Mechanism by which mammalian target of rapamycin inhibitors sensitize multiple myeloma cells to dexamethasone-induced apoptosis.
Mammalian target of rapamycin (mTOR) inhibitors curtail cap-dependent translation. However, they can also induce post-translational modifications of proteins. We assessed both effects to understand the mechanism by which mTOR inhibitors like rapamycin sensitize multiple myeloma cells to dexamethasone-induced apoptosis. Sensitization was achieved in multiple myeloma cells irrespective of their PTEN or p53 status, enhanced by activation of AKT, and associated with stimulation of both intrinsic and extrinsic pathways of apoptosis. The sensitizing effect was not due to post-translational modifications of the RAFTK kinase, Jun kinase, p38 mitogen-activated protein kinase, or BAD. Sensitization was also not associated with a rapamycin-mediated increase in glucocorticoid receptor reporter expression. However, when cap-dependent translation was prevented by transfection with a mutant 4E-BP1 construct, which is resistant to mTOR-induced phosphorylation, cells responded to dexamethasone with enhanced apoptosis, mirroring the effect of coexposure to rapamycin. Thus, sensitization is mediated by inhibition of cap-dependent translation. A high-throughput screening for translational efficiency identified several antiapoptotic proteins whose translation was inhibited by rapamycin. Immunoblot assay confirmed rapamycin-induced down-regulated expressions of XIAP, CIAP1, HSP-27, and BAG-3, which may play a role in the sensitization to apoptosis. Studies in a xenograft model showed synergistic in vivo antimyeloma effects when dexamethasone was combined with the mTOR inhibitor CCI-779. Synergistic effects were associated with an enhanced multiple myeloma cell apoptosis in vivo. This study supports the strategy of combining dexamethasone with mTOR inhibitors in multiple myeloma and identifies a mechanism by which the synergistic effect is achieved.
Department of Medicine, Greater Los Angeles VA Healthcare Center, University of California at Los Angeles School of Medicine, Los Angeles, California, USA.
Huajun
H
Yan
Patrick
P
Frost
Yijiang
Y
Shi
Bao
B
Hoang
Sanjai
S
Sharma
Myrna
M
Fisher
Joseph
J
Gera
Alan
A
Lichtenstein
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D002352:Carrier Proteins / Q000378:metabolism; D018797:Cell Cycle Proteins; D045744:Cell Line, Tumor; D003907:Dexamethasone / Q000494:pharmacology*; D004357:Drug Synergism; D051418:Focal Adhesion Kinase 2 / Q000378:metabolism; D006801:Humans; D048670:MAP Kinase Kinase 4 / Q000378:metabolism; D051379:Mice; D016688:Mice, Inbred NOD; D016513:Mice, SCID; D009101:Multiple Myeloma / Q000188:drug therapy / Q000201:enzymology / Q000473:pathology; D010750:Phosphoproteins / Q000378:metabolism; D047428:Protein Kinase Inhibitors / Q000494:pharmacology*; D011494:Protein Kinases / Q000378:metabolism*; D011965:Receptors, Glucocorticoid / Q000378:metabolism; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D020123:Sirolimus / Q000031:analogs & derivatives / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D051025:bcl-Associated Death Protein / Q000378:metabolism; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; C087000:EIF4EBP1 protein, human; D010750:Phosphoproteins; D047428:Protein Kinase Inhibitors; D011965:Receptors, Glucocorticoid; D051025:bcl-Associated Death Protein; C401859:temsirolimus; D003907:Dexamethasone; D011494:Protein Kinases; C546842:MTOR protein, human; C546843:mTOR protein, mouse; D051418:Focal Adhesion Kinase 2; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases; D048051:p38 Mitogen-Activated Protein Kinases; D048670:MAP Kinase Kinase 4; D020123:Sirolimus