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. (C) 2000 by The American Society of Hematology.