miR-21 plays a dual role in tumor formation and cytotoxic response in breast tumors

Breast cancer (BrCa) relies on specific microRNAs to drive disease progression. Oncogenic miR-21 is upregulated in many cancers including BrCa and is associated with poor survival and treatment resistance. We sought to determine the role of miR-21 in BrCa tumor initiation, progression and treatment response. In a triple-negative BrCA model, radiation exposure increased miR-21 in both primary tumor and metastases. In vitro, miR-21 knockdown decreased survival in all BrCa subtypes in presence of radiation. The role of miR-21 to BrCa initiation was evaluated by implanting wild-type miR-21 BrCa cells into genetically-engineered mouse models where miR-21 was intact, heterozygous or globally ablated. Tumors were unable to grow in mammary fat pads of miR-21-/- mice, grew in ~50% of miR-21+/- and 100% in miR-21+/+ mice. The contribution of miR-21 in progression and metastases was tested by crossing miR-21-/- mice with mice that spontaneously develop BrCa. Global ablation of miR-21 significantly decreased tumorigenesis and metastases of BrCa while sensitizing tumors to radio- and chemotherapeutic agents by a Fas/FasL-dependent apoptosis. Therefore, targeting miR-21 alone or in combination with various radio or cytotoxic therapies may represent novel and efficacious therapeutic modalities for the future treatment of BrCa patients. We used microarray to identify gene expression associated with miR-21 function in breast development and metastasis. We analyzed miR-21+/+;MMTV-PyMT(WT) and miR-21-/-;MMTV-PyMT(KO) tumors with or without radiation treatment (RT) using Affymetrix gene expression array