Inhibition of breast cancer cell extracellular matrix degradative activity by chemically modified tetracyclines

BACKGROUND: Inhibition of tumour cell proliferation, invasion and metastasis by chemically modified tetracyclines has been ascribed to inhibition of matrix metalloproteinase (MMP) activity. METHODS: Exposure of the human breast carcinoma cell line MDA-MB-231 or its MMP-9-overproducing transfected clone (E-10) to 6-demethyl, 6-deoxy, 4-de [dimethylamino]-tetracycline (CMT-3), a chemically modified non-antimicrobial tetracycline followed by analysis using gelatinase activity assay, zymography, degradation of radiolabelled extracellular matrix (ECM), Western blotting, TNF-alpha ELISA and cell viability assays. RESULTS: CMT-3 treatment results in diminution in extracellular MMP-9 protein levels as well as inhibition of gelatinase activity. This prevents cell-mediated ECM degradation without inducing general cytostasis or cytotoxicity. Culturing E-10 cells in 10 or 20 microM CMT-3 diminished secreted MMP-9 levels by 45% or 60%, respectively, but did not affect levels of most other secreted proteins, including tissue inhibitor of Metalloproteinases (TIMP-1). ECM degradation by E-10 cells or their conditioned medium was inhibited by approximately 20%-30% in the presence of 20 microM CMT-3, reflecting inhibition of MMP-9 activity in addition to diminution of released MMP-9 levels. TNF-alpha levels were also diminished in E-10 conditioned medium in the presence of CMT-3, but cell viability, measured by MTS reduction and cytosolic LDH retention, was unaffected. CONCLUSIONS: It is proposed that the reduction in ECM-degradative activity reflects diminished levels of expression as well as inhibition of enzymatic activity of MMPs released by cells in the presence of CMT-3. These multiple effects of CMT-3 may offer promise for use in suppressing tumour invasion, and if used in conjunction with other chemotherapy agents, may lead to more successful treatment of cancer.