Autophagy and apoptosis in acute myeloid leukemia

Open Access
Author:
Liu, Qiang
Graduate Program:
Pharmacology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 07, 2016
Committee Members:
  • Hong-Gang Wang, Dissertation Advisor
  • Hong-Gang Wang, Committee Chair
  • Jennifer Xavier, Committee Member
  • Shantu Amin, Committee Member
  • Qing Yang, Outside Member
Keywords:
  • Bcl-2
  • Mcl-1
  • leukemia
  • autophagy
  • apoptosis
  • acute myeloid leukemia
  • small molecule inhibitors
Abstract:
AML is a clonal hematopoietic malignancy characterized by the uncontrolled proliferation of aberrantly differentiated myeloid cells in the bone marrow. This leads to the suppression of normal hematopoiesis in the patient that, if left unchecked, can eventually be fatal. AML is the deadliest form of leukemia in the United States, and mainstay therapy for AML has remained largely unchanged for the past four decades. Relapse and refractory disease are major hurdles to the success of treatment, and there is an urgent need for new targets and therapies. Autophagy is a catabolic process where cytoplasmic contents are encapsulated by double-membraned vesicles and delivered to the lysosome for degradation. Recent research efforts have elucidated a possible role of autophagy in cancer malignant transformation and progression, and have further led to the development of a number of selective autophagy modulators. It has thus become important to specifically determine the role of autophagy in the initiation and development of AML. Here, we employ a mouse model of AML expressing the fusion oncogene MLL-AF9 and explore the effects of Atg5 deletion, a key autophagy protein, on the malignant transformation and progression of AML. The in vivo deletion of Atg5 in MLL-AF9-transduced bone marrow cells during primary transplantation prolonged the survival of recipient mice, suggesting that autophagy plays a role in MLL-AF9-driven leukemia initiation. In contrast, deletion of Atg5 in malignant AML cells during secondary transplantation did not influence the survival or chemotherapeutic response of leukemic mice. Thus, Atg5-dependent autophagy may contribute to the development but not chemotherapy sensitivity of murine AML induced by MLL-AF9. Deregulated apoptosis is a hallmark of cancer. The anti-apoptotic group of Bcl-2 family proteins is frequently found to be over-expressed in numerous cancers, causing both evasion of apoptosis and resistance to treatment. Mcl-1 is an anti-apoptotic member of the Bcl-2 family that has long been hypothesized as a promising therapeutic target. To this end, we characterized a series of small molecules based on maritoclax, a novel small molecule antagonist of Mcl-1, in the context of AML. We determined that maritoclax caused the selective proteasome-dependent down-regulation on Mcl-1 and subsequent cytotoxicity in AML cells with elevated Mcl-1. We further performed structure-activity relationship analysis and uncovered a number of pyoluteorin derivatives that demonstrated superior potency towards Mcl-1-dependent cancers as well possessing in vivo efficacy. In order to expand the strategies by which to antagonize Mcl-1-dependent cancers, we designed and validated a robust high-throughput approach to uncover small molecules which post-transcriptionally modulate Mcl-1 stability. We uncovered a number of hits which down-regulate Mcl-1, among which is niclosamide with a hitherto unknown mechanism of action. We similarly performed structure-activity relationship studies on niclosamide and uncover a number of salicylamide derivatives based on niclosamide that potently and selectively induce proteasome-dependent Mcl-1 down-regulation. Overall, we present a novel high-throughput approach to discover novel post-transcriptional modulators of Mcl-1 and were able to describe the pharmacophore of salicylamide derivatives which selectively antagonize Mcl-1. Thus, we were able to characterize a series of small molecule Mcl-1 antagonists with the potential to be developed for the treatment of AML. Taken together, these studies have dissected the role of autophagy and the pharmacologic inhibition of Mcl-1 in the context of AML. While autophagy did not play a significant role in the progression or chemotherapeutic response in a mouse model of AML, the selective inhibition of Mcl-1 by inducing its proteasomal degradation might be a promising therapeutic strategy. We believe that this knowledge will significantly contribute to the understanding of AML pathogenesis and treatment, will serve as a platform for additional drug discovery efforts to elucidate novel compounds for the treatment of AML.