Menthol Enhances an Antiproliferative Activity of 1α,25-Dihydroxyvitamin D(3) in LNCaP Cells

1α,25-dihydroxyvitamin D(3) [1α,25(OH)(2)D(3)], the most active form of vitamin D(3), and its analogues have therapeutic benefits for prostate cancer treatment. However, the development of hypercalcemia is an obstacle to clinical applications of 1α,25(OH)(2)D(3) for cancer therapy. In this study, we provide evidence that menthol, a key component of peppermint oil, increases an anti-proliferation activity of 1α,25(OH)(2)D(3) in LNCaP prostate cancer cells. We found that menthol per se does not exhibit antiproliferative activity, but it is able to enhance 1α,25(OH)(2)D(3)-mediated growth inhibition in LNCaP cells. Fluorometric assays using Fura-2 showed that 1α,25(OH)(2)D(3) does not induce acute Ca(2+) response, whereas menthol evokes an increase in [Ca(2+)](i), which suggests that cross-talks of menthol-induced Ca(2+) signaling with 1α,25(OH)(2)D(3)-mediated growth inhibition pathways. In addition, Western blot analysis revealed that 1α,25(OH)(2)D(3) and menthol cooperatively modulate the expression of bcl-2 and p21 which provides the insight into the molecular mechanisms underlying the enhanced 1α,25(OH)(2)D(3)-mediated growth inhibition by menthol. Thus, our findings suggest that menthol may be a useful natural compound to enhance therapeutic effects of 1α,25(OH)(2)D(3).