Intracellular Thiol Oxidation Is Linked with Loss of Δψm and Disease Progression in Acute Promyelocytic Leukaemia

Acute promyelocytic leukaemia (APL) is a type of myeloid malignancy defined by the chromosomal translocation t(15;17) and subsequent expression of the PML-RARα fusion protein. Long term remission in APL is achieved through a combination of high dose all-trans retinoic acid (ATRA) and arsenic trioxide (ATO), the latter of which has potential to induce mitochondrial derived oxidative stress and initiate the intrinsic apoptosis pathway. ATO is particularly effective in the treatment of APL when compared with acute myeloid leukaemia (AML), suggesting that the mitochondrial membrane potential (Δψm) of APL cells is compromised. Here we show that loss of Δψm is associated with intracellular thiol oxidation and disease progression in APL.

The mitochondrial probe 3,3'-diethyloxacrbocyanine iodide (DiOC₂(3)) was used to evaluate MRP8-PML/RARA transgenic mice Δψm in bone marrow mononuclear cells (BMMC) at three disease stages: 1) pre-APL, 2) APL and 3) ATRA treated APL. Our data show that MRP8-PML/RARA BMMC cells during the APL stage accumulate significantly less DiOC₂(3) compared with pre-APL cells (0.05±0.02 vs 0.24±0.05, p<0.05; data are normalised FL1/FL3 MFI ratio). The DiOC₂(3) accumulation in BMMC from ATRA treated APL mice was equivalent to that of pre-APL cells (p>0.05). This finding indicates a loss of Δψm in APL. This may result in free radical leakage from the mitochondria, which in turn could oxidise intracellular thiols.

To evaluate intracellular thiol oxidation, a previously optimised flow cytometric assay that incorporates the thiol reactive probe fluorescein-5 maleimide (F5M) was used. Our data show a significant decrease in F5M MFI for MRP8-PML/RARA BMMC cells during the APL stage, compared with pre-APL cells (219.01±47.67 vs 672.66±131.04, p<0.05; data are F5M MFI relative to unstained controls). This finding signifies that there is more thiol oxidation at the APL stage. ATRA treated APL mice showed an equivalent F5M MFI to that of pre-APL cells. Confocal microscopy confirmed that the F5M signal was intracellular. This observation shows that there is an increase in intracellular thiol oxidation in APL, which may be tracked during disease progression and treatment course. This finding may explain why APL cells are more sensitive to pro-oxidant treatments i.e. ATO.

To understand these observations further, the APL cell line (NB4) was subjected to glucose oxidase (GOX) mediated oxidative stress in cell culture over a 4 hour time-course. F5M MFI signal was compared with the THP1 and Kasumi-1 AML cell lines subjected to the same treatment. All cell lines showed similar significant increases in F5M MFI after 1 hour GOX treatment relative to control (p<0.05), indicating an increase in 'reduced' cellular thiol i.e. reductive stress. However, after 4 hours NB4 cells showed a significant decrease in F5M MFI relative to control (p<0.05), indicating an increase in 'oxidised' cellular thiol i.e. oxidative stress. In contrast, THP1 cells showed no difference in MFI after 4 hours (p>0.05), whereas F5M MFI Kasumi-1 remained significantly increased after 4 hours (p<0.05) relative to control. This finding indicates differences in the sensitivity to oxidative stress between APL and AML cells.

Taken together, this study shows that APL cells are more sensitive to oxidative stress compared with AML cells. Our F5M flow cytometric assay illustrates that an increase in cellular thiol oxidation is linked to disease progression in APL, with loss of Δψm associated with this process. Our F5M flow cytometric assay may therefore have clinical utility in monitoring treatment efficacy in APL.