Human melatonin MT1 receptor induction by valproic acid and its effects in combination with melatonin on MCF-7 breast cancer cell proliferation

We have reported that valproic acid upregulates melatonin MT1 receptor expression in rat C6 glioma cells. In addition to its anticonvulsant and mood stabilizing properties, valproic acid can also inhibit the growth of cancer cells. Since the melatonin MT1 receptor has been implicated in the oncostatic action of melatonin on human MCF-7 breast cancer cells, the effect of valproic acid on its expression was examined in this cell line. Treatment of MCF-7 cells with valproic acid (0.5 or 1 mM) for 24 or 72 h caused a significant increase in melatonin MT1 receptor mRNA or protein expression, as shown by reverse transcription-polymerase chain reaction (RT-PCR) analysis and western blotting, respectively. MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assays revealed a concentration-dependent inhibition of MCF-7 cell proliferation by valproic acid (0.5, 1.0 or 5 mM), but melatonin (1 or 10 nM) was ineffective alone or in combination with valproic acid, in the first (MCF-7A) subline examined. However, in subsequent experiments using a different (MCF-7B) subline, which expressed higher levels of MT1 receptor mRNA and showed modest sensitivity to melatonin, a combination of this hormone with valproic acid produced a significant synergistic inhibition of cell proliferation. These findings indicate that clinically relevant concentrations of valproic acid upregulate melatonin MT1 receptor expression in human breast cancer cells. Moreover, the enhanced antiproliferative effect observed with a combination of valproic acid and melatonin suggests that a similar therapeutic approach may be beneficial in breast cancer.