MicroRNA-7 Inhibits Multiple Oncogenic Pathways to Suppress HER2Δ16 Mediated Breast Tumorigenesis and Reverse Trastuzumab Resistance

The oncogenic isoform of HER2, HER2 Delta 16, is expressed with HER2 in nearly 50% of HER2 positive breast tumors where HER2 Delta 16 drives metastasis and resistance to multiple therapeutic interventions including tamoxifen and trastuzumab. In recent years microRNAs have been shown to influence multiple aspects of tumorigenesis and tumor cell response to therapy. Accordingly, the HER2 Delta 16 oncogene alters microRNA expression to promote endocrine resistance. With the goal of identifying microRNA suppressors of HER2 Delta 16 oncogenic activity we investigated the contribution of altered microRNA expression to HER2 Delta 16 mediated tumorigenesis and trastuzumab resistance. Using a gene array strategy comparing microRNA expression profiles of MCF-7 to MCF-7/HER2 Delta 16 cells, we found that expression of HER2 Delta 16 significantly altered expression of 16 microRNAs by 2-fold or more including a 4.8 fold suppression of the miR-7 tumor suppressor. Reestablished expression of miR-7 in the MCF-7/HER2 Delta 16 cell line caused a G1 cell cycle arrest and reduced both colony formation and cell migration activity to levels of parental MCF-7 cells. Suppression of miR-7 in the MCF-7 cell line resulted in enhanced colony formation activity but not cell migration, indicating that miR-7 suppression is sufficient to drive tumor cell proliferation but not migration. MiR-7 inhibited MCF-7/HER2 Delta 16 Delta cell migration through a mechanism involving suppression of the miR-7 target gene EGFR. In contrast, miR-7 inhibition of MCF-7/HER2 Delta 16 cell proliferation involved a pathway where miR-7 expression resulted in the inactivation of Src kinase independent of suppressed EGFR expression. Also independent of EGFR suppression, reestablished miR-7 expression sensitized refractory MCF-7/HER2 Delta 16 cells to trastuzumab. Our results demonstrate that reestablished miR-7 expression abolishes HER2 Delta 16 induced cell proliferation and migration while sensitizing HER2 Delta 16 expressing cells to trastuzumab therapy. We propose that miR-7 regulated pathways, including EGFR and Src kinase, represent targets for the therapeutic intervention of refractory and metastatic HER2 Delta 16 driven breast cancer.