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 similar to7.6, 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, pHi maintained at similar to7.6 and COX-2 expression was not inhibited. Additionally, analysis utilizing protein synthesis inhibitor cycloheximide demonstrated that pHi 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. (C) 2003 Wiley-Liss, Inc.