Colorectal cancer, the second leading cause of cancer deaths in the United States, has a high probability of metastasizing to the liver. Matrix metalloproteinases (MMPs) are a family of zinc dependent endopeptidases that are implicated in cancer metastasis and many aspects of tumor progression. Using a splenic injection experimental metastasis model, mice that are genetically deficient in MMP-9 demonstrated a nearly 2-fold decrease in liver weight compared with wild type (WT) mice following injection with MC38 syngeneic colorectal cancer cells. Bioluminesence imaging data demonstrates an early negative effect on tumor growth in MMP-9 null mice when compared with WT controls. Reconstitution of the bone marrow in MMP-9-/- mice with cells competent to produce MMP-9 did not recapitulate the WT phenotype of overwhelming burden of metastatic disease in the liver. In situ hybridization revealed the presence of MMP-9 mRNA in both MC38 tumor cells and in surrounding stromal cells, and immunostaining with anti MMP-9 was consistent with MMP-9 expression by resident liver Kupffer cells. Stromal-derived MMP-9 contributes to the establishment and growth of colorectal metastases in the liver. Stromal MMP-9 was derived from resident cells, most likely Kupffer cells, as opposed to infiltrating bone marrow-derived cells and the effect of stromal MMP-9 was independent of expression of MMP-9 by tumor cells. Stromal-derived MMP-9 may represent an important target for selective inhibition in the treatment of metastases. (c) 2007 Wiley-Liss, Inc.