A proteolytic pathway to genomic instability and malignant transformation.

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An elevated expression of membrane type-1 matrix metalloproteinase (MT1-MMP) is closely associated with malignancies. MT1-MMP exhibits distinctive and important pericellular cleavage functions. Recently, we determined that MT1-MMP was trafficked along the tubulin cytoskeleton to the centrosomal compartment during endocytosis. After reaching the plasma membrane, MT1-MMP is internalized in the Rab-4-positive recycling endosomes and the Rab-11-positive pericentrosomal recycling endosomes. The microtubular trafficking results in the accumulation of cellular MT1-MMP in the pericentrosomal compartment. The presence of the transmembrane domain is required for the microtubular vesicular trafficking of MT1-MMP because the soluble mutants are not presented at the cell surface and thus they are not delivered to the centrosomes. Our data suggests that the functionally important, integral, centrosomal pericentrin-2 is a cleavage target of MT1-MMP in humans but not in mice. We determined that the D948G mutant of murine pericentrin that exhibited the cleavage sequence of human pericentrin was sensitive to MT1-MMP, while unmodified murine pericentrin was resistant to proteolysis by MT1-MMP. Pericentrin is essential to the normal functioning of centrosomes and to mitotic spindle formation. The expression of MT1-MMP stimulates pericentrin cleavage and this event results in mitotic spindle aberrations and aneuploidy in non-malignant cells. Volumes of data indicate that chromosome instability is an early event of carcinogenesis. In agreement, MT1-MMP induces tumorigenicity in normal human breast epithelial cells. Following transfection with MT1-MMP, these normal cells generates tumors in an orthotopic tumor model in nude mice. Gene array analyses of MT1-MMP-transfected cells demonstrate a significant shift in gene expression including upregulation of oncogenes when compared to normal cells. We believe that our data point to a novel intracellular proteolytic pathway to chromatin instability and to the essential mechanism involved in the transition from normalcy to malignancy.