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

// Juliana Velez 1, 2 , Rongqing Pan 6 , Jason T.C. Lee 7 , Leonardo Enciso 2 , Marta Suarez 2 , Jorge Eduardo Duque 3 , Daniel Jaramillo 1 , Catalina Lopez 4 , Ludis Morales 1 , William Bornmann 5 , Marina Konopleva 6 , Gerald Krystal 7 , Michael Andreeff 6 , Ismael Samudio 1, 2, 8 1 Grupo de Terapia Celular y Molecular, Pontificia Universidad Javeriana, Bogotá, Colombia 2 Programa de Investigación e Innovación en Leucemia Aguda y Crónica (PILAC), Bogotá, Colombia 3 Centro Oncológico de Antioquia, Medellín, Colombia 4 Grupo de Terapia Regenerativa, Universidad de Caldas, Manizales, Colombia 5 Department of Experimental Therapeutics, The University of Texas MD Anderson Cancer Center, Houston, TX, USA 6 Section of Molecular Hematology and Therapy, The University of Texas MD Anderson Cancer Center, Houston, TX, USA 7 Terry Fox Laboratory, British Columbia Cancer Agency, Vancouver, Canada 8 Centre for Drug Research and Development, Vancouver, Canada Correspondence to: Ismael Samudio, email: ismael.samudio@gmail.com Keywords: metformin, phenformin, leukemia, mitochondria, ABT 737 Received: February 16, 2016 Accepted: May 22, 2016 Published: June 06, 2016 ABSTRACT 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.