Requirement for ER-mitochondria Ca²⁺ transfer, ROS production and mPTP formation in L-asparaginase-induced apoptosis...

Acute lymphoblastic leukemia (aLL) is a malignant cancer in the blood and bone marrow characterized by rapid expansion of lymphoblasts. It is a common pediatric cancer and the principal basis of cancer death in children. Previously, we reported that L-asparaginase, a key component of acute lymphoblastic leukemia chemotherapy, causes IP3R-mediated ER Ca²⁺ release, which contributes to a fatal rise in [Ca²⁺]_cyt, eliciting aLL cell apoptosis via upregulation of the Ca²⁺-regulated caspase pathway (Blood, 133, 2222-2232). However, the cellular events leading to the rise in [Ca²⁺]_cyt following L-asparaginase-induced ER Ca²⁺ release remain obscure. Here, we show that in acute lymphoblastic leukemia cells, L-asparaginase causes mitochondrial permeability transition pore (mPTP) formation that is dependent on IP3R-mediated ER Ca²⁺ release. This is substantiated by the lack of L-asparaginase-induced ER Ca²⁺ release and loss of mitochondrial permeability transition pore formation in cells depleted of HAP1, a key component of the functional IP3R/HAP1/Htt ER Ca²⁺ channel. L-asparaginase induces ER Ca²⁺ transfer into mitochondria, which evokes an increase in reactive oxygen species (ROS) level. L-asparaginase-induced rise in mitochondrial Ca²⁺ and reactive oxygen species production cause mitochondrial permeability transition pore formation that then leads to an increase in [Ca²⁺]_cyt. Such rise in [Ca²⁺]_cyt is inhibited by Ruthenium red (RuR), an inhibitor of the mitochondrial calcium uniporter (MCU) that is required for mitochondrial Ca²⁺ uptake, and cyclosporine A (CsA), an mitochondrial permeability transition pore inhibitor. Blocking ER-mitochondria Ca²⁺ transfer, mitochondrial ROS production, and/or mitochondrial permeability transition pore formation inhibit L-asparaginase-induced apoptosis. Taken together, these findings fill in the gaps in our understanding of the Ca²⁺-mediated mechanisms behind L-asparaginase-induced apoptosis in acute lymphoblastic leukemia cells.