Role of nitric oxide in pathogenesis of tumor growth and its possible application in cancer treatment.

In this review, the role of nitric oxide (NO) in the pathogenesis of the tumor growth and possibilities of its application in the treatment of cancer patients are analyzed. NO is one of the most important mediators of physiological processes being involved in the regulation of practically all body functions in health and disease. The role of NO in the development of many pathological conditions has been extensively studied and debated in recent years. Today it is clear that NO in relation to malignant tumors may exhibit a dual activity - can stimulate tumor growth and cause an opposite antitumor effect. Effects of NO are mostly dependent on its concentration. At low concentrations, NO could inhibit apoptosis and cause mutations that potentially lead to the formation of malignant growth loci. However, a high concentration of NO appears to be detrimental to malignant cells, in particular under conditions of simultaneous exposure to ionizing radiation. In humans, the inducible NO synthase (iNOS, type II) is the most powerful form of NO synthases (NOS) and has the ability to synthesize large amounts of NO for a long time and exert a protective function. iNOS is expressed in macrophages, monocytes, neutrophils, fibroblasts, hepatocytes, and other cell types. In tissue of malignant tumors, the macrophagal iNOS is the main form. Experimental data provide an evidence that activated macrophages and leukocytes, which are the part of peritumorous inflammatory infiltrate, can provide radiosensitization of tumors by direct synthesis of NO and indirectly - through the secretion of cytokines stimulating iNOS activity in cancer cells. Such approach could be useful for the development of new schemes and methods of anticancer therapy based on the activation of endogenous NO biosynthesis pathways.