**7.1 Therapeutic potential of microRNAs in cancer**

Rapidly growing evidence supports that miRNAs play key roles in the pathogenesis of cancer and many miRNAs can function either as oncogenes or tumor suppressors [55]. MiRNAs can influence the development, progression, and metastasis of cancers [29, 30]. Their functional effect may differ depending on their expression levels. They have either an oncogenic potential or tumor-suppressor effect depending on their downstream impact on target genes and thereby controlling the biologic manifestations of cancers. The activity of a lost or down-regulated tumor suppressor miRNA can be restored by using miRNA mimics [56]. To date, there are some miRNA-based trials for treatment of cancers. For examples, miR-34 is one of the tumor suppressor miRNAs and it is significantly downregulated in many kinds of cancer [57]. Therefore, a cancer therapy synthetic miR-34 (MRX34) has been developed and has entered phase I clinical trial for liver cancer and metastasis from other cancers (NCT01829971) [57]. In lung cancer, miR-27a has been reported to be a potential targeted therapy for lung cancer [58]. MicroRNA-loaded minicells (miR-16-based mimic miRNA) are designed to counteract the loss of the miR-15 and miR-16 family and are used in clinic trials for small-cell lung cancer and mesothelioma [59]. The miR-205BP/S3 is a possible promising therapeutic modality for melanoma [60]. Let-7 is well recognized as one of the important tumor suppressors. So re-expression of the tumor-suppressor let-7 is another proposed miRNA therapeutic strategy to upregulate tumor-suppressor miRNA by exogenously transfecting with pre-let-7 that led to the inhibition of growth [27]. In addition to tumor suppressor miRNAs, some of the miRNAs can be served as oncogenes and used as therapeutic targets for cancer. For example, miR-21 is significantly overexpressed in many types of human cancers, thus miR-21 is a potential therapeutic target for a certain cancer [47].
