A cellular screening assay to test the ability of PKR to induce cell death in mammalian cells

PKR and TLR3 trigger distinct signals that coordinate the induction of antiviral apoptosis

RIG-I-like receptors (RLRs), protein kinase R (PKR), and endosomal Toll-like receptor 3 (TLR3) sense viral non-self RNA and are involved in cell fate determination. However, the mechanisms by which intracellular RNA induces apoptosis, particularly the role of each RNA sensor, remain unclear. We performed cytoplasmic injections of different types of RNA and elucidated the molecular mechanisms underlying viral dsRNA-induced apoptosis. The results obtained revealed that short 5'-triphosphate dsRNA, the sole ligand of RIG-I, induced slow apoptosis in a fraction of cells depending on IRF-3 transcriptional activity and IFN-I production. However, intracellular long dsRNA was sensed by PKR and TLR3, which activate distinct signals, and synergistically induced rapid apoptosis. PKR essentially induced translational arrest, resulting in reduced levels of cellular FLICE-like inhibitory protein and functioned in the TLR3/TRIF-dependent activation of caspase 8. The present results demonstrated that PKR and TLR3 were both essential for inducing the viral RNA-mediated apoptosis of infected cells and the arrest of viral production.

Relevance of Translation Initiation in Diffuse Glioma Biology and its Therapeutic Potential

Cancer cells are continually exposed to environmental stressors forcing them to adapt their protein production to survive. The translational machinery can be recruited by malignant cells to synthesize proteins required to promote their survival, even in times of high physiological and pathological stress. This phenomenon has been described in several cancers including in gliomas. Abnormal regulation of translation has encouraged the development of new therapeutics targeting the protein synthesis pathway. This approach could be meaningful for glioma given the fact that the median survival following diagnosis of the highest grade of glioma remains short despite current therapy. The identification of new targets for the development of novel therapeutics is therefore needed in order to improve this devastating overall survival rate. This review discusses current literature on translation in gliomas with a focus on the initiation step covering both the cap-dependent and cap-independent modes of initiation. The different translation initiation protagonists will be described in normal conditions and then in gliomas. In addition, their gene expression in gliomas will systematically be examined using two freely available datasets. Finally, we will discuss different pathways regulating translation initiation and current drugs targeting the translational machinery and their potential for the treatment of gliomas.

Regulation of p53 expression and apoptosis by vault RNA2-1-5p in cervical cancer cells

nc886 or VRNA2-1 has recently been identified as a noncoding RNA instead of a vault RNA or a pre-microRNA. Several studies have reported that pre-miR-886 plays a tumor-suppressive role in a wide range of cancer cells through its activity as a cellular protein kinase RNA-activated (PKR) ligand and repressor. However, by sequencing stem-PCR products, we found that a microRNA originating from this precursor, vault RNA2-1-5p (VTRNA2-1-5p), occurs in cervical cancer cells. The expression levels of the predicted targets of VTRNA2-1-5p are negatively correlated with VTRNA2-1-5p levels by quantitative reversion transcription PCR (qRT-PCR). Previous results have shown that VTRNA2-1-5p is overexpressed in human cervical squamous cell carcinomas (CSCCs) compared with adjacent healthy tissues. Inhibition of VTRNA2-1-5p increases Bax protein expression and apoptotic cell death in cervical cancer cells. Our findings suggest that VTRNA2-1-5p has oncogenic activity related to the progression of cervical cancer. Here, we report that VTRNA2-1-5p directly targeted p53 expression and functioned as an oncomir in cervical cancer. VTRNA2-1-5p inhibition decreased cervical cancer cell invasion, proliferation, and tumorigenicity while increasing apoptosis and p53 expression. Interestingly, VTRNA2-1-5p inhibition also increased cisplatin-induced apoptosis of HeLa and SiHa cells. In human clinical cervical cancer specimens, low p53 expression and high VTRNA2-1-5p expression were positively associated. In addition, VTRNA2-1-5p was found to directly target the 5′ and 3′ untranslated regions (UTRs) of p53. We propose that VTRNA2-1-5p is a direct regulator of p53 and suggest that it plays an essential role in the apoptosis and proliferation of cervical cancer cells.

Use of a negative selectable marker for rapid selection of recombinant vaccinia virus

Vaccinia virus has been a powerful tool in molecular biology and vaccine development. The relative ease of inserting and expressing foreign genes combined with its broad host range has made it an attractive antigen delivery system against many heterologous diseases. Many different approaches have been developed to isolate recombinant vaccinia virus generated from homologous recombination; however, most are time-consuming, often requiring a series of passages or specific cell lines. Herein we introduce a rapid method for isolating recombinants using the antibiotic coumermycin and the interferon-associated PKR pathway to select for vaccinia virus recombinants. This method uses a negative selection marker in the form of a fusion protein, GyrB-PKR, consisting of the coumermycin dimerization domain of Escherichia coli gyrase subunit B fused to the catalytic domain of human PKR. Coumermycin-dependent dimerization of this protein results in activation of PKR and the phosphorylation of translation initiation factor, eIF2α. Phosphorylation of this factor leads to an inhibition of protein synthesis, and an inhibition of virus replication. In the presence of coumermycin, recombinants are isolated due to the loss of this coumermycin-sensitive gene by homologous recombination. We demonstrate that this method of selection is highly efficient and requires limited rounds of enrichment to isolate recombinant virus.

Selective cell death mediated by small conditional RNAs

Cancer cells are characterized by genetic mutations that deregulate cell proliferation and suppress cell death. To arrest the uncontrolled replication of malignant cells, conventional chemotherapies systemically disrupt cell division, causing diverse and often severe side effects as a result of collateral damage to normal cells. Seeking to address this shortcoming, we pursue therapeutic regulation that is conditional, activating selectively in cancer cells. This functionality is achieved using small conditional RNAs that interact and change conformation to mechanically transduce between detection of a cancer mutation and activation of a therapeutic pathway. Here, we describe small conditional RNAs that undergo hybridization chain reactions (HCR) to induce cell death via an innate immune response if and only if a cognate mRNA cancer marker is detected within a cell. The sequences of the small conditional RNAs can be designed to accept different mRNA markers as inputs to HCR transduction, providing a programmable framework for selective killing of diverse cancer cells. In cultured human cancer cells (glioblastoma, prostate carcinoma, Ewing's sarcoma), HCR transduction mediates cell death with striking efficacy and selectivity, yielding a 20- to 100-fold reduction in population for cells containing a cognate marker, and no measurable reduction otherwise. Our results indicate that programmable mechanical transduction with small conditional RNAs represents a fundamental principle for exploring therapeutic conditional regulation in living cells.

Increased susceptibility of breast cancer cells to stress mediated inhibition of protein synthesis

Protein synthesis is a tightly controlled process, and its deregulation plays an important role in tumorigenesis. Protein synthesis remains poorly understood with very few well-identified validated targets for therapeutic purposes. In this study, we use nitric oxide (NO), which suppresses protein synthesis by inactivating eukaryotic initiation factor 2-alpha (eIF2-alpha), to examine the mechanism by which low and high oxidative stress inhibits protein synthesis. In breast cancer cells, low NO stress induced heme-regulated inhibitor (HRI) activation, which facilitated gradual decline in short half-life proteins. High NO stress induced HRI and protein kinase R (PKR) activation, leading to a sharp decline in protein synthesis as accessed by a decline in short and long half-life proteins and dramatic morphologic changes. In contrast, human mammary epithelial (HME) and Ras transfected untransformed HME (MCF-10A1 neo N) cells were less susceptible to NO-induced inhibition of protein synthesis and cytostasis. Our results suggest that NO-induced cytostasis in breast cancer cells was due to PKR activation and increased phosphorylation of eIF2-alpha, whereas the reduced susceptibility of normal mammary epithelial cells to NO could be due to the inaccessibility of PKR, which is bound to inhibitor p58.

EGF receptor-targeted synthetic double-stranded RNA eliminates glioblastoma, breast cancer, and adenocarcinoma tumors in mice

BACKGROUND

Glioblastoma multiforme (GBM) is the most lethal form of brain cancer. With the available treatments, survival does not exceed 12-14 mo from the time of diagnosis. We describe a novel strategy to selectively induce the death of glioblastoma cells and other cancer cells that over-express the EGF receptor. Using a non-viral delivery vector that homes to the EGF receptor, we target synthetic anti-proliferative dsRNA (polyinosine-cytosine [poly IC]), a strong activator of apoptosis, selectively to cancer cells.

METHODS AND FINDINGS

Poly IC was delivered by means of a non-viral vector: 25kDa polyethylenimine-polyethyleneglycol-EGF (PEI25-PEG-EGF). EGFR-targeted poly IC induced rapid apoptosis in the target cells in vitro and in vivo. Expression of several cytokines and "bystander killing" of untransfected tumor cells was detected in vitro and in vivo. Intra-tumoral delivery of the EGFR-targeted poly IC induced the complete regression of pre-established intracranial tumors in nude mice, with no obvious adverse toxic effects on normal brain tissue. A year after treatment completion the treated mice remain cancer-free and healthy. Similarly, non-viral delivery of poly IC completely eliminated pre-established breast cancer and adenocarcinoma xenografts derived from EGFR over-expressing cancer cell lines, suggesting that the strategy is applicable to other EGFR-over-expressing tumors.

CONCLUSION

The strategy described has yielded an effective treatment of EGFR over-expressing GBM in an animal model. If this strategy is translated successfully to the clinical setting, it may actually offer help to GBM patients. Moreover the elimination of two additional EGFR over-expressing cancers in vivo suggests that in principle this strategy can be applied to treat other tumors that over-express EGFR.