Biguanides sensitize leukemia cells to ABT-737-induced apoptosis by inhibiting mitochondrial electron transport.

Metformin displays antileukemic effects partly due to activation of AMPK and subsequent inhibition of mTOR signaling. Nevertheless, Metformin also inhibits mitochondrial electron transport at complex I in an AMPK-independent manner, Here we report that Metformin and rotenone inhibit mitochondrial electron transport and increase triglyceride levels in leukemia cell lines, suggesting impairment of fatty acid oxidation (FAO). We also report that, like other FAO inhibitors, both agents and the related biguanide, Phenformin, increase sensitivity to apoptosis induction by the bcl-2 inhibitor ABT-737 supporting the notion that electron transport antagonizes activation of the intrinsic apoptosis pathway in leukemia cells. Both biguanides and rotenone induce superoxide generation in leukemia cells, indicating that oxidative damage may sensitize toABT-737 induced apoptosis. In addition, we demonstrate that Metformin sensitizes leukemia cells to the oligomerization of Bak, suggesting that the observed synergy with ABT-737 is mediated, at least in part, by enhanced outer mitochondrial membrane permeabilization. Notably, Phenformin was at least 10-fold more potent than Metformin in abrogating electron transport and increasing sensitivity to ABT-737, suggesting that this agent may be better suited for targeting hematological malignancies. Taken together, our results suggest that inhibition of mitochondrial metabolism by Metformin or Phenformin is associated with increased leukemia cell susceptibility to induction of intrinsic apoptosis, and provide a rationale for clinical studies exploring the efficacy of combining biguanides with the orally bioavailable derivative of ABT-737, Venetoclax.

Platelets promote mitochondrial uncoupling and resistance to apoptosis in leukemia cells: a novel paradigm for the bone marrow microenvironment.

Here we report that leukemia cell lines and primary CD34+ leukemic blasts exposed to platelet rich plasma (PRP) or platelet lysates (PL) display increased resistance to apoptosis induced by mitochondria-targeted agents ABT-737 and CDDO-Me. Intriguingly, leukemia cells exposed to platelet components demonstrate a reduction in mitochondrial membrane potential (ΔΨM) and a transient increase in oxygen consumption, suggestive of mitochondrial uncoupling. Accompanying the ranolazine-sensitive increase in oxygen consumption, a reduction in triglyceride content was also observed in leukemia cells cultured with platelet components indicating that lipolysis and fatty acid oxidation may support the molecular reduction of oxygen in these cells. Mechanistically, platelet components antagonized Bax oligomerization in accordance with previous observations supporting an antiapoptotic role for fatty acid oxidation in leukemia cells. Lastly, substantiating the notion that mitochondrial uncoupling reduces oxidative stress, platelet components induced a marked decrease in basal and rotenone-induced superoxide levels in leukemia cells. Taken together, the decrease in ΔΨM, the transient increase in ranolazine-sensitive oxygen consumption, the reduction in triglyceride levels, and the reduced generation of superoxide, all accompanying the increased resistance to mitochondrial apoptosis, substantiate the hypothesis that platelets may contribute to the chemoprotective sanctuary of the bone marrow microenvironment via promotion of mitochondrial uncoupling.

Mitochondrial uncoupling and the reprograming of intermediary metabolism in leukemia cells.

Nearly 60 years ago Otto Warburg proposed, in a seminal publication, that an irreparable defect in the oxidative capacity of normal cells supported the switch to glycolysis for energy generation and the appearance of the malignant phenotype (Warburg, 1956). Curiously, this phenotype was also observed by Warburg in embryonic tissues, and recent research demonstrated that normal stem cells may indeed rely on aerobic glycolysis - fermenting pyruvate to lactate in the presence of ample oxygen - rather than on the complete oxidation of pyruvate in the Krebs cycle - to generate cellular energy (Folmes et al., 2012). However, it remains to be determined whether this phenotype is causative for neoplastic development, or rather the result of malignant transformation. In addition, in light of mounting evidence demonstrating that cancer cells can carry out electron transport and oxidative phosphorylation, although in some cases predominantly using electrons from non-glucose carbon sources (Bloch-Frankenthal et al., 1965), Warburg's hypothesis needs to be revisited. Lastly, recent evidence suggests that the leukemia bone marrow microenvironment promotes the Warburg phenotype adding another layer of complexity to the study of metabolism in hematological malignancies. In this review we will discuss some of the evidence for alterations in the intermediary metabolism of leukemia cells and present evidence for a concept put forth decades ago by lipid biochemist Feodor Lynen, and acknowledged by Warburg himself, that cancer cell mitochondria uncouple ATP synthesis from electron transport and therefore depend on glycolysis to meet their energy demands (Lynen, 1951; Warburg, 1956).