Covalent Peroxisome Proliferator-activated Receptor γ Adduction by Nitro-fatty Acids

Abstract

The peroxisome proliferator-activated receptor-␥ (PPAR␥) binds diverse ligands to transcriptionally regulate metabolism and inflammation. Activators of PPAR␥ include lipids and antihyperglycemic drugs such as thiazolidinediones (TZDsThe rapidly expanding global burden of type II diabetes mellitus (DM) 3 and the concomitant increased risk for cardiovascular disease (1, 2) have motivated better understanding of relevant cell signaling pathways and potential therapeutic strategies. One major characteristic of metabolic syndrome and DM is insulin resistance, leading to hyperglycemia and dyslipidemia. Following initial clinical use of TZDs as anti-hyperglycemic agents to treat DM in the late 1990s, the nuclear receptor PPAR␥ was discovered as their molecular target. This receptor is expressed primarily in adipose tissue, muscle, and macrophages, where it regulates glucose uptake, lipid metabolism/ storage, and cell proliferation/differentiation (3-5). Thus, PPAR␥ ligands and allied downstream signaling events play a pivotal role in both the development and treatment of DM (6, 7). This is underscored by the observation that mutations in the C-terminal helix 12 of the ligand-binding domain (LBD) of PPAR␥ (e.g. P467L or V290M) are linked with severe insulin resistance and the onset of juvenile DM (8).The oxidizing inflammatory milieu contributing to the pathogenesis of obesity, diabetes, and cardiovascular disease also promotes diverse biomolecule oxidation, nitrosation, and nitration reactions by O 2 and ⅐ NO-derived species. Although oxidized fatty acids typically propagate proinflammatory conditions, the recently detected class of NO 2 -FA act as anti-inflammatory mediators. Nitroalkene derivatives of oleic acid (OA-NO 2 ) and linoleic acid (LNO 2 ) have been detected in healthy human blood and murine cardiac tissue. The levels of free/unesterified OA-NO 2 are ϳ1-3 nM in human plasma (9, 10), with OA-NO 2 produced at increased rates and present at higher concentrations during inflammatory and metabolic stress (11-13). The signaling actions of NO 2 -FA are primarily ascribed to the electrophilic olefinic carbon situated ␤ to the electron-withdrawing NO 2 substituent, facilitating kinetically rapid and reversible Michael addition with nucleophilic amino acids (i.e. Cys and His) (14). NO 2 -FA adduction of proteins and GSH occurs in model systems and clinically, with this reaction

Modulation of Nitro-fatty Acid Signaling

“…During acute metabolic and inflammatory stress, free nitroalkene tissue levels can increase in murine hearts from undetectable to 9.5 and 17.3 nM for NO 2 -OA and NO 2 -CLA respectively, following an episode of focal cardiac ischemia-reperfusion (11). These concentrations are well within the range of those required for physiological signaling (12,13).…”

Dietary inorganic nitrate reverses features of metabolic syndrome in endothelial nitric oxide synthase-deficient mice

“…Although nitrite is clearly an intermediate in bioactivation of nitrate (10,16,24), the terminal effector may be one of several related bioactive nitrogen oxide species, including NO (10), Snitrosothiols, and nitrated fatty acids (25). In addition to eliciting prototypical cGMP-mediated effects, such as vasodilatation, nitrite (22,26), NO (27), or their reaction products also signal via redox-dependent modification of critical protein thiols.…”

Conjugated Linoleic Acid Is a Preferential Substrate for Fatty Acid Nitration

“…The consideration of the endogenous generation, metabolism, and reactions of electrophilic NO 2 -FA is of relevance because these species potently limit inflammation via multiple mechanisms, including PPAR␥ activation, the inhibition of expression of pro-inflammatory NF-B-regulated genes, and up-regulation of HO-1 and other phase 2 genes regulated by Keap1/Nrf2. Thus, in murine models of metabolic and inflammatory injury, fatty acid nitroalkene administration at nanomolar concentrations prevents restenosis after vessel injury (49), limits weight gain and loss of insulin sensitivity in murine models of metabolic syndrome (14), protects against ischemia-reperfusion injury (17,47,50), reduces plaque formation in a rodent model of atherosclerosis (51), and inhibits the onset of chemically induced inflammatory bowel disease (52).…”

“…In human coronary artery endothelium for example, fatty acid nitroalkenes significantly influence the expression of ϳ400 metabolic and anti-inflammatory-related genes (13). Specific cellular nitroalkylation targets include functionally significant thiol residues in the transcriptional regulatory proteins PPAR␥ (14), Keap1/ Nrf2 (Kelch-like ECH-associated protein 1 (Keap1)/regulator of nuclear factor (erythroid-derived-2)-like 2 (Nrf2)) (15), heat shock factor-1 (HSF-1), and NF-B (4).…”