<p>Cancer metastasis is the cause of about 90% of cancer patients' deaths. Despite significant improvements in the past three decades in understanding the molecular bases of oncogenic transformation of cancer cells, little is known about the molecular mechanisms underlying tumour cells' alteration of their microenvironment, entrance into the circulation, and colonization of distant organs. In recent years, accumulating evidence has indicated that tumour microenvironment, which consists of a variety of cell types and extracellular matrix components&#65292;plays an important role in regulating the metastatic abilities of carcinoma cells. Co-opted by cancer cells, those stromal cells promote tumour progression via multiple mechanisms, including enhancement of tumour invasiveness, elevation of angiogenesis, and suppression of immune surveillance activity. </p><p>Using a series of human breast cancer cell lines with different metastatic potentials <italic>in vivo</italic>, we performed an unbiased screen examining expression of miRNAs, and found that miR-126 and miR-126*, whose expression are regulated by methylation of the promoter of their host gene Egfl7 inside tumour cells, were significantly negatively correlated with metastatic potential. Using both mouse xenograft models and <italic>in vitro</italic> assays, we showed that this pair of miRNAs suppressed breast cancer metastasis through shaping the tumour microenvironment without changing tumour cell autonomous properties. Specifically, miR-126 and miR-126* act independently to suppress the sequential recruitment of mesenchymal stem cells (MSCs) and inflammatory monocytes into the primary tumour stroma, consequently inhibiting lung metastasis by breast tumour cells. Mechanistically, these miRNAs directly inhibit the production of stromal cell-derived factor-1 alpha (Sdf-1&alpha;, also known as Cxcl12), and indirectly suppress the expression of chemokine (C-C motif) ligand 2 (Ccl2) by the cancer cells within the tumour mass in an Sdf-1&alpha;-dependent manner. In addition, in contrast with the majority of reports which have shown incorporation of only the guiding strand of the miRNA duplex into the mRNA-targeting RNA induced silencing complex (RISC), both strands of the miR-126 RNA duplex are maintained at a similar level and suppress Sdf-1&alpha; expression independently. </p><p>Collectively, we have determined a dynamic process by which the composition of the primary tumour microenvironment could be altered via a change in the expression of two tumour-suppressive miRNAs derived from a single miRNA precursor to favor metastasis by breast cancer cells. Importantly, this work provides a prominent mechanism to explain the clinical correlation between reduced expression of miR-126/126* and poor metastasis-free survival of breast cancer patients.</p> / Dissertation

abstract: Stromal cell-derived factor-1α (SDF-1α) and its key receptor, CXCR4 are ubiquitously expressed in systems across the body (e.g. liver, skin, lung, etc.). This signaling axis regulates a myriad of physiological processes that range from maintaining of organ homeostasis in adults to, chemotaxis of stem/progenitor and immune cell types after injury. Given its potential role as a therapeutic target for diverse applications, surprisingly little is known about how SDF-1α mediated signaling propagates through native tissues. This limitation ultimately constrains rational design of interventional biomaterials that aim to target the SDF-1α/CXCR4 signaling axis. One application of particular interest is traumatic brain injury (TBI) for which, there are currently no means of targeting the underlying biochemical pathology to improve prognosis. Growing evidence suggests a relationship between SDF-1α/CXCR4 signaling and endogenous neural progenitor/stem cells (NPSC)-mediated regeneration after neural injury. Long-term modulation of the SDF-1α/CXCR4 signaling axis is thus hypothesized as a possible avenue for harnessing and amplifying endogenous regenerative mechanisms after TBI. In order to understand how the SDF-1α/CXCR4 signaling can be modulated in vivo, we first developed and characterized a sustained protein delivery platform in vitro. We were the first, to our knowledge, to demonstrate that protein release profiles from poly(D,L,-lactic-co-glycolic) acid (PLGA) particles can be tuned independent of particle fabrication parameters via centrifugal fractioning. This process of physically separating the particles altered the average diameter of a particle population, which is in turn was correlated to critical release characteristics. Secondly, we demonstrated sustained release of SDF-1α from PLGA/fibrin composites (particles embedded in fibrin) with tunable burst release as a function of fibrin concentration. Finally, we contrasted the spatiotemporal localization of endogenous SDF-1α and CXCR4 expression in response to either bolus or sustained release of exogenous SDF-1α. Sustained release of exogenous SDF-1α induced spatially diffuse endogenous SDF-1/CXCR4 expression relative to bolus SDF-1 administration; however, the observed effects were transient in both cases, persisting only to a maximum of 3 days post injection. These studies will inform future systematic evaluations of strategies that exploit SDF-1α/CXCR4 signaling for diverse applications. / Dissertation/Thesis / Doctoral Dissertation Bioengineering 2016