Vorinostat exhibits anticancer effects in triple-negative breast cancer cells by preventing nitric oxide-driven histone deacetylation.

Triple-negative breast cancers (TNBC) that produce nitric oxide (NO) are more aggressive, and the expression of the inducible form of nitric oxide synthase (NOS2) is a negative prognostic indicator. In these studies, we set out to investigate potential therapeutic strategies to counter the tumor-permissive properties of NO. We found that exposure to NO increased proliferation of TNBC cells and that treatment with the histone deacetylase inhibitor Vorinostat (SAHA) prevented this proliferation. When histone acetylation was measured in response to NO and/or SAHA, NO significantly decreased acetylation on histone 3 lysine 9 (H3K9ac) and SAHA increased H3K9ac. If NO and SAHA were sequentially administered to cells (in either order), an increase in acetylation was observed in all cases. Mechanistic studies suggest that the "deacetylase" activity of NO does not involve S-nitrosothiols or soluble guanylyl cyclase activation. The observed decrease in histone acetylation by NO required the interaction of NO with cellular iron pools and may be an overriding effect of NO-mediated increases in histone methylation at the same lysine residues. Our data revealed a novel pathway interaction of Vorinostat and provides new insight in therapeutic strategy for aggressive TNBCs.

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).

Nitric oxide inhibits FTO demethylase activity to regulate N6-methyladenosine mRNA methylation.

N6-methyladenosine (m6A) is the most abundant internal modification on eukaryotic mRNAs. Demethylation of m6A on mRNA is catalyzed by the enzyme fat mass and obesity-associated protein (FTO), a member of the nonheme Fe(II) and 2-oxoglutarate (2-OG)-dependent family of dioxygenases. FTO activity and m6A-mRNA are dysregulated in multiple diseases including cancers, yet endogenous signaling molecules that modulate FTO activity have not been identified. Here we show that nitric oxide (NO) is a potent inhibitor of FTO demethylase activity by directly binding to the catalytic iron center, which causes global m6A hypermethylation of mRNA in cells and results in gene-specific enrichment of m6A on mRNA of NO-regulated transcripts. Both cell culture and tumor xenograft models demonstrated that endogenous NO synthesis can regulate m6A-mRNA levels and transcriptional changes of m6A-associated genes. These results build a direct link between NO and m6A-mRNA regulation and reveal a novel signaling mechanism of NO as an endogenous regulator of the epitranscriptome.

Nitric oxide and hydrogen sulfide: Sibling rivalry in the family of epigenetic regulators.

Nitric oxide (NO) and hydrogen sulfide (H2S) were previously only known for their toxic properties. Now they are regarded as potent gaseous messenger molecules (gasotransmitters) that rapidly transverse cell membranes and transduce cellular signals through their chemical reactions and modifications to protein targets. Both are known to regulate numerous physiological functions including angiogenesis, vascular tone, and immune response, to name a few. NO and H2S often work synergistically and in competition to regulate each other's synthesis, target protein activity via posttranslational modifications (PTMs), and chemical interactions. In addition to their canonical modes of action, increasing evidence has demonstrated that NO and H2S share another signaling mechanism: epigenetic regulation. This review will compare and contrast biosynthesis and metabolism of NO and H2S, their individual and shared interactions, and the growing body of evidence for their roles as endogenous epigenetic regulatory molecules.