Differential requirement for the stress-activated protein kinase/c-Jun NH(2)-terminal kinase in RNAdamage-induced apoptosis in primary and in immortalized fibroblasts

Role of the Ribonuclease ONCONASE in miRNA Biogenesis and tRNA Processing: Focus on Cancer and Viral Infections

The majority of transcribed RNAs do not codify for proteins, nevertheless they display crucial regulatory functions by affecting the cellular protein expression profile. MicroRNAs (miRNAs) and transfer RNA-derived small RNAs (tsRNAs) are effectors of interfering mechanisms, so that their biogenesis is a tightly regulated process. Onconase (ONC) is an amphibian ribonuclease known for cytotoxicity against tumors and antiviral activity. Additionally, ONC administration in patients resulted in clinical effectiveness and in a well-tolerated feature, at least for lung carcinoma and malignant mesothelioma. Moreover, the ONC therapeutic effects are actually potentiated by cotreatment with many conventional antitumor drugs. This review not only aims to describe the ONC activity occurring either in different tumors or in viral infections but also to analyze the molecular mechanisms underlying ONC pleiotropic and cellular-specific effects. In cancer, data suggest that ONC affects malignant phenotypes by generating tRNA fragments and miRNAs able to downregulate oncogenes expression and upregulate tumor-suppressor proteins. In cells infected by viruses, ONC hampers viral spread by digesting the primer tRNAs necessary for viral DNA replication. In this scenario, new therapeutic tools might be developed by exploiting the action of ONC-elicited RNA derivatives.

Strengths and Challenges of Secretory Ribonucleases as AntiTumor Agents

Approaches to develop effective drugs to kill cancer cells are mainly focused either on the improvement of the currently used chemotherapeutics or on the development of targeted therapies aimed at the selective destruction of cancer cells by steering specific molecules and/or enhancing the immune response. The former strategy is limited by its genotoxicity and severe side effects, while the second one is not always effective due to tumor cell heterogeneity and variability of targets in cancer cells. Between these two strategies, several approaches target different types of RNA in tumor cells. RNA degradation alters gene expression at different levels inducing cell death. However, unlike DNA targeting, it is a pleotropic but a non-genotoxic process. Among the ways to destroy RNA, we find the use of ribonucleases with antitumor properties. In the last few years, there has been a significant progress in the understanding of the mechanism by which these enzymes kill cancer cells and in the development of more effective variants. All the approaches seek to maintain the requirements of the ribonucleases to be specifically cytotoxic for tumor cells. These requirements start with the competence of the enzymes to interact with the cell membrane, a process that is critical for their internalization and selectivity for tumor cells and continue with the downstream effects mainly relying on changes in the RNA molecular profile, which are not only due to the ribonucleolytic activity of these enzymes. Although the great improvements achieved in the antitumor activity by designing new ribonuclease variants, some drawbacks still need to be addressed. In the present review, we will focus on the known mechanisms used by ribonucleases to kill cancer cells and on recent strategies to solve the shortcomings that they show as antitumor agents, mainly their pharmacokinetics.

A novel strategy for engineering of a smart generation of immune ribonucleases against EGFR+ cells

The overexpression of epidermal growth factor receptor (EGFR) could result in the development of solid tumors of prostate, breast, gastric, colorectal, ovarian, and head and neck, leading to carcinoma. Antibody therapies are ideal methods to overcome malignant diseases. However, immunoribonucleases are a new generation of antibodies in which an RNase binds to a specific antibody and shows a stronger ability to terminate cancer cells. In this study, we engineered Rana pipiens RNase to bind to the scFv of human antiepidermal growth factor receptor antibody. The molecular dynamic simulations confirmed protein stability and the ability of scFv-ranpirnase (rantoxin) to bind to epidermal growth factor receptor protein. Then, the rantoxin construct was synthesized in a pCDNA 3.1 Neo vector. CHO-K1 cells were used as expression hosts and the construct was transfected. Cells were selected by antibiotic therapies using neomycin, 120 mg/ml, and the high-yield colony was screened by real-time polymerase chain reaction (PCR) methods. Then, the recombinant protein production was confirmed using the sodium dodecyl sulfate polyacrylamide gel electrophoresis and western blot analyses. The molecular dynamic simulation (MDS) confirmed that the I467, S468, Q408, and H409 amino acids of EGFR bonded well to rantoxin. As revealed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and western blot analyses, the rantoxin production and PCR analysis showed that the T3 colony can produce rantoxin messenger RNA fourfold higher than the GAPDH gene. The immunotoxin function was assessed in A431 cancer cells and EGFR-negative HEK293 cells, and IC₅₀  values were estimated to be 22.4 ± 3 and >620.4 ± 5 nM, respectively. The results indicated that the immunotoxins produced in this study had the potential for use as anticancer drugs.

Antiviral Ranpirnase TMR-001 Inhibits Rabies Virus Release and Cell-to-Cell Infection In Vitro

Currently, no rabies virus-specific antiviral drugs are available. Ranpirnase has strong antitumor and antiviral properties associated with its ribonuclease activity. TMR-001, a proprietary bulk drug substance solution of ranpirnase, was evaluated against rabies virus in three cell types: mouse neuroblastoma, BSR (baby hamster kidney cells), and bat primary fibroblast cells. When TMR-001 was added to cell monolayers 24 h preinfection, rabies virus release was inhibited for all cell types at three time points postinfection. TMR-001 treatment simultaneous with infection and 24 h postinfection effectively inhibited rabies virus release in the supernatant and cell-to-cell spread with 50% inhibitory concentrations of 0.2–2 nM and 20–600 nM, respectively. TMR-001 was administered at 0.1 mg/kg via intraperitoneal, intramuscular, or intravenous routes to Syrian hamsters beginning 24 h before a lethal rabies virus challenge and continuing once per day for up to 10 days. TMR-001 at this dose, formulation, and route of delivery did not prevent rabies virus transit from the periphery to the central nervous system in this model (n = 32). Further aspects of local controlled delivery of other active formulations or dose concentrations of TMR-001 or ribonuclease analogues should be investigated for this class of drugs as a rabies antiviral therapeutic.

Role of the MAPK/cJun NH2-terminal kinase signaling pathway in starvation-induced autophagy

Autophagy is required for cellular homeostasis and can determine cell viability in response to stress. It is established that MTOR is a master regulator of starvation-induced macroautophagy/autophagy, but recent studies have also implicated an essential role for the MAPK8/cJun NH₂-terminal kinase 1 signal transduction pathway. We found that MAPK8/JNK1 and MAPK9/JNK2 were not required for autophagy caused by starvation or MTOR inhibition in murine fibroblasts and epithelial cells. These data demonstrate that MAPK8/9 has no required role in starvation-induced autophagy. We conclude that the role of MAPK8/9 in autophagy may be context-dependent and more complex than previously considered.

Abbreviations: AKT: thymoma viral proto-oncogene;ALB: albumin; ATG4: autophagy related 4; BCL2: B cell leukemia/lymphoma 2; BECN1: beclin 1, autophagy related; BNIP3: BCL2/adenovirus E1B interacting protein 3; CQ: chloroquine diphosphate; DMEM: Dulbecco’s modified Eagle’s medium; EDTA: ethylenediaminetetraacetic acid; EBSS: Earle’s balanced salt solution; FBS: fetal bovine serum; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GFP: green fluorescent protein; HRAS: Harvey rat sarcoma virus oncogene; IgG: Immunoglobulin G; MAPK3/ERK1: mitogen-activated protein kinase 3; MAPK8/JNK1: mitogen-activated protein kinase 8; MAPK9/JNK2: mitogen-activated protein kinase 9; MAPK10/JNK3: mitogen-activated protein kinase 10; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MEFs: mouse embryonic fibroblasts; MTOR: mechanistic target of rapamycin kinase; RPS6KB1/p70: ribosomal protein S6 kinase, polypeptide 1; PPARA: peroxisome proliferator activated receptor alpha; SEM: standard error of the mean; SQSTM1/p62: sequestosome 1; TORC1: target of rapamycin complex 1; TORC2: target of rapamycin complex 2; TRP53: transforming related protein 53; TUBA: tubulin alpha; UV: ultraviolet; WT: wild-type

Activating transcription factor 3 is crucial for antitumor activity and to strengthen the antiviral properties of Onconase

Onconase is a ribonuclease that presents both antitumor and antiviral properties linked to its ribonucleolytic activity and represents a new class of RNA-damaging drugs. It has reached clinical trials for the treatment of several cancers and human papilloma virus warts. Onconase targets different RNAs in the cell cytosol but Onconase-treated cells present features that are different from a simple arrest of protein synthesis. We have used microarray-derived transcriptional profiling to identify Onconase-regulated genes in two ovarian cancer cell lines (NCI/ADR-RES and OVCAR-8). RT-qPCR analyses have confirmed the microarray findings. We have identified a network of up-regulated genes implicated in different signaling pathways that may explain the cytotoxic effects exerted by Onconase. Among these genes, activating transcription factor 3 (ATF3) plays a central role in the key events triggered by Onconase in treated cancer cells that finally lead to apoptosis. This mechanism, mediated by ATF3, is cell-type independent. Up-regulation of ATF3 may also explain the antiviral properties of this ribonuclease because this factor is involved in halting viral genome replication, keeping virus latency or preventing viral oncogenesis. Finally, Onconase-regulated genes are different from those affected by nuclear-directed ribonucleases.

RNase activity of sialic acid-binding lectin from bullfrog eggs drives antitumor effect via the activation of p38 MAPK to caspase-3/7 signaling pathway in human breast cancer cells

Sialic acid-binding lectin obtained from bullfrog eggs (SBL) induces cell death in cancer cells but not in normal cells. This antitumor effect is mediated through its ribo-nuclease (RNase) activity. However, the underlying molecular mechanisms remain unclear. We found that the p38 mitogen-activated protein kinase (MAPK) signaling pathway was activated when SBL induced cell death in three human breast cancer cell lines: SK-BR-3, MCF-7, and MDA-MB231. The suppression of p38 MAPK phosphorylation by a p38 MAPK inhibitor as well as short interference RNA knockdown of p38 MAPK expression significantly decreased cell death and increased the cell viability of SBL-treated MDA-MB231 cells. H103A, an SBL mutant lacking in RNase activity, showed decreased SBL-induced cell death compared with native SBL. However, the loss of RNase activity of SBL had no effect on its internalization into cells. The H103A mutant also displayed decreased phosphorylation of p38 MAPK. Moreover, SBL promoted caspase-3/7 activation followed by a cleavage of poly (ADP-ribose)-polymerase, whereas the SBL mutant, H103A, lost this ability. The SBL-induced caspase-3/7 activation was suppressed by the p38 MAPK inhibitor, SB203580, as well as pan-caspase inhibitor, zVAD-fmk. In the presence of zVAD-fmk, the SBL-induced cell death was decreased. In addition, the cell viability of SBL-treated MDA-MB231 cells recovered by zVAD-fmk treatment. Taken together, our results suggest that the RNase activity of SBL leads to breast cancer cell death through the activation of p38 MAPK followed by the activation of caspase-3/7.

Enteropathogenic Escherichia coli inhibits type I interferon- and RNase L-mediated host defense to disrupt intestinal epithelial cell barrier function

Enteropathogenic Escherichia coli (EPEC) primarily infects children in developing countries and causes diarrhea that can be deadly. EPEC pathogenesis occurs through type III secretion system (T3SS)-mediated injection of effectors into intestinal epithelial cells (IECs); these effectors alter actin dynamics, modulate the immune response, and disrupt tight junction (TJ) integrity. The resulting compromised barrier function and increased gastrointestinal (GI) permeability may be responsible for the clinical symptoms of infection. Type I interferon (IFN) mediates anti-inflammatory activities and serves essential functions in intestinal immunity and homeostasis; however, its role in the immune response to enteric pathogens, such as EPEC, and its impact on IEC barrier function have not been examined. Here, we report that IFN-β is induced following EPEC infection and regulates IEC TJ proteins to maintain barrier function. The EPEC T3SS effector NleD counteracts this protective activity by inhibiting IFN-β induction and enhancing tumor necrosis factor alpha to promote barrier disruption. The endoribonuclease RNase L is a key mediator of IFN induction and action that promotes TJ protein expression and IEC barrier integrity. EPEC infection inhibits RNase L in a T3SS-dependent manner, providing a mechanism by which EPEC evades IFN-induced antibacterial activities. This work identifies novel roles for IFN-β and RNase L in IEC barrier functions that are targeted by EPEC effectors to escape host defense mechanisms and promote virulence. The IFN-RNase L axis thus represents a potential therapeutic target for enteric infections and GI diseases involving compromised barrier function.

RC-6 ribonuclease induces caspase activation, cellular senescence and neuron-like morphology in NT2 embryonal carcinoma cells

Frog ribonucleases have been demonstrated to have anticancer activities. However, whether RC-6 ribonuclease exerts anticancer activity on human embryonal carcinoma cells remains unclear. In the present study, RC-6 induced cytotoxicity in NT2 cells (a human embryonal carcinoma cell line) and our studies showed that RC-6 can exert anticancer effects and induce caspase-9 and -3 activities. Moreover, to date, there is no evidence that frog ribonuclease-induced cytotoxicity effects are related to cellular senescence. Therefore, our studies showed that RC-6 can increase p16 and p21 protein levels and induce cellular senescence in NT2 cells. Notably, similar to retinoic acid-differentiated NT2 cells, neuron-like morphology was found on some remaining live cells after RC-6 treatment. In conclusion, our study is the first to demonstrate that RC-6 can induce cytotoxic effects, caspase-9/-3 activities, cellular senescence and neuron-like morphology in NT2 cells.

Sialic acid-binding lectin (leczyme) induces caspase-dependent apoptosis-mediated mitochondrial perturbation in Jurkat cells

Sialic acid binding lectin (SBL) isolated from Rana catesbeiana oocytes is a multifunctional protein which has lectin activity, ribonuclease activity and antitumor activity. However, the mechanism of antitumor effects of SBL is unclear to date and the validity for human leukemia cells has not been fully studied. We report here that SBL shows cytotoxicity for some human leukemia cell lines including multidrug-resistant (MDR) cells. The precise mechanisms of SBL-induced apoptotic signals were analyzed by combinational usage of specific caspase inhibitors and the mitochondrial membrane depolarization detector JC-1. It was demonstrated that SBL causes mitochondrial perturbation and the apoptotic signal is amplified by caspases and cell death is executed in a caspase-dependent manner. The efficacy of this combinational usage was shown for the first time, to distinguish the apoptotic pathway in detail. SBL selectively kills tumor cells, is able to exhibit cytotoxicity regardless of P-glycoprotein expression and has potential as an alternative to conventional DNA-damaging anticancer drugs.

Ranpirnase and its potential for the treatment of unresectable malignant mesothelioma

Ribonucleases are a superfamily of enzymes which operate at the crossroads of transcription and translation, catalyzing the degradation of RNA; they can be cytotoxic because the cleavage of RNA renders indecipherable its information. Ranpirnase is a novel ribonuclease which preferentially degrades tRNA, thus leading to inhibition of protein synthesis and, ultimately, to cytostasis and cytotoxicity. Ranpirnase has demonstrated antitumor activity both in vitro and in vivo in several tumor models. The maximum tolerated dose emerging from phase I studies was 960 g/m², with renal toxicity as the main dose-limiting toxicity. A large phase II trial showed that ranpirnase has disease-modifying activity against malignant mesothelioma. Ranpirnase proved to be superior to doxorubicin in a phase III trial, while preliminary results of another large, phase III trial, suggest that the combination of ranpirnase and doxorubicin could be more effective than doxorubicin alone. In all the above studies, ranpirnase seems to act mainly as a cytostatic rather than a cytotoxic drug, stabilizing progressive disease and potentially prolonging patients’ survival. Ranpirnase may thus find its niche in combination with doxorubicin for mesothelioma as a second-line therapy, where no standard of care presently exists.

A novel vascular disrupting agent plinabulin triggers JNK-mediated apoptosis and inhibits angiogenesis in multiple myeloma cells

Previous studies have established a role of vascular-disrupting agents as anti- cancer agents. Plinabulin is a novel vascular-disrupting agent that exhibits potent interruption of tumor blood flow because of the disruption of tumor vascular endothelial cells, resulting in tumor necrosis. In addition, plinabulin exerts a direct action on tumor cells, resulting in apoptosis. In the present study, we examined the anti-multiple myeloma (MM) activity of plinabulin. We show that low concentrations of plinabulin exhibit a potent antiangiogenic action on vascular endothelial cells. Importantly, plinabulin also induces apoptotic cell death in MM cell lines and tumor cells from patients with MM, associated with mitotic growth arrest. Plinabulin-induced apoptosis is mediated through activation of caspase-3, caspase-8, caspase-9, and poly(ADP-ribose) polymerase cleavage. Moreover, plinabulin triggered phosphorylation of stress response protein JNK, as a primary target, whereas blockade of JNK with a biochemical inhibitor or small interfering RNA strategy abrogated plinabulin-induced mitotic block or MM cell death. Finally, in vivo studies show that plinabulin was well tolerated and significantly inhibited tumor growth and prolonged survival in a human MM.1S plasmacytoma murine xenograft model. Our study therefore provides the rationale for clinical evaluation of plinabulin to improve patient outcome in MM.

JNK1 is required for lentivirus entry and gene transfer

Although a lot of progress has been made in development of lentiviral vectors for gene therapy, the interactions of these vectors with cellular factors have not been explored adequately. Here we show that lentivirus infection phosphorylates JNK and that blocking the kinase activity of JNK decreases gene transfer in a dose-dependent manner, regardless of the viral envelope glycoprotein. Knockdown by small interfering RNA (siRNA) revealed that JNK1 but not JNK2 was required for productive gene transfer. The effect of JNK on gene transfer was not due to changes in the cell cycle, as JNK knockdown did not affect the cell cycle profile of target cells and even increased cell proliferation. In addition, confluent cell monolayers also exhibited JNK phosphorylation upon lentivirus infection and a dose-dependent decrease in gene transfer efficiency upon JNK inhibition. On the other hand, JNK activation was necessary for lentivirus internalization into the cell cytoplasm, while inhibition of JNK activity decreased virus entry without affecting binding to the cell surface. These experiments suggest that JNK is required for lentivirus entry into target cells and may have implications for gene transfer or for development of antiviral agents.

A human ribonuclease induces apoptosis associated with p21WAF1/CIP1 induction and JNK inactivation

Background

Ribonucleases are promising agents for use in anticancer therapy. Among the different ribonucleases described to be cytotoxic, a paradigmatic example is onconase which manifests cytotoxic and cytostatic effects, presents synergism with several kinds of anticancer drugs and is currently in phase II/III of its clinical trial as an anticancer drug against different types of cancer. The mechanism of cytotoxicity of PE5, a variant of human pancreatic ribonuclease carrying a nuclear localization signal, has been investigated and compared to that of onconase.

Methods

Cytotoxicity was measured by the MTT method and by the tripan blue exclusion assay. Apoptosis was assessed by flow cytometry, caspase enzymatic detection and confocal microscopy. Cell cycle phase analysis was performed by flow cytometry. The expression of different proteins was analyzed by western blot.

Results

We show that the cytotoxicity of PE5 is produced through apoptosis, that it does not require the proapoptotic activity of p53 and is not prevented by the multiple drug resistance phenotype. We also show that PE5 and onconase induce cell death at the same extent although the latter is also able to arrest the cell growth. We have compared the cytotoxic effects of both ribonucleases in the NCI/ADR-RES cell line by measuring their effects on the cell cycle, on the activation of different caspases and on the expression of different apoptosis- and cell cycle-related proteins. PE5 increases the number of cells in S and G₂/M cell cycle phases, which is accompanied by the increased expression of cyclin E and p21^WAF1/CIP1 together with the underphosphorylation of p46 forms of JNK. Citotoxicity of onconase in this cell line does not alter the cell cycle phase distribution and it is accompanied by a decreased expression of XIAP

Conclusions

We conclude that PE5 kills the cells through apoptosis associated with the p21^WAF1/CIP1 induction and the inactivation of JNK. This mechanism is significantly different from that found for onconase.

Crystal structure of RNase A tandem enzymes and their interaction with the cytosolic ribonuclease inhibitor

Because of their ability to degrade RNA, RNases are potent cytotoxins. The cytotoxic activity of most members of the RNase A superfamily, however, is abolished by the cytosolic ribonuclease inhibitor (RI). RNase A tandem enzymes, in which two RNase A molecules are artificially connected by a peptide linker, and thus have a pseudodimeric structure, exhibit remarkable cytotoxic activity. In vitro, however, these enzymes are still inhibited by RI. Here, we present the crystal structures of three tandem enzymes with the linker sequences GPPG, SGSGSG, and SGRSGRSG, which allowed us to analyze the mode of binding of RI to the RNase A tandem enzymes. Modeling studies with the crystal structures of the RI-RNase A complex and the SGRSGRSG-RNase A tandem enzyme as templates suggested a 1 : 1 binding stoichiometry for the RI-RNase A tandem enzyme complex, with binding of the RI molecule to the N-terminal RNase A entity. These results were experimentally verified by analytical ultracentrifugation, quantitative electrophoresis, and proteolysis studies with trypsin. As other dimeric RNases, which are comparably cytotoxic, either evade RI binding or potentially even bind two RI molecules, inactivation by RI cannot be the crucial limitation to the cytotoxicity of dimeric RNases.

Cellular uptake of ribonuclease A relies on anionic glycans

Bovine pancreatic ribonuclease (RNase A) can enter human cells, even though it lacks a cognate cell-surface receptor protein. Here, we report on the biochemical basis for its cellular uptake. Analyses in vitro and in cellulo revealed that RNase A interacts tightly with abundant cell-surface proteoglycans containing glycosaminoglycans, such as heparan sulfate and chondroitin sulfate, as well as with sialic acid-containing glycoproteins. The uptake of RNase A correlates with cell anionicity, as quantified by measuring electrophoretic mobility. The cellular binding and uptake of RNase A contrast with those of Onconase, an amphibian homologue that does not interact tightly with anionic cell-surface glycans. As anionic glycans are especially abundant on human tumor cells, our data predicate utility for mammalian ribonucleases as cancer chemotherapeutic agents.

TAT-mediated delivery of a DNA repair enzyme to skin cells rapidly initiates repair of UV-induced DNA damage

UV light causes DNA damage in skin cells, leading to more than one million cases of non-melanoma skin cancer diagnosed annually in the United States. Although human cells possess a mechanism (nucleotide excision repair) to repair UV-induced DNA damage, mutagenesis still occurs when DNA is replicated before repair of these photoproducts. Although human cells have all the enzymes necessary to complete an alternate repair pathway, base excision repair (BER), they lack a DNA glycosylase that can initiate BER of dipyrimidine photoproducts. Certain prokaryotes and viruses produce pyrimidine dimer-specific DNA glycosylases (pdgs) that initiate BER of cyclobutane pyrimidine dimers (CPDs), the predominant UV-induced lesions. Such a pdg was identified in the Chlorella virus PBCV-1 and termed Cv-pdg. The Cv-pdg protein was engineered to contain a nuclear localization sequence (NLS) and a membrane permeabilization peptide (transcriptional transactivator, TAT). Here, we demonstrate that the Cv-pdg-NLS-TAT protein was delivered to repair-proficient keratinocytes and fibroblasts, and to a human skin model, where it rapidly initiated removal of CPDs. These data suggest a potential strategy for prevention of human skin cancer.

JNK phosphorylates beta-catenin and regulates adherens junctions

The c-Jun amino-terminal kinase (JNK) is an important player in inflammation, proliferation, and apoptosis. More recently, JNK was found to regulate cell migration by phosphorylating paxillin. Here, we report a novel role of JNK in cell adhesion. Specifically, we provide evidence that JNK binds to E-cadherin/beta-catenin complex and phosphorylates beta-catenin at serine 37 and threonine 41, the sites also phosphorylated by GSK-3beta. Inhibition of JNK kinase activity using dominant-negative constructs reduces phosphorylation of beta-catenin and promotes localization of E-cadherin/beta-catenin complex to cell-cell contact sites. Conversely, activation of JNK induces beta-catenin phosphorylation and disruption of cell contacts, which are prevented by JNK siRNA. We propose that JNK binds to beta-catenin and regulates formation of adherens junctions, ultimately controlling cell-to-cell adhesion.

Ribonucleases as novel chemotherapeutics : the ranpirnase example

Ranpirnase, a cytotoxic ribonuclease from the frog Rana pipiens, is the archetype of a novel class of cancer chemotherapeutic agents based on homologs and variants of bovine pancreatic ribonuclease (RNase A). Ranpirnase in combination with doxorubicin is in clinical trials for the treatment of unresectable malignant mesothelioma and other cancers. The putative mechanism for ranpirnase-mediated cytotoxicity involves binding to anionic components of the extracellular membrane, cytosolic internalization, and degradation of transfer RNA leading to apoptosis. The maintenance of ribonucleolytic activity in the presence of the cytosolic ribonuclease inhibitor protein is a key aspect of the cytotoxic activity of ranpirnase. The basis for its specific toxicity for cancer cells is not known. This review describes the development of ranpirnase as a cancer chemotherapeutic agent.

c-Jun NH2-terminal Kinase Promotes Apoptosis by Down-regulating the Transcriptional Co-repressor CtBP

Genetic knock out of the transcriptional co-repressor carboxyl-terminal-binding protein (CtBP) in mouse embryonic fibroblasts results in up-regulation of several genes involved in apoptosis. We predicted, therefore, that a propensity toward apoptosis might be regulated through changes in cellular CtBP levels. Previously, we have identified the homeodomain-interacting protein kinase 2 as such a regulator and demonstrated that HIPK2 activation causes Ser-422 phosphorylation and degradation of CtBP. In this study, we found that c-Jun NH2-terminal kinase 1 activation triggered CtBP phosphorylation on Ser-422 and subsequent degradation, inducing p53-independent apoptosis in human lung cancer cells. JNK1 has previously been linked to UV-directed apoptosis. Expression of MKK7-JNK1 or exposure to UV irradiation reduced cellular levels of CtBP via a proteasome-mediated pathway. This effect was prevented by JNK1 deficiency. In addition, sustained activation of the JNK1 pathway by cisplatin similarly triggered CtBP degradation. These findings provide a novel target for chemotherapy in cancers lacking p53.

On the track of antitumour ribonucleases

Ribonucleases (RNases) are potential alternatives to non-mutagenic antitumour drugs. Among these enzymes, onconase, bovine-seminal ribonuclease and the Rana catesbeiana and Rana japonica lectins exert a cytotoxic activity that is selective for tumour cells. A model for the mechanism of cytotoxicity of these RNases which involves different steps is generally accepted. The model predicts that cytotoxicity requires interaction of the RNases with the cell membrane and internalisation to occur by endocytosis. Then, at a precise point, the RNases are translocated to the cytosol where they cleave cellular RNA if they have been able to preserve their ribonucleolytic activity. The cleavage of cellular RNA induces apoptosis but there is evidence suggesting that RNase-triggered apoptosis does not entirely result from the inhibition of protein synthesis. How efficiently a particular RNase carries out each of the steps determines its potency as a cytotoxin.

An Apoptotic Signaling Pathway in the Interferon Antiviral Response Mediated by RNase L and c-Jun NH2-terminal Kinase

Cellular stress responses induced during viral infections are critical to the health and survival of organisms. In higher vertebrates, interferons (IFNs) mediate the innate antiviral response in part through the action of RNase L, a uniquely regulated enzyme. RNase L is activated by 5'-phosphorylated, 2'-5' oligoadenylates (2-5A) produced from IFN-inducible and double stranded RNA-dependent synthetases. We show that viral activation of the c-Jun NH2-terminal kinases (JNK) family of MAP kinases and viral induction of apoptosis are both deficient in mouse cells lacking RNase L. Also, JNK phosphorylation in response to 2-5A was greatly reduced in RNase L-/- mouse cells. In addition, 2-5A treatment of the human ovarian carcinoma cell line, Hey1b, resulted in specific ribosomal RNA cleavage products coinciding with JNK activation. Furthermore, suppression of JNK activity with the chemical inhibitor, SP600125, prevented apoptosis induced by 2-5A. In contrast, inhibition of alternative MAP kinases, p38 and ERK, failed to prevent 2-5A-mediated apoptosis. Short interfering RNA to JNK1/JNK2 mRNAs resulted in JNK ablation while also suppressing 2-5A-mediated apoptosis. Moreover, Jnk1-/- Jnk2-/- cells were highly resistant to the apoptotic effects of IFN and 2-5A. These findings suggest that JNK and RNase L function in an integrated signaling pathway during the IFN response that leads to elimination of virus-infected cells through apoptosis.

Requirement for SAPK-JNK signaling in the induction of apoptosis by ribosomal stress in REH lymphoid leukemia cells

The present studies examined performance of SAPK cascades and apoptotic commitment following ribosomal trauma in REH lymphoid leukemia cells. Ribostatic insults included disruption of ribosomal activity by mechanistically dissimilar agents such as blasticidin-S (BCS) (which binds 28S-rRNA to block peptidyl bond formation), kasugamycin (KSM) (which binds 18S-rRNA to prevent translational initiation), and cycloheximide (CHX) (which blocks A-site to P-site translocation of peptidyl-tRNA). Exposure of REH cells to BCS elicited DNA degradation and apoptotic cytolysis. BCS stimulated JNK1/JNK2 and p38, and their shared targets c-Jun and ATF2. Inhibition of JNK1/JNK2 (but not of p38) antagonized blasticidin-induced apoptosis, whereas targeting alternative ribosomal sites with KSM or CHX limited translation, but failed to activate the SAPK cascade or initiate apoptosis. Our findings indicate that interference with 28S-rRNA by BCS initiates apoptosis in REH cells through recruitment of SAPK-JNK signaling. Disparities between the lethal actions of BCS, KSM, and CHX appear to reflect established differences in the subribosomal targets of these agents. We propose that the SAPK cascade comprises an essential mechanism for the transduction of specific lethal stress signals emanating from active ribosomes, and that interference with the 28S-rRNA, rather than the peptidyl transfer center of the large subunit, is critical to apoptotic commitment.

Changing the net charge from negative to positive makes ribonuclease Sa cytotoxic

Ribonuclease Sa (pI = 3.5) from Streptomyces aureofaciens and its 3K (D1K, D17K, E41K) (pI = 6.4) and 5K (3K + D25K, E74K) (pI = 10.2) mutants were tested for cytotoxicity. The 5K mutant was cytotoxic to normal and v-ras-transformed NIH3T3 mouse fibroblasts, but RNase Sa and 3K were not. The structure, stability, and activity of the three proteins are comparable, but the net charge at pH 7 increases from -7 for RNase Sa to -1 for 3K and to +3 for 5K. These results suggest that a net positive charge is a key determinant of ribonuclease cytotoxicity. The cytotoxic 5K mutant preferentially attacks v-ras-NIH3T3 fibroblasts, suggesting that mammalian cells expressing the ras-oncogene are potential targets for ribonuclease-based drugs.

The UV (Ribotoxic) stress response of human keratinocytes involves the unexpected uncoupling of the Ras-extracellular signal-regulated kinase signaling cascade from the activated epidermal growth factor receptor

In mammals, UVB radiation is of biological relevance primarily for the cells of the epidermis. We report here the existence of a UVB response that is specific for proliferating human epidermal keratinocytes. Unlike other cell types that also display a UVB response, keratinocytes respond to UVB irradiation with a transient but potent downregulation of the Ras-extracellular signal-regulated kinase (ERK) signaling cascade. The downregulation of ERK precedes a profound decrease in the steady-state levels of cyclin D1, a mediator of the proliferative action of ERK. Keratinocytes exhibit high constitutive activity of the Ras-ERK signaling cascade even in culture medium lacking supplemental growth factors. The increased activity of Ras and phosphorylation of ERK in these cells are maintained by the autocrine production of secreted molecules that activate the epidermal growth factor receptor (EGFR). Irradiation of keratinocytes increases the phosphorylation of EGFR on tyrosine residues Y845, Y992, Y1045, Y1068, Y1086, Y1148, and Y1173 above the basal levels and leads to the increased recruitment of the adaptor proteins Grb2 and ShcA and of a p55 form of the regulatory subunit of the phosphatidylinositide 3-kinase to the UVB-activated EGFR. Paradoxically, however, UVB causes, at the same time, the inactivation of Ras and a subsequent dephosphorylation of ERK. By contrast, the signaling pathway leading from the activated EGFR to the phosphorylation of PKB/Akt1 is potentiated by UVB. The UVB response of keratinocytes appeared to be a manifestation of the more general ribotoxic stress response inasmuch as the transduction of the UVB-generated inhibitory signal to Ras and ERK required the presence of active ribosomes at the time of irradiation.