Anti-estrogen therapies are not effective in ER− breast cancers, thus identifying mechanisms underlying lack of ER expression in ER− breast cancers is imperative. We have previously demonstrated that hyperactivation of MAPK (hMAPK) downstream of overexpressed EGFR or overexpression/amplification of Her2 represses ER protein and mRNA expression. Abrogation of hMAPK in ER− breast cancer cell lines and primary cultures causes re-expression of ER and restoration of anti-estrogen responses. This study was performed to identify mechanisms of hMAPK-induced transcriptional repression of ER. We found that ER promoter activity is significantly reduced in the presence of hMAPK signaling, yet did not identify specific promoter sequences responsible for this repression. We performed an epigenetic compound screen in an ER− breast cancer cell line that expresses hMAPK yet does not exhibit ER promoter hypermethylation. A number of HDAC inhibitors were identified and confirmed to modulate ER expression and estrogen signaling in multiple ER− cell lines and tumor samples lacking ER promoter methylation. siRNA-mediated knockdown of HDACs 1, 2, and 3 reversed the mRNA repression in multiple breast cancer cell lines and primary cultures and ER promoter-associated histone acetylation increased following MAPK inhibition. These data implicate histone deacetylation downstream of hMAPK in the observed ER mRNA repression associated with hMAPK. Importantly, histone deacetylation appears to be a common mechanism in the transcriptional repression of ER between ER− breast cancers with or without ER promoter hypermethylation.
Bibliographical noteFunding Information:
Acknowledgments This work was supported by grants from the National Institutes of Health (NIH 1R01 CA113674 to DEA and NIH 1R01NS067289 to NA), the Bankhead Coley Foundation (BC-09BW-04-RC1 to DEA), and by the Sylvester Comprehensive Cancer Center at the University of Miami Miller School of Medicine (to DEA). We would like to thank the Center for Therapeutic Innovation at the University of Miami Miller School of Medicine for the use of their epigenetic compound library and laboratory equipment, and Laura Parsons and Joeli Brinkman for technical contributions. We would also like to thank Drs. Marc Lippman, Zafar Nawaz, and Kerry Burnstein of the University of Miami for thoughtful discussion.
© 2014, Springer Science+Business Media New York.
- Breast cancer
- Estrogen receptor
- Histone deacetylase inhibitor
- Hormonal status
- Transcriptional repression