Abstract
MicroRNA-mediated post-transcriptional regulation plays key roles in stem cell self-renewal and tumorigenesis. However, the in vivo functions of specific microRNAs in controlling mammary stem cell (MaSC) activity and breast cancer formation remain poorly understood. Here we show that miR-31 is highly expressed in MaSC-enriched mammary basal cell population and in mammary tumors, and is regulated by NF-κB signaling. We demonstrate that miR-31 promotes mammary epithelial proliferation and MaSC expansion at the expense of differentiation in vivo. Loss of miR-31 compromises mammary tumor growth, reduces the number of cancer stem cells, as well as decreases tumor-initiating ability and metastasis to the lung, supporting its pro-oncogenic function. MiR-31 modulates multiple signaling pathways, including Prlr/Stat5, TGFβ and Wnt/β-catenin. Particularly, it activates Wnt/β-catenin signaling by directly targeting Wnt antagonists, including Dkk1. Importantly, Dkk1 overexpression partially rescues miR31-induced mammary defects. Together, these findings identify miR-31 as the key regulator of MaSC activity and breast tumorigenesis.
Highlights
mammary stem cell (MaSC) are controlled by the dynamic interplay of multiple molecular pathways such as hormone, Notch and Wnt signaling[7, 10, 11]
We demonstrate that miR-31 plays an important role in MaSC self-renewal and tumorigenesis by regulating Wnt pathway activation
This study is the first to demonstrate the in vivo, physiological role of a specific microRNA in regulating MaSCs and mammary gland development using a loss-of-function mouse model (Supplementary Fig. 9)
Summary
MaSCs are controlled by the dynamic interplay of multiple molecular pathways such as hormone, Notch and Wnt signaling[7, 10, 11]. Wnt/β-Catenin signaling is important for promoting MaSC activity and determining a basal cell fate. By utilizing miR-31 gain− and loss-of-function mouse models, coupled with the MMTVPyVT mammary tumor model, here we demonstrate that miR-31 promotes MaSC activity and breast tumorigenesis by regulating multiple signaling pathways. Chromatin immunoprecipitation (ChIP) assay revealed that p65 binds to its predicted cognate sites in miR-31 promoter, and that RANKL siRNA reduced this binding (Fig. 1h) Together, these data suggest that miR-31 expression is directly activated by NF-κB pathway in the mammary gland. In miR-31 promoter driven-luciferase reporter assays, treatment of estradiol and progesterone enhanced luciferase reporter activity, while mutation of p65-binding sites blocked it (Supplementary Fig. 1f). MiR-31 expression is hormone-responsive, likely through an indirect mechanism that involves the RNAKL/RANK/NF-κB pathway
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