A positive TGF‐β/miR‐9 regulatory loop promotes the expansion and activity of tumour‐initiating cells in breast cancer

Abstract

Background and PurposeMicroRNA‐9 (miR‐9) has previously been described as a dual‐functional RNA during breast cancer progression and its roles need to be clarified thoroughly.Experimental ApproachA miR‐9 knockout mode of mouse breast cancer, the MMTV‐PyMT model (PyMT‐miR‐9−/−), combined with different human breast cancer cell lines were used to evaluate the effects of miR‐9 on breast cancer initiation, progression and metastasis. Lin‐NECs (Neoplastic mammary epithelial cells) and pNECs (Pre‐neoplastic mammary epithelial cells) were isolated and subjected to tumour‐initiation assay. Whole‐mount staining of mammary gland and histology was performed to determine mammary gland growth. Tumour‐initiating analysis combining a series of in vitro experiments were carried out to evaluate miR‐9 roles in tumour‐initiating ability. RNA‐sequencing of human breast cancer cells, and mammary glands at hyperplastic stages and established tumours in PyMT and PyMT‐miR‐9−/− mice, ChIP and luciferase report assays were conducted to reveal the underlying mechanisms.Key ResultsMiR‐9 is ectopically expressed in breast cancer and its level is negatively correlated with the prognosis, especially in basal‐like breast cancer patients. Additionally, miR‐9 is essential for breast cancer progression by promoting the expansion and activity of tumour‐initiating cells (TICs) in preneoplastic glands, established tumours and xenograft modes. Mechanistically, the activity of TICs hinges on a positive TGF‐β/miR‐9 regulatory loop mediated by the STARD13/YAP axis.Conclusions and ImplicationsThese findings demonstrate that miR‐9 is an oncogenic miRNA rather than a tumour‐suppressor in breast cancer, calling for rectification of the model for this conserved and highly abundant miRNA.