Dicarbonyl-modified lipoproteins contribute to proteinuric kidney injury.

Lipoprotein modification by reactive dicarbonyls, including isolevuglandin (IsoLG), produces dysfunctional particles. Kidneys participate in lipoprotein metabolism, including tubular uptake. However, the process beyond the proximal tubule is unclear, as is the effect of kidney injury on this pathway. We found that patients and animals with proteinuric injury have increased urinary apolipoprotein AI (apoAI), IsoLG, and IsoLG adduct enrichment of the urinary apoAI fraction compared with other proteins. Proteinuric mice, induced by podocyte-specific injury, showed more tubular absorption of IsoLG-apoAI and increased expression of lipoprotein transporters in proximal tubular cells compared with uninjured animals. Renal lymph reflects composition of the interstitial compartment and showed increased apoAI and IsoLG in proteinuric animals, supporting a tubular cell-interstitium-lymph pathway for renal handling of lipoproteins. IsoLG-modified apoAI was not only a marker of renal injury but also directly damaged renal cells. IsoLG-apoAI increased inflammatory cytokines in cultured tubular epithelial cells (TECs), activated lymphatic endothelial cells (LECs), and caused greater contractility of renal lymphatic vessels than unmodified apoAI. In vivo, inhibition of IsoLG by a dicarbonyl scavenger reduced both albuminuria and urinary apoAI and decreased TEC and LEC injury, lymphangiogenesis, and interstitial fibrosis. Our results indicate that IsoLG-modified apoAI is, to our knowledge, a novel pathogenic mediator and therapeutic target in kidney disease.

Myeloperoxidase-induced modification of HDL by isolevuglandins inhibits paraoxonase-1 activity.

Reduced activity of paraoxonase 1 (PON1), a high-density lipoprotein (HDL)-associated enzyme, has been implicated in the development of atherosclerosis. Post-translational modifications of PON1 may represent important mechanisms leading to reduced PON1 activity. Under atherosclerotic conditions, myeloperoxidase (MPO) is known to associate with HDL. MPO generates the oxidants hypochlorous acid and nitrogen dioxide, which can lead to post-translational modification of PON1, including tyrosine modifications that inhibit PON1 activity. Nitrogen dioxide also drives lipid peroxidation, leading to the formation of reactive lipid dicarbonyls such as malondialdehyde and isolevuglandins, which modify HDL and could inhibit PON1 activity. Because isolevuglandins are more reactive than malondialdehyde, we used in vitro models containing HDL, PON1, and MPO to test the hypothesis that IsoLG formation by MPO and its subsequent modification of HDL contributes to MPO-mediated reductions in PON1 activity. Incubation of MPO with HDL led to modification of HDL proteins, including PON1, by IsoLG. Incubation of HDL with IsoLG reduced PON1 lactonase and antiperoxidation activities. IsoLG modification of recombinant PON1 markedly inhibited its activity, while irreversible IsoLG modification of HDL before adding recombinant PON1 only slightly inhibited the ability of HDL to enhance the catalytic activity of recombinant PON1. Together, these studies support the notion that association of MPO with HDL leads to lower PON1 activity in part via IsoLG-mediated modification of PON1, so that IsoLG modification of PON1 could contribute to increased risk for atherosclerosis, and blocking this modification might prove beneficial to reduce atherosclerosis.

Kidney injury-mediated disruption of intestinal lymphatics involves dicarbonyl-modified lipoproteins.

Kidney disease affects intestinal structure and function. Although intestinal lymphatics are central in absorption and remodeling of dietary and synthesized lipids/lipoproteins, little is known about how kidney injury impacts the intestinal lymphatic network, or lipoproteins transported therein. To study this, we used puromycin aminoglycoside-treated rats and NEP25 transgenic mice to show that proteinuric injury expanded the intestinal lymphatic network, activated lymphatic endothelial cells and increased mesenteric lymph flow. The lymph was found to contain increased levels of cytokines, immune cells, and isolevuglandin (a highly reactive dicarbonyl) and to have a greater output of apolipoprotein AI. Plasma levels of cytokines and isolevuglandin were not changed. However, isolevuglandin was also increased in the ileum of proteinuric animals, and intestinal epithelial cells exposed to myeloperoxidase produced more isolevuglandin. Apolipoprotein AI modified by isolevuglandin directly increased lymphatic vessel contractions, activated lymphatic endothelial cells, and enhanced the secretion of the lymphangiogenic promoter vascular endothelial growth factor-C by macrophages. Inhibition of isolevuglandin synthesis by a carbonyl scavenger reduced intestinal isolevuglandin adduct level and lymphangiogenesis. Thus, our data reveal a novel mediator, isolevuglandin modified apolipoprotein AI, and uncover intestinal lymphatic network structure and activity as a new pathway in the crosstalk between kidney and intestine that may contribute to the adverse impact of kidney disease on other organs.

Mitochondrial Isolevuglandins Contribute to Vascular Oxidative Stress and Mitochondria-Targeted Scavenger of Isolevuglandins Reduces Mitochondrial Dysfunction and Hypertension.

Hypertension remains a major health problem in Western Societies, and blood pressure is poorly controlled in a third of patients despite use of multiple drugs. Mitochondrial dysfunction contributes to hypertension, and mitochondria-targeted agents can potentially improve treatment of hypertension. We have proposed that mitochondrial oxidative stress produces reactive dicarbonyl lipid peroxidation products, isolevuglandins, and that scavenging of mitochondrial isolevuglandins improves vascular function and reduces hypertension. To test this hypothesis, we have studied the accumulation of mitochondrial isolevuglandins-protein adducts in patients with essential hypertension and Ang II (angiotensin II) model of hypertension using mass spectrometry and Western blot analysis. The therapeutic potential of targeting mitochondrial isolevuglandins was tested by the novel mitochondria-targeted isolevuglandin scavenger, mito2HOBA. Mitochondrial isolevuglandins in arterioles from hypertensive patients were 250% greater than in arterioles from normotensive subjects, and ex vivo mito2HOBA treatment of arterioles from hypertensive subjects increased deacetylation of a key mitochondrial antioxidant, SOD2 (superoxide dismutase 2). In human aortic endothelial cells stimulated with Ang II plus TNF (tumor necrosis factor)-α, mito2HOBA reduced mitochondrial superoxide and cardiolipin oxidation, a specific marker of mitochondrial oxidative stress. In Ang II-infused mice, mito2HOBA diminished mitochondrial isolevuglandins-protein adducts, raised Sirt3 (sirtuin 3) mitochondrial deacetylase activity, reduced vascular superoxide, increased endothelial nitric oxide, improved endothelium-dependent relaxation, and attenuated hypertension. Mito2HOBA preserved mitochondrial respiration, protected ATP production, and reduced mitochondrial permeability pore opening in Ang II-infused mice. These data support the role of mitochondrial isolevuglandins in endothelial dysfunction and hypertension. We conclude that scavenging of mitochondrial isolevuglandins may have therapeutic potential in treatment of vascular dysfunction and hypertension.

Scavenging of reactive dicarbonyls with 2-hydroxybenzylamine reduces atherosclerosis in hypercholesterolemic Ldlr-/- mice.

Lipid peroxidation generates reactive dicarbonyls including isolevuglandins (IsoLGs) and malondialdehyde (MDA) that covalently modify proteins. Humans with familial hypercholesterolemia (FH) have increased lipoprotein dicarbonyl adducts and dysfunctional HDL. We investigate the impact of the dicarbonyl scavenger, 2-hydroxybenzylamine (2-HOBA) on HDL function and atherosclerosis in Ldlr-/- mice, a model of FH. Compared to hypercholesterolemic Ldlr-/- mice treated with vehicle or 4-HOBA, a nonreactive analogue, 2-HOBA decreases atherosclerosis by 60% in en face aortas, without changing plasma cholesterol. Ldlr-/- mice treated with 2-HOBA have reduced MDA-LDL and MDA-HDL levels, and their HDL display increased capacity to reduce macrophage cholesterol. Importantly, 2-HOBA reduces the MDA- and IsoLG-lysyl content in atherosclerotic aortas versus 4-HOBA. Furthermore, 2-HOBA reduces inflammation and plaque apoptotic cells and promotes efferocytosis and features of stable plaques. Dicarbonyl scavenging with 2-HOBA has multiple atheroprotective effects in a murine FH model, supporting its potential as a therapeutic approach for atherosclerotic cardiovascular disease.

Pro-inflammatory HDL in women with obesity and nonalcoholic steatohepatitis.

BACKGROUND: Individuals with non-alcoholic fatty liver disease (NAFLD), which includes non-alcoholic steatohepatitis (NASH), are at increased risk for cardiovascular events, independent of traditional risk factors. Limited data on pro-inflammatory high density lipoprotein (HDL) in NASH exists in the literature. We hypothesized that HDL from individuals with NASH would be more pro-inflammatory than HDL from individuals without NASH. METHODS: Study participants were individuals with obesity who had undergone bariatric surgery with wedge liver biopsy. Using HDL isolated from serum obtained from study participants at the time of surgery, HDL-elicited macrophage cytokine expression (TNF-α, IL-1ß, and IL-6) from THP-1 macrophages, HDL-associated receptor expression (ABCA1 and ABCG1) from apolipoprotein E deficient (apo E-/-) mouse peritoneal macrophages, and isolevuglandin (isoLG) modified HDL were measured. RESULTS: 11 women with NASH and 15 women without NASH were included in the study. Both TNF-α (P = 0.032) and IL-1ß (P = 0.029) were significantly more expressed by THP-1 macrophages exposed to HDL from women with NASH compared to women without NASH. ABCA1 and ABCG1 expression by apo E-/- mouse peritoneal macrophages was not significantly different when exposed to HDL from either women with NASH or women without NASH. IsoLG-modified HDL isolated from the serum of women with NASH trended higher than women without NASH. CONCLUSION: Our study suggests a more pro-inflammatory HDL in women with obesity and NASH compared to women with obesity and without NASH.

Modified sites and functional consequences of 4-oxo-2-nonenal adducts in HDL that are elevated in familial hypercholesterolemia.

The lipid aldehyde 4-oxo-2-nonenal (ONE) is a highly reactive protein crosslinker derived from peroxidation of n-6 polyunsaturated fatty acids and generated together with 4-hydroxynonenal (HNE). Lipid peroxidation product-mediated crosslinking of proteins in high-density lipoprotein (HDL) causes HDL dysfunction and contributes to atherogenesis. Although HNE is relatively well-studied, the role of ONE in atherosclerosis and in modifying HDL is unknown. Here, we found that individuals with familial hypercholesterolemia (FH) had significantly higher ONE-ketoamide (lysine) adducts in HDL (54.6 ± 33.8 pmol/mg) than healthy controls (15.3 ± 5.6 pmol/mg). ONE crosslinked apolipoprotein A-I (apoA-I) on HDL at a concentration of > 3 mol ONE per 10 mol apoA-I (0.3 eq), which was 100-fold lower than HNE, but comparable to the potent protein crosslinker isolevuglandin. ONE-modified HDL partially inhibited HDL's ability to protect against lipopolysaccharide (LPS)-induced tumor necrosis factor α (TNFα) and interleukin-1ß (IL-1ß) gene expression in murine macrophages. At 3 eq, ONE dramatically decreased apoA-I exchange from HDL, from ∼46.5 to ∼18.4% (p < 0.001). Surprisingly, ONE modification of HDL or apoA-I did not alter macrophage cholesterol efflux capacity. LC-MS/MS analysis revealed that Lys-12, Lys-23, Lys-96, and Lys-226 in apoA-I are modified by ONE ketoamide adducts. Compared with other dicarbonyl scavengers, pentylpyridoxamine (PPM) most efficaciously blocked ONE-induced protein crosslinking in HDL and also prevented HDL dysfunction in an in vitro model of inflammation. Our findings show that ONE-HDL adducts cause HDL dysfunction and are elevated in individuals with FH who have severe hypercholesterolemia.

Simplified LC/MS assay for the measurement of isolevuglandin protein adducts in plasma and tissue samples.

Isolevuglandins (IsoLGs) are a family of highly reactive 4-ketoaldehydes formed by lipid peroxidation that modify the lysyl residues of cellular proteins. Modification of proteins by IsoLGs have been shown to contribute to disease processes such as the development of hypertension. Accurate quantitation of the extent of protein modification by IsoLGs is essential for understanding the mechanisms whereby these modifications contribute to disease and the efficacy of interventions designed to prevent this modification. The previously described LC/MS assay to quantitate IsoLG protein adducts was extremely labor-intensive and time consuming, and while it offered reasonably low intra-day variation for replicate samples, variation when replicate samples were processed on separate days was significant. These limitations significantly restricted utilization of this approach. We therefore performed a series of studies to optimize the assay. We now report a significantly simplified LC/MS assay for measurement of IsoLG protein adducts with increased sensitivity and lower intra-day and inter-day variability.

Modification by isolevuglandins, highly reactive γ-ketoaldehydes, deleteriously alters high-density lipoprotein structure and function.

Cardiovascular disease risk depends on high-density lipoprotein (HDL) function, not HDL-cholesterol. Isolevuglandins (IsoLGs) are lipid dicarbonyls that react with lysine residues of proteins and phosphatidylethanolamine. IsoLG adducts are elevated in atherosclerosis. The consequences of IsoLG modification of HDL have not been studied. We hypothesized that IsoLG modification of apoA-I deleteriously alters HDL function. We determined the effect of IsoLG on HDL structure-function and whether pentylpyridoxamine (PPM), a dicarbonyl scavenger, can preserve HDL function. IsoLG adducts in HDL derived from patients with familial hypercholesterolemia (n = 10, 233.4 ± 158.3 ng/mg) were found to be significantly higher than in healthy controls (n = 7, 90.1 ± 33.4 pg/mg protein). Further, HDL exposed to myeloperoxidase had elevated IsoLG-lysine adducts (5.7 ng/mg protein) compared with unexposed HDL (0.5 ng/mg protein). Preincubation with PPM reduced IsoLG-lysine adducts by 67%, whereas its inactive analogue pentylpyridoxine did not. The addition of IsoLG produced apoA-I and apoA-II cross-links beginning at 0.3 molar eq of IsoLG/mol of apoA-I (0.3 eq), whereas succinylaldehyde and 4-hydroxynonenal required 10 and 30 eq. IsoLG increased HDL size, generating a subpopulation of 16-23 nm. 1 eq of IsoLG decreased HDL-mediated [3H]cholesterol efflux from macrophages via ABCA1, which corresponded to a decrease in HDL-apoA-I exchange from 47.4% to only 24.8%. This suggests that IsoLG inhibits apoA-I from disassociating from HDL to interact with ABCA1. The addition of 0.3 eq of IsoLG ablated HDL's ability to inhibit LPS-stimulated cytokine expression by macrophages and increased IL-1ß expression by 3.5-fold. The structural-functional effects were partially rescued with PPM scavenging.

Identification of phosphorylation sites in glycine N-methyltransferase from rat liver.

Previous studies have shown that rat glycine N-methyltransferase (GNMT) is phosphorylated in vivo, and could be phosphorylated in vitro on serine residues with a significant increase of enzyme activity, but no phosphorylation sites were identified. In this work the identification of the specific phosphorylation sites of rat GNMT is reported. Three different preparations of rat GNMT were analyzed: (1) purified from liver by standard methods of protein purification, (2) prepared from isolated hepatocytes and from liver tissue by immunoprecipitation, and (3) recombinant protein expressed in Escherichia coli. We measured the molecular weights of protein isoforms using electrospray mass spectrometry and used liquid chromatography-tandem mass spectrometry (LC-MS/MS) of peptides resulting from tryptic and chymotryptic digests. We also performed chemical analysis of phosphoamino acids and protein sequencing. In all samples, the phosphorylated serine residues 71, 182, and 241 were found. In GNMT prepared from liver tissue and hepatocytes an S9 additional residue was found to be phosphorylated. In hepatocytes and in recombinant GNMT S139 was detected. Serine 9 was also identified as a target for cAMP-dependent protein kinase in vitro. The positions of these phosphorylated residues in the tertiary structure of GNMT indicate their possible effect on enzyme conformation and activity.

A second FMN binding site in yeast NADPH-cytochrome P450 reductase suggests a mechanism of electron transfer by diflavin reductases.

NADPH-cytochrome P450 reductase transfers two reducing equivalents derived from a hydride ion of NADPH via FAD and FMN to the large family of microsomal cytochrome P450 monooxygenases in one-electron transfer steps. The mechanism of electron transfer by diflavin reductases remains elusive and controversial. Here, we determined the crystal structure of truncated yeast NADPH-cytochrome P450 reductase, which is functionally active toward its physiological substrate cytochrome P450, and discovered a second FMN binding site at the interface of the connecting and FMN binding domains. The two FMN binding sites have different accessibilities to the bulk solvent and different amino acid environments, suggesting stabilization of different electronic structures of the reduced flavin. Since only one FMN cofactor is required for function, a hypothetical mechanism of electron transfer is discussed that proposes shuttling of a single FMN between these two sites coupled with the transition between two semiquinone forms, neutral (blue) and anionic (red).

A biosynthetic pathway generating 12-hydroxy-5,8,14-eicosatrienoic acid from arachidonic acid is active in mouse skin microsomes.

The epidermis expresses cyclooxygenases, lipoxygenases, and cytochromes P450, which utilize arachidonic acid to generate a diverse array of lipid mediators affecting epidermal cellular differentiation and functions. Recent studies show that mouse epidermis expresses CYP2B19, a keratinocyte-specific epoxygenase that generates 11,12- and 14,15-epoxyeicosatrienoic (EET) acids from arachidonate. We studied CYP2B19-dependent metabolism in mouse epidermal microsomes, reconstituted in the presence of [1-(14)C]arachidonic acid. The majority of the (14)C products formed independently of NADPH, indicative of robust epidermal cyclooxygenase and lipoxygenase activities. We studied two NADPH-dependent products generated in a highly reproducible manner from arachidonate. One of these (product I) coeluted with the CYP2B19 product 14,15-EET on a reversed-phase high-performance liquid chromatography (HPLC) system; there was no evidence for other regioisomeric EET products. Further analyses proved that product I was not an epoxy fatty acid, based on different retention times on a normal-phase HPLC system and failure of product I to undergo hydrolysis in acidic solution. We analyzed purified epidermal (14)C products by liquid chromatography negative electrospray ionization mass spectrometry. Structures of the NADPH-dependent products were confirmed to be 12-oxo-5,8,14-eicosatrienoic acid (I) and 12-hydroxy-5,8,14-eicosatrienoic acid (II). This was the first evidence for a 12-hydroxy-5,8,14-eicosatrienoic acid biosynthetic pathway in mouse epidermis. Epidermal microsomes also generated 12-hydroperoxy, 12-hydroxy, and 12-oxo eicosatetraenoic acids from arachidonate, possible intermediates in the 12-hydroxy-5,8,14-eicosatrienoic acid biosynthetic pathway. These results predict that hydroxyeicosatrienoic acids are synthesized from arachidonate in human epidermis. This would have important implications for human skin diseases given the known pro- and anti-inflammatory activities of stereo- and regioisomeric hydroxyeicosatrienoic acids.

Evidence that cytochrome P450 CYP2B19 is the major source of epoxyeicosatrienoic acids in mouse skin.

CYP2B19 is an arachidonic acid monooxygenase highly expressed in the outer, differentiated cell layers of mouse epidermis. We aimed to establish whether CYP2B19 is the source of epidermal epoxyeicosatrienoic acids (EETs), which are implicated in mechanisms regulating epidermal cornification. We show that primary cultures of mouse epidermal keratinocytes expressed native CYP2B19, as determined by mass spectrometry. Differentiation upregulated CYP2B19 mRNA levels ( approximately 39-fold) detected by real-time PCR, CYP2B19 immunoreactivity detected by Western blotting, and cellular levels of the CYP2B19 product 11,12-EET. Cellular 11,12-EET formed from endogenous arachidonic acid increased preferentially (4- to 12-fold) at Day 4 or 5 of differentiation, compared with undifferentiated (Day 0) keratinocyte cultures. Temporally, these results concur with the maximal levels of CYP2B19 mRNA measured at Day 2 and CYP2B19 immunoreactivity at Day 4. We conclude that while mouse epidermis likely expresses multiple cytochrome P450 enzymes, existing evidence supports native CYP2B19 as being the major source of epidermal EET formation.