THE ROLE OF MIR-340 IN POST-TRANSCRIPTIONAL REGULATION OF THE UPA-SYSTEM IN BREAST CANCER

The urokinase-type plasminogen activator system (uPA-system), whose main components are the serine protease uPA (PLAU), the cell surface receptor uPAR (PLAUR) and the uPA inhibitor PAI-1 (SERPINE1), has been extensively studied for its involvement in cancer pathogenesis. Specifically, nowadays the components of the uPA-system are well-characterised determinants for the prognosis of breast cancer. The regulation of the gene expression of the uPA-system components is very complex and depends on a plethora of stimuli acting both at transcriptional and post-transcriptional level. The uPA-system components are often over expressed in breast cancer but the detailed molecular mechanisms regulating the expression are still to uncover. In an expression analysis conducted on a cohort of unselected breast cancer patients, we found that the expression of PLAU and PLAUR is highly correlated. Meta-analyses of published experimental data and in silico studies pointed out the possibility that PLAU, PLAUR and also SERPINE1 might be negatively regulated at post-transcriptional level by a microRNA, the miR-340. We experimentally validated the role of miR-340 as negative regulator of the expression of the three uPA-system components using MDA-MB-231, a triple negative breast cancer cell line. Microarray experiments, performed to characterise the global transcriptome changes induced by miR-340 in MDA-MB-231 cells, showed that miR-340 down regulates also the expression of desmoplastic reaction-related genes underlining a possible role of miR-340 in regulating tumour-associated genes. Notably, most of the identified miR-340 target genes were found indeed to be associated with poor clinical outcome in breast cancer. Functional studies carried out in MDA-MB-231 cells suggested that miR-340 might modulate cell proliferation, even if this effect was not confirmed in vivo. In order to better define the functional role of miR-340, we generated a miR-340 deficient mouse model, taken advantage of the zinc finger nuclease technology. Overall these data identify, for the first time, a single microRNA that is able to down regulate the expression of the three main components of the uPA-system together with desmoplastic reaction and breast cancer prognosis-related genes, thus representing a new potential player in the pathogenesis of breast cancer.