Increased stromal expression of murine urokinase plasminogen activator in a human breast cancer xenograft model following treatment with the matrix metalloprotease inhibitor, batimastat.

The matrix metalloprotease (MMP) family of enzymes and the urokinase plasminogen activator (uPA) pathway have both been implicated in tumor invasion and metastasis and in poor prognosis of cancer. We have previously shown that treatment with batimastat, a synthetic MMP inhibitor, leads to significant retardation but not regression of tumor growth in a human breast cancer xenograft model. In addition, batimastat treatment did not inhibit local tumor invasion, nor did it encourage stromal encapsulation of the tumor, suggesting the additional involvement of non-MMP proteolytic mechanisms. To investigate the presence of an alternative extracellular matrix protease whose activity is known to be important in breast cancer, but which is not inhibited by batimastat, expression of murine and human uPA were examined by in situ hybridization and ELISA. No differences were observed between untreated and batimastat-treated tumors regarding human uPA mRNA and protein. In contrast, murine uPA mRNA expression was increased at the tumor-stromal junction in batimastat-treated tumors in comparison with the control tumors. In agreement with these results, batimastat treatment was shown to significantly induce murine uPA protein content in the tumors. Inoculating MDA435/LCC-6 cells into immunodeficient, uPA-deficient mice resulted in tumor growth retardation as compared to tumor growth in littermate wild-type controls, while addition of batimastat treatment to uPA-/- mice did not result in further growth inhibition. The increased expression of stromal uPA may represent a cellular response to MMP inhibition and may demonstrate a new level of plasticity in the malignant progression of the disease. These results may have important implications for the clinical applications of MMP inhibitors, as well as for development of other anti-invasion drugs.