Previously, we demonstrated that
malignant glioma cell lines have increased intracellular pH (pHi) as a result of increased activities of the type I
sodium/hydrogen exchanger (NHE1). This alkalotic pHi of 7.2 to 7.4 is favorable for augmented glycolysis,
DNA synthesis, and cell cycle progression. Conversely, reductions in pHi have been associated with reduced rates of proliferation in transformed cell types. The effects of reducing pHi directly and by NHE1 inhibition on human
malignant glioma cells were systematically compared with those on primary rat astrocytes. Neither
cariporide, nor direct acidification to pHi 6.9 altered the proliferative rates or viabilities of human U87 or U118
malignant glioma cell lines. However,
amiloride significantly impaired
glioma cell proliferation and viability while not affecting astrocytes at concentrations (500 microM) that exceeded its inhibition of NHE1 in
glioma cells (IC50 = 17 microM). Preventing a reduction of pHi did not alter the
drug's antiproliferative and cytotoxic effects on
glioma cells. These findings indicated that
amiloride's cytotoxic effects on
glioma cells are independent of its ability to inhibit NHE1 or to reduce intracellular pHi. The
amiloride derivative 2,4
dichlorobenzamil (DCB) inhibits the
sodium-calcium exchanger (NCX) and was both antiproliferative and cytotoxic to
glioma cells at low doses (20 microM). By contrast,
KB-R7943 [(2-[2-[4-nitrobenzyloxy]phenyl]ethyl)-isothioureamethanesulfonate] preferentially blocks
sodium-dependent
calcium influx by NCX (reverse mode) and was nontoxic to
glioma cells. It is proposed that DCB (20 microM) and
amiloride (500 microM) impair
calcium efflux by NCX, leading to elevations of intracellular
calcium that initiate a morphologically necrotic, predominantly
caspase-independent
glioma cell death.