let-7 Regulates Self Renewal and Tumorigenicity of Breast Cancer Cells

SUMMARY Cancers may arise from rare self-renewing tumor-initiating cells (T-IC). However, how T-IC self renewal, multipotent differentiation, and tumorigenicityaremaintainedremainsobscure. Because miRNAs can regulate cell-fate decisions, we compared miRNA expression in selfrenewing and differentiated cells from breast cancer lines and in breast T-IC (BT-IC) and non-BT-ICfrom1 � breastcancers.let-7miRNAs were markedly reduced in BT-IC and increased with differentiation. Infecting BT-IC with let-7lentivirus reduced proliferation, mammosphere formation, and the proportion of undifferentiated cells in vitro and tumor formation and metastasis in NOD/SCID mice, while antagonizing let-7 by antisense oligonucleotides enhanced in vitro self renewal of non-T-IC. Increased let-7 paralleled reduced H-RAS and HMGA2, known let-7 targets. Silencing H-RAS in a BTIC-enriched cell line reduced self renewal but had no effect on differentiation, while silencing HMGA2 enhanced differentiation but did not affect self renewal. Therefore let-7 regulates multiple BT-ICstem cell-like properties by silencing more than one target.