Five lines of evidence suggest that melatonin works via epigenetic processes: (1) melatonin influences transcriptional activity of nuclear receptors (ERalpha, GR and RAR) involved in the regulation of breast cancer cell growth; (2) melatonin down-regulates the expression of genes responsible for the local synthesis or activation of estrogens including aromatase, an effect which may be mediated by methylation of the CYP19 gene or deacetylation of CYP19 histones; (3) melatonin inhibits telomerase activity and expression induced by either natural estrogens or xenoestrogens; (4) melatonin modulates the cell cycle through the inhibition of cyclin D1 expression; (5) melatonin influences circadian rhythm disturbances dependent on alterations of the light/dark cycle (i.e., light at night) with the subsequent deregulation of PER2 which acts as a tumor suppressor gene.
Overall, we found no evidence that these common polymorphisms of the MTHFR and MTR genes are associated with promoter methylation of E-cadherin, p16, and RAR-beta2 genes in breast cancer.
Forced expression of RAR-beta2 reverts the malignant phenotype of RAR-beta2-negative breast cancer cells and reconstitutes retinoid sensitivity in these cells.
Several groups demonstrated that this is also true for the RAR-beta2 in breast cancer by treating breast cancer cell lines with a demethylating agent and examining expression of the RAR-beta2 gene in response to a challenge with retinoic acid.
Using methylation-specific polymerase chain reaction and LOH analysis, we found that biallelic inactivation via epigenetic changes of both maternal and paternal alleles, or epigenetic modification of one allele combined with genetic loss of the remaining allele, could completely suppress RAR beta2 expression in breast cancer.
RAR-beta2 expression was studied by reverse transcription-polymerase chain reaction (RT-PCR) analysis in eight breast cancer cell lines that were either treated with the demethylating agent 5-aza-2'-deoxycytidine and subsequently with all-trans-retinoic acid (ATRA) or left untreated.