Autophagic degradation of the BCR-ABL oncoprotein and generation of antileukemic responses by arsenic trioxide.

We provide evidence that arsenic trioxide (As(2)O(3)) targets the BCR-ABL oncoprotein via a novel mechanism involving p62/SQSTM1-mediated localization of the oncoprotein to the autolysosomes and subsequent degradation mediated by the protease cathepsin B. Our studies demonstrate that inhibitors of autophagy or cathepsin B activity and/or molecular targeting of p62/SQSTM1, Atg7, or cathepsin B result in partial reversal of the suppressive effects of AS(2)O(3) on BCR-ABL expressing leukemic progenitors, including primitive leukemic precursors from chronic myelogenous leukemia (CML) patients. Altogether, these findings indicate that autophagic degradation of BCR-ABL is critical for the induction of the antileukemic effects of As(2)O(3) and raise the potential for future therapeutic approaches to target BCR-ABL expressing cells by modulating elements of the autophagic machinery to promote BCR-ABL degradation.

Critical roles for mTORC2- and rapamycin-insensitive mTORC1-complexes in growth and survival of BCR-ABL-expressing leukemic cells.

mTOR-generated signals play critical roles in growth of leukemic cells by controlling mRNA translation of genes that promote mitogenic responses. Despite extensive work on the functional relevance of rapamycin-sensitive mTORC1 complexes, much less is known on the roles of rapamycin-insensitive (RI) complexes, including mTORC2 and RI-mTORC1, in BCR-ABL-leukemogenesis. We provide evidence for the presence of mTORC2 complexes in BCR-ABL-transformed cells and identify phosphorylation of 4E-BP1 on Thr37/46 and Ser65 as RI-mTORC1 signals in primary chronic myelogenous leukemia (CML) cells. Our studies establish that a unique dual mTORC2/mTORC1 inhibitor, OSI-027, induces potent suppressive effects on primitive leukemic progenitors from CML patients and generates antileukemic responses in cells expressing the T315I-BCR-ABL mutation, which is refractory to all BCR-ABL kinase inhibitors currently in clinical use. Induction of apoptosis by OSI-027 appears to negatively correlate with induction of autophagy in some types of BCR-ABL transformed cells, as shown by the induction of autophagy during OSI-027-treatment and the potentiation of apoptosis by concomitant inhibition of such autophagy. Altogether, our studies establish critical roles for mTORC2 and RI-mTORC1 complexes in survival and growth of BCR-ABL cells and suggest that dual therapeutic targeting of such complexes may provide an approach to overcome leukemic cell resistance in CML and Ph+ ALL.

Protein kinase R as mediator of the effects of interferon (IFN) gamma and tumor necrosis factor (TNF) alpha on normal and dysplastic hematopoiesis.

IFNγ and TNFα are potent inhibitors of hematopoiesis and have been implicated in the pathophysiology of bone marrow failure and myelodysplastic syndromes (MDS). We examined the role of protein kinase R (PKR) in the generation of the inhibitory effects of these myelosuppressive cytokines on hematopoiesis. Our data demonstrate that PKR is rapidly phosphorylated/activated in response to engagement of IFNγ or TNFα receptors in normal human hematopoietic progenitors. Such engagement of PKR is important for the suppressive effects of these cytokines on normal hematopoiesis. Pharmacological targeting of PKR using a specific inhibitor or siRNA-mediated PKR knockdown results in partial reversal of the suppressive effects of IFNγ and TNFα on normal human CD34+-derived myeloid (colony-forming unit-granulocyte-monocytic) and erythroid (burst-forming unit-erythroid) progenitors. Importantly, inhibition of PKR activity or expression increases hematopoietic colony formation from human MDS progenitors, suggesting that drugs that target PKR may provide a novel approach for the treatment of MDS and marrow failure syndromes. Altogether, our data establish that beyond its key role in the induction of IFN-antiviral responses, PKR plays important roles in signaling for IFNγ and other myelosuppressive cytokine receptors as a common mediator of signals for hematopoietic suppression.

Autophagy is a critical mechanism for the induction of the antileukemic effects of arsenic trioxide.

Arsenic trioxide (As(2)O(3)) exhibits potent antitumor effects in vitro and in vivo, but the precise mechanisms by which it generates such responses are not well understood. We provide evidence that As(2)O(3) is a potent inducer of autophagy in leukemia cells. Such induction of autophagy by As(2)O(3) appears to require activation of the MEK/ERK pathway but not the AKT/mammalian target of rapamycin or JNK pathways. In efforts to understand the functional relevance of arsenic-induced autophagy, we found that pharmacological inhibitors of autophagy or molecular targeting of beclin 1 or Atg7 results in reversal of the suppressive effects of As(2)O(3) on leukemic cell lines and primary leukemic progenitors from acute myelogenous leukemia patients. Altogether, our data provide direct evidence that autophagic cell death is critical for the generation of the effects of As(2)O(3) on acute myelogenous leukemia cells and raise the potential of modulation of elements of the autophagic machinery as an approach to enhance the antitumor properties of As(2)O(3) and possibly other heavy metal derivatives.

Arsenic trioxide-dependent activation of thousand-and-one amino acid kinase 2 and transforming growth factor-beta-activated kinase 1.

Arsenic trioxide (As(2)O(3)) has potent antileukemic properties in vitro and in vivo, but the mechanisms by which it generates its effects on target leukemic cells are not well understood. Understanding cellular mechanisms and pathways that are activated in leukemic cells to control the generation of As(2)O(3) responses should have important implications in the development of novel approaches using As(2)O(3) for the treatment of leukemias. In this study, we used immunoblotting and immune complex kinase assays to provide evidence that the kinases thousand-and-one amino acid kinase 2 (TAO2) and transforming growth factor-beta-activated kinase 1 (TAK1) are rapidly activated in response to treatment of acute leukemia cells with As(2)O(3). Such activation occurs after the generation of reactive oxygen species and regulates downstream engagement of the p38 mitogen-activated protein kinase. Our studies demonstrate that siRNA-mediated knockdown of TAO2 or TAK1 or pharmacological inhibition of TAK1 enhances the suppressive effects of As(2)O(3) on KT-1-derived leukemic progenitor colony formation and on primary leukemic progenitors from patients with acute myelogenous leukemia. These results indicate key negative-feedback regulatory roles for these kinases in the generation of the antileukemic effects of As(2)O(3). Thus, molecular or pharmacological targeting of these kinases may provide a novel approach to enhance the generation of arsenic-dependent antileukemic responses.

IRF8 directs stress-induced autophagy in macrophages and promotes clearance of Listeria monocytogenes.

Autophagy, activated by many stresses, plays a critical role in innate immune responses. Here we show that interferon regulatory factor 8 (IRF8) is required for the expression of autophagy-related genes in dendritic cells. Furthermore in macrophages, IRF8 is induced by multiple autophagy-inducing stresses, including IFNγ and Toll-like receptor stimulation, bacterial infection, starvation and by macrophage colony-stimulating factor. IRF8 directly activates many genes involved in various steps of autophagy, promoting autophagosome formation and lysosomal fusion. Consequently, Irf8(-/-) macrophages are deficient in autophagic activity, and excessively accumulate SQSTM1 and ubiquitin-bound proteins. We show that clearance of Listeria monocytogenes in macrophages requires IRF8-dependent activation of autophagy genes and subsequent autophagic capturing and degradation of Listeria antigens. These processes are defective in Irf8(-/-) macrophages where uninhibited bacterial growth ensues. Together these data suggest that IRF8 is a major autophagy regulator in macrophages, essential for macrophage maturation, survival and innate immune responses.

Resveratrol enhances the suppressive effects of arsenic trioxide on primitive leukemic progenitors.

Efforts to enhance the antileukemic properties of arsenic trioxide are clinically relevant and may lead to the development of new therapeutic approaches for the management of certain hematological malignancies. We provide evidence that concomitant treatment of acute myeloid leukemia (AML) cells or chronic myeloid leukemia (CML) cells with resveratrol potentiates arsenic trioxide-dependent induction of apoptosis. Importantly, clonogenic assays in methylcellulose demonstrate potent suppressive effects of the combination of these agents on primitive leukemic progenitors derived from patients with AML or CML. Taken together, these findings suggest that combinations of arsenic trioxide with resveratrol may provide an approach for targeting of early leukemic precursors and, possibly, leukemia initiating stem cells.

Autophagy is a survival mechanism of acute myelogenous leukemia precursors during dual mTORC2/mTORC1 targeting.

PURPOSE: To examine whether induction of autophagy is a mechanism of leukemic cell resistance to dual mTORC1/mTORC2 inhibitors in acute myelogenous leukemia (AML) leukemic progenitors. EXPERIMENTAL DESIGN: Combinations of different experimental approaches were used to assess induction of autophagy, including immunoblotting to detect effects on LC3II and p62/SQTM1 expression and on ULK1 phosphorylation, immunofluorescence, and electron microscopy. Functional responses were assessed using cell viability and apoptosis assays, and clonogenic leukemic progenitor assays in methylcellulose. RESULTS: We provide evidence that treatment of AML cells with catalytic mTOR inhibitors results in induction of autophagy, which acts as a regulatory mechanism to promote leukemic cell survival. Such induction of autophagy by dual mTORC1/mTORC2 inhibitors partially protects primitive leukemic precursors from the inhibitory effects of such agents and limits their activities. Simultaneous blockade of the autophagic process using chloroquine or by knockdown of ULK1 results in enhanced antileukemic responses. CONCLUSIONS: Dual targeting of mTORC2 and mTORC1 results in induction of autophagy in AML cells. Combinations of catalytic mTOR targeting agents and autophagy inhibitors may provide a unique approach to target primitive leukemic precursors in AML.

BCR-ABL1-induced leukemogenesis and autophagic targeting by arsenic trioxide.

We have recently shown that arsenic trioxide (As 2O 3) is a potent inducer of autophagic degradation of the BCR-ABL1 oncoprotein, which is the cause of chronic myeloid leukemia (CML) and Ph+ acute lymphoid leukemia (Ph+ ALL). Our recently published work has shown that pharmacological inhibition of autophagy or molecularly targeting of elements of the autophagic machinery partially reverses the suppressive effects of As 2O 3 on primitive leukemic precursors from CML patients. Altogether, our studies have provided direct evidence that arsenic-induced, autophagy-mediated, degradation of BCR-ABL1 is an important mechanism for the generation of the effects of As 2O 3 on BCR-ABL1 transformed leukemic progenitors. These studies raise the potential of future clinical-translational efforts employing combinations of arsenic trioxide with autophagy-modulating agents to promote elimination of early leukemic progenitors and, possibly, leukemia-initiating stem cells.

Regulation of the kinase RSK1 by arsenic trioxide and generation of antileukemic responses.

Arsenic Trioxide (As2O3) is one of the most effective agents in the treatment of acute promyelocytic leukemia (APL), but has no significant efficacy in other forms of AML. The mechanisms of relative resistance of non-APL cells are not well understood, but emerging evidence suggests that activation of negative feedback regulatory loops and pathways contributes to such resistance. We provide evidence that a signaling cascade involving the kinase RSK1 is engaged in a negative feedback manner during arsenic-treatment of cells and exhibits regulatory effects on growth and survival of AML cells in response to treatment with As2O3. Our data demonstrate that pharmacological inhibition or molecular disruption of expression of RSK1 enhances As2O3-dependent apoptosis and/or growth inhibition of AML cells. Importantly, combination of a pharmacological inhibitor of RSK and As2O3 results in enhanced suppression of primary AML leukemic progenitors. Altogether, our findings suggest an important regulatory role for RSK1 in the generation of the effects of As2O3 in AML cells. They also raise the potential of RSK1 targeting in combination with As2O3 as a novel approach to promote antileukemic responses.

Arsenic trioxide and the phosphoinositide 3-kinase/akt pathway in chronic lymphocytic leukemia.

Simultaneous targeting of the phosphoinositide 3-kinase (PI3K)/Akt pathway increases arsenic trioxide (ATO)-dependent cytotoxicity of chronic lymphocytic leukemia (CLL) cells, whereas it has no significant effects on normal lymphocytes. Combinations of ATO with small molecules that target PI3K and/or Akt may provide a novel approach for the treatment of CLL.