Modulation of 4E-BP1 function as a critical determinant of enzastaurin-induced apoptosis

• Published in: Molecular cancer therapeutics Vol. 9; no. 12; pp. 3158 - 3163
• Main Authors: Dumstorf, Chad A, Konicek, Bruce W, McNulty, Ann M, Parsons, Stephen H, Furic, Luc, Sonenberg, Nahum, Graff, Jeremy R
• Format: Journal Article
• Language: English
• Published: United States 01.12.2010
• Subjects: Animals; Apoptosis - drug effects; Carrier Proteins - metabolism; Cell Line, Tumor; Drug Screening Assays, Antitumor; Eukaryotic Initiation Factor-4F - metabolism; Gene Knockout Techniques; Humans; Indoles - pharmacology; Mice; Mice, Knockout; Phosphoproteins - metabolism; Phosphorylation - drug effects; Proto-Oncogene Proteins c-akt - metabolism; Signal Transduction - drug effects; TOR Serine-Threonine Kinases - metabolism
• DOI: 10.1158/1535-7163.mct-10-0413
• ISSN: 1535-7163; 1538-8514

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.