Aurora kinase A and B as new treatment targets in aromatase inhibitor-resistant breast cancer cells.

Aromatase inhibitors (AIs) are used for treatment of estrogen receptor α (ER)-positive breast cancer; however, resistance is a major obstacle for optimal outcome. This preclinical study aimed at identifying potential new treatment targets in AI-resistant breast cancer cells. Parental MCF-7 breast cancer cells and four newly established cell lines, resistant to the AIs exemestane or letrozole, were used for a functional kinase inhibitor screen. A library comprising 195 different compounds was tested for preferential growth inhibition of AI-resistant cell lines. Selected targets were validated by analysis of cell growth, cell cycle phase distribution, protein expression, and subcellular localization. We identified 24 compounds, including several inhibitors of Aurora kinases e.g., JNJ-7706621 and barasertib. Protein expression of Aurora kinase A and B was found upregulated in AI-resistant cells compared with MCF-7, and knockdown studies showed that Aurora kinase A was essential for AI-resistant cell growth. In AI-resistant cell lines, the clinically relevant Aurora kinase inhibitors alisertib and danusertib blocked cell cycle progression at the G2/M phase, interfered with chromosome alignment and spindle pole formation, and resulted in preferential growth inhibition compared with parental MCF-7 cells. Even further growth inhibition was obtained when combining the Aurora kinase inhibitors with the antiestrogen fulvestrant. Our study is the first to demonstrate that Aurora kinase A and B may be treatment targets in AI-resistant cells, and our data suggest that therapy targeting both ER and Aurora kinases may be a potent treatment strategy for overcoming AI resistance in breast cancer.

New cell culture model for aromatase inhibitor-resistant breast cancer shows sensitivity to fulvestrant treatment and cross-resistance between letrozole and exemestane.

Aromatase inhibitor (AI) treatment is first-line systemic treatment for the majority of postmenopausal breast cancer patients with estrogen receptor (ER)-positive primary tumor. Although many patients benefit from treatment, some will develop resistance, and models mimicking acquired resistance will be valuable tools to unravel the resistance mechanisms and to find new treatments and biomarkers. Cell culture models for acquired resistance to the three clinically relevant AIs letrozole, anastrozole and exemestane were developed by selection and expansion of colonies of MCF-7 breast cancer cells surviving long-term AI treatment under conditions where endogenous aromatase-mediated conversion of androgen to estrogen was required for growth. Four cell lines resistant to each of the AIs were established and characterized. Maintenance of ER expression and function was a general finding, but ER loss was seen in one of twelve cell lines. HER receptor expression was increased, in particular EGFR expression in letrozole-resistant cell lines. The AI-resistant cell lines had acquired ability to grow without aromatase-mediated conversion of testosterone to estradiol, but upon withdrawal of AI treatment, testosterone induced minor growth stimulation. Letrozole, exemestane and tamoxifen were able to abrogate the testosterone stimulation but could not reduce growth to below the level in standard growth medium with AI, demonstrating cross-resistance between letrozole, exemestane and tamoxifen. In contrast, fulvestrant totally blocked growth of the AI resistant cell lines both after withdrawal of AI and with AI treatment. These data show that ER is the main driver of growth of the AI-resistant cell lines and indicate ligand-independent activation of ER. Fulvestrant is an efficient treatment option for these AI-resistant breast cancer cells, and the cell lines will be useful tools to disclose the underlying molecular mechanism for resistance to the different AIs.

Sorafenib and nilotinib resensitize tamoxifen resistant breast cancer cells to tamoxifen treatment via estrogen receptor α.

Tamoxifen­resistant breast cancer is a major clinical problem and new treatment strategies are highly warranted. In this study, the multitargeting kinase inhibitors sorafenib and nilotinib were investigated as potential new treatment options for tamoxifen­resistant breast cancer. The two compounds inhibited cell growth, reduced expression of total estrogen receptor α (ER), Ser118-phosphorylated ER, FOXA1 and AIB1 and resensitized tamoxifen­resistant cells to tamoxifen. The ER downmodulator fulvestrant exerted strong growth inhibition of tamoxifen­resistant cells and addition of sorafenib and nilotinib could not further suppress growth, showing that sorafenib and nilotinib exerted growth inhibition via ER. In support of this, estradiol prevented sorafenib and nilotinib mediated growth inhibition. These results demonstrate that sorafenib and nilotinib act via ER and ER-associated proteins, indicating that these kinase inhibitors in combination with tamoxifen may be potential new treatments for tamoxifen­resistant breast cancer.