Dichloroacetate enhanced the antiproliferative
effect of melatonin
The above results suggest that melatonin antitumor
effects are dependent on cellular oxidative metabolism
and are caspase-3 activity-independent. Therefore,
Glu-CSCs presented a resistant profile which might be
related to their predominantly glycolytic metabolism.
As melatonin appears to affect only cells relying on
mitochondrial metabolism, we next questioned whether
a combined treatment with dichloroacetate could affect
Glu-CSCs. As shown in Figure 4A, co-treatment with
10 mM dichloroacetate and 1 mM melatonin resulted
in decreased Glu-CSCs cell mass (p < 0.01 and p <
0.001 for 48 and 72 hours of treatment, respectively).
Although the calculated combination index with 10 mM
dichloroacetate and 0.1 mM melatonin suggested
additive and antagonistic effects in Gal-CSCs and
Gal-dCCs respectively, the combination of 10 mM
dichloroacetate with 1 mM melatonin clearly exerted
a synergistic effect in both groups of galactose mediagrown
cells (Table 1). As dichloroacetate activates
pyruvate dehydrogenase (PDH) by inhibiting pyruvate
dehydrogenase kinase (PDHK), we evaluated phospho(
Ser293)-PDH after 72 hours treatment with 1
mM melatonin and 10 mM dichloroacetate. However,
treatment with dichloroacetate alone was able to
significantly reduce phospho(Ser293)-PDH in Glu-CSCs,
Glu-dCCs and Gal-dCCs. Furthermore, treatment with
melatonin, alone or in combination with dichloroacetate,
did not alter the phosphorylation status of PHD in all
types of P19 cells with the exception of Gal-dCCs where
a significant decrease of phospho(Ser293)-PDH (p