Resistance to anticancer drugs can sometimes be overcome by combination treatment with other therapeutic drugs. Here, we showed that
phytosphingosine treatment in combination with
arsenic trioxide (
As(2)O(3)) enhanced cell death of naturally As(2)O(3)-resistant human
myeloid leukemia cells. The combination treatment induced an increase in intracellular
reactive oxygen species level, mitochondrial relocalization of Bax,
poly(ADP-ribose) polymerase-1 (PARP-1) activation, and
cytochrome c release from the mitochondria.
N-acetyl-l-cysteine, a
thiol-containing
antioxidant, completely blocked Bax relocalization, PARP-1 activation, and
cytochrome c release. Pretreatment of 3,4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone, a PARP-1 inhibitor, or PARP-1/
small interfering RNA partially attenuated
cytochrome c release, whereas the same treatment did not affect Bax relocalization. The combination treatment induced selective activation of
p38 mitogen-activated protein kinase (MAPK). Inhibition of
p38 MAPK by treatment of
SB203580 or expression of dominant-negative forms of
p38 MAPK suppressed the combination treatment-induced Bax relocalization but did not affect PARP-1 activation. In addition,
antioxidant N-acetyl-l-cysteine completely blocked
p38 MAPK activation. These results indicate that
phytosphingosine in combination with
As(2)O(3) induces synergistic apoptosis in As(2)O(3)-resistant
leukemia cells through the
p38 MAPK-mediated mitochondrial translocation of Bax and the PARP-1 activation, and that
p38 MAPK and PARP-1 activations are
reactive oxygen species dependent. The molecular mechanism that we elucidated in this study may provide insight into the design of future combination
cancer therapies to cells intrinsically less sensitive to
As(2)O(3) treatment.