RT Journal Article SR Electronic T1 Sequence-Dependent Potentiation of Paclitaxel-Mediated Apoptosis in Human Leukemia Cells by Inhibitors of the Mitogen-Activated Protein Kinase Kinase/Mitogen-Activated Protein Kinase Pathway JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 143 OP 154 DO 10.1124/mol.60.1.143 VO 60 IS 1 A1 Chunrong Yu A1 Shujie Wang A1 Paul Dent A1 Steven Grant YR 2001 UL http://molpharm.aspetjournals.org/content/60/1/143.abstract AB Effects of inhibitors of the mitogen-activated protein kinase kinase/mitogen-activated protein kinase (MEK/MAPK) cascade have been examined in relation to paclitaxel-induced apoptosis in human monocytic leukemia cells (U937). Cells treated with paclitaxel (250 nm; 6 h) followed by PD98059 (40 μM; 15 h) exhibited a significant increase in mitochondrial dysfunction (e.g., cytochromec release), caspase activation, poly ADP-ribose polymerase cleavage, and apoptosis, whereas pretreatment of cells with PD98059 reduced lethality. Similar results were obtained with other MEK/MAPK inhibitors (e.g., U0126 and PD184352). Subsequent exposure of paclitaxel-treated cells to PD98059 did not enhance dephosphorylation/activation of p34cdc2 but diminished expression of the antiapoptotic protein Mcl-1. The caspase inhibitor ZVAD-fmk opposed potentiation of paclitaxel-induced loss of mitochondrial membrane potential (Δψm) and apoptosis by PD98059, but not cytochrome c release. Paclitaxel treatment induced sustained phosphorylation/activation of MAPK, an effect prevented by subsequent, but not prior, exposure to PD98059. Paclitaxel treatment also induced c-Jun N-terminal kinase phosphorylation, but this effect was enhanced only slightly by subsequent PD98059 administration. Although paclitaxel alone failed to induce p38 MAPK activation, subsequent (but not prior) exposure to PD98059 induced a dramatic increase in p38 MAPK phosphorylation. Moreover, coadministration of the p38 MAPK inhibitors SB203580 and SB202190 abrogated the increase in paclitaxel-mediated apoptosis induced by PD98059. Finally, subsequent PD98059 exposure increased, whereas prior exposure decreased inhibition of clonogenicity by paclitaxel. Together, these findings suggest that subsequent exposure of paclitaxel-treated U937 cells to MEK/MAPK inhibitors induces perturbations in signaling pathways, particularly the p42/44 MAPK and p38 MAPK cascades, that lower the threshold for mitochondrial injury and induction of cell death. MAPKmitogen-activated protein kinaseERKextracellular signal-regulated kinaseJNKc-Jun N-terminal kinaseSAPKstress-activated protein kinasePKCprotein kinase CMEKmitogen-activated protein kinase kinaseFCSfetal calf serumDMSOdimethyl sulfoxideTNFtumor necrosis factorPBSphosphate-buffered saline7-AAD7-amino actinomycin DPBS-Tphosphate-buffered saline-Tween 20NP-40Nonidet P-40