Mouse
macrophage metalloelastase (
MME) has been associated with the generation of
angiostatin, an internal fragment of
plasminogen, which inhibits angiogenesis. To clarify whether
tumor cells that consistently generate
MME can suppress angiogenesis and, therefore, inhibit the growth of primary
tumors in vivo, we transfected a
cDNA coding for
MME into murine B16-BL6
melanoma cells that grow rapidly and are
MME deficient. The generation of active
MME in
MME-transfected clones was confirmed by immunoprecipitation followed by in vitro cleavage of
plasminogen. Subcutaneous implantation of these stable clones in C57BL/6 mice inhibited primary
tumor growth by an average of 73% (P = 0.00002), which directly correlated with a significant reduction of blood vessel formation (approximately 76%) in such
tumors. Microangiography revealed massive angiogenesis in control
tumors (mock and vector); however, in
MME-transfected primary
tumors it demonstrated a decreased and disrupted vascular network. Western blot analysis using a specific anti-mouse
angiostatin antibody demonstrated a strong 38-kDa immunoreactive band in
MME-transfected
tumors and in the serum of mice bearing those
tumor cells. These results show that placing
MME gene directly into B16-BL6
melanoma cells is an effective approach to suppress primary
tumor growth in vivo because it halts angiogenesis. Our data provide a feasible and promising strategy for gene therapy of
cancer by targeting
tumor vasculature.