MEK1/2 inhibitors promote ara-C-induced apoptosis but not loss of Delta Psi(m) in HL-60 cells

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.