134 Espinosa and Slack: The Role of MicroRNAs in Cancer There are 12 let-7 homologs in the human genome, organized in eight clusters. These clusters map to fragile sites associated with lung, breast, urothelial, and cervical cancers, suggesting that let-7 family mem- bers may act as tumor suppressors. At least four of these clusters have been confirmed to be commonly lost in malignancies [44]. Studies by Takamizawa et al. and Yanaihara et al. have presented evidence that tran- scripts of certain let-7 homologs are signif- icantly downregulated in human lung cancer and that low levels of let-7 correlate with poor prognosis [45,46]. Moreover, it has been shown that transient expression of miRNAs of the let-7 family in cell lines de- rived from lung adenocarcinomas inhibited colony formation, suggesting that these miRNAs could have growth-suppressing properties and opening the possibility that let-7 might be used as a therapeutic tool. Important insights into the mechanism through which let-7 is capable of controlling cellular proliferation came from our labora- tory, derived from the observation that the let-7 family member miR-84 negatively reg- ulates let-60, the ortholog of human HRAS, KRAS, and NRAS in C. elegans [47]. We have shown that certain members of the let- 7 family are capable of genetically interact- ing with RAS, and let-7 and RAS are reciprocally expressed in lung tumor sam- ples. We also found that let-7 is capable of negatively regulating RAS in human cells, and overexpression of let-7 in human cancer cell lines results in reduced levels of RAS protein, compared to untreated cells. As ex- pected, if let-7 levels are reduced in cancer cell lines, levels of RAS protein increase sig- nificantly. Moreover, through the use of re- porter constructs, it was confirmed that the regulation of RAS by let-7 takes place through the direct interaction of the miRNA with the 3’UTR of Ras mRNA. The human RAS family of proteins is one of the most important components of a myriad of signaling cascades, has been shown to possess oncogenic activity, and is often mutated in tumors [48]. As such, these studies position let-7 as a promising thera- peutic tool for the treatment of lung cancer, as well as other malignancies resulting from the overexpression of RAS. It will be impor- tant to more carefully analyze the specific contributions of the individual members of this miRNA family, in order to better under- stand the let-7-mediated regulation of cell growth. Other miRNAs acting as tumor suppressors Studies have implicated other miRNAs in tumorigenesis. MiR-143 and miR-145, for instance, have been shown to be con- stantly down-regulated in colorectal tumors [49], and recent studies by Croce et al. also have shown that the downregulation of these miRNAs is a common occurrence in breast carcinomas and breast cancer lines [50]. Moreover, the location of these miRNAs is known to be in a genomic locus frequently deleted in myelodysplastic syndrome [44]. While some studies have ascribed the down- regulation of miR-143 and miR-145 in colon cancer to a block in Dicer processing [49], further studies are required to determine whether the expression levels of these miRNAs are specifically altered. miRNAS AS ONCOGENES Several microRNAs may regulate MYC in a dynamic manner MicroRNAs can also act as oncogenes, either directly by down-regulating tumor suppressors or indirectly by down-regulat- ing genes that can act to restrict the activity of known oncogenes. An example of this is found in the dynamics of the interaction be- tween miR-155 and the MYC oncogene, a transcription factor with the ability to regu- late cell growth through the induction of both cell proliferation and apoptosis [51]. Notably, MYC is often mutated or amplified in human cancers. The miRNA miR-155 has been linked with MYC overexpression and B-cell cancers, suggesting it may be playing a role in the regulation of this oncogene. miR-155 is encoded by nucleotides 241-262 of BIC, which was originally iden- tified as a transcript derived from an integra- tion site for the avian leukosis virus and