Synthesis of All Stereoisomers of Monomeric Spectomycin A1/A2 and Evaluation of Their Protein SUMOylation-Inhibitory Activity.

A synthetic methodology to access all possible stereoisomers of spectomycin A1 (SMA1) and A2 (SMA2) has been established through late-stage diversification. The key reaction for the construction of all four diastereomers is an intramolecular cyclization based on the umpolung of π-allyl palladium species with bis(pinacolato)diborane (B2 (pin)2 ). Silyl group assisted direct benzylic oxidation of each isomer enabled construction of the fragile ß-hydroxytetralone skeleton to provide the SMAs. The relative and absolute stereochemistry of SMA2 was also determined, and the absolute stereochemistry of SMA1 was extrapolated based on the optical rotation of SMA2. The axial chirality of SMAs is discussed based on circular dichroism spectra and DFT calculations, and it is concluded that the M isomer is predominant in solution. Biochemical assessment of all isomers in vitro revealed that the C9 hydroxyl group and dimeric structure were both important for protein SUMOylation-inhibitory activity.

Incorporation of Acid-Labile Masking Groups for the Traceless Synthesis of C-Terminal Peptide α-Ketoacids.

An optimized protocol for the masking of α-ketoacids with acid-labile cyclic acetal protecting groups is reported. Unlike prior approaches, these new conditions allow the synthesis of protected α-ketoacids bearing aromatic, hindered alkyl, and protected polar side chains. Attachment to a Wang-type linker and solid support provides a resin that delivers fully unprotected C-terminal peptide α-ketoacids upon resin cleavage. These peptides are the key starting materials for chemical protein synthesis using the α-ketoacid-hydroxylamine ligation.

Chemical Synthesis of the 20†kDa Heme Protein Nitrophorin†4 by α-Ketoacid-Hydroxylamine (KAHA) Ligation.

The chemical synthesis of the 184-residue ferric heme-binding protein nitrophorin 4 was accomplished by sequential couplings of five unprotected peptide segments using α-ketoacid-hydroxylamine (KAHA) ligation reactions. The fully assembled protein was folded to its native structure and coordinated to the ferric heme b cofactor. The synthetic holoprotein, despite four homoserine residues at the ligation sites, showed identical properties to the wild-type protein in nitric oxide binding and nitrite dismutase reactivity. This work establishes the KAHA ligation as a valuable and viable approach for the chemical synthesis of proteins up to 20 kDa and demonstrates that it is well-suited for the preparation of hydrophobic protein targets.

Assessment of flavaglines as potential chikungunya virus entry inhibitors.

Chikungunya virus (CHIKV) is a re-emerging mosquito-borne alphavirus that recently caused large epidemics in islands in, and countries around, the Indian Ocean. There is currently no specific drug for therapeutic treatment or for use as a prophylactic agent against infection and no commercially available vaccine. Prohibitin has been identified as a receptor protein used by chikungunya virus to enter mammalian cells. Recently, synthetic sulfonyl amidines and flavaglines (FLs), a class of naturally occurring plant compounds with potent anti-cancer and cytoprotective and neuroprotective activities, have been shown to interact directly with prohibitin. This study therefore sought to determine whether three prohibitin ligands (sulfonyl amidine 1 m and the flavaglines FL3 and FL23) were able to inhibit CHIKV infection of mammalian Hek293T/17 cells. All three compounds inhibited infection and reduced virus production when cells were treated before infection but not when added after infection. Pretreatment of cells for only 15 minutes prior to infection followed by washing out of the compound resulted in significant inhibition of entry and virus production. These results suggest that further investigation of prohibitin ligands as potential Chikungunya virus entry inhibitors is warranted.

KAHA ligations that form aspartyl aldehyde residues as synthetic handles for protein modification and purification.

Aldehydes are widely recognized as valuable synthetic handles for the chemoselective manipulation of peptides and proteins. In this report, we show that peptides and small proteins containing the aspartic acid semialdehyde (Asa) side chain can be easily prepared by a chemoselective amide-forming ligation that results in the formation of the Asa residue at the ligation site. This strategy employs the α-ketoacid-hydroxylamine (KAHA) ligation in combination with a new isoxazolidine monomer that forms a side-chain aldehyde upon ligation. This monomer is easily prepared on a preparative scale by a catalytic, enantioselective approach and is readily introduced onto the N-terminus of a peptide segment by solid phase peptide synthesis. The ligated product can be further functionalized by bioorthogonal reactions between the aldehyde residue and alkoxyamines or hydrazides. We demonstrated that glucagon aldehyde, an unprotected 29-mer peptide prepared by KAHA ligation, can be site specifically and chemoselectively modified with biotin, dyes, aliphatic oximes, and hydroxylamines. We further describe a simple and high recovery one-step purification process based on the capture of a 29-mer glucagon aldehyde and a 76-mer ubiquitin aldehyde by an alkoxyamine-functionalized polyethylene glycol resin. The peptide or protein was released from the resin by addition of a hydroxylamine to provide the corresponding oximes.

eIF4F is a nexus of resistance to anti-BRAF and anti-MEK cancer therapies.

In BRAF(V600)-mutant tumours, most mechanisms of resistance to drugs that target the BRAF and/or MEK kinases rely on reactivation of the RAS-RAF-MEK-ERK mitogen-activated protein kinase (MAPK) signal transduction pathway, on activation of the alternative, PI(3)K-AKT-mTOR, pathway (which is ERK independent) or on modulation of the caspase-dependent apoptotic cascade. All three pathways converge to regulate the formation of the eIF4F eukaryotic translation initiation complex, which binds to the 7-methylguanylate cap (m(7)G) at the 5' end of messenger RNA, thereby modulating the translation of specific mRNAs. Here we show that the persistent formation of the eIF4F complex, comprising the eIF4E cap-binding protein, the eIF4G scaffolding protein and the eIF4A RNA helicase, is associated with resistance to anti-BRAF, anti-MEK and anti-BRAF plus anti-MEK drug combinations in BRAF(V600)-mutant melanoma, colon and thyroid cancer cell lines. Resistance to treatment and maintenance of eIF4F complex formation is associated with one of three mechanisms: reactivation of MAPK signalling, persistent ERK-independent phosphorylation of the inhibitory eIF4E-binding protein 4EBP1 or increased pro-apoptotic BCL-2-modifying factor (BMF)-dependent degradation of eIF4G. The development of an in situ method to detect the eIF4E-eIF4G interactions shows that eIF4F complex formation is decreased in tumours that respond to anti-BRAF therapy and increased in resistant metastases compared to tumours before treatment. Strikingly, inhibiting the eIF4F complex, either by blocking the eIF4E-eIF4G interaction or by targeting eIF4A, synergizes with inhibiting BRAF(V600) to kill the cancer cells. eIF4F not only appears to be an indicator of both innate and acquired resistance but also is a promising therapeutic target. Combinations of drugs targeting BRAF (and/or MEK) and eIF4F may overcome most of the resistance mechanisms arising in BRAF(V600)-mutant cancers.

Benzofuran derivatives as anticancer inhibitors of mTOR signaling.

A series of 32 derivatives and isosteres of the mTOR inhibitor 2 were synthesized and compared for their cytotoxicity in radioresistant SQ20B cancer cell line. Several of these compounds, in particular 30b, were significantly more cytotoxic than 2. Importantly, 30b was shown to block both mTORC1 and Akt signaling, suggesting insensitivity to the resistance associated to Akt overactivation observed with rapamycin derivatives currently used in clinic.

RE12 derivatives displaying Vaccinia H1-related phosphatase (VHR) inhibition in the presence of detergent and their anti-proliferative activity against HeLa cells.

New derivatives of Vaccinia H1-related phosphatase (VHR) inhibitor RE12 (5) were designed by replacing the long straight alkyl chain with other hydrophobic functionalities containing two aromatic rings, with the aim of obtaining potent, cell-active inhibitors. We established a direct coupling reaction between tetronic acid derivative and thioimidate to prepare the RE derivatives 6a-6i efficiently. These compounds all showed VHR-inhibitory activity in the presence of 0.001% NP-40, whereas RE12 (5) was inactive under this condition, even at 100 µM. Further structure-activity studies focused on terminal substitution afforded trifluoromethyl derivative 6k (RE176) and nitro derivative 6l (RE177). The IC50 value of 6l in the presence of NP-40 was almost equivalent to that of RE12 (5) in its absence. Compound 6k (RE176) potently inhibited proliferation of HeLa cells.

Selective anticancer effects of a synthetic flavagline on human Oct4-expressing cancer stem-like cells via a p38 MAPK-dependent caspase-3-dependent pathway.

Cancer stem cells (CSCs) are considered as the initiators of the carcinogenic process and are therefore emerging targets for innovative anticancer therapies. In order to evaluate the anticancer chemopreventive activity of flavagline derivatives, we used the pluripotent teratocarcinomal cell as a model of Oct4-expressing cancer stem-like cell and determined the underlying cellular and molecular mechanisms induced by a synthetic flavagline. We precisely investigated the effects of the flavagline derivative FL3 on the human embryonal carcinoma (EC) cell line NT2/D1 and compared the responses to those of a normal more restrictive pluripotent stem cell line (i.e. BJ fibroblast cell line). FL3 selectively inhibited the proliferation of NT2/D1 cells by inducing G1 phase cell cycle arrest in a dose-dependent manner. Moreover, FL3 treatment specifically triggered apoptosis in association with an induction of the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and caspase-3 activation followed by a drastic downregulation of the master regulator of stemness Oct4. Forced inhibition of p38 MAPK activity by the specific pharmacological inhibitor SB203580 or by p38 MAPK gene knockdown using small-interfering RNA (siRNA) counteracted the effects of FL3, demonstrating that its chemopreventive action is related to growth inhibition and a p38-dependent caspase-3-dependent induction of apoptosis in Oct4-expressing CSCs. This study also shows that FL3 selectively kills poorly differentiated and highly aggressive carcinomal cells, but has little effect on normal stem-like cells. Thus FL3 offers great promise for cancer treatment since it is able to target the carcinogenic process without affecting normal cells.

Benzofuran derivatives as a novel class of inhibitors of mTOR signaling.

High-throughput screening (HTS) hit 1 was previously identified as an inhibitor of the Akt/mTOR (Akt/mammalian target of rapamycin) signaling, which is a major target in oncology. The cytotoxicity of 1 was determined on a panel of human cancer cells lines with an IC50 comprised between 30 and 140 µM. Subsequent structure--activity relationship (SAR) studies led us to the identification of compounds that displayed an enhanced cytotoxicity. We demonstrated also that these molecules directly bind to mTOR complex 1 (mTORC1) and inhibit its kinase activity.

Flavaglines: potent anticancer drugs that target prohibitins and the helicase eIF4A.

Flavaglines are complex natural products that are found in several medicinal plants of Southeast Asia in the genus Aglaia; these compounds have shown exceptional anticancer and cytoprotective activities. This review describes the significance of flavaglines as a new class of pharmacological agents and presents recent developments in their synthesis, structure-activity relationships, identification of their molecular targets and modes of action. Flavaglines display a unique profile of anticancer activities that are mediated by two classes of unrelated proteins: prohibitins and the translation initiation factor eIF4A. The identification of these molecular targets is expected to accelerate advancement toward clinical studies. The selectivity of cytotoxicity towards cancer cells has been shown to be due to an inhibition of the transcription factor HSF1 and an upregulation of the tumor suppressor TXNIP. In addition, flavaglines display potent anti-inflammatory, cardioprotective and neuroprotective activities; however, the mechanisms underlying these activities are yet to be elucidated.

Prohibitin ligands in cell death and survival: mode of action and therapeutic potential.

Prohibitins (PHBs) are scaffold proteins that modulate many signaling pathways controlling cell survival, metabolism, and inflammation. Several drugs that target PHBs have been identified and evaluated for various clinical applications. Preclinical and clinical studies indicate that these PHB ligands may be useful in oncology, cardiology, and neurology, as well as against obesity. This review covers the physiological role of PHBs in health and diseases and current developments concerning PHB ligands.

N-[3a-(4-Bromo-phen-yl)-8b-hy-droxy-6,8-dimeth-oxy-3-phenyl-2,3,3a,8b-tetra-hydro-1H-cyclo-penta-[b]benzofuran-1-yl]formamide monohydrate.

In the title compound, C26H24BrNO5·H2O, a synthetic analogue of natural flavagline, the cyclo-pentane ring adopts an envelope conformation (the flap atom bearing the phenyl group) and the vicinal phenyl and bromo-phenyl groups are slightly shifted relative to each other [CPh-C-C-CPhBr = 36.3 (2)°]. Intra-molecular N-H⋯O and C-H⋯O hydrogen bonds form S(5) motifs. In the crystal, the organic and the water mol-ecules are linked by an O-H⋯O hydrogen bond. Pairs of organic and water mol-ecules, located about inversion centers, inter-act through O-H⋯O hydrogen bonds, forming R4(4)(20) and R4(4)(26) motifs, which together lead to C2(2)(9) motifs. The crystal packing is also characterized by N-H⋯O and C-H⋯O hydrogen bonds between neighbouring organic mol-ecules, forming R2(2)(10) and R2(2)(18) motifs, respectively.

Flavaglines as potent anticancer and cytoprotective agents.

Flavaglines represent a family of plant natural products that display potent anticancer, cardioprotective, and neuroprotective activities. Novel flavagline derivatives were synthesized and examined for their cytotoxicity on a panel of human cancer cell lines, their cardioprotection against doxorubicin-induced apoptosis in cardiomyocytes, and their neuroprotection in culture models of Parkinson's disease and cisplatin-induced neurotoxicity. The structural requirements of flavaglines for cardio- and neuroprotection were for the first time unraveled and appeared to be slightly different from those for cytotoxicity on cancer cells. We provide also the first evidence that flavaglines may alleviate cisplatin-induced neurotoxicity, suggesting a prophylactic potential of these compounds to prevent this frequently encountered adverse effect of cancer chemotherapies.

The natural anticancer compounds rocaglamides inhibit the Raf-MEK-ERK pathway by targeting prohibitin 1 and 2.

Rocaglamides are potent natural anticancer products that inhibit proliferation of various cancer cells at nanomolar concentrations. We have recently shown that these compounds prevent tumor growth and sensitize resistant cancer cells to apoptosis by blocking the MEK-ERK-eIF4 pathway. However, their direct molecular target(s) remain(s) unknown. In this study, using an affinity chromatography approach we discovered that prohibitin (PHB) 1 and 2 are the direct targets of rocaglamides. Binding of rocaglamides to PHB prevents interaction between PHB and CRaf and, thereby, inhibits CRaf activation and subsequently CRaf-MEK-ERK signaling. Moreover, knockdown of PHB mimicked the effects of rocaglamides on the CRaf-MEK-ERK pathway and cell cycle progression. Thus, our finding suggests that rocaglamides are a new type of anticancer agent and that they may serve as a small-molecular tool for studying PHB-mediated cellular processes.

Recent advances in the biology and chemistry of the flavaglines.

The flavaglines are a family of plant natural products that induce potent anticancer and neuroprotective activities. This review summarizes recent synthetic approaches to flavaglines and the current status of their pharmacological properties.

Flavaglines alleviate doxorubicin cardiotoxicity: implication of Hsp27.

BACKGROUND: Despite its effectiveness in the treatment of various cancers, the use of doxorubicin is limited by a potentially fatal cardiomyopathy. Prevention of this cardiotoxicity remains a critical issue in clinical oncology. We hypothesized that flavaglines, a family of natural compounds that display potent neuroprotective effects, may also alleviate doxorubicin-induced cardiotoxicity. METHODOLOGY/PRINCIPAL FINDINGS: Our in vitro data established that a pretreatment with flavaglines significantly increased viability of doxorubicin-injured H9c2 cardiomyocytes as demonstrated by annexin V, TUNEL and active caspase-3 assays. We demonstrated also that phosphorylation of the small heat shock protein Hsp27 is involved in the mechanism by which flavaglines display their cardioprotective effect. Furthermore, knocking-down Hsp27 in H9c2 cardiomyocytes completely reversed this cardioprotection. Administration of our lead compound (FL3) to mice attenuated cardiomyocyte apoptosis and cardiac fibrosis, as reflected by a 50% decrease of mortality. CONCLUSIONS/SIGNIFICANCE: These results suggest a prophylactic potential of flavaglines to prevent doxorubicin-induced cardiac toxicity.

Novel flavaglines displaying improved cytotoxicity.

Novel flavagline analogues were synthesized and examined with respect to their cytotoxicity. Structural features critical to the potential of this class of anticancer natural products were unraveled. We demonstrated, in particular, that the introduction of substituants at C-2 has a deleterious effect on multidrug resistance. Replacement of the hydroxy at C-1 by an aminoformyl with the opposite configuration enhances the cytotoxicity and led to a compound that reduces tumors growth in an allograft model at nontoxic doses.

Synthetic analogue of rocaglaol displays a potent and selective cytotoxicity in cancer cells: involvement of apoptosis inducing factor and caspase-12.

Flavaglines constitute a family of natural anticancer compounds. We present here 3 (FL3), the first synthetic flavagline that inhibits cell proliferation and viability (IC(50) approximately 1 nM) at lower doses than did the parent compound, racemic rocaglaol. Compound 3 enhanced doxorubicin cytotoxicity in HepG2 cells and retained its potency against adriamycin-resistant cell lines without inducing cardiomyocyte toxicity. Compound 3 induced apoptosis of HL60 and Hela cells by triggering the translocation of Apoptosis Inducing Factor (AIF) and caspase-12 to the nucleus. A fluorescent conjugate of 3 accumulated in endoplasmic reticulum (ER), suggesting that flavaglines bind to their target in the ER, where it triggers a cascade of events that leads to the translocation of AIF and caspase-12 to the nucleus and probably inhibition of eIF4A. Our studies highlight structural features critical to their antineoplastic potential and suggest that these compounds would retain their activity in cells refractory to caspase activation.