Melatonin mitigates mitochondrial dysfunction
in healthy cells, maintaining membrane potential and
optimizing electron transport within the respiratory chain
[8, 43, 44]. However, in some melatonin-sensitive cancer
cells, mitochondrial depolarization is observed [45, 46].
Although this effect is not observed in other tumor
cells [39], it suggests, according to our results, that the
disruption of mitochondrial membrane potential is not
essential for melatonin-induced antiproliferative actions.
Despite this, P19 cells relying on oxidative metabolism
showed a decrease in cell respiration after treatment with
melatonin, an effect also identified in Glu-CSCs when
maximal respiration is induced with FCCP. Although Glu-
CSCs present depolarized mitochondria and decreased
oxygen consumption, mitochondrial respiration is not
completely impaired as cells possess functional OXPHOS
machinery but probably decouple respiration from ATP
production [23]. Then, when cells undergo differentiation,
mitochondria became polarized suggesting a different
modulation of mitochondrial potential between stem
and differentiated cells. While mitochondrial membrane
potential in differentiated cells is maintained by the electron
transport chain, in undifferentiated cells this process
depends mostly on glycolysis and on ATP hydrolase activity
of the F1Fo-ATPase [23]. Our results suggest a direct effect
of melatonin in the electron transport chain, although the
observed increase of mitochondrial membrane potential
may not result from an inhibition of the respiratory chain
alone but instead from effects on ATP synthase.