Nitric Oxide Donors as Neuroprotective Agents after an Ischemic Stroke-Related Inflammatory Reaction

Figure 2

Cerebral ischemia-reperfusion activates two major signaling pathways which exert an effect on NOD. (A) NF-κB pathway. Oxidative stress (OS) and inflammatory stimuli phosphorylate NIK. Subsequently, IKK phosphorylates NIK, which in turn phosphorylates IkB, resulting in Iκb degradation and NF-κB translocation and activation. This action exerts different effects depending on the cell line. In endothelial cells, NF-κB promotes a proinflammatory phenotype, with the expression of cellular adhesion molecules and proinflammatory cytokines that induce leukocyte migration to the ischemic territory and promote acute inflammation. In glial cells, NF-κB leads to the expression of iNOS, COX-2, and proinflammatory cytokines. These effects contribute to nitrosative, oxidative, and inflammatory damage. Finally, in neurons, NF-κB induces the expression of apoptosis pathways. NOD can act at different levels in this pathway: NOD-derived NO diffuses across target cell membranes, where it is able to nitrosylate kinases located upstream of NF-κB, such as IKK, thereby suppressing their ability to phosphorylate [1]. This inhibition prevents Iκb phosphorylation and its degradation; thus the release of NF-κB. NO can also increase Iκb gene transcription and stabilize the complex formed by Iκb and NF-κB [2]. Furthermore, NOD-derived NO is capable of nitrosylating directly into the p50 and p65 subunits of NF-κB, which blocks their ability to migrate to the nucleus [3]. All of these actions prevent the nuclear translocation of NF-κB, therefore the expression of their target genes [4]. (B) Cerebral ischemia-reperfusion increases nNOS activity, which enhances its NO production. This NO can react with free radicals to produce ONOO⁻ and also activates the JNK3 pathway. The result is c-Jun phosphorylation and mitochondrial dysfunction, with an increase in Bcl2 phosphorylation and cytochrome C release into the cytoplasm. In addition, this activates caspase-3 and leads to neuronal apoptosis. NOD-derived NO downregulates neuronal apoptosis by inhibiting increased phosphorylation of JNK, c-Jun, and Bcl-2 [5]. This is achieved by S-nitrosylation of nNOS, which interferes with its NO production [6]. NO is also capable of nitrosylating caspase-3 directly [7]. All of these effects, along with an increase in CBF, reduce brain damage after the ischemia-reperfusion event. Bcl2, B-cell lymphoma 2; CBF, cerebral blood flow; COX-2, cyclooxygenase 2; Cytoc, cytochrome; ICAM, intercellular adhesion molecule; IkB, inhibitors of kB; IKK, Iκb kinase; IL-1β, interleukin 1 beta; iNOS, inducible nitric oxide synthase; JNK3, c-Jun N-terminal kinases 3; NF-κB, nuclear factor kappa B; NIK, NF-𝜅B-inducing kinase; nNOS, neuronal nitric oxide synthase; NO, nitric oxide; NOD, nitric oxide donors; ONOO⁻, peroxynitrite; TNF-α, tumor necrosis factor alpha; [n] S-nitrosylation.