5 x 10^4 cells were seeded per well of 24 well plates and transfected 24 h later with 12 pmol of siRNA and 3 µl of INTERFERin.

Solid tumor growth triggers a wound healing response. Similar to wound healing, fibroblasts in the tumor stroma differentiate into myofibroblasts (also referred to as cancer-associated fibroblasts) primarily, but not exclusively, in response to transforming growth factor-beta (TGF-beta). Myofibroblasts in turn enhance tumor progression by remodeling the stroma. Among proteases implicated in stroma remodeling, matrix metalloproteinases (MMPs), including MMP-9, play a prominent role. Recent evidence indicates that MMP-9 recruitment to the tumor cell surface enhances tumor growth and invasion. In the present work, we addressed the potential relevance of MMP-9 recruitment to and activity at the surface of fibroblasts. We show that recruitment of MMP-9 to the fibroblast cell surface occurs through its fibronectin-like (FN) domain and that the molecule responsible for the recruitment is lysyl hydroxylase 3 (LH3). Functional assays suggest that both pro- and active MMP-9 trigger alpha-smooth muscle actin expression in cultured fibroblasts, reflecting myofibroblast differentiation, possibly as a result of TGF-beta activation. Moreover, the recombinant FN domain inhibited both MMP-9-induced TGF-beta activation and alpha-smooth muscle actin expression by displacing MMP-9 from the fibroblast cell surface. Together our results uncover LH3 as a new docking receptor of MMP-9 on the fibroblast cell surface and demonstrate that the MMP-9 FN domain is essential for the interaction. They also show that the recombinant FN domain inhibits MMP-9-induced TGF-beta activation and fibroblast differentiation, providing a potentially attractive therapeutic reagent toward attenuating tumor progression where MMP-9 activity is strongly implicated.