Abstract
Acute Lymphoblastic Leukemia (ALL) is the most common hematological malignancy and the main cause of cancer-related death in children. Current chemotherapy treatments based mainly on antifolate regimens have lead to high cure rates (≈80%), however event-free survival (EFS) for children and adults diagnosed with chemotherapy resistant phenotypes of ALL or after the relapse, continues to be dismal (EFS ∼10-20%). High-risk chemotherapy treatment based on intensification strategies and/or the use of stem cell transplantation have led to marginal improvements with limited impact on cure rates of resistant/refractory or relapsed ALL phenotypes. Our studies attempted to discover highly effective treatment strategies for ALL by targeting novel molecular pathways with existing agents that represent a backbone of current effective chemotherapeutic regimens for ALL. Our findings for the first time demonstrated that ALL exhibited sensitivity to reactive oxygen species (ROS) -induced endoplasmic reticulum (ER) stress and the unfolded protein response (UPR)-mediated cell death. These findings suggest a novel use for methotrexate (MTX), the backbone of current antifolate-based ALL chemotherapy, in combination with exogenous 5-aminoimidazole- 4-carboxamide-1-beta-4-ribofuranoside (AICAr) as part of the treatment intensification to target ER stress/UPR signaling in ALL. The sensitivity of ALL cell lines and primary patient cells to ER stress-induced cytotoxicity was further confirmed by using the glycolytic inhibitor 2-deoxy-D-glucose (2-DG) in ALL cell models. We found that 2-DG induced significant cell death under normoxia in Bp-ALL and T-ALL cells. This finding makes ALL one of the few tumor cell lines that exhibit sensitivity to glycolytic inhibitor 2-DG under normoxia. Using D-mannose, a sugar essential for N-linked glycosylation, our laboratory has postulated that 2-DG inhibits N-linked glycosylation and interferes with glycan-dependent folding and quality control of nascent ER proteins, which leads to ER stress-induced cytotoxicity in ALL. Our data also demonstrate for the first time that Akt and AMP-activated protein kinase (AMPK) differentially modulate the UPR function in ALL and determine the cellular responses (survival vs. death) depending on a type of ER stress-causing chemotherapeutic injury. In the case of AICAr plus MTX treatment we postulate that AMPK blocks the induction of the proapoptotic UPR signaling caused by the accumulation of ROS and the prolonged ER stress. In that case, Akt, a cellular antagonist of AMPK, plays a paradoxical proapoptotic role. However, in the case of 2- DG treatment, the effects of AMPK and Akt on cell cytotoxicity are reversed. We postulate that it is AMPK-mediated inhibition of the pro-survival glucose regulated protein 78 (GRP78)-regulated UPR signaling that synergistically potentiates apoptosis in ALL via induction of proteotoxicity. Our data underscore the complexity of the interactions within these pathways, and suggests that the contextual relationship between these signaling proteins and cross-talk with related pathways are critical determinants of cellular responses and fate in ALL cells.