Melatonin showed a pro-oxidant effect,
reduced BCL-2 expression and induced a
caspase-3-independent cell death in P19 cells
with oxidative metabolism
The disruption of the mitochondrial electron transport
chain results in a higher reactive oxygen species (ROS)
production [26]. Differentiated P19 cells (Glu-dCCs and
Gal-dCCs) presented higher malondialdehyde (MDA)
content, a classical marker of oxidative stress, when
compared to their stem counterparts (p < 0.001). Similarly,
cells cultured in galactose (glucose-free), glutamine/
pyruvate- containing media (Gal-CSCs and Gal-dCCs) also
presented higher MDA content than their high glucosecultured
counterparts. Despite these basal differences, 1 mM
melatonin significantly increased MDA levels in all P19 cell
groups compared to their corresponding controls (p < 0.001),
especially in cells cultured in galactose medium (Figure
2D). This effect may be related with the ability of melatonin
to decrease the proliferation rate of highly oxidative cells.
Intriguingly, although 0.1 mM melatonin also impacted
oxygen consumption of cells cultured in galactose media,
this concentration also exhibited an antioxidant effect.
Since excessive ROS production by mitochondria
plays a preponderant role in mitochondrial outer
membrane permeabilization, we investigated alterations
in BCL-2 and BAX protein content. We found no
changes in BAX content in whole-cell extracts in any
of the analyzed cell groups. However, melatonin-treated
cells cultured in galactose media showed a decreased
content of the antiapoptotic protein BCL-2, while this
effect was not found when cells were grown in high
glucose media (Figure 3A). The observed decrease of
BCL-2 content in melatonin-treated and galactosecultured
cells suggests that the intrinsic apoptotic
pathway may have been activated. However, our data
did not show the expected increase in caspase-3-like
activity (Figure 3B). Interestingly, untreated Gal-CSCs
show the highest activity of caspase-3 when compared
to Glu-CSCs (p < 0.001). This observation may occur
as consequence of the forced metabolic remodeling and
its associated differentiation process induced by the
galactose (glucose-free), glutamine/pyruvate- containing
medium. Nonetheless, the calcein-AM and propidium
iodide Live/Dead assay confirmed that 1 mM melatonin
increases the percentage of dead cells (calcein-/
propidium iodide+) in cell populations with higher
mitochondrial metabolism (Gal-CSCs and Gal-dCCs;
p < 0.001). Moreover, this effect was also detected in
Glu-dCCs (p