induces MMP-9 production. Sub-confluent cells were cultured overnight in serum-free medium at pH 7.4 and stimulated with serum-free medium at the indicated pH. After 24 h, the conditioned medium was collected, concentrated, and analyzed by gelatin zymography. A. A clear zone indicates gelatinolytic activity. B. Intensity of zymogram was quantified by Scion Image (Scion corp., Frederick, MD, USA). Representative results were shown from three independent experiments. *P < 0.01; **P < 0.01; ***P < 0.001; NS, not significant.