Poly-ADP-Ribosylation of Estrogen Receptor-Alpha by PARP1 Mediates Antiestrogen Resistance in Human Breast Cancer Cells.

Nicholas Pulliam,Kathy D Miller,Fang Fang,Heather M O’Hagan,Kenneth P Nephew,Robert Clarke,Weini Wang,Riddhi Sood,Jessica Tang

doi:10.3390/cancers11010043

Abstract

Therapeutic targeting of estrogen receptor-α (ERα) by the anti-estrogen tamoxifen is standard of care for premenopausal breast cancer patients and remains a key component of treatment strategies for postmenopausal patients. While tamoxifen significantly increases overall survival, tamoxifen resistance remains a major limitation despite continued expression of ERα in resistant tumors. Previous reports have described increased oxidative stress in tamoxifen resistant versus sensitive breast cancer and a role for PARP1 in mediating oxidative damage repair. We hypothesized that PARP1 activity mediated tamoxifen resistance in ERα-positive breast cancer and that combining the antiestrogen tamoxifen with a PARP1 inhibitor (PARPi) would sensitize tamoxifen resistant cells to tamoxifen therapy. In tamoxifen-resistant vs. -sensitive breast cancer cells, oxidative stress and PARP1 overexpression were increased. Furthermore, differential PARylation of ERα was observed in tamoxifen-resistant versus -sensitive cells, and ERα PARylation was increased by tamoxifen treatment. Loss of ERα PARylation following treatment with a PARP inhibitor (talazoparib) augmented tamoxifen sensitivity and decreased localization of both ERα and PARP1 to ERα-target genes. Co-administration of talazoparib plus tamoxifen increased DNA damage accumulation and decreased cell survival in a dose-dependent manner. The ability of PARPi to overcome tamoxifen resistance was dependent on ERα, as lack of ERα-mediated estrogen signaling expression and showed no response to tamoxifen-PARPi treatment. These results correlate ERα PARylation with tamoxifen resistance and indicate a novel mechanism-based approach to overcome tamoxifen resistance in ER+ breast cancer.

Highlights

Estrogen receptor-α (ERα) is a nuclear receptor [1,2,3] that responds to estrogen stimulation by translocating to the nucleus and regulating the transcription of target genes, including ERα and genes involved in regulating tumor progression [4]
We observed that tamoxifen-resistant (MCF7-T) cells showed increased levels of oxidative damage (Figure 1A), as indicated by immunofluorescence staining for the oxidative-damage marker
We observed greater interaction between ERα and PAR in the tamoxifen-resistant MCF7-T cell line compared to the sensitive MCF7 cells (Figure 3E). These results demonstrate that oxidative damage is sufficient to promote ERα PARylation in breast cancer and correlate ERα PARylation with tamoxifen resistance

Summary

Introduction

Estrogen receptor-α (ERα) is a nuclear receptor [1,2,3] that responds to estrogen stimulation by translocating to the nucleus and regulating the transcription of target genes, including ERα and genes involved in regulating tumor progression [4]. 70% of breast cancers express ERα and rely on estrogen binding for growth and promotion of tumorigenesis [4]. Many ERα+ breast cancer patients are treated with the anti-estrogen tamoxifen, which binds to the receptor within the ligand-binding domain (LBD). Highly effective in ER+ breast cancers, many of these patients develop resistance to tamoxifen, relapse and eventually succumb to the disease, despite continued ERα expression in their tumors [9]. In this regard, a better underlying mechanisms mediating tamoxifen resistance is of foremost clinical significance [10]

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Conclusion