During metastasis, carcinoma cells acquire the ability to invade surrounding tissues and intravasate through the endothelium to enter systemic circulation. Both the invasion and intravasation processes require degradation of basement membrane and extracellular matrix (ECM). Although proteolytic activity is associated with increased metastasis and poor clinical outcome, the molecular triggers for matrix degradation in tumor cells are largely unknown.
Invadopodia are specialized actin-based membrane protrusions found in cancer cells that degrade ECM via localization of proteases (Tarone et al., 1985
, Chen, 1989
). Their ability to mediate focal ECM degradation suggests a critical role for invadopodia in tumor invasion and metastasis. However, a definitive role for invadopodia in local invasion and metastasis in vivo has not yet been clearly demonstrated. As actin-based structures, invadopodia contain a primarily branched F-actin core and actin regulatory proteins, such as cortactin, WASp, and the Arp2/3 complex (Linder, 2007
). The SH3-domain-rich proteins Tks4 (Buschman et al., 2009
) and Tks5 (Seals et al., 2005
) function as essential adaptor proteins in clustering structural and enzymatic components of invadopodia. The matrix degradation activity of invadopodia has been associated with a large number of proteases, including membrane type MMPs (MT1-MMP) (Linder 2007
). Invadopodia formation requires tyrosine phosphorylation of several invadopodia components including cortactin (Ayala et al., 2008
), Tks4 (Buschmann et al., 2009
), and Tks5 (Seals et al., 2005
) by Src family kinases.
Our previous study found that the Twist1 transcription factor, a key regulator of early embryonic morphogenesis, was essential for the ability of tumor cells to metastasize from the mammary gland to the lung in a mouse breast tumor model and was highly expressed in invasive human lobular breast cancer (Yang et al., 2004
). Since then, studies have also associated Twist1 expression with many aggressive human cancers, such as melanomas, neuroblastomas, prostate cancers, and gastric cancers (Peinado et al., 2007
). Twist1 can activate a latent developmental program termed the epithelial-mesenchymal transition (EMT), thus enabling carcinoma cells to dissociate from each other and migrate.
The EMT program is a highly conserved developmental program that promotes epithelial cell dissociation and migration to different sites during embryogenesis. During EMT, cells lose their epithelial characteristics, including cell adhesion and polarity, and acquire a mesenchymal morphology and the ability to migrate (Hay, 1995
). Biochemically, cells downregulate epithelial markers such as adherens junction proteins E-cadherin and catenins and express mesenchymal markers including vimentin and fibronectin (Boyer and Thiery, 1993
). In addition to Twist1, the zinc-finger transcription factors, including Snail, Slug, ZEB1, and ZEB2 (Peinado et al., 2007
), can also activate the EMT program by directly binding the E-boxes of the E-cadherin promoter to suppress its transcription. However, it is unclear how Twist1, as a bHLH transcription factor, controls the EMT program. In this study, we test the hypothesis that Twist1 plays a major role in regulating ECM degradation to promote tumor metastasis.