Evidence of epigenetic changes affecting the chromatin state of the retinoic acid receptor beta 2 promoter in breast cancer cells

Retinoic acid (RA)-resistance in breast cancer cells has been associated wi th irreversible loss of retinoic acid receptor beta, RAR beta, gene express ion. Search of the causes affecting RAR beta gene activity has been oriente d at identifying possible differences either at the level of one of the RAR beta promoters, RAR beta 2, or at regulatory factors. We hypothesized that loss of RAR beta 2 activity occurs as a result of multiple factors, includ ing epigenetic modifications, which can pattern RAR beta 2 chromatin state. Using methylation-specific PCR, we found hypermethylation at RAR beta 2 in a significant proportion of both breast cancer cell lines and primary brea st tumors. Treatment of cells with a methylated RAR beta 2 promoter, by mea ns of the DNA methyltransferase inhibitor 5-Aza-2'-deoxycytidine (5-Aza-CdR ), led to demethylation within RAR beta 2 and expression of RAR beta indica ting that DNA methylation is at least one factor, contributing to RAR beta inactivity. However, identically methylated promoters can differentially re spond to RA, suggesting that RAR beta 2 activity may be associated to diffe rent repressive chromatin states. This supposition is supported by the find ing that the more stable repressive RAR beta 2 state in the RA-resistant MD A-MB-231 cell line can be alleviated by the HDAC inhibitor, trichostatin A (TSA), with restoration of RA-induced RAR beta transcription. Thus, chromat in-remodeling drugs might provide a strategy to restore RAR beta activity, and help to overcome the hurdle of RA-resistance In breast cancer.