Aurora-A mitotic kinase induces endocrine resistance through down-regulation of ERα expression in initially ERα+ breast cancer cells

Abstract

Development of endocrine resistance during tumor progression represents a major challenge in the management of estrogen receptor alpha (ERα) positive breast tumors and is an area under intense investigation. Although the underlying mechanisms are still poorly understood, many studies point towards the 'cross-talk' between ERα and MAPK signaling pathways as a key oncogenic axis responsible for the development of estrogen-independent growth of breast cancer cells that are initially ERα+ and hormone sensitive. In this study we employed a metastatic breast cancer xenograft model harboring constitutive activation of Raf-1 oncogenic signaling to investigate the mechanistic linkage between aberrant MAPK activity and development of endocrine resistance through abrogation of the ERα signaling axis. We demonstrate for the first time the causal role of the Aurora-A mitotic kinase in the development of endocrine resistance through activation of SMAD5 nuclear signaling and down-regulation of ERα expression in initially ERα+ breast cancer cells. This contribution is highly significant for the treatment of endocrine refractory breast carcinomas, because it may lead to the development of novel molecular therapies targeting the Aurora-A/SMAD5 oncogenic axis. We postulate such therapy to result in the selective eradication of endocrine resistant ERαlow/- cancer cells from the bulk tumor with consequent benefits for breast cancer patients.

Figure 1. Endocrine Resistant Breast Cancer Cells.

(A) Immumohistochemistry staining of low-grade tubular tumors for MCF-7 and high-grade vMCF-7^ΔRaf-1 Primary and Metastatic tumors. Breast cancer xenografts were stained with a polyclonal antibody targeting the ERα (Abcam, Cambridge, Massachusetts, USA). (B) Immunofluorescence analysis showing down-regulation of ERα expression in vMCF-7^ΔRaf-1 1GX cancer cells compared to parental MCF-7 and vMCF-7^ΔRaf-1 cells. (C) Graph showing the percentage of cancer cells harboring an ERα^low/− phenotype from three independent experiments (+/− s.d.; *p<0.0705 vs. MCF-7; **p<0.0001 vs. vMCF-7^ΔRaf-1). (D) Graph showing the percentage of cancer cells in the S phase of the cell cycle during starvation from 17-β estradiol and following treatment with 17-β estradiol (10⁻¹⁰ M) alone or in combination with 4-OH-tamoxifen (10⁻⁷ M) for 48 hours from three independent experiments (+/− s.d.; *p<0.0008 vs. MCF-7; **p<0.0009 vs. vMCF-7^ΔRaf-1).

Figure 2. Molecular characterization of endocrine resistant breast cancer cells.

(A) Immunoblot analysis showing Aurora-A (Cell Signaling Technology, Boston, MA, USA) over-expression in vMCF-7^ΔRaf-1 1GX cancer cells compared to parental MCF-7 and vMCF-7^ΔRaf-1 cells. (B) In Vitro Real Time Cell Proliferation Assay showing stronger activity of Fulvestrant (50 nM) in combination with alisertib (50 nM) in tamoxifen resistant vMCF-7^ΔRaf-1 1GX cancer cells. Experiments were performed in triplicate. (C) Immunofluorescence analysis showing inhibition of nuclear SMAD5 (Cell Signaling Technology, Boston, MA, USA) phosphorylation in breast cancer cells treated with alisertib. p∼SMAD5 was labeled in red and nuclei were labeled in blue with DAPI. (D) Immunoblot analysis showing selective alisertib-induced down-regulation of SMAD5 phosphorylation.

Figure 3. Mechanistic Linkage Between Aurora-A Over-expression, SMAD5 Activation And ERα Down-Regulation In Initially ERα+ Breast Cancer Cells.

(A) Immunoblot analysis showing p∼Aurora-A in breast cancer cells. (B) Immunofluorescence analysis showing that Aurora-A-induced ERα down-regulation is linked to p∼SMAD5 nuclear activation. ERα (Abcam, Cambridge, Massachusetts, USA) was labeled in green, p∼SMAD5 (Cell Signaling Technology, Boston, MA, USA) was labeled in red and nuclei were labeled in blue with DAPI. (C) Graph showing the percentage of cells expressing ERα and p∼SMAD5 in breast cancer cells. Experiments were performed in triplicate (+/− s.d.; *p<0.0083 vs. vMCF-7^{ΔRaf-1/Aurora-A}; **p<0.0048 vs. vMCF-7^{ΔRaf-1/Aurora-A}; ***p<0.0083 vs. vMCF-7^{ΔRaf-1/Aurora-A}; ****p<0.0021 vs. vMCF-7^{ΔRaf-1/Aurora-A}).

Figure 4. Role Of SMAD5 Over-Expression In ERα Down-Regulation.

(A) Immunoblot analysis showing parental and MCF-7 cells engineered to over-express SMAD5. (B) Immunofluorescence analysis showing that SMAD5 over-expression induces ERα down-regulation in ERα+MCF-7 cells. ERα (Abcam, Cambridge, Massachusetts, USA) was labeled in green, p∼SMAD5 (Cell Signaling Technology, Boston, MA, USA) was labeled in red and nuclei were labeled in blue with DAPI. (C) Graph showing the percentage of cells expressing p∼SMAD5 and ERα in vMCF-7^SMAD5 and parental cells. Experiments were performed in triplicate (+/− s.d.; *p<0.0001 vs. MCF-7; **p<0.0001 vs. MCF-7).

Figure 5. Model of endocrine resistance and breast cancer progression.

Aberrant activation of MAPK signaling stabilizes and activates Aurora-A kinase that in turn induces down-regulation/loss of ERα expression through phosphorylation and activation of SMAD5 nuclear signaling leading to endocrine resistance and tumor progression.