1α,25-Dihydroxyvitamin D synergistically enhances anticancer effects of ginsenoside Rh2 in human prostate cancer cells.

1α,25-dihydroxyvitamin D (1,25(OH)D, commonly known as calcitriol), the most active metabolite of vitamin D, and ginsenoside Rh2 can regulate cellular differentiation and proliferation proteins. The purpose of the present study was to assess the effect of 1,25(OH)D on the anticancer activities of Rh2 in human prostate cancer cells such as androgen-dependent LNCaP and androgen-independent C4-2 in vitro. The effects of treatment with 1,25(OH)D or Rh2, either alone or in combination, on prostate cancer cells were evaluated through tetrazolium-based cell viability assay, BrdU cell proliferation rate estimation assay, and Western blot protein expression analyses of nuclear receptors (androgen receptor and vitamin D receptors) and apoptotic proteins (Bcl-2, Bax, and Caspase 3). The Combination Indices (CI) and Dose Reduction Indices (DRI) of 1,25(OH)D and Rh2 were calculated to determine synergistic anticancer activity using Calcusyn software (Biosoft, Cambridge, UK). The cell viability assay data indicate that Rh2 treatment alone inhibited cell viability in a concentration-dependent manner and the addition of 10 nM 1,25(OH)D to Rh2 significantly enhanced its ability to reduce cell viability up to 80 % in both the cell lines. Similarly, addition of 10 nM 1,25(OH)D to Rh2 significantly lowered its IC values for cell proliferation from the range of 32-65 μM to 14-8 μM in LNCaP and C4-2 cells. In addition, protein expression analyses indicated that the combined treatment with Rh2 and 1,25(OH)D led to greater downregulation of androgen receptor expression compared to single agent exposure. Similarly, the presence of 1,25(OH)D synergistically increased the pro-apoptotic actions of Rh2 in both the cell lines. Overall, 1,25(OH)D augments the Rh2-mediated anticancer effects through stimulating apoptosis and reduced cell proliferation which suggests that synergism of this combination may lead to potential lower need of the active vitamin D and limited toxicity from it.