oa Editorial [Hot topic: Targeting Nitric Oxide for Tumor Therapy (Executive Editor: Antonio Contestabile)]

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.