

To determine how extensive this property is among flavaglines, we tested members of the cyclopentabenzofurans, cyclopentbenzopyrans, and benzooxepines family for their potential to inhibit protein synthesis. We have previously characterized silvestrol as an inhibitor of translation initiation in vitro and in vivo.

Results Structure-activity relationships of flavaglines Our results are consistent with the idea that silvestrol's anticancer activity is linked to its ability to preferentially block translation of highly structured, malignancy-related mRNAs. We provide further insight into the mechanism of action of silvestrol and show that it depletes the eIF4F complex of eIF4A and this is associated with a preferential reduction in the translation of mRNAs with structured 5′untranslated regions. Herein, we report that silvestrol is effective against two human xenograft models as a single agent. The deregulation of the PI3k/Akt/mTOR signaling axis in human cancers, the finding that ectopic expression of eIF4E is oncogenic, , and the demonstration that targeted down-regulation of eIF4E displays therapeutic benefit in xenograft mouse models suggest that the process of translation initiation is a potential anti-cancer target.

Hence, altering levels with flavaglines can exert profound gene specific effects. These mRNAs typically harbor lengthy, G+C rich, highly-structured 5′ UTRs that encumber efficient RNA unwinding by the eIF4F complex and subsequently prevent efficient ribosome loading. By comparison, a select group of mRNAs is extremely sensitive to, and dependent upon, eIF4F for translation. These mRNAs are efficiently translated when eIF4F activity is limiting.

β-actin, GAPDH) that require a minimal amount of eIF4F for 43S pre-initiation complexes recruitment. Many cellular mRNAs are characterized by relatively short, unstructured 5′ UTRs (e.g. The extent to which translation of specific mRNAs is altered in response to changes in mTOR activity and eIF4F levels varies substantially among different transcripts and is largely dependent upon sequence elements within each mRNA, such as the presence of discrete hairpin structures in the 5′ untranslated regions. Levels of cellular eIF4F are regulated by the target of rapamycin, mTOR. Silvestrol acts as a chemical inducer of dimerization (CID) to force an engagement between eIF4A f and RNA, although how this inhibits translation initiation is not known. The helicase activity of eIF4A c is ∼20-fold more efficient than eIF4A f and during initiation, eIF4A f likely cycles through the eIF4F complex.
MG MEASUREMENT COMPUTING INSTACAL FREE
eIF4A is an abundant translation factor that exists in a free form (referred to herein as eIF4A f) and as a subunit of the heterotrimeric eIF4F complex (eIF4A c). eIF4F is composed of three subunits: eIF4E, which binds to the cap structure present at the 5′ end of mRNAs eIF4A, a DEAD-box RNA helicase implicated in preparing a ribosome landing pad for 43S pre-initiation complexes (40S ribosomal subunit and associated factors) by unwinding 5′ mRNA structure and eIF4G, a large scaffolding protein involved in recruiting the 43S pre-initiation complex via its interaction with 40S-associated eIF3. Translation initiation is regulated by eIF4F at the level of the ribosome recruitment step. Silvestrol inhibits translation initiation by targeting the RNA helicase, eukaryotic initiation factor (eIF) 4A, and prevents ribosome loading onto mRNA templates. We have previously shown that the flavagline silvestrol can re-sensitize tumor cells to standard-of-care agents, such as doxorubicin, in the Eμ-myc lymphoma model. These compounds show in vitro activity against tumor cell lines, , promising activity in xenograft cancer models, , and appear to block G2/M cell cycle progression. Cyclopentabenzofuran flavaglines are inhibitors of translation initiation isolated from Asian plants of the genus Aglaia of the family Meliacae –.
