Abstract B52: The epithelial-mesenchymal transition factor Snail represses the tumor suppressor microRNA let-7 and contributes to invasiveness of ovarian cancer cells.

Introduction: Cell fate choices are made throughout development, and in adult organisms whenever stem cells divide. We study the interrelation of transcription factors (TF) and microRNAs in cell fate decisions. Metastasis of ovarian and other solid tumors involves epithelial-mesenchymal transition (EMT), brought about by TF such as Snail, Twist, Slug, and Zeb1. Cells undergoing reprogramming paradoxically upregulate the EMT TF Snail and downregulate let-7, and Snail binds the promoters of several let-7 family members consistent with direct regulation. Let-7 is a microRNA highly expressed in differentiated cells, and absent in pluripotent cells. Its targets include the oncogenes Myc, Ras, and HMGA2. Many aspects of somatic cell reprogramming are seen as analogous to the acquisition of stem cell phenotypes observed in cancer. Let-7 levels are decreased in many ovarian tumors, and low let-7 correlates with poor prognosis. In breast cancer, let-7 downregulation is associated with metastasis. Results: Let-7 levels are known to be regulated by control of processing by the RNA binding protein and pluripotency factor Lin28, but transcriptional regulators of let-7 have not been described. We show that Snail represses let-7 transcription in cancer-related EMTs. ChIP assays demonstrate binding of Snail to let-7 promoters, and luciferase assays demonstrate direct regulation of Let-7i by Snail. Thus we are exploring two avenues for metastasis inhibition in OC: decreasing Snail and increasing let-7. We have characterized several OC cell lines for their phenotype with regard to EMT and cancer stem cell markers, in order to select agents to advance to testing in our orthotopic patient-derived xenograft model. We have examined the role of Snail in OC metastasis by manipulating expression levels and carrying out invasiveness assays. In epithelial and cancer cell lines, stimulation with TGF-beta or EGF caused morphological and gene expression changes consistent with EMT. The OC line OVSAHO underwent EMT when stimulated with EGF, but not with TGF-b. In breast, pancreatic, and OC cell lines, increases in Snail expression correlate with decreased let-7 expression. Conclusions: We propose that the inverse relationship between Snail and let-7 plays an important role at decision points crucial for cell fate decisions. In cancer metastasis, derepression of let-7 targets (many of them oncofetal genes) leads to dedifferentiation and a more aggressive phenotype. In addition to its role in directly causing EMT and affording cells with the migratory and invasive phenotype necessary for metastasis, our model places Snail at an important control point for initiation of dedifferentiation events normally held in check by let-7. Citation Format: Yevgeniya Ioffe, Alyse Hill, Michael McCarthy, Linda Sanderman, Hugo Campos, Tise Suzuki, Andrew Caster, Juli Unternaehrer-Hamm. The epithelial-mesenchymal transition factor Snail represses the tumor suppressor microRNA let-7 and contributes to invasiveness of ovarian cancer cells. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research: Exploiting Vulnerabilities; Oct 17-20, 2015; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(2 Suppl):Abstract nr B52.