1. Introduction
Mitochondria control the biological processes occurring in almost all
cells of the human body, such as the synthesis of iron-sulfur clusters,
apoptosis, cell signaling, maintenance of calcium homeostasis, and
maintaining reactive oxygen species levels.[1, 2]Above all, mitochondria are renowned for synthesizing high energy
phosphate bonds in the form of ATP during oxidative phosphorylation
(OXPHOS). During the series of oxidation and reduction reactions within
the mitochondrial complexes, electrons follow one of the two major
electron transport pathways to finally reduce oxygen to water. One
pathway is through complexes I, III, and IV using electrons from NADH,
and the other is through complexes II, III, and IV using
FADH2 from succinic acid.[3] The
continuous pumping of protons increases the hydrogen ion concentration
and reduces pH, eventually generating a membrane potential across the
inner mitochondrial membrane. This membrane potential is utilized by
complex V (ATP synthase) to generate ATP.[4-6]
The significance of OXPHOS function can again be emphasized by the fact
that mitochondrial diseases arise from dysfunctional OXPHOS
systems.[7] Defective complex I was found to cause
inflammation and cell death.[8] Despite the
significant influence of OXPHOS in many diseases, its function in cancer
has been overlooked because it was previously thought that cancer cells
mostly utilize aerobic glycolysis as their main energy-synthesizing
process.[9] However, growing evidence has shown
that OXPHOS function is upregulated in lung
cancer,[10, 11] estrogen receptor (ER)-positive
breast cancer,[12] pancreatic
cancers,[13] Hodgkin’s
lymphoma,[14] and other
cancers.[15] In addition, blocking mitochondrial
respiration resulted in suppressed tumor growth in xenograft
models,[16] further proving that mitochondrial
function is essential for tumor cell proliferation. Mitochondrial ATP
synthesis function through the OXPHOS system can no longer be
overlooked, even in cancer, for it can be representative of the
mitochondrial robustness.
Despite the apparent significance of the mitochondrial OXPHOS system and
ATP, there is not yet a single assay to measure the mitochondrial
function related to ATP production. This study was thus designed to
propose a new method for an accurate, direct, and real-time detection of
mitochondrial function through sensitivity to activators and inhibitors
in isolated mitochondria, cell lines, and peripheral blood mononuclear
cells (PBMCs).