Publications

MicroRNAs have been increasingly implicated in human cancer and interest has grown about the potential to use microRNAs to combat cancer. Lung cancer is the most prevalent form of cancer worldwide and lacks effective therapies. Here we have used both in vitro and in vivo approaches to show that the let-7 microRNA directly represses cancer growth in the lung. We find that let-7 inhibits the growth of multiple human lung cancer cell lines in culture, as well as the growth of lung cancer cell xenografts in immunodeficient mice. Using an established orthotopic mouse lung cancer model, we show that intranasal let-7 administration reduces tumor formation in vivo in the lungs of animals expressing a G12D activating mutation for the K-ras oncogene. These findings provide direct evidence that let-7 acts as a tumor suppressor gene in the lung and indicate that this miRNA may be useful as a novel therapeutic agent in lung cancer.

The modulation of gene expression by small non-coding RNAs is a recently discovered level of gene regulation in animals and plants. In particular, microRNAs (miRNAs) and Piwi-interacting RNAs (piRNAs) have been implicated in various aspects of animal development, such as neuronal, muscle and germline development. During the past year, an improved understanding of the biological functions of small non-coding RNAs has been fostered by the analysis of genetic deletions of individual miRNAs in mammals. These studies show that miRNAs are key regulators of animal development and are potential human disease loci.

Little is known about the protein complexes required for microRNA formation and function. Here we used native gel electrophoresis to identify miRNA ribonucleoprotein complexes (miRNPs) in Caenorhabditis elegans. Our data reveal multiple distinct miRNPs that assemble on the let-7 miRNA in vitro. The formation of these complexes is affected but not abolished by alg-1 or alg-2 null mutations. The largest complex (M*) with an estimated molecular mass of >669 kDa cofractionates with the known RISC factors ALG-1, VIG-1, and TSN-1. The M* complex and two complexes, M3 and M4, with similar molecular weights of approximately 500 kDa, also assemble on all other miRNAs used in our experiments. Two smaller complexes, M1 (approximately 160 kDa) and M2 (approximately 250 kDa), assemble on the members of the let-7 miRNAs family but not lin-4 or mir-234, and their formation is highly dependent on specific sequences in the 5' seed region of let-7. Moreover, an unidentified protein, p40, which only appears in the M1 and M2 complexes, was detected by UV triggered cross-linking to let-7 but not to lin-4. The cross-linking of p40 to let-7 is also dependent on the let-7 sequence. Another unidentified protein, p13, is detected in all let-7 binding complexes and lin-4 cross-linked products. Our data suggest that besides being present in certain large miRNPs with sizes similar to reported RISC, the let-7 miRNA also assembles with specific binding proteins and forms distinct small complexes.

MicroRNAs (miRNAs) are small noncoding RNAs, approximately 22 nucleotides in length, that repress target messenger RNAs (mRNAs) through an antisense mechanism. The let-7 miRNA was originally discovered in the nematode Caenorhabditis elegans, where it regulates cell proliferation and differentiation, but subsequent work has shown that both its sequence and its function are highly conserved in mammals. Recent results have now linked decreased let-7 expression to increased tumorigenicity and poor patient prognosis. Moreover, during normal development, accumulation of let-7 can be prevented by LIN28, a promoter of pluripotency. Based on these findings, we propose that let-7 regulates 'stemness' by repressing self-renewal and promoting differentiation in both normal development and cancer. A more complete understanding of its function will thus provide insights into these processes and might yield diagnostic and therapeutic advances for cancer treatment.

Cellular stress responses are potent and dynamic, allowing cells to effectively counteract diverse stresses. These pathways are crucial not only for maintaining normal cellular homeostasis, but also for protecting cells from what would otherwise lead to their demise. A novel class of genes, termed miRNAs, has recently been implicated in the cellular stress response. For example, it has been demonstrated that a cardiac-specific miRNA that is not required for normal development is requisite for a normal cardiac stress response in mice. In addition, we have found that a miRNA family is able to modulate the cellular response to cytotoxic cancer treatment both in vitro and in vivo. In this review, we will discuss these and other important developments in the field. In particular, we will focus on studies that have linked miRNAs to the genotoxic stress response and will suggest how this connection may be both important for our understanding of biology and pertinent for the development of novel cancer therapies.

MicroRNAs (miRNAs) are noncoding RNAs that regulate numerous target genes through a posttranscriptional mechanism and thus control major developmental pathways. The phylogenetically conserved let-7 miRNA regulates cell proliferation and differentiation, thus functioning as a key regulator of developmental timing in C. elegans and a tumor suppressor gene in humans. Using a reverse genetic screen, we have identified genetic interaction partners of C. elegans let-7, including known and novel potential target genes. Initial identification of several translation initiation factors as suppressors of a let-7 mutation led us to systematically examine genetic interaction between let-7 and the translational machinery, which we found to be widespread. In the presence of wild-type let-7, depletion of the translation initiation factor eIF3 resulted in precocious cell differentiation, suggesting that developmental timing is translationally regulated, possibly by let-7. As overexpression of eIF3 in humans promotes translation of mRNAs that are also targets of let-7-mediated repression, we suggest that eIF3 may directly or indirectly oppose let-7 activity. This might provide an explanation for the opposite functions of let-7 and eIF3 in regulating tumorigenesis.

The first two known microRNAs (miRNAs), lin-4 and let-7, were originally discovered in the nematode Caenorhabditis elegans and control the timing of stem-cell division and differentiation. let-7 was subsequently found as the first known human miRNA. let-7 and its family members are highly conserved across species in sequence and function, and misregulation of let-7 leads to a less differentiated cellular state and the development of cell-based diseases such as cancer. Although much research has been devoted to let-7 target prediction and to understanding its biological role, research into what regulates let-7 has only just begun. Here, we review let-7-family conservation and the recent advances in understanding how let-7-expression is regulated at the transcriptional and post-transcriptional levels across species. A greater understanding of what controls let-7 expression might enable the development of treatments to fight or prevent many cancers.

MicroRNAs (miRNAs) are a recently discovered class of small RNA molecules that negatively regulate gene expression at the post-transcriptional level. MiRNAs play key roles in development and establishment of cell identity and aberrant metabolism/expression of miRNAs has been linked to human diseases including cancer. Components of the miRNA machinery and miRNAs themselves are involved in many cellular processes that are altered in cancer, such as differentiation, proliferation and apoptosis. Some miRNAs exhibit differential expression levels in cancer and have demonstrated capability to affect cellular transformation, carcinogenesis and metastasis acting either as oncogenes or tumour suppressors. We are only beginning to comprehend the functional repercussions of the gain or loss of particular microRNAs on cancer. Nonetheless, although microRNAs have been discovered in humans a mere eight years ago, a host of promising potential applications in the diagnosis, prognoses and therapy of cancer are emerging at a rapid pace.

A circuit of transcription factors has been discovered in Caenorhabditis elegans that could provide a link between laboratory-defined intracellular 'longevity pathways', gene dysregulation and the process of normal aging.