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

Abstract

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 ca- IntroductionElements of the autophagic machinery have attracted recently considerable attention as a potential target for the development of novel approaches for the treatment of malignancies. 1,2 Similar to apoptosis, autophagy is a programmed cell death mechanism, but it is distinguished by a self-catabolic process involving lysosomal proteolytic degradation of cellular components. 3 This is initiated by the formation of a double-membrane enclosed structure, known as the autophagosome. 4 On fusion with a lysosome, a cellular organelle characterized by low pH and hydrolytic enzymes, 5,6 such structure eventually develops into the autophagolysosome where degradation of organelles occurs.Under different circumstances, autophagy can either inhibit or promote malignant cell survival, but its precise role in tumorigenesis remains to be established. 7,8 The role of inducible autophagy in BCR-ABL expressing leukemia cells is poorly understood. For example, there is previous evidence suggesting that autophagy may play regulatory roles in BCR-ABL leukemogenesis, 9 whereas other studies have shown that pharmacologic inhibition of autophagy enhances the effects of imatinib mesylate and other targeted therapies in CML. [10][11][12] There are also opposing lines of evidence, pointing toward tumor inhibitory effects of autophagy, 13,14 although a recent study demonstrated that BCR-ABL exerts suppressive effects on autophagy via engagement of the PI3K/FoxO4/ATF5/ mTOR pathway. 15 Arsenic trioxide (AS 2 O 3 ) exhibits potent antileukemic effects in vitro and in vivo and has major clinical activity in the treatment of patients with acute promyelocytic leukemia (APL). [16][17][18] This agent was previously shown to target and eliminate leukemia initiating stem cells (LICs) in mouse models in vivo via PML targeting. 19 Notably, there is evidence that AS 2 O 3 degrades BCR-ABL, 20 raising the possibility that this agent may provide an approach to target CML LICs. However, a major limiting factor for the incorporation of arsenic in non-APL malignancies has been the requirement of high concentrations for induction of cell death in non-APL cells and the incomplete understanding of the mechanisms by which it promotes antileukemic responses.A major mechanism contributing to the antineoplastic effects of arsenic is induction of apoptosis, 16-18 with upstream JNK activation being a prominent regulatory mechanism. 21 In a recent study, we provided evidence that arsenic trioxide induces autophagy in AML leukemic progenitors and demonstrated that such autophagy is essential for generation of the inhibitory effects of arsenic on primitive leukemic precursors. 22 However, the key downstream cellular events by which such arsenic-de...

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

“…1A and B). Similarly, and as expected, 16,17 treatment with As 2 O 3 resulted in upregulation of expression of LC3II (Fig. 1C).…”

“…We were particularly interested in examining whether resveratrol can potentiate arsenic trioxide-induced autophagy or apoptosis, as our recent studies have established that in addition to apoptotic cell death, induction of autophagy is a key mechanism for the generation of arsenic-induced antileukemic effects on primary leukemic precursors. 16,17 As expected, both agents were found to induce an autophagic state, but resveratrol does not further enhance induction of autophagy by As 2 O 3 . On the other hand, resveratrol potently enhances As 2 O 3 -dependent apoptotic cell death and promotes the suppressive effects of As 2 O 3 on primary leukemic progenitors from patients with AML and CML, suggesting a developmental therapeutics approach to target early leukemic precursors and leukemia initiating stem cells.…”

p62/SQSTM1 upregulation constitutes a survival mechanism that occurs during granulocytic differentiation of acute myeloid leukemia cells

“…8,9 At the molecular level, p62/SQSTM1 acts as a pro-tumoral molecule by ensuring efficient and selective activation of cell signaling axes involved in cell survival, proliferation, and metabolism (i.e., NF-kB, mTOR, and Nrf-2 pathways). 3,[5][6][7][10][11][12][13] p62/ SQSTM1 can also signal anti-tumoral responses either by inactivating the pro-oncogenic signaling through BCR-ABL 14 and Wnt pathways 15,16 or by inducing the activation of caspase 8, a pro-death protein. 17,18 Interestingly, in response to stress, autophagy promotes the degradation of p62, thus limits the activation of p62-regulatory pathways that control tumorigenesis.…”

L’autophagie, acteur clé de la leucémogenèse et cible thérapeutique dans les hémopathies malignes

“…montré dans différents types cellulaires pulmonaires (cellules épithéliales, fibroblastes, macrophages) de même qu'après exposition pulmonaire de souris, ou de rats, à des NP, d'oxydes métalliques ou carbonées (noir de carbone, nanotubes de carbone), majoritairement, mais aussi à des dendrimères ou encore des nanooxydes de terre rare [9]. Autophagie et dégradation de protéines de fusion associées aux leucémies Une autre spécificité de l'autophagie est la dégradation des protéines codées par des translocations spécifiques des leucé-mies [7,8]. L'inhibition de l'autophagie permettrait ainsi la stabilisation de ces protéines de fusion oncogéniques.…”

“…L'inhibition de l'autophagie permettrait ainsi la stabilisation de ces protéines de fusion oncogéniques. L'acide rétinoïque (ATRA), seul ou en combinaison avec le trioxide d'arsenic (ATO), restaure la différenciation des cellules de leucémie promyélocytaire (LAM3) porteuses de la translocation PML-RARA [8]. La rémission complète des LAM3 nécessite la dégra-dation totale de PML-RARA, un processus dépendant de l'autophagie.…”