- Proteins and Peptides
- Lysates and Cell Lines
By Jamshed Arslan Pharm.D.
Adenosine triphosphate (ATP) is life’s energy-carrying molecule. It is typically produced by mitochondrial ATP synthase (MAS) through oxidative phosphorylation (Oxphos). For example, Oxphos of a glucose molecule generates about 26-28 molecules of ATP. In Oxphos, MAS catalyzes the transfer of electrons from energy-rich molecules (NADH or FADH2) to oxygen (O2) by a series of protein complexes. If oxygen is not available to pick these electrons, as happens in ischemia, two interesting phenomena occur: MAS reverses its function and starts hydrolyzing ATP; and endogenous inhibitor protein of the mitochondrial F1F0-ATPase (IF1) protein inhibits MAS to prevent energy/ATP dissipation. However, the role of MAS 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 survive in hypoxia/anoxia. They found that in cancer cells experiencing low oxygen tension, IF1 plays a role in preventing MAS from switching to ATP hydrolysis, thereby preserving cellular ATP and promoting malignant growth.
To determine the role of IF1 in cancer cell metabolism, the team exposed IF1-silenced and control osteosarcoma cells to hypoxia. They found a decrease in mitochondrial mass in all the cells. To further explore this phenomenon, the researchers loaded cells with TMRM fluorescent dye and oligomycin. The basis for this approach is that TMRM is sequestered only in the active mitochondria and facilitates measurement of mitochondrial membrane potential (MMP), while the oligomycin blocks a MAS channel necessary for Oxphos of ADP to ATP. Therefore, this approach sheds light on the consequences of MAS inhibition in cancer cells.
The team measured MMP, which is one of the markers of mitochondrial functioning, and cellular ATP levels. Flow cytometry and fluorescence microscopy revealed that both MAS inhibition (by oligomycin) and IF1 presence made no difference on mitochondrial function in TMRM-loaded cells. Likewise, ATP levels under hypoxia remained similar in all cases (irrespective of the presence of MAS and/or IF1). Overall, osteosarcoma cells were found to maintain mitochondrial function, partly because hypoxia/anoxia could not initiate MAS-induced ATP hydrolysis.
The researchers then went on to explore how this can lead to cancer cells adapting to low oxygen.
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 function by disrupting ATP synthesis, and stimulates oxygen consumption without a corresponding increase in ATP production.
Metabolic analysis revealed that IF1 reduced FCCP’s uncoupling effect and only IF1-silenced clones showed a decline in cellular ATP levels under hypoxia/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 during hypoxia/anoxia.
Dissecting the role of IF1 in hypoxia/anoxia 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 energy needed for their growth and survival.
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.
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