Mouse macrophage metalloelastase generates angiostatin from plasminogen and suppresses tumor angiogenesis in murine colon cancer

Previous studies showed that the mouse macrophage metalloelastase (MME) generates the angiogenesis inhibitor angiostatin from plasminogen in vitro. This study aimed to determine whether tumor cells engineered with MME could generate angiostatin and suppress tumor angiogenesis in vivo. Murine CT-26 colon cancer cells stably transfected with MME were inoculated subcutaneously. A radioisotope tracer, immunoblotting, immunofluorescence and immunohistochemistry were used to explore the pathway of the angiostatin generation. The results showed that tumors derived from MME-transfected cells demonstrated a less microvessel density compared with control tumors derived from vector-transfected and non-transfected cells (P<0.001). The expression of vascular endothelial growth factor (VEGF) was significantly lower in the MME-transfected group compared with that of the controls. The growth of MME-transfected tumors was significantly retarded compared with the control tumors (P<0.001). Western blot analysis, using a specific anti-mouse plasminogen (1-4 Kringle) antibody, demonstrated two strong immunoreactive bands (38- and 35-kDa) in MME-transfected tumors. gamma-ray counting data demonstrated that plasminogen cleavage occurred mostly in tumors formed by cells forced to express. We concluded that MME was demonstrated to be an efficient angiostatin-producing MMP and its presence was negatively correlated with the growth of colon cancer in tumor-bearing mice. These findings provide direct evidence that MME generates angiostatin in tumor-bearing mice and the therapeutic application of MME against tumors.