IFITM1 suppression blocks proliferation and invasion of aromatase inhibitor-resistant breast cancer in vivo by JAK/STAT-mediated induction of p21

• Interferon induced transmembrane protein 1 (IFTM1) overexpression correlates with poor clinical outcome in breast cancer.
• IFITM1 knockdown diminishes AI-resistant MCF-7:5C tumor growth and invasiveness.
• IFITM1 loss in MCF-7:5C cells decreases CD31+ blood vessel density and matrix metalloproteinase 1 (MMP1) expression in vivo.
• IFITM1 overexpression in MCF-7 cells drives estrogen-independent proliferation in vitro, and tumor growth and invasion in vivo.
• Loss of IFITM1 in MCF-7:5C cells induces death due to JAK/STAT mediated increases in p21 expression and nuclear localization.

Interferon induced transmembrane protein 1 (IFITM1) belongs to a family of interferon stimulated genes (ISGs) that is associated with tumor progression and DNA damage resistance; however, its role in endocrine resistance is not known. Here, we correlate IFITM1 expression with clinical stage and poor response to endocrine therapy in a tissue microarray consisting of 94 estrogen receptor (ER)-positive breast tumors. IFITM1 overexpression is confirmed in the AI-resistant MCF-7:5C cell line and not found in AI-sensitive MCF-7 cells. In this study, the orthotopic (mammary fat pad) and mouse mammary intraductal (MIND) models of breast cancer are used to assess tumor growth and invasion in vivo. Lentivirus-mediated shRNA knockdown of IFITM1 in AI-resistant MCF-7:5C cells diminished tumor growth and invasion and induced cell death, whereas overexpression of IFITM1 in wild-type MCF-7 cells promoted estrogen-independent growth and enhanced their aggressive phenotype. Mechanistic studies indicated that loss of IFITM1 in MCF-7:5C cells markedly increased p21 transcription, expression and nuclear localization which was mediated by JAK/STAT activation. These findings suggest IFITM1 overexpression contributes to breast cancer progression and that targeting IFITM1 may be therapeutically beneficial to patients with endocrine-resistant disease.

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