Apoptosis of leukemic cells accompanies reduction in intracellular pH after targeted inhibition of the Na(+)/H(+) exchanger

The Na(+)/H(+) exchanger isoform 1 (NHE1) is primarily responsible for the regulation of intracellular pH (pH(i)). It is a ubiquitous, amiloride-sensitive, growth factor-activatable exchanger whose role has been implicated in cell-cycle regulation, Apoptosis, and neoplasia. Here we demonstrate that leukemic cell lines and peripheral blood from primary patient leukemic samples exhibit a constitutively and statistically higher pH(i) than normal hematopoietic tissue. We then show that a direct correlation exists between pH(i) and cell-cycle status of normal hematopoietic and leukemic cells. Advantage was taken of this relationship by treating leukemic cells with the Na(+)/H(+) exchanger inhibitor, 5-(N, N-hexamethylene)-amiloride (HMA), which decreases the pH(i) and induces Apoptosis. By incubating patient leukemic cells in vitro with pharmacologic doses of HMA for up to 5 hours, we show, using flow cytometry and fluorescent ratio imaging microscopy, that when the pH(i) decreases, apoptosis-measured by annexin-V and TUNEL methodologies-rapidly increases so that more than 90% of the leukemic cells are killed. The differential sensitivity exhibited between normal and leukemic cells allows consideration of NHE1 inhibitors as potential antileukemic agents. (Blood. 2000;95:1427-1434)