription Factors/metabolism , Treatment Outcome , Xenograft Model Antitumor AssaysABSTRACT
Stem cell factor (SCF) and erythropoietin are strictly required for preventing apoptosis and stimulating proliferation, allowing the differentiation of erythroid precursors from colony-forming unit-E to the polychromatophilic stage. In contrast, terminal maturation to generate reticulocytes occurs independently of cytokine signaling by a mechanism not fully understood. Terminal differentiation is characterized by a sequence of morphological changes including a progressive decrease in cell size, chromatin condensation in the nucleus and disappearance of organelles, which requires transient caspase activation. These events are followed by nucleus extrusion as a consequence of plasma membrane and cytoskeleton reorganization. Here, we show that in early step, SCF stimulates the Rho/ROCK pathway until the basophilic stage. Thereafter, ROCK-1 is activated independently of Rho signaling by caspase-3-mediated cleavage, allowing terminal maturation at least in part through phosphorylation of the light chain of myosin II. Therefore, in this differentiation system, final maturation occurs independently of SCF signaling through caspase-induced ROCK-1 kinase activation.
