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By Jamshed Arslan Pharm.D.
Adenosine triphosphate (ATP) is the major life’s energy-carrying molecule. It is mainly produced by mitochondrial ATP synthase (Complex V) through oxidative phosphorylation (Oxphos). For example, Oxphos-dependent oxidation of a glucose molecule generates about 30 molecules of ATP. In Oxphos, respiratory chain (r.c.) complexes catalyze the transfer of electrons from energy-rich molecules (NADH or FADH2) to oxygen (O2) and Complex V generates ATP exploiting the energy released by the r.c. complexes. If oxygen is not available to pick up these electrons, as it happens in ischemia, two interesting phenomena occur: Complex V reverses its function and starts hydrolyzing ATP, and endogenous IF1 protein inhibits Complex V to prevent energy/ATP dissipation. However, the role of Complex V and IF1 in the context of cancer cells experiencing hypoxia/anoxia has remained unclear.
A research group at the University of Bologna, Italy, has recently shown that cancer cells overexpress IF1 to be able to survive in anoxia/pseudo-anoxia. They found that in cancer cells experiencing low oxygen tension (down to 0.1%), Complex V can still synthesize ATP.
To determine the role of IF1 in cancer cell metabolism, the team exposed IF1-silenced and control osteosarcoma cells to hypoxia. They found a slight decrease in mitochondrial mass in all the cells. To further explore this phenomenon, the researchers loaded cells with TMRM fluorescent dye in presence or absence of Oligomycin. The basis for this approach is that TMRM is accumulated in mitochondria depending on its membrane potential (MMP), therefore it can be assayed by evaluating the TMRM fluorescence. The Oligomycin blocks the proton transport through the channel of Complex V, inhibiting the synthesis or hydrolysis of ATP. Therefore, since the authors observed a slight increase of the MMP of cancer cells upon Oligomycin addition, hypoxic cancer cells were still synthesizing ATP. However, ATP levels under hypoxia slightly decreased compared to normoxic values (irrespective of the low oxygen concentration).
Inhibitor of mitochondrial ATP synthase and uncoupler of oxidative phosphorylation. Antibiotic; exhibits anti-tumor activity.
To study energy transformation and ATP preservation in cancer cells, the team exposed osteosarcoma cells to different concentrations of carbonyl cyanide-4-(trifluoromethoxy) phenylhydrazone (FCCP). FCCP is a mitochondrial uncoupler, a chemical that compromises mitochondrial functioning by uncoupling Oxphos (i.e. disconnecting transport of electrons down the respiratory chain from ADP phosphorylation by Complex V), therefore mimicking a condition of ischemia/anoxia. Analysis revealed that IF1-silencing reduced FCCP’s uncoupling effect and only IF1-silenced clones showed a major decline in cellular ATP level under pseudo-anoxia/anoxia. As expected, FCCP inhibited cell proliferation, but the presence of IF1 enabled cancer cells to keep growing even with the highest concentration of FCCP. In other words, IF1 protected cancer cells from FCCP’s anti-proliferative effects by preserving ATP as it occurs in pseudo-anoxia/anoxia.
Dissecting the role of IF1 in hypoxia/anoxia in this study is crucial in cancer biology since solid tumors are often known to have necrotic anoxic regions. From a therapeutic perspective, reducing the overexpression of IF1 in tumors may deprive cancer cells of the energy needed for their growth and survival.
Learn more about hypoxia adaptations in cancer
Jamshed Arslan, Pharm D.
University of Alabama at Birmingham, School of Medicine
Dr. Arslan studies cell signaling in mitochondrial defects in C. elegans
and transgenic mice.
References
Sgarbi, Gianluca, et al. “The Role of the ATPase Inhibitor Factor 1 (IF1) in Cancer Cells Adaptation to Hypoxia and Anoxia.” Biochimica et Biophysica Acta (BBA) – Bioenergetics, vol. 1859, no. 2, 2018, pp. 99 – 109. https://doi.org/10.1016/j.bbabio.2017.10.007
