NO can stimulate pathways resulting in either cell growth or cell death, depending on the relative level of NO and a variety of associated factors [2]. In tumors, hyponitroxia is relative rather than absolute: low levels of NO (< 100 nM) [3] are produced by three NOS enzymes described above [4]) and associated with the oxidative burst of macrophages. At the low concentrations of NO found in tumors, NO mediates redox signaling pathways AZD2281 ic50 linked to the proangiogenic activities of vascular endothelial growth factor and inhibition of thrombospondin 1 [5], promoting malignant conversion, tumor progression [6], and resistance to therapy in multiple cancers including prostate

[7], colonic, lung [8], and mammary adenocarcinomas [8] and [9]. Other candidate

oncogenic functions of NO include cell proliferation, invasion and metastasis, and stem cell renewal [3]. Hyponitroxia thus represents a modified form of hormesis [10], a dose-response model characterized by a beneficial effect at low doses and a detrimental effect at high doses. NO also exerts a direct effect on responses to hypoxia through changes in expression of hypoxia inducible factor, alpha subunit Topoisomerase inhibitor (HIF-1α). Mimicking and attenuating hypoxia [11], NO drives HIF-1α signaling, by inhibition of prolyl hydroxylase 2 [12], resulting in a more aggressive and resistant phenotype (Figure 2). Hypoxia catalyzes the oncogenicity of NO: in addition to l-arginine, molecular oxygen is an essential substrate for the activity of NOSs, Casein kinase 1 and exposure to low-oxygen tension limits endogenous NO production by these enzymes [13] and [14]. However, in the absence of complete anoxia, a rare state even in tumors, NO synthesis is only inhibited rather than abrogated [14], resulting in the constitutive induction of the enzyme guanyl cyclase (GC) [15] and the accumulation of its downstream mitogenic effector cyclic guanosine monophosphate. S-nitrosylation of caspases, leading to their inactivation, has also been proposed as a mechanism by which NO can block apoptosis and result in tumorigenesis [16]. In addition, hypoxia also redirects macrophage l-arginine metabolism

from NOS to arginase [17], an enzyme that converts l-arginine to urea, leading to decreased arginine availability as a substrate for NO production. Thus, as an inactivating mechanism for endogenous NO production, hypoxia acts as a protumorigenic stimulus, potentiating the destructive potential of NO [18], separate from its effects on nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) [19] and HIF-dependent transcriptional pathways. However, the reverse is true as well: hyponitroxia exacerbates hypoxia through alterations in blood flow and oxygen consumption through NO mitochondria-mediated pathways [20] and [21]. Therefore, hypoxia and hyponitroxia are closely related and can affect a variety of downstream targets—either simultaneously or sequentially.