The effects of pharmacologic MEK1/2 inhibitors on ara-C-mediated mitochondr
ial injury, caspase activation, and apoptosis have been examined in HL-60 l
eukemic cells. Coadministration of subtoxic concentrations of the MEK1/2 in
hibitors U0126 (20 muM), PD98059 (40 muM), or PD184352 (10 muM) with 10-100
muM ara-C (6 h) potentiated apoptosis (i.e., by approx twofold), and pro-c
aspase 3, pro-caspase 8, Bid, and PARP cleavage. Unexpectedly, MEK1/2 inhib
itors failed to enhance ara-C-mediated loss of mitochondrial membrane poten
tial (Delta psi (m)), but instead induced substantial increases in cytosoli
c release of cytochrome c and Smac/DIABLO. U0126/ara-C-mediated apoptosis a
nd procaspase 3 activation, but not cytochrome c or Smac/DIABLO release, we
re blocked by the pan-caspase inhibitor ZVAD-fmk. Together, these findings
indicate that potentiation of ara-C-mediated lethality in HL-60 cells by ME
K1/2 inhibitors involves enhanced cytosolic release of cytochrome c and Sma
c/DIABLO but not discharge of Delta psi (m), implicating activation of an a
poptotic pathway that differs, at least with respect to the nature of the a
ccompanying mitochondrial injury, from that triggered by ara-C alone. (C) 2
001 Academic Press.