DNA methylation is considered as a potential cause of aberrations in regulation of gene expression during carcinogenesis. Therefore, changes in DNA methylation patterns may be targets for chemoprevention. In the present study, we investigated effects of all-trans retinoic acid (ATRA), vitamin D(3), and resveratrol alone and in combination with adenosine analogues: 2-chloro-2'-deoxyadenosine (2CdA) and 9-beta-D-arabinosyl-2-fluoroadenine (F-ara-A), on methylation and expression of retinoic acid receptor beta 2 (RAR beta 2) in MCF-7 and MDA-MB-231 breast cancer cell lines. Alterations in methylation and expression levels after treatment of cells with the tested compounds were evaluated by methylation-sensitive restriction analysis (MSRA) and real-time PCR, respectively. RAR beta 2 promoter in the tested fragment was partially methylated in MCF-7 cells and non-methylated in MDA-MB-231 cells. In MCF-7 cells, all compounds, except for resveratrol, inhibited promoter methylation and increased expression of RAR beta 2. All natural compounds improved the action of 2CdA and F-ara-A on RAR beta 2 methylation and/or expression. Combination of ATRA or vitamin D(3) with 2CdA was the most effective. In MDA-MB-231 cells, only 2CdA, F-ara-A, and ATRA induced RAR beta 2 expression without any notable effects in combined treatment. Our results demonstrate that both natural compounds and adenosine analogues are able to reduce promoter methylation and/or induce expression of RAR beta 2 in non-invasive MCF-7 cells. Furthermore, the natural compounds improve effects of adenosine analogues, however only at early non-invasive stages of carcinogenesis.
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