Translation inhibitors induce cell death by multiple mechanisms and Mcl-1 reduction is only a minor contributor

Homoharringtonine in the treatment of acute myeloid leukemia: A review

Acute myeloid leukemia (AML) is a hematological malignancy characterized by the accumulation of immature myeloid precursor cells. Over half of AML patients fail to achieve long-term disease-free survival under existing therapy, and the overall prognosis is poor, necessitating the urgent development of novel therapeutic approaches. The plant alkaloid homoharringtonine (HHT), which has anticancer properties, was first identified more than 40 years ago. It works in a novel method of action that prevents the early elongation phase of protein synthesis. HHT has been widely utilized in the treatment of AML, with strong therapeutic effects, few toxic side effects, and the ability to enhance AML patients' prognoses. In AML, HHT can induce cell apoptosis through multiple pathways, exerting synergistic antitumor effects, according to clinical and pharmacological research. About its modes of action, some findings have been made recently. This paper reviews the development of research on the mechanisms of HHT in treating AML to offer insights for further research and clinical therapy.

Harnessing high-throughput OMICS in emerging zoonotic virus preparedness and response activities

Emerging and re-emerging viruses pose unpredictable and significant challenges to global health. Emerging zoonotic infectious diseases, which are transmitted between humans and non-human animals, have been estimated to be responsible for nearly two-thirds of emerging infectious disease events and emergence events attributed to these pathogens have been increasing in frequency with the potential for high global health and economic burdens. In this review we will focus on the application of highthroughput OMICS approaches to emerging zoonotic virus investigtations. We highlight the key contributions of transcriptome and proteome investigations to emerging zoonotic virus preparedness and response activities with a focus on SARS-CoV-2, avian influenza virus subtype H5N1, and Orthoebolavirus investigations.

Combined HDAC and eIF4A inhibition: A novel epigenetic therapy for pancreatic adenocarcinoma

Pancreatic ductal adenocarcinoma-(PDAC) needs innovative approaches due to its 12% 5-year survival despite current therapies. We show marked sensitivity of pancreatic cancer cells to the combination of a novel eIF4A inhibitor, des-methyl pateamine A (DMPatA), and a histone deacetylase inhibitor, romidepsin, inducing epigenetic reprogramming as an innovative therapeutic strategy. Exploring the mechanistic activity of this combination showed that with a short duration of romidepsin at low doses, robust acetylation persisted up to 48h with the combination, while histone acetylation rapidly faded with monotherapy. This represents an unexpected mechanism of action against PDAC cells that triggers transcriptional overload, metabolic stress, and augmented DNA damage. Structurally different class I HDAC inhibitors exhibit the same hyperacetylation patterns when co-administered with DMPatA, suggesting a class effect. We show efficacy of this combination regimen against tumor growth in a MIA PaCa-2 xenograft model of PDAC with persistent hyperacetylation confirmed in tumor samples.

STATEMENT OF SIGNIFICANCE

Pancreatic ductal adenocarcinoma, a significant clinical challenge, could benefit from the latent potential of epigenetic therapies like HDAC inhibitors-(HDIs), typically limited to hematological malignancies. Our study shows that a synergistic low dose combination of HDIs with an eIF4A-inhibitor in pancreatic cancer models results in marked pre-clinical efficacy, offering a promising new treatment strategy.

Homoharringtonine promotes heart allograft acceptance by enhancing regulatory T cells induction in a mouse model

BACKGROUND

Homoharringtonine (HHT) is an effective anti-inflammatory, anti-viral, and anti-tumor protein synthesis inhibitor that has been applied clinically. Here, we explored the therapeutic effects of HHT in a mouse heart transplant model.

METHODS

Healthy C57BL/6 mice were used to observe the toxicity of HHT in the liver, kidney, and hematology. A mouse heart transplantation model was constructed, and the potential mechanism of HHT prolonging allograft survival was evaluated using Kaplan-Meier analysis, immunostaining, and bulk RNA sequencing analysis. The HHT-T cell crosstalk was modeled ex vivo to further verify the molecular mechanism of HHT-induced regulatory T cells (Tregs) differentiation.

RESULTS

HHT inhibited the activation and proliferation of T cells and promoted their apoptosis ex vivo . Treatment of 0.5 mg/kg HHT for 10 days significantly prolonged the mean graft survival time of the allografts from 7 days to 48 days ( P <0.001) without non-immune toxicity. The allografts had long-term survival after continuous HHT treatment for 28 days. HHT significantly reduced lymphocyte infiltration in the graft, and interferon-γ-secreting CD4 + and CD8 + T cells in the spleen ( P <0.01). HHT significantly increased the number of peripheral Tregs (about 20%, P <0.001) and serum interleukin (IL)-10 levels. HHT downregulated the expression of T cell receptor (TCR) signaling pathway-related genes ( CD4 , H2-Eb1 , TRAT1 , and CD74 ) and upregulated the expression of IL-10 and transforming growth factor (TGF)-β pathway-related genes and Treg signature genes ( CTLA4 , Foxp3 , CD74 , and ICOS ). HHT increased CD4 + Foxp3 + cells and Foxp3 expression ex vivo , and it enhanced the inhibitory function of inducible Tregs.

CONCLUSIONS

HHT promotes Treg cell differentiation and enhances Treg suppressive function by attenuating the TCR signaling pathway and upregulating the expression of Treg signature genes and IL-10 levels, thereby promoting mouse heart allograft acceptance. These findings may have therapeutic implications for organ transplant recipients, particularly those with viral infections and malignancies, which require a more suitable anti-rejection medication.

Nuclear release of eIF1 globally increases stringency of start-codon selection to preserve mitotic arrest physiology

Regulated start-codon selection has the potential to reshape the proteome through the differential production of uORFs, canonical proteins, and alternative translational isoforms. However, conditions under which start-codon selection is altered remain poorly defined. Here, using transcriptome-wide translation initiation site profiling, we reveal a global increase in the stringency of start-codon selection during mammalian mitosis. Low-efficiency initiation sites are preferentially repressed in mitosis, resulting in pervasive changes in the translation of thousands of start sites and their corresponding protein products. This increased stringency of start-codon selection during mitosis results from increased interactions between the key regulator of start-codon selection, eIF1, and the 40S ribosome. We find that increased eIF1-40S ribosome interactions during mitosis are mediated by the release of a nuclear pool of eIF1 upon nuclear envelope breakdown. Selectively depleting the nuclear pool of eIF1 eliminates the changes to translational stringency during mitosis, resulting in altered mitotic proteome composition. In addition, preventing mitotic translational rewiring results in substantially increased cell death and decreased mitotic slippage following treatment with anti-mitotic chemotherapeutics. Thus, cells globally control translation initiation stringency with critical roles during the mammalian cell cycle to preserve mitotic cell physiology.

Single Cell Profiling Distinguishes Leukemia-Selective Chemotypes

A central challenge in chemical biology is to distinguish molecular families in which small structural changes trigger large changes in cell biology. Such families might be ideal scaffolds for developing cell-selective chemical effectors - for example, molecules that activate DNA damage responses in malignant cells while sparing healthy cells. Across closely related structural variants, subtle structural changes have the potential to result in contrasting bioactivity patterns across different cell types. Here, we tested a 600-compound Diversity Set of screening molecules from the Boston University Center for Molecular Discovery (BU-CMD) in a novel phospho-flow assay that tracked fundamental cell biological processes, including DNA damage response, apoptosis, M-phase cell cycle, and protein synthesis in MV411 leukemia cells. Among the chemotypes screened, synthetic congeners of the rocaglate family were especially bioactive. In follow-up studies, 37 rocaglates were selected and deeply characterized using 12 million additional cellular measurements across MV411 leukemia cells and healthy peripheral blood mononuclear cells. Of the selected rocaglates, 92% displayed significant bioactivity in human cells, and 65% selectively induced DNA damage responses in leukemia and not healthy human blood cells. Furthermore, the signaling and cell-type selectivity were connected to structural features of rocaglate subfamilies. In particular, three rocaglates from the rocaglate pyrimidinone (RP) structural subclass were the only molecules that activated exceptional DNA damage responses in leukemia cells without activating a detectable DNA damage response in healthy cells. These results indicate that the RP subset should be extensively characterized for anticancer therapeutic potential as it relates to the DNA damage response. This single cell profiling approach advances a chemical biology platform to dissect how systematic variations in chemical structure can profoundly and differentially impact basic functions of healthy and diseased cells.

Homoharringtonine enhances cytarabine-induced apoptosis in acute myeloid leukaemia by regulating the p38 MAPK/H2AX/Mcl-1 axis

Acute myeloid leukaemia (AML) is a fatal haematopoietic malignancy and is treated with the conventional combination of cytarabine (Ara-C) and daunorubicin (Dau). The survival rate of AML patients is lower due to the cardiotoxicity of daunorubicin. Clinically, homoharringtonine (HHT) plus Ara-C has been reported to be equally effective as Dau plus Ara-C in some types of AML patients with less toxic effects. We utilized the clinical use of homoharringtonine in combination with Ara-C to test its combination mechanism. We found that the insensitivity of AML cells to cytarabine-induced apoptosis is associated with increased Mcl-1 stability and p38 inactivation. HHT downregulates Mcl-1, phosphorylates H2AX and induces apoptosis by activating p38 MAPK. Inactivation of p38 through inhibitors and siRNA blocks apoptosis, H2AX phosphorylation and Mcl-1 reduction. HHT enhances Ara-C activation of the p38 MAPK signalling pathway, overcoming Ara-C tolerance to cell apoptosis by regulating the p38/H2AX/Mcl-1 axis. The optimal ratio of HHT to Ara-C for synergistic lethality in AML cells is 1:4 (M/M). HHT synergistically induces apoptosis in combination with Ara-C in vitro and prolongs the survival of xenografts. We provide a new mechanism for AML treatment by regulating the p38 MAPK/H2AX/Mcl-1 axis to improve cytarabine therapy.

Birinapant selectively enhances immunotoxin-mediated killing of cancer cells conditional on the IAP protein levels within target cells

Immunotoxins (ITs) target cancer cells via antibody binding to surface antigens followed by internalization and toxin-mediated inhibition of protein synthesis. The fate of cells responding to IT treatment depends on the amount and stability of specific pro-apoptotic and pro-survival proteins. When treated with a pseudomonas exotoxin-based immunotoxin (HB21PE40), the triple-negative breast cancer (TNBC) cell line MDA-MB-468 displayed a notable resistance to toxin-mediated killing compared to the epidermoid carcinoma cell line, A431, despite succumbing to the same level of protein synthesis inhibition. In a combination screen of ~1912 clinically relevant and mechanistically annotated compounds, we identified several agents that greatly enhanced IT-mediated killing of MDA-MB-468 cells while exhibiting only a modest enhancement for A431 cells. Of interest, two Smac mimetics, birinapant and SM164, exhibited this kind of differential enhancement. To investigate the basis for this, we probed cells for the presence of inhibitor of apoptosis (IAP) proteins and monitored their stability after the addition of immunotoxin. We found that high levels of IAPs inhibited immunotoxin-mediated cell death. Further, TNFα levels were not relevant for the combination's efficacy. In tumor xenograft studies, combinations of immunotoxin and birinapant caused complete regressions in MDA-MB-468tumor-bearing mice but not in mice with A431 tumors. We propose that IAPs constitute a barrier to immunotoxin efficacy which can be overcome with combination treatments that include Smac mimetics.

Synthesis of portimines reveals the basis of their anti-cancer activity

Marine-derived cyclic imine toxins, portimine A and portimine B, have attracted attention because of their chemical structure and notable anti-cancer therapeutic potential1-4. However, access to large quantities of these toxins is currently not feasible, and the molecular mechanism underlying their potent activity remains unknown until now. To address this, a scalable and concise synthesis of portimines is presented, which benefits from the logic used in the two-phase terpenoid synthesis5,6 along with other tactics such as exploiting ring-chain tautomerization and skeletal reorganization to minimize protecting group chemistry through self-protection. Notably, this total synthesis enabled a structural reassignment of portimine B and an in-depth functional evaluation of portimine A, revealing that it induces apoptosis selectively in human cancer cell lines with high potency and is efficacious in vivo in tumour-clearance models. Finally, practical access to the portimines and their analogues simplified the development of photoaffinity analogues, which were used in chemical proteomic experiments to identify a primary target of portimine A as the 60S ribosomal export protein NMD3.

High expression of ABCF1 is an independent predictor of poor prognosis in bladder cancer

ABCF1, a member of the ATP-binding cassette (ABC) transporter family, is involved in the malignant progression of tumors. However, the role of ABCF1 in bladder cancer is poorly understood. In our study, we explored the differential expression of ABCF1 in bladder cancer and normal bladder tissues based on bioinformatic analysis and immunohistochemical results. GSEA was performed to ascertain the potential related signaling pathways of ABCF1. The relationship between ABCF1 expression and bladder cancer progression was analyzed using the GSE13507 dataset. In addition, the differential expression of ABCF1 in the cell lines was verified by quantitative real-time polymerase chain reaction (qRT‒PCR) and Western blotting. ABCF1 was upregulated in bladder cancer, and the high expression of ABCF1 was closely related to sex (P = 0.00056), grade (P = 0.00049), T stage (P = 0.00007), and N stage (P = 0.0076). High expression of ABCF1 was correlated with poor overall survival in bladder cancer patients (P < 0.001). In addition, univariate and multivariate Cox regression analyses showed that high ABCF1 expression was an independent factor for poor prognosis in bladder cancer patients. Therefore, ABCF1 expression is closely related to the progression of bladder cancer and can be used as a potential indicator of poor prognosis and a therapeutic target for bladder cancer.

Targeting the translational machinery in gastrointestinal stromal tumors (GIST): a new therapeutic vulnerability

Although KIT-mutant GISTs can be effectively treated with tyrosine kinase inhibitors (TKIs), many patients develop resistance to imatinib mesylate (IM) as well as the FDA-approved later-line agents sunitinib, regorafenib and ripretinib. Resistance mechanisms mainly involve secondary mutations in the KIT receptor tyrosine kinase gene indicating continued dependency on the KIT signaling pathway. The fact that the type of secondary mutation confers either sensitivity or resistance towards TKIs and the notion that secondary mutations exhibit intra- and intertumoral heterogeneity complicates the optimal choice of treatment in the imatinib-resistant setting. Therefore, new strategies that target KIT independently of its underlying mutations are urgently needed. Homoharringtonine (HHT) is a first-in-class inhibitor of protein biosynthesis and is FDA-approved for the treatment of chronic myeloid leukemia (CML) that is resistant to at least two TKIs. HHT has also shown activity in KIT-mutant mastocytosis models, which are intrinsically resistant to imatinib and most other TKIs. We hypothesized that HHT could be effective in GIST through downregulation of KIT expression and subsequent decrease of KIT activation and downstream signaling. Testing several GIST cell line models, HHT led to a significant reduction in nascent protein synthesis and was highly effective in the nanomolar range in IM-sensitive and IM-resistant GIST cell lines. HHT treatment resulted in a rapid and complete abolishment of KIT expression and activation, while KIT mRNA levels were minimally affected. The response to HHT involved induction of apoptosis as well as cell cycle arrest. The antitumor activity of HHT was confirmed in a GIST xenograft model. Taken together, inhibition of protein biosynthesis is a promising strategy to overcome TKI resistance in GIST.

Inhibition of Protein Synthesis Induced by CHK1 Inhibitors Discriminates Sensitive from Resistant Cancer Cells

The DNA-damage-activated checkpoint protein CHK1 is required to prevent replication or mitosis in the presence of unrepaired DNA damage. Inhibitors of CHK1 (CHK1i) circumvent this checkpoint and enhance cell killing by DNA-damaging drugs. CHK1i also elicit single-agent cytotoxicity in a small subset of cell lines. Resolving the mechanisms underlying the single-agent activity may permit patient stratification and targeted therapy against sensitive tumors. Our recent comparison of three CHK1i demonstrated that they all inhibited protein synthesis only in sensitive cells. LY2606368, the most selective of these CHK1i, was used in the current study. Comparison across a panel of cell lines demonstrated that sensitive cells died upon incubation with LY2606368, whereas resistant cells underwent growth inhibition and/or cytostasis but failed to die. Sensitive cells exhibited inhibition of protein synthesis, elevated DNA damage, impaired DNA repair, and subsequently death. The consequence of CHK1 inhibition involved activation of cyclin A/CDK2 and MUS81, resulting in DNA damage. This damage led to activation of AMPK, dephosphorylation of 4E-BP1, and inhibition of protein synthesis. Inhibition of MUS81 prevented activation of AMPK, while inhibition of AMPK enhanced DNA repair and cell survival. The activation of AMPK may involve a combination of LKB1 and CaMKKβ. This study raises questions concerning the potential importance of the inhibition of protein synthesis in response to other drugs, alone or in combination with CHK1i. It also highlights the importance of clearly discriminating among growth inhibition, cytostasis, and cell death, as only the latter is likely to result in tumor regression.

Natural products as drugs and tools for influencing core processes of eukaryotic mRNA translation

Eukaryotic protein synthesis is the highly conserved, complex mechanism of translating genetic information into proteins. Although this process is essential for cellular homoeostasis, dysregulations are associated with cellular malfunctions and diseases including cancer and diabetes. In the challenging and ongoing search for adequate treatment possibilities, natural products represent excellent research tools and drug leads for new interactions with the translational machinery and for influencing mRNA translation. In this review, bacterial-, marine- and plant-derived natural compounds that interact with different steps of mRNA translation, comprising ribosomal assembly, translation initiation and elongation, are highlighted. Thereby, the exact binding and interacting partners are unveiled in order to accurately understand the mode of action of each natural product. The pharmacological relevance of these compounds is furthermore assessed by evaluating the observed biological activities in the light of translational inhibition and by enlightening potential obstacles and undesired side-effects, e.g. in clinical trials. As many of the natural products presented here possess the potential to serve as drug leads for synthetic derivatives, structural motifs, which are indispensable for both mode of action and biological activities, are discussed. Evaluating the natural products emphasises the strong diversity of their points of attack. Especially the fact that selected binding partners can be set in direct relation to different diseases emphasises the indispensability of natural products in the field of drug development. Discovery of new, unique and unusual interacting partners again renders them promising tools for future research in the field of eukaryotic mRNA translation.

Inhibitors of Eukaryotic Translational Machinery as Therapeutic Agents

Inhibiting eukaryotic protein translation with small molecules is emerging as a powerful therapeutic strategy. The advantage of targeting cellular translational machinery is that it is required for the highly proliferative state of many neoplastic cells, replication of certain viruses, and ultimately the expression of a wide variety of protein targets. Although, this approach has been exploited to develop clinical agents, such as homoharringtonine (HHT, 1), used to treat chronic myeloid leukemia (CML), inhibiting components of the translational machinery is often associated with cytotoxic phenotypes. However, recent studies have demonstrated that certain small molecules can inhibit the translation of specific subsets of proteins, leading to lower cytotoxicity, and opening-up therapeutic opportunities for translation inhibitors to be deployed in indications beyond oncology and infectious disease. This review summarizes efforts to develop inhibitors of the eukaryotic translational machinery as therapeutic agents and highlights emerging opportunities for translation inhibitors in the future.

Multi-kinase targeted therapy as a promising treatment strategy for ovarian tumors expressing sfRon receptor

The sfRon kinase is an important therapeutic target in ovarian cancer that contributes to prominent tumor growth and disease progression. We reasoned that a multi-kinase inhibition of sfRon pathway might be an effective strategy to achieve a sustained anti-tumor response, while simultaneously preventing treatment resistance. We performed a detailed dissection of sfRon signaling in vitro and demonstrated that S6K1 is a key component of a multi-kinase targeting strategy in sfRon expressing ovarian tumors. We selected AD80 compound that targets several kinases within sfRon pathway including AKT and S6K1, and compared its efficacy with inhibitors that selectively target either sfRon or PI3 kinase. Using human ovarian xenografts and clinically relevant patient-derived xenografts (PDXs), we demonstrated that in vivo treatment with single agent AD80 shows superior efficacy to a standard-care chemotherapy (cisplatin/paclitaxel), or to the direct inhibition of sfRon kinase by BMS777607. Our findings indicate that ovarian tumors expressing sfRon are most effectively treated with multi-kinase inhibitors simultaneously targeting AKT and S6K1, such as AD80, which results in long-term anti-tumor response and prevents metastasis development.

The multifaceted functions of RNA helicases in the adaptive cellular response to hypoxia: From mechanisms to therapeutics

Hypoxia is a hallmark of cancer. Hypoxia-inducible factor (HIF), a master player for sensing and adapting to hypoxia, profoundly influences genome instability, tumor progression and metastasis, metabolic reprogramming, and resistance to chemotherapies and radiotherapies. High levels and activity of HIF result in poor clinical outcomes in cancer patients. Thus, HIFs provide ideal therapeutic targets for cancers. However, HIF biology is sophisticated, and currently available HIF inhibitors have limited clinical utility owing to their low efficacy or side effects. RNA helicases, which are master players in cellular RNA metabolism, are usually highly expressed in tumors to meet the increased oncoprotein biosynthesis demand. Intriguingly, recent findings provide convincing evidence that RNA helicases are crucial for the adaptive cellular response to hypoxia via a mutual regulation with HIFs. More importantly, some RNA helicase inhibitors may suppress HIF signaling by blocking the translation of HIF-responsive genes. Therefore, RNA helicase inhibitors may work synergistically with HIF inhibitors in cancer to improve treatment efficacy. In this review, we discuss current knowledge of how cells sense and adapt to hypoxia through HIFs. However, our primary focus is on the multiple functions of RNA helicases in the adaptive response to hypoxia. We also highlight how these hypoxia-related RNA helicases can be exploited for anti-cancer therapeutics.

The Mitochondrial Apoptotic Effectors BAX/BAK Activate Caspase-3 and -7 to Trigger NLRP3 Inflammasome and Caspase-8 Driven IL-1β Activation

Intrinsic apoptosis resulting from BAX/BAK-mediated mitochondrial membrane damage is regarded as immunologically silent. We show here that in macrophages, BAX/BAK activation results in inhibitor of apoptosis (IAP) protein degradation to promote caspase-8-mediated activation of IL-1β. Furthermore, BAX/BAK signaling induces a parallel pathway to NLRP3 inflammasome-mediated caspase-1-dependent IL-1β maturation that requires potassium efflux. Remarkably, following BAX/BAK activation, the apoptotic executioner caspases, caspase-3 and -7, act upstream of both caspase-8 and NLRP3-induced IL-1β maturation and secretion. Conversely, the pyroptotic cell death effectors gasdermin D and gasdermin E are not essential for BAX/BAK-induced IL-1β release. These findings highlight that innate immune cells undergoing BAX/BAK-mediated apoptosis have the capacity to generate pro-inflammatory signals and provide an explanation as to why IL-1β activation is often associated with cellular stress, such as during chemotherapy.

RITA requires eIF2α-dependent modulation of mRNA translation for its anti-cancer activity

Tumor protein 53 (p53, encoded by the TP53 gene) is a key tumor suppressor regulating cell fates in response to internal and external stresses. As TP53 is mutated or silenced in a majority of tumors, reactivation of p53 by small molecules represents a promising strategy in cancer therapeutics. One such agent is RITA (reactivation of p53 and induction of tumor cell apoptosis), which restores p53 expression in cells with hyperactive HDM2 and induces apoptosis. Yet, mechanisms underlying the anticancer activity of RITA are incompletely understood. Here we show that RITA suppresses mRNA translation independently of p53 by inducing eIF2α phosphorylation. Surprisingly, reactivation of p53 following RITA treatment is critically dependent on eIF2α phosphorylation. Moreover, inhibition of eIF2α phosphorylation attenuates pro-apoptotic and anti-neoplastic effects of RITA, while inducing phosphorylation of eIF2α enhances the anticancer activity of RITA. Collectively, these findings demonstrate that the translational machinery plays a major role in determining the antineoplastic activity of RITA, and suggest that combining p53 activators and translation modulators may be beneficial.

New Thalidomide-Resembling Dicarboximides Target ABC50 Protein and Show Antileukemic and Immunomodulatory Activities

We identified novel dicarboximides that were selectively cytotoxic towards human leukemia cells. Using chemical and biological methods, we characterized the biological activity, identified cellular protein targets and defined the mechanism of action of the test dicarboximides. The reported IC₅₀ values (concentration required to reduce cell survival fraction to 50% of control) of selected dicarboximides were similar or lower than IC₅₀ of registered anticancer drugs, for example cytarabine, sorafenib, irinotecan. Test compounds induced apoptosis in chronic myelogenous (K562) and acute lymphoblastic (MOLT-4) leukemia cells by activation of receptor and mitochondrial apoptotic pathways and increased the expression of proapoptotic genes (BAX, NOXA, HTRA2, TNFRSF10B, ESRRBL1). Selected dicarboximides displayed immunomodulatory activity and downregulated IKZF1 and IKZF3 transcription factors in K562 and MOLT-4 leukemia cells. ATP-binding cassette protein 50 (ABC50) was identified as a target for dicarboximides. Cancer cells with knocked down ABC50 showed increased resistance to dicarboximides. Based on the structure of dicarboximides and thalidomide, novel proteolysis-targeting chimeras (PROTACs) were synthesized and used as tools to downregulate ABC50 in leukemia cells.

Redox modifications of cysteine-containing proteins, cell cycle arrest and translation inhibition: Involvement in vitamin C-induced breast cancer cell death

Vitamin C (VitC) possesses pro-oxidant properties at high pharmacologic concentrations which favor repurposing VitC as an anti-cancer therapeutic agent. However, redox-based anticancer properties of VitC are yet partially understood. We examined the difference between the reduced and oxidized forms of VitC, ascorbic acid (AA) and dehydroascorbic acid (DHA), in terms of cytotoxicity and redox mechanisms toward breast cancer cells. Our data showed that AA displayed higher cytotoxicity towards triple-negative breast cancer (TNBC) cell lines in vitro than DHA. AA exhibited a similar cytotoxicity on non-TNBC cells, while only a minor detrimental effect on noncancerous cells. Using MDA-MB-231, a representative TNBC cell line, we observed that AA- and DHA-induced cytotoxicity were linked to cellular redox-state alterations. Hydrogen peroxide (H2O2) accumulation in the extracellular medium and in different intracellular compartments, and to a lesser degree, intracellular glutathione oxidation, played a key role in AA-induced cytotoxicity. In contrast, DHA affected glutathione oxidation and had less cytotoxicity. A "redoxome" approach revealed that AA treatment altered the redox state of key antioxidants and a number of cysteine-containing proteins including many nucleic acid binding proteins and proteins involved in RNA and DNA metabolisms and in energetic processes. We showed that cell cycle arrest and translation inhibition were associated with AA-induced cytotoxicity. Finally, bioinformatics analysis and biological experiments identified that peroxiredoxin 1 (PRDX1) expression levels correlated with AA differential cytotoxicity in breast cancer cells, suggesting a potential predictive value of PRDX1. This study provides insight into the redox-based mechanisms of VitC anticancer activity, indicating that pharmacologic doses of VitC and VitC-based rational drug combinations could be novel therapeutic opportunities for triple-negative breast cancer.

Molecular targets for modulating the protein translation vital to proteostasis and neuron degeneration in Parkinson's disease

Parkinson's disease (PD) is the most common neurodegenerative movement disorder, which is characterized by the progressive loss of dopaminergic neurons in the Substantia Nigra pars compacta concomitant with Lewy body formation in affected brain areas. The detailed pathogenic mechanisms underlying the selective loss of dopaminergic neurons in PD are unclear, and no drugs or treatments have been developed to alleviate progressive dopaminergic neuron degeneration in PD. However, the formation of α-synuclein-positive protein aggregates in Lewy body has been identified as a common pathological feature of PD, possibly stemming from the consequence of protein misfolding and dysfunctional proteostasis. Proteostasis is the mechanism for maintaining protein homeostasis via modulation of protein translation, enhancement of chaperone capacity and the prompt clearance of misfolded protein by the ubiquitin proteasome system and autophagy. Deregulated protein translation and impaired capacities of chaperone or protein degradation can disturb proteostasis processes, leading to pathological protein aggregation and neurodegeneration in PD. In recent years, multiple molecular targets in the modulation of protein translation vital to proteostasis and dopaminergic neuron degeneration have been identified. The potential pathophysiological and therapeutic significance of these molecular targets to neurodegeneration in PD is highlighted.

UBXN2A enhances CHIP-mediated proteasomal degradation of oncoprotein mortalin-2 in cancer cells

Overexpression of oncoproteins is a major cause of treatment failure using current chemotherapeutic drugs. Drug-induced degradation of oncoproteins is feasible and can improve clinical outcomes in diverse types of cancers. Mortalin-2 (mot-2) is a dominant oncoprotein in several tumors, including colorectal cancer (CRC). In addition to inactivating the p53 tumor suppressor protein, mot-2 enhances tumor cell invasion and migration. Thus, mot-2 is considered a potential therapeutic target in several cancer types. The current study investigated the biological role of a ubiquitin-like protein called UBXN2A in the regulation of mot-2 turnover. An orthogonal ubiquitin transfer technology followed by immunoprecipitation, in vitro ubiquitination, and Magnetic Beads TUBE2 pull-down experiments revealed that UBXN2A promotes carboxyl terminus of the HSP70-interacting protein (CHIP)-dependent ubiquitination of mot-2. We subsequently showed that UBXN2A increases proteasomal degradation of mot-2. A subcellular compartmentalization experiment revealed that induced UBXN2A decreases the level of mot-2 and its chaperone partner, HSP60. Pharmacological upregulation of UBXN2A using a small molecule, veratridine (VTD), decreases the level of mot-2 in cancer cells. Consistent with the in vitro results, UBXN2A^+/- mice exhibited selective elevation of mot-2 in colon tissues. An in vitro Anti-K48 TUBE isolation approach showed that recombinant UBXN2A enhances proteasomal degradation of mot-2 in mouse colon tissues. Finally, we observed enhanced association of CHIP with the UBXN2A-mot-2 complex in tumors in an azoxymethane/dextran sulfate sodium-induced mouse CRC model. The existence of a multiprotein complex containing UBXN2A, CHIP, and mot-2 suggests a synergistic tumor suppressor activity of UBXN2A and CHIP in mot-2-enriched tumors. This finding validates the UBXN2A-CHIP axis as a novel and potential therapeutic target in CRC.

A Review of Omacetaxine: A Chronic Myeloid Leukemia Treatment Resurrected

The paradigm of targeted therapy was pioneered for chronic myeloid leukemia (CML). The advent of tyrosine kinase inhibitors (TKIs) has led to marked improvements in responses and overall survival; however, there is still a subset of patients that are either resistant through a multitude of mechanisms or intolerant to standard TKI therapy. Omacetaxine mepesuccinate (omacetaxine), a semisynthetic purified homoharringtonine compound, has been studied for over 40 years and was approved in 2012 by the Food and Drug Administration (FDA) for patients with CML refractory or intolerant to two or more TKIs. Omacetaxine has a novel mechanism of action-inhibition of protein synthesis, which does not overlap with kinase inhibition. Multiple studies have demonstrated that omacetaxine can achieve responses in heavily treated patients with either chronic-phase or accelerated-phase CML, regardless of the presence of mutations in the tyrosine kinase domain. This review will outline the tortuous story of omacetaxine, including preclinical and clinical studies of homoharringtonine, current indications, and management guidelines.

Cross-talk between protein synthesis, energy metabolism and autophagy in cancer

Translation is a pivotal step in the regulation of gene expression as well as one of the most energy consuming processes in the cell. Dysregulation of translation caused by the aberrant function of upstream signaling pathways and/or perturbations in the expression or function of components of the translation machinery is frequent in cancer. In this review, we discuss emerging findings that highlight hitherto unappreciated aspects of signaling to the translation apparatus with the particular focus on emerging connections between protein synthesis, autophagy and energy homeostasis in cancer.

Essential role for Bim in mediating the apoptotic and antitumor activities of immunotoxins

Protein synthesis is crucial for regulating cell homeostasis and, when unrestricted, it can lead to tumorigenesis. Immunotoxins derived from Pseudomonas exotoxin are antibody-toxin fusion proteins that inhibit protein synthesis of mammalian cells via ADP-ribosylation of the eukaryotic elongation factor-2. Here we investigate the role of the Bcl-2 family proteins in the response of cancer cells to immunotoxin challenge. Besides the well-known reduction of the prosurvival Bcl-2 family member, Mcl-1, following inhibition of protein synthesis, we show for the first time that immunotoxins also reduce the levels of selected proapoptotic BH-3-only proteins. Among these, only Bim protein levels correlated with the ability of immunotoxins to induce an apoptotic response. To support our findings, we verified that a Bim knockout completely abolished immunotoxin-mediated apoptosis. Further, mice bearing either wild-type or Bid knockout tumors responded to immunotoxin treatment with a decrease in growth kinetics, whereas mice engrafted with Bim knockout tumors showed no reduction in tumor size or prolongation of survival following immunotoxin treatment. From these results, we conclude that Bim expression is a major susceptibility factor for tumor cell death and, as such, constitutes a potential biomarker that could be evaluated before immunotoxin treatment. In support of this hypothesis, clinically, we analyzed patient cells before immunotoxin treatment and report that samples of hairy cell leukemia with high levels of Bim protein responded with a greater decrease in leukemic cell count compared with those samples expressing a low level of Bim.

Deletion of eIF2β lysine stretches creates a dominant negative that affects the translation and proliferation in human cell line: A tool for arresting the cell growth

BACKGROUND

Eukaryote initiation factor 2 subunit β (eIF2β) plays a crucial role in regulation protein synthesis, which mediates the interaction of eIF2 with mRNA. eIF2β contains evolutionarily conserved polylysine stretches in amino-terminal region and a zinc finger motif in the carboxy-terminus.

METHODS

The gene eIF2β was cloned under tetracycline transcription control and the polylysine stretches were deleted by site-directed mutagenesis (eIF2βΔ3K). The plasmid was transfected into HEK 293 TetR cells. These cells were analyzed for their proliferative and translation capacities as well as cell death rate. Experiments were performed using gene reporter assays, western blotting, flow cytometry, cell sorting, cell proliferation assays and confocal immunofluorescence.

RESULTS

eIF2βΔ3K affected negatively the protein synthesis, cell proliferation and cell survival causing G2 cell cycle arrest and increased cell death, acting in a negative dominant manner against the native protein. Polylysine stretches are also essential for eIF2β translocated from the cytoplasm to the nucleus, accumulating in the nucleolus and eIF2βΔ3K did not make this translocation.

DISCUSSION

eIF2β is involved in the protein synthesis process and should act in nuclear processes as well. eIF2βΔ3K reduces cell proliferation and causes cell death. Since translation control is essential for normal cell function and survival, the development of drugs or molecules that inhibit translation has become of great interest in the scenario of proliferative disorders. In conclusion, our results suggest the dominant negative eIF2βΔ3K as a therapeutic strategy for the treatment of proliferative disorders and that eIF2β polylysine stretch domains are promising targets for this.

Cycloheximide Can Induce Bax/Bak Dependent Myeloid Cell Death Independently of Multiple BH3-Only Proteins

Apoptosis mediated by Bax or Bak is usually thought to be triggered by BH3-only members of the Bcl-2 protein family. BH3-only proteins can directly bind to and activate Bax or Bak, or indirectly activate them by binding to anti-apoptotic Bcl-2 family members, thereby relieving their inhibition of Bax and Bak. Here we describe a third way of activation of Bax/Bak dependent apoptosis that does not require triggering by multiple BH3-only proteins. In factor dependent myeloid (FDM) cell lines, cycloheximide induced apoptosis by a Bax/Bak dependent mechanism, because Bax^-/-Bak^-/- lines were profoundly resistant, whereas FDM lines lacking one or more genes for BH3-only proteins remained highly sensitive. Addition of cycloheximide led to the rapid loss of Mcl-1 but did not affect the expression of other Bcl-2 family proteins. In support of these findings, similar results were observed by treating FDM cells with the CDK inhibitor, roscovitine. Roscovitine reduced Mcl-1 abundance and caused Bax/Bak dependent cell death, yet FDM lines lacking one or more genes for BH3-only proteins remained highly sensitive. Therefore Bax/Bak dependent apoptosis can be regulated by the abundance of anti-apoptotic Bcl-2 family members such as Mcl-1, independently of several known BH3-only proteins.

4EBP1/c-MYC/PUMA and NF-κB/EGR1/BIM pathways underlie cytotoxicity of mTOR dual inhibitors in malignant lymphoid cells

The mammalian target of rapamycin (mTOR), a kinase that regulates proliferation and apoptosis, has been extensively evaluated as a therapeutic target in multiple malignancies. Rapamycin analogs, which partially inhibit mTOR complex 1 (mTORC1), exhibit immunosuppressive and limited antitumor activity, but sometimes activate survival pathways through feedback mechanisms involving mTORC2. Thus, attention has turned to agents targeting both mTOR complexes by binding the mTOR active site. Here we show that disruption of either mTOR-containing complex is toxic to acute lymphocytic leukemia (ALL) cells and identify 2 previously unrecognized pathways leading to this cell death. Inhibition of mTORC1-mediated 4EBP1 phosphorylation leads to decreased expression of c-MYC and subsequent upregulation of the proapoptotic BCL2 family member PUMA, whereas inhibition of mTORC2 results in nuclear factor-κB-mediated expression of the Early Growth Response 1 (EGR1) gene, which encodes a transcription factor that binds and transactivates the proapoptotic BCL2L11 locus encoding BIM. Importantly, 1 or both pathways contribute to death of malignant lymphoid cells after treatment with dual mTORC1/mTORC2 inhibitors. Collectively, these observations not only provide new insight into the survival roles of mTOR in lymphoid malignancies, but also identify alterations that potentially modulate the action of mTOR dual inhibitors in ALL.

Small molecules targeting Mcl-1: the search for a silver bullet in cancer therapy

Progress towards the development of potent and selective inhibitors of the pro-survival protein Mcl-1 is reviewed.

Total synthesis of a biotinylated rocaglate: Selective targeting of the translation factors eIF4AI/II

The total synthesis of a biotinylated derivative of methyl rocaglate is described. This compound was accessed from synthetic methyl rocaglate (2) via formation of the propargyl amide and subsequent click reaction with a biotin azide. Affinity purification revealed that biotinylated rocaglate (8) and methyl rocaglate (2) bind with high specificity to translation factors eIF4AI/II. This remarkable selectivity is in line with that found for the more complex rocaglate silvestrol (3).

Efficacy and safety of homoharringtonine plus cytarabine and aclarubicin for patients with myelodysplastic syndrome-RAEB

The aim of the present study was to evaluate the treatment outcome of homoharringtonine, cytarabine (AraC) and aclarubicin combination therapy as induction treatment for myelodysplastic syndromes-refractory anemia with excess blasts (MDS-RAEB). A total of 24 patients with MDS-RAEB who were aged between 18 and 66 years were treated with homoharringtonine, AraC and aclarubicin (HAA regimen). The HAA regimen consisted of homoharringtonine (2 mg/m² intramuscularly twice daily, days 1-3), AraC (75 mg/m² injected subcutaneously twice daily, days 1-7) and aclarubicin (12 mg/m², days 1-7). The overall response rate was 79% with a complete remission rate of 58.3% and partial remission rate of 20.7%. There was no evidence of early mortality in this group of patients. The median overall survival (OS) was 36.2 months (95% confidence interval, 24.6-47.4 months), and the estimated three year overall survival rate was 45.8%. In conclusion, HAA combination therapy is a suitable induction regimen for patients with MDS-RAEB, which may improve the outcome for de novo higher-risk MDS patients, particularly of those with favorable and intermediate cytogenetics.

Depletion of Amyloid Precursor Protein (APP) causes G0 arrest in non-small cell lung cancer (NSCLC) cells

We recently reported that Amyloid Precursor Protein (APP) regulates global protein synthesis in a variety of human dividing cells, including non-small cell lung cancer (NSCLC) cells. More specifically, APP depletion causes an increase of both cap- and IRES-dependent translation. Since growth and proliferation are tightly coupled processes, here, we asked what effects artificial downregulation of APP could have elicited in NSCLC cells proliferation. APP depletion caused a G0/G1 arrest through destabilization of the cyclin-C protein and reduced pRb phosphorylation at residues Ser802/811. siRNA to cyclin-C mirrored the cell cycle distribution observed when silencing APP. Cells arrested in G0/G1 (and with augmented global protein synthesis) increased their size and underwent a necrotic cell death due to cell membrane permeabilization. These phenotypes were reversed by overexpression of the APP C-terminal domain, indicating a novel role for APP in regulating early cell cycle entry decisions. It is seems that APP moderates the rate of protein synthesis before the cell clears growth factors- and nutrients-dependent checkpoint in mid G1. Our results raise questions on how such processes interact in the context of (at least) dividing NSCLC cells. The data presented here suggest that APP, although required for G0/G1 transitions, moderates the rate of protein synthesis before the cell fully commits to cell cycle progression following mechanisms, which seem additional to concurrent signals deriving from the PI3-K/Akt/mTORC-1 axis. APP appears to play a central role in regulating cell cycle entry with the rate of protein synthesis; and its loss-of-function causes cell size abnormalities and death.

Apoptosis inducers in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is characterized by a typical defect in apoptosis and is still an incurable disease. Numerous apoptosis inducers have been described. These synthetic compounds and natural products (mainly derived from plants) display antileukemic properties in vitro and in vivo and some have even been tested in the clinic in CLL. They act through several different mechanisms. Most of them involve proteins of the Bcl-2 family, which are the key regulators in triggering the mitochondrial pathway of caspase-dependent apoptosis. Thus, the Mcl-1/Noxa axis appeared as a target. Here I overview natural and synthetic apoptosis inducers and their mechanisms of action in CLL cells. Opportunities for developing novel, apoptosis-based therapeutics are presented.

Rocaglamide, silvestrol and structurally related bioactive compounds from Aglaia species

Covering: 2006 to 2013. Investigations on the chemistry and biology of rocaglamide, silvestrol and structurally related bioactive compounds from Aglaia species during the period 2006-2013 are reviewed. Included are new phytochemical studies of naturally occurring rocaglamide derivatives, an update on synthetic methods for cyclopenta[b]benzofurans, and a description of the recent biological evaluation and mechanism-of-action studies on compounds of this type.

Prosurvival Bcl-2 family members affect autophagy only indirectly, by inhibiting Bax and Bak

Antiapoptotic B-cell lymphoma 2 (Bcl-2) family members such as Bcl-2, myeloid cell leukemia 1 (Mcl-1), and B-cell lymphoma-X large (Bcl-xL) are proposed to inhibit autophagy by directly binding to the BH3 domain of Beclin 1/Atg6. However, these Bcl-2 family proteins also block the proapoptotic activity of Bcl-2-associated X (Bax) and Bcl-2 homologous antagonist/killer (Bak), and many inducers of autophagy also cause cell death. Therefore, when the mitochondrial-mediated apoptosis pathway is functional, interpretation of such experiments is complicated. To directly test the impact of the endogenous antiapoptotic Bcl-2 family members on autophagy in the absence of apoptosis, we inhibited their activity in cells lacking the essential cell death mediators Bax and Bak. We also used inducible lentiviral vectors to overexpress Bcl-2, Bcl-xL, or Mcl-1 in cells and subjected them to treatments that promote autophagy. In the absence of Bax and Bak, Bcl-2, Bcl-xL, and Mcl-1 had no detectable effect on autophagy or cell death in myeloid or fibroblast cell lines. On the other hand, when Bax and Bak were present, inhibiting the prosurvival Bcl-2 family members stimulated autophagy, but this correlated with increased cell death. In addition, inhibition of autophagy induced by amino acid starvation, etoposide, or interleukin-3 withdrawal did not affect cell death in the absence of Bax and Bak. These results demonstrate that the antiapoptotic Bcl-2 family members do not directly inhibit components of the autophagic pathway but instead affect autophagy indirectly, owing to their inhibition of Bax and Bak.

Simplified silvestrol analogues with potent cytotoxic activity

The complex natural products silvestrol (1) and episilvestrol (2) are inhibitors of translation initiation through binding to the DEAD-box helicase eukaryotic initiation factor 4A (eIF4A). Both compounds are potently cytotoxic to cancer cells in vitro, and 1 has demonstrated efficacy in vivo in several xenograft cancer models. Here we show that 2 has limited plasma membrane permeability and is metabolized in liver microsomes in a manner consistent with that reported for 1. In addition, we have prepared a series of analogues of these compounds where the complex pseudo-sugar at C6 has been replaced with chemically simpler moieties to improve drug-likeness. Selected compounds from this work possess excellent activity in biochemical and cellular translation assays with potent activity against leukemia cell lines.

Flavaglines target primitive leukemia cells and enhance anti-leukemia drug activity

Identification of agents that target human leukemia stem cells is an important consideration for the development of new therapies. The present study demonstrates that rocaglamide and silvestrol, closely related natural products from the flavagline class of compounds, are able to preferentially kill functionally defined leukemia stem cells, while sparing normal stem and progenitor cells. In addition to efficacy as single agents, flavaglines sensitize leukemia cells to several anticancer compounds, including front-line chemotherapeutic drugs used to treat leukemia patients. Mechanistic studies indicate that flavaglines strongly inhibit protein synthesis, leading to the reduction of short-lived antiapoptotic proteins. Notably though, treatment with flavaglines, alone or in combination with other drugs, yields a much stronger cytotoxic activity toward leukemia cells than the translational inhibitor temsirolimus. These results indicate that the underlying cell death mechanism of flavaglines is more complex than simply inhibiting general protein translation. Global gene expression profiling and cell biological assays identified Myc inhibition and the disruption of mitochondrial integrity to be features of flavaglines, which we propose contribute to their efficacy in targeting leukemia cells. Taken together, these findings indicate that rocaglamide and silvestrol are distinct from clinically available translational inhibitors and represent promising candidates for the treatment of leukemia.

Omacetaxine: a protein translation inhibitor for treatment of chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is driven by the Bcr-Abl fusion protein, which is a result of a (9;22) chromosomal translocation. Imatinib, dasatinib, and nilotinib (tyrosine kinase inhibitors, TKI) have revolutionized how CML is treated. Although the majority of patients respond to these kinase inhibitors, a subset becomes resistant to these therapeutics. Synribo (omacetaxine mepesuccinate) was recently approved by the U.S. Food and Drug Administration for Philadelphia-positive CML either in the chronic or the accelerated phase whose disease failed two prior TKIs. With omacetaxine 1.25 mg/m(2) twice daily for 14 days during induction and for 7 days during maintenance, a major cytogenetic response occurred in 20% of patients in the chronic phase and major hematologic response in 27% of patients in the accelerated phase. Laboratory investigations unraveled the mechanism of action and effectiveness of this agent. Bcr-Abl protein is intrinsically programmed to turn over with a short half-life that makes it susceptible to protein translation inhibitors. Omacetaxine (homoharringtonine) inhibits total protein biosynthesis by binding to A-site cleft of ribosomes. As a corollary to this action, there is a diminution of short-lived proteins, such as Bcr-Abl, followed by cell death. Approval of this first-in-class protein translation inhibitor opens up new avenues for its use in other diseases as well as mechanism-based combinations.

Omacetaxine mepesuccinate in the treatment of intractable chronic myeloid leukemia

In a significant proportion of patients with chronic myeloid leukemia, resistance to BCR-ABL tyrosine kinase inhibitors develops due to acquisition of BCR-ABL kinase domain mutations and insensitivity of leukemia stem cells to tyrosine kinase inhibitors. Omacetaxine mepesuccinate (formerly called homoharringtonine) is a natural alkaloid that inhibits protein synthesis and induces cell death. Omacetaxine mepesuccinate has been recently approved by the US Food and Drug Administration to treat patients with chronic myeloid leukemia who failed to respond to multiple tyrosine kinase inhibitors and/or acquired the BCR-ABL-T315I mutation. In this review, we discuss the use and effectiveness of omacetaxine mepesuccinate in the treatment of chronic myeloid leukemia, with coverage of its pharmacology, mode of action, and pharmacokinetics. We believe that omacetaxine mepesuccinate will be beneficial to many patients with chronic myeloid leukemia who do not respond well to tyrosine kinase inhibitors.

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

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

Combination of the PI3K inhibitor ZSTK474 with a PSMA-targeted immunotoxin accelerates apoptosis and regression of prostate cancer

The phosphoinositide 3-kinase (PI3K) pathway is activated in most advanced prostate cancers, yet so far treatments with PI3K inhibitors have been at best tumorostatic in preclinical cancer models and do not show significant antitumor efficacy in clinical trials. Results from tissue culture experiments in prostate cancer cells suggest that PI3K inhibitors should be combined with other cytotoxic agents; however, the general toxicity of such combinations prevents translating these experimental data into preclinical and clinical models. We investigated the emerging concept of tumor-targeted synthetic lethality in prostate cancer cells by using the pan-PI3K inhibitor ZSTK474 and the immunotoxin J591PE, a protein chimera between the single-chain variable fragment of the monoclonal antibody J591 against the prostate-specific membrane antigen (PSMA) and the truncated form of the Pseudomonas aeruginosa exotoxin A (PE38QQR). The combination of ZSTK474 and J591PE increased apoptosis within 6 hours and cell death (monitored at 24-48 hours) in the PSMA-expressing cells LNCaP, C4-2, and C4-2Luc but not in control cells that do not express PSMA (PC3 and BT549 cells). Mechanistic analysis suggested that induction of apoptosis requires Bcl-2-associated death promoter (BAD) dephosphorylation and decreased expression of myeloid leukemia cell differentiation protein 1 (MCL-1). A single injection of ZSTK474 and J591PE into engrafted prostate cancer C4-2Luc cells led to consistent and stable reduction of luminescence within 6 days. These results suggest that the combination of a PI3K inhibitor and a PSMA-targeted protein synthesis inhibitor toxin represents a promising novel strategy for advanced prostate cancer therapy that should be further investigated.

Synthesis of biotinylated episilvestrol: highly selective targeting of the translation factors eIF4AI/II

Silvestrol (1) and episilvestrol (2) are protein synthesis inhibitors, and the former has shown efficacy in multiple mouse models of cancer; however, the selectivity of these potent cytotoxic natural products has not been described. Herein, it is demonstrated that eukaryotic initiation factors eIF4AI/II were the only proteins detected to bind silvestrol (1) and biotinylated episilvestrol (9) by affinity purification. Our study demonstrates the remarkable selectivity of these promising chemotherapeutics.