VDR microRNA expression and epigenetic silencing of vitamin D signaling in melanoma cells

Malignant melanoma cells express the vitamin D receptor (VDR). However, some melanoma cell lines fail to respond to the antiproliferative effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). We reported previously that out of seven melanoma cell lines analyzed, three cell lines (MeWo, SK-Mel28, SM) respond to the antiproliferative effects of 1,25(OH)2D3, while the others (SK-Mel5, SK-Mel25, IGR, Meljuso) are resistant. It was the aim of this study to investigate whether epigenetic mechanisms are of importance for the abrogation of vitamin D signaling in vitamin D resistant melanoma cells. We used the histone deacetylase inhibitor (HDACI) trichostatin A (TSA) and the DNA methyltransferase inhibitor (DNMTI) 5-azacytidine (5-Aza) to elucidate the effects of protein acetylation and of DNA hypermethylation on 1,25(OH)2D3-induced effects on cell proliferation, respectively. Additionally we analyzed the expression of VDR microRNA in 1,25(OH)2D3-responding and resistant melanoma cells. TSA and 5-Aza exerted dose- and time-dependent antiproliferative effects on melanoma cell lines. Interestingly, combination therapy with 1,25(OH)2D3 and TSA exerted synergistic antiproliferative effects in a 1,25(OH)2D3-resistant melanoma cell line (IGR) (p < 0.05). Combination therapy with 1,25(OH)2D3 and 5-Aza resulted in synergistic (MeWo after 72 h; p < 0.05) or additive (other melanoma cell lines analyzed) antiproliferative effects. Additionally, we could show that VDR mRNA expression is relatively high in two of three 1,25(OH)2D3-responsive melanoma cells as compared to resistant cells, moreover this relatively high VDR expression is associated with low expression of miRNA125b in MeWo and SK-Mel28 cells. Our results suggest that the endogenous VDR mRNA level is inversely associated with expression of miRNA125b in melanoma cell lines analyzed. Moreover, miRNA125b may be involved in the regulation of VDR expression and in the resistance against 1,25(OH)2D3 in melanoma cells. It can be speculated whether miRNA125b may be of prognostic importance and/or may represent a therapeutic target for malignant melanoma. Drugs that influence epigenetic mechanisms might be promising therapeutics for the treatment of metastasized malignant melanoma, alone or in combination with antiproliferative or cytotoxic agents such as 1,25(OH)2D3.