Inhibition of the proliferation of acquired aromatase inhibitor-resistant breast cancer cells by histone deacetylase inhibitor LBH589 (panobinostat)

Aromatase inhibitors (AIs) are important drugs for treating postmenopausal patients with hormone receptor-positive breast cancer. However, acquired resistance to AI therapies is a significant problem. Our study has revealed that the histone deacetylase inhibitor LBH589 treatment abrogated growth of AI-resistant cells in vitro and in vivo, causing cell cycle G2/M arrest and induced apoptosis. LBH589 treatment also reduced the level of NF-kappa B1 which is overexpressed when AI resistance develops. Analyzing paired tumor specimens from 12 patients, we found that NF-kappa B1 expression was increased in recurrent AI-resistant tumors as compared to the paired primary tumors before AI treatment. This finding was consistent with up-regulated NF-kappa B1 expression seen in a collection of well-established AI-resistant cell lines. Furthermore, knockdown of NF-kappa B1 expression significantly suppressed the proliferation of AI-resistant cells. Treatment of AI-resistant cell lines with LBH589 suppressed NF-kappa B1 mRNA and protein expression. In addition, LBH589 treatment abrogated growth of AI-resistant tumors in mice, and was associated with significantly decreased levels of NF-kappa B1 in tumors. In all, our findings strongly support further investigation of LBH589 as a novel therapeutic strategy for patients with AI-resistant breast cancer, in part by suppressing the NF-kappa B1 pathway.