Low concentrations of
As(2)O(3) (</=1 micromol/L) induce long-lasting remission in patients with
acute promyelocytic leukemia (APL) without significant myelosuppressive side effects. Several groups, including ours, have shown that 0.5 to 1 micromol/L
As(2)O(3) induces apoptosis in APL-derived NB4 cells, whereas other leukemic cells are resistant to
As(2)O(3) or undergo apoptosis only in response to greater than 2 micromol/L
As(2)O(3). In this report, we show that the ability of
As(2)O(3) to induce apoptosis in leukemic cells is dependent on the activity of the
enzymes that regulate cellular H(2)O(2) content. Thus, NB4 cells have relatively low levels of
glutathione peroxidase (GPx) and
catalase and have a constitutively higher H(2)O(2) content than U937 monocytic
leukemia cells.
Glutathione-S-transferase pi (GSTpi), which is important for cellular efflux of
As(2)O(3), is also low in NB4 cells. Moreover,
As(2)O(3) further inhibits GPX activity and increases cellular H(2)O(2) content in NB4 but not in U937 cells.
Selenite pretreatment of NB4 cells increases the activity of GPX, lowers cellular H(2)O(2) levels, and renders NB4 cells resistant to 1 micromol/L
As(2)O(3). In contrast, concentrations of
As(2)O(3) that alone are not capable of inducing apoptosis in NB4 cells induce apoptosis in the presence of the GPx inhibitor
mercaptosuccinic acid. Similar effects are observed by modulating the activity of
catalase with its inhibitor, aminotriazol. More important from a therapeutic point of view, U937 and HL-60 cells, which require high concentrations of
As(2)O(3) to undergo apoptosis, become sensitive to low, clinically acceptable concentrations of
As(2)O(3) when cotreated with these GPx and
catalase inhibitors. The induction of apoptosis by
As(2)O(3) involves an early decrease in cellular mitochondrial membrane potential and increase in H(2)O(2) content, followed by
cytochrome c release,
caspase 3 activation, DNA fragmentation, and the classic morphologic changes of apoptosis.