Nitric oxide and BCNU chemoresistance in C6 glioma cells: role of S-nitrosoglutathione

Protective Role of Natural Products in Glioblastoma Multiforme: A Focus on Nitric Oxide Pathway

In spite of therapeutic modalities such as surgical resection, chemotherapy, and radiotherapy, Glioblastoma Multiforme (GBM) remains an incurable fatal disease. This necessitates further therapeutic options that could enhance the efficacy of existing modalities. Nitric Oxide (NO), a short-lived small molecule, has been revealed to play a crucial role in the pathophysiology of GBM. Several studies have demonstrated that NO is involved in apoptosis, metastasis, cellular proliferation, angiogenesis, invasion, and many other processes implicated in GBM pathobiology. Herein, we elaborate on the role of NO as a therapeutic target in GBM and discuss some natural products affecting the NO signaling pathway.

Nitric Oxide as a Potential Adjuvant Therapeutic for Neuroblastoma: Effects of NO on Murine N2a Cells

Neuroblastoma, the most common extracranial solid tumor in children, accounts for 15% of all pediatric cancer deaths. Pharmaceutical applications of S-Nitrosylation, which, under normal conditions is involved with a host of epigenetic and embryological development pathways, have exhibited great potential for use as adjuvant therapeutics in the clinical management of cancer. Herein, an evaluation of the impact of nitric oxide (NO) as a potent anticancer agent on murine neuroblastoma cells is presented. Excitingly cell viability, colony formation, and non-carcinogenic cell analysis illustrate the significance and practicality of NO as a cytotoxic anticancer therapeutic. Resazurin, WST-8 (2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium, monosodium salt), and MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphyltetrazolium bromide) assays consistently displayed a moderate, ~20-25% reduction in cell viability after exposure to 1 mM S-Nitrosoglutathione (GSNO). A colony formation assay demonstrated that treated cells no longer exhibited colony formation capacity. Identically GSNO-treated Adult Human Dermal Fibroblasts (HDFa) exhibited no decrease in viability, indicating potential discrimination between neoplastic and normal cells. Collectively, our findings indicate a potential application for NO as an adjuvant therapeutic in the clinical management of neuroblastoma.

Improvement of conventional anti-cancer drugs as new tools against multidrug resistant tumors

Multidrug resistance (MDR) is the dominant cause of the failure of cancer chemotherapy. The design of antitumor drugs that are able to evade MDR is rapidly evolving, showing that this area of biomedical research attracts great interest in the scientific community. The current review explores promising recent approaches that have been developed with the aim of circumventing or overcoming MDR. Encouraging results have been obtained in the investigation of the MDR-modulating properties of various classes of natural compounds and their analogues. Inhibition of P-gp or downregulation of its expression have proven to be the main mechanisms by which MDR can be surmounted. The use of hybrid molecules that are able to simultaneously interact with two or more cancer cell targets is currently being explored as a means to circumvent drug resistance. This strategy is based on the design of hybrid compounds that are obtained either by merging the structural features of separate drugs, or by conjugating two drugs or pharmacophores via cleavable/non-cleavable linkers. The approach is highly promising due to the pharmacokinetic and pharmacodynamic advantages that can be achieved over the independent administration of the two individual components. However, it should be stressed that the task of obtaining successful multivalent drugs is a very challenging one. The conjugation of anticancer agents with nitric oxide (NO) donors has recently been developed, creating a particular class of hybrid that can combat tumor drug resistance. Appropriate NO donors have been shown to reverse drug resistance via nitration of ABC transporters and by interfering with a number of metabolic enzymes and signaling pathways. In fact, hybrid compounds that are produced by covalently attaching NO-donors and antitumor drugs have been shown to elicit a synergistic cytotoxic effect in a variety of drug resistant cancer cell lines. Another strategy to circumvent MDR is based on nanocarrier-mediated transport and the controlled release of chemotherapeutic drugs and P-gp inhibitors. Their pharmacokinetics are governed by the nanoparticle or polymer carrier and make use of the enhanced permeation and retention (EPR) effect, which can increase selective delivery to cancer cells. These systems are usually internalized by cancer cells via endocytosis and accumulate in endosomes and lysosomes, thus preventing rapid efflux. Other modalities to combat MDR are described in this review, including the pharmaco-modulation of acridine, which is a well-known scaffold in the development of bioactive compounds, the use of natural compounds as means to reverse MDR, and the conjugation of anticancer drugs with carriers that target specific tumor-cell components. Finally, the outstanding potential of in silico structure-based methods as a means to evaluate the ability of antitumor drugs to interact with drug transporters is also highlighted in this review. Structure-based design methods, which utilize 3D structural data of proteins and their complexes with ligands, are the most effective of the in silico methods available, as they provide a prediction regarding the interaction between transport proteins and their substrates and inhibitors. The recently resolved X-ray structure of human P-gp can help predict the interaction sites of designed compounds, providing insight into their binding mode and directing possible rational modifications to prevent them from becoming P-gp drug substrates. In summary, although major efforts were invested in the search for new tools to combat drug resistant tumors, they all require further implementation and methodological development. Further investigation and progress in the abovementioned strategies will provide significant advances in the rational combat against cancer MDR.

Synthetic 2-aryl-3-((piperidin-1-yl)ethyl)thiazolidin-4-ones exhibit selective in vitro antitumoral activity and inhibit cancer cell growth in a preclinical model of glioblastoma multiforme

Glioblastoma multiforme (GBM) is the worst form of primary brain tumor, which has a high rate of infiltration and resistance to radiation and chemotherapy, resulting in poor prognosis for patients. Recent studies show that thiazolidinones have a wide range of pharmacological properties including antimicrobial, anti-inflammatory, anti-oxidant and anti-tumor. Here, we investigate the effect antiglioma in vitro of a panel of sixteen synthetic 2-aryl-3-((piperidin-1-yl)ethyl)thiazolidin-4-ones where 13 of these decreased the viability of glioma cells 30-65% (100 μM) compared with controls. The most promising compounds such as 4d, 4l, 4m and 4p promoted glioma reduction of viability greater than 50%, were further tested at lower concentrations (12.5, 25, 50 and 100 μM). Also, the data showed that the compounds 4d, 4l, 4m and 4p induced cell death primarily through necrosis and late apoptosis mechanisms. Interestingly, none of these 2-aryl-3-((piperidin-1-yl)ethyl)thiazolidin-4-ones were cytotoxic for primary astrocytes, which were used as a non-transformed cell model, indicating selectivity. Our results also show that the treatment with sub-therapeutic doses of 2-aryl-3-((piperidin-1-yl)ethyl)thiazolidin-4-ones (4d, 4l and 4p) reduced in vivo glioma growth as well as malignant characteristics of implanted tumors such as intratumoral hemorrhage and peripheral pseudopalisading. Importantly, 2-aryl-3-((piperidin-1-yl)ethyl)thiazolidin-4-ones treatment did not induce mortality or peripheral damage to animals. Finally, 2-aryl-3-((piperidin-1-yl)ethyl)thiazolidin-4-ones also changed the nitric oxide metabolism which may be associated with reduced growth and malignity characteristics of gliomas. These data indicates for the first time the therapeutic potential of synthetic 2-aryl-3-((piperidin-1-yl)ethyl)thiazolidin-4-ones to GBM treatment.

β-galactosyl-pyrrolidinyl diazeniumdiolate: an efficient tool to investigate nitric oxide functions on promoting cell death

Nitric oxide (NO) is an active free radical gas that plays crucial roles in a broad range of biological processes. Extremely short half-life makes it difficult to use NO directly in research. It has been suggested that different concentrations of NO may lead to quite opposite results on cytotoxicity. However, the net effect of intracellular NO on tumor cell death has been controversial, partly because it is hard to precisely control the amount of NO generated exclusively within the target cells. Therefore, we have developed a cell-specific NO donor, β-galactosyl-pyrrolidinyl diazeniumdiolate (β-Gal-NONOate), in hopes of simulating the actual effects of intracellularly derived NO on the patterns of cell death as well as investigating its underlying mechanisms. In this study, by using three different tumor cell models, we showed that β-Gal-NONOate could steadily transport NO into the target cells with similar delivery efficiencies and exerted a determinative effect on cell death. In addition, β-Gal-NONOate-derived intracellular NO could provoke both apoptosis and necrosis in a concentration-dependent manner. While lower NO concentration primarily induced apoptosis, higher NO concentration mainly triggered necrosis. Moreover, the intrinsic apoptotic pathway, characterized by rapid Ca²⁺ overload and subsequent mitochondrial damage, was the collective mechanism responsible for the apoptotic death in all the three tumor cell lines. Taken together, since this cell-specifically derived NO is cheap to use and easy to quantify, β-Gal-NONOate might be used as a novel and ideal tool to standardize intracellular NO generation and evaluate its net effects in different cellular and experimental settings in the coming future.

Pro-apoptotic role of integrin β3 in glioma cells

Malignant gliomas are the most destructive type of brain cancer. In order to gain a better understanding of the molecular mechanisms of glioma cell death and survival, we previously established an alkylating agent 1, 3-bis(2-chloroethyl)-1-nitrosourea (BCNU)-resistant variant of C6 rat glioma cells. Proteomic analysis indicated a significant down-regulation of integrin beta 3 (ITGB3) in the BCNU-resistant C6R cells. Re-expression of ITGB3 in C6R cells restored the BCNU sensitivity. In U87MG, U373MG, and T98G human glioma cells, there was a positive correlation between ITGB3 expression and the sensitivity to BCNU and etoposide, suggesting an important role of ITGB3 in glioma cell death. Over-expression of ITGB3 cDNA significantly increased the sensitivity of the human glioma cells to the anticancer drug-induced apoptosis. Nitric oxide showed an additive effect on the anticancer drug-induced glioma cell death by increasing ITGB3 expression. Subsequent dissection of signaling pathways indicated that extracellular signal-regulated kinase and unligated integrin-mediated cell death pathway may be involved in the pro-apoptotic role of ITGB3 in glioma cells. These results implicate ITGB3 in glioma cell death/survival and drug resistance.

HDMX regulates p53 activity and confers chemoresistance to 3-bis(2-chloroethyl)-1-nitrosourea

Glioblastoma multiforme (GBM) is one of the deadliest tumors afflicting humans, and the mechanisms of its onset and progression remain largely undefined. Our attempts to elucidate its molecular pathogenesis through DNA copy-number analysis by genome-wide digital karyotyping and single nucleotide polymorphism arrays identified a dramatic focal amplification on chromosome 1q32 in 4 of 57 GBM tumors. Quantitative real-time PCR measurements revealed that HDMX is the most commonly amplified and overexpressed gene in the 1q32 locus. Further genetic screening of 284 low- and high-grade gliomas revealed that HDMX amplifications occur solely in pediatric and adult GBMs and that they are mutually exclusive of TP53 mutations and MDM2 amplifications. Here, we demonstrate that HDMX regulates p53 to promote GBM growth and attenuates tumor response to chemotherapy. In GBM cells, HDMX overexpression inhibits p53-mediated transcriptional activation of p21, releases cells from G0 to G1 phase, and enhances cellular proliferation. HDMX overexpression does not affect the expression of PUMA and BAX proapoptotic genes. While in GBM cells treated with the chemotherapeutic agent 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), HDMX appears to stabilize p53 and promote phosphorylation of the DNA double-stranded break repair protein H2AX, up-regulate the DNA repair gene VPX, stimulate DNA repair, and confer resistance to BCNU. In summary, HDMX exhibits bona fide oncogenic properties and offers a promising molecular target for GBM therapeutic intervention.

Targeting nitric oxide for cancer therapy

A blueprint for the ideal anticancer molecule would include most of the properties of nitric oxide (NO•), but the ability to exploit these characteristics in a therapeutic setting requires a detailed understanding of the biology and biochemistry of the molecule. These properties include the ability of NO• to affect tumour angiogenesis, metastasis, blood flow and immuno surveillance. Furthermore NO• also has the potential to enhance both radio- and chemotherapy. However, all of these strategies are dependent on achieving appropriate levels of NO•, since endogenous levels of NO• appear to have a clear role in tumour progression. This review aims to summarize the role of NO• in cancer with particular emphasis on how the properties of NO• can be exploited for therapy.

Protective effects of lipopolysaccharide preconditioning against nitric oxide neurotoxicity

We have characterized lipopolysaccharide (LPS) preconditioning-induced neuroprotective mechanisms against nitric oxide (NO) toxicity. Pretreatment of rat cortical cultures with LPS attenuated neurotoxicity of NO donors, including sodium nitroprusside (SNP) and diethylamine NONOate (NONOate). A transiently increased expression of endothelial nitric oxide synthase (eNOS) accompanied by an increase in NO production was observed during LPS preconditioning. Application of NOS inhibitors including L-N(5)-(1-iminoethyl)-ornithine (L-NIO) and L-nitroarginine methylester (L-NAME) abolished LPS-dependent protection against SNP toxicity. The LPS effect was also blocked by KT5823, an inhibitor of cGMP-dependent protein kinase (PKG). Consistently, application of 8-bromo-cyclic GMP (8-Br-cGMP), a slowly degradable cGMP analogue capable of PKG activation, was neuroprotective. LPS preconditioning resulted in a heightened neuronal expression of Bcl-2 protein that was abolished by L-NAME and KT5823, the respective inhibitors of NOS and PKG. Together, our results reveal the signaling cascade of "LPS --> eNOS --> NO --> cGMP/PKG --> Bcl-2" that might have contributed to the LPS protective effects in cortical neurons.

Enteric glia regulate intestinal barrier function and inflammation via release of S-nitrosoglutathione

BACKGROUND & AIMS

Barrier functions across epithelia and endothelia are essential for homeostatic tissue regulation. Astroglia interact with cerebral endothelia to maintain the blood-brain barrier. Whether similar interactions between astrocyte-like enteric glia and epithelia regulate intestinal barrier function is not known.

METHODS

Fluorescent permeability markers were used to measure intestinal barrier function in vivo after conditional ablation of enteric glia in transgenic mice. Enteric glial cell regulation of epithelial barrier integrity then was modeled in vitro using coculture. Glial-derived barrier-inducing factors were characterized using size-exclusion chromatography and mass spectrometry. Epithelial barrier integrity was assessed by transepithelial resistance readings and by quantitative measurement of tight-junction-associated protein expression by quantitative polymerase chain reaction and Western blot.

RESULTS

We show that ablation of enteric glial cells in transgenic mice causes intestinal mucosal barrier dysfunction, resulting in inflammation. Glial-derived s-nitrosoglutathione (GSNO) was identified as a potent inducer of mucosal barrier function in vitro and in vivo and of attenuated tissue inflammation after ablation of enteric glia in transgenic mice. GSNO regulation of mucosal barrier function was associated directly with an increased expression of perijunctional F-actin and tight-junction-associated proteins zonula occludens-1 and occludin. GSNO also significantly restored mucosal barrier function in colonic biopsy specimens from patients with Crohn's disease, a well-described inflammatory permeability disorder associated with enteric glial-cell disruption.

CONCLUSIONS

Enteric glia therefore share the ability of astrocytes to regulate tight-junction integrity, and cellular interactions comparable with those maintaining blood-brain barrier function also regulate epithelial permeability at mucosal surfaces.

Nitric oxide donors attenuate clongenic potential in rat C6 glioma cells treated with alkylating chemotherapeutic agents

1,3-Bis(2-chloroethyl)-1-nitrosourea (BCNU) kills tumor cells via multiple actions including alkylation and carbamoylation. Previously, we have reported that formation of S-nitrosoglutathione (GSNO) in glioma cells overexpressing inducible nitric oxide synthase (iNOS) contributed to nitric oxide (NO)-dependent carbamoylating chemoresistance against BCNU. To further characterize the effects of NO on alkylating cytotoxicity, colony formation assay was applied to evaluate the effects of various NO donors on rat C6 glioma cells challenged with alkylating agents. We demonstrate that NO donors including GSNO, diethylamine NONOate (DEA/NO), and sodium nitroprusside (SNP) substantially reduced the extent of colony formation in glioma cells treated with alkylating agents, namely methyl methanesulfonate (MMS), N-methyl-N-nitrosourea (MNU), and N-ethyl-N-nitrosourea (ENU). Without alkylating agents these NO-releasing agents alone had no effects on clongenic potential of rat C6 glioma cells. Among these three NO donors used, the effectiveness in potentiating alkylating cytotoxicity is in the order of "GSNO>DEA/NO>SNP" when applied at the same dosages. GSNO also exerted similar synergistic actions reducing the extents of colony formation when co-administrated with 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-hydrazine (compound #1), another alkylating agent that mimics the chloroethylating action of BCNU. Together with our previous findings, we propose that NO donors may be used as adjunct chemotherapy with alkylating agents for such malignant brain tumors as glioblastoma multiforme (GBM). In contrast, production of NO as a result of iNOS induction, such as that occurring after surgical resection of brain tumors, may compromise the efficacy of carbamoylating chemotherapy.

S-nitrosoglutathione and hypoxia-inducible factor-1 confer chemoresistance against carbamoylating cytotoxicity of BCNU in rat C6 glioma cells

BCNU (1,3-bis[2-chloroethyl]-1-nitrosourea) is the mainstay in glioblastoma multiform chemotherapy with only minimal effects. BCNU may kill tumor cells via carbamoylating cytotoxicity, which irreversibly inhibits glutathione reductase with resultant accumulation of oxidized form of glutathione causing oxidative stress. S-nitrosoglutathione (GSNO) is a product of glutathione and nitric oxide interaction. We report that GSNO formation may underlie carbamoylating chemoresistance mediated by activation of inducible nitric oxide synthase. Transactivation of hypoxia-inducible factor-1 (HIF-1)-responsive genes reduces oxidative stress caused by glutathione depletion. We also noted that preconditioning of C6 glioma cells to induce HIF-1 and its downstream genes confers chemoresistance against carbamoylating cytotoxicity of BCNU.

Nitric oxide donor drugs: an update on pathophysiology and therapeutic potential

The discovery of the multiple physiological and pathophysiological processes in which nitric oxide (NO) is involved has promoted a great number of pharmacological researches to develop new drugs that are capable of influencing NO production directly and/or indirectly for therapeutic purposes (i.e, NO-releasing drugs, NO-inhibiting drugs, and phosphodiesterase V inhibitors). In particular, the so-called NO donor drugs could actually have an important therapeutic effect in the treatment of many diseases such as arteriopathies (atherosclerosis and its sequelae, arterial hypertension and some forms of male sexual impotence), various acute and chronic inflammatory conditions (colitis, rheumatoid arthritis and tissue remodelling), and several degenerative diseases (Alzheimer's disease and cancer). The old organic nitrates show some well-known pitfalls including the induction of tolerance and acute side effects related to abrupt vasodilation such as cephalea and hypotension, which limit their therapeutic indications. A low therapeutic index (i.e., peroxynitrite toxicity) has always characterised the sydnonimines class. A series of interesting new classes of NO donors are under intense pharmacological investigation and scrutiny (S-nitrosothiols, diazeniumdiolates and NO hybrid drugs), each characterised by a particular pharmacokinetic and pharmacodynamic profile. The most important obstacle in the field of NO donor drugs is represented by the difficulty in targeting NO release, and thereby its effects, to a particular tissue.

Protective effects of S-nitrosoglutathione against amyloid beta-peptide neurotoxicity

Amyloid beta-peptide (Abeta) is a major constituent of senile plaques in the brains of Alzheimer's disease (AD) patients. We have previously demonstrated ceramide production secondary to Abeta-induced activation of neutral sphingomyelinase (nSMase) in cerebral endothelial cells and oligodendrocytes, which may contribute to cellular injury during progression of AD. In this study, we first established the "Abeta --> nSMase --> ceramide --> free radical --> cell death" pathway in primary cultures of fetal rat cortical neurons. We also provided experimental evidence showing that S-nitrosoglutathione (GSNO), a potent endogenous antioxidant derived from the interaction between nitric oxide (NO) and glutathione, caused dose-dependent protective effects against Abeta/ceramide neurotoxicity via inhibition of caspase activation and production of reactive oxygen species (ROS). This GSNO-mediated neuroprotection appeared to involve activation of cGMP-dependent protein kinase (PKG), phosphatidylinositol 3-kinase (PI3K), and extracellular signal-regulated kinase (ERK). Activation of the cGMP/PKG pathway induced expression of thioredoxin and Bcl-2 that were beneficial to cortical neurons in antagonizing Abeta/ceramide toxicity. Consistently, exogenous application of thioredoxin exerted remarkable neuroprotective efficacy in our experimental paradigm. Results derived from the present study establish a neuroprotective role of GSNO, an endogenous NO carrier, against Abeta toxicity via multiple signaling pathways.