Depletion of intracellular Ca2+ stores, phosphorylation of eIF2alpha, and sustained inhibition of translation initiation mediate the anticancer effects of clotrimazole

Translatomics reveals the role of dietary calcium addition in regulating muscle fat deposition in pigs

Intramuscular fat (IMF) in pork holds significant importance for economic performance within the pig industry and dietary calcium supplementation enhances the accumulation of intramuscular fat. Additionally, calcium ions inhibit translation and reduce protein synthesis. However, the mechanism by which calcium regulates IMF deposition in muscle through translation remains largely unknown. In this study, we compared the ribosome profiles of the longissimus dorsi muscles of Duroc × Landrace × Large white pigs from the normal calcium (NC) group or calcium supplement (HC) group by Ribo-seq, and RNA-seq. By integrating multiple-omics analysis, we further discovered 437 genes that were transcriptionally unchanged but translationally altered and these genes were significantly enriched in the oxidative phosphorylation signaling pathway. Furthermore, experimental data showed that inhibiting the expression of COX10 and mtND4L increased triglyceride accumulation in C2C12 cells, providing new targets for intramuscular fat deposition. Finally, this work links dietary calcium, translation regulation and IMF deposition, providing a new strategy for both meat quality and economic performance within the pig industry.

Repurposing antifungal drugs for cancer therapy

BACKGROUND

Repurposing antifungal drugs in cancer therapy has attracted unprecedented attention in both preclinical and clinical research due to specific advantages, such as safety, high-cost effectiveness and time savings compared with cancer drug discovery. The surprising and encouraging efficacy of antifungal drugs in cancer therapy, mechanistically, is attributed to the overlapping targets or molecular pathways between fungal and cancer pathogenesis. Advancements in omics, informatics and analytical technology have led to the discovery of increasing "off-site" targets from antifungal drugs involved in cancerogenesis, such as smoothened (D477G) inhibition from itraconazole in basal cell carcinoma.

AIM OF REVIEW

This review illustrates several antifungal drugs repurposed for cancer therapy and reveals the underlying mechanism based on their original target and "off-site" target. Furthermore, the challenges and perspectives for the future development and clinical applications of antifungal drugs for cancer therapy are also discussed, providing a refresh understanding of drug repurposing.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This review may provide a basic understanding of repurposed antifungal drugs for clinical cancer management, thereby helping antifungal drugs broaden new indications and promote clinical translation.

Cross-linkable star-hyperbranched unimolecular micelles for the enhancement of the anticancer activity of clotrimazole

Clotrimazole, a hydrophobic drug routinely used in the treatment of vaginal candidiasis, also shows antitumor activity. However, its use in chemotherapy has been unsuccessful to date due to its low solubility in aqueous media. In this work, new unimolecular micelles based on polyether star-hyperbranched carriers of clotrimazole are presented that can enhance solubility, and consequently the bioavailability, of clotrimazole in water. The amphiphilic constructs consisting of a hydrophobic poly(n-alkyl epoxide) core and hydrophilic corona of hyperbranched polyglycidol were synthesized in a three-step anionic ring-opening polymerization of epoxy monomers. The synthesis of such copolymers, however, was only possible by incorporating a linker to facilitate the elongation of the hydrophobic core with glycidol. Unimolecular micelles-clotrimazole formulations displayed significantly increased activity against human cervical cancer HeLa cells compared to the free drug, along with a weak effect on the viability of the normal dermal microvascular endothelium cells HMEC1. This selective activity of clotrimazole on cancer cells with little effect on normal cells was a result of the fact that clotrimazole targets the Warburg effect in cancer cells. Flow cytometric analysis revealed that the encapsulated clotrimazole significantly blocks the progression of the HeLa cycle in the G0/G1 phase and induces apoptosis. In addition, the ability of the synthesized amphiphilic constructs to form a dynamic hydrogel was demonstrated. Such a gel facilitates the delivery of drug-loaded single-molecule micelles to the affected area, where they can form a continuous, self-healing layer.

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.

Targeting translation regulators improves cancer therapy

Deregulation of protein synthesis may be involved in multiple aspects of cancer, such as gene expression, signal transduction and drive specific cell biological responses, resulting in promoting cancer growth, invasion and metastasis. Study the molecular mechanisms about translational control may help us to find more effective anti-cancer drugs and develop novel therapeutic opportunities. Recently, the researchers had focused on targeting translational machinery to overcome cancer, and various small molecular inhibitors targeting translation factors or pathways have been tested in clinical trials and exhibited improving outcomes in several cancer types. There is no doubt that an insight into the class of translation regulation protein would provide new target for pharmacologic intervention and further provide opportunities to develop novel anti-tumor therapeutic interventions. In this review, we summarized the developments of translational control in cancer survival and progression et al, and highlighted the therapeutic approach targeted translation regulation to overcome the cancer.

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.

eIF2 alpha phosphorylation alleviates UVA-induced HO-1 expression in mouse epidermal cells

Ultraviolet A (UVA) irradiation is a potential environmental stressor, which contributes to inflammation, photoaging, and carcinogenesis. UVA causes endoplasmic reticulum stress, hence phosphorylates the α subunit of eIF2. Meanwhile, UVA also induces expression of haem oxygenase-1 (HO-1) and nuclear factor erythroid-derived two related factor 2 (Nrf2) in human skin cells. In mouse JB6 cell, we found high dose UVA could change cell morphology, cause cell viability loss. UVA irradiation activated phosphorylation of eIF2α and Nrf2-HO-1 pathway in a dose-dependent manner. Besides, modulation of eIF2α phosphorylation status could alter expression pattern of Nrf2-HO-1 signalling. Salubrinal, a selective inhibitor of eIF2α dephosphorylation, increased the S phase in cell cycle of JB6 cells after UVA irradiation, suggesting phosphorylation status of eIF2α may affect cellular homeostasis under UVA irradiation. The study directed to further acknowledge about the relationship of UVA-induced eIF2α phosphorylation and Nrf2-HO-1 pathway, which may play a role in phototherapy and photo protection.

Boric Acid Activation of eIF2α and Nrf2 Is PERK Dependent: a Mechanism that Explains How Boron Prevents DNA Damage and Enhances Antioxidant Status

Boron is abundant in vegetables, nuts, legumes, and fruit and intake is associated with reduced risk of cancer and DNA damage and increased antioxidant status. Blood boric acid (BA) levels are approximately 10 μM BA in men at the mean US boron intake. Treatment of DU-145 human prostate cancer cells with 10 μM BA stimulates phosphorylation of elongation initiation factor 2α (eIF2α) at Ser51 leading to activation of the eIF2α/ATF4 pathway which activates the DNA damage-inducible protein GADD34. In the present study, we used MEF WT and MEF PERK (±) cells to test the hypothesis that BA-activated eIF2α phosphorylation requires protein kinase RNA-like endoplasmic reticulum kinase (PERK) and activates Nrf2 and the antioxidant response element (ARE). BA (10 μM) increased phosphorylation of eIF2α Ser51 in MEF WT cells at 1 h, but not in MEF Perk -/- cells exposed for as long as 6 h. GCN2 kinase-dependent phosphorylation of eIF2α Ser51 was activated in MEF PERK -/- cells by amino acid starvation. Nrf2 phosphorylation is PERK dependent and when activated is translocated from the cytoplasm to the nucleus where it acts as a transcription factor for ARE. DU-145 cells were treated with 10 μM BA and Nrf2 measured by immunofluorescence. Cytoplasmic Nrf2 was translocated to the nucleus at 1.5-2 h in DU-145 and MEF WT cells, but not MEF PERK -/- cells. Real-time PCR was used to measure mRNA levels of three ARE genes (HMOX-1, NQO1, and GCLC). Treatment with 10 μM BA increased the mRNA levels of all three genes at 1-4 h in DU-145 cells and HMOX1 and GCLC in MEF WT cells. These results extend the known boric acid signaling pathway to ARE-regulated genes. The BA signaling pathway can be expressed using the schematic [BA + cADPR → cADPR-BA → [[ER]_i Ca²⁺↓] → 3 pathways: PERK/eIF2αP → pathways ATF4 and Nrf2; and [[ER]_i Ca²⁺↓] → ER stress → ATF6 pathway. This signaling pathway provides a framework that links many of the molecular changes that underpin the biological effects of boron intake.

Clotrimazole inhibits the Wnt/β-catenin pathway by activating two eIF2α kinases: The heme-regulated translational inhibitor and the double-stranded RNA-induced protein kinase

The Wnt/β-catenin signaling pathway controls cell proliferation and differentiation, and therefore, when this pathway is excessively activated, it causes tumorigenesis. Our chemical suppressor screening in zebrafish embryos identified antifungal azoles including clotrimazole, miconazole, and itraconazole, as Wnt/β-catenin signaling inhibitors. Here we show the mechanism underlying the Wnt/β-catenin pathway inhibition by antifungal azoles. Clotrimazole reduced β-catenin revels in a proteasome-independent fashion. By gene knockdown of two translational regulators, heme-regulated translational inhibitor and double-stranded RNA-induced protein kinase, we show that they mediate the clotrimazole-induced inhibition of the Wnt/β-catenin pathway. Thus, clotrimazole inhibits the Wnt/β-catenin pathway by decreasing β-catenin protein levels through translational regulation. Antifungal azoles represent genuine candidate compounds for anticancer drugs or chemopreventive agents that reduce adenomatous polyps.

Translation Control by p53

The translation of mRNAs plays a critical role in the regulation of gene expression and therefore, in the regulation of cell proliferation, differentiation and apoptosis. Unrestricted initiation of translation causes malignant transformation and plays a key role in the maintenance and progression of cancers. Translation initiation is regulated by the ternary complex and the eukaryotic initiation factor 4F (eIF4F) complex. The p53 tumor suppressor protein is the most well studied mammalian transcription factor that mediates a variety of anti-proliferative processes. Post-transcriptional mechanisms of gene expression in general and those of translation in particular play a major role in shaping the protein composition of the cell. The p53 protein regulates transcription and controls eIF4F, the ternary complex and the synthesis of ribosomal components, including the down-regulation of rRNA genes. In summary, the induction of p53 regulates protein synthesis and translational control to inhibit cell growth.

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.

eIF2α Phosphorylation Mediates IL24-Induced Apoptosis through Inhibition of Translation

IL24 is an immunomodulatory cytokine that also displays broad cancer-specific suppressor effects. The tumor-suppressor activities of IL24 include inhibition of angiogenesis, sensitization to chemotherapy, and cancer-specific apoptosis. Supra-physiologic activation and/or overexpression of translation initiation factors are implicated in the initiation and progression of cancer animal models as well as a subset of human cancers. Activation and/or overexpression of translation initiation factors correlate with aggressiveness of cancer and poor prognosis. Two rate-limiting translation initiation complexes, the ternary complex and the eIF4F complex, are regulated by eIF2α and 4E-BP1 phosphorylation, respectively. The work reported here provides direct evidence that IL24 induces inhibition of translation initiation leading to apoptosis in squamous cell carcinoma. A dominant constitutively active mutant of eIF2α, which is resistant to phosphorylation, was used to determine the involvement of eIF2α in IL24-induced apoptosis. Treatment with IL24 resulted in inhibition of protein synthesis, expression of downstream biomarkers of ternary complex depletion such as CHOP, and induction of apoptosis in cancer cells. The constitutively active nonphosphorylatable mutant of eIF2α, eIF2α-S51A, reversed both the IL24-mediated translational block and IL24-induced apoptosis. Intriguingly, IL24 treatment also caused hypophosphorylation of 4E-BP1, which binds to eIF4E with high affinity, thus preventing its association with eIF4G and therefore preventing elF4F complex assembly.Implications: These results demonstrate a previously unrecognized role of IL24 in inhibition of translation, mediated through both phosphorylation of eIF2α and dephosphorylation of 4E-BP1, and provide the first direct evidence for translation control of gene-specific expression by IL24. Mol Cancer Res; 15(8); 1117-24. ©2017 AACR.

Targeting translation: eIF4E as an emerging anticancer drug target

The translation initiation factor eIF4E mediates a rate-limiting process that drives selective translation of many oncongenic proteins such as cyclin D1, survivin and VEGF, thereby contributing to tumour growth, metastasis and therapy resistance. As an essential regulatory hub in cancer signalling network, many oncogenic signalling pathways appear to converge on eIF4E. Therefore, targeting eIF4E-mediated cap-dependent translation is considered a promising anticancer strategy. This paper reviews the strategies that can be used to target eIF4E, highlighting agents that target eIF4E activity at each distinct level.

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.

Role of the K(Ca)3.1 K+ channel in auricular lymph node CD4+ T-lymphocyte function of the delayed-type hypersensitivity model

BACKGROUND AND PURPOSE

The intermediate-conductance Ca(2+)-activated K(+) channel (K(Ca)3.1) modulates the Ca(2+) response through the control of the membrane potential in the immune system. We investigated the role of K(Ca)3.1 on the pathogenesis of delayed-type hypersensitivity (DTH) in auricular lymph node (ALN) CD4(+) T-lymphocytes of oxazolone (Ox)-induced DTH model mice.

EXPERIMENTAL APPROACH

The expression patterns of K(Ca)3.1 and its possible transcriptional regulators were compared among ALN T-lymphocytes of three groups [non-sensitized (Ox-/-), Ox-sensitized, but non-challenged (Ox+/-) and Ox-sensitized and -challenged (Ox+/+)] using real-time polymerase chain reaction, Western blotting and flow cytometry. KCa 3.1 activity was measured by whole-cell patch clamp and the voltage-sensitive dye imaging. The effects of K(Ca)3.1 blockade were examined by the administration of selective K(Ca)3.1 blockers.

KEY RESULTS

Significant up-regulation of K(Ca)3.1a was observed in CD4(+) T-lymphocytes of Ox+/- and Ox+/+, without any evident changes in the expression of the dominant-negative form, K(Ca)3.1b. Negatively correlated with this, the repressor element-1 silencing transcription factor (REST) was significantly down-regulated. Pharmacological blockade of K(Ca)3.1 resulted in an accumulation of the CD4(+) T-lymphocytes of Ox+/+ at the G0/G1 phase of the cell cycle, and also significantly recovered not only the pathogenesis of DTH, but also the changes in the K(Ca)3.1 expression and activity in the CD4(+) T-lymphocytes of Ox+/- and Ox+/+.

CONCLUSIONS AND IMPLICATIONS

The up-regulation of K(Ca)3.1a in conjunction with the down-regulation of REST may be involved in CD4(+) T-lymphocyte proliferation in the ALNs of DTH model mice; and K(Ca)3.1 may be an important target for therapeutic intervention in allergy diseases such as DTH.

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.

The in vitro and in vivo antitumor effects of clotrimazole on oral squamous cell carcinoma

Background

Clotrimazole is an antifungal imidazole derivative showing anti- neoplastic effect in some tumors, but its anticancer potential is still unclear in oral squamous cell carcinoma (OSCC). The aim of this study was to evaluate the antitumor effect of clotrimazole, and to investigate the possible mechanism of clotrimazole-mediated antitumor activity in OSCC.

Methodology

In vitro experiments, the cell viability and clonogenic ability of three human OSCC cell lines CAL27, SCC25 and UM1 were detected after clotrimazole treatment by CCK8 assay and colony formation assay. Cell cycle progression and apoptosis were assessed by flow cytometry, and the involvement of several mediators of apoptosis was examined by western blot analysis. Then, the in vivo antitumor effect of clotrimazole was investigated in CAL27 xenograft model. Immunohistochemistry and western blot analysis were performed to determine the presence of apoptotic cells and the expression of Bcl-2 and Bax in tumors from mice treated with or without clotrimazole.

Results

Clotrimazole inhibited proliferation in all three OSCC cell lines in a dose-and time-dependent manner, and significantly reduced the colony formation of OSCC cells in vitro. Clotrimazole caused cell cycle arrest at the G₀/G₁ phase. In addition, clotrimazole induced apoptosis in OSCC cells, and significantly down-regulated the anti-apoptotic protein Bcl-2 and up-regulated the pro-apoptotic protein Bax. Notably, clotrimazole treatment inhibited OSCC tumor growth and cell proliferation in CAL27 xenograft model. Clotrimazole also markedly reduced Bcl-2 expression and increased the protein level of Bax in tumor tissues of xenograft model.

Conclusion

Our findings demonstrated a potent anticancer effect of clotrimazole by inducing cell cycle arrest and cellular apoptosis in OSCC.

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.

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.

Low nanomolar thapsigargin inhibits the replication of vascular smooth muscle cells through reversible endoplasmic reticular stress

Thapsigargin (TG), an inhibitor of Ca(2+) ATPase pumps in the endoplasmic reticulum (ER), inhibits replication of human vascular smooth muscle cell (hVSMC) at low nM concentrations. TG blocks replication of other cell types through promotion of ER stress (ERS). In order to determine whether ERS may mediate the cytostatic effect of TG in hVSMCs, the effect of TG on ERS in hVSMCs was studied by assessing markers of ERS: Immunoglobulin Heavy Chain Binding Protein (BiP), growth inhibitory transcription factor, GADD153, phosphorlylated eukaryotic initiation factor 2α (p-eIF2α) and phosphorlylated protein kinase R (p-PKR). hVSMCs derived from saphenous veins were rendered quiescent with serum-free medium for 96 h incubated with 10 nM TG at 37 °C for 24 h, then washed free of TG and incubated with 10% foetal calf serum (FCS) for a further 24 h. At selected times, BiP, GADD153, p-eIF2α, p-PKR and cyclin D1 expression was assessed. TG promoted a marked increase in BiP and GADD153, but suppressed cyclin D1 mRNA and protein expression. Under serum-free conditions p-eIF2α and p-PKR expression was not enhanced by TG. 15-24 h After removal of TG all these factors returned to levels seen in control cells. These data demonstrate that the inhibitory effect of 10nM TG on hVSMC replication is mediated through induction of ERS and associated factors that cessate replication and is reversible. These observations have implications in the aetiology and treatment of diseases that include atherogenesis, vein graft failure and restenosis.

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.

Androgen signaling promotes translation of TMEFF2 in prostate cancer cells via phosphorylation of the α subunit of the translation initiation factor 2

The type I transmembrane protein with epidermal growth factor and two follistatin motifs 2 (TMEFF2), is expressed mainly in brain and prostate. Expression of TMEFF2 is deregulated in prostate cancer, suggesting a role in this disease, but the molecular mechanism(s) involved in this effect are not clear. Although androgens promote tmeff2 transcription, androgen delivery to castrated animals carrying CWR22 xenografts increases TMEFF2 protein levels in the absence of mRNA changes, suggesting that TMEFF2 may also be post-transcriptionally regulated. Here we show that translation of TMEFF2 is regulated by androgens. Addition of physiological concentrations of dihydrotestosterone (DHT) to prostate cancer cell lines increases translation of endogenous TMEFF2 or transfected TMEFF2-Luciferase fusions, and this effect requires the presence of upstream open reading frames (uORFs) in the 5′-untranslated region (5′-UTR) of TMEFF2. Using chemical and siRNA inhibition of the androgen receptor (AR), we show that the androgen effect on TMEFF2 translation is mediated by the AR. Importantly, DHT also promotes phosphorylation of the α subunit of the translation initiation factor 2 (eIF2α) in an AR-dependent manner, paralleling the effect on TMEFF2 translation. Moreover, endoplasmic reticulum (ER) stress conditions, which promote eIF2α phosphorylation, also stimulate TMEFF2 translation. These results indicate that androgen signaling promotes eIF2α phosphorylation and subsequent translation of TMEFF2 via a mechanism that requires uORFs in the 5′-UTR of TMEFF2.

Modulation of the oxidative stress in malaria infection by clotrimazole

Antimycotic clotrimazole (CTZ) has demonstrated remarkable activity against Plasmodium falciparum in vitro and in vivo. Hemoglobin degradation by Plasmodium parasites makes amino acids available for protein synthesis, inducing oxidative stress in infected cells and producing free heme. These events represent biochemical targets for potential antimalarials. In this study, we have tested the ability of CTZ to modify the oxidative status in Plasmodium berghei-infected erythrocytes. After hemolysis, activities of superoxide dismutase (SOD), catalase (CAT), glutathione cycle and NADPH+H+-producing dehydrogenases were investigated using UV-visible spectrophotometry. Thiobarbituric acid reactive substances (TBARS) were evaluated as a marker of lipid damage. Results showed that CTZ significantly decreased the overall activity of 6-phosphagluconate dehydrogenase (6PGD) compared to infected and non-treated cells; consequently, the glutathione cycle was inhibited, leaving the parasite vulnerable to the oxidative stress originating from hemoglobin degradation. As a compensatory response, CTZ prevented some loss of SOD and CAT activities in infected cells. The infection triggered lipid peroxidation in erythrocytes, which was decreased by CTZ. These results suggest the presence of a redox unbalance in cells treated with CTZ, discussing a possible effect of this compound disturbing the oxidative status in a Plasmodium berghei-infection.

The effect of clotrimazole on energy substrate uptake and carcinogenesis in intestinal epithelial cells

Clotrimazole has anticarcinogenic activity in several cell types. Our aims were to investigate the anticarcinogenic effect of clotrimazole in a tumoral intestinal epithelial (Caco-2) cell line, to compare it with the effect in a nontumoral intestinal epithelial cell line (IEC-6 cells), and to investigate inhibition of energy substrate uptake as a mechanism contributing to it. The effect of clotrimazole on cell proliferation, viability and differentiation, H-deoxyglucose (H-DG), H-O-methyl-glucose (H-OMG), and C-butyrate uptake, as well as mRNA expression levels of glucose transporters was assessed. In Caco-2 cells, clotrimazole decreased cellular viability and proliferation and increased cell differentiation. The effect on cell proliferation and viability was potentiated by rhodamine123. Clotrimazole also decreased cellular viability and proliferation in IEC-6 cells, but increased the cellular DNA synthesis rate and had no effect on cell differentiation. Exposure of Caco-2 cells to clotrimazole (10 µmol/l) for 1 and 7 days increased (by 20-30%) the uptake of H-DG and H-OMG, respectively, but had no effect on C-butyrate uptake. The effect on H-DG and H-OMG transport was maximal at 10 µmol/l, and the pharmacological characteristics of transport were not changed. However, clotrimazole changed the mRNA expression levels of the facilitative glucose transporter 2 and the Na-dependent glucose cotransporter. Clotrimazole exhibits comparable cytotoxic effects in tumoral and nontumoral intestinal epithelial cell lines. In Caco-2 cells, the cytotoxic effect of clotrimazole was strongly potentiated by the inhibition of oxidative phosphorylation. Moreover, stimulation of glucose uptake might be a compensation mechanism in response to the glycolysis inhibition caused by clotrimazole.

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.

Disturbance in hemoglobin metabolism and in vivo antimalarial activity of azole antimycotics

Plasmodium parasites degrade host hemoglobin to obtain free amino acids, essential for protein synthesis. During this event, free toxic heme moieties crystallize spontaneously to produce a non-toxic pigment called hemozoin or ß-hematin. In this context, a group of azole antimycotics, clotrimazole (CTZ), ketoconazole (KTZ) and fluconazole (FCZ), were investigated for their abilities to inhibit ß-hematin synthesis (IßHS) and hemoglobin proteolysis (IHbP) in vitro. The ß-hematin synthesis was recorded by spectrophotometry at 405 nm and the hemoglobin proteolysis was determined by SDS-PAGE 12.5%, followed by densitometric analysis. Compounds were also assayed in vivo in a malaria murine model. CTZ and KTZ exhibited the maximal effects inhibiting both biochemical events, showing inhibition of β-hematin synthesis (IC50 values of 12.4 ± 0.9 µM and 14.4 ± 1.4 µM respectively) and inhibition of hemoglobin proteolysis (80.1 ± 2.0% and 55.3 ± 3.6%, respectively). There is a broad correlation to the in vivo results, especially CTZ, which reduced the parasitemia (%P) of infected-mice at 4th day post-infection significantly compared to non-treated controls (12.4 ± 3.0% compared to 26.6 ± 3.7%, p = 0.014) and prolonged the survival days post-infection. The results indicated that the inhibition of the hemoglobin metabolism by the azole antimycotics could be responsible for their antimalarial effect.

Clotrimazole induces a late G1 cell cycle arrest and sensitizes glioblastoma cells to radiation in vitro

Tumor cells are characterized by their high rate of glycolysis and clotrimazole has been shown to disrupt the glycolysis pathway thereby arresting the cells in the G1 cell cycle phase. Herein, we present data to support our hypothesis that clotrimazole arrests tumor cells in a radiosensitizing, late G1 phase. The effects of clotrimazole were studied using the glioblastoma cell line, U-87 MG. Flow cytometry was used to analyze cell cycle redistribution and induction of apoptosis. Immunoblots were probed to characterize a late G1 cell cycle arrest. Nuclear and cytoplasmic fractions were collected to follow the clotrimazole-induced translocation of hexokinase II. Clonogenic assays were designed to determine the radiosensitizing effect by clotrimazole. Our studies have shown a dose-dependent and time-dependent clotrimazole arrest in a late G1 cell cycle phase. Concurrent with the late G1 arrest, we observed an overexpression of p27 along with a decreased expression of p21, cyclin-dependent kinase 1, cyclin-dependent kinase 4, and cyclin D. Clotrimazole induced the translocation of mitochondrial-bound hexokinase II to the cytoplasm and the release of cytochrome c into the cytoplasm. Clotrimazole-induced apoptosis was enhanced when combined with radiation. Clotrimazole was shown to sensitize tumor cells to radiation when the cells were irradiated for 18 h post-clotrimazole treatment. The disruption of the glycolysis pathway by clotrimazole leads to cell cycle arrest of U-87 MG cells in the radiosensitizing late G1 phase. The use of clotrimazole as a radiosensitizing agent for cancer treatment is novel and may have broad therapeutic applications.

The antidepressant sertraline inhibits translation initiation by curtailing mammalian target of rapamycin signaling

Sertraline, a selective serotonin reuptake inhibitor, is a widely used antidepressant agent. Here, we show that sertraline also exhibits antiproliferative activity. Exposure to sertraline leads to a concentration-dependent decrease in protein synthesis. Moreover, polysome profile analysis of sertraline-treated cells shows a reduction in polysome content and a concomitant increase in 80S ribosomes. The inhibition in translation caused by sertraline is associated with decreased levels of the eukaryotic initiation factor (eIF) 4F complex, altered localization of eIF4E, and increased eIF2alpha phosphorylation. The latter event leads to increased REDD1 expression, which in turn impinges on the mammalian target of rapamycin (mTOR) pathway by affecting TSC1/2 signaling. Sertraline also independently targets the mTOR signaling pathway downstream of Rheb. In the Emu-myc murine lymphoma model where carcinogenesis is driven by phosphatase and tensin homologue (PTEN) inactivation, sertraline is able to enhance chemosensitivity to doxorubicin. Our results indicate that sertraline exerts antiproliferative activity by targeting the mTOR signaling pathway in a REDD1-dependent manner.

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

The cyclin-dependent kinase inhibitor p27(Kip1) plays a critical role in regulating entry into and exit from the cell cycle. Post-transcriptional regulation of p27(Kip1) expression is of significant interest. The embryonic lethal abnormal vision (ELAV)-like RNA-binding protein HuR is thought be important for the translation of p27(Kip1), however, different reports attributed diametrically opposite roles to HuR. We report here an alternative mechanism wherein HuR regulates stability of the p27(Kip1) mRNA. Specifically, human and mouse p27(Kip1) 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 p27(Kip1) mRNA and play a critical role in its accumulation. Deleting HuR binding sites or knocking down HuR expression destabilizes p27(Kip1) mRNA and reduces its accumulation. We also identified a CT repeat in the 5' UTR of full-length p27(Kip1) mRNA isoforms that interact with a approximately 41-kDa protein and represses p27(Kip1) expression. This CT-rich element and diffuse elements in the 3' UTR regulate post-transcriptional expression of p27(Kip1) 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 p27(Kip1) expression.

Repression of PKR mediates palmitate-induced apoptosis in HepG2 cells through regulation of Bcl-2

In the present study we found that double-stranded RNA-dependent protein kinase (PKR) regulates the protein expression level and the phosphorylation of Bcl-2 and exploits an anti-apoptotic role in human hepatocellular carcinoma cells (HepG2). Saturated free fatty acids (FFAs), e.g. palmitate, have been shown to induce cellular apoptosis in various types of cells by different mechanisms. We found palmitate down-regulates the activity of PKR, and thereby decreases the protein level of Bcl-2, mediated, in part, by the NF-κB transcription factor. In addition to the protein level of Bcl-2, the phosphorylation of Bcl-2 at different amino acid residues, such as Ser70 and Ser87, is also important in regulating cellular apoptosis. The decrease in the phosphorylation of Bcl-2 at Ser70 upon exposure to palmitate is mediated by PKR and possibly JNK, while the phosphorylation of Bcl-2 at Ser87 is not affected by palmitate or PKR. In summary, PKR mediates the regulation of the protein level and the phosphorylation status of Bcl-2, providing a novel mechanism of palmitate-induced apoptosis in HepG2 cells.

Clotrimazole enhances lysis of human erythrocytes induced by t-BHP

Clotrimazole (CLT) is an antifungal and antimalarial agent also effective as a Gardos channel inhibitor. In addition, CLT possesses antitumor properties. Recent data provide evidence that CLT forms a complex with heme (hemin), which produces a more potent lytic effect than heme alone. This study addressed the effect of CLT on the lysis of normal human erythrocytes induced by tert-butyl hydroperoxide (t-BHP). For the first time, it was shown that 10 microM CLT significantly enhanced the lytic effect of t-BHP on erythrocytes in both Ca(2+)-containing and Ca(2+)-free media, suggesting that the effect is not related to Gardos channels. CLT did not affect the rate of free radical generation, the kinetics of GSH degradation, methemoglobin formation and TBARS generation; therefore, we concluded that CLT does not cause additional oxidative damage to erythrocytes treated with t-BHP. It is tempted to speculate that CLT enhances t-BHP-induced changes in erythrocyte volume and lysis largely by forming a complex with hemin released during hemoglobin oxidation in erythrocytes: the CLT-hemin complex destabilizes the cell membrane more potently than hemin alone. If so, the effect of CLT on cell membrane damage during free-radical oxidation may be used to increase the efficacy of antitumor therapy.

Cyclic ADP-ribose links metabolism to multiple fission in the dinoflagellate Crypthecodinium cohnii

Cellular metabolism is required for cell proliferation. However, the way in which metabolic signals are conveyed to cell cycle decisions is unclear. Cyclic ADP-ribose (cADPR), the NAD(+) metabolite, mobilizes calcium from calcium stores in many cells. We found that dinoflagellate cells with higher metabolic rate underwent multiple fission (MF), a division mode in which cells can exceed twice their sizes at G1. A temperature shift-down experiment suggested that MF involves a commitment point at late G1. In fast-growing cells, cADPR level peaked in G(1) and increased with increasing concentrations of glucose in the medium. Addition of glycolytic poison iodoacetate inhibited cell growth, reduced cADPR levels as well as the commitment of cell cycles in fast-growing cells. Commitment of MF cell cycles was induced by a cell permeant cADPR agonist, but blocked by a specific antagonist of cADPR-induced Ca(2+) release. Our results establish cADPR as a link between cellular metabolism and cell cycle control.

Proteomic and transcriptomic study on the action of a cytotoxic saponin (Polyphyllin D): induction of endoplasmic reticulum stress and mitochondria-mediated apoptotic pathways

Polyphyllin D (PD) is a potent cytotoxic saponin found in Paris polyphylla. In the present study, bioinformatic, proteomic and transcriptomic analyses were performed to study the mechanisms of action of PD on human nonsmall cell lung cancer (NSCLC) cell line (NCI-H460). Using a gene expression-based bioinformatic tool (connectivity map), PD was identified as a potential ER stress inducer. Our proteomic and transcriptomic analyses revealed that PD treatment led to upregulation of typical ER stress-related proteins/genes including glucose-regulated protein 78 (BiP/GRP78) and protein disulfide isomerase (PDI). In particular, elevated expression of C/EBP homologous transcription factor (chop) and activation of caspase-4 occurred at early time point (8 h) of PD treatment, signifying an initial ER stress-mediated apoptosis. Induction of tumor suppressor p53, disruption of mitochondrial membrane, activation of caspase-9 and caspase-3 were detected upon prolonged PD treatment. Collectively, these data revealed that PD induced the cytotoxic effect through a mechanism initiated by ER stress followed by mitochondrial apoptotic pathway. The ability of activating two major pathways of apoptosis makes PD an attractive drug lead for anticancer therapeutics.

11-Phenyl-[b,e]-dibenzazepine compounds: novel antitumor agents

A series of 11-phenyl-[b,e]-dibenzazepine compounds were synthesized and shown to be inhibitors of tumor cell proliferation with IC(50) values ranging from submicromolar to micromolar concentrations. Flow cytometric analyses of several active compounds demonstrated inhibition of cell cycle progression at the G(0)-G(1) phase transition resulting in G(0)-G(1) arrest.

17alpha-Hydroxylase/17,20 lyase inhibitor VN/124-1 inhibits growth of androgen-independent prostate cancer cells via induction of the endoplasmic reticulum stress response

Inhibitors of the enzyme 17alpha-hydroxylase/17,20 lyase are a new class of anti-prostate cancer agents currently undergoing preclinical and clinical development. We have previously reported the superior anticancer activity of our novel 17alpha-hydroxylase/17,20 lyase inhibitor, VN/124-1, against androgen-dependent cancer models. Here, we examined the effect of VN/124-1 on the growth of the androgen-independent cell lines PC-3 and DU-145 and found that the compound inhibits their growth in a dose-dependent manner in vitro (GI50, 7.82 micromol/L and 7.55 micromol/L, respectively). We explored the mechanism of action of VN/124-1 in PC-3 cells through microarray analysis and found that VN/124-1 up-regulated genes involved in stress response and protein metabolism, as well as down-regulated genes involved in cell cycle progression. Follow-up real-time PCR and Western blot analyses revealed that VN/124-1 induces the endoplasmic reticulum stress response resulting in down-regulation of cyclin D1 protein expression and cyclin E2 mRNA. Cell cycle analysis confirmed G1-G0 phase arrest. Measurements of intracellular calcium levels ([Ca2+]i) showed that 20 micromol/L VN/124-1 caused a release of Ca2+ from endoplasmic reticulum stores resulting in a sustained increase in [Ca2+]i. Finally, cotreatment of PC-3 cells with 5, 10, and 20 micromol/L VN/124-1 with 10 nmol/L thapsigargin revealed a synergistic relationship between the compounds in inhibiting PC-3 cell growth. Taken together, these findings show VN/124-1 is endowed with multiple anticancer properties that may contribute to its utility as a prostate cancer therapeutic.

Effect of clotrimazole on the pump cycle of the Na,K-ATPase

The effect of the antimycotic drug clotrimazole (CLT) on the Na,K-ATPase was investigated using fluorescence and electrical measurements. The results obtained by steady-state fluorescence experiments with the electrochromic styryl dye RH421 were combined with those achieved by a pre-steady-state method based on fast solution exchange on a solid supported membrane that adsorbs the protein. Both techniques are suitable for monitoring the electrogenic steps of the pump cycle and are in general complementary, yielding distinct kinetic information. The experiments show clearly that CLT affects specific partial reactions of the pump cycle of the Na,K-ATPase with an affinity in the low micromolar range and in a reversible manner. All results can be consistently explained by proposing the CLT-promoted formation of an ion-occluded-CLT-bound conformational E(2) state, E(2)(CLT)(X(2)) that acts as a "dead-end" side track of the pump cycle, where X stands for H+ or K+. Na+ binding, enzyme phosphorylation, and Na+ transport were not affected by CLT, and at high CLT concentrations approximately (1/3) of the enzyme remained active in the physiological transport mode. The presence of Na+ and K+ destabilized the inactivated form of the Na,K-ATPase.

Involvement of ERKs, RSK2 and PKR in UVA-induced signal transduction toward phosphorylation of eIF2alpha (Ser(51))

Double-stranded RNA-dependent protein kinase R (PKR) has been implicated in anti-viral (antitumor) and apoptotic responses. PKR is activated by extracellular stresses and phosphorylates the alpha subunit of protein synthesis initiation factor eIF2, thereby inhibiting protein synthesis and impeding virus multiplication. Phosphorylation of eIF2alpha in mammalian cells has been shown to be increased after ultraviolet (UV) stress and to be required for UV-induced repression of protein translation. UVA is an important etiological factor in skin carcinogenesis and we observed that UVA induced phosphorylation of PKR (Thr(451)) and eIF2alpha (Ser(51)) in mouse skin epidermal JB6 Cl41 cells. The induction was suppressed by the MEK1 inhibitor, PD 98059. UVA stimulation of PKR and eIF2alpha phosphorylation was also inhibited by a dominant-negative mutant (DNM) of ERK2- or RSK2-deficient cells (RSK2(-)). An inhibitor of p38, SB 202190 or a DNM of p38alpha kinase (DNM-p38alpha) suppressed UVA-induced phosphorylation of eIF2alpha (Ser(51)) but had no effect on phosphorylation of PKR (Thr(451)). Our data indicated that phosphorylation of PKR at Thr(451) is mediated through ERK2 and RSK2, but not through p38 kinase, and is involved in the regulation of Ser(51) phosphorylation of eIF2alpha in UVA-irradiated JB6 cells. In vitro and in vivo kinase assays indicated that phosphorylation of eIF2alpha at Ser(51) occurred indirectly through ERK2, RSK2 or p38 kinase in the cellular response to UVA. These data may lead to the use of these signaling molecules as targets to develop more effective chemopreventive agents with fewer side effects to control UV-induced skin cancer.

Clotrimazole inhibits and modulates heterologous association of the key glycolytic enzyme 6-phosphofructo-1-kinase

Clotrimazole is an antifungal azole derivative recently recognized as a calmodulin antagonist with promising anticancer effects. This property has been correlated with the ability of the drug to decrease the viability of tumor cells by inhibiting their glycolytic flux and consequently decreasing the intracellular concentration of ATP. The effects of clotrimazole on cell glycolysis and ATP production are considered to be due to the detachment of the glycolytic enzymes from the cytoskeleton. Here, we show that clotrimazole directly inhibits the key glycolytic enzyme 6-phosphofructo-1-kinase (PFK). This property is independent of the anti-calmodulin activity of the drug, since it is not mimicked by the classical calmodulin antagonist compound 48/80. However, the clotrimazole-inhibited enzyme can be activated by calmodulin, even though calmodulin has no effect on PFK activity in the absence of the drug. Clotrimazole alone induces the dimerization of PFK reducing the population of tetramers, which is not observed when calmodulin is also present. Since PFK dimers are less active than PFK tetramers, this can explain the inhibitory effect of clotrimazole on the enzyme. Additionally, clotrimazole positively modulates the association of PFK with erythrocyte membranes. Altogether, our data support a hitherto unrecognized action of clotrimazole as a negative modulator of glycolytic flux through direct inhibition of the key enzyme PFK.

Identifying small molecule inhibitors of eukaryotic translation initiation

In eukaryotes, translation initiation is rate-limiting with much regulation exerted at the ribosome recruitment and ternary complex (eIF2.GTP.Met-tRNA(i)(Met)) formation steps. Although small molecule inhibitors have been extremely useful for chemically dissecting translation, there is a dearth of compounds available to study the initiation phase in vitro and in vivo. In this chapter, we describe reverse and forward chemical genetic screens developed to identify new inhibitors of translation. The ability to manipulate cell extracts biochemically, and to compare the activity of small molecules on translation of mRNA templates that differ in their factor requirements for ribosome recruitment, facilitates identification of the relevant target.

Brasilicardin A, a Natural Immunosuppressant, Targets Amino Acid Transport System L

Lymphocytes in T cell activation require extracellular nutrients to provide energy for cellular proliferation and effector functions. Therefore, inhibitors of nutrient transporters are expected to be a new class of immunosuppressant. Here, we report that the molecular target of brasilicardin A (BraA), an immunosuppressive compound, is the amino acid transporter system L. BraA inhibited the cell-cycle progression of murine T cell lymphocyte CTLL-2 cells in G1 phase, and potently inhibited the uptake of amino acids that are substrates for amino acid transport system L. Moreover, BraA stimulated the GCN2 activation and, subsequently, the phosphorylation of eIF2alpha. These results suggest that the immunosuppressive activity of BraA is induced by amino acid deprivation via the inhibition of system L and that the amino acid transporter is a target for immunosuppressant.

Functional KCa3.1 K+ channels are required for human lung mast cell migration

BACKGROUND

Mast cell recruitment and activation are critical for the initiation and progression of inflammation and fibrosis. Mast cells infiltrate specific structures in many diseased tissues such as the airway smooth muscle (ASM) in asthma. This microlocalisation of mast cells is likely to be key to disease pathogenesis. Human lung mast cells (HLMC) express the Ca2+ activated K+ channel K(Ca)3.1 which modulates mediator release, and is proposed to facilitate the retraction of the cell body during migration of several cell types. A study was undertaken to test the hypothesis that blockade of K(Ca)3.1 would attenuate HLMC proliferation and migration.

METHODS

HLMC were isolated and purified from lung material resected for bronchial carcinoma. HLMC proliferation was assessed by cell counts at various time points following drug exposure. HLMC chemotaxis was assayed using standard Transwell chambers (8 microm pore size). Ion currents were measured using the single cell patch clamp technique.

RESULTS

K(Ca)3.1 blockade with triarylmethane-34 (TRAM-34) did not inhibit HLMC proliferation and clotrimazole had cytotoxic effects. In contrast, HLMC migration towards the chemokine CXCL10, the chemoattractant stem cell factor, and the supernatants from tumour necrosis factor alpha stimulated asthmatic ASM was markedly inhibited with both the non-selective K(Ca)3.1 blocker charybdotoxin and the highly specific K(Ca)3.1 blocker TRAM-34 in a dose dependent manner. Although K(Ca)3.1 blockade inhibits HLMC migration, K(Ca)3.1 is not opened by the chemotactic stimulus, suggesting that it must be involved downstream of the initial receptor-ligand interactions.

CONCLUSIONS

Since modulation of K(Ca)3.1 can inhibit HLMC chemotaxis to diverse chemoattractants, the use of K(Ca)3.1 blockers such as TRAM-34 could provide new therapeutic strategies for mast cell mediated diseases such as asthma.

Role of simvastatin and methyl-beta-cyclodextrin [corrected] on inhibition of poliovirus infection

Cells exposed to simvastatin or to methyl-beta-cyclodextrin show reduced poliovirus infection, without alteration in virus binding or on the kinetics of genome entry, suggesting that the steps which are altered are those post uncoating and genome entry. Reduction of infection by cyclodextrin is reversed by increasing MOI whereas that produced by simvastatin treatment is not, suggesting that the effects on infection are not due to a reduction in cholesterol. The differences in the characteristics of inhibition can be explained by the differential effects of the compounds. Cyclodextrin inhibits the store-operated calcium channels, suggesting that reduction in infection is through translational inhibition. Simvastatin produces vesicles from internal membranes which cannot sustain viral RNA synthesis, reducing infection through reduced transcription. The results indicate that the impact on viral infection by the cholesterol-modifying agents is due to the cellular changes produced rather than due to disruption of the cholesterol-rich domains.

Production of infectious human cytomegalovirus virions is inhibited by drugs that disrupt calcium homeostasis in the endoplasmic reticulum

We previously reported that human cytomegalovirus (HCMV) infection induces endoplasmic reticulum (ER) stress, resulting in activation of the unfolded protein response (UPR). Although some normal consequences of UPR activation (e.g., translation attenuation) are detrimental to viral infection, we have previously shown that HCMV infection adapts the UPR to benefit the viral infection (14). For example, UPR-induced translation attenuation is inhibited by viral infection, while potentially beneficial aspects of the UPR are maintained. In the present work, we tested the ability of HCMV to overcome a robust induction of the UPR by the drugs thapsigargin and clotrimazole (CLT), which disrupt ER calcium homeostasis. A 24-h treatment with these drugs beginning at 48, 72, or 96 h postinfection (hpi) completely inhibited further production of infectious virions. HCMV could not overcome the inhibition of global translation by CLT; however, between 48 and 72 hpi, HCMV overcame translational inhibition caused by thapsigargin. Despite the restoration of translation in thapsigargin, the accumulation of immediate-early and early gene products was modestly retarded (50% or less), whereas the accumulation of an early-late and late gene product was significantly retarded. Electron microscopic analysis shows that the drugs severely disrupt the maturation of HCMV virions. This can be accounted for by both the retarded accumulation of late gene products and the drug-induced depletion of ER calcium, which disrupts critical cellular functions needed for maturation.

Recent advances in Ca2+-dependent Ras regulation and cell proliferation

Our understanding of the mechanisms whereby growth factors stimulate cell proliferation through the Ras pathway stems largely from studies of the canonical pathway involving recruitment of Ras activators and inhibitors to the vicinity of receptor tyrosine kinases via phosphotyrosine-binding adaptor proteins. Ca(2+) has seldom joined the party, despite the identification of phospholipase Cgamma and Ca(2+) entry as receptor tyrosine kinase-dependent signals. Mechanisms by which Ca(2+) can directly influence Ras activity have remained relatively elusive. Similarly, the mechanisms whereby Ca(2+) modulates the cell cycle have been equally murky, and yet there are some interesting parallels in the role of Ras and Ca(2+) in cell cycle re-entry. This review focuses on a number of novel mechanisms that link Ca(2+) with the regulation of Ras activity and signaling output. Their collective discovery adds to the complexities of Ras regulation and raises further questions about the role of Ca(2+) signals in Ras-dependent cell proliferation.

Clotrimazole Inhibits Hemoperoxidase of Plasmodium falciparum and Induces Oxidative Stress

The mechanism of antimalarial activity of clotrimazole was studied placing emphasis on its role in inhibiting hemoperoxidase for inducing oxidative stress in Plasmodium falciparum. Clotrimazole, in the presence of H2O2, causes irreversible inactivation of the enzyme, and the inactivation follows pseudo-first order kinetics, consistent with a mechanism-based (suicide) mode. The pseudo-first order kinetic constants are ki = 2.85 microM, k(inact) = 0.9 min(-1), and t(1/2) = 0.77 min. The one-electron oxidation product of clotrimazole has been identified by EPR spectroscopy as the 5,5'-dimethyl-1-pyrroline N-oxide (DMPO) adduct of the nitrogen-centered radical (aN = 15 G), and as DMPO protects against inactivation, this radical is involved in the inactivation process. Binding studies indicate that the clotrimazole oxidation product interacts at the heme moiety, and the heme-clotrimazole adduct has been dissociated from the inactivated enzyme and identified (m/z 1363) by mass analysis. We found that the inhibition of hemoperoxidase increases the accumulation of H2O2 in P. falciparum and causes oxidative stress. Furthermore, the inhibition of hemoperoxidase correlates well with the inhibition of parasite growth. The results described herein indicate that the antimalarial activity of clotrimazole might be due to the inhibition of hemoperoxidase and subsequent development of oxidative stress in P. falciparum.