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 regu lation of intracellular pH (pH(i)), It is a ubiquitous, amiloride-sensitive , growth factor-activatable exchanger whose role has been implicated in cel l-cycle regulation, apoptosis, and neoplasia, Here we demonstrate that leuk emic cell lines and peripheral blood from primary patient leukemic samples exhibit a constitutively and statistically higher pH(i) than normal hematop oietic 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+/Hexchanger inhibitor, 5-(N, N-hexamethylene) amiloride (HMA), which decrease s 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 flo w cytometry and fluorescent ratio imaging microscopy, that when the pH(i) d ecreases, apoptosis-measured by annexin-V and TUNEL methodologies-rapidly i ncreases so that more than 90% of the leukemic cells are killed, The differ ential sensitivity exhibited between normal and leukemic cells allows consi deration of NHE1 inhibitors as potential antileukemic agents. (C) 2000 by T he American Society of Hematology.