Dissecting the role of microRNAs during metastasis

Cancer is one of the major causes of death in the Western world. While frequently cured if detected at early stages, cancer therapies often fail in patients suffering from progressed, metastatic tumors. Consequently, there is an urgent necessity for dissecting the molecular mechanisms leading to metastasis (Gupta and Massague 2006). In the last few years microRNAs (miRNAs), ~20-23 nt long non-coding RNAs, have emerged as novel regulators of gene expression that bind to cognate regions in the 3`UTR of mRNAs (He and Hannon 2004) (Ambros 2004) (Engels and Hutvagner 2006). miRNAs have been shown to be crucially involved in the fine-tuning of gene expression, in cell differentiation and development (Alvarez-Garcia and Miska 2005) (Giraldez, Mishima et al. 2006) (Kloosterman and Plasterk 2006). Importantly, a disregulation of their expression can result in tumorigenic transformation and therefore some miRNAs can even be used as biomarkers to monitor cancer (Hahn, Counter et al. 1999) (Voorhoeve and Agami 2006) (Yanaihara, Caplen et al. 2006) (Mattie, Benz et al. 2006). Although cancers can be classified and prognosed according to their miRNA profile (Lu, Getz et al. 2005) (Yanaihara, Caplen et al. 2006), functional characterisation of miRNA-regulated pathways and their role during metastasis is lacking. The main goal of this project is to identify and functionally characterise miRNAs involved in the metastatic process. In particular, we want to analyse how miRNA expression changes during epithelial-to- mesenchymal-transition (EMT) and metastasis, identify miRNAs crucially involved in these processes and characterise the metastasis-relevant pathways regulated by those miRNAs. Achieving these goals will extend our knowledge towards metastasis and might result in the identification of novel biomarkers and potential new targets for therapeutic intervention.

Our aim was to identify miRNAs dysregulated during cancer progression, to reveal their downstream targets and pathways and to analyse their contribution to tumorigenesis. We successfully identified three miRNAs and could analyse their impact on tumor progression (1 paper published, 2 resubmitted): 1.) miR-29a: miR-29a suppresses tristetraprolin, which is a regulator of epithelial polarity and metastasis (published in EMBO Reports, April 2009). We showed that miR-29a is often upregulated in human breast tumors and suppresses the expression of tristetraprolin (TTP), a regulator of mRNAs with AU-rich 3`-UTRs (many oncogenes belong to this class). This induced epithelial-to-mesenchymal-transition (EMT) and promoted metastatic lung colonisation. 2.) miR-100: miRNA-100-dependent IGF2 suppression inhibits breast tumorigenesis by interfering with proliferation and survival signaling (resubmitted to EMBO Mol Med, February 2011) We found a strong reduction in miR-100 levels in human breast cancer. Oncogenic Ras/Erk signaling can downregulate miR-100. Loss of miR-100 led to a very strong increase in IGF2 levels and a promotion of tumorigenesis. Overexpression of miR-100 could inhibit breast tumorigenesis in mouse experiments and might therefore be an interesting candidate for miRNA restoration therapies. 3.) miR-193a: miR-193a promotes breast cancer by downregulating Sef, an inhibitor of FGF signaling (resubmitted to Nature Cell Biology, February 2011, recently rejected; will be modified according to the reviews and resubmitted in the near future). We demonstrated upregulation of miR-193a in human breast carcinomas. This caused suppression of Sef, a negative regulator of FGF signaling concomitantly with changes in FGF target genes including Mdm2, which was strongly increased. Mdm2 is a master regulator of p53 and apoptosis and an important oncogene. We generated a transgenic mouse with inducible miR-193 expression (in collaboration with TaconicArtemis) and could show that miR-193a promoted surival in tumors, whereas its inhibition interfered with tumor growth and metastatic lung colonisation.