Triptolide (1) is a structurally unique diterpene triepoxide isolated from a traditional Chinese medicinal plant with anti-inflammatory, immunosuppressive, contraceptive and antitumor activities. Its molecular mechanism of action, however, has remained largely elusive to date. We report that triptolide covalently binds to human XPB/ERCC3, a subunit of the transcription factor TFIIH, and inhibits its DNA-dependent ATPase activity, which leads to the inhibition of RNA Polymerase II mediated transcription and likely nucleotide excision repair. The identification of XPB as the target of triptolide accounts for the majority of the known biological activities of triptolide. These findings also suggest that triptolide can serve as a novel molecular probe for studying transcription and, potentially, as a new type of anticancer agents through inhibition of the ATPase activity of XPB.

Translation initiation in eukaryotes is accomplished through the coordinated and orderly action of a large number of proteins, including the eIF4 initiation factors. Herein, we report that pateamine A (PatA), a potent antiproliferative and proapoptotic marine natural product, inhibits cap-dependent eukaryotic translation initiation. PatA bound to and enhanced the intrinsic enzymatic activities of eIF4A, yet it inhibited eIF4A-eIF4G association and promoted the formation of a stable ternary complex between eIF4A and eIF4B. These changes in eIF4A affinity for its partner proteins upon binding to PatA caused the stalling of initiation complexes on mRNA in vitro and induced stress granule formation in vivo. These results suggest that PatA will be a valuable molecular probe for future studies of eukaryotic translation initiation and may serve as a lead compound for the development of anticancer agents.

Stress granules are aggregates of small ribosomal subunits, mRNA, and numerous associated RNA-binding proteins that include several translation initiation factors. Stress granule assembly occurs in the cytoplasm of higher eukaryotic cells under a wide variety of stress conditions, including heat shock, UV irradiation, hypoxia, and exposure to arsenite. Thus far, a unifying principle of eukaryotic initiation factor 2␣ phosphorylation prior to stress granule formation has been observed from the majority of experimental evidence. Pateamine A, a natural product isolated from marine sponge, was recently reported to inhibit eukaryotic translation initiation and induce the formation of stress granules. In this report, the protein composition and fundamental progression of stress granule formation and disassembly induced by pateamine A was found to be similar to that for arsenite. However, pateamine A-induced stress granules were more stable and less prone to disassembly than those formed in the presence of arsenite. Most significantly, pateamine A induced stress granules independent of eukaryotic initiation factor 2␣ phosphorylation, suggesting an alternative mechanism of formation from that previously described for other cellular stresses. Taking into account the known inhibitory effect of pateamine A on eukaryotic translation initiation, a model is proposed to account for the induction of stress granules by pateamine A as well as other stress conditions through perturbation of any steps prior to the rejoining of the 60S ribosomal subunit during the entire translation initiation process. Stress granules (SGs)2 were first observed as cellular bodies visible by microscopy in tomato cells subjected to heat shock (1-3). Subsequently, SGs were identified in mammalian cells exposed to a variety of stress conditions, including oxidative stress, energy depletion, UV irradiation, and hypoxia (4). SG assembly is part of an adaptive response that recruits selected mRNAs and associated proteins for storage or triage to processing bodies (PBs) (5) that are sites of mRNA decay, allowing survival under adverse conditions. Sequestration of these components may help cells to recover post-stress by replenishing the cellular pool of mRNA without the need for new transcription. The physiological relevance of SGs is underscored by the presence of SGs in tissues of animals under stress (4), and SGs have been implicated in radioresistance of tumor cells (6) and tumor necrosis factor ␣ signaling (7). The study of SGs, their mechanism of formation, and biological role is a relatively new field in cell biology. Thus, a deeper understanding of the mechanism of SG formation and cellular functions may be clinically relevant.A critical step in SG formation shared by most stress conditions is phosphorylation of the ␣ subunit of eukaryotic initiation factor 2 (eIF2) (8), which is a component of the eIF2-GTPtRNAi Met ternary complex. The ternary complex is part of the 43S complex (40S particle, eIF3, and ternary complex) that is recruited to mRNA by ...

A series of pateamine A (1) derivatives were synthesized for structure/activity relationship (SAR) studies and a selection of previous generation analogs were re-evaluated based on current information regarding the mechanism of action of these translation inhibitors. Structural modifications in the new generation of derivatives focused on alternations to the C19-C22 Z, E-diene and the trienyl side chain of the previously described simplified, des-methyl, des-amino pateamine A (DMDAPatA, 2). Derivatives were tested for anti-proliferative activity in cell culture and for inhibition of mammalian cap-dependent translation in vitro. Activity was highly dependent on the rigidity and conformation of the macrolide and the functionality of the side chain. The only well tolerated substitutions were replacement of the N,N-dimethyl amino group found on the side chain of 2 with other tertiary amine groups. SAR reported here suggests that this site may be modified in future studies to improve serum stability, cell-type specificity, and/or specificity towards rapidly proliferating cells.

Nonsense-mediated mRNA decay (NMD) in mammalian cells is a key mechanism for the removal of mRNA containing premature stop codons and is mediated by the coordinated function of numerous proteins that dynamically associate with the exon junction complex. The information communicated by these interactions and the functional consequences from a mechanistic perspective, however, are not completely documented. Herein, we report that the natural product pateamine A (PatA) is capable of inhibiting NMD through direct interaction with eIF4AIII, which is independent of its inhibition of translation initiation. Furthermore, we have characterized the mechanisms by which PatA and cycloheximide modulate NMD. Unlike CHX, PatA was found to inhibit NMD by a novel mechanism that is independent of the phosphorylation of Up-frameshift protein 1.In mammalian cells, nonsense-mediated mRNA decay (NMD) 2 is one of the key RNA surveillance mechanisms to specifically degrade mRNA with premature stop codons (PTCs) located more than 50 -55 nucleotides upstream of the final exon-exon junction. PTCs can be formed in genes containing a nonsense mutation or frameshift mutation or as a result of errors that occur during transcription or RNA splicing (1-4). After splicing, the exon junction complex (EJC) imprints mature mRNAs 20 -24 nucleotides upstream of the exon-exon junction (5). The EJC is a dynamic multiprotein complex that plays an essential role in NMD. The core EJC proteins eIF4AIII, Y14, Magoh, and MLN51 form a platform to interact with several other proteins in a dynamic fashion to regulate NMD (6). The spatial-temporal regulation of NMD by the EJC and its partner proteins has been extensively investigated, leading to the proposition of the "linear interaction model" (4, 7). According to this model, deposition of the EJC onto mRNA causes the Y14-Magoh and eIF4AIII complex to effectively recruit Upf3 that interacts with Upf2. Although the mechanism of loading Upf1 onto EJC is poorly understood, it has been shown that phosphorylation and dephosphorylation of Upf1 by SMG-1, -5, -6, and -7 affect the interaction with the EJC complex through

Central to cap-dependent eukaryotic translation initiation is the eIF4F complex, which is composed of the three eukaryotic initiation factors eIF4E, eIF4G, and eIF4A. eIF4A is an RNA-dependent ATPase and an ATP-dependent helicase that unwinds local secondary structure in mRNA to allow binding of the 43S ribosomal complex. The marine natural product pateamine A (PatA) has been demonstrated to inhibit cap-dependent initiation by targeting eIF4A and disrupting its protein-protein interactions while increasing its enzymatic activities. Here we demonstrate that the increased activity is caused by the induction of global conformational changes within eIF4A. Furthermore, binding of PatA is dependent on substrate (RNA and ATP) binding, and the increased activity upon PatA binding is caused by relief of a negative regulatory function of the eIF4A unique domain linker.

Cyclosporin A (CsA) is a widely used immunosuppressant drug. Its immunosuppressive activity occurs through the inhibition of the protein phosphatase calcineurin via formation of a ternary complex with cyclophilin A (CypA). CsA also inhibits endothelial cell proliferation and angiogenesis. This has been thought to occur through calcineurin inhibition as well. However, CsA is also a potent inhibitor of cyclophilins, a class of prolyl isomerases. Because calcineurin inhibition requires binding, and therefore inhibition of CypA, the relative contributions of calcineurin and cyclophilin inhibition in antiangiogenesis have not been addressed. We have taken a chemical biology approach to explore this question by dissociating the two activities of CsA at the molecular level. We have identified a nonimmunosuppressive analog of CsA that does not inhibit calcineurin but maintains inhibition of endothelial cell proliferation and in vivo angiogenesis. The same analog also maintains inhibition of all cyclophilin isoforms tested. We also show that a second, structurally distinct, cyclophilin inhibitor is sufficient to block endothelial cell proliferation. These results suggest that the inhibition of cyclophilins may play a larger role in the antiangiogenic activity of CsA than previously believed, and that cyclophilins may be potential antiangiogenic drug targets.

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