The dissemination of cancer cells from the primary tumor to distant sites, known as metastasis, is the main cause of mortality in cancer patients and is a hallmark of cancer. Metastasis is a very complex multicellular process that involves many biological pathways and steps. One essential important step in metastasis is the breakdown and remodeling of the extracellular matrix (ECM), a specialized network of fibrous proteins and proteoglycans surrounding many cells. The ECM not only provides an essential physical scaffolding for cellular constituents, but also initiates important biochemical and biomechanical cues or signals that are required for cancer metastasis. The breakdown and remodeling of the ECM is mediated by the formation of structures known as invadopodia, which are dynamic actin-rich membrane protrusions that degrade the ECM through the secretion of the matrix-degrading proteins, matrix metalloproteinases (MMPs). MMPs are a family of proteases that degrade various components of the ECM and have been strongly implicated in multiple stages of cancer progression including the acquisition of invasive and metastatic properties. Defects in secretion of MMPs have been implicated in many cancer cell types and are generally attributed to mutations in cellular transport and trafficking machinery necessary for exocytosis of cargo, like MMPs from secretory vesicles.

The release of the MMPs from cells by exocytosis from secretory vesicles leads directly to the breakdown of the components of the ECM and is involved in metastasis. To investigate the role MMPs have on metastasis, we study the storage, transport and exocytosis of secretory vesicles that release the proteases, into the ECM in human cancer cells. We study exocytosis in these cancer cells using fluorescence microscopy to image secretory vesicles containing MMPs in living cells. To do this, we use high-resolution imaging of fluorescently-labeled MMPs to monitor the fusion of secretory vesicles and exocytosis of its cargo through the plasma membrane. Live-cell imaging of cancer cells reveals thousands of secretory vesicles that store MMPs and that undergo regulated exocytosis and release into the ECM. This has allowed us to begin to identify the critical trafficking steps and exocytic proteins and regulators that are associated with MMP-containing secretory vesicles the mediate metastasis. This includes cell shape changes like migration/mobility that are accompanied by the formation of invasive MMP-dependent structures in cells. The goal of our research is to understand the molecular basis of exocytosis and how the plasma membrane continuously changes the shape and composition of cells (eg: proteins and lipids) during trafficking events that lead to exocytosis of MMPs during cancer metastasis.