Modulation of 4E-BP1 function as a critical determinant of enzastaurin-induced apoptosis.
Enzastaurin (LY317615.HCl) is currently in a phase III registration trial for diffuse large B-Cell lymphoma and numerous phase II clinical trials. Enzastaurin suppresses angiogenesis and induces apoptosis in multiple human tumor cell lines by inhibiting protein kinase C (PKC) and phosphoinositide 3-kinase (PI3K)/AKT pathway signaling. PI3K/AKT pathway signaling liberates eukaryotic translation initiation factor 4E (eIF4E) through the hierarchical phosphorylation of eIF4E binding proteins (4E-BP). When hypophosphorylated, 4E-BPs associate with eIF4E, preventing eIF4E from binding eIF4G, blocking the formation of the eIF4F translation initiation complex. Herein, we show that enzastaurin treatment impacts signaling throughout the AKT/mTOR pathway leading to hypophosphorylation of 4E-BP1 in cancer cells of diverse lineages (glioblastoma, colon carcinoma, and B-cell lymphoma). Accordingly, enzastaurin treatment increases the amount of eIF4E bound to 4E-BP1 and decreases association of eIF4E with eIF4G, thereby reducing eIF4F translation initiation complex levels. We therefore chose to evaluate whether this effect on 4E-BP1 was involved in enzastaurin-induced apoptosis. Remarkably, enzastaurin-induced apoptosis was blocked in cancer cells depleted of 4E-BP1 by siRNAs, or in 4EBP1/2 knockout murine embryonic fibroblasts cells. Furthermore, eIF4E expression was increased and 4E-BP1 expression was decreased in cancer cells selected for reduced sensitivity to enzastaurin-induced apoptosis. These data highlight the importance of modulating 4E-BP1 function, and eIF4F complex levels, in the direct antitumor effect of enzastaurin and suggest that 4E-BP1 function may serve as a promising determinant of enzastaurin activity.
Dumstorf
Chad A
CA
Lilly Research Labs, Eli Lilly and Company, Indianapolis, Indiana 46285. graff_jeremy@lilly.com.
Konicek
Bruce W
BW
McNulty
Ann M
AM
Parsons
Stephen H
SH
Furic
Luc
L
Sonenberg
Nahum
N
Graff
Jeremy R
JR
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; D004354:Drug Screening Assays, Antitumor; D039562:Eukaryotic Initiation Factor-4F / Q000378:metabolism; D039642:Eukaryotic Initiation Factors; D055786:Gene Knockout Techniques; D006801:Humans; D007211:Indoles / Q000494:pharmacology*; D051379:Mice; D018345:Mice, Knockout; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; C499527:Eif4ebp1 protein, mouse; D039562:Eukaryotic Initiation Factor-4F; D039642:Eukaryotic Initiation Factors; D007211:Indoles; D010750:Phosphoproteins; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; C504878:enzastaurin