UC San Diego

Breast cancer metastasis, the dissemination of primary tumor cells to distant organs, is the leading cause of death among breast cancer patients. During metastasis, tumor cells must invade through the basement membrane and local tissues, intravasate into the circulatory system, travel to a distant organ, extravasate into the new tissue, and establish micrometastases, which eventually become secondary tumors. The basic helix-loop-helix transcription factor Twist1 is expressed in a variety of aggressive cancers, and is essential for metastasis to take place. Twist1 activates a developmental program known as the Epithelial-Mesenchymal Transition (EMT), during which tumor cells lose their cell-cell junctions and become more migratory and invasive. Twist1 endows tumor cells with an increased invasive capacity by inducing formation of subcellular structures known as invadopodia. Located on the ventral surface of invasive cells, invadopodia are actin-rich protrusions of the cell membrane that recruit proteases to points of cell-matrix contact, and thus degrade the surrounding extracellular matrix (ECM). It was found that Twist1-mediated formation of invadopodia is required for metastasis to take place, but the precise role that Twist1 plays in invadopodia assembly was not completely understood. This dissertation presents data demonstrating that Twist1 is involved in regulation of a serine protease known as Fibroblast Activation Protein alpha (FAPα). FAPα is located at invadopodia, and while not required for EMT or cellular migration, is essential for metastasis to take place. Further investigation found that FAPα expression is required for ECM degradation, but its protease activity is not. Although ECM degradation is impaired when FAPα expression is suppressed, early stages of invadopodia formation still take place, suggesting that FAPα may be involved in the later steps of invadopodia assembly, when proteases are recruited. Indeed, results show that the matrix metalloprotease MT1-MMP is mislocalized in the absence of FAPα expression, providing additional evidence that the role of FAPα at invadopodia is more structural than proteolytic. Further studies must be undertaken to examine how FAPα is trafficked to invadopodia, and which domain of the protein is important for its function.