Current Pharmaceutical Design - Volume 16, Issue 4, 2010

Nitric oxide (NO) is in itself a very simple and labile chemical entity, being one of the 10 smallest molecules known to exist in nature, whose half life is estimated to be in the range from a few seconds to a few tens of seconds in living organisms [1, 2]. From more than 20 years of research, NO has emerged as an important messenger molecule in almost every tissue and in particular in the cardiovascular, immune and nervous system [3-6]. The “physiologic” concentration of NO in tissues has been the matter of long debates and controversial opinions. A recent conclusion, based on extensive measurements and revision of literature data, proposed by one of the “fathers” of studies on NO kinetics, John Garthwaite, puts physiologic NO concentrations in the range 0.1-5 nM [7], orders of magnitude less than previous estimates. As the reader will easily appreciate from the collection of present contributions, this issue is extremely relevant to understand the role of NO in tumors because many of the dichotomous actions of this molecule, either beneficial or detrimental, are ascribed to differences in concentration, particularly when physiology turns into pathology. In tumors, indeed, NO can both promote or inhibit progression of growth and metastasis [8-10]. As recently highlighted [11], notwithstanding the complexity of its basic chemistry and the multiplicity of its biochemical actions, NO studies offer great opportunities both for better understanding of fundamental cellular mechanisms and for devising novel therapies. Papers comprised in the present issue constitute a significant step towards these goals as they not only address the state of the art of both consensus knowledge and controversial points, but also devise potential therapeutic strategies for the future. Another important aspect of these contributions is the fact that some of them adopt a comprehensive approach involving presentation and discussion of data derived from a multiplicity of different tumors and models while others deal with more focused examination of the role of NO in specific types of tumors. This allows the reader to better understand mechanisms and therapeutic prospects and to evaluate controversial issues stemming from observations made among different tumors or within the same class of tumors. The first paper of the collection, from Hickok and Thomas [12], highlights a host of often underestimated methodological problems and possible pitfalls of experimental design that explain how controversial results may be primarily derived from investigators not being fully aware of these problems. The complex relationship between different amounts of NOS protein expression in a given tumor and actual production of NO, for example, is heavily dependent on the dual role of oxygenation on NO bioavailability with high oxygen concentrations stimulating NO production while concomitantly accelerating its metabolic conversion and vice versa. Thus, NOS expression alone says little about NO concentration especially in hypoxic tissues like tumors. Down-stream targets of NO also have a complex relationship with NO concentration and duration of tissue exposure. Some of these targets are readily activated by very low concentrations while others are slowly activated by high concentrations with a multitude of others being activated at intermediate conditions. These, as well as the other methodological problems addressed, will not only help researchers make their experimental design more rigorous, but also constitute important caveats against being too confident when generalizing results and defining correlations between hypothetical NO concentrations and the phenotypic response of tumors. The essential feature of the paper of Ma and collaborators [13] is to offer a comprehensive survey of the literature concerning main actions of NO on tumor biology: proliferation, apoptosis, metastasis and angiogenesis. Particular emphasis is devoted to the signaling pathways promoted or regulated by NO, which underlie these different functions. This in depth examination allows to be highlighted two essential features of the complex relationships between NO and cancer. The first is the “double-edged sword” characteristic of NO which may exert in all these functions opposite effects depending on the concentration reached and on the signaling pathways regulated. The second is the fact that the enzymatic NOS isoform producing NO is not neutral with respect to the fact that the generated NO promotes one of the possible opposite effects on target cells. This apparently counterintuitive evidence has been repeatedly reported not only for cancer cells, but also in the pathophysiology of many tissues and is here brought back to the logical explanation that these opposite responses are the consequence of the different constitutive or substrate-regulated catalytic potencies of NOS isoforms, as well as of the different NO-regulated signaling pathways preferentially expressed in different cell types. Central to the contribution of Hirst and Robson [14] is to critically review what we know about NO in cancer therapy and what we can devise as future therapeutic strategies, with particular emphasis on the positive interactions between NO and chemotherapeutics. Besides exhaustively considering the results originating from many in vitro and in vivo studies based on the rationale of enhancing or inhibiting NO activity in tumors, the additional role of NO as chemotherapy adjuvant is considered. Review of literature data reveal many cases of increased anti-tumoral potency through combined administration of chemotherapeutics and NO-releasing procedures. Mechanisms of this positive interaction are probably multiple in nature and only poorly understood at present: among them, nitrosation reactions, potentiation of DNAdamaging properties of chemotherapeutics and attenuation of acquired chemoresistance are supported by some experimental evidence (see also reference 18). The exciting possibility to more efficaciously couple conventional chemotherapy to NO administration through appropriate conjugate chemistry is also addressed in the paper, which convincingly advocates for increasing efforts to combine conventional chemotherapy with NO-related chemistry from researchers and companies. The paper of Wang and Xie [15] addresses the role of NO in the pathogenesis, prevention and treatment of one of the most aggressive and intractable tumors, pancreatic cancer. Through a critical in depth evaluation of the literature, the authors masterly review the molecular basis of this tumor and the extensive amount of data that strongly link it to NO function and dysfunction. Interest is, in particular, focused on the inducible form of NOS and on its role in contrasting or favoring tumor growth and progression. The main endpoint of the survey is that also in this specific tumor NO action is of dual nature, antitumor activity being related to “physiologic” levels and promotion of tumor progression to deregulated NO production. An original explanation of this effect of NO overproduction is that it may determine a selective pressure on cells through both genetic and epigenetic mechanisms, thus ensuring in the tumor environment an advantage to malignant cells in terms of survival and growth. The level of knowledge on molecular interactions involving NO in pancreatic cancer, renders this tumor a promised candidate for the development of effective strategies targeting NO in cancer therapy. The paper of Siesjo [16] deals with the role of NO in another type of cancer characterized by high resistance to therapies and poor prognosis, glioma. As expected, actions of NO on glioma are multifarious spanning modulation of cell proliferation, invasiveness and immune response to tumor vasculogenesis and additive effects to chemotherapy and radiotherapy For this last point, it is interesting to recall the radiation-induced bystander effect that results in increased NO toxicity towards pre-irradiated cells and the fact that an additonal way through which NO may favor chemotherapy against glioma is its damaging action of blood brain barrier which could increase drug entry into the brain. Another important lesson that is derived from the study of this tumor, is the synergy between immunotherapy and induction of iNOS in glioma, which adds a further element, potentially important for future therapies, to the vast picture of possible interactions of NO with multiple conventional antitumoral approaches.

The role of nitric oxide (NO•) as a mediator of cancer phenotype has led researchers to investigate strategies for manipulating in vivo production and exogenous delivery of this molecule for therapeutic gain. Unfortunately, NO• serves multiple functions in cancer physiology. In some instances, NO• or nitric oxide synthase (NOS) levels correlate with tumor suppression and in other cases they are related to tumor progression and metastasis. Understanding this dichotomy has been a great challenge for researchers working in the field of NO• and cancer therapy. Due to the unique chemical and biochemical properties of NO•, it's interactions with cellular targets and the subsequent downstream signaling events can be vastly different based upon tumor heterogeneity and microenvironment. Simple explanations for the vast range of NO-correlated behaviors will continue to produce conflicting information about the relevance of NO• and cancer. Paying considerable attention to the chemical properties of NO• and the methodologies being used will remove many of the discrepancies in the field and allow for in depth understanding of when NO-based chemotherapeutics will have beneficial outcomes.

The action of L-arginine is mainly dependent on its end-product, nitric oxide (NO). The L-arginine/NO pathway has been confirmed to play an important role in tumor development. Recent findings indicate that NO derived from L-arginine could influence angiogenesis factors, vascular permeability, perivascular-cell recruitment and vessel remodeling and maturation. Additionally, the Larginine/ NO pathway could activate a broad array of genes that are functionally involved in proliferation, metastasis and apoptosis. Interestingly, this pathway plays roles in both tumorigensis and tumor killing. The role of the L-arginine/NO pathway in tumor therapy has been well-studied. Members of this pathway have been reported to be promising therapeutic molecules in tumor therapy. This review article summarizes research data on the roles of the L-arginine/NO pathway in cancer biology and cancer therapy.

Nitric oxide is a key second messenger in most tissues, where it is generated at low concentrations, predominantly by the catalytic action of two constitutively expressed isoforms of nitric oxide synthase (NOS). Both of these are found in tumours, but malignancy is also associated with the expression of high levels of the inducible isoform of NOS, which is responsible for generation of high NO´ concentrations, not associated with normal physiology. This has profound consequences for the aetiology and malignant progression of primary cancer and metastatic dissemination. It also ensures that tumour vasculature remains highly dilated, so maintaining the abnormally high growth rates, characteristic of malignant disease. This dependency on NO´ can be targeted therapeutically by administering NOS inhibitors to block NO´ production, so reducing the availability of metabolic substrates and slowing tumour growth. However, there is now clear evidence that the effects of NO´ in tumours are bimodal such that intermediate levels optimise tumour growth, and interventions to raise or lower NO´ concentrations can inhibit it. Concentrations in the high μM range generated by NOS gene therapy or NO´ donor drugs induce apoptosis in solid tumours in vivo and slow their growth dramatically. These interventions are also potent enhancers of the anticancer effects of cytotoxic chemotherapy, particularly with the anthracyclines and platinum compounds. There is also clear evidence for specificity against malignant compared with normal cells, associated with the specific generation of peroxynitrite. Recent clinical trials have demonstrated both the safety and efficacy of nitric oxide therapy against lung and prostate cancer.

Pancreatic cancer is hallmarked by aggressive biology and extreme lethality and very high mortality rates. The underlying molecular mechanism of its rapid development and progression is unclear, however. Recent identification and functional validation of nitric oxide (NO) production in pancreatic cancer suggest a role for NO in the pathogenesis of this disease. Conceivably, overproduction of NO imposes an adverse selection pressure on the tumor microenvironment, which causes genetic and epigenetic changes in tumor and tumor stromal cells and promotes the evolution of these cells into more malignant cells conferred with a tremendous survival and growth advantage. Designing effective strategies targeting NO to control pancreatic cancer development and progression requires a full understanding of the molecular mechanisms of signaling of NO production and action in the tumor microenvironment.

Malignant gliomas bear the most dismal prognosis of all human cancers despite the progress in therapy of many other tumors. The search for alternative and complementary treatments has therefore a high priority. Emerging knowledge of the dual and diverging role of nitric oxide in glioma biology has focused on possibilities to achieve anti-glioma effects by modulation of nitric oxide (NO) release and function in these tumors. NO has been shown to influence proliferation of glioma cells, vascular function in glioams, invasive capacity of gliomas, effects of chemo and radiotherapy and also immune reactivity against these tumors. The mechanisms behind the reported diverse and dual effects of NO in glioma biology are multiple. Some of the diversity can be explained by different experimental setups as in vitro versus in vivo models but the cellular sources, timing, absolute levels and gradients play a decisive role for the effects of NO on glioma biology. Current research in this field is hampered by the lack of inhibitors and donors approved for clinical use.

Nitric oxide (NO) exerts its function in several cell and organ compartments. Recently, several lines of evidence have been accrued showing that NO can play a critical role in oncogenesis. Here we summarize some of these findings and highlight the role of NO as a possible target for antineoplastic drugs. Specifically, NO appears to affect some aspects of neuronal tumour progression, particularly the chemoresistance phenotype, through inhibition of MYC activity and expression of a large set of ATP binding cassette transporters. Here we provide lines of evidence supporting the view that MYCN can alter expression of several members of the ABC transporter family thus influencing the chemoresistance phenotype of neuroblastoma cells. Furthermore, we show that increased intracellular NO concentration either through addition of NO donors to culture medium or through forced expression of nNOS in neuroblastoma cells leads to decreased expression of MYCN and ABC drug transporter genes. Overall, data reviewed here and novel results presented, unveil a NO-MYCN-ABC transporters axis with important implication on development and control of the chemoresistance phenotype in neuronal tumours.

Recent evidence suggests that nitric oxide (NO) has a remarkable anti-proliferative action towards dividing neural precursor cells as well as towards cells giving rise to neural-derived tumors. The present paper summarizes essential literature-derived information on this issue and provides novel experimental evidence for these NO-mediated actions regarding a well characterized population of neuronal precursors, the cerebellar granule cell precursors and a cell line of medulloblastoma, a pediatric tumor originating from these same precursor cells undergoing deregulated proliferation. Evidence is presented regarding the NO-mediated regulation of proliferation of neuronal precursor cells both during developmental and adult neurogenesis. Then, the role of NO in the control of proliferation of neural-derived tumor cells, such as PC12 and neuroblastoma cells, is discussed. Novel experimental data are provided documenting the anti-proliferative action of NO towards basal and mitogen-stimulated division of rat cerebellar granule cell precursors, as well as towards medulloblastoma DAOY cells. Finally, some molecular correlates of NO action on cell cycle regulation are discussed. Overall, the data presented and discussed here highlight similarities at the molecular level between physiologic processes regulating normal proliferation of neural precursors and pathologic deregulation of these processes leading to tumor formation.

Nitric oxide is a pleiotropic ancestral molecule, which elicits beneficial effect in many physiological settings but is also tenaciously expressed in numerous pathological conditions, particularly breast tumors. Nitric oxide is particularly harmful in adipogenic milieu of the breast, where it initiates and promotes tumorigenesis. Epidemiological studies have associated populations at a greater risk for developing breast cancer, predominantly estrogen receptor positive tumors, to express specific polymorphic forms of endothelial nitric oxide synthase, that produce sustained low levels of nitric oxide. Low sustained nitric oxide generates oxidative stress and inflammatory conditions at susceptible sites in the heterogeneous microenvironment of the breast, where it promotes cancer related events in specific cell types. Inflammatory conditions also stimulate inducible nitric oxide synthase expression, which dependent on the microenvironment, could promote or inhibit mammary tumors. In this review we re-examine the mechanisms by which nitric oxide promotes initiation and progression of breast cancer and address some of the controversies in the field.

Obesity in young people is a form of malnutrition that is found more and more in industrialized countries known both for its association with obesity in adult age and the chronic-degenerate pathologies correlated to it. In 1998 in the United States the prevalence of obesity in 4-12 year olds was 22% for the Hispanics and Afro-Americans and 12.3% for the non Hispanic Caucasians Also in children as in adults, the consequence of obesity is hyperinsulinemia, in direct proportion to the body weight. In this review we discuss the effects of hyperinsulinemia and obesity in young people, and evaluated the difference of cardiovascular complication in young and in adults.

The global impact of human immunodeficiency virus and acquired immunodeficiency syndrome (HIV/AIDS) is increasing and traditional preventative ‘safe sex’ strategies do not seem to be slowing the spread of this virus. With an efficacious vaccine at least a decade away, the only strategy to avoid the ever-increasing cost of highly active antiretroviral therapy (HAART) is to develop new methods that prevent virus transmission. Microbicides are topicallyapplied molecules that disrupt the HIV cycle and block infection. This review discusses the current state of the art in microbicide development, looking at the most clinically advanced microbicides and those at earlier development stages based on their mechanisms of action. The socioeconomic impact of microbicide use is also considered, as this will determine whether microbicides are taken up and used consistently by the target population.