Cells of four groups were homogenized in lysis buffer containing 50 mM Tris-HCl (pH 7.4), 150 mM NaCl, 5 mM CaCl2, 0.2 mM NaN3, and 0.01% Triton. Soluble extracts were separated by centrifugation and stored at –20°C. Gelatin zymography was performed according to the manufacturer's instructions (Genmed Scientifics Inc, MA, USA).