Epigenetics: The third pillar of nitric oxide signaling.

Nitric oxide (NO), the endogenously produced free radical signaling molecule, is generally thought to function via its interactions with heme-containing proteins, such as soluble guanylyl cyclase (sGC), or by the formation of protein adducts containing nitrogen oxide functional groups (such as S-nitrosothiols, 3-nitrotyrosine, and dinitrosyliron complexes). These two types of interactions result in a multitude of down-stream effects that regulate numerous functions in physiology and disease. Of the numerous purported NO signaling mechanisms, epigenetic regulation has gained considerable interest in recent years. There is now abundant experimental evidence to establish NO as an endogenous epigenetic regulator of gene expression and cell phenotype. Nitric oxide has been shown to influence key aspects of epigenetic regulation that include histone posttranslational modifications, DNA methylation, and microRNA levels. Studies across disease states have observed NO-mediated regulation of epigenetic protein expression and enzymatic activity resulting in remodeling of the epigenetic landscape to ultimately influence gene expression. In addition to the well-established pathways of NO signaling, epigenetic mechanisms may provide much-needed explanations for poorly understood context-specific effects of NO. These findings provide more insight into the molecular mechanisms of NO signaling and increase our ability to dissect its functional role(s) in specific micro-environments in health and disease. This review will summarize the current state of NO signaling via epigenetic mechanisms (the "third pillar" of NO signaling).