Nitrated fatty acids: from diet to disease
Introduction
The need for fatty acids goes far beyond the obligatory role of building blocks for fat and dietary sources for fuel in our bodies. The discovery of leukotrienes and prostaglandins, as enzymatic oxygenation products of unsaturated fatty acids, advanced the field demonstrating that fatty acid mediators are potent autocrine and paracrine signaling molecules. The technical advances provided by mass spectrometry-based approaches led to renewed interest and discoveries in this area of research. It has become clear that this large group of newly discovered bioactive molecules derived from polyunsaturated fatty acids executes physiological responses that are central to tissue homeostasis. Among them, NO2-FAs are formed that participate in anti-inflammatory and tissue protective actions through activation of Nrf2-dependent-signaling and concomitant inhibition of NF-κB-derived inflammation. The latest advances are provided in areas of NO2-FA formation and role in fibrotic disorders with a focus on unresolved inflammation, which appears to be a common feature of fibrosis across all tissues, and examining the signaling actions of NO2-FAs as a novel therapeutic.
Section snippets
Formation of nitro-fatty acids
Dietary interventions have been shown to be effective in a broad range of pathologies including diabetes and metabolic syndrome [1]. In addition to providing the nutrients to support metabolism, certain dietary components modulate metabolic processes. Moreover, these can be further metabolized and processed by the microbiome, providing a new set of modified molecules with unique signaling properties that regulate metabolism. These microbial transformations are important [2,3•], the microbiome
Stomach as a bioreactor
The acidic conditions of the stomach during digestion result in its protonation and the secondary generation of nitric oxide (NO) through reductive reactions, the nitrosating species dinitrogen trioxide (N2O3) and nitrating species nitrogen dioxide (NO2) [9]. Initial studies evaluating the intake effects of high doses of nitrite demonstrated the formation of these reactive species in the stomach to be responsible for the formation of tumorigenic nitrosamines [10], prompting stringent standards
Endogenous NO2-FA
While NO2-CLA detection is well established and its levels are modulated by diet and inflammation, the remaining classes of NO2-FAs still need to be explored [13••,14]. Nitro-oleic acid (NO2-OA), which has long been used as a surrogate to investigate the actions of endogenous NO2-FA [15], has been reported in the subnanomolar level in plasma [16] but is usually below the limit of detection and/or quantification. In contrast to NO2-CLA, NO2-OA and its metabolites are normally not detected in
Nitro-fatty acid reactivity
The interest in the physiology and pharmacology of NO2-FA is fueled by the investigation of their anti-inflammatory and protective actions reported in several preclinical animal models of disease by NO2-OA, which is used as a surrogate to interrogate the effects of endogenous NO2-FA [15,18]. NO2-FAs are electrophilic molecules that contain a nitroalkene group that reacts mainly with cysteines through Michael addition reactions [19]. While cysteine adducts are the most relevant to NO2-FA
Nitro-fatty acid signaling
The reversibility of the NO2-FA reaction is central to their pleiotropic signaling activity. While initially nuclear factor (erythroid-derived 2)-like 2 (Nrf2), heat shock response (HSR) activation and nuclear factor kappa B (NF-κB) inhibition were proposed as main drivers of their signaling mechanisms, emerging evidence reveals new pathways that are inhibited, specifically STING, epoxide hydrolase and angiotensin II receptor [23, 24, 25]. The current understanding of the signaling of NO2-FA
Complexities of fibrosis
Fibrosis is a complex and vital program to repair injured tissue. It occurs after repetitive insults to the epithelium and is defined by the accumulation of extracellular matrix (ECM) molecules such as collagen and fibronectin [30]. Under normal conditions that lead to wound healing following an injury, the fibrotic ECM is degraded, the epithelium is repaired and fibrosis is resolved. In a fibrotic state, however, the normal repair and resolution mechanisms are dysfunctional leading to scarring
The need for novel therapeutics: NO2-FA
Electrophilic drugs inhibit pro-inflammatory signaling mediators and have been shown to provide a valuable approach in several pre-clinical fibrosis models. NO2-OA and other electrophiles, such as dimethyl fumarate and 2-cyano-3,12-dioxoolean-1,9-dien-28-oic acid (CDDO), protect against kidney and pulmonary animal models of fibrosis [32, 33, 34, 35]. In cardiovascular disease, NO2-OA reverses hypoxia-induced right ventricular (RV) pressure and fibrotic RV remodeling in a pulmonary arterial
NF-κB inhibition
Under basal conditions, NF-κB is inactive as it is complexed with inhibitory κB (IκB)-α subunit, which contains a strong nuclear export signal, in the cytoplasm. In the canonical pathway, most stress signals activate NF-κB by controlling the redox-sensitive serine-specific IkB kinase (IKK). The activation of the IKK complex facilitates IκB-α phosphorylation (Ser-32 and Ser-36) by IκB kinase β (IKK-β) leading to IκB-α ubiquitination and proteasomal degradation resulting in NF-κB transactivation [
Nrf2 activation
NO2-FA derivatives activate Nrf2-dependent gene transcription by alkylating two functionally significant cysteine residues (Cys-273 and Cys-288) of Kelch-like ECH-associated protein (Keap)-1 in the cytoplasm. This inhibits Keap1-dependent Nrf2 degradation and results in nuclear translocation of de novo synthesized Nrf2 protein, binding to antioxidant response element and transactivation of gene signaling [41,42]. Nrf2-regulated genes are critical in protecting against oxidative stress, by
Crosstalk between Nrf2 and NF-κB pathways
Not only does Nrf2 activate antioxidant gene expression but Nrf2 directly limits inflammation by binding to promoter regions of pro-inflammatory cytokines [46]. Additionally, Nrf2 inhibits RNA polymerase II recruitment to the transcription start site of IL-6 and IL-1β genes without altering the required recruitment of NF-κB [46]. In this case, Nrf2 and NF-κB signaling appear to be acting individually. However, there is considerable crosstalk between these two redox-regulated transcription
Summary and perspective
Unresolved inflammation is the backbone of many diseases, especially during the progression to fibrosis. Electrophilic NO2-FAs have been proven safe in Phase I clinical trials and provide new dietary and pharmacological approaches to target inflammatory and fibrotic disorders by reversibly regulating signaling pathways. The decrease of inflammation and oxidative stress through Nrf2 and NF-κB pathways provides the conditions to return to homeostasis. The use of electrophiles is not free of
Conflict of interest statement
F.J.S. has financial interest in Complexa Inc. N.K.H.K. has no conflicts of interest to declare.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We would like to thank Ms. Sonia Salvatore for input and graphic design. This work was supported by R01-GM125944 and R01-DK112854 (F.J.S), AHA17GRN33660955 (F.J.S), P01-HL103455 (N.K.H.K.) and University of Pittsburgh Medical Center Competitive Medical Research Fund Award (N.K.H.K.).
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