Functional Activation of the Estrogen Receptor-a and Aromatase by the HDAC Inhibitor Entinostat Sensitizes ER-Negative Tumors to Letrozole

Approximately 25% of breast cancers do not express the estrogen receptor-a (ERa) and consequently do not respond to endocrine therapy. In these tumors, ERa repression is often due to epigenetic modifications such as methylation and histone deacetylation. For this reason, we investigated the ability of the histone deacetylase inhibitor entinostat (ENT) to trigger reexpression of ERa and aromatase in breast cancer cells, with the notion that this treatment would restore sensitivity to the aromatase inhibitor (AI) letrozole. ENT treatment of tumor cells increased expression of ERa and aromatase, along with the enzymatic activity of aromatase, in a dosedependent manner both in vitro and in vivo. Notably, ERa and aromatase upregulation resulted in sensitization of breast cancer cells to estrogen and letrozole. Tumor growth rate was significantly lower in tumor xenografts following treatment with ENT alone and in combination with letrozole than in control tumors (P > 0.001). ENT plus letrozole also prevented lung colonization and growth of tumor cells, with a significant reduction (P > 0.03) in both visible and microscopic foci. Our results show that ENT treatment can be used to restore the letrozole responsiveness of ER-negative tumors. More generally, they provide a strong rationale for immediate clinical evaluation of combinations of histone deacetylase and aromatase inhibitors to treat ER-negative and endocrineresistant breast cancers. Cancer Res; 71(5); 1893–903. 2011 AACR.