Editors' ChoiceDevelopment

Protease-Independent Invasion

Science Signaling  08 Jan 2013:
Vol. 6, Issue 257, pp. ec5
DOI: 10.1126/scisignal.2003942

Matrix metalloproteinases (MMPs) are important for the invasion of epithelial cells into the surrounding stroma because their proteolytic activities enable the degradation of extracellular matrix (ECM) and the release of signaling molecules into the extracellular space. Mori et al. report that Mmp14 functions in mammary gland epithelial tube branching independently of its proteolytic domain. Whereas Mmp2 and Mmp3 are primarily expressed in the stroma, Mmp14 was expressed in the stroma and the tips of epithelial branches in developing mouse mammary glands in vivo. Primary mammary organoids and aggregates of the mammary gland cell line EpH4 were cultured in dense (3 mg/ml) three-dimensional collagen matrices to represent the premalignant mammary gland microenvironment or in sparse (1 mg/ml) three-dimensional collagen matrices, which mimic the normal mammary gland microenvironment. In both dense and sparse matrices, fibroblast growth factor 2 (FGF2) induced invasion into the collagen matrix and branching of the epithelial cells, and treatment with a short hairpin RNA (shRNA) targeting Mmp14 prevented FGF2-induced epithelial cell invasion. However, inhibitors of the proteolytic activity of MMPs only prevented invasion in dense gels. Whereas treatment of EpH4 cells with MMP inhibitors had no effect on the activity of the mitogen-activated protein kinase (MAPK) Erk, silencing Mmp14 reduced the abundance of phosphorylated (active) Erk as well as the abundance of integrin β1 (Itgb1) and phosphorylated (active) Itgb1. Silencing Itgb1 by shRNA prevented invasion and reduced the abundance of both Mmp14 and phosphorylated Erk in EpH4 cells cultured in sparse matrix. The abundance of Itgb1 was also reduced in mammary gland tissue from Mmp14 knockout mice compared with control heterozygotes. Similarly, treatment with a MAPK inhibitor reduced invasion and the abundance of both Mmp14 and Itgb1 in EpH4 cells cultured in sparse matrix. Coimmunoprecipitation and fluorescence resonance energy transfer (FRET) experiments revealed that Mmp14 and Itgb1 associated with one another in EpH4 cells. Transgenic expression of full-length, membrane-tethered Mmp14 or a mutant version of Mmp14 lacking the extracellular catalytic and hemopexin domains restored both invasion and the abundance of phosphorylated Itgb1 in EpH4 cells in which Mmp14 was knocked down. FRET analysis confirmed that this mutant version of Mmp14, which consists of only the transmembrane and cytoplasmic domains, associated with Itgb1. Together, these data suggest a model in which the transmembrane or cytoplasmic domains of Mmp14, or both, promote signaling to the MAPK pathway through Itgb1 to stimulate epithelial tube branching during normal mammary gland development. It is not clear from this report whether noncatalytic activities of Mmp14 play a role in the context of tumor progression, but these results suggest that the noncatalytic domains of MMPs should not be overlooked in efforts to target MMPs in cancer treatment.

H. Mori, A. T. Lo, J. L. Inman, J. Alcaraz, C. M. Ghajar, J. D. Mott, C. M. Nelson, C. S. Chen, H. Zhang, J. L. Bascom, M. Seiki, M. J. Bissell, Transmembrane/cytoplasmic, rather than catalytic, domains of Mmp14 signal to MAPK activation and mammary branching morphogenesis via binding to integrin β1. Development 140, 343–352 (2013). [Abstract] [Full Text]