High‐throughput screens identify microRNAs essential for HER2 positive breast cancer cell growth

MicroRNAs (miRNAs) are non‐coding RNAs regulating gene expression post‐transcriptionally. We have characterized the role of miRNAs in regulating the human epidermal growth factor receptor 2 (HER2)‐pathway in breast cancer. We performed miRNA gain‐of‐function assays by screening two HER2 amplified cell lines (KPL‐4 and JIMT‐1) with a miRNA mimic library consisting of 810 human miRNAs. The levels of HER2, phospho‐AKT, phospho‐ERK1/2, cell proliferation (Ki67) and apoptosis (cPARP) were analyzed with reverse‐phase protein arrays. Rank product analyses identified 38 miRNAs (q < 0.05) as inhibitors of HER2 signaling and cell growth, the most effective being miR‐491‐5p, miR‐634, miR‐637 and miR‐342‐5p. We also characterized miRNAs directly targeting HER2 and identified seven novel miRNAs (miR‐552, miR‐541, miR‐193a‐5p, miR‐453, miR‐134, miR‐498, and miR‐331‐3p) as direct regulators of the HER2 3′UTR. We demonstrated the clinical relevance of the miRNAs and identified miR‐342‐5p and miR‐744* as significantly down‐regulated in HER2‐positive breast tumors as compared to HER2‐negative tumors from two cohorts of breast cancer patients (101 and 1302 cases). miR‐342‐5p specifically inhibited HER2‐positive cell growth, as it had no effect on the growth of HER2‐negative control cells in vitro. Furthermore, higher expression of miR‐342‐5p was associated with better survival in both breast cancer patient cohorts. In conclusion, we have identified miRNAs which are efficient negative regulators of the HER2 pathway that may play a role in vivo during breast cancer progression. These results give mechanistic insights in HER2 regulation which may open potential new strategies towards prevention and therapeutic inhibition of HER2‐positive breast cancer.