Bryostatin 1 enhances paclitaxel-induced mitochondrial dysfunction and apoptosis in human leukemia cells (U937) ectopically expressing Bcl-xL.
The effects of the protein kinase C (PKC) activator and down-regulator bryostatin 1 were examined with respect to paclitaxel-induced apoptosis and antiproliferative activity in human myeloid leukemia cells (U937) displaying enforced expression of the anti-apoptotic protein Bcl-xL. Overexpression of Bcl-xL blocked various aspects of paclitaxel-mediated apoptosis, including caspase-3 activation, degradation of poly(ADP-ribose) polymerase (PARP), loss of mitochondrial membrane potential (Delta Psim), and release of cytochrome c. However, subsequent (but not prior) exposure of paclitaxel-treated U937/Bcl-xL cells (500 nM; 6 h) to bryostatin 1 (10 nM; 15 h) restored the extent of apoptosis, caspase activation, and mitochondrial damage to levels approximating those in paclitaxel-treated empty-vector control cells (U937/Neo). Potentiation of paclitaxel-induced apoptosis by bryostatin 1 in U937/Bcl-xL cells occurred primarily in the G2M cell population, and was associated with alterations in Bcl-xL gel mobility and a reduction in paclitaxel-mediated stimulation of CDK1 activity. Enhancement of paclitaxel-induced apoptosis by bryostatin 1 in Bcl-xL overexpressors was accompanied by a corresponding reduction in clonogenic potential. In contrast to its effects on apoptosis, bryostatin 1 failed to restore paclitaxel-mediated increases in free Bax levels in U937/Bcl-xL cells. Lastly, the actions of bryostatin 1 were mimicked by a pharmacologic inhibitor of the MEK1/MAP kinase pathway (PD98059), but not by SB203580, an inhibitor of p 38 MAP kinase. Moreover, sequential exposure of both U937/Neo or/Bcl-xL cells to paclitaxel followed by bryostatin 1 or PD98059 was associated with a net reduction in MAP kinase activity. Collectively, these findings indicate that protection against paclitaxel-mediated mitochondrial dysfunction and apoptosis in human U937 leukemia cells conferred by Bcl-xL overexpression can be substantially overcome by bryostatin 1 and possibly other agents that interrupt the MAP kinase signal transduction pathway.
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Department of Medicine, Medical College of Virginia, Virginia Commonwealth University, Richmond, VA 23298, USA.
Wang
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Boise
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Dent
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Grant
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D000971:Antineoplastic Combined Chemotherapy Protocols / Q000494:pharmacology*; D054713:Bryostatins; D002828:Choristoma / Q000378:metabolism; D006801:Humans; D007783:Lactones / Q000494:pharmacology*; D007951:Leukemia, Myeloid / Q000188:drug therapy* / Q000378:metabolism; D018942:Macrolides; D008928:Mitochondria / Q000187:drug effects*; D017239:Paclitaxel / Q000494:pharmacology*; D011518:Proto-Oncogene Proteins / Q000096:biosynthesis*; D020298:U937 Cells
D054713:Bryostatins; D007783:Lactones; D018942:Macrolides; D011518:Proto-Oncogene Proteins; C046785:bryostatin 1; D017239:Paclitaxel