Bertal H. Aktas

Chemical genetics identify eIF2α kinase heme-regulated inhibitor as an anticancer target

Translation initiation plays a critical role in cellular homeostasis, proliferation, differentiation and malignant transformation. Consistently, increasing the abundance of the eIF2·GTP·Met-tRNAi translation initiation complex transforms normal cells and contributes to cancer initiation and the severity of some anemia. The chemical modifiers of the eIF2·GTP·Met-tRNAi ternary complex are therefore invaluable tools for studying its role in the pathobiology of human disorders and for determining if this complex can be pharmacologically targeted for therapeutic purposes. Using a cell based assay, we identified N,N’-diarylureas as novel inhibitors of the ternary complex abundance. Direct functional-genetics and biochemical evidence demonstrated that the N,N’-diarylureas activate heme regulated inhibitor kinase, thereby phosphorylate eIF2α and reduce abundance of the ternary complex. Using tumor cell proliferation in vitro and tumor growth in vivo as paradigms, we demonstrate that N,N’-diarylureas are potent and specific tools for studying the role eIF2·GTP·Met-tRNAi ternary complex in the pathobiology of human disorders.

Human CD59 Inhibitor Sensitizes Rituximab-Resistant Lymphoma Cells to Complement-Mediated Cytolysis

Rituximab efficacy in cancer therapy depends in part on induction of complement-dependent cytotoxicity (CDC). Human CD59 (hCD59) is a key complement regulatory protein that restricts the formation of the membrane attack complex, thereby inhibiting induction of CDC. hCD59 is highly expressed in B-cell non-Hodgkins lymphoma (NHL), and upregulation of hCD59 is an important determinant of the sensitivity of NHL cells to rituximab treatment. Here, we report that the potent hCD59 inhibitor rILYd4 enhances CDC in vitro and in vivo, thereby sensitizing rituximab-resistant lymphoma cells and primary chronic lymphocytic leukemia cells (CLL) to rituximab treatment. By defining pharmcokinetic/pharmacodynamic profiles of rILYd4 in mice, we showed that by itself rILYd4 does not adversely mediate in vivo hemolysis of hCD59-expressing erythrocytes. Increasing expression levels of the complement regulators CD59 and CD55 in rituximab-resistant cells occur due to selection of preexisting clones rather than de novo induction of these proteins. Moreover, lymphoma cells overexpressing CD59 were directly responsible for the resistance to rituximab-mediated CDC therapy. Our results rationalize the use of rILYd4 as a therapeutic adjuvant for rituximab treatment of rituximab-resistant lymphoma and CLL. Furthermore, they suggest that preemptive elimination of CD59-overexpressing subpopulations along with rituximab treatment may be a useful approach to ablate or conquer rituximab resistance.

Structure of the eukaryotic translation initiation factor eIF4E in complex with 4EGI-1 reveals an allosteric mechanism for dissociating eIF4G

Significance eIF4E is critical for protein synthesis and becomes hyperactive in cancer cells. Small-molecule inhibitors of the eIF4E/eIF4G initiation factor complex have recently been found to exhibit antitumor activity in vitro and in vivo. However, their mode of action at the atomic level has remained elusive. Here, we report high-resolution crystal structures of complexes of 4EGI-1 analogue inhibitors with eIF4E. We find that inhibition of eIF4G binding must be allosteric, because the 4EGI-1 and eIF4G bind at distant epitopes on eIF4E. Compound binding induces extension of an α-helix that stretches between the two binding sites. Indeed, mutations increasing helix propensity in this region reduce eIF4G affinity in the absence of the inhibitor, which is consistent with the proposed allosteric model. The interaction of the eukaryotic translation initiation factor eIF4E with the initiation factor eIF4G recruits the 40S ribosomal particle to the 5′ end of mRNAs, facilitates scanning to the AUG start codon, and is crucial for eukaryotic translation of nearly all genes. Efficient recruitment of the 40S particle is particularly important for translation of mRNAs encoding oncoproteins and growth-promoting factors, which often harbor complex 5′ UTRs and require efficient initiation. Thus, inhibiting the eIF4E/eIF4G interaction has emerged as a previously unpursued route for developing anticancer agents. Indeed, we discovered small-molecule inhibitors of this eIF4E/eIF4G interaction (4EGIs) that inhibit translation initiation both in vitro and in vivo and were used successfully in numerous cancer–biology and neurobiology studies. However, their detailed molecular mechanism of action has remained elusive. Here, we show that the eIF4E/eIF4G inhibitor 4EGI-1 acts allosterically by binding to a site on eIF4E distant from the eIF4G binding epitope. Data from NMR mapping and high-resolution crystal structures are congruent with this mechanism, where 4EGI-1 attaches to a hydrophobic pocket of eIF4E between β-sheet2 (L60-T68) and α-helix1 (E69-N77), causing localized conformational changes mainly in the H78-L85 region. It acts by unfolding a short 310-helix (S82-L85) while extending α-helix1 by one turn (H78-S82). This unusual helix rearrangement has not been seen in any previous eIF4E structure and reveals elements of an allosteric inhibition mechanism leading to the dislocation of eIF4G from eIF4E.

Embryonic Lethal Abnormal Vision-like HuR-dependent mRNA Stability Regulates Post-transcriptional Expression of Cyclin-dependent Kinase Inhibitor p27Kip1

The cyclin-dependent kinase inhibitor p27Kip1 plays a critical role in regulating entry into and exit from the cell cycle. Post-transcriptional regulation of p27Kip1 expression is of significant interest. The embryonic lethal abnormal vision (ELAV)-like RNA-binding protein HuR is thought be important for the translation of p27Kip1, however, different reports attributed diametrically opposite roles to HuR. We report here an alternative mechanism wherein HuR regulates stability of the p27Kip1 mRNA. Specifically, human and mouse p27Kip1 mRNAs interact with HuR protein through multiple U-rich elements in both 5′ and 3′ untranslated regions (UTR). These interactions, which occur in vitro and in vivo, stabilize p27Kip1 mRNA and play a critical role in its accumulation. Deleting HuR binding sites or knocking down HuR expression destabilizes p27Kip1 mRNA and reduces its accumulation. We also identified a CT repeat in the 5′ UTR of full-length p27Kip1 mRNA isoforms that interact with a ∼41-kDa protein and represses p27Kip1 expression. This CT-rich element and diffuse elements in the 3′ UTR regulate post-transcriptional expression of p27Kip1 at the level of translation. This is the first demonstration that HuR-dependent mRNA stability and HuR-independent mRNA translation plays a critical role in the regulation of post-transcriptional p27Kip1 expression.

Dual activators of protein kinase R (PKR) and protein kinase R-like kinase PERK identify common and divergent catalytic targets.

Chemical genetics has evolved into a powerful tool for studying gene function in normal and pathobiology. PKR and PERK, two eukaryotic translation initiation factor 2 alpha (eIF2α) kinases, play critical roles in the maintenance of cellular hemostasis, metabolic stability, and anti‐viral defenses. Both kinases interact with and phosphorylate additional substrates including tumor suppressor p53 and nuclear protein 90. Loss of function of both kinases has been studied by reverse genetics and with recently identified inhibitors. In contrast, no activating probes for studying the catalytic activity of these kinases are available. We identified 3‐(2,3‐dihydrobenzo[b][1,4]dioxin‐6‐yl)‐5,7‐dihydroxy‐4H‐chromen‐4‐one (DHBDC) as a specific dual activator of PKR and PERK by screening a chemical library of 20 000 small molecules in a dual luciferase surrogate eIF2α phosphorylation assay. We present here extensive biological characterization and a preliminary structure–activity relationship of DHBDC, which phosphorylates eIF2α by activating PKR and PERK but no other eIF2α kinases. These agents also activate downstream effectors of eIF2α phosphorylation by inducing CEBP homologue protein, suppressing cyclin D1 expression, and inhibiting cancer cell proliferation, all in a manner dependent on PKR and PERK. Consistent with the role of eIF2α phosphorylation in viral infection, DHBDC inhibits the proliferation of human hepatitis C virus. Finally, DHBDC induces the phosphorylation of IκBα and activates the NF‐κB pathway. Surprisingly, activation of the NF‐κB pathway is dependent on PERK but independent of PKR activity. These data indicate that DHBDC is an invaluable probe for elucidating the role of PKR and PERK in normal and pathobiology.

Synthesis of Rigidified eIF4E/eIF4G Inhibitor-1 (4EGI-1) Mimetic and Their in Vitro Characterization as Inhibitors of Protein–Protein Interaction

The 4EGI-1 is the prototypic inhibitor of eIF4E/eIF4G interaction, a potent inhibitor of translation initiation in vitro and in vivo and an efficacious anticancer agent in animal models of human cancers. We report on the design, synthesis, and in vitro characterization of a series of rigidified mimetic of this prototypic inhibitor in which the phenyl in the 2-(4-(3,4-dichlorophenyl)thiazol-2-yl) moiety was bridged into a tricyclic system. The bridge consisted one of the following: ethylene, methylene oxide, methylenesulfide, methylenesulfoxide, and methylenesulfone. Numerous analogues in this series were found to be markedly more potent than the parent prototypic inhibitor in the inhibition of eIF4E/eIF4G interaction, thus preventing the eIF4F complex formation, a rate limiting step in the translation initiation cascade in eukaryotes, and in inhibition of human cancer cell proliferation.

The integrated endoplasmic reticulum stress response in Leishmania amazonensis macrophage infection: the role of X-box binding protein 1 transcription factor

Endoplasmic reticulum (ER) stress triggers the integrated ER‐stress response (IERSR) that ensures cellular survival of ER stress and represents a primordial form of innate immunity. We investigated the role of IERSR during Leishmania amazonensis infection. Treatment of RAW 264.7 infected macrophages with the ER stress‐inducing agent thapsigargin (TG; 1 μM) increased L. amazonensis infectivity in an IFN1‐α receptor (IFNAR)‐dependent manner. In Western blot assays, we showed that L. amazonensis activates the inositol‐requiring enzyme (IRE1)/ X‐box binding protein (XBP)‐1‐splicing arms of the IERSR in host cells. In chromatin immunoprecipitation (ChIP) assays, we showed an increased occupancy of enhancer and promoter sequences for the Ifnb gene by XBP1 in infected RAW 264.7 cells. Knocking down XBP1 expression by transducing RAW 264.7 cells with the short hairpin XBP1 lentiviral vector significantly reduced the parasite proliferation associated with impaired translocation of phosphorylated IFN regulatory transcription factor (IRF)‐3 to the nucleus and a decrease in IFN1‐β expression. Knocking down XBP1 expression also increased NO concentration, as determined by Griess reaction and reduced the expression of antioxidant genes, such as heme oxygenase (HO)‐1, that protect parasites from oxidative stress. We conclude that L. amazonensis activation of XBP1 plays a critical role in infection by protecting the parasites from oxidative stress and increasing IFN1‐β expression.—Dias‐Teixeira, K. L., Calegari‐Silva, T. C., Dos Santos, G. R. R. M., Vitorino dos Santos, J., Lima, C., Medina, J. M., Aktas, B. H., Lopes, U. G. The integrated endoplasmic reticulum stress response in Leishmania amazonensis macrophage infection: the role of X‐box binding protein 1 transcription factor. FASEB J. 30, 1557–1565 (2016). www.fasebj.org

3-substituted indazoles as configurationally locked 4EGI-1 mimetics and inhibitors of the eIF4E/eIF4G Interaction

4EGI‐1, the prototypic inhibitor of eIF4E/eIF4G interaction, was identified in a high‐throughput screening of small‐molecule libraries with the aid of a fluorescence polarization assay that measures inhibition of binding of an eIF4G‐derived peptide to recombinant eIF4E. As such, the molecular probe 4EGI‐1 has potential for the study of molecular mechanisms involved in human disorders characterized by loss of physiological restraints on translation initiation. A hit‐to‐lead optimization campaign was carried out to overcome the configurational instability in 4EGI‐1, which stems from the E‐to‐Z isomerization of the hydrazone function. We identified compound 1 a, in which the labile hydrazone was incorporated into a rigid indazole scaffold, as a promising rigidified 4EGI‐1 mimetic lead. In a structure–activity relationship study directed towards probing the structural latitude of this new chemotype as an inhibitor of eIF4E/eIF4G interaction and translation initiation we identified 1 d, an indazole‐based 4EGI‐1 mimetic, as a new and improved lead inhibitor of eIF4E/eIF4G interaction and a promising molecular probe candidate for elucidation of the role of cap‐dependent translation initiation in a host of pathophysiological states.

Synthesis and SAR study of novel 3,3-diphenyl-1,3-dihydroindol-2-one derivatives as potent eIF2·GTP·Met-tRNAiMet ternary complex inhibitors.

The growing recognition of inhibition of translation initiation as a new and promising paradigm for mechanism-based anti-cancer therapeutics is driving the development of potent, specific, and druggable inhibitors. The 3,3-diaryloxindoles were recently reported as potential inhibitors of the eIF2·GTP·Met-tRNAi(Met) ternary complex assembly and 3-{5-tert-butyl-2-hydroxyphenyl}-3-phenyl-1,3-dihydro-2H-indol-2-one #1181 was identified as the prototypic agent of this chemotype. Herein, we report our continuous effort to further develop this chemotype by exploring the structural latitude toward different polar and hydrophobic substitutions. Many of the novel compounds are more potent than the parent compound in the dual luciferase ternary complex reporter assay, activate downstream effectors of reduced ternary complex abundance, and inhibit cancer cell proliferation in the low μM range. Moreover, some of these compounds are decorated with substituents that are known to endow favorable physicochemical properties and as such are good candidates for evaluation in animal models of human cancer.

Inhibition of Translation Initiation as a Novel Paradigm for Cancer Therapy

Publisher Summary This chapter discusses the role and expression of translation initiation factors in cancer. The regulation of gene expression at the level of translation initiation is critical for proper control of cell growth, proliferation, differentiation, and apoptosis. Deregulation of translation initiation is frequently observed in tumors and plays an important role in the genesis, progression, and maintenance of some cancers. This is because unrestricted translation favors expression of genes that promote cell proliferation, malignant transformation, and cancer progression. Restricting the translation initiation by molecular and chemical genetic approaches reverts the malignant phenotype because it preferentially reduces translation of messenger ribonucleic acids (mRNAs) that code for proteins important for the genesis and progression of cancer. The growing understanding of the structural biology and the mechanistic insight into the translation initiation cascade led to the identification of pharmacological targets for the development of mechanism-specific anticancer agents—a new paradigm for anticancer therapy. Targeting cancer pathways downstream of their cellular signaling networks at the level of translation initiation is more effective and could offer fewer side effects than inhibiting their upstream targets.