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Histone deacetylase inhibitor entinostat in combination with a retinoid downregulates HER2 and reduces the tumor initiating cell population in aromatase inhibitor-resistant breast cancer

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Abstract

Resistance to aromatase inhibitors (AIs) involves increased HER2. One mechanism by which HER2 may mediate resistance is through expansion of the tumor initiating cell (TIC) population. This study investigates whether combining all-trans retinoic acid (ATRA) and histone deacetylase inhibitor entinostat (ENT) can inhibit TICs and HER2 in AI-resistant cells and tumors. Modulation of cell viability and HER2 expression were assessed in AI-resistant cells treated with ATRA + ENT. Letrozole-resistant LTLT-Ca cells treated with ATRA + ENT were assayed for changes in TIC characteristics, such as TIC markers (BCRP, ALDH, and BMI-1), side population (SP), and mammosphere formation. Xenograft tumors of MCF-7Ca cells made resistant to letrozole were treated with ATRA, ATRA + letrozole, ATRA + ENT, or ATRA + ENT + letrozole. Resulting tumors were assayed for changes in TIC characteristics. Patient samples taken pre- and post-AI treatment were analyzed for changes in ERα and HER2 protein expression. Treatment with ATRA + ENT reduced HER2 expression and viability (P < 0.001) in AI-resistant cells, as well as decreased SP (P < 0.0001), mammosphere formation (P < 0.01), and expression of TIC molecular markers (P < 0.01) in LTLT-Ca. A reduction in tumor growth rate was observed in mice treated with ENT + ATRA + letrozole when compared to mice treated with single agents (P < 0.0001) or ENT + ATRA (P = 0.02). Decreased TIC characteristics, including mammosphere formation (P < 0.05), were observed in tumors from the triple combination. An increase in HER2 and downregulation in ERα protein expression was observed in patients upon resistance to AI (P < 0.005). These studies indicate that the combination of ATRA and ENT inhibits the TIC population of AI-resistant cells and may be effective in reducing tumor recurrence.

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Abbreviations

AI:

Aromatase inhibitor

ERα:

Estrogen receptor-alpha

HER2:

Human epidermal growth factor-2

TIC:

Tumor initiating cells

ALDH:

Aldehyde dehydrogenase

SP:

Side population

ENT:

Entinostat

HDAC:

Histone deacetylase

BCRP:

Breast cancer resistance protein

ATRA:

All-trans retinoic acid

Δ4A:

3,17-Androstenedione

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Acknowledgments

This work was supported by grants to G Sabnis (KG10037 from Susan G Komen), A Kazi (BC1039031 from Department of Defense Breast Cancer Research Program) and to A Brodie (CA-62483 from NCI/NIH and SAC100010 from Susan G Komen). The authors would like to thank the University of Maryland School of Medicine Flow Core for their assistance.

Conflict of interest

The authors declare that they have no conflict of interest.

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Authors and Affiliations

  1. Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore, MD, 21201, USA

    Amanda J. Schech, Preeti Shah, Stephen Yu, Gauri J. Sabnis & Armina Kazi

  2. Division of Biostatistics, University of Maryland School of Medicine and University of Maryland Marlene and Stewart Greenebaum Cancer Center, Baltimore, MD, 21201, USA

    Olga Goloubeva

  3. Department of Medicine, University of Maryland School of Medicine and University of Maryland Marlene and Stewart Greenebaum Cancer Center, Baltimore, MD, 21201, USA

    Paula Rosenblatt

  4. Department of Biological Sciences, Towson University, Towson, MD, USA

    Preeti Shah

  5. Department of Biology, Loyola University Maryland, Baltimore, MD, USA

    Armina Kazi

  6. Mayo Clinic, Jacksonville, FL, USA

    Saranya Chumsri

  7. Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, 685 W. Baltimore St, HSF-I, Room 580G, Baltimore, MD, 21201, USA

    Angela Brodie

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  1. Amanda J. Schech
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  2. Preeti Shah
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  4. Gauri J. Sabnis
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  5. Olga Goloubeva
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  7. Armina Kazi
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  8. Saranya Chumsri
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  9. Angela Brodie
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Correspondence to Angela Brodie.

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Schech, A.J., Shah, P., Yu, S. et al. Histone deacetylase inhibitor entinostat in combination with a retinoid downregulates HER2 and reduces the tumor initiating cell population in aromatase inhibitor-resistant breast cancer. Breast Cancer Res Treat 152, 499–508 (2015). https://doi.org/10.1007/s10549-015-3442-z

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  • DOI: https://doi.org/10.1007/s10549-015-3442-z

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