Reduction of intracellular pH inhibits constitutive expression of Cyclooxygenase‐2 in human colon cancer cells

Cyclooxygenase‐2 (COX‐2) over‐expression is critically involved in tumor formation. Intracellular pH (pH i ) has been shown to be alkaline in cancer cells, and to be an important trigger for cell proliferation. This study therefore analyzed the relationship between pH i and COX‐2 expression. HRT‐18 and Caco‐2 cells cultured in medium with bicarbonate maintained a pH i of ∼7.6, which is higher than that of non‐neoplastic cells. Cells grown in bicarbonate‐free medium with a pH at 6.8 showed a reduction in pH i to approximately 7.0. Importantly, reduction of pH i resulted in a complete inhibition of COX‐2 mRNA and protein expression. When cells were grown in bicarbonate‐supplemented medium at pH 6.8, pH i maintained at ∼7.6 and COX‐2 expression was not inhibited. Additionally, analysis utilizing protein synthesis inhibitor cycloheximide demonstrated that pH i mediated inhibition of COX‐2 mRNA expression requires de novo protein synthesis of regulatory protein(s). These data strongly suggest that an alkaline pH i is an important trigger for constitutive COX‐2 expression. Defining pH i ‐mediated mechanisms that govern the constitutive COX‐2 expression may help in developing new strategies to block COX‐2 over‐expression in cancer cells. J. Cell. Physiol. 198: 295–301, 2004© 2003 Wiley‐Liss, Inc.