Embryonic lethal abnormal vision-like HuR-dependent mRNA stability regulates post-transcriptional expression of cyclin-dependent kinase inhibitor p27Kip1

The human antigen R as an actionable super-hub within the network of cancer cell persistency and plasticity

In this perspective article, a clinically inspired phenotype-driven experimental approach is put forward to address the challenge of the adaptive response of solid cancers to small-molecule targeted therapies. A list of conditions is derived, including an experimental quantitative assessment of cell plasticity and an information theory-based detection of in vivo dependencies, for the discovery of post-transcriptional druggable mechanisms capable of preventing at multiple levels the emergence of plastic dedifferentiated slow-proliferating cells. The approach is illustrated by the author's own work in the example case of the adaptive response of BRAFV600-melanoma to BRAF inhibition. A bench-to-bedside and back to bench effort leads to a therapeutic strategy in which the inhibition of the baseline activity of the interferon-γ-activated inhibitor of translation (GAIT) complex, incriminated in the expression insufficiency of the RNA-binding protein HuR in a minority of cells, results in the suppression of the plastic, intermittently slow-proliferating cells involved in the adaptive response. A similar approach is recommended for the validation of other classes of mechanisms that we seek to modulate to overcome this complex challenge of modern cancer therapy.

Role of symmetry in 3,3-diphenyl-1,3-dihydroindol-2-one derivatives as inhibitors of translation initiation

The ternary complex (eIF2·GTP·Met-tRNAiMet) and the eIF4F complex assembly are two major regulatory steps in the eukaryotic translation initiation. Inhibition of the ternary complex assembly is therefore a promising target for the development of novel anti-cancer therapeutics. Building on the finding that clotrimazole (CLT), a molecular probe that depletes intracellular Ca2+ stores and subsequently induce eIF2α phosphorylation, inhibit translation initiation, and reduce preferentially the expression of oncoproteins over "housekeeping" ones,1-3 we undertook structure activity relationship (SAR) studies that identified 3,3-diarylindoline-2-one #1181 as an interesting scaffold. Compound #1181 also induce phosphorylation of eIF2α thereby reducing the availability of the ternary complex, which leads to inhibition of translation initiation.4 Our subsequent efforts focused on understanding SAR iterative lead optimization to enhance potency and improve bioavailability. Herein, we report a complementing study focusing on heavily substituted symmetric and asymmetric 3,3-(o,m-disubstituted)diarylindoline-2-ones. These compounds were evaluated by the dual luciferase reporter ternary complex assay that recapitualates phosphorylation of eIF2α in a quantitative manner. We also evaluated all compounds by sulforhodamine B assay, which measures the overall effect of compounds on cell proliferations and/or viability.

New activators of eIF2α Kinase Heme-Regulated Inhibitor (HRI) with improved biophysical properties

Heme-regulated inhibitor (HRI), a eukaryotic translation initiation factor 2 alpha (eIF2α) kinase, is critically important for coupling protein synthesis to heme availability in reticulocytes and adaptation to various environmental stressors in all cells. HRI modifies the severity of several hemoglobin misfolding disorders including β-thalassemia. Small molecule activators of HRI are essential for studying normal- and patho-biology of this kinase as well as for the treatment of various human disorders for which activation of HRI or phosphorylation of eIF2α may be beneficial. We previously reported development of 1-((1,4-trans)-4-aryloxycyclohexyl)-3-arylureas (cHAUs) as specific HRI activators and demonstrated their potential as molecular probes for studying HRI biology and as lead compounds for treatment of various human disorders. To develop more druglike cHAUs for in vivo studies and drug development and to expand the chemical space, we undertook bioassay guided structure-activity relationship studies replacing cyclohexyl ring with various 4-6-membered rings and explored further substitutions on the N-phenyl ring. We tested all analogs in the surrogate eIF2α phosphorylation and cell proliferation assays, and a subset of analogs in secondary mechanistic assays that included endogenous eIF2α phosphorylation and expression of C/EBP homologous protein (CHOP), a downstream effector. Finally, we determined specificity of these compounds for HRI by testing their anti-proliferative activity in cells transfected with siRNA targeting HRI or mock. These compounds have significantly improved cLogPs with no loss of potencies, making them excellent candidates for lead optimization for development of investigational new drugs that potently and specifically activate HRI.

Tandem RNA isolation reveals functional rearrangement of RNA-binding proteins on CDKN1B/p27Kip1 3'UTRs in cisplatin treated cells

Post-transcriptional control of gene expression is mediated via RNA-binding proteins (RBPs) that interact with mRNAs in a combinatorial fashion. While recent global RNA interactome capture experiments expanded the repertoire of cellular RBPs quiet dramatically, little is known about the assembly of RBPs on particular mRNAs; and how these associations change and control the fate of the mRNA in drug-treatment conditions. Here we introduce a novel biochemical approach, termed tobramycin-based tandem RNA isolation procedure (tobTRIP), to quantify proteins associated with the 3ʹUTRs of cyclin-dependent kinase inhibitor 1B (CDKN1B/p27^Kip1) mRNAs in vivo. P27^Kip1 plays an important role in mediating a cell’s response to cisplatin (CP), a widely used chemotherapeutic cancer drug that induces DNA damage and cell cycle arrest. We found that p27^Kip1 mRNA is stabilized upon CP treatment of HEK293 cells through elements in its 3ʹUTR. Applying tobTRIP, we further compared the associated proteins in CP and non-treated cells, and identified more than 50 interacting RBPs, many functionally related and evoking a coordinated response. Knock-downs of several of the identified RBPs in HEK293 cells confirmed their involvement in CP-induced p27 mRNA regulation; while knock-down of the KH-type splicing regulatory protein (KHSRP) further enhanced the sensitivity of MCF7 adenocarcinoma cancer cells to CP treatment. Our results highlight the benefit of specific in vivo mRNA-protein interactome capture to reveal post-transcriptional regulatory networks implicated in cellular drug response and adaptation.

An Oligonucleotide-based Tandem RNA Isolation Procedure to Recover Eukaryotic mRNA-Protein Complexes

RNA-binding proteins (RBPs) play key roles in the post-transcriptional control of gene expression. Therefore, biochemical characterization of mRNA-protein complexes is essential to understanding mRNA regulation inferred by interacting proteins or non-coding RNAs. Herein, we describe a tandem RNA isolation procedure (TRIP) that enables the purification of endogenously formed mRNA-protein complexes from cellular extracts. The two-step protocol involves the isolation of polyadenylated mRNAs with antisense oligo(dT) beads and subsequent capture of an mRNA of interest with 3'-biotinylated 2'-O-methylated antisense RNA oligonucleotides, which can then be isolated with streptavidin beads. TRIP was used to recover in vivo crosslinked mRNA-ribonucleoprotein (mRNP) complexes from yeast, nematodes and human cells for further RNA and protein analysis. Thus, TRIP is a versatile approach that can be adapted to all types of polyadenylated RNAs across organisms to study the dynamic re-arrangement of mRNPs imposed by intracellular or environmental cues.

Modulation of the mRNA-binding protein HuR as a novel reversal mechanism of epirubicin-triggered multidrug resistance in colorectal cancer cells

HuR (ELAVL1), a RNA-binding protein, plays a key role in posttranscriptional regulation of multidrug resistance (MDR)-related genes. Among various HuR-regulated oncogenic transcripts, the activation of galectin-3/β-catenin survival pathway is critical to induce transcription of cyclin D1, P-glycoprotein (P-gp) and/or multidrug resistance-associated proteins (MRPs). In this study, we aim to elucidate the HuR-regulating pathways related to epirubicin-mediated resistance in human colorectal carcinoma cells. The effects and mechanisms of epirubicin treatment on the expressions of upstream survival signals (e.g., β-catenin) and downstream MDR transporters (e.g., P-gp) and anti-apoptotic pathways (e.g., Bcl-2) were assessed with or without HuR knockdown (siHuR) or overexpression (overHuR; ectopic HuR or pcDNA3/HA-HuR). Our results showed that siHuR decreased transcriptional expressions of galectin-3, β-catenin, cyclin D1, Bcl-2, P-gp, MRP1, and MRP2 in epirubicin-treated colon cancer cells. Consistently, the co-treatment of epirubicin and siHuR diminished the expressions of galectin-3, ß-catenin, c-Myc, P-gp and MRP1. HuR silencing enhanced the intracellular accumulation of epirubicin in colon cancer cells. On the other hand, overHuR abolished such effects. Furthermore, siHuR significantly intensified epirubicin-mediated apoptosis via increasing reactive oxygen species and thus promoted the cytotoxic effect of epirubicin. The combined treatments of siHuR and epirubicin significantly reduced the expression of Bcl-2, but increased the expression of Bax, as well as activity and expression levels of caspase-3 and -9. In contrast, overHuR abrogated these effects. Our findings provide insight into the mechanisms by which siHuR potentiated epirubicin-induced cytotoxicity via inhibiting galectin-3/β-catenin signaling, suppressing MDR transporters and provoking apoptosis. To our best knowledge, this is an innovative investigation linking the post-transcriptional control by HuR silencing to survival signaling repression, efflux transporter reversal and apoptosis induction. Our study thus provides a powerful regimen for circumventing MDR in colon cancer cells.

Development of 1-((1,4-trans)-4-Aryloxycyclohexyl)-3-arylurea Activators of Heme-Regulated Inhibitor as Selective Activators of the Eukaryotic Initiation Factor 2 Alpha (eIF2α) Phosphorylation Arm of the Integrated Endoplasmic Reticulum Stress Response

Heme-regulated inhibitor (HRI), an eukaryotic translation initiation factor 2 alpha (eIF2α) kinase, plays critical roles in cell proliferation, differentiation, adaptation to stress, and hemoglobin disorders. HRI phosphorylates eIF2α, which couples cellular signals, including endoplasmic reticulum (ER) stress, to translation. We previously identified 1,3-diarylureas and 1-((1,4-trans)-4-aryloxycyclohexyl)-3-arylureas (cHAUs) as specific activators of HRI that trigger the eIF2α phosphorylation arm of ER stress response as molecular probes for studying HRI biology and its potential as a druggable target. To develop drug-like cHAUs needed for in vivo studies, we undertook bioassay-guided structure-activity relationship studies and tested them in the surrogate eIF2α phosphorylation and cell proliferation assays. We further evaluated some of these cHAUs in endogenous eIF2α phosphorylation and in the expression of the transcription factor C/EBP homologous protein (CHOP) and its mRNA, demonstrating significantly improved solubility and/or potencies. These cHAUs are excellent candidates for lead optimization for development of investigational new drugs that potently and specifically activate HRI.

A versatile tandem RNA isolation procedure to capture in vivo formed mRNA-protein complexes

We describe a tandem RNA isolation procedure (TRIP) that enables purification of in vivo formed messenger ribonucleoprotein (mRNP) complexes. The procedure relies on the purification of polyadenylated mRNAs with oligo(dT) beads from cellular extracts, followed by the capture of specific mRNAs with 3'-biotinylated 2'-O-methylated antisense RNA oligonucleotides, which are recovered with streptavidin beads. TRIP was applied to isolate in vivo crosslinked mRNP complexes from yeast, nematodes and human cells for subsequent analysis of RNAs and bound proteins. The method provides a basis for adaptation to other types of polyadenylated RNAs, enabling the comprehensive identification of bound proteins/RNAs, and the investigation of dynamic rearrangement of mRNPs imposed by cellular or environmental cues.

Depletion of eIF2·GTP·Met-tRNAi translation initiation complex up-regulates BRCA1 expression in vitro and in vivo

Most sporadic breast and ovarian cancers express low levels of the breast cancer susceptibility gene, BRCA1. The BRCA1 gene produces two transcripts, mRNAa and mRNAb. mRNAb, present in breast cancer but not in normal mammary epithelial cells, contains three upstream open reading frames (uORFs) in its 5′UTR and is translationally repressed. Comparable tandem uORFs are characteristically seen in mRNAs whose translational efficiency paradoxically increases when the overall translation rate is decreased due to phosphorylation of eukaryotic translation initiation factor 2 α (eIF2α). Here we show fish oil derived eicosopanthenoic acid (EPA) that induces eIF2α phosphorylation translationally up-regulates the expression of BRCA1 in human breast cancer cells. We demonstrate further that a diet rich in EPA strongly induces expression of BRCA1 in human breast cancer xenografts.

Small-Molecule targeting of translation initiation for cancer therapy

Translation initiation plays a critical role in the regulation of cell growth and tumorigenesis. We report here that inhibiting translation initiation through induction of eIF2α phosphorylation by small-molecular-weight compounds restricts the availability of the eIF2·GTP·Met-tRNA_i ternary complex and abrogates the proliferation of cancer cells in vitro and tumor growth in vivo. Restricting the availability of the ternary complex preferentially down-regulates the expression of growth-promoting proteins and up-regulates the expression of ER stress response genes in cancer cells as well as in tumors excised from either animal models of human cancer or cancer patients. These findings provide the first direct evidence for translational control of gene-specific expression by small molecules in vivo and indicate that translation initiation factors are bona fide targets for development of mechanism-specific anti-cancer agents.

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.

Conditional knockout of the RNA-binding protein HuR in CD4⁺ T cells reveals a gene dosage effect on cytokine production

The posttranscriptional mechanisms by which RNA binding proteins (RBPs) regulate T-cell differentiation and cytokine production in vivo remain unclear. The RBP HuR binds to labile mRNAs, usually leading to increases in mRNA stability and/or translation. Previous work demonstrated that HuR binds to the mRNAs encoding the Th2 transcription factor trans-acting T-cell-specific transcription factor (GATA-3) and Th2 cytokines interleukin (IL)-4 and IL-13, thereby regulating their expression. By using a novel conditional HuR knockout (KO) mouse in which HuR is deleted in activated T cells, we show that Th2-polarized cells from heterozygous HuR conditional (OX40-Cre HuR(fl/+)) KO mice had decreased steady-state levels of Gata3, Il4 and Il13 mRNAs with little changes at the protein level. Surprisingly, Th2-polarized cells from homozygous HuR conditional (OX40-Cre HuR(fl/fl)) KO mice showed increased Il2, Il4 and Il13 mRNA and protein via different mechanisms. Specifically, Il4 was transcriptionally upregulated in HuR KO T cells, whereas Il2 and Il13 mRNA stabilities increased. Additionally, when using the standard ovalbumin model of allergic airway inflammation, HuR conditional KO mice mounted a robust inflammatory response similar to mice with wild-type HuR levels. These results reveal a complex differential posttranscriptional regulation of cytokines by HuR in which gene dosage plays an important role. These findings may have significant implications in allergies and asthma, as well as autoimmune diseases and infection.

Structure-activity relationship study of 4EGI-1, small molecule eIF4E/eIF4G protein-protein interaction inhibitors

Protein-protein interactions are critical for regulating the activity of translation initiation factors and multitude of other cellular process, and form the largest block of untapped albeit most challenging targets for drug development. 4EGI-1, (E/Z)-2-(2-(4-(3,4-dichlorophenyl)thiazol-2-yl)hydrazono)-3-(2-nitrophenyl)propanoic acid, is a hit compound discovered in a screening campaign of small molecule libraries as an inhibitor of translation initiation factors eIF4E and eIF4G protein-protein interaction; it inhibits translation initiation in vitro and in vivo. A series of 4EGI-1-derived thiazol-2-yl hydrazones have been designed and synthesized in order to delineate the structural latitude and improve its binding affinity to eIF4E, and increase its potency in inhibiting the eIF4E/eIF4G interaction. Probing a wide range of substituents on both phenyl rings comprising the 3-phenylpropionic acid and 4-phenylthiazolidine moieties in the context of both E- and Z-isomers of 4EGI-1 led to analogs with enhanced binding affinity and translation initiation inhibitory activities.

Explorations of substituted urea functionality for the discovery of new activators of the heme-regulated inhibitor kinase

Heme-regulated inhibitor kinase (HRI), a eukaryotic translation initiation factor 2 alpha (eIF2α) kinase, plays critical roles in cell proliferation, differentiation, and adaptation to cytoplasmic stress. HRI is also a critical modifier of hemoglobin disorders such as β-thalassemia. We previously identified N,N'-diarylureas as potent activators of HRI suitable for studying the biology of this important kinase. To expand the repertoire of chemotypes that activate HRI, we screened a ∼1900 member N,N'-disubstituted urea library in the surrogate eIF2α phosphorylation assay, identifying N-aryl,N'-cyclohexylphenoxyurea as a promising scaffold. We validated hit compounds as a bona fide HRI activators in secondary assays and explored the contributions of substitutions on the N-aryl and N'-cyclohexylphenoxy groups to their activity by studying focused libraries of complementing analogues. We tested these N-aryl,N'-cyclohexylphenoxyureas in the surrogate eIF2α phosphorylation and cell proliferation assays, demonstrating significantly improved bioactivities and specificities. We consider these compounds to represent lead candidates for the development of potent and specific HRI activators.

MAPK kinase 3 is a tumor suppressor with reduced copy number in breast cancer

Cancers are initiated as a result of changes that occur in the genome. Identification of gains and losses in the structure and expression of tumor-suppressor genes and oncogenes lies at the root of the understanding of cancer cell biology. Here, we show that the mitogen-activated protein kinase (MAPK) MKK3 suppresses the growth of breast cancer, in which it varies in copy number. A pervasive loss of MKK3 gene copy number in patients with breast cancer is associated with an impairment of MKK3 expression and protein level in malignant tissues. To assess the functional role of MKK3 in breast cancer, we showed in an animal model that MKK3 activity is required for suppression of tumor growth. Active MKK3 enhanced expression of the cyclin-dependent kinase inhibitors p21(Cip1/Waf1) and p27(Kip1), leading to increased cell-cycle arrest in G1 phase of the cell cycle. Our results reveal the functional significance of MKK3 as a tumor suppressor and improve understanding of the dynamic role of the MAPK pathway in tumor progression.

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.

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.

Tumor suppression by small molecule inhibitors of translation initiation

Translation initiation factors are over-expressed and/or activated in many human cancers and may contribute to their genesis and/or progression. Removal of physiologic restraints on translation initiation causes malignant transformation. Conversely, restoration of physiological restrains on translation initiation reverts malignant phenotypes. Here, we extensively characterize the anti-cancer activity of two small molecule inhibitors of translation initiation: #1181, which targets the eIF2-GTP-Met-tRNA_i ternary complex, and 4EGI-1, which targets the eIF4F complex. In vitro, both molecules inhibit translation initiation, abrogate preferentially translation of mRNAs coding for oncogenic proteins, and inhibit proliferation of human cancer cells. In vivo, both #1181 and 4EGI-1 strongly inhibit growth of human breast and melanoma cancer xenografts without any apparent macroscopic- or microscopic-toxicity. Mechanistically, #1181 phosphorylates eIF2α while 4EGI-1 disrupts eIF4G/eIF4E interaction in the tumors excised from mice treated with these agents. These data indicate that inhibition of translation initiation is a new paradigm in cancer therapy.

miR-9 controls the timing of neurogenesis through the direct inhibition of antagonistic factors

The timing of commitment and cell-cycle exit within progenitor populations during neurogenesis is a fundamental decision that impacts both the number and identity of neurons produced during development. We show here that microRNA-9 plays a key role in this process through the direct inhibition of targets with antagonistic functions. Across the ventricular zone of the developing zebrafish hindbrain, miR-9 expression occurs at a range of commitment stages. Abrogating miR-9 function transiently delays cell-cycle exit, leading to the increased generation of late-born neuronal populations. Target protection analyses in vivo identify the progenitor-promoting genes her6 and zic5 and the cell-cycle exit-promoting gene elavl3/HuC as sequential targets of miR-9 as neurogenesis proceeds. We propose that miR-9 activity generates an ambivalent progenitor state poised to respond to both progenitor maintenance and commitment cues, which may be necessary to adjust neuronal production to local extrinsic signals during late embryogenesis.

A complex 'mRNA degradation code' controls gene expression during animal development

Current understanding of the molecular mechanisms underlying mRNA degradation indicates that specific mRNA degradation rates are primarily encoded within the mRNA message itself in the form of cis-regulatory elements bearing particular primary sequences and/or secondary-structures. Such control elements are operated by RNA-binding proteins (RBPs) and/or miRNA-containing complexes. Based on the large number of RBPs and miRNAs encoded in metazoan genomes, their complex developmental expression and that specific RBP and miRNA interactions with mRNAs can lead to distinct degradation rates, I propose that developmental gene expression is shaped by a complex 'mRNA degradation code' with high information capacity. Localised cellular events involving the modification of RBP and/or miRNA target sequences in mRNAs by alternative polyadenylation added to the activation of specific RBP and miRNA activities via cell signalling are predicted to further expand the capacity of the mRNA degradation code by coupling it to dynamic events experienced by cells at specific spatiotemporal coordinates within the developing embryo.

In vitro inhibition of translation initiation by N,N'-diarylureas--potential anti-cancer agents

Symmetrical N,N'-diarylureas: 1,3-bis(3,4-dichlorophenyl)-, 1,3-bis[4-chloro-3-(trifluoromethyl)phenyl]- and 1,3-bis[3,5-bis(trifluoromethyl)phenyl]urea, were identified as potent activators of the eIF2α kinase heme regulated inhibitor. They reduce the abundance of the eIF2·GTP·tRNA(i)(Met) ternary complex and inhibit cancer cell proliferation. An optimization process was undertaken to improve their solubility while preserving their biological activity. Non-symmetrical hybrid ureas were generated by combining one of the hydrophobic phenyl moieties present in the symmetrical ureas with the polar 3-hydroxy-tolyl moiety. O-alkylation of the later added potentially solubilizing charge bearing groups. The new non-symmetrical N,N'-diarylureas were characterized by ternary complex reporter gene and cell proliferation assays, demonstrating good bioactivities. A representative sample of these compounds potently induced phosphorylation of eIF2α and expression of CHOP at the protein and mRNA levels. These inhibitors of translation initiation may become leads for the development of potent, non-toxic, and target specific anti-cancer agents.

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-tRNA_i 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-tRNA_i 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-tRNA_i ternary complex in the pathobiology of human disorders.