Reactive oxygen species-mediated regulation of the Na+/H+ exchanger, NHE-1 gene expression: a new mechanism for tumor cells' resistance to apoptotic cell death

Abstract

Reactive Oxygen Species have long been known to cause cellular damage. But recently, ROS have been implicated as signaling molecules. Tumor cells display an altered redox status. Our lab has recently shown that expression of a constitutively active form of Rac1 inhibits tumor cell death by apoptosis through intracellular production of superoxide anion (O2-). Another characteristic of transformed cells is a shift towards alkaline intracellular pH. NHE-1, one of the major pHi regulators, has been shown to be of particular importance in tumor cells. Our data shows that production of intracellular O2- induces transcription of NHE-1 while increase in H2O2 inhibited it. Using Rac mutants, which have differential ability to produce O2- in the cell, and drugs that affect the intracellular ROS levels, we were able to show that NHE1 gene is redox-responsive. Changes in NHE1 gene expression were translated into NHE1 protein expression. By over-expressing or silencing NHE-1 gene we show that cell response to apoptotic triggers such as staurosporin and etoposide correlates with the amount of NHE-1 protein expression on the cell surface. Moreover, down-regulation of NHE-1 gene expression in tumor cell lines tested reverted their resistant phenotype.