Nitric oxide promotes p53 nuclear retention and sensitizes neuroblastoma cells to apoptosis by ionizing radiation

Nitric oxide-induced resistance or sensitization to death in tumor cells

This report summarizes the present state of our knowledge pertaining to the NO-induced resistance or sensitization of tumor cell death. The effects of NO and its synergy with members of the TNF family, with cytotoxic drugs, and with ionizing radiations have been investigated. The dual effect of NO-induced resistance or sensitization and the underlying molecular mechanisms are discussed.

Influence of NO synthase inhibitor L-NAME on parasitemia and survival of Plasmodium berghei infected mice

Accelerated suicidal death or eryptosis of infected erythrocytes may delay development of parasitemia in malaria. Eryptosis is inhibited by nitric oxide (NO). The present study has been performed to explore, whether inhibition of NO synthase by L-NAME modifies the course of malaria. We show here that L-NAME (>or=10 microM) increased phosphatidylserine exposure of Plasmodium falciparum infected human erythrocytes, an effect significantly more marked than in noninfected human erythrocytes. We further show that parasitemia in Plasmodium berghei infected mice was significantly decreased (from 50% to 18% of circulating erythrocytes 20 days after infection) by addition of 1 mg/ml L-NAME to the drinking water. According to CFSE labelling L-NAME treatment accelerated the clearance of both, noninfected and infected, erythrocytes from circulating blood, but did not significantly extend the life span of infected animals. In conclusion, treatment with L-NAME shortens the life span of circulating erythrocytes and thus delays development of parasitemia during malaria.

Protein/DNA arrays identify nitric oxide-regulated cis-element and trans-factor activities some of which govern neuroblastoma cell viability

Toxic nitric oxide (NO) levels can regulate gene expression. Using a novel protein/DNA array, we show that toxic NO levels regulate the binding of trans-factors to various cis-elements in neuroblastoma cells, including CRE and those recognized by the transcription factors AP1, AP2, Brn-3a, EGR, E2F1 and SP1. Functionality of some of the cis-elements was confirmed by electro mobility shift and reporter assays. Interestingly, CREB, AP-1, Brn-3a, EGR and E2F1 can control mammalian cell viability. NO induced the anti-apoptotic Bcl-2 protein and its mRNA prior to the onset of death of 30-60% of the cells. Promoter analysis of the bcl-2 gene confirmed the involvement of a CRE in NO-dependent bcl-2 transcription. Neuroblastoma cells over-expressing bcl-2 became much more resistant to NO-induced apoptosis; conversely, Bcl-2 knockdown cells were rendered markedly more sensitive to NO. Together these results suggest that Bcl-2 counteracts NO-induced apoptosis in a fraction of the cell population. Thus, NO stimulates the binding of many trans-factors to their cognate cis-elements, some of which can regulate cell viability through transcriptional activation of target genes. Our results emphasize that a DNA/protein array approach can reveal novel, global transcription factor activities stimulated by cell death-regulating molecules.

Up-regulation of Fas (CD95) expression in tumour cells in vivo

Both the function and regulation of Fas expression in tumours is poorly understood. Our laboratory has reported that cultured, low Fas-expressing tumours undergo massive, yet reversible, up-regulation of cell surface Fas expression when injected into mice. The present study was aimed at determining what causes this enhanced Fas expression and whether the newly expressed Fas functions as a death receptor. Newly expressed Fas is indeed capable of inducing apoptosis. Based on our observation that Fas induction is reduced when tumour cells are injected into immune-deficient mice, we propose that Fas up-regulation in vivo involves the host's immune system. Accordingly, Fas up-regulation occurs in vitro when low Fas-expressing tumour cells are cocultured with lymphoid cells. Furthermore ascitic fluid extracted from tumour-bearing mice trigger Fas up-regulation in low Fas expressing tumours. This last finding suggests that a soluble factor(s) mediates induction of Fas expression. The best candidate for this soluble factor is nitric oxide (NO) based on the following observations: the factor in the ascites is unstable; Fas expression is induced to a lesser degree after injection into inducible NO synthase (NOS)-deficient (iNOS(-/-)) mice when compared to control mice; similarly, coculture with iNOS(-/-) splenocytes induces Fas less effectively than coculture with control splenocytes; and finally, the NO donor SNAP induces considerable Fas up-regulation in tumours in vitro. Our model is that host lymphoid cells in response to a tumour increase NO synthesis, which in turn causes enhanced Fas expression in the tumour.

Hyperthermic enhancement of tumor radiosensitization strategies

Local hyperthermia of living tissue can cause significant increases in blood flow and oxygenation depending on time-temperature history. Increases in perfusion of the abnormal and insufficient vasculature found in solid tumors may increase tumor oxygenation, thereby increasing the radiation sensitivity of the tumor. We hypothesized that local heating of tumor would increase the oxygenation of the tumor tissue and allow other oxygenating agents to further modify tumor oxygenation and radiation response. In the present study the effect of moderate temperature hyperthermia (MTH) at 41.5-42.5 degrees C for 30-60 min, 250 mg/kg nicotinamide, or carbogen breathing (95% O2/5% CO2) on the radiation sensitivity of FSaII murine fibrosarcomas or R3230 AC rat adenocarcinomas was studied. Individually, these treatments increased the tumor cell sensitivity to single dose 10-15 Gy X-irradiation by 1-5 fold on average, as measured by the in vivo/in vitro tumor excision assay. The combination of tumor MTH with nicotinamide or carbogen breathing increased the radiation sensitivity by 3-5 fold in FSaII tumors and 10-30 fold in R3230 tumors with varying levels of statistical significance. Finally, the triple combination of adjuvant MTH, nicotinamide and carbogen breathing increased the radiation-induced cell death in FSaII tumors to a similar extent as the dual combinations of MTH, nicotinamide or heat, carbogen breathing. However, in R3230 AC tumors the triple adjuvant combination significantly increased radiation-induced cell killing compared to all other dual adjuvant treatments (p < 0.04). To interrogate the mechanism by which heating alters tumor physiology, nitric oxide production in tumor and endothelial cells in culture and tumor tissue after heating was studied. Heating caused an increase in nitric oxide production over a 24 h period after treatment. Subsequently, inhibiting the enzymatic production of NO with L-NAME was found to increase heat-induced growth delay of FSaII tumors. The cause and effect of increased nitric oxide production and the response of the tumor vasculature to heat are discussed in the context of the tumor radiosensitization achieved by heating, carbogen breathing and nicotinamide.

Hyperubiquitylation of wild-type p53 contributes to cytoplasmic sequestration in neuroblastoma

Neuroblastoma (NB) is the most common solid malignancy in childhood and its prognosis is still generally poor. In contrast to many other cancers, mutations of the p53 tumor suppressor are rare. Instead, significant cytosolic sequestration of wtp53 is one of several mechanisms that attenuate p53 function in this cancer. Here, we report that aberrant p53 hyperubiquitylation contributes to p53 cytoplasmic sequestration in NB. NB lines constitutively harbor an elevated portion of wtp53 as stable ubiquitylated species confined to the cytoplasm. p53 hyperubiquitylation is not due to dysregulation by Hdm2 or proteasomal dysfunction. Instead, the defect lies in p53 regulation by HAUSP, a major p53-deubiquitylating enzyme. In contrast to non-NB cancer cells with nuclear p53 and normal ubiquitylation, p53 from NB cells shows impaired HAUSP interaction. Conversely, interference with p53 hyperubiquitylation in NB cells by Nutlin 3a or by a C-terminal p53 peptide (aa 305-393) results in p53 relocalization from the cytoplasm to the nucleus, and in case of Nutlin, in reactivation of p53's transcriptional and apoptotic functions. Moreover, nutlin and camptothecin act synergistically in inducing NB cell apoptosis. Hence, this study strengthens the rationale for targeting p53 deubiquitylation by drugs like Nutlin as a promising new strategy in NB therapy.

Biological fate and clinical implications of arginine metabolism in tissue healing

Since its discovery in 1987, many biological roles (including wound healing) have been identified for nitric oxide (NO). The gas is produced by NO synthase using the dibasic amino acid L-arginine as a substrate. It has been established that a lack of dietary L-arginine delays experimental wound healing. Arginine can also be metabolized to urea and ornithine by arginase-1, a pathway that generates L-proline, a substrate for collagen synthesis, and polyamines, which stimulate cellular proliferation. Herein, we review subjects of interest in arginine metabolism, with emphasis on the biochemistry of wound NO production, relative NO synthase isoform activity in healing wounds, cellular contributions to NO production, and NO effects and mechanisms of action in wound healing.

A combination of radiotherapy, nitric oxide and a hyperoxygenation sensitizing protocol for brain malignant tumor treatment

Brain malignant tumor such as glioblastoma is a challenging medical and surgical problem. In spite of surgery, radiotherapy and chemotherapy, the prognosis is still very poor. The limitations of currently available treatment modalities to cure or significantly prolong and improve the quality of life should stimulate rigorous research and studies to combat brain malignant tumors. While precision radiotherapy to reduce tumor size and ameliorate symptoms is still the standard of care, tumor sensitivity to radiation is compromised by low oxygen tensions and a necrotic tumor center. We propose to take advantage of the fact that elevated oxygen increases sensitivity of tumor cells to radiation. A specific application of hyperbaric oxygen (HBO(2)), using nitric oxide (NO) donors and inducers (such as L-arginine, dinitrite or tocopheryl succinate) and ascorbic acid to dilate blood vessels, should permit oxygen tensions in the range of 1000 mmHg to diffuse into the cells and thus increase sensitivity to radiation. This should permit doses that are low enough to cause the death of tumors cells yet minimize injury to brain tissue near the tumor and induced neurological sequelae.

Nitric oxide, a double edged sword in cancer biology: Searching for therapeutic opportunities

Nitric oxide (NO) is a pleiotropic molecule critical to a number of physiological and pathological processes. The last decade has witnessed major advances in dissecting NO biology and its role in cancer pathogenesis. However, the complexity of the interactions between different levels of NO and several aspects of tumor development/progression has led to apparently conflicting findings. Furthermore, both anti-NO and NO-based anticancer strategies appear effective in several preclinical models. This paradoxical dichotomy is leaving investigators with a double challenge: to determine the net impact of NO on cancer behavior and to define the therapeutic role of NO-centered anticancer strategies. Only a comprehensive and dynamic view of the cascade of molecular and cellular events underlying tumor biology and affected by NO will allow investigators to exploit the potential antitumor properties of drugs interfering with NO metabolism. Available data suggest that NO should be considered neither a universal target nor a magic bullet, but rather a signal transducer to be modulated according to the molecular makeup of each individual cancer and the interplay with conventional antineoplastic agents.

Cyclooxygenase inhibitors modulate the p53/HDM2 pathway and enhance chemotherapy-induced apoptosis in neuroblastoma

Cyclooxygenase-2 (COX-2) is upregulated in many tumors including neuroblastoma, and its overexpression has been implicated in resistance to p53-dependent apoptosis. Although p53 is rarely mutated in neuroblastoma, the p53 protein is rendered inactive via several mechanisms including sequestration in the cytoplasm. Here, we show that COX inhibitors inhibit the growth of neuroblastoma and when combined with low doses of chemotherapy, exert synergistic effects on neuroblastoma cells. Following COX inhibitor treatment, HDM2, which targets p53 for ubiquitin-mediated degradation, is downregulated, resulting in an attenuation of p53 ubiquitination and an increase in p53 half-life. The level of HDM2 phosphorylation at ser166, which influences both HDM2 and p53 subcellular distribution, is markedly diminished in response to COX inhibitors and is associated with increased p53 nuclear localization. Combining COX inhibitors with low-dose chemotherapy potentiates apoptosis and p53 stability, nuclear localization, and activity. p53 knockdown by siRNA resulted in the rescue of COX-inhibitor-treated cells, indicating that COX inhibitor-induced apoptosis is, at least in part, p53-dependent. Taken together, these results provide the first evidence that COX inhibitors enhance chemosensitivity in neuroblastoma via downregulating HDM2 and augmenting p53 stability and nuclear accumulation.

p73, the “Assistant” Guardian of the Genome?

Although p53 is clearly involved in the salvage pathway to DNA damage, its frequent mutations do not explain the efficacy of radiotherapy and chemotherapy. Indeed, around 50% of all human cancers show mutations in p53, and a further fraction show a functional inactivation of the protein. Nevertheless, patients seem to respond to therapy that would otherwise require a functional p53. At least in part, these responses could be explained by the pathway mediated by p73. This mechanism is parallel to, but independent of the p53 pathway. Several pieces of evidence show a significant interaction between these two proteins. Therefore, while p53 can be rightly defined as the guardian of the genome, we could think of p73 as the "assistant" guardian of the genome!

Regulation of p53 by activated protein kinase C-delta during nitric oxide-induced dopaminergic cell death

Selective cell death of dopaminergic neurons in the substantia nigra is the major cause of Parkinson disease. Current evidence suggests that this cell death could be mediated by nitric oxide by-products such as nitrate and peroxynitrite. Because protein kinase C (PKC)-delta is implicated in apoptosis of various cell types, we studied its roles and activation mechanisms in nitric oxide (NO)-induced apoptosis of SN4741 dopaminergic cells. When cells were treated with sodium nitroprusside (SNP), a NO donor, endogenous PKC-delta was nitrated and activated. Immunoprecipitation revealed that p53 co-immunoprecipitated with PKC-delta and was phosphorylated at the 15th serine residue in SNP-treated cells. An in vitro kinase assay revealed that p53 was directly phosphorylated by SNP-activated PKC-delta. The p53 Ser-15 phosphorylation was suppressed in SNP-treated cells when the NO-mediated activation of PKC-delta was inhibited by rottlerin or (-)-epigallocatechin gallate. Within 3 h of p53 phosphorylation, its protein levels increased because of decreased ubiquitin-dependent proteosomal proteolysis, whereas the protein levels of MDM2, ubiquitin-protein isopeptide ligase, were down-regulated in a p53 phosphorylation-dependent fashion. Taken together, these results demonstrate that nitration-mediated activation of PKC-delta induces the phosphorylation of the Ser-15 residue in p53, which increases its protein stability, thereby contributing to the nitric oxide-mediated apoptosis-like cell death pathway. These findings may be expanded to provide new insight into the cellular mechanisms of Parkinson disease.

Control of p53 nuclear accumulation in stressed cells

Wild-type p53 accumulates in the nucleus following stress. Current models suggest this nuclear accumulation involves phosphorylation at p53 N-terminal sites, and inhibition of murine double minute (MDM)2-dependent nuclear export. We monitored the effects of stress on MDM2-dependent nuclear export of wild-type p53 and a mutant lacking N-terminal phosphorylation sites. Etoposide and ionizing radiation inhibited nuclear export of wild-type p53 and the phosphor-mutant to comparable extents, indicating nuclear export inhibition does not require N-terminal phosphorylation. Cytoplasmic p53 accumulated in the nucleus of transfected cells treated with the nuclear export-inhibitor leptomycin B (LMB). Interestingly, LMB caused less p53 nuclear accumulation than stress treatment, suggesting stress-induced nuclear accumulation of p53 does not result solely from inhibited nuclear export.

Nitric Oxide–Induced Apoptosis in Lymphoblastoid and Fibroblast Cells Dependent on the Phosphorylation and Activation of p53

When nitric oxide (NO) is produced at micromolar concentrations, as during inflammation, exposure to surrounding cells is potentially cytotoxic. The NO-dependent signaling pathways that initiate cell death are thought to involve the tumor suppressor protein p53, but the degree to which this factor contributes to NO-induced cell death is less clear. Various reports either confirm or negate a role for p53 depending on the cell type and NO donor used. In this study, we have used several pairs of cell lines whose only differences are the presence or absence of p53, and we have treated these cell lines with the same NO donor, spermineNONOate (SPER/NO). Treatment with SPER/NO induced such apoptotic markers as DNA fragmentation, nuclear condensation, poly(ADP-ribose) polymerase cleavage, cytochrome c release, and Annexin V staining. p53 was required for at least 50% of SPER/NO-induced apoptotic cell death in human lymphoblastoid cells and for almost all in primary and E1A-tranformed mouse embryonic fibroblasts, which highlights the possible importance of DNA damage for apoptotic signaling in fibroblasts. In contrast, p53 did not play a significant role in NO-induced necrosis. NO treatment also induced the phosphorylation of p53 at Ser15; pretreatment with phosphoinositide-3 kinase (PI3K) family inhibitors, wortmannin, LY294002, and caffeine, blocked such phosphorylation, but the p38 mitogen-activated protein kinase inhibitor, SB203580, did not. Pretreatment with the PI3K family inhibitors also led to a switch from NO-induced apoptosis to necrosis, which implicates a PI3K-related kinase such as ataxia telangiectasia mutated (ATM) or ATR (ATM and Rad3 related) in p53-dependent NO-induced apoptosis.

High levels of endogenous nitric oxide produced after burn injury in rats arrest activated T lymphocytes in the first G1 phase of the cell cycle and then induce their apoptosis

Major physical traumas provoke a systemic inflammatory response and immune dysfunction. In a model of thermal injury in rats, we previously showed that an overproduction of nitric oxide (NO) was responsible for the collapse of lymphoproliferative responses. In the present work, we performed a time-course analysis of cell proliferation and cell death parameters in order to establish the sequence of events triggered by the high NO output in Wistar/Han rat splenocytes activated with Con A, 10 days after burn injury. We demonstrate that activated T cells from burned rats never divided whereas normal T cells underwent four division cycles. However, T cells from both burned and normal rat entered the G1 phase as shown by increase of cell size, mitochondria hyperpolarization, and expression of cyclin D1. Burned rat T cells progressed to the late G1 phase as shown by expression of the nuclear Ki-67 antigen, but they never entered the S phase. They underwent apoptosis as shown by morphological parameters, disruption of transmembrane mitochondrial potential, and DNA fragmentation. Persistent accumulation of the p53 protein accompanied these phenomena. NO synthase inhibitors antagonize alterations of cell proliferation and cell death parameters in burned rat T cells and accelerated p53 turnover.

The intimate relation between nitric oxide and superoxide in apoptosis and cell survival

Intra- and intercellular communication in or between cells allows adaptation to changes in the environment. Formation of reactive oxygen (ROS) and nitrogen (RNS) species in response to external insults gained considerable attention in provoking cell demise along an apoptotic subroute of cell death, thus attributing radical formation to pathologies. In close association, stabilization of the tumor suppressor p53 and activation of caspases convey proapoptotic signaling. Complexity was added with the notion that ROS and RNS signals overlap and/or produce synergistic as well as antagonistic effects. With respect to nitric oxide (NO) signaling, it became clear that the molecule is endowed with pro- or antiapoptotic signaling capabilities, depending to some extend on the concentration and cellular context, i.e., ROS generation. Here, some established concepts are summarized that allow an explanation of p53 accumulation under the impact of NO and an understanding of NO-evoked cell protection at the level of caspase inhibition, cyclic GMP formation, or expression of antiapoptotic proteins. In addition, the overlapping sphere of ROS and RNS signaling is recapitulated to appreciate cell physiology/pathology with the notion that marginal changes in the flux rates of either NO or superoxide may shift vital signals used for communication and cell survival into areas of pathology in close association with apoptosis/necrosis.

Nonresponsiveness of cerebral p53-MDM2 functional circuit in newborn rat pups rendered IUGR via uteroplacental insufficiency

Severe uteroplacental insufficiency causes cerebral apoptosis in the fetus. Moderate uteroplacental insufficiency causes intrauterine growth retardation (IUGR) and increases the risk of postnatal neurological morbidity. In the rat, uteroplacental insufficiency and IUGR affect cerebral gene expression of Bcl-2 and predispose the newborn IUGR rat toward cerebral apoptosis when challenged with perinatal hypoxia. Expression of Bcl-2, as well as the proapoptotic protein Bax, is regulated by p53. p53 also induces MDM2 transcription, which functions to limit further p53-induced apoptosis. The predisposition of the IUGR fetus toward cerebral apoptosis suggests that the p53-MDM2 "functional" circuit may be perturbed in the newborn IUGR rat brain. We hypothesized that MDM2 cerebral expression does not increase in response to increased p53 expression or increased levels of phospho-p53 (Ser15), an activated form of p53. To prove this hypothesis, we induced IUGR through bilateral uterine ligation of the pregnant rat. Uteroplacental insufficiency significantly increased p53 mRNA, total p53 protein, and phospho-p53 (Ser15) protein levels in the brain at term. Increased expression of phospho-p53 (Ser15) and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling-positive cells were localized to the CA1 region of the hippocampus, the subcortical and periventricular white matter, and the amygdala of the IUGR rat brain. In contrast, uteroplacental insufficiency decreased cerebral MDM2 mRNA and phospho-MDM2 (Ser166) protein levels in the IUGR rat pups. We conclude that the cerebral MDM2 response to increased p53 expression is not present in the newborn IUGR rat pup, and we speculate that this contributes to the predisposition of the IUGR fetus toward perinatal and long-term neurodevelopmental morbidities.

N/A

N/A

Nitric oxide-induced cell death of cerebrocortical murine astrocytes is mediated through p53- and Bax-dependent pathways

We have investigated the mechanism by which nitric oxide (NO) induces the death of mouse astrocytes. We show that NO (from donor diethylenetriamine-NO adduct) induces death with several features of apoptosis, including chromatin condensation, phosphatidylserine exposure on the outer leaflet of the plasma membrane, Bax translocation to the mitochondria and cytochrome c release, but no caspase activation or nuclear fragmentation is observed. Nitric oxide also elevates p53 expression, causing a concomitant increase in p53 serine 18 phosphorylation and p53 translocation from the cytoplasm to the nucleus. Activation of Bax and p53 is important for NO-induced apoptosis-like cell death because Bax- or p53-deficient astrocytes are much more resistant than wild-type cells to the same NO treatment. We further demonstrate that LY294002-sensitive kinases are responsible for controlling serine 18 phosphorylation of p53, thereby regulating the pro-apoptotic activity of p53 in astrocytes. While apoptosis is suppressed in the presence of LY294002, however, death by necrosis is increased, suggesting that LY294002-sensitive kinases additionally suppress a latent necrotic response to NO. We conclude that NO-induced death in astrocytes is mediated by p53- and Bax-dependent mechanisms, although full manifestation of apoptosis is aborted by concomitant inhibition of caspase activation. More generally, our data suggest that apoptotic mediators should be evaluated as the cause of cell death even in cases where a full apoptotic phenotype is lacking.

Regulation of the apoptosis-necrosis switch

Execution of the apoptotic program involves a relatively limited number of pathways. According to a general view, these would converge to activate the caspase family of proteases. However, there is increasing evidence that apoptotic-like features can also be found when caspases are inhibited. Moreover, under pathological conditions, apoptosis and nonapoptotic death paradigms are often interwined, which suggest that, in vivo, cells may use diverging execution pathways. Molecular switches between apoptosis and necrosis include adenosine triphosphate-dependent steps in the activation of caspases or steps sensitive to reactive oxygen/nitrogen species. In turn, caspase activation can cause necrosis by promoting ion overload.

The importance of p53 location: nuclear or cytoplasmic zip code?

The regulation of p53 functions is tightly controlled through several mechanisms including p53 transcription and translation, protein stability, post-translational modifications, and subcellular localization. Despite intensive study of p53, the regulation of p53 subcellular localization although important for its function is still poorly understood. The regulation of p53 localization depends on factors that influence its nuclear import and export, subnuclear localization and cytoplasmic tethering and sequestration. In this review, we will focus on various proteins and modifications that regulate the location and therefore the activity of p53. For example, MDM2 is the most important regulator of p53 nuclear export and degradation. Cytoplasmic p53 associates with the microtubule cytoskeleton and the dynein family of motor proteins; while Parc and mot2 are involved in its cytoplasmic sequestration. Finally, a portion of p53 is localized to the mitochondria as part of the non-transcriptional apoptotic response. In this review we strive to present the most recent data on how the activity of p53 is regulated by its location.

Nitric oxide and wound healing

Nitric oxide is a short-lived free radical that acts at the molecular, cellular, and physiologic level. Since its discovery almost 20 years ago it has proven itself as an important element in wound healing. This review highlights many of the important aspects of nitric oxide in wound healing, including a review of the basic biology of nitric oxide, its role as part of the cytokine cascade and as a promoter of angiogenesis, as well as its more recently elucidated role in apoptosis.

Nitric oxide activates diverse signaling pathways to regulate gene expression

Nitric oxide signaling is crucial for effecting long lasting changes in cells, including gene expression, cell cycle arrest, apoptosis, and differentiation. We have determined the temporal order of gene activation induced by NO in mammalian cells and have examined the signaling pathways that mediate the action of NO. Using microarrays to study the kinetics of gene activation by NO, we have determined that NO induces three distinct waves of gene activity. The first wave is induced within 30 min of exposure to NO and represents the primary gene targets of NO. It is followed by subsequent waves of gene activity that may reflect further cascades of NO-induced gene expression. We verified our results using quantitative real time PCR and further validated our conclusions about the effects of NO by using cytokines to induce endogenous NO production. We next applied pharmacological and genetic approaches to determine the signaling pathways that are used by NO to regulate gene expression. We used inhibitors of particular signaling pathways, as well as cells from animals with a deleted p53 gene, to define groups of genes that require phosphatidylinositol 3-kinase, protein kinase C, NF-kappaB, p53, or combinations thereof for activation by NO. Our results demonstrate that NO utilizes several independent signaling pathways to induce gene expression.